A cadaveric investigation of the Motec total wrist arthroplasty.

The wrist is a highly complex anatomical joint comprising relative motion of the radius, carpals and metacarpals. Wrist arthritis and injury can be treated surgically with arthrodesis (fusion), which relieves pain but restricts motion. Therefore, motion preserving procedures such as total wrist arthroplasty (TWA) are in development. The Motec TWA is a ball-and-socket style prosthesis that replaces all anatomic articulations at the wrist. While follow-up studies of Motec patients report improved range of motion (ROM), quantitative descriptions of the functional Dart Thrower's Motion (DTM) and circumduction are yet to be published. Additionally the change in wrist centre of rotation (COR) induced by Motec TWA and its relationship to functional joint biomechanics has not yet been studied. These avenues will be explored in this thesis following the development and evaluation of methods to compare wrist biomechanics before and after surgery. A cadaveric approach was taken to facilitate detailed comparison between surgical conditions using invasive X-Ray Reconstruction of Moving Morphology (XROMM) techniques. An active wrist motion simulator was designed to move cadaveric wrists by actuating five agonist tendons. Control between different surgical conditions was achieved by applying identical tendon displacements in anatomic and Motec conditions. Tendon forces were recorded by the simulator while bone motion was recorded using biplanar videoradiography and later reconstructed using XROMM. An evaluation of the methods was performed using six wrists. Flexion/extension (FE), radioulnar deviation (RUD), DTM and circumduction were successfully simulated in both conditions. Simulated wrist angles and tendon forces were repeatable within individual specimen trials, but not across the trials of different specimens. It was concluded that simulated wrist motions could be compared between surgical conditions to determine potential mechanical effects of a surgery. Two sets of experiments using the cadaveric simulator are reported in this thesis. The first investigated ROM, tendon forces and wrist COR in pre- and post-TWA wrists (n = 14). The second investigated ROM and tendon forces in six different Motec COR positions (n = 5). Pre- and post-TWA comparisons revealed no significant increase in peak tendon forces and no significant differences in circumduction area or in-plane FE and RUD range. However, altered wrist joint biomechanics were implied by the differences in FE/RUD coupling behaviour during DTM and circumduction as well as significantly decreased flexor tendon forces. Indeed, Motec COR was located proximally, radially and dorsally to that of the anatomic wrist. Radioulnar and volar/dorsal COR shift were linearly related to post-operative changes in the ROM and tendon forces of RUD and extension respectively. Proximal COR shift was linearly related to decreased tendon forces during extension and DTM. In the comparison of different Motec COR positions, limitations pertaining to sample-size, inter-specimen repeatability and impingement precluded strong conclusions. However, the most distal COR positions appeared to be associated with increased tendon forces and decreased ROM in extension and flexion. In conclusion this thesis provides supporting evidence that the wrist can retain functional ROM following Motec TWA. Furthermore, while Motec COR position does affect wrist ROM and tendon forces, strict anatomic COR reproduction may not be necessary to achieve a functional ROM without exceeding anatomic tendon forces.