Bursitis is inflammation of a bursa caused by repetitive use, trauma, infection, or systemic inflammatory disease. Bursae are flattened sacs that serve as a protective buffer between bones and overlapping muscles (deep bursae) or between bones and tendons/skin (superficial bursae). These synovial-lined sacs are filled with minimal amounts of fluid to facilitate movement during muscle contraction. Deep bursae, such as the subacromial and iliopsoas bursae, are located in the fascia. Superficial bursae, such as the olecranon and prepatellar bursae, are located in the subcutaneous tissue. Humans have approximately 160 bursae. Bursitis most commonly affects the subacromial, olecranon, trochanteric, prepatellar, and infrapatellar bursae. Symptoms of bursitis may include localised tenderness, oedema, erythema, and/or reduced movement.

Inflammation causes synovial cells to multiply and thereby increases collagen formation and fluid production. A more permeable capillary membrane allows entrance of high protein fluid. The bursal lining may be replaced by granulation tissue followed by fibrous tissue. Haemorrhage may occur. One study suggests this process may be mediated by cytokines, metalloproteases, and cyclooxygenases. In septic arthritis, local trauma usually causes inoculation of bacteria into the bursa, which triggers the inflammatory process.