The fluid-filled sacs in joints that absorb shock and reduce friction are called bursae.
A bursa is a small sac lined with synovial membrane and filled with synovial fluid. It usually sits between structures that move against one another, such as bone and tendon, bone and muscle, or skin and bone. By creating a low-friction cushion, bursae help joints move more smoothly and protect nearby tissues from repeated pressure.
Bursae are found around many major joints, including the shoulder, elbow, hip, and knee. The prepatellar bursa lies in front of the kneecap, while the olecranon bursa is located at the point of the elbow. Inflammation of one of these sacs is called bursitis and can cause pain, swelling, and restricted movement.
A common mix-up is confusing bursae with cartilage, ligaments, or tendons. Cartilage cushions and shapes joint surfaces, ligaments connect bone to bone, and tendons connect muscle to bone. Bursae are separate fluid-filled structures whose main job is to reduce rubbing between moving tissues.