The sesamoid bones of the foot are two tiny bones found under the first metatarsal bone, near the big toe joint (first metatarsophalangeal joint). Even though they are small, they can cause a significant amount of pain when they are injured. Inflammation of the tissues surrounding the sesamoid bones or a fracture of either bone usually causes significant pain under the ball of the foot, in the area under the big toe joint. Pain is often felt with every step, as the big toe flexes and weight-bearing increases under the big toe joint.
Sesamoid Bone Function
It's hard to believe two tiny bones such as the sesamoids would serve any purpose, but they do have an important function. The two bones sit within the tendon of the flexor hallucis brevis muscle, which stabilizes the big toe as we flex and push-off during gait. The sesamoid bones give that muscle a mechanical advantage, helping to increase the power it needs to help the big toe joint function efficiently. If one or both sesamoids is removed, the big toe will likely develop structural problems, such as a hammertoe or bunion of the big toe joint.
In some people, a sesamoid bone may be split, or bipartite, which is a normal anatomical variant that makes it appear as though it is fractured on an X-ray. Another anatomical variant that is seen on some X-rays is two tiny sesamoids in the area of the 5th metatarsal bone, at the pinkie toe joint.
Types of Injuries that Affect the Sesamoids
Sudden or chronic injury in this area of the foot can result in a sesamoid bone fracture or injury to the soft tissue surrounding the bones. Symptoms of a fractured sesamoid or torn ligament may include swelling, bruising, and pain while applying pressure to that area. Sesamoid fractures can occur acutely, such as after a fall from a height. Chronic injury to the ball of the foot can cause a sesamoid stress fracture, which has an increased incidence in activities that require a lot of weight-bearing on the toes, such as ballet dancing and basketball. Diagnostic tests to identify a sesamoid fracture include X-ray and less often, MRI or CT scan. MRI is also indicated for identifying soft tissue damage, such as a torn tendon or a ligament sprain.
A condition similar to sesamoiditis is turf toe, a sprain of the first metatarsophalangeal (MTPJ) joint. Turf toe refers to any degree of ligament injury to the first MTPJ, ranging from overstretching to tearing of the toe's ligament(s). The injury occurs when the toe endures excessive force while flexing, such as when an athlete pushes off from the big toe when cutting and running. Like sesamoiditis, pain and other symptoms are located on the underside of the first MTPJ, at the ball of the foot. Depending on the degree of injury, treatment for sesamoiditis and turf toe will often be similar, and center on decreasing stress on the first MTPJ in order to allow healing.
Treatment of Sesamoiditis
Sesamoiditis can be a stubborn condition because of the amount of weight-bearing pressure on the ball of the foot during gait. More severe cases of sesamoiditis, turf toe, or fracture of a sesamoid bone may require non-weight bearing with crutches or a cast. Sesamoid fractures and severe cases of turf toe may require surgery.
Sesamoiditis that is not associated with a fracture or soft tissue tear may be treated with rest from activity, padding around the ball of the foot, and shoe orthotics (arch supports) to relieve ground pressure from the ball of the foot. Shoe recommendations may be given, such as a hard-soled shoe or a rocker-bottom shoe. A rocker-bottom shoe is similar to a toning shoe, in that they both have a curved sole which decreases pressure on the bottom of the toes.
Castro, Michael D. and Pomeroy, Gregory. Traumatic Injuries to the Midfoot and Forefoot. Sammarco MD, James G. and Cooper MD, Paul S (Eds.), Foot and Ankle Manual, 2nd ed. Williams and Wilkins, 1998. pp. 125-30.