Investigations

A work up of an irritable hip (transient synovitis) should seek both to exclude alternative diagnoses and to demonstrate confirmatory features.

Haematological tests such as FBC, WCC, ESR and CRP are usually normal. Throat swab and mid-stream urine specimens rarely reveal an infection.

Radiographs are poor at demonstrating effusions and are frequently normal.

Ultrasound is the most useful investigation and can detect an effusion in 95% of cases where one is present. The anterior hip capsule is displaced 2.5 mm from the femoral head in a normal hip, but by up to 6 mm in an irritable hip. Any effusion must be aspirated and cultured to exclude septic arthritis. However, an effusion is not demonstrable in all cases of irritable hip.

There are four independent clinical predictors to differentiate between septic arthritis and transient synovitis. In transient synovitis;

• temperature should be <38.5°C (<101.3°F)
• the child may limp but should be able to bear weight
• ESR should be well under 40 mm/hour
• serum white blood cell count <12,000 cells/mm³.

If all of the above criteria are met, the predicted probability of septic arthritis is less than 0.2% for zero predictors, and 3.0% for one predictor (2)

A C-reactive protein over 20 is a strong independent predictor of septic arthritis of the hip. (3)

References

1. Krul M et al. Acute non-traumatic hip pathology in children: incidence and presentation in family practice. Fam Pract. 2010 Apr;27(2):166-70.
2. Luhmann SJ et al. Differentiation between septic arthritis and transient synovitis of the hip in children with clinical prediction algorithms. J Bone Joint Surg Am. 2004 May;86-A(5):956-62.
3. Swarup I et al. Septic Arthritis of the Hip in Children: A Critical Analysis Review. JBJS Rev. 2020 Feb;8(2):e0103.