The abductor pollicis longus: relation between innervation, muscle bellies and number of tendinous slips.

The aim of this study was to assess the relation between the insertions of the distal tendinous slips, the muscle bellies and the innervation pattern of the abductor pollicis longus (APL) muscle and of the extensor pollicis brevis (EPB). The upper extremities of 31 frozen cadavers were dissected under magnifying lenses to describe the distribution of the posterior interosseous nerve (PION). The number and the distribution of distal tendinous slip insertions of the APL muscle were variable. Two superficial and deep distal tendon groups were noted. The separation into superficial and deep muscular parts of the APL was frequent (87%). The EPB muscle was generally constituted by one muscle belly and one tendinous slip (93.5%). The innervation by the PION to the APL and EPB muscles was classified into five types. The specific innervation between superficial and deep muscular parts of the APL muscle, the specific innervation of the deep muscle bellies and the independence of the superficial and deep distal tendon groups of the APL muscle are arguments in favor of a complex functional role of the APL motor unit in thumb mechanics. However, no independence of the tendinous slips in the two distal tendon groups and no correlation between the number of tendinous slips and muscle bellies or innervation were observed. These limit the functional role of the two independent superficial and deep musculotendinous APL motor units. The use of the APL tendon for interposition arthroplasty, for tendon transfer or tendon translocation seems logical, particularly if using one of the two distal tendon groups.