Acute calcific periarthiritis of the knee presenting with calcification within the lateral collateral ligament.

A 71-year-old woman was admitted with acute swelling of the right knee, pain on the lateral aspect and restricted movement. There was no instability or locking. She had no history of trauma and was generally in good health. Plain radiographs demonstrated a calcific opacity adjacent to the lateral femoral condyle. This was shown to be within the lateral collateral ligament (LCL) at ultrasound and MRI. A diagnosis of acute calcific periarthritis (ACP) was made. The patient's symptoms resolved within a few weeks with simple analgaesia. ACP presenting with calcification within the LCL is rare. It is important to recognise the clinical and imaging findings of this condition as it may mimic other more serious pathologies such as infection and gout. This may result in unnecessary investigations, misdiagnoses and incorrect treatments.