The shoulder joint is the most mobile joint in the body and allows the arm to move in many directions. This ability to move makes the joint inherently unstable and also makes the shoulder the most often dislocated joint in the body.

In the shoulder joint, the head of the humerus (upper arm bone) sits in the glenoid fossa, an extension of the scapula, or shoulder blade. Because the glenoid fossa (fossa = shallow depression) is so shallow, other structures within and surrounding the shoulder joint are needed to maintain its stability.

Within the joint, the labrum (a fibrous ring of cartilage) extends from the glenoid fossa and provides a deeper receptacle for the humeral head. The capsule tissue that surrounds the joint also helps maintain stability. The rotator cuff muscles and the tendons that move the shoulder provide a significant amount of protection for the shoulder joint.

Cause

Dislocations of the shoulder occur when the head of the humerus is forcibly removed from its socket in the glenoid fossa. It’s possible to dislocate the shoulder in many different directions, and a dislocated shoulder is described by the location where the humeral head ends up after it has been dislocated. Ninety percent or more of shoulder dislocations are anterior dislocations, meaning that the humeral head has been moved to a position in front of the joint.

Posterior dislocations are those in which the humeral head has moved backward toward the shoulder blade. Other rare types of dislocations include luxatio erecta, an inferior dislocation below the joint, and intrathoracic, in which the humeral head gets stuck between the ribs

A partial dislocation (subluxation) means the head of the upper arm bone (humerus) is partially out of the socket (glenoid). A complete dislocation means it is all the way out of the socket. Both partial and complete dislocation cause pain and unsteadiness in the shoulder

The shoulder joints are the most commonly dislocated joints in the body.

Approximately 25% of shoulder dislocations or instability injuries will have associated fractures.

Symptoms

Symptoms to look for include:

• Swelling
• Numbness
• Weakness
• Bruising
• Sometimes dislocation may tear ligaments or tendons in the shoulder or damage nerves

Diagnosis

The muscles may have spasms from the disruption, and this can make it hurt more. When the shoulder dislocates time and again, there is shoulder instability.

The doctor will examine the shoulder and may order an X-ray. It is important that the doctor know how the dislocation happened and whether the shoulder had ever been dislocated before.