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Medical Definition of Extensor expansion
1. A triangular tendinous aponeurosis including the tendon of the extensor digitorum centrally, interosseus tendons on each side, and a lumbrical tendon laterally. It covers the dorsal aspect of the metacarpophalangeal joint and the proximal phalanx. Synonym: dorsal hood, extensor aponeurosis, extensor expansion. (05 Mar 2000)