AN EVALUATION OF MARGINAL ALVEOLAR BONE IN THE ANTERIOR MANDIBLE USING PRE- AND POST-TREATMENT COMPUTED TOMOGRAPHY IN CASES TREATED NON-EXTRACTION

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1 AN EVALUATION OF MARGINAL ALVEOLAR BONE IN THE ANTERIOR MANDIBLE USING PRE- AND POST-TREATMENT COMPUTED TOMOGRAPHY IN CASES TREATED NON-EXTRACTION David T. Garlock, D.M.D. An Abstract Presented to the Graduate Faculty of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2012

2 ABSTRACT Objective: To evaluate marginal alveolar bone height in the anterior mandible after orthodontic treatment and to assess correlations that exist between morphological and treatment changes. Materials and Methods: Using 57 preand post-treatment CBCTs (17 males and 40 females, 22 Class I and 35 Class II, with an average age of 18.7 ±10.8 years, and an average treatment time of 22.7 ±7.3 months), the cortical bone thickness, ridge thickness, distance from the apex to the labial cortical bone, and the distance from the cemento-enamel-junction (CEJ) to marginal bone crest (MBC) were measured. Changes in the CEJ-MBC distance were correlated with pre-treatment measurements and the treatment changes. Results: While there was great variation, the average facial and lingual vertical bone losses were 1.16 ±2.26 mm and 1.33 ±2.50 mm, respectively. IMPA changes were also highly variable, averaging 2.4 degrees. Facial CEJ to MBC distance change was negatively correlated with lingual CEJ-MBC change, pre-treatment apex level cortical bone thickness (both labial and lingual), pre-treatment apex level ridge thickness, change in midroot level labial cortical bone thickness, and the apex moving closer to the labial cortical bone. Facial CEJ-MBC 1

3 distance was positively correlated with the apex moving forward, change in apex level lingual cortical bone thickness, and change in midroot level lingual cortical bone thickness. Lingual CEJ-MBC distance change was negatively correlated with pre-treatment midroot level labial cortical bone thickness, change in apex level lingual cortical bone thickness, and change in midroot level lingual cortical bone thickness. There was a positive correlation between lingual vertical bone loss and change in midroot level labial cortical bone thickness. Conclusions: Orthodontic treatment causes changes in alveolar bone height and cortical bone thickness around the mandibular incisors. While pre-treatment cortical bone thickness, ridge width thickness and specific tooth movements all played a role in what happens to the bone during treatment, incisor inclination was not correlated with alveolar bone height changes. 2

4 AN EVALUATION OF MARGINAL ALVEOLAR BONE IN THE ANTERIOR MANDIBLE USING PRE- AND POST-TREATMENT COMPUTED TOMOGRAPHY IN CASES TREATED NON-EXTRACTION David T. Garlock, 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 2012

5 COMMITTEE IN CHARGE OF CANDIDANCY: Adjunct Professor Peter H. Buschang, Chairperson and Advisor Professor Eustaquio Araujo Professor Rolf G. Behrents Professor Ki Beom Kim i

6 DEDICATION I dedicate this thesis to my always supportive and loving family. To my wife, Lisa, for her love, support and patience. She has made me into the person I am today. Her sacrifices over the last 8 years, both the seen and unseen, have carried us and allowed us to eat, live, laugh and love. To my children, Sawyer and Sydney, who have brought so much joy and unconditional love to my life, especially during the stressful times. To my parents, Mary and the late Tim Garlock, who are always there for me, and whose love, support and guidance have shaped me into the person I am today. ii

7 ACKNOWLEDGEMENTS My gratitude is first to God, my Father, for life and for all the blessings and opportunities He has given to me. I want to thank my wife, for all her support, especially for managing our home, finances and everything else in our lives, providing an atmosphere and feeling in our home that makes it a sanctuary from the rigors of work. Thank you to Dr. Paquette and Kim Foster of Paquette Orthodontics, for providing the sample for this study and putting in extra time to organize the sample. Lastly, a great thanks to each member of my committee for their help, guidance and knowledge. Thanks to Dr. Behrents for injecting his experience, knowledge and experience into the development of this project. Thanks to Dr. Araujo, for being more than a professor to me, as a friend and a true mentor. Lastly, I want to give a special thanks to Dr. Peter Buschang, for all his assistance with the project design, statistics, brainstorming sessions, and for his expertise and patience. Without his support, none of this would have been possible. He took time out of his busy schedule (even when he was overseas), to help me through this project. iii

8 TABLE OF CONTENTS List of Tables...v List of Figures...vi-vii CHAPTER 1 : INTRODUCTION...1 CHAPTER 2 : REVIEW OF THE LITERATURE Bone Biology...3 Cortical Bone Anatomy...10 Ridge Width Anatomy...16 Incisor Inclination and Symphysis Anatomy..17 Effects of Orthodontics on the Periodontium...19 Cortical Bone Thickness Changes with Treatment..20 Ridge Thickness...24 Alveolar Bone Height Loss...26 Alveolar Bone Loss with Orthodontic Treatment...28 Anterior Mandibular Alveolar Bone Height Loss: Experimental Model...30 Anterior Mandibular Alveolar Bone Height Loss: Clinical Model...33 Incisor Inclination...34 Conclusion...36 References...38 CHAPTER 3 : JOURNAL ARTICLE Abstract...45 Introduction...47 Materials and Methods...49 Sample and Composition...49 Method of Analysis...50 CBCT Analysis...52 Statistical Analysis...54 Results...55 Descriptive statistics and t-test...55 Inter-correlations...59 Pre-treatment inter-correlations...60 Changes in treatment inter-correlations...62 Discussion...67 Conclusions...74 References...75 Appendix...79 Vita Auctoris...88 i

9 LIST OF TABLES Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 - Mean values of mandibular labial alveolar bone width after lower incisor retraction Mean values of mandibular lingual alveolar bone width after retraction of lower incisors Definitions of variables, their associated abbreviations and method error Descriptive statistics and one sample t-test values for tooth position Descriptive statistics and one sample t-test values for bony changes Distance change from CEJ to MBC Correlations with age, treatment time and Angle classification Correlations of pre-treatment variables Correlations of treatment changes variables...63 i

10 LIST OF FIGURES Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure A1 Figure A2 - Cortical bone thickness in cross-sections Mandibular cortical bone thickness in upper buccal area Mandibular cortical bone thickness in upper lingual area Means and ranges of cortical bone thickness in the mandibular buccal region Mandibular width for the upper mandibular third area Linear and angular variables measured Pre- and post-treatment cephalometric superimposition landmarks Averages and standard deviations for tooth movement Scatter plot of facial CEJ-MBC changes and pre-treatment facial cortical bone thickness at apex Scatter plot of facial CEJ-MBC changes and pre-treatment ridge thickness at apex Scatter plot of facial CEJ-MBC changes and lower incisor apex distance change Scatter plot of facial CEJ-MBC changes and apex to cortical bone distance change Scatter plot of facial CEJ-MBC changes and midroot level facial cortical bone thickness change CBCT sagittal orientation of the lower right incisor CBCT coronal orientation of the lower right incisor...79 ii

11 Figure A3 Figure A4 Figure A5 Figure A6 Figure A7 Figure A8 Figure A9 Figure A10 Figure A11 - CBCT axial orientation of lower right incisor All three planes of space on the CBCT oriented simultaneously Distance from the CEJ to Marginal Bone Crest measured on the labial and lingual sides...81 Method for measuring and calculating midroot height...82 Method for measuring midroot ridge thickness...83 Method for measuring midroot level cortical bone thickness on both the labial and lingual...84 Method for measuring the apex level ridge thickness Method for measuring apex level cortical bone thickness on both the labial and lingual Method for measuring distance from apex to internal border of the labial cortical bone...87 iii

12 CHAPTER 1: INTRODUCTION When dental students graduate, they take upon themselves the sacred Hippocratic Oath, vowing to maintain a certain ethical standard and duty to those they treat. 1 In addition to the oath, the philosophical doctrine of do no harm is engraved on their minds. Both these principles should guide orthodontists treatment plans and execution. As en vogue treatment modalities surge and retreat in the clinical orthodontic community, clinicians are faced with the question of what does the literature say and what effects will this treatment have on the patient? Currently, the non-extraction treatment philosophy is gaining popularity. With the obvious limitations of basal bone in cases with large arch length tooth size discrepancy, orthodontists are forced to accept one of two solutions to justify non-extraction treatments, less tooth or more bone. The follow-up question is, if more bone is not created, what happens to the bone when the basal limits are encroached upon? The problem that clinicians face today, in this non-extraction era, is that no one knows what happens to the alveolar bone height when teeth are moved beyond the initial anatomic borders of the mandible. 1

13 With advances in technology, it is now possible to answer this question. Computed tomography has enabled clinicians and researchers to see things they had never been able to see before. The aim of this study was to evaluate how changes of incisor position effect marginal alveolar bone heights, using cone beam computed tomography. A secondary purpose of the study was to evaluate if a correlation exists between initial bone characteristics and amount of vertical alveolar bone loss. Specifically the cortical bone thickness (midroot level and apex level), ridge thickness (midroot level and apex level), incisor angulation, and distance from the apex to the internal border of the cortical bone, will be correlated to marginal alveolar bone loss. In order to comprehend these associations, an understanding of the bone biology and normal anatomy of the mandible is important. This knowledge can then be applied to the effects that orthodontic treatment has on the periodontium. The areas of the periodontium that need to be most closely reviewed are the cortical bone thickness, ridge thickness, and alveolar bone height. Finally, studies on incisor inclination will also be analyzed in order to see what effect it has on the alveolar bone. 2

14 CHAPTER 2 - REVIEW OF THE LITERATURE Bone Biology A solid understanding of the biology behind tooth movement helps clinicians realize the physiological and anatomical limitations of the periodontium. The imperative cells that make orthodontic tooth movement possible are the osteoblasts and osteoclasts. In fact, the matrix producing osteoblasts, tissue resorbing osteoclasts and osteocytes (which are essentially highly specialized and fully differentiated osteoblasts) account for 90% of all cells in the human skeleton. 2 These cells are found along the socket walls nearest the periodontal membrane, on the endosteal side of the cortical bone toward the marrow spaces, and on the surface of the bone trabeculae in the cancellous bone. 3 Osteoblasts are responsible for regulating bone mineralization and are capable of producing microns of new osteoid per day. 4 On the surface of newly deposited bone, osteoid is always present. 3 It is only through specific receptors and transmembrane proteins that osteoblasts are able to respond to the metabolic and mechanical stimuli that turn them on or off. 5,6 On the other hand, osteoclasts are very efficient at resorbing 3

15 bone. In fact, an active osteoclast can resorb the same amount of bone in one day that it would take seven to ten generations of osteoblasts to form. 7 Osteoblasts and osteoclasts are constantly being turned on and off, working in concert to meet the metabolic and mechanical needs of the craniofacial complex. Traditionally, orthodontics has accepted two theories that explain how the cellular components of bone are activated and suppressed. 3 The pressure tension theory correlates with the alterations of blood flow, generated by an orthodontic force, to a change in chemical messengers which consequentially produces tooth movement. The companion theory involves the piezoelectric phenomenon, which is created by the bending of crystalline structures in bone when a force is applied. Slight changes in configuration of the structure are thought to influence bone metabolism and the apposition and resorption process. 8 There also exists a more progressive theory of why teeth move. Henneman et al 9 described a theoretical model involving four stages to tooth movement, starting with the matrix strain and fluid flow stage. Just after the application of force, strain in the PDL matrix and the alveolar bone results in fluid flow in both tissues. It is thought that fluid flow, on the eventual resorption side, 4

16 occludes the canaliculi of the lacunae that lead to the osteocytes. This occlusion leads to osteocyte apoptosis which results in recruitment of bone resorbing osteoclasts. This process is termed the fluid shear stress theory. Stage two is the cell strain stage. Due to the matrix strain and fluid flow, the cells deform, which activates different mediators that in turn activate many different cell types. The third stage is the cell activation and differentiation stage. Responding to deformation, the fibroblast and osteoblasts in the PDL and osteocytes in the bone are activated. The fourth and final stage in the induction of tooth movement is the remodeling stage. This includes a combination of PDL remodeling and alveolar apposition and resorption, all allowing tooth movement. Part of Henneman s theoretical model involves Melsen s 10 assertion that the pressure side of the bone and PDL are not under pressure. Melsen believes that the collagen fibers of the PDL, which connect the tooth with the alveolar bone, are in reality unloaded when pressure is applied and this results in resorption. Because of this, Henneman 9 feels it is more appropriate to term the pressure and tension sides, resorption and apposition sides, respectively. 5

17 The activation of the osteoblasts and osteoclasts necessary for bone resorption and remodeling to occur is essential in the cortical bone remodeling process. In fact, these two cells work together in what is called the Basic Multicellular Unit or BMU to remodel bone. 11 In cortical bone the BMU forms a cylindrical canal via a cutting cone (made up of osteoclasts) digging in the dominant loading direction. 12 The circular tunnel created is then filled in by thousands of trailing osteoblasts that produce an osteon of new bone. 13 Understanding the BMU of bone remodeling helps explain possible mechanism by which bony dehiscence or fenestration might occur. The fact that mechanical forces (stress and strain) on bone influence the resorption and apposition process is well accepted. What is less understood in bone mechanobiology, are the mechanism by which these forces function. Frost 14 explained one potential mechanism in his mechanostat theory. This theory suggests that if local strain levels exceed a mechanical set-point and fall in the microstrains range, bone modeling occurs and cortical bone mass will increase. If microstrain levels are below the microstrain level, bone is removed. Therefore, it seems on the spectrum of stress levels, there 6

18 is stress that can form bone and stress that can remove bone. If microstrain forces exceed a certain level, microfractures can occur in the bone. It has been suggested that the microdamage that occurs in bone can also induce bone formation. 15 The microdamage theory stems from a hypothesis that due to the material fatigue of bone, microcracks form. This then leads to apoptosis of osteocytes near the cracks, which consequentially attracts osteoclasts to the site. 16 Microcracks in bone represent the initial damage that precludes bone being remodeled. If microcracks form in the alveolar process due to orthodontic forces moving teeth into the alveolar bone, this could provide a possible explanation of what happens to bone. In addition to bone biology, the unique qualities of the periodontium allow teeth to move through alveolar bone when orthodontic forces are placed on the teeth. 17 From the teeth, the force is then transmitted through a collagenous membrane called the periodontal ligament (PDL). The PDL provides a nutritive and functional purpose to the tissues to which it attaches. It contains blood vessels and undifferentiated stem cells that have the potential to become osteoblasts, cementoblasts and fibroblasts. 18 The 7

19 principle fibers in the PDL are embedded in the bone surrounding the roots, called bundle bone or the alveolar bone proper. 3 Alveolar bone surrounds the tooth to a vertical level about 1 mm apical to the cementoenamel junction. 3 After reaching a certain thickness and maturity, parts of the bundle bone are reorganized into lamellated bone. 3 Consequentially, when a force is applied to a tooth, through the PDL and then to the bone, apposition and resorption zones are created within the PDL. These two processes results in permanent tooth movement through the alveolar bone. 19 The periosteum, which is a thin outer tissue layer of the bone, also contains cellular components which are activated during bone apposition and resorption. The matrix-producing and proliferating cells in the cambium layer (of the periosteum) are subject to mechanical influence. Whenever the pressure exceeds a certain threshold, reducing the blood supply to these cells, osteogenesis ceases. However, if the periosteum is exposed to tension, it responds with bone deposition. 3 Viewing the maturation of bone histologically, two types of bone can be differentiated based on their molecular structure. Woven bone and lamellar bone are defined by their microscopic appearance. Woven bone is 8

20 found during the embryonic and fetal stages of life, in ligament and tendon insertions of healthy adults and in regions where the structure of bone has been compromised by pathology or fracture. In general, woven bone is immature and poorly developed bone. 20 Mechanical stimulation perpetuates the rapid production of woven bone which ultimately remodels into dense lamellar bone. 21 Lamellar or mature bone can be found in both trabecular and cortical bone. It materializes within a few weeks after woven bone is deposited. Understanding the structure of bone also helps to better understand the biology of tooth movement. The two basic structural types of bone are cortical and cancellous bone. Cancellous bone, otherwise known as trabecular or spongy bone, is softer, weaker and the less dense than cortical bone. It is also highly vascular. These qualities make it able to hold a reservoir of red bone marrow, which is the source of blood cell production for the body. Cortical bone, also known as compact bone, forms the outer shell of most bones and has a significantly higher density than cancellous bone. As it relates to the alveolus, it is cortical bone that lines the outer most buccal and lingual surfaces of the mandible and has been coined by some as orthodontic walls, signifying the 9

21 anatomic limits of tooth movement. 22 When moving teeth into the thin cortical plates, Graber advises that a high degree of caution should be used, especially in adult patients. 3 Cortical Bone Anatomy Cortical bone thickness varies throughout the maxilla and mandible. Ono et al 23 evaluated buccal cortical bone thickness around the first molars and premolars of the maxilla and mandible. CT scans of 43 adult patients were taken. They evaluated cross sections of bone mesial and distal to the first molar. Cortical thicknesses at various heights, ranging from 1 to 15 mm below the alveolar crest, were measured. The average cortical bone thickness ranged between 1.09 mm and 1.62 mm in the maxilla and from 1.59 mm and 2.66 mm in the mandible. Similar to the findings of Park and Cho, 24 cortical bone thickness in both jaws tended to increase from the CEJ to the apex, with a greater increase seen in the mandible than in the maxilla. Cortical bone distal to the first molar was significantly thicker than the cortical bone mesial to the first molar in both the maxilla and mandible (Fig. 1). 10

22 Figure 1. Cortical bone thickness in cross-sections mesial and distal to the maxillary and mandibular first molar (mesial: 5-6, distal 6-7) at vertical heights 1-15 mm at 1 mm intervals (Adapted from Ono et al 23 ) While most studies measure interproximal cortical bone thickness of the posterior teeth, some studies have included portions of the anterior mandible. Park and Cho 24 measured the thickness of cortical bone using three dimensional images of 60 adult patients. They measured bone from the mesial of the mandibular second molar to the distal of the canine, at vertical heights 5 mm, 7 mm and 9 mm from the CEJ. The average cortical bone thicknesses 5 mm from the CEJ distal to the canines were 1.28 mm and 1.26 mm on the right and left sides, respectively. The average cortical bone thicknesses 9 mm from the CEJ distal to the canines were 1.44 mm on both the right and left sides. There was no difference in cortical bone thickness between the right and left sides. 11

23 Schwartz-Dabney and Dechow 25 evaluated variations in cortical material properties throughout the mandible using fresh cadaver specimens. Many properties were evaluated, including cortical thickness, which was defined as the thickness from the periosteum to the cortical-trabecular interface. They removed 31 samples of facial and lingual bone from 10 fresh adult dentate mandibles. It was found that the cortical plate was significantly thicker on the facial side than the lingual side. The most anterior and coronal cortical bone had a mean facial thickness of 2.2 mm ±0.7 mm. The most anterior and coronal cortical bone had a mean lingual thickness of 1.7 mm ±0.7 mm. Because the exact locations of the measurements were not specified, these results can be considered helpful, but not conclusive. A study using computed tomography to view cortical bone thickness at various levels was performed by Swasty et al. 26 Based on 111 subjects with high, normal and low mandibular plane angles, 13 cross sections were made through the mandible, including one down the midline. From the constructed cross sections, measurements of the cortical bone thickness at three different locations were recorded, including one third and two thirds the distance of the ridge height, as well as at the symphysis. The 12

24 results demonstrated that the cortical bone thickness in the midline ranged from 1.65 mm to 3.64 mm, depending on the vertical location. Cortical bone was the thinnest at the symphysis in all the facial types; it was thinner than all sites excluding the lower lingual and lower buccal (Fig. 2-3). Their study also showed that there were no statistically significant differences between the three different facial types in cortical bone thickness in the upper facial and upper lingual regions. Figure 2. Mandibular cortical thickness in upper buccal area with each of the 13 coronal sections, divided by the 3 facial types for vertical facial dimension (average = blue; high and long = red; low and short = green). (Adapted from Swasty et al 26 ) 13

25 Figure 3. Mandibular cortical thickness in upper lingual area with each of the 13 coronal sections, divided by the 3 facial types for vertical facial dimension (average = blue; high and long = red; low and short = green). (Adapted from Swasty et al 26 ) The work of Farnsworth et al 27 showed that there are differences in cortical bone thickness between adolescents and adults. They measured and compared cortical bone thickness in common mini-screw implant sites of 26 adults (ages 20-45) and 26 teenagers, with equal numbers of males and females. Their findings (Fig. 4) showed that there was a significant difference in cortical bone thickness among adults and teens in all areas excluding the infrazygomatic crest, mandibular buccal aspect between the first and second molars, and the posterior palate. The differences in the interradicular regions between adolescents and adults tended to increase from anterior to posterior. As a result, there was less of a discrepancy of cortical bone thickness between adults and adolescents in the anterior portion of the mandible. The most anterior measurement was 14

26 on the buccal between the lateral incisor and canine, where the mean thickness for adolescents was 0.86 ±0.07 mm and the mean thickness for adults was 1.2 ±0.18 mm. They also confirmed what many investigators had previously found, that there was no sex difference in cortical bone thickness for the mandible. 23,25,27,28 Figure 4. Means and ranges of cortical bone thickness in the mandibular buccal region (Adapted from Farnsworth et al 27 ) 7=second molar, 6=first molar, 5=second premolar, 4=first premolar, 3=canine, 2=lateral incisor 15

27 Ridge Width Anatomy As stated previously, Swasty et al 26 used computed tomography to evaluate differences in cortical bone thickness and ridge width in patients with various facial heights. They measured ridge width at the occlusal and apical third of the mandible. They found that ridge width of the occlusal third of the mandible was the thinnest at the midline. They also found that the apical third at the midline was the thickest compared to the apical third thickness of the entire mandible. There also was a statistically significant difference in ridge width between long-faced and short-faced individuals, with the long-faced individuals having a much thinner ridge. The widths of the mandibular cross sections were the same for males and females, except for four sites in the upper third of the mandible

28 Figure 5. Mandibular width at each of the 13 coronal sections across the mandible for the upper mandibular third area among the three facial types and across the lower third of the mandible. (Adapted from Swasty et al 26 ) Incisor Inclination and Symphysis Anatomy Yamada et al 29 studied the spatial relationship of the mandibular incisors and the supporting bone in untreated adults with mandibular prognathism using cone beam computed tomography. They also looked at the relationship of the mandibular central incisor root apex in the cancellous bone. The distances from the apex to the internal cortical borders on both the buccal and lingual sides, and the 17

29 alveolar bone angles were measured as represented in figure 6. Figure 6. Linear and angular variables measured. (Adapted from Yamada et al 29 ) No differences between male and females were found. They did find a positive correlation between the labial alveolar bone angle and the incisor angle, lingual alveolar bone angle and incisor angle, the central incisor angle and cancellous bone thickness, and the central incisor angle and the apex-to-lingual cortical plate distance. It was also found that the apex to labial cortical bone (L1a-D) distance was consistently smaller than apex to lingual cortical bone (L1a-E), which is consistent with what clinicians see for a Class III dental compensation. In another study, Yu et al 30 found very similar results. The only difference was that they found a positive correlation between the incisor inclination and the distance from both the buccal and lingual cortices to 18

30 the apex. Given the results of the two studies, it can be said that incisor inclination is associated with alveolar bone morphology and apex position. Effects of Orthodontics on the Periodontium The tissue response to orthodontic forces allows teeth to move through bone, 17 but it can also result in adverse side effects. These side effects include gingival inflammation, alveolar bone loss, marginal bone recession, damage to the tooth enamel surfaces, pulpal reactions and root resorption. Many factors may affect the alveolar bone. The amount of force used, 31 treatment involving the closing of extraction spaces, 32,33,34 and the retention of plaque from fixed appliance therapy, 35 all can play a role in alveolar bone height changes. Also, due to the fact that their PDL is reportedly more quiescent, adults might experience more root resorption and bone loss than adolescents. 31 There might also be differences between adults and adolescents due to growth of the jaws and development of the alveolus. In adults, correction is achieved via teeth moving through the alveolus only

31 Another factor that plays a role in the periodontium response to orthodontic treatment is the anatomy and characteristics of the bone. Fuhrmann 37 evaluated 11 patients who had before and after treatment cone beam computed tomography images taken. From those images he measured symphysis width, cortical bone thickness and presence of bony dehiscence. He found that bone dehiscence or fenestrations were common at the mandibular incisors when ridge width and cortical bone thickness were thin. He suggested that a small symphysis with reduced labiolingual bone width, frontal crowding, and thin facial or lingual cortical bone were risk factors for bone dehiscence. Unfortunately, no specific data points were provided, nor were any explanations given as to the statistics that were used to support the claimed correlations. Cortical Bone Thickness Changes with Treatment One alveolar bone change due to orthodontic treatment that has been studied is the alveolar bone thickness. When teeth in the anterior portion of the mandible are moved labially or lingually through the trabecular bone and toward cortical bone, caution must be exercised. It has been suggested and will be discussed later, that when such 20

32 movement is attempted, dehiscense and fenestration in the buccal and lingual cortical plates (depending on the type of tooth movement) can occur. 22,38,39,40,41 However, De Angelis 42 believes that mechanotherapy induces alveolar distortion, much like the process that is seen in other bones undergoing active migration or drift. The distorted alveolus is thought to alter the electric environment via the piezoelectricity of bone. This in turn is thought to coordinate apposition and resorption. The alveolar bone is thought to retain its structural characteristics and size as it moves. While De Angelis s theory is interesting, the majority of clinical studies suggest that a violation of the cortical plates will result in a short term fenestrations or dehiscences. Many investigators have attempted to observe whether or not bone can regenerate once the cortical plate has been perforated. Remmelink and van der Molen 43 found, using laminagrams, that locations that showed dehiscence in the upper anterior incisor region were covered by a dense cortical plate 5-7 years post orthodontic treatment. Wainwright 38 histologically evaluated what occurs to the cortical bone when the root apex is placed outside the cortical plate, then replaced back into the cancellous bone. He found that the buccal root surface had no 21

33 cortical bone once it penetrated the cortical plate. However, after a 4-month retention period, some osteogenesis occurred, but it was insufficient to completely cover the root surface. It was only after the teeth had relapsed that he began to see repair of the perforations. Sarikaya et al, 44 evaluated bimaxillary protrusion cases requiring the extraction of four premolars in order to determine the effect that anterior tooth retraction had on alveolar bone thickness. Using cephalograms and cone beam computed tomography before treatment and 3 months after retraction of the incisors, they looked at the labial and the lingual alveolar plates at the crest, midroot, and apical levels of 19 adolescent patients. They found that after controlled tipping of the mandibular incisors, the labial bone maintained its original thickness, except at the crest level where it actually decreased. The lingual alveolar bone of the mandible decreased significantly over the central incisors at all three levels measured (crest, midroot and apical levels) (Table. 1-2). Another significant finding was that 11 of the 19 patients evaluated had at least one tooth out of the alveolar bone at the crest level. This study demonstrates the inherent 22

34 risk to the integrity of the cortical plate if teeth are moved outside the cortical bone. Table 1. Comparison of mean values of mandibular labial alveolar bone width measured from CT scans before and after retraction of lower incisors. S1, S2 and S3 represent the crest, midroot and apical levels respectively (Adapted from Sarikaya et al 44 ) T1 T2 Mean SD Mean SD P Mandibular right central incisor S S S Mandibular left central incisor S S S Table 2. Comparison of mean values of mandibular lingual alveolar bone width measured from CT scans before and after retraction of lower incisors. S1, S2 and S3 represent the crest, midroot and apical levels respectively (Adapted from Sarikaya et al 44 ) T1 T2 Mean SD Mean SD P Mandibular right central incisor S S S Mandibular left central incisor S S S While the majority of the literature focuses on cortical bone thickness in areas where mini-screws can be placed, a few studies have evaluated anterior mandibular cortical bone. However, many of these studies examined the 23

35 effect of retracting incisors after extraction of premolars. No studies could be found in human subjects using computed tomography that attempted to correlate cortical bone thickness to incisor flaring and the effect it has on cortical bone. Ridge Thickness The next characteristic of alveolar bone that needs to be discussed is ridge thickness and the effect that the size of the symphysis may have on treatment. In 1976, Mulie and Hoeve 45 attempted to better understand the limitations of tooth movement as it relates to the size of the symphysis, using laminagraphy and occlusal films. They classified three types of symphyses and how each reacted to leveling the curve of Spee via intrusion mechanics. In symphysis type 1, the mandibular incisors were in the center of a relatively wide symphysis and did not contact the lingual cortical plate after intrusion. In symphysis type 2, the symphysis was narrower and the incisors contacted the lingual cortical plate after intrusion mechanics were applied. In symphysis type 3, the mandibular incisors barely fit in the alveolar process and the apex was outside of the symphysis post intrusion. 24

36 Their findings suggest that not only does the anatomy of the mandibular symphysis vary from patient to patient, but that the size of the symphysis as it relates to the size of the incisors is significant with regards to what treatment is possible, and what the possible adverse affects of treatment might be. They observed that it was more difficult to intrude the incisors in patients with thin symphyses. They also noted that the root apex perforated through the cortical plate more frequently in patients with thin symphyses. Strahm et al 46 attempted to apply a force that would achieve lower incisor translation via a reverse pull face mask coupled with labial root torque. A sample of 27 patients was compared to a sample of 26 patients treated with activators and conventional headgears. Both groups had a second phase of treatment. They concluded that the use of reverse head gear in comparison to the activator group appeared to decrease bone apposition in the anterior part of the symphysis, leading to a 0.7 mm reduction in width, while the activator group had an increase of 0.5 mm. Ridge widths were measured near the lower border of the mandible. Symphyseal widths measured at the level of B point showed an increase of 0.1 mm for the headgear group. They also found that bodily movement of the incisors did 25

37 not occur and noted that the width limit of the lower apical base should be respected during orthodontic treatment planning. It is important to emphasize that, like the findings of Wendell et al, 47 the authors noted that a reduction in symphysis width is most likely due to pressure exerted from the external chin cup of the reverse pull headgear. Nonetheless, the study demonstrated the possibility that mandibular width can effect treatment and that the symphysis can change due to treatment. A question that has not been answered in the literature regarding the symphysis is whether or not there is a correlation between symphysis width and the amount of alveolar bone loss after non extraction orthodontic treatment. Until the recent advent of cone beam computed tomography, such a study would have been very difficult to perform. Alveolar Bone Height Loss The alveolar bone response to orthodontic mechonotherapy and tooth movements depends on various factors. Factors that may affect alveolar bone loss include the amount of force used for tooth movements, 31 the presence of dental plaque, 48 and the type and amount of 26

38 tooth movement. 49,50,51 No correlation exists between treatment time and alveolar bone resorption 52 or whether or not extractions are performed. 53 Some controversy exists as to whether there is a sex difference in alveolar bone loss. Studies of untreated malocclusions state that males have a larger CEJ to marginal bone crest distance than females, 54,55 while other studies of orthodontically treated patients identified no differences between sexes. 32,56,57 As mentioned previously, a loss in alveolar or marginal bone height is an adverse side effect of orthodontic tooth movement. Based on histologic observations, Schei et al 58 defined bone loss as a distance of more than one millimeter from the CEJ to the crest of the alveolar bone. The height of bone in any individual is very dependent on their age. Once a patient has reached adulthood, bone loss normally occurs, even without undergoing orthodontic treatment. The idea that bone loss occurs in adults who do not undergo orthodontic treatment is consistent with the findings of Albandar et al, 54 who observed bone loss in adults over a 2 year period. Their study found that subjects 32 years of age and younger had little bone loss, but those from age 33 to 45 years old had bone loss of 0.2 mm per year. 27

39 Harris and Baker 59 also compared alveolar bone loss of adults and adolescents. Using lateral cephalometric and panoramic radiographs, they evaluated the crestal bone loss of 24 adolescents and 36 adult orthodontic patients. They reported somewhat greater bone loss in adults, and recognized the limitations of measuring bone loss from panoramic and cephalometric radiographs. Nonetheless, it can be concluded from this study and the other aforementioned research, that adults will generally have more alveolar bone loss than adolescents at the beginning of orthodontic treatment. Alveolar Bone Loss with Orthodontic Treatment Most studies evaluating how orthodontics affects alveolar bone height have used bitewing and/or periapical radiographs and have only looked at the posterior dentition. 33,52,57,60,61 Aass and Gjermo 57 found that 16.2 percent of orthodontically treated patients and 4.3 percent of untreated subjects, had vertical bone loss greater than 2 mm. However, widening of the periodontal ligament space was recorded as bone loss, which potentially increases the incidence of bone loss in the treated group. Bondemark 52 demonstrated that no marginal bone loss greater than 2 mm occurred over a 5 year period after initial treatment. 28

40 However, he did find that treated patients had more bone loss at the maxillary molars than untreated patients. Baxter 62 found less than 0.5 mm of vertical alveolar bone loss after active orthodontic treatment. He also showed no statistical difference in alveolar bone height in extraction and non extraction cases. A systematic review done by Bollen et al 63 looked at the effects of orthodontic therapy on the periodontium. They looked at three studies 52,61,64 and found that the average amount of vertical bone loss was 0.13 mm. Zachrisson and Alnaes 33 also used conventional radiographs to evaluate alveolar bone loss in both treated and untreated groups. Using posterior bitewings, they looked at 51 patients treated with extraction of four first premolars and 54 untreated individuals. They found an average of 1.1 mm between the cemento-enamel junction (CEJ) to the crest of the interdental alveolar bone in treated individuals and 0.88 mm in the untreated group. These differences were statistically significant. It can be concluded that on average 0.3 mm of bone loss occurred in their treated sample. The highest figures for CEJ to crest of the interdental alveolar bone were seen in the closed extraction spaces, especially distal to the canine. 29

41 While most of the previously mentioned studies evaluating bone loss near extraction sites used records taken at the end of treatment, Reed et al 53 performed a more long term study. They evaluated the periodontal status adjacent to teeth that had been moved orthodontically into extraction sites. Evaluating 12 patients who had bilateral premolar extractions of the maxilla and had completed orthodontic therapy a minimum of 10 years previously, they found no differences in bone heights between the extraction sites and other tooth surfaces. Although the evidence seems to be contradictory, it is safe to say that orthodontics could have an effect on the alveolar bone height. In addition, the studies previously sited all evaluated the posterior dentition. In order to properly view the alveolar bone height in the anterior mandibular region an animal must be sacrificed or computed tomography (which is relatively new to orthodontics) must be used. Anterior Mandibular Alveolar Bone Height Loss: Experimental Model A few experiments evaluating alveolar bone height using animal models have been performed. Thilander et al 49 observed what happens to the alveolar bone if the incisors 30

42 are moved too far labially. Using six dogs, three experimental and three controls, the investigators moved the right lower incisors labially, causing alveolar bone loss to approximately the mid root level. The teeth were immediately moved back to their original position over a five months time period and held there for an additional five months. The conclusions drawn from the study were that dehiscence can occur in the labial alveolar plate by moving the teeth too far labially and that bone will reform if teeth are moved back to their original position. Steiner et al 50 used monkeys (Macaca nemistrina) to evaluate changes of the marginal periodontium as a result of labial tooth movement. In five monkeys, the central incisors were moved labially 3.05 mm on average. An exploratory surgery was performed during which they found significant recession of the marginal bone. The average marginal bone level on the incisors was 5.48 mm, with the control cuspid having a marginal bone level of 1.52 mm, demonstrating a 3.96 mm greater amount of bone loss in the treated group of teeth. A statistically significant difference existed between displaced and control teeth. Batenhorst et al 51 investigated the effects of facial tipping of incisors on the periodontium using monkeys as a model. In two monkeys, the left or right central and 31

43 lateral incisors were tipped 6 mm labially, with the control central and lateral not being moved at all. The teeth were then maintained for 240 days in the proclined position. The animals were then sacrificed and measurements were made from the CEJ to the crest of the alveolar bone. The amount of bone loss on the facial surface was 7.98 mm and 6.78 mm for the central and lateral incisors, which was approximately 5 mm greater than for the control teeth. This model clearly shows bone loss occurs when teeth are excessively proclined. Interestingly enough, Wingard and Bowers 65 performed a similar study using four monkeys but found different results. After moving the incisors labially 2-5 mm, they sacrifice the animals and evaluated the periodontium for any dehiscence or alveolar bone loss. They found that there was no difference in bone loss between the treated and untreated monkeys, and that there were no dehiscences or fenestrations produced from the labial movement of the incisors. All of the aforementioned studies regarding labial incisor movement and associated bone loss claim the teeth were tipped forward by advancing the wire forward. However, some studies used round wire and others used rectangular wire to advance the incisors forward. Because 32

44 none of the studies actually measured the amount of angulation change that occurred, it is not possible to state whether or not the teeth were tipped, translated or a combination of the two. Anterior Mandibular Alveolar Bone Height Loss: Clinical Model Studies using computed tomography to look at marginal alveolar bone before and after orthodontic treatment have also been performed in human patients. Lund et al 66 evaluated the distance between the CEJ and the marginal bone crest (MBC) at the buccal, lingual, mesial and distal surfaces of adolescent incisors before and after orthodontics in conjunction with premolar extractions using computed tomography. They found that 84 percent of lingual surfaces of the mandibular central incisors demonstrated a bone-height decreases greater than 2 mm. The average increase in distance between CEJ and MBC on the lingual aspect of the mandibular central incisors was 5.7 mm, with a 0.8 mm increase on the buccal aspect of the same tooth. It should be noted that the lower incisors in premolar extraction cases are moved lingually to close extraction spaces. 33

45 Knowing that moving mandibular incisors lingually can cause bone loss, one logically wonders what would happen to the alveolar bone height if teeth were moved labially? With the increased use of CBCT in orthodontics, studies evaluating the effects of moving teeth beyond the pre-treatment cortical plate are becoming more common and are yielding valuable clinical information. Incisor Inclination While incisor inclination plays an important functional role in overbite stability 67 the focus of this review will be on the role incisor inclination has in alveolar bone morphology and alveolar bone change. As previously stated, studies have shown that when the root apex is moved against the cortical plate or further, severe root resorption and bony dehiscence may occur. 22,68 What has not been discussed is whether an association exists between lower incisor inclination and morphology or loss of the supporting alveolar bone. It was previously established that incisor inclination is correlated to morphology in untreated individuals. It is now important to evaluate what occurs to the alveolus when inclination is part of treatment. A case report 34

46 performed by Wehrbein et al 40 described the mandible of a deceased 19-year-old female who had been treated orthodontically for 19 months. They attempted to evaluate what happened to the incisors, alveolar bone and symphysis after orthodontic treatment. The initial lateral cephalograms revealed a very narrow symphysis with the incisors straight above the thin bone. Treatment included aligning and putting lingual root torque on the lower incisors. Morphologic evaluation of the dry mandible revealed the sagittal interproximal bone was thinner than the buccal/lingual width of the incisors. Measurements of the alveolar bone heights on the lingual of the incisors decreased ranging from 2.3 mm to 6.9 mm. The root axis changed by about 12 degrees. All of the findings suggest that given a thin symphysis, extreme caution should be used when torquing or moving the incisors sagittally. Raposo et al 69 attempted to determine if incisor inclination provided a good estimate of alveolar bone level using cone beam computed tomography. Using cephalometric radiographs they performed various measurements, including the IMPA. Two groups of patients were formed based on pre-treatment IMPA; one group with an IMPA greater than 92 degrees and another group with an IMPA less than 92 degrees. From the CBCT images, measurements from the CEJ 35

47 to the marginal bone were measured. The authors found no statistical difference between the two IMPA groups for the CEJ to marginal bone variable. While IMPA is a good general means of quantifying incisor proclination, it does not account for translation verses tipping quantities of the tooth movement. As seen in the aforementioned animal studies, the type of tooth movement is an important factor in alveolar bone dehiscence formation. Conclusion After a review of the literature, it is apparent that while much has been written about the potential effects of treatment on vertical bone height, very little is conclusive. Understanding bone biology and how osteoclasts and osteoblasts function in response to external forces, causing bone resorption or deposition, helps to know the possible reasons of how bone could be lost. It has been shown that when teeth encroach on the labial or lingual cortical bone, a thinning of the bone occurs and in some instances dehiscences or fenestrations may occur. A thin ridge has also been linked to increased occurrence of dehiscence and fenestration. It has been also established that bone loss does occur after orthodontic treatment and 36