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Efficacy of a Low-Profile Functional Splint for Protecting the Thumb Metacarpophalangeal Joint Ulnar Collateral Ligament
Joseph A Gil, MD; Kerry Ebert, MEd, OTR/L, CHT2; Keri Blanchard, OTR/L, CHT; Avi D Goodman, MD; Joseph Crisco, PhD; Julia A. Katarincic, MD
Brown University, Providence, RI

Introduction
Thumb metacarpophalangeal (MCP) joint ulnar collateral ligament (UCL) injury results from a sudden thumb hyperextension or hyperabduction force. A repaired UCL has been demonstrated to be significantly weaker compared to the intact UCL, and further, the repaired UCL must be protected for approximately 10 weeks, until adequate ligament healing has occurred. However, Division I collegiate football players have been allowed to return to sport as early as 4 weeks. The purpose of our study was to determine the efficacy of a low-profile functional splint in resisting abduction across the UCL of the thumb and protecting it from reinjury.

Methods
Ten fresh frozen cadaver thumbs were tested by implanting a threaded Kirschner wire (K-wire) into the proximal phalanx of the thumb, leaving the distal aspect protruding from the first phalanx. A second K-wire was implanted into the thumb metacarpal for a reference. The distal radius and ulna were mounted to a board in a standardized fashion. An anteroposterior (AP) radiograph of the thumb was obtained with a 2 N preload valgus force applied to the thumb, and the angle between the K-wires was measured as a baseline. Subsequently, 20 N, 40 N, 60 N, 80N and 100N valgus forces were applied to the distal K-wire through an adjustable pulley that provided a stable MCP joint position. AP radiographs of the thumb were then obtained after each force was applied. The angle of displacement between the wires was measured and compared to baseline. A custom low-profile functional splint, immobilizing the thumb MCP but leaving the carpometacarpal and interphalangeal joints free, was fashioned for each cadaveric thumb, and the above protocol repeated. A repeated measures 2-way ANOVA with a post-hoc Sidak's multiple comparisons test was performed to assess mean differences in the abduction angle between the control and splinted specimens at each applied load.

Results
The tested low-profile functional splint significantly reduced mean abduction angles at each applied load (P< 0.05). There was 41% reduction in abduction angle at 20 N, 35% reduction at 40 N, 31% reduction at 60N, 30% reduction at 80 N, and 25% reduction at 100N.

Conclusion
The low-profile functional splint significantly reduces abduction of the thumb MCP UCL up to 100N. We have previously demonstrated that the maximum load to failure of a repaired UCL is 84N; therefore, this splint effectively protects the UCL up to at least 100N after UCL repair.


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