Your hip is one of the largest weight-bearing joints in your body. When it's working properly, it lets you walk, sit, bend, and turn without pain. To keep it moving smoothly, a complex network of bones, cartilage, muscles, ligaments, and tendons must all work in harmony.
The hip is a very stable ball-and-socket joint: A ball (femoral head) at the top of the thighbone (femur) fits into a rounded socket or cup-like cavity (acetabulum) in your pelvis. Bands of tissues called ligaments form a capsule connecting the ball to the socket and holding the bones in place.
A layer of smooth tissue called cartilage cushions the surface of the bones, helping the ball to rotate easily in the socket. Fluid-filled sacs (bursae) cushion the area where muscles or tendons glide across bone. The capsule surrounding the joint also has a lining (synovium) that secretes a clear liquid called synovial fluid. This fluid lubricates the joint, further reducing friction and making movement easier.
Injury or disease can damage your hip in several ways, resulting in a broken or deteriorated bone, irritated bursae, or worn cartilage. The most common cause of hip pain is osteoarthritis (OA). Other causes of hip pain include rheumatoid arthritis, osteoporosis, osteonecrosis (death of bone caused by insufficient blood supply), injury, infection, and bone tumors.