The shoulder, or glenohumeral joint, consists of a ball at the head of the humerus, the upper arm bone, and a socket called the glenoid fossa. The ball is two-thirds larger than the shallow socket, a design that makes the shoulder joint the most mobile in the body. The mobility of the glenohumeral joint, however, comes at the cost of stability. Dislocations occur in the shoulder more frequently than in any other joint, according to The American Orthopaedic Society for Sports Medicine. A subluxation is a partial dislocation; the humeral head or ball is displaced but does not completely come out of the socket.
Types
Traumatic subluxation results from an injury that forces the ball to the rim of the socket at the front, back or bottom of the joint. According to the American Academy of Orthopaedic Surgeons, the displacement is commonly toward the front when the arm has been placed in a vulnerable position -- as in when throwing a ball, for example.
Atraumatic subluxation occurs in the absence of significant injury. UW Orthopaedics and Sports Medicine points out that this type of instability tends to occur in both shoulders, to be multidirectional and to be familial.
Causes
University Sports Medicine lists the following causes of traumatic subluxation: falls on an outstretched arm, a direct blow or an arm being forced into an awkward position. Examples of these risks are skiing, contact sports and motor vehicle accidents.
The cause of an atraumatic subluxation may be as benign as reaching into the back seat of a car, according to UW Orthopaedics and Sports Medicine. Contributing to atraumatic instability are developmental factors, such as lax ligaments or a flat glenoid fossa; neuromuscular factors, such as the muscle imbalance resulting from a stroke; or predisposing factors, such as an unhealed injury.
Symptoms
Pain, numbness and muscle weakness result from tears to the joint capsule or its lining and from stretching of ligaments, muscles and other soft tissue. In addition, a patient may experience a sensation of looseness or joint instability, as if the joint goes in and out.
Diagnosis
A physician bases a diagnosis on the symptoms, a patient's description of the injury and a physical examination. An X-ray is generally required only when complications are suspected.
Treatment
In most cases, the glenohumeral joint rolls back into place without any medical intervention. An exception to this is subluxation due to neuromuscular imbalance. In the case of a stroke patient, for example, subluxation is an ongoing problem that requires continued management.
A physician may prescribe ice and medications to relieve pain and inflammation. A sling or other type of immobilizer may be necessary to rest and stabilize the joint. In some cases, surgery may be necessary to repair torn or lax ligaments.
Rehabilitation
Increasing mobility and strengthening muscles are critical to restoring the function of the glenohumeral joint, without which many activities of daily living are severely restricted. Once tissues have healed and with a physician's permission, exercises may be started. Strengthening the rotator cuff muscles is particularly important in preventing recurrent subluxation.
