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Extensor expansions of hand
The extensor expansions of the hand are complex, triangular aponeurotic sheaths, with a hood-like appearance, that function as the compound tendinous attachment of the extensor digitorum, lumbrical, palmar interossei, and dorsal interossei muscles. The hood-like base of this triangular aponeurosis covers the metacarpophalangeal joint dorsally and wraps around the lateral aspects of the joint. This broad base tapers to the apex of the triangular aponeurosis to attach to the base of the distal phalanx. The central axis of the aponeurosis is thick and contains the strong, flat tendon of the extensor digitorum muscle. The borders of the triangle are also thick, as the tendons of the lumbricals and interossei enter the aponeurosis from the sides. Between these thickened regions, the aponeurosis is thin and translucent. The central extensor digitorum tendon gives rise to two collateral slips at the level of the proximal interphalangeal joint, which angle to the sides of the expansion to join with the thick margins and then converge again to the midline to attach to the distal phalanx. Each of the fingers has an extensor expansion.