Background: Thumb ulnar collateral ligament (UCL) injuries are common, but the kinematics of these injuries have not been comprehensively described, especially regarding kinematic changes with progressive UCL injury. Methods: Eleven cadaveric thumbs underwent kinematic testing under 4 conditions: intact, partial tear (50%) of the proper UCL, full tear of the proper UCL, and complete tear of both the proper and accessory UCL. Kinematic testing parameters included varus/valgus stress, pronation/supination, and volar/dorsal translation at −10 degree, 0 degree, 15 degree, and 30 degree of metacarpophalangeal flexion. Results: Partial tear of the proper UCL did not result in significant increases in laxity in any direction compared with intact (P ≥ .132). Full tear of the proper UCL resulted in a significant increase in valgus angulation (18.8° ± 1.7° vs 11.5° ± 1.5°; P = .024) and pronation (15.4° ± 2.5° vs 12.6° ± 2.3°; P = .034) at 30 degree of flexion relative to intact. Complete tear of both the proper and accessory collateral ligaments resulted in increased valgus angulation at all degrees of flexion (P < .001). Complete tear also resulted in a significant volar translation at 0 degree, 15 degree, and 30 degree of flexion (P ≤ .016). Conclusion: Partial tear of the proper UCL does not significantly affect the stability of the joint, but full tear of the proper UCL increases valgus instability at 30 degree of flexion. Complete tear of the UCL is necessary for increased varus/valgus instability at all degrees of flexion and results in significant increases in pronation/supination and volar translation.
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