Illustrated Encyclopedia of Human Anatomic Variation: Opus I: Muscular System: Alphabetical Listing of Muscles: F

Flexor Hallucis Longus

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

Complete division of the tendons is very rare. A connecting slip from flexor hallucis is distributed, in more than 50% of cases, to the tendons of the second and third toes; less frequently, it passes to the second toe alone, or to the second, third or fourth, or, rarely, to all five toes. A doubled flexor hallucis longus has been reported, with both tendons inserting on the terminal phalanx of the great toe.

Macalister discusses the flexor tendons of the foot, having summarized the work of several anatomists, as follows. Turner provided an analysis of 50 specimens and has found that no two are alike. That in all, the flexor hallucis provides a slip to flexor digitorum longus, which is flat, membranous, or rounded. In eleven, this slip went to the second toe alone, forming about one half of its tendon in four cases, as described by Arnold, Henle, and Theile. It forms more than half in six cases, and nearly the whole, as described by Diemerbröck and Meckel, in one. In twenty cases it bifurcated, and ended in the long tendons of the second and third toes (Church, Rolleston, Huxley). Of these, the second toe slip was larger than the third, in twelve cases; in one of which it formed, with the accessorium (quadratus plantae), the whole of the second toe tendon. In six cases it was equally distributed to the second and third toes; and in two cases formed nearly all the tendons of both toes. In eighteen it went to the second, third, and fourth toes with a larger share going to the second in seven; to the second and third in five; to the second and fourth in one; and equally to all three in five. In one case it went to all four outer (lateral) toes; and in this case the slips for the second and third were larger than those for the fourth and fifth. In all cases the slip from the flexor hallucis never joined the tendon of the long flexor before the division of the latter into its digital slips; in nine specimens a band passed from the tendon of the long flexor before its division to join the flexor hallucis beyond the origin of the connecting link for the common flexor. Schultze has provided an analysis of 100 tendon arrangements. In thirty-two of these the slip from flexor hallucis went to the second toe flexor alone; in fifty-eight to two toes, second and third; in ten to the second, third. and fourth; in twenty-nine cases the slip from flexor longus to the hallucis was present. (Wood also found this six times in thirty-six cases, 1938). Mr. Wood found in one case the first tendon of flexor digitorum longus absent, and its place taken by a slip from flexor hallucis. Perfect separateness of the tendons is also rare, but has been seen by Wood and by Macalister. Flexor digitorum sent slips to flexor hallucis, and the latter gave off tendons to the second and third toes; that to the second was the only perforating tendon for it, as there was none from the common flexor. A slip from the flexor communis joined the third toe tendon. In a paper on flexor digitorum and on the union of the flexor tendons, Gies describes in one case, a tendon from the flexor digitorum joining that of the hallucis opposite the metatarso-phalangeal joint, the flexor digitorum giving off before it joined the accessorius, a slip from which arose two bellies, each one ending in a tendon which joined the perforating tendon of the fourth and fifth toes respectively.

Other modes of union, the following have been described or seen by Macalister. He saw the great toe supplied by the hallucis, the second by a tendon composed of:

1. A slip from extensor hallucis;
2. One tendinous from accessorius;
3. Two thin threads from the communis;

the third toe supplied by a slip from the communis, and one from the accessorius; the fourth and fifth by the long toe flexor alone. In one subject, Brugnone found the flexor longus with a fifth tendon, which acted as the perforated flexor of the little toe. Flower and Murie saw the following arrangement in a bushwoman; the flexor digitorum supplied the fifth, and partly the fourth, third. and second, the flexor hallucis assisted it in supplying these, and exclusively acted on the hallux. See also Flexor digitorum longus.

A muscle closely related to flexor hallucis longus is fibulotarsalis, which arises from the fibula and inserts, via the sustentaculum, to the underside of calcaneus and the navicular bones.

In the normal arrangement of the long flexor muscles, several features indicate that their degree of differentiation is not the same as that between the corresponding muscles in the upper limb. In the leg, both flexors are closely connected; in the foot, the flexors of the great toe do not act alone, but also send slips to other toes. In a few instances, however, progressive variations have been observed in the form of special muscular slips for the second and fifth toes. Supplementary fasciculi of varying size have been described arising from the fascia of the leg and joining the tendon of flexor digitorum longus at the ankle. A slip has also been observed arising from the fascia over flexor digitorum and joining the tendon of flexor hallucis longus in the sole of the foot.

Flexor hallucis longus itself, other than the tendon unions, has very few variations. Bergmann found a single tendon from a bicipital muscle which arose from the outer layer of the tibial fascia going to flexor hallucis (quoted by Henle).

A more frequent variation (1-5% of bodies) is flexor accessorius longus, also known as longus, accessorius ad quadratum plante, accessorius ad flexor accessorium, accessorius longus ad flexor digitorum longum, accessorius flexori hallucis longi superior (Reinhart), accessorius ad accessorium (Turner) or peroneocalcaneus internus (Macalister). Its origin in the leg can vary widely (from the tibia or fibula, or from both bones, the deep fascia, or one of the muscles); it ends in a tendon which, after passing beneath the flexor retinaculum of the foot, joins the accessorius (quadratus plantae) or flexor digitorum longus in the sole. This muscle, when present, is located in the pre-Achilles fat pad of the lower calf and may appear radiographically as a soft tissue mass.

Flexor Accessorius Longus.
from Nidecker, Hochstetter, and Fredenhagen

References

Focacci, M. (1903) Etude morphologique sur les Fléchisseurs longs du pied. Arch. Ital. Biol. 39:486-487.

Henle, J. (1871) Handbuch der Muskellehre des Menschen, in Handbuch der systematischen Anatomie des Menschen. Verlag von Friedrich Vieweg und Sohn, Braunschweig.

Körner, F, (1949-50) Eine Verdoppelung des Musculus flexor hallucis longus. Anat. Anz. 97:16-22.

LeDouble, A.F. (1897) Traité des Variation du Système Musculaire de l'Homme et Leur Signification au Point de l'Anthropologie Zoologique. Libraire C. Reinwald, Schleicher Freres, Paris.

Macalister, A. (1875) Additional observations on muscular anomalies in human anatomy (third series), with a catalogue of the principal muscular anomalies hitherto published. Trans Roy. Irish Acad. Sci. 25:1-134.

Nidecker, A.C., Hochstetter, A. von., and H. Fredenhagen . (1984) Accessory muscles of the lower calf. Radiology. 151:47-48

Turner, W. (1864-67) On variability in human structure, with illustrations, from the flexor muscles of the fingers and toes.Trans. Roy. Soc. Edinburgh. 24:175-189.

Testut, L. (1884) Les Anomalies Musculaires Chez L'Homme Expliques par L'Anatomie Comparée, Masson, Paris.

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