The abductor pollicis brevis (APB) is a flat, thin muscle located just under the skin. It is a thenar muscle and contributes to the bulk of the palm’s thenar eminence. It originates from the flexor retinaculum of the hand, the tubercle of the scaphoid bone and sometimes from the tubercle of the trapezium. The APB is responsible for abduction of the thumb i.e. movement of the thumb anteriorly, a direction perpendicular to the palm (it also assists in opposition and extension of the thumb). The APB is innervated by the recurrent branch of the median nerve (C5- C7, C8-T1). [1]
The abductor pollicis longus (APL) is one of the extrinsic muscles of the hand and its tendon forms the anterior border of the anatomical snuffbox. The major function of the APL is to abduct the thumb at the wrist. it also assists in extending and rotating the thumb. It also helps to abduct the wrist (radial deviation) and flex the hand. The APL is innervated by the posterior interosseous nerve, which is a continuation of the deep branch of the radial nerve after it passes through the supinator muscle. The posterior interosseous nerve is derived from spinal segments C7 & C8. [2]
In the event of a peripheral nerve lesion (median nerve), the following muscles substitute for APB actions; opponens pollicis, flexor pollicis brevis and abductor pollicis longus (APL). [3]
Therefore, the APL does in fact substitute for the function of an impaired, injured or otherwise compromised APB muscle.
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