Septic arthritis is an orthopaedic emergency which requires timely management to prevent joint destruction and poor outcome. Differentiating septic arthritis from transient synovitis in pediatric patients is aided by the use of Kocher criteria which have excellent sensitivity but lack specificity. In addition to these two disorders, primary pyomyositis is bacterial infection of skeletal muscle that most commonly affects children. Patients present with pain, swelling, fever, and elevated inflammatory markers which mimics septic arthritis. If left untreated, pyomyositis can lead to abscess formation and sepsis.
Due to potential for nearly identical presentations of septic arthritis and pyomyositis, differentiation of these two disorders can be aided with the use of MRI which has a high sensitivity for detecting muscle edema and abscess formation. In this case series, we discuss the use of MRI to assist with the diagnosis of pyomyositis versus septic arthritis. The authors advocate the use of MRI in questionable or complicated cases of septic arthritis or where synovial fluid aspiration is unable to be obtained promptly.