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POSTERIOR OCCLUSAL CHANGES WITH A HAWLEY VS PERFECTOR/HAWLEY RETAINER: A FOLLOW-UP STUDY Elizabeth M. Bauer, D.M.D. An Abstract Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2008 Abstract The purpose of this study is to characterize postorthodontic settling of the posterior occlusion by comparing patients wearing Hawley retainers against patients initially wearing Perfector retainers and then switching to Hawley retainers. This follow-up study was based on 40 patients (25 Perfector and 15 Hawley) evaluated at four time points over eight months. They were part of a larger sample of 50 patients randomly assigned to wear either Hawley or Perfector retainers. The Perfector patients were given Haley retainers two months post-debond. Objective and subjective measures were collected at the end of treatment (T1), two months post debond (T2), six months post-debond (T3) and eight months post-debond (T4). Occlusal bite registrations were scanned and traced to quantify posterior areas of contact and near contact (ACNC). A seven-item questionnaire was used to assess the patient’s perception of occlusion. Results showed that ACNC increased during the first six months of retainer wear. The Hawley group showed a 129% increase in ACNC, while the Perfector/Hawley group showed a 104.9% increase in ACNC over eight months of retention. No significant differences in ACNC were found between the Hawley and 1 Perfector/Hawley groups. Both the Hawley and Perfector/Hawley groups showed the greatest rates of increase in ACNC between T1-T2. While the Perfector/Hawley group perceived greater improvements in their occlusion than the Hawley group, group differences after eight months were small and related to the changes that occurred during the first two months of retention. 2 POSTERIOR OCCLUSAL CHANGES WITH A HAWLEY VS PERFECTOR/HAWLEY RETAINER: A FOLLOW-UP STUDY Elizabeth M. Bauer, D.M.D. A Thesis Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2008 COMMITTEE IN CHARGE OF CANDIDACY: Adjunct Professor Peter H. Buschang, Chairperson and Advisor Professor Rolf G. Behrents, Assistant Clinical Professor Donald R. Oliver i DEDICATION I dedicate this project to my husband Ben, who has supported and encouraged me throughout this entire process. ii ACKNOWLEDGEMENTS I would like to acknowledge the following individuals: Dr. Peter Buschang for chairing my thesis committee. Dr. Rolf Behrents and Dr. Donald Oliver for serving on my thesis committee. Dr. Jennifer Horton, Laura Wiley, Celia Giltinan, Daniel Kilfoy, and Dr. Heidi Israel for their assistance with this project. Parkell Bio-Materials for supplying the Blu Mousse impression material used in this experiment. The Orthodontic Education and Research Foundation for contributing to the funding of this project. iii TABLE OF CONTENTS List of Tables...........................................vi List of Figures.........................................vii CHAPTER 1: INTRODUCTION...................................1 CHAPTER 2: REVIEW OF THE LITERATURE Review of Topics..........................................6 Ideal Occlusion...........................................7 Introduction.........................................7 Definitions..........................................9 Methods to Measure Posterior Occlusion.......... ........13 Introduction........................................13 Articulating Indicators.............................14 Photoocclusion......................................16 T-Scan..............................................18 Polyether Impression Material.......................19 Silicone-based Impression Material..................20 Optical Scanning....................................22 Summary.............................................23 Importance of Posterior Occlusion: Function..............24 Importance of Posterior Occlusion: Stability.............27 Immediate Post-orthodontic Occlusion.....................31 Retention................................................34 Introduction........................................34 Significance of Retention...........................35 Retention Devices...................................36 Post-Orthodontic Occlusion and Retention.................40 Introduction........................................40 Past Studies........................................41 Conclusion..........................................55 References...............................................59 CHAPTER 3: JOURNAL ARTICLE Abstract.................................................66 Introduction.............................................67 iv Materials and Methods....................................71 Statistical Analyses................................74 Results..................................................74 Discussion...............................................78 Conclusions..............................................84 Literature Cited.........................................94 Vita Auctoris............................................97 v LIST OF TABLES Table 1: Comparison of areas of contact and near contact (ACNC) of posterior occlusion with Hawley and Perfector groups at T1 (posttreatment), T2, T3 and T4 (approximately eight months later).......................86 Table 2: Comparison of areas of contact and near contact (ACNC) of posterior occlusion with Hawley and Perfector groups from T1-T2, T2T3, T3-T4 and T1-T4.......................87 Table 3: Patient perception of posterior occlusion with Hawley and Perfector groups at T1 (post-treatment),T2, T3 and T4 (approximately eight months later)using a visual analogue scale.....................88 Table 4: Changes in patient perception of occlusion in Hawley and Perfector groups from T1-T2, T2-T3, T3-T4, and T1-T4 using a visual analogue scale............................89 vi LIST OF FIGURES Figure 1: Median cumulative areas of contact and near contact measured in 50µm thickness intervals between T1(end of treatment) and T4 (eight months)for patients initially wearing Hawley retainers.................................90 Figure 2: Median cumulative areas of contact and near contact measured at 50µm thickness intervals between T1 (end of treatment) and T4 (eight months) for patients initially wearing Perfector retainers.......................91 Figure 3: Median absolute changes in areas of contact and near contact at 50µm thickness intervals between T1 (end of treatment) and T4 (eight months) for patients initially wearing Hawley and Perfector retainers............92 Figure 4: Median cumulative changes in areas of contact and near contact at 50µm thickness intervals between T1 (end of treatment) and T4 (eight months) for patients initially wearing Hawley and Perfector retainers....93 vii CHAPTER 1: INTRODUCTION The goal of orthodontics is an esthetic result that establishes proper occlusion so as to maximize masticatory health, function and stability. Through active orthodontic treatment, the teeth are brought into proper occlusion. Once the appliances are removed, the teeth are held in the attained position with some type of device during the retention phase of treatment. Although active tooth movement is not usually occurring during this phase, there may be slight or even large changes in the position of the teeth. Some of these changes, such as settling of the occlusion, are desirable, while other changes, such as rotational relapse, are not. Since post-orthodontic changes are anticipated, it is an accepted procedure for the orthodontist to prescribe a retainer and continue observation until the teeth have settled into their final position. Most orthodontists develop a “retention protocol” based on their own beliefs concerning posttreatment changes and how long these changes may be expected to occur. The Hawley retainer and the Perfector ® , which is a modified version of the Tooth Positioner, are two ® TP Orthodontics, Inc. 1 appliances used in orthodontics during the retention phase of treatment. Several retention studies have been conducted with the intention of clarifying the advantages of various retention devices; however, most of them have not objectively evaluated the effects of the Hawley and Perfector ® teeth. retainers on the occlusion of the posterior Furthermore, it is unclear which retention method is more effective in encouraging post-orthodontic settling. It is important for orthodontists to know the effectiveness of different retention designs, the nature of postorthodontic changes that occur, and how long these changes are expected to continue after active treatment in order to prescribe the optimal retention protocol. A previous study by Horton et al.1 evaluated changes in the posterior occlusion over two months of retention in an attempt to determine if there were quantitative or qualitative differences in the posterior occlusion in cases where the Perfector ® and Hawley retainer were used. No differences were found between the two groups after two months of retention. Although the differences between the Hawley and Perfector ® appliance after two months has been examined by Horton et al.1, it has not yet been determined ® TP Orthodontics, Inc. 2 what changes may occur thereafter in the Perfector ® group, especially after the occlusal coverage of the Perfector ® appliance is removed. Furthermore, it is currently not clear what happens after Perfector ® use is discontinued and retention is continued with a Hawley retainer. During the time the Perfector ® is worn, the patient is instructed to perform chewing exercises for several hours a day and then sleep with the Perfector ® at night. This process is supposed to align the teeth into a more ideal position, which in theory, should result in increased occlusal contacts. However, the Perfector ® by design has occlusal coverage, which may inhibit actual settling. There has not been a study that examines what happens to the amount of settling after the Perfector ® patient is given a Hawley retainer. The teeth, in theory have been moved into a more ideal position. By removing the occlusal coverage of the Perfector ® , it is possible that the teeth may then be able to settle, and actually produce increased occlusal contacts months later. ® There has not yet been a study that compares TP Orthodontics, Inc. 3 the Hawley and Perfector ® group’s months later, after both groups are given identical Hawley retainers. There is a possibility that although there were no apparent differences initially, the Perfector ® group may provide better occlusion in the long run. Knowing the nature of occlusal changes over time and the qualitative and quantitative differences between the Hawley and Perfector ® retainers will assist the orthodontist in making an informed decision when prescribing a retention protocol. The current study is a follow-up study to Horton et al.1 While the same sample of patients will be used, each patient will be given identical Hawley retainers at the beginning of the current follow-up study (2 months postdebond). Identical materials and methods as Horton et al.1 will be used to maintain continuity of the data. The goal of this follow-up study is to further investigate whether and how the retention device used after active orthodontic treatment affects posterior occlusion after two different retention groups (Perfector ® and Hawley) are given identical Hawley retainers at two months and again assessed six and eight months post-debond. ® TP Orthodontics, Inc. 4 Occlusal bite registrations of silicone-based impression material will be analyzed using optical scanning and Image Tool® software to assess 1) area of occlusal contacts (actual and near contacts) at 6 and 8 months post-debond, 2) differences in settling between the Hawley and Perfector ® groups after both retention groups are given identical retainers, 3) quantify the rates of post-orthodontic settling to determine the nature of the changes that occur over time and 4) patient perception of occlusion. It is hypothesized that post-orthodontic settling will continue to increase over time; however, the nature of these changes may differ between the two retention groups. It is expected that both the Hawley and Perfector ® patients will both show increased occlusal contacts over time, with the Perfector ® patients showing more overall settling later in the post-orthodontic phase. ® TP Orthodontics, Inc. 5 CHAPTER 2: REVIEW OF THE LITERATURE Review of Topics This review of the literature will begin by discussing the importance of ideal occlusion, as this concept has been a cornerstone of orthodontics since the specialty was founded. To thoroughly understand this topic, the definition of ideal occlusion will be reviewed along with a description of anterior and posterior occlusion. While anterior occlusion is often the focus in orthodontics, this study will focus on posterior occlusion for two reasons: 1) the important role of posterior occlusion in overall masticatory health, function and stability and 2) the lack of orthodontic literature pertaining to post-orthodontic posterior occlusion. Next, the review will review the different methods that have been used to measure posterior occlusion. These concepts are fundamental for understanding the research methods used in the present study and will also aid in illustrating the drawbacks and benefits of previously used methods. This section will illustrate the importance of using a more objective and quantifiable method of measuring posterior occlusion. The review will then describe the importance of occlusion as it 6 relates to function and stability. It will then explore the condition of the occlusion immediately following orthodontic treatment. This is important because it is useful to know how these discrepancies will change over time, and how long these changes take place. sections will focus on retention. The final two First, a brief history of retention will be provided, followed by descriptions of various retention methods. Finally, there will be a review of retention studies evaluating post-orthodontic posterior occlusion. It will be shown that we currently do not have a clear understanding of post-orthodontic occlusal changes produced by Hawley and Perfector retainers. Ideal Occlusion Introduction Throughout the history of orthodontics the importance of occlusion has been emphasized and well documented. The definition of ideal occlusion has been a topic of debate among dental professionals for several years and still remains an issue today. The concept of ideal occlusion has changed over the years and has been influenced by many theories. In 1900, Angle published Treatment of Malocclusion of the Teeth and Fractures of the Maxillae, 7 edition 6, which greatly assisted dentists in their diagnosis and treatment of various malocclusions. In this textbook, Angle described the antero-posterior relationship of the maxillary and mandibular molars and established the Angle classification system that is still widely used today among orthodontists.2 Another innovator in orthodontics, Charles Tweed, published the article titled “Indications for the extraction of teeth in orthodontic procedures” in 1944, which suggested that the stability of orthodontic treatment was influenced by the attainment of good occlusal contacts.3 In 1972, Andrews introduced the six keys to normal occlusion.4 These ideas further emphasized the importance of ideal occlusion and aided in the development of the straight-wire appliance that many orthodontists use today. The concept of ideal occlusion is multifactorial, with occlusal contacts being one of the major contributors. Through thorough evaluation of the various aspects of occlusion, Okeson5 describes ideal occlusion as: 1)the presence of even and simultaneous contacts of all teeth when the mandibular condyles are in their most superoanterior position resting on the posterior slopes of the articular eminences with the discs properly interposed, the anterior teeth should contact but more lightly than the posterior teeth 2) each tooth should contact so that occlusal forces are directed through the long axis of the tooth, 3) the presence of canine guidance in laterotrusive movements, 4) the presence of 8 anterior guidance in protrusive movements and 5) heavier posterior than anterior contacts in the alert feeding position. These concepts of ideal occlusion have played an important role in orthodontics since the specialty was established and are still valued and strived for by orthodontists today. It was recently stated in a modern orthodontic textbook that the “operational goal” of orthodontics is to obtain optimal occlusion.6 On this basis, it is important that future research continues to focus on developing methods for identifying, describing and better understanding occlusion. Definitions Occlusion can be divided into two types: anterior and posterior. Anterior occlusion involves the incisors and canines and exists when the incisal edges of the mandibular incisors contact the lingual surfaces of the maxillary incisors.5 The maxillary and mandibular incisors act to guide the mandible into protrusive movements while the maxillary and mandibular canines guide the mandible during excursive movements, providing anterior guidance.5 In comparison to posterior occlusion, the contacts on the anterior teeth are much lighter, and sometimes may not even 9 be present.5 Ideal occlusion has often been described based on the complete absence of anterior occlusal contact.7-9 The anterior teeth, mainly the incisors, are inclined labially, as they are not designed to withstand heavy occlusal forces. The posterior teeth consist of maxillary and mandibular premolars and molars, and have a much different function than the anterior teeth. The posterior occlusion, as described by Okeson,5 pertains to the relationships between cusp tips and their opposing central fossae and marginal ridges. The posterior teeth establish and maintain the vertical dimension of occlusion and are designed to withstand the heavy forces of mastication.5 The posterior occlusion has been closely related to masticatory function and occlusal stability. As previously stated, Angle developed a classification system using the antero-posterior relationship of the maxillary and mandibular first molars to define occlusion. According to Angle, the mesiobuccal cusp of the maxillary first molar should occlude with the buccal groove of the mandibular first molar. This was classified as a “Class I” molar relationship. If the maxillary and mandibular first molars have a normal relationship, but the teeth are crowded or rotated, a “Class I malocclusion” exists. A “Class II” relationship exists if the lower first molar is 10 positioned distally in relation to the maxillary first molar. Finally, if the mandibular first molar is positioned mesially in relation to the maxillary first molar, a “Class III” relationship exists.2 The American Board of Orthodontics has implemented a grading system to evaluate the quality of finished orthodontic cases. The objective grading system (OGS) was implemented in 1999 in order to provide a fair and objective means of evaluating occlusion for the ABO phase III examination.10 In the ABO OGS, seven occlusal criteria are measured on plaster models to assess the patient’s final occlusion. The criteria included tooth alignment, vertical positioning of marginal ridges, buccolingual inclination of posterior teeth, occlusal relationship, occlusal contacts, overjet, and interproximal contacts.10 The eighth criterion is root angulation, which is measured using a panoramic film.10,11 Points are assigned for each of the eight criteria. A case will pass if fewer than 20 points are deducted and fail if more than 30 points are deducted.10,11 On this basis, occlusion is a major component in determining the acceptability of a case for the ABO phase III examination as well as diagnosis, treatment planning and overall case management.10 11 Depending on the extent of anterior contact and whether the cusp tips contact one or two opposing marginal ridges, the total number of ideal occlusal contacts may vary. ideal occlusions, if a cusp tip contacts two In opposing marginal ridges, two contacts on the cusp tip will result, but if the cusp tip only contacts one opposing ridge only one contact on the cup tip will result.5 Ricketts12 reported that an ideal occlusion will produce 48 contacts, while Okeson5 reported ideal occlusions as having between 34 and 48 contacts, with each cusp tip contacting either one or two marginal ridges. Haydar et al.13 reported 40.50 +/- 9.68 contacts in patients with ideal occlusion. Maximum intercuspation occurs when the maxillary and mandibular teeth are brought together into occlusion. The posterior occlusion is often evaluated by having the patient bite into maximum intercuspation, which has been shown to be a repeatable position.14 Occlusal contacts of maxillary and mandibular teeth during function is referred to as functional occlusion, which is important during mastication, swallowing and speech.15 12 Methods to Measure Posterior Occlusion Introduction Orthodontists commonly evaluate posterior occlusion using Angle’s classification, which helps to determine the relationship between the upper and lower first molars in the saggital plane. While this method is helpful in evaluating how well the maxillary and mandibular teeth relate to each other, it does not quantify the amount of occlusal contact, which can only be assessed in the vertical plane between the maxillary and mandibular teeth. Occlusal contacts, which can vary in size and number, provide a measure of how forces may dissipate during function. This also cannot be measured in the saggital plane. Studies measuring posterior occlusion have used various methods of evaluating tooth contact including photoocclusion, the T-scan method, articulating indicators (paper, film or silk), and polyether or silicone bite registrations. Despite the information these methods can provide, the accuracy and reliability of some of these methods remain questionable. The following section will explore the various methods of measuring posterior occlusion along with the problems and benefits associated 13 with each. The section will conclude by describing optical scanning of silicon-based impression materials, which is a newer and more reliable method of measuring posterior occlusal contacts. Articulating Indicators Articulating indicators are routinely used in dentistry to register occlusal contacts and to assess the patient’s occlusion. They are made of various materials such as paper, nylon or silk that transfer color from the articulating indicator to the teeth marking occlusal contacts.16 Articulating indicators are also fabricated with different surface characteristics (smooth vs. coarse) and thicknesses which might affect the marking ability of the indicator.1 An advantage of using articulating indicators is that they are very easy to use. After the indicator is placed over the teeth the patient is instructed to bite together onto the articulating indicator. This action transfers color to the areas of occlusal contact. After the markings are registered, the number and location of occlusal contacts can be evaluated. While articulating indicators allow for gross assessment of contact size and location, it has been reported that this method is less reliable than other methods because of 14 variations in film thickness, the character of the ink, the surface texture of the indicators, and the condition of the occlusal surfaces in contact.17,18 Carossa et al.19 examined the effect of articulating strip thickness on the number of posterior contacts. Two different thicknesses (8μm and 40 µm) of articulating indicators were used to register the contacts. The force and time of biting were also recorded. Operator-dependent factors were evaluated by using thirteen dental students and thirteen expert dental professionals to register and record the data. It was found that when using the thinner articulating strips, dental students applied stronger and shorter bites than experts. Furthermore, the biting time impacted the number of occlusal markings when the thinner strips were used. However, the thicker articulating strips produced greater contact areas, regardless of the operator. Overall, the results showed that when using articulating indicators to register occlusal contacts, paper thickness, operator experience, biting force, and time all have an effect on the occlusal markings. Similar results were reported by Millstein and Maya,16 who compared the effects of articulating indicators consisting of different materials (paper, film, silk and nylon), colors and thicknesses on occlusal markings. Images were recorded using articulator-mounted casts and a 15 video camera. It was found that the size of occlusal markings was significantly affected by the color, thickness and material of the articulating indicator. Based on ten indicators, it was demonstrated that the markings were not repeatable, suggesting that the use of articulating indicators may not be the most dependable method to record and measure posterior contacts. Photoocclusion The photoocclusion technique was introduced in 1985 by Gazit and Lieberman as a method of measuring occlusal contacts using an occlusal wafer and a polariscope, which measures the polarization of light.1 Using the photocclusion method, subjects are instructed to bite onto an occlusal wafer with continuous pressure for 10 seconds. After two consecutive bite registrations are taken, the occlusal wafer is projected onto the polariscope and analyzed.18 Each contact that appears on the polariscope is traced and recorded. This information serves as a quantitative analysis. The occlusal contacts can also be evaluated qualitatively by interpretation of the colored stress patterns, which are dependent on the degree of occlusal penetration into the occlusal wafer. 16 These patterns can then be translated into percentages using a visual interpretation chart.18 Gazit and Lieberman considered light contact to be up to 40% penetration into the wafer, medium between 40% and 60% penetration into the wafer, and heavy contact as over 60% penetration into the occlusal wafer.18 However, in 1986, Gazit, Lieberman and Fitzig20 conducted a follow-up study in which they examined the reproducibility and reliability of the photocclusion technique. Results of their study indicated that the photocclusion technique was a fairly inaccurate method of evaluating occlusal contacts. The photocclusion technique was also compared to articulating indicators in recording occlusal contacts. Using both the photoocclusion technique and the articulating indicator technique, occlusal records were taken on 11 male dental students with Class I occlusion at the beginning of the study and again one month later. It was found that neither the photoocclusion nor the articulating indicator technique were highly reproducible. However, the photocclusion technique was more reliable than the articulating indicator technique.20 17 T-Scan The T-Scan, a computerized occlusal analysis device, was introduced in 1987 by Maness et al.21 The T-Scan is a computerized system for recording occlusal contacts and the force of occlusal contact.21 This system consists of a 60 μm thick piezoelectric foil sensor made from conductive ink on a polyester film substrate, a sensor handle, hardware and software for recording, viewing and analyzing the data.22 film. A plastic intra-oral carrier is used to hold the The sensor is connected to a computer, which provides information regarding position, strength, frequency and timing of occlusal contacts.23 Information pertaining to bite force can be measured using the T-scan system software. This information can be measured as either a force movie, which is a 3-second recording of force, or a force snap shot, which is an immediate measurement.22 The sensitivity and reliability of the T-Scan system for occlusal analysis was tested by Hsu et al.23 The sensitivity threshold of the sensor was analyzed and the sensor’s reliability in recording occlusal contacts was measured in an articulator using reproducible forces. It was determined that the T-Scan sensor did not have a uniform sensitivity throughout its surface. 18 Furthermore, the number of registered occlusal contacts varied from sensor to sensor, within the same sensor and between trials.23 Finally, it was found that the T-Scan continually recorded fewer occlusal contacts than actual existed;23 Similar results were reported by Lyons et al.22 After examining the sensitivity and reliability of the T-Scan method, several authors have concluded that the T-Scan system is an inconsistent method for recording occlusal contacts.22,23 Polyether Impression Material Impression materials such as Polyether have been widely used to register posterior occlusion in maximum intercuspation. For instance, Durbin and Sadowsky24 measured the posterior occlusion using polyether impression material to fabricate occlusal bite registrations. Polyether impression material was injected onto the occlusal surfaces of the mandibular teeth and patients were instructed to “bite firmly on your back teeth and hold the position” for two minutes.24 Using the polyether bite registrations, perforations in the material were transferred to study models with a pencil. These markings were counted to determine the number of contacts. In a follow-up to Durbin and Sadowsky’s study, Razdolsky et al.25 also measured posterior occlusion by injecting 19 polyether impression material onto the mandibular occlusal surfaces. As in the previous study, the patients were instructed to “bite firmly on their back teeth and hold the position” for two minutes.25 In this follow-up study, near contacts were evaluated as well as actual contacts. However, the method used to identify near contacts was very subjective; they were identified as a “change in color from yellow to transparent white.”25 Near contacts were transferred to the study models with a black marker and counted. The actual contacts, defined as “perforations” in the polyether bite registration, were evaluated using a radiographic viewing screen in a dark room and transferred to study models with a red marker. Silicone-based Impression Material Several studies have used silicone-based impression materials to assess occlusal contacts. For example, using a silicone putty impression material, Dincer et al.26 counted the number of contacts and near contacts by evaluating perforations and translucent areas in the silicone material. Actual contacts were classified as perforations and near contacts were identified as translucencies. Using the bite registrations, the contacts 20 and near contacts were then transferred onto study models for further evaluation. Sauget et al.27 also fabricated occlusal bite registrations using polysiloxane impression material to register and assess occlusal contacts. They instructed subjects to “bite firmly” in maximum intercuspation after injecting the material over the occlusal surfaces of the mandibular teeth. Similar to the previous study, perforations in the bite registration were identified as actual contacts. However, a caliper was used to measure near contacts, which consisted of a material thickness of 0.20 mm or less. Haydar et al.13 also evaluated occlusal contacts using silicone-based impression material. As in previous studies, the patients were instructed to bite “firmly” onto their back teeth for 30 seconds after the impression material was injected onto the mandibular occlusal surfaces. To evaluate occlusal contacts, the bite registrations were simply held up to the light. Perforations in the material that allowed light to pass through were counted as contacts, while the thin/transparent areas without perforations were classified as near contacts. The actual contacts were transferred to the maxillary cast; the near contacts were recorded by 21 marking their midpoints on the cast. Occlusal photographs of the models were then taken, the photographs were standardized and the contacts were traced on acetate paper. Finally, all contacts were counted and combined for the evaluations. Optical Scanning Several recent studies have abandoned counts of occlusal contacts and have used optical scanning to quantify areas of contacts.17,28,29 Optical scanning of occlusal bite registrations provides a more objective method of evaluating contacts and near contacts. A silicone-based material, called Blu Mousse is applied to the mandibular posterior teeth and, as in previous studies, the subjects were instructed to bite down “firmly” and hold in order to fabricate an occlusal bite registration. The occlusal registrations are then placed onto a scanner with the mandibular occlusal surface facing downward. After the bite registration has been scanned, the platform area of the mandibular first molar and premolars is traced using the software program Image Tool, (University of Texas Health Science Center, San Antonio) which computes the platform area and determines the frequency distributions of pixels delimited by the platform area in gray scales. 22 A calibration curve, which relates known thicknesses of Blu Mousse to a gray scale, is developed using calibration wedges of Blu Mousse impression material. Using this curve, the investigators were able to determine the thickness of sample registrations based on gray scale values of the area scanned. Therefore, instead of subjectively classifying contacts and near contacts, they were quantified “optically” on the basis of light transmitting through the registration material and thickness was measured up to 350μm.28,29 When scanning silicone-based impression materials, it has been shown that pixel densities are only detectible in increments of 50μm.30 In other words, a thickness of 40μm can be distinguished from 90μm and a thickness of 90μm can be distinguished from 140μm, but a thickness of 90μm cannot be distinguished from 100μm. Summary Dental research has used several different methods for measuring the posterior occlusion and areas of contact and near contact, including articulating paper or film, silk, the photo-occlusion technique, visual assessment and the TScan method. Although these techniques have provided 23 valuable information, they are not as accurate as more recently developed methods of evaluating occlusal contacts. Studies evaluating occlusion and occlusal contacts often use silicone-based impression materials because they provide more accurate registrations.31 To date, most studies have used counts to evaluate the bite registrations. Optical scanning of occlusal registrations provides a more objective and precise alternative for measuring occlusal contact areas.1 Importance of Posterior Occlusion: Function Achieving an ideal posterior occlusion is a very important factor in orthodontic treatment because it directly affects masticatory function. Occlusal contact area has been shown to be among the most important factors determining masticatory performance.32 Although the occlusal contact area is only a fraction of the total occlusal surface area, it represents the most important fraction of the total area involved in mastication.32 Yurkstas et al.32 found that the occlusal contact area was the major factor in accounting for variation in masticatory performance. They also showed that masticatory performance 24 was related to the occlusal contact area in a curvilinear manner. Because the correlation between occlusal contact area and masticatory performance is highly significant, performance can be predicted with a relative degree of certainty.32 It has also been shown that the contact and near contact areas form the actual food platform and that there is a high correlation between this small area and chewing performances.33 Toro et al.34 and English et al.35 demonstrated that subjects with normal occlusion are able to break down foods more efficiently than those with malocclusion. When comparing the masticatory efficiency and ability of subjects with normal occlusion to patients with Class II malocclusions, Henrikson et al.36 found that subjects with normal occlusion presented significantly better masticatory efficiency and ability in comparison to the Class II subjects. It was concluded that fewer occlusal contacts and a larger overjet were the factors most closely associated with reduced masticatory efficiency. Owens et al.29 found that patients with malocclusions had smaller areas of contact and near contact than patients with normal occlusions and that the patients with smaller areas of contact and near contact were less efficient in their ability to break down food during mastication. Near contacts are important because they 25 provide a measure of near occlusion, which may be a more important factor in masticatory performance than actual contacts.29 During mastication, near contacts may become contacts with flexure of the mandible and movements of the teeth within the PDL space. Omar et al.37 found a significant correlation between masticatory efficiency scores and occlusal analysis scores, showing that deviations from normal occlusion affect masticatory efficiency. When investigating the effects of age, posterior occlusal contacts, occlusal force, and salivary flow on masticatory performance in 328 adults, Ikebe et al.38 found that masticatory performance was significantly associated with posterior occlusal contacts, occlusal force and salivation. It was concluded that decreases in posterior occlusal contacts and occlusal force were associated with reduction of masticatory performance. Mastication involves heavy forces that sometimes exceed 100 pounds and pressures that have been estimated at over 10,000 pounds per square inch.32 Such forces emphasize the importance of ideal occlusion. It is important that these forces are vertically directed down the long axis of the teeth. This is why both contacts and near contacts are important during function; near contacts maximize the total contact area. Maximizing tooth contacts minimizes the 26 stresses distributed among the teeth and ideally located contacts help to distribute forces of mastication parallel to the long axis of the dentition providing maximum periodontal support.26 Therefore, the location of occlusal contacts and increased number and size of occlusal contacts are important. A strong correlation between bite force and occlusal contacts in adults was found by Bakke et al.39 A positive correlation between bite force and the number of interocclusal contacts was also reported by Ingervall and Minder.40 Julien et al.28 demonstrated that masticatory performance in adults was affected by posterior ramus height, contact area, and bite force, and that those with greater contact areas had better masticatory performance. Collectively, these studies emphasize the importance of ideal occlusion as it relates to occlusal function and the role of occlusal contacts in masticatory performance and efficiency. Importance of Posterior Occlusion: Stability Maintaining post treatment alignment has been a goal of orthodontics since the specialty came into being. been hypothesized It has that post treatment crowding is not 27 necessarily due to relapse, but to a phenomenon known as instability.41 Relapse occurs when the teeth move back toward their original position after orthodontic treatment is finished. Driscoll-Gilliland et al.41 attribute instability to vertical growth of the craniofacial complex, and the consequent eruption and migration of teeth necessary to reestablish occlusion. For instance, as the patient grows there is vertical eruption of the teeth which in turn, changes the relationship between the teeth. Therefore, the occlusion is less stable, and as a result, the relationships between adjacent and opposing teeth change and crowding results.35 It has been suggested that good intercuspation and occlusal contacts may be essential for stable orthodontic results. Parkinson et al.17 assessed the relationship between posterior occlusion and post treatment changes by examining pretreatment, post treatment and post retention records of 49 Class I and Class II extraction cases. Overbite, overjet, mandibular incisor irregularity, right and left molar deviations, midline deviation, and mandibular arch length were measured and occlusal registrations were made of each set of dental casts. It was concluded that post treatment contact and near contact areas may be factors in overbite and overjet stability. 28 Deng and Fu42 examined occlusal contacts before and after orthodontic treatment in 32 eight to seventeen year old patients by utilizing silicone occlusal bite registrations. The registrations were analyzed using a computer imaging system called Add-Picture. After comparing the images before and after treatment, significant differences were noted in the number, type, location and size of contacts. Results indicated that orthodontic treatment has a positive effect on the distribution of occlusal contacts in intercuspal position. They interpret the findings to suggest that treatment contributes to the establishment of occlusal stability and that the location of contacts in intercuspation is one of the main factors responsible for maintaining correct alignment of the teeth and stabilization of the mandible. It was also noted that failure to provide adequate centric stops may cause occlusal instability, resulting in a shifting of the teeth as well as disturbances in the muscles and joints. Dincer et al.26 hypothesized that one of the most important factors in occlusal stability is the existence of centric stops. Centric stops on functional cusps were thought to ensure the stability of orthodontic treatment. Upper and lower removable Hawley retention appliances were 29 given to 20 treated orthodontic patients and their occlusal contact points were determined from occlusal registrations taken at the beginning and end of 9 months of retention. Subjects were compared to 20 untreated controls with ideal occlusion. Results showed a significant increase in the number of posterior contacts and a significant increase in the number of contacts in ideal locations during the 9 month retention period. The authors suggested that these results provide important signs of occlusal stability. It was concluded that in order to maintain the occlusal stability that is needed for the success of orthodontic treatments, ideal occlusal contacts and localization of contacts in centric and eccentric occlusion should be considered. Conversely, Ormiston et al.43 suggest that finishing occlusal relationships to perfection might not ensure postretention stability. When evaluating ABO and post-treatment PAR scores for stable and unstable groups, the analysis of the ABO scores at T2 demonstrated a regression to the mean, in which subjects with low T2 scores tended to increase their score (deteriorate) at T3, and those with high T2 scores tended to decrease their score (improve) at T3. Although the post treatment PAR, incisor irregularity and ABO scores were all lower for the stable group, the 30 differences were not large. These results show that both the stable and unstable groups were finished to comparable levels and that the post treatment condition was not the most important or influential factor in long-term stability. If this is true, other factors might play more important roles in stability, such as treatment and retention methods, compliance, the tendency to return to pre-treatment condition, and growth. Only a small number of studies have addressed the important contributions of occlusal contacts to maintaining functional and structural stability. Although it is commonly accepted by orthodontists that ideal occlusion enhances long-term stability, there is little scientific support of the relationship between occlusion and stability. Furthermore, it is a possibility that the studies that show increased stability in untreated subjects may not have finished the treated patients to an ideal occlusion. Immediate Post-orthodontic Occlusion Based on the premise that ideal occlusion is necessary for optimal function and stability, it is important to know 31 the quality of occlusion after active orthodontic treatment. Using the photocclusion technique on 12 orthodontically treated patients, Gazit and Lieberman18 showed widely varying numbers of occlusal contacts. However, the study casts showed close to ideal buccolingual and mesiodistal relationships for all the cases. This study illustrates a common mistake orthodontists may make; a patients’ occlusion may look satisfactory upon clinical observation, but when closely examined, the occlusion may not be as ideal as it appears. After measuring 138 post-treatment models of Class I non-extraction patients using the ABO OGS criteria (interproximal contacts and root angulations were excluded), Fleming et al.44 reported a mean overall OGS score of 24.9 +/- 8.0. The highest point deduction (6.25 +/- 3.75) from the model analysis was due to occlusal contacts. The lowest point deduction was associated with occlusal relationships (1.74 +/- 1.83). Overall, occlusal contacts contributed to approximately 26% of the total deductions. Approximately 56.2% of the variation in the total score was due to the occlusal contact component.44 This illustrates that occlusal contacts were the most important component contributing to and explaining 32 variability in the overall OGS scores in the study. Previous studies have also shown occlusal contacts to be important determinants of overall post-treatment OGS scores.45-48 When comparing treatment results between orthodontists and general dentists, Abei et al.45 found that neither group consistently produced the level of quality required to pass the ABO phase III examination. According to ABO standards, when using the OGS method of grading, patients who score more than 30 points are likely to fail the ABO phase III examination. When comparing treatment outcomes between two orthodontic programs, Deguchi et al.46 found that only 45.1% of finished patients from Okayama University (OU) and 46.6% of patients from Indiana University (IU) had OGS scores of less than 30. These results show that fewer than half of the patients had passable immediate post-treatment results according to ABO examination criteria. After examining 115 consecutively finished cases obtained from nine Board certified orthodontists, Cook49 found that according to ABO OGS standards, 18% of the cases passed, 47% were undetermined and 35% failed. In that study, alignment and root angulation contributed most to the overall score. Interestingly, Parkinson et al.17 found that despite excellent treatment results, the proximity of posterior 33 occlusal surfaces lessened during orthodontic treatment. Haydar et al.13 reported that the total mean number of contacts at the end of active treatment was 21.20 for the Hawley group and 24.80 for the Perfector group, which were much less than the 39.4 contacts of the untreated group. Assuming that occlusion is not ideal immediately posttreatment, it is important to know how certain aspects of occlusion will change over the long term. Existing studies suggest that little attention has been given to this aspect of finishing. This emphasizes the importance of post- orthodontic settling. It leads to the important question of how much settling can we expect and rely on over time. Retention Introduction The retention phase is a very important and challenging part of orthodontic treatment. Retention devices are typically given to patients following orthodontic treatment to prevent relapse, promote further “settling” of the occlusion and to make minor corrections in the dentition.50 Different types of retainers can be used to accomplish these goals including removable, fixed, passive and active 34 retention devices, all of which will be discussed in the following sections. Significance of Retention Several factors contribute to orthodontic relapse. After active appliances are removed, the teeth must be retained in their new positions long enough for the periodontal and gingival fibers to reorganize. For example, by studying the periodontal tissues of dogs with orthodontically rotated teeth, Reitan51 found that the connective tissue fibers in the supracrestal periodontal tissues continued to persist after 232 days. Furthermore, it has been shown that tissue changes and post treatment tooth position contribute to relapse after active orthodontic treatment.51 Bone also needs time for reorganization and maturation. It has been shown that after the teeth have been moved through the bone they have a strong tendency to relapse.52 For this reason it is important to hold the teeth in their new position to enhance future stability. Another important factor in retention is growth. It has been shown that certain growth patterns can contribute to a patient’s malocclusion.53 Growth has been shown to continue into adulthood, which could affect a patients occlusion after orthodontic treatment is finished.53 35 For instance, it has been shown that post-treatment relapse may be affected by continued Class II, Class III, open bite or deep bite growth patterns.54 Furthermore, it has been demonstrated in both treated and untreated subjects that there is a significant amount of growth in lower facial height even after adolescence, which may contribute to lower incisor irregularity in late adolescents through middle adulthood.41 As such, retention is required to ensure that growth does not alter the final orthodontic result. Retention Devices The Hawley retainer is one of the most commonly used retainers today. It consists of a stainless steel labial bow with adjustment loops at the canines.55 Various retentive clasps may be utilized in the molar and premolar regions to enhance retention of the device, which is especially important if the Hawley retainer is used to actively move a tooth, such as a rotated incisor. The wire extensions from the labial bow and accessory clasps are embedded in acrylic, which covers the patient’s palate and further aid in maintenance of the retainer. Hawley retainers allow for occlusal contact between the maxillary and mandibular teeth because they do not cover the occlusal 36 surfaces of the teeth. Following removal of fixed appliances, patients are typically instructed to wear removable Hawley retainers full-time for the first few months, and at nighttime only thereafter.56 Another type of removable retainer, the wrap-around retainer, consists of a circumferential labial bow with full coverage acrylic over the palate.57 The design of the wrap-around retainer is very similar to the Hawley retainer; however the clasps and labial bow do not cross over the occlusion, which further enhances interocclusal contact between the maxillary and mandibular teeth. The clear overlay retainer, in contrast to the Hawley and wrap-around retainers, provides full occlusal coverage of the maxillary and mandibular teeth. It is fabricated by vacuum forming a heated plastic sheet over a stone model.58 It is recommended that patients wear the clear overlay retainer full time for a few months, followed by nighttime only wear.56 When long term retention is needed in cases with expected instability, such as severely rotated incisors or large diastemas, fixed retention is often indicated. Fixed retainers are usually fabricated from 0.030 round steel wire which is adapted to the lingual surfaces of the teeth from canine to canine. The wire is then either bonded to 37 the lingual surfaces of the canines and rests passively against the incisors, or is bonded to each tooth from canine to canine. Another commonly used appliance is the tooth positioner. Unlike the previously described retention devices which hold the teeth in their debanded position, the positioner is used to move the teeth after the orthodontic appliances are removed. This appliance was developed by Kesling, who believes that by using “functional forces,” which are generated by chewing exercises, the teeth can be successfully repositioned into a more ideal occlusion.59 This can only be accomplished if the appliance is properly worn by the patient and the prescribed clenching exercises are performed.60 The positioner is fabricated from a model in which the teeth have been dissected and reset in wax with improved positions, arch form, axial inclinations, and occlusion.60 The tooth positioner is then constructed from the reset models as a single rubber appliance that fits over the maxillary and mandibular teeth. The patient is instructed to perform clenching exercises into the appliance and, according to Kesling,60 the “resiliency” of the rubber material allows the teeth to move toward their new ideal position. 38 The Perfector ® is a newer, modified version of the positioner. It is fabricated in the same manner as the positioner; however some changes have been made to its overall design. Unlike the positioner, which is made out of a rubber material, The Perfector ® was made out of silicone, which, according to the manufacturer, is a more pliable material. To increase retention of the device, a wire bow, seating springs and labial acrylic were added to the design. These added features not only help with retention, but are also thought to aid in controlling overjet. While wearing the Perfector ® , the patient must perform clenching exercises during the day and sleep with it at night. The protocol is as follows: For the first 2 weeks, the patient is instructed to perform clenching exercises for 3-4 hours a day; for the next 4 weeks, the patient is instructed to perform clenching exercises for 2-3 hours a day, and; for the final 2 weeks, the patient is instructed to perform 2 hours of clenching exercises a day. Throughout, the Perfector ® is worn at night. Corrections with the Perfector ® are usually obtained during the first ® TP Orthodontics, Inc. . 39 six weeks.61 After 2-3 months of Perfector ® wear, patients are given Hawley retainers or other, more low profile retention devices. It has not yet been determined what occlusal changes occur after the occlusal coverage of the Perfector ® appliance has been removed. Post-Orthodontic Occlusion and Retention Introduction The nature of orthodontic treatment produces changes in a patient’s occlusion. Although most of the changes are positive, some studies have shown that occlusion is not ideal immediately following orthodontic treatment and that the number of occlusal contacts might actually decrease.17,42,44 Post-orthodontic occlusion studies remain controversial. While most indicate that contacts increase during the retention period,13,18,24,25 some show that there are no improvements or the occlusion may worsen or regress to the mean.17,62-64 In addition to holding the teeth in a desired position, another aim of the retention phase is to allow settling of the occlusion. The available data suggest that the amount of settling that occurs during retention is ® TP Orthodontics, Inc. 40 largely affected by the type of appliance prescribed and the duration that the appliance is worn.13,18,24,25 Past Studies The majority of post-orthodontic studies have shown that occlusal contacts increase during the retention phase. Post-orthodontic settling typically improves the occlusion and the interdigitation of the dentition over time. However, this remains a controversial issue, as other studies have produced contradictory results. Various studies support the findings that ideal occlusal relationships should be established during active orthodontic treatment and should not be expected to change during the retention phase. Sultana et al.65 reported that occlusal force and occlusal contact area in the total dental arch significantly increased during retention, due primarily to increases in the molar region. These findings support previous reports showing an increase in the number of occlusal contacts in the molar region in the retention phase.25 However, Sultana et al.65 reported no changes in the occlusal force and occlusal contact area in the premolar regions during the retention period and after retention. Therefore, eruption and settling in the premolar region after orthodontic treatment should not be expected to 41 occur. If this is true, it suggests that occlusal contacts in the premolar region should be established during active orthodontic treatment. When examining pre-treatment, post-treatment and post-retention (average of 14.2 years post-treatment) models of 49 subjects to assess the relationship between posterior occlusion and post-treatment changes, Parkinson et al.17 found that the area of actual and near contacts at or below 300um did not significantly increase during the post-treatment period. This indicates that practitioners should not expect the proximity of posterior occlusal surfaces to improve post-treatment with long term settling. A study by Nett and Huang,62 who examined long-term (minimum 10 years post retention) post-treatment changes, found a statistically significant improvement in overall occlusal relationships. On average, alignment scores worsened, while scores for marginal ridge, bucco-lingual inclination, occlusal contacts and overjet criteria all significantly improved during the post-retention period. However, based on a sample of 100 subjects, it was concluded that well finished cases tended to worsen over time while acceptable or poorly finished cases tend to improve in the long term, illustrating a regression to the mean.66 These results are supported by similar findings by 42 Ormiston et al.43 A study by Otuyemi and Jones67 reported that PAR scores in well treated patients tend to get worse over time, with only 38% of patients maintaining treatment results at T3. However, it has been shown by Weiland68 that occlusal shifts decrease from T2 to T3, suggesting that some discrepancies at T2 will diminish in severity by T3. Dincer et al.26 found that there was an increase in the number of contacts in “not ideal” locations throughout the retention period. These contacts represented one-third of the total posterior contacts. They concluded that rather than expecting the occlusion to settle into correct contact positions, the occlusion should be finished as close to the ideal as possible before the orthodontic appliances are removed. These findings are supported by Haydar et al.13 who found no major changes in occlusion after orthodontic treatment. The findings of these studies suggest that the final occlusion should be as ideal as possible at the end of orthodontic treatment and that only minimal changes can be expected to occur during the retention phase. Although there are studies that do not demonstrate increased settling and improved occlusal relationships during the retention phase, the majority of retention studies show improvements. 43 Gazit and Lieberman18 measured occlusal contacts on 12 patients between the ages of 14 and 16 years of age who had been orthodontically treated for approximately 18 months. Occlusal recordings were taken at debond, one month postdebond, and one year post-debond. Anterior and posterior occlusal contacts were recorded and analyzed using the photo-occlusion technique. Prior to the last recording (one year), all patients were retainer-free for 3 months. Importantly, the authors did not report the type of retention that was used in the study. Results showed that there were an average of 11.2 contacts on the day of debond. One month later, four patients demonstrated decreases in the number of contacts and eight demonstrated increases in number of contacts. When records were taken one year after debond, the average number of contacts had increased to 17.4. This represents a gain of 6.2 anterior and posterior contacts. Throughout the study, only 3 of the 12 subjects had anterior contacts. Similar results were reported by Dincer et al.,26 who evaluated occlusal contacts of 20 orthodontically treated patients at debond and after 9 months of retention. All of these patients had Class I malocclusion, four premolar extractions and had been treated with standard edgewise mechanics. The treatment group was compared to a 44 nontreatment group of 20 subjects with ideal occlusion, who served as controls. The treated patients were instructed to wear the maxillary and mandibular Hawley retainers fulltime for the first six months of retention and then nighttime only for the last three months of the study. Occlusal records were taken in centric and eccentric occlusion with silicone putty (Optosil plus) impression material. Actual contacts were identified as perforations in the material and translucent areas were counted as near contacts. Using different colors, both actual and near contacts of the premolar, first molar and second molar regions were transferred to study models. During the 9 month retention period, a total gain of 7.6 posterior contacts was reported. Before retention, patients registered a total number of 11.4 posterior contacts (actual and near) and at the end of the 9 month retention period, there were a total of 19 posterior contacts (actual and near). The gain of 7.6 posterior contacts during nine months of retention was considered a significant increase. However, as previously mentioned, one third of these posterior contacts occurred in “not ideal” locations. Not only is it important to evaluate post-orthodontic occlusion and the gain in occlusal contacts over time, but it is also important to evaluate if the type of retention 45 device used has an affect on occlusal contacts and posttreatment settling. Durbin and Sadowsky24 accomplished this by evaluating post-treatment occlusal changes of 38 patients at the time of debond and three months postdebond. The patients were divided into two groups; 23 patients were given upper and lower Hawley retainers while the other 15 patients were instructed to wear tooth positioners. It is important to note that, due to specific preferences of the instructors and patient acceptance, the subjects were not randomly assigned to the two retention groups. This suggests that there may have been group differences in the quality of post-orthodontic occlusion. Occlusal contacts were recorded using polyether rubber impression bites and the locations of the contacts were subsequently transferred to study models. It is also important to note that areas of near contact were not measured. They reported that the total number of contacts in the combined sample increased from 10.11 at the end of orthodontic treatment to 11.5 during the 3 month post treatment period, which represents a total increase of 14.1%. This change was due entirely to an increase in the number of contacts on posterior teeth, which increased from 8.7 to 10.1, representing a 16.3% increase in posterior contacts. Over the three months of retention, the number 46 of anterior teeth in contact and the number of contacts on the anterior teeth decreased in 37% of the cases. It was also shown that those cases with fewer teeth in contact at the end of treatment developed more contacts over the 3 month post-treatment period.24 The positioner group showed an increase in posterior contacts from 7.8 to 8.9, which represented a gain of 1.0 posterior contacts. The posterior contacts in the Hawley group increased from 9.3 to 10.9, representing a gain of 1.6 posterior contacts. When examining the total number of posterior teeth in contact before and after 3 months of retention, Durbin and Sadowsky24 found that the positioner group increased from 9.7 to 10.7 while the Hawley group increased from 10.7 to 11.4. Although the authors concluded that the group retained with the tooth positioner demonstrated a greater gain in the total number of teeth in contact over time and was statistically more effective than the Hawley retainer (p<.05), the differences were small. To evaluate the continued settling of the occlusion, Razdolsky et al.25 conducted a follow-up study to Durbin and Sadowsky’s study.24 In addition to recording the actual contacts, Razdolsky et al.25 also recorded near contacts and the location of contacts. Because only 28 of the 38 patients from the original study were available for this 47 follow up study, an additional 12 patients were included. The retention protocols for this follow-up study varied. Maxillary Hawley retainers along with mandibular fixed lingual retainers were worn by 28 patients, tooth positioners were worn by 3 patients who then changed to a maxillary Hawley retainer, 8 patients wore maxillary and mandibular Hawley retainers and one patient wore a maxillary retainer only. Following the removal of orthodontic appliances, the patients were instructed to wear the retainers full-time for the first 6 to 12 months and then at night only. The study did not compare the different retention devices and their effects on occlusal contacts. Occlusal bite registrations were taken using polyether rubber impression material at two timepoints; T1 was within two hours after removal of fixed appliances and T2 was obtained after approximately 21 months of retention. The time between T1 and T2 ranged from 11 to 28 months. The number of actual and near contacts on the second molars, first molars, premolars, canines and incisors were registered by positioning the polyether bite registrations on a radiographic viewing screen in a dark room. Areas of actual contact were counted as perforation in material and were transferred to a study model with red marker, while the areas of near contacts were counted as a change in the 48 registration material from yellow to transparent white, and were transferred onto the models with a black marker. From the day of debond (T1) to the follow up visit an average of 21 months later/(T2), the total number of contacts (actual and near) increased from 36.6 to 58.2, showing a total increase of 21.6 total contacts. This overall gain in occlusal contacts can be explained as an increase in the number of actual and near contacts on the posterior teeth.25 Haydar et al.13 also compared the tooth positioner to the Hawley retainer. Twenty patients were equally divided into two groups, a Hawley group and a Positioner group, and were compared to a control group of 10 subjects with normal occlusion. It was not stated whether the subjects were randomly assigned to their respective retention groups. Anterior and posterior contacts were registered at the end of active orthodontic treatment (T1) and 3 months into the retention period (T2) by having patients bite down into a silicone-based impression material. These bite registrations were then held up to daylight, the number of contacts were counted, and the perforations were transferred to maxillary casts. At T1, the total number of contacts was 21.2 for the Hawley group, 24.8 for the Positioner group and 39.4 for the control group. The mean number of contacts at T2 was 22.4 for the Hawley group, 49 27.0 for the Positioner group and 40.5 for the control group, resulting in a gain of 2.2 contacts in the positioner group and 1.2 contacts in the Hawley group. However, the authors reported that there were no statistically significant differences between the Hawley and positioner groups with regard to the increase of contacts, concluding that there were no group differences based on post-orthodontic occlusion. The Hawley retainer has also been compared to the clear overlay retainer based on changes in the number of occlusal contacts following orthodontic treatment. Sauget et al.27 examined 30 patients who were randomly assigned at the time of appliance removal into either the Hawley group or the clear overlay retainer group. Occlusal bite registrations in maximum intercuspation were taken with vinyl polysiloxane at three timepoints; at the time of debond (T1), at retainer delivery 1 week post-debond (T2), and after three months of retention (T3). Occlusal contacts were evaluated and classified as follows; contacts were defined as actual perforations in the material while near contacts appeared as thin translucencies and had a material thickness of 0.20 mm or less. contacts were evaluated. Both anterior and posterior At the time of debond (T1), the Hawley group had a total of 34.3 contacts (actual and near, 50 anterior and posterior) while the clear overlay retainer group had a total of 31.8 contacts. One week later, at the time of retainer delivery (T2), there were no significant differences between the two retainer groups. After three months of retention (T3), there were 45.7 total contacts in the Hawley group and 36.7 total contacts in the clear overlay retainer group. Based on posterior contacts only (actual and near), there was a reported change in the Hawley group from 25.3 at T1 to 35.9 contacts at T3 and a change in the clear overlay retainer group from 23.7 at T1 27.9 at T3. It was concluded that, when compared to the clear overlay retainer, the Hawley retainer allowed more settling of the occlusion during the retention phase. These findings suggest that if one of the objectives of retention is to allow for relative vertical tooth eruption and settling, particularly in the posterior teeth, a Hawley retainer should be prescribed, as the clear overlay retainers tend to hold the teeth in their debond position. These differences between the Hawley and clear overlay retainer were statistically significant. Basciftci et al.69 conducted a study comparing a modified wraparound Hawley retainer to the Jenson plate. This was a follow up study to evaluate the number of contacts in centric occlusion during a 1-year retention period in 51 subjects using two different retention protocols. In this study, 20 patients received modified wraparound Hawley retainers and 20 patients received maxillary Jensen plates along with a mandibular fixed retainer. A Jensen plate consists of a removable palatal plate of acrylic with cclasps on the second molars and an outer labial bow extending between the lateral incisors. Due to the fact that some clinicians in the postgraduate clinic had a preference between retention devices, the patients were not randomly assigned to their retention groups. Patients were instructed to wear the retainers full time for 6 months and then nighttime only for the next six months. The two retention groups were also compared to a control group of 20 subjects with normal occlusions. Occlusal bite registrations were taken with Zeta plus, a soft siliconebased impression material, at the following timepoints; In the treated groups, T1 was taken within 2 hours after debond and T2 was obtained approximately 14 months later. In the control sample, the two sets of occlusal records were taken 12 months apart. Occlusal records were taken by having the patient bite into the silicon based impression material in maximum intercuspation for one minute. The interocclusal bite registration was held up to a light box to view the occlusal contacts. 52 Actual contacts were identified as perforations in the bite registration that let the light through, and thin, transparent areas that did not perforate the material were recorded as near contacts. After the contacts were classified, they were transferred to a model of the maxillary teeth. At each of the two timepoints, the following variables were recorded from the study models: total number of contacts (actual and near), number of actual contacts on second molars, first molars, premolars, canines, and incisors; and the number of near contacts on the previously mentioned teeth. Results of the study indicate that when compared to the control group, total and posterior contacts (actual and near) increased during the retention period. In the Hawley group, there were statistically significant increases in actual contacts on the second molars, near contacts on the premolars, and total contacts on the first molars and premolars. In the maxillary Jensen plate and mandibular fixed retainer group, there were statistically significant increases in actual contacts on the first molars, second molars, premolars and canines, and total contacts on the first and second molars. The control sample demonstrated slight occlusal changes which may have been due to growth and development. Overall, Basciftci et al.69 concluded that both retention protocols allow settling of the occlusion and that no 53 statistically significant differences exist between the groups. Horton et al.1 evaluated the short term changes in posterior occlusion when 50 patients were given either a Hawley retainer or a Perfector ® and mandibular spring aligner. Patients who had full orthodontic treatment were randomly assigned to one of the two retention groups. Objective and subjective measures were taken at two timepoints: the day the retainers were delivered (T1) and approximately two months later (T2). To quantify posterior areas of contact (less than 50um) and near contact (50350um), occlusal bite registrations were taken with Blu Mousse at both timepoints. Each occlusal bite registration was scanned and traced using Image Tool software. It was shown that in the Hawley group, areas of contact and near contact (ACNC) was 6.7 mm2 at T1 and 11 mm2 at T2. ACNC for the Perfector ® /Spring Aligner group were 8.4 mm2 at T1 and 13 mm2 at T2. There were statistically significant increases in ACNC in both the Hawley and Perfector ® retention groups after two months. To assess the patient’s perception of occlusion, a seven item questionnaire was given at both timepoints. ® Results from this qualitative analysis showed TP Orthodontics, Inc. 54 that patients who wore the Perfector ® /Spring aligner retainers reported significantly greater improvements in how well their back teeth fit together, how well they could chew tough meats, and how much pain they felt when biting down.1 It was concluded by Horton et al.1 that there were no differences between the Hawley and Perfector ® retainers in terms of post-orthodontic settling. similar amounts of improvement. Both produced However, patients wearing the Perfector ® perceived greater and more beneficial occlusal changes over time. Overall, these studies tend to show conflicting results on post-orthodontic changes, indicating the need for further research in this area. Conclusion Various studies have evaluated post-orthodontic occlusion and the effects of different retention devices. However, these studies use varying methods to record and measure occlusal contacts. Some of these methods have proven to be more reliable than others. Considering the differences in protocol among the studies, it is difficult to draw 55 concrete conclusions. Only broad statements regarding postorthodontic settling and the affects of different retention devices can be made. These include that: 1) during the retention phase of treatment the number of posterior occlusal contacts tend to increase and 2) contacts increase for several months after removal of fixed appliances and 3) different types of retention devices may have different effects on post-orthodontic occlusion and settling. It is currently not clear what happens after the Perfector ® patients are given Hawley retainers. As stated earlier, the patient is instructed to perform chewing exercises for several hours a day and then sleep with the Perfector ® at night. This process is supposed to align the teeth into a more ideal position, which should result in increased occlusal contacts. However, the Perfector ® by design has occlusal coverage which may inhibit actual settling. There has not been a study examining what happens to the amount of settling after the Perfector ® patient is given a Hawley retainer. With proper use of the Perfector ® , the teeth, in theory, have been moved into a more ideal position. ® By removing the occlusal coverage of TP Orthodontics, Inc. 56 the Perfector ® , there is a possibility that the teeth will then be able to further settle, and in turn produce increased occlusal contacts months later. There have been no studies comparing the effects of Hawley and Perfector ® retainers months after both groups are given identical Hawley retainers. There is a possibility that although there were no apparent differences initially, the Perfector ® group may provide better occlusion in the long run. Knowing the nature of occlusal changes over time and the qualitative and quantitative differences between the Hawley and Perfector ® retainers will assist the orthodontist in making an informed decision when prescribing his or her retention protocol. The goal of this follow-up study is to further investigate whether and how the retention device used after active orthodontic treatment affect posterior occlusion after two different retention groups (Perfector ® and Hawley) are given identical Hawley retainers at two months and are assessed six and eight months post-debond. Occlusal bite registrations of silicone-based impression material will be analyzed using optical scanning and Image Tool software to assess the differences in 1) area of occlusal contacts ® TP Orthodontics, Inc. 57 (actual and near contacts) at eight months post-debond, 2) differences in settling between the Hawley and Perfector ® groups after both retention groups are given identical retainers, and 3) patient perception of occlusion. ® TP Orthodontics, Inc. 58 References 1. Horton J, Buschang P, Behrents R, Oliver D (in press). Comparison of the effects of a hawley and perfector/spring aligner retainers on post orthodontic occlusion . Am J Orthod Dentofacial Orthop. 2. Angle E. Treatment of malocclusion of the teeth and fractures of the maxillae. Philadelphia: SS White Dental Manufacturing; 1900. 3. Tweed C. Indications for the extraction of teeth in orthodontic procedures. Am J Orthod 1944;30:405-408. 4. Andrews LF. The six keys to normal occlusion. Am J Orthod 1972;62:296-309. 5. Okeson J. Management of temporomandibular disorders and occlusions. St. Louis: CV Mosby; 2003. 6. Sarver D, Proffit W, Ackerman J. Diagnosis and treatment planning in orthodontics. In: Graber T, Vanarsdall R, editors. Orthodontics; current principles and techniques. St. Louis: Mosby; 2000. 7. Roth RH. Functional occlusion for the orthodontist. J Clin Orthod 1981;15:32-40, 4451 8. McHorris WH. Occlusion with particular emphasis on the functional and parafunctional role of anterior teeth. Part 2. J Clin Orthod 1979;13:684-701. 9. Beyron H. Occlusal relations and mastication in Australian Aborigines. Acta Odontol Scand 1964;22:597-678. 10. Costalos P, Sarraf K, Cangialosi T, Efstratiadis S. Evaluation of the accuracy of diital model analysis for the American Board of Orthodontics objective grading system for dental casts. Am J Orthod Dentofacial Orthop 2005;128:624-629. 59 11. Casko J, Vaden J, Kokich V, Domone J, James R, Cangialosi T. Objective grading system for dental casts and panoramic radiographs. Am J Orthod Dentofacial Orthop 1998;114:589-599. 12. Ricketts RM. Occlusion--the medium of dentistry. J Prosthet Dent 1969;21:39-60. 13. Haydar B, Ciger S, Saatci P. Occlusal contact changes after the active phase of orthodontic treatment. Am J Orthod Dentofacial Orthop 1992;102:22-28. 14. Moller E, Bakke M. Occlusal harmony and disharmony: frauds in clinical dentistry? Int Dent J 1988;38:7-18. 15. Clark JR, Evans RD. Functional occlusion: I. A review. J Orthod 2001;28:76-81. 16. Millstein P, Maya A. An evaluation of occlusal contact marking indicators. A descriptive quantitative method. J Am Dent Assoc 2001;132:1280-1286; quiz 1319. 17. Parkinson CE, Buschang PH, Behrents RG, Throckmorton GS, English JD. A new method of evaluating posterior occlusion and its relation to posttreatment occlusal changes. Am J Orthod Dentofacial Orthop 2001;120:503-512. 18. Gazit E, Lieberman MA. Occlusal contacts following orthodontic treatment. Measured by a photocclusion technique. Angle Orthod 1985;55:316-320. 19. Carossa S, Lojacono A, Schierano G, Pera P. Evaluation of occlusal contacts in the dental laboratory: influence of strip thickness and operator experience. Int J Prosthodont 2000;13:201-204. 20. Gazit E, Fitzig S, Lieberman MA. Reproducibility of occlusal marking techniques. J Prosthet Dent 1986;55:505-509. 21. Maness WL, Benjamin M, Podoloff R, Bobick A, Golden RF. Computerized occlusal analysis: a new technology. Quintessence Int 1987;18:287-292. 22. Lyons MF, Sharkey SW, Lamey PJ. An evaluation of the T-Scan computerised occlusal analysis system. Int J Prosthodont 1992;5:166-172. 60 23. Hsu M, Palla S, Gallo L. Sensitivity and reliability of the T-scan system for occlusal anaysis. J Craniomandib Disord Facial Oral Pain 1992;6:17-23. 24. Durbin DS, Sadowsky C. Changes in tooth contacts following orthodontic treatment. Am J Orthod Dentofacial Orthop 1986;90:375-382. 25. Razdolsky Y, Sadowsky C, BeGole EA. Occlusal contacts following orthodontic treatment: a follow-up study. Angle Orthod 1989;59:181-185; discussion 186. 26. Dincer M, Meral O, Tumer N. The investigation of occlusal contacts during the retention period. Angle Orthod 2003;73:640-646. 27. Sauget E, Covell DA, Jr., Boero RP, Lieber WS. Comparison of occlusal contacts with use of Hawley and clear overlay retainers. Angle Orthod 1997;67:223-230. 28. Julien KC, Buschang PH, Throckmorton GS, Dechow PC. Normal masticatory performance in young adults and children. Arch Oral Biol 1996;41:69-75. 29. Owens S, Buschang PH, Throckmorton GS, Palmer L, English J. Masticatory performance and areas of occlusal contact and near contact in subjects with normal occlusion and malocclusion. Am J Orthod Dentofacial Orthop 2002;121:602-609. 30. Sakaguchi RL, Anderson GC, DeLong R. Digital imaging of occlusal contacts in the intercuspal position. J Prosthodont 1994;3:193-197. 31. Wright PS. Image analysis and occlusion. J Prosthet Dent 1992;68:487-491. 32. Yurkstas AA. The Masticatory Act. a Review. J Prosthet Dent 1965;15:248-262. 33. Yurkstas AA, Manly R. Measurement of occlusal contact area effective in mastication. Am J Orthod 1949;35:185-195. 34. Toro A, Buschang PH, Throckmorton G, Roldan S. Masticatory performance in children and adolescents with Class I and II malocclusions. Eur J Orthod 2006;28:112119. 35. English JD, Buschang PH, Throckmorton GS. Does malocclusion affect masticatory performance? Angle Orthod 2002;72:21-27. 61 36. Henrikson T, Ekeberg E, Nilner M. Masticatory efficiency and ablility in relation to occlusion and mandibular dysfunction in girls. Int J Prosthodont 1998;11:125-132. 37. Omar SM, McEwen JD, Ogston SA. A test for occlusal function. The value of a masticatory efficiency test in the assessment of occlusal function. Br J Orthod 1987;14:85-90. 38. Ikebe K, Matsuda K, Morii K, Furuya-Yoshinaka M, Nokubi T, Renner R. Association of masticatory performance with age, posterior occlusal contacts, occlusal force and salivary flow in older adults. Int J Prosthodont 2006;19:475-481. 39. Bakke M, Holm B, Jensen BL, Michler L, Moller E. Unilateral, isometric bite force in 8-68-year-old women and men related to occlusal factors. Scand J Dent Res 1990;98:149-158. 40. Ingervall B, Minder C. Correlation between maximum bite force and facial morphology in children. Angle Orthod 1997;67:415-422; 423-414. 41. Driscoll-Gilliland J, Buschang PH, Behrents RG. An evaluation of growth and stability in untreated and treated subjects. Am J Orthod Dentofacial Orthop 2001;120:588-597. 42. Deng Y, Fu MK. Occlusal contact changes before and after orthodontic treatment of a group of child & adolescent patients with TMJ disturbance. Aust Orthod J 1995;13:231237. 43. Ormiston J, Huang G, Little R, Decker J, Seuk G. Retrospective analysis of long-term stable and unstable orthodontic treatment outcomes. Am J Orthod Dentofacial Orthop 2005;128:568-574. 44. Fleming J. An analysis of variability in Class I non-extraction treatment outcomes in a resident clinic using the American Board of Orthodontics Objective Grading System. Masters Thesis. Saint Louis: Saint Louis University; 2006. 45. Abei Y, Nelson S, Amberman B, Hans M. Comparing orthodontic treatment outcome between orthodontists and general dentists with the ABO index. Am J Orthod Dentofacial Orthop 2004;126:544-548. 62 46. Deguchi T, Honjo T, Fukunaga T, Miyawaki S, Roberts W, Takano-Yamamoto T. Clinical assessment of orthodontic outcomes with the peer assessment rating, discrepancy index, objective grading system, and comprehensive clinical assessment. Am J Orthod Dentofacial Orthop 2005;127:434-443. 47. Djeu G, Shelton C, Maganzini A. Outcome assessment of Invisalign and traditional orthodontic treatment compared with the American Board of Orthodontics objective grading sytem. Am J Orthod Dentofacial Orthop 2005;128:292-298. 48. Yang-Powers L, Sadowsky C, Rosenstein S, BeGole E. Treatment outcome in a graduate orthodontic clinic using the American Board of Orthodontics grading system. Am J Orthod Dentofacial Orthop 2002:451-455. 49. Cook M. Evaluation of Board-certified orthodontist's sequential finished cases with the ABO objective grading system. Masters Thesis. Saint Louis: Saint Louis University; 2003. 50. Binder RE. Retention and post-treatment stability in the adult dentition. Dent Clin North Am 1988;32:621-641. 51. Reitan K. Principles of retention and avoidance of posttreatment relapse. Am J Orthod 1969;55:776-790. 52. Riedel RA. A review of the retention problem. Angle Orthod 1960;30:179-199. 53. Behrents R. A treatise on the continuum of growth in the aging craniofacial skeleton: University of Michigan Center for Human Growth and Development; 1984. 54. Nanda RS, Nanda SK. Considerations of dentofacial growth in long-term retention and stability: is active retention needed? Am J Orthod Dentofacial Orthop 1992;101:297302. 55. Hawley C. A removable retainer. Int J Orthod 1919;5:291-298. 56. Lindauer SJ, Shoff RC. Comparison of Essix and Hawley retainers. J Clin Orthod 1998;32:95-97. 57. Collett T. A rationale for removable retainers. J Clin Orthod 1998;32:667-669. 63 58. Ponitz RJ. Invisible retainers. Am J Orthod 1971;59:266-272. 59. Kesling H. The tooth positioner as the means of final positioning of teeth to a predetermined pattern. J Dental Child 1944;11:103-105. 60. Kesling H. The philosophy of the tooth positioning appliance. Am J Ortho and Oral Surg 1945;31:297-304. 61. TP Orthodontics. La Porte, Indiana. 62. Nett BC, Huang GJ. Long-term posttreatment changes measured by the American Board of Orthodontics objective grading system. Am J Orthod Dentofacial Orthop 2005;127:444-450. 63. Otuyemi O, Jones S. Long term evaluation of treated Class II Division I malocclusions utilizing the PAR index. Br J Orthodont 1995;22:171-180. 64. Ormiston J, Huang G, Little R, Decker J, Seuk G. Retrospective analysis of long-term stable and unstable orthodontic treatment outcomes. Am J Orthod Dentofacial Orthop 2005;128:568-574. 65. Sultana M, Yamada K, Hanada K. Changes in occlusal force and occlusal contact area after active orthodontic treatment: a pilot study using pressure-sensitive sheets. Journal of Oral Rehabilitation 2002;29:484-491. 66. Nett BC, Huang GJ. Long-term posttreatment changes measured by the American Board of Orthodontics objective grading system. Am J Orthod Dentofacial Orthop 2005;127:444-450. 67. Otuyemi O, Jones S. Long term evaluation of treated Class II Division I malocclusions utilizing the PAR index. Br J Orthodont 1995;22:171-180. 68. Weiland F. The role of occlusal discrepancies in the long-term stability of the mandibular arch. Eur J Orthod 1994;16:521-529. 64 69. Basciftci FA, Uysai T, Sari Z, Inan O. Occlusal contacts with different retention procedures in 1-year follow-up period. Am J Orthod Dentofacial Orthop 2007;131:357362. 65 CHAPTER 3: JOURNAL ARTICLE Abstract Purpose: To characterize post-orthodontic settling of the posterior occlusion by comparing patients wearing Hawley retainers against patients initially wearing Perfector retainers and then switching to Hawley retainers. Methods: This follow-up study was based on 40 patients (25 Perfector and 15 Hawley) evaluated at four time points over eight months. They were part of a larger sample of 50 patients randomly assigned to wear either Hawley or Perfector retainers. The Perfector/Hawley patients were given Hawley retainers two months post-debond. Objective and subjective measures were collected at the end of treatment (T1), two months post debond (T2), six months post-debond (T3) and eight months post-debond (T4). Occlusal bite registrations were scanned and traced to quantify posterior areas of contact and near contact (ACNC). A seven-item questionnaire was used to assess the patient’s perception of occlusion. Results: ACNC increased during the first six months of retainer wear. Both the Hawley and Perfector/Hawley groups showed the greatest rates of increase in ACNC between T1-T2. Overall, the Hawley group showed a 129% increase in ACNC, while the Perfector/Hawley 66 group showed a 104.9% increase in ACNC over eight months of retention. While the Hawley group showed greater increases in ACNC from T2-T3 and the Perfector/Hawley group showed slightly greater increases from T3-T4, overall group differences were not statistically significant. While the Perfector/Hawley group perceived greater improvements in their occlusion than the Hawley group, group differences after eight months were small and related to the changes that occurred during the first two months of retention. Conclusions: Settling occurred at decelerating rates until six months after which they stabilized. No significant differences in ACNC were found between the Hawley and Perfector/Hawley groups after eight months of retainer wear. Introduction Achieving an ideal posterior occlusion is an important outcome of orthodontic treatment. Posterior occlusion pertains to the relationships between cusp tips and their opposing central fossa and marginal ridges.1 These relationships are important because the posterior teeth establish and maintain the vertical dimension of occlusion and are designed to withstand the heavy forces of mastication.1 Posterior occlusal contact area has been shown 67 to be among the most important factors determining masticatory performance.2-5 relatively small, Occlusal contact area, albeit represents the most important fraction of the total area involved in mastication.2 This explains why subjects with normal occlusion are able to break down foods more efficiently than subjects with malocclusion.6,7 Good intercuspation and occlusal contacts may also be essential for stable orthodontic results, especially for maintaining overbite and overjet.8,9 Furthermore, the location of contacts in intercuspation is one of the main factors responsible for stabilization of the mandible; failure to provide adequate centric stops may cause occlusal instability.9 Our understanding of posterior occlusion during the retention phase of orthodontic treatment remains limited. While most studies indicate that contacts increase and that occlusion improves,10-14 some studies show that there are no improvements, others show that the occlusion worsens, and some show that occlusion regresses to the mean.8,15-17 Because most studies have evaluated posterior occlusion at only two timepoints, the patterns of occlusal change that occur remain unknown. Does the settling occur rapidly; does it increase at a regular rate; when might settling be expected to stop? It is also controversial how the type of 68 retention device used affects post-orthodontic settling. Sauget et al.18 reported that Hawley retainers allowed more settling of the occlusion than clear overlay retainers after three months of retention. In contrast, Basciftci et al.14 found no differences in the number of posterior occlusal contacts between Hawley retainers and the Jenson plate 1 year post-treatment. Based on changes in the number of occlusal contacts three months post-debond, Durbin and Sadowsky12 reported significantly more settling with the active positioner retainers than with passive Hawley retainers; Haydar and coworkers10 found no differences between these two retainers over the same time period. The controversies pertaining to post-orthodontic occlusion may relate to the study designs. First, most studies could not adequately control for selection bias because patients have not been randomly assigned to their respective retention groups. For instance, Durbin and Sadowsky12 allocated subjects to either the Hawley or positioner groups depending on the preferences of the instructors and patient acceptance. Both factors could introduce group differences in the quality of postorthodontic occlusion. Furthermore, most studies have used counts or visual assessments of occlusal contacts, which 69 might be expected to be less discriminating than areas of occlusal contact and near contact(ACNC).6,7,19,20 In a recent randomized controlled trial, Horton et al19 found no differences in ACNC between patients wearing Perfector or Hawley retainers, but their study was limited to two months. It remains unknown whether ACNC change when patients switch from active Perfector ® retainers to passive retainers, as they commonly do after two or three months. Comparisons of active and passive retainers are important for determining whether noticeable treatment effects are limited to the anterior dentition only, or if they produce differences in posterior occlusion as well. Based on the foregoing, a follow-up study was designed to evaluate the long-term occlusal changes that occurred for the patients evaluated by Horton et al.19 this are twofold: The goals of 1) to evaluate the time-course of the occlusal changes that occur over the first eight months of retention, and 2) to determine if the posterior occlusion of patients who switch from the Perfector to the Hawley retainers after two months differs from the occlusion of patients who wore only Hawley retainers for eight months. ® TP Orthodontics, Inc. 70 Materials and Methods Fifty subjects who had completed full orthodontic treatment to class I molar and canine relationships were recruited by Horton et al.19 Subjects were excluded if they had any history of temporomandibular dysfunction, large restorations on the posterior teeth, allergies to any materials used in the study, periodontal disease and/or muscular dysfunction or if they were non-compliant with retainer wear.19 They were randomly assigned, with 22 receiving Hawley retainers (9 males and 13 females) and 28 patients receiving Perfector/Spring Aligners (13 males 15 females) and followed for two months. Forty-four patients completed the first part of the study and consented to participate in this follow-up study, which lasted for an additional six months. Due to patient dropout and missed appointments, 40 patients completed the second part of the study, including 25 patients (11 males and 14 females) originally assigned to the Perfector/Spring aligner group and 15 patients (5 males and 10 females) assigned to the Hawley group. The Hawley group wore the same retainers throughout the eight month observation period. After the first two months, maxillary and mandibular alginate impressions were taken for each of the subjects in the Perfector group and 71 Hawley retainers were delivered within two weeks. This group (Perfector/Hawley) was instructed to wear the maxillary and mandibular Hawley retainers full-time for the duration of the follow-up study. Data were collected at four timepoints: Immediate posttreatment (T1),two month post-debond (T2), 6 months postdebond (T3), and 8 months post-debond (T4). At each timepoint, duplicate bilateral posterior occlusal bite registrations were taken in maximum intercuspation using Blu Mousse,® a silicone impression material (Parkell BioMaterials, Farmingdale, NY). The Blu Mousse® was applied to the occlusal surfaces of the mandibular premolars and first molars and the patient was instructed to bite firmly on their back teeth for 30 seconds.10,18,19 Using a holder, each registration was placed in a standardized position and scanned at 300 DPI with the mandibular occlusal surfaces facing downward. The mandibular premolars and first molars on the scanned image were traced using Image Tool® (University of Texas Health Science Center, San Antonio) software, which calculated the platform area of the outlined teeth and the frequency distribution of pixels within the platform area based on 256 possible gray scales. Because all methods and 72 procedures were the same, the calibration curve developed by Horton et al.19 was used to establish the relationship between the 256 gray scales and ACNC at thicknesses ranging from 0μm to 350μm. The upper limit for scanned light penetration through the impression was approximately 400µm. The thicknesses were recorded in increments of 50µm, with 0-50µm representing contacts and the other increments representing areas of near contacts. Patient perception of occlusion was assessed using the seven item questionnaire developed by Horton et al19 to measure how well their teeth fit together, their level of occlusal discomfort, and their masticatory function. A 148mm visual analogue scale was used with the following seven questions: Q1. How well do your back teeth fit together when you bite down hard? Q2. Do your back teeth contact each other evenly when you bite down hard? Q3. How well can you chew tough meats, such as steak or chops, with your back teeth? Q4. How well can you chew fresh vegetables, such as carrots or celery, with your back teeth? Q5. How much pain do you feel when you bite down hard on your back teeth? 73 Q6. How much discomfort do you experience when you bite down hard on your back teeth? Q7. When you bite down hard, do you feel your back teeth slide? The terms “very well” or “very poor”, “none” or “very much”, and “no slide” or “large slide” served as anchors for the visual analogue scale. Statistical Analysis Based on their skewness and kurtosis, the variables showed significant (p<.05) departures from normality. As such, they were described by medians (50th percentile) and interquartile ranges (25th and 75th percentiles). Wilcoxon signed ranks tests evaluated changes over time; the MannWhitney U was used to compare the two retainers. Results Hawley Group: Wilcoxon signed ranks tests showed significant (p<.05) increases in ACNC with Hawley retainers between T1-T2 and T2-T3 for all thickness intervals except 50-100 um (Tables 74 1 &2; Figure 1). The changes that occurred after 6 month (i.e. T3-T4) were not statistically significant. The greatest overall absolute increase (1.49 mm2) in ACNC occurred between T2-T3 at the 300-350 um interval. The majority of changes in ACNC occurred during the first six months. At the 350um level the total cumulative ACNC increased from 7.01 mm2 immediately post-treatment to 10.7 mm2 at T2, to 16.6 mm2 at T3, to 16.0 mm2 at T4, with all changes being significant (p<.05) except those from T3-T4 (Table 2, Figure 1). At the thinnest ACNC (<150 um), increases tended to be greater from T1-T2, while they tended to be greater at the thickest (>300 um) ACNC from T2-T3 (Figure 1). Perfector/Hawley Group: The Perfector/Hawley group showed increases in ACNC similar to those of the Hawley group. Increases in ACNC were significant (p<.05) at all thickness levels between T1-T2 and T2-T3. The Perfector/Hawley group also showed small but significant increases in ACNC > 200 um from T3-T4 (Table 2). Increases were greatest from T1-T2 at the 300350 um level, with an increase of 0.74 mm2. The total cumulative ACNC increased from 8.3 mm2 at T1, to 13.2 mm2 at T2, to 16.3 mm2 at T3, and to 17.0 mm2 at T4 (Figure 2). The 75 cumulative increases in ACNC were significant at all levels between T1-T2 and T2-T3. Group Comparisons: Absolute and cumulative ACNC showed no significant group differences at any of the four time points (Table 1). Changes in ACNC were significantly greater between T2-T3 for the Hawley than for the Perfector/Hawley at the 200-250 um, 250-300um and 300-350 um levels (Table 2). In contrast, the Perfector/Hawley showed significantly greater increases in contact areas (<50µm) between T3-T4. Cumulative changes in areas of near contact showed no significant group differences. Even though there was a tendency for the Hawley group to show greater increases than the Perfector/Hawley group in absolute (Figure 3) and cumulative areas (Figure 4) of near contact between T1-T4, the differences were not statistically significant. Questionnaire Data: The only significant (p=0.035) change of perception for the patients in the Hawley group occurred between T2-T3 for Q6, indicating that they experienced increased discomfort when they bit down hard on their back teeth. The Perfector/Hawley group showed significant improvements 76 from T1-T2 in how well their back teeth fit together (Q1;p=.001), how well their back teeth contact each other when they bite down hard (Q2;p=.012) and how well they can chew tough meats with their back teeth (Q3;p=.026). From T3-T4, the Perfector/Hawley patients also perceived less of a slide when they bit on their back teeth (p=.026;Q7). From T1-T4 Perfector/Hawley patients showed significant (p<.05) improvements in all questions except Q5. At the time of debond (T1), the Perfector/Spring Aligner group perceived significantly more difficulty chewing tough meats (Q3;p=.029)and more pain when biting(Q5;p=.004)than the Hawley group (Table 3). The only other significant group difference at the other time points was at T4, with the Perfector/Hawley group reporting significantly more pain when biting (Q5;p=.047)than the Hawley group. With respect to the changes in perception over time (Table 4), the Perfector/Hawley group perceived greater improvements than the Hawley group from T1-T2 in how well their back teeth fit together (Q1;p=.009), how well they could chew tough meats (Q3;p=.021), and how much pain they felt when they bite down (Q5;p=.025). The only other significant (p<.05) group difference occurred from T1-T4, with the Perfector/Hawley group showing greater 77 improvements in how well their back teeth fit together (Q1;p=.026). Discussion The Hawley group demonstrated a substantial settling during the first six months of retention. ACNC increased from 7.0mm² to 16.6mm²,representing an increase of over 130%. The changes were greatest at the thicker levels of near contact and least at the level of contacts (Figure 3). The increases observed are substantially larger than previously reported for Hawley retainers, which range from 6%-67%.10,12,18,21 The differing results can be partially explained by the duration of time that the patients were followed. Most retention studies of shorter duration (3 months or less) have reported increases ranging from only 6% to 42%.10,12,18 In contrast, Gazit and Lieberman11 reported a 55% gain in anterior and posterior contacts over one year while Dincer et al.21 reported a 67% gain of posterior ACNC over nine months. In addition, the different methods used to measure posterior occlusion could explain some of the variability across studies. Horton et al,19 showed an overall increase in posterior ACNC of 63% over two months, which is substantially higher than relative increases based on counts over comparable time periods.10,12,18 78 This suggests that ACNC are more discriminating for evaluating posterior occlusion than counting the number of contacts and near contacts. The Perfector/Hawley group also demonstrated substantial increases in settling over the first eight months of retention, with ACNC increasing over 104%. The changes were also greater at the thicker levels of near contact and least at the level of contacts (50 µm)(Figure 3). The overall increases observed were again substantially greater than previously reported. Haydar et al.10 reported a 9% gain in anterior and posterior ACNC after three months of positioner wear; Durbin and Sadowsky12 reported a 14% gain in posterior occlusal contacts after three months of retention; Horton et al.19 reported a 53% increase in posterior ACNC after two months of Perfector wear. The differences between studies may again be explained by the different methods used to evaluate posterior occlusion and the study duration. The differences could also be partially due to the materials used. The Perfector appliance is fabricated from silicone material, which is a softer, more pliable material than the rubber used to fabricate the tooth positioner. The added resiliency may increase the movements of the teeth under functional forces, resulting in a gain in posterior contacts. 79 The Perfector’s seating springs and a labial bow may have also led to increased amount of settling. The rates of increase in ACNC for both the Hawley and Perfector retainers were greater from T1-T2 than T2-T3. In other words, ACNC increased at decelerating rates until six months, after which they stabilized (Figure 1 and 2). It is possible that rates were greater during the second two months than during the third two months, but the data do not allow such a comparison. Unfortunately, there are no longitudinal data with multiple observations available for comparison. The decreasing rates of settling observed may be explained by the law of diminishing returns. Immediately after debond, the teeth are the furthest away from their “settling” point, and therefore have a further distance to potentially move. In other words, the greatest potential for increasing ACNC is when braces are initially removed; as ACNC decrease, their potential to increase decreases proportionately. This may explain why the majority of settling was observed early in the retention phase, and also why the majority of increases occurred at the thicker near contact level (350µm). It was hypothesized at the beginning of the study that the Perfector group would show a greater amount of settling after the occlusal coverage of the Perfector was 80 no longer present. There was a tendency for the Hawley group to show greater increases in ACNC during the first six months, while the Perfector/Hawley group tended to show greater increases during the last two months. In fact, the Perfector/Hawley group showed significant changes between T3 and T4 at thickness levels >200um, while the Hawley showed no changes over the same time period. Over the same time periods, the Perfector/Hawley group also showed significantly greater increases in contact areas (<50um) than the Hawley group. This shows that even though the differences are small, a certain amount of settling continued to occur late in the retention phase for the Perfector/Hawley group. The lack of statistical significance between groups could be partially due to patient compliance. While all patents were repeatedly instructed to wear their Hawley retainers full-time, it was difficult to assess how long the patients had actually worn their retainer. Sample sizes could also have reduced the power of the comparisons. During the first part of the study, six Hawley patients were dropped and the Perfector patients were over-sampled due to expected non-compliance. The combination of over-sampling and dropouts produced an unequal numbers of patients in each retention group, which 81 may have had an effect on the ability to identify differences. Overall post-treatment increases in posterior ACNC were similar for the Hawley and Perfector groups. This suggests that during the retention phase, the teeth have a limited amount of settling, no matter if the Hawley or Perfector retention device is used. These findings are similar to Haydar et al.,10 who after comparing the tooth positioner to the Hawley retainer after three months, found that there were no statistically significant differences in the number of contacts. In contrast, Durbin and Sadowski12 reported that the positioner produced a greater increase in the total number of teeth in contact over time than the Hawley retainer, but the differences were small. Although there were no significant differences between the Hawley and Perfector groups in regards to the ACNC, other dimensions of occlusion may benefit from Perfector wear, such as axial inclination, rotations, etc. These factors could affect the patients’ perception of occlusion and help explain the overall improvements observed for the Perfector/Hawley group. Whereas patients wearing the Hawley retainer generally did not perceive any changes over time, patients initially wearing the Perfector/Hawley noted significant improvements in how well their back teeth fit 82 together. The Perfector/Hawley patients demonstrated significant improvements for all questions except one over the eight month observation period. However, it must be emphasized that the overall improvements that occurred in the Perfector/Hawley group closely reflect the changes that occurred during the first two months of Perfector wear. To better understand the lack of group differences and the changes in patient perception that occurred, it is important to emphasize that, as reported by Horton,19 the Perfector/Hawley group initially reported more pain and discomfort than the patients assigned to the Hawley group. This suggests a potential sampling bias, with the Perfector/Hawley group having a greater potential to improve than the Hawley group. The initial group differences in perception could also explain, at least partially, the lack of group differences after eight months of retention. The lack of long-term group differences may be further explained by the slight improvements that the Hawley group showed over time, which decreased the overall group differences. Nevertheless, it is important to emphasize that the Perfector is designed to influence many aspects of occlusion such as rotations, axial inclinations, alignment, interproximal space closure, crossbite correction, arch coordination and overjet correction.22 83 Such changes may have altered patients’ perceptions of occlusion, which could also explain the improvements identified for those patients who initially wore the Perfector. This suggests that the Perfector/Hawley combination may be a useful retention protocol for patients needing additional corrections other than settling of the occlusion. The Hawley appliance, on the other hand, maintains the buccolingual positions of the teeth, but allows vertical movements. This may explain why the Hawley showed an increased amount of settling, but did not show improvements in patient perception of occlusion and comfort over eight months of retention. Conclusions 1. Areas of contact and near contact (ACNC) increased at decelerating rates over the first six months, with little or no change thereafter. 2. The greatest increases in ACNC occurred at the thicker near contact levels (300-350um). 3. Overall, ACNC increased 130% for the Hawley group and 104% for the Perfector/Hawley group after 8 months, but the group differences were not statistically significant. 84 4. While the Perfector/Hawley group perceived greater improvements in their occlusion than the Hawley group, the differences after eight months of retention were not statistically significant. Because the majority of post-treatment settling of the occlusion occurs during the first six months, it is recommended that Hawley and Perfector/Hawley retainers should be worn for at least this amount of time before switching to other retention protocols. Moreover, many practitioners prefer to perform equilibrations after orthodontic treatment, but do not know when settling is complete. Based on the results of this study, it is recommended that the practitioner wait six months before occlusal equilibration. 85 Table 1. Areas of contact and near contact (ACNC) of posterior the occlusion at T1, T2, T3 and T4. T1 areas (mm2) Hawley Thickness (µm) Median T2 areas (mm2) Perfector IQR Median Hawley IQR Median T3 areas (mm2) Perfector IQR Media n IQR Hawley Median T4 areas (mm2) Perfector IQR Median IQR Hawley Median Perfector IQR Median IQR Absolute Thickness ≤50 0.66 0.23/1.42 0.71 0.28/1.21 1.35 0.71/2.27 1.25 0.76/1.96 1.80 1.25/3.51 1.84 0.88/2.60 1.74 1.35/2.73 1.83 1.15/3.43 50-100 0.73 0.30/1.88 1.14 0.44/1.82 1.67 1.17/2.96 1.76 1.08/2.26 2.22 1.51/3.84 2.38 1.41/3.21 2.33 1.89/3.90 2.29 1.77/3.53 100-150 0.89 0.37/2.00 1.17 0.44/1.83 1.74 1.28/3.00 1.72 1.04/2.47 2.03 1.53/3.24 2.22 1.42/3.12 2.35 1.64/3.08 2.37 1.80/3.29 150-200 0.90 0.46/1.92 1.03 .49/1.63 1.54 1.28/2.77 1.73 1.11/2.60 1.98 1.47/3.28 2.21 1.44/2.90 2.12 1.58/3.24 2.44 1.75/3.02 200-250 1.08 0.46/2.02 1.18 0.46/1.58 1.71 1.33/2.73 1.87 1.14/2.43 2.09 1.60/3.74 2.34 1.51/2.65 2.13 1.60/3.62 2.42 1.91/3.12 250-300 1.30 0.45/2.17 1.33 0.55/1.73 1.77 1.46/2.98 1.99 1.22/2.66 2.29 1.82/4.42 2.57 1.75/3.21 2.47 1.95/4.0 2.87 2.18/3.82 300-350 1.60 0.63/2.62 1.62 0.69/2.01 1.94 1.66/3.56 2.30 1.38/2.90 2.97 2.21/5.85 3.19 2.09/3.73 3.14 2.43/5.33 3.16 2.83/5.14 Cumulative Thickness ≤50 0.66 0.23/1.42 0.71 0.28/1.21 1.35 0.71/2.27 1.25 0.76/1.96 1.80 1.25/3.51 1.84 0.88/2.60 1.74 1.35/2.73 1.83 1.15/3.43 ≤100 1.23 0.63/3.10 1.96 0.69/2.97 3.02 1.80/5.12 2.92 1.96/4.21 4.24 2.76/7.67 4.37 2.21/5.95 4.40 2.72/8.06 4.17 2.85/7.16 ≤150 2.12 1.15/5.22 2.96 1.15/4.73 4.46 3.30/8.24 4.89 2.79/6.20 6.11 4.32/11.66 6.36 3.67/9.45 6.17 5.41/11.14 6.25 4.96/10.21 ≤200 3.01 1.63/7.37 4.12 1.73/6.47 5.72 4.65/11.10 6.68 3.63/8.69 8.03 5.92/15.16 8.45 5.20/11.97 8.50 7.48/14.38 8.13 6.79/13.83 ≤250 4.10 2.15/9.41 5.36 2.19/8.03 7.26 6.20/13.85 8.56 4.85/11.12 10.59 7.74/18.91 10.68 6.72/14.45 10.53 8.91/18.13 10.63 8.73/17.10 ≤300 5.38 2.77/11.47 6.69 2.67/10.03 8.89 7.72/16.72 10.83 6.18/13.79 13.40 9.77/23.32 13.42 8.48/17.53 13.10 10.73/22.30 13.50 10.99/20.87 ≤350 7.01 3.37/13.90 8.30 3.40/11.80 10.67 9.37/20.26 13.16 7.62/16.64 16.61 12.57/29.17 16.33 10.68/22.29 16.03 12.84/27.64 17.00 13.89/26.43 86 Table 2. Changes in areas of contact and near contact (ACNC) from T1-T2, T2-T3, T3-T4 and from T1-T4. T1-T2 areas (mm2) Hawley Thickness (µm) Median T2-T3 areas (mm2) Perfector IQR Median T3-T4 areas (mm2) Hawley IQR Median Perfector IQR Median Hawley IQR Median T1-T4 areas (mm2) Perfector IQR Median Hawley IQR Median Perfector IQR Median IQR Absolute Thickness ≤50 0.49 0.25/1.24 0.52 0.05/1.10 0.65 0.55/2.29 0.29 -0.09/0.76 -0.12 -0.60/0.24 0.36 -0.19/0.80 1.02 0.56/2.04 1.19 50-100 1.10 -0.09/1.49 0.38 -0.02/0.73 0.66 0.47/1.39 0.69 0.17/1.01 0.37 -0.21/0.92 0.18 -0.43/0.71 1.60 1.23/2.01 1.17 0.33/2.67 0.64/2.15 100-150 0.91 0.15/1.42 0.21 -0.06/1.20 0.43 0.14/0.99 0.65 0.21/0.81 0.19 -0.12/0.46 .04 -0.25/0.33 1.54 0.68/2.12 1.11 0.42/1.84 150-200 0.81 0.06/1.29 0.29 -0.02/1.03 0.98 0.24/1.13 0.51 0.11/0.94 0.07 -0.14/0.33 0.14 -0.17/0.29 1.36 0.66/1.81 1.09 0.43/1.45 200-250 0.98 0.14/1.38 0.53 -0.01/0.96 0.86 0.40/1.45 0.42 0.01/0.71 0.19 -0.38/0.34 0.19 -0.06/0.47 1.48 0.76/1.79 1.29 0.63/1.51 250-300 1.03 0.11/1.29 0.57 -0.04/0.96 0.99 0.54/1.75 0.52 0.10/0.98 0.04 -0.65/0.44 0.18 -0.05/0.51 1.70 0.75/2.45 1.38 0.65/2.04 300-350 0.97 0.04/1.41 0.74 0.15/1.11 1.49 0.63/2.88 0.71 0.20/1.37 -0.26 -0.70/0.64 0.19 -0.14/0.74 2.00 0.90/3.65 1.76 0.95/2.89 ≤50 0.49 0.25/1.24 0.52 0.05/1.09 0.65 0.55/2.29 0.29 -0.09/0.76 -0.12 -0.60/0.24 0.36 -0.19/0.80 1.02 0.56/2.04 1.19 0.33/2.67 ≤100 1.51 0.12/3.06 0.81 0.22/1.95 1.37 0.83/3.67 0.94 -0.02/1.73 0.02 -0.62/1.16 0.48 -0.52/1.50 2.99 1.89/5.25 2.41 1.19/3.84 ≤150 2.96 0.35/4.13 1.19 0.27/2.77 1.65 0.91/4.01 1.69 0.24/2.37 0.21 -0.76/1.66 0.51 -0.49/1.75 4.82 2.18/7.40 3.55 1.51/6.28 ≤200 4.07 0.45/5.05 1.43 0.48/4.18 2.11 1.36/4.99 1.94 0.69/3.26 0.26 -0.88/2.01 0.56 -0.57/1.68 6.18 2.83/9.07 4.80 2.13/7.36 ≤250 5.33 0.57/6.43 1.90 0.60/5.14 3.27 1.75/6.64 2.08 1.10/2.08 -0.16 -0.78/2.32 0.73 -0.64/1.89 7.83 3.60/10.80 6.28 2.57/8.78 ≤300 6.47 0.64/8.11 2.66 0.60/5.93 5.13 2.37/8.86 2.68 1.35/4.79 -0.22 -1.03/2.28 1.00 -0.67/2.39 9.60 4.32/13.12 7.86 3.22/10.81 ≤350 7.43 0.68/10.46 3.77 0.32/6.67 6.42 3.16/11.69 3.34 1.57/5.79 -0.27 -1.75/2.71 1.36 -0.93/3.15 11.28 5.08/17.69 9.62 4.04/13.70 Cumulative Thickness 87 Table 3. Patient perception of posterior occlusion at T1, T2, T3 and T4 using a visual analogue scale. T1 areas (mm2) Hawley Questio n number Median 1 2 T2 areas (mm2) Perfector Hawley Median T3 areas (mm2) Perfector IQR Hawley Perfector Median Hawley IQR Median IQR 124.75 110.25/139.31 118.75 101.13/131.88 125.25 112.75/138.08 125.00 118.13/136.50 119.50 97.88/137.19 129.25 124.25/139.38 3 141.88 139.13/145.50 134.50 128.13/142.50 139.00 130.38/144.38 4 139.25 125.75/144.00 132.50 127.63/143.13 139.50 131.13/144.63 139.25 128.50/144.75 145.00 127.00/147.50 142.75 135.50/146.00 5 145.25 143.00/147.75 138.00 131.88/143.75 143.00 130.75/145.63 143.25 137.00/146.75 143.00 131.00/148.00 141.25 132.75/145.00 6 137.75 111.50/145.94 135.00 115.50/142.38 142.13 117.19/143.75 141.75 124.56/143.50 142.50 134.00/148.00 142.63 133.25/145.63 7 138.00 125.19/144.88 140.00 125.75/142.50 139.38 125.38/145.00 138.75 131.00/142.75 145.00 140.00/148.00 136.50 131.38/143.38 88 Median T4 areas (mm2) IQR Median Perfector IQR Media n IQR IQR Median IQR 134.75 127.00/140.00 138.00 124.50/146.50 137.13 128.00/142.50 135.50 124.25/147.50 138.50 131.25 120.38/138.00 130.00 111.00/147.50 134.75 125.31/140.00 142.00 117.75/146.75 138.00 132.00/141.50 139.50 131.50/143.19 146.00 140.00/147.50 140.00 136.00/144.50 143.00 131.50/148.00 139.50 137.00/146.00 145.00 138.00/148.00 141.00 137.50/145.00 145.00 141.50/148.00 141.00 134.50/145.50 143.00 126.50/147.00 143.00 131.00/144.00 143.00 132.00/148.00 141.00 136.00/145.00 136.00/142.50 Table 4. Changes in patient perception of occlusion from T1-T2, T2-T3, T3-T4 and from T1-T4 using a visual analogue scale. T1-T2 areas (mm2) Hawley Question Number Median T2-T3 areas (mm2) Perfector IQR Median IQR Hawley Median T3-T4 areas (mm2) Perfector IQR Median IQR Hawley Median T1-T4 areas (mm2) Perfector IQR Median IQR Hawley Median Perfector IQR Median IQR 1 -0.75 -9.94/11.75 13.63 1.75/36.88 3.50 0.00/21.50 0.25 -5.19/3.19 1.00 0.00/5.00 0.13 -1.94/8.75 3.50 -4.63/18.50 13.75 10.00/29.00 2 -0.50 -7.75/14.25 8.50 -4.38/34.38 0.50 -0.00/4.00 0.25 -8.50/10.25 0.00 -1.00/4.00 3.50 -2.13/6.25 2.00 -6.75/23.25 11.00 3.25/24.50 3 -0.25 -7.88/1.38 3.50 -1.69/11.00 1.00 0.00/8.00 1.63 -2.50/6.75 0.00 -2.00/1.00 0.25 -5.00/3.25 0.00 -0.50/3.25 5.00 -0.75/11.00 4 0.50 -5.38/4.25 2.00 -1.75/10.88 1.00 -1.00/8.00 2.00 -0.88/7.88 0.50 -0.50/4.50 0.38 -0.63/1.00 4.00 -0.25/17.25 8.25 2.50/14.50 5 -0.50 -5.81/0.75 2.75 -1.75/9.00 0.00 -0.50/7.00 -1.50 -8.00/1.50 0.50 0.00/8.00 1.00 -1.38/3.75 0.00 -0.50/1.50 2.00 -3.00/9.00 6 0.25 -10.25/10.75 8.00 -2.50/23.63 2.00 0.00/17.75 -0.25 -5.00/4.81 0.00 -7.50/1.75 0.25 -3.25/2.63 2.50 -0.13/23.25 8.00 1.00/23.75 7 -0.25 -7.06/12.94 -0.13 -7.75/8.50 0.00 -1.00/19.25 -1.25 -4.63/8.63 0.00 -5.50/4.00 2.00 -1.00/7.63 3.50 -1.00/20.13 2.50 -1.00/10.00 89 Area (mm2) ≤50 18.5 16.5 14.5 12.5 10.5 8.5 6.5 4.5 2.5 0.5 0 ≤100 2 ≤150 ≤200 4 ≤250 6 ≤300 ≤350 8 10 Time Interval (months) Figure 1. Median cumulative areas of contact and near contact measured at 50 um thickness intervals between T1 (end of treatment) and T4 (8 months) for patients initially wearing Hawley retainers. 90 2 Area (mm ) ≤50 ≤100 ≤150 ≤200 ≤250 ≤300 ≤350 18.5 16.5 14.5 12.5 10.5 8.5 6.5 4.5 2.5 0.5 0 2 4 6 8 10 Time Interval (months) Figure 2. Median cumulative areas of contact and near contact measured at 50 um thickness intervals between T1 (end of treatment) and T4 (8 months) for patients initially wearing Perfector retainers. 91 Areas (mm2) 2.1 Hawley 1.9 1.7 1.5 1.3 Perfector 1.1 0.9 0.7 0.5 ≤50 50-100 100-150 150-200 200-250 250-300 300-350 Thickness (um) Figure 3. Median changes in areas of contact and near contact at 50 um thickness intervals between T1 (end of treatment) and T4 (8 months) for patients initially wearing Hawley and Perfector retainers. 92 12.5 Hawley 2 Area (mm ) 10.5 Perfector 8.5 6.5 4.5 2.5 0.5 ≤50 ≤100 ≤150 ≤200 ≤250 ≤300 ≤350 Thickness Interval Figure 4. Median cumulative changes in areas of contact and near contact at 50 um thickness intervals between T1 (end of treatment) and T4 (8 months) for patients initially wearing Hawley and Perfector retainers. 93 Literature Cited 1. Okeson J. Management of temporomandibular disorders and occlusions. St. Louis: CV Mosby; 2003. 2. Yurkstas AA. The Masticatory Act. a Review. J Prosthet Dent 1965;15:248-262. 3. Yurkstas AA, Manly R. Measurement of occlusal contact area effective in mastication. Am J Orthod 1949;35:185-195. 4. Owens S, Buschang PH, Throckmorton GS, Palmer L, English J. Masticatory performance and areas of occlusal contact and near contact in subjects with normal occlusion and malocclusion. Am J Orthod Dentofacial Orthop 2002;121:602609. 5. Ikebe K, Matsuda K, Morii K, Furuya-Yoshinaka M, Nokubi T, Renner R. Association of masticatory performance with age, posterior occlusal contacts, occlusal force and salivary flow in older adults. Int J Prosthodont 2006;19:475-481. 6. 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Sauget E, Covell DA, Jr., Boero RP, Lieber WS. Comparison of occlusal contacts with use of Hawley and clear overlay retainers. Angle Orthod 1997;67:223-230. 95 19. Horton J, Buschang P, Comparison of the effects aligner retainers on post Orthodontics. Am J Orthod Behrents R, of a hawley orthodontic Dentofacial Oliver D (in press). and perfector/spring occlusion Orthop. 20. Julien KC, Buschang PH, Throckmorton GS, Dechow PC. Normal masticatory performance in young adults and children. Arch Oral Biol 1996;41:69-75. 21. Dincer M, Meral O, Tumer N. The investigation of occlusal contacts during the retention period. Angle Orthod 2003;73:640-646. 22. Kesling H. The philosophy of the tooth positioning appliance. Am J Ortho and Oral Surg 1945;31:297-304. 96 VITA AUCTORIS Elizabeth Bauer was born in Edwardsville, IL on October 21, 1977. She graduated from the University of Alabama in 1999 with a B.S. degree in Psychology. In 2005, she graduated from Southern Illinois School of Dental Medicine and began her residency in orthodontics at Saint Louis University. She married Benjamin R. Hite on September 29, 2007. 97