Relationship among Types of Growth Patterns, Buccolingual Molar Inclination and Cortical Bone Thickness of the Mandible: A CT Scan Study

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1 JIOS /jp-journals ORIGINAL ARTICLE Relationship among Types of Growth Patterns, Buccolingual Molar Inclination and Cortical Bone Thickness of the Mandible Relationship among Types of Growth Patterns, Buccolingual Molar Inclination and Cortical Bone Thickness of the Mandible: A CT Scan Study 1 Narendra Shriram Sharma, 2 Sunita S Shrivastav, 3 Pushpa V Hazarey ABSTRACT Objective: The purpose of this study was to evaluate the relationship between different types of growth patterns, the buccolingual molar inclination and cortical bone thickness of the mandible. Materials and methods: The material consisted of total 30 cases within the age range of 18 to 25 years. They all demonstrated normal occlusion with a minimal dental discrepancy without any cross bite or facial asymmetry. The selected subjects were differentiated clinically into horizontal growth pattern, normal growth pattern and vertical growth pattern based on the parameters like clinical FMA and antegonial notch. Results: The cortical bone thickness of the first permanent mandibular molar (M1) and second permanent mandibular molar (M2) sections was thicker in short-faced subjects than in average and long-faced subjects. The buccolingual inclination of the first permanent mandibular molar (M1) and second permanent mandibular molar (M2) sections in the short-faced subjects was significantly smaller than the same dimension in the average and long-faced subjects. Conclusion: The results of this study provide evidence that a significant, but complex relationship exists between structures of the mandibular body and types of growth pattern. The morphological features that relate to masticatory function and types of growth pattern are associated with the cortical bone thickness of the mandibular body and the buccolingual inclination of the first and second permanent mandibular molars. Keywords: Molar inclination, Cortical bone thickness, CT scan. How to cite this article: Sharma NS, Shrivastav SS, Hazarey PV. Relationship among Types of Growth Patterns, Buccolingual Molar Inclination and Cortical Bone Thickness of the Mandible: A CT Scan Study. J Ind Orthod Soc 2012;46(2): INTRODUCTION Many factors such as the tongue, buccinator muscle and mastication 1 involved in the position and inclination of the teeth, are important in orthodontic treatment. Regarding the tooth position in the mandibular structure, there have been many reports on the relationship between the lower incisor and symphysis based solely on evaluation of cephalograms. Kanazawa and Kasai et al 1 investigated the differences in tooth axis between ancient and modern people and Tsunori et al 2 reported the characteristics of the mandible in Asiatic Indians using computed tomography. However, the relationship between the buccolingual position of the molars and dentofacial morphology has not been fully investigated. In a previous study, 2 1 Assistant Professor, 2 Reader, 3 Professor and Head 1-3 Department of Orthodontics, Sharad Pawar Dental College, Wardha Maharashtra, India Corresponding Author: Narendra Shriram Sharma, Assistant Professor, Department of Orthodontics, Sharad Pawar Dental College Wardha, Maharashtra, India, sharmanarendra047@gmail.com Received on: 29/9/11 Accepted after Revision: 9/11/11 Asiatic Indians skulls were used, but the sex and age of these skulls were estimated by linear discriminate analysis (the probability of correct discrimination was approximately 80%). With males being, on an average, larger than females, a generalization that is commonly employed is based on skeletal morphology, that the cortical bone of males will be thicker and overall individual bones will be more massive and heavier. It has been long accepted that there is a correlation between masticatory muscle orientation, thickness, bite force and its effect on the dentoskeletal morphology. The cause of such a morphology is largely attributed to the abnormal masticatory pattern or an additive combination of deleterious biomechanical factors along with a genetic predisposition. Often in order to improve the dental occlusion, clinicians plan a method of treatment based on inadequate or improper diagnostic criteria. Facial morphology is an important factor in orthodontic treatment, mainly because it influences the anchorage system, growth prediction of maxillofacial structure, goal of orthodontic treatment along with assessment of bite force and masticatory function. The purpose of this study was to evaluate the relationship between different types of growth pattern, thickness of mandibular cortical bone and mandibular molar inclination (buccolingually) using computed tomography (CT) scans. The Journal of Indian Orthodontic Society, April-June 2012;46(2):

Narendra Shriram Sharma et al MATERIALS AND METHODS The present study was done after ethical approval from an institutional ethical committee.

2 Narendra Shriram Sharma et al MATERIALS AND METHODS The present study was done after ethical approval from an institutional ethical committee. In the present study, total 30 cases were selected within the age group of 18 to 25 years at random. Criteria for selecting the cases are as follows: 1. None of the subjects had undergone any orthodontic therapy. 2. Clinically, intraoral examination was conducted to assess the occlusion. Cases were selected with the minimal dental discrepancy, i.e. minimal crowding, spacing and rotations. 3. The selected subjects were differentiated clinically into horizontal growth pattern, normal growth pattern and vertical growth pattern based on the parameters like clinical FMA and antegonial notch. 4. Cephalometric analysis was conducted to group the cases into normal growth pattern, horizontal growth pattern and vertical growth pattern, i.e. groups 1, 2 and 3 respectively by using following parameters (Fig. 1): Frankfort mandibular plane angle (between FH and MP plane) Gonial angle (Ar-Go-Me) Facial axis (line connecting Ptm-Gn point, perpendicular to Ba-N plane) Mandibular arc (DC-XI and XI-Pm). Fig. 1: Reference points and angular measurements utilized on lateral cephalogram Cephalogram of all the subjects were taken with the teeth in maximum intercuspation with the head positioned (F-H plane) parallel to floor. Group FMA Gonial angle Group Normal growth pattern Group 2 < 20 < 120 Horizontal growth pattern Group 3 > 30 > 135 Vertical growth pattern Group 1 was taken as a control group to which the readings of groups 2 and 3 were compared. A computed tomogram was obtained for each individual in the coronal plane. Right side of the mandible was used to obtain coronal section in the first permanent mandibular molar region and the second permanent mandibular molar region. The standard planes for positioning consisted of midpoint of both central permanent maxillary incisors and mesiobuccal cusps of the first permanent maxillary molars. The standard plane was positioned perpendicular to the vertical line of the positioning light in the CT scan machine. For each permanent mandibular molar, the guideline was made parallel to its axis through the mesial cusp to obtain tomograms of each mesial root (Fig. 2). 60 Fig. 2: Guidelines of mandibular sections used by computer tomography CT scan was conducted on X-vision/GX-P/C-2100 machine at 120 kv and 150 mas in the Department of Radiology, AVBRH, DMIMS (DU) Sawangi, Wardha. CT scan slides of 1 mm thickness were taken to evaluate coronal section in centric occlusion. Once the image was obtained, cortical bone thickness was measured with the help of a reference point which was the midtooth point (MT) (Fig. 3). The MT was formed by joining the highest margins of the buccal and lingual alveolar process and the center of this line was marked as MT. Line was drawn from MT to the most inferior point on the lower border of a mandible. The center of this line was set as origin point (O). The MT-O line was taken as 0 and 19 such measurements from an external point on the cortical bone surface were placed every 15 clockwise from the point O. On these 19 JAYPEE

4: Reference lines for angular measurement of the tooth inclination (degree) points, the cortical bone thickness was measured as the shortest horizontal distance between the external cortical bone

3 JIOS Relationship among Types of Growth Patterns, Buccolingual Molar Inclination and Cortical Bone Thickness of the Mandible RESULTS Fig. 3: Reference points and measurements of cortical bone thickness (mm) Fig. 4: Reference lines for angular measurement of the tooth inclination (degree) points, the cortical bone thickness was measured as the shortest horizontal distance between the external cortical bone and the inner cortical bone (Fig. 3). The tracings were then digitized with the computerized program AutoCAD Tooth inclination was measured as an angle between the basal line (right and left inferior border of the mandibular section) and tooth axis (central fossa and midpoint at one-third of the distance from the root apex) (Fig. 4). The division of cases into groups 1, 2 and 3 based on the cephalograms and the differences in tooth inclinations of M1 and M2 are shown in Table 1. The inclination of M2 in group 2 subjects was significantly smaller than in groups 3 and 1 subjects. However, this indicated that the molars of group 2 subjects were positioned more lingually than those of the group 3 subjects. There was no significant difference in M1 inclination. Tables 2 and 3 show the difference in cortical bone thickness among the groups 1, 2 and 3. The buccal regions (from 45 to 120 degree areas) of group 2 subjects were thicker than those of groups 1 and 3 subjects. The basal regions (from 135 to 225 degree areas) of group 2 subjects were thicker than those of group 3 subjects. In addition, the lingual regions (from 240 to 315 degree areas) of group 2 subjects were thicker than those of groups 1 and 3 subjects. Tables 4 and 5 show the correlation coefficients between cortical bone thickness of each section and cephalometric parameters. Significant correlation coefficients in M1 sections were found between FMA and basal and lingual cortical bone thickness and between mandibular arc and buccal and lingual cortical bone thickness. Significant correlation coefficients in M2 sections were found between FMA and basal and lingual cortical bone thickness, between facial axis and cortical bone thickness and between the gonial angle and lingual cortical bone thickness. Table 6 show the correlation coefficients between cephalometric measurements and inclination of mandibular molars at each section. A significant correlation was found in the first molar region when the molar inclination was correlated to mandibular arc, indicating that as mandibular arc increases, molar inclination decreases. There was also a significant correlation coefficient when the molar inclination was compared in the second molar region. It showed a significant correlation with FMA and gonial angle. DISCUSSION In orthodontic research, there are many reports on the relationship between a lower incisor and mandibular symphysis evaluated on lateral cephalogram. The mesiodistal changes in the mandibular molar axis during treatment have also been Table 1: Mean measurements of facial types, tooth inclination and maxillary arch width and depth Variables Short face Average face Long face Student s t-test Mean SD Mean SD Mean SD SF-AF AF-LF SF-LF FMA Gonial angle Mandibular arc * Facial axis * * Tooth inclination M M * * *Significance of F < 0.05 The Journal of Indian Orthodontic Society, April-June 2012;46(2):

4 Narendra Shriram Sharma et al Table 2: Cortical bone thickness at the first molar region among facial types (mm) Sr. no. Region Degrees SF (n = 10) AF (n = 10) LF (n = 10) Student s t-test Mean SD Mean SD Mean SD SF-AF AF-LF SF-LF 1 Buccal Basal * * Lingual * * *Significance of F < Table 3: Cortical bone thickness at the second molar region among facial types (mm) Sr. no. Region Degrees SF (n = 10) AF (n = 10) LF (n = 10) Student s t-test Mean SD Mean SD Mean SD SF-AF AF-LF SF-LF 1 Buccal Basal * * 14 Lingual * * * *Significance of F < evaluated by lateral radiographic tomography. 3 However, the buccolingual inclination of molar is not yet fully understood. Computed tomography of the coronal section is necessary when examining the buccolingual inclination. On the measurement accuracy of computed tomography, Kawamura et al 4 indicated that the reliability of tooth inclination should be assessed by a paired t-test of initial and repeated measurements of the first and second molar. In this study, the means and standard deviations of absolute errors computed for linear and angular measurements remained less than 0.1 mm and 0.14 degree. There was no significant difference between the initial and repeated measurements. Thus, the method was confirmed to be useful and reliable, and might contribute to the evaluation of tooth inclination and cortical bone thickness. In this study, rightsided data was used for analysis because the guideline of the mandibular section was defined by right side of the mandible on the preliminary view. The results of this study provide evidence that buccal cortical bone thickness is associated with the type of growth pattern. A thicker buccal cortical bone is associated with a smaller gonial angle and mandibular plane angle. On the other hand, with respect to tooth inclination, the subjects with a smaller gonial angle and mandibular plane angle were found to have lingually positioned molar. Kawamura 5 also reported that the buccolingual inclination of the mandibular molar was associated with the type of growth pattern. 62 JAYPEE

5 JIOS Relationship among Types of Growth Patterns, Buccolingual Molar Inclination and Cortical Bone Thickness of the Mandible Table 4: Correlation coefficients between facial measurements and cortical bone thickness at the first molar region Sr. no. Region Degrees FMA Gonial angle Mandibular Facial axis arc 1 Buccal ** ** ** ** ** ** * ** ** 0.48** 0.40* ** ** 7 Basal * 0.37* * * Lingual ** ** 0.44* ** 0.53** ** 0.39* ** 0.44* 0.21 Significance level of t-value of correlation coefficient:* p < 0.05; ** p < Table 5: Correlation coefficients between facial measurements and cortical bone thickness at the second molar region Sr. no. Region Degrees FMA Gonial angle Mandibular Facial axis arc 1 Buccal * ** 0.49** ** * 0.40* 0.57** ** 0.63** Basal * 0.39* ** * * 0.56** ** ** 0.43* 0.38* ** 0.61** 0.41* 14 Lingual * * ** ** 0.01 Significance level of t-value of correlation coefficient:* p < 0.05; ** p < Table 6: Correlation coefficients for facial measurements and inclination of mandibular molars at each section FMA Gonial Mandibular Facial axis angle arc Molar inclination M * 0.21 M2 0.35* * 0.01 Significance level of t-value of correlation coefficient: *p < 0.05 From the perspective of functional anatomy, the characteristics of the gonial and mandibular plane angles have been investigated with regard to the relationship between dentofacial morphology and masticatory function. Varrela 6 correlated muscle activity during maximal clenching with mandibular prognathism, anterior inclination of the mandible, and a small gonial angle. Patients with a myotonic dystrophy show weak masticatory muscles and their craniofacial morphology is characterized by a large angle between the mandibular and palatal planes and a steep mandible. 7 Most studies have reported that bite force is associated with a large posterior facial height, a long mandible, a flat mandibular plane and a small gonial angle, 8-12 which characterize SF types. These relationships are independent of the overall size and their specificity argues for differences in the tension-generating capacity of muscle according to type of growth pattern (long, The Journal of Indian Orthodontic Society, April-June 2012;46(2):

6 Narendra Shriram Sharma et al average and short face). Ishida and Soma 13 reported that the direction of force applied to the molar in the occlusal terminal phase showed a buccal direction of the lower molar. Originally, the mandibular molars erupt lingually and then incline toward the buccal. Furthermore, the molars move buccally because of tongue pressure and masticatory function. Finally, the molars reach a balanced position between tongue and buccal pressure, and also are in the position of adaptability of masticatory function. The changes in dental arch width are greater in the mandibular first molar than in the second molar. 14 The bite force of the second molar is also greater than that of the first molar as a result of jaw muscle orientation and moment arms. 15 Kanazawa and Kasai 4 reported that the lingual inclination of the mandibular molar of ancient people was less than that of modern people, and the mandibular cortical bone of the ancient people was thicker because of the supporting strong bite force and masticatory function. The standing position of teeth might therefore be changed by masticatory function. The cortical bone thickness of the molar sections seems to be influenced by masticatory function. The mandibular body of the molar region had a structure resistance to torsional moments. The width of the cortical bone in the buccal-alveolar part of the mandibular molar region was 2.35 to 5 mm. The widths of cortical bone and spongy bone on the buccal side were relatively thicker than on the lingual side. The buccal cortical bone thickness of the mandibular molar was influenced by the masticatory muscle and mandibular movement. The bite force or masticatory function caused by the masticatory muscles influences not only tooth position and dental arch form, but also mandibular shape and structure. 16,17 Investigation of the mandibular structure, including the teeth, is important for understanding occlusal stability and tooth position in orthodontic treatment. The relationship between type of growth pattern and structures of the mandibular body would also be interesting from the view of functional anatomy. The results of this study suggest that the mandibular body of the second molar region has a structure resistant to applied force from the buccal direction. Therefore, the cortical bone becomes thicker to support the second molar. In addition, the results of this investigation might indicate that the buccolingual inclination of the mandibular second molar is related to the masticatory function associated with type of growth pattern. Recently, with the advent of implant supported treatment modalities, the envelope of correction amenable to conventional mechanics has expanded. Popular among the implants are microscrew implants as their size permits placement in areas previously inaccessible. Placement of miniscrew too close to the root can also result in insufficient bone remodeling around the screw and transmission of occlusal forces through the teeth to the screw which can lead to implant failure. Therefore, it is important to select insertion sites carefully using clinical and radiographic evaluation of their anatomical details. The knowledge of the present study is important for clinical placement of mini-implants or other temporary anchorage devices. REFERENCES 1. Kanazawa E, Kasai K. A comparative study of vertical sections of the Jomon and modern Japanese mandibles. Anthropological Science 1998;106(Supplement): Tsunori M, Mashita M, Kasai K. Relationship between facial types and tooth and bone characteristics of the mandible obtained by CT scanning. Angle Ortho 1998;68: Rebellato J, Lindauer SJ, Rubenstein LK, Isaacson RJ, Davidovitch M, Vroom K. Lower arch perimeter preservation using the lingual arch. Am J Orthod and Dentofac Orthop 1997;112: Kawamura A, Kanazawa E, Kasai K. Relationship between teeth positions and morphological characteristics of vertical sections of the mandible obtained by CT scanning. Orthodontic Waves 1998;57: Kawamura A. Relationship between buccolingual inclination of mandibular molars and dentofacial morphology. J of Nihon University School of Dentistry 1999;25: Varrela J. Dimensional variation of craniofacial structures in relation to changing masticatory-functional demands. Euro J of Orthod 1992;14: Ringqvist M. Isometric bite force and its relation to dimensions of the facial skeleton. Acta Odontologica Scandinavica 1973; 31: Møller E. The chewing apparatus. Acta Physiologica Scandinavica 1966;69(Supplement 280): Ingervall B, Helkimo E. Masticatory muscle force and facial morphology in man. Arch of Oral Bio 1978;23: Ingervall B, Bitsanis E. A pilot study of the effect of masticatory muscle train in on facial growth in long-face children. Euro J of Orthod 1987;9: Kiliaridis S, Mejersjö C, Thilander B. Muscle function and craniofacial morphology: A clinical study in patients with myotonic dystrophy. Euro J of Orthod 1989;11: Ingervall B. Correlation between maximum bite force and facial morphology in children. Angle Orthodontist 1997;67: Ishida T, Soma K. Stress analysis of the space between the upper and lower first molars during the final stage of occlusion. J of Japan Orthod Soc 1993;52: Moyers RE, Linden FPGM van der, Riolo ML, McNamara JA. Standards of human occlusal development, Monograph No. 5, Craniofacial Growth Series. Center for Human Growth and Development, University of Michigan, Ann Arbor, 1976; Van Spronsen PH, Koolstra JH, Van Ginkel FC, Weijs WA, Valk J, Prahl-Andrersen B. Relationship between the orientation and moment arms of the human jaw muscles and normal craniofacial morphology. Eur J of Orthod 1997;19: Fogle LL, Glaros AG. Contributions of facial morphology, age, and gender to EMG activity under biting and resting conditions: A canonical correlation analysis. J of Dent Res 1995;74: Kiliaridis S, Johansson A, Haraldson T, Omar R, Carlsson GE. Craniofacial morphology, occlusal traits, and bite force in persons with advanced occlusal tooth wear. Am J of Orthod and Dentofac Orthop 1995;107: JAYPEE

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