Ball-and-Socket Joints

The most range of movement by the joints is provided by a "ball-and- socket" joint, in which the spherical head of one bone lodges in the spherical cavity of another. In the shoulder joint, the humerus (upper arm bone) fits into the socket of the shoulder blade. Because the socket is shallow and the joint loose, the shoulder is the body's most mobile joint. The hip joint is less mobile than the shoulder, but it is more stable. The ball of the femur's head fits tightly into a deep socket in the hip bone. A rim of cartilage lining the socket helps grip the femur firmly; the ligament binding the two bones is among the strongest in the human body.