Shapes and Sizes of Common Silicone Metacarpophalangeal Arthroplasties: Clinical Implications.

Background: Silastic metacarpophalangeal joint (MCPJ) arthroplasty is a recognised treatment for painful finger arthritis. There are two commonly used, albeit different, designs; the Swanson and the NeuFlex© . Which design is optimal is unclear. The purpose of this study was to evaluate the radiological differences relative to the bones following implantation. Methods: We examined the radiological features of these implants up to 1 year of follow-up. We reviewed the postoperative radiographs of 42 patients with 113 MCPJ arthroplasties and assessed the implant body anatomical 'fit' relative to the widths of the cut metacarpals and proximal phalanges and resection lengths of the metacarpal heads. We also looked for potential axial implant rotation. Results: The Swanson implants were consistently and statistically significantly wider than the NeuFlex© implants and almost always overhung the margins of the native MCPJ. Four of 33 (12%) of the Swanson and 1 of 80 NeuFlex© implants had rotated axially, the difference was statistically significant. One NeuFlex© implant had fractured at its hinge. Conclusions: The appreciable difference in the positions of the implant bodies relative to the bones may be important. The overhang of the Swanson implants may confer some stability to the arthroplasty helping to resist lateral deviation forces, but concomitant ligament reconstruction may increase the risk of implant rotation which is likely to reduce the postoperative ranges of motion. Axial silastic implant rotation has not previously been reported. It may influence joint biomechanics; future implant designs should consider the risks of implant rotation. Level of Evidence: Level IV (Therapeutic).