Mechanical Energy Densities in Human Temporomandibular Joints

Mechanical fatigue failure may account, in part, for clinical differences in human temporomandibular joints (TMJs). Therefore, energy input to tissues should be considered. Objectives: This pilot study measured and compared energy densities (ED) in TMJs of 18 subjects (ten females, eight males) with TMJ disc displacement (+DD) and 14 subjects (seven females, seven males) without TMJ disc displacement (-DD) in order to investigate if diagnostic group and sex differences exist. Methods: Subjects provided informed consent and anatomic and kinematic data (cephalometric and magnetic resonance images, jaw tracking) to characterize individual-specific intra-capsular contact mechanics during symmetrical jaw closing. Numerical modeling was used to determine TMJ compressive loads (F_normal) and dynamic stereometry was used to link anatomy and movement data for the ED estimations, where: ED=W/Q (W=work done or mechanical energy input=tractional forceXdistance of stress-field translation, Q=volume of cartilage). ANOVA (group, sex), ANCOVA (sex, single variable effects), and regression analyses (single variable contributions) were performed with p<0.05 considered significant. Results: Mean ED (±standard error) were: females -DD=3.8 (1.6), males -DD=1.9 (1.6), females +DD=6.4 (1.3), and males +DD=2.5 (1.5) mJ/mm³. Thus, mean ED were relatively higher for females and the +DD group and were significantly greater by 3.4-fold in females +DD compared to males –DD (p<0.04). Within the +DD group, ANCOVA showed mean ED adjusted for F_normal were significantly greater (p<0.05) by 3.6-fold in females (4.3 mJ/mm³) compared to males (1.2 mJ/mm³) and the variables: distance and velocity of stress-field translation and tractional coefficient (tractional force/F_normal) were significantly related to ED (p<0.02). Conclusion: Differences in mechanical energy densities may be important to DD and sex differences in the TMJ. This work was supported in part by NIH (R01 DE016417-01A1).