Pain in the shoulder is a common problem affecting all ages of the general population. It is the second most common cause of musculoskeletal pain. Radiographic diagnosis of the disease entity causing nonspecific pain begins with evaluation of how the shoulder joint has been affected. There are three areas in the shoulder joint to be observed: (1) the glenohumeral joint, (2) the subacromial space, and (3) the acromioclavicular joint.
Glenohumeral joint involvement
Narrowing of the glenohumeral joint space with lack of involvement of the acromioclavicular (AC) joint or the subacromial space is usually accompanied by radiographic changes of osteoarthritis. It must be remembered that although the shoulder is not a weight-bearing joint, mechanical contact forces across the joint can actually be quite high with lifting of any weight. Primary osteoarthritis of the shoulder can be seen particularly in elderly women. However, because osteoarthritic changes of the glenohumeral joint are relatively rare, the primary underlying abnormality in the cartilage must be considered. This abnormality may be disruption, deformity, or deposition.
Disruption of the cartilage can occur either in chronic repetitive trauma, such as recurrent dislocations, or in late-stage osteonecrosis. In the posttraumatic shoulder, a Hill-Sachs deformity, a “trough sign,” or a Bankart lesion may be identified in addition to the glenohumeral joint space narrowing and osteoarthritic changes. In late-stage osteonecrosis, the humeral head will be flattened and often fragmented.
Distortion of the underlying cartilage occurs in epiphyseal dysplasia or dysplasia of the scapular neck. In both instances the glenohumeral joint space narrowing and osteoarthritic changes will be superimposed on a recognizably dysplastic humeral head or flattened glenoid ( Fig. 6-1 ).
Deposition of a foreign substance into the cartilage is the most common cause of cartilage degeneration. This is observed in calcium pyrophosphate dihydrate (CPPD) crystal deposition disease, acromegaly, and ochronosis. The most common of these is CPPD crystal deposition disease.
CPPD Crystal Deposition Disease ( Fig. 6-2 )
Observation of osteoarthritis involving both glenohumeral joints in a patient strongly suggests CPPD arthropathy. Early, before the joint is narrowed, chondrocalcinosis may be identified. With glenohumeral joint space narrowing, one will see subchondral sclerosis, osteophytosis, and occasionally cyst formation. The osteophyte will be seen best on the external rotation anteroposterior (AP) view. One may be able to identify calcification in the cartilage of the AC joint or glenohumeral joint, making the diagnosis more definitive.
Subacromial space involvement
Isolated loss of the subacromial space occurs in a chronic rotator cuff tear and in certain positions in the shoulder impingement syndrome. If there is less than 7 mm between the undersurface of the acromion and the top of the humeral head, this space is considered narrowed.
Chronic Rotator Cuff Tear ( Figs. 6-3 and 6-4 )
The glenohumeral joint space is initially preserved. The humeral head appears superior to its normal articulation with the glenoid, and the space between the acromion and humeral head measures less than 7 mm ( Fig. 6-3 ). There is osseous erosion of the undersurface of the acromion, with adjacent bone sclerosis. There may be sclerosis of the articulating humeral head as well. These radiographic changes are seen only in a chronic tear. There are no plain film findings in an acute rotator cuff tear; the radiographic diagnosis must be made through another modality, such as computed tomography (CT) arthrography, ultrasonography, or magnetic resonance (MR) imaging.
Indicators of rotator cuff tendon disease are subcortical cyst formation and cortical irregularity of the greater tuberosity ( Fig. 6-4 ). These findings are only manifestations of rotator cuff degeneration and are not specifically associated with rotator cuff tear as is narrowing of the humeral acromial space.
Shoulder Impingement Syndrome ( Figs. 6-5 and 6-6 )
In the shoulder impingement syndrome, pain is caused when the periarticular soft tissues, such as the rotator cuff, biceps tendon, or subacromial bursa, are trapped between the greater tuberosity of the humeral head and the coracoacromial ligamentous arch. Pain is produced on abduction or elevation of the externally rotated arm. On the normal AP view of the shoulder, bone excrescences are seen on the undersurface of the acromion ( Fig. 6-5 ). These excrescences may be better visualized on an AP view in which the tube is angled 30 degrees caudally. Often there is some flattening, bone sclerosis, and bone proliferation at the greater tuberosity. If the shoulder is radiographed in external rotation and abduction, the greater tuberosity appears to abut the acromion ( Fig. 6-6 ). Frequently, a coexistent chronic rotator cuff tear is present.
