Shoulder Injuries: Instability (Dislocation and Subluxation)
The shoulder joint is a ball and socket joint that connects the bone of the upper arm (humerus) with a shallow socket (glenoid cavity) of the shoulder blade (scapula). The glenoid labrum is a rim of cartilage attached to the glenoid rim. The capsule is a broad ligament that surrounds and stabilizes the joint. If the arm is pulled out of its socket, the capsule and glenoid labrum tear, usually from the rim of the glenoid cavity. A dislocation occurs when the humerus comes completely out of the socket and stays out.
A subluxation occurs when the humerus comes partly out of the socket and then slips back in. When the capsule tears from the glenoid rim, the shoulder can become unstable and dislocate or subluxate repeatedly. The most common direction for the humeral head to dislocate is toward the front of the body (anteriorly); this typically occurs if the arm goes too far behind the body when the arm is in an overhead position (such as when throwing a ball). The humeral head can also dislocate toward the back of the body (posteriorly) when force is directed toward the back of the shoulder; this can occur when falling forward on an outstretched arm or blocking with the arm straight ahead in football.
Diagnosis of Shoulder Instability
The direction of shoulder dislocation or subluxation can usually be made by physical examination. It is possible for the shoulder to be unstable in more than one direction. Multidirectional instability is more common in loose-jointed (double-jointed) individuals.
If the diagnosis of instability or its direction is in doubt, additional tests that can be helpful are:
- Magnetic resonance imaging (MRI) or computed tomography scan (CT scan)
- MRI or CT scan can be performed after dye is injected into the shoulder joint (arthro-MRI or arthro-CT)
- Examination under anesthesia followed by arthroscopy
Treatment of Shoulder Instability
Some patients who dislocate their shoulder do well after the injury and do not have recurrent instability. They tend to be older in age and not active in sports. Young people, especially athletes, are prone to have recurrent dislocations and subluxations and usually need surgery to correct the shoulder problem.
The unstable shoulder joint can be repaired by reattaching the torn capsule to the glenoid rim. This is called a Bankart repair. The muscles are separated to expose the shoulder capsule. If the capsule is found to have torn away from the bone, three holes are made in the glenoid rim. Stitches are passed through each hole and through the capsule and tied, securing the capsule to the glenoid rim. The capsule heals back to the bony rim and prevents the shoulder from re-dislocating. It takes several months for the capsule to heal back to the bone. During this time, extremes of shoulder motion should be avoided so that the stitches are not torn.
The torn capsule can also be repaired with the aid of an arthroscope without opening the shoulder. If the size of the tear is small and the physical demands that will be placed on the shoulder are low, this may be a good alternative. However, if the tear is large and the shoulder is very unstable, open repair is stronger, especially in an athlete.
Risks & Results of Surgery
The success rate of the Bankart repair is approximately 97%. If there is a fracture of the glenoid rim (Bankart lesion) and a compression fracture of the humeral head (Hill-Sachs lesion), there is an increased risk of recurrent instability following repair. Two nerves are at risk during surgery since they are near the operative field, but they are rarely injured. As with any surgical procedure, there are potential risks. The incidence of infection is less than 0.5%. The shoulder can lose some motion after surgery, especially if the shoulder has to be over-tightened because of excess laxity.