Method: Research Diagnostic Criteria and magnetic resonance TMJ images were used by calibrated examiners to classify 93 subjects into 4 diagnostic groups based on presence(+) or absence (-) of disc displacement(DD) and pain [group, n=females, males: -DD/-pain, n=15, 13; -DD/+pain, n=8, 8; +DD/-pain, n=16, 13; +DD/+pain, n=15, 13). Subjects gave informed consent to participate in IRB-approved protocols. Three-dimensional geometries described positions of masticatory muscles, dental arches, and mandibular condyles for all subjects from lateral and posteroanterior cephalometric radiographs. The range of anteroposterior variations of masticatory muscle orientations represented (>1300 possible combinations) were used in 2 numerical models to identify key combinations of muscle orientations that produced the highest predicted lateral pterygoid muscle forces. These muscle orientations were compared amongst diagnostic groups using effect-size (Cohen’s d) and ANOVA-Tukey-Kramer post-hoc tests.
Result: Lateral pterygoid muscle forces were highest (2.5X applied bite-force) for the combination of relatively upright masseter, posteriorly-directed temporalis, and medially-directed lateral pterygoid muscles. Diagnostic group differences were found. Mean masseter muscle orientation in –DD/+pain subjects was 7° more upright compared to the 3 other diagnostic groups. These overall differences were significant and inter-group effect sizes were large (Table).
Table: Between Group Masseter Muscle Orientation Comparisons by Cohen’s d Effect Size (Tukey-Kramer adjusted p-value) |
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|
+DD/+pain |
+DD/-pain |
-DD/-pain |
-DD/+pain |
2.86 (<0.03) |
2.56 (<0.02) |
2.92 (<0.04) |
Mean temporalis muscle orientation was more posteriorly-directed in -DD/+pain subjects compared to –DD/-pain subjects by 9°. This difference was significant and effect size was large (p<0.03, Cohen’s d= 2.45).
Conclusion: Masseter and temporalis muscle orientations were significantly different in -DD/+pain subjects and in part explain predicted group differences in lateral pterygoid muscle forces.