Sesamoiditis is a common ailment that affects the forefoot, typically in young people who engage in physical activity like running or dancing. Its most common symptom is pain in the ball-of-the-foot, especially on the medial or inner side. The term is a general description for any irritation of the sesamoid bones, which are tiny bones within the tendons that run to the big toe. Like the kneecap, the sesamoids function as a pulley, increasing the leverage of the tendons controlling the toe. Every time you push off against the toe the sesamoids are involved, and eventually they can become irritated, even fractured. Because the bones are actually within the tendons, sesamoiditis is really a kind of tendinitis - the tendons around the bones become inflamed as well.
Sesamoiditis typically can be distinguished from other forefoot conditions by its gradual onset. The pain usually begins as a mild ache and increases gradually as the aggravating activity is continued. It may build to an intense throbbing. In most cases there is little or no bruising or redness. One of the major causes of sesamoiditis is increased activity. You've probably stepped up your activity level lately, which has forced you to put more pressure on the balls of your feet. Speedwork, hillwork, or even increased mileage can cause this. If you have a bony foot, you simply may not have enough fat on your foot to protect your tender sesamoids. Also, if you have a high arched foot, you will naturally run on the balls-of-your-feet, adding even more pressure.