Abductor Pollicis Longus – Anatomy, Nerve Supply, Function

Abductor Pollicis Longus (APL) is the tendon of the first extensor compartment of the wrist. It helps in movement and stabilization of the thumb. Multiple tendinous insertions of APL has been reported by many authors.^[rx] The anatomical variation may remain asymptomatic or may present with painful conditions like de Quervain tenosynovitis (DQT).^[rx,rx] The treatment of such conditions in severe cases involves either an injection therapy or surgical decompression of the first extensor compartment. Causes of treatment failure in DQT can be due to the variations in the anatomy tendons of the APL.^[rx],The other clinical implications of multiple tendinous slips insertion of APL is its association with first carpometacarpal arthritis or subluxation and its use as a tendon graft.^[rx]It seems essential to understand the anatomic variation of APL by the orthopedic surgeons as it has been associated with many orthopedic conditions. This cadaveric study was designed to look for tendinous variations of APL in the Indian population.

Structure of Abductor Pollicis Longus

The abductor pollicis longus lies immediately below the supinator and is sometimes united with it. It arises from the lateral part of the dorsal surface of the body of the ulna below the insertion of the anconeus, from the interosseous membrane, and from the middle third of the dorsal surface of the body of the radius.^[rx]

Passing obliquely downward and lateralward, it ends in a tendon, which runs through a groove on the lateral side of the lower end of the radius, accompanied by the tendon of the extensor pollicis brevis.^[rx]

The insertion is divided into a distal, superficial part and a proximal, deep part. The superficial part is inserted with one or more tendons into the radial side of the base of the first metacarpal bone, and the deep part is variably inserted into the trapezium, the joint capsule and its ligaments, and into the belly of abductor pollicis brevis (APB) or opponens pollicis.^[rx]

Origin and Insertion of Abductor Pollicis Longus

Abductor pollicis longus originates from the posterior surface of the proximal half of the radius, ulna and intervening interosseous membrane. The site of origin is located just inferior to the insertion of the anconeus muscle. From here, the muscle courses inferolateral towards the radial side of the hand, becoming more superficial as it enters the distal third of the forearm.

Just proximal to the wrist, the muscle gives off a narrow tendon. This tendon passes through a groove carved on the lateral surface of the distal end of the radius, where it is joined by the tendon of extensor pollicis brevis muscle.

Abductor pollicis longus tendon then passes beneath the extensor retinaculum. In doing so, it traverses the first dorsal (extensor) compartment of the retinacular space. Once inside the dorsum of the hand, the tendon splits into two slips that insert onto the base of first metacarpal and trapezium bones, respectively. Some fibers may also insert into surrounding structures; opponents pollicis, abductor pollicis brevis and the fascia of the thenar eminence.

Nerve Supply of Abductor Pollicis Longus

The abductor pollicis longus muscle 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.^[rx]

Abductor pollicis longus is innervated by the posterior interosseous nerve (C7, C8), which is a continuation of the deep branch of the radial nerve. The radial nerve is a branch of the posterior cord of the brachial plexus.

Blood Supply of Abductor Pollicis Longus

Abductor pollicis longus is supplied by the posterior interosseous artery.^[rx]

Blood supply to the abductor pollicis longus muscle comes from the interosseous branches of the ulnar artery;

• The proximal part of the muscle is supplied by the lateral branch of the posterior interosseous artery.
• The distal part is vascularised by a perforating branch of the anterior interosseous artery.

The Function of Abductor Pollicis Longus

The chief action of abductor pollicis longus is to abduct the thumb at the carpometacarpal joint, thereby moving the thumb anteriorly. It also assists in extending and rotating the thumb.^[rx]

By its continued action it helps to abduct the wrist (radial deviation) and flex the hand.^[rx]

The APL insertion on the trapezium and the APB origin on the same bone is the only connection between the thumb’s intrinsic and extrinsic muscles.^[rx] As the thumb is brought into action, these two muscles must coordinate to keep the trapezium stable in the carpus,^[rx] which is important for the proper functioning of the thumb (i.e. precision and power grip.)^[rx]

References

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