The hand has two constant sesamoid bones at the thumb MCP joint — one in the flexor pollicis brevis (radial sesamoid) and one in the adductor pollicis (ulnar sesamoid) — and variable sesamoids at other MCP joints and the thumb IP joint. The thumb MCP sesamoids distribute the enormous pinch forces at the thumb MCP level and protect the FPL tendon beneath them. The radial sesamoid (in the FPB tendon) is the more commonly fractured of the two.
Thumb MCP sesamoid fractures from hyperextension injuries produce palmar MCP joint pain with point tenderness over the affected sesamoid. They are distinguished from the normal sesamoids on radiograph by their acuity of margin and surrounding oedema on MRI. Management is conservative with splinting and modification of thumb pinch activities.
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