Aberrant Extensor Digitorum Tendon Lies Superficial to Extensor Retinaculum

BACKGROUND: The extensor digitorum muscle (EDM) is located on the posterior aspect of the forearm. The tendons of this muscle typically run deep to the extensor retinaculum (ER) and a fibrous sheath that, not only stabilizes and aligns the extensor tendons but prevents them from bowstringing with extension of the wrist. The ER has been used in the past as a graft source to treat Boxer's knuckle, an injury of the metacarpophalangeal (MP) joint capsule. This indicates the minor role the ER plays in physiologic function. EDM is innervated by the posterior interosseous nerve, a branch of the radial nerve, and is used in extending digits 2 through 5. CASE INFORMATION: This case report documents an anatomical variant that was found during routine dissection of a 66-year-old male cadaver's EDM. The most medial tendon of the EDM, attached distally to digit 5, was discovered to be passing superficial to the ER. CONCLUSION: To our knowledge, this anatomic variation has yet to be documented. The donor's medical history reports no motor dysfunction to the phalanges; thus it is suspected that this variation likely resulted in little to no complications for the donor. The lack of previous documentation on this variant, both in scientific literature and the donor's personal medical history, leads us to suspect that the variant is either uncommon or not of mechanical significance. We have theorized that this anatomical variance would present itself in a similar manner as the palmaris longus tendon.