It consists of a shallow "ball-and-saucer" and is based on the upper, outer corner of the shoulder blade. Four small muscles and a joint capsule surround the joint and connect the shoulder blade to the arm bone. A large muscle covers the top of the joint and other large muscles connect the arm bone to the rest of the skeleton. The shoulder blade forms the base of the shoulder and floats over the rib cage. Several large muscles control its position.

Shoulder Injuries
Anyone who experiences shoulder pain that is severe or does not improve in a few days should see a family doctor or an orthopaedic surgeon.

Most shoulder injuries involve the rotator cuff tendons or the ligaments that connect the ball-and-saucer joint. Other shoulder injuries result from instability, which means the ball does not remain centered in the saucer.

Rotator Cuff Injuries
Injuries to rotator cuff tendons may occur as the result of an accident, the growth of bone spurs, or certain movements of the arm or joint. An example: overhead motion, especially repetitive lifting, places stress on the rotator cuff and may result in injuries.

Whatever the cause, a rotator cuff injury causes pain, weakness, loss of motion, or even an inability to lift the arm.

Shoulder Instability
A dislocated shoulder is one example of an injury that occurs because of shoulder instability. The ball totally leaves the saucer and may become trapped in front, below, or behind it. Medical treatment to put the shoulder back in place or surgery to repair damaged joint surfaces or ligaments may be necessary.

Another example of an injury caused by shoulder instability is "subluxation." With this injury, the ball doesn't quite slip out of socket. It may go back into place by itself, or medical treatment may be necessary.

Sports Injuries
The collarbone acts as a support between the shoulder and the rest of the skeleton. Collarbone fractures are common in sports.

The area where this bone connects to the shoulder blade, called the AC joint, may also be a source of trouble. Injuries to the AC joint typically result from a fall or a blow to the shoulder and are common in football.

Arthritis
Arthritis in the AC joint becomes a problem for some people as they age. The condition causes pain and pressure to the rotator cuff muscles below it. Arthritis of the main shoulder joint, while not as common as hip or knee arthritis, occurs primarily in older people.

Treatments
Rest, rehab exercises, and anti-inflammatory medicines take care of many shoulder injuries. When conservative treatments are not effective, surgery may be necessary.

Surgeons use an instrument called an arthroscope to perform many shoulder procedures. An arthroscope consists of a tube, a lens, and a fiber optic light source. This instrument allows a surgeon to visualize the surgical area through very small incisions, resulting in less pain and faster recoveries for patients than in the past.