X-ray of the left hand showing periarticular soft tissue swelling of the proximal inter-phalangeal joints (arrow) in a case of juvenile idiopathic arthritis (JIA)
X-rays however have low sensitivity for assessment of early structural damage in joints. Bone erosions are less commonly seen in children on X-rays 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 on plain radiographs. Thus, in contrast to adult rheumatoid arthritis where validated radiographic scoring systems are available and are used in clinical trials for effectiveness of new therapies, no such validated score has been used in JIA trials [3, 4].
Early signs of inflammation such as synovitis and osteitis are undetectable on radiographs. In addition, the complexities of the maturing skeleton limit standardization of radiographic scoring for JIA . Although multiple radiographic scoring systems for JIA have been proposed, none has been widely accepted for routine clinical use, due to significant inter and intra-observer variation [6, 7]. Thus, clinical assessment of joint function and disability takes precedence over radiographic findings.
Specific joint findings in juvenile idiopathic arthritis will depend on the underlying abnormality, the chronicity of the disease, and the treatment effect. The earliest abnormalities include soft tissue swelling, osteopenia, and effusion. Periosteal reaction may be seen occasionally. Typically, the osteopenia is initially periarticular, becoming more diffuse with time (Fig. 11.2). Joint effusions are encountered commonly and can be seen in inflammatory or noninflammatory joint disease. Radiographs are useful for identifying late complications of arthritis, such as accelerated bone growth, premature epiphyseal fusion, and limb length discrepancy. However, routine surveillance radiographs are not predictive of disease course and should be obtained during follow-up only when there is a change in symptoms in JIA.
X-ray of the ankle showing decreased density of bones in a known case of juvenile rheumatoid arthritis
Joint effusion is an early sign of joint disorders and is visible as a soft tissue shadow. A sign of knee effusion is fullness in the supra-patellar region seen on the lateral view of knee. In the elbow, knee, and ankle, adjacent fat lines and fat pads may be displaced by fluid.
Periosteal reaction, when present, is commonly seen in the phalanges, metacarpals, and metatarsals but may also occur in the long bones. Joint space narrowing is caused by cartilage loss and is usually uniform in JIA. In some patients with rheumatoid factor positive polyarthritis or systemic arthritis, early erosive disease can occur. Bone erosions are typically located at joint margins in the bare areas but also may occur at tendon insertions  due to reduced cartilage thickness at these regions .
Deformity of the fingers, such as Boutonniere or swan-neck deformity, can be seen in a variety of disorders, including JIA or systemic lupus erythematosus (SLE) (Fig. 11.3). Enlarged or irregular epiphyseal ossification centers are seen in patients with hemophilia, JIA, and tuberculous arthritis. Atlantoaxial subluxation or cervical vertebrae pseudo-subluxation and ankylosis may be noted in patients with JIA, Down syndrome, dysostosis multiplex, and SLE.
X-ray of the hand in a case of chronic juvenile arthritis showing ankylosis of carpal bones, erosions, compression and flexion deformities of the proximal interphalangeal joints and periosteal bone formation along the proximal phalanges
Enlargement of ossification centers and epiphyses, contour irregularity, trabecular changes, and squaring (typically of the patella) are seen in JIA, hemophilia, etc. Tibiotalar slant can also be noted in JIA. Late sequelae of JIA include epiphyseal deformity, abnormal angular carpal bones, widening of the intercondylar notch of knees, and premature fusion of the growth plates.
Growth disturbances are more frequent if disease onset is early. At the hip, protrusio acetabuli, 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, wrist joint, and rarely hip joint. Growth disturbance of the temporomandibular joint may lead to micrognathia and temporomandibular disk abnormality.
Enthesitis-related arthritis or juvenile spondyloarthropathies mainly involve lower limb joints in an asymmetrical fashion. Involvement of interphalangeal joint of the hallux can also be seen. New bone formation may be seen at the margins of bones. Affected joints show swelling, effusion, epiphyseal overgrowth, erosions, osteopenia, cartilage space narrowing, and rarely fusion. Swelling and periosteal new bone formation is seen in fingers and toes . Asymmetrical involvement of sacroiliac joints is seen in early disease which may later become symmetrical (Fig. 11.4). Erosions occur first on the iliac side of the sacroiliac joint. Pseudo widening occurs as a result of erosion. Sclerosis and finally ankylosis can develop.
Axial image through the sacroiliac joints showing erosions predominating in the iliac bone (arrow)
Thus except for joint effusions which can be detected early on, significant disease progression occurs for the X-ray abnormalities to become apparent in patients with pediatric rheumatic disorders. X-rays may pick up other etiologies of joint pain such a tumor or systemic findings of a skeletal dysplasia, but it can be difficult to distinguish between changes of diseases such as hemophilia and JIA, both of which show epiphyseal squaring and osteopenia as mentioned above.
Computed Tomography (CT)
CT can give valuable information that may not be apparent on a plain film because of its sensitivity to minor variations in radiographic density and its ability to eliminate overlapping of structures. CT however should be avoided in children due to risk of radiation exposure, to circumvent that a low-dose CT has been developed. CT is superior to plain X-ray in diagnosis of TMJ, sacroiliac joint, and cranio-vertebral joint (CVJ) pathologies. In TMJ the mandibular growth plate lies under a thin layer of fibrocartilage located at the surface of the condylar head. Orthopantomogram (OPT) and CT scanning are both useful in delineating the extent of condylar damage . CT is generally preferred to OPT because of the shorter exam time and lower radiation dose. However, these modalities cannot distinguish ongoing active disease from damage due to past disease activity and cannot pick up early changes such as synovial inflammation.
Among the acquired causes of CVJ abnormality, rheumatoid arthritis (RA) is the most frequent . Synovial hypertrophy eventually leads to destruction of articular cartilage and bone, along with the development of synovial cysts and ligamentous laxity. RA has a unique propensity to adversely affect the complex joints of the upper cervical spine. Due to the complexity of these joints and their corresponding articular surfaces, substantial arthropathy with ligament, joint, and bone destruction can occur. CT provides the ability to detect and characterize calcification, cortical disruption, and periosteal reaction. Though CT can show bony abnormalities very well and can profile joints with complex anatomy including the CV junction, MRI has now largely superseded CT in the overall assessment of JIA due to high radiation dose in CT.
Musculoskeletal ultrasound (US) is being increasingly used for the diagnosis and follow-up of patients with rheumatic diseases. US is helpful in the assessment of soft tissues, fluid collection, and cartilage and bone surfaces. US allows precise evaluation of synovial hyperplasia, joint effusion, cartilage damage, bone erosion, tenosynovitis, and enthesopathy (Figs. 11.5 and 11.6). US helps in knowing the integrity of the cartilage in the immature skeleton as it demonstrates the cartilage of unossified epiphysis and the ossific nuclei earlier than radiographs. The real-time capability of US allows dynamic assessment of joint and tendon movements, which can often aid the detection of structural abnormalities .
Ultrasound of the hip in a child with transient synovitis showing anechoic effusion (arrow) and synovial thickening (solid arrow)
Ultrasound image of right shoulder showing erosions in the head of the humerus (arrow) in a case of juvenile rheumatoid arthritis
Synovial hypertrophy is seen on US as hypoechoic, non-displaceable, and poorly compressible intra-articular tissue that may have a Doppler signal. In contrast synovial fluids are more anechoic and displaceable and show no Doppler signal. Tenosynovitis is seen as hypoechoic or anechoic thickened tissue with or without fluid within the tendon sheath that is visible in two perpendicular planes and may exhibit Doppler signal. Enthesopathy on US appears as abnormally hypoechoic or thickened tendon or ligament at its bone attachment that is visible in two perpendicular planes and may exhibit Doppler signal or bone changes. US may also detect erosions on bone surface. On US, erosions are seen as an intra-articular discontinuity of the bone surface that is visible in two perpendicular planes.
US has many advantages such as being noninvasive, relatively inexpensive, lack of radiation, and ability to repeat it as often as necessary, making it particularly useful for the monitoring of treatment. It does not require sedation for scanning in younger children. In addition due to its portability, it can be used at the point of care. US can also be helpful in precise aspiration and biopsy for diagnostic purposes from joint, muscle, or soft tissue collections . It also helps in accurate placement of the needle for intra-articular corticosteroid injections.
Power Doppler (PD) ultrasound detects synovial blood flow, which is a sign of increased synovial vascularization and active inflammation  (Fig. 11.7). Assessment of synovial vascularization on PD is more sensitive than serum markers of inflammation in the identification of active disease. The degree of vascularity detected by PD strongly correlates with serum IL-6 levels. The sensitivity of Doppler may be further enhanced by intravascular microbubble contrast agents . Doppler enables differentiation between inactive fibrotic tissue from pannus and quantification of synovitis. US can detect tendon disease including widening of the flexor tendon sheath, loss of the normal fibrillar architecture, tears, and synovial cysts.
Color Doppler image of the wrist showing increased vascularity suggesting active synovitis
US is helpful in detecting subclinical synovitis and may thus help to change the classification of disease from monoarticular to polyarticular. However, US is an operator-dependent modality, needs training, and at times may lack objectivity. Recognition of the normal pediatric anatomy is essential while performing ultrasound in this age group. The cartilaginous physis in children appears hypoechoic as compared to the echogenic epiphysis in adults and may be mistaken for pathology or synovial hypertrophy. It has limited applicability in joints like temporomandibular and sacroiliac joints. It is also not good at detecting subchondral changes in bone.
Microbubble-specific imaging modes such as harmonic imaging, extended field of view, and transmission US, as well as 3D and 4D US, offer exciting possibilities for the future.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) plays an important role in musculoskeletal imaging in children. It is the preferred modality over CT as it gives excellent soft tissue characterization, has no radiation, and hence is safe in children. It can also define physiological processes such as edema, loss of circulation as in avascular necrosis, and increased vascularity as in tumors.