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What Is Extensor Pollicis Longus?/Extensor Pollicis Longus is part of the deep extensors of the forearm together with extensor pollicis brevis, abductor pollicis longus, extensor indicis and supinator muscles. It is located on the posterior aspect of forearm, extending from the middle third of the ulna, and adjacent interosseous membrane, to the distal phalanx of the thumb.
In human anatomy, the extensor pollicis longus muscle (EPL) is a skeletal muscle located dorsally on the forearm. It is much larger than the extensor pollicis brevis, the origin of which it partly covers and acts to stretch the thumb together with this muscle.
Structure of Extensor Pollicis Longus
Passing through the third tendon comparxrtment,[rx] lying in a narrow, oblique groove on the back of the lower end of the radius,[rx] it crosses the wrist close to the dorsal midline before turning towards the thumb using Lister’s tubercle on the distal end of the radius as a pulley.[rx]
It obliquely crosses the tendons of the extensores carpi radialis longus and brevis, and is separated from the extensor pollicis brevis by a triangular interval, the anatomical snuff box in which the radial artery is found.[rx]
6.7 to 9.7 centimetres (2.6 to 3.8 in) in length, the tendon passes through a long and superficial synovial sheath which, passing obliquely from the radial border of the forearm into the thumb, extends from the proximal border of the extensor retinaculum to the first carpometacarpal joint. In the synovial sheath a proximal and a distal mesotendon connect the tendon to the floor of the sheath.[rx]
Extensor pollicus longus
- Function: Extension of the thumb by acting on the carpometacarpal joint, the metacarpophalangeal joint, and the interphalangeal joint.
- Origin: Dorsal aspects of middle ulna and interosseous membrane
- Insertion: Distal phalanx of 1st finger
- Innervation: Posterior interosseous nerve (C7, C8)
Origin and Insertion of Extensor Pollicis Longus
Extensor pollicis longus originates from the middle third of the posterior surface of ulna, mostly along its radial border. This attachment extends onto the adjacent interosseous membrane, and is situated proximal to the origin of extensor indicis muscle. From here, the muscle belly runs obliquely in a radial direction, towards the lateral aspect of the wrist joint. The muscle ends in a tendon that passes deep to extensor retinaculum and travels across the posterolateral aspect of the hand to insert onto the base of the distal phalanx of the thumb.
More specifically, the tendon of extensor pollicis longus forms what is referred to as an extensor expansion (or dorsal aponeurosis) as it is joined by the tendon of abductor pollicis brevis laterally and adductor pollicis medially. Together, this aponeurosis forms the extensor mechanism of the thumb.
The tendon of extensor pollicis longus is supplied by branches from various arteries. Before the tendon enters its synovial sheath, arteries from the anterior interosseous artery or its muscular branches enter the tendon. The sheath itself is supplied by the posterior ramus of the same artery. In the metacarpal region, beyond the synovial sheath, the tendon is supplied directly from the radial artery. At the phalanges, the tendon forms a dorsal aponeurosis which is supplied by a digital branch of the first dorsal metacarpal artery.[rx]
Extensor pollicis brevis receives its blood supply from the posterior interosseous artery and perforating branches of the anterior interosseous artery.
The extensor pollicis longus muscle receives innervation from the posterior interosseous nerve (C7 and C8) which is the continuation of the deep branch of the radial nerve.
Extensor digitorum is innervated by posterior interosseous nerve which is a continuation of a deep branch of radial nerve (root value C7 and C8).
Extensor pollicis longus extends the terminal phalanx of the thumb. While abductor pollicis brevis and adductor pollicis, both attached to the extensor pollicis longus tendon, can extend the thumb’s interphalangeal joint to the neutral position, only extensor pollicis longus can achieve full hyperextension at the interphalangeal joint. This complete extension at the interphalangeal joint is not possible, or considerably more difficult, with the carpal, carpometacarpal, and metacarpophalangeal joints simultaneously extended. Likewise, flexion at the interphalangeal joint by flexor pollicis longus is considerably reduced in wrist flexion.[rx]
It also applies an extensor force at the metacarpophalangeal joint together with the extensor pollicis brevis and extends and adducts at the carpometacarpal joint of the thumb.[rx]