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Increased Rotation Instability in Ulnar Collateral Ligament Injuries of the Thumb: A Biomechanical Study
Patrick Marinello, MD, Xavier Simcock, MD; Steven Maschke, MD
Cleveland Clinic Foundation, Cleveland, OH

Introduction: In a normal thumb, there is minimal rotation about the thumb metacarpal phalangeal (MCP) joint, but with a complete injury to the medial ligamentous structures a clinically increased rotation about the MCP joint can be detected. The purpose of our study is to substantiate our clinical finding in a cadaveric model. Our hypothesis was that sequentially disrupting the dorsal capsule, the proper ulnar collateral ligament (pUCL) and the accessory ulnar collateral ligament (aUCL) of a thumb will lead to significant progressive rotational instability.
Materials and Methods: Twelve fresh from adult cadaveric upper extremities with intact thumbs and no prior history of thumb injury were used for this study. The mean age was 65 (range 49-76) with 4 females and 8 males specimens. The specimens were thawed to room temperature and dissected to expose the dorsal capsule, the pUCL and the aUCL. With fluoroscopic assistance two 0.45 Kirshner wires were placed through the distal metacarpal and proximal phalanx approximately 2 cm from the MCP joint, both parallel to the joint line and parallel to the horizon. The dorsal capsule, pUCL and aUCL were sequentially sectioned and the MCP joint was ranged manually in pronation and supination in the natural resting position of the hand. Digital photographs captured the extremes of motion (pronation and supination) at baseline and after each structure were sectioned. A digital goniometer was used to measure the relative pronation and supination using the k-wires to mark rotation. A student’s paired t-test was used for statistical analysis with a threshold of p<0.05.
Results: Mean baseline pronation and supination were 16°(SDą4.3°) and 12.8° (SDą4.2°), respectively. After sectioning the dorsal capsule and pUCL the mean pronation and supination were 23.4°(SDą8.8°) and 21.6° (SDą3.4°), respectively and increased to 28.2°(SDą9.2°) and 29.7° (SDą2.6°) in pronation and supination, respectively, after sectioning the aUCL. These were both significant changes from baseline (p<0.05). With the pUCL sectioned, there was a mean increase in percent pronation and supination of 29.2% (SDą14.8%) and 39.2% (SDą22.4%), respectively compared to baseline. With the aUCL also sectioned, the increase in percent pronation and supination from baseline was 41.6% (SDą12.8%) and 57% (SDą12.9%), respectively.
Conclusions: This biomechanical study demonstrates a significant increase in the rotational instability (pronation and supination) of the metacarpal phalangeal joint after UCL injury. Clinically, this increase in rotational instability can help more accurately diagnose gamekeeper’s thumb and supplement traditional valgus stress testing of this injury.

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