does not preclude the possibility of a bacterial bone or joint infection, and many infections are preceded by a history of minor trauma Septic arthritis is the most serious infectious cause of joint pain and limp It is more common in younger children and typically presents with a warm, swollen joint (although swelling in the hip is very difficult to detect clinically) Exquisite pain with attempts to flex or extend the joint is characteristic of septic arthritis, and the degree of pain with motion serves as a helpful clinical sign in distinguishing bacterial joint infection from inflammatory conditions A common diagnostic challenge is differentiating septic arthritis, transient (or toxic) synovitis, and Lyme disease (in endemic areas) in a young child with fever, limp, and pain localized to the hip Transient synovitis, a postinfectious reactive arthritis, follows a milder course It is usually preceded by a recent viral respiratory or gastrointestinal illness Acute-phase reactants may be elevated in each of these conditions, although usually less so in synovitis A unilateral joint effusion, which is better visualized with ultrasound than plain films, may be present in each of these conditions Bilateral effusions are more suggestive of an inflammatory synovitis Joint aspiration may be required for a definitive diagnosis because a septic hip is a surgical emergency requiring open drainage Osteomyelitis is another potentially serious infectious cause of limp, although the presentation is typically more chronic than that of a septic joint Osteomyelitis, which is also more common in younger children, presents with pain and occasionally warmth and swelling, usually over the metaphysis of a long bone A reactive joint effusion may be present Occasionally, osteomyelitis and septic arthritis will coexist Chronic recurrent multifocal osteomyelitis (CRMO), also known as chronic nonbacterial osteomyelitis (CNO), is a rare inflammatory condition that should be considered in cases of suspected osteomyelitis that seem atypical in their response to treatment More detailed discussions of both septic joint and osteomyelitis are found in Chapters 94 Infectious Disease Emergencies and 121 Musculoskeletal Emergencies