The Effect of Metacarpal Stem Configuration on Load Transfer to the Long-fingermetacarpal: an In-vitro Biomechanical Study

Introduction: Failures in total wrist arthroplasty (TWA) can be most often attributed to three main factors: improper prosthesis centering, component loosening and soft tissue imbalance (1). Of these, one of the most often reported complications is metacarpal loosening with dorsal protrusion of the stem (2). Abnormal load transfer through the prosthesis is believed to contribute to component loosening (3). Although a variety of distal stem configurations exist, few studies report quantitative data to justify an optimal stem design (4-7). The distal component of many wrist arthroplasties consists of a single stem for insertion into the long-finger metacarpal. In cases where revision is required or poor bone stock is present, the option for a double-pronged stem is available for insertion into the long and index metacarpals (8). The purpose of this study was to compare the force transmission to the long-finger metacarpal in response to anterior-posterior (AP) bending applied to a single versus a double-pronged metacarpal stem. The added effect of stem fixation through the capitate was also investigated. Methods: A custom designed metacarpal implant was instrumented with strain gauges for measuring load transfer to the long-finger metacarpal (Figure 1). The implant could be connected or disconnected via a clamp to simulate the double or single-pronged configurations respectively. The strain gauge wires were secured with a metal cap and exited the specimen distally.