[Relationship of the flexor hallucis longus and flexor digitorum longus tendons in the plantar midfoot. An anatomical cadaver study].

Purpose Of The Study: To verify, in cadaver specimens, the precise anatomic structure of cross-connection between the flexor hallucis longus (FHL) and flexor digitorum longus (FDL) tendons and to identify its position in relation to orientation points in the foot in view of the surgical technique for tendon transfer, including the necessity of subsequent tendon suture.

Material And Methods: A hundred cadaver legs (50 cadavers of 29 men and 21 women, all Caucasians) were dissected by an extensive medio-plantar approach. The anatomical relationship of the FHL and FDL tendons was studied. The distances measured were as follows: between the first metatarsophalangeal joint line and the FHL origin of a tendinous slip; between the slip and the calcaneal tubercle; and the total distance between the first metatarsophalangeal joint line and the calcaneal tubercle.

Results: In all cadaver legs, a cross-connection between the FHL and FDL tendons was present in the plantar midfoot. There was always a tendinous slip branching from the FHL tendon and running lateral and distal to the FDL tendon. The slip was attached to the FDL tendon distal to its branching for the toes; it either attached to only the FDL tendon for the second toe or it split to attach to the FDL tendon branches for the second and the third toe. The two configurations were referred to as type II and type II,III, respectively. Type II was found in both legs of 31 cadavers (62 %), and type II,III in 14 cadavers (28 %). In five cadavers (10 %) attachment was different in the right and the left foot. Expressed in absolute numbers, there were 67 type II attachments (two-thirds of the findings) and 33 type II,III attachments (one-third of the findings). The average distance between the first metatarsophalangeal joint line and the origin of an FHL tendinous slip was 7.3 (± 0.9 ) cm, the average distance between the origin of an FHL tendinous slip and the calcaneal tubercle was 9.2 (± 1.1) cm and the average distance between the first metatarsophalangeal joint line and the calcaneal tubercle was 16.5 (± 1.6) cm.

Discussion: Various presentations of the anatomical relationship between the FHL and FDL tendons in the midfoot are found in the literature. Some describe it as a mere cross-connection between the tendons; others report certain forms of attachment. Only two papers on this issue have recently been available in the international literature and their findings markedly differ from our observations made on a significantly higher number of specimens.

Conclusions: The FHL and FDL tendons in the midfoot are cross-connected with a tendinous slip branching proximal to the FHL tendon to be attached to the FDL tendon distal to its branching for the second or the third toe. Transfer of the FHL and FDL tendons is often used in the treatment of tibialis posterior muscle dysfunction, extensive inveterated ruptures of the Achilles tendon and, less frequently, posterior foot balancing in neuromuscular foot deformities. The anatomic pattern of a tendon cross-connection in relation to orientation points in the foot varies to such an extent that, during surgery, it is impossible to keep strictly to the recommended halves of the distances between the calcaneal tubercle and the first metatarsophalangeal joint. The length of a FDL graft is limited by the level of tendon branching for the toes; it is not necessary to cut through the slip. Because of the anatomical structure of tendon cross-connection, we consider it necessary to suture the distal stump of one tendon to the other tendon to retain their correct function.