Calcific deposits are more easily identified and characterized with radiographs, computed tomography (CT), and ultrasonography (US) than with magnetic resonance (MR). On radiographs, most calcific deposits appear as homogeneous and amorphous densities, ovoid, linear, or triangular in shape, with and without internal trabeculations (Fig. 5-1A). The precise appearance and location varies with the phase of the disease process and specific anatomical structure involved. The supraspinatus is the most frequently affected tendon. US is highly sensitive for detecting even very small calcific deposits and may be used to guide puncture, aspiration, and lavage as therapeutic options. Hyperechoic foci with minimal or no significant posterior shadowing are identified, sometimes as ill defined and fluffy or as discrete, well-defined calcifications that are linear or rounded (Fig. 5-1B). Calcific deposits may be seen on MR as nodular foci of low signal intensity in all pulse sequences (Fig. 5-1C), and may be easier to identify on gradient-echo sequences, as they may induce blooming artifact. Inflammatory changes in surrounding soft tissues may be present and identified as heterogeneous hyperintensity in fluid-sensitive (T2-weighted and short T1 inversion recovery [STIR]) sequences.

Figure 5-1 Calcific bursitis. A, External rotation view of the right shoulder in an adult patient presenting to the emergency room with acute exacerbation of right shoulder pain demonstrates periarticular mineralization (arrow) adjacent to the greater tuberosity of the humerus. B, Oblique longitudinal US image of the shoulder in a different patient also presenting with shoulder pain shows a curvilinear hyperechoic focus (arrow) within the supraspinatus (SS) tendon near its humeral (H) insertion. C, Oblique coronal proton density MR image in the same patient as A shows an amorphous focus of low signal intensity in the subacromial-subdeltoid bursa (arrow) superficial to the SS tendon (asterisk).

Rotator Cuff Abnormalities and Impingement

The concept of extrinsic subacromial impingement was initially described by Neer in 1972 and continues to be a very controversial topic even today. The notion of impingement is based on the anatomical arrangement of the shoulder joint (fixed component) and on the motion of the humeral head relative to the other components of the shoulder joint (dynamic component). In essence, the rotator cuff tendons (primarily the supraspinatus) and muscles, as well as the subacromial-subdeltoid bursa located between the coracoacromial arch and the humeral head, may be impinged with motion of the arm. It is thought that this repetitive microtrauma from friction will lead to tendon degeneration, bursal inflammation, and, ultimately, a tear of the cuff. The “critical zone” is particularly susceptible to this pathophysiological process; this critical zone has been described as a hypovascular area located on the anterior aspect of the supraspinatus tendon, approximately 1 cm from its insertion. Other factors described as potential contributors to the impingement process are narrowing of the subacromial space by enthesophytes arising from the undersurface of the acromion, advanced acromioclavicular joint osteoarthritis with hypertrophic changes, a type III acromion (anteriorly hooked), and the presence of an os acromiale (secondary ossification center at the tip of the acromion that persists after skeletal maturity as a separate bone). However, impingement can occur without any visible anatomic predisposing factor, and the presence of anatomic variations does not necessarily indicate that there is impingement. Thus, the diagnosis of shoulder impingement is made mainly on clinical grounds, rather than on the basis of the radiographs alone. The radiologist should focus on describing, characterizing, and grading rotator cuff disease, as well as on identifying potential contributing sources to the impingement process.

The spectrum of rotator cuff pathology includes tendinopathy or tedinosis, partial- and full-thickness tendon tears, and subacromial-subdeltoid bursitis. Most rotator cuff tears are chronic and occur as the result of repetitive microtrauma. Acute rotator cuff tears are rare but do occur, especially in older patients with preexisting chronic impingement and degenerative tendon changes. It is not unusual, however, for patients with acute or chronic rotator cuff tears to seek care at the emergency room on an urgent basis because of severe or acutely worsening shoulder pain.

Radiographs should always be performed initially for evaluation of shoulder pain. However, they are often not contributory. The presence of gas from vacuum phenomenon in the glenohumeral joint strongly suggests the absence of a full-thickness rotator cuff tear. The rotator cuff tendons cannot be directly seen on radiographs; rather, there are a number of radiographic findings that serve as indirect evidence of cuff pathology and impingement (Fig. 5-2). These include superior subluxation of the humerus with a decreased subacromial space (less than 8 mm) and secondary changes in the humeral head, such as sclerosis, flattening, surface irregularity, and cystic changes. Radiographs may also demonstrate potential anatomic causes of impingement (Fig. 5-3).

Figure 5-2 Chronic rotator cuff tear indirect signs. External rotation view of the shoulder shows superior subluxation of the humeral head with decreased subacromial space to 4 mm (white arrow), sclerosis of the humeral head (asterisk), and surface irregularity at the greater tuberosity (black arrow).

Figure 5-3 Potential anatomic sources of impingement. A, Internal rotation view of the shoulder shows an enthesophyte at the undersurface of the acromion (white arrow) and hypertrophic changes at the acromioclavicular joint (black arrow). B, Y-view of the shoulder reveals a type III (anteriorly hooked) acromion (arrows).

Direct visualization of the rotator cuff tendons is achieved with MR or US. One of these two methods is usually required to accurately diagnose and characterize rotator cuff tears. Selection of the modality depends on availability, individual expertise, and preference of the interpreting radiologist. Both offer relative advantages and disadvantages, which determine preference, and practice trends vary among different world countries. For example, in the United States, MR is used more commonly than US, perhaps because of its faster learning curve and because the method is easily reproducible and less operator dependent than US. On the other hand, US is a great cost-effective alternative in experienced hands and is the first alternative in many institutions throughout the world.

Rotator cuff tendinopathy (tendon degeneration) is characterized on MR by increased signal within the tendon on low TE sequences (T1 and proton density). The tendon may demonstrate associated focal or diffuse thickening, but this is not a constant finding (Fig. 5-4A). Abnormal signal within the rotator cuff tendons in low TE sequences may be seen in a variety of normal situations, and thus there is need for close clinical correlation: most commonly, magic angle artifact as an area of increased signal at 55 degrees from the main magnetic field, which, on oblique coronal planes, coincides with the supraspinatus “critical zone.” Cuff tendon tears present as disruption (interruption) of fibers and may be either partial or full-thickness in the craniocaudal plane. High-signal fluid is seen separating the disrupted fibers. This fluid may extend from the articular (inferior) surface superiorly in varying degrees to the bursal (superior) surface (Fig. 5-4B). Partial thickness tears affecting the articular surface are more common than isolated bursal surface tears. In full-thickness tears, fluid invariably extends across the tendon (Fig. 5-4C) into the subacromial-subdeltoid bursa. Subacromial-subdeltoid bursitis may occur in isolation or in conjunction with rotator cuff tears.

Figure 5-4 Spectrum of rotator cuff pathology seen on MR. A, Tendinopathy or tendinosis is characterized in this proton-density image as increased signal and thickening (arrow) of the supraspinatus (SS) tendon. B, Partial thickness, articular surface tear of the SS tendon on an oblique coronal STIR image characterized by fluid signal (arrow) between the disrupted articular surface and interstitial fibers and the intact bursal surface fibers (arrowheads).C, Full-thickness tear of the SS tendon seen on an oblique coronal T2-weighted sequence as fluid signal (arrow) extending across the entire thickness of the tendon.

Figure 5-5A shows an intact supraspinatus and its relation to the humeral head and deltoid muscle. The primary or direct signs of full-thickness supraspinatus tear in US include nonvisualization of the tendon and a hypoechoic or anechoic full-thickness defect filling the gap of the torn tendon (Fig. 5-5B). Secondary or indirect signs that are helpful to correlate with the primary signs include sagging of the peribursal fat, cortical irregularity at the greater tuberosity, fluid in the subacromial-subdeltoid bursa, and muscle atrophy. Partial-thickness tears manifest sonographically as focal areas of hypoechoic or anechoic tendon defects involving the bursal or articular surface (Fig. 5-5C). An adequate exam requires evaluation of the extension of the defect on two orthogonal planes to confirm the findings. Tendon degeneration is demonstrated as internal heterogeneous echogenicity.

Figure 5-5 Spectrum of rotator cuff pathology on US. A, Normal longitudinal image of the left shoulder demonstrating an intact supraspinatus (SS) and its relation to the humeral head (H) and deltoid muscle (D). B, Longitudinal US image of the left shoulder with nonvisualization of a fully torn and retracted SS tendon. There is a gap in the expected location of the tendon (arrows). C, Longitudinal US image of the left shoulder demonstrating a partial-thickness articular surface tear of the SS tendon as a focal hypoechoic area at its insertion (arrow).

(Courtesy of Alda Cossi, MD.)

Acromioclavicular Joint Disease (Osteolysis and Osteoarthritis)

The incidence of acromioclavicular joint pathology as a cause of shoulder pain is higher than generally realized. The acromioclavicular joint is often ignored, to the point that it has been termed the “forgotten” or “overlooked” joint. Osteolysis and osteoarthritis are two of the most common causes of shoulder pain arising from the acromioclavicular joint.

Osteolysis

Destruction of bone (osteolysis) may be seen as the result of multiple localized or generalized conditions. Destruction of the distal end of the clavicle is not uncommon and may be idiopathic, post-traumatic, or caused by rheumatoid arthritis, hyperparathyroidism, metastatic disease, multiple myeloma, primary osteolysis syndromes, and infection.

Post-traumatic osteolysis deserves special attention because of its relative frequency as a cause of debilitating pain and painful shoulder motion, and because it very often goes unrecognized. This entity can occur after a single or multiple episodes of minimal or major injury to this region. Most important is the recognition that post-traumatic osteolysis can occur without an obvious acute or known traumatic episode, and in those situations it is thought to be due to repetitive stress, as seen in weightlifters, judo practitioners, and pneumatic tool workers. The pathogenesis is poorly understood, and several theories have been formulated, including a neurologic and/or vascular mechanism, hyperemia, and autonomic phenomenon. More recently, it has been proposed that osteolysis may be the result of a reactive process originating in the synovium or a subchondral fracture.

When advanced, resorption of the distal clavicle may be easily recognized radiographically (Fig. 5-6), with loss of up to 3 cm of bone and widening of the acromioclavicular joint. The radiologist, however, should focus on identifying early signs, as immobilization seems to diminish the amount of bone loss and shorten the natural course of the lytic phase. Early radiographic signs include soft tissue swelling, demineralization, and loss of the subarticular sclerotic cortex at the distal end of the clavicle. MR findings usually precede radiographic findings. Initially, there is periarticular soft tissue swelling/edema, and a bone marrow edema pattern may be evident. The marrow signal abnormality can be limited to the distal end of the clavicle or involve the acromion as well, albeit to a lesser degree. There may be an associated joint effusion, although this finding is variable. The disease process then progresses to bone erosion and frank destruction. Other MR findings include cortical irregularity, subchondral erosion or cystic changes, and a subchondral line suggestive of a subchondral fracture.

Figure 5-6 Post-traumatic osteolysis. A 28-year-old weightlifter with no recognizable acute trauma presented with acute shoulder pain. Anteroposterior radiograph of the right shoulder revealed a widened acromioclavicular joint and resorption of the distal end of the clavicle (arrow).

Osteoarthritis

Any arthropathy can involve the synovial acromioclavicular joint. Osteoarthritis is, however, the one most commonly found in clinical practice and a very important cause of disability and loss of work hours. The acromioclavicular joint is almost universally affected in the elderly population with osteoarthritis. Shoulder pain caused by osteoarthritis can be severe and incapacitating and can cause affected patients to seek emergency medical attention.

The radiographic hallmarks of osteoarthritis include joint space narrowing, subchondral sclerosis (“eburnation”), subchondral cyst formation, osteophytes, deformity, and malalignment. Although MR is not necessary to detect acromioclavicular osteoarthritis, the acromioclavicular joint should be included in the field of view of all shoulder MR examinations. All imaging findings described on radiographs can be seen on MR. Additionally, MR may depict joint effusions, capsular hypertrophy, and subchondral bone marrow edema pattern. The clinical significance of MR and radiographic findings is poorly understood, as these abnormalities are present in both symptomatic and asymptomatic patients. Some studies suggest that the presence of bone marrow edema pattern and an effusion is associated with pain and that the thickness of the capsule (greater than 3 mm) may be a predictor of a good response to intra-articular injections.

Glenohumeral Joint Disease (Arthropathy and Adhesive Capsulitis)

The glenohumeral joint may be affected by a multitude of conditions. The discussion in this section is limited to adhesive capsulitis, because of its frequency and complexity, and to rheumatoid arthritis, as this is a very common arthropathy affecting this joint. Only osteoarthritis is more common, and the imaging features of osteoarthritis have already been described for the acromioclavicular joint.

Rheumatoid Arthritis

Rheumatoid arthritis is a symmetric inflammatory arthropathy affecting predominantly the small joints of the hand, wrist, and feet. Involvement of the glenohumeral joint is not infrequent and occurs later in the disease course. Approximately half of patients with rheumatoid arthritis have shoulder symptoms during the first 2 years of their disease.

Conventional radiographs are still important for diagnosis and classification of rheumatoid arthritis. Classically, there is uniform joint space narrowing, periarticular demineralization, subchondral cystic changes, and marginal erosions. Erosions in the shoulder have a predilection for the lateral portion of the humeral head and may resemble a Hill-Sachs deformity (Fig. 5-7). Characteristically, there is lack of productive bone changes. Superior subluxation of the humeral head can also be seen, as chronic rotator cuff tear or cuff atrophy occurs frequently in patients with rheumatoid arthritis. The role of other imaging tests for rheumatoid arthritis is still evolving. US and MR are more sensitive for detection of erosions and soft tissue findings. There may be a role for these modalities in early detection and for evaluation of disease activity or response to therapy. MR can demonstrate erosions earlier than plain radiographs, as well as subchondral cystic changes on both sides of the joint. Erosions are commonly located in the humeral head near the insertion of the rotator cuff tendons. MR can also show common findings not identifiable in radiographs, like joint effusion, signs of synovitis, tears or atrophy of the rotator cuff muscles and tendons, synovial cysts, bursitis, and rice bodies. Routine MR is limited for evaluation of glenohumeral articular cartilage.

Figure 5-7 Rheumatoid arthritis of the glenohumeral joint. Anteroposterior (A) and axillary (B) views of the right shoulder demonstrate uniform joint space narrowing and large humeral head erosions (arrows).

Adhesive Capsulitis

Also known as a “frozen shoulder,” adhesive capsulitis is a commonly recognized but poorly understood disorder causing severe shoulder pain, stiffness, and disability. Even though the diagnosis of this condition is made clinically, rather than on the basis of imaging findings, radiographs and other tests are often performed to exclude other conditions that can cause shoulder pain and to identify features that may support the diagnosis. Although most imaging is performed on a nonemergent basis, patients with adhesive capsulitis often present in an urgent setting with significant shoulder pain.

Adhesive capsulitis is a synovitis that may be idiopathic or the result of other conditions including multiple arthropathies and trauma. Pathologically, it is shown to be a synovial inflammation with preferential involvement of the rotator interval, axillary pouch, and subscapularis recess. Imaging findings parallel histological findings. Radiographs are not contributory, as might be expected from the description of the underlying pathology. Likewise, MR examinations may show no abnormalities. Findings that support the diagnosis include synovial prominence at the rotator interval with obliteration of the subcoracoid fat, thickening of the coracohumeral and/or superior glenohumeral ligaments, and capsular thickening at the axillary pouch. After intravenous administration of gadolinium chelates, enhancement of the rotator interval and/or axillary recess may be demonstrated. MR arthrography has been advocated because of the added information obtained from estimating the volume of fluid necessary to achieve joint distention (typically decreased in adhesive capsulitis), and the potentially decreased capacity of the axillary pouch and bicipital tendon sheath. However, these findings have not been reproduced consistently and there is probably limited added value from MR arthrography as a diagnostic tool for this condition.

THE PAINFUL HIP

As with most other joints, the imaging evaluation of hip pain typically begins with conventional radiographs, even in the absence of history of trauma. The routine examination should include an anteroposterior and a frog leg lateral view of the hip along with an anteroposterior view of the pelvis. The view of the pelvis is essential for comparison with the opposite hip and for detection of subtle abnormalities. The anteroposterior view should be obtained with internal rotation to optimally evaluate the femoral neck. Unfortunately, most patients with hip pain tend to flex and externally rotate the hip for pain relief. This position tends to shorten the femoral neck and makes the evaluation more difficult. The frog leg lateral radiograph is obtained with the hip in abduction and provides a tangential view of the anterior and posterior surfaces of the femoral head.

A systematic evaluation of the radiographs should begin with the soft tissues for swelling, displacement of fat pads, and calcifications. Overall bone alignment, including the acetabular coverage of the femoral head, should be assessed. The width of the symptomatic joint space, as well as symmetry with the contralateral hip, may provide clues about presence of an articular process. The pattern of bone mineralization and trabeculation is also important. Finally, it is mandatory to carefully evaluate for preservation of the pelvic lines and bony contours, particularly of the femoral neck.

Insufficiency Fractures

Insufficiency fractures are a type of stress fracture that occurs when a usual strength or physiologic force is applied to an abnormal or weakened bone. Most insufficiency fractures are caused by osteoporosis. In the pelvis, subcapital neck fractures (Fig. 5-8) are by far the most common. However, these fractures are usually associated with some degree of trauma. Common locations in the pelvis not usually associated with trauma include the sacrum, pubic rami, and supra-acetabular region. Undisplaced insufficiency fractures are very difficult to diagnose on conventional radiographs. Good radiographic technique and a high index of suspicion, especially when evaluating demineralized bones, are essential. If a stress fracture is suspected clinically and radiographs are not diagnostic, an MR examination should be obtained if available. MR is more accurate than scintigraphy. Additionally, MR may demonstrate coexisting conditions or alternative diagnoses that may influence management.