The shoulder joint is a connection between two bones, which form the ball and the socket. The shoulder is the most flexible of all the joints in the body. The rounded head or ball of the upper arm bone (humerus) and the cup like formation of the shoulder blade (scapula) meet to form the shoulder.
The rotator cuff is a group of muscles and tendons in the shoulder, connecting the upper arm to the shoulder blade. The rotator cuff helps to lift the arm over the head and also helps to hold the ball (humeral head) in the socket (glenoid) of the shoulder.
A healthy shoulder has smooth cartilage covering where the bones of the upper arm and the shoulder meet. Cartilage can wear over time from arthritis or injury. This causes the ends of the bones to rub against each other and leads to pain, stiffness, limited movement, and a decreased ability to perform activities of daily living.