Bilateral extensor pollicis et indicis accessorius: clinical and anatomical perspectives

The extensor muscles of the hand play a crucial role in thumb and finger movement. Thumb extension is primarily mediated by the extensor pollicis longus muscle and extensor pollicis brevis muscle, and extension of the index finger is facilitated by the extensor indicis muscle and extensor digitorum muscle. This report describes a rare anatomical variation, the extensor pollicis et indicis accessorius, in which a single tendon bifurcated to insert on both the index finger and thumb.

This variation was identified bilaterally during routine dissection of a 92-year-old female donor. The donor had been embalmed with a formaldehyde-based fixation method.

On each side, an accessory extensor pollicis et indicis accessorius muscle was present in the deep posterior compartment of the forearm, giving rise to a single tendon that bifurcated distally to insert on both the dorsal aspect of the thumb and the dorsal expansion of the index finger. Measurements of the extensor pollicis et indicis accessorius muscle were not significantly different. Several case studies have documented the existence of such anomalies, but bilateral descriptions are rare.

Historical and contemporary sources differ in defining extensor pollicis et indicis accessorius. Based on a comprehensive literature review and the present bilateral anatomical finding, this report recommends using the term extensor pollicis et indicis accessorius muscle for this configuration and proposes revising existing classification systems to include accessory extensor muscles originating from the extensor pollicis longus muscle, the extensor indicis muscle, or the ulna, rather than limiting them to extensor pollicis longus muscle anomalies.