Benefits of Additional Procedures for Metacarpophalangeal Hyperextension on Simple Trapeziectomy in Thumb Basal Osteoarthritis: A Biomechanical Cadaver Study.

Background: Patients with advanced osteoarthritis of the first carpometacarpal joint (CMC-1) may develop hyperextension of the first metacarpophalangeal joint (MCP-1). No clear clinical benefice has been reported consecutively to the surgical treatment of the MCP-hyperextension combined to a trapeziectomy. The reason of the missing benefit may be due to changes in the thumb position impairing the thumb stability secondary to the surgical procedures. We assessed changes in the transmission of forces at the thumb's end phalanx following a trapeziectomy combined with the surgical adjustment of the hyperextension of the MCP-1-joint in a biomechanical investigation using cadavers. Methods: The thumb muscles were loaded with nylon cables connected to a tension meter in 8 forearm cadavers. A 6-axis force sensor assessed the termino-lateral key-pinch orthogonal strength vectors at the level of the thumb distal phalanx prior to any surgery, and following a simple trapeziectomy, a trapeziectomy combined to a MCP-1-capsulodesis and the transfer of the extensor pollicis brevis over the metacarpal-1 head, or to an MCP-1-arthrodesis. Results: Combination of the trapeziectomy with the MCP-1- joint palmar capsulodesis and EPB-transfer or with a MCP-arthrodesis in neutral pronation-supination resulted in a significant shift of the thumb in pronation-abduction with respect to the preoperative assessment. The lowest shift was achieved when performing the arthrodesis in 20° supination or by overloading of the adductor pollicis. Conclusions: Combining the trapeziectomy with surgeries addressing the MCP-1-joint hyperextension induced a shift of the thumb in pronation-abduction that could impair the key-pinch stability. When considering additional procedures for MCP-1-joint hyperextension deformities, it should be recommended to fix the EPB-tendon on the radial aspect of the metacarpal head if a tendon transfer is considered, otherwise the MCP joint arthrodesis should be performed in supinated position, in order to achieve lateral key-pinch stability.