Okajimas Folia Anat. Jpn. , 73(2-3): 139-142, August, 1996

The Accessory Tendon of the Extensor Indicis Muscle

By

S. CAVDAR and U. SEHIRLI

Marmara University, Faculty of Medicine, Department of Anatomy, Haydarpasa 81326, Istanbul-Turkiye

-Received for Publication, August 9, 1995-

Key Words: Accessory tendon, Extensor Indicis Muscle, Variation

Summary: The extensor indicis and the extensor pollicis longus muscles differentiates from the extensor digitorum profundus muscle. The extensor indicis musde is an unstable muscle concerning its variations. Kosugi (1989) found the frequency of variations of this muscle to be 20% and described 18 different types of variations of this muscle. This study describes a rare case of the extensor indicis muscle. The extensor indicis muscle develops an accessory tendon in between the extensor indicis and extensor pollicis longus muscle. It passes under the extensor retinaculum. At the level of 2nd metacarpal bone, the accessory extensor indicis tendon is connected to the tendon of the extensor pollicis longus muscle by a intertendinous connection.

The extensor indicis (proprius) is a narrow quadrupeds have been classified by Straus (1941) elongated muscle located in the deep extensor group into three groups. of the . It arises from the posterior surface of 1. Brachio-antebrachial group the distal to the extensor pollicis longus and 2. Antebrachio-manual group from the interosseus membrane. Its tendon passes 3. Manual group under the extensor retinaculum in company with the The brachio-antebrachial group basically takes its tendons of extensor digitorum communis. At the origin from the dorsal aspect of the brachial bone level of the it joins the ulnar and inserts onto the antebrachial bones. It acts mainly side of the tendon of extensor digitorum (Williams, on the elbow joint. The antebrachio-manual group 1989). basically takes its origin from the dorsal aspects of Cauldwell, Anson and Wright (1943), in a study the antebrachial bones (including the interosseus of the extensor indicis muscle in 263 consecutive membrane) and inserts onto the manual bones. It specimens, found three cases in which the muscles acts mainly on the joint. The manual group had an abnormal origin. In the first case, the muscle takes its origin from the dorsal aspect of the carpus had the usual origin from the ulna, became tendinous, and/or metacarpus and inserts on the digits (thumb and then again became muscular with a secondary and fingers). It acts mainly on the joints of the attachment to the region of the . In the thumb and fingers. second case, a muscle with a rudimentary origin at With the evolution of species, the brachia. the normal site was inserted into a second muscle antebrachial group has extended its insertion to the arising from two heads from the proximal carpal distal portion of the . On the other , bones. In the third case, a short muscle was only the origin of the antebrachio-manual group has present, arising from the distal end of the , undergone proximal migration during the phylo- proximal carpal bones and related ligaments. This genetic course (Ribbing, 1938; Straus, 1941). The muscle was refered to as "extensor digiti manus brachio-antebrachial group has been referred to as brevis". Ogura (1987) considered the extensor digi- the superficial layer of the forearm extensor muscu- torum manus to be a variant of the extensor indicis lature, and the antebrachio-manual group as its deep muscle due to its common arterial and supply. layer in mammals. Yoshida (1990) cited in 11 out of 22 upper limbs The developmental relations of the extensor which did not have the extensor indicis proprius, in muscles were classified phylogenetically by Straus stead the extensor digiti brevis was present. (1941) and modified by Kosugi et al. , 1989 (Table 1). Kuwabata (1959)reported a case in which the ex- In the mammals, the extensor indicis proprius and tensor indicis muscle was absent. the extensor pollicis longus differentiate from the ex- The extensor muscles of the forearm and hand in tensor digitorum profundus muscle phylogenetically .

139 140 S. cavdar and U. Sehirli

Table 1. The developmental relations of the extensor muscles from the point of phylogeny. This table was classified by Straus (1941) and modified by Kosugi (1989)

The extensor pollicis longus is the lateral compo- Kosugi (1984) studied 375 cadaveric upper limbs nent of the extensor digitorum profundus and is and found 5 different types of variations, each type referred to as the first part. The medial component having subgroups, according to the insertion of the of the extensor digitorum profundus is the extensor tendon (Figure 1). Yoshida (1990) found 12 vari- indicis proprius and is referred to as the second part ations of the extensor indicis proprius muscle. by Straus (1941). Both Yoshida's (1990) 832 and A rare variation of the extensor indicis was ob- Kosugi's (1989) 516 series of upper limbs exhibited a served in one of our cadavers which have not been marked stability in the first part, while the second reported in literature. Detailed 'description of this part appeared to be highly unstable. rare case, the possible function of the accessory The most commonvariations of the extensor indicis tendon and further, phylogenetic development of muscle are accessory slips, accessory tendons or the muscle are discussed. other anomalies (Kosugi, 1984; Yoshida, 1990).

Materials and Methods

The skin and the subcutaneous fat was removed from the extensor aspect of the forearm and the dorsum of the hand of a 54 year old formaldehyde fixed cadaver. An accessory tendon of the extensor indicis muscle was revealed by fine dissection.

Results

The extensor indicismuscle developedan accessory tendon in between extensor indicis and extensor pollicis longus muscles 5 cm above the extensor retinaculum. The accessory tendon did not contain a seperate muscle. At the lower aspect of the extensor Fig. 1. A picture taken from the right hand of a 54 year old forearm the accessory tendon passed under the cadavers, showing an accessory tendon of the extensor extensor retinaculum, lateral to the main tendon of indicis muscle which is attached to the tendon of the the extensor indicis and medial to the main tendon extensor pollicis longus by a intertendinous connection. Abbreviations; ED: extensor digitorum, AEI: accessory extensor of the extensor pollicis longus muscles. At the level indicis, EPL: extensor pollicis longus. of second metacarpal bone the accessory extensor Accessory Tendon 141

Fig. 2. Schematic illustration showing the classification of the variations of the extensor indicis muscle by Kosuri (19891. indicis tendon was connected to the ulnar side of the Discussion extensor pollicis longus muscle by way of a inter- tendinous connection. The accessory tendon con- The extensor indicis and extensor pollicis longus tinued downwards and inserted to the ulnar side of develop from the extensor digitorum profundus the distal portion of the second metacarpal bone and muscle in man (Straus, 1941). Yoshida (1990), de- to the base of the proximal phalanx (Figure 1). scribed 11 cases out of 832 which the cleavage between The main tendon of the extensor indicis muscle the extensor pollicis longus and extensor indicis was normal anatomically. proprius did not exist and the fusion of the two 142 S. Cavdar and U. Sehirli muscles were observed. connection between the tendons of these two muscles Kosugi (1989) described anomaly as the past may affect the independent functions of the muscles. form (reversion) or the future form on the way to This can be restrictive to people who use fine vol- differentiation. untary finger movements such as pianists. Kosugi (1989)divided the extensors of the forearm This study shows one of the variations of the into 2 groups. The first group possessed many vari- on going differentiation procedure of the extensor ations which are referred as muscles on the way indicis muscle. to differentiation (extensor indicis, extensor carpi The knowledge of the variations of the region can radialis longus and brevis, and extensor digiti minimi) be helpful in the success of the surgery of the hand. and the second group having not so many variations, in other words stable differentiated muscles(extensor pollicis longus and extensor carpi ulnaris). References The variations of the extensor indicis muscles are quite common. Papers have been published on the 1) Yoshida Y. Anatomical study on the extensor digitorum extensor musculature of the forearm and hand. profundus muscle in the Japanese. Okajimas Folia Anat Jpn 1990; 66:339-354. However, the variation described in this case was 2) Kosugi K, Shibata S and Kamashita H. Anatomical study on not reported in the literature. the variation of the extensor muscle of human forearm the The intertendinous connections are bands located relation between differentiation and variation. Tokyo Jikeikai on the dorsum of the hand between adjacent tendons Med J 1989; 36:93-111. of the extensor digitorum muscle. They are thought 3) Jones BV. An anomalous extensor indicis muscle. J. Bone and Joint Surg. vol 41-B: 763-765, 1959. to affect independent extension of the fingers 4) Lewke J. Zur funktionellen Bedeutung der connexus inter- (Williams, 1989). An intertendinous connection as tendinei (Juncture tendinum). Arch Orthop Unfallchirurgie in this case between the extensor pollicisand extensor 1960; 52:122-124. indicishas not been described in the literature. Lewke 5) Williams PL, Warwick R, Dyson M and Bannister LH, Gray's Anatomy. 37th ed. 622-625. Churchill Livingstone, (1960) states the function of the intertendinous con- New York 1989. nections to play an essential role in the 3rd and 4th 6) Claudwell EW, Anson BJ and Wright RR. The extensor digits since they lack proper extensor tendons. For indicis propius muscle. A study of 263 consecutive specimens. example, stretching of the extensor in the course of Q Bull Northwest Univ Med Sch 1943; 17:267. flexion of the 3rd digit would inhibit any voluntary 7) Straus WL. The phylogeny of the human forearm extensors. extension of the 4th digit. However, the intertendin- Hum Biol 1941; 13:203-238. 8) Ribbing L. Die Muskuln und Nerven der Extremitaten. In ous connections moderate this stretching, thereby Bolk, L et al (eds) Handb. vergleich. Anat. Wirbeltiere. Bd. allowing to maintain a certain degree of mobility of 5, 543-656, Urban, Berlin, 1938. the unflexed finger. The index and the thumb are the 9) Ogura T, Inoue H and Tanabe G. Anatomical and clinical two fingers which can produce fine movements due studies of the extensor digitorum brevis. J Hand Surg 1987; to the extra muscles devoted to these two Angers. A 12-A:100-107.