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A fibular sesamoid is one of two tiny bones found along the underside of the first metatarsophalangeal joint in the foot, the joint at the base of the big toe. Along with the tibial sesamoid, it is nestled among the fibers of a tendon crossing the joint, the flexor hallucis brevis (FHB) tendon. In doing so, these bones hold the FHB tendon and the tendon of the flexor hallucis longus (FHL) muscle, which act as a pulley on the joint to flex or curl the big toe downward, farther away from the joint. The tibial and fibular sesamoid serve the dual functions of improving the effectiveness of the tendons at moving the joint by increasing the moment arm, and shielding the tendons from damage by forces caused by the foot pushing off the ground during gait movements.
Forming the first metatarsophalangeal joint are the first metatarsal bone, the long bone of the foot that runs between the base of the big toe and the arch, and the first proximal phalanx, the near bone of the big toe. This is a condyloid or ellipsoidal joint, which means that the convex distal or far end of the metatarsal bone fits against the concave proximal or near end of the phalanx. Though the toe can move from side to side in the joint, its major motions are those of flexion and extension: curling downward and lifting upward. The motion of flexion is initiated by the flexor hallucis brevis and longus tendons, which run from their corresponding muscles in the foot and calf to the sesamoid and phalanx bones of the big toe.
Where the flexor hallucis brevis, itself dividing into two portions that attach via two tendons, attaches to the base of the proximal phalanx is where the fibular sesamoid and tibial sesamoid are found. Both on the plantar or underside of the foot, the sesamoids are smooth, round bones, with the fibular sesamoid situated closer to the second toe. Each portion of the FHB tendon runs over and around one of these small bones, attaching at the base of the proximal phalanx. In the space between the two sesamoids is a groove through which passes the tendon of the flexor hallucis longus before attaching to the distal, or far, phalanx bone. Additionally, a portion of the tendon of the adductor hallucis muscle, which pulls the big toe inward toward the second toe, inserts on the fibular sesamoid.
The sesamoid bones have an effect on the mechanics of the foot when pushing off the ground. They prevent the FHB and FHL tendons from being pinched by the metatarsophalangeal joint as the toe flexes downward during push-off. They provide protection for the FHL tendon, which sits in a furrow between the sesamoid bones rather than be crushed by the metatarsal and phalangeal bones against the ground.
These bones also act as a fulcrum for the metatarsophalangeal joint, increasing the moment arm of toe flexion. The moment arm is simply the distance between the center of the joint and the muscle or tendon passing the joint, also known as the “line of force." In other words, the presence of the tibial and fibular sesamoid places the FHB tendon farther from the bones it crosses. This increased moment arm improves the mechanical effectiveness of the flexor hallucis brevis at rotating the big toe about the joint when the muscle contracts, much in the same way that opening a door is more effective when one pushes on the side of the door farthest from the hinge rather than the side nearest the hinge.