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COMPARISON OF THE EFFECTS OF AN ESSIX AND HAWLEY RETAINER ON POST-ORTHODONTIC OCCLUSION Collin Tsai An Abstract Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirement for the Degree of Master of Science in Dentistry 2010 Abstract Purpose: To evaluate and compare the posterior occlusal changes between the Essix and Hawley retainer during the first 3 months of retention. Methods: This randomized trial evaluated 40 patients who had completed full orthodontic treatment and been assigned to wear either Essix or Hawley retainer. They were evaluated at three time points, debond (T1), one month after debond (T2) and three months after debond (T3). Duplicate bite registrations were used to quantity posterior areas of contact and near contact (ACNC). The patient’s perceptions about their occlusion and compliance with retainers wear were evaluated using a 5 item questionnaire. Results: Both the Essix and the Hawley retainers produced slight decreases in ACNC from T1 to T2. At T2, there were significant (p<.05) group differences of cumulative ACNC at ≤50 µm and ≤100 µm, with the Hawley group having greater ACNC. showed increases in ACNC from T2 to T3. Both groups At T3, significant group differences of cumulative ACNC were observed at ≤50 µm, ≤100 µm, ≤150 µm and ≤200 µm, again with the Hawley group showing greater ACNC than the Essix group. Total cumulative ACNC of the Essix group decreased from 19.3 mm² at T1 to 17.5 mm² at T3; ACNC of the Hawley 1 increased from 21.2 mm² at T1 to 22.1 mm² at T3. Group differences in ACNC at T3 were not statistically significant. Conclusion: The Hawley retainer produced greater ACNC than the Essix retainer, especially at or below the 200 µm level, but the differences are small. 2 COMPARISON OF THE EFFECTS OF AN ESSIX AND HAWLEY RETAINER ON POST-ORTHODONTIC OCCLUSION Collin Tsai A Thesis Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirement for the Degree of Master of Science in Dentistry 2010 COMMITTEE IN CHARGE OF CANDIDACY: Adjunct Professor Peter H. Buschang, Chairperson and Advisor Professor Estaquio Araujo Assistant Professor Ki Beom Kim i DEDICATION I would like to dedicate to the two most important people in my life. To my mother, Helen Chan, for motivating me, showing me that nothing is unachievable with hard work and giving me the best on the little that she had. To my fiancée, Angeline Chow, for always supporting me, giving me great advice, and making me laugh when life was stressful. ii ACKNOWLEDGEMENTS Dr. Buschang, for helping me substantially in the design, the statistical analysis and the interpretation of my results. Dr. Araujo and Dr. Kim, for their time, interest, and help in this study. Dynaflex for their sponsor in making retainers for this study. iii Table of Contents List of Tables..........................................v List of Figures........................................vi CHAPTER 1: INTRODUCTION.................................1 CHAPTER 2: LITERATURE REVIEW Ideal Occlusion and Posterior Occlusal Contacts.........3 Methods to Measure Posterior Occlusion..................7 Visual Assessment..................................7 Articulating Indicators............................8 Photo-occlusion...................................10 T-Scan............................................11 Polyether Impression Material.....................13 Silicone-based Impression Material................14 Optical Scanning..................................16 Retention..............................................17 Retention Appliances...................................19 Hawley Retainer...................................19 Wrap Around Retainer..............................20 Positioner........................................20 Perfector.........................................21 Spring Retainer...................................22 Fixed Lingual Retainer............................22 Essix/Vacuum Formed Retainer/Clear Overlay Retainer ..................................................23 Post-Orthodontics Occlusion: Settling..................24 Past Studies......................................25 Studies Comparing Hawley Retainer to Essix (Vacuum Formed) Retainer.......................................33 Conclusion.............................................40 References.............................................43 CHAPTER 3: JOURNAL ARTICLE Abstract...............................................48 Introduction...........................................50 Materials and Methods..................................53 Statistical Analyses...................................56 Results................................................57 Discussion.............................................60 Conclusions............................................66 References.............................................67 Appendix...............................................69 Vita Auctoris..........................................81 iv List of Tables Table 1: Areas of contact and near contact (ACNC) of posterior occlusion with Essix and Hawley retainers at debond (T1), 1 month after debond (T2) and 3 months after debond (T3)...........70 Table 2: Changes in the areas of contact and near contact (ACNC) of posterior occlusion with Essix and Hawley retainers at debond (T1), 1 month after debond (T2) and 3 months after debond (T3)....71 Table 3: Patient perception of posterior occlusion at at debond (T1), 1 month after debond (T2) and 3 months after debond (T3) using a visual analogue scale.........................................72 Table 4: Changes in patient perception of posterior occlusion at at debond (T1), 1 month after debond (T2) and 3 months after debond (T3) using a visual analogue scale.......................73 v LIST OF FIGURES Figure 1: Photo of registration.......................74 Figure 2: Scan of registration........................74 Figure 3: Tracing of occlusal table...................74 Figure 4: Calibration Curve: step-wedge thickness (Y) estimated from grey scales (X0.........................75 Figure 5: Median cumulative areas of contact and near contact of Essix and Hawley retainers at ≤350 µm thickness at T1 (debond), T2 (1 month post-debond) and T3 (3 months post-debond).................................76 Figure 6: Median cumulative areas of contact and near contact of Essix and Hawley retainers at ≤50 µm and ≤100 µm thickness at T1 (debond), T2 (1 month post-debond) and T3 (3 months post-debond) .............................77 Figure 7: Median absolute areas of contact and near contact of Essix and Hawley retainers at T1 (debond) and T2 (1 month post-debond)...............................78 Figure 8: Median cumulative areas of contact and near contact of Essix and Hawley retainers at ≤150 µm and ≤200 µm thickness at T1 (debond), T2 (1 month post-debond) and T3 (3 months post-debond)................. ............79 Figure 9: Median absolute areas of contact and near contact of Essix and Hawley retainers at T1 (debond) and T3 (3 month post-debond).............. ................80 vi Chapter 1: Introduction One of the many goals of orthodontic treatment is to establish proper occlusion, allowing the patient to have healthy masticatory function and stability. Once the teeth are brought into an esthetically pleasing alignment and proper occlusion, the appliance is removed. A retainer is then prescribed to the patient by the orthodontist. This marks the beginning of the retention phase of orthodontic treatment. changes occur. During this phase, many The purpose of prescribing a retainer is to prevent undesirable changes such as rotational relapse, while allowing desirable changes, such as establishing stability and allowing settling of the occlusion. Many different types of retainers are available for the orthodontist to prescribe. Currently, the Essix and the Hawley retainer are the two most commonly prescribed retainers in the United States and the United Kingdom. It has been determined from previous studies that both Hawley and Essix retainers are equally as effective in preventing post-treatment irregularity and relapse. However, there are no studies that have objectively attempted to determine which type of retention method is more effective in allowing settling of the posterior 1 occlusion using the same retention protocol. The purpose of this study is to objectively quantify the posterior occlusal changes that occur with Essix and Hawley retainers from debond to one and three months post-debond and to determine if one is superior in allowing settling to occur in the dentition. 2 Chapter 2: Review of the Literature Ideal Occlusion and Posterior Occlusal Contacts Crooked and irregular teeth have been around for over 50,000 years. Since 1000 BC, there have been many attempts and successes in correcting the irregularity of teeth, but little emphasis was placed on correcting the occlusion.1,2 It was not until about 100 years ago that orthodontists focused on and realized the importance of correcting the occlusion. In 1900, Angle devised a way to classify the occlusion based on the antero-posterior position of the maxillary first molar relative to the mandibular first molar. According to Angle, a normal occlusion occurred when the mesiobuccal cusp of the maxillary first molar occluded with the buccal groove of the mandibular first molar. Three types of malocclusion existed under Angle’s classification. A Class I malocclusion is when there is a normal molar relationship, but the teeth are not in alignment. A Class II malocclusion is present when the mandibular first molar is distally positioned relative to the maxillary first molar. A Class III malocclusion exists when the mandibular first molar is mesially positioned to the maxillary first molar.3 In 1972, Andrews added to Angle’s definition of normal occlusion by noting that the 3 distal surface of the distobuccal cusp of the maxillary first molar should occlude with the mesial surface of the mesiobuccal cusp of the mandibular second molar.4 Okeson added that posterior occlusion should focus on the relationship between cusp tips and central fossae, as well as the relationship between cusp tips and marginal ridges.5 The function of the posterior teeth is to effectively break up food when chewing and to maintain vertical dimension. The function of the anterior teeth is to incise food and guide the mandible through excursive and protrusive movements. The incisors guide the mandible into protrusive contact, thereby disocclusing the posterior teeth. The canines guide the mandible during excursive movements. For this reason, contacts between the anterior teeth are much lighter than the contacts for posterior teeth and can even be absent.5,6 The anterior tooth contacts that guide the mandible through protrusive and extrusive movements are called anterior guidance.5 Besides esthetics, another goal of orthodontic treatment is ideal occlusion. Ideal occlusion can be defined as Class I molar relationship, anterior guidance, maximum occlusal contacts and no occlusal 4 interferences.6-8 Maximum intercuspation is the occlusal position when the teeth are in maximum contact and can also be called centric occlusion or the patient’s most comfortable bite. Retruded contact position is the occlusal position when the first tooth contact occurs on the mandibular path of closure with the condyles in the retruded axis position.7 Some orthodontists believe that an ideal treatment goal is to have maximum intercuspation coincident with retruded contact position.8-10 An association between non-coincidence of maximum intercuspation, retruded contact position and temporomandibular disorders has been shown.11 Other studies support the notion that the non-coincidence of the two can be considered normal.12,13 Clarke concludes that there is no evidence to show any disadvantage in patients when maximum intercuspation that does not coincide with a retruded contact position.7 In maximum intercuspation, Ricketts described the ideal occlusion having a maximum of 48 contacts between the cusp tips and opposing marginal ridges, with third molars excluded. According to Rickets, 32-48 contacts are adequate for a good balanced occlusion.14 This is in agreement with Okeson who states that there are 34-48 contacts in ideal occlusion.5 5 Providing patients with an ideal occlusion is important in orthodontics because it directly affects masticatory performance and function. Occlusal contacts of maxillary and mandibular teeth during mastication, swallowing and speech is called functional occlusion.7 English et al. found that the ability to process and break down food is dependent upon the molar relationship.15 They found that a normal Class I molar relationship was most efficient in breaking down food, followed by Class I and II malocclusions, with Class III malocclusion being the most inefficient in mastication. Toro and co-workers also showed that children with a Class I malocclusion do not break down food as well as those with normal occlusion and children with severe malocclusions cannot break down food as well as those with less severe Class I malocclusions.16 Besides molar relationship, the number of tooth contacts and area of tooth contacts also affects masticatory function. Yurkstas and Manly were the first to identify a correlation between occlusal contact areas and masticatory performance.17 Bakke and collegues have also shown a positive correlation between adult bite force and occlusal contact area.18 Positive correlations between bite force and occlusal contact area also hold true for children as well, with stronger correlations for 6 girls than boys.19 A study by Julien et al. concluded that greater height of the ramus, larger occlusal contact area and stronger bite force all contribute to increased masticatory performance.20 Using a silicone based impression material to record the bite and measure areas of contact and near contact (ACNC), Owens et al. defined actual contact areas at a thickness at or below 50 µm and areas of near contact at thicknesses between 50 µm and 350 µm.21 They demonstrated that Class III subjects had the smallest ACNC and also had the most difficultly breaking down food. Conversely, subjects with normal occlusion had the largest ACNC and were able to break down food into the smallest pieces. This led them to the conclusion that subjects with more closely fitting posterior occlusion and larger ACNC were better able to break down food. Methods to Measure Posterior Occlusion Visual Assessment The most basic method to evaluate posterior occlusion is gross visual assessment. As simple and unsophisticated as this may be, it is still routinely used by orthodontist to evaluate the posterior dentition. Orthodontists commonly use gross visualization to 7 determine the relationship of the molars based on Angle’s classification. Orthodontists also rely on visual assessment to evaluate tooth contact of the posterior occlusion. This may be the quickest way to grossly evaluate the posterior occlusion, but it is neither accurate nor reliable. Besides visual assessment, researchers have developed and used various other methods to measure posterior occlusion and tooth contact. Among these methods used include articulating indicators (paper, film or silk), photo-occlusion, T-scan, polyether impression material and silicone-based impression material. Although advances have been made in terms of methodology to measure posterior occlusion, the accuracy and reliability of these methods remain questionable. Articulating Indicators Articulating indicators are routinely used in all fields of dentistry to record contacts between the teeth. Articulating indicators can be made out of paper, silk or plastic. They can be found in various thickness, colors, and texture. They can be single-sided or double-sided.22 Articulating indicators are typically placed between the maxillary and mandibular teeth and the patient is instructed to bite down with the indicator between the 8 teeth. This transfers color to the area(s) on the teeth that are in occlusal contact with their antagonists. The clinician can then visualize and evaluate the marked areas to make gross adjustments as needed. Although articulating indicators may seem simple to use, Carossa et al. demonstrated that the assessment of interocclusal contacts is significantly affected by operator experience.23 They also showed that the thickness of the articulating indicators, bite force of the patient and duration of the bite all have an influence on the contact area obtained from articulating indicators. This is in agreement with Millstein’s results, who found that depending on thickness, color and type of material, articulating indicators can produce various quantities of markings.24 A more recent study by Millstein and Maya yielded similar results. They noted that near occlusal contacts may appear as actual contacts depending on the thickness of the marking indicator, suggesting that articulating indicators differ and that physical difference among articulating indicators will affect their ability to mark areas of contact.22 The most recent article on articulating indicators demonstrated that the occlusal markings obtained by articulating indicators are not reflective of the occlusal load and operator subjectiveness is “tantamount to clinical 9 guessing.”25 The available evidence suggests that articulating indictors may not be a reliable and accurate mean of measuring posterior occlusion. Photo-occlusion In 1985, Gazit and Lieberman developed the photoocclusion technique to measure posterior occlusion. The technique consisted of biting into an occlusal wafer for 10 seconds. This was repeated twice to enhance accuracy. The occlusal wafer, with the bite indentations, was then viewed on a polariscope and traced. The quantitative analysis was based on the number of contacts as viewed on the polariscope. The qualitative analysis was based on the colored stress pattern produced by the occlusal wafer. This pattern depends on the degree of light penetration into the occlusal wafer. The authors considered 40% penetration as light contact, 60% penetration as medium contact and over 60% as heavy contact.26 A later study by Gazit et al. examined the reproducibility and reliability of the photo-occlusion technique and compared it to the articulating markers. They measured the posterior occlusion and tooth contacts of 11 subjects with Class I occlusion using both techniques.27 Measurements were taken at the beginning of 10 the study and one month later. They concluded that the photo-occlusion technique had a higher degree of reproducibility than articulating markers. The photo- occlusion technique also produced more contacts than articulating markers, especially in the premolar areas. However, the results indicated that neither the photoocclusion technique nor articulating makers were highly reproducible.27 T-Scan In 1987, Maness et al. introduced the T-Scan, a computerized device developed to capture and record occlusal contacts.28 The T-Scan system is composed of a piezoelectric foil sensor, a sensor handle, the computerized system unit and software to view, record and analyze the data. The disposable sensor, which is less than 100 µm thick, is made of conductive ink on a polyester film substrate. The polyester film substrate is thin, yet strong, resistant to tear and possess elastic properties to resist permanent deformation. According to Maness et al., these attributes makes it superior to other types of polyester registration strips. The patient bites onto, but not through the disposable sensor, and the information is relayed to the computer.28 The software provides a three-dimensional representation 11 of tooth contacts, along with when they occlude. This makes it possible to determine the first tooth contact and the sequence of tooth contacts thereafter. The software also allows comparison of occlusal patterns at different time points, such as before and after treatment. Hsu and collegues conducted a study to evaluate the sensitivity and reliability of the T-Scan.29 The occlusal patterns of 47 patients were analyzed by the T-Scan. The sensitivity and reliability of the sensor in recording occlusal contact was tested with an articulator using reproducible forces. The results showed that the number of occlusal contacts varied within the same patient. On average, the number of occlusal contacts obtained from the T-Scan was less than the actual number of contacts present. They concluded that the sensor of the T-Scan lacked the sensitivity to reliably measure the number of tooth contacts in occlusion.29 This is in agreement with the findings of Lyons et al., who compared the force applied to the sensor for subjects at increasing forces.30 Lyons et al. also found that the number of contacts recorded by the T-scan was not consistent with the force being applied.30 12 Polyether Impression Material Utilizing impressions to obtain information about the dentition is common practice in dentistry. Impression material, such as polyether can also be used to measure the posterior occlusion in maximum intercuspation. Durbin and Sadowsky used polyether rubber impression material to record the posterior occlusion.31 The patient was seated upright and polyether impression material was injected onto the occlusal surfaces of the mandibular teeth. The patient was then instructed to bite firmly onto their posterior teeth and to hold the position for 2 minutes. The bite registration was then transferred to study models with a pencil, using the perforations as a guide. The total numbers of tooth contacts were quantified. However, no attempt was made to investigate areas of near occlusal contact. Razdolsky et al. also used polyether to measure posterior occlusion in a follow up study to the one conducted by Durbin and Sadowsky, using a similar protocol.32 The bite registrations were positioned on a light box in a dark room. Areas of actual contact were identified as perforations in the polyether occlusal registrations and transferred to the study models using a red marker. Razdolsky et al. found no significant 13 differences between the contacts of the polyether bite registration and the actual patient.32 In this follow-up, areas of near occlusal contact were also counted. Areas of near contact were identified by a change in color of the Ramitec material from yellow to a transparent white. The white transparent centers were transferred onto the study models with a black marker. However, Razdolsky et al. did not have a method to check for errors in identifying areas of near contact.32 The evaluation of color change also introduces subjectivity to the evaluation of near contacts. Silicone-based Impression Material Silicone-based impression materials have been used in several studies to evaluate and measure occlusal contacts. In 1992, Haydar et al. used a soft silicone- based impression material to take occlusal records of 20 patients.33 The silicone based impression material was loaded into two separate syringes and applied on the occlusal surfaces of the mandibular teeth. The patients were then asked to bite firmly for 30 seconds. The bite registration was held up to sun light, perforations signified actual contacts, while very thin transparent sections without perforations were considered areas of near occlusal contact. The bite registration was then 14 transferred onto the upper cast of the patients. Photographs of the occlusal view of the models were taken and standardized. Actual contacts and near contacts were then traced on acetate paper. Sauget et al. used vinyl polysiloxane to record the occlusal contacts.34 Again, the impression material was applied on the occlusal surface of all mandibular teeth. The patient was told to bite firmly in maximum intercuspation for 2 minutes. The bite registrations were examined on a light box. Perforations in the impression material were counted as actual contacts. Near contacts were counted only if the size of the thin translucencies on the bite registration were under 0.20 mm, when measured with an Iwanson caliper. Dincer and co-wokers used a silicon putty impression material to measure the occlusal contacts. The material was placed over the mandibular teeth and the patients were instructed to close tightly on their back teeth and also slide the lower jaw to the right, to the left and slide their jaw forward.35 Perforations in the impression material were identified as actual contacts. Translucent areas in the impression material were identified as near contacts. All contacts were transferred to lower study models using different colors for each type of contact. 15 Optical Scanning Manually counting perforations and evaluating translucencies in bite registrations can introduce subjectivity and error. Recently, several researchers have utilized optical scanning of occlusal bite registration to quantify contacts and near contacts. Optical scanning of occlusal bite registrations provides a more objective and accurate method to measure and evaluate contacts and near contacts.20,21,36 The method of optical scanning involves a silicone-based impression material, called Blu Mousse, to obtain the bite registration. Blu Mousse is applied on the occlusal surface of mandibular posterior teeth (1st molars and both premolars) and the patient is instructed to bite down firmly and maintain maximum intercuspation until the material has set. The occlusal registrations are then placed onto the scanner with the mandibular surface facing down. The scanned image of the bite registration is then traced using Image Tool software developed by the University of Texas Health Science Center, San Antonio. The software calculates the platform area of the teeth traced and determines the frequency distributions of the pixels corresponding to each of the 256 gray scales. Calibration step wedges of Blu Mousse impression material of known thickness are used to establish a calibration 16 curve corresponding to the thickness of the impression material and each of the 256 gray scales. This calibration curve, allows the software to analyze the thickness of the occlusal bite registration and determine the thickness based on gray scale values of the area scanned. The thickness of the impression material is typically measured in 50 µm increments, from 0 µm to 350 µm. The increments of 50 µm are significant because pixel densities are only detectable in increments of 50 µm.37 The upper limit of light penetration is between 350 µm and 400 µm. Actual contacts were defined as 50 µm or less and near contacts were defined as greater than 50 µm, but less than 350 µm. Retention Even after the active phase of orthodontic treatment, during which the teeth are properly aligned with fixed appliances, treatment is far from over. Removal of appliances marks the beginning of the most challenging aspect of orthodontics, the retention phase. The primary purpose of retention is to maintain teeth in their corrected positions. Without a retention phase, there is a tendency to the teeth to go back to their pretreatment positions. This tendency is called relapse. The importance of retention was recognized as early as 17 1919, when Hawley wrote that, “if anyone would take my cases when they are finished, retain them and be responsible for them afterwards, I would gladly give them half the fee.”38 The goal of the retention phase is to prevent relapse, maintain the alignment and esthetics of the dentition and allow for post-treatment settling of the occlusion. After the removal of appliances, retention is needed because the gingival and periodontal fibers, which are disrupted by orthodontic movement, require time to reorganize. Reitan demonstrated that fibers of the gingival attachment requires 4-6 months to reorganize, while supracrestal periodontal fibers may take over 232 days to adapt to the new teeth position.39 Retention is critical during and even after the first post-treatment year to prevent relapse of the dentition. Even with retention, it has been shown that incisor crowding and irregularities increased and alignment of teeth was variable and unpredictable in the long run when appliances for retention have been removed.40,41 Another important reason for retention is growth. Not only does growth or lack of growth have the potential to affect the initial malocclusion, it also has the potential to cause malocclusion post-treatment. Class II, Class III, open-bite and deep-bite growth patterns at the start of treatment can still persist when treatment 18 is finalized.42 Selection of retention devices must be based on the patients’ maturational status and any anticipated future growth to prevent unfavorable changes of the dentition. Discoll-Gilliland and collegues has shown that there is significant vertical growth in posterior and lower anterior face height after completion of orthodontic treatment, which was related to increases in lower incisor irregularities in both orthodontically treated and untreated populations.43 Melrose and Meillett concluded that total stability does not exist in the craniofacial skeleton or dentition after treatment, and relapse in saggital, vertical or lateral skeletal dimensions may occur.44 Retention Appliances Hawley Retainer Perhaps the most common type of retainer used is the Hawley retainer or some variation of it. and introduced in the 1920s. It was designed The Hawley retainer is a removable retainer and consists of an adjustable labial bow at the canines and an acrylic base lingual to the teeth.45 Retentive features to hold the retainer include ball clasps, Adam’s clasp or circumferential clasps. 19 The labial bow can be shortened to close small spaces and bent to correct minor rotations. If retention clasps are bent properly and do not cross the occlusal table, maxillary and mandibular Hawley retainers may allow the posterior teeth to contact.46 Hawley retainers are typically worn 24 hours per day for the first 6 months after active orthodontic treatment, and then only at night for 12 hours. Wrap-around Retainer Another type of removable retainer is the wraparound retainer. It is similar to the Hawley retainer except there is a plastic bar along the labial and lingual surfaces of maxillary and mandibular teeth.46 There are no clasps that extend over the occlusal surface, therefore permitting the posterior teeth to come into occlusion. Positioner The tooth positioner is a removable appliance that functions as a finishing appliance, a retainer or both. After a cast is made from the impression, the teeth can be reset to ideal positions to improve arch form, angulation of teeth and minor irregularities. 20 It is a one-piece rubber appliance for both the maxillary and mandibular teeth. Positioners are worn 24 hours per day during the first two days. The patient then wears it for 4 hours during the day and while sleeping. During the 4 hours, the patient is instructed to clench onto the appliance every 30 seconds, repeatedly.47 According to Kesling, clenching into the rubber moves the teeth into position dictated by the appliance and also into more ideal occlusion to allow post-treatment settling.48 However, a common complaint among patients is that the positioner is too bulky, uncomfortable and interferes with speech.47 Perfector The Perfector is a modified version of the positioner. It is made from silicone in order to enhance patient comfort and acceptance. The Perfector also has an anterior labial bow covered with acrylic and seating springs to enhance retention. Fabrication of the Perfector is similar to the Positioner and also has a patient wear protocol very similar to the positioner. It also involves active clenching for 4 hours during the day and passive wear at night.49 21 Spring Retainer The spring retainer is a removable retainer that resembles the Hawley retainer in structure and the positioner in function. Like the Hawley retainer, it has an acyclic base and an adjustable labial bow. Like the positioner, the anterior teeth can be reset to ideal positions. Acrylic covers the labial bow, actively positioning the teeth into the preset alignment. Spring retainers, depending on design, may also have clasps that prevent posterior teeth from contacting. Spring retainers are typically worn 24 hours per day until irregularities are corrected. After that they are worn only at night, for 12 hours per day. Fixed Lingual Retainer Permanent or fixed retention can be used in situations where there is doubt about patient compliance in removable retainer wear. A fixed retainer can be fabricated a number of different ways. Knierim first reported the use of a 0.030 mm stainless steel wire, adapted to the teeth and bonded to the lingual surface of the teeth with composite.50 Since then, many clinicians have incorporated the same technique, but with different wires. Alexander uses a 0.0215-inch multistraned 22 stainless steel wire as a fixed retainer for the mandibular arch.51 A lighter 0.0175 or 0.0195-inch wire can be used to retain closure of maxillary diastema.46 Booth et al. have shown that patients who have worn a bonded lingual retainer for over 20 years exhibited no periodontal problems.52 Essix/Vacuum Formed Retainer/Clear Overlay Retainer The vacuum formed retainer was introduced by Ponitz in 1971. Ponitz called it the “invisible retainers” because it is made from clear plastic.53 Three decades after it was first introduced, the vacuum formed retainer is still fabricated the same way. the impressions of the patient. Casts are poured from A sheet of plastic is heated until the plastic slumps and the vacuum unit exerts negative pressure to adapt the plastic over the patient’s cast. In 1993, Sheridan introduced the Essix retainer, which is essentially the same as the vacuum formed retainer of previous years, except the material is thinner and stronger.54 Essix retainers are made from 0.030 inch sheets of copolyester plastic. It covers the occlusal surface of all teeth and does not allow the posterior teeth to contact. Sheridan recommends that the Essix retainer be worn full-time for a short period and 23 then only at night.54 However, there is no consensus among clinicians concerning Essix retainer wear. Post-Orthodontics Occlusion: Settling The active phase of orthodontic treatment can produce many positive changes such as better alignment of teeth, correction of crossbites, correction of impacted teeth, enhanced esthetics and Class I occlusion.46 Even though the occlusion may be corrected to Class I relationship, the active phase of orthodontic treatment can also have a negative effect on the occlusion. While esthetics generally improves, studies have shown that orthodontic treatment can decrease the number of occlusal contacts.36,55 The number of occlusal contacts then increase during the retention phase. This is a type of “beneficial” relapse commonly referred to as “settling.”26,31-33 The literature suggests that the type of retention device used and the duration of wear also has an effect on the amount of settling of the posterior teeth.26,31,32,34 24 Past Studies Most post-orthodontic occlusal studies demonstrate that occlusal contacts increase during the retention phase, after fixed appliance removal and completion of orthodontic treatment. Using the photo-occlusion technique, Gazit and Lieberman measured the occlusal contacts of the anterior and posterior teeth immediately after completion of orthodontic treatment on 12 patients between 14 and 16 years of age.26 Measurements were made at time of appliance removal, one month after, and one year after appliance removal. They reported an average of 11.2 contacts at the time of appliance removal. After one month, 4 patients showed a decrease in the number of contacts, while 8 showed increases. At one year after appliance removal and completion of orthodontic treatment, all 12 patients showed increases in the number of contacts, with an average of 17.4. The 6.2 gain represents a 56% increase in contacts of both anterior and posterior teeth. However, the authors report that no retention device was used for 3 months prior to the measurement at one year. The authors did not specify the type of retention device used. A study by Dincer et al. in 2003 produced similar results pertaining to post-orthodontic occlusal contact changes.35 In this study, 20 individuals with ideal 25 occlusion served as the control group and were compared to 20 patients with Class I malocclusion who all had four premolar extractions and had been treated with standard edgewise mechanics. The treated patients were all given maxillary and mandibular Hawley retainers at time of appliance removal and were instructed to wear them fulltime for the first six months and nighttime only for the last three months of the study. Silicone putty was used to take the bite registration and measure the posterior occlusion. Perforations in the bite registration were counted as actual contacts and translucent areas were considered near contacts. Dincer et al. reported that at the time of appliance removal, the average number of posterior contacts (both actual and near) was 11.4. At the end of the nine month period after appliance removal, they reported an average of 19 posterior contacts (both actual and near), an increase of 67%. According to the authors, this was considered a significant increase in the number of posterior contacts. However, they noted that one-third of the increases in posterior contacts were not in ideal locations.35 Post-orthodontic occlusal studies have also investigated occlusal contact changes comparing different retention devices. In 1986, Durbin and Sadowsky investigated the post-treatment occlusal changes in 38 patients, 3 months after appliance removal.31 26 The patients were divided into two groups; 23 were given maxillary and mandibular Hawley retainers, and 15 were given tooth positioners as retention device. The patients given Hawley retainers were instructed to wear the appliance at all times except when eating. The patients given tooth positioners were instructed to wear the positioner while sleeping and as much as possible while awake for the first 2 days; followed by 4 hours, while awake, for the first 2 weeks; 3 hours for the next 4 weeks, and 2 hours for the last 6 weeks of the study, while awake. The authors reported that the patients were compliant to a large degree and the that the three who were not compliant were removed from the study.31 The occlusal bite registration was obtained using polyether rubber impression material. The authors reported an increase in the average number of total contacts from 10.11, at the end of treatment to, 11.53, at 3 months after appliance removal, regardless of the type of retention device given. This represents an overall increase of 14.05% and was due mostly to increase in posterior contacts from an average of 8.70 to 10.12 contacts. The study also showed that patients who had fewer teeth in contact at the end of treatment developed more contacts over the 3 month post-treatment period than those who had more contacts. 27 In the same study, Durbin and Sadowsky also reported an increase in the average total number of contacts from 7.77 to 8.87 in the tooth positioner group. This represents a total increase of 1.10 contacts or 14%. Hawley group increased from 9.3 to 10.93. The This represents a total increase of 1.63 or 18% from appliance removal to 3 months post-orthodontic treatment.31 The average number of posterior teeth in contact increased from 9.73 to 10.66 (total increase of 0.93 or 10%) in the positioner group, while the increase in the Hawley retainer group was from 10.65 to 11.39 (total increase of 0.73 or 7%). Even though the average number of total contacts increased significantly more in the Hawley group (p<.05), the authors reported that the tooth positioner was statistically more effective than the Hawley retainer (p<.05) in establishing additional teeth in contact, but the gain was minimal. The author concluded that the interdigitation of teeth improved after orthodontic appliance removal as settling occurs. A follow-up to the study by Durbin and Sadowsky was conducted by Razdolsky et al. in 1989. In this study, 28 of the 38 patients originally evaluated by Durbin and Sadowsky, along with an additional 12 patients were evaluated from 11 to 21 months after orthodontic appliance removal.32 During the period of the study, 28 patients had a maxillary Hawley retainer and a mandibular 28 fixed lingual retainer. Three patients had tooth positioners, followed by maxillary Hawley retainers and 8 patients had maxillary and mandibular Hawley retainers. One patient had only a maxillary Hawley retainer. Patients with removable retainers were instructed to wear the retainers full-time for 6-12 months and then at night only, for the remainder of the study. Similar to the original study, the occlusal bite registration was obtained using polyether rubber impression material and the actual contacts were recorded as perforations in the bite registration. Unlike the original study, Razdolsky et al. also recorded areas of near contact as a change in the color of the bite registration material from yellow to transparent white. The results showed that the mean number of teeth in contact increased from 17.5 at the time of appliance removal to 20.7 (18% increase), approximately 21 months later, which was statistically significant (p<0.01). They also reported that the mean number of contacts (actual and near contacts combined) increased from 36.6 to 58.2, a total increase of 59% or 21.6 total contacts, between the two time points of measurement (p<.001). Haydar et al. also compared the tooth positioner to the Hawley retainer.33 In this study, 10 patients were assigned maxillary and mandibular Hawley retainers and 10 patients were assigned tooth positioners after completion 29 of orthodontic treatment. A non-treated control group of 10 people was also examined. The authors did not specify whether or not the patients were randomly assigned. Bite registrations, using a soft silicone-based impression material, were obtained at the time of appliance removal and again at 3 months after appliance removal. The bite registrations were evaluated visually by holding them up to daylight. Perforations were counted as actual contacts and thin transparent sections were counted as near contacts. The total number of contacts at time of appliance removal was 21.2 for the Hawley group, 24.8 for the tooth positioner group and 39.4 for the untreated control group. At 3 months post-appliance removal, the total number of contacts was 22.4 for the Hawley group, 27.0 for the tooth positioner group and 40.5 for the untreated control group. This represents an increase of 6%, 9%, and 3% respectively. The authors concluded that there were no statistically significant differences in posterior contacts between the 3 groups studied.33 In 2009, Horton et al. published a study comparing the short-term changes in posterior occlusion produced by the Perfector and Hawley retainer.56 In this study, 50 patients who completed orthodontic treatment were randomly assigned to either a Perfector/Spring aligner group or a Hawley retainer group. Twenty-two patients received maxillary and mandibular Hawley retainers and 30 instructed to wear the retainer full-time. Twenty-eight patients received a Perfector and mandibular spring retainer and instructed to wear both devices at night, while sleeping. According to the manufacturer’s instructions, the patients were to perform clenching exercises 3-4 hours during the first two weeks, followed by 2-3 hours during the next 4 weeks, and for 2 hours the final two weeks of the study. However, during the interim period (about 4 weeks), while the retainers were being made, both groups were where given Essix retainers and instructed to wear them full-time. Occlusal bite registrations were taken with Blu Mousse at time of orthodontic appliance removal and 2 months postorthodontic appliance removal. The bite registrations were scanned and evaluated using Image Tool software. The results indicated that the areas of contact and near contact (ACNC) in the Hawley group increased from 6.7 mm², at time of appliance removal, to 11 mm² at 2 months post-orthodontic treatment; a 64% increase. The ACNC for the Perfector/Spring Aligner group also increased from 8.4mm² at time of appliance removal to 13 mm² at 2 months post-orthodontic treatment; a 55% increase. Although, the gain in number of ACNC were statistically significant (p<.05) in both groups during the 2 months of this study, there was no significant difference (p<.05) between the two groups. The patient’s perception of occlusion was 31 also assessed using a seven-item questionnaire. Patients in the Perfector/Spring Aligner group 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. Horton et al. concluded that ACNC increased substantially and similarly in patients wearing Hawley retainers and Perfector/Spring Aligners and there was no difference between the two groups in post-orthodontic settling.56 Bauer et al. conducted a follow-up study to the original study by Horton et al.56,57 In this study, Bauer et al. recalled 40 of the 50 patients from the original study. Twenty-five were from the Perfector/Spring aligner group and 15 were from the Hawley group. Patients in the Perfector/Spring aligner group were given Hawley retainers at 2 months post-orthodontic treatment. Methods to obtain the bite registration and quantitative analysis of ACNC were identical to the original study by Horton et al. However, measurements were taken at 4 time points: (T1), at the end of orthodontic treatment, (T2), 2 months post-orthodontic treatment, (T3), 6 months postorthodontic treatment and (T4), 8 months post-orthodontic treatment. The results indicated that the Hawley group showed a 129% increase in ACNC, while the Perfector/Hawley group showed a 105% increase in ACNC during the 8 months of the study. 32 The greatest increase in ACNC was between T1-T2. The Hawley group showed greater increases in ACNC from T2-T3, while the Perfector/Hawley group showed slightly greater increases from T3-T4. In agreement with Horton et al., the authors of this study concluded that the gain in ACNC were not significantly difference between the two groups after 8 months of retention wear.56,57 Studies Comparing Hawley Retainer to Essix (Vacuum Formed) Retainer The Hawley retainer and the vacuum formed retainer (Essix) are the 2 most commonly prescribed retainers in the United States and the United Kingdom.58 Despite their popularity and frequency of use, only a few studies have been conducted comparing the effectiveness of these two retention devices. In 1998, Lindauer and Shoff published the first study comparing Hawley and Essix retainers.59 In this study, 19 patients were given Essix retainers after full orthodontic treatment. They were instructed to wear the mandibular Essix retainer full-time and the maxillary Essix retainers at least half-time for the first 4 weeks, and both retainers only at night afterwards. Twenty-one patients were given Hawley retainers and instructed to wear the retainers full-time for the first 3 months and only at night, or 12 hours per 33 day afterwards. Study casts were taken at the end of orthodontic treatment and 6 months post-orthodontic treatment. The casts were used to measure anterior crowding using Little’s Irregularity Index, overbite and overjet. The results showed that at 6 months post- orthodontic treatment, patients in the Hawley retainer group showed slightly more incisor irregularity in both arches than the Essix, but the difference was statistically significant only in the maxillary arch (p<.05). They also reported that there were no significant differences between groups in the amount of change in overbite or overjet that occurred. No patients in either group developed anterior open bites. The authors concluded that Essix retainers are just as effective as Hawley retainers in maintaining orthodontic corrections and preventing relapse of the dentition.59 A more recent study comparing the effectiveness of Hawley to Essix retainers in maintaining orthodontic corrections and preventing relapse was conducted in 2007 by Rowland et al.58 This study was a randomized trial and used a much larger sample size than Lindauer and Shoff’s.59 After completion of orthodontic treatment, 196 patients were randomly assigned Hawley retainers and instructed to wear them full-time for the first 3 months, and only at night for the next 3 months. A total of 201 patients were randomly assigned Essix retainers, which 34 they were instructed to wear full-time for the first week, and only at night, or 12 hours per day for the remainder of the study. At the end of the study, only 155 patients remained in each group. The others were dropped from the study due to loss, damaged, noncompliance, or withdrawal from the study. Study models of the patients were taken at orthodontic appliance removal and at 6 months post-orthodontic appliance removal. The results showed no statistically significant difference between the two retainer groups in the changes that occurred in overbite, overjet, intercanine width, and intermolar width. However, a statistically significant difference was found between the retainers in the maintenance of incisor irregularity; the Hawley group demonstrated almost twice as much the change in incisor irregularity over the 6 month period. The Hawley group had a 0.51 mm change in incisor irregularity, while the Essix group had a 0.26 mm change in incisor irregularity. The authors concluded that the Essix or vacuum formed retainers were more effective than Hawley retainers at preventing incisor irregularity, but that the two retainers were equally effective in maintaining overjet, overbite, intercanine width, and intermolar width.58 Hichens et al. conducted a study comparing effectiveness and patient satisfaction of Essix and Hawley retainers.60 A total of 196 patients were randomly 35 assigned to the Hawley group, and 201 patients were randomly assigned into the Essix group. assessed after 3 months and 6 months. time points, all patients were given Each group was At the respective questionnaires to fill out; cost analysis was based on patient records and national databases. A total of 352 questionnaires (Hawley 171, Essix 181) were completed at the 3 month time point. No significant difference was found between the two retainer groups at that time. At the 6 month time point, a total of 350 questionnaires (Hawley 168, Essix 182) were completed. At the 6 month time point, the results showed that patients in the Essix group wore their retainers as instructed more often than the Hawley group. Patients also indicated that the Hawley retainers caused more embarrassment in terms of speech and esthetics. More patients in the Hawley group broke their retainers than patients in the Essix group. It was also determined through cost analysis that the Essix retainer was more cost-effective than the Hawley retainer to the clinician and the patients over the 6 months period. The authors concluded that Essix retainers are less likely to be broken, cause less embarrassment, and are more cost effective for the clinician and the patient than Hawley retainers over the first 6 months posttreatment. 36 A very limited amount of research exists comparing the posterior occlusal changes of the Hawley and Essix retainer. A clinical trial of unknown duration by Sheridan et al. reported that 11 out of 407 patients given Essix retainers developed a slight anterior open bite.54 Sauget et al. conducted a study in 1995 comparing the occlusal contacts of clear overlay retainers and Hawley retainers.34 Thirteen patients received maxillary and mandibular Hawley retainers; two patients received maxillary Hawley retainers with mandibular fixed lingual retainers, and 15 patients received maxillary and mandibular clear overlay retainers. The patients who received Hawley retainers were instructed to wear them full-time. The patients who received clear overlay retainers were instructed to wear them full-time for the first 3 days then nightly afterwards. Vinyl polysiloxane was used to record occlusal contacts at 3 time points; at the time of appliance removal (T1), at the time of retainer delivery, one week later (T2), and at 3 months after retainer delivery (T3). The bite registrations were viewed using a light box; perforations were counted as actual contacts and thin translucencies were counted as near contacts if they were 0.20 mm or less thick. This evaluation of translucencies may be affected by the subjectivity of the authors. At T1, the Hawley retainer group had a mean of 37 34.3 total contacts (actual and near), while the clear overlay retainer group had 31.8 total contacts. There were no significant changes between T1 and T2. At T3, after 3 months retention, the Hawley group had a mean of 45.7 total contacts and the clear overlay retainer group had a mean of 36.7 total contacts. Based on posterior contacts only, the Hawley group had an average increase of 10.6 contacts from T1 to T3; a 33% increase. The clear overlay retainer group only had an average increase of 4.2 contacts; an increase of 13%. These changes in posterior occlusion were statistically significant. It was concluded that the Hawley retainer allowed significantly more settling of the occlusion than the clear overlay retainer.34 The authors did not discuss whether the two patients given maxillary Hawley retainers and mandibular fixed lingual retainers skewed the data. They also failed to mention whether the patients in the two retainer groups were randomly allocated. Dincer and Aslan published the most recent study investigating the effects of Essix retainers on posterior occlusion.61 They investigated the long term changes in occlusal contacts with Essix retainers. Fifteen Class I patients with four premolar extractions completed orthodontic treatment and were given Essix retainers. Fifteen Class I untreated individuals served as the controls. Patients were given maxillary and mandibular 38 Essix retainers at the time of appliance removal and were instructed to wear them full-time for 6 months and then only at night for the next 3 months. Occlusal records were taken using the methods described by Razdolsky et al. at the time of retainer delivery (T0), at the end of the 9 month retention period (T1) and 2.5 years after removal of appliance (T2).32 viewed under a light box. The bite registration was Perforations and thin transparent sections without perforations were also counted. was 21.80. The average number of posterior contacts at T0 At T2 the average number of posterior contacts was 22.13 indicating no significant changes compared to T0. However, at T3, the average number of posterior contacts increased to 27.67, a 27% increase; exceeding the number of average posterior contacts in the untreated control group. It was concluded that in the long term, the number of occlusal contacts increased with Essix retainers.61 The authors did not take records at any time during full-time Essix wear, nor did they indicate whether patients continued to wear the Essix retainer at night after the 9 month retention period. Similar to the study by Sauget et al., this study was also affected by the subjectivity of the authors in counting the contacts they considered to be actual and near. 39 Overall, there have been very few studies evaluating the Essix retainer as a retention device. Two studies indicate that the Essix retainer is slightly superior to the Hawley retainer in preventing incisor irregularities.58,59 However, this may be due to the Essix being more readily accepted by patients than the Hawley retainer. One study showed that Essix retainers increased the number of occlusal contacts over the long term, but they did not indicate whether patients continued to wear the Essix retainers after the retention period.61 The study may also have been affected by the subjectivity of the authors in quantifying actual and near occlusal contacts. Only one study exists comparing the Essix retainer to the Hawley retainer and the method of evaluation may also be affected by the subjectivity of the authors and lack of random allocation of the patients.34 Conclusion The goals of the retention phase are to prevent relapse, maintain the alignment and esthetics of the dentition and allow for post-treatment settling of the occlusion. Currently, there are many different types of retention devices available for the orthodontist to prescribe. It is up to the orthodontist to prescribe a 40 retainer that will best achieve this goal. Clinical experience gives orthodontists a preference in retention device, but that preference is subject to bias. A review of the literature shows that there is very limited evidence based research that directly compares different types of retention devices. Among these studies, there is even less that reaches a definite conclusion as to which retainer is best for the retention phase.62 As previously stated, one of the goals of the retention phase is to allow for post-treatment settling of the posterior occlusion. Many of the past studies used unreliable methods, such as articulating indicators, photo-occlusion method and T-scan to evaluate the settling of the occlusion during retention. Also, the samples in the study were not randomly allocated. More importantly, these studies relied on visually evaluating the bite registration and counting the contacts, introducing bias and subjectivity in to the study. Despite the fact that Hawley retainers and Essix retainer remain the two most commonly prescribed retainers, there is little evidence as to which is best for the posterior occlusion. Two studies have shown that the Essix and Hawley retainer are as equally effective in maintaining alignment of the dentition; the Essix may even be slightly superior in preventing incisor irregularity when compared to the Hawley retainer.58,59 41 Only one study compared settling of the occlusion with the Essix retainer and the Hawley retainer.34 Although it was determined by the authors of this study that the Hawley retainer allowed more posterior settling than the Essix retainer, the method of visually counting and evaluating the contacts introduces subjectivity and bias. Also, this study placed patients who received Essix retainers and Hawley retainers on different wearing protocols. While, most clinicians agree that the Hawley retainer requires full-time wear, there is no agreement on the protocol for Essix wear.62-64 Another aspect that has not been investigated is the patient’s perception of the posterior occlusion during retention. The goal of the present study was to evaluate and compare the effects of two different types of retainers on the posterior occlusion 1 month and 3 months postorthodontic treatments using the same wear regimen. Occlusal bite registration were obtained using siliconebased impression material and analyzed using optical scanning and Image Tool to avoid subjectivity of the investigator. The area of occlusal contacts and near contacts were measured to access whether or not a there is a difference in the posterior settling between the Essix retainer and the Hawley retainer given identical wear regimen. The patient’s perception of their posterior occlusion were also evaluated. 42 Reference List 1. Weinberger B. Historical resume of the evolution and growth of orthodontia. J Am Dent Assoc. 1934;21:20012021. 2. Wahl N. Orthodontics in 3 millennia. Chapter 16: Late 20th century appliances. Am J Orthod Dentofacial Orthop. 2008;134:827-830. 3. Angle E. Treatment of malocclusion of the teeth and fractures of the maxillae. 6th ed. Philadelphia: SS White Dental Manufacturing; 1900. 4. Andrews L. The six keys to normal occlusion. Am J Orthod. 1972; 62:296-309. 5. Okeson J. Management of temporomadibular disorders and occlusions. 5th ed. St. Louis: CV Mosby; 2003. 6. McHorris W. Occlusion with particular emphasis on the functional and parafunctional role of anterior teeth. Part 2. J Clin Orthod. 1979;13:684-701. 7. Clark J, Evans R. Functional occlusion: I. A review. J Orthod. 2001;28:76-81 8. Roth R. Functional occlusion for the orthodontist. J Clin Orthod. 1981;15:32-40,44-51. 9. Williams R. Occlusal treatment for the postorthodontic patient. Am J Orthod. 1971;59:431-442. 10. Aubrey R. Occlusal objectives in orthodontic treatment. Am J Orthod. 1978;74:162-175. 11. Solberg W, Woo M, Houston J. Prevalence of mandibular dysfunction in young adults. J Am Dent Assoc. 1979;98:2534 12. Pullinger A, Seligman D, Solberg W. Temporomandibular disorders. Part II: Occlusal factors associated with temporomandibular joint tenderness and dysfunciton. J Prosthet Dent. 1988;59:363-367. 13. Pullinger A, Seligman D, Gornbein J. A multiple logistic regression analysis of the risk and relative odds of temporomandibular disorders as a function of common occlusal features. J Dent Res. 1993; 72:968-979. 43 14. Ricketts R. Occlusion--the medium of dentistry. J Prosthet Dent. 1969;21:39-60. 15. English J, Buschang P, Throckmorton G. Does malocclusion affect masticatory performance? Angle Orthod. 2002;72:21-27. 16. Toro A, Buschang P, Thorckmorton G, Roldan S. Masticatory performance in children and adolescents with Class I and II malocclusions. Eur J Orthod. 2006;28:112119. 17. Yurkstas A, Manly R. Measurement of occlusal contact area effective in mastication.Am J Orthod. 1949;35:185195. 18. Bakke M, Holm B, Jensen B, 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. 19. Ingervall B, Minder C. Correlation between maximum bite force and facial morphology in children. Angle Orthod. 1997;67:415-422. 20. Julien K, Buschang P, Throckmorton G, Dechow P. Normal masticatory performance in young adults and children. Arch Oral Biol. 1996;41:69-75. 21. Owens S, Buschang P, Throckmorton G, 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. 22. Millstein P, Maya A. An evaluation of occlusal contact marking indicators. A descriptive quantitative method. J Am Dent Assoc. 2001; 132:1280-1286. 23. 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. 24. Millstein P. An evaluation of occlusal contact marking indicators: a descriptive, qualitative method. Quintessence Int Dent Dig. 1983; 14:813-836. 25. Kerstein R. Articulating paper mark misconceptions and computerized occlusal analysis technology. Dent Implantol Update. 2008; 19:41-46. 44 26. Gazit E, Lieberman M. Occlusal contacts following orthodontic treatment. Measured by a photocclusion technique. Angle Orthod. 1985;55:316-320. 27. Gazit E, Lieberman M, Fitzig S. Reproducibility of occlusal marking techniques. J Prosthet Dent. 1986; 55:505-509. 28. Maness W, Benjamin M, Podoloff R, Bobick A, Golden R. Computerized occlusal analysis: a new technology. Quintessence Int. 1987;18:287-292. 29. Hsu M, Palla S, Gallo L. Sensitivity and reliability of the T-scan system for occlusal analysis. J Craniomandib Disord Facial Oral Pain. 1992;6:17-23. 30. Lyons M, Sharkey S, Lamey P. An evaluation of the TScan computerised occlusal analysis system.Int J Prosthodont. 1992;5:166-172. 31. Durbin D, Sadowsky C. Changes in tooth contacts following orthodontic treatment. Am J Orthod Dentofacial Orthop. 1986;90:375-382. 32. Razdolsky Y, Sadowsky C, BeGole E. Occlusal contacts following orthodontic treatment: a follow-up study. Angle Orthod. 1989; 59:181-186. 33. 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. 34. Sauget E, Covell DJ, Boero R, Lieber W. Comparison of occlusal contacts with use of Hawley and clear overlay retainers. Angle Orthod. 1997;67:223-230. 35. Dincer M, Meral O, Turner N. The investigation of occlusal contacts during the retention period. Angle Orthod. 2003;73:640-646. 36. Moller E, Bakke M. Occlusal harmony and disharmony: frauds in clinical dentistry? Int Dent J. 1988; 38:7-18. 37. Sakaguchi R, Anderson G, Delong R. Digital imaging of occlusal contacts in the intercuspal positon. J Prosthodont. 1994;3:193-197. 38. Riedel R. A review of the retention problem. Angle Orthod. 1960;30:179-199. 45 39. Reitan K. Clinical and histologic observations on tooth movement during and after orthodontic treatment. Am J Orthod. 1967;53:721-745. 40. Little R, Riedel R, Artun J. An evaluation of changes in mandibular anterior alignment from 10-20 years postretention. Am J Orthod Dentofacial Orthop. 1988;93:423-428. 41. Little R, Wallen T, Riedel R. Stability and relapse of mandibular anterior alignment-first premolar extraction cases treated by traditional edgewise orthodontics. Am J Orthod. 1981;80:349-365. 42. Nanda R, Nanda S. Considerations of dentofacial growth in long-term retention and stability: is active retention needed?. Am J Orthod Dentofacial Orthop. 1992;101:297-302. 43. Driscoll-Gilliland J, Buschang P, Behrents R. An evaluation of growth and stability in untreated and treated subjects. Am J Orthod Dentofacial Orthop. 2001;120:588-597. 44. Melrose C, Meillett D. Toward a perspective on orthodontic retention? Am J Orthod Dentofacial Orthop. 1998; 113:507-514. 45. Hawley C. A removable retainer. Dent. Cosmos. 1919;61:449. 46. Proffit W, Fields H, Sarver D. Contemporary Orthodontics 4th edition. St. Louis: Mosby;2007. 47. Binder R. Retention and post-treatment stability in the adult dentition. Dent Clin North Am. 1988;32:621-641. 48. Kesling H. The philosophy of the tooth positioning appliance. Am J Ortho and Oral Surg. 1945;31:297-304. 49. Elasser W. Some observations on the history and uses of the Kesling positioner. 1950;36: 368-374. 50. Knierim R. Invisible lower cuspid to cuspid retainer. Angle Orthod. 1973;43:218-220. 51. Alexander R. The 20 Principles of the Alexander Discipline. Chicago: Quintessence; 2008. 52. Booth F, Edelman J, Profitt W. Twenty-year follow-up of patients with permanently bonded mandibular canine-to46 canine retainers. Am J Orthod Dentofacial Orthop. 2008;133:70-76. 53. Ponitz R. Invisible retainers. Am J Orthod. 1971;59:266-272. 54. Sheridan J, LeDoux W, McMinn R. Essix retainers: fabrication and supervision for permanent retention. J Clin Orthod. 1993;27:35-45. 55. Deng Y, Fu M. Occlusal contact changes before and after orthodontic treatment of a group of children & adolescent patients with TMJ disturbance. Aust Orthod J. 1995;13:231-237. 56. Horton J, Buschang P, Oliver D, Behrents R. Comparison of the effects of Hawley and perfector/spring aligner retainers on postorthodontic occlusion. Am J Orthod Dentofacial Orthop. 2009;135:729-736. 57. Bauer E, Buschang P, Oliver D, Behrents R. Posterior occlusal changes with a Hawley vs Perfector/Hawley retainer. 2007. Angle Orthod. (pending). 58. Rowland H, Hichens L, Williams A, et al. The effectiveness of Hawley and vacuum-formed retainers: a single randomized controlled trial. Am J Orthod Dentofacial Orthop. 2007;132:730-737. 59. Lindauer S, Shoff R. Comparison of Essix and Hawley retainers. J Clin Orthod. 1998;65:95-97. 60. Hichens L, Rowland H, Williams A, et al. Costeffectiveness and patient satisfaction: Hawley and vacuum-formed retainers. Eur J Orthod. 2007;29:372-378. 61. Dincer M, Aslan B. Effects of thermoplastic retainers on occlusal contacts. Eur J Orthod. 2009; Epub ahead of print. 62. Littlewood S, Millet D, Doubleday B, Bearn D, Worthington H. Orthodontic retention: a systematic review. J Orthod. 2006; 33:205-212. 63. Ledvinka J. Vacuum-formed retainers more effective than Hawley retainers0. Evid Based Dent. 2009;10:47. 64. Kau C. Orthodontic retention regimes: will we ever have the answer? Evid Based Dent. 2006;7:100. 47 Chapter 3: Journal Article Abstract Purpose: To evaluate and compare the posterior occlusal changes between the Essix and Hawley retainer during the first 3 months of retention. Methods: This randomized trial evaluated 40 patients who had completed full orthodontic treatment and been assigned to wear either Essix or Hawley retainer. They were evaluated at three time points, debond (T1), one month after debond (T2) and three months after debond (T3). Duplicate bite registrations were used to quantity posterior areas of contact and near contact (ACNC). The patient’s perceptions about their occlusion and compliance with retainers wear were evaluated using a five item questionnaire. Results: Both the Essix and the Hawley retainers produced slight decreases in ACNC from T1 to T2. At T2, there were significant (p<.05) group differences of cumulative ACNC at ≤50 µm and ≤100 µm, with the Hawley group having greater ACNC. showed increases in ACNC from T2 to T3. Both groups At T3, significant group differences of cumulative ACNC were observed at ≤50 µm, ≤100 µm, ≤150 µm and ≤200 µm; again, with the Hawley group showing greater ACNC than the Essix 48 group. Total cumulative ACNC of the Essix group decreased from 19.3 mm² at T1 to 17.5 mm² at T3; ACNC of the Hawley increased from 21.2 mm² at T1 to 22.1 mm² at T3. Group differences in ACNC at T3 were not statistically significant. Conclusion: The Hawley retainer produces greater ACNC than the Essix retainer, especially at or below the 200 µm level, but the differences are small. 49 Introduction Ideal occlusion has been defined as Class I molar and canine relationships, along with a maximum number of contacts between the cusp tips and marginal ridges of maxillary and mandibular teeth.1,2 Obtaining ideal occlusion is an important goal of orthodontic treatment because patients with Class I molar relationship are able to break down foods better than patients with Class II or Class III molar relationship.3 Individuals who possess normal occlusion are able to break down foods better than those with malocclusion because they have larger areas of contact and near contact (ACNC) between opposing teeth.4,5 Masticatory performance and bite force are also directly related to the number of occlusal contacts.6-8 While there is no debate regarding the importance of ideal occlusion, post-orthodontic retention remains as one of the most controversial issues in orthodontics. The goal of the retention phase is to prevent relapse, maintain esthetics of dentition and allow for posttreatment settling of the occlusion. It is well established that relapse of the dentition relates to periodontal factors, the stability of the occlusion and growth.9,10 While there is no disagreement that retainers are needed to prevent relapse of the dentition, a spectrum of wavering opinions exists as to the which 50 retention device will be most efficient in allowing settling of the occlusion.11,12 Studies have shown that occlusal contacts decrease during orthodontic treatment, but increase during retention.13-15 The type of retainer used has been shown to have a direct influence on the amount of occlusal contacts gained during retention. For example, Dincer et al. reported a 67% increase in contact and near contact numbers for patients wearing Hawley retainer nine months post-orthodontic treatment.16 Durbin and Sadowsky showed a greater increase in the number of tooth contacts for subjects wearing tooth positoners(18% increase) than those wearing Hawley retainers(14% increase) at three months post-debond.15 In contrast, Haydar et al. found no statistically significant difference between the positioner (6% increase) and the Hawley retainer (3% increase) three months after orthodontic treatment.17 Importantly, the samples in each of these studies were not randomized, making the results susceptible to bias, and the data were subjectively tallied based on the researchers’ visual assessment of occlusal contacts and near contacts. Horton et al. recently conducted a randomized study comparing the occlusal changes between patients wearing Perfector/Spring Aligner or Hawley retainers.18 They found that areas of contact and near contact (ACNC) 51 increased less with the Perfector/Spring Aligner (55%) than with the Hawley retainer (64%) two months postdebond, but the group differences were not statistically significant. However, there was an interim period of 4 weeks, from debond of braces to delivery of retainers, during which the subjects in both groups wore Essix retainers while their retainers were being fabricated. Despite the fact that the Essix and Hawley retainer are the two most commonly prescribed retainers in the United States and the United Kingdom19, there has been only one study directly comparing their effects on the posterior occlusion. Sauget et al. found that Hawley retainers (33%) produce significantly greater contact and near contact numbers than Essix retainers (13%) three months post-debond.20 However, their patients were not randomized and the authors visually counted the contacts and near contacts rather than quantifying the total ACNC. The purpose of this study was to objectively evaluate and compare the posterior occlusal changes in patients who have been randomly allocated to wear either an Essix or Hawley retainer. This information is important because the Essix and Hawley retainer are the two most commonly prescribed retainers, even though their effect on the posterior occlusion have not been well established. 52 Materials and Methods Sixty subjects were recruited from Saint Louis University, Center for Advanced Dental Education, Department of Orthodontics, between March 1, 2009 and August 15, 2009. All subjects had completed full orthodontic treatment to Class I molar and canine relationships. Subjects were excluded from the study if they had any history of temporomandibular disorder, large restorations on posterior teeth, allergies to any materials used this study and periodontal disease and /or muscular dysfunction. Subjects were also withdrawn from the study if they were non-compliant with regards to retainer wear, lost their retainers, or did not have longitudinal records. This study was approved by the Biomedical Institutional Review Board at Saint Louis University. The 60 subjects were randomly assigned to either to the Hawley retainer group or to the Essix retainer group; 30 in the Hawley group and 30 in the Essix group. Those in the Hawley group received maxillary and mandibular Hawley retainers; those in the Essix group received maxillary and mandibular Essix retainers. Ten subjects in the Essix group and 10 subjects in the Hawley group, who either lost their retainers or failed to show up for their follow-up appointment, were excluded from the 53 study. A total of 20 subjects in Essix group and 20 subjects in the Hawley group fully participated in the study. Each group had two subjects with 4 premolar extractions. Maxillary and mandibular Essix retainers for the patients in the Essix group were immediate fabricated and delivered at the same appointment. All Essix retainers were fabricated using Essix C+ and were trimmed 2 mm below the gingival margins. Subjects assigned to the Hawley group had alginate maxillary and mandibular impressions taken one appointment prior to debonding the braces. The Hawley retainers were all fabricated by the same laboratory with ball clasps mesial to the first molar. Subjects in the Hawley group also received their retainers at the debonding appointment. Both groups were instructed to wear their retainers full-time, except when eating and brushing. Data were collected at three time points: At debond of braces (T1), one month post-debond (T2) and three months post-debond (T3). At each time point, two sets of bilateral posterior bite registrations were taken in maximum intercuspation using Blu Mousse (Parkell BioMaterials, Farmingdale, NY), a silicone bite registration material. Blue Mousse was applied to the occlusal surface of mandibular 1st and 2nd premolars and 1st molars. 54 The subjects were instructed to bite down firmly and hold in maximum intercuspation for approximately 1 minute. Each occlusal registration was placed in holders with the mandibular occlusal surface facing down and scanned at 300 DPI. The mandibular premolars and 1st molars were traced using Image Tool (University of Texas Health Science Center, San Antonio) software, which calculated the platform area of the traced teeth and the frequency distribution of pixels within the platform based on 256 possible gray scales (See Appendix for all tables and figures; Fig. 1-3). The calibration curve developed by Horton et al. was used to correlate the 256 gray scales obtained from Image Tool and ACNC thickness of the occlusal registration obtained from the subjects(Fig. 4).18 The thickness of the occlusal registration ranges from 0 µm to 350 µm, with the upper limit for scanned light to be at approximately 350 µm. The thicknesses were analyzed in increments of 50 µm. thickness of 0-50 µm represents actual contact of teeth A and any increments above that represents areas of near contacts. Patient perception of their occlusion was also accessed at the 3 time points (T1, T2 and T3) using questions from a seven-item questionnaire developed by Horton et al to assess opinions about their occlusion, level of discomfort with their occlusion, their 55 masitcatory function and compliance with retainer wear.18 A 148 mm analogue scale was used with the following questions: Q1. How often do you wear your retainer as instructed by your orthodontist? (Asked at T2 and T3 only) Q2. How well do your back teeth fit together when you bite down hard? Q3. Do your back teeth contact each other evenly when you bite down hard? Q4. How well can you chew tough meats, such as steak or chops, with your back teeth? Q5. How much pain do you feel when you bite down hard on your back teeth? The terms “very often” or “never,” “very well or very poor,” and “none” or “very much” served as anchors for the visual analog scale. Statistical Analysis Central tendencies and dispersions were described using medians and interquartile ranges due to the skewness and kurtosis of the data. The five perception questions were changed to percents ranging from 0-100 by dividing each response by 148. The Wilcoxon signed ranked test was used to evaluate changes over time and 56 the Mann-Whitney test was used to evaluate group differences. Results At debond, there were no significant differences between the Hawley and the Essix retainer groups in ACNC or cumulative ACNC. Cumulative ACNC 350 µm in the Essix and Hawley groups decreased 24% and 3%, respectively, during the first month, but neither of the changes was statistically significant (Table 1,2). The decreases in ACNC during the first month were followed by a 20% increase in cumulative ACNC for the Essix group and an 8% increase for the Hawley group during the second and third month (Table 1, Fig. 5). One month after debond, cumulative ACNC showed statistically significant (p<.05) group differences between the Essix and Hawley retainer groups at the ≤50 µm and the ≤100 µm levels (Table 1, Fig 6). Areas of contact (≤50 µm) of the Essix group (0.3 mm²) were significantly smaller than the areas of contact of the Hawley group (0.9 mm²). For ACNC, the Essix group also had significantly smaller ACNC ≤100 µm than the Hawley group (1.2 mm² versus 2.4 mm²). There were also statistically significant group differences at the 50-100 µm and the 300-350 µm levels, with the Hawley group 57 having greater ACNC than the Essix group at each level (Table 1, Fig. 6, 7). At three months post-debond, there were statistically significant group differences in cumulative ACNC at the ≤50 µm, ≤100 µm, ≤150 µm and ≤200 µm levels, with consistently greater ACNC for the Hawley than the Essix retainers (Table 1, Fig. 8). The Hawley group had significantly greater ACNC at the 0-50 µm and 50-100 µm levels than the Essix retainer group (Table 1, Fig 9). Total cumulative areas of contact and near contact (ACNC 350 µm) of the Essix group decreased from 19.3 mm² at debond to 17.5 mm² three month post-debond, representing a decrease of 9%. In contrast, the Hawley showed a slight (5%) increase in total cumulative ACNC over the same three month period, from 21.2 mm² at debond to 22.1 mm² (Table 1, Fig. 1). However, group differences in the total cumulative changes ACNC that occurred between debond and three months post-debond were not statistically significant. The only statistically significant group difference was at the 100-150 µm level, where the Essix group demonstrated a 0.4 mm² decrease and the Hawley showed a 0.1 mm² increase (Table 2). At the time of debond, patients in both the Essix and Hawley retainer group reported that their back teeth fit together very well when they bit down hard (Q2), contacted each other very evenly when they bit down hard 58 (Q3), they were able to chew tough meats very well (Q4), and had little or no pain when they bite down hard (Q5). Patients in the Hawley group reported that their teeth fit together significantly better than the patients in the Essix group; the other questions showed no statistically significant group differences at the time of debond (Table 3). Both groups reported small improvements during the first month. Patients in the both groups indicated a worsening of their occlusion during the second and third months (Table 3). However, there were no statistically significant group differences at T2 or T3 in changes that occurred (Table 4). After the first month, both the Essix group (93%) and the Hawley group (91%) reported that they wore their retainers as instructed by their orthodontist (Q1). Little or no change in compliance of retainer wear occurred during the second and third months. Reported compliance for the Essix and Hawley groups were 88% and 90%, respectively, at the three month assessment. There were no statistically significant group differences in the patients’ perception of how well they wore their retainers at either of the time points or in the changes that occurred over time (Table 4). 59 Discussion Three months after orthodontic appliance removal, the ACNC of the Essix group showed an overall decrease of 1.8mm², representing 9% decrease. This differs from the 13% increase in ACNC reported by Sauget et al. at three months post-appliance removal.20 While both studies evaluated the subjects at the same time point. Sauget et al. instructed their subjects to wear the Essix retainers only at night, while the subjects in the present study instructed the subjects to wear their retainers fulltime.20 A greater increase for subjects wearing the Essix only at night might be expected, due to increase in the posterior occlusion as subjects transition from full-time to part-time wear. The decrease in ACNC between the time of debond and three months later is likely a due to the fact that Essix retainer covers the occlusal surface and not only prevents the posterior teeth from settling, but actually disoccluded the teeth slightly. In fact, the patients in the Essix group experienced a substantial (24%) decrease in ACNC during the first month after debond, followed by an increase of 20% during the subsequent three months. At the end of the three months of retention, the ACNC were still below, but closely approximated, the ACNC at debond. Dincer and Aslan found no significant changes overtime, but their 60 measurements were take nine months after debond, the last three months of which patients only wore their Essix retainers part-time.21 It is not unreasonable to assume that if measurements were taken while patients were still wearing Essix retainers full-time that they would also observe an initial rate of decrease in ACNC, followed by an increase of ACNC close to the value at debond. The initial, decreases in ACNC may have been due to the posterior teeth being unaccustomed to the Essix retainers, as the dentition adjusted to the retainers, the ACNC returned near the amount at debond. The Hawley group demonstrated an overall increase of 0.9 mm² over the three months of retention, representing a 5% increase. The findings compares well with those of Haydar et al who reported a 6% increase over a period of 3 months in subjects wearing Hawley retainers full-time.17 Dincer et al. reported a 67% increase in contacts and near contacts, but their measurements were taken nine months after debond, the last three months of which the patients only wore their Hawley retainers part-time.16 Despite using similar methods to measure the posterior occlusion, a disparity exists between the present results and those of Horton et al., who reported a 63% increase in ACNC after 2 months.18 The difference could be due to the fact that Horton and colleagues kept their patients in Essix retainers for approximately 4 61 weeks while the retainers for the study were being made. Based on the present results, which indicate that there is an initial 24% decrease in ACNC for patients wearing Essix retainers during the first month, the 66% increase in ACNC reported by Horton et al. may include a compensation for the initial decrease, along with the lesser increase associated with Hawley retainers. This further supports the finding that Essix retainers may initially decrease ACNC because it covers the occlusal surface of all teeth. In addition, the Hawley retainers in the present study were made in a different lab and it is also possible that the lab used different size ball clasps for retention of the Hawley retainers. According to Sheridan et al., the minimum bulk thickness of the first Essix retainer was 0.015”. then, the material has become thicker. in the present study is 0.040”. Since The Essix C+ used According to Raintree Essix, after the plastic conforms to the impression model, the thickness of the retainer is 0.030” (762 µm), yielding a bulk thickness of 1524 µm that covers the occlusal surface of all maxillary and mandibular teeth.22 The ball clasps used for the Hawley retainers in the present study has a maximum thickness of 0.032” (812 µm), yielding a maximum thickness of 1624 µm that can potentially lie across the occlusal surface for both arches. However, since ball clasps seat in the occlusal 62 embrasure between teeth and are also staggered, the maximum thickness may decrease substantially. This might explain why the Hawley retainer had significantly greater cumulative ACNC at ≤50 µm and ≤100 µm levels at one month and three months post-debond, as well as greater ACNC at ≤150 µm and ≤200 µm levels at the third month. The ball clasps of the Hawley retainers are staggered and can also conform to the occlusal embrasure of the teeth minimizing the distance the posterior teeth are disoccluded. The full coverage of all teeth of the Essix retainer – the plastic is minimally 1624 µm thick – may prevent the teeth from contacting each other and settling at levels less ≤200 µm. Patients wearing the Essix retainers and patients wearing the Hawley retainers all perceived that their teeth fit together well, contacted each other evenly, they could chew tough meats well and felt little or no pain when biting down hard at the time of debond. was no significant changes overtime. There Horton et al. also reported no changes over time in the perception of occlusion for patients wearing Hawley retainers.18 This suggests that patients are insensitive to small changes in their occlusion if their occlusion fits well. Initially, patients in the Essix group perceived that their teeth did not fit together as well when they bit down hard compared to the Hawley group. 63 This difference might explain the differences in ACNC observed for Essix (19.3 mm²) versus the Hawley group (21.2 mm²), supporting the notion that individuals are able to perceive differences in ACNC. Similar to all studies involving removable retainers, a limitation to this study is patient compliance. Even though all patients reported from the questionnaire that they wore their retainers full-time consistently, it is unknown whether or not their answers actually reflected the time they wore their retainers. Due to the inability to accurately assess amount of time patients actually wore their retainers and the veracity in their answers, a larger sample size was needed. Sixty patients were recruited, but due to dropouts and lost retainers, only 40 were used in the study. Another limitation is the lack of consistency in retainer wear protocol among clinicians, especially with the Essix retainer. There is no consensus with respect to the protocol for Essix wear; it can range from full-time wear to part-time wear only. 11,12,22,23 Since patients were instructed to wear the Essix retainers full-time throughout this study, it is unknown whether part-time wear would have yielded a different result. Clinically, the results imply that the Essix retainer may prevent settling of the dentition at interocclusal distances of ≤200 µm during the first three 64 months of retention compared with the Hawley retainer. However, at interocclsual distances of ≤350 µm, no statistically significant differences between the two retainers exist. Occlusal equilibration should not be performed prior to three months post-debond as the ACNC can increase or decrease depending on the type of retention device. 65 Conclusions 1. After one month of retention, the Hawley retainer produced significantly greater ACNC than the Essix at cumulative ACNC ≤200 µm; after three months of retention the Hawley retainer produced significantly greater ACNC at ≤50 µm, ≤100 µm, ≤150 µm and ≤200 µm levels. The full coverage of all teeth by the Essix retainer may prevent tooth contacts and settling at occlusal distances of ≤200 µm. 2. During the first three months of full-time retainer wear, areas of contact and near contact (ACNC) decreased 9% in patients wearing the Essix retainers and increased 5% in patients wearing the Hawley retainers. Because the absolute differences were small (4.6 mm²) and the variability was larger, the group differences was not statistically significant. 3. Both groups reported that they wore their retainers consistently throughout the study, their teeth fit together well, their teeth contacted each other evenly, they could chew tough meats well, and they felt little or no pain when biting down hard at debond. No significant changes in the patients’ subjective perceptions of retainer wear occurred. 66 Literature Cited 1. Okeson J. Management of temporomadibular disorders and occlusions. 5th ed. St. Louis: CV Mosby; 2003. 2. Roth R. Functional occlusion for the orthodontist. J Clin Orthod. 1981;15:32-40,44-51. 3. English J, Buschang P, Throckmorton G. Does malocclusion affect masticatory performance? Angle Orthod. 2002;72:21-27. 4. Toro A, Buschang P, Thorckmorton G, Roldan S. Masticatory performance in children and adolescents with Class I and II malocclusions. Eur J Orthod. 2006;28:112119. 5. Owens S, Buschang P, Throckmorton G, 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. 6. Yurkstas A, Manly R. Measurement of occlusal contact area effective in mastication.Am J Orthod. 1949;35:185195. 7. Bakke M, Holm B, Jensen B, 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. 8. Julien K, Buschang P, Throckmorton G, Dechow P. Normal masticatory performance in young adults and children. Arch Oral Biol. 1996;41:69-75. 9. Melrose C, Meillett D. Toward a perspective on orthodontic retention? Am J Orthod Dentofacial Orthop. 1998; 113:507-514. 10. Reitan K. Clinical and histologic observations on tooth movement during and after orthodontic treatment. Am J Orthod. 1967;53:721-745. 11. Littlewood S, Millet D, Doubleday B, Bearn D, Worthington H. Orthodontic retention: a systematic review. J Orthod. 2006; 33:205-212. 12. Kau C. Orthodontic retention regimes: will we ever have the answer? Evid Based Dent. 2006;7:100. 67 13. Moller E, Bakke M. Occlusal harmony and disharmony: frauds in clinical dentistry? Int Dent J. 1988; 38:7-18. 14. Deng Y, Fu M. Occlusal contact changes before and after orthodontic treatment of a group of children & adolescent patients with TMJ disturbance. Aust Orthod J. 1995;13:231-237. 15. Durbin D, Sadowsky C. Changes in tooth contacts following orthodontic treatment. Am J Orthod Dentofacial Orthop. 1986;90:375-382. 16. Dincer M, Meral O, Turner N. The investigation of occlusal contacts during the retention period. Angle Orthod. 2003;73:640-646. 17. 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. 18. Horton J, Buschang P, Oliver D, Behrents R. Comparison of the effects of Hawley and perfector/spring aligner retainers on postorthodontic occlusion. Am J Orthod Dentofacial Orthop. 2009;135:729-736. 19. Rowland H, Hichens L, Williams A, et al. The effectiveness of Hawley and vacuum-formed retainers: a single randomized controlled trial. Am J Orthod Dentofacial Orthop. 2007;132:730-737. 20. Sauget E, Covell DJ, Boero R, Lieber W. Comparison of occlusal contacts with use of Hawley and clear overlay retainers. Angle Orthod. 1997;67:223-230. 21. Dincer M, Aslan B. Effects of thermoplastic retainers on occlusal contacts. Eur J Orthod. 2009; Epub ahead of print. 22. Sheridan J, LeDoux W, McMinn R. Essix retainers: fabrication and supervision for permanent retention. J Clin Orthod. 1993;27:35-45. 23. Ledvinka J. Vacuum-formed retainers more effective than Hawley retainers0. Evid Based Dent. 2009;10:47. 68 Appendix 69 Table 1. Areas of contact and near contact (ACNC) of posterior occlusion with Essix and Hawley retainers at debond (T1), 1 month after debond (T2) and 3 months after debond (T3). Thickness (µm) Debond Areas (mm²) Essix Hawley Median IQ Range Median IQ Range 1 Month Post-Debond Areas (mm²) Essix Hawley Median IQ Range Median IQ Range 3 Months Post-Debond Areas (mm²) Essix Hawley Median IQ Range Median IQ Range 70 Individual ≤ 50 50-100 100-150 200-250 250-300 300-350 0.9 1.5 1.9 2.7 3.9 7.6 0.6/1.7 1.0/2.8 1.3/3.4 1.9/4.3 2.9/5.9 5.4/11.0 1.1 2.0 1.9 2.6 4.0 7.7 1.0/1.9 1.3/2.4 1.2/2.6 1.5/3.6 2.2/5.7 5.1/10.4 0.3 0.7 1.2 2.2 3.2 5.4 0.2/0.7 0.3/1.8 0.5/2.4 0.8/3.2 1.2/4.3 3.4/8.3 0.9 1.5 1.7 2.8 4.0 7.3 0.4/1.7 1.1/2.2 1.3/2.5 2.0/3.6 3.0/5.2 6.2/10.4 0.4 0.8 1.3 2.3 3.5 7.2 0.2/0.7 0.4/2.0 0.4/2.6 0.6/3.8 1.2/5.8 3.4/11.4 1.1 2.0 2.1 3.0 4.0 7.9 0.8/1.7 1.5/2.6 1.6/3.2 2.4/4.7 3.3/5.8 5.5/9.9 Cumulative ≤ 50 ≤ 100 ≤ 150 ≤ 200 ≤ 250 ≤ 300 ≤ 350 0.6 2.4 4.1 6.3 9.2 12.6 19.3 0.6/1.7 1.3/4.8 2.6/7.8 4.1/11.8 6.2/15.8 9.2/22.3 15.2/34.1 1.2 3.2 5.2 7.1 9.6 13.0 21.2 1.0/1.9 2.4/4.5 3.6/6.9 4.8/10.1 6.3/13.8 8.5/19.6 13.0/30.0 0.3 1.2 2.2 3.9 6.1 9.4 14.6 0.2/0.7 0.5/2.4 1.0/1.8 1.6/7.8 2.4/10.9 3.8/15.2 6.8/23.4 0.9 2.4 4.2 6.4 9.1 12.7 20.5 0.4/1.7 1.5/3.8 2.7/6.2 4.4/9.1 6.8/12.1 9.8/17.2 17.0/27.1 0.4 1.4 2.2 4.3 6.8 10.3 17.5 0.2/0.7 0.6/2.7 1.1/4.9 1.6/8.4 2.3/11.9 3.4/17.1 6.9/28.8 1.1 3.2 4.8 7.1 10.2 14.2 22.1 0.8/1.7 2.6/4.0 4.5/7.1 6.6/11.1 9.0/16.0 12.3/22.2 17.3/32.1 Table 2. Changes in the areas of contact and near contact (ACNC) of posterior occlusion with Essix and Hawley retainers at debond (T1), 1 month after debond (T2) and 3 months after debond (T3). Thickness (µm) Debond to 1 Month After Debond Changes (mm²) Essix Hawley Median IQ Range Median IQ Range 1 Month to 3 Months After Debond Changes (mm²) Essix Hawley Median IQ Range Median IQ Range Debond to 3 Months After Debond Changes (mm²) Essix Hawley Median IQ Range Median IQ Range 71 Individual ≤ 50 50-100 100-150 150-200 200-250 250-300 300-350 -0.3 -0.2 -0.4 -0.3 -0.5 -0.6 -0.8 -0.9/0.0 -1.4/0.3 -1.1/0.3 -1.0/0.0 -1.1/0.0 -1.6/0.3 -3.8/1.1 -0.3 -0.3 -0.1 -0.1 -0.1 -0.1 0.4 -0.6/0.0 -0.6/0.2 -0.5/0.1 -0.6/0.4 -0.7/0.5 -1.1/1.6 -1.6/1.6 0.1 0.1 0.0 0.0 0.0 0.3 1.5 0.0/0.2 -0.3/0.4 -0.3/0.6 -0.3/0.7 -0.4/0.7 -0.5/1.4 -1.1/2.5 0.3 0.2 0.3 0.1 0.2 0.1 -0.2 0.0/0.5 0.0/0.9 -0.2/0.8 -0.3/0.8 -0.4/0.7 -0.6/0.9 -1.3/1.4 -0.3 -0.5 -0.4 -0.5 -0.3 -0.5 0.9 -0.7/0.0 -1.5/0.3 -1.1/0.1 -0.9/0.5 -0.9/0.5 -1.4/0.8 -3.0/2.4 0.1 0.0 0.1 0.2 0.2 0.0 0.4 -0.2/0.2 -0.2/0.5 -0.2/0.5 -0.3/0.8 -0.5/1.0 -0.6/1.2 -2.0/1.7 Cumulative ≤ 50 ≤ 100 ≤ 150 ≤ 200 ≤ 250 ≤ 300 ≤ 350 -0.3 -0.5 -0.7 -1.0 -1.6 -2.5 -3.0 -0.9/0.0 -2.4/0.2 -3.6/0.4 -4.9/0.2 -6.0/0.0 -7.7/0.3 -10.9/1.3 -0.3 -0.7 -0.8 -0.7 -0.7 -0.6 -1.1 -0.6/0.0 -1.1/0.4 -1.8/0.3 -2.3/0.2 -3.2/0.5 -4.4/0.9 -4.2/2.1 0.1 0.2 0.1 0.4 0.6 1.0 1.5 0.0/0.2 -0.2/0.7 -0.6/1.1 -1.1/1.8 -1.4/2.4 -1.7/3.5 -2.5/7.7 0.3 0.5 0.8 0.8 0.9 1.0 1.1 0.0-0.5 -0.1/1.7 0.0/2.3 -0.3/3.0 -0.6/3.7 -1.3/4.4 -3.1/7.1 -0.3 -0.6 -0.9 1.5 -2.3 -2.9 -2.2 -0.7/0.0 -2.2/0.2 -3.4/0.1 -4.5/0.5 -4.8/1.5 -4.9/2.6 -7.8/5.3 0.1 0.0 0.4 0.5 0.5 0.8 0.7 -0.2/0.2 -0.3/0.3 -0.6/0.8 -0.8/1.2 -1.2/2.4 -1.7/2.7 -1.6/4.7 Table 3. Patient perception of posterior occlusion at debond (T1), 1 month after debond (T2) and 3 months after debond (T3) using a visual analogue scale, converted to percentages. Debond Values (mm) Essix Hawley Question Medium IQ Medium IQ Range Range 72 Teeth Fit Teeth Contact Evenly Chew Tough Meats Pain Biting 1 Month After Debond values (mm) Essix Hawley Medium IQ Medium IQ Range Range 3 Months After Debond values (mm) Essix Hawley Medium IQ Medium IQ Range Range 24 19/43 24 14/32 17 10/23 8 6/20 17 6/46 13 8/29 31 17/41 16 7/24 19 6/44 10 7/21 17 9/38 16 8/34 10 2/30 9 7/15 5 2/24 6 3/9 10 3/19 8 2/12 6 1/10 5 1/17 2 1/12 3 2/13 4 2/22 5 1/30 Table 4. Changes in patient perception of posterior occlusion at debond (T1), 1 month after debond (T2) and 3 months after debond (T3) using a visual analogue scale, converted to percentages. Debond to 1 Month After Debond Changes (mm) Essix Hawley Question Medium IQ Medium IQ Range Range 73 Teeth Fit Teeth Contact Evenly Chew Tough Meats Pain Biting 1 Month After Debond to 3 months After Debond Changes (mm) Essix Hawley Medium IQ Medium IQ Range Range Debond to 3 Months After Debond Changes (mm) Essix Hawley Medium IQ Medium IQ Range Range -1 -18/2 -13 -23/0 0 -7/8 1 -3/13 -8 -18/7 -5 -18/6 -8 -21/23 -2 -10/-13 0 -7/8 6 0/13 -10 -21/0 3 -7/14 -3 -23/8 -3 -8/0 0 -9/5 -1 -3/8 0 -20/6 -3 -8/1 0 -5/6 -1 -8/2 2 -1/9 0 -2/5 1 -4/14 0 -4/4 74 Fig. 1 Photo of registration. Fig. 2 Scan of registration. Fig. 3 Tracing of occlusal table. 75 Fig. 4 Calibration Curve: step-wedge thickness (Y) estimated from grey scales (X). 76 Fig. 5 Median cumulative areas of contact and near contact of Essix and Hawley retainers at ≤350 µm thickness at T1 (debond), T2 (1 month post-debond) and T3 (3 months post-debond). 77 Fig. 6 Median cumulative areas of contact and near contact of Essix and Hawley retainers at ≤50 µm and ≤100 µm thickness at T1 (debond), T2 (1 month post-debond) and T3 (3 months post-debond). 78 Fig. 7 Median absolute areas of contact and near contact of Essix and Hawley retainers at T1 (debond) and T2 (1 month postdebond). 79 Fig. 8 Median cumulative areas of contact and near contact of Essix and Hawley retainers at ≤150 µm and ≤200 µm thickness at T1 (debond), T2 (1 month post-debond) and T3 (3 months post-debond). 80 Fig. 9 Median absolute areas of contact and near contact of Essix and Hawley retainers at T1 (debond) and T3 (3 month postdebond). 81 Fig. 10 Median absolute areas of contact and near contact of Essix and Hawley retainers at T2 (1 month post-debond) and T3 (3 month post-debond). Table 1. Areas of contact and near contact (ACNC) of posterior occlusion with Essix and Hawley retainers at debond (T1), 1 month after debond (T2) and 3 months after debond (T3). Thickness (µm) Individual ≤ 50 50-100 100-150 150-200 200-250 250-300 300-350 Cumulative ≤ 50 ≤ 100 ≤ 150 ≤ 200 ≤ 250 ≤ 300 ≤ 350 Debond Areas (mm²) Essix Hawley Median IQ Median IQ Range Range 1 Months After Debond Areas (mm²) Essix Hawley Median IQ Median IQ Range Range 3 Months After Debond Areas (mm²) Essix Hawley Median IQ Median IQ Range Range 0.9 1.5 1.9 2.2 2.7 3.9 7.6 0.6/1.7 1.0/2.8 1.3/3.4 1.5/3.6 1.9/4.3 2.9/5.9 5.4/11.0 1.1 2.0 1.9 2.0 2.6 4.0 7.7 1.0/1.9 1.3/2.4 1.2/2.6 1.4/2.8 1.5/3.6 2.2/5.7 5.1/10.4 0.3 0.7 1.2 1.7 2.2 3.2 5.4 0.2/0.7 0.3/1.8 0.5/2.4 0.6/2.7 0.8/3.2 1.2/4.3 3.4/8.3 0.9 1.5 1.7 2.2 2.8 4.0 7.3 0.4/1.7 1.1/2.2 1.3/2.5 1.6/3.2 2.0/3.6 3.0/5.2 6.2/10.4 0.4 0.8 1.3 1.8 2.3 3.5 7.2 0.2/0.7 0.4/2.0 0.4/2.6 0.6/3.1 0.6/3.8 1.2/5.8 3.4/11.4 1.1 2.0 2.1 2.3 3.0 4.0 7.9 0.8/1.7 1.5/2.6 1.6/3.2 1.9/3.6 2.4/4.7 3.3/5.8 5.5/9.9 0.6 2.4 4.1 6.3 9.2 12.6 19.3 0.6/1.7 1.3/4.8 2.6/7.8 4.1/11.8 6.2/15.8 9.2/22.3 15.2/34.1 1.2 3.2 5.2 7.1 9.6 13.0 21.2 1.0/1.9 2.4/4.5 3.6/6.9 4.8/10.1 6.3/13.8 8.5/19.6 13.0/30.0 0.3 1.2 2.2 3.9 6.1 9.4 14.6 0.2/0.7 0.5/2.4 1.0/1.8 1.6/7.8 2.4/10.9 3.8/15.2 6.8/23.4 0.9 2.4 4.2 6.4 9.1 12.7 20.5 0.4/1.7 1.5/3.8 2.7/6.2 4.4/9.1 6.8/12.1 9.8/17.2 17.0/27.1 0.4 1.4 2.2 4.3 6.8 10.3 17.5 0.2/0.7 0.6/2.7 1.1/4.9 1.6/8.4 2.3/11.9 3.4/17.1 6.9/28.8 1.1 3.2 4.8 7.1 10.2 14.2 22.1 0.8/1.7 2.6/4.0 4.5/7.1 6.6/11.1 9.0/16.0 12.3/22.2 17.3/32.1 100 Table 2. Changes in the areas of contact and near contact (ACNC) of posterior occlusion with Essix and Hawley retainers at debond (T1), 1 month after debond (T2) and 3 months after debond (T3). 95 Thickness (µm) Individual ≤ 50 50-100 100-150 150-200 200-250 250-300 300-350 Cumulative ≤ 50 ≤ 100 ≤ 150 ≤ 200 ≤ 250 ≤ 300 ≤ 350 Debond to 1 Month After Debond Changes (mm²) Essix Hawley Median IQ Range Median IQ Range 1 Month to 3 Months After Debond Changes (mm²) Essix Hawley Median IQ Median IQ Range Range Debond to 3 Months After Debond Changes (mm²) Essix Hawley Median IQ Median IQ Range Range -0.3 -0.2 -0.4 -0.3 -0.5 -0.6 -0.8 -0.9/0.0 -1.4/0.3 -1.1/0.3 -1.0/0.0 -1.1/0.0 -1.6/0.3 -3.8/1.1 -0.3 -0.3 -0.1 -0.1 -0.1 -0.1 0.4 -0.6/0.0 -0.6/0.2 -0.5/0.1 -0.6/0.4 -0.7/0.5 -1.1/1.6 -1.6/1.6 0.1 0.1 0.0 0.0 0.0 0.3 1.5 0.0/0.2 -0.3/0.4 -0.3/0.6 -0.3/0.7 -0.4/0.7 -0.5/1.4 -1.1/2.5 0.3 0.2 0.3 0.1 0.2 0.1 -0.2 0.0/0.5 0.0/0.9 -0.2/0.8 -0.3/0.8 -0.4/0.7 -0.6/0.9 -1.3/1.4 -0.3 -0.5 -0.4 -0.5 -0.3 -0.5 0.9 -0.7/0.0 -1.5/0.3 -1.1/0.1 -0.9/0.5 -0.9/0.5 -1.4/0.8 -3.0/2.4 0.1 0.0 0.1 0.2 0.2 0.0 0.4 -0.2/0.2 -0.2/0.5 -0.2/0.5 -0.3/0.8 -0.5/1.0 -0.6/1.2 -2.0/1.7 -0.3 -0.5 -0.7 -1.0 -1.6 -2.5 -3.0 -0.9/0.0 -2.4/0.2 -3.6/0.4 -4.9/0.2 -6.0/0.0 -7.7/0.3 -10.9/1.3 -0.3 -0.7 -0.8 -0.7 -0.7 -0.6 -1.1 -0.6/0.0 -1.1/0.4 -1.8/0.3 -2.3/0.2 -3.2/0.5 -4.4/0.9 -4.2/2.1 0.1 0.2 0.1 0.4 0.6 1.0 1.5 0.0/0.2 -0.2/0.7 -0.6/1.1 -1.1/1.8 -1.4/2.4 -1.7/3.5 -2.5/7.7 0.3 0.5 0.8 0.8 0.9 1.0 1.1 0.0-0.5 -0.1/1.7 0.0/2.3 -0.3/3.0 -0.6/3.7 -1.3/4.4 -3.1/7.1 -0.3 -0.6 -0.9 1.5 -2.3 -2.9 -2.2 -0.7/0.0 -2.2/0.2 -3.4/0.1 -4.5/0.5 -4.8/1.5 -4.9/2.6 -7.8/5.3 0.1 0.0 0.4 0.5 0.5 0.8 0.7 -0.2/0.2 -0.3/0.3 -0.6/0.8 -0.8/1.2 -1.2/2.4 -1.7/2.7 -1.6/4.7 101 Table 3. Patient perception of posterior occlusion at at debond (T1), 1 month after debond (T2) and 3 months after debond (T3) using a visual analogue scale. 96 Debond Values (mm) Essix Hawley Question Medium IQ Medium IQ Range Range Teeth Fit Teeth Contact Evenly Chew Tough Meats Pain Biting 1 Month After Debond values (mm) Essix Hawley Medium IQ Medium IQ Range Range 3 Months After Debond values (mm) Essix Hawley Medium IQ Medium IQ Range Range 24 19/43 24 14/32 17 10/23 8 6/20 17 6/46 13 8/29 31 17/41 16 7/24 19 6/44 10 7/21 17 9/38 16 8/34 10 2/30 9 7/15 5 2/24 6 3/9 10 3/19 8 2/12 6 1/10 5 1/17 2 1/12 3 2/13 4 2/22 5 1/30 102 Table 4. Changes in patient perception of posterior occlusion at at debond (T1), 1 month after debond (T2) and 3 months after debond (T3) using a visual analogue scale. Debond to 1 Month After Debond Changes (mm) Essix Hawley Question Medium IQ Medium IQ Range Range Teeth Fit Teeth Contact Evenly Chew Tough Meats Pain Biting 1 Month After Debond to 3 months After Debond Changes (mm) Essix Hawley Medium IQ Medium IQ Range Range Debond to 3 Months After Debond Changes (mm) Essix Hawley Medium IQ Medium IQ Range Range -1 -18/2 -13 -23/0 0 -7/8 1 -3/13 -8 -18/7 -5 -18/6 -8 -21/23 -2 -10/-13 0 -7/8 6 0/13 -10 -21/0 3 -7/14 -3 -23/8 -3 -8/0 0 -9/5 -1 -3/8 0 -20/6 -3 -8/1 0 -5/6 -1 -8/2 2 -1/9 0 -2/5 1 -4/14 0 -4/4 103 Vita Auctoris Collin Tsai was born in San Francisco, California on the 26th of May 1979. He graduated from the University in San Francisco in 2001 with a B.S. degree in Biology and a minor in Chemistry. He entered dental school at the University of California, Los Angeles in 2003 and graduated in 2007. After dental school, he began his residency in orthodontics at Saint Louis University and obtained a certificate in orthodontics and a Master’s Degree in 2009. 100