Many people have a bunion — a deformity of the metatarsophalangeal (MTP) joint at the base of the big toe. A bunion develops when the first metatarsal bone of the foot turns outward and the big toe points inward (toward the other toes), causing the joint to jut out (see "Anatomy of a bunion"). The Latin name for the deformity is hallux valgus (hallux means big toe, and valgus means turned away from the midline of the body). Most shoes don't accommodate the resulting protrusion and so put pressure on the misaligned joint. Eventually, the bursa (a fluid-filled sac that surrounds and cushions the joint) becomes inflamed, and the entire joint becomes stiff and painful.

A bunion develops when susceptible feet are repeatedly squeezed into narrow, pointed-toe footwear. The big toe pushes against the other toes, sometimes diving over or under them. As a result, the base of the big toe, juts or angles out from the foot.