Background: Length of flexor hallucis longus (FHL), localization of master knot of Henry (MKH) and relationship between MKH and neurovascular bundle are essential for the achievement of FHL tendon transfer. The purpose of this study is to define the localization of MKH in reference to bony landmarks of the foot, its relationship to plantar neurovascular bundle and to investigate in situ and ex vivo length of FHL tendon in single incision, double incision and minimally invasive techniques. Materials and methods: Foot length was examined in sixty-two feet of thirty-one soft cadavers (9 males, 22 females). Various parameters including the relationship between MKH and neurovascular bundle, the distances from MKH to medial malleolus (MM), navicular tuberosity (NT) and the first interphalangeal joint of great toe (IP) were measured. Surface localization of MKH in relation to a line joining the medial end of plantar flexion crease at the base of great toes (MC) to NT (MC-NT line) was determined. Lengths of FHL tendon graft from three surgical techniques were examined. In situ length was measured in the plantar surface of foot and ex vivo length was measured after tendon was cut from its insertion. Results: The mean length of foot was 230.98±15.35 mm with a statistically significant difference between genders in both sides (p<0.05). No distance was found between medial plantar neurovascular bundle (MPNVB) and MKH. Mean distance of 17.13±3.55 mm was found between lateral plantar neurovascular bundle (LPNVB) and MKH. MKH was located at a mean distance of 117.11±1.00 mm proximal to IP, 26.28±4.75 mm under NT and 59.58±7.51mm distal to MM with a statistically significant difference of MKH-IP distance between genders in both sides and MKH-NT in right side. MKH was located anterior to NT (66.1 %), at NT (27.4%) and posterior to NT (6.5%) on the MC-NT line. Surface localization of MKH was 94.75+8.43% of MC-NT line from MC with a perpendicular distance of 25.11±5.37mm below MC-NT line. The in situ and ex vivo tendon lengths from MTJ to ST, to MKH and to IP were 39.05±10.88 mm and 34.43±10.23 mm, 73.45±9.91 mm and 68.63±9.43 mm, 197.98±13.89 and 191.79±14.00 mm, respectively. A statistically significant difference between genders was found in MTJ-IP of in situ and ex vivo length of both sides (p < 0.05). The mean length of tendon between in situ and ex vivo was significantly different in all techniques (p < 0.05). A moderate positive correlation between foot length and tendon length was found in MTJ-IP of both in situ and ex vivo tendon length. Conclusions: A statistically significant difference between in situ and ex vivo tendon length was shown in all harvesting techniques. Surface location of MKH was approximately at 95%of MC-NT line from MC with a perpendicular distance of 25 mm from MC-NT line.

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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 and lateral bellies of FHL (MB and LB) were identified and traced until the end of the most distal muscle fiber to determine the medial and lateral MTJs. MTJ was classified into 4 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

Tendinous interconnections between flexor hallucis longus (FHL) and flexor digitorum longus (FDL) have functional importance in toe movement and act as a natural tenodesis after harvesting FHL tendon proximal to master knot of Henry (MKH). This anatomical study aimed to clarify the type, location and distribution to lesser toes of tendinous interconnection between FHL and FDL tendons in 102 embalmed and 62 soft cadaveric feet. According to the classification of Beger et al., three types of interconnection (type I, II, V) were found in 85.4 %, 0.6%, and 7.9% of specimen, respectively. In addition, a new type of connection (6.1%) in which is FHL tendon bifurcated into one tendon to the first toe and the other tendon fused with FDL tendon. The tendinous interconnection was located either proximal (−) or distal (+) to MKH. The mean distance from MKH in type I was1.38±10.40 mm (from −59.47 to 20.52 mm). The proximal and distal interconnection slips in type V were located at 6.61±4.69 mm (from 0.00 to 12.47mm) and 12.95±3.48mm (from 7.05 to 16.13 mm) from MKH. In the new type, the interconnection slip was located at −6.59±8.93 mm (from −20.81 to 5.48 mm) from MKH. The distribution of tendinous slip to lesser toes was determined by pulling the FHL tendon and observing the lesser toes’ movement. According to the classification of Plaass et al., the prevalence of distribution of tendinous slip to lesser toes in type a, b, c and d was 19.5%, 67.7%, 11.0%, and 1.8%, respectively. Knowledge of the tendinous interconnection and its location are important for enhancing the clinical efficacy of FHL tendon transfer and minimizing of the functional loss of toes in post‐operative period. Support or Funding Information 1. The ๑ ๐ ๐ th Anniversary Chulalongkorn University Fund for Doctoral Scholarship from the Graduate School, Chulalongkorn University 2. Overseas Academic Presentation Scholarship for Graduate Students from the Graduate School, Chulalongkorn University