Implications for the anatomy of the flexor hallucis brevis insertion

Excessive hallux proximal phalangeal base removal during first MTP resection arthroplasty, implant arthroplasty, or amputation can compromise the flexor hallucis brevis (FHB) insertion leading to first MTP joint plantarflexion weakness, cock-up toe deformity, and altered forefoot loading.

The insertional anatomy of the FHB tendon was measured in 30 fresh frozen cadaver specimens. The FHB was then cyclically loaded and bone sequentially removed from the proximal phalangeal base until specimen failure occurred.

The mean length of the lateral and medial FHB insertions measured 9.0 +/- 0.6 mm and 8.0 +/- 0.5 mm, respectively (p < 0.0001). The mean width of the lateral and medial FHB insertions measured 7.1 +/- 1.0 mm and 8.8 +/- 1.4 mm, respectively (p < 0.0001). FHB insertion length as a percentage of total proximal phalangeal length was 26% +/- 3% medially and 30% +/- 3% laterally. Twenty-two specimens failed at the FHB insertion site after an average of 7.8 +/- 1.0 mm or 24% +/- 3% of the total length of the proximal phalanx had been resected from the base.

Six millimeters of bone or 20% of the proximal phalangeal length could be safely removed from the hallux proximal phalangeal base without compromising the integrity of the FHB insertion to a physiologic load. Retaining 10 mm of the hallux proximal phalangeal base during amputation would preserve the medial FHB insertion and sacrifice none to less than 0.4 mm of the lateral insertion in our specimens.

This study demonstrates how much hallux proximal phalangeal base can be resected during first MTP arthroplasty or needs to be retained during amputation to preserve FHB function.