Primary shoulder dislocation

Shoulder dislocations are relatively common. They typically occur as the result of a traumatic injury, such as from a fall or motor vehicle accident.Other potential causes of a shoulder dislocation include seizures and electrocutions.

• Most commonly, the shoulder dislocates by sliding forward (anterior) out of the socket. This occurs when the shoulder dislocates while the arm is raised away from the body.
• The shoulder can also dislocate by sliding backward (posterior). This can occur when the shoulder dislocates while the arm is raised in front of the body. Following a seizure, the shoulder often dislocates backward.

The shoulder is stable because of the combined stabilizing effects of bone surfaces, ligaments, and muscles. When a shoulder dislocates, any or all of these structures can be injured to different degrees.

• When the head of the humerus dislocates, the glenoid socket and the ligaments in the front of the shoulder are often injured.
• The labrum — the cartilage rim around the edge of the glenoid — may also tear. This is known as a Bankart lesion. Learn more: 
• In some instances, the trauma may be severe enough that there is also a break in either the glenoid socket or upper arm bone.

In very severe dislocations, such as those that result from motor vehicle accidents, damage to nerves or blood vessels can also occur. 

Risk Factors

• Some people are born with greater laxity or looseness in their ligaments. These people are at greater risk of dislocating their shoulders.
• People who have had shoulder dislocations in the past also have a high risk of future dislocations — and these can happen even without an injury. When your shoulder has recurrent dislocations— this leads to a condition known as chronic shoulder instability.