The small fluid-filled sac that cushions and reduces friction around a joint is a bursa.
A bursa is lined by synovial membrane and contains a small amount of viscous synovial fluid. It acts as a low-friction cushion between moving structures, such as bone and tendon, muscle and bone, or skin and bone. Bursae are especially common around major joints, including the shoulder, elbow, hip, and knee.
Their names often reflect their locations. The prepatellar bursa lies over the kneecap, while the olecranon bursa sits at the tip of the elbow. The subacromial bursa helps tendons move beneath the shoulder’s acromion. Repeated pressure or irritation can inflame a bursa, a condition called bursitis.
A bursa is not the same as a tendon, ligament, or cartilage. Tendons connect muscle to bone, ligaments connect bone to bone, and cartilage provides smooth structural support at joints. The word bursa comes from Medieval Latin for “purse,” referring to its small, bag-like form.