Bursitis can occur in nearly any joint when the bursa sac becomes swollen with fluid and inflamed. Bursae are small, slippery, fluid-filled sacs, much like tiny water balloons, that act like a cushion over a joint to decrease friction. They also act as a lubricant between the bones, muscles, ligaments, and tendons of the joint.
Each bursa sac consists of a thin-walled membrane (called the synovial membrane or synovium) and the synovial fluid produced by the membrane. There are a few different bursae within the feet that can develop bursitis due to overuse, injury, bone deformity, or medical conditions (such as arthritis).