Case Report: A Unique Unilateral Variation of a Divided Extensor Digitorum Muscle in a Subject with Bilateral Absence of Extensor Indicis

Reports of variations in the extensor digitorum muscle have been documented since at least 1813. These variations have fallen into 6 groups: (1) deficiencies in tendon number – usually the tendon to the 5thdigit being absent, (2) doubling of tendons to digits, (3) difference in number of muscle bellies, (4) presence of tendinous slips that join other muscles, (5) presence and arrangement of intertendinous connections, (6) presence of separate muscle bellies for each tendon, and (7) accessory muscle bellies. The present case report describes a unilateral variation of the extensor digitorum muscle, observed in a 73 year‐old Caucasian male with bilateral absence of the extensor indicis muscles. During anatomical dissection of this individual, we observed that the extensor digitorum muscle on the subject’s right side appeared to be divided into two distinct muscle masses. The lateral muscle mass (1 cm wide at its thickest point and 11 cm in length) originated from the medial aspect of the extensor carpi radialis brevis and provided a tendon (20 cm in length) to the index finger. The medial muscle mass had the usual origin to the lateral epicondyle via the common extensor tendon and provided tendons to the third and fourth digits. Interestingly, the extensor indicis muscles were also absent bilaterally. While cases of absent extensor indicis muscles have been reported previously, we are not aware of any report in the literature that describes a divided extensor digitorum muscle or such a divided extensor digitorum muscle occurring in the presence of bilaterally missing extensor indicis muscles. Indeed, to our knowledge, this is the first such report of this occurrence. Thus, to the 6 groups of variant types commonly recognized for extensor digitorum as listed above, we now add a 7th new category of “divided muscle mass” (which in our case is “bipartite”, but future studies may discover other arrangements). Developmental and functional considerations of this variation will be further discussed. Support or Funding Information Department of Pathology and Laboratory Medicine, David Geffen School of Medicine at UCLA