General Considerations. Radiographs are often initially normal, but in the early phase there may be nonspecific consequences of hypervascularity and inflammation, including joint effusion, soft tissue swelling, epiphyseal widening, and periarticular osteopenia. This is partly because in the pediatric skeleton thick cartilage protects bone, so erosive change is encountered only with advanced disease. In addition, the synovial hypertrophy and cartilaginous changes typical of early JIA are radiographically occult. However, radiographs performed early in the course of disease do both allow exclusion of other causes of pain and establish a baseline. US and MRI are able to demonstrate early findings of synovial hypertrophy and hyperemia.

During the intermediate phase, cartilage destruction results in joint space narrowing, cortical erosions, and epiphyseal overgrowth; these findings are demonstrable on both radiographs and MRI and to a lesser extent US. Finally, ankylosis, muscle atrophy, growth disturbance, angular deformity at the joints, and contractures may be seen with late disease.

During active disease, clinical examination generally provides adequate assessment of articular synovitis. US and MRI are, however, useful for early staging and for monitoring response to therapy. Indeed, techniques such as T2 mapping may demonstrate progressive cartilage deterioration even when clinical findings are stable [5]. Numerous attempts have been made to develop radiographic scoring systems, none of which have been widely accepted [6, 7]. Radiographs therefore do not play an important role in monitoring progression, but they are important for evaluating deformities and growth abnormalities.

Radiographs may demonstrate joint effusions early in JIA, especially at the knee (Fig. 20.1) The small joints of the hands and feet may show fusiform soft tissue swelling, especially common with psoriatic arthritis. US and MRI are more sensitive for evaluation of the small joints than are radiographs.

Osteopenia is another relatively early finding (Fig. 20.2). In the acute setting, this results from juxta-articular hyperemia and resultant trabecular resorption, whereas in chronic disease it is more generalized and results from steroid use and disuse atrophy.

Periostitis occurs at any stage of disease and in any long bone but is most common in the short tubular bones of the hands and feet (Fig. 20.3). This usually develops at para-articular locations, due to inflammation of the joint capsule and tendinous insertions. Bones may enlarge and become square as a result (especially common with psoriatic arthritis) (Fig. 20.4).

Erosions are evident relatively late, after thick protective cartilage has been destroyed by prolonged synovial proliferation (Fig. 20.5). Although erosions may occur anywhere along the articular surface, they are most common at locations where there is less cartilage—insertion sites of intraosseous ligaments and synovial reflections. Radiographs show decreased joint space due to cartilage loss before erosions become apparent, but they are relatively insensitive for showing trabecular bone loss [8]. In patients with systemic JIA, joint space narrowing and erosions may be found relatively early in the course of disease [9].

Growth disturbance, a relatively late finding, is both more common and more severe in children with early onset of disease. It may be generalized, resulting from chronic disease and long-term steroid use. Alternatively, localized growth disturbance may result from periarticular inflammation and localized hyperemia, which lead to early growth acceleration and subsequent epiphyseal enlargement as well as early physeal closure and accelerated maturation. This accounts for the typical appearance at the knee (Figs. 20.6 and 20.7): ballooned epiphyses, wide intercondylar notch, and squared inferior patella, which may in turn lead to subluxation. Although hyperemia leads to initial growth acceleration and longitudinal overgrowth, premature bone maturation and physeal fusion/epiphyseal destruction may eventually cause decreased limb length. A similar process takes place in the carpal and tarsal bones, which may demonstrate premature ossification and then osseous coalition (Fig. 20.8). Growth disturbance is also common in the mandible (see discussion below).

Ankylosis (Fig. 20.9), erosions, synovial hypertrophy, effusions, and muscle imbalance may cause abnormal stress at the joints and lead to flexion and extension deformities as well as joint subluxation and dislocation (see subsequent paragraph). The knee may develop varus or valgus deformity, in part due to asymmetric epiphyseal overgrowth (see Fig. 20.7). This may contribute to deformity at the hip, including enlargement of the femoral head, acetabular protrusion, coxa vara, and hip subluxation. A similar process may occur in the upper extremities, with asymmetric epiphyseal enlargement causing deformity at the elbow and shoulder (Figs. 20.10 and 20.11).

Deformities at the hand and wrist are most common with polyarticular JIA. These include such finger deformities as flexion at the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints, Boutonniere deformity (flexion at PIP with hyperextension at DIP) (Fig. 20.12), and swan neck deformity (extension at PIP and flexion at DIP). Ligamentous laxity and sometimes mass-like hypertrophied synovium may lead to carpal subluxation or dislocation, especially common at the hamate. The wrist may deviate in a radial direction [7].

Patients with the psoriatic or enthesitis-related subtypes may demonstrate overgrowth and inflammation of a digit (dactylitis) (Fig. 20.13). ERA may manifest with thickening of tendons and ligaments, showing wispy periostitis along their osseous attachments.

Ultrasound. Both US and MRI may be used for early assessment of the disease process (Fig. 20.14). US depicts the severity of disease by allowing assessment of joint effusions, synovial thickening, cartilage destruction/thinning, associated synovial cysts, erosions, and tenosynovitis (Figs. 20.15 and 20.16). It may identify clinically occult knee effusions as well as subclinical synovitis in multiple joints [10, 11]. Later on, it may identify enthesitis, inflammation of tendons or ligaments where they insert into the bone (most common at the calcaneal insertion of the Achilles tendon). US is especially useful for evaluating the joints of the hand and feet. It may be employed to guide joint aspiration or therapeutic injection, and it helps assess response to intra-articular treatment [7].

Synovial proliferation appears as hypoechoic, irregular synovial membrane thickening, easily distinguishable from joint fluid (see Chap. 22). With cartilage involvement, the normal smooth contour of hypoechoic cartilage is altered, and the typically sharp margins become blurred and obliterated [10]. Erosions appear as a break in the cortex that measures at least 2 mm; the floor is irregular, and adjacent marrow demonstrates acoustic enhancement [12].

If power Doppler demonstrates hypervascular pannus, disease is considered active (see Fig. 20.14). Serial US may then be used to monitor disease activity and demonstrate response to therapy, showing decreased thickness and vascularity of the synovium, as well as a decrease in joint fluid [13]. However, US provides only limited visualization of some joints (such as the sacroiliac and temporomandibular joints), and a narrow acoustic window may preclude complete assessment of large joints. The efficacy of US is also limited by difficulty in positioning and examining children whose joints are swollen and painful.

Magnetic Resonance Imaging. MRI offers optimal assessment of joint disease in JIA, and contrast-enhanced MRI is the most sensitive technique for detecting synovitis, demonstrating pathology even before the physical exam becomes abnormal. It is superior to US in depicting early inflammation and synovitis, as well as in evaluation of the cartilage. MRI readily shows marrow edema, which predicts future erosions [14]. It also accurately evaluates the late manifestations of the disease, including erosions, joint space loss, and ligamentous involvement (Figs. 20.17, 20.18, and 20.19) [7, 13, 15]. MRI thus plays an important role in determining prognosis.

Joint effusion and acute synovitis constitute the earliest MRI manifestations in JIA (Fig. 20.20) [16]. Joint fluid and thickened synovium appear similar on conventional spin echo images. However, heavily T2-weighted (T2-W) images and fast spin echo (FSE) techniques differentiate hyperintense joint fluid from relatively hypointense synovium [13]. Normal synovium (Fig. 20.21) is at most 2 mm thick and appears hypointense on T1-weighted (T1-W) and T2-W sequences. Abnormal synovium appears as a thick, irregular, wavy layer that is hypo- to isointense on T1-W and hyperintense on T2-W sequences (Fig. 20.22). T2-W sequences may delineate frond-like pannus, formed when progressive inflammation leads to synovial proliferation (Fig. 20.23). Hypertrophic synovium may slough and undergo fibrinoid necrosis, forming intra-articular loose bodies (“rice bodies”), readily apparent on heavily T2-W images (Fig. 20.24). These loose bodies preclude the use of intra-articular steroids.