Background: Anatomy of flexor hallucis longus (FHL) is essential for the achievement of tendon transfer and several procedures performed in the foot and ankle. The aim of this study was to evaluate the anatomical knowledge of FHL including the type and location of musculotendinous junction (MTJ), tendinous interconnections (TIC) morphology, its location related to Master Knot of Henry (MKH), and the pattern of TIC distribution.
Materials and methods: One hundred and sixty-six legs from 52 embalmed and 31 soft cadavers were assessed. The medial (MB) and lateral (LB) bellies of FHL were identified and traced until the end of the most distal muscle fibre to determine the medial and lateral MTJs. MTJ was classified into four types based on the existence and length of MB and LB: type 1, long LB and shorter MB; type 2, equal length of both bellies; type 3, only LB and no MB; type 4, long MB and shorter LB. Low lying muscle belly was defined as muscle extending beyond the zero point (the point of intersection between distal osseous part of tibia and FHL tendon). The distance between MTJ and zero point was measured. TIC was classified into seven types based on the direction and number of slip: type I, one slip from FHL to flexor digitorum longus (FDL); type II, crossed connection: type III, one slip from FDL to FHL; type IV, no connection; type V, two slip from FHL to FDL; type VI, two slip from FHL to FDL and one slip from FDL to FHL; type VII, two slips from FDL to FHL and one slip from FHL to FDL. The distance between the TIC and MKH was measured. TIC distribution was defined into four types based on slip distribution to lesser toes: type a, distributed to second toe; type b, distributed to second and third toes; type c, distributed to second to fourth toes, and type d, distributed to second to fifth toes.
Results: Type 1 and type 3 of MTJ morphology were found in 87.3% and 12.7%, respectively. Low lying LB was detected in 66.13% of cases with a mean distance of 13.10 ± 4.51 mm. All MBs ended proximal to the zero point with a mean distance of –21.99 ± 13.21 mm. Three types of TIC (I, II, V) were identified. The highest frequency was type I (82.93%). In addition, a new type of TIC was depicted in 8.53% of cases. Part of the FHL tendon in this type fused with FDL tendon and the rest extended directly to the first toe. TIC could be located either proximal, distal or at the MKH. The highest prevalence was distal to MKH in 51.67% of cases with a mean distance of 11.23 ± 5.13 mm and 8.73 ± 4.2 mm in low lying and non-low-lying groups, respectively. Four types of slip distribution to lesser toes were defined, mostly in type b. No statistically significant differences were detected among all parameters including genders, sides, and groups.
Conclusions: Knowledge of this investigation might enhance the clinical efficacy of tendon harvesting and transfer in foot and ankle surgery.