Zone 3 of the extensor pollicis longus corresponds to the dorsal thumb MCP joint level, where the EPL tendon runs within a portion of the extensor hood and can be involved in the junctura tendinum connections, synovitis, or direct laceration. At this zone the EPL is the primary extensor of the proximal phalanx and its disruption produces inability to extend the thumb IP joint (more critical) and impaired MCP extension.
EPL rupture at zone 3 (MCP dorsum) from laceration or synovitis in rheumatoid arthritis produces inability to extend the thumb IP joint, the most critical functional deficit. EPL rupture at zone 4 (dorsal wrist, from distal radius fracture attrition on Lister's tubercle) is more common and also presents as thumb IP joint extension loss. Repair of zone 3 EPL lacerations is performed primarily if the ends are accessible. When primary repair is not feasible, EDM transfer (extensor digiti minimi transferred to EPL distal stump) restores thumb IP extension.
Sharp laceration at the thumb MCP dorsum divides the EPL in zone 3, producing inability to extend the thumb IP joint while MCP extension is partially preserved by EPB contribution; primary repair under loupe magnification with 4-0 or 3-0 monofilament suture and 6-week thumb spica splinting allows tendon healing.
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