Assessing changes in wrist biomechanics during ageing and optimal design of total wrist arthroplasty and assistive rehabilitation devices

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 or eliminates motion at the joint. Therefore, motion preserving procedures, e.g. total wrist arthroplasty (TWA), are increasingly being attempted to maintain patients' joint function. The Motec wrist is a ball and socket style TWA prosthesis which replaces all anatomical articulations at the wrist. While follow-up studies of Motec patients have been positive, detailed assessments of post-operative range of motion (ROM) and efficiency are yet to be performed. Particularly those investigating circumduction and the functionally important Dart Thrower's Motion (DTM). Hence, this project has the following objectives. First to compare functional ROM and efficiency of motion in pre- and post- TWA wrists. Second to investigate optimal placement of the Motec centre of rotation (COR) within the wrist joint space. Third to compare pre- and post- operative moment arms of muscles crossing the joint.

Due to invasive surgical and experimental techniques, a cadaveric approach was taken. 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, and tendon forces were recorded as a measure of motion efficiency. In an evaluation of the simulator, biplanar X-ray videoradiography (BPVR) and X-ray reconstruction of moving morphology (XROMM) were used to measure the overall wrist motions of six cadaveric wrists before and after a Motec surgery. Flexion/extension, radioulnar deviation, DTM and circumduction were successfully simulated in both conditions. Additionally, simulated wrist angles and tendon forces were repeatable within single specimens but not between different specimens. The evaluation suggested that simulated wrist motions could be compared between surgical conditions to determine potential mechanical effects of the surgery.

Two sets of experiments were performed using the cadaveric simulator, during which wrist angles and tendon forces were recorded throughout seven motion patterns. The first set studied pre- and post-TWA wrists (n = 14). The second set studied the Motec wrist with six different COR positions as well as a pre-TWA condition (n = 6). Both sets of experiments measured tendon forces as an indicator of wrist motion efficiency, captured simulated motions using BPVR and measured wrist angles using XROMM animations. In the final stage of the project, biomechanical wrist joint models will be created to estimate muscle moment arms in anatomical and Motec wrists. Although this work is yet to be completed.

Pre/post-TWA comparison revealed no significant differences in ranges of flexion, extension and radioulnar deviation, or in the area of circumduction. Importantly, the Motec wrist was able to produce DTM. Peak flexor tendon forces were significantly lower in Motec wrists during flexion and did not otherwise differ from the pre-TWA condition. Peak extensor tendon forces did not significantly differ apart from a post-TWA decrease in extensor carpi ulnaris force during extension. Data collected during the second set of experiments is yet to be fully processed. We are aiming to determine whether there is a Motec COR position in which the most optimal ROM and efficiency of motion are achieved. Position of the Motec COR will also be compared with the best-fit location of the anatomical wrist COR.

Results thus far suggest that Motec TWAs have the potential to recreate functional wrist motion with similar efficiency to the pre-surgical state. Further studies are required to determine whether these conclusions are supported in vivo.