Total wrist arthroplasty (TWA) is a surgical solution that provides pain relief and preserves some wrist motion for patients with severe wrist pathology. However, TWA designs suffer from high complication rates, and to date, the in vivo biomechanics have not been assessed. Therefore, this study aimed to evaluate the biomechanics of a TWA design and compare it to the biomechanics of healthy wrists with the hope of improving future designs. To do so, we first developed a mathematical model of healthy carpal bone kinematics as a function of wrist flexion-extension and radial-ulnar deviation. Then, the accuracy of an advanced imaging technique, biplane videoradiography (BVR), for studying the healthy wrist and replaced wrist motion was evaluated. Finally, we compared four primary outcome measures (range-of-motion (ROM), the center of rotation (COR), the alignment of the components, and the articular contact pattern) between TWA and healthy subjects. We found two distinct motion patterns for the proximal and distal rows. We verified the submillimeter and subdegree accuracy of the BVR system. We demonstrated a 65% smaller envelope of the ROM for TWA compared to the wrist. As expected, the COR of the TWA was located at the center of the curvature of its articulating surface; however, it was observed that the COR of the TWA shifts about two times more than a healthy wrist during functional tasks. Furthermore, a potential association between increased wrist ROM and increased volar tilt and offset of the radial component was observed, which demonstrated that the alignment parameters might influence the functional outcomes for patients. Lastly, it was observed that the center of contact is located on the radial side of the convex surface of the carpal component and the ulnar side of the concave surface of the radial component resulting in the non-uniform articulation pattern of the TWA components. The mismatch could potentially lead to abnormal stresses at the implant-bone interface, a possible mechanism causing the high complication rate of current TWA designs. These findings can be used to improve future designs, determine protocols for wear and stress testing of implants in the laboratory, and validate computational models.
