The EDC tendons share a muscle belly for the middle, ring, and little fingers, with interconnections (juncturae tendinum) that prevent independent ring finger extension when other fingers are flexed. The EDC is centralised over the MCP joint by the sagittal bands — sagittal band rupture (boxer's knuckle) allows the EDC to sublux into the valley between metacarpal heads during gripping.
MCP, PIP, and DIP extension of fingers 2-5; the sagittal bands maintain the tendon centralised over the MCP joint
Sagittal band rupture (boxer's knuckle) from direct MCP joint impact causes EDC subluxation to the ulnar side during MCP flexion, producing painful snapping. Acute injuries (less than 6 weeks) are splinted with the MCP in extension for 6 weeks; chronic subluxation requires surgical sagittal band repair or reconstruction.
EDC tendon subluxation from sagittal band tear producing MCP snapping during flexion managed with extension splinting or surgical repair.
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