Chapter 104

• Staphylococcus aureus predominates as the cause of septic arthritis and osteomyelitis. Other bacteria commonly implicated, especially in younger patients, include Kingella kingae, pneumococcus, and group A Streptococcus.
• Neonates and young infants are particularly vulnerable to infection of the hip. In older infants and children, the knee is more commonly affected.
• No single test or finding is sufficient to predict the presence of a septic joint but a history of fever, the inability to bear weight, elevated erythrocyte sedimentation rate or C-reactive protein, and an elevated white blood cell count are suggestive of the diagnosis.
• The mainstay in the diagnosis of septic arthritis is analysis of joint fluid. Fluid is usually obtained by percutaneous aspiration. In the hip, aspiration is facilitated by sonographic guidance.
• Treatment of septic arthritis consists of antibiotic therapy and drainage of the involved joint. Drainage may be by aspiration or surgical intervention.
• Ultrasound is able to detect subperiosteal abscesses early in osteomyelitis and may be the only imaging test required in uncomplicated cases. Magnetic resonance imaging has the highest sensitivity and specificity for detecting osteomyelitis.
• Intervertebral diskitis is an acute infection of the vertebral disk usually seen in children younger than 5 years. The lumbar area is most commonly involved.

Musculoskeletal diseases are frequently encountered in pediatric patients. They vary in significance from minor, self-limited illnesses to serious systemic diseases. Limb-threatening complications can occur. In the case of infants and young children, the evaluation of musculoskeletal complaints is complicated by the patient's inability to articulate the problem and the inherent difficulty of performing a sufficient physical examination in an uncooperative patient.

Septic arthritis is an infection within a joint space. Bacterial pathogens are common in patients with acute septic arthritis, whereas fungal and mycobacterial pathogens tend to be associated with chronic septic arthritis. Acute septic arthritis occurs in all age groups but is more common in children; 75% of cases occur in children younger than 5 years, with the peak incidence being between 6 and 24 months of age. Boys are affected twice as frequently as girls.1 The infection involves a joint of the lower extremity in 75% of cases, with the knee and hip being most commonly involved. Other affected joints, in order of involvement, include the ankle, elbow, shoulder, and wrist. More than 90% of cases are monoarticular.

### Etiology

Seeding of the joint with bacteria occurs either by hematogenous spread, direct inoculation of infected material into the joint capsule, or from an adjacent site of infection. In children, it most commonly results from hematogenous spread as bacteria pass into the synovial space through the highly vascular synovial membrane. Infection secondary to trauma most commonly affects the knee. Contiguous spread of infection from osteomyelitis to the joint space occurs in approximately 10% of cases and is more common in newborns and young infants. In these children, blood vessels cross the physis and thereby connect the metaphysis and epiphysis ...

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