The shoulder is a ball and socket joint. Unlike the hip, which is a deep ball and socket joint, the shoulder is a very shallow joint. The shallowness of the joint allows for great mobility of the shoulder, but it can also create problems when the joint becomes too mobile. Shoulder instability arises when the ball moves around too easily in the socket and starts to cause symptoms.
The Normal Shoulder
The socket of the shoulder, or the glenoid cavity, is circled by a ring of soft tissue called the labrum that serves to deepen the cavity of the glenoid. This is important because the labrum helps to keep the “ball” – the head of the arm bone (the humerus) – in place. The joint is also stabilized by a capsule, which contains a group of ligaments surrounding the joint. These ligaments stretch just enough to allow normal movement of the shoulder, but tighten at the extremes of motion to prevent too much motion. The muscles and tendons surrounding the shoulder also play a role in adding stability to the shoulder.
The joint capsule surrounds the ball and socket joint of the shoulder and provides stability to the joint.
The joint capsule in a very flexible person will naturally be more loose. This natural laxity in itself may not be a problem, but with overuse in certain sports, such as swimming, volleyball or with throwing or pitching in baseball, the capsule may start to stretch and become too lax. If the joint capsule gets stretched out and the shoulder muscles become weak, the ball of the humerus begins to slip around too much within the shoulder.
The most extreme example of shoulder instability is a shoulder dislocation. The first time a shoulder dislocation occurs, it is usually due to a traumatic event. During a dislocation, as the head of the humerus is forced out of the socket, it can tear the front portion of the labrum, also known as a Bankart tear. As this portion of the labrum is the structure that acts as a bumper to keep the humeral head in place, a Bankart tear makes it easier for the shoulder to dislocate again.
Symptoms of Shoulder Instability
Symptoms of shoulder instability include a sensation of looseness in the shoulder joint. The shoulder may feel as if it comes partially out of place (also known as subluxation). The shoulder may start to become painful with activity, and you may start to avoid certain activities that cause pain. Secondary conditions such as inflammation, tendinitis and bursitis may develop from the excessive motion irritating other structures in the shoulder.
Shoulder dislocations are very apparent when they occur. A shoulder dislocation is painful, and the shoulder looks deformed. Many times the dislocation has to be returned to the normal position, or reduced, in the emergency room. In a very unstable shoulder, shoulder dislocations may actually occur when you are asleep. When a shoulder dislocates, it can stretch out the nerves in the front of the shoulder, and it may cause numbness and tingling in the arm.
A dislocated shoulder, with the displaced humeral
head stretching the nerves in the arm, which can
cause some numbness or tingling.
Instability of the shoulder can usually be diagnosed with a history and physical examination. You may or may not describe a sensation of looseness in the shoulder, but a physical examination will pick up excessive laxity of the shoulder, as well as secondary conditions that occur as a result of shoulder instability. The exam will also show if you have any apprehension as you approach the position that can elicit a shoulder dislocation. A history of a shoulder dislocation clearly shows shoulder instability.
X-Rays in the office can show a small deformity in the humerus, or arm bone, that may occur after a shoulder dislocation. A Magnetic Resonance Imaging scan, or MRI, will show if a joint capsule is excessively stretched out, or if there is tearing of the labrum, or a Bankart tear.
Shoulder instability without a history of dislocation is usually best treated with physical therapy. Physical therapy will strengthen the muscles of the shoulder girdle, which are part of the stabilizing forces of the shoulder. Strengthening will also create a more efficient shoulder and reduce the stretching of the joint capsule with activity. A physical therapist will also use modalities such as massage, ultrasound and electrical stimulation to help reduce pain.
Icing the shoulder twenty minutes twice daily will also help reduce inflammation and relieve pain. If necessary, a corticosteroid injection into the shoulder may be helpful in further reducing pain and inflammation.
After a dislocated shoulder is reduced, a single dislocation is often treatable without surgery. If there is continued pain after physical therapy and strengthening, or if there are multiple dislocations of the shoulder, an arthroscopy may be necessary to fix the structures in the shoulder. During an arthroscopy, the torn labrum is reattached to the glenoid using some anchors with stitches in them, recreating the “bumper” that prevents the humeral head from slipping out. This is known as a Bankart repair. Additionally, if the capsule is stretched out and loose, stitches are placed in the capsule to make it tighter.
After a Bankart repair and tightening of the capsule, the tissues must be able to heal in place. While it is important to move the shoulder to prevent scar tissue, or a frozen shoulder, from developing, there should only be passive motion of the shoulder. Passive motion is when a physical therapist moves the shoulder for the patient – the patient should not make any active effort to move the shoulder by himself. Usually, these passive motion only restrictions apply for four weeks. After the tissues have had time to heal, the physical therapist will advance your activities, and you will start strengthening exercises. While recovery is different for every patient, you should expect about four months of physical therapy, two to three times weekly, with additional exercises at home after you are done with therapy.