Shoulder Dislocation, Posterior

Shoulder Dislocation, Posterior

The usual story is that the patient visits the emergency room and comes back to see the doctor because the patient is having constant shoulder pain and is unable to move the shoulder. When examining the patient, the patient will have limitation of external rotation of the shoulder. You may be shown an x-ray, an AP view of the shoulder, and the interpretation of the x-ray is that the shoulder appears normal. You need to get two x-ray views (orthogonal views): AP view and axillary view. An AP view x-ray alone will not diagnose posterior shoulder dislocation. When you have posterior dislocation of the shoulder, the AP x-ray view will show the classic “lightbulb” humeral head due to internal rotation of the shoulder.

The humeral head takes on a rounded appearance. The axillary view x-ray will show dislocation of the shoulder posteriorly. It is the best view to show the posterior shoulder dislocation. After reduction, always get an axillary view and check concentric reduction. Locate the coracoid (anteriorly) and outline it. Locate the acromion (posteriorly). Then locate the glenoid and determine whether the dislocation is posterior or anterior. In posterior dislocation of the shoulder, the axillary view will show the humeral head going posteriorly away from the coracoid and in the direction of the acromion. With posterior shoulder dislocation, the shoulder is locked in the internal rotation position with prominence of the posterior shoulder, prominence of the coracoid process, and flattening of the anterior shoulder. Posterior shoulder dislocation may be associated with fracture of the lesser tuberosity. 50% of posterior shoulder dislocations will have a Reverse Hill-sachs lesion or impaction fracture next to the lesser tuberosity. When you examine the patient and you see limitation of the range of motion, especially external rotation of the shoulder, you may think it is adhesive capsulitis (frozen shoulder). Frozen shoulder can start by limiting the external rotation, however it is usually a global restriction of the range of motion.

Posterior dislocation of the shoulder is rare (about 5%) and it is usually stable after reduction if no fracture is present. Posterior dislocation of the shoulder usually occurs after seizures or electric shock. Why is it that dislocation of the shoulder most commonly occurs as a posterior shoulder dislocation with seizures and electric shock? This is a controversial subject. Some physicians believe that this is due to the fact that the shoulder internal rotator muscles (pectoralis major, latissimus dorsi, and subscapularis) are stronger than the external rotator muscles. Up to 50% of posterior dislocations of the shoulder can go undiagnosed when the patient is examined in the emergency room, especially if dislocation results from seizures. If posterior dislocation of the shoulder occurs due to seizures, the patient should be examined carefully and neurology consult should be done to control the patient’s seizures. Any future treatment of posterior dislocation of the shoulder may fail due to lack of controlling seizures. Closed reduction is not difficult in the acute setting and can be done up to 3 months. Instability is rare with absence of fracture. Immobilize the arm in neutral rotation with the elbow at the side and posterior to the plane of the body. Impaction less than 20%, do closed reduction and immobilize in external rotation. Open reduction is done when posterior dislocation is chronic or locked. In locked posterior dislocation, the deltopectoral approach to the shoulder is usually used. If the defect is between 20%-40%, transpose the lesser tuberosity or the subscapularis tendon into the defect. More than 45% defect or if the dislocation is more than 6 months, do arthroplasty and place the prosthesis in less retroversion.