Effects on the metatarsophalangeal joint after simulated first tarsometatarsal joint arthrodesis.

A first tarsometatarsal (TMT) arthrodesis is a common procedure; however; the biomechanical effects on the first metatarsophalangeal (MTP) joint are not well understood. Instant centers of rotation range of motion have been used as biomechanical parameters to determine function of the first MTP joint. The effects that a simulated first TMT joint arthrodesis has on the distribution of instant centers of rotation and resistance to dorsiflexion the first MTP joint were investigated. Five lower extremity limbs were mounted onto a custom-loading frame. A 3-dimensional tracking system was placed along the first ray. A tilting platform that simulates propulsion was used to calculate the instant centers of rotation. A hinged platform was used to determine the motion of the first MTP joint at 40 N of force. Both parameters were measured before and after simulated first TMT joint arthrodesis. Instant centers of rotation were mathematically calculated with a modified Reuleaux method. The standard deviation between instant centers of rotation was found to be significantly reduced (P = .05) after the simulated first TMT arthrodesis. There was an average of a 25% (P = .01) increase in dorsiflexion of the MTP joint after a simulated first TMT arthrodesis. The findings of this study suggest that first TMT arthrodesis does not have a negative effect on the first MTP joint. There was no reduction of the intrametatarsal angle and plantar flexion or shortening of the metatarsal. Thus, the change in biomechanics of the first MTP joint can only be attributed to elimination of the first TMT joint motion.