A variant extensor indicis muscle and the branching pattern of the deep radial nerve could explain hand functionality and clinical symptoms in the living patient

Abstract

The purpose of this study is to document the topographic anatomy of an extensor indicis (EI) muscle with a double tendon and the associated distribution of the deep branch of the radial nerve (DBRN). Both EI tendons were positioned deep to the tendons of the extensor digitorum as they traversed the dorsal osseofibrous tunnel. They then joined the medial slips of the extensor expansion of the second and third digits. In all other dissected forearms, a tendon of the EI muscle joined the medial slip of the extensor expansion to the index finger. The DBRN provided short branches to the superficial extensor muscles, long branches to the abductor pollicis longus and extensor pollicis brevis muscles, and terminated as the posterior interosseous nerve. Descending deep to the extensor pollicis longus muscle, the posterior interosseous nerve sent branches to the extensor pollicis brevis and EI muscles. Understanding of the topographic anatomy of an EI with a double tendon, and the associated distribution of the DBRN, may contribute to accurate diagnosis and treatment of hand lesions.

Keywords: extensor indicis; neuropathy; radial nerve; variation; wrist pain.

Figure 1

Topography of the extensor indicis (EI) muscle with a double tendon within the muscular layers of the extensor compartment of the forearm. A – The superficial muscular layer includes ECU, extensor carpi ulnaris; EDM, extensor digiti minimi; ED, extensor digitorum. The deep layer includes EPL, extensor pollicis longus; EPB, extensor pollicis brevis; APL, abductor pollicis longus; EI with a double tendon. B – Schematic representation illustrates the deep muscular layer. EI2, extensor indicis to the second digit; EI3, extensor indicis to the third digit; SS, supinator superficial part; SD, supinator deep part; APL; EPL; EPB; IM, interosseous membrane. C – The extensor indicis lateral part (EIL) of the muscle belly becomes tendinous proximal to the extensor retinaculum (ER). The extensor indicis medial part (EIM) is longer with the musculotendinous junction descending deep to the superior margin of the ER. Both tendons of the EI are positioned medial and deep to the tendons of the ED, as they traverse the osseofibrous tunnel under the ER.

Figure 2

The distal attachments of the EI double tendon. On the dorsum of the hand, opposite the heads of the second metacarpal (M2) and the third metacarpal (M3) bones, the tendons of the EI to the second (EI2), and third (EI3) digits join the ulnar sides of the ED, enhancing the medial slips (MS) of the extensor expansion (EE) of the second and third digits.

Figure 3

The branching pattern of the deep branch of the radial nerve (DBRN) in the forearm with the variant EI muscle. A – Schematic representation illustrates the DBRN which traverses the supinator (S) muscle, and immediately provides the short branches (SB). The long branches include the abductor pollicis longus branch (APLB), the extensor pollicis longus branch (EPLB), and the posterior interosseous nerve (PIN). The PIN provides the abductor pollicis brevis branch (APBB), and the extensor indicis branches (EIB). B – The DBRN descends superficial to the APL muscle providing the APLB and EPLB, and then as the PIN, descends deep to the EPL muscle. C – The PIN descends deep to the EPL muscle on the posterior surface of the IM. Being located between the EPL and the lateral part of the EI muscle, the PIN provides the two to three EIB.