= OBSERVATION HIP = TRANSITORY SYNOVITIS = TOXIC SYNOVITIS = COXITIS FUGAX

= nonspecific inflammatory reaction

◊Most common nontraumatic cause of acute limp in a child!

Etiology: unknown; no organism on joint aspiration

Average age: 6 (range, 5–10) years; M÷F = 2÷1

• history of recent viral illness (65%)
• mild fever (25%), mildly elevated ESR (50%)
• developing limp over 1–2 days; pain in hip, thigh, knee
• serosanguinous joint fluid
• radiographs usually normal
• joint effusion:
  • displacement of femur from acetabulum
  • displacement of psoas line
  • lateral displacement of gluteal line (least sensitive + least reliable)
• regional osteoporosis (? hyperemia, disuse)

US:

• fluid in anterior recess between capsule + femoral neck

MR:

• joint effusion
• intense enhancement of hypertrophied synovium (DDx: septic arthritis)

NUC:

• normal / slight increase in activity → excluding osteomyelitis + avascular necrosis

Prognosis: complete recovery within a few weeks

Dx: per exclusion

Rx: non–weight-bearing treatment

DDx:

1. Septic arthritis
2. Trauma
3. Legg-Perthes disease
4. Acute rheumatoid arthritis
5. Acute rheumatic fever
6. Tuberculosis
7. Malignancy