Joints are points in the body where two separate bones meet. The ends of the bones are connected by thick bands of tissue called ligaments. The knee joint, for instance, is formed by the lower leg bone (called the tibia or shinbone) and the thighbone (called the femur). The hip joint is a ball-and-socket joint formed by the ball (or femoral head) at the upper end of the thighbone, and the rounded socket (or acetabulum) in the pelvis.
Cartilage, a smooth, plastic-like tissue, coats the ends of some bones and lines the joint sockets to prevent the bones from rubbing against each other. A delicate membrane called the synovium acts as a lubricant to reduce friction and wear in the joint. Normally, all parts of the joint work together, and the joint moves easily without pain.