Frozen Shoulder

Anatomy

The shoulder is a ball-and-socket joint with great freedom of movement. The head of the humerus is a relatively large ball, and the glenoid is a relatively small socket. The relationship between the two is commonly compared to a golf ball sitting on a golf tee. Because there are few bony restrictions on the motion of the humerus at the shoulder, the joint is typically very free moving, with approximately 180° of forward flexion and abduction and 90° of internal and external rotation. However, this freedom of movement also causes instability in the shoulder. Several redundant anatomic structures provide the shoulder with stability. The glenoid labrum is a ring of cartilage that extends from the rim of the glenoid and wraps around the head of the humerus. Outside the labrum is the shoulder capsule, a loose bag of synovium that defines the joint space and prevents extremes of motion. The glenohumeral ligaments are fibrous thickenings of the interior capsule wall that attach the glenoid to the humerus and provide the majority of the stabilizing properties of the capsule. Fifteen individual muscles and tendons cross the shoulder joint. Of these, four short muscles, collectively called the rotator cuff, contribute significant stability to the shoulder joint. When the rotator cuff muscles contract, they pull the humeral head onto the glenoid. In addition, they contribute to internal and external rotation and abduction of the shoulder.

Causes

Frozen shoulder is an idiopathic illness. It occurs most commonly among women between the ages of 30 and 60 years. Apart from age and sex, having diabetes mellitus is the strongest risk factor for frozen shoulder. Occasionally, it occurs in patients who have had shoulder pain or loss of function due to other pathology, such as rotator cuff strains or tendinosis, fracture, paralytic stroke, or seemingly minor trauma. Whether or not these ailments contribute to the development of frozen shoulder is controversial. Rarely, thyroid disease, Parkinsonism, or antiretroviral medications (e.g., for human immunodeficiency virus [HIV] infection) can be associated with the development of frozen shoulder. Some authors have argued that patients with low pain thresholds or poor compliance with physical therapy exercises seem to be at higher risk, but this is not well established. Using a shoulder or arm sling can also lead to frozen shoulder if it is used for too long. Typically, 1 to 3 days of shoulder immobilization following injury is safe. More prolonged shoulder immobilization should be accompanied by at least daily shoulder range of motion or pendulum stretch exercises if they can be safely performed.

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