This is by far the most common type of dislocated shoulder representing 95–98% of all cases(1). This type of injury is almost always as a result of trauma or direct force on the arm or outstretched hand, such as a fall or sports injury. Basically, in an anterior dislocation the humeral head pops out of the glenoid socket in a forward or anterior direction. This process tears the shoulder capsule and detaches the labrum from the glenoid socket bone; the labrum is an important type of cartilage that lies between the humerus head and the glenoid socket. The labrum forms a sort of cushion, deepens the socket and assists in keeping the femoral head in position; it is essential in the role of stabilising the joint.
Posterior shoulder dislocations are not as common as anterior dislocations and account for only 2-4% of all shoulder dislocations(2). Basically, in a posterior dislocation the humeral head pops out of the glenoid socket in a backward or posterior direction. This injury commonly occurs in adults during convulsions or seizures. Electrocution or lightning are other cause for this type of dislocation, although not so common.
Inferior Dislocation (or Luxatio Erecta)
Inferior or Luxatio Erecta is the least common type of dislocated shoulder injury representing less than 1% of all cases(3). This is a rare injury caused by a hyper-abduction of the arm. In an inferior dislocation, the humeral head is forced downwards against the outer tip of the shoulder blade, (the acromion). As you can see from the X-Ray above the inferior dislocation presentation is quite unique in that classically the shoulder is in abduction, with the elbow flexed and the forearm raised either straight up or behind the head.
This X-ray above, shows unusually wide separation of the humeral head and glenoid. Something must be inside that joint space, and usually, this finding indicates a large hematoma is present in the joint.
This scary X-ray shows erosions of the surfaces of the humeral head and glenoid. Extensive infection, septic arthritis, is probably inside that joint.
The human shoulder is a ‘ball and socket’ joint that consists of three principal bones: the humerus or upper arm bone, the clavicle or collarbone and the scapula or shoulder blade and their correlating ligaments, muscles and tendons. The shoulder joint is the most mobile and flexible joint of the human body and allows a diverse range of movements of the arms and hands. Stability can sometimes be compromised for flexibility in the case of this joint and that makes it the most frequently dislocated joint in the body.
Dislocations of the shoulder are divided into two categories (as illustrated by the X-rays above) Complete or Luxation and Partial or Subluxation:-
- Complete or Luxation: The ball or the head of the humerus comes completely out of the socket. (See Image 1)
- Partial or Subluxation: The ball or head of the humerus is only partially out of the socket. The humeral head slides across the shallow joint surface but does not come completely out of the glenoid socket. (see Image 2)
There are several types of dislocations of the shoulder joint and they are classified according to the direction that the femoral head comes out of the glenoid socket. They are:-
- Anterior dislocation
- Posterior dislocation
- Inferior dislocation
1. Wilson SR et al; Dislocation, Shoulder, eMedicine, Dec 2009
2. Gor DM. The trough line sign. Radiology. 2002;224: 485-6.doi:10.1148/radiol.2242010352 – Pubmed citation
3. Grate I, Jr. Luxatio erecta: a rarely seen, but often missed shoulder dislocation. Am J Emerg Med 2000;18(3):317-21.