An early-teen boy with spastic bilateral cerebral palsy predominantly affecting the lower limbs was admitted to hospital with a 12-hour history of fever and pain in the left thigh. Two months previously the patient had a biceps femoris and semimembranosus muscle fasciotomy and a tendon shortening procedure and had been on anticoagulation therapy. The pain presented following his initial mobilisation after this procedure. His previous medical history was remarkable for a low-grade fever 3 weeks previously, which resolved after a 7-day course of amoxicillin-clavulanic acid.

The patient had a temperature of 38.2°C with a severe limitation in all ranges of movement of his knee, but he was able to weight bear as normal.

Blood tests showed a white blood cells (WBC) of 13 560 x 10ˆ9/L, increased C reactive protein (CRP) level of 43.9 mg/L, a normal erythrocyte sedimentation rate (ESR), a haemoglobin of 103 gr/L and a platelets of 381 x 10ˆ9/L. Creatine kinase and coagulation tests (including D-dimer) were within the normal range.

X-rays of the left hip and knee were unremarkable. An ultrasound scan showed …