Anatomy and histology of the A5 pulley.

Since the anatomy and actual existence of the A5 pulley have been variably reported in the literature, we sought to better define its macroscopic and microscopic structure. Thirty-one A5 pulleys were dissected from 32 fingers. The average proximal to distal length was 3.8 mm; the average width was 8.9 mm. The distal edge of the pulley was proximal to the distal interphalangeal joint, 7.7 mm from the profundus tendon insertion. On histologic study by light microscopy, 3 distinct layers were noted; fibrofatty tissue was noted in the outermost layer, hyaluronic acid-secreting cells were noted in the innermost layer, and connective tissue containing collagen bundles, fibrocytes, and interspersed elastin fibers was noted in the middle layer. The A5 pulley is a discrete structure, with measurements as noted as well as a histologic composition consistent with that reported for the other annular pulleys.