Methods: IRB approved Research Diagnostic Criteria and MRI imaging of the TMJs were used by investigators to classify 93 subjects in 4 diagnostic groups based on presence (+) or absence (-) of disc displacement (DD) and pain [-DD/-pain, n=28; -DD/+pain, n=16; +DD/-pain, n=29; +DD/+pain, n=28). Three-dimensional geometries of the positions of the muscles of mastication, dental arches, and mandibular condyles were produced for each subject using lateral and posteroanterior cephalometric radiographs. The 3D geometry of each subject was used in validated numerical models to calculate masticatory muscle forces for a range of bite-force angulations on molar and incisor teeth. ANOVA and Holm-simulated step-down procedure post-hoc tests were used to test for significant diagnostic group differences in masticatory muscle forces.
Results: For a range of biting angles on the molar teeth, +P/-DD subjects had >14% higher ipsilateral muscle forces and >11% higher contralateral forces compared the other groups (all P<0.01). Average lateral pterygoid muscle forces in the +P-DD group were >25% greater than in the other diagnostic groups (all P<0.01). During incisor biting, for a range of biting angles, +P-DD ipsilateral muscle forces were >30% greater compared to the other groups (all P<0.01). Lateral pterygoid muscle forces in the +P-DD group were >50% higher compared to +P+DD group (P<0.05), and >65% higher compared to the –P-DD group (P<0.01).
Conclusions: There were significant differences in masticatory muscle forces amongst TMD diagnostic subgroups and healthy subjects for specified biting angles on molar teeth. +P/-DD subjects were mechanically different compared to the other diagnostic groups. Supported by the NIH (R01DE16417, JN-PI) and UMKC Student Research.
Keywords: Computers, Human, Mechanics, Muscle and TMJ and masticatory muscles