Structure and function of the abductor pollicis longus muscle

J. Anat. (1991), 174, pp. 221-227 221 With 5 figures Printed in Great Britain Structure and function of the abductor pollicis longus muscle ELVIRE VAN OUDENAARDE* University of Nijmegen, Department of Anatomy and Embryology, The Netherlands (Accepted 2 August 1990) INTRODUCTION The shape of the joint surfaces and the arrangement of the ligamentous system of the carpometacarpal joint of the thumb (CMC I) can explain the great mobility of the thumb in relation to the hand. The opposing facets of the trapezium and the first metacarpal (MC I) are incongruent in all positions of the thumb and in a bone-ligament specimen it can be seen that the ligaments themselves are insufficient to ensure the stability of the joint as is described by Batmanabane (1982), Bojsen- Moller (1976), Cooney, Lucca, Chao & Lindscheid (1981), Kaplan & Spinner (1984), Kuczynski (1974, 1983), Pieron (1973), Zancolli, Ziddenberg & Zancolli (1987). It is a general principle that the most important factors for the stability of the mobile joints are the muscles and tendons passing over such a joint. For the proper functioning of the thumb, as it is used in precision and power grips and in pushing movements, it is necessary to ensure good stability. We have analysed the abductor pollicis longus muscle (APL) because of its multiple insertions around the CMC I joint, with separate tendons to the bones of the joint. Remarkable is the great variety in the number of tendons and the varied sites of insertion, as has been described by Baba (1954), Chopra, Singh & Sing Makhni (1958), Coleman, Afee & Anson (1953), Lacey, Goldstein & Tobin (1951), Loomis (1951), Rayan (1989) and Stein (1951). The tendons can be grouped into a superficial and a deep group (Kauer, 1987) with possible separate functioning of the various divisions. There has been no systematic description of the structure of the muscle or the relationship between the tendons and the muscle belly. With this in mind we have dissected the whole muscle and analysed a series of embryonic transverse histological sections to search for indications of possible different functions of the various parts of the muscle and for the investigation of their stabilising functions. MATERIAL AND METHODS The anatomy of the APL A series of 84 formaldehyde-fixed forearms was dissected, 22 of them with the aid of a dissecting microscope. The relationship was analysed between the tendons of the APL and the CMC I joint on one side and, on the other, between the muscle bellies and the tendons. Histological studies Serial transverse and frontally sectioned specimens of 5 mid-term fetal hands (110-140 mm crown-rump length, 13-18 weeks) were studied to investigate the * Reprint requests to Mrs Drs E. v. Oudenaarde, Weezenhof 62-37, 6536 AP Nijmegen, The Netherlands

222 ELVIRE VAN OUDENAARDE 2 Fig. 1. The muscle belly of the APL. Drawing of cadaver specimen, showing the divisions of the APL. The superficial and ulnar parts pulled aside (arrows) to show the deep part between them. 1, deep part; 2, ulnar superficial part; 3, radial superficial part. Table 1. Sites of insertion of the APL tendons Number of specimens: 84 Site of insertion Number of specimens Division I (Deep) Trapezium 51 Abductor pollicis brevis (or its fascia) 62 Opponens pollicis 8 Capsule 32 Anterior oblique ligament 7 Division II (Superficial) First metacarpal by: one tendon 44 two tendons 14 multiple tendons 26 tendons and three of these series were used to study the whole muscle from origin to insertion. RESULTS The APL can be divided into a superficial and a deep head, each terminating in one or more tendons. The muscle belly consists fundamentally of three parts (Fig. 1). In spite of the great variability in their arrangements, a clear basic pattern can be identified. The three parts result in two divisions of tendons as follows. The first part is more proximally situated and originates from the ulna, the interosseous membrane and the radius and is covered by the extensor digitorum longus. This is the deep part with numerous subdivisions of the belly, all parallel to one another. They all terminate in a central tendon. The fibres are short, obliquely attached to the tendon in a pennate fashion and close together. After its passage through the first compartment of the extensor retinaculum, the deep tendon separates into several branches and is inserted

Abductor pollicis longus 223 Fig. 2. Bursa. A synovial bursa is visible between the superficial and deep tendons. 1, superficial tendon; 2, deep tendon; 3, cavity of bursa. Fig. 3. Synovial layer. Between the superficial and deep tendons can be seen a synovial web. 1, superficial tendon; 2, deep tendon; 3, synovial web. into the trapezium, the APB and other sites as shown in Table 1. The other two parts are superficial. One of them originates from the ulna and interosseous membrane, the other from the deep fascia of the forearm and the radius. These two parts are connected to each other superficial to the deep part by a small area of loose connective

224 ELVIRE VAN OUDENAARDE Fig. 4. The APL in a transverse section of a fetal hand in the distal part of the radius. The arrow indicates the small area of loose connective tissue. The black areas represent the oval deep tendon and the drawn out superficial tendon. 1, deep tendon; 2, superficial tendon. 11% W" mow.w 10.1A* AI q.-w -11. N. 4FL, 1-- Fig. 5. Bursa in a front section of a fetal hand. X, bursa between the tendons; R, radius and proximal end; D, distal end. tissue and they terminate in a tendon to MC I. This tendon is broad and flat and often divides into multiple smaller tendons (Table 1). In this part of the muscle, in contrast to the deep part, the fibres are longer, lying in a thin layer and are arranged in a more parallel manner. As a rule there are no connections between the fibres of the deep and

Table 2. Abductor pollicis longus Number of tendons of the APL Number of specimens: 84 Number of tendons Number of specimens 225 1 6 2 20 3 29 4 17 5 10 6 2 the superficial divisions. Within the ulnar and radial superficial heads are myo-fibre bridges between the muscle fibre bundles and connective tissue fibres between the tendons. There is often a large bursa, originating from, and communicating with, the synovial part of the tendon sheath, between the superficial and deep tendons (Fig. 2). As well as the bursa, a synovial layer extends between and under the tendons proximal to the bursa and directly distal to the first compartment of the extensor retinaculum (Fig. 3). The findings by dissection were confirmed by histological analysis of the fetal material (Fig. 4). The APL was traced on transverse sections from origin to insertion. A short distance from the origin, the separation of the superficial and the deep parts can be identified and this remains permanently visible throughout the course of the muscle from proximal to distal. At the level of the distal quarter of the radius, the proximal division of the muscle continues as a tendon. A layer of loose connective tissue is visible between the radial and ulnar parts before the formation of the tendon. The bursa is well seen in frontal sections (Fig. 5). Anatomical variations in the shapes of the muscle are occasionally seen. In six cases we found only one tendon for the whole muscle, inserting into MC I (Table 2). In these cases the three origins were present, but they were all inserted into the central tendon, starting proximally from the deep part of the muscle. In these cases there were connections between the tendon sheath and the capsular ligaments. Sometimes we found an early separate belly on the radial side. This detached itself from the proximal deep part and became superficial at the same level as the whole muscle. Then it passed deep to the other tendons and was inserted into the trapezium or the APB. DISCUSSION The structure of the APL has been studied because of the importance of its stabilising function in relation to the CMC I joint. The APL separates into a deep and a superficial head, each of which terminates in one or more tendons. The parts of the muscle are interwoven, but as a rule without connections between muscle fibres or tendon fibres of the divisions. A great deal is known about the tendons but very little about the muscle bellies of the APL. The great variability in the arrangement of the tendons and in the sites of insertion of the APL is mentioned by Baba (1954), Chopra et al. (1958), Coleman et al. (1953), Kauer (1987), Lacey et al. (1951), Loomis (1951), Rayan (1989) and Stein (1951). The most important statements about the muscle are as follows: "It may be stated that it is exceptional to find a single tendon or insertion of this muscle " (Coleman

226 ELVIRE VAN OUDENAARDE et al. 1953). It is shown in Tables I and 2 that we have found mostly more than one tendon. We confirm the notion of Pieron (1973), that if there is only one tendon, then there is a connection between the tendon sheath and the capsule as well as the ligaments. " If accessory tendons were present they were joined to the main tendon for a short distance by a web of synovium" (Stein, 1951). We found the same web, as well as a bursa between the tendons. This bursa facilitates gliding between the tendons. Concerning the muscle bellies there is little further to be discussed. There is only a short comment by Williams & Warwick (1975), Kaneff (1969) and Lacey et al. (1951). "The APL has two bellies, each of which terminates in a tendon" (Lacey et al. 1951). Warwick & Williams (1975) state that occasionally the whole muscle is divided into two. Kaneff (1969) describes an extra muscle, 'the abductor manus' (as he calls it), on the radial side of the APL with a separate insertion. This is equivalent to the radial part of the superficial head of the APL. In all our comparable cases the ulnar part is attached to the extensor pollicis brevis muscle (EPB). As regards a differentiated function for the divisions of the APL the following observations are relevant. There are multiple tendons with different places ofinsertion, in two layers, and with a bursa between the tendons. The muscle bellies are separated into a superficial and a deep division without connections between the two divisions. There is a marked difference in muscle structure between the two divisions. The superficial part has relatively long, parallel fibres arranged in a thin layer. This suggests the possibility of considerable shortening, but not a great deal of strength. The fibres of the deep part are short, obliquely arranged in a pennate fashion and are packed closely together. The deep division is therefore arranged for strength rather than for range of shortening. Concerning the contractions of the APL in thumb movements there are two possibilities. Firstly the superficial part contracts and at the same time the deep part is relaxed. This deep part probably functions, for example, only in movements of the wrist. Secondly both parts are active during all the thumb motions, for example the superficial head with a concentric contraction and the deep head with an isometric contraction. A consideration of the evidence for a differentiated function of the divisions mentioned above leads us to believe that the second possibility is the most probable. It is not plausible that the deep division, with the greater number of fibres, will be inactive during thumb movements. For the confirmation of isometric contraction an EMG study is necessary. EMG activity is present in all movements of the thumb (Cooney, An, Daube & Askew, 1985; Ebskov, 1970; Weatherby, Sutton & Krusen, 1963), but the APL is only described as a whole in these reports. No EMG study of the individual parts of the muscle has been carried out. From the structure, the arrangement of the fibres and the sites of insertion it may be expected that the deep division of the APL is very important in order to support the trapezium as a platform upon which the first metacarpal moves. The superficial division will be active in moving MC I. SUMMARY The abductor pollicis longus muscle was examined in dissections and histologically to study the insertions around the CMC I joint. The APL consists fundamentally of a superficial and a deep division, both terminating in one or more tendons. The deep division is proximally situated, it is covered by the extensor digitorum muscle and consists of several muscle bellies; it terminates in a central tendon. The fibres are short, obliquely attached to the tendon in a pennate manner and close together. After the

Abductor pollicis longus 227 passage through the extensor retinaculum the tendon separates into many branches. The superficial division is more distally situated, not covered by other muscles, lying superficial to the tendon of the deep part. The fibres are long, parallel to one another and form a thin layer. The tendon passes, together with the deep division, through the same compartment of the extensor retinaculum and inserts into MC I. If the muscle contracts, then the structures around the CMC I joint will be tensed by the deep division and MC I will be affected by the superficial division. It is to be expected that in the appropriate thumb movements the superficial part will show an isotonic contraction and the deep part, an isometric action. The superficial part, with long thin fibres, presumably has the least strength while the deep part, with its larger number of fibres, is the most powerful. The functional analysis gives the impression that the deep head will mainly support the trapezium as a platform upon which MC I moves. The superficial head will be active in moving MC I. I wish to thank Professor J. M. G. Kauer M.D., Ph.D., Department of Anatomy and Embryology, University of Nijmegen, The Netherlands, for stimulating this project, for his contribution in supplying some material and for the hospitality of his laboratory of anatomy. REFERENCES BABA, M. A. (1954). The accessory tendon of the abductor pollicis longus muscle. Anatomical Record 119, 541-546. BATMANABANE, M. (1982). Whether mobility influences the osteometric features of the articular end of the metacarpal bones. Acta morphologica neerlandica scandinavica 20, 111-115. BOJSEN-M6LLER, F. (1976). Osteoligamentous guidance of the movements of the human thumb. American Journal of Anatomy 147, 71-79. CHOPRA, R. P., SINGH, S. & SING MAKHNI, S. (1958). The insertion of the abductor pollicis longus and extensor pollicis brevis. Journal of the Anatomical Society of India 7(2), 105-109. COLEMAN, S. S., AFEE, D. K. & ANSON, B. J. (1953). The insertion of the abductor pollicis longus muscle. Quarterly Bulletin of Northwestern University Medical School 27, 117-122. COONEY, W. P., LUCCA, M. J., CHAo, E. Y. S. & LINDSCHEID, R. L. (1981). The kinesiology of the thumb trapeziometacarpal joint. Journal of Bone and Joint Surgery 63A(9), 1371-1381. COONEY, W. P., AN, K. N., DAUBE, J. R. & AsKEw, L. J. (1985). Electromyographic analysis of the thumb; a study of isometric forces in pinch and grasp. Journal of Hand Surgery 10(2), 202-210. EBSKOV, B. (1970). De motibus motoribusque pollicis humani. Thesis, University of Copenhagen, Denmark. KANEFF, A. VON (1969). Sehr seltene Kombination von Muskelvarietaten der radialen und dorsalen Muskelgruppen eines menschlichen Vorderarmes. Anatomischer Anzeiger 125, 535-538. KAPLAN, E. B. & SPINNER, M. (1984). Kaplan's Functional and Surgical Anatomy of the Hand. New York: Lippincott. KAUER, J. M. G. (1987). Functional anatomy of the carpometacarpal joint of the thumb. Clinical Orthopaedics and Related Research 220, 7-13. KUCZYNSKI, K. (1974). Carpometacarpal joint of the human thumb. Journal of Anatomy 118, 119-126. KUCZYNSKI, K. (1983). Configuration of the articular surfaces of the CMC joint of the thumb. Handchirurgie 15, 191-193. LACEY II, T., GOLDSTEIN, L. A. & TOBIN, C. E. (1951). Anatomical and clinical study of the variations in the insertions of the abductor pollicis longus tendon, associated with stenosing tendovaginitis. Journal of Bone and Joint Surgery 33A(2), 347-350. LOOMIs, L. K. (1951). Variations of stenosing tenosynovitis at the radial styloid process. Journal of Bone and Joint Surgery 33A(2), 340-346. PIERON, A. P. (1973). The mechanism of the first CMC joint. Thesis, Leiden, The Netherlands. RAYAN, G. M. J. (1989). Anomalous abductor pollicis longus insertion in the thenar muscles. Journal ofhand Surgery 14A, 550-552. STEIN, H. (1951). Variations of the tendons of insertion of the abductor pollicis longus and the extensor pollicis brevis. Anatomical Record 110, 49-53. WARWICK, R. & WILLIAMS, P. L. (ed.) (1975). Gray's Anatomy, 35th ed. Edinburgh: Longman Group Ltd. WEATHERBY, H. T., SUTTON, L. R. & KRUSEN, U. L. (1963). The kinesiology of muscles of the thumb, an EMG study. Archives of Physical Medicine and Rehabilitation 44, 321-326. ZANCOLLI, E. A., ZIDDENBERG, L. & ZANCOLLI JR, E. (1987). Biomechanics of the trapeziometacarpal joint. Clinical Orthopaedics and Related Research 220, 14-26.