Acute Arthritis/Osteomyelitis in Children

OSTEOMYELITIS IN CHILDREN: Osteomyelitis (OM) is defined as an infection of the bone marrow and adjacent osseous structures with potential surrounding soft tissue extent. It can occur at any age, generally in children 1–16 years old. In children, it is predominantly caused by hematogenous spread of infection, typically bacterial. Generally, the metaphysis is involved. Transphyseal spread to the epiphysis and joint is rare but can occur in children younger than 18 months, or in older children with closed growth plates, due to a different blood perfusion to the epiphysis. The first imaging modality of choice is conventional radiography (CR), but CR has limited sensitivity in early stages. Osseous destruction is evident only 10–14 days after the initial infection. In low-grade osteomyelitis, nonspecific soft tissue swelling and subtle bone resorption or a lytic lesion at the metaphysis may be seen, as in more advanced stages, lytic bone destruction, and periosteal formation are seen. Magnetic resonance imaging (MRI) is the preferred modality for early detection of lesions. Brodie’s abscess is a subtype of osteomyelitis typically seen in children, with intraosseous abscess formation (Figure 1). Another specific subtype of OM is chronic recurrent multifocal osteomyelitis (CRMO), which is characterized by multiple sterile inflammatory bone lesions with a relapsing and remitting course. Whole-body MRI is increasingly being used for evaluation. OM has a variable imaging appearance and often mimics other bone diseases. It is important for radiologists to be able to differentiate because the prognosis varies widely. Differential diagnoses include primary bone neoplasms, with Ewing sarcoma being the major differential diagnosis. ACUTE ARTHRITIS: Arthritis, which is simply defined as inflammation of a joint, may affect one or more joints and often is accompanied by swelling, redness, tenderness, warmth, and pain with movement. Arthritis has many etiologies. In a child with acute-onset monoarthritis, the differential diagnosis must always include septic arthritis as this is a medical urgency and requires prompt treatment. Arthritis can also be related to trauma, hematologic diseases (haemophilia, leukemia, etc.), but multiple other etiologies can be differentiated as well. The differential diagnosis of a child suspected of having systemic juvenile arthritis (JIA) is often difficult, early in the disease course. It is a significant cause of short- and long-term disabilities. Imaging in JIA serves to exclude alternative diagnoses, for classification of JIA, and for follow-up of treatment. Since late treatment can cause severe damage to joints and impair skeletal maturation, early detection is critical. Historically, imaging was based on radiography, but CR mainly detects late changes. MRI can detect both early and late changes, and is the modality of choice, especially for imaging of complex joints (Figure 2). US can evaluate multiple joints at the same time and can assist needle aspiration or joint infiltration.