Two sesamoid bones (radial and ulnar) are consistently present at the palmar aspect of the thumb MCP joint, embedded within the volar plate and the flexor pollicis brevis tendons. The radial sesamoid lies within the flexor pollicis brevis (abductor pollicis brevis) insertion and the ulnar sesamoid within the adductor pollicis insertion. The flexor pollicis longus tendon passes between them in the inter-sesamoid groove.
Thumb MCP sesamoid injuries occur in hyperextension (turf toe equivalent) and direct blows. The ulnar sesamoid is more commonly fractured. Sesamoid fractures are distinguished from bipartite sesamoids (a normal variant) by MRI showing acute marrow oedema. In gamekeeper's (skier's) thumb UCL rupture, the ulnar sesamoid can avulse with the volar plate. Sesamoid resection (sesamoidectomy) for chronic sesamoiditis requires meticulous repair of the corresponding FPB or adductor tendon to prevent MCP joint instability and thumb deformity.
Acute UCL rupture of the thumb MCP may avulse the ulnar sesamoid along with the volar plate insertion, visible on radiograph as a small avulsion fragment at the MCP palmar ulnar corner; the Stener lesion (UCL interposition) requires surgical repair and sesamoid fragment fixation or excision with FPB reconstruction.
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