Cartilage energy density, muscle duty factor, and TMJ disc displacement

Objectives: Cartilage fatigue is thought to involve 1) work energy density (ED) and 2) frequency of muscle activity (ie: duty factor, DF). We tested in women whether or not there were diagnostic group differences in ED, DF, and ED•DF.
Methods: Subjects gave informed consent. Research Diagnostic Criteria and CBCT/MR images were used by calibrated examiners to classify 36 women based on presence (+) or absence (-) of disc displacement (DD) and pain (P) [-DD-P, n=16; +DD/-P, n=8; +DD/+pain, n=12). Subjects provided anatomic and kinematic data of intra-capsular contact mechanics. Subjects performed EMG/bite-force (BF) calibrations and were instructed how to use portable EMG recorders to collect awake- and sleep-state masseter and temporalis muscle activities. Dynamic stereometry was used to estimate average ED per closing cycle of the mandible, where: ED=mechanical energy input per cartilage volume. Masseter and temporalis DFs (DF=duration of muscle activity/total recording time, %) were determined based on EMG/BF calibrations. ANOVA identified significant diagnostic group differences in ED, DF, and ED•DF. Cohens d was used to determine effect size.
Results: Significant diagnostic group differences in mean EDs occurred between +DD+P (13.4 mJ/mm³) and –DD-P subjects (6.3 mJ/mm³, P=0.0073). Night masseter muscle DFs were largest in –P+DD (5.1%) and smallest in –P-DD (0.3%) subjects, but were not significantly different. Mean ED•DF for sleep-state masseter in +DD-P (79.8 mJ/mm³•%) was 40x larger than in –P-DD (1.7 mJ/mm³•%) subjects. Although Cohens d (2.4) indicated a large diagnostic group effect, approximately 110 subjects are required for β>0.80.
Conclusions: A distinguishing feature of +P+DD subjects was significantly higher EDs imposed on the TMJ disc. In –P+DD women, a large diagnostic group effect was produced by the combination of ED and DF. Reducing the possibility of Type II error requires the number of subjects must to be increased.