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RACP DWE Paediatrics study materials
RACP DWE Paediatrics study resources

The 4-Year-Old With a Limp and Knee Pain

RACP DWE Paediatrics · Put your knowledge to work in context.

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The 4-Year-Old With a Limp and Knee Pain

A 4-year-old boy presents with 3 weeks of intermittent right knee pain and a limp. He is afebrile. He points to the anterior knee, not the hip. There is no history of trauma. He has been less keen to walk and has started crawling at home. Examination: temperature 36.9°C, weight on the 50th centile, no rash. There is mild restriction of internal rotation of the right hip and the knee has a small effusion. FBC: Hb 108 g/L, WCC 8.2 ×10⁹/L, platelets 340 ×10⁹/L, ESR 42 mm/h, CRP 18 mg/L. X-ray of the knee is reported as normal. The family recently moved from a country with a high tuberculosis burden.

1. What is your differential diagnosis and what is the first investigation you would arrange?

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The key clinical clue is that knee pain in a child can be referred from the hip. Restricted internal rotation of the hip with a limp and a normal knee X-ray should make you think of hip pathology first.

Differential:

  1. Septic arthritis of the hip - less likely with 3 weeks of symptoms and no fever, but must be excluded. A child who is afebrile and systemically well with a chronic course is less typical, but never exclude it on history alone.
  2. Transient synovitis - usually post-viral, short-lived, and self-limiting; 3 weeks is long.
  3. Perthes disease (Legg-Calvé-Perthes) - avascular necrosis of the femoral head, typically boys aged 4–8, with a limp and hip/groin/knee pain. This fits well.
  4. Juvenile idiopathic arthritis - possible with a joint effusion and raised inflammatory markers, but usually more than one joint and a longer history.
  5. Osteomyelitis - possible but less likely without fever or focal bony tenderness.
  6. Tuberculosis of the hip - relevant given the family's origin from a high-burden country; chronic monoarthritis with a limp and elevated ESR is a classic presentation.
  7. Malignancy - leukaemia or bone tumour can present with a limp and bone pain; the mild anaemia and raised ESR warrant consideration.

First investigation: Anteroposterior and frog-leg lateral X-rays of the hips, not just the knee. If the hip X-ray is normal and symptoms persist, MRI of the hip is the most sensitive test for early Perthes, osteomyelitis, or soft-tissue pathology.

Additional tests: FBC and film, ESR/CRP (already done), blood cultures if febrile, and consider hip aspiration if septic arthritis is suspected. Given the TB risk, arrange a chest X-ray and discuss TB testing (IGRA or Mantoux) with the infectious diseases team.

2. The hip X-ray shows a small, sclerotic femoral head with widening of the joint space. How would you explain this to the parents and what is the management plan?

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The X-ray findings - a small, sclerotic femoral head with joint space widening - are consistent with Perthes disease (Legg-Calvé-Perthes), avascular necrosis of the proximal femoral head. The natural history is variable and depends on the age of the child and the extent of head involvement.

Explanation to parents:

  • Explain that the blood supply to the ball of the hip joint has been interrupted, causing the bone to soften and fragment. It is not an infection and is not caused by anything the parents did.
  • The knee pain is referred from the hip - this is why the knee X-ray was normal.
  • The condition usually runs a course of 18–36 months and then heals, but the shape of the healed head determines long-term hip function.
  • Younger children (under 6) tend to do better because they have more growth remaining and the head can remodel.

Management plan:

  • Refer urgently to a paediatric orthopaedic surgeon. The main aims are to maintain hip range of motion, contain the femoral head within the acetabulum, and avoid deformity.
  • Initial treatment is usually conservative: physiotherapy, avoidance of high-impact activity, and sometimes a period of bed rest or traction if the hip is irritable. Analgesia as needed.
  • Regular review with X-rays to monitor the stage of the disease (initial, fragmentation, reossification, healed).
  • Surgery (e.g., femoral osteotomy or pelvic osteotomy) may be considered if there is progressive deformity or loss of containment, particularly in older children or those with severe involvement.
  • Long-term follow-up into adulthood is needed because of the risk of early osteoarthritis.
  • Address the TB question: the chest X-ray and TB testing should still be arranged given the family's origin, because TB hip can mimic Perthes. Discuss with the respiratory/infectious diseases team.