Sensitivity of the jaw-jerk reflex in patients with myogenous temporomandibular disorder.

Changes in the activity of human jaw-elevator muscles related to the mandibular stretch (jaw-jerk) reflex could be involved in the aetiology of temporomandibular disorders (TMD). In order to investigate whether there are differences in the sensitivity of the jaw-jerk reflex between myogenous TMD patients (n = 10) and gender- and age-matched controls (n = 10), jaw-jerk reflexes were elicited under standardized conditions. By measuring the reflex with bipolar surface electromyography (EMG), reflex sensitivity was determined from relations between reflex amplitude and jaw displacement from the masseter and the anterior temporalis muscles. Reflex amplitude and background EMG activity were normalized with respect to the maximal voluntary contraction (MVC) to correct for differences in the thickness of soft tissues overlying the muscle or in electrode placement. In addition to normalization with respect to MVC, for the patients, normalization was also applied with respect to a MVC that was scaled by multiplying values by the ratio of the mean MVC of controls to the mean MVC of patients. At a constant level of background EMG activity, the reflex sensitivity can be determined from the slope (reflex gain) and x-intercept (reflex threshold) of the reflex amplitude-jaw displacement relation. No significant differences between patients and controls were found for the gain or threshold values of either the masseter or the anterior temporalis muscles with a univariate analysis of variance. It is concluded that jaw-jerk reflex sensitivity is not significantly changed in myogenous TMD patients. Therefore, the fusimotor system probably does not play a part in the perpetuating myogenous TMD.