The thumb A2 pulley is the distal annular pulley of the thumb flexor sheath, located over the proximal phalanx of the thumb. It is the primary constraint for the FPL tendon at the MCP level.
Maintains FPL tendon close to the thumb proximal phalanx, prevents bowstringing during thumb flexion, and provides the annular constraint that becomes the site of trigger thumb in flexor tenosynovitis.
Trigger thumb (stenosing tenosynovitis of the FPL) occurs at the A1 pulley of the thumb, not A2 — the A2 is involved in rheumatoid pannus-induced pulley destruction producing FPL bowstringing. In FPL tendon repair, the A2 pulley may need partial resection for tendon retrieval, then reconstruction.
Rheumatoid pannus eroding the thumb A2 pulley allowing FPL bowstringing and progressive thumb flexion deformity, requiring pulley reconstruction.
The thumb A2 pulley may require partial incision for FPL retrieval during zone II laceration repair, reconstructed using a pulley remnant suture or tendon graft.
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