A normal, healthy hip allows you to walk, turn, and squat without pain. Full function of the hip joint depends on the successful coordination of many interrelated parts, including bones, muscles, tendons, ligaments, and nerves. Like other free-moving synovial (movable) joints, the hip contains a small amount of clear, oil-like fluid that lubricates the joint whenever you move it.
Smooth soft tissue called cartilage helps prevent the joint from dislocating and assists both the muscles that power hip and leg movement and the tendons that attach those muscles to the bones.
Age, injury and disease can cause the hip's cartilage to wear down. Without sufficient cartilage to serve as a cushion, the bones begin to rub together, gradually becoming rough and irregular. As a result, the ball eventually grinds in the hip socket when you move your leg, causing pain and stiffness.