Masticatory Muscle Duty Factors in TMD Diagnostic Groups

Method: MRI and Research Diagnostic Criteria were used to categorize 71 informed and consenting subjects according to TMJ disc position (+DD = disc displacement) and pain status (+P = presence of pain). Male (n= 28) and female (n=42) subjects (+DD/+Pain, n=18; +DD/-Pain, n =14; -DD/-Pain, n=38) participated in two laboratory EMG/bite-force calibrations to determine subject-specific masseter and temporalis muscle threshold activities per 20 N bite-force (RMS_20N, µV). Over 3 days subjects recorded day and night masseter and temporalis muscle activities using portable EMG recorders. Masseter and temporalis Duty Factors (DF = duration of muscle activity/total recording time, %) were determined based on muscle-specific thresholds of magnitude (5% to 80% RMS_20N). ANOVA and Tukey-Kramer post-hoc tests were used to determine independent variable effects on DF.

Result: Subjects produced 333 ambulatory EMG recordings, with average durations of 6.9 and 7.6 hrs. for awake and night recordings respectively. Most intra-group and inter-group muscle DF differences occurred at 5%RMS_20N thresholds. Significant inter-group differences were seen for masseter muscle DFs during awake-state recording where +DD groups had mean DFs of approximately 6.1%. This equated to 23.5 minutes of an average awake-state recording, and was 1.7x greater compared to the –DD/-P subjects (adjusted P<0.01). Masseter night DFs were significantly lower in the +DD/+Pain and –DD/-Pain groups (adjusted P<0.001), but remained elevated (5.5%) in the +DD/-P group (adjusted P<0.001). No inter-group differences were seen in awake- or night-state Temporalis DFs, but intra-group differences were seen where night-state Temporalis DFs in the +DD groups were on average 5x lower compared to Temporalis DFs in awake-state recordings (all P≤ 0.05).

Conclusion: Increased DFs of low level masseter muscle activity appears to be characteristic of +DD subjects.