Mechanobehavior in TMD Diagnostic Groups of Women

Objectives: Cartilage fatigue may account for the precocious development of degenerative joint disease in the TMJ. This study estimated TMJ work energy densities (EDs, mJ/mm3), and muscle duty factors (DFs, %) which were used to calculate Mechanobehavioral Scores (MBS) in TMD diagnostic groups of women.
Methods: Forty-seven women gave informed consent. RDC, and MRI and CBCT imaging determined grouping of subjects according to presence/absence (+/-) of bilateral disc displacement (DD). Subjects (+DD n= 29; -DD n= 18), provided cephalometric and magnetic resonance images, and jaw tracking data. Dynamic stereometry was used to characterize individual-specific data of stress-field dynamic mechanics during 10 symmetrical jaw closing cycles. TMJ disc energy densities were then calculated (ED=W/Q (W=work done or mechanical energy input = tractional force X distance of stress-field translation, Q=volume of cartilage). Subjects were trained to record masticatory muscle activities over 3 days and nights. EMG/bite-force calibrations determined subject-specific masseter and temporalis muscle threshold activities per 20 N bite-force (RMS20N, µV). Masseter and temporalis DFs (DF = duration of muscle activity/total recording time, %) were determined based on muscle-specific thresholds of magnitudes ranging from ≥5% to ≥25% RMS20N). MBS were calculated (ED2 x DF), and ANOVA and Bonferonni/Tukey HSD post hoc analyses tested for intergroup differences in DFs, EDs, and MBS.
Results: +DD subjects had averaged DFs which were 1.9 (Overall), 1.7 (Awake), and 2.5 (Night) times higher compared to –DD subjects (all P< 0.01). Average EDs was 1.4 times larger (P= 0.012) in +DD subjects, but the differences were limited to the contralateral TMJ. MBS were predominantly higher during awake periods, where MBS in +DD subjects peaked at 8.7 times larger compared to –DD subjects (all P<0.04).
Conclusions: Mechanobehavior scores may explain TMJ disc displacement in women. This work was supported by NIH (R01 DE016417).