Five Days of Hip Pain with a Normal Radiograph: Infection, Inflammation, or Something Else
A 4-year-old girl has 5 days of right thigh and knee pain and refuses to walk. Temperature is 37.9 °C, CRP 62 mg/L, ESR 45 mm/h, and white cell count 10.2 × 10⁹/L. She will not allow the right hip to be flexed past 30 degrees. Radiographs of the pelvis, right hip, and right knee are normal. Blood cultures have been drawn but are not yet back. There is no preceding trauma and no recent antibiotic use. The team is unsure whether this is transient synovitis, septic arthritis, osteomyelitis, or chronic recurrent multifocal osteomyelitis (CRMO), and the on-call resident asks whether a bone scan would be quicker than MRI and whether gadolinium is needed.
1. Compare ultrasound, MRI, and bone scan for this child and recommend a study, including timing and how to handle sedation.
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Recommendation: MRI of the pelvis and both femora with fat-suppressed fluid-sensitive sequences and post-gadolinium imaging, within 24–48 hours and after blood cultures are drawn.
- Ultrasound. Excellent and immediate for joint effusion and for guiding aspiration, but it cannot assess marrow signal, subperiosteal collections, or the extent of osseous involvement. A positive ultrasound showing a large effusion in this clinical setting should prompt emergent aspiration, not replace MRI.
- Bone scan. Sensitive for multifocal disease but nonspecific, delivers a higher dose than a focused MRI, and takes hours. It is a reasonable problem-solving tool when MRI is unavailable or when multifocal disease is the question.
- MRI. Best characterization of marrow edema, abscess, subperiosteal collection, and adjacent joint involvement. Include the whole femur and pelvis, not just the hip, because adjacent and multifocal disease is common and because the field of view determines whether skip lesions are seen.
- Contrast. Gadolinium helps distinguish drainable abscess and subperiosteal collection from diffuse edema and helps identify septic arthritis. Give it unless renal function or prior reaction precludes it.
- Sedation. Feed-and-sleep or Child Life preparation for a 4-year-old; reserve general anesthesia for failure of those measures. Delaying the study for a full anesthesia slot is the wrong trade-off in suspected musculoskeletal infection.
2. How do you distinguish osteomyelitis from CRMO in this child, and does the distinction change immediate management?
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Acute hematogenous osteomyelitis.
- Fever, focal point tenderness, markedly elevated CRP/ESR, positive blood culture in a minority, and rapid onset over days.
- MRI shows a single, often metaphyseal, focus with surrounding edema and possible subperiosteal abscess or joint involvement.
CRMO.
- Characteristically a well-appearing child with insidious pain, minimal or no fever, modest inflammatory markers, and frequently multiple symmetric or metaphyseal lesions (clavicle, tibia, femur, pelvis).
- MRI shows multifocal bone edema and sometimes sclerosis or hyperostosis, with no abscess.
- Diagnosis is one of exclusion: infection, malignancy, and Langerhans cell histiocytosis must be considered.
Does it change management? Yes, and immediately. In this child, fever plus a CRP of 62 mg/L favors infection. Blood cultures should be obtained, and empiric intravenous antibiotics directed at Staphylococcus aureus should not be delayed for imaging; the MRI is obtained to define drainable collections and the extent of disease. If instead the MRI showed multiple symmetric non-purulent lesions in a child who looks well, the correct move is to stop or avoid prolonged antibiotics, obtain cultures to exclude infection, and involve rheumatology for NSAID therapy and evaluation for associated inflammatory disease.
3. MRI shows a 2-cm subperiosteal abscess along the proximal femur and a moderate hip joint effusion. What is the role of interventional radiology, and what is your follow-up imaging strategy?
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Immediate management.
- Ultrasound-guided aspiration of the hip joint is both diagnostic and therapeutic and should be performed urgently; a purulent aspirate confirms septic arthritis and triggers surgical washout.
- A subperiosteal abscess of 2 cm in a child usually requires drainage. Image-guided percutaneous drainage by interventional radiology is appropriate when the collection is accessible and there is no extensive soft-tissue necrosis; open surgical debridement is preferred for complex, multiloculated, or necrotic collections. Communicate directly with orthopedics and the interventionalist to decide, rather than sending the child in two directions.
- Continue culture-directed intravenous antibiotics.
Follow-up imaging.
- Do not routinely re-image a child who is improving. Clinical and laboratory response (defervescence, falling CRP) is the primary endpoint.
- Re-image if there is persistent fever beyond 48–72 hours, rising inflammatory markers, new focal findings, or failure to improve - to look for an undrained collection, a second focus, or a complication such as pathologic fracture.
- At the end of therapy, radiographs are usually sufficient; MRI is reserved for persistent pain or concern for chronic osteomyelitis, where it may show persistent edema for months and must be interpreted with the clinical picture.