Structure of the Flexor Digitorum Superficialis

Transcription

1 Okajimas Folia Anat. Jpn., 56(5) : , 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, Forearm musculature. Summary. Fifty-two forearms 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 tendons for the third and fourth digits, respectively. The deep layer is composed of the deep part of the humeroulnar head. After forming an intermediate tendon, 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 The organization of the flexor digitorum superficialis has been studied by many workers such as Wood (1867), Rauber-Kopsch (1955), Inoue (1935) and Frohse and Fraenkel (1908). Its anomalies have also been reported by various authors such as Le Double (1897), Graeper (1917), Mainland (1927), Iwami (1951), Chowdhary (1951), Takeshige et al. (1959), Mori (1964), Dahlgard and Kawth (1965), Ko and Takahashi (1965), Jones (1966), Dylevsky (1968), Vichare (1970), Rojek Solarska and Solarski (1971), Shrewsbury and Kuczynski (1974), Hayes (1974), Carr 277

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

3 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.

4 Ohtani deep aspect of the muscle belly for the third digit. This fasciculus occurred in 19 out of 52 muscles (36.5%). Based on the occurrence of these two communicating fasciculi, the flexor digitorum superficialis can be classified into four types. Type I: ThisRis the simplest type as indicated above, there being no communicating fasciculi between the superficial layer and the deep layer. Type II : This type has only fasciculus A. 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 : uppermost 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.

5 Structure of the Flexor Digitorum Superficialis 281 A and fasciculus B. Type IV : This type has fasciculus B only. These four types are illustrated schematically in Fig. 1. In the present study, type II was the most common (29/52, 55.8%), followed by type III (18/52, 34.6 %). The other types, I (4/52, 7.7%) and IV (1/52, 1.9%), were rare. The correlation between the occurrence of fasciuli A and B was insignificant : the X2 value for the 2 x 2 table was as small as 0.65 without Yates' correction. No significant differences in occurrence were detected between the sexes, or between the right and left muscles. Innervation : The flexor digitorum superficialis is usually innervated by four to five branches from the median nerve. The uppermost one (Npd in the textfigures) arises before the median nerve traverses the pronator teres, and passes mainly to the deep part of the humeroulnar head. One of its branches penetrates through both parts of the humeroulnar head to reach the palmaris longus (Npl in the text-figures). The middle branch (Nsf in the textfigures) arises just before the median nerve passes behind the aponeurotic membrane which bridged between the humeroulnar head and the radial head. It enters the superficial part of the humeroulnar head near its ulnar border and is ramified to supply the whole superficial layer of the humeroulnar head, the communicating muscle fasciculus A and then to the distal belly for the fifth digit, the communicating muscle fasciculus B and the radial head. Sometimes at the middle of the forearm, a branch (NI- in the text-figures) to the radial head and the radial and distal parts of the superficial layer of the humeroulnar head, i. e. to the belly for the third digit, arises separately from the median nerve. At about the junction of the middle and lower thirds of the forearm, one or two distal branches (Ndd in the textfigures) arise from the median nerve and pass into the distal belly of the deep layer, mainly in the distal belly for the second digit (Figs. 2-A, B, C). In one case, a slender branch from the ulnar nerve supplied a small part of the deep layer of the humeroulnar head. The other part of this muscle, however, was innervated by the median nerve. Fuchino (1960) has reported that at the middle of the forearm the upper part of the muscle belly for the index finger of the flexor digitorum superficialis is sometimes innervated by a branch from the ulnar nerve (12%). In our case, however only a small part of the humeroulnar head was innervated by the ulnar branch (Fig. 3). The brachial plexus of this case was composed of C5, C6, C7, C8 and Thl as usual. A small root from the lateral fasciculus derived from C5, C6 and C7 fused with the main root from the medial fasciculus composed of C8 and Thl to form the ulnar nerver. However, the detailed relation between this unusual root and the incidental peripheral ulnar innervation of the muscle requires further investigation. Other variations : Six minor kinds of variations were observed in the flexor digitorum superficialis ; two of them, however, were from materials other than the systematically examined 52 muscles, and cannot be treated statistically. (1) The first type of variation which was found in three muscles (5.8%) had small accessory deep radial heads which fused with the deep layer of the muscle. One case showed two such muscle slips which arose from the radius 2 cm and 5 cm distal from the humeroradial joint, respectively, and fused with the intermediate tendon of the deep layer. The proximal one was 0.6 cm wide and 4 cm

6 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 wide and 4 cm long. Both were supplied by a branch from the median nerve which went to the flexor digitorum profundus (Fig. 4). This suggests that these muscle slips may possibly derive from the latter muscle. In the other two cases, a thin accessory muscle slip about 1 cm wide arose from the middle of the radius just behind the normal radial head of this muscle and fused with the muscle belly for the

7 Structure of the Flexor Digitorum Superficialis 283 second digit at its radial side. This accessory muscle slip was innervated by a separate branch from the median nerve to the distal part of the radial head (Fig. 5). (2) In the second type of variation, the radial head was completely absent. This variation was observed in two out of 52 cases (3.8%). Iwami has reported such variations in 10% and Mori in 7.5% of cases. (3) The third type of variation was observed in one case. The muscle had an anomalous muscle slip derived from the radial part of the common flexor tendon. This muscle slip gave rise to a tendon 3 cm long and 0.5 cm wide at the middle of the forearm. A fleshy belly 5 cm long then arose from this intermediate 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.

8 Ohtani belly destined for the fourth digit at its radial border. The intermediate tendon also had a weak connection with the upper border of the radial head. This proximal belly was innervated by a branch from the median nerve which went to the flexor carpi radialis and a small part of the humeroulnar head. The distal belly was innervated by a twig of the middle branch (Nsf in the text-figures) which went to the superficial part of the humeroulnar head and a small part of the radial head (Fig. 6). This anomalous case may represent a combination of a separated slip of the flexor carpi radialis and a poorly developed accessory superficial radial head (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 : brachioradialis, FCU : flexor carpi ulnaris, Ra: radial artery. CHU, CR, Npd, Npl, Ndd and A: see legend to Figure 2.

9 Structure of the Flaxor Digitorum Superficialis 285 (5)). (4) in the fourth type of variation, the tendon of the fifth digit was absent. Such variations have been reported by many authors including Iwami (2%), Inoue (3%), Frohse and Fraenkel, and Rauber-Kopsch. In the present study, only one case (1.9%) was observed. (5) The single case of the fifth type was an additional case to the present statistics. The muscle belly destined for the fouth digit had an additional broad but thin radial head which covered almost the whole of the belly of the third digit. This additional radial head as well as the normal deep radial head was supplied by a twig of the middle branch (Nsf) from the median nerve (Fig. 7). According to Outi (1969), such a superficial radial head is not of rare occurrence, but we encountered only one case. (6) This case was also additional to 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.

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

11 Structure of the Flexor Digitorum Superficialis 287 Ref ereces 1) Carr, N.D. and O'Callaghan, D.: An unusual flexor of the fifth finger. Acta anat. 98: , ) Chowdhary, D.S. : A rare anomaly of m. flexor digitorum sublimis. J. Anat. 85 : , ) Corning, H.K. : Lehrbuch der Entwicklungsgeschichte des Menschen. Verlag von J.F. Bergman, Miinchen und Wiesbaden, ) Dahlgard, D.L. and Kawth, G.E. : An anomalous arrangement of the flexor musculature of the forearm and hand. Anat. Rec. 153: , ) Dylevsky, I.: The origin and the developmental explanation of some known variation of the flexor digitorum superficialis and profundus. Anthropologic (Brno) 6: 19-23, ) Frohse, F. und Frankel, M.: Die Muskeln des menschlichen Armes. In Handbuch der Anatomic des Menschen. ed. by K. von Bardeleben, 2 Bd. Verlag von Gustav Fischer, Jena, ) Fuchino, K.: Studies on muscles of the upper arm and forearm of the Japanese. Part II. Studies on the nerve distribution of the flexor muscles of the forearm (Japanese text with English abstract). Kagoshima Igaku Z. 12: 32-53, ) Haines, R.W. : The flexor muscles of the forearm and hand in lizards and mammals. J. Anat. 84: 13-29, ) Hayes, C.W. : Anomalous flexor sublimis muscle with incipient carpal tunnel syndrome. Plastic Reconstr. Surg. 53: , ) Graper, R.: Eine sehr seltene Varietal des M. flexor digitorum sublimis. Anat. Anz , ) Inoue, R. On the forearm muscles of the Japanese and their relationships to the nerves and blood vessels distributed in them. (Japanese text). Kaibo-Gaku Z. 7: , ) Iwami, S.: On the musculature of the forearm and the hand of the fetuses. (Japanese text with English abstract). Igaku Kenkyu 21 : , ) Iwami, S.: On the anomalous muscle originated from the fascia of the m. flexor digitorum sublimis (Japanese text with English abstract). ibid. : , ) Jones, E.G.: Some unusual muscular a- nomalies explained embryologically. Acta anat. 64 : , ) Ko, K. und Takahashi, T.: Ueber einen Ausnahmefall des M. flexor digitorum superficialis (Japanese text). Kurume Igaku. Z. 28 : , ) Le Double, A.F. : Traite des variations du systeme musculaire de l'homme. 2. Scleicher, Paris, ) Lewis, W.H.: The development of the arm in man. Amer. J. Anat. 1: , ) Mainland, D.: An uncommon abnormality of the flexor digitorum sublimis. J. Anat. 62: 86-89, ) Mori, M.: Statistics on the musculature of the Japanese. Okajimas Folia anat. jpn. 40 : , ) Outi, H.: Myology. In Anatomy (Japanese text) ed. by Mori, 0. et al. 10th ed., Kanehara Shuppan, Tokyo, ) Kopsch, F.R. Lehrbuch und Atlas der Anatomic des Menschen. 1 Bd., ) Ribbing, L.: Die Muskeln und Nerven der Extremitaten. In Handbuch der vergleichenden Anatomic der Wirbeltiere. ed. by Bolk, L., Gäppert, E., Kallius, E. and Lubosch, W. 5 Bd. Urban & Schwarzenberg, Berlin-Wien, ) Rojek Solarska, A. and Solarski, J.: A case of double middle tendon of the flexor digitorum superficialis muscle. Folia Morphol. (Warszawa) 30: , ) Shrewsbury, M.M. and Kuczynski, K.: Flexor digitorum superficialis tendon in the fingers of the human hand. Hand, 6: , ) Straus, W.L. : The homologes of the forearm flexors : Urodeles, Lizards, Mammals. Amer. J. Anat. 70: , ) Takeshige, Y., Kitano, Y., Kono, T., Kurematsu, K. und Oka, S.: Ueber die beiderseitige Anomalie des Musculus digiti secundi et quinti des M. flexor digi-

12 Ohtani torum superficialis (Japanese text with German abstract). Kurume Igaku. Z. 22: 22-32, ) Vichare, N.A. : Anomalous muscle belly of the flexor digitorum superficialis. Report df a case. J. Bone JT. Surg. (Edinburgh) 52B: , ) Wood, J.: On the muscular variations and their relation to comparative anatomy. J. Anat., Land. 1: 44-59, 1867.