Purpose of the study: The flexor hallucis brevis (FHB) is one of the short muscles of the foot. It divides in front into two portions, which are inserted into the medial and lateral sides of the base of the first phalanx of the great toe. The detailed knowledge of its insertion into the proximal phalangeal base is decisive in any surgery involving the first metatarsophalangeal (MTP) joint, such as implant arthroplasty, resection arthroplasty or amputation. Complications resulting from injury to this insertion are commonly known. The aim of this study was to describe in detail the morphology of FHB insertion sites and to determine a safe extent of resection to be done at the base of the proximal phalanx.
Material and methods: In 36 cadaver specimens we measured FHB insertion length from the plantar side of the proximal phalangeal base and from the plane perpendicular to the long axis of the phalanx and passing through the base of the concave articular surface of the phalanx.
Results: Measured from the plantar base of the proximal phalanx, the mean length of the medial insertion site was 11.5 ± 0.9 mm (range, 9.5 to 13.0 mm) and that of the lateral insertion site was 9.5 ± 1.1 mm (range, 8.0 to 11.5 mm). After resection, the mean values for medial and lateral FBH insertion sites were 8.5 ± 1.7 mm (range, 6.5 to 11.0 mm) and 7.1 ± 1.4 mm (ran- ge, 5.5 to 9.5 mm), respectively. The mean total proximal phalangeal length was 33.1 ± 2.2 mm (range, 28.5 to 37.0 mm), reduced after resection to 30.5 ± 2.1 mm (range, 26.0 to 34.5 mm).
Discussion: Our results show that the medial insertion site, where the medial FHB tendon and distal part of the abductor hallucis muscle are joining, is longer than the lateral site. Therefore the length of the lateral site is decisive for preserving FHB function. Since the plane perpendicular to the long axis of the phalanx and passing through the base of the concave articular surface of the phalanx is almost identical with the beginnings of FHB insertions, it seems optimal for clinical practice to perform the initial resection along this plane.
Conclusions: To preserve at least one third of the FHB insertion, the final resection should not exceed 4 mm or 13 % of the proximal phalangeal length, as measured from the reference plane defined above.