Sesamoiditis is painful inflammation of the sesamoid apparatus, which is located in the forefoot. It is a common condition that typically affects physically active young people. Sesamoiditis causes pain in the ball of the foot, especially on the inner (medial) side. The foot pain may be constant, or it may occur with or be aggravated by, movement of the big toe joint. It may be accompanied by swelling (edema) throughout the bottom (plantar aspect) of the forefoot.
Anatomy of the Forefoot
The forefoot consists of the five toes and their connecting long bones, the metatarsals. Each toe (phalanx) is made up of several small bones called phalanges. The phalanges of all five toes are connected to the metatarsals by metatarsophalangeal joints at the ball of the foot. The forefoot bears half the body’s weight and balances pressure on the ball of the foot.
The big toe, or hallux, has two phalanges and two joints (interphalangeal joints); it also has two tiny, round, sesamoid bones that enable it to move up and down. On an x-ray of the foot, they appear as a pair of distinctive oval dots near the first metatarsal head (front end of the first long bone of the forefoot). The other four toes each have three phalanges, two joints, and no sesamoid bones.
The sesamoid bones closest to the inner side of the foot are called medial sesamoid bones; the ones closest to the outside of the foot are called lateral sesamoid bones. The sesamoids are embedded in the flexor hallucis brevis tendon, one of several tendons that exert pressure from the big toe against the ground and help initiate the act of walking. The sesamoid bones have two principal functions.
- They absorb impact forces in the forefoot during walking through a series of attachments to other structures in the forefoot. Although they are separated by a bony ridge called the crista at the bottom (plantar aspect) of the first metatarsal head, they are connected to one another by an intersesamoid ligament. They also are attached to other tendons and ligaments in the forefoot (e.g., tendons of the abductor and abductor hallucis muscles, sesamophalangeal ligament, metatarsosesamoid ligament). This array of attachments enables the sesamoids to disperse some of the impact of the foot striking the ground during walking.
- The connecting ligaments, the first metatarsophalangeal joint capsule, and the sesamoid bones (known collectively as the sesamoid apparatus) act as a fulcrum, providing the flexor tendons a mechanical advantage as they pull the big toe down against the ground during walking.