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CORTICAL BONE THICKNESS OF BLACK AND WHITE AMERICAN ADOLESCENTS Ningning Zhang D.D.S. 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 ABSTRACT Purpose: To assess the differences in maxillary and mandibular cortical bone thickness between black and white American adolescents using CBCT images. Methods: Pre- treatment cone-beam computed tomography (CBCT) images of 69 patients, including 34 black adolescents (17 females; 17 males) and 35 white adolescents (18 females; 17 males) were studied. The Digital Imaging and Communication in Medicine (DICOM) multifiles of each CBCT scan were imported into the Dolpin Imaging 11 3D software. Standardized orientations were used to measure the buccal and lingual cortical bone thickness in both maxilla and mandible at 16 interradicular sites. Total alveolar ridge thickness and medullary space thickness were also measured at the same interradicular sites. Results: Two-way analysis of variants showed (P‹0.05) significant group differences at 3 of the 16 sites, with black adolescents having thicker cortical bone. Alveolar ridge thicknesses were significantly greater in black adolescents, except at the distal of the first molar in the mandible. Medullary thickness was also significantly greater in black adolescents, except between the first and second mandibular premolars. No statistically significant gender differences in cortical thickness were found. Males 1 had significantly thicker alveolar ridge and medullary space than females except for distal to the first molar in the mandible. Conclusion: Black adolescents have thicker cortical bone than whites but the difference is small. There is no significant sex difference in cortical bone thickness. There is a significant difference in alveolar ridge thickness between blacks and whites. The difference in alveolar ridge thickness is due to the difference in medullary space. 2 CORTICAL BONE THICKNESS OF BLACK AND WHITE AMERICAN ADOLESCENTS Ningning Zhang D.D.S. A Thesis Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2012 3 COMMITTEE IN CHARGE OF CANDIDACY: Adjunct Professor Peter H. Buschang Chairperson and Advisor Professor Rolf G. Behrents Associate Professor Ki Beom Kim Associate Clinical Professor Donald R. Oliver i DEDICATION This thesis is dedicated to my parents, my husband and my teachers who have supported and guided me throughout my life to achieve my dreams. ii ACKNOWLEDGEMENTS The author would like to acknowledge: Dr. Peter Buschang for the tremendous dedication and time he put into this project, guiding me through every step of the thesis process. My gratitude is beyond words; Dr. Rolf Behrents for providing advice and direction not only throughout the thesis process but throughout my entire orthodontic education; Dr. Donald Oliver and Dr. Ki Beom Kim for guidance with this thesis and teaching me their knowledge during my orthodontic education; Dr. Derid Ure and Dr. Joe Mayes for use of their records in this study. iii TABLE OF CONTENTS List of Tables . . . . . . . . . . . . . . . . . . . . . .v List of Figures . . . . . . . . . . . . . . . . . . . . .vi CHAPTER 1: REVIEW OF THE LITERATURE . . . . . . . . . . . 1 Overview. . . . . . . . . . . . . . . . . .1 Bone development and adaptation to loads. .4 Recent studies on source of variation in cortical bone thickness . . . . . . . . . 11 Differences between American white and black population . . . . . . . . . . . . .21 Clinical implications. . . . . . . . . . .29 Summary and statement of thesis. . . . . .31 References . . . . . . . . . . . . . . . .33 CHAPTER 2: JOURNAL ARTICLE . . . . . . . . . . . . . . . 39 Abstract . . . . . . . . . . . . . . . . .39 Introduction . . . . . . . . . . . . . . .40 Materials and methods. . . . . . . . . . .42 Sample size and composition . . . . .42 Method of analysis . . . . . . . . . 43 Statistical analysis . . . . . . . . 47 Results. . . . . . . . . . . . . . . . . .47 Comparisons between black and white adolescents . . . . . . . . . .47 Comparisons between female and male adolescents . . . . . . . . . .48 Inter- and intra-arch comparisons. . 48 Discussion . . . . . . . . . . . . . . . .57 Cortical bones . . . . . . . . . . . 57 Alveolar ridge and medullary thickness . . . . . . . . . . . . . .59 Conclusions. . . . . . . . . . . . . . . .61 References . . . . . . . . . . . . . . . .62 Appendix . . . . . . . . . . . . . . . . . . . . . . . . 67 Vita Auctoris . . . . . . . . . . . . . . . . . . . . . .71 iv List of Tables Table 1: Summary of articles. . . . . . . . . . . .12 Table 2: Difference in cortical bone thickness in posterior region . . . . . . . . . . . . .14 Table 3: Values for appendicular skeletal muscle and regional bone mineral for black and white females matched for age, height, weight and menstrual status. . . . . . . . . . . . . 23 Table 4: Body composition variables for the core group by gender and race. . . . . . . . . 24 Table 5: Bone mineral density in the core group by gender and race . . . . . . . . . . . . . 25 Table 6: Summary of the mean difference in cortical bone thickness at the mandibular left 6mm site. . . . . . . . . . . . . . . . . . . 28 Table 7: Summary of the mean difference in cortical bone thickness at the mandibular left 9mm site. . . . . . . . . . . . . . . . . . . 28 Table 8: Maxillary and mandibular cortical bone thickness (mm) in black and white American adolescents. . . . . . . . . . . . . . . .49 Table 9: Maxillary and mandibular alveolar ridge and medullary thickness (mm) in black and white American adolescents. . . . . . . . . . . 50 Table 10: Maxillary and mandibular cortical bone thickness (mm) in female and male American adolescents. . . . . . . . . . . . . . . .51 Table 11: Maxillary and mandibular alveolar ridge and medullary thickness (mm) in black and white American adolescents. . . . . . . . . . . 52 v List of Figures Figure 1: A diagram illustrating the relationship of strains and adaptive responses . . . . . . 8 Figure 2: Cortical bone thickness in sections mesial and distal to the first molar. . . . . . .15 Figure 3A: Means and ranges of cortical bone thickness in various areas. . . . . . . . . . . . . 17 Figure 3B: Means and ranges of cortical bone thickness in various areas. . . . . . . . . . . . . 18 Figure 4: Maxillary and mandibular cortical bone thicknesses of hypodivergent and hyperdivergent young adults measured 5mm from the crest of the alveolar ridge . . .20 Figure 5: Mean PMI of men and women by age group and racial group . . . . . . . . . . . . . . .26 Figure 6A: Axial slice showing the sagittal plane runs through all posterior teeth on the side of measurement. . . . . . . . . . . . . . . .45 Figure 6B: Sagittal CBCT slice showing the axial plane orientated parallel to the alveolar ridge crest . . . . . . . . . . . . . . . . . . 45 Figure 6C: Sagittal CBCT slice showing the axial plane positioned 5mm from the crest of the alveolar ridge. . . . . . . . . . . . . . 46 Figure 6D: Axial CBCT slice showing buccal and lingual cortical bone thickness measured. Alveolar ridge thickness was measured at the same level. . . . . . . . . . . . . . . . . . .46 Figure 7: Means of buccal cortical bone thickness in the maxilla. . . . . . . . . . . . . . . .53 Figure 8: Means of lingual cortical bone thickness in the maxilla. . . . . . . . . . . . . . . .53 Figure 9: Means of buccal cortical bone thickness in the mandible. . . . . . . . . . . . . . . 54 vi Figure 10: Means of lingual cortical bone thickness in the mandible. . . . . . . . . . . . . . . 54 Figure 11: Means of alveolar ridge thickness in the maxilla. . . . . . . . . . . . . . . . . .55 Figure 12: Means of medullary space thickness in the maxilla. . . . . . . . . . . . . . . . . .55 Figure 13: Means of alveolar ridge thickness in the mandible. . . . . . . . . . . . . . . . . 56 Figure 14: Means of medullary space thickness in the mandible. . . . . . . . . . . . . . . . . 56 Figure 15: Means of buccal cortical bone thickness in the maxilla. . . . . . . . . . . . . . . .67 Figure 16: Means of lingual cortical bone thickness in the maxilla. . . . . . . . . . . . . . . .67 Figure 17: Means of buccal cortical bone thickness in the mandible. . . . . . . . . . . . . . . 68 Figure 18: Means of lingual cortical bone thickness in the mandible. . . . . . . . . . . . . . . 68 Figure 19: Means of alveolar ridge thickness in the maxilla. . . . . . . . . . . . . . . . . .69 Figure 20: Means of medullary space thickness in the maxilla. . . . . . . . . . . . . . . . . .69 Figure 21: Means of alveolar ridge thickness in the mandible. . . . . . . . . . . . . . . . . 70 Figure 22: Means of medullary space thickness in the mandible. . . . . . . . . . . . . . . . . 70 vii CHAPTER 1: REVIEW OF THE LITERAURE Overview Bone is one of the most important biological tissues affecting orthodontic treatment. In clinical practice, mini-screw implants (MSIs) have been widely used for anchorage in orthodontics. Their success depends on the length and type of the MSI used, as well as the cortical bone available at the placement site. It has been shown that the quality and quantity of cortical bone is better at predicting the stability of MSI than the length and type of MSI.1 Understanding the variations in maxillary and mandibular cortical bone may be beneficial for placing MSIs. One potentially important source of variation in cortical thickness is the ethnicity of the patient. It is widely accepted that bone adapts to the mechanical loads applied to it. The Frost mechanostat theory outlines the process of bony adaptation by osteoblasts and osteoclasts.2,3 Bone adaptation to strains can also be explained by the fluid flow theory, which states that osteocytes are excited by mechanical loading influenced bone fluid to induce apposition while the reduction of canalicular fluid flow causes apoptosis of osteocytes.4 Applying these theories to the craniofacial 1 region, the cortical bone thickness might be expected to be related to the stresses and strains applied to the maxilla and mandible. For example, biting forces generated by the muscles of mastication might be expected to influence the modeling of the maxilla and mandible. The specific modeling changes that occur depend on craniofacial morphology, age, gender and presence of dental and temporomandibular joint pathology.5 For example, adults have thicker cortical bone than children,6 and they also have stronger biting forces. Hypodivergent patients have thicker cortical bone than hyperdivergent patients7,8 and they also have stronger muscle of mastication.9,10 In contrast the gender difference in cortical thickness has been found to be minimal6,7,11 while maximum bite forces are higher in males.5 Many studies have compared post cranial bone of black and white populations.12–15 They all show that blacks have thicker bones at all skeletal sites than their white counterparts. Relatively few studies have been performed that evaluate black and white differences in the craniofacial region. Benson et al16 developed the panoramic mandibular index (PMI) to relate cortical bone thickness with the relative constant distance between mental foramen and the inferior mandibular boarder measured on panoramic 2 radiograph. They found that black Americans have thicker cortical bone than their white counterparts in both the maxilla and the mandible, but the differences were not significant. A recent study by Humphries17 studied the cortical bone thickness between second premolars and first molars from cone-beam computed tomography (CBCT) slices at four vertical sites. The results indicated that cortical bone was thicker in blacks than whites at all mandibular sites except for mandibular left 3mm away from the alveolar crest. However, the differences between blacks and whites were not statistically significant. The author suspected that the distortion in the CBCTs may have masked the ethnic differences. The purpose of the present study was to measure the thickness of cortical bone in the posterior regions of the maxilla and mandible of black and white American adolescents. The measurements were taken from pre-Phase II treatment CBCT slices. The subjects were classified by ethnicity and compared to determine whether there were ethnic differences in alveolar cortical bone morphology. If differences could be demonstrated between black and white 3 American adolescents, the patient’s ethnicity will have to be considered when using MSIs as a modality of treatment. The following will start with a review of the literature pertaining to bone development and its adaptation to function because mechanical loads produce stress and strains in the bone affecting the modeling and remodeling process. The masticatory force is the major load to the craniofacial region. The second section will review studies on how variables, such as age, gender, vertical facial pattern, affect cortical bone thickness. The last section will discuss the known post-craniofacial and craniofacial bone differences in whites and blacks. Bone’s development and adaptation to loads Bone is an important tissue. It functions as support and protection for the body and organs, while producing red and white blood cells, and storing minerals. Bone is also a specialized matrix composed of collagen and ground substance. Osteoblasts form bone and partially mineralize it. They are formed by perivascular connective tissue cells surrounding venules and capillaries. Osteoclasts resorb bone by a lytic process involving acid and enzymes. They 4 are formed by circulating preosteoclasts produced in bone marrow. Bones grow and adapt by surface apposition and resorption.18 Bone can be classified as woven, lamellar, composite and bundle bone according to its histology.18 Woven bone varies considerably in structure. It serves a critical role in wound healing and provides initial stability for fractures. Lamellar bone is strong, highly organized and well mineralized. It forms the majority of the adult human skeleton. In orthodontic treatment, mature lamellar bone is formed approximately one year after tooth movement.18 Both compact and trabecular bone are lamellar bone. Composite bone is formed by the deposition of lamellar within the woven bone lattice. It is generally high-quality and loadbearing. It eventually is remodeled into secondary osteons. Bundle bone is a functional adaption of lamellar bone with attached ligaments and tendons. Alveolar bone holds teeth for mastication and withstands the masticatory force. It consists of buccal and lingual cortical plates and medullary space in between. Cortical bone consists of compact bones, made of osteons. Each osteon has a central Haversian canal and peripheral concentric layers of lamellae. Medullary bone is the 5 central cavity of a bone where bone marrow exists. It is composed of trabecular bone. It is generally accepted that bone adapts to mechanical loads. Bone adapts through changes in bone mass, geometric distribution, matrix organization and the collagen orientation of the lamella. The two distinct mechanisms in an adaptation process are modeling and remodeling.18 The process is influenced by genetics, biomechanical environment and force application. The modeling and remodeling processes rely on the actions of osteoblasts and osteoclasts. In cortical bone, they form a unit called basic multicellular unit (BMU). The BMU forms a cylindrical canal in cortical bone. Osteoclasts form a circular tunnel in the loading direction while osteoblasts follow them forming secondary osteons.19 BMUs provide the main postnatal modifications affecting bone strength. When the remodeling process starts, osteoclasts start to attach to the bone tissue matrix and acidfy the microenvironment dissolving the organic and inorganic matrices of the bone. Osteoblasts then appear at the same surface depositing osteoids and mineralizing them to form new bones.20 Global modeling increases bone mass and strength. Global remodeling by BMUs turns over bones in either the 6 conservation or disuse modes. In the conservation mode resorption equals formation. In the disuse mode, the remodeling BMUs make less bone than they resorb causing a net loss of bone. Remodeling also repairs bone microdamage by removing and replacing the damaged bone with new bone.21 The detailed mechanism by which the mechanical forces influencing bone modeling and remodeling is a very complex process and yet to be fully discovered. One widely accepted theory is Frost’s mechanostat hypothesis. Frost stated that there is a minimum effective strain (MES). Any external mechanical load must exceed the MES to elicit an adaptive response in bone. There is a range of MES strain values that will not evoke any response. Strains above the range will have a positive adaptive response causing bone gain while strains below the MES range will have a negative adaptive response, causing bone loss. The modelingdependent bone gain happens at strain values above 1500 microstrains, while the remodeling-dependent bone loss occurs at strain values below 100 microstrains.2,3,21 Therefore modeling and remodeling are not stimulated at the same time on the same surface (Figure 1). The mechanical factors dominate the control of bone growth and development. Nonmechanical agents could influence the mechanical factor 7 and thus influence the bone modeling and remodeling process but would not replace the mechanical factors.21 Figure 1. A diagram illustrating the relationship of strains and adaptive responses. MESr: minimum effective strain range at and above which BMUs begin decreasing towards normal. MESm: minimum effective strain range at and above which mechanically controlled lamellar modeling drifts begins. MESp: minimum effective strain range above which woven bone drifts turn on and suppresses local lamellar bone drifts (Adapted from Jee21) An alternative theory, the fluid flow theory describes the cellular mechanisms by which bone adapts to mechanical loads. The theory states that when mechanical load was applied to the bone, strains were produced.4,19,22 The strains alter the fluid flow in the cannaliculi in the bone connecting osteocytes. The fluid produces shear stress on the membranes of osteocytes.4 Osteocytes were activated producing anabolic paracine factors, recruiting 8 osteoblasts.23 When fluid flow in the cannaliculi is reduced, osteoclasts resorb bone.22,24 In vivo studies were done to test this theory. Tan et al24 studied the effect of pulsating fluid flow on TNF-α induced apoptosis in chicken osteocytes, osteoblasts and periosteal fibroblasts. TNF-α is a pro-inflammatory cytokine with apoptotic potency. They found that when fluid was static, TNF-α increased apoptosis by more than two-folds in osteocytes and osteoblasts. One hour of pulsating flow inhibited TNF-α induced apoptosis. Owan et al25 studied the fluid forces and the expression of osteopontin (OPN) mRNA expression. The expression of OPN was used to measure the anabolic response of MC3TC-E1 cells, mouse osteoblast-like cells. They found that high magnitude fluid forces significantly increased OPN message level. It indicated that fluid forces influence OPN expression in osteoblasts and the fluid forces within the bone matrix may play an important role in bone’s modeling response to mechanical loading. Examples of bone adapting to function can be found in daily life such as tennis players with much stronger racquet-holding arm than the other arm. Studies have shown a positive correlation between the size the masseter and medial pterygoid muscles and the remodeling process of the 9 bone at the gonial angle. When masseters are hypertrophied, it causes the gonial angles to be hypertrophied as well26. The atrophied or absence of the temporal muscle also results in a smaller and modified coronoid process.27 Murray28 stated that bones, when separated from function during growth, developed the general features of that particular bone but the specific morphological details were missing. Genetics determines the formation and general morphology of a bone. However, the combination of genetics and its mechanical environment can make the same bone differ from each other. Tsai29 et al studied the mandible after botulinum neurotoxin type A (BoTx/A) injections into the temporalis and masseter muscles of growing rats to induce masticatory hypoactivity. One-way analysis of variance was used to analyze muscle volume and bone mineral density (BMD). They found reductions in cortical bone thickness and BMD in the skull and mandible of rats injected with BoTx/A. The volumes of the temporalis and masseter muscles injected with BoTx/A were also smaller. These findings are consistent with the notion that both direct and indirect loadings, such as tension or bending from the function of muscles, contribute to 10 alterations in skeletal development.30 With respect to the craniofacial complex, weaker muscles might be expected to produce weaker bite forces, leading to smaller functional effects on the alveolar bone of maxilla and mandible, causing less bony adaptation. On the other hand it is reasonable to propose that individuals with larger mechanical load to the jaws and stronger masticatory force will have thicker mandible and cortical bone. Recent studies on sources of variations in cortical bone thickness Studies6–8,11,31,32 (Table 1) have been performed to measure differences in maxillary and mandibular cortical bone thickness. Between individuals, age, sex, and vertical growth patterns have been studied. It has been shown that adults have thicker cortical bone than children.6 Such differences can be explained by the fact that bone adapts to function. During growth, individuals grow in body size, muscles size and occlusal force.33–36 Higher occlusal forces in adults cause alveolar bone to adapt, inducing formation of thicker cortical bone. Hypodivergent individuals have also been 11 shown to have higher masticatory forces than hyperdivergent individuals.9 Hypodivergent patients have shown to have thicker cortical bone than hyperdivergent patients.7,8 However, there are no differences between males and females in cortical bone thickness. Within the same individual, no side differences in cortical thickness have also been found.31,32 The posterior aspect of the mandible has been shown to have thicker cortical bone than the anterior aspect.6,11 Mandibular cortical bone has also been shown to be thicker than maxillary cortical bone.6,8,11 Table 1 Summary of articles comparing cortical bone thickness 1) between individuals 2) within an individual Adult vs. adolescent Male vs. Female Hypodivergent vs. hyperdivergent Deguchi et al31 no age differences no difference ― Ono et al32 ― thinner in female in the maxilla mesial to the first molar ― Farnsworth et al6 thicker in adults no difference ― Swasty et al7 ― Wider mandible in male, but no difference in cortex thicker in hypodivergent patients Horner et al8 ― ― thicker in hypodivergent patients Variables 12 Table 1 Continued Right vs. Left Maxilla vs. Mandible Anterior vs. Posterior Mandible ― ― Posterior is thicker Deguchi et al31 no differences Thicker in mandible ― Ono et al32 no differences Thicker in mandible ― Farnsworth et al6 ― Thicker in mandible thicker in the posterior region Horner et al8 ― Thicker in mandible thicker in the posterior region Variables Carter et al11 Carter et al11 studied human cadaver hemimandibles that were subjected to sagittal split ramus osteotomies. They found a difference between anterior and posterior mandible in the cortical thickness in the proximal segment. The areas of the mandible closest to the sagittal split, the posterior segments were the areas exhibiting the thickest cortical bone. Deguchi et al31 quantitatively evaluated cortical bone thickness in various common mini-screw implant (MSI) placement locations in the maxilla and the mandible using three-dimensional computed tomographic images reconstructed from 10 adults. Cortical bone thicknesses were measured in the buccal and lingual regions mesial and distal to the first molar, distal to the second molar and in the 13 premaxillary region at two different vertical levels. It was found that there were no significant differences in either cortical bone thickness or root proximity due to sex, age, or side. Significantly less cortical bone was found in the maxillary buccal region at the occlusal level distal to the second molar, than in other areas in the maxilla. Significantly more cortical bone was observed on the lingual side of the second molar compared with the buccal side. In the mandible, mesial and distal to the second molar, significantly more cortical bone was observed compared with the maxilla. No significant differences between vertical locations were noted within either the mandible or the maxilla (Table 2). However, there was a significant difference in cortical thickness between the mandible and maxilla at the same vertical heights. There was significantly thicker cortical bone in the mandibular molar area than the same area of the maxilla. Table 2.Difference in cortical bone thickness in posterior region (Aapted from Deguchi et al31) 5-6 (O) Maxilla Mean SD 5-6 (A) Mean SD 6-7 (O) Mean SD 6-7 (A) Mean SD 7 (O) Mean SD Buccal 1.8 0.6 1.6 0.6 1.5 0.5 1.6 0.5 1.3 0.5 Lingual 1.7 0.9 — — 1.7 0.7 — — 1.7 0.6 Mandible Buccal 1.9 0.6 1.8 0.5 2.0 0.6 1.8 0.5 1.9 0.7 Significant difference between 5-6, 6-7 and lingual (P‹0.05) Significant difference between buccal (P‹0.05) Significant difference between jaws (P‹0.05) 5 Second premolar; 6 first molar; 7 second moalr; O,occlusal level;A, apical level 14 Ono et al32 evaluated cortical bone thickness in the buccal posterior region mesial and distal to the first molar in 43 patients. Computed tomography was used to evaluate the sites. The average cortical bone thicknesses ranged from 1.1 to 2.1 mm in the maxilla and 1.6 to 3.0 mm in the mandible. The mandibular cortical bone sites were significantly thicker than the corresponding maxillary cortical bone sites. The further the distance away from the alveolar crest, the thicker the cortical bone tended to be. Cortical bone thickness did not differ significantly between the right and left sides. However, it was thinner in females than males in the region of attached gingival in the maxilla mesial to the first molar (Figure 2). Figure 2. Cortical bone thickness in sections mesial and distal to the first molar; 5-6: mesial to the first molar; 6-7: distal to the first molar; *: significant difference (P‹0.05)between 5-6 and 6-7 (Adapted from Ono et al32) 15 Farnsworth et al6 studied age, sex, and regional differences in cortical bone thickness at commonly used maxillary and mandibular MSI placement sites. Cone-beam computed tomography (CBCT) slices of 26 adolescents and 26 adults were evaluated. They found that there was no significant difference in cortical bone thickness between the sexes, but that there were significant differences between adolescents and adults. Adult cortices were significantly thicker in all areas except the infrazygomatic crest, the mandibular buccal first molarsecond molar site, and the posterior palate site (Figure 3). It was also found that there were differences in cortical bone thickness between and within regions of the jaw. Cortical bone was thicker in the posterior than the anterior regions of both the maxilla and mandible. Thicker cortical bone was also present in the mandible than in the maxilla. 16 Means and ranges of cortical bone thickness (mm) Mandibular Buccal 3 2.5 2 1.5 Adolescents 1 Adults 0.5 0 2-3 4-5 5-6 6-7 Site Means and ranges of cortical bone thickness (mm) Maxillary Buccal 2 1.8 1.6 1.4 1.2 1 Adolescents 0.8 Adults 0.6 0.4 0.2 0 2-3 4-5 5-6 6-7 Site Figure 3A Means and ranges of cortical bone thickness in various areas (Adapted from Farsworth6 et al) 17 Means and ranges of cortical bone thickness (mm) Maxillary Lingual 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Adolescents Adults 2-3 4-5 5-6 6-7 Site Means and ranges of cortical bone thickness (mm) Palatal and Infrazygomatic Crest 2.5 2 1.5 Adolescents 1 Adults 0.5 0 3 mm 6 mm 9 mm IZ Palatal Site Figure 3B Means and ranges of cortical bone thickness in various areas (Adapted from Farsworth6 et al) Swasty et al7 studied CBCT slices to determine the differences in cortical plate thicknesses in patients with different vertical facial dimensions. Both the facial height index (ratio of posterior facial height to anterior facial height) and mandibular plane angle were used to 18 categorize patients. The long-face group was found to have slightly thinner cortical bone than the other two groups. The difference was statistically significant at some sites in the mandible. The long-face group also had somewhat thinner widths in the upper third of the mandible than the other two facial groups. The mandible was wider and taller in males than females. There were no statistically significant differences in cortical plate thickness between males and females. Horner et al8 assessed differences in maxillary and mandibular cortical bone thickness between hyperdivergent and hypodivergent young adults. Pre-treatment CBCT from 57 patients were studied. Hypodivergent subjects were found to have significantly thicker buccal cortices than the hyperdivergent subjects except for the bone between the maxillary first molars and second premolars, and the bone between mandibular canines and first premolars (Figure 4). Medullary thickness was largely unaffected by facial divergence. A study by Proffit et al9 showed that adults with long faces have weaker biting forces during swallowing, simulated chewing and maximum biting. The results agree with the previous discussion related to the function of masticatory muscles and their influence on the bony shape and structure of the maxilla and mandible. 19 Figure 4: Maxillary and mandibular cortical bone thicknesses (mean ± 1SD) of hypodivergent and hyperdivergent young adults measured 5mm from the crest of the alveolar ridge. (Adapted from Horner et al8) The above studies demonstrate that the maxillary and mandibular cortical bone thicknesses vary at different sites and among different groups. These studies suggest that the stress and strains of the masticatory muscular apparatus may be a determinant of the amount and density of 20 the cortical and trabecular bone of the mandible and maxilla. The only exception to the relationship between muscle force and cortical thickness pertains to sex differences. It is reasonable to assume that males would have thicker cortical bone in alveolus due to their larger masticatory muscles and greater maximum biting forces than females33,36. Maximum biting force, however, is rarely required during daily mastication. Although sex differences in diet were shown by studies,37,38 the force required to masticate modern diet was far below the maximum biting force.39 The similarity in cortical bone thickness of two sexes indicates that the strains associated with daily masticatory forces are more important in determining group differences than maximum bite forces or muscle mass. Differences between American white and black populations American blacks comprised an increasingly larger minority group in the U.S. In the 2000 census, black American was the largest minority group, comprising of 12.0% of the population. Black American held its largest minority position in the 2010 census comprising 12.6% of the population. As such, American black patients could comprise 21 a large part of the orthodontic patient base. Understanding the differences and similarities between American black and white population should help practitioners make appropriate treatment decisions. Studies12–14 have demonstrated that black Americans have higher skeletal mass and denser skeletal bones than their white counterparts in all skeletal sites. However, relatively few studies have been performed evaluating differences in the craniofacial region. Cohn et al12 studied skeletal mass and radial bone mineral content in black and white women. Twenty-six black women were studied using total-body neuron activation analysis. It was found that black women had a greater skeletal mass and bone mineral content of the radius than age-matched white female subjects, when controlling for differences in statue. The mean absolute total-body calcium of black females was 16.7% higher than that of age-matched white women. Controlling for body size showed that more than half of this differences was due to the larger muscle mass. The larger muscle mass in black women was a determinant of their increased skeletal mass. Ortiz et al13 studied body composition difference between black and white females. Twenty-eight matched pairs were studied. Body composition was evaluated by means of 22 anthropometry, dual-photon-absorptiometry (DPA), dilution, underwater weighting and whole-body 3 H2O 40 K counting. The results of these procedures were used to calculate compartmental masses and other relevant body-composition indices. Total bone density of the black females was 1.18±0.14 g/cm2, which was significantly higher than the bone density of white females (1.09±0.09 g/cm2). Black females had greater skeletal muscle and bone mineral in the upper and lower and combined extremities than the white females (Table 3). The results confirmed that black and white females of equivalent age, weight, height and menstrual status differed significantly in body composition. The black females had 10-15% more skeletal muscle and total bone mineral mass than their white counterparts. Table 3. Values for appendicular skeletal muscle (SM) and regional bone mineral (bone) for black and white females matched for age, height, weight and menstrual status (Adapted from Ortiz et al13) Upper Extremities SM Black 6.1±1.3 (4.09.4) White 4.8±1.2 (2.68.3) Bone 0.31±0.06 (0.190.42) 0.25±0.04 (0.160.32) Lower Extremities SM Bone 11.9±2.1 (8.117.0) 0.91±0.19 (0.501.31) 10.9±1.6 (8.514.6) 0.80±0.13 (0.521.01) Total Appendicular SM 18.0±3.0 (12.725.0) 15.7±2.2 (12.019.4) Bone 1.22±0.25 (0.691.76) 1.05±0.17 (0.681.32) Ettinger et al14 performed a cohort study of 402 black and white men and women. Dual energy x-ray absorptiometry (DXA) of total body, hip and lumbar spine were performed in the array scanning mode. Mean hip and spine size, 23 determined by DXA, was smaller in black than in white women. There were no statistically significant racial differences between black and white men at these sites. Black men were found to have 3% greater total skeletal area than their white counterparts due to larger appendicular bone area. Similar trends were observed between black and white women (Table 4). Bone density at all skeletal sites were significantly greater in blacks than in whites (Table 5). Young, adult, black men and women had substantially greater bone-mineral-density than whites at all skeletal sites. Table 4. Body composition variables for group by gender and race (Adapted from Ettinger et al14) Men Women Black White Black White Spine, posterior anterior 65.3 65.7 55.2 57.6 hip, total 43.7 43.7 32.9 34.3 arms 47.4 441 388 356 legs 907 861 792 770 Skeletal area, cm² 2436 2355 2096 2075 L3 Volume, cm² whole body 39.6 40.5 31.6 35 Lean mass, kg 64.2 60.2 43.7 42.6 16 18.6 27.9 23.5 Trunk: leg fat ratio 0.92 1.24 0.92 0.86 Waist:hip ratio 0.82 0.85 0.75 0.72 Sum of 4 skinfold measurements, mm 52.9 53.4 77 58.7 Fat mass, kg 24 Table 5. Bone mineral density (BMD) in the core group by gender and race (Adapted from Ettinger et al14) Men Women Black White Black White posterioranterior, g/cm² 1.148 lateral L3,g/cm² 0.909 1.03 1.13 1.045 0.8 0.862 0.812 volumtric, L3, g/cm² 0.246 0.223 0.256 0.241 neck, g/cm² 1.068 0.891 0.962 0.862 trochanter, g/cm² 0.903 0.783 0.778 0.728 Ward's triangle, g/cm² 0.961 0.769 0.894 0.789 total, g/cm² 1.187 1.034 1.036 0.955 1.295 1.177 1.163 1.09 Spine: Femur Total body, g/cm² In one of the few studies evaluating ethnic differences in the craniofacial complex, Benson16 developed panoramic mandibular index to study mandibular cortical bone thickness. The index was calculated as a ratio of the cortical thickness to the distance between the mental foramen and the inferior mandibular border on panoramic radiograph. Greater index indicates a greater cortical bone mass. The index was applied to 353 subjects, evenly distributed among black, Hispanic, white and male and females. All panoramic radiographs were obtained from the same radiograph unit. It was found that blacks had greater mean PMI than Hispanics or whites (Figure 5). However, the differences in the index were only significantly different among elderly men. 25 0.45 0.4 0.35 PMI 0.3 0.25 Black Male 0.2 Hispanic Male 0.15 White Male 0.1 0.05 0 30-39 40-49 50-59 60-69 70-79 Age Group (years) 0.45 0.4 0.35 PMI 0.3 0.25 Black Female 0.2 Hispanic Female 0.15 White Female 0.1 0.05 0 30-39 40-49 50-59 60-69 70-79 Age Group (years) Figure 5. Mean panoramic mandibular index of men and women by age group and racial group. Standard error of the mean are indicated. Significant differences (p ‹0.05) were noted among black and Hispanic in 60-69 age group.(Adapted from Benson et al16) A recent master’s thesis evaluated ethnic differences in the cortical bone of the craniofacial region.17 CBCT scans of 154 consecutively scanned patients were studied. 26 There were 18 African Americans, including nine males and nine females. The Caucasian sample included 21 females and 22 males. The age of the samples was not disclosed. The cortical bone thickness between the second premolars and first molars were measured at 3, 6, 9, and 12mm increments from the crest of the alveolar ridge. At the mandibular left quadrant at 6 mm (LL6), African-Americans had significantly greater cortical bone thickness than Asians and Hispanics. No significant differences were observed between African-Americans and Caucasians at this site (Table 6). At the mandibular left quadrant at 9 mm African American cortical bone thickness was significantly greater than all other ethnic groups (Table 7). Although not statistically significant, the data indicated that cortical bone thickness was greater in the African Americans than Caucasians. Humphries stated that there was distortion in the CBCT data which may have masked some of the ethnic differences. He suggested that ethnicity of the patients should be considered when planning orthodontic treatment using MSIs. The theoretical basis for the speculated ethnic difference in cortical bone thickness was not mentioned. Age6 and vertical facial pattern7,8 have been proved to affect cortical bone thickness. Neither variable was controlled in this study. 27 Table 6: Summary of the mean difference in cortical bone thickness at the mandibular left 6mm site. The mean difference is significant at the 0.05 level. (Adapted Humphries17) Ethinicity Ethinicity (I) (I) Mean Differences (I-J) A AA C AA -0.22* C -0.15* H -0.05 A 0.22* C 0.08 H 0.17* A 0.15* AA H -0.08 H 0.09 A 0.05 AA -0.17* C -0.09 Table 7: Summary of the mean difference in cortical bone thickness at the mandibular left 9mm site. The mean difference is significant at the 0.05 level.(Adapted from Humphries17) Ethinicity Ethinicity (I) (I) Mean Differences (I-J) A AA C H AA -0.25* C -0.03 H -0.02 A 0.25* C 0.21* H 0.23* A 0.03 AA -0.21* H 0.02 A 0.02 AA -0.23* C -0.02 The thickness of alveolar ridge is often called alveolar ridge width in literature. The width of alveolar ridge is closely related to the tooth supported. The ridge develops both vertically and horizontally as tooth erupts.40 28 Alveolar ridges resorb when a tooth is extracted41–43 or periodontal disease is present.44,45 It is therefore an important consideration factor in dental implants placement.46,47 Studies have shown that males have wider alveolar ridge than females at selected sites7 and mandibular ridges are thicker than maxillary ridges.46 The sex difference may be due to the larger tooth size in males.48,49 No study has been published on the ethnic difference in alveolar ridge thickness. It also has been demonstrated blacks are larger than whites in most body composition.50,51 According to the allometric scaling, the larger alveolar ridge may be expected in blacks. Clinical implication Multiple sites for MSIs have been utilized to obtain desired stability including the basal bone below the roots of the teeth,1,52 infrazygomatic crest of the maxilla53 and palatal alveolar bone and paramedical plate.54,55 Cortical bone thickness has been shown to affect both the primary and secondary stability of MSIs and dental implants. 29 The strength of primary stability can be evaluated by pull-out test. Huja et al56 studied the pull-out strength of 56 titanium screws in bone at various sites in the maxilla and mandible of four beagle dogs. It was found that screws placed in the anterior mandibular region had significantly lower pull-out strength than those placed in the posterior mandibular region. The cortical bone in the anterior mandible was also thinner and less dense than the cortical bone in the posterior region. Their regression analysis showed a weak but significant correlation between the pullout force and cortical bone thickness. Motoyoshi et al57 examined the relationship between cortical bone thickness and the success rate of MSIs placed for orthodontic anchorage. They studied 32 patients using pre-treatment CT scans to measure the cortical bone thickness around the proposed implant sites. The success of the MSIs was defined as being able to withstand the force application for at least six months. The results show that the cortical bone thickness was significantly greater in the successful MSI group than in the failure group. The success rates of MSIs in different ethnic groups are yet to be studied. If the results of this thesis show that black Americans have thicker cortical bone than white 30 American, the patient’s ethnicity will have to be considered when using MSIs as a modality of treatment. If the results of this study indicate no differences, then the patient’s ethnicity will play a less important role in orthodontic MSIs treatment planning. Summary and Statement of Thesis It has been well established that that the function of muscles of mastication influence the bony shape and structure of the maxilla and mandible. Ethnic differences in masticatory forces have yet to be established. Studies suggest that black Americans have thicker cortical bone than whites but the difference are usually not significant16,17. A more comprehensive study of cortical bone thickness is necessary to determine if ethnic differences actually exist. It is also important to evaluate ethnic differences in alveolar ridge thickness, which holds important implications in implant and MSI placements. Ethnic differences in alveolar ridge thickness have not previously been evaluated. If current study shows there is a significant difference between blacks and whites in cortical bone thickness and alveolar ridge thickness, patient’s ethnicity will have to be given consideration 31 while treatment planning for MSI and dental implants. 32 References 1. 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Cross-sectional human mandibular morphology as assessed in vivo by cone-beam computed tomography in patients with different vertical facial dimensions. Am J Orthod Dentofacial Orthop. 2011;139:e377–e389. 8. Horner KA, Behrents RG, Kim KB, Buschang PH. Cortical bone and ridge thickness of hyperdivergent and hypodivergent adults. Am J Orthod Dentofacial Orthop. 2012;142:170–8. 9. Proffit WR, Fields HW, Nixon WL. Occlusal forces in normal- and long-face adults. J Dent Res. 1983;62:566–70. 10. Buschang P, Throckmorton G. Influence of jaw muscle strength on malocclusion. In: Orthodontics for the Next Millennium. Ormco, CA 1997. 33 11. Carter TB, Frost DE, Tucker MR, Zuniga JR. Cortical thickness in human mandibles: clinical relevance to the sagittal split ramus osteotomy. Int J Adult Orthodon Orthognath Surg. 1991;6:257–60. 12. Cohn SH, Abesamis C, Yasumura S, Aloia JF, Zanzi I, Ellis KJ. Comparative skeletal mass and radial bone mineral content in black and white women. 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[Los Angeles, California]: University of Southern California; 2007. 18. Roberts E. Chapter 10 in Orthodontics Current Principle and Techniques. 5th ed. St. Louis, Missouri: Elsevier Mosby; 2012. 19. Henneman S, Von den Hoff JW, Maltha JC. Mechanobiology of tooth movement. Eur J Orthod. 2008;30:299–306. 20. Ruimerman R. Modeling and Remodeling in Bone Tissue. Eindhoven, Netherlands: Technische Universiteit Eindhoven; 2005. 34 21. Jee WS. Principles in bone physiology. J Musculoskelet Neuronal Interact. 2000;1:11–3. 22. Burger EH, Klein-Nulend J, Smit TH. Strain-derived canalicular fluid flow regulates osteoclast activity in a remodelling osteon--a proposal. J Biomech. 2003;36:1453–9. 23. Burger EH, Klein-Nulend J. Mechanotransduction in bone--role of the lacuno-canalicular network. FASEB J. 1999;13:S101–112. 24. Tan SD, Kuijpers-Jagtman AM, Semeins CM, Bronckers ALJJ, Maltha JC, Von den Hoff JW, et al. Fluid shear stress inhibits TNFalpha-induced osteocyte apoptosis. J Dent Res. 2006;85:905–9. 25. Owan I, Burr DB, Turner CH, Qiu J, Tu Y, Onyia JE, et al. Mechanotransduction in bone: osteoblasts are more responsive to fluid forces than mechanical strain. Am J Physiol. 1997;273:C810–815. 26. Scott J, Symons N. Introduction to Dental Anatomy. 8th ed. Edinburgh: Churchill Livingston; 1977. 27. Avis V. The relation of the temporal muscle to the form of the coronoid process. Am J Phys Anthropol. 1959;17:99–104. 28. Murray PDF. Bones: A Study of the Development and Structure of the Vertebrate Skeleton. 1936. 29. Tsai C-Y, Shyr Y-M, Chiu W-C, Lee C-M. Bone changes in the mandible following botulinum neurotoxin injections. Eur J Orthod. 2011;33:132–8. 30. Bresin A, Kiliaridis S, Strid KG. Effect of masticatory function on the internal bone structure in the mandible of the growing rat. Eur J Oral Sci. 1999;107:35–44. 31. Deguchi T, Nasu M, Murakami K, Yabuuchi T, Kamioka H, Takano-Yamamoto T. Quantitative evaluation of cortical bone thickness with computed tomographic scanning for orthodontic implants. Am J Orthod Dentofacial Orthop. 2006;129:721.e7–12. 35 32. Ono A, Motoyoshi M, Shimizu N. Cortical bone thickness in the buccal posterior region for orthodontic mini-implants. Int J Oral Maxillofac Surg. 2008;37:334–40. 33. Usui T, Uematsu S, Kanegae H, Morimoto T, Kurihara S. Change in maximum occlusal force in association with maxillofacial growth. Orthod Craniofac Res. 2007;10:226– 34. 34. Pancherz H. Temporal and masseter muscle activity in children and adults with normal occlusion. An electromyographic investigation. Acta Odontol Scand. 1980;38:343–8. 35. Raadsheer MC, Kiliaridis S, Van Eijden TM, Van Ginkel FC, Prahl-Andersen B. Masseter muscle thickness in growing individuals and its relation to facial morphology. Arch Oral Biol. 1996;41:323–32. 36. Braun S, Hnat WP, Freudenthaler JW, Marcotte MR, Hönigle K, Johnson BE. A study of maximum bite force during growth and development. Angle Orthod. 1996;66:261– 4. 37. Bates CJ, Prentice A, Finch S. Gender differences in food and nutrient intakes and status indices from the National Diet and Nutrition Survey of people aged 65 years and over. Eur J Clin Nutr. 1999;53:694–9. 38. Wardle J, Haase A, Steptoe A, Nillapun M, Jonwutiwes K, Bellisie F. Gender differences in food choice: The contribution of health beliefs and dieting. Ann Behav Med. 2004;27:107–16. 39. Scully C. Oxford Handbook of Applied Dental Sciences. Oxford University Press; 2002. 40. Hinds KF. Alveolar ridge development with forced eruption and distraction of retained natural dentition. Oral Maxillofac Surg Clin North Am. 2004;16:75–89, vi–vii. 41. Pietrokovski J, Massler M. Alveolar ridge resorption following tooth extraction. J Prosthet Dent. 1967;17:21–7. 42. Devlin H, Ferguson MW. Alveolar ridge resorption and mandibular atrophy. A review of the role of local and systemic factors. Br Dental J. 1991;170:101–4. 36 43. Araújo MG, Lindhe J. Dimensional ridge alterations following tooth extraction. An experimental study in the dog. J Clin Periodontol. 2005;32:212–8. 44. Page RC, Schroeder HE. Pathogenesis of inflammatory periodontal disease. A summary of current work. Lab Invest. 1976;34:235–49. 45. Grossi SG, Genco RJ, Machtet EE, Ho AW, Koch G, Dunford R, et al. Assessment of Risk for Periodontal Disease. II. Risk Indicators for Alveolar Bone Loss. J Periodontol. 1995;66:23–9. 46. Eufinger H, König S, Eufinger A. The role of alveolar ridge width in dental implantology. Clin Oral Investig. 1998;1:169–77. 47. Iasella JM, Greenwell H, Miller RL, Hill M, Drisko C, Bohra AA, et al. Ridge Preservation with Freeze-Dried Bone Allograft and a Collagen Membrane Compared to Extraction Alone for Implant Site Development: A Clinical and Histologic Study in Humans. J Periodontol. 2003;74:990–9. 48. Doris JM, Bernard BW, Kuftinec MM, Stom D. A biometric study of tooth size and dental crowding. Am J Orthod. 1981;79:326–36. 49. Schwartz GT, Dean MC. Sexual dimorphism in modern human permanent teeth. Am J Phys Anthropol. 2005;128:312– 7. 50. Nelson DA, Jacobsen G, Barondess DA, Parfitt AM. Ethnic differences in regional bone density, hip axis length, and lifestyle variables among healthy black and white men. J Bone Miner Res. 1995;10:782–7. 51. Wagner DR, Heyward VH. Measures of body composition in blacks and whites: a comparative review. Am J Clin Nutr. 2000;71:1392–402. 52. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod. 1997;31:763–7. 53. Lin JCY, Liou EJW, Yeh C-L. Intrusion of overerupted maxillary molars with miniscrew anchorage. J Clin Orthod. 2006;40:378–383. 37 54. Janssen KI, Raghoebar GM, Vissink A, Sandham A. Skeletal anchorage in orthodontics--a review of various systems in animal and human studies. Int J Oral Maxillofac Implants. 2008;23:75–88. 55. Jung BA, Harzer W, Gedrange T, Kunkel M, Moergel M, Diedrich P, et al. Spectrum of indications for palatal implants in treatment concepts involving immediate and conventional loading. J Orofac Orthop. 2010;71:273–80. 56. Huja SS, Litsky AS, Beck FM, Johnson KA, Larsen PE. Pull-out strength of monocortical screws placed in the maxillae and mandibles of dogs. Am J Orthod Dentofacial Orthop. 2005;127:307–13. 57. Motoyoshi M, Yoshida T, Ono A, Shimizu N. Effect of cortical bone thickness and implant placement torque on stability of orthodontic mini-implants. Int J Oral Maxillofac Implants. 2007;22:779–84. 38 CHAPTER 2: JOURNAL ARTICLE Abstract: Purpose: To assess the differences in maxillary and mandibular cortical bone thickness between black and white American adolescents. Methods: Pre-treatment cone-beam computed tomography (CBCT) images of 69 patients, including 34 black (17 females; 17 males) and 35 white (18 females; 17 males) adolescents were used to evaluate cortical bone and alveolar ridge thickness 5 mm below the alveolar crest. The shortest distances between the periosteal and endosteal aspects of cortical bone were measured in both maxilla and mandible at 16 interradicular sites. Total alveolar ridge thickness was measured at the same interradicular sites. Results: Two-way analysis of variance showed no significant sex differences and significant (P‹0.05) group differences at three of the 16 sites, with black adolescents having thicker cortical bone. Alveolar ridges were significantly greater in black adolescents, except distal to the first molasr in the mandible. Medullary thickness was significantly greater in black adolescents, except between the first and second mandibular premolars. Males had significantly thicker alveolar ridges and medullary spaces than females, except distal to the first mandibular molars. 39 On the buccal surface, the more posterior cortical bone of the mandible was thicker than the more anterior cortical bone. The cortical bone was thicker in the maxilla than in the mandible. Conclusion: There were only limited differences in cortical bone thicknesses between black and white adolescents, but there were significant differences favoring blacks in alveolar ridge thickness. There was no sex difference in cortical bone thickness but definite differences favoring males in alveolar ridge and medullary space. Introduction Mini-screw implants (MSIs) are widely used by orthodontists, with approximately 80% of the AAO orthodontists having used at least one MSI.1 The success of MSIs depends on their stability. Cortical bone quality and quantity are major factors associated with stability of MSIs,2–4 with failures being linked to thin cortical bones.3– 5 It has even been suggested that the cortical bone thickness is more important than the implant length in determining MSIs stability.6 40 Differences in cortical bone thickness are thought to be related to function; cortical plate thickness adapts to masticatory forces.7–10 Hyperdivergent individuals have thinner cortical bone,12,13 which could be due to lower masticatory forces. It has also been shown that adults have thicker cortical bone than children,11 which could be explained by increases in body size, muscle size and occlusal forces during growth.15–18 Differences in post-cranial bone indicate that there could also be differences between blacks and whites in cortical bone surrounding the teeth. Blacks have thicker post-cranial cortical bones, stronger muscles and greater muscle mass.20–22 Benson et al23 reported thicker cortical bone among blacks than whites, but the differences were only significantly different among elderly men. Humphries,24 who studied the cortical bone thickness at four vertical sites between second premolar and first molars, found that blacks have thicker mandibular cortical bone thickness at two sites. This study had limited sample sizes and the ages of the samples were not given. The purpose of the present study was to measure the thickness of cortical bone in the posterior regions of the maxilla and mandible of black and white American 41 adolescents. If differences exist, patient’s ethnicity has to be considered as a factor in using MSI as a treatment modality. Materials and Methods Sample size and composition Pre-treatment cone beam computed tomography (CBCT) scans (i-CAT Classic, Imaging Science International, Hatfield, PA) were taken with a single 360° rotational scan time of 20 seconds, 120 kVp, and 0.4 mm voxel size. The sample and the side of the mouth used for the measurements were selected based on: 1) no missing or unerupted permanent teeth in the areas being measured; 2) no periapical or periradicular pathology; 3) no apparent facial or dental asymmetries on photographs; 4) no vertical or horizontal periodontal bone loss; 5) no medical or dental diseases affecting bone metabolism. The side that best met the inclusion criteria was measured; if both sides met the criteria, the side was randomly chosen because studies have shown no side differences in cortical bone thicknesses in either the maxilla or mandible.25,26 42 The scans were selected based on sex, age (patients had to be 10-17 year old), vertical growth pattern (measured based on the Frankfort to mandibular plane or FMA angle) and race (either white or black). The white subjects were matched to the black subjects based on age, sex and FMA. Samples were obtained from a private practice in Lubbock Texas. The samples included 34 blacks and 35 whites. The black group was 12.7 ±1.8 years of age and had an FMA of 24.7 ±5.2°. The white group was 12.8 ±1.6 years old and had a mean FMA of 24.2 ±5.8°. Methods of analysis The Digital Imaging and Communication in Medicine (DICOM) multifiles of each CBCT scan were imported into Dolphin Imaging 11 3D software (Dolphin Imaging Systems LLC, Chatsworth, CA) for analysis. Each image as oriented in three planes of place so that the buccal and lingual cortical plates could be measured 5 mm apical to the alveolar crest, approximately at the level of mucogingival junction. The height of 5 mm was chosen because it has been commonly used for MSI placement.27,28 The images were orientated so that the sagittal plane in the axial slice went through all posterior teeth on the side of measurement (Figure 6A). The alveolar crest was 43 then orientated parallel to the axial plane in the sagittal slice (Figure 6B). The measurement site was selected 5 mm from the alveolar ridge crest (Figure 6C). The cortical bone thickness was measured on the axial slice. The slices were oriented so that the shortest distance between the buccal and lingual was measured (Figure 6D). Cortical bone thickness was measured on the buccal and lingual sides of maxilla and mandible 1) 1 mm distal to the first molars (distal of 6); 2) between the second premolars and first molars (5-6); 3) between the first premolars and second premolars (4-5); and 4) 1 mm mesial to the first premolars (mesial of 4). These are the sites commonly used for MSI placement.1,2,28–30 Alveolar ridge thickness was measured at the same sites as cortical bone thickness in both the maxilla and mandible (Figure 6C). Alveolar ridge thickness was defined as the distance between the buccal and lingual cortical periosteal sites. Medullary thickness was calculated as the differences between alveolar ridge thickness and the sum of the buccal and lingual cortical bone thickness. Twenty subjects were chosen randomly to be measured a second time at all sites by the same operator. Method 44 Figure 6A. Axial CBCT slice showing the sagittal plane running through all posterior teeth on the side of measurement. Figure 6B. Sagittal slice showing the axial plane orientated parallel to the alveolar ridge crest. 45 Figure 6C. Sagittal slice showing the axial plane positioned 5 mm from the crest of the alveolar ridge. Figure 6D. Axial slice showing buccal and lingual cortical bone thickness measured. Alveolar ridge thickness was measured at the same level. 46 errors (√(∑differences2/2n) of the replicate measures ranged from 0.15-0.24 mm. There were no statistically significant systematic errors. Statistical analysis Descriptive statistics indicated that the data were normally distributed. Two-way ANOVA were used to evaluate groups and sex differences in cortical bone, alveolar ridge and medullary space thickness. Results Comparisons of blacks and whites Cortical bone was generally thicker in black than in white adolescents, but the differences were statistically significant at only three sites: maxillary buccal between first molar and second premolar, between the first premolar and second premolar and maxillary lingual mesial of the first premolar. No statistically significant differences between blacks and whites were observed in the mandible (Table 8; Figures 7-10). Alveolar ridge thickness was greater in black than in white adolescents at all sites. The differences were 47 statistically significant at all sites except distal to the first mandibular molar (Table 9; Figures 11, 13). Medullary thickness was also significantly greater in blacks than in whites at all maxillary sites, and at the two most posterior mandibular sites (Table 9; Figures 12, 14). Comparison of females and males While cortical bone tended to be thicker in males than in females, none of the sex differences were statistically significant (Table 10; Figures 15-18). The alveolar ridges were consistently thicker in males than females, with differences being statistically significant at all sites except distal to the mandibular first molar (Table 11; Figures 19, 21). Medullary thickness was also significantly greater in males than females, except distal to the first mandibular molars (Table 11; Figures 20, 22). Inter- and intra-arch comparisons The cortical bone was significantly thicker in the maxilla than in mandible, except on the buccal surface mesial to the first premolars and between first and second premolars in blacks. The alveolar ridges and medullary thickness were also significantly thicker in the maxilla 48 than in the mandible, except mesial to first premolars and distal to the first molars (Tables 8, 9). The cortical bone thickness increased from posterior to anterior on the buccal side of the mandible. The thickest cortical bone on the lingual side was between first and second premolar. There was no clear pattern of differences observed for cortical bone thickness in the maxilla. Alveolar ridge and medullary thickness were greater at the more posterior sites than at the more anterior sites of both jaws (Tables 8, 9). Table 8. Maxillary and mandibular cortical bone thickness (mm) in black and white American adolescents (* indicates statistically significant group difference (p≤0.05)). Jaw Cortex Maxilla Buccal Lingual Mandible Buccal Lingual Site Black White Differences Mean SD Mean SD Mean Sig. Distal of 6 1.85 0.34 1.78 0.36 0.07 0.44 5-6 1.85 0.36 1.67 0.26 0.18 0.02* 4-5 1.89 0.33 1.67 0.32 0.22 0.01* Mesial of 4 1.77 0.40 1.63 0.29 0.14 0.11 Distal of 6 2.04 0.46 1.97 0.38 0.07 0.56 5-6 1.85 0.38 1.87 0.32 -0.02 0.82 4-5 1.96 0.46 1.81 0.42 0.15 0.17 Mesial of 4 2.14 0.47 1.87 0.33 0.27 0.01* Distal of 6 3.23 0.81 3.25 0.59 -0.02 0.87 5-6 2.20 0.52 2.11 0.52 0.09 0.43 4-5 1.91 0.42 1.85 0.41 0.06 0.52 Mesial of 4 1.79 0.44 1.67 0.37 0.12 0.21 Distal of 6 2.75 0.46 2.86 0.53 -0.11 0.38 5-6 2.79 0.41 2.63 0.51 0.16 0.13 4-5 3.17 0.68 2.73 0.61 0.44 0.07 Mesial of 4 2.91 0.60 2.57 0.45 0.34 0.08 49 Table 9. Maxillary and mandibular alveolar ridge and medullary thickness (mm) in black and white American adolescents (* indicates statistically significant group difference (p≤0.05)). Jaw Maxilla Site Alveolar Ridge Medullary Thickness Mandible Alveolar Ridge Medullary Thickness Black White Differences Mean SD Mean SD Mean Sig. Distal of 6 17.31 2.00 15.88 1.22 1.43 <0.01* 5-6 15.30 1.62 13.72 1.34 1.58 <0.01* 4-5 13.35 1.83 11.66 1.42 1.69 <0.01* Mesial of 4 12.10 2.13 10.48 1.53 1.62 <0.01* Distal of 6 13.42 2.09 12.13 1.35 1.29 <0.01* 5-6 11.60 1.58 10.18 1.13 1.42 <0.01* 4-5 9.51 1.70 8.18 1.22 1.33 <0.01* Mesial of 4 8.19 1.91 6.97 1.32 1.22 <0.01* Distal of 6 15.98 1.92 15.33 1.98 0.65 0.17 5-6 13.39 1.78 12.48 1.60 0.91 0.03* 4-5 11.99 1.83 10.84 1.91 1.15 0.01* Mesial of 4 11.13 1.97 10.14 1.75 0.99 0.02* Distal of 6 10.00 1.26 9.22 1.68 0.78 0.03* 5-6 8.40 1.43 7.73 1.33 0.67 0.04* 4-5 6.91 1.30 6.32 1.37 0.59 0.06 Mesial of 4 6.43 1.60 5.92 1.54 0.51 0.14 50 Table 10. Maxillary and mandibular cortical bone thickness (mm) in female and male American adolescents (* indicates statistically significant group difference (p≤0.05)). Jaw Cortex Maxilla Buccal Site Female Mean Lingual Mandible Buccal Lingual SD Male Mean SD Differences Mean Sig Distal of 6 1.78 0.38 1.86 0.28 -0.08 0.57 5-6 1.76 0.36 1.79 0.27 -0.03 0.97 4-5 1.74 0.35 1.81 0.33 -0.07 0.41 Mesial of 4 1.63 0.36 1.78 0.35 -0.15 0.10 Distal of 6 1.96 0.43 2.04 0.45 -0.08 0.17 5-6 1.84 0.35 1.85 0.36 -0.01 0.64 4-5 1.86 0.37 1.91 0.50 -0.05 0.66 Mesial of 4 1.97 0.46 2.04 0.37 -0.07 0.52 Distal of 6 3.18 0.76 3.31 0.63 -0.13 0.47 5-6 2.16 0.53 2.16 0.48 0.00 0.90 4-5 1.82 0.39 1.94 0.46 -0.12 0.25 Mesial of 4 1.67 0.35 1.77 0.47 -0.10 0.15 Distal of 6 2.85 0.48 2.75 0.51 0.10 0.40 5-6 2.40 0.40 2.70 0.52 -0.30 0.91 4-5 2.88 0.61 3.02 0.74 -0.14 0.38 Mesial of 4 2.66 0.58 2.81 0.50 -0.15 0.23 51 Table 11. Maxillary and mandibular alveolar ridge and medullary thickness (mm) in female and male American adolescents (* indicates statistically significant group difference (p≤0.05)). Jaw Maxilla Site Alveolar Ridge Medullary Thickness Mandible Alveolar Ridge Medullary Thickness Female Male Differences Mean SD Mean SD Mean Sig Distal of 6 15.79 1.22 17.40 1.95 -1.61 <0.01* 5-6 13.44 1.22 15.18 1.82 -1.74 <0.01* 4-5 11.92 1.14 13.15 2.18 -1.23 <0.01* Mesial of 4 10.38 1.28 12.17 2.20 -1.79 <0.01* Distal of 6 12.05 1.35 13.58 2.03 -1.53 <0.01* 5-6 10.24 1.11 11.54 1.67 -1.30 <0.01* 4-5 8.31 1.11 9.43 1.87 -1.12 <0.01* Mesial of 4 6.78 1.00 8.36 1.95 -1.58 <0.01* Distal of 6 15.31 1.91 16.00 1.91 -0.69 0.14 5-6 12.62 1.46 13.31 1.88 -0.69 0.02* 4-5 10.89 1.68 11.95 2.02 -1.06 0.01* Mesial of 4 9.85 1.56 11.45 1.83 -1.60 <0.01* Distal of 6 9.28 1.52 9.95 1.40 -0.67 0.06 5-6 7.73 1.30 8.44 1.39 -0.71 0.02* 4-5 6.25 1.24 7.02 1.31 -0.77 0.01* Mesial of 4 5.53 1.37 6.86 1.44 -1.33 <0.01* 52 Black White 2.50 2.00 mm 1.50 1.00 0.50 0.00 Distal of 6 5-6* 4-5* Mesial of 4 Figure 7. Means of buccal cortical bone thickness in the maxilla. (* indicates statistically significant sites(p≤0.05)) Black White 3.00 2.50 mm 2.00 1.50 1.00 0.50 0.00 Distal of 6 5-6 4-5 Mesial of 4* Figure 8. Means of lingual cortical bone thickness in the maxilla. (* indicates statistically significant sites (p≤0.05)) 53 Black White 4.50 4.00 3.50 mm 3.00 2.50 2.00 1.50 1.00 0.50 0.00 Distal of 6 5-6 4-5 Mesial of 4 Figure 9. Means of buccal cortical bone thickness in the mandible. (* indicates statistically significant sites(p≤0.05)) Black White 4.50 4.00 3.50 mm 3.00 2.50 2.00 1.50 1.00 0.50 0.00 Distal of 6 5-6 4-5 Mesial of 4 Figure 10. Means of lingual cortical bone thickness in the mandible. (* indicates statistically significant sites(p≤0.05)) 54 Black White 25.00 20.00 mm 15.00 10.00 5.00 0.00 Distal of 6* 5-6* 4-5* Mesial of 4* Figure 11. Means of alveolar ridge thickness in the maxilla. (* indicates statistically significant sites (p≤0.05)) Black White 18.00 16.00 14.00 mm 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Distal of 6* 5-6* 4-5* Mesial of 4* Figure 12. Means of medullary space thickness in the maxilla. (* indicates statistically significant sites (p≤0.05)) 55 Black White 20.00 18.00 16.00 14.00 mm 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Distal of 6 5-6* 4-5* Mesial of 4* Figure 13. Means of alveolar ridge thickness in the mandible. (* indicates statistically significant sites (p≤0.05)) Black White 12.00 10.00 mm 8.00 6.00 4.00 2.00 0.00 Distal of 6* 5-6* 4-5 Mesial of 4 Figure 14. Means of medullary space thickness in the mandible. (* indicates statistically significant sites (p≤0.05)) 56 Discussion Cortical bones Only limited differences in cortical bone thickness were observed between blacks and whites. The differences were statistically significant at only three maxillary sites. This is consistent with previous studies,23,24 showing limited differences between blacks and whites. The CBCT used in the study had a 0.4 mm voxel size, which was larger or equivalent to some of the differences calculated. The mean method error was 0.15-0.24 mm, which increased the variability of the measurement. The present study may not have enough power to detect all the differences. However, the pattern was for blacks to have thicker (4-14%) cortical bone than whites. Blacks are known to have larger postcranial muscle mass and strength than whites.20–22 There have been no comparative craniofacial muscle forces studies published. The similarity in cortical bone indicates similar muscle force of blacks and whites. Future study for craniofacial muscle force may be needed. Males and females had similar cortical bone thicknesses. Males had thicker cortical bone than females, but the differences were even smaller than the black and white differences, and not statistically significant. This 57 lack of sex difference in cortical bone thickness has been previously demonstrated.11,25 Males have larger masticatory muscles and greater maximum biting forces than females.14,15 Although sex differences in diet have been reported31,32 maximum biting forces rarely occur in daily mastication. The forces required to masticate modern diets is far below the maximum biting force.33 The similarity in cortical bone thickness indicates that the strains associated with daily masticatory forces are more important in determining group differences than maximum bite forces or muscle mass. The mandible has thicker buccal and lingual cortical bone than the maxilla. The differences ranged from 0.03 mm to 1.42 mm. Previous studies have also shown thicker cortical bone in the mandible than maxilla.11,12,25,26 During mastication more stress and strains are applied to the mandible because it is a single bone. The maxilla is connected to the midface and is able to dissipate any of the force produced. Consequently, the mandible undergoes greater bending than the maxilla during mastication. In clinical practice, it is difficult to anesthetize mandibular teeth through infiltration due to the thick cortical plate preventing the diffusion of anesthetics into the pulp.34 58 Cortical bone thickness increased from posterior to anterior on the buccal side of the mandible. The thickest cortical bone on the lingual side was found between first and second premolar. These findings are consistent with studies by Farnsworth et al11 and Horner et al.12 The pattern can be explained by masticatory force distribution within the mandible. The force developed during biting increases from anterior teeth to molars.7,35 Therefore, bone in the molar areas is subject to the higher levels of stress and strains, necessitating more bony adaptation than in the anterior region. The thicker cortical bone in the posterior mandible makes it well suited for MSI placement. Alveolar ridge and medullary thickness Black adolescents have thicker alveolar ridges than white adolescents, primarily due to increased medullary thickness. The results indicated that medullary thickness was approximately 8-17% greater in black than in white adolescents, which is relatively only slightly greater than the differences observed in cortical thickness. Alveolar and medullary width might be expected to be related to tooth size. Studies have demonstrated that blacks have bigger teeth than whites.36,37,38,39 Differences in ridge and 59 medullary thickness may also be related to body size because blacks are larger than whites in most body composition.40,41 The body size difference may explain the larger alveolar ridge and medullary thickness according the allometric scaling. Males have thicker alveolar ridge than females due to greater medullary bone. The results indicated that the difference was 4-19%. Males have larger teeth than females in all dimensions.42,43 In addition, males are bigger than females in most dimensions.44 The difference in tooth size and body size may explain the differences in alveolar ridge width. Swasty et al13 reported that males have thicker ridges than females only in the premolar and canine regions of the upper third of the mandible. However, their study was based on a wide age-range (10-65 years old) and the number of males and females were not indicated. Thicker alveolar bone has important indications in dental implant placement and denture prosthesis fabrication. 60 Conclusions Based on pre-treatment CBCT images of 69 adolescent patients, the following conclusions can be made about interradibular bone 5 mm from the alveolar crest: 1) The thickness of cortical bone is similar or slightly thicker in blacks than in whites. 2) Alveolar ridge thickness is thicker in blacks than in whites due primarily to thicker medullary bone. 3) The cortical bone thickness is similar in males and females. 4) Alveolar ridge thickness is thicker in male than in females due to the differences in medullary thickness. 5) Buccal cortical bone in the mandible increases from more anterior to the more posterior sites. 6) The cortical bone was thicker in the mandible than in the maxilla. 61 References 1. Buschang PH, Carrillo R, Ozenbaugh B, Rossouw PE. 2008 survey of AAO members on miniscrew usage. J Clin Orthod. 2008;42:513–8. 2. Costa A, Raffainl M, Melsen B. 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Ethnic differences in regional bone density, hip axis length, and lifestyle variables among healthy black and white men. J Bone Miner Res. 1995;10:782–7. 41. Wagner DR, Heyward VH. Measures of body composition in blacks and whites: a comparative review. Am J Clin Nutr. 2000;71:1392–402. 42. Doris JM, Bernard BW, Kuftinec MM, Stom D. A biometric study of tooth size and dental crowding. Am J Orthod. 1981;79:326–36. 43. Schwartz GT, Dean MC. Sexual dimorphism in modern human permanent teeth. Am J Phys Anthropol. 2005;128:312– 7. 44. Ogden CL, Fryar CD, Carroll MD, Flegal KM. Mean body weight, height, and body mass index, United States 19602002. Adv Data. 2004;347:1–17. 66 Appendix Female Male 2.50 2.00 mm 1.50 1.00 0.50 0.00 Distal of 6 5-6 4-5 Mesial of 4 Figure 15. Means of buccal cortical bone thickness in the maxilla. (* indicates statistically significant sites(p≤0.05)) Female Male 3.00 2.50 mm 2.00 1.50 1.00 0.50 0.00 Distal of 6 5-6 4-5 Mesial of 4 Figure 16. Means of lingual cortical bone thickness in the maxilla. (* indicates statistically significant sites(p≤0.05)) 67 Female Male 4.50 4.00 3.50 mm 3.00 2.50 2.00 1.50 1.00 0.50 0.00 Distal of 6 5-6 4-5 Mesial of 4 Figure 17. Means of buccal cortical bone thickness in the mandible. (* indicates statistically significant sites(p≤0.05)) Female Male 4.00 3.50 3.00 mm 2.50 2.00 1.50 1.00 0.50 0.00 Distal of 6 5-6 4-5 Mesial of 4 Figure 18. Means of lingual cortical bone thickness in the mandible. (* indicates statistically significant sites(p≤0.05)) 68 Female Male 25.00 20.00 mm 15.00 10.00 5.00 0.00 Distal of 6* 5-6* 4-5* Mesial of 4* Figure 19. Means of alveolar ridge thickness in the maxilla. (* indicates statistically significant sites (p≤0.05)) Female Male 18.00 16.00 14.00 mm 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Distal of 6* 5-6* 4-5* Mesial of 4* Figure 20. Means of medullary space thickness in the maxilla. (* indicates statistically significant sites (p≤0.05)) 69 Female Male 20.00 18.00 16.00 14.00 mm 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Distal of 6 5-6* 4-5* Mesial of 4* Figure 21. Means of alveolar ridge thickness in the mandible. (* indicates statistically significant sites (p≤0.05)) Female Male 12.00 10.00 mm 8.00 6.00 4.00 2.00 0.00 Distal of 6 5-6* 4-5* Mesial of 4* Figure 22. Means of medullary space thickness in the mandible. (* indicates statistically significant sites (p≤0.05)) 70 VITA AUTORIS Ningning Zhang was born to Mr. Feng Zhang and Mrs. Min Li in SuZhou city, People’s Republic of China in 1983. She moved to the United States in 2001 for education. She received her undergraduate education at University of Rochester in Rochester New York. She finished her dental education at Columbia University in New York City in 2009 with a degree of Doctor of Dental Surgery. She then completed one year residency in advanced education in general dentistry at New York Presbyterian Hospital. She is currently doing postgraduate education in Orthodontics at Saint Louis University. She plans to graduate in December 2012 with Master of Science in Dentistry. She is married to Dr. Somnang Lim. 71