Okajimas Folia Anat. Jpn., 56(5) : 277-288, December 1979

Structure of the Flexor Digitorum Superficialis

By

OSAMU OHTANI

Department of Anatomy, Okayama University Medical School, Okayama 700, Japan (Director : Prof. Dr. H. Outi) —Received for Publication, March 9, 1979—

Key Words: Flexor digitorum superficialis, Striated muscle, musculature.

Summary. Fifty-two of 26 Japanese adult cadavers were examined. The flexor digitorum superficialis can be separated into two layers: a superficial layer and a deep layer. The superficial layer is composed of the radial head and the superficial part of the humeroulnar head. It forms two muscle bellies which give rise to the for the third and fourth digits, respectively. The deep layer is composed of the deep part of the humeroulnar head. After forming an inter- mediate , the deep layer also divides into two fleshy bellies which give rise to the tendons for the second and fifth digits, respectively. Based on the mode of occurrence of the communicating muscle fasciculi between the superficial layer and the deep layer, the flexor digitorum superficialis can be classified into four types. Type I muscle has no communicating fasciculus (4/52, 7.7%). Type II muscle has a communicating muscle fasciculus between the intermediate tendon and the muscle belly for the fourth digit (muscle fasciculus A) (29/52, 55.8%). Type III muscle has the muscle fasciculus A as well as another communicating muscle fasciculus between the intermediate tendon and the belly for the third digit (muscle fasciculus B) (18/52, 34.6%). Type IV muscle has the muscle fasciculus B only (1/52, 1.9%). Six other kinds of variations were also recognized. The flexor digitorum superficialis is innervated by four to five branches from the median nerve. These are usually grouped into uppermost, middle and distal branches. In one case, a small part of the humeroulnar head was innervated by a slender branch from the ulnar nerve.

Introduction authors such as Le Double (1897), Graeper (1917), Mainland (1927), Iwami (1951), The organization of the flexor digi- Chowdhary (1951), Takeshige et al. (1959), torum superficialis has been studied by Mori (1964), Dahlgard and Kawth (1965), many workers such as Wood (1867), Ko and Takahashi (1965), Jones (1966), Rauber-Kopsch (1955), Inoue (1935) and Dylevsky (1968), Vichare (1970), Rojek Frohse and Fraenkel (1908). Its anomalies Solarska and Solarski (1971), Shrewsbury have also been reported by various and Kuczynski (1974), Hayes (1974), Carr

277 278 0. Ohtani

et al. (1977), etc. from the soacalled oblique line) of the Such descriptions, however, were not radius just medial and distal to the inser- sufficiently comprehensive to satisfy our tion of the spinator and the pronator teres. observations in the course of routine The muscle belly of the flexor digi- student dissection. The author therefore torum superficialis can be separated into investigated the flexor digitorum super- two Myers : a superficial layer and a ficialis in Japanese subjects and attempted deep layer. The superficial layer is com- to make a morphological classification. posed of two muscle bellies, which are closely associated with one another but can be easiry separated. The greater Materials and Methods part of the superficial layer of the hu- meroulnar head forms a muscle belly The flexor digitorum superficialis which gives rise to the tendon for the muscles of 26 Japanese adult cadavers, fourth digit. The remainder of the super- or 52 forearms which were left undis- ficial part of the humeroulnar head and the sected in the course of routine student radial head unite to form a muscle belly dissection in 1976, were used. The bel- which gives rise to the tendon for the lies of each muscle were carefully third digit. When these two heads con- separated from one another according to verge to unite in the upper part of the the positions of the tendons, directions forearm, an aponeurotic membrane bridges of the muscle fasciculi, their origins and between the two heads. Behind this fibrous their insertions. The whole muscle with its vessels and nerves was then severed arch pass the median nerve and the ulnar artery. The deep layer arises as the at its origins and its tendons destined deep part of the humeroulnar head. It for the digits in the lower part of the forms a cylinder-like belly near its origin, forearm or at its insertions, respectively. becomes gradually flattened and in the The muscle was finally carefully dis- middle of the forearm forms an interme- sected out and the nerves were followed diate tendon. From this tendon originate up to each muscle bundle. two distal bellies which give rise to the tendons for the second and fifth digits, Results and Discussion respectively. This represents the simplest organiza- Organization : The flexor digitorum tion of the flexor digitorum superficialis. superficialis stretches across the forearm However, two kinds of communicating between the medial epicondyle of the fasciculi (A and B) are often observed and the anterior border of the between the superficial layer and the radius. It is covered by the palmaris deep layer. Fasciculus A arises from longus, the flexor carpi radialis and the the ulnar border of the intermediate flexor carpi ulnaris. The large, thick, tendon, usually at its upper part, and somewhat cylinder-like humeroulnar head runs obliquely to unite with the ulnar arises from the medial epicondyle of the border of the muscle belly for the fourth humerus by means of the common flexor digit. This fasciculus was found in 47 tendon, and from the ulnar colateral out of 52 muscles examined (90.4%). ligament and the medial border of the Fasciculus B, which is generally smaller coronoid process. The thin but broad and lower in origin than the former, radial head arises from about the upper arises from the radial border of the two thirds of the anterior border (mainly intermediate tendon and fuses with the Structure of the Flexor Digitorum Superficialis 279

Fig. 1. Schematic representation of the four types of flexor digitorum superficialis viewed from the posterior side. Type I has no communicating muscle fasciculus between the superficial layer (muscle belly for the 3rd and 4th digit) and the deep layer (muscle belly for the 2nd and 5th digits). Type II has the communicating muscle fasciculus A (A) between the intermediate tendon (IT) of the deep layer and the belly for the 4th digit. Type III has the communicating muscle fasciculus A as well as the communicating muscle fasciculus B (B) between the intermediate tendon of the deep layer and the muscle belly for the 3rd digit. Type IV has the communicating muscle fasciculus B (B) only. 280 0. Ohtani deep aspect of the muscle belly for the Type I: ThisRis the simplest type as third digit. This fasciculus occurred in indicated above, there being no com- 19 out of 52 muscles (36.5%). municating fasciculi between the super- Based on the occurrence of these two ficial layer and the deep layer. communicating fasciculi, the flexor digi- Type II : This type has only fasciculus torum superficialis can be classified into A. four types. Type III : This type has both fasciculus

Fig. 2. The most usual pattern of innervation of the flexor digitorum superficialis viewed from the posterior side. 2-A : type I (left side), 2-B: type II (right side), 2-C type III (left side). CHU : caput humeroulnare, CR : caput radiale, IT : intermediate tendon of the deep layer, A: communicating muscle fasciculus A, B: communicating muscle fasciculus B, Npd : upper- most branch from the median nerve to the proximal belly of the deep layer, Npl : branch from Npd to the palmaris longus, Nsf : middle branch from the median nerve distributed in the superficial layer, fasciculus A, belly for the 2nd digit, and fasciculus B, Nr : separate branch to the muscle belly for the 3rd digit, Ndd : distal branch from the median nerve to the distal belly of the deep layer, i. e. the distal belly for the 2nd digit. Structure of the Flexor Digitorum Superficialis 281

A and fasciculus B. At about the junction of the middle Type IV : This type has fasciculus B and lower thirds of the forearm, one or only. two distal branches (Ndd in the text- These four types are illustrated sche- figures) arise from the median nerve and matically in Fig. 1. In the present study, pass into the distal belly of the deep type II was the most common (29/52, layer, mainly in the distal belly for the 55.8%), followed by type III (18/52, 34.6 second digit (Figs. 2-A, B, C). %). The other types, I (4/52, 7.7%) and In one case, a slender branch from the IV (1/52, 1.9%), were rare. The correla- ulnar nerve supplied a small part of the tion between the occurrence of fasciuli deep layer of the humeroulnar head. A and B was insignificant : the X2 value The other part of this muscle, however, for the 2 x 2 table was as small as 0.65 was innervated by the median nerve. without Yates' correction. No significant Fuchino (1960) has reported that at the differences in occurrence were detected middle of the forearm the upper part of between the sexes, or between the right the muscle belly for the index finger of and left muscles. the flexor digitorum superficialis is some- Innervation : The flexor digitorum times innervated by a branch from the superficialis is usually innervated by four ulnar nerve (12%). In our case, however to five branches from the median nerve. only a small part of the humeroulnar The uppermost one (Npd in the text- head was innervated by the ulnar branch figures) arises before the median nerve (Fig. 3). The brachial plexus of this traverses the pronator teres, and passes case was composed of C5, C6, C7, C8 and mainly to the deep part of the hume- Thl as usual. A small root from the roulnar head. One of its branches pene- lateral fasciculus derived from C5, C6 trates through both parts of the hume- and C7 fused with the main root from roulnar head to reach the palmaris the medial fasciculus composed of C8 and longus (Npl in the text-figures). Thl to form the ulnar nerver. However, The middle branch (Nsf in the text- the detailed relation between this unusual figures) arises just before the median root and the incidental peripheral ulnar nerve passes behind the aponeurotic innervation of the muscle requires fur- membrane which bridged between the ther investigation. humeroulnar head and the radial head. Other variations : Six minor kinds of It enters the superficial part of the variations were observed in the flexor humeroulnar head near its ulnar border digitorum superficialis ; two of them, and is ramified to supply the whole however, were from materials other than superficial layer of the humeroulnar head, the systematically examined 52 muscles, the communicating muscle fasciculus A and cannot be treated statistically. and then to the distal belly for the (1) The first type of variation which fifth digit, the communicating muscle was found in three muscles (5.8%) had fasciculus B and the radial head. small accessory deep radial heads which Sometimes at the middle of the fore- fused with the deep layer of the muscle. , a branch (NI- in the text-figures) to One case showed two such muscle slips the radial head and the radial and distal which arose from the radius 2 cm and 5 parts of the superficial layer of the cm distal from the humeroradial joint, humeroulnar head, i. e. to the belly for respectively, and fused with the inter- the third digit, arises separately from mediate tendon of the deep layer. The the median nerve. proximal one was 0.6 cm wide and 4 cm 282 0. Ohtani

Fig. 3 Fig. 4 Fig. 5 Fig. 3. A case (left side) supplied by the median nerve and the ulnar nerve. A branch from the ulnar nerve (Npdu) supplied a small proximal part of the deep layer. Npd, Nsf, Nr, Ndd, IT, A, CHU and CR : see legend to Figure 2. Fig. 4. A case (left side) with two accessory deep radial heads (ams1 and ams2) which fused with the intermediate tendon of the deep layer. Note that these accessory muscle slips are supplied by a branch from the median nerve which passes to the flexor digitorum profundus. CHU, Npd, Npl, Nsf, Ndd, CR, IT, A and B: see legend to Figure 2. Fig. 5. A case (right side) with an accessory deep muscle slip (ams) which arose from the radius just behind the normal radial head and fused with the distal muscle belly of the 2nd digit. Nr : a branch from the median nerve which supplied the distal part of the radial head and the abnormal muscle slip. CHU, CR, Npd, Nsf and Ndd : see legend to Figure 2. long, while the distal one was 0.4 cm the latter muscle. wide and 4 cm long. Both were supplied In the other two cases, a thin acces- by a branch from the median nerve sory muscle slip about 1 cm wide arose which went to the flexor digitorum pro- from the middle of the radius just behind fundus (Fig. 4). This suggests that these the normal radial head of this muscle muscle slips may possibly derive from and fused with the muscle belly for the Structure of the Flexor Digitorum Superficialis 283 second digit at its radial side. This of cases. accessory muscle slip was innervated by (3) The third type of variation was a separate branch from the median nerve observed in one case. The muscle had to the distal part of the radial head an anomalous muscle slip derived from (Fig. 5). the radial part of the common flexor (2) In the second type of variation, tendon. This muscle slip gave rise to a the radial head was completely absent. tendon 3 cm long and 0.5 cm wide at the This variation was observed in two out middle of the forearm. A fleshy belly of 52 cases (3.8%). Iwami has reported 5 cm long then arose from this inter- such variations in 10% and Mori in 7.5% mediate tendon to fuse with the muscle

Fig. 6. A case (left side) with an abnormal digastric muscle slip. This arose from the common flexor tendon, was covered by the palmaris longus and the flexor carpi radialis and fused with the muscle belly of the 4th digit. 6-A : anterior aspect. Note that the muscle slip has an intermediate tendon (it). 6-B: posterior aspect showing the innervation. The proximal belly (pams) of this abnormal muscle was supplied by a branch from the median nerve (Nfcr) which also passed to the proximal part of the superficial layer and the flexor carpi radialis. The distal belly (dams) was innervated by a branch to the superficial layer and the muscle belly of the 4th digit. CHU, Npd, Npl, Nsf, Nr, Ndd, IT, A and CR : see legend to Figure 2. 284 0. Ohtani

belly destined for the fourth digit at its twig of the middle branch (Nsf in the radial border. The intermediate tendon text-figures) which went to the super- also had a weak connection with the ficial part of the humeroulnar head and upper border of the radial head. This a small part of the radial head (Fig. 6). proximal belly was innervated by a This anomalous case may represent a branch from the median nerve which combination of a separated slip of the went to the flexor carpi radialis and a flexor carpi radialis and a poorly deve- small part of the humeroulnar head. loped accessory superficial radial head (a The distal belly was innervated by a well developed example is described in

Fig. 7. A case (left side) with a well-developed, unusual superficial head. This covers almost the whole of the belly of the 3rd digit, and participates in the formation of the belly for the 4th digit. 7-A : anterior aspect. 7-B: posterior aspect showing the innervation. U: ulnar nerve, NI: median nerve, Ba : brachial artery, PT : pronator teres, BR : , FCU : flexor carpi ulnaris, Ra: radial artery. CHU, CR, Npd, Npl, Ndd and A: see legend to Figure 2. Structure of the Flaxor Digitorum Superficialis 285

(5)). broad but thin radial head which covered (4) in the fourth type of variation, almost the whole of the belly of the the tendon of the fifth digit was absent. third digit. This additional radial head Such variations have been reported by as well as the normal deep radial head many authors including Iwami (2%), was supplied by a twig of the middle Inoue (3%), Frohse and Fraenkel, and branch (Nsf) from the median nerve Rauber-Kopsch. In the present study, (Fig. 7). According to Outi (1969), such only one case (1.9%) was observed. a superficial radial head is not of rare (5) The single case of the fifth type occurrence, but we encountered only one was an additional case to the present case. statistics. The muscle belly destined (6) This case was also additional to for the fouth digit had an additional the present statistics. An accessory

Fig. 8. A case (right side) of an accessory muscle slip (ams) running down to the wrist. It arose from the radius as part of the normal radial head and was inserted into the flexor retinaculum. 8-A : anterior aspect. 8-B: posterior aspect showing the innervation. Note the accessory muscle slip innervated by a separate branch from the median nerve (Nams). CHU, CR, A, B, IT, Npd, Nsf, Nr and Ndd : see legend to Figure 2. 286 0. Ohtani

muscle slip arose from the anterior travaginal flexors of Haines), while border of the radius as a deep part of Straus (1942) postulates a dual origin the normal radial head of the flexor from the and the forearm muscles. digitorum superficialis. It separated from On the other hand, according to the the normal radial head 2 cm distal from embryological study of Lewis (1902), the the origin, and formed a thin tendon 0.2 flexor digitorum superficialis in the cm wide and 5 cm long. It passed human embryo develops from the com- downwards to develop a fusiform fleshy mon flexor premuscle mass in the 11 mm belly 1 cm wide and 7 cm long, along embryo which is continuous with the the radial side of the tendon to the tissue Of the digits. During develop- third finger. The muscle belly was ment, this muscle is attached to the replaced immediately above the carpal developing radius and ulna. In the 16 tunnel by a flat tendon 5 cm long. The mm embryo the flexor digitorum super- distal tendon passed through the carpal ficialis arises from the medial epicondyle tunnel and was inserted into the deep of the humerus and coronoid process, aspect of the flexor retinaculum. It was and stretches over the forearm and the supplied by a separate branch from the middle of the carpus. At this stage, the median nerve 4 cm proximal to the wrist muscle fibers extend to the middle of joint. In this case, the tendon for the the carpus where they give rise to four fifth digit was completely absent (Fig. tendons. Even in the 20 mm embryo, the 8). Several accessory muscles arising muscle fibers of the flexor digitorum from the anterior border of the radius superficialis reach the level of the base have previously been, reported by dif- of the metacarpus before giving rise to ferent authors. These muscles, however, the digital tendons. According to this were described either as being fused study, various patterns of the flexor with the tendon to the fifth digit (Mori, digitorum superficialis could have arisen 1964) or as having a usual insertion from an incomplete or complete division (Jones, 1966). The muscle slip reported of the flexor premuscle mass or failure here was not inserted into structures of the muscle fibers to retreat from the such as those cited above but fused with palm. Corning (1921) has already pos- the flexor retinaculum. tulated that the Muscle variations could The results of comparative studies in- have arisen from incomplete or abnormal dicate that the flexor digitorum super- splitting due to incompleteness or failure ficialis represents a rather newly de- in development at a given stage of dif- veloped muscle. It appears as such only ferentiation. Since the superficial flexor in mammals and is especially well de- premuscle mass begins division and re- veloped in primates, although diverse gression from the palm in the 11 mm opinions exist as to its possible genetic embryo, any disturbance could have oc- equivalent in lower vertebrates. Ribbing curred at, or probably before this stage (1938) and Haines (1950) are apparently (Jones, 1966). The present study provides of the same opinion, that the lacertilian further evidence of disturbed develop- homolog of the muscle is a series of ment ; namely, abnormal splitting or short hand muscles (the layer lb of the failure of regression from the palm of flexores digitorum breves superficiales the superficial flexor mass. of Ribbing, and the paratendinous in- Structure of the Flexor Digitorum Superficialis 287

Ref ereces 13) Iwami, S.: On the anomalous muscle originated from the of the m. 1) Carr, N.D. and O'Callaghan, D.: An un- flexor digitorum sublimis (Japanese text usual flexor of the fifth finger. Acta with English abstract). ibid. : 1086-1087, anat. 98: 376-379, 1977. 1951. 2) Chowdhary, D.S. : A rare anomaly of 14) Jones, E.G.: Some unusual muscular a- m. flexor digitorum sublimis. J. Anat. nomalies explained embryologically. Acta 85 : 100-101, 1951. anat. 64 : 517-530, 1966. 3) Corning, H.K. : Lehrbuch der Entwick- 15) Ko, K. und Takahashi, T.: Ueber einen lungsgeschichte des Menschen. Verlag Ausnahmefall des M. flexor digitorum von J.F. Bergman, Miinchen und Wies- superficialis (Japanese text). Kurume baden, 1921. Igaku. Z. 28 : 1447-1449, 1965. 4) Dahlgard, D.L. and Kawth, G.E. : An 16) Le Double, A.F. : Traite des variations anomalous arrangement of the flexor du systeme musculaire de l'homme. 2. musculature of the forearm and hand. Scleicher, Paris, 1897. Anat. Rec. 153: 251-256, 1965. 17) Lewis, W.H.: The development of the 5) Dylevsky, I.: The origin and the de- arm in man. Amer. J. Anat. 1: 145-185, velopmental explanation of some known 1902. variation of the flexor digitorum superfi- 18) Mainland, D.: An uncommon abnormal- cialis and profundus. Anthropologic ity of the flexor digitorum sublimis. J. (Brno) 6: 19-23, 1968. Anat. 62: 86-89, 1927. 6) Frohse, F. und Frankel, M.: Die Mus- 19) Mori, M.: Statistics on the musculature keln des menschlichen Armes. In Hand- of the Japanese. Okajimas Folia anat. buch der Anatomic des Menschen. ed. jpn. 40 : 195-300, 1964. by K. von Bardeleben, 2 Bd. Verlag von 20) Outi, H.: Myology. In Anatomy (Jap- Gustav Fischer, Jena, 1908. anese text) ed. by Mori, 0. et al. 10th 7) Fuchino, K.: Studies on muscles of the ed., Kanehara Shuppan, Tokyo, 1969. upper arm and forearm of the Japanese. 21) Kopsch, F.R. Lehrbuch und Atlas der Part II. Studies on the nerve distribu- Anatomic des Menschen. 1 Bd., 1955. tion of the flexor muscles of the forearm 22) Ribbing, L.: Die Muskeln und Nerven (Japanese text with English abstract). der Extremitaten. In Handbuch der ver- Kagoshima Igaku Z. 12: 32-53, 1960. gleichenden Anatomic der Wirbeltiere. 8) Haines, R.W. : The flexor muscles of ed. by Bolk, L., Gäppert, E., Kallius, E. the forearm and hand in lizards and and Lubosch, W. 5 Bd. Urban & Sch- mammals. J. Anat. 84: 13-29, 1950. warzenberg, Berlin-Wien, 1938. 9) Hayes, C.W. : Anomalous flexor sub- 23) Rojek Solarska, A. and Solarski, J.: A limis muscle with incipient carpal tun- case of double middle tendon of the nel syndrome. Plastic Reconstr. Surg. flexor digitorum superficialis muscle. 53: 479-483, 1974. Folia Morphol. (Warszawa) 30: 275-277, 10) Graper, R.: Eine sehr seltene Varietal 1971. des M. flexor digitorum sublimis. Anat. 24) Shrewsbury, M.M. and Kuczynski, K.: Anz. 50 80-84, 1917. Flexor digitorum superficialis tendon in 11) Inoue, R. On the forearm muscles of the fingers of the human hand. Hand, the Japanese and their relationships to 6: 121-133, 1974. the nerves and blood vessels distributed 25) Straus, W.L. : The homologes of the in them. (Japanese text). Kaibo-Gaku forearm flexors : Urodeles, Lizards, Mam- Z. 7: 1155-1207, 1934. mals. Amer. J. Anat. 70: 281-316, 1942. 12) Iwami, S.: On the musculature of the 26) Takeshige, Y., Kitano, Y., Kono, T., forearm and the hand of the fetuses. Kurematsu, K. und Oka, S.: Ueber die (Japanese text with English abstract). beiderseitige Anomalie des Musculus di- Igaku Kenkyu 21 : 1073-1085, 1951. giti secundi et quinti des M. flexor digi- 288 0. Ohtani •

torum superficialis (Japanese text with port df a case. J. Bone JT. Surg. (Edin- German abstract). Kurume Igaku. Z. burgh) 52B: 757-759, 1970. 22: 22-32, 1959. 28) Wood, J.: On the muscular variations 27) Vichare, N.A. : Anomalous muscle belly and their relation to comparative anato- of the flexor digitorum superficialis. Re- my. J. Anat., Land. 1: 44-59, 1867.