Design and implementation of an instrumented ulnar head prosthesis to measure loads in vitro

The development of a novel instrumented implant for ulnar head replacement is presented in this study. This implant was instrumented with strain gauges to quantify bending moments about the anatomic axes of the distal ulna, and subsequently the distal radioulnar joint (DRUJ) reaction force magnitude. The implant was surgically inserted in seven cadaveric upper extremities, which were subsequently mounted in a custom joint simulator. Simulated active unresisted pronation and supination motion trials were conducted using computer-controlled pneumatic actuators to simulate forearm musculature. Passive (unloaded) trials were also conducted. The reaction force across the DRUJ ranged from 2 to 10 N in magnitude during this unresisted motion. Increased bending moment magnitudes were measured when the forearm was positioned in supination compared to pronation. The magnitude of joint bending moments showed a consistent pattern with forearm position, regardless of simulated active or passive rotation, or supination and pronation motion trials. This result illustrates that the primary influence on joint load is likely the position and contact with the radial articulation. This study of DRUJ loading should be useful for biomechanical modeling, implant design considerations and improved knowledge of articular mechanics.