The shoulder joint joins three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone). The shoulders are the most movable joints in the body. They can also be unstable because the ball of the upper arm is larger than the shoulder socket that holds it. To remain in a stable or normal position, the shoulder must be anchored by muscles, tendons, and ligaments.
The rotator cuff is located firmly in the shoulder socket. The rotator cuff keeps the shoulder stable when the arm is moved in any direction.
Because your shoulder can be unstable, it can be easily injured. Common problems include:
Treatments for shoulder injuries often include physical therapy exercises to strengthen the shoulder and improve movement. The best individual treatment plan depends on age, how much the injury causes pain, and whether the injury was sudden or happened from wear and tear.
NIH: National Institute of Arthritis and Musculoskeletal and Skin Diseases