31 - Variations in Abductor Pollicis Longus Tendons and Sites of Distal Attachment

Poster Board Number: 31
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Odd poster #s – first hour
Even poster #s – second hour

Abstract Body : Introduction: Extrinsic muscles of the pollex can develop accessory tendons from uncharacteristic muscle cleavage and tissue attachments during development. The abductor pollicis longus (APL) is the most common muscle known to exhibit such variations. The typical description of APL depicts a single distal tendon that arises from its muscle belly, travels through the 1st extensor compartment, and inserts on the base of the 1st metacarpal. However, multiple tendons and varying insertion sites have been reported to regularly occur for APL. While these non-typical morphologies are usually asymptomatic, clinical sequelae may result and affect pollex function. The objective of this study was to investigate the prevalence of accessory APL tendons, their sites of distal attachment, and relevant clinical implications.

Methods: One hundred and ten APL muscles were dissected and examined on ethically acquired, anatomically embalmed human cadaveric donors. The APL muscle, tendons, and site(s) of distal attachment were cleaned of extraneous tissue to clearly visualize the APL’s course and insertion site(s). The number of tendons present and their site(s) of distal attachment were recorded. Select examples of various specimens were photographed in situ for representative demonstrations, and color replacement was utilized to aid with structure identification.

Results: The typical description of APL was present in only 8 (7.2%) of specimens. Seventy-seven (70.0%) APL muscles presented with two tendons, 24 (21.8%) with three tendons, and 1 (0.9%) with four tendons. Of the 102 APLs with multiple tendons, at least one tendon inserted at the base of the 1st metacarpal. Of the 77 APLs with two total tendons, the insertion of the accessory tendon was variable: 11 (10.0%) to the base of the 1st metacarpal, 45 (40.9%) to the trapezium, 12 (10.9%) to the abductor pollicis brevis muscle (APB), and 9 (8.2%) to the thenar fascia. Of the 24 APLs with three total tendons, the two accessory tendons inserted at the base of the 1st metacarpal and trapezium in 9 (8.2%) cases, base of the 1st metacarpal and thenar fascia in 6 (5.5%) cases, base of the 1st metacarpal and APB in 4 (3.6%) cases, trapezium and APB in 3 (2.7%) cases, the opponens pollicis and thenar fascia in 1 (0.9%) case, and both to the trapezium in 1 (0.9%) case. In the single APL with four total tendons, one accessory tendon attached to the base of the 1st metacarpal while the other two attached to the trapezium.

Conclusions: While the typical APL is widely regarded to present a single tendon that inserts to the base of the 1st metacarpal, the present study suggests that this configuration seldomly exists (7.2% prevalence) in isolation but is instead more often accompanied by an accessory tendon with distal attachment to the trapezium. The presence of accessory APL tendons may crowd the 1st dorsal compartment and cause stenosing tenosynovial inflammation (i.e., de Quervain’s tenosynovitis), and forceful pull of APL tendons on the trapezium may lead to subluxation of the trapeziometacarpal joint. On the contrary, the frequent presence of long, robust accessory APL tendons may represent suitable candidates for tendon transfer or graft surgeries. This study serves as a valuable contribution to literature regarding APL variations and may help clinicians and medical educators better anticipate frequent APL tendon and distal attachment site morphologies and their clinical implications.