An accessory extensor pollicis longus tendon: an anatomical variant.

The extensor pollicis longus (EPL) tendon lies within the third dorsal compartment of the wrist. It originates from the middle third of the posterior surface of the ulna and the adjacent interosseous membrane, proximal to the origin of extensor indicis propius (EIP), and forms the dorsal border of the anatomical snuffbox, curving around the ulnar border of Lister's tubercle before inserting onto the base of the distal phalanx of the thumb.1 Because it is responsible for extension of the interphalangeal joint (IPJ) and retropulsion of the thumb, and is as a secondary adductor of the thumb, EPL dysfunction can prove problematic, affecting hand function and activities of daily living. Anatomical variations of the EPL tendon are rare, with only a few anomalies incidentally discovered and reported in the literature.2, 3 Chiu4 during a cadaveric dissection, discovered a supernumerary muscle during a cadaveric dissection which arose from the dorsal surface of the ulna immediately proximal to the EIP, ran between the EIP and EPL, and became tendinous within the fourth dorsal compartment before merging with the EPL tendon over the first metacarpal. Cohen5 found an accessory tendon coursing from the radial aspect of the fourth compartment, parallel to the EPL, and inserting onto the distal phalanx of the thumb, whilst performing tenosynovectomy on a patient with rheumatoid arthritis, although he did not explore the origin of the muscle itself. These variations led Turker6 to devise a classification system of anomalies of the EPL, with Type 1 comprising supernumerary accessory tendons and type 2 consisting of tendinous interconnections. We present a case of an EPL variant, with dissection to the proximal tendon revealing previously undescribed anatomy. A 76 right-hand dominant (RHD) male underwent a left wrist arthrodesis with proximal row carpectomy, as well as a trapeziectomy and suspensionplasty, for end-stage radiocarpal arthritis with concomitant base of thumb arthritis. A dorsal midline approach to the wrist was utilized, an a z-shaped division of extensor retinaculum was performed to allow later repair. Two discrete tendons were identified coursing around Lister's tubercle toward the thumb (Fig. 1). Further proximal dissection revealed that the radial tendon passed through the third compartment, and had a course, thick and distal muscle belly matching that of the primary EPL (Fig. 2). The ulnar tendon split from the common tendon of the extensor digitorum comminis (EDC) muscle, radial to the tendon to the index finger, within the fourth dorsal compartment, before passing radial around Lister's tubercle to the thumb. Traction of this tendon resulted in retropulsion of the thumb, with extension at the IPJ, and no extension of the index finger (distinguishing it from a extensor pollicis et indicis7). Traction on the radial tendon resulted in the same action. No juncturae tendinum were noted between the primary and accessory EPL tendon (Figs. 2 and 3).2 Access to the wrist joint was obtained using the interval between accessory EPL and remaining EDC tendons, and the procedure was completed without complication. The primary EPL tendon was transposed superficial to the retinaculum on completion to prevent an attritional rupture. Postoperative clinical examination revealed independent thumb retropulsion and index finger extension, with no hyperextension of the thumb IPJ noted on either side.8 An ultrasound was performed to further define the anatomy, and confirmed separate origins, with independent muscle bellies of the primary and accessory EPL tendons. Whilst previous authors have described an accessory EPL tendons arising from the fourth dorsal compartment, these have risen contiguous to the EIP.4, 9 To the best of our knowledge, this is the first report of an anomalous tendon arising from the common origin of the EDC. Anatomical musculotendinous variations are thought to result from erroneous division of mesoderm or aberrant deposition of myogenic tissue during embryogenesis.10 It is more likely that the variant described in this case arose from aberrant myogenic tissue deposition, as the EPL and EDC normally arise from discrete precursor skeletal masses. An understanding of anatomical variations is important when considering tendon repair, reconstruction or transfers.11 Open access publishing facilitated by The University of Sydney, as part of the Wiley - The University of Sydney agreement via the Council of Australian University Librarians. Raymond G. Kim: Conceptualization; data curation; methodology; resources; software; supervision; validation; visualization; writing – original draft; writing – review and editing. David Graham: Formal analysis; methodology; project administration; supervision; writing – original draft; writing – review and editing. Richard Lawson: Methodology; project administration; supervision; validation; visualization; writing – original draft; writing – review and editing. Brahman Sivakumar: Conceptualization; investigation; methodology; project administration; resources; supervision; validation; visualization; writing – original draft; writing – review and editing.