Development of an Actuator Driven Cadaveric Wrist Testing System For Carpal Kinematic Studies

Cadaver based joint simulators have been a backbone of orthopedic biomechanical experiments to simulate in vivo loading conditions and injury models in order to better understand intact joint kinematics, or develop new repair techniques. The purpose of this study was to develop a cadaver based wrist testing system capable of simulating grip in order to understand carpal kinematics under conditions that mimic the loading conditions seen in vivo. The testing system is based on linear actuators with in series load cells controlled by custom coded Labview software that are connected via sutures to the muscle tendons of wrist flexors/extensors and finger flexors, allowing fully customizable loading protocols. Carpal kinematics are measured using custom posts inserted into the carpus and digitized with a Microscribe 3DLX. Post-experimental dissection and digitization were used to create 3D models of the carpus at each testing position.The system was validated by comparing the kinematics of the intact carpus in wrist flexion/extension and radial/ulnar deviation to data previously reported in the literature. Data from the new system was comparable to previous reports. Next the effect of grip on intact carpal kinematics was investigated by loading flexor digitorum profundus. Grip caused significant flexion of the scaphoid and lunate with an average flexion of 0.8° for the scaphoid and 0.4° for the lunate. The last phase was to analyze the effect of repetitive grip on a wrist with a scapholunate interosseous ligament tear, an injury known to cause progressive degenerative changes following the initial tear. Following 500 cycles of grip, there were significant changes to the carpus including: increased scaphoid flexion, ulnar deviation, and pronation, lunate extension, widening of the scapholunate gap, and proximal translation of the head of the capitate. Utilizing these kinematic changes seen in cadavers, future studies can be performed to develop new repair strategies for this challenging injury. In conclusion, a validated cadaveric wrist testing system has been developed that has elucidated the effects of grip on the intact wrist, and provided significant information on the effect of functional stresses, such as repetitive grip, have on a wrist with a scapholunate interosseous ligament tear