The four extensor digitorum tendons pass through the fourth extensor compartment at the wrist under the extensor retinaculum and fan out across the dorsal hand to the extensor expansions. Juncturae tendinum (tendinous connections between adjacent tendons at the level of the metacarpals) limit independent finger extension, particularly of the ring finger, and prevent complete extension tendon rupture from retracting proximally. The EDC tendons merge with the interosseous and lumbrical contributions at the extensor expansion.
Finger MCP extension and assists in IP joint extension through the extensor expansion
Sagittal band rupture at the MCP joint allows the EDC tendon to sublux ulnarward off the metacarpal head during finger flexion, producing a snapping knuckle and extension lag from the flexed position. Tenosynovitis of the EDC at the wrist under the extensor retinaculum produces dorsal wrist pain and crepitus. Extensor lag at the MCP joint from EDC injury or sagittal band rupture is managed with extension splinting for acute injuries.
EDC tendon subluxation from the metacarpal head producing snapping and MCP extension lag managed with extension splinting acutely and surgical repair for chronic instability.
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