Muscle and the Extensor Indicis Radialis Muscle in Japanese

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1 Okajimas Folia Anat. Jpn., 71(6): , March, 1995 Anatomical Studies on the Extensor Pollicis et Indicis Accessorius Muscle and the Extensor Indicis Radialis Muscle in Japanese By Yukio YOSHIDA Department of Anatomy, Aichi Medical University, Nagakute, Aichi , Japan - Received for Publication, September 30, Key Words: Macroscopic anatomy, Skeletal muscle, Extensor pollicis et indicis accessorius muscle, Extensor indicis radialis muscle, Extensor digitorum profundus muscle Summary: Anatomical studies on the extensor pollicis et indicis accessorius and extensor indicis radialis muscles were conducted on 952 upper limbs from 476 Japanese adult cadavers. Each anomalous muscle occurred between the extensor pollicis longus proprius and extensor indicis proprius muscles in the dorsum of the forearm and hand. The extensor pollicis et indicis accessorius muscle was present in 13 of the limbs (1.4%). It was not coexistent with another anomalous muscle of the extensor digitorum profundus mass in 9 cases, and it was coexistent with that muscle in 4. The extensor indicis radialis muscle was present in 34 limbs (3.6%). It was accompanied by another anomaly of the extensor digitorum profundus mass in 5 cases, and not accompanied by that muscle in 29. The extensor pollicis et indicis accessorius and extensor indicis radialis muscles were innervated by the posterior interosseus nerve of the radial nerve. It seems that the former anomalous muscle supplies the extensor control of the thumb and index finger, and the latter one of the index finger only. Each anomalous muscle was considered to be differentiating on the radial side of the extensor digitorum profundus mass in humans. It is well known that the forearm extensor musculature of primates exhibits much variation in differentiation of the thumb and fingers. From a phylogenetical standpoint the extensor digitorum profundus mass is the most interesting among the forearm muscles. It, the deep layer of the forearm extensor musculature, shows many anomalies in primates including humans. Anatomical studies of the extensor digitorum profundus mass have been published by Ribbing (1938) and Kaneff (1959b) in mammals, Straus (1941) and Aziz and Dunlap (1986) in primates, and Cauldwell et al. (1943), Yoshida (1978), Yoshida and Fukuyama (1979), Kosugi et al. (1984), Yoshida (1985), Aziz and Dunlap (1986) and Yoshida (1990) in humans. However there have been few descriptions concerned with the supernumerary muscles (accessory slips) which occurred between the extensor pollicis longus proprius and extensor indicis proprius muscles in humans. For that reason their variation and their relationship to other anomalies of the extensor digitorum profundus mass have remained unknown. Therefore the morphology of the extensor pollicis et indicis accessorius and extensor indicis radialis muscles of man is discussed in this paper. Materials and Methods This work is based on gross anatomical dissection of 952 upper limbs from 476 cadavers. They were all adults, 252 Japanese males and 224 Japanese females. The materials were examined at the Departments of Anatomy of Chiba University, Kanazawa Medical University and Aichi Medical University, between 1973 and The extensor pollicis longus proprius muscle, extensor indicis proprius muscle, and additional slips (accessory muscles) which occurred between them were examined. Particular attention was paid to the points where the accessory muscles arose and were inserted, and to their relationship to two proprius muscles supplying the thumb and index finger. Observations Both the extensor pollicis longus proprius and extensor indicis proprius muscles were always present in all upper limbs in which the extensor pollicis et indicis accessorius or extensor indicis radialis muscle occurred. Each anomalous muscle and two proprius muscles lay immediately below the extensor digitorum communis muscle in the dorsum of the forearm and 355

2 356 Y. Yoshida hand. The extensor pollicis longus proprius muscle was regularly inserted on the dorsal surface of the base of the distal phalanx of the thumb, and the extensor indicis proprius muscle on the medial side of the back of the index finger. The extensor pollicis et indicis accessorius muscle in general arises from the lateral part of the posterior surface of the ulna in its distal third and also from the interosseus membrane between the extensor pollicis longus proprius and extensor indicis proprius muscles. It is a narrow elongated muscle, situated lateral to and parallel with the extensor indicis proprius muscle. The extensor pollicis et indicis accessorius muscle ends in a single tendon near the wrist joint. Its tendon, in the distal half, bifurcates into two compartments, the radial and the ulnar. The radial compartment is attached to the medial side of the thumb or fused with the extensor pollicis longus proprius tendon, and the ulnar one to the lateral side of the index finger or rarely fused with the extensor indicis proprius tendon. The extensor pollicis et indicis accessorius muscle was present in 13 limbs (Nos. 1-13). Nos The extensor pollicis et indicis accessorius muscle occurred between the extensor pollicis longus proprius and extensor indicis proprius muscles. Another accessory muscle, belonging to the extensor digitorum profundus mass, was not present in the upper limb. The radial compartment of the anomalous muscle tendon was attached to the medial side of the thumb, and the ulnar one to the lateral side of the index finger, at a distance from the extensor pollicis longus proprius and extensor indicis proprius tendons, in Nos. 1-4 (Fig. 2).The radial compartment of the tendon was fused with the extensor pollicis longus proprius tendon, and the ulnar one was attached to the lateral side of the index finger in Nos. 5-8 (Fig. 3). The radial compartment was united with the extensor pollicis longus proprius tendon, and the ulnar one with the extensor indicis proprius tendon in No. 9. Nos There were two supernumerary muscles derived from the extensor digitorum profundus mass, the extensor pollicis et indicis accessorius and extensor medii proprius, in the upper limb. The extensor medii proprius muscle originated from the dorsal ulnar shaft medial to the extensor indicis proprius muscle and was inserted on the medial side of the dorsum of the middle finger. The radial compartment of the tendon of the extensor pollicis et indicis accessorius was fused with the extensor pollicis longus proprius tendon, and the ulnar one was attached to the lateral side of the index finger in each case (Fig. 4). No. 13. The extensor pollicis et indicis accessorius and extensor digiti brevis manus muscles were present in the upper limb. The extensor digiti brevis manus, a small muscle, originates from the wrist joint and carpus, ulnar and distal to the extensor indicis proprius muscle. Its tendon was united with the extensor indicis proprius tendon at the distal portion. The muscle bellies of the extensor pollicis et indicis accessorius and extensor indicis proprius combined to form a single mass. The radial compartment of the tendon of the extensor pollicis et indicis accessorius was fused with the extensor pollicis longus proprius tendon, and the ulnar one was attached to the lateral side of the index finger (Fig. 5). The extensor indicis radialis muscle, which was named by Yoshida (1990), has its origin in the posterior surface of the ulna between the extensor pollicis longus proprius and extensor indicis proprius muscles, similar to the former anomalous muscle. Its single tendon, not bifurcating, is inserted on the dorsal surface of the index finger lateral to the tendons of the extensor indicis proprius and extensor digitorum communis muscles. The extensor indicis radialis muscle was present in 34 of the limbs (Nos ). Nos The extensor indicis radialis muscle occurred between the extensor pollicis longus proprius and extensor indicis proprius muscles in the upper limb. An additional muscle slip, belonging to the extensor digitorum profundus mass, was not seen in the upper limb. The extensor indicis radialis was separated from the extensor indicis proprius in Nos (Fig. 6), and the muscle bellies of the extensor indicis radialis and extensor indicis proprius combined to form a single mass in Nos Nos There were two anomalous muscles derived from the extensor digitorum profundus mass, the extensor indicis radialis and extensor medii proprius, in the upper limb. The muscle bellies of the extensor indicis radialis and extensor indicis proprius were combined to form a single mass in each case (Fig. 7). Nos The extensor indicis radialis and extensor carpi profundus muscles were present in the upper limb. The extensor carpi profundus muscle was named by Yoshida (1990). A small muscle, it arose from the posterior surface of the ulna in its distal part, ulnar to the extensor indicis proprius. The extensor carpi profundus muscle was inserted on the carpus adjacent to the fourth metacarpal bone. Four muscle bellies, of two anomalies and two proprius, were separated from each other in No. 45 (Fig. 8). Two muscle bellies of the extensor indicis radialis and extensor indicis proprius combined to form a single mass in No. 46. No. 47. Three anomalous muscles, the extensor indicis radialis, extensor indicis et medii accessorius and extensor medii proprius, occurred in the upper

3 Extensor Pollicis et Indicis Accessorius Muscle and Extensor Indicis Radialis Muscle in Japanese 357 limb. The extensor indicis et medii accessorius muscle, which was named by Yoshida (1990), was situated ulnar to the extensor indicis proprius and radial to the extensor medii proprius. Its tendon, being membranous at the distal portion, fanned out between the medial side of the index finger and the lateral side of the middle finger. These three anomalous and two proprius muscles were separated from each other. The extensor indicis radialis in this case was small (Fig. 9). The nerve supply of the above mentioned muscles was as follows. All the anomalous muscles and two proprius muscles were always innervated by the posterior interosseus nerve of the radial nerve. Discussion The posterior (extensor) musculature of the forearm and hand in quadrupeds phylogenetically split first into three groups, the brachio-antebrachial, antebrachio-manual and manual (Straus, 1941). The brachio-antebrachial group basically originates from the brachium (arm) and is inserted on the antebrachium (forearm). Its origin and insertion have migrated to the distal portion of the forelimb (upper limb). The antebrachio-manual group has its origin in the antebrachium and is attached to the manus (hand). Its origin has shown proximal migration. The manual group originates from the carpus and/or metacarpus and is inserted on the digits (fingers) in amphibians and reptiles. The brachio-antebrachial group later differentiated into the superficial layer, and the antebrachiomanual group into the deep layer of the forearm extensor musculature. In mammals the manual group (proper extensor muscle of the hand) has degenerated and is absent in the hand. The antebrachio-manual group consists of the abductor pollicis longus and extensor digitorum profundus masses in basic mammals. The former mass forms the abductor pollicis longus (wide sense) muscle, and the latter mass the extensor pollicis et indicis communis muscle in lower forms (rabbit, dog etc:). The extensor pollicis et indicis communis muscle originates from the ulnar shaft medial to the abductor pollicis longus and ends in a single tendon. This muscle generally occurs in the forearm of a mammal which possesses diminutive fingers (Ribbing, 1938; Kaneff, 1959b). Its tendon bifurcates at the distal portion to supply the first and second fingers. In this case neither the extensor pollicis longus proprius muscle nor the extensor indicis proprius muscle is present in the forelimb, so that the whole extensor digitorum profundus mass forms a single extensor pollicis et indicis communis muscle. In primates the extensor digitorum profundus mass differentiates into two parts. Straus (1941) termed them the first and second parts. The first part folios the extensor pollicis longus proprius muscle, and the second part the extensor digitorum proprius muscle. The former muscle is inserted on the pollex (thumb), and the latter one on the second to fourth digits (fingers) in the lemur, gibbon etc. The first part is fundamentally conservative among the primates including humans. On the other hand, the second part exhibits considerable variation among the species with the evolution of the fingers. In many platyrrhine and catarrhine monkeys the tendons of the second part are attached to the second and third fingers. The association of two tendons with the index and middle fingers is the most common primate arrangement of the second part. Its ulnar segment to the third finger is reduced in apes and humans. As a result, the second part of the extensor digitorum profundus mass forms only the extensor indicis proprius muscle in many human upper limbs. In humans the function of the second part of the extensor digitorum profundus mass (only the extensor indicis proprius) diminishes the force of extension, but it consequently enables us to supply the extensor control of the index finger. This reduction on the ulnar side of the second part seems to be the differentiation in the extensor moving the index finger. Such a muscle mass similar to the second part of the extensor digitorum profundus mass occurs in the brachio-antebrachial group (Kaneff, 1959a; Kaneff and Cihak, 1970; Yoshida, 1985). The extensor digitorum lateralis muscle, which belongs to the humeroulnaris sector of the brachio-antebrachial group, has a few tendons to the third (or fourth) to fifth digits in animals (rabbit, dog etc.). Its radial segment, to the third and fourth digits, has become reduced with the evolution of the fingers. In consequence the extensor digitorum lateralis supplies only the fifth finger. It is termed the extensor digiti quinti muscle in primates or the extensor digiti minimi muscle in man. So we humans can extend the little finger independently of other fingers. From the abductor pollicis longus (wide sense) the abductor pollicis longus (narrow sense) and extensor pollicis brevis muscles later differentiate in humans. The extensor pollicis longus proprius and extensor indicis proprius muscles form the important functional couple during pinch gripping. The tendon of the extensor pollicis longus proprius is inserted on the base of the distal phalanx of the thumb, and that of the extensor indicis proprius on the dorsal aponeurosis of the index finger ulnar to the corresponding tendon of the extensor digitorum communis muscle. As the extensor digitorum communis and extensor digiti minimi muscles (these two belonging

4 358 Y. Yoshida Table 1. Arrangement of the muscle slips derived from the extensor digitorum profundus. ): proximal combination, : distal fusion. to the brachio-antebrachial group) are inserted on the second to fifth fingers, the extensor control of the thumb is supplied only by the antebrachio-manual group (abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus proprius muscles) in man. Furthermore, proper extensor movement of the index finger also is mainly supplied by the extensor indicis proprius muscle of the antebrachiomanual group. The extensor pollicis et indicis accessorius or extensor indicis radialis muscle always appears in company with the extensor pollicis longus proprius and extensor indicis proprius muscles, situated between them, in the human upper limb. So neither anomalous muscle is compensatory for two proprius muscles. Furthermore, the extensor pollicis et indicis accessorius muscle of humans clearly is different from the extensor pollicis et indicis communis muscle of animals, which is formed by the whole extensor digitorum profundus mass. When the radial compartment of the tendon of the extensor pollicis et indicis accessorius is not formed, the muscle becomes the extensor indicis radialis. The extensor pollicis et indicis accessorius muscle shows much evolutionary modification in the independent extensor moving the thumb and index finger, and the extensor indicis radialis muscle moving the index finger only. Varieties of the extensor digitorum profundus muscle in the current series are shown in Table 1 and Figure. 1. The frequency of occurrence of each anomalous muscle slip derived from the extensor digitorum profundus mass has already been summarized in an article by Yoshida (1990). Of 832 extensor digitorum profundus masses, 216 (26.0%) showed anomalies (including double tendons of the extensor pollicis longus proprius and extensor indicis proprius, and total absence of the extensor indicis proprius). The most frequent additional slip was the extensor medii proprius muscle, which was situated ulnar (medial) to the extensor indicis proprius muscle. Its tendon was attached to the ulnar side of the dorsum of the middle finger. The extensor medii proprius muscle occurred in 9.6% of the upper limbs. The next most frequent ones were the extensor indicis radialis and extensor indicis ulnaris muscles. Each was encountered in 2.9% of the limbs. The extensor indicis ulnaris muscle was named by Yoshida (1990). The muscle was situated ulnar (medial) to the extensor indicis proprius from origin to insertion and radial (lateral) to the extensor medii proprius when it was present. The differentiating muscles, the extensor pollicis et indicis accessorius and extensor indicis radialis, occurred in 4.1% of the upper limbs. The additional muscle slips which are present ulnar to the extensor indicis proprius are generally in the direction of muscles normally present in primates (atavism or primitive type). There have been many reports of the cases of the duplication or double tendon of the extensor indicis muscle (Cauldwell et al., 1943 etc.). The frequent anomaly of the second part of the extensor digitorum profundus mass is famous, but each case may include the coexistence of the extensor indicis proprius and extensor indicis radialis. The extensor indicis radialis muscle clearly is different from the extensor indicis

5 Extensor Pollicis et Indicis Accessorius Muscle and Extensor Indicis Radialis Muscle in Japanese 359 Fig. 1. Diagrams showing various types of the extensor pollicis et indicis accessorius and extensor indicis radialis muscles. 1 to 47 indicate numbers of the cases as shown in Table 1. proprius muscle. The genuine duplication of the extensor indicis arises from the coexistence of the extensor indicis proprius and extensor indicis ulnaris, and its double tendon is formed by the proximal combination of them (Aziz and Dunlap, 1986; Yoshida, 1990). The extensor digitorum profundus mass of humans exhibits additional muscles on both sides of the extensor indicis proprius in the same upper limb, the differentiating one on the radial side and the primitive one on the ulnar side. Such anomalies were seen in 3 out of 13 extensor pollicis et indicis accessonus (Nos ), and 5 out of 34 extensor indicis radialis (Nos ). Cases similar to the author's Nos (Fig. 4) have been reported by Kaneff (1959c) and Aziz and Dunlap (1986). Although Kaneff (1959c) noted that this arrangement of the extensor digitorum profundus mass was very rare, it was present in 3 out of 952 upper limbs (0.3%) in the current series. These cases in man indicate that the differentiating muscle slip of the radial segment and the primitive one of the ulnar segment of the second part coexist in the same upper limb. As the human thumb and index finger are developing in the extensor moving, the radial segment of the second part may have a tendency to divide into small muscle slips. The manual group (proper extensor muscle of hand) is not present in almost all mammals. But in man the extensor digiti brevis manus muscle appears in the dorsum of some hands. The extensor digiti brevis manus muscle was present in 10 out of 20 (50.0%) human upper limbs in which the extensor indicis proprius muscle did not occur, only in 5 out

6 Figs. 360 Y. Yoshida of 534 (0.9%) upper limbs in which that muscle occurred (Yoshida et al., 1984). So it seems that the extensor digiti brevis manus muscle compensates for the extensor indicis proprius muscle in man. Additionally it and the extensor indicis proprius were always innervated by the dorsal interosseus nerve of the radial nerve. The extensor digiti brevis manus muscle has a single tendon to the index finger in many cases, and rarely to the middle finger. And the single tendon may be fused with the extensor indicis proprius tendon. Cauldwell et al. (1943) described a case of the extensor digiti brevis manus muscle which had a double tendon to the index and middle fingers. The author has never seen such a case, but Yoshida et al. (1984) observed a case of the extensor digiti brevis muscle only to the middle finger in company with the extensor indicis proprius and extensor medii proprius muscles. The extensor digiti brevis manus muscle of man possibly represents the atavistic element of the second part of the extensor digitorum profundus which has not ascended into the forearm. In the case of No. 13 (Fig. 5) the differentiating extensor pollicis et indicis accessorius and the primitive extensor digitorum brevis manus were coexistent on both sides of the extensor indicis proprius. In comparing the incidence of these anomalous muscles in males and females, and the right and left limbs, a significant difference was not clear in this study. Therefore, I am not discussing it. The extensor pollicis et indicis accessorius and the extensor indicis radialis were innervated by the dorsal interosseus nerve of the radial nerve. The author was unable to actually follow it, so did not mention it. References 1) Aziz MA and Dunlap SS. The human extensor digitorum profundus muscle with comments on the evolution of the primate hand. Primates 1986; 27: ) Cauldwell EW, Anson BJ and Wright RR. The extensor indicis proprius muscle. A study of 263 consecutive specimens. Q Bull NWest Univ Med Sch 1943; 17: ) Kaneff A. Beitrag zur Morphologie und Ausbildung des M. ext. digitorum lateralis bei einigen fiinffingerigen Saugern (Felis domestica, Canis familiaris, Orictolagus cuniculus und Rattus rattus). Gegenbaurs Morphol Jb 1959a; 100: ) Kaneff A. Beitrag zur Morphologie des M. extensor digitorum profundus bei manchen fiinffingerigen Saugern (Felis domestica, Canis familiaris, Orictolagus cuniculus und Rattus rattus). Gegenbaurs Morphol Jb 1959b; 100: ) Kaneff A. Ober eine sehr seltene Kombination von Varietaten des tiefen Fingerstreckers, M. extensor digitorum profundus, beim Menschen Anat Anz 1959c; 107: ) Kaneff A and Cihak R. Die Umbildung des M. extensor digitorum lateralis in der Phylogenese und in der menschlichen Ontogenese. Acta Anat 1970; 77: ) Kosugi K, Meguri S, Fukushima 0 and Koda M. Anatomical study on the variation of the extensor muscles. 1. M. extensor indicis. Tokyo Jikeikai Med J 1984; 99: (in Japanese) 8) Ribbing L. Die Muskeln und Nerven der Extremitaten. In Bolk L et al. (eds.) Handb vergleich Anat Wirbeltiere Bd. 5, 1938: , Urban, Berlin. 9) Straus WL. The phylogeny of the human forearm extensors. Hum Biol 1941; 13: ) Yoshida Y. Studies on the extensor indicis muscle in man. Acta Anat Nippon 1978; 53:1-15. (in Japanese) 11) Yoshida Y. A study on the extensor digiti minimi muscle in man. Acta Anat Nippon 1985; 60: (in Japanese) 12) Yoshida Y. Anatomical study on the extensor digitorum profundus muscle in the Japanese. Okajimas Folia Anat Jpn 1990; 66: ) Yoshida Y and Fukuyama U. Phylogeny of forearm extensor muscles in tetrapods. J Kanazawa Med Univ 1979; 4: (in Japanese) 14) Yoshida Y, Yasutaka S and Seki Y. A study on the extensor digitorum brevis manus muscle in man. Acta Anat Nippon 1984; 59: (in Japanese) Explanation of Figures Abbreviations: A, Extensor pollicis longus proprius; B, Extensor pollicis et indicis accessorius; C, Extensor indicis radialis; D, Extensor indicis proprius; E, Extensor indicis et medii accessorius; F, Extensor medii proprius; G, Extensor carpi profundus; H, Extensor digiti brevis manus; I, thumb; II, index finger; III, middle finger. Plate I 2-5. Both the extensor pollicis longus proprius and extensor indicis proprius are seen in each case. Fig. 2. No. 1 on the right side. The extensor pollicis et indicis accessorius is present between the extensor pollicis longus proprius and extensor indicis proprius. Its tendon is bifurcating to supply the thumb and index finger (arrow). Fig. 3. No. 5 on the right side. The extensor pollicis et indicis accessorius is seen. The radial compartment of the tendon is fused with the tendon of the extensor pollicis longus proprius (arrow). Fig. 4. No. 10 on the right side. Two additional muscle slips of the extensor digitorum profundus, the extensor pollicis et indicis accessorius and extensor medii proprius, occur in the upper limb. Fig. 5. No. 13 on the right side. The extensor pollicis et indicis accessorius and extensor digiti brevis manus arc present in the upper limb. The extensor digiti brevis manus is fused with the extensor indicis proprius at the distal portion (arrow).

7 Extensor Pollicis et Indicis Accessorius Muscle and Extensor Indicis Radialis Muscle in Japahese 361 Plate 1

8 362 Y. Yoshida Plate II Figs Both the extensor pollicis longus proprius and extensor indicis proprius are seen in each case. Fig. 6. No. 14 on the right side. The extensor indicis radialis is situated radial to the extensor indicis proprius. It is inserted on the lateral side of the dorsum of the index finger. Fig. 7. No. 43 on the right side. Two additional muscle slips of the extensor digitorum profundus, the extensor indicis radialis and extensor medii proprius, occur in the upper limb. Fig. 8. Fig. 9. No. 45 on the left side. The extensor indicis radialis and extensor carpi profundus are seen in the upper limb. The latter muscle is situated ulnar and distal to the extensor indicis proprius and is inserted on the carpus adjacent to the fourth metacarpus. No. 47 on the left side. Three anomalous muscles, the extensor indicis radialis (small), extensor indicis et medii accessorius and extensor medii proprius, are present in the upper limb. The tendon of the extensor indicis et medii accessorius becomes membranous at the distal portion to supply between the medial side of the index finger and the lateral side of the middle finger (arrow).

9 Extensor Pollicis et Indicis Accessorius Muscle and Extensor Indicis Radialis Muscle in Japanese 363 Plate 11