ball-and-socket

Whereas THR replaces the ball-and-socket interface at the top of the femur with artificial components, which can include plastic, ceramic and metal materials, resurfacing keeps more of the femur intact, instead placing a rounded metal cap over the head of the bone and lining the socket with a metal shell.

The ball-and-socket joint of the shoulder is secured by the four muscles and tendons of the rotator cuff, which can become irritated, torn or overstretched.

While fingers in the animal kingdom are almost always made up of hinge joints that bend front and back, the aye-aye’s middle finger joint is a ball-and-socket, allowing it to rotate and turn almost like a human shoulder, Hartstone-Rose says.

Dr. Günther compares it to a ball-and-socket joint, except there is no socket.

Any sort of twist or imbalance can put stress on ball-and-socket joints.