The two sesamoid bones (tibial and fibular) beneath the first metatarsal head each form a small synovial joint with the plantar surface of the first metatarsal head. The tibial sesamoid articulates with the medial metatarsal head facet and the fibular sesamoid with the lateral facet. Together with the intersesamoid ridge and the crista, these articulations guide the sesamoid complex during first MTP joint flexion and extension.
Sesamoiditis involves pain and inflammation of these sesamometatarsal articulations, most common in runners and dancers from repetitive first MTP loading. The tibial sesamoid is the larger and more commonly injured of the two. Bipartite sesamoid, a normal variant in up to 30% of individuals, must be distinguished from fracture on plain radiograph by comparing with the contralateral foot and noting smooth corticated edges. Avascular necrosis of the sesamoid (Renander disease) causes progressive collapse visible on MRI.
Inflammation and stress injury at the sesamoid-metatarsal articulation produces plantar first MTP pain under the metatarsal head, worsened by toe-off and relieved by metatarsal pad offloading, managed with rest, orthotics, and occasionally cortisone injection or surgical sesamoidectomy.
Acute fracture from landing on the foot or chronic stress fracture from repetitive loading produces sharp plantar pain with MTP extension, differentiated from bipartite sesamoid by comparing with the contralateral side and using MRI to confirm bone oedema.
