Anatomical study for flexor hallucis longus tendon transfer in treatment of Achilles tendinopathy

The purpose of the study was to describe the anatomical variations of the connection between the flexor hallucis longus (FHL) and flexor digitorum longus (FDL) tendons in the knot of Henry in Asians, and quantify the length of FHL tendon graft with different incisions.

Sixty-four embalmed feet of 32 cadavers were analyzed anatomically with respect to the individual cross-links in the planta pedis. Single incision technique graft length was measured from the musculotendinous junction of FHL and the point at sustentaculum tali. Double incision technique was measured from musculotendinous junction of FHL and the level of the master knot of Henry. Additionally, minimally invasive incision technique was measured from musculotendinous junction of FHL to the first interphalangeal joint. These three techniques were then combined to determine the total potential tendon graft length obtainable using different approach.

Only two different configurations were found. Type 1, a tendinous slip branched from the FHL to the FDL (62 of 64 feet). Type 2, a slip branched from the FHL to the FDL and another slip from the FDL to FHL (2 of 64). The average length of the FHL graft available from a single incision measured 5.08 cm (range 3.32-10.35, SD = 1.09), double incision technique measured 6.72 cm (range 4.69-12.09, SD = 1.03), and minimally invasive incision measured 17.49 cm (range 13.51-20.52, SD = 1.80). The difference between the lengths obtained from these three techniques was statistically significant (p < 0.001).

The absence of no attachment and FDL tendon to the FHL between the two tendons in the foot may be more frequent than previously reported. Only two configurations of the anatomical relationship were found in this study. In over 96 % of the feet, a proximal to distal connection from the FHL to the FDL was found, which might contribute to the residual function of the lesser toes after FDL transfer.