Sesamoid injury is a common and often missed cause of pain under the 1st metatarsal head, seen mainly in runners, dancers, footballers and military recruits.
Sesamoiditis is a generic term rather than a precise diagnosis. It covers stress reaction, stress fracture, acute fracture, osteonecrosis, a symptomatic bipartite sesamoid, chondromalacia and sesamoid injury with turf toe. Persistent pain needs imaging, because management differs between these.
The 2 hallux sesamoids sit within the medial and lateral heads of flexor hallucis brevis, beneath the 1st metatarsal head. The flexor hallucis longus (FHL) tendon runs between them, and the intersesamoid ligament binds the pair. A ridge on the metatarsal head (the crista) separates them; wear of the crista lets them sublux, typically laterally with hallux valgus.
The sesamoids act as a pulley for flexor hallucis brevis, protect the FHL tendon and take load through the 1st ray at push-off, up to 300% of body weight according to BOFAS. The medial (tibial) sesamoid sits more directly under the metatarsal head and is injured more often. The blood supply enters proximally, so the distal pole is relatively poorly vascularised, which predisposes to delayed union and osteonecrosis after fracture.
A bipartite sesamoid, most often medial, is a normal variant. Reported prevalence varies widely, from about 2% to 15% of feet in radiographic series, and it is not always bilateral.
Risk factors:
Examine with the hallux passively dorsiflexed: this exposes the sesamoids and separates sesamoid tenderness from joint-line tenderness.
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