The development of strains, forces and nociceptor activity in retrodiscal tissues of the temporomandibular joint of male and female goats.

Chronic pain in the temporomandibular (TM) joint is predominantly manifested in women. We examined biomechanical and neural factors that could contribute to this differential representation. Relationships between jaw rotation, soft tissue strains and soft tissue forces were examined in the goat TM joint. Strains were minimal until the jaw was rotated beyond the normal range of motion (7.25 deg). There were no significant differences in rotation-strain patterns in males and females. Stress developed as strains were introduced by jaw rotation. Gender differences were observed. Males manifested higher failure loads (15.94 +/- 1.98 and 11.37 +/- 2.02 N, for males and females respectively) and higher elastic stiffness than females (5.62 +/- 1.19 N/mm and 1.64 +/- 0.31 N/mm, for males and females respectively). Recordings were made from cell bodies in the trigeminal ganglion whose distal processes innervated the retrodiscal tissue of the temporomandibular joint of the goat (n = 48). Nociceptor reactivity was characterized with respect to the capacity to transduce mandibular rotation (rotation-interval functions; n = 29). On the basis of established relationships between rotation, strain and tissue forces, rotation-interval functions were transformed into strain-interval and force-interval functions. Comparisons were made between nociceptor properties grouped by gender. No differences in properties were observed when nociceptors were characterized with respect to jaw rotation; however, gender differences were obtained when nociceptor reactivity was characterized with respect to retrodiscal strains or forces. Consistent with smaller failure loads, nociceptors of retrodiscal tissues of females manifested a smaller range (1.12 vs 4.33 N), force to average (1.51 vs 4.64 N), force to minimum (0.95 vs 2.48 N) and force to asymptotic discharge (2.07 vs 6.81 N). Consistent with lower elastic stiffness, nociceptors of female tissues manifested higher average strain (54.4% vs 41.9%) and peak strain (74.0% vs 58.1%) to asymptotic discharge relative to those sampled from male tissues. The implications of these findings for TM joint injury and chronic pain are considered.