Ulnar collateral ligament strain of the thumb metacarpophalangeal joint: biomechanical comparison of two postoperative immobilization techniques.

BACKGROUND The aim of this study was to compare postoperative immobilization techniques of the thumb metacarpophalangeal (MP) ulnar collateral ligament (UCL) in a cadaver model of a noncompliant patient. METHODS A cadaveric model with fresh-frozen forearms was used to simulate pinch under two immobilization conditions: (1) forearm-based thumb spica splint alone and (2) forearm-based thumb spica splint with supplemental transarticular MP Kirschner wire fixation. Pinch was simulated by thumb valgus loading and flexor pollicis longus (FPL) loading. Ulnar collateral ligament displacements were measured and strain values calculated. Statistical analysis was performed using a repeated measures analysis of variance model. RESULTS With valgus thumb loading, we noted a significantly lower UCL strain in the splint and pin group compared to splint immobilization alone. Increased load was associated with a statistically significant increase in UCL strain within each immobilization condition. FPL loading resulted in negative displacement, or paradoxical shortening, of the UCL in both immobilization groups. CONCLUSIONS While immobilized, valgus thumb force, as opposed to MP flexion, is a likely contributor to UCL strain during simulated pinch representing noncompliance during the postoperative period. Supplemental thumb MP pin fixation more effectively protects the UCL from valgus strain. UCL shortening with FPL loading likely represents paradoxical MP extension due to flexion of the distal phalanx against the distal splint, suggesting attempted thumb flexion with splint immobilization alone does not jeopardize UCL repair. CLINICAL RELEVANCE This study provides a foundation to aid clinical decision-making after UCL repair. It reinforces the practice of surgeons who routinely pin their MP joints, but also brings to attention that the use of temporary MP pin fixation may be considered in difficult cases, such as those with potential noncompliance or tenuous repair.