[The Arthur Vick Award: kinematics of the metacarpophalangeal joint after surface replacement arthroplasty].

Aim: Prosthetic replacement in the hand must address such unique challenges as preservation of the collateral ligaments, tendon balancing,and Stability. Surface replacement arthroplasty can be an alternative to other current implants. The purpose of this study was to evaluate the metacarpophalangeal joint kinematics after surface replacement arthroplasty.
Method: The kinematics of pyrolytic carbon as a surface replacement implant for the metacarpophalangeal joint (MCP) was compared with the intact MCP joint in eight fresh cadaver long fingers by means of an electromagnetic tracking system (Polhemus, Colchester, VT). The eight human cadaver MCP joints were tested before implantation, after implantation, after collateral ligaments resection, and after collateral ligaments reconstruction.
Results: The kinematics of the MCP joint is reproduced by the joint surface replacement arthroplasty when normal ligament tension was present. The maximum angular displacement of the pyrocarbon implant was 378 for lateral deviation and 338 for rotation during the passive flexion and extension motion. The instantaneus center of rotation (ICR) after implant insertion was nearly identical to the center of rotation of the normal joint. The results also indicated that the collateral ligaments provide the primary stability of the MCP joint. No significant differences in lateral and rotational stability after surface replacement arthroplasty were noted. While collateral ligaments resection significantly affected the stability of the MCP joint.
Conclusion: The ICR of the pyrocarbon implant most closely matched that of the intact MCP joint. The pyrocarbon implant provides suitable stability to radio-ulnar deviation and rotational stresses as a resurfacing implant and it simulates the kinematics of the intact MCP joint. By using new materials and taking the anatomical and biomechanical requirements into consideration, the endoprosthesis of the finger joints has created an option to achieve good long-term results. The inadequate results of earlier and current prostheses are a consequence of their mechanical construction and their materials. The success of the new implants could be proven by preferably long-term, controlled studies.