Orientations of wrist axes of rotation influence torque required to hold the hand against gravity: a simulation study of the nonimpaired and surgically salvaged wrist.

The wrist is a complex kinematic link connecting the forearm and hand. The kinematic design of the wrist is permanently altered during surgical salvage procedures, such as proximal row carpectomy (PRC) and scaphoid-excision four-corner fusion (SE4CF), which have the unintended consequence of long-term functional impairments to both the wrist and hand. We developed simulations of the nonimpaired, PRC, and SE4CF wrists to evaluate if surgically altered wrist kinematics contribute to functional impairments. Specifically, as a step toward understanding the connection between kinematics and function, we examined the torque necessary to statically maintain functional postures. All simulations included only bone geometry and joint kinematics; soft tissues were excluded. Our simulations demonstrate that the torque necessary to maintain a functional posture is influenced by the orientations of the flexion and deviation axes of rotation relative to each other and the anatomical planes of the radius. The magnitude of torque required to hold the hand against gravity decreased in simulations of the PRC wrist compared to the nonimpaired wrist. In contrast, the torque required increased relative to the nonimpaired wrist in simulations of the SE4CF wrist. These divergent results are directly related to how motion is coupled between the flexion-extension and deviation axes of rotation. This study highlights that, even without considering the effects of soft tissues, changing the kinematic design of the wrist influences function; therefore, kinematics should be considered when surgically redesigning the wrist.