The shoulder is a ball and socket joint. The head of the humerus (upper arm bone) is a round ball that fits into the acetabulum or socket. Without the rotator cuff to keep the joint in balance, the humerus tends to move up too close to the shoulder blade. The patient ends up with a painful impingement as the soft tissues get pinched during movement.

A RSP takes care of this problem by changing the direction of the force in the shoulder. The top portion of the humeral head is cut off and a cup-shaped socket is inserted into the bone. A flat plate is placed on the socket side and a round three-quarter spherical ball called a glenosphere is attached.

The first design of reverse shoulder replacement did have some complications related to mechanical failure of the implant. Researchers found that getting the right size and fit for each patient is important. Once the implant came in a variety of sizes, then soft-tissue tension could be altered for each patient's shoulder.

Having a better understanding of what happens in a shoulder with a deficient rotator cuff helped surgeons change the operative technique. They also changed from a smaller nonlocking screw to a larger locking cortical screw to hold the baseplate in place against the bone.

They also found that when the baseplate is put in with a slight tilt, the shear forces across the bone were evened out more. Changing the center of rotation for the shoulder by screwing the glenospere closer to the bone also improved stabilization and reduced stress. All of these changes have reduced the problems with this procedure.