ANOMALIES OF THE FLEXOR DIGITORUM SUPERFICIALIS MUSCLE

ANOMALIES OF THE FLEXOR DIGITORUM SUPERFICIALIS MUSCLE D. ELLIOT, A. R. KHANDWALA and M. KULKARNI From the Hand Surgery Department, St Andrew's Centre for Plastic Surgery, Broom®eld Hospital, Chelmsford, UK

Three anomalies of the human ¯exor digitorum super®cialis are presented. The normal development of this muscle from the amphibian to the human is discussed and the described anomalies of the muscle in humans classi®ed. Journal of Hand Surgery (British and European Volume, 1999) 24B: 5: 570±574 nerve from the ulnar to the radial side to be inserted into the undersurface of the most distal and radial part of the carpal ligament, passing super®cial to the normal tendons and the median nerve. It seems unlikely that this small anomaly was the cause of the carpal tunnel syndrome.

Although many rare anatomical variations of the ¯exor digitorum super®cialis (FDS) muscle itself were described by the anatomists of the last century (Le Double, 1897; Testut, 1884; Turner, 1864), most appear to have no clinical signi®cance. In more recent times, further variants, mostly involving the muscle and tendon of the index ®nger more distally, have come to notice because of their relationship with clinical problems requiring surgery. It is no longer believed that the human embryo evolves through a series of representations of our ancestral predecessors and that such anomalies represent arrests of development, or atavisms, in which anomalous parts retain the form they had in species lower in the evolutionary tree (Kaplan, 1981). However, the actual number of anomalous variations which occur is small, with the same anomalies recurring regularly and appearing curiously similar to the FDS muscle at various stages in the evolution of this muscle from the amphibians to the higher primates. It is dicult to explain these observations simply by chance and, as yet, they have no explanation. As much of the writing in the hand surgery literature has discussed anomalies of the FDS in terms of such arrests of development, and the anomalies themselves have been upheld as supportive evidence of the evolutionary process, this writing requires reinterpretation. Nevertheless, the papers recording them remain a valuable record of the anomalous development of this muscle. We report a further two patients with anomalies of the FDS to the index ®nger and one with an anomaly of the middle ®nger super®cialis tendon which has not been described previously. This study also attempts to collate the literature on the anomalous development of the human FDS muscle.

Case 2 A 31-year-old hairdresser presented with a swelling in the right palm which had been increasingly painful for over 1 year and had been interfering with her work for 4 months, during which time it had increased in size. She had also noticed a similar, although smaller and painless, swelling in her left palm. The clinical ®ndings suggested tenosynovitis of the ¯exor tendons of the right index ®ngers. Ultrasound of the right hand was reported as showing gross synovial thickening around the index ¯exor tendons with a small ¯uid collection adjacent. Surgery revealed an anomalous muscle attached proximally and distally to the super®cialis tendon of the index ®nger (Fig 2). In the light of previous experience with a similar case in which division of the muscle fascia had relieved the pain, the muscle fascia was divided and the

CASE REPORTS Case 1 A 42-year-old housewife had persistent symptoms of left median nerve compression. During carpal tunnel decompression an abnormal tendon was found to run across the median nerve proximal to the carpal canal (Fig 1). The tendon originated as a small muscle attached to the FDS muscle of the middle ®nger, 8 cm proximal to the wrist crease, and crossed the median

Fig 1 View of the left carpal tunnel and distal forearm at surgery showing an anomalous muscle belly and tendon (shown by arrow) arising from the FDS muscle of the middle ®nger and crossing the other long ¯exor tendons to insert into the radial aspect of the carpal ligament on its deep surface. 570

ANOMALIES OF FDS

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®nger was found to be abnormally long, extending into the carpal tunnel when the ®ngers were fully extended (Fig 3). DISCUSSION

Fig 2 View of the right palm at surgery showing an anomalous muscle arising from, and reinserting into, the FDS tendon of the index ®nger.

Fig 3 View of the wrist during exploration of a median nerve which had been divided 18 months previously and had not been repaired primarily. The FDS muscle belly to the index ®nger extends into the carpal canal when the ®ngers are in the extended position. The muscle is seen protruding between the overlying tendons of the super®cialis ¯exors to the middle and ring ®ngers just proximal to the carpal tunnel.

muscle was not excised. Although there remained a swelling in the line of the index ¯exor tendons postoperatively, the pain resolved quickly. The left palmar swelling remained asymptomatic and was not investigated or explored. From its shape and position, it was presumed to be a similar anomaly. Case 3 A 21-year-old man presented 18 months after a complete division of the median nerve. At exploration of the nerve, the muscle belly of the FDS to the index

Anomalies of the ¯exor digitorum super®cialis come to the attention of clinicians for various reasons. The palmar anomalies, and occasionally the longer of the forearm anomalies, may present clinically as painless or painful palmar masses. Palmar anomalies may also present with symptoms of carpal tunnel syndrome, often intermittent and only present during activity, presumably as a result of palmar muscle bellies moving proximally to lie under the ¯exor retinaculum during ®nger ¯exion. The forearm anomalies are also found in association with carpal tunnel syndrome, although it is questionable whether these anomalies, which had been present from birth, could be the sole cause of a de novo nerve compression in adult life. Bunnell (1948) discussed the evolution of the human ®nger ¯exors and, more recently, Still and Kleinert (1973) reviewed some of the literature on the evolution of the muscles of the hand and forearm. However, Mainland (1927) identi®ed the de®nitive works on this subject as having been written by Eisler (1895), McMurrich (1903a; 1903b) and GraÈfenberg (1905). The work of these authors has been summarized in English by Congdon (1920) and, more recently, by Strauss (1942). In order to simplify future discussion of these FDS anomalies, we have classi®ed them (Fig 4) and collated the cases reported in previous studies in date order (Table 1). The upper limb evolved for terrestrial use in the amphibians, in the forearms of which the ¯exor muscles are di€erentiated into a super®cial and a deep layer. In amphibians, both forearm muscle layers act as wrist ¯exors and neither ever inserts distal to the metacarpals. The mammalian ¯exor digitorum profundus and pronator quadratus muscles evolved from the deeper layer of the amphibian forearm muscles and the ¯exor digitorum super®cialis and palmaris longus from the central part of the super®cial layer. The lateral and medial parts of the super®cial layer have become the pronator teres, ¯exor carpi radialis and ¯exor carpi ulnaris muscles in mammals. Amphibian digits are ¯exed by several layers of brevis muscles which take origin within the hand, from the multiple layers of the palmar aponeurosis. The mammalian musculature appears to have evolved by the forearm muscle bellies retreating proximally from the carpus and the palmar aponeurosis, while the adjacent insertions of the forearm muscles and origins of the brevis muscles became free from the carpus centrally to become those parts of the long profundus and super®cialis tendons passing through the carpal canal into the proximal palm. The brevis muscles of the borders of the hand evolved to

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Table 1ÐCases reported in previous studies of FDS anomalies Authors Macalister (1866) Fromont (1895) GraÈper (1916) Mainland (1927) Tanzer (1959) Case (1966) Wesser et al. (1969) Gardner (1970) Vichare (1970) Smith (1971) Inglis et al. (1972) Still and Kleinert (1973)

Fig 4 Diagram showing the amphibian and human super®cial ¯exor muscles and the observed human anomalies of the FDS muscle. Type I FDS tendon to FDS tendon attachment. Type II Flexor retinaculum to FDS tendon attachment. Type III Digastric muscle in the FDS tendon. Type IV Distal extension of FDS muscle belly. Type V Anomalies of the FDS in the forearm.

become the thenar and hypothenar muscles and those originating from the deeper central part of the aponeurosis became the lumbricals. The central super®cial brevis muscles either disappeared or became the palmar parts of the super®cialis tendons. While the amphibian pattern of insertion of the forearm ¯exors is seen in most reptiles and these muscles remain wrist ¯exors only, the ¯exor profundus has evolved to be a ®nger ¯exor in some reptiles. However, the ¯exor super®cialis only became a ®nger ¯exor in early mammals. This increase in the part played in ®nger ¯exion by the forearm ¯exors and the disappearance of independent ¯exor brevis muscles in the upper limb of higher mammals re¯ects the di€erentiation of the hand into a grasping device. The mammalian foot remains a structure in which the ¯exor brevis muscles are retained to allow the toes to obtain a good grip before the long and short extensors act, ®rst to dorsi¯ex the ankle then the metatarsophalangeal joints, so transferring the body weight forwards. As the hand became emancipated from locomotor function, coincident with the ¯exor muscle changes described above, the short extensors were lost in the hand in the arboreal primates. The power to bend the digits backwards at the metacarpophalangeal joints more than a short way, still present in most monkeys, has largely disappeared in all but the most supple humans. In the human hand, the digital extensors have become mere correctors of the ¯exors (Wood Jones, 1941).

Sex/Age Patients Hands Anomaly

* * * M/45 * F/16 M/11 M/21 F/19 F/28 * F/38 F/28 F/33 Hayes (1974) F/29 Neviaser (1974) M/22 Das and Brown (1975) F/17 Probst and Hunter (1975) F/49 Tountas and Halikman (1976) F/14 Baruch and Hass (1977) F/37 Christensen (1977) Fx3/20** Aghasi et al. (1980) F/22 Bourrel and Chickly (1980) * Martelo-Villar (1980) F/14 Figueiredo and Hooper (1980) F/61 Hutton et al. (1981) F/23 Elias and Schulter-Ellis (1985) F/18 M/18 Ametewee et al. (1985) F/23 Coenen and Biltjes (1991) F/28 Sanger et al. (1991) F/23 Kostakoglu et al. (1997) F/17 Elliot et al. (1999) F/42 F/31 M/21

1** 1** 1** 1** 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 1 6 1 1 1 1 1 1 1 1 1 1 1 1

1 2 1 1 1 1 1 1 1 1 1 1 1 2 2 2 1 1 1 1 5 2 8 2 1 1 1 2 1 2 1 1 1 2 1

III II III V IV III II IV I III IV IV or V IV or V II III IV III III I IV ? IV IV II V I III III ?IV II II I V I IV

*Details not given **Cadaver studies

Our ®rst and third cases, and others described previously, illustrate FDS muscle anomalies in the forearm. These fall into two groups. The ®rst (Type IV, Fig 4) are super®cialis muscle bellies extending more distally than normal and impinging on the median nerve just proximal to (Baruch and Hass, 1977; Gardner, 1970), or actually in, the carpal tunnel (Fig 3) (Aghasi et al., 1980; Bourrel and Chickly, 1980; Inglis et al., 1972; Neviaser, 1971; Tanzer, 1959) to cause median nerve compression. The case reported by Ametewee et al. (1985) was probably also a Type IV anomaly but the detail of this case was not clear. The distal extents of the super®cialis muscle bellies are very variable (Turner, 1864) and the latter anomalies would seem to be extreme examples of normal variation. The second (Type V, Fig 4) are true anomalies of the super®cial muscle layer in the distal forearm. These are less common and often incompletely described because their dissection was limited to the incision, usually at the wrist, made to

ANOMALIES OF FDS

treat the clinical needs of the case. Mainland (1927) described one case in which a single super®cialis tendon to the ring and little ®ngers had three anomalous muscle bellies along its length, two of which were in the forearm. Figueiredo and Hooper (1980) described another anomalous muscle arising deep to the other super®cialis tendons and lying under the median nerve. Traction on the muscle at the wrist ¯exed the PIP joint of the index ®nger. Case 1 of this study illustrates an anomaly which has not been described previously. Still and Kleinert (1973) reported two cases with index super®cialis muscle bellies passing into the carpal canal and described as ``aberrant'', although it is not clear whether these were additional muscle bellies or normal muscles extending more distally than usual. The anomalous muscle in case 2 above is identical to several described previously (Hutton et al., 1981; Kostakoglu et al., 1997; Tountas and Halikman, 1976; Vichare, 1970). These Type I anomalies arise from the carpal part of the FDS tendon and reinsert into the same FDS tendon in the distal palm. In the case described by Tountas and Halikman (1976), a proximal tendon of the anomalous muscle persisted proximally into the forearm and was not explored to its origin. The case described by Christensen (1977) presented simply with painless swellings of the radial aspect of the palm. Although reported as a palmar anomaly of the index FDS recurring in various members of the same family, this case was not investigated or explored surgically and may have been an anomaly of the ®rst lumbrical muscle and not an anomalous FDS of the index ®nger. Coenen and Biltjes (1991), Sanger et al. (1991) and Still and Kleinert (1973) all reported single cases of the Type II anomaly in which the muscle originated from the ¯exor retinaculum and inserted into the long super®cialis tendon in the distal palm, in this respect resembling the amphibian ¯exor brevis more closely than the other anomalies. Martelo-Villar (1980) reported a similar case in which an anomalous muscle was attached proximally to the base of the thenar eminence. Wesser et al. (1969) reported a case of an anomalous index super®cialis muscle arising from the ¯exor retinaculum with no long index super®cialis muscle and tendon in the forearm. The most extensive example of this particular anomaly was that presented by Fromont (1895) in which a cadaveric hand had four anomalous super®cialis muscles arising from the ¯exor retinaculum and acting to ¯ex the PIP joints of the four ®ngers. Long forearm super®cialis muscles were only present for the middle and ring ®ngers. The other hand also had forearm ¯exor super®cialis muscles of the middle and ring ®ngers only and these inserted into the ¯exor retinaculum. A functional palmar super®cialis muscle extended from the retinaculum to the PIP of the index ®nger. Small palmar super®cialis muscles also extended from the retinaculum to the bases of the middle and ring ®ngers but without distal attachment or function.

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A number of authors have reported cases of the Type III anomaly in which the index ®nger super®cialis tendon included a true digastric muscle in the palm, with normal tendons both proximal and distal to the muscle mass (Case, 1966; Das and Brown, 1975; Elias and Schulter-Ellis, 1985; GraÈper, 1916; Macalister, 1866; Probst and Hunter, 1975; Smith, 1971). Hayes (1974) presented a bilateral example of this anomaly. In this case, traction on both proximal ends of the super®cialis tendons after excision of the palmar super®cialis muscles suggested that there were no super®cialis muscle bellies in the forearm but this was not con®rmed by extension of the surgical wounds into the forearms. In the past, there has been considerable debate whether the presence of these digastric muscles indicated that the proximal palmar part of the super®cialis tendon evolved from the amphibian brevis muscle or whether the brevis simply disappeared in the hand during mammalian evolution at some period later than that during which the proximal palmar part of the super®cialis tendon formed from another source, probably the ¯exor retinaculum. Mainland (1927) attempted to present an all encompassing theory of this evolutionary process but this was too complicated and was never accepted and the precise evolution of this part of the ¯exor super®cialis tendon remains obscure. Recent genetic research suggests that modi®cation of the gene pool of lower mammals, such as rats and mice, can induce muscle or tendon anomalies such as are seen in the human forearm. In time, this work may explain why anomalies recur in a very limited number of forms and why they can bear striking resemblance to the same parts in species lower in the evolutionary tree (Fig 4). Acknowledgements The authors wish to thank Ms. Diane Harris for help in collecting the research material, Mr N.S. Niranjan FRCS for his artwork and Dr Peter Ashby of the University of Dundee for allowing us to quote his research material and for his advice on the genetic aspects of this study.

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Received: 6 April 1999 Accepted: 21 April 1999 D. Elliot MA FRCS, St Andrew's Centre for Plastic Surgery, Broom®eld Hospital, Court Road, Chelmsford, Essex CM1 7ET, UK. # 1999 The British Society for Surgery of the Hand Article no. jhsb.1999.0237