The 6-Week-Old with a Limp and a Normal-Looking Radiograph

Differential:

• Infection: osteomyelitis, septic arthritis of the hip or knee, or a soft-tissue infection. A 6-week-old with poor feeding, irritability and a swollen limb is unwell until proven otherwise; afebrile does not exclude sepsis in neonates.
• Non-accidental injury (NAI): a 6-week-old cannot mobilise, so any injury is suspicious. Metaphyseal corner fractures and rib fractures are classic but may be radiographically occult early.
• Metabolic bone disease of prematurity if he was preterm (not stated - check).
• Toddler's fracture is unlikely at 6 weeks as he is not weight-bearing, but a spiral femoral fracture from a twisting injury is possible.
• Neoplastic or haematological causes are rare but considered.

Why a normal radiograph is not reassuring:

• In early osteomyelitis, radiographic changes (periosteal reaction, lucency, sclerosis) take 10–14 days to appear. A normal film at day 3 of symptoms is expected and does not exclude infection.
• In neonates, the physis and epiphyses are largely cartilaginous and poorly seen on plain film, so joint involvement and metaphyseal injury can be missed.
• Therefore the child should not be discharged on the basis of a normal radiograph alone.

Immediate management:

• Do not discharge. Escalate to the paediatric team and the on-call consultant.
• Sepsis screen: full blood count, CRP, blood cultures, and joint aspiration if septic arthritis is suspected. Start empirical intravenous antibiotics after cultures if infection is suspected - do not delay for imaging.
• Analgesia and immobilisation of the limb.
• Safeguarding: if NAI is a possibility, involve the paediatric safeguarding lead and document carefully; do not confront the parents.

Further imaging:

• Ultrasound of the hip and thigh - quick, no radiation, and can detect a joint effusion, subperiosteal collection, or soft-tissue swelling. This is the first-line test in a neonate with a limp.
• MRI of the whole femur and adjacent joints (with or without contrast) is the most sensitive test for osteomyelitis, joint effusion, and soft-tissue involvement, and can detect metaphyseal injury. It requires sedation or feed-and-wrap in a 6-week-old, so coordinate with paediatric anaesthesia.
• Skeletal survey if NAI is suspected, performed according to local safeguarding protocol, ideally within 24 hours and repeated at 2 weeks for occult fractures.
• Repeat radiograph at 10–14 days if the initial film was normal but clinical suspicion persists, to look for periosteal reaction.

This changes the case from a probable infection to a probable safeguarding concern, and both must be managed in parallel.

• Safeguarding is now the priority. A metaphyseal corner fracture in a non-mobile 6-week-old is a highly specific marker of inflicted injury. Notify the paediatric safeguarding lead immediately and follow the local child protection pathway; a formal skeletal survey and a retinal examination by ophthalmology should be arranged.
• Do not assume the effusion is septic, but do not exclude it. A corner fracture can produce a sympathetic effusion, but a joint effusion in a neonate still warrants aspiration and culture if there is any clinical concern. Continue antibiotics until cultures are negative.
• Document objectively. Record the imaging findings, the discussion with the safeguarding team, and the chronology. Avoid speculative language about who caused the injury.
• Communicate with the family carefully. Explain that further tests are needed to protect the child, without accusing them; the safeguarding team leads that conversation.
• Follow-up imaging at 2 weeks to look for healing fractures and to complete the skeletal survey, and involve the paediatric radiology consultant in the reporting.