Aims: To examine the jaw-stretch reflex after injection of local anesthetic (LA) into painful temporomandibular joints (TMJs), since the functional role of jaw-stretch reflexes in patients with painful temporomandibular disorders is still not well understood.
Methods: Thirteen female patients with a clinical diagnosis of disc displacement without reduction and TMJ pain participated in this open study. Reflex responses were evoked by fast stretches at 15% of the maximal voluntary contraction level before and after injection of 1 mL carbocaine into the painful TMJ. Electromyographic (EMG) activity was recorded from the left and right masseter and temporalis muscles, and the mean level of prestimulus EMG activity and peak-to-peak amplitude of the stretch reflex were measured. Visual analog scale ratings of TMJ pain and TMJ pressure pain thresholds (PPTs) were also obtained. Eleven healthy women were examined with the same protocol (except for PPT determinations) before and after injection of LA into the TMJ.
Results: In patients, injection of LA reduced the TMJ pain during jaw movements and maximum clenching (P < .021) and increased the PPTs in the painful TMJ (P < .01). The prestimulus EMG activity in the masseter on the painful side (feedback muscle) was unaffected by the injection of LA (P > .262). There were no significant side-to-side asymmetries of latency or amplitude measures of the stretch reflex in the patient group. Both the peak-to-peak amplitude and the normalized peak-to-peak amplitude of the stretch reflex were reduced in the masseter and temporalis muscles on the painful side and in the masseter on the nonpainful side after LA injections (P < .048). There were no effects of LA injections into the TMJ in the healthy group on any EMG or stretch parameters.
Conclusion: These results do not support the notion of asymmetries in the jaw-stretch reflex in patients with TMJ pain, but they do suggest that the reflex sensitivity can be influenced by nociceptive activity from the TMJ area.