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.
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.
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.
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.
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.
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