The shoulder is a "ball-and-socket" joint where the "ball" is the rounded top of the arm bone (humerus) and the "socket" is the cup (glenoid) of the shoulder blade. A layer of cartilage called the labrum cushions and deepens the socket. A dislocation occurs when the humerus pops out of its socket, either partially or completely. As the body's most mobile joint, able to move in many directions, the shoulder is most vulnerable to dislocation.
A fracture is a partial or complete crack through a bone. It is usually caused by an impact injury such as a direct blow or a fall, and can result in severe pain and limited movement. Shoulder fractures may involve the clavicle (collarbone), proximal humerus (top of the arm bone), and scapula (shoulder blade). Other bones and soft tissues may be affected as well.
One of the most common causes of shoulder pain, impingement occurs when the front of the shoulder blade rubs against the rotator cuff as a person lifts his/her arm. The rotator cuff is a group of muscles and tendons that stabilizes the shoulder and permits lifting and rotating movements. If the rotator cuff weakens or is injured, the bone of the upper arm (humerus) can lift up, pinching the rotator cuff against the shoulder blade. The muscles can then swell further, creating a vicious cycle of pain and weakness that will not improve without intervention.
A shoulder is deemed unstable when it frequently dislocates or slips partially out of joint (subluxation). People with unstable shoulders may experience pain and limited motion in the joint and feel that moving in the wrong way will cause their shoulder to dislocate, which in turn may discourage them from participating in sports such as swimming, volleyball and baseball. Shoulder instability most often develops from a traumatic injury such as a football tackle that stretches or tears the ligaments in the shoulder, or from a naturally loose joint capsule that does not hold the ball of the humerus tightly in its socket. Younger patients who experience a traumatic dislocation are more likely to develop shoulder instability.