An unusual configuration of two anomalies in the extensor digitorum profundus complex in a human.

The extensor digitorum profundus complex underwent degeneration of the ulnar segments during primate adaptation and evolution. This process resulted in the preservation of only the extensor pollicis longus and extensor indicis in some apes, including humans. Consequently, anatomical variations within the digitorum profundus complex in modern humans have been well-documented, with detailed reports on their frequency and patterns in previous studies. Here, we report an unusual arrangement involving two anomalies in the extensor digitorum profundus complex, identified in a 66-year-old Japanese male cadaver. In this cadaver, two accessory muscles differentiated from both the extensor pollicis longus and extensor indicis. Notably, the latter muscle featured a tendon bifurcating towards both the thumb and index fingers, referred to as the extensor pollicis et indicis communis. Under the extensor retinaculum, the tendon of the accessory extensor pollicis longus passed through an independent compartment, whereas that of the extensor pollicis et indicis communis traversed a compartment shared by the extensor indicis and the extensor digitorum communis. Both muscles were innervated by the posterior interosseous nerve. Previous studies have reported that the accessory slip of the extensor pollicis longus and extensor pollicis et indicis communis appear at frequencies of 0.6% and 0.4-1.4%, respectively. However, to the best of our knowledge, a configuration in which both appear simultaneously has not been reported. The data from this case could provide essential insights into the variations in the extensor digitorum profundus complex in humans and non-human primates.