1. X-rays. Obtain two views in perpendicular planes. Particularly helpful when child is younger than 2 years of age, because screening X-rays of bilateral lower extremities reveal fractures in 20% of cases. In children, growth plates are weaker than ligamentous attachments; thus, occult fractures through the growth plate are more common. X-rays show sclerotic changes of some bone tumors and osteomyelitis. Periosteal reaction or lytic changes can be seen in infections (> 10 days).
Ultrasound. Quick and easy method of evaluating for joint effusion, particularly in the hip joint.
Bone scan. More sensitive than other tests for early osteomyelitis and useful if multiple areas are suspected or when site of problem is unclear.
CT and MRI scans. Usually not needed acutely. Obtain CT scan for definition of bones, MRI scan for definition of soft tissues and effusions. Useful if presentation is atypical or diagnosis is difficult.
V. Plan. Start with careful history of limp and constitutional symptoms.
A. If patient has high fever or other features of infection, locate the source: bone, joint, muscle, skin. Consider CBC with differential, ESR, CRP; abnormalities may also suggest malignancy. Culture potential sources of infection and start empiric antibiotics if warranted.
B. Determine if there was trauma. Could this be abuse? Point tenderness on growth plate may suggest fracture. Consider obtaining x-rays of the area. Bone pain suggests fracture, local bone tumor, or leukemia. X-rays showing fractures, sclerotic rings, or leukemic lines will support the diagnosis.
C. Carefully evaluate hip joint. Remember that referred pain to the knee may originate in the hip. Permanent damage may result from increased pressure within the joint capsule. Consider DDH in younger kids, LCPD in school-age boys, SCFE in overweight adolescents, and septic arthritis in everyone (medical emergency).
VI. Problem Case Diagnosis. This 4-year-old girl had fever of 5 days' duration and point tenderness over the left distal tibia. A plain x-ray was normal, but increased uptake was seen on bone scan in the left tibial metaphysis. Diagnosis of osteomyelitis was made. Bone biopsy was performed, and culture was positive for Staphylococcus aureus.
VII. Teaching Pearl: Question. Why does knee pain occur in patients with hip pathology?
VIII. Teaching Pearl: Answer. The anterior branch of the obturator nerve passes close to the hip joint and, if irritated, may send a painful sensation to the medial side of the knee.
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