Extensor Indicis Radialis and Extensor Medii Proprius Associated with an Unknown Fibromuscular Slip: a Case Report

Posterior compartment of forearm and hand is among the most variable parts of the human body. During a routine dissection, we found two anatomical variants in the right forearm and hand of an 84-year-old male cadaver including the extensor indicis radialis and the extensor medii proprius associated with a fibromuscular slip. Histological analysis using hematoxylin and eosin staining revealed that this unusual fibromuscular slip associated with the extensor medii proprius was composed of dense connective tissue along with small amount of skeletal muscle fibers, and the innervation from the posterior interosseous nerve. Based on the existing literature, a fibromuscular slip associated with a hand extensor tendon on the dorsum of hand has never been reported. Further studies are needed to clarify the evolutionary origin of this structure, although it resembles to a certain extent the dorsometacarpales, a group of muscles that were once present in our mammalian ancestors.