A bunion forms when the bursa (a sac of fluid at friction points between the tendons and bone in some areas and between bone and the skin in others) becomes inflamed along the edge of the joint at the base of the big toe.

There are two types of bunions. The acute bunion causes the sharper pain. It develops from a bursitis, a sudden outcropping of a fluid-filled sac.

An acute bunion can progress into the second type of bunion, the hallux valgus, a chronic but often painless deformity involving...