THE ANATOMICAL RECORD 250:475–479 (1998) Biomechanical Stability of Abductor Pollicis Longus Muscles With Variable Numbers of Tendinous Insertions MAHMOUD MELLING,1* ROLAND REIHSNER,2 MARTIN STEINDL,3 DANIELA KARIMIAN-TEHERANI,1 MARTINA SCHNALLINGER,1 AND MARK BEHNAM1 1Department of Anatomy I, University of Vienna, Vienna, Austria 2Ludwig Boltzmann Institute for Experimental Plastic Surgery, Vienna, Austria 3Martin Steindl, Department of Orthopedics, Orthopädisches Krankenhaus der Stadt Wien-Gersthof, Vienna, Austria ABSTRACT Background: In the course of a study of different variants of the tendon of the abductor pollicis longus (APL) muscle, the unusual finding of a tendon with six subdivisions was observed in the first compartment. Using this preparation and others with various numbers of tendons (2, 3, 4, and 6), we intended to establish whether the multitendoned muscles increased the strength of the thumb. Methods: Fifty-two upper extremities were examined with attention to the tendons of the APL muscle. The strain on each tendon was biomechanically determined using a microcomputer and potentiometer. The stress was measured continuously and plotted against strain. Results: Following treatment with elastase, it is seen that the significance of changes in stability and Young’s modulus is inversely proportional to the number of tendons arising from the APL. Conclusions: Division of the tendon into several parts leads to better mechanical distribution of stress and extension on thumb abduction. In addition, palmar-flexion in the wrist may be supported. Anat. Rec. 250:475– 479, 1998. r 1998 Wiley-Liss, Inc. Key words: abductor pollicis longus muscle tendon; pancreas elastase; stress extension relationship; Young’s modulus; transplant material Mainly clinical interests, and in particular surgical ones, led to the present investigations. Repeated observations of supernumerary abductor pollicis longus (APL) muscle tendons in the first compartment and of variants in the forearm opened questions about the frequency of such anomalies. Meckel (1832), e.g., described the division of the tendon of the APL muscle into three tendon slips and Schmidt (1987), its division into four. This study is a detailed report of a form of this anomaly that has not yet been described in the literature. The importance of these observations cannot be assessed without some idea of the frequency of such anomalies. MATERIALS AND METHODS In all, 52 upper extremities were prepared, taken from cadavers of persons who had died at ages ranging from 78 to 93 years. All were obtained from the dissection material held by the First School of Anatomy, University of Vienna. Formol-carbol fixation had already been performed on 46 of the cadavers, whereas six were in a nonfixed, i.e., fresh condition. Great care was taken to prepare only material on which there were no surgical scars. r 1998 WILEY-LISS, INC. The machine as well as the measuring device and control circuit is shown in Figure 3. All specimens were loaded at a constant strain rate of 10% per minute, achieved by a thyristor controlled gear-box (0.04–1 RPM). During the tests the specimens (original length for all specimens: 10 mm) were kept in a bath of phosphate-buffered saline at room temperature. Small sheets of abrasive paper were fixed to the jaws of the clamps to prevent the samples from slipping out. The strain was measured using a potentiometer (range: 100 mm; sensitivity: 0.025 mm). Because of the very small cross-sectional area, we developed a load cell for small loads (0.005–100N). Length and load were recorded by a digital-multimeter (DMM). Strain was calculated by the ratio: increase in length/original length and stored together with the measured load. The control unit was adjusted by a microcompuer (Hewlett-Packard HP 86). The stress was measured continuously and plotted against the strain. The curve for stress plotted against extension allows determination of the stability (max, maximal load capac- *Correspondence to: Dr. M. Melling, Haslingergasse 29/1/12, A-1170 Vienna, Austria. Received 8 July 1997; Accepted 11 November 1997 476 M. MELLING ET AL. Fig. 1. Schematic illustrating preparation of the tendon of the APL muscle of a fresh (nonfixed) female left hand. 1. APL tendons marked by 3 arrows; 2. main tendon of the APL muscle; 3. APB muscle was detached at the origin to reveal the supernumerary tendon of the APL muscle. ity), maximal extension (the extension corresponding to the stability, emax) and the rigidity (Emax, tangents at maximum slope of the stress-extension curve). In addition, the extension was determined where the onset point of the linear portion of the stress-extension curve begins (eon). To obtain characteristic values for the material that are independent of the cross section, it is necessary to standardize on the primary cross section. To this end, the dry weight per unit of length was determined. This represents the collagen content in the cross-sectional area, which in turn represents the loadbearing portion (Weinans et al., 1992). If the stability and rigidity are now divided by the dry weight per unit of length, the respective quotients are max and Emax; that is to say, Youngs’s modulus in the steepest section of the stress-extension relationship. To establish the influence of the elastic fibers, which admittedly have a much smaller effect on the stability than do the collagen fibers, the preparations were divided into two groups: those in one group were left untreated, wheras the preparations in the other group were digested with pancreas elastase (Sigma 1250, St. Louis, MO). This involved treating 10 mg of tissue in 1.0 ml phosphatebuffered saline with 10 U elastase (3 h at 37°C). The elastase activity was 80 U/mg protein and 1.7 U/mg trypsin (Fyhrie and Schaffler, 1995). On the basis of an earlier study, no trypsin inhibitor was used, as from the mechanical aspect no differences were seen between preparations with and without use of the inhibitor (Fyhrie and Schaffler, 1995). RESULTS The variant of the tendon of the APL muscle (Fig. 1) described here was observed during preparation of a fresh (nonfixed) left arm taken from the cadaver of a woman who died at the age of 83 years. The tendon of this well-developed APL muscle, which itself followed a normal course, was divided ,2.7 cm proximal to the first metacarpal bone into six parts; some of these were round in section and others flattened, and their individual thickness varied from 1.7-4.2 mm. All the accessory tendons were to the ulnar side of the main tendon, but with it in the common tendon sheath. Precise separation of the accessory tendons was only present in the distal segment of the tendon ,3.2 cm before the insertion point; in contrast, there was no clear differentiation in the proximal segment. The accessory tendons were quite thick in parts and the same length and thickness as the main tendon in other parts. 477 ABDUCTOR POLLICIS LONGUS MUSCLES TABLE 1. Number of supernumerary tendons of the APL muscle in the upper extremities Male Female Total Right Left No. of tendons 3 5 4 7 — — 19 5 6 7 11 3 1 33 2 3 2 3 4 6 52 TABLE 2. Mechanical properties of preparations with the tendon of the APL muscle split into 2, 3, 4, and 6 slips [emax elasticity in %, eon onset of linear portions of stressstrain relations in %, smax stability (Nm/g), Emax (Nm/g) maximal Young’s modulus in the linear portion of the curve] No. emax eon smax Emax 1a 1b 2a 2b 2c 3a 3b 3c 3d 4a 4b 4c 4d 5a 5b 6a 6b 6c 6d 6e 6f 9.5 10.0 9.8 10.3 8.8 12.5 11.8 8.0 6.0 13.8 9.8 10.0 12.5 11.3 13.3 10.5 11.3 9.8 10.8 9.0 9.5 1.00 0.75 0.50 0.50 0.50 0.75 0.50 1.00 1.00 0.75 0.50 0.50 0.50 2.00 1.00 1.00 1.25 1.00 1.00 0.75 1.00 13.2 25.2 10.1 28.6 38.3 9.40 27.1 9.58 10.1 27.9 55.4 24.9 17.6 50.0 14.2 12.7 13.0 15.7 11.1 22.8 28.6 166.2 511.0 692.4 655.8 533.3 126.6 274.2 151.5 215.9 315.7 641.0 302.3 141.2 538.2 127.0 186.5 195.9 247.3 181.2 385.8 461.2 Of the six divisions of the split abductor tendon (described in order from palmar to dorsal), one inserted in the radial margin of the abductor pollis brevis (APB) muscle immediately adjacent to its origin at the tubercle of the scaphoid bone. The next four tendon slips, which had the same thickness in parts, took a course to the palmoradial third of the base of the first metacarpal bone. A further tendon slip followed the classic course to the base of the first metacarpal bone, which made it seem logical to designate this one as the main tendon. Only these six slips were found together with the extensor pollicis brevis (EPB) muscle tendon in the first compartment. In Table 1 we also provide the distributions of the number of supernumerary tendons of the APL muscle in 52 upper extremities by sex and localization. The individual values revealed by the mechanical analysis are displayed in Tables 2 (untreated tendons) and 3 (elastase-treated). A typical measurement curve is shown in Figure 2, and the mechanical parameters are explained there as well. With regard to elasticity (emax) and the extension at the point where the linear portion of the stress- Fig. 2. Typical graph of a stress-strain relation of an APL muscle tendon, where max is the stability of the tendon, emax the extensibility, and Emax the Young’s modulus in the steepest portion of the curve. The transition point between the initial area and the linear part of the curve, i.e., the upper limit of the physiological range of the biomechanical stability, is marked with eon. extension relationship begins (eon), there were no great differences among the subjects, that is to say among the different numbers of tendon slips. After elastase treatment, there were significant differences (p , 0.05) between the elasticity and the onset of the linear portion of the stress-strain relationship (Table 4). With regard to the stability (max) and the maximal Young’s modulus (Emax), there were significant differences between the proband with the tendon split into six slips and the others. The influence of the elastase treatment was minimal for the tendon split into six. This was not true for the other cases, in which a reduction of the stability was observed after the elastase treatment (Table 5). DISCUSSION Duplication of the APL muscle was reported previously (Henle, 1871; Krause, 1880; Walsh, 1897; von Bardeleben, 1906; Stein, 1951; Coleman et al., 1953; Baba, 1954; Williams et al., 1989). Division of the abductor tendon into three slips was also described 478 M. MELLING ET AL. Fig. 3. Computer-assisted tensile testing machine and control circuit. TABLE 3. Mechanical properties of the preparations with the tendon of the APL muscle into 2, 3, 4, and 6 slips after elastase treatment [emax elasticity in %, eon onset of linear portions of stress-strain relations in %, smax stability (Nm/g), Emax (Nm/g) maximal Young’s modulus in the linear portion of the curve] No. emax eon smax Emax 1a 1b 2a 2b 2c 3a 3b 3c 3d 4a 4b 4c 4d 5a 5b 6a 6b 6c 6d 6e 6f 9.0 9.3 8.3 9.8 9.0 11.3 11.5 8.8 7.5 11.5 7.8 8.8 10.5 9.5 11.0 9.5 10.0 8.3 9.3 8.8 8.0 0.50 0.50 0.25 0.50 0.25 0.50 0.50 0.75 0.50 0.50 0.50 0.25 0.50 1.25 0.75 0.50 0.75 0.50 0.25 0.50 0.50 13.1 36.9 9.36 24.9 22.3 10.1 27.1 10.9 9.62 28.9 45.4 23.6 21.3 38.6 14.7 14.8 16.3 18.3 8.96 30.8 28.2 145.5 562.3 651.9 507.6 365.3 137.9 316.3 148.0 189.4 287.6 497.5 297.9 176.6 383.9 130.0 222.9 224.6 282.0 158.1 423.1 304.0 (Meckel, 1832; Lacey et al., 1951; Loomis, 1951; Lapidus and Fenton, 1952; Walsh, 1955; Bergman et al., 1988) as well as division into four slips (Wood, 1867 and Ders 1868, according to Henle, 1871; Rauber and Kopsch, 1914; Bell and Bell, 1811, according to Lacey et al., 1951; Verdan, 1952, according to Wulle, 1974; Backhouse, 1981; Schmidt, 1987) and into five slips (Bunnell, 1948; Bunnel and Böhler, 1958; Schmidt and TABLE 4. Differences between the mechanical properties of tendons of the APL muscle with and without elastase treatment, with special reference to elasticity and onset of the linear portion of the stress-strain relationship Untreated Elastase-treated No. emax (%) eon (%) emax (%) eon (%) 21 10.42 6 0.4 0.85 6 0.08 9.41 6 0.26 0.52 6 0.05 TABLE 5. Differences between the mechanical properties of tendons with and without elastase treatment, with special reference to stability (smax in Nm/g) and Young’s modulus (Emax in Nm/g) measured in the linear portion of the stress-strain relationship Untreated Elastase-treated No. smax Emax smax Emax All 21 22.2 6 0.22 335.7 6 41.7 21.6 6 2.3 296.0 6 35.6 A. Without 6-fold split of the tendon of the APL muscle 15 39.2 6 13.3 359.5 6 46.5 22.5 6 0.21 320.5 6 37.3 B. With 6-fold split of the tendon of the APL muscle 6 17.3 6 2.8 276.3 6 48.6 19.6 6 3.39 269.3 6 37.4 Lahl, 1988); Melling et al., (1996) reported seven slips. However, division of the APL muscle tendon into as many as six slips in this form has been neither reported nor investigated before this study. It is our opinion that the division described here is clinically significant, especially with reference to the diagnosis and treatment of de Quervain’s stenosing tenosynovitis affecting the radius. 479 ABDUCTOR POLLICIS LONGUS MUSCLES Unexplained persistence of postsurgical pain following a revision of the first tunnel could be avoided by a thorough initial inspection resulting in the immediate recognition of any multiple tendons enclosed in separate compartments and ensuring their free movement. Clinical experience has shown that the lateral position of supernumerary tendons also increases the risk of injury (Schmidt et al., 1968). One disadvantage of the aberrant tendons is that they lead to the condition first described by de Quervain (1895) as chronic stenosing tenosynovitis. From a mechanical point of view, however, division into several parts leads to better distribution of stress and extension on abduction of the thumb. In the extreme case of the sixfold split, an improvement of the stress distribution on abduction of the thumb might be obtained, and in addition, the palmar flexion in the wrist may be supported. Even within the physiological range, then, there are smaller loads on the individual tendon slips with resultant better distribution of the stress when the tendons are considered together as a stresstransmitting complex. The relation between extension and load capacity of the tendons is also contingent on the insertions of the musculature. Lacey et al. (1951) and Neviaser et al. (1980) have also pointed out the clinical significance of accessory end tendons of the APL muscle inserting into the trapezoid bone or the abductor pollicis brevis muscle. It is perfectly feasible to use these in plastic reconstruction of the interosseuse-dorsalis -I muscle or the extensor pollicis brevis muscle. The thumb is useless without the tendon of the abductor pollicis muscle, because the metacarpal phalangeal joint will overextend whenever the pincer grip between the thumb and index finger is attempted. This means that the arch of the thumb is destroyed, the entire thenar eminence sinks into the concave palm of the hand, and a right-angle bend arises along the thenar fold (Bunnell and Böhler, 1958). Although Bunnell (1948) regards the deviant mechanism of the aberrant tendon as causing the development of pain in de Quervain’s chronic tenosynovitis, the accessory tendon also should be considered. He explains the occurrence of a supernumerary, aberrant tendon in the human as an example of atavism, since in most primates—e.g., chimpanzees, gorillas, and gibbons—this muscle normally exhibits two tendons, one inserting at the first metacarpal bone and the other at the trapezoid bone. The observations of Bunnell (1948) in 22 patients indicate the frequency of aberrant APL muscle tendons. He reported accessory tendons in 12 of 22 patients who underwent surgery. Lacey et al. (1951) found accessory tendons in 82% of their cases and Stein (1951) in 68%, whereas Baba (1954) found aberrant tendons in 132 (98,5%) of 134 wrists examined. Considering the aforementioned research, continuing descriptions of aberrant tendons of the APL muscle as a variant seems unwarranted. Our notes on the number, thickness, course, and insertions of these supernumerary tendons are meant to aid in determining the anatomical properties of the APL muscle as well as to assist clinicians by highlighting the great significance of tendon deviations as a potential cause of chronic tenosynovitis. LITERATURE CITED Baba, M.A. 1954 The accessory tendon of the abductor pollicis longus muscle. Anat. Rec., 199:541–547. Backhouse, K.M. 1981 Abductor pollicis longus musculo-tendinous split at a replacement motor for ruptured extensor pollicis longus. Hand, 13:271–275. Bell, J., and C. Bell 1811 Anatomy. Codell & Davies, London (cited by Lacey et al., 1951). Bergman, R.A., S.A. Thompson, A.K. Afifi, and F.A. Saadeh 1988 Compendium of Human Anatomic Variation. Urban & Schwarzenberg, Baltimore, p. 16. Bunnell, S. 1948 Surgery of the Hand 2nd ed. J.B. Lippincott, Philadelphia, pp. 455–457. Bunnell, S., and J. Böhler 1958 Die Chirurgie der Hand. Parts 1 and 2, Maudrich, Vienna. Coleman, S.S., D.K. McAffe, and B.J. Anson 1953 The insertion of the abductor pollicis longus muscle: An anatomical study of 175 specimens. Q. Bull. Northwest. Univ. Med. School, 27:117–122. De Quervain, F. 1895 über eine Form von chronischer Tendovaginitis. Korrespondenzbl. Schweizer Ärzte, 25:389–394. Ders. 1868 (cited by Henle, J. 1871, pp. 226–228). Fyhrie, D.P., and M.B. Schaffler 1995 The adaptation of bone apparent density to applied load. J. Biomech., 28:135–146. Henle, J. 1871 Handbuch der systematischen Anatomie des Menschen, 2nd ed., vol. 1., part 2. Vieweg & Sohn, Braunschweig, pp. 226–228. Krause, W. 1880 Anatomische Varietäten Tabellen; in Krause C.F.T., and Krause, W. (eds): Handbuch der menschlichen Anatomie, 3rd ed., Vol. 3. Hahnsche Buchhandlung, Hannover, pp. 104–105. Lacey, T., L.A. Goldstein, and C.E. Tobin 1951 Anatomical and clinical study of the variations in the insertions of the abductor pollicis longus tendon, associated with stenosing tendovaginitis. J. Bone Joint. Surg. [A], 33:347–350. Lapidus, P.W., and R. Fenton 1952 Stenosing tenovaginitis at the wrist and fingers. Arch. Surg., 64:475–477. Loomis, L.T. 1951 Variation of stenosing tenosynovitis at the radial styloid process. J. Bone Joint. Surg. [A], 33:340–347. Meckel, J.F. 1832 Anatomy. Carey & Lea, Philadelphia. Melling, M., J. Wilde, M. Schnallinger, W. Schweighart, and M. Panholzer 1996 Supernumerary Tendons of the Abductor pollicis. Acta. Anat., 155:291–294. Neviaser, R.J., J.N. Wilson, and M.M. Gardner 1980 Abductor pollicis longus transfer for replacement of first dorsal interosseus. J. Hand. Surg., 5:53–57. Rauber, A., and F. Kopsch 1914 Rauber’s Lehrbuch der Anatomie des Menschen. III. Muskeln, Gefässe, 10th ed. Thieme, Leipzig. Schmidt, H.M., and J. Lahl 1988 Untersuchungen an den Sehnenfächern der streckmuskeln am menschlichen Handrücken und ihrer Sehnenscheiden. Gegenbaurs Morphol. Jahrb., 134:155–173. Schmidt, R. 1987 Zur funktionellen und klinischen Bedeutung der accessorischen Sehnen des M. abductor pollicis longus. Verh. Anat. Ges., 81:319–321. Schmidt, R., R. Schultka, R. Hammer, and A. Dorn 1968 Untersuchungen zur Häufigkeit accessorischer Sehnen des M. abductor pollicis longus, unter Berücksichtigung ihrer Bedeutung für die Praxis. Gegenbaurs Morphol. Jahrb., 112:139–149. Stein, A.H., Jr. 1951 Variations of the tendons of insertion of the abductor pollicis longus and the extensor pollicis brevis. Anat. Rec., 110:49–55. Verdan, C. 1952 Chirurgie reparatrice et fonctionnelle des tendons de la main. Vol. 1. L’Expansion Scientifique Francaise, Paris, p. 244. Von Bardeleben, K. 1906 Lehrbuch der systematischen Anatomie des Menschen. Urban & Schwarzenberg, Berlin, pp. 373–375. Walsh, J.F. 1897 The Anatomy and Functions of the Muscles of the Hand and of the Extensor of the Tendons of the Thumb. Walsh, Philadelphia. Walsh, A.C. 1955 Variation in the abductor polllicis longus tendon. Canad. Med. Assoc. J., 73:741–743. Weinans, H., R. Huiskes, and Grootenboer, H.J. 1992 The behavior of adaptive bone-remodeling simulation models. J. Biomech, 25:1425– 1441. Williams, P.L., R. Warwick, M. Dyson, and L.H. Bannister (eds) 1989 Gray’s Anatomy, 37th ed. Churchill/Livingstone, Edinburgh, p. 624. Wood, J. 1867 (cited by Henle, J. 1871, pp. 226–228). Wulle, C. 1974 Eine Variante des ersten Strecksehnenscheidenfaches und seine Bedeutung für die Behandlung des Morbus de Quervain. Handchirurgie, 6:189–190.