Dislocations of the shoulder occur when the head of the humerus is forcibly removed from its socket in the glenoid fossa. 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.
Patients with a previous shoulder dislocation are more prone to redislocation. This occurs because the tissue does not heal properly and/or because the tissue stretches out and becomes more lax. Younger patients (teenagers and those aged 20 years) have a much higher frequency of redislocation than patients in their 50s and 60s. Many physicians believe that age is less of a predisposing risk factor for redislocation than activity level.