The adductor pollicis has two heads (oblique from the carpal bones and metacarpal bases, and transverse from the third metacarpal shaft) that converge on a shared tendon inserting on the ulnar base of the thumb proximal phalanx via the ulnar sesamoid. The tendon also sends a slip into the extensor mechanism of the thumb, contributing to the medial side of the extensor hood.
The adductor pollicis tendon is the primary deforming force in Stener lesion, where the ulnar collateral ligament of the thumb MCP ruptures and the adductor aponeurosis interpositions between the ligament and its insertion, blocking spontaneous healing and requiring surgical repair. Assessment of adductor pollicis integrity uses the Froment sign (flexion of the thumb IP joint to substitute for weak adduction) in ulnar nerve palsy, where the adductor is denervated. The tendon is transferred in opponensplasty procedures for severe thenar weakness.
Complete rupture of the thumb UCL with interposition of the adductor aponeurosis over the torn ligament end produces a palpable mass at the thumb MCP medial side and unstable key pinch, requiring surgical repair as spontaneous healing is blocked by the adductor aponeurosis.
Denervation of the adductor pollicis in ulnar nerve palsy causes the patient to flex the thumb IP joint via FPL when attempting key pinch, substituting FPL for the paralysed adductor, a positive Froment sign indicating loss of the adductor pollicis tendon function.
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