The hip joint is one of our most flexible joints, allowing for a greater range of motion than almost all other joints in the body, except the shoulder. It is a ball-and-socket synovial joint formed between the os coxa (hip bone) and the femur. The acetabulum, a cup-shaped structure on the os coxa, forms the socket for the hip joint, while the rounded head of the femur forms the ball of the joint. A thin tissue called the synovial membrane surrounds the hip joint. In a healthy functioning hip, this membrane produces a small amount of fluid that lubricates the cartilage and eliminates almost all friction during movement. Problems and hip pain occur when the joint surface wears out, causing the bone ends to rub painfully against each other.