Mandibular Tori Are Associated With Mechanical Stress and Mandibular Shape



Mandibular Tori Are Associated With Mechanical Stress and Mandibular Shape




Journal of Oral and Maxillofacial Surgery, 2014-11-01, Volume 72, Issue 11, Pages 2115-2125, Copyright © 2014 American Association of Oral and Maxillofacial Surgeons


Purpose

The influence of mechanical stimulation on the formation of torus mandibularis (TM) is still poorly understood. We sought to understand the etiology of TMs by investigating the role of parafunctional activity and mandibular morphology on the formation of TMs.

Materials and Methods

We designed a case-control study for patients attending the dental clinic of the present study (University of São Paulo School of Dentistry, São Paulo, SP, Brazil). Patients presenting with TMs were defined as cases, and those without TMs were defined as controls. Finite element analysis (FEA) was used in 3-dimensional mandibular models to examine the stress distribution in the mandibles with and without TMs. In addition, the associations of mandibular arch shape, mandibular cortical index, and parafunctional activity with the presence of T were assessed using odds ratio analysis.

Results

A total of 10 patients with TMs and 37 without TMs were selected (22 men and 25 women, mean age 54.3 ± 8.4 years). FEA showed a stress concentration in the region in which TMs form during simulation of parafunctional activity. The radiographic assessment showed that those with TMs were more likely to have a square-shaped mandible with sharp angles ( P = .001) and a normal mandibular cortex ( P = .03). The subjects without TMs had a round-shaped mandible with obtuse angles and an eroded mandibular cortex.

Conclusions

Parafunctional activity could be causing the formation of TMs by concentrating mechanical stress in the region in which TMs usually form. Thus, mandibular geometries that favor stress concentration, such as square-shaped mandibles, will be associated with a greater prevalence of TMs.

Harnessing the human body's capability for ectopic oral bone formation, such as occurs with torus mandibularis (TM), promises the potential of an alternative to surgical bone and soft tissue augmentation for dental rehabilitation. Understanding the etiology of ectopic oral bone formation would improve planning of TM treatment and management strategies. It would also allow the exploitation of the mechanism of TM formation to create new bone for regenerative purposes.

TM is one of the most commonly encountered oral bone exostoses. It consists primarily of dense cortical bone without marrow and can often present bilaterally on the lingual aspect of the mandible from the retromolar region to the symphysis. The most common site, however, is in the canine and premolar region above the mylohyoid line. TM has a nearly equal gender predilection and tends to grow slowly and continuously from the peripubertal period onward. Typically, the TM gradually enlarges into adulthood but has been found to spontaneously stop growing and even to regress in size, in the absence of teeth.

Numerous studies have indirectly implicated a relationship between TMs and signs of parafunctional habits, such as abfraction, dental attrition, and temporomandibular dysfunction. These studies have suggested that the pattern of masticatory stress might influence the occurrence of ectopic oral bone formation, specifically TM.

Bone adapts to mechanical challenges by the biologic machinery operating at the tissue level, such that bone deposition and bone resorption occur in concert to increase bone strength and alter bone morphology. Functional adaptation of bone to mechanical loading is best exemplified by the strengthening of load-bearing bones. Accordingly, it could be hypothesized that the forces exerted on the lingual bone during excursive parafunction trigger a cascade of molecular events leading to TM formation; therefore, because the canine and premolar teeth bear greater occlusal loads during excursive movements, they might exert a force on the bone that results in periosteal stretching and subsequent cortical bone deposition.

The distribution of mechanical stress within an object depends on the makeup and morphology of the object in question. Accordingly, we hypothesized that the excursive parafunctional stresses would be concentrated in the lingual canine and premolar region of the mandible, congruent with the typical location of TM formation. We also hypothesized that the mandibular morphology in subjects with TMs allows for a concentration of parafunctional stresses in the anterior lingual area of the mandible. To test our hypotheses, we implemented a case-control study using finite element analysis (FEA) to analyze the parafunctional stress distribution within the mandible of subjects with and without TMs. Moreover, we evaluated the mandibular morphology of the case and control subjects to identify the associations with the presence of TMs.


Materials and Methods


Study Design and Sample

To address the research purpose, we designed and implemented a case-control study, which followed the Strengthening the Reporting of Observational Studies in Epidemiology guidelines for observational studies and the Declaration of Helsinki for medical ethics. The ethics committee of the University of São Paulo School of Dentistry (São Paulo, SP, Brazil) approved the present study (protocol no. N105/11). The patients presenting with TMs were defined as cases, and those without TMs were defined as controls. The cases and controls were comparable in terms of age and gender (no significant differences). All subjects willing to participate in the present study signed an informed consent form allowing the use of their treatment data and radiographic images.

The study population was composed of subjects attending the dental clinic at the University of São Paulo School of Dentistry (São Paulo, SP, Brazil) from January 2013 to July 2013.


Inclusion and Exclusion Criteria

The subjects included in the study sample had dental study models, digital orthopantomographs, and cone beam computed tomographic (CBCT) scan data, obtained from the archives of the dental clinic for analysis. All patients attending the clinic undergo routine digital orthopantomography in the initial examination. In addition, all patients requiring oral surgery or diagnosis of an unrelated pathologic condition underwent CBCT. The demographic parameters (ie, age and gender) were recorded for all subjects. The subjects' medical records were also assessed for any history of parafunctional habits, including tooth grinding, nail biting, and/or clenching.

The subjects with systemic factors known to interfere with bone or soft tissue healing were excluded from the present study. Such factors included diabetes mellitus (types 1 and 2), hyperthyroidism, smoking history, rheumatoid arthritis, cancer with bone metastases, chemotherapy, chronic steroid use, metabolic bone disease, major renal disease, and/or any immunocompromised state. Furthermore, the subjects taking medication known to affect bone metabolism were excluded from the present study.


Variables

The primary outcome variable of the present study was the presence of TMs. A history of parafunctional activity was analyzed as a predictor variable and as a secondary outcome variable to assess the direct associations between this variable and other predictor variables. Accordingly, case subjects were defined as those with discernable protrusions of bone on the lingual aspect of the mandible, consistent with TMs and detected on CBCT axial images. Control subjects were defined as those with a uniformly contoured lingual aspect of the mandible consistent with the lack of TMs.

Furthermore, 5 predictor variables were assessed: age, gender, mandibular arch shape, mandibular cortical index, and history of parafunctional activity. Complementary variables such as mandibular angle measurements and FEA were also compared between the case and control groups.


Data Collection


History of Parafunctional Activity

The subjects' medical records were assessed for any history of parafunctional habits, including tooth grinding, nail biting, and/or clenching. The report of the presence of any of these parameters was considered positive for a history of parafunctional activity.


Mandibular Arch Shape Classification

Dental study casts were used to classify the mandibular arch shape as either square-shaped or round-shaped using a previously described classification for arch morphology. In brief, the references points, lines, and angles were identified according to the teeth position in the mandibular study cast. A square-shape mandible will have a dental arch with a protruded canine and the anterior teeth in an almost linear arrangement. The posterior teeth will also be aligned, and both posterior hemiarchs will be parallel to each other. A round-shaped mandible will have a semicircle dental arch with no canine protrusion.


Radiographic Methods

All preoperative digital orthopantomographic images were obtained using the Veraviewepocs 2D (Morita, Tokyo, Japan) set at 60 kV and 4 mA, with a 0.5-mm copper filter. Images were provided in tagged image file format and analyzed using ImageJ software (National Institutes of Health, Bethesda, MD).

All CBCT images were obtained with the scan unit i-CAT Classic (Image Sciences International, Hatfield, PA), configured using a diagnostic protocol for dental implants (0.25-mm voxel, 120 kVp, 8 mA, 16 cm field of view in diameter and 6 cm in height, and 1-mm slice thickness). The CBCT images were provided in the Digital Imaging Communications in Medicine (DICOM) format and analyzed using OsiriX imaging software (open-source, DICOM viewer OsiriX 3.9.4 version, Pixmeo, Geneva, Switzerland).


Radiographic Measurements Assessed


Mandibular cortical index

The mandibular cortical index (MCI) is a measurement used for the approximation of bone mineral density (BMD) from an assessment of orthopantomographic images. The MCI classifies the appearance of the inferior cortex of the mandible distal to the mental foramina, as viewed on the orthopantomograph. A normal cortex is indicative of high systemic bone density. This classification uses a 3-point scale:

  • C1—normal: the endosteal margin of the cortex is even and sharply defined on both sides of the mandible

  • C2—mild to moderate erosion: the endosteal margin shows semilunar defects (lacunar resorption) or seems to form endosteal cortical residues (1 to 3 layers) on 1 or both sides of the mandible

  • C3—severe erosion: the cortical layer forms heavy endosteal residues and is clearly porous


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Mandibular Tori Are Associated With Mechanical Stress and Mandibular Shape Arthur Rodriguez Gonzalez Cortes DDS, MS , Zhaoyu Jin MEng , Matthew Daniel Morrison DMD, MSc , Emiko Saito Arita DDS, PhD , Jun Song PhD and Faleh Tamimi BDS, PhD Journal of Oral and Maxillofacial Surgery, 2014-11-01, Volume 72, Issue 11, Pages 2115-2125, Copyright © 2014 American Association of Oral and Maxillofacial Surgeons Purpose The influence of mechanical stimulation on the formation of torus mandibularis (TM) is still poorly understood. We sought to understand the etiology of TMs by investigating the role of parafunctional activity and mandibular morphology on the formation of TMs. Materials and Methods We designed a case-control study for patients attending the dental clinic of the present study (University of São Paulo School of Dentistry, São Paulo, SP, Brazil). Patients presenting with TMs were defined as cases, and those without TMs were defined as controls. Finite element analysis (FEA) was used in 3-dimensional mandibular models to examine the stress distribution in the mandibles with and without TMs. In addition, the associations of mandibular arch shape, mandibular cortical index, and parafunctional activity with the presence of T were assessed using odds ratio analysis. Results A total of 10 patients with TMs and 37 without TMs were selected (22 men and 25 women, mean age 54.3 ± 8.4 years). FEA showed a stress concentration in the region in which TMs form during simulation of parafunctional activity. The radiographic assessment showed that those with TMs were more likely to have a square-shaped mandible with sharp angles ( P = .001) and a normal mandibular cortex ( P = .03). The subjects without TMs had a round-shaped mandible with obtuse angles and an eroded mandibular cortex. Conclusions Parafunctional activity could be causing the formation of TMs by concentrating mechanical stress in the region in which TMs usually form. Thus, mandibular geometries that favor stress concentration, such as square-shaped mandibles, will be associated with a greater prevalence of TMs. Harnessing the human body's capability for ectopic oral bone formation, such as occurs with torus mandibularis (TM), promises the potential of an alternative to surgical bone and soft tissue augmentation for dental rehabilitation. Understanding the etiology of ectopic oral bone formation would improve planning of TM treatment and management strategies. It would also allow the exploitation of the mechanism of TM formation to create new bone for regenerative purposes. TM is one of the most commonly encountered oral bone exostoses. It consists primarily of dense cortical bone without marrow and can often present bilaterally on the lingual aspect of the mandible from the retromolar region to the symphysis. The most common site, however, is in the canine and premolar region above the mylohyoid line. TM has a nearly equal gender predilection and tends to grow slowly and continuously from the peripubertal period onward. Typically, the TM gradually enlarges into adulthood but has been found to spontaneously stop growing and even to regress in size, in the absence of teeth. Numerous studies have indirectly implicated a relationship between TMs and signs of parafunctional habits, such as abfraction, dental attrition, and temporomandibular dysfunction. These studies have suggested that the pattern of masticatory stress might influence the occurrence of ectopic oral bone formation, specifically TM. Bone adapts to mechanical challenges by the biologic machinery operating at the tissue level, such that bone deposition and bone resorption occur in concert to increase bone strength and alter bone morphology. Functional adaptation of bone to mechanical loading is best exemplified by the strengthening of load-bearing bones. Accordingly, it could be hypothesized that the forces exerted on the lingual bone during excursive parafunction trigger a cascade of molecular events leading to TM formation; therefore, because the canine and premolar teeth bear greater occlusal loads during excursive movements, they might exert a force on the bone that results in periosteal stretching and subsequent cortical bone deposition. The distribution of mechanical stress within an object depends on the makeup and morphology of the object in question. Accordingly, we hypothesized that the excursive parafunctional stresses would be concentrated in the lingual canine and premolar region of the mandible, congruent with the typical location of TM formation. We also hypothesized that the mandibular morphology in subjects with TMs allows for a concentration of parafunctional stresses in the anterior lingual area of the mandible. To test our hypotheses, we implemented a case-control study using finite element analysis (FEA) to analyze the parafunctional stress distribution within the mandible of subjects with and without TMs. Moreover, we evaluated the mandibular morphology of the case and control subjects to identify the associations with the presence of TMs. Materials and Methods Study Design and Sample To address the research purpose, we designed and implemented a case-control study, which followed the Strengthening the Reporting of Observational Studies in Epidemiology guidelines for observational studies and the Declaration of Helsinki for medical ethics. The ethics committee of the University of São Paulo School of Dentistry (São Paulo, SP, Brazil) approved the present study (protocol no. N105/11). The patients presenting with TMs were defined as cases, and those without TMs were defined as controls. The cases and controls were comparable in terms of age and gender (no significant differences). All subjects willing to participate in the present study signed an informed consent form allowing the use of their treatment data and radiographic images. The study population was composed of subjects attending the dental clinic at the University of São Paulo School of Dentistry (São Paulo, SP, Brazil) from January 2013 to July 2013. Inclusion and Exclusion Criteria The subjects included in the study sample had dental study models, digital orthopantomographs, and cone beam computed tomographic (CBCT) scan data, obtained from the archives of the dental clinic for analysis. All patients attending the clinic undergo routine digital orthopantomography in the initial examination. In addition, all patients requiring oral surgery or diagnosis of an unrelated pathologic condition underwent CBCT. The demographic parameters (ie, age and gender) were recorded for all subjects. The subjects' medical records were also assessed for any history of parafunctional habits, including tooth grinding, nail biting, and/or clenching. The subjects with systemic factors known to interfere with bone or soft tissue healing were excluded from the present study. Such factors included diabetes mellitus (types 1 and 2), hyperthyroidism, smoking history, rheumatoid arthritis, cancer with bone metastases, chemotherapy, chronic steroid use, metabolic bone disease, major renal disease, and/or any immunocompromised state. Furthermore, the subjects taking medication known to affect bone metabolism were excluded from the present study. Variables The primary outcome variable of the present study was the presence of TMs. A history of parafunctional activity was analyzed as a predictor variable and as a secondary outcome variable to assess the direct associations between this variable and other predictor variables. Accordingly, case subjects were defined as those with discernable protrusions of bone on the lingual aspect of the mandible, consistent with TMs and detected on CBCT axial images. Control subjects were defined as those with a uniformly contoured lingual aspect of the mandible consistent with the lack of TMs. Furthermore, 5 predictor variables were assessed: age, gender, mandibular arch shape, mandibular cortical index, and history of parafunctional activity. Complementary variables such as mandibular angle measurements and FEA were also compared between the case and control groups. Data Collection History of Parafunctional Activity The subjects' medical records were assessed for any history of parafunctional habits, including tooth grinding, nail biting, and/or clenching. The report of the presence of any of these parameters was considered positive for a history of parafunctional activity. Mandibular Arch Shape Classification Dental study casts were used to classify the mandibular arch shape as either square-shaped or round-shaped using a previously described classification for arch morphology. In brief, the references points, lines, and angles were identified according to the teeth position in the mandibular study cast. A square-shape mandible will have a dental arch with a protruded canine and the anterior teeth in an almost linear arrangement. The posterior teeth will also be aligned, and both posterior hemiarchs will be parallel to each other. A round-shaped mandible will have a semicircle dental arch with no canine protrusion. Radiographic Methods All preoperative digital orthopantomographic images were obtained using the Veraviewepocs 2D (Morita, Tokyo, Japan) set at 60 kV and 4 mA, with a 0.5-mm copper filter. Images were provided in tagged image file format and analyzed using ImageJ software (National Institutes of Health, Bethesda, MD). All CBCT images were obtained with the scan unit i-CAT Classic (Image Sciences International, Hatfield, PA), configured using a diagnostic protocol for dental implants (0.25-mm voxel, 120 kVp, 8 mA, 16 cm field of view in diameter and 6 cm in height, and 1-mm slice thickness). The CBCT images were provided in the Digital Imaging Communications in Medicine (DICOM) format and analyzed using OsiriX imaging software (open-source, DICOM viewer OsiriX 3.9.4 version, Pixmeo, Geneva, Switzerland). Radiographic Measurements Assessed Mandibular cortical index The mandibular cortical index (MCI) is a measurement used for the approximation of bone mineral density (BMD) from an assessment of orthopantomographic images. The MCI classifies the appearance of the inferior cortex of the mandible distal to the mental foramina, as viewed on the orthopantomograph. A normal cortex is indicative of high systemic bone density. This classification uses a 3-point scale: C1—normal: the endosteal margin of the cortex is even and sharply defined on both sides of the mandible C2—mild to moderate erosion: the endosteal margin shows semilunar defects (lacunar resorption) or seems to form endosteal cortical residues (1 to 3 layers) on 1 or both sides of the mandible C3—severe erosion: the cortical layer forms heavy endosteal residues and is clearly porous Mandibular angle measurements The mandibular angle measurements (in degrees) were recorded separately in random order. Three buccal and three lingual angles were labeled on the axial CBCT images of the mandibles (at the level of the mental foramina; Fig 1 ). These angles were as follows: Angle A: the vertex at the right mental foramen connects the anterior aspect of the right mandibular symphysis with the apex of the mental protuberance Angle B: the vertex at the apex of the mental protuberance connects with points at the right and left mental foramina Angle C: contralateral to angle A—the vertex at the left mental foramen connects the anterior aspect of the left mandibular symphysis with the apex of the mental protuberance Angle D: the vertex on the lingual aspect of the right mental foramen connects with 2 points: 1 at the posterior aspect of the alveolar lingual plate and 1 in the lingual foramina Angle E: the vertex on the lingual foramina connects with points at the lingual aspect of the right and left mental foramina Angle F: Contralateral to angle D—the vertex at the lingual aspect of the left mental foramen connects the posterior aspect of the alveolar lingual plate and lingual foramina Figure 1 Schematic 3-dimensional presentation of the A, buccal and B, lingual mandibular angles (see text for details). Data Analysis Finite Element Analysis To assess the stress distribution within the case ( Figs 2A,C ) and control ( Figs 2B,D ) mandibles during functional and parafunctional loading, von Mises stress calculations were performed on FEA simulations ( Figs 3B,D ). The von Mises stress, denoted as σVM σ V M in the equation, is a quantity commonly used in engineering to predict the yield of materials under multiaxial loading conditions. It is defined as a function of the individual stress components (ie, σij σ i j and σkk σ k k with i, j, and k = 1, 2, and 3, respectively): σVM=32σijσij−12σ2kk−−−−−−−−−−−−√ σ V M = 3 2 σ i j σ i j − 1 2 σ k k 2 Figure 2 Surface models (isotropic and axial views) of A,C, a mandible with torus mandibularis and B,D, a mandible without torus mandibularis. Figure 3 Boundary conditions applied during finite element analysis simulations on a mandible with A, torus mandibularis (TM) and C, without TM, where Ω d and Ω f denote the displacement and traction boundary conditions, respectively. The force components of Ω f are shown for their respective Cartesian coordinates as F x , F y , and F z . The corresponding von Mises stress fields (σ) for the forces applied are depicted by color mapping for the mandible B, with TM (B) and D, without TM. Stress analysis using FEA simulations has been shown to be effective in the study of the biomechanical behavior of bones. In the present study, for simplicity, we assumed that the material constituting the volume model was homogenous, isotropic, and linear-elastic. We note that actual mandibular bone is a heterogeneous material, with the elastic moduli, Poisson's ratio, different in the longitudinal, radial, and tangential directions (ie, anisotropic). Nonetheless, we adopted the idealized FEA model to avoid complicated parameterization and to qualitatively illustrate the influence of the mandible geometry, rather than perform a quantitative assessment. A comprehensive review of the accuracy of FEA in describing the biomechanical behavior of bones can be found studies by Wong et al, Choi et al, Meyer et al, and Vollmer et al. In our FEA simulations, the Young modulus and Poisson ratio chosen for the material was E = 13 GPa and ν = 0.3, respectively, the values often used for cortical bone. Two FEA material models were constructed from the 3-dimensional CBCT scan data: 1 representative of the case group ( Figs 3A,B ) and 1 representative of the control group ( Figs 3C,D ). The model generation process was as follows. First, the CBCT scan images in DICOM format were used to construct surface models of the mandibles ( Fig 2 ). These were converted to stereolithography (STL) files containing the coordinates for the triangular facets and vertices. Second, the STL files were meshed again with HyperMesh (Altair Engineering, Auckland, New Zealand) using solid tetrahedral elements to create the corresponding volume models ( Figs 2A,C ) onto which the material properties were added. For the 2 FEA models shown in Figure 3, 20,898 nodes (85,842 linear tetrahedral elements in total) and 25,373 nodes (105,585 linear tetrahedral elements in total) were used for Figure 3B and Figure 3D , respectively. Mesh convergence studies were performed for both models to ensure that the simulation results were stable according to those mesh densities. Finally, the meshes generated by HyperMesh (Altair Engineering) were imported into Abaqus (Simulia, Providence, RI) to perform the stress analysis. The boundary conditions (BCs) were designed to simulate the typical occlusal constraints and muscle responses experienced during mastication and parafunctional loading. Displacement and traction BCs were applied to the mesh to compute the deformation and stress states within the mandibles. The 0-displacement BC, denoted as Ω d , was applied to the working side condyle and canine, simulating canine guidance on parafunctional loading, and a traction BC, denoted as Ω f , was applied to the medial aspect of the contralateral gonial angle, simulating contraction of the contralateral medial pterygoid muscle ( Figs 3A,C ). The force vector, F→ F → , acting on Ω f had force components (F x , F y , F z ) prescribed as 100 N, 0, 100 N. It mimics the force exerted by the lateral pterygoid muscle and was in line with that described by Schindler et al. Statistical Analysis All radiographic measurements were performed by 2 trained observers (ie, dentists with expertise in oral radiology), who were unaware of the presence or absence of TMs. Intraobserver reliability was assessed between measurements performed 2 weeks apart to eliminate memory bias. Intra- and interobserver agreement was assessed using the intraclass correlation coefficient (ICC) for the CBCT angulations and the kappa test for MCI. Conditional logistic regression analysis was performed using the IBM SPSS Statistics, version 17 software (IBM, Armonk, NY). Risk estimates are presented as odds ratios (ORs), with 95% confidence intervals (CIs). In addition, the Mann-Whitney U test was used to assess the angle differences between the cases and controls. A P value < .05 was considered statistically significant. Results The CBCT data from 53 subjects were initially selected during the study period. Of the 12 subjects identified with TMs, 2 were excluded because of metabolic bone disease. Of the 41 subjects identified as not having TMs, 4 were excluded (3 with diabetes mellitus and 1 with metabolic bone disease). Thus, 47 patients (22 men and 25 women, mean age 54.3 ± 8.4 years) fulfilled the inclusion criteria. Intraobserver reproducibility and interobserver reliability were confirmed for the CBCT angulations (ICC 0.82 to 0.89, P = .001) and mandibular shape and MCI measurements (kappa index 0.78 to 0.84, P = .01). Unlike physiologic mastication, parafunctional loading generated a considerable amount of mechanical stress in the mandible ( Fig 3 ). The largest von Mises stresses were concentrated on the lingual aspect of the mandible in the canine and premolar apical regions, corresponding to the typical site of TMs. The stress values were the greatest in the case subjects ( Figs 3B,D ). The presence of TMs was strongly associated with a square-shaped mandibular arch (OR 26.44, 95% CI 4.39 to 115.21, P = .001; Table 1 ), a history of parafunctional habits (OR 5.44, 95% CI 1.22 to 24.04, P = .046), and a normal mandibular cortex (MCI C1; OR 6.57, 95% CI 1.21 to 35.43, P = .030). A history of parafunctional habits, however, was not significantly associated with a square-shaped mandibular arch ( P = .137) or with MCI C1 ( P = .524; Table 2 ). No significant associations were found between the subjects' age or gender and the presence of TMs. Table 1 Associations Between Subject Characteristics and Presence of Tori Mandibularis Characteristic TM OR (95% CI) P value No Yes Age (yrs) .297 ≤60 18 07 1 >60 19 03 0.40 (0.09-1.82) Gender .154 Female 22 03 1 Male 15 07 3.42 (0.76-15.38) Mandibular arch shape .001 ∗ Round 34 03 1 Square 03 07 26.44 (4.39-115.21) MCI .030 ∗ Eroded cortex (C2-C3) 23 02 1 Normal cortex (C1) 14 08 6.57 (1.21-35.43) History of parafunctional habits .046 ∗ No 29 04 1 Yes 08 06 5.44 (1.22-24.04) Abbreviations: C1, normal mandibular cortex; C2, mild to moderate erosion; C3, severe erosion; CI, confidence interval; MCI, mandibular cortical index; OR, odds ratio; TM, torus mandibularis. ∗ Statistically significant ( P < .05). Table 2 Associations Between Subject Characteristics and History of Parafunctional Habits Characteristic History of Parafunctional Habits OR (95% CI) P value No Yes Mandibular arch shape .137 Round 28 09 1 Square 05 05 3.11 (0.73-13.25) MCI .524 Eroded cortex (C2-C3) 14 08 1 Normal cortex (C1) 19 06 0.55 (0.15-1.95) Abbreviations: CI, confidence interval; MCI, mandibular cortical index; OR, odds ratio. To further confirm the relationships between the mandibular geometry and the presence of TMs, the buccal and lingual angles of the case and control subjects were analyzed ( Fig 4 ). A statistically significant difference between the case and control subjects was observed for the buccal angles A ( P = .007) and C ( P = .022), such that these angles were more acute for the case subjects ( Table 3 ). No statistically significant associations were observed between the remaining mandibular angle measurements for the case and control subjects. Figure 4 Axial computed tomography scans of a mandible A, with torus mandibularis (TM) ( Inset, pronounced TM bilaterally) and B, without TM. Table 3 Comparison Between Buccal and Lingual Angles for Mandibles With and Without TM Angle TM Mean SD P Value ( t Test) Left buccal angle .009 No 153.268 5.25 Yes 148.200 3.63 Central buccal angle .825 No 116.403 6.52 Yes 115.833 8.36 Right buccal angle .021 No 152.874 6.09 Yes 147.578 5.46 Left lingual angle .234 No 149.529 6.41 Yes 146.722 5.64 Central lingual angle .342 No 118.524 9.40 Yes 115.333 6.46 Right lingual angle .136 No 150.418 6.60 Yes 146.856 4.85 Abbreviations: SD, standard deviation; TM, torus mandibularis. Discussion The present study strove to test the hypothesis that excursive parafunctional stresses would be concentrated in the ridge area in which TMs usually form. A second hypothesis was that the stress accumulation provided by parafunction could be favored by mandibular morphology. Both hypotheses were confirmed. Additionally, parafunctional activity and mandibular morphology were independent factors influencing TM formation. Numerous studies have shown associations between signs of parafunctional activity (ie, dental attrition and temporomandibular joint dysfunction) and the presence of TMs. A paucity of evidence, however, is available to support the mechanism behind this association. The results of our investigations have confirmed our hypothesis (ie, unlike physiologic mastication, parafunctional activity results in the accumulation of mechanical stress in the typical location of TMs). In addition, stress accumulates to a greater extent in individuals with TMs, suggesting a morphologic pattern for these individuals. Thus, our data have confirmed that parafunctional stress is associated with ectopic bone formation in the mandible, offering concrete evidence of the mechanism involved in TM formation. FEA of the case and control mandibles in our study showed that parafunctional loading produced greater levels of mechanical stress in the lingual apical region of the canine and premolar teeth compared with physiologic loading. The focal area of stress concentration corresponded with that of the typical region of TM development. Compared with the control subjects, the case subjects had a more pronounced area of stress concentration in this region for the same force, Ω d and BCs applied. These findings suggest that subjects with TMs possess specific mandibular geometric conditions that allow for focal stress concentrations (during parafunctional loading) in the typical area of TM formation. Although direct correlations between high overall BMD and the presence of oral TMs have been made, our investigation is the first to present the relationship between MCI and the presence of TMs. The use of MCI as a surrogate for BMD measurement has been previously validated. Our investigations showed an MCI value of C1 (normal mandibular cortex) was associated with the presence of TMs, suggesting that bone metabolism could play a role in TM formation. Occlusal forces transmitted to the mandible during mastication follow force vectors according to the masticatory muscles and the position and angulation of the teeth. The tooth position, however, will be largely dictated by the shape of the dental arches, which, in turn, is affected by the geometric shape of the upper and lower jaws. We quantified the various angulations on the buccal and lingual aspects of the case and control subjects' mandibles and observed that subjects with TM were more likely to have sharper, more acute, buccal mandibular angulations. To our knowledge, the present study is the first to demonstrate that a square-shaped mandibular arch form is associated with the presence of TMs. Our results have also indicated that the mandibular geometry and anatomy (ie, MCI, angulations, and shape) were not influenced significantly by parafunctional activity. This indicates that the mandibular geometry and parafunctional activity probably act as synergic independent predisposing factors associated with a greater risk of TM formation. Of the various, albeit poorly understood, factors—environmental, genetic, and metabolic— believed to play a role in the development of TMs, our investigations have provided evidence for environmental and genetic influences (ie, mechanical stress and mandibular geometry, respectively). Our findings have also implicated the involvement of bone metabolism in TM formation, which was shown by the increased risk of the presence of TMs in subjects lacking erosion of the inferior mandibular cortex (ie, an MCI of C1). In the present study, we used an FEA model meshed from the patient's mandible scan data, which allowed us to accurately represent all geometric aspects of the mandible. Furthermore, using FEA, it was possible to directly visualize the deformation and stress state and the distribution within the mandible under different loads. Coupling the FEA modeling with clinical findings enabled us to better identify key biomechanical components and subsequently perform targeted clinical tests and characterizations. One limitation of our current FEA study was that we used a simplified description of the mandible material. Thus, the possible contributions from 1) the material heterogeneity and anisotropy and 2) differences in material properties owing to gender, age, and so forth, were not captured in the analysis. Furthermore, no quantitative variables were taken from the FEA performed. Future studies should address this issue with more refined and targeted FEA analysis. Another limitation of the present study was that its case-control design allowed for detection of associations, but not causality. Future cohort or randomized clinical trials are needed to address this issue. Furthermore, no direct quantitative evaluation of bone metabolism was performed in our study. Therefore, additional studies on bone turnover are needed to assess the factors involved in the biochemical etiology of TM. The results of our investigations have collectively provided insight into the mechanism of TM formation. First, parafunctional activity generates mechanical stress on the lingual aspect of the anterior mandible in the typical TM location. Second, a morphologic pattern exists for the mandibular geometry of subjects with TMs, such that stress accumulation is heightened in subjects with a square-shaped mandible with sharper buccal angles. In conclusion, parafunctional activity could be causing the formation of TMs by concentrating mechanical stress in the region in which TMs usually form. Thus, mandibular geometries that favor stress concentration, such as square-shaped mandibles, will be associated with a greater prevalence of TM. A PhD scholarship was given to Dr Cortes by the National Council for Scientific and Technological Development (CNPq – Brazil, No. 140291/2011-3). 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