The extensor hood (dorsal expansion, extensor apparatus) is the complex dorsal fibrous structure of each finger that integrates the extrinsic extensor digitorum communis with the intrinsic contributions from the lumbricals and interossei. The EDC central slip inserts on the dorsal middle phalanx base at the PIP joint; the lateral bands from intrinsic and extrinsic contributions run lateral to the PIP joint and converge beyond it to insert via the conjoint lateral band on the dorsal distal phalanx base. The sagittal bands at the MCP level prevent bowstringing of the EDC and centralise it over the metacarpal head.
Sagittal band injury at the MCP joint produces extensor tendon subluxation to the ulnar side of the metacarpal head, causing a painful palpable snap during MCP flexion (the boxer's knuckle). Boutonniere deformity results from central slip disruption at the PIP joint, allowing the lateral bands to migrate palmarly (becoming PIP flexors rather than extensors) while the distal bands overpull the DIP into hyperextension. Swan neck deformity (opposite pattern) results from volar plate laxity at the PIP joint, allowing dorsal lateral band migration.
Acute flexion force at the MCP joint or chronic attrition from rheumatoid arthritis ruptures the radial sagittal band, allowing the EDC tendon to sublux into the ulnar intermetacarpal valley producing a painful palpable snap during MCP flexion; primary sagittal band repair for acute injuries and sagittal band reconstruction for chronic subluxation restores tendon centralisation.
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