Abductor pollicis longus
The Abductor pollicis longus (APL) is one of a deep extensor of the forearm and is responsible for facilitating movement and stabilization of the thumb. It's tendon is present in the first extensor compartment of the wrist. It lies immediately below the supinator and sometimes unite with it. The muscle belly consists fundamentally of three parts which results into two divisions of tendons. 
Origin & Insertion
The first part of the abductor pollicis muscle is a deep part which originates from the ulna, interosseous membrane and radius and is covered by extensor digitorum longus. It has numerous subdivisions of belly (all arranged in a parallel fashion) which terminates in a central tendon. The fibres of first part are short, obliquely attached to a tendon in a pennate fashion. After its passage through the first compartment of extensor retinaculum, the deep tendon seperates into several branches and is ultimately inserted into trapezium, abductor pollicis brevis, opponens pollicis, capsule and anterior oblique ligament.
The other two parts of abductor pollicis longus are superficial. One of them originates from the ulna and interosseous membrane whereas the other originates from the deep fascia of forearm and the radius. These two parts are connected to the third deep part by a small area of connective tissues and terminates in a tendon to metacarpal 1.
It is supplied by the Posterior Interosseous nerve (C7-C8) which is a continuation of the deep branch of Radial nerve.
Posterior interosseous artery
The main functions of APL include abduction of the thumb and extension of the first carpo-metacarpal joint. It also assists in radial deviation and flexion of the wrist.
The most important clinical condition involving the APL, usually along with the Extensor pollicis brevis is the DeQuervain's Syndrome. It is characterized by thickening and and inflammation of tendons of APL and EPB resulting in pain and swelling in the first extensor compartment of wrist. Pain increases on movements of thumb making the tendons more susceptible to degeneration and lesions.
The strength of APL is assessed by asking the subject to abduct the thumb with the forearm in neutral psotioin. Resistance is applied against the lateral aspect ofg the distal end of first metacarpal in the direction of adduction of thumb.
The test used for clinical diagnosis of DeQuervains is known as the Finklestein's test. The patient is asked to amke a fist with the thumb inside the fingers. The therapist/examiner passively deviates the wrist to the ulnar side. A positive test is indicated by pain over the radial styloid process at the site of the tendons of APL and EPB.
Management of DeQuervain's syndrome would depend on the severity of the cindition and can include:
- Electrotherapy modalities including local Ultrasound and TENS application
- Cold packs/heat packs
- Manual therapy including myofascial release of the tendons and stretching
- Stregnth training for long standing conditions
- Thumb splinting, medication and taping can be done for very painful or acute cases
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