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A Variant Extensor Indicis Muscle and the Branching Pattern of the Deep

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A Variant Extensor Indicis Muscle and the Branching Pattern of the Deep ISSN 0008-3194 (p)/ISSN 1715-6181 (e)/2015/64–71/$2.00/©JCCA 2015 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 Myroslava Kumka, MD, PhD* The purpose of this study is to document the topographic L’objectif de cette étude est de documenter l’anatomie anatomy of an extensor indicis (EI) muscle with a double topographique du musculus extensor indicis (MEI), tendon and the associated distribution of the deep ou muscle extenseur de l’index, avec un tendon double branch of the radial nerve (DBRN). Both EI tendons et la distribution de la grosse branche du nerf radial were positioned deep to the tendons of the extensor (DGBNR) qui lui est associée. Les deux tendons du digitorum as they traversed the dorsal osseofibrous MEI sont positionnés profondément dans les tendons du tunnel. They then joined the medial slips of the extensor extensor digitorum alors qu’ils traversent le tunnel osso- expansion of the second and third digits. In all other fibreux dorsal. Ils joignent ensuite les portions internes dissected forearms, a tendon of the EI muscle joined the de l’expansion de l’extenseur au niveau du deuxième medial slip of the extensor expansion to the index finger. et troisième doigt. Dans tous les autres avant-bras The DBRN provided short branches to the superficial disséqués, un tendon du MEI joint la portion interne extensor muscles, long branches to the abductor de l’expansion de l’extenseur à l’index. La DGBNR pollicis longus and extensor pollicis brevis muscles, fournit de petites branches aux muscles d’extenseur and terminated as the posterior interosseous nerve. superficiels, de grosses branches aux muscles du long Descending deep to the extensor pollicis longus muscle, abducteur du pouce et du court extenseur du pouce, the posterior interosseous nerve sent branches to the pour finir en tant que nerf interosseux postérieur. Le extensor pollicis brevis and EI muscles. Understanding nerf interosseux postérieur descend profondément dans of the topographic anatomy of an EI with a double le muscle du long extenseur du pouce et envoie des tendon, and the associated distribution of the DBRN, branches au court extenseur du pouce et au MEI. Une bonne compréhension de l’anatomie topographique d’un MEI avec double tendon, et la DGBNR associée, * Department of Anatomy, Canadian Memorial Chiropractic College, Ontario, Canada. Correspondence should be addressed to: Myroslava Kumka, MD, PhD, Canadian Memorial Chiropractic College, Department of Anatomy, 6100 Leslie Street, Toronto, ON M2H 3J1, Canada Tel: (416) 482-2340 ext:175 Email: [email protected] © JCCA 2015 64 J Can Chiropr Assoc 2015; 59(1) M Kumka may contribute to accurate diagnosis and treatment of peut favoriser des diagnostics précis et améliorer le hand lesions. traitement de lésions aux mains. (JCCA 2015; 59(1):64-71) (JCCA 2015; 59(1):64-71) key words: extensor indicis, neuropathy, radial mots clés : musculus extensor indicis, neuropathie, nerve, variation, wrist pain nerf radial, variation, douleur au poignet Introduction lesions. It may also be useful for modeling of neurovascu- Anatomical variations of the extensor muscles of the hand, lar and muscular compartments of the forearm and hand. including the extensor indicis (EI), are common, and no The following questions are posed: i) Could the pre- standard pattern has been described.1-13 These variations sented variant in the morphology of the EI muscle explain are often a matter of concern to clinicians as they may lead hand functionality and clinical symptoms in the living to a misdiagnosis. For example, a variant EI muscle may patient?, ii) What branching pattern of the radial nerve give rise to dorsal wrist pain and hence may be diagnosed should be expected in the case of an EI with a double ten- incorrectly as a ganglion, soft tissue tumor, synovial cyst don?, iii) What are the clinical implications of the branch- or tenovaginitis.14-18 For surgeons, the variations of EI ten- ing pattern of the DBRN? dons are of particular clinical importance, since this is the We present the topographic anatomy of the EI muscle muscle that is commonly used to perform graft and tendon with a double tendon and discuss its functional and clin- transfer operations and should be considered during ex- ical implications. The distribution of the DBRN within tensor tendon exploration for trauma.5,7,19 Since hand sur- forearms possessing single or double tendons of the EI gery relies mainly on applied anatomy, it is essential to re- muscle is investigated and its clinical significance is dis- visit not only the anatomy of the presented variance of the cussed. EI muscle but also the distribution of the deep branch of the radial nerve (DBRN) as it may help to explain clinical Material and methods symptoms developed by the partial or complete entrap- This study utilized 16 embalmed cadavers (32 upper ment of the main trunk of this nerve or its branches.20-24 limbs) of both genders and of different ages using ana- To our knowledge, no studies have investigated the cor- tomical macro-and microdissections. An 89-year-old relation between the variant EI and the branching pattern male had a double tendon of the extensor indicis muscle of the radial nerve. Even though the distribution of the bilaterally. This variant muscle was dissected carefully to DBRN is complicated and inconsistent,23 enhancing our expose its origin, course and insertion. All of the fore- understanding of the relationship between the radial nerve arms were dissected to examine the distribution of the and the variant EI may contribute to accurate diagnosis, radial nerve branches within the posterior forearm to de- effective surgical procedures, and the modeling of neuro- termine whether the branching pattern of the radial nerve vascular compartments.25 Familiarity with the variations varies significantly within the forearms possessing single of extensor muscles and their innervation are crucial for or double tendons of the EI muscle. The specimens were assessment and treatment of pathologic conditions with documented and photographed. magnetic resonance imaging, diagnostic sonography, and magnetic resonance neurography.26-30 Results The rationale of the presented study is to document In an 89-year–old male cadaver, the EI muscle with a the existence and topographic anatomy of the EI muscle double tendon to the second and third digits was found bi- with a double tendon and the associated distribution of the laterally within the deep layer of the extensor muscle com- DBRN. This is done to enhance clinicians’ awareness, so partment of the forearm (Figure 1A). This variant muscle as to provide adequate assessment and treatment of hand originated from the posterior surface of the distal third of J Can Chiropr Assoc 2015; 59(1) 65 A variant extensor indicis muscle and the branching pattern of the deep radial nerve could explain hand functionality EI EI2 EI3 ER, cut EPL EIM EPB EI EIL APL EPB EPB EI EPL APL ED IM EDM APL ECU EPL, reflected IM SD ECU, reflected SS EDM, reflected ED, reflected Figure 1A Figure 1B Figure 1C 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. the shaft of the ulna, the adjacent interosseous membrane, tendons of the EI were positioned medial and deep to the and from the internal surface of the antebrachial fascia tendons of the extensor digitorum (ED) muscle as they occupying the interface of the superficial-deep forearm traversed the fourth osseofibrous tunnel under the exten- extensors (Figure 1B). The lateral part of the muscle belly sor retinaculum (Figure 1C). On the dorsum of the hand, was shorter and became tendinous proximal to the ex- opposite the heads of the second and third metacarpal tensor retinaculum. However, the medial part was long- bones, the two tendons of the EI joined the ulnar sides of er, with the musculotendinous junction descending deep the tendons of the ED, enhancing the medial slips of the to the superior margin of the extensor retinaculum. Both extensor expansion (EE) for the second and third digits 66 J Can Chiropr Assoc 2015; 59(1) M Kumka 32 MS MS Following the international anatomical terminology, we classify the PIN as the terminal long branch of the DBRN. The DBRN traversed the supinator muscle between EE the superficial and deep layers, emerging in the posterior EE compartment deep to the superficial layer of the exten- EE sor muscles, immediately providing the short branches to ED the ED, extensor digiti minimi, and extensor carpi ulnaris muscles. Descending superficial to the abductor pollicis EE longus (APL), the DBRN provided the long branches to M3 the APL and the extensor pollicis brevis (EPB), and ter- minated as the PIN (Figures 3A and 3B).
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