The diagnosis of complete tears of the ulnar collateral ligament of the thumb metacarpophalangeal (MP) joint depends on demonstration of excessive laxity of the ligament, but there is controversy on whether laxity greater than a certain cut-off value or laxity greater than the opposite thumb is the criterion for diagnosis. We examined 200 thumbs of 100 normal individuals in extension and in 30deg of flexion. In 34% of subjects there was a difference of 10deg or more between right and left thumbs in extension, and 12% had a difference of at least 15deg. In flexion, 22% of thumbs differed by 10deg or more and 3% by 15deg or more. Comparison with the uninjured contralateral thumb is unreliable in many individuals. We recommend the lack of a definite end point on stress examination as indicating complete rupture of the ulnar collateral ligament.
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