Arthritis and Differential Inflammatory Joint Disorders

Radiography

Conventional radiography is not effective in the evaluation of soft tissue abnormalities, which are precursors of cartilage degeneration in persons with JIA. Moreover, available radiographic scoring systems have poor internal consistency and poor criterion and construct validity because they do not take into consideration patients' sex and age. As mentioned earlier, there is low sensitivity (50%) and moderate specificity (85%) in the detection of cartilage destruction with radiography, hence the expanding role for ultrasound and MRI in the evaluation of JIA.

A variety of radiographic features can be encountered with joint disease. Specific joint findings will depend on the underlying abnormality, the chronicity of the disease, and the response to therapy. A systematic approach to the imaging interpretation of any joint is highly recommended. One popular approach is the “ABCDS” of joint disease, featuring assessment of joint A lignment, B one density and other bone changes, C artilage loss, D istribution of joint disease (whether monoarticular, oligoarticular, or polyarticular), and S oft tissue abnormalities ( Box 136.3 ).

The earliest abnormalities include soft tissue swelling, osteopenia, and effusion. Periosteal reaction occasionally may be seen. Typically, the osteopenia is initially periarticular ( Fig. 136.1 ), becoming more diffuse with time. Osteopenia may be subtle and better recognized by comparison with the contralateral extremity (if it is unaffected). With long-standing disease, uniform bone loss may occur with a thin cortex. Uncommonly, a linear subphyseal demineralization can be observed, but this finding is nonspecific and can also be seen in persons with other conditions such as leukemia.

Joint effusions are encountered commonly and can be seen in inflammatory or noninflammatory joint disease. A sign of knee effusions is fullness in the suprapatellar region, which is best seen on the lateral view. In the elbow, knee, and ankle, adjacent fat lines and fat pads are displaced. Periosteal reaction, when present, is commonly seen in the phalanges, metacarpals, and metatarsals but also can occur in the long bones. Joint space narrowing may be caused by cartilage loss (see Fig. 136.1 ). In persons with JIA, the joint space narrowing is usually uniform. In some patients with rheumatoid factor positive polyarthritis or systemic arthritis, early erosive disease can occur ( Fig. 136.2 ).

Bone erosions are typically located at joint margins in the bare areas but also may occur at tendinous insertions. Bone erosions also can be seen in persons with septic arthritis or hemophilic arthritis related to the inflammatory reaction caused by intraosseous hemorrhage. Large erosions can also be seen in the camptodactyly arthropathy coxa vara pericarditis syndrome ( e-Fig. 136.3 ). Deformity of the fingers, whether with boutonniere (proximal interphalangeal [PIP] flexion with distal interphalangeal [DIP] extension) or swan neck (PIP extension with DIP flexion) deformity, can be seen in a variety of disorders, including JIA ( Fig. 136.4 ), camptodactyly arthropathy coxa vara pericarditis syndrome, or systemic lupus erythematosus. Enlarged or irregular epiphyseal ossification centers can be seen in persons with hemophilia, JIA, and tuberculous arthritis. Atlantoaxial subluxation or cervical vertebrae pseudosubluxation and ankylosis ( e-Fig. 136.5 ) may be noted in persons with JIA, the arthropathy of Down syndrome, dysostosis multiplex, and systemic lupus erythematosus.

In contrast to adult patients with inflammatory arthritis, bone erosions are less commonly seen in children because the epiphyseal ossification center is surrounded not only by articular cartilage but also by epiphyseal cartilage and the spherical growth plate. As a result, significant cartilage loss must occur before osseous erosions are visible with radiography. Therefore the role of MRI is relatively more important in children to detect articular or epiphyseal cartilage erosions before actual bony erosions are visible on radiography.

Changes in bone growth and maturation with changes in the normal size of ossification centers and alteration of normal bone modeling can be seen in persons with JIA, and with infections and hemophilia. Enlargement of ossification centers and epiphyses (see Fig. 136.4 ), contour irregularity, trabecular changes, and squaring (typically of the patella) can be seen. Tibiotalar slant (ankle valgus) can also be seen.

Late sequelae of JIA include epiphyseal deformity, abnormal angular carpal bones, widening of the intercondylar notch of knees (see Fig. 136.4 ), and premature fusion of the growth plates. Growth disturbances are more frequent if disease onset is early. Joint space narrowing and osseous erosions are usually late manifestations. At the hip, protrusio acetabuli (see Fig. 136.1 ), premature degenerative changes, coxa magna, and coxa valga can be seen. Joint space loss can progress to ankylosis, particularly in the apophyseal joints of the cervical spine (see e-Fig. 136.5 ) and wrist. Rarely, ankylosis also can be seen in larger joints, including the hips. Subluxation of the joints, especially at the wrist, may be evident. Growth disturbance of the temporomandibular joint may lead to micrognathia and temporomandibular disk abnormality.

Although radiography should be used initially in evaluation of joints, cross-sectional imaging techniques have provided a significant improvement in anatomic delineation and diagnosis.

Magnetic Resonance Imaging

MRI is an optimal tool for evaluation of both soft tissues and osteochondral abnormalities with superb tissue contrast. Contrast-enhanced MRI is extremely sensitive for detecting active disease and for early detection of cartilage loss, bone erosions, and synovial hypertrophy in children and adolescents. MRI provides multiplanar evaluation with a combination of available imaging sequences, including T1 and fast spin echo T2-weighted sequences, gradient echo sequences, and postcontrast studies all tailored to the specific clinical problem. High cost, limited availability, and frequent need for sedation in very young patients are limitations.

Various forms of cartilage can be visualized on MRI, including articular, unossified epiphyseal, and the physeal cartilage. Gadolinium-enhanced MRI can demonstrate normal vessels present within the cartilaginous epiphysis. MRI also is helpful in detecting synovial abnormalities and can be used to assess for changes with therapy.

Without contrast material, proliferating synovium on MRI appears as soft tissue thickening, which is typically intermediate signal on T1-weighted and increased signal intensity on T2-weighted images ( Fig. 136.6 ). It may have slightly higher signal intensity than adjacent fluid on unenhanced T1-weighted images. At times, pannus may appear as intermediate to dark signal intensity on T2-weighted images outlined by bright signal joint fluid. Its variable signal intensity reflects the relative amount of fibrous tissue and hemosiderin. Intravenous administration of gadolinium-based contrast agents improves visualization of thickened synovium, especially with use of fat-suppression techniques. Proliferating synovium appears as enhancing linear, villous, or nodular tissue. Images should be obtained immediately after contrast injection because diffusion of contrast material from the synovium into joint fluid occurs over time. Hypervascular inflamed pannus enhances significantly (see Fig. 136.2E ), whereas fibrous inactive pannus shows much less enhancement. Subchondral cysts and bone erosions (see Fig. 136.2 ) appear as low signal areas on T1-weighted sequences, with overlying articular and epiphyseal cartilage loss better delineated on fluid-sensitive sequences. Meniscal hypoplasia can be seen in some cases of JIA ( e-Fig. 136.7 ).

Quantitative techniques have been developed for synovial volume. MRI is more sensitive than clinical evaluation in detecting some specific joint involvement, including the temporomandibular joint, which often demonstrates inflammatory change in the absence of clinical symptoms.

With prolonged synovial inflammation, well-defined intraarticular nodules termed “rice bodies” ( Fig. 136.8 ) may be present. Rice bodies likely arise from detached fragments of hypertrophied synovial villi. On MRI, rice bodies are low signal on T2-weighted images because of their fibrous tissue composition and are associated with joint effusion, synovial hypertrophy, and synovial enhancement after gadolinium administration.

Bone marrow edema is represented by areas of low T1-weighted and high T2-weighted signal intensity ( e-Fig. 136.9 ) and should be differentiated from normal marrow speckling seen on fluid-sensitive sequences, most frequently in the ankle and foot. Studies in adults have shown an association between presence of subchondral bone marrow edema in persons with inflammatory arthritis and radiographic erosive progression.

Because cartilage is one of the earliest sites of damage in persons with JIA, it is an important area to be evaluated with MRI. Standard T2-weighted and proton density magnetic resonance (MR) sequences can assess articular cartilage injury, from increased signal intensity within cartilage to articular cartilage thinning, erosions, and areas of full-thickness cartilage loss. Fat-saturated fluid-sensitive sequences will also depict subchondral bone marrow edema. Advanced sequences for cartilage evaluation are discussed later.

Although MRI has been extensively investigated in JIA, standardized measures for data acquisition and interpretation are not widely used ; hence this technique is underutilized both in clinical practice and in research. Few MRI scales have been designed to specifically assess morphologic changes with JIA.

Ultrasonography

Recent advances in ultrasonography, including better transducers and more pediatric musculoskeletal experience, have stimulated increased use of ultrasound in the assessment of pediatric joint disease. Ultrasound is ideal for assessing the pediatric musculoskeletal system because of its ability to visualize intraarticular structures such as cartilage and thickened synovium without the need for radiation. Ultrasound is very sensitive in detecting joint effusion, even in areas where radiographs are insensitive, such as the hip (see Fig. 136.2C ) and shoulder. Intraarticular masses may be detected with ultrasound, although their appearance is often nonspecific. Tendons and ligaments also can be assessed with higher-frequency transducers. Fluid within the synovial sheath appears as an anechoic halo surrounding the tendon ( Fig. 136.10 ), whereas synovial thickening appears as a hypoechoic thickening around the tendon. Synovial hyperemia leads to increased signal on Doppler evaluation. Power Doppler has been shown to detect residual disease activity more sensitively than clinical examination and/or MRI both in active disease and when JIA is in remission and could be used to predict short-term relapse in patients with JIA who appear to be in remission clinically.

Sonography can also be used to assess other periarticular soft tissue abnormalities, including popliteal cysts ( e-Fig. 136.11 ) or other soft tissue masses, and to guide aspiration or injection of joints.

Disadvantages of ultrasound include lack of standardization of ultrasound techniques for assessment of growing joints and normative data, lack of capability to visualize the central aspect of some joints, and difficulty in assessing some joints such as the temporomandibular joints ( Fig. 136.12 ).

Erosions and focal or diffuse thinning of the articular cartilage also can be detected but only peripherally in the joint. Color Doppler ultrasound enables the detection of perisynovial hyperemia. Studies in children have demonstrated the ability of color and power Doppler sonography, with or without intravenous injection of contrast agents, to estimate synovial activity in JIA. Resistive indices and fraction of color pixels may be used as quantitative measurements of the blood flow. Contrast-enhanced sonography holds potential for detecting active synovial inflammatory disease in persons with subclinical JIA and may help guide early treatment.

Very limited information is available about the diagnostic performance and accuracy of ultrasound compared with MRI or clinical examination (in knees, sensitivity for joint effusion is 62%, ranging between 60% and 90% for clinically active joints and approximately 70% for clinically inactive joints ; for superficial cartilage destruction, overall sensitivity is 60%). Ultrasound-determined synovial thickness of the knee seems to correlate with clinical and laboratory (sedimentation rate and C-reactive protein levels) disease activity scores and with biomarkers of disease activity. In ankles, however, very poor agreement was observed comparing clinical and ultrasound scores.

Imaging in the Assessment of Response to Therapy

Once therapy has begun for patients with JIA, imaging can be a helpful adjunct to assess disease activity and response to therapy. To date, several studies have examined radiographic changes before and after initiation of therapy, whereas more recent studies have used CT and/or MRI to describe joint changes and to use more quantitative measures of disease activity.