The Effect of Locking Screws on Distal Component Fixation in Total Wrist Arthroplasty Using a Cadaver Model

PURPOSE: The advent of total wrist arthroplasty has allowed for motion-sparing surgical treatment for wrist arthritis. The Integra Freedom Total Wrist Arthroplasty recently incorporated locking caps into its distal component fixation to minimize implant micromotion and improve osseous integration. The purpose of this study was to assess the kinematic effect of locking caps in a cadaveric model. METHODS: The Integra Freedom was implanted in 4 matched-pair cadavers and tested with and without the use of the locking caps, with the testing order randomized. Each specimen was tested on a custom testing system in a position of 15° of radial deviation, neutral position, and 15° of ulnar deviation with 25 N, 50 N, 75 N, and 100 N of compressive force. The rotation of the capitate, trapezoid, and hamate at all positions was measured using a 3-dimensional digitizer. RESULTS: Statistical analysis showed no difference in carpal rotation between the nonlocking cap and locking cap groups at all testing loads and wrist positions. The absolute motion of the distal row was minimal. However, of the total 216 loads/positions tested, only 4 (1.8%) showed a rotation of greater than 2° and only 34 (15.7%) showed a rotation of greater than 1°. CONCLUSIONS: This study shows that in a time zero cadaveric model, the initial osseous fixation of the distal component in the Integra Freedom is robust with or without locking caps. The addition of locking caps did not have a kinematic effect on distal carpal row fixation. However, further investigation into its clinical role is necessary. CLINICAL RELEVANCE: At time zero, there is minimal carpal motion after implantation of the Integra Freedom Total Wrist with functional loading. The addition of locking caps did not lead to any decrease in carpal motion.