Effect of chronic and experimental jaw muscle pain on pain- pressure thresholds and stimulus-response curves

Pain-pressure thresholds (PPTs) and stimulus-response (S-R) curves in masseter muscles and index dingers of 11 female patients with chronic jaw-muscle pain were compared with that of 11 matched control subjects. Experimental hyperalgesic and hypoalgesic conditions in the masseter muscles of control subjects were induced by intramuscular injuction of 5% saline and of local anesthetic, respectively. The PPTs were found to be significantly lower in the masseter muscles of apin patients than in those of control subjects. The mean slopes of the S-R curves were significantly lower in the masseter muscles of pain patients than in those of control subjects. The mean slopes of the S-R curves were significantly steeper for the masseter muscles of pain patients (0.481 ± 0.213) than of control subjects (0.274 ± 0.201, P.0256). There were no statistically significant differences in PPTs or S-R curves for the index finger. The PPTs in masseter muscles of control subjects were not significantly affected by injection of 5% saline; however, the slopes of the S-R curves for the masseter muscles were significantly steeper for saline-injection values compared to baseline values (21.7% ± 29.6%, P.037). Injection of local anesthetic into masseter muscles of control subjects increased the PPTs significantly and reduced the slopes of the S-R curves significantly as compared to baseline values (-22.9% ± 34.6%, P.155). The present results suggest that PPTs and S-R curves are valuable tools for quantitative description of chronic and experimental jaw muscle pain.