Abductor Pollicis Longus distal muscle belly variation in a male human cadaver

Background Typically, five muscles of the posterior forearm intersect the wrist joint and contribute tendons to the first two digits. Purpose Depict the anatomical variation in the muscles and tendons of the extrinsic musculature of the thumb in one human cadaver. Methods One specimen was dissected and reported in this work. During dissection, an extra muscle belly was found embedded within the tendon of abductor pollicis longus. This variation was discovered near the distolateral attachment site at the base of the first metacarpal joint. Results The additional muscle belly is located between abductor pollicis brevis and extensor carpi radialis muscles. Typically, the proximal muscle belly originates at the posterior proximal half of the ulna and middle one‐third of the radius inserting at the typical site of attachment, lateral aspect of the base of the first metatarsal, via the tendon. However, the tendon continues distally to a second muscle belly embedded within the distolateral one‐third of the abductor pollicis longus tendon where the attachment site can be found at the traditional lateral aspect of the base of the first metacarpal bone. Conclusion The extrinsic extensors of the thenar region in this cadaver had an additional distal muscle belly embedded within the distal (extra) portion of the tendon differing from the standard structures. Knowledge of this additional structure may help medical professionals understand possible functional differences among individuals with this morphological variation. Picture of thumb with extra tendon anatomy variation. Figure 1