Human Cadaver Anatomical Variation of the Extrinsic Musculature of the Thumb
Authors: Hanrahan Clare , Correa Ashley , Junious Victoria , Norman Angela , Guthrie Kylee , Slack Thomas , Rosario Martin G. 
Published: June 2021

Background

Typically, there are five muscles in the posterior forearm that cross the wrist joint and give tendons to the first two fingers. They include the extensor pollicis longus, abductor pollicis longus, and extensor pollicis brevis, which all originate on the posterior forearm and insert on the first digit. Additionally, the extensor digitorum originates on the posterior forearm and inserts on digits 2–5, and the extensor indicis inserts on the tendon of the extensor digitorum to the second digit.

Purpose

Describe the anatomical variation in the musculature and tendons of the extrinsic musculature of the thumb.

Methods

Two specimens were dissected and reported in this work. The dissection of the posterior forearm and dorsum of the hand were completed by students in the Physical Therapy program at Texas Woman's University Dallas under the supervision of the Anatomist and Lab Coordinator of that program. Due to the frailty of the skin, it was removed easily using blunt dissection and by peeling it off manually. Once the skin was removed and the underlying fascia cleaned, the muscles of the posterior forearm and dorsum of the hand were identified and exposed. During this process, an extra tendon (for the first specimen) was noticed near the distal posterolateral radius that could be followed proximally to the muscle belly. The tendon spit and inserted at the base of the first and second metacarpophalangeal joints. The second specimen had 5 extrinsic tendons (two extra) that inserted at the thumb.

Results

Specimen 1; the additional muscle is located on the posterior forearm, in between the Extensor Pollicis Longus and Extensor Pollicis Brevis muscles. The muscle originates from the middle posterior surface of the ulna and the interosseous membrane. The thumb muscle inserts into the base of the metacarpophalangeal joints of the first (similarly as the tendon of the Extensor Pollicis Longus muscle) and second phalange. The function appears to be an extensor of the pollex. Specimen 2; it appears that the extensor digitorum give an extra tendon to the pollicis, similar insertion as the extensor pollicis longus was observed. Also, the tendon of the Abductor pollicis longus splits in two and was inserted at the base of the first metacarpal. Both extra muscles, because of their location at the posterior forearm are innervated by the radial nerve and irrigated by the posterior interosseous artery.

Conclusion

It appears that the extensors of the forearm muscles of these cadavers had additional muscles and tendons that is different from the standard structures. These muscles could improve the motor function of the pollex. However, the small muscle size could provide very little improvements or weak movements. The knowledge of this additional muscle might help surgeons understand the possible functional differences amongst individuals with this structural difference.

Support or Funding Information

Texas Woman's University Physical Therapy Program

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This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.