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THE EFFECT OF ANTEROPOSTERIOR MOVEMENT OF THE MAXILLARY DENTITION ON FACIAL HEIGHT Arthur A. Jones, D.M.D. A Thesis Presented to the Graduate Faculty of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2015 COMMITTEE IN CHARGE OF CANDIDACY: Professor Rolf G. Behrents, Chairperson and Advisor Professor Eustaquio A. Araujo Professor Emeritus Lysle E. Johnston Jr. Associate Clinical Professor Donald R. Oliver i Dedication To my wife, Ashley, for her patience and understanding during the many months I devoted to this project, and for her love and support throughout this entire residency experience. To my parents, whom I credit for every one of my achievements in life. I would not be the person I am today without their unwavering love and support. ii Acknowledgement I wish to express my sincere gratitude to the following individuals: Dr. Behrents, for his guidance throughout my time in residency and his time spent on this project; Dr. Lysle E. Johnston Jr., for his assistance with the structuring of this project and his time spent on corrections; Dr. Araujo, for his valuable input on the project design and time spent reviewing corrections; Dr. Oliver, for your incredible attention to detail and time spent on corrections; Dr. Kevin Walde, for providing the sample used in this thesis; and Dr. Heidi Israel, for her patience and assistance with the statistics for this study. iii Table of Contents List of Tables.............................................v List of Figures...........................................vi CHAPTER 1: INTRODUCTION....................................1 CHAPTER 2: REVIEW OF THE LITERATURE........................2 Contemporary Reasons for Extractions.............2 The Wedge Hypothesis.............................3 Vertical Changes in Untreated Subjects...........5 Vertical Changes During Treatment................7 Extractions.................................7 Cervical Pull Headgear.....................13 Intraoral Molar Distalizers................16 Distalization vs Upper Premolar Extraction.18 Summary and Statement of Thesis.................19 Literature Cited................................22 CHAPTER 3: JOURNAL ARTICLE................................26 Abstract........................................26 Introduction....................................28 Literature Review..........................28 Materials and Methods...........................32 Sample.....................................32 Cephalometric Analysis.....................34 Error Study................................40 Statistical Analysis.......................40 Results.........................................42 Discussion......................................48 Limitations................................48 Effects of Treatment.......................49 Molar Displacement.........................50 Mandibular Rotation........................51 Vertical Changes in Menton.................53 Summary and Conclusions.........................55 Literature Cited................................57 Vita Auctoris.............................................61 iv List of Tables Table 2.1: Studies reporting differential vertical effects resulting from extraction treatment...........12 Table 2.2: Studies reporting vertical changes due to cervical pull headgear treatment..............15 Table 2.3: Studies analyzing vertical effects resulting from molar distalization......................17 Table 3.1: Comparison of T1 measurements between groups..42 Table 3.2: Descriptive statistics for T1, T2, and T2-T1 changes for Frog group........................43 Table 3.3: Descriptive statistics for T1, T2, and T2-T1 changes for U4s extraction group..............44 Table 3.4: Comparison of T2 measurements between groups..45 Table 3.5: Comparison of T2-T1 changes between groups....47 Table 3.6: Mandibular basal bone rotation compared to the literature....................................52 v List of Figures Figure 3.1: Diagram of the Frog appliance................33 Figure 3.2: Vertical measurements relative to SN-7°......36 Figure 3.3: AP measurements relative to SN-7° perpendicular................................36 Figure 3.4: Mandibular basal bone rotation relative to SN-7°........................................38 Figure 3.5: Movement of U6 relative to maxillary basal bone, and rotation of mandibular basal bone relative to maxilla..........................39 Figure 3.6: Movement of L6 relative to mandibular basal bone.........................................40 vi CHAPTER 1: INTRODUCTION Many factors are considered in the decision to extract or not extract in an orthodontic treatment plan. Among these is a belief that facial vertical dimension may be altered by this decision. This belief has come to be known as the “wedge” hypothesis. This concept encompasses supp- osed effects resulting from distal molar movement, as well as mesial molar movement. Accordingly, this literature review will attempt to clarify the nature of changes in facial vertical measurements resulting from treatments that cause anteroposterior (AP) molar movement within “the wedge.” It will begin with a brief review of contemporary reasons for extraction and an introduction to the wedge hypothesis. A necessary discussion of vertical growth in untreated subjects will lead into a progression through the literature reporting vertical effects resulting from extraction treatment, cervical pull headgear, and various intraoral distalizers. In the final section, the short list of studies encompassing upper premolar extraction and distalization treatment will be discussed. 1 CHAPTER 2: REVIEW OF THE LITERATURE Contemporary Reasons For Extraction In his article titled “To extract or not extract: A diagnostic decision, not a marketing decision,” S. Jack Burrow wrote: “Evidence-based treatment is certainly the direction that modern orthodontics should be taking. But unfortunately much orthodontic research evidence is being ignored because of profit considerations, ease of treatment, and marketing efforts.”1 Clinicians take many factors into account when deciding to extract or not extract in a given case, but true to Burrow’s statement, many of the current trends in nonextraction treatment seem to be the result of marketing efforts focused on increasing profits and not necessarily doing what is best for the patient. Numerous personal beliefs and biases unique to each clinician also likely play a role this decision. Some of these beliefs are based on sound evidence, whereas others may simply be based on personal opinions or anecdotal evidence. It is necessary to examine these beliefs to deter- mine what warrants consideration in the decision to extract or not extract. A 1996 study conducted by Baumrind and coworkers2 offers a somewhat current view of orthodontists’ reasons for tooth extraction. The records of 72 subjects were 2 examined and treatment planned by a panel of five experienced clinicians. Reasons for their decision to extract or not extract in each case were recorded and ranked for importance. Crowding was cited as the most important factor favoring extractions, followed closely by incisor protrusion, desire to improve profile, severity of Class II or large anteroposterior (AP) discrepancy, and stability. One of these five factors was considered as the main reason for extraction in 84% of cases. These five factors can be viewed as interconnected entities in orthodontic treatment planning, and the benefits of extraction treatment for their correction is well documented in the literature. However, another factor, “closing the bite,” was cited as a reason for extraction in 7% of the cases in the study. For many clinicians, the so- called “wedge effect” is responsible for this bite closure following extractions. The Wedge Hypothesis The wedge hypothesis is based on an idea that the anatomical relationship of the maxilla to the mandible is wedged apart to a greater or lesser extent by anteroposterior (AP) movement of the interposed dentition. Basic geometry would suggest that anterior movement of the 3 dentition would result in retraction of the wedge, and conversely, posterior movement would cause a greater insertion of the wedge. Although the term “wedge” was not used, Schudy is often referenced as one of the first to describe a disproportionate effect of molar extrusion on anterior facial height and overbite. His papers in the 1960s focused on the delicate balance of condylar growth and molar eruption as the critical factor controlling vertical and horizontal chin position.3 According to Schudy, when treating hyperdivergent patients, “we must not contribute toward a greater open bite by moving the molars occlusally and/or distally,” and concerning hypodivergent patients, “maxillary molars should be moved distally as much as possible.”4 He also strongly favored extraction treatment in hyperdivergent phenotypes and nonextraction in hypodivergent phenotypes. Other prominent authors5 have agreed with this philosophy. These seminal papers laid the groundwork for a belief that changes in vertical dimension could be dictated by the decision to extract or not extract. With the birth of a plausible hypothesis, the supposed wedge effect from AP molar movement went seemingly unquestioned. Any appliance that moved molars distally was immediately deemed to 4 increase facial height, while extraction was automatically deemed to decrease facial height. Vertical Changes in Untreated Subjects Before exploring the effect of orthodontic treatment on vertical dimension, it is necessary to have an understanding of the normal changes in facial height due to growth alone. Among the changes in the facial dimensions from childhood into adulthood, changes in the vertical dimension are perhaps the most significant. The rate of vertical growth peaks during the adolescent growth spurt, but continues at a lower level throughout adulthood.6 Maxillary vertical changes result mainly from sutural lowering along with an increase in height of the alveolar processes. Osseous modeling tends to be resorptive on the nasal floor and depository on the palatal surface, also contributing to the lowering of the palatal plane. The mandibular condyle is the chief site of vertical growth in the mandible, with the height of the ramus expected to increase 1 mm to 2 mm each year from childhood to adolescence.7 Significant rotational changes are also occurring in each jaw at the same time. Björk and Skieller’s8 implant studies in the 1960s showed that overall mandibular changes 5 result from rotation of basal bone relative to the cranial base, along with concurrent remodeling of the exterior surfaces. This distinction between basal, or core rotation, in contrast with apparent rotation of external surfaces is an important concept when evaluating changes. Although, on average, from childhood through adolescence the mandibular basal bone rotates 15 degrees forward, backward and compensatory external remodeling of around 11-12 degrees results in an apparent decrease of the mandibular plane (MP) of only 2 to 4 degrees. Björk and Skieller’s9,10 implant studies also showed that the maxillary basal bone undergoes significant rotation during growth as well, but compensatory remodeling tends to equal the internal rotation. This balance usually results in maintenance of the palatal plane angle relative to the cranial base. Compens- atory tooth eruption serving to maintain occlusal contacts also occurs during this vertical and rotational growth. Deviations from the normal internal or external remodeling patterns in either jaw can result in abnormal vertical facial proportionality. Although somewhat rare, backward rotation of the mandibular basal bone in conjunction with backwards surface remodeling is said to result in 6 a hyperdivergent phenotype with increased vertical proportions.7 A longitudinal study published in 1974 by Riolo et al.11 at the University of Michigan followed numerous cephalometric measures from six to sixteen years of age from an untreated sample of 47 males and 36 females. Lin- ear measures of total facial height (TFH) and lower anterior facial height (LAFH) consisted of the distances from nasion to menton (Na-Me), and anterior nasal spine to menton (ANS-Me), respectively. In males, over the entire age range, mean yearly increases of TFH and LAFH were 3.0 mm, and 1.8 mm, respectively. In females, over the entire age range, mean yearly increases of TFH and LAFH were 1.8 mm, and 0.8 mm, respectively. Vertical Changes During Treatment Extractions The reported effects of extractions on facial height in the literature are mixed, with no solid evidence to support a consistent effect. In 1990, Garlington and Logan12 studied the effect of enucleation of mandibular second premolars in hyperdivergent subjects. Using Björk and Skieller’s9,10 method of structural superimposition, they observed a forward rotation of the mandible in 17 of the 23 7 subjects. When the LAFH measurements were compared to hyperdivergent norms described by Isaacson and co-workers13 the treatment had resulted in a statistically significant decrease in LAFH. In an attempt to control for growth as a confounding factor, Chua et al.14 used data from the Michigan growth standards to convert pre-treatment and post-treatment LAFH measurements to standardized Z scores for 174 Class I and Class II subjects. Changes in Z scores for LAFH were tested for significant differences between a four premolar extraction group and a nonextraction group. The nonextr- action group exhibited a significant increase in LAFH, while the extraction group exhibited no significant change. The mechanics used in the nonextraction group were poorly defined, however, and actual molar movements were not measured in either group. Kocadereli15 compared two groups of 40 Class I subjects treated either nonextraction or with four premolar extractions. The upper first molar (U6) movement relative to PTV was also measured. Following extraction, the U6 came for- ward 3 mm more than in the nonextraction group. Interest- ingly, even with this significant difference in anterior molar movement, no vertical differences were found to be statistically significant between the groups. 8 A consensus in orthodontics is that more anchorage loss can be expected with second premolar extraction when compared to first premolar extraction, suggesting more of an effect on vertical dimension. Kim et al.16 compared four first premolar extraction cases to four second premolar extraction cases as a test of the theory that greater anchorage loss in the second premolar extraction cases should have a more significant effect on vertical dimension. Importantly, arch length discrepancies were matched between the groups to allow for a meaningful comparison of molar movements, and no extraoral anchorage, elastics, expansion devices, or transpalatal arches were used during treatment. No significant between-groups differences were found when the linear changes in facial vertical dimension were compared. There were significant differences in the maxilloma- ndibular angle (MMA) and SN to palatal plane angle (SN-PP), but these differences amounted to roughly 1°. In clinical practice, changes of this magnitude can be considered insignificant, and perhaps cannot be distinguished from routine tracing error. If vertical changes due to the wedge effect following extractions are difficult to demonstrate in growing patients, studying adults with minimal growth potential provides an opportunity to elucidate such an effect. 9 Ramesh and others17 examined 45 adult subjects with SN-GoGn measures of >32. The subjects were treated with upper and lower first premolar extractions. They found no signif- icant change in vertical facial dimensions. The upper and lower molars came forward 2.3 mm, and 2.2 mm, respectively. The upper molar also extruded 2.2 mm; however, a movement that may have negated any potential vertical effects resulting from anterior molar movement. With the exception of Garlington and Logan,12 several of the previous studies suffer from the inadequacy of relying on surface landmarks to detect mandibular rotational The studies using Björk and Skieller’s9,10 method changes. of structural superimposition may paint a clearer picture of the actual changes occurring in each jaw during treatment. A recent study from 2013 conducted by Bayirli et al.18 used this method to compare four premolar extraction treatment to a matched group of untreated controls. Maxillary and mandibular core rotations relative to the cranial base were measured. There was essentially no rotation of the mandibular core relative to the cranial base in the treatment group, whereas in the untreated controls roughly 1° of forward rotation was observed. In 2015, Charalampakis19 conducted a similar comparison of four first premolar 10 extractions, four second premolar extractions, and nonextraction treatment by way of structural superimposition and a cartesian grid. No significant differences in vertical changes were found between the groups. Although a clinically significant wedge effect should be immediately evident at the completion of treatment, data is conflicting regarding changes in facial height in longterm post-treatment comparisons of subjects treated with or without extractions. Interestingly, in a study of 63 bord- erline subjects with a mean post-treatment follow up of 14.5 years, Paquette and coworkers20 found that the overall changes in LAFH during the observation period were significantly less in subjects treated with extractions than those treated without extractions; however, the changes during treatment alone were not significantly different. A simi- lar study of 62 extraction and nonextraction subjects by Luppanapornlarp and Johnston,21 with a mean post-treatment follow up time of 15.3 years, showed no significant differrences in vertical changes during treatment or in the posttreatment period. Table 2.1 summarizes the literature reporting differential vertical effects between extraction treatment and nonextraction treatment. 11 Table 2.1. Studies reporting differential vertical effects resulting from extraction treatment. Author Year Sample(s) Garlington & Logan12 1990 23 U4 L5 Paquette et al.20 1992 30 non-ext Differential Vertical Effect? Yes Yes 33 U4 L4 Luppanapornlarp & Johnston21 1993 29 non-ext No 33 extraction* Chua et al.14 1993 89 non-ext Yes 89 extraction* Kocadereli15 1999 40 non-ext No 40 U4 L4 Kim et al.16 2005 27 U4 L4 No 27 U5 L5 Kumari & Fida22 2010 41 non-ext No 40 U4 L4 Ramesh et al.17 2012 45 U4 L4 Bayirli et al.18 2013 36 untreated No Yes 36 U4 L4 Charalampakis19 2015 67 non-ext No 61 U4 L4 63 U5 L5 non-ext = non-extraction, U4 = upper first premolars, L4 = lower first premolars, U5 = upper second premolars, L5 = lower second premolars, *extraction pattern not specified 12 In summary, few studies have been able to document a significant effect of extraction treatment on vertical dimension or mandibular rotation. While the previously mentioned studies of extraction treatment focus on the effects of relative anterior movement of the dentition, the belief that molar distalization “into the wedge” would lead to increased facial height was also embedded into the minds of many. Cervical Pull Headgear If moving molars distally and occlusally caused opening of the mandibular plane, the Kloehn type cervical pull headgear was seen as the perfect appliance for such an effect. In a large sample of Class II subjects treated in the mixed dentition with partial fixed appliances and headgear, Baumrind et al.23 found that the rate of increase in LAFH was 1.5 times that of untreated controls. The authors suggested that this treatment modality should be avoided when an increase in LAFH is undesirable. However, in an even larger study of 200 cases, Boecler et al.24 found that mean differences in vertical changes are negligible among groups treated with cervical headgear, combination headgear, or no extraoral force at all. Despite limited support for an effect on facial height, the cervical pull headgear controversy still cont- 13 inued to resurface. A prospective randomized clinical trial in 1998 by Ghafari et al.25 comparing cervical headgear to the functional regulator provided some interesting data relative to the wedge hypothesis. In the headgear group the mandibular plane rotated backwards 1° relative to the functional regulator group, a difference not considered clinically significant. The interesting finding, however, was that the palatal plane to mandibular plane angle (PPMP) decreased even with this small amount of backward mandibular rotation. This finding is precisely opposite the expected increase of this angle in a true wedge effect Other recent studies26,27 resulting from distalization. also fail to find significant differences in facial height following the use of cervical pull headgear. Once again, using the structural method of superimposition would provide a better picture of the actual changes occurring during treatment. Haralabakis28 used this method to measure differences in actual mandibular rotation between high and low angle patients treated with cervical pull headgear. Interestingly, both high and low angle groups still showed a marked counterclockwise rotation of the mandible relative to SN, with a difference between them of only .86°. This result represented another finding that counters the expected wedge effect. 14 The study by Baumrind et al.23 in 1981 seemed to offer some solid evidence supporting a wedge effect following the use of cervical pull headgear, but it was followed by less convincing evidence in the years after. Table 2.2 summarizes studies reporting changes in facial height or mandibular rotation due to treatment with cervical pull headgear. Table 2.2 Studies reporting vertical changes due to cervical pull headgear treatment. Author Year Baumrind et al.23 1981 Sample(s) 74 Cervical Differential Vertical Effect? Yes 50 Untreated Boecler et al.24 1989 89 Cervical No 33 No Headgear Ghafari et al.25 1998 35 Headgear No 28 Functional Regulator Alvanos26 2004 30 High angle No 30 Low angle Haralabakis & Sifakakis28 2004 31 High angle No 29 Low angle Freitas et al.27 2008 25 Cervical 16 Untreated 15 No Intraoral Molar Distalizing Appliances Numerous intraoral distalizing appliances have been developed over the years. Almost universally, some variat- ion of a Nance button with wire attachments bonded to premolars serves as the anchorage unit, with active components ranging from nickel titanium coil springs, to larger diameter spring extensions embedded in the acrylic button. Given the similarity of effects of headgear and intraoral distalizers on molar position,29 both appliances would be expected to have similar effects on vertical dimension. Of importance is the fact that the majority of studies on intraoral distalizers are conducted immediately after the distalization phase, not after the completion of treatment. Many of these studies30-32 attribute significant changes in lower facial height or mandibular plane angle to driving molars distally “into the wedge,” although no comparisons were made to untreated controls. When data is examined after completion of comprehensive treatment any significant differences in vertical dimension might be lost. Table 2.3 displays the data from studies of intraoral distalizers reporting changes in LAFH or MP after the completion of treatment with full fixed appliances. 16 Table 2.3 Studies analyzing vertical effects resulting from molar distalization. Author(s) Chiu et al.33 Sample Appliance N=32 Pendulum Molar Movement (mm) -0.5 N=22 Pendulum N=32 N=43 ΔMP (°) ΔLAFH (mm) 0.9 4.4 0.1 0.4 3.2 Distal Jet 0.5 0.1 3.9 Bone -1.9 0.3 .. 2005 Angelieri et al.34 2006 Chiu et al.33 2005 Hourfar et al.35 2014 anchored Frog The changes shown in table 2.3 following comprehensive treatment are unimpressive if one assumes an average treatment time of 2.5 years. Extrapolation of mean yearly incr- eases in LAFH of 1-2 mm from the Michigan growth standards11 provides relative values in untreated controls. Reported changes in MP are also very similar to those of untreated controls. Also important to note in table 2.1, are the modest overall changes in molar position after comprehensive treatment. Anchorage loss after distalization is frequently observed in the next phase of treatment, leaving 17 final molar positions very similar to the pre-treatment position.34 Importantly, none of the previously mentioned studies used structural superimposition to measure changes in mandibular core rotation, all relied on changes in surface landmarks. Distalization vs. Upper Premolar Extraction All of the previously mentioned studies present a gap in the literature when testing the wedge hypothesis. Comp- arisons of different extraction patterns, or different distalization appliances alone explore only that respective aspect of the wedge hypothesis. Comparison of upper first premolar extraction treatment with distalization, however, simultaneously explores both aspects of the wedge hypothesis and could elucidate any differences in vertical dimension due to AP movement of the dentition. Comparisons of this nature are relatively scarce in the literature. In 2009, de Almeida-Pedrin et al.36 compared the pendulum appliance to cervical headgear, and maxillary premolar extractions. The study failed to find any significant differ- ences between the changes in SN-GoGn or LAFH in any of the groups. Two recent master’s theses37,38 compared groups treated with upper premolar extraction to the distal jet appliance. When the changes in SN-GoGn were compared bet- ween the groups, no statistically significant differences 18 were found. A significant weakness is that none of these studies employed structural superimposition to evaluate mandibular rotation. Summary and Statement of Thesis If a clinician is faced with the dilemma of choosing between molar distalization or and extraction of upper first premolars, is it valid to base the decision on the belief that the two treatments lead to different vertical effects? Despite attempts to clarify changes in vertical dimension resulting from nonextraction or extraction treatment, the literature thus far is inconsistent with regard to a clinically significant wedge effect. Evidence thus far does not support a decrease in vertical dimension resulting from anterior movement of posterior teeth during extraction treatment, although some studies do show differential changes between nonextraction treatment and extraction treatment. Support for an increase in vertical dimension or mandibular plane angle following molar distalization “into the wedge” is also inconsistent. Some studies with data immediately following distalization report an effect on vertical dimension, with overall treatment changes 19 appearing very similar to those due to normal vertical growth alone. A weakness in all of the previously mentioned studies is the fact that the focus is on just one aspect of the wedge hypothesis, such as studying varying amounts of distalization or mesial molar displacement alone. A study encompassing both aspects of the wedge hypothesis will provide a clearer picture of differences if the effect can be demonstrated. Starting with similar pre-treatment cond- itions, one treatment should lead to relative distal movement of the upper first molar, and the other should lead to relative anterior movement of the upper first molar. This study will explore the possibilities of increasing or decreasing vertical dimension as a result of the treatment decision. The lack of studies comparing rotational changes with structural superimposition also presents a significant gap in the literature when distalization is compared to upper premolar extraction treatment. Structural superimposition will be used to obtain valid measures of actual mandibular basal bone rotation relative to the maxilla and SN-7°. Superimposition will also provide a clear picture of molar movements relative to each respective jaw. Vertical and horizontal reference planes will also be established to 20 allow measurement of components of change in various surface landmarks. 21 Literature Cited 1. Burrow SJ. To extract or not to extract: a diagnostic decision, not a marketing decision. Am J Orthod Dentofac Orthop. 2008;133:341-2. 2. Baumrind S, Korn EL, Boyd RL, Maxwell R. The decision to extract: Part II. Analysis of clinicians' stated reasons for extraction. Am J Orthod Dentofac Orthop. 1996;109:393–402. 3. Schudy FF. The control of vertical overbite in clinical orthodontics. Angle Orthod. 1968;38:19-39. 4. Schudy FF. Vertical growth versus anteroposterior growth as related to function and treatment. Angle Orthod. 1964;34:75–93. 5. Sassouni V, Nanda S. Analysis of dentofacial vertical proportions. Am J Orthod 1964;50:801-23. 6. Behrents RG. A treatise on the continuum of growth in the aging craniofacial skeleton(PhD Dissertation). University of Michigan, Ann Arbor; 1984. 7. Proffit WR, Fields HW, Sarver DM. Contemporary Orthodontics. 4th ed. St. Louis: Mosby; 2007. 8. Björk A, Skieller V. Normal and abnormal growth of the mandible. A synthesis of longitudinal cephalometric implant studies over a period of 25 years. Eur J Orthod. 1983;5:1-46. 9. Björk A, Skieller V. Growth of the maxilla in three dimensions as revealed radiographically by the implant method. Br J Orthod. 1977;4:53–64. 10. Björk A, Skieller V. Postnatal growth and development of the maxillary complex. Ann Arbor, MI: The University of Michigan; 1976. 11. Riolo ML, Moyers RE, McNamara JA Jr, Hunter WS. An atlas of craniofacial growth. Monograph 2, Craniofacial Growth Series. Ann Arbor: Center for Human Growth and Development, The University of Michigan, 1974:379. 22 12. Garlington M, Logan LR. Vertical changes in high mandibular plane cases following enucleation of second premolars. Angle Orthod. 1990;60:263-7. 13. Isaacson JR, Isaacson RJ, Speidel TM, Worms FW. Extreme variation in vertical facial growth and associated variation in skeletal and dental relations. Angle Orthod. 1971;41:219-29. 14. Chua AL, Lim JY, Lubit EC. The effects of extraction versus nonextraction orthodontic treatment on the growth of the lower anterior face height. Am J Orthod Dentofac Orthop. 1993;104:361-8. 15. Kocadereli I. The effect of first premolar extraction on vertical dimension. Am J Orthod Dentofac Orthop. 1999;116:41-5. 16. Kim TK, Kim JT, Mah J, Yang WS, Baek SH. First or second premolar extraction effects on facial vertical dimension. Angle Orthod. 2005;75:177-82. 17. Ramesh GC, Pradeep MC, Kumar GA, Girish KS, Suresh BS. Over-bite and vertical changes following first premolar extraction in high angle cases. J Contemp Dent Pract. 2012;13:812-8. 18. Bayirli B, Vaden JL, Johnston LE, Jr. Long-term mandibular skeletal and dental effects of standard edgewise treatment. Am J Orthod Dentofac Orthop. 2013;144:682-90. 19. Charalampakis V. Effects of extraction vs nonextraction orthodontic treatment on vertical dental and skeletal measurements. Master’s Thesis, Saint Louis University, 2015. 20. Paquette DE, Beattie JR, Johnston LE, Jr. A long-term comparison of nonextraction and premolar extraction edgewise therapy in "borderline" Class II patients. Am J Orthod Dentofac Orthop. 1992;102:1-14. 21. Luppanapornlarp S, Johnston LE, Jr. The effects of premolar-extraction: a long-term comparison of outcomes in "clear-cut" extraction and nonextraction Class II patients. Angle Orthod. 1993;63:257-72. 23 22. Kumari M, Fida M. Vertical facial and dental arch dimensional changes in extraction vs. non-extraction orthodontic treatment. J Coll Physicians Surg Pak. 2010;20:17-21. 23. Baumrind S, Korn EL, Molthen R, West EE. Changes in facial dimensions associated with the use of forces to retract the maxilla. Am J Orthod. 1981;80:17-30. 24. Boecler PR, Riolo ML, Keeling SD, TenHave TR. Skeletal changes associated with extraoral appliance therapy: an evaluation of 200 consecutively treated cases. Angle Orthod. 1989;59:263-70. 25. Ghafari J, Shofer FS, Jacobsson-Hunt U, Markowitz DL, Laster LL. Headgear versus function regulator in the early treatment of Class II, Division 1 malocclusion: a randomized clinical trial. Am J Orthod Dentofac Orthop. 1998;113:51-61. 26. Alvanos A. A study of the effect of rapid maxillary expansion and cervical pull headgear on patients having Class II malocclusion with low or high mandibular plane angles. Master’s Thesis, Saint Louis University, 2004. 27. Freitas MR, Lima DV, Freitas KMS, Janson G, Henriques JFC. Cephalometric evaluation of Class II malocclusion treatment with cervical headgear and mandibular fixed appliances. Eur J Orthod. 2008;30:477-82. 28. Haralabakis NB, Sifakakis IB. The effect of cervical headgear on patients with high or low mandibular plane angles and the "myth" of posterior mandibular rotation. Am J Orthod Dentofac Orthop. 2004;126:310-17. 29. Taner TU, Yukay F, Pehlivanoglu M, Çakırer B. A comparative analysis of maxillary tooth movement produced by cervical headgear and pend-x appliance. Angle Orthod. 2003;73:686-91. 30. Chaques-Asensi J, Kalra V. Effects of the pendulum appliance on the dentofacial complex. J Clin Orthod. 2001;35:254–7. 24 31. Ghosh J, Nanda RS. Evaluation of an intraoral maxillary molar distalization technique. Am J Orthod Dentofac Orthop. 1996;110:639–46. 32. Kircelli BH, Pektas ZO, Kircelli C. Maxillary molar distalization with a bone-anchored pendulum appliance. Angle Orthod. 2006;76:650-9. 33. Chiu PP, McNamara Jr JA , Franchi L. A comparison of two intraoral molar distalization appliances: distal jet versus pendulum. Am J Orthod Dentofac Orthop. 2005;128:353–65. 34. Angelieri F, Almeida RR, Almeida MR, Fuziy A. Dentoalveolar and skeletal changes associated with the pendulum appliance followed by fixed orthodontic treatment. Am J Orthod Dentofac Orthop. 2006;129:520-7. 35. Hourfar J, Ludwig B, Kanavakis G.. An active, skeletally anchored transpalatal appliance for derotation, distalization and vertical control of maxillary first molars. J Orthod 2014;41:S24-32. 36. De Almeida-Pedrin RR, Henriques JF, de Almeida RR, de Almeida MR, McNamara JA Jr. Effects of the pendulum appliance, cervical headgear, and 2 premolar extractions followed by fixed appliances in patients with Class II malocclusion. Am J Orthod Dentofac Orthop. 2009;136:833-42. 37. Fotakido E. Comparison of soft tissue and dentoalveolar effects of the tad-supported distal jet versus upper premolar extractions. Master’s Thesis, Saint Louis University, 2015. 38. Kaplan NL. Comparison of effects of the distal Jet appliance, upper premolar extraction and headgear in patients with Class II malocclusion. Master’s Thesis, Saint Louis University, 2011. 25 CHAPTER 3: JOURNAL ARTICLE Abstract Purpose: This investigation seeks to determine if antero- posterior movement of the maxillary dentition during orthodontic treatment affects facial vertical changes by nature of a “wedge effect.” Materials and Methods: A sample of 28 Class II subjects treated with maxillary first premolar extraction (U4s) were compared to a sample of 28 Class II subjects treated with the SMD (Frog) molar distalizer using a retrospective study design. The pre- treatment and post-treatment lateral cephalograms for each subject were hand traced. Measurements of landmark positions were made from established vertical and horizontal reference planes. Structural superimposition was also used to measure AP and vertical molar movements within each jaw, as well as actual mandibular core rotation relative to the cranial base and maxilla. Results: When measured from maxillary superimposition, upper premolar extraction treatment resulted in a mean mesial movement of the U6 of 3.31 mm, and distalization with the Frog resulted in a mean distal movement of the U6 of -0.79 mm. The U6 extruded 1 mm more in the U4s group than the Frog group. Despite the statistically significantly different changes in molar position, independent t-tests revealed no 26 significant differences in facial vertical measurements or mandibular rotation between the two groups. Small forward rotation of mandibular basal bone was noted in both treatment groups. Conclusions: Differences in molar extrusion weaken the validity of the test, but likely represent an inherent barrier to achieving a clinically significant wedge effect. This study does not support the idea that anteroposterior movement of molars within “the wedge” causes changes in facial vertical dimension. 27 Introduction Many factors are considered in the decision to treat with or without extractions. For many clinicians, a belief in the so-called “wedge hypothesis” is an appealing concept that may tip the scales toward extraction or nonextraction. The wedge like anatomical relationship of the maxilla and mandible in the lateral view forms the basis for this hypothesis. Basic geometry would suggest that AP movement of the dentition within this dental configuration should alter lower anterior facial height (LAFH) through hinging of the mandible. This hypothesis most often surfaces when consid- ering extractions in hopes of controlling, or decreasing vertical dimension, although it is also appealing when trying to increase facial height through molar distalization in hypodivergent patients. Literature Review Among the changes occurring in the facial structures during childhood and adolescence, those in the vertical dimension are some of the most significant. These changes are largely the result of a balance between sutural growth of the maxilla and growth of the mandibular condyle.1 Comp- lex rotational changes, masked to some extent by external remodeling also occur in each jaw during this time.2-4 28 The usual pattern of mandibular rotation is forward, or counterclockwise, in relation to the maxilla and cranial base, but compensatory remodeling of the external processes can significantly alter the appearance of the changes. Alth- ough the rate of change is greatest in childhood and adolescence, vertical facial growth continues at a lower rate throughout life with slight differences in directional tendencies existing between males and females.5 The extent that orthodontic treatment can alter these vertical changes has long been debated. An ability to ach- ieve predictable and stable alterations in vertical dimension or mandibular rotation through nonsurgical treatment modalities would be an invaluable skill for orthodontists, as mandibular rotation impacts facial esthetics by changing the vertical and horizontal position of the chin. AP occl- usal relationships are also directly affected by this rotation. Schudy is often referenced as one of the first to describe a disproportionate effect of molar movement on incisor relationships. In the 1960s, Schudy6,7 suggested avoi- ding occlusal or distal movement of molars in open bite cases, and encouraged it in hypodivergent patients. Other prominent authors8 have agreed with this concept, recommending extractions in high angle cases and nonextraction 29 treatment in low angle cases. These influential papers laid the groundwork for a belief that vertical dimension can be altered by the decision whether or not to extract. Some studies9,10 suggest that vertical dimension is increased with nonextraction treatment relative to extraction treatment. Others11-14 fail to find significant diff- erences in vertical changes over the course of the two treatments. Although a wedge effect should be evident immediately post-treatment, studies of long-term differences after extraction and nonextraction therapy are inconclusive as well. One study15 found that in the overall observation period changes in LAFH were significantly less in subjects treated with extractions, while another16 showed no significant differences in vertical changes during a similar period of observation. By the nature of a wedge effect, virtually any appliance causing distal molar movement was automatically assumed to cause “clockwise” rotation of the mandible. The Kloehn type headgear, with its posterior and inferior direction of pull, was one of the first modalities to be implicated in causing a wedge effect. Early studies17,18 reported significant increases in vertical dimension with cervical traction, but more recent studies19-22 fail to support this effect. 30 The majority of studies on intraoral distalizing appliances focus on data immediately after the distalization phase,23-25 attributing increases in LAFH and mandibular plane (MP) to distalization “into the wedge.” However, studies reporting changes after comprehensive treatment with fixed appliances26-28 show overall vertical changes that are comparable to those due to normal growth alone. Studies in Class II subjects comparing molar distalization to upper premolar extraction treatment are relativeely scarce in the literature, however, they provide an opportunity to contrast the supposed effects resulting from anterior or posterior movement of the dentition. The eff- ectiveness of modern intraoral distalizing appliances now allows for nonextraction treatment of AP discrepancies that at one time almost certainly required extractions. This option often creates a dilemma when cases fall into a gray area where distalization and extraction of upper premolars are competing treatment options. For some clinicians, dev- iations in LAFH or MP may tip the scales to favor one or the other. One study29 compared the changes in SN-GoGn and LAFH among groups of Class II subjects treated with headgear, distal jet, and upper first premolar extraction and found no significant differences between the groups. 31 Two similar investigations30,31 reporting only SN-GoGn as a vertical indicator also failed to find significant differences between the two treatments. Importantly, none of these stu- dies used structural superimposition to determine the actual rotation of the mandibular basal bone relative to the cranial base or maxilla. The intent of this study was to compare molar distalization treatment to upper first premolar extraction treatment, with the goal of elucidating differential effects on facial vertical dimension due to AP movement of the maxillary dentition. To the author’s knowledge, few studies have used structural superimposition to compare upper premolar extraction treatment to molar distalization. The null hypothesis of the study was that there are no differences in facial vertical changes or mandibular rotation between Class II subjects treated with molar distalization or extraction of upper first premolars. Materials and Methods Sample The data for this retrospective study were obtained from 112 lateral cephalograms from 56 subjects divided into 2 groups based on treatment. For the molar distalization group, the tooth-borne Simplified Molar Distalizer (SMD), 32 or “Frog”, was selected somewhat arbitrarily from the numerous available appliance designs. Developed by Walde,32 this appliance shares the same Nance button and bonded occlusal rests as other distalizers, but incorporates a sagittally oriented jackscrew connected to a transpalatal arch as the active element. A diagram of the Frog appliance can be seen below in figure 3.1. Figure 3.1. Diagram of the Frog appliance (photo used by permission of Dynaflex) The 28 subjects for the SMD (Frog group) treatment group were obtained from the private office of a single clinician. The 28 subjects in the upper first premolar extraction (U4s group) treatment group were obtained from the archives of Saint Louis University Center for Advanced Dental Education. 33 Subjects were initially selected based on the following inclusion criteria: 1. adolescent 2. Caucasian 3. bilateral Class II molar relationship 4. no expansion devices 5. no congenitally missing teeth, and 6. diagnostic quality pre (T1) and post (T2) treatment lateral cephalograms. The subjects in the 2 groups were selected to match pre-treatment age and sex as closely as possible. Each group consisted of 14 males and 14 females, with a mean starting age of 11 years and 9 months in the Frog group, and 12 years and 3 months in the U4s group. The mean treatment duration in each group was 2 years and 11 months. Cephalometric Analysis Pre-treatment (T1) and post-treatment (T2) digital lateral cephalograms for each subject in the Frog group were printed on high quality paper and hand-traced on acetate paper. For the U4s group, pre-treatment and post- treatment lateral cephalograms were hand-traced on acetate paper. Magnification values for all cephalograms were known, allowing for adjustment of all measurements to zero magnification. The first part of the analysis involved the construction of vertical and horizontal reference planes for each tracing. The horizontal reference plane, used for vertical 34 measurements, was established at the level of nasion using the averaged sella-nasion line minus seven degrees (SN-7°). The vertical reference plane, used for AP measurements, was established perpendicular to SN-7° at the anterior-most point on the wall of sella turcica. Reference planes were first constructed on the pre-treatment radiograph, then transferred to the post-treatment radiograph after cranial base superimposition based on stable structures decribed by Björk and Skieller.2 Five vertical landmarks (posterior nasal spine [PNS], anterior nasal spine [ANS], upper molar cusp tip [UMT], lower molar cusp tip [LMT], and menton [Me]) were identified for vertical linear measurements. Four landmarks (subspinale [A], upper molar mesial contact [UMC], lower molar mesial contact [LMC], and supramentale [B]) were identified for AP measurements relative to the SN-7° perpendicular. Vertical and AP measurements were made to various landmarks as shown in figures 3.2 and 3.3. 35 90° 7° N S ANS PNS UMT LMT Me Figure 3.2. Vertical measurements relative to SN-7°. 90° N S A UMC LMC B Figure 3.3. AP measurements relative to SN-7° perpendicular. 36 7° The palatal plane was established using a line through the landmarks ANS and PNS. The mandibular plane (MP) was drawn as a tangent to the lower border of the mandible through Me. The angular relationship formed by the palatal plane and mandibular plane formed the palatal plane to mandibular plane angle (PP-MP°). The mandibular plane angle was measured relative to the SN-7° reference line. Structural superimposition as described by Björk and Skieller2-4 was also used to measure AP and vertical movements of molars relative to each respective jaw, as well as actual mandibular basal bone rotation relative to the cranial base and maxilla. Superimposing on stable structures in the cranial base allows for measurement of the angular change in mandibular fiducial lines, revealing the basal bone rotation (Δ Mand fiducial). Figure 3.4 illustrates the measurement of mandibular basal bone rotation relative to the cranial base. 37 Pre-treatment Post-treatment Δ Mand. fiducial Figure 3.4. Mandibular basal bone rotation relative to cranial base. The maxillary superimposition was used to measure the angular change in mandibular fiducial lines relative to the maxilla (ΔMand/Max fiducial). Forward, or counterclockwise rotations were assigned a negative value, and backward, or clockwise rotations were assigned positive values. The landmarks UMT and LMT were used in the measurement of vertical displacements of the upper first molar (U6) and lower first molar (L6) to each respective jaw. Vertical displacements of the U6 and L6 were measured perpendicular to the palatal and mandibular planes, respectively. Extru- sive displacements were represented by positive values, while intrusive displacements were represented by negative 38 values. The landmarks UMC and LMC were used for the measurement of AP displacements of the U6 and L6 relative to each respective jaw. AP molar movements were measured parallel to the mean functional occlusal plane (MFOP), as described by Johnston.33 Anterior, or mesial molar displac- ements, were represented by positive values, while posterior, or distal displacements, were represented by negative values. Figures 3.5 and 3.6 illustrate these measurements made by way of the structural superimposition technique. U6 V U6 AP Pre-treatment Post-treatment Δ Mand./Max. fiducial Figure 3.5. Movement of U6 relative to maxillary basal bone, and rotation of mandibular basal bone relative to maxilla. 39 L6 AP L6 V Pre-treatment Post-treatment Figure 3.6. Movement of L6 relative to mandibular basal bone. Error Study Ten cephalograms were randomly selected for retracing for a reliability analysis based on Cronbach’s alpha. The lowest alpha value obtained was .92 for U6 vertical displacement, indicating a high degree of reliability. Statistical Analysis All statistical analysis was performed with the Statistical Package for the Social Science (IBM SPSS, Version 2.0, Armonk, NY). Descriptive data were calculated for T1 and T2 measurements in each group, and then compared at each time point with independent samples t-tests to detect T1 and T2 differences. Mean changes during treat- 40 ment (T2-T1) within each group were compared using paired t-tests. Independent samples t-tests were used to detect between group differences in mean treatment changes (T2T1). A significance level of p <.05 was set for all statistical analyses. 41 Results Descriptive statistics for T1 measurements are shown in table 3.2. At T1, statistically significant differences were found in the AP position of A and overjet (OJ). In the U4s group, the mean T1 position of A point was 2.56 mm anteriorly displaced relative to the Frog group. The mean OJ in the Frog group was 3.19 mm, in contrast to 6.95 mm in the U4s group. Table 3.1. Comparison of T1 measurements between groups. Measure Frog U4s P Mean SD Mean SD OJ-Overjet (mm) 3.19 1.80 6.95 2.41 <.001 MP (°) 26.59 5.40 24.53 5.54 .166 PP-MP (°) 25.05 4.92 24.70 5.54 .800 V-ANS (mm) 49.46 3.89 48.03 2.76 .118 V-PNS (mm) 47.84 2.90 47.85 2.52 .992 V-Me (mm) 105.07 6.64 105.10 4.90 .984 V-UMT (mm) 67.44 4.03 67.86 3.28 .668 V-LMT (mm) 67.27 3.97 67.31 3.23 .972 AP-A (mm) 58.57 4.53 61.13 4.70 .043 AP-UMC (mm) 35.09 4.90 36.78 4.82 .197 AP-LMC (mm) 33.18 4.80 33.69 4.68 .687 AP-B (mm) 50.16 5.91 51.77 6.07 .317 Descriptive statistics for T1 and T2 measurements, along with the results of paired t-tests for T1 and T2 42 measurements within each group are shown in tables 3.2 and 3.3. Table 3.2. Descriptive statistics for T1, T2, and T2-T1 changes for Frog group. Measure T1 Mean SD T2 Mean SD T2-T1 Mean SD MP (°) 26.59 5.40 26.21 6.17 -0.37 2.89 .498 PP-MP (°) 25.05 4.92 24.45 6.05 -0.61 3.31 .341 V-ANS (mm) 49.46 3.89 51.60 3.88 2.14 3.13 .001 V-PNS (mm) 47.84 2.90 49.87 3.73 2.03 2.32 <.001 V-Me (mm) 105.07 6.64 111.04 8.78 5.97 4.68 <.000 V-UMT (mm) 67.44 4.03 70.80 5.46 3.36 3.20 <.000 V-LMT (mm) 67.27 3.97 71.03 5.69 3.76 3.59 <.000 AP-A (mm) 58.57 4.53 60.26 5.22 1.69 3.35 .013 AP-UMC (mm) 35.09 4.90 36.13 5.38 1.04 3.44 .121 AP-LMC (mm) 33.18 4.80 37.05 5.97 3.88 3.94 <.001 AP-B (mm) 50.16 5.91 52.43 7.36 2.28 4.89 .020 U6AP (mm) Structural Superimposition Method .. .. .. .. -0.79 1.68 P .. U6V (mm) .. .. .. .. 1.36 1.13 .. L6AP (mm) .. .. .. .. 1.54 1.24 .. L6V (mm) .. .. .. .. 1.93 1.59 .. ΔMand. fiducial(°) .. .. .. .. -1.46 3.76 .. ΔMand./Max. fiducial (°) .. .. .. .. -0.89 3.33 .. 43 Table 3.3. Descriptive statistics for T1, T2, and T2-T1 changes for U4s extraction group. Measure T1 Mean SD T2 Mean SD T2-T1 Mean SD MP (°) 24.54 5.54 24.48 5.94 -0.05 2.10 .894 PP-MP (°) 24.70 5.54 23.98 6.91 -0.71 2.55 .149 V-ANS (mm) 48.03 2.76 50.66 3.11 2.64 2.70 <.001 V-PNS (mm) 47.85 2.52 49.87 1.80 2.02 2.26 <.001 105.10 4.90 111.48 6.76 6.38 4.81 <.001 V-UMT (mm) 67.86 3.28 72.37 3.69 4.51 3.03 <.001 V-LMT (mm) 67.31 3.23 71.72 3.52 4.42 2.94 <.001 AP-A (mm) 61.13 4.70 60.68 5.03 -0.45 2.99 .433 AP-UMC (mm) 36.78 4.82 41.12 6.58 4.34 2.99 <.001 AP-LMC (mm) 33.69 4.68 36.33 6.22 2.64 3.24 <.001 AP-B (mm) 51.77 6.07 52.40 7.00 0.63 2.70 .230 U6AP (mm) Structural Superimposition Method .. .. .. .. 3.31 1.67 .. V-Me (mm) P U6V (mm) .. .. .. .. 2.27 1.73 .. L6AP (mm) .. .. .. .. 1.51 1.26 .. L6V (mm) .. .. .. .. 2.88 2.22 .. ΔMand. fiducial(°) .. .. .. .. -0.48 2.80 .. ΔMand./Max. fiducial (°) .. .. .. .. -2.05 3.20 .. In the Frog group, no statistically significant changes occurred in MP° or PP-MP° during treatment. Stati- stically significant changes occurred in the positions of all vertical landmarks, as well as AP landmarks, with the exception of AP-UMC. In the U4s group, no statistically 44 significant changes occurred in MP° or PP-MP°. Stat- istically significant changes occurred in the positions of all vertical landmarks. Among AP measurements, statis- tically significant changes occurred in AP-UMC, AP-LMC. No significant changes occurred in AP-A or AP-B in the U4s group. A between group comparison of T2 final measurements relative to vertical and horizontal reference planes, seen in table 3.4, revealed a statistically significant difference only in the AP position of UMC. Table 3.4. Comparison of T2 measurements between groups. Measure Frog U4s P Mean SD Mean SD MP (°) 26.21 6.17 24.48 5.94 .289 PP-MP (°) 24.45 6.05 23.98 6.91 .790 V-ANS (mm) 51.60 3.88 50.66 3.11 .323 V-PNS (mm) 49.87 3.73 49.87 1.80 .992 V-Me (mm) 111.04 8.78 111.48 6.76 .834 V-UMT (mm) 70.80 5.46 72.37 3.69 .211 V-LMT (mm) 71.03 5.69 71.72 3.52 .588 AP-A (mm) 60.26 5.22 60.68 5.03 .764 AP-UMC (mm) 36.13 5.38 41.12 6.58 .003 AP-LMC (mm) 37.05 5.97 36.33 6.22 .661 AP-B (mm) 52.43 7.36 52.40 7.00 .986 45 Descriptive statistics along with the results of independent samples t-tests of changes brought about from growth and treatment between groups are shown in table 3.5. Comparison of changes relative to reference planes revealed statistically significant differences between the Frog group and U4s group only in the AP changes of A (p<.014) and UMC (p<.001). T2-T1 changes measured by structural superimposition revealed statistically significant differences between groups in the AP displacement of the U6 (p<.001). The U6 moved distally 0.79 mm relative to the maxilla in the Frog group, and moved mesially 3.31 mm relative to the maxilla in the U4s group. A statistically significant difference in the vertical displacement of the U6 was also noted between the groups when measured with maxillary superimposition (p<.023). In the Frog group, the U6 exhibited a mean extrusion of 1.36 mm, while in the U4s group the U6 exhibited a mean extrusion of 2.27 mm. Struc- tural superimposition revealed no statistically significant differences in the rotation of the mandibular fiducial line relative to the maxilla or SN-7°. 46 Table 3.5. Comparison of T2-T1 changes between groups. Measure Frog U4s P Mean SD Mean SD MP (°) -0.37 2.89 -.054 2.10 .636 PP-MP (°) -0.61 3.31 -0.71 2.55 .893 V-ANS (mm) 2.14 3.13 2.64 2.70 .531 V-PNS (mm) 2.03 2.32 2.02 2.26 .980 V-Me (mm) 5.97 4.68 6.38 4.81 .748 V-UMT (mm) 3.36 3.20 4.51 3.03 .173 V-LMT (mm) 3.76 3.59 4.42 2.94 .458 AP-A (mm) 1.69 3.35 -0.45 2.99 .015 AP-UMC (mm) 1.04 3.44 4.34 2.99 <.001 AP-LMC (mm) 3.88 3.94 2.64 3.24 .207 AP-B (mm) 2.28 4.89 0.63 2.70 .124 Structural Superimposition Method U6AP (mm) -0.79 1.68 3.31 1.67 <.001 U6V (mm) 1.36 1.13 2.27 1.73 .024 L6AP (mm) 1.54 1.24 1.51 1.26 .912 L6V (mm) 1.93 1.59 2.88 2.22 .071 ΔMand. fiducial(°) ΔMand./Max. fiducial (°) -1.46 3.76 -0.48 2.80 .273 -0.89 3.33 -2.05 3.20 .189 47 Discussion Limitations For a valid comparison of vertical changes resulting solely from anteroposterior displacement of the upper molars, vertical molar displacements should be equal in each group. In this regard, structural superimposition revealed a limitation of the study, with a statistically significant difference in U6 extrusion noted between groups. In the U4s group, the U6 extruded 1 mm more than in the Frog group. It is possible that in the U4s group the increased extrusion during mesial displacement of the U6 negated potential vertical effects. This extrusion is in agreement with other studies13 of upper premolar extraction, and may represent an inherent barrier to causing a clinically significant wedge effect during protraction of the dentition. While the sample treated with the Frog was obtained from a single experienced clinician, the U4s sample was obtained from a mix of several clinicians and a variety of treatment mechanics may have been used in the cases. This heterogeneity in mechanics is considered a weakness in some study designs; however, the purpose of this study was to focus on the skeletal response to defined molar movements and not the mechanics used to achieve them. 48 Some argue that a wedge effect only happens in hyperdivergent phenotypes with high mandibular plane angles. In consideration, it is important to note that the Frog group (mean, 26.59°; SD, 5.40°) and U4s group (mean, 24.53°; SD, 5.54°) were characterized as having average mandibular plane angles as a whole, but were distributed over a fairly wide range. Studies of cervical pull headgear in subjects with high mandibular plane angles22 have, thus far, failed to show significant differences when compared to low angles, but perhaps future studies could compare upper premolar extraction treatment to molar distalization in subjects with high mandibular plane angles. Effects of Treatment Comparison of T2 final measurements of both groups revealed strikingly similar positions of all landmarks, with the only statistically significant difference being in the AP position of UMC. The AP positional difference of UMC at T2 between groups amounted to 5 mm, roughly the difference between the expected Class I molar relationship following successful distalization, and the full step Class II molar relationship following the extraction of upper first premolars. 49 Given the increased protrusion and OJ at T1 in the U4s group, the significant difference in the change of point A relative to the Frog group was likely due to greater anterior retraction requirements and subsequent remodeling of cortical bone around incisor apices.34,35 A study by de Almeida-Pedrin and coworkers29 found that A point moved 2 mm posteriorly in the group treated with upper first premolar extractions. The insignificant change in B point in the U4s group was an unexpected finding, however, and may have represented a difference in mandibular growth in this group. Molar Displacement By nature of the study design, AP changes of the U6 relative to both the maxilla, and horizontal reference planes were significantly different between the groups. Relative to the maxillary regional superimposition, the U6 in the Frog group exhibited a mean distal displacement of 0.79 mm, while the U4s group exhibited a mesial displacement of 3.31 mm. The distal displacement noted with the tooth-borne frog appliance used in this study compares favorably to other studies of tooth-borne distalizers.26,27,31 The small net anterior displacement of UMC (1.04 mm) measured relative to the vertical reference plane in the 50 Frog group was likely due to a combination of forward maxillary growth and a loss of anchorage after distalization.27 The mesial molar displacement of 3.31 mm in the U4s treatment group measured by regional superimposition compares favorably with the findings of Luecke and Johnston,36 who reported a mean mesial displacement of 3.2 mm. This mesial displacement of the upper first molar falls in between that reported following the extraction of four first premolars and four second premolars in other studies.14,37 Mandibular Rotation Importantly, in both treatment groups the mandibular plane exhibited small mean forward rotations relative to SN-7°, as well as the palatal plane. This apparent rota- tion of the mandibular plane compares favorably to the findings of Kaplan,31 as well as de-Almeida-Pedrin and coworkers.29 Fotakido30 found small backwards rotations in SN- GoGn (U4s= 0.03°, Distal Jet= 0.91°), although the differences in these changes relative to those of the present study are clinically insignificant. These insignificant changes of the mandibular plane suggest the decision bet- 51 ween these two treatments has little affect on mandibular rotation. The forward rotational pattern of the mandibular basal bone observed in this study also seems consistent in the literature and appears to be unaffected by different treatment modalities. Table 3.6 compares the mandibular basal bone rotation observed in the present study to other literature employing structural superimposition to evaluate the effect of various treatment modalities. Table 3.6. Mandibular basal bone rotation compared to the literature. Year N Present Study 2015 28 U4 extract 28 Distalization 13 U4 extract -0.5 13 Untreated -1.0 31 Cervical HG* -1.3 29 Cervical HG** -2.4 67 Nonext. -0.22 61 4 extract -0.53 63 5 extract 0.06 Meral et al.38 N. Haralabakis & Sifakakis22 Charalampakis14 2004 2004 2015 Treatment Mandibular Basal Rotation (°) -0.48 Author * = FMA> 28°, ** = FMA< 22° 52 -1.46 Vertical Changes in Menton For an analysis of vertical changes in menton, age at start, treatment duration, and method of measurement must be taken into account. The study by Charalampakis14 was most similar in design to the present study, however, treatment duration was shorter and the subjects were older at the start of treatment. Vertical changes in Me ranged from 5.04 mm to 5.18 mm in the study by Charalampakis. Consid- ering the longer duration of treatment in the present study, vertical changes in Me of 5.97 mm and 6.38 mm can be considered comparable. A study by Standerwick39 in 2014 provides some perspective on the vertical changes in menton observed in this study relative to untreated controls. Using data from the Michigan growth standards,40 Standerwick converted the traditional measurements of LAFH and AFH into vertical vectors relative to an SN-7° reference plane. The yearly vertical component of change in pogonion during puberty was 2.8 mm in males, and 1.5 mm in females. Averaging these values to represent the equal distribution of males and females in the present study, followed by multiplication of the mean treatment time (2.9 yrs), gives an expected vertical change of 6.2 mm relative to SN-7°. This value is strikingly 53 similar to the changes (Frog= 5.97 mm, U4s= 6.38 mm) observed in this study. In consideration of these results, we were unable to reject the null hypothesis that there are no differences in facial vertical changes or mandibular rotation between Class II subjects treated with molar distalization or extraction of upper first premolars. 54 Summary and Conclusions Differential vertical effects were not observed between two treatments featuring different AP molar movements. The effects on vertical dimension and mandibular rotation were similar with molar distalization and the extraction of maxillary first premolars. Although the goal was to test the wedge hypothesis, differences in molar extrusion noted between the groups weakened the validity of the test. However, the mechanics used in each group are likely representative of those commonly used in clinical practice, exposing an inherent barrier to achieving a clinically significant wedge effect. While pure anterior translation could theoretically result in a wedge effect, it is highly unlikely that such an effect would be clinically achievable on a consistent basis. In addition, any actual wedge effect is likely masked by measurement error, dentoalveolar compensations, and normal variations in facial growth. This study does not lend support to the idea that changes in vertical dimension are dictated by the decision to distalize molars or extract upper first premolars in Class II patients. It appears that various orthodontic treatments have little impact on facial growth tendencies, 55 and factors other than vertical dimension should guide the decision to pursue extractions or molar distalization. 56 Literature Cited 1. Proffit WR, Fields HW, Sarver DM. Contemporary Orthodontics. 4th ed. St. Louis: Mosby; 2007. 2. Björk A, Skieller V. Normal and abnormal growth of the mandible. A synthesis of longitudinal cephalometric implant studies over a period of 25 years. 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Am J Orthod Dentofac Orthop. 1992;101:4–12. 37. Komolpis R. Cephalometric comparison between first premolar and second premolar extraction. Ann Arbor, MI: University of Michigan; 1998. 38. Meral O, Işcan HN, Okay C, Gürsoy Y. Effects of bilateral upper first premolar extraction on the mandible. Eur J Orthod. 2004;26:223-31. 39. Standerwick R. Analysis of average horizontal and vertical mandibular growth components relative to the horizontal plane as determined from cross-sectional cephalometric normal standards. J Dent App. 2014;1:5560. 40. Riolo ML, Moyers RE, McNamara JA Jr, Hunter WS. An atlas of craniofacial growth. Monograph 2, Craniofacial Growth Series. Ann Arbor: Center for Human Growth and Development, The University of Michigan, 1974:379. 60 Vita Auctoris Arthur Alvin Jones IV was born on June 16, 1982 in Mobile, Alabama. He received his D.M.D degree from the University of Alabama at Birmingham School of Dentistry in May of 2013. In June of 2013 he began his post-graduate training in orthodontics at Saint Louis University, in Saint Louis, Missouri. He is currently a candidate for the degree of Master of Science in Dentistry. 61