A 61-year-old man presented to the emergency department with a sudden onset of atraumatic pain and swelling in the left knee that had lasted for 3 hours. His height was 175 cm, his weight was 74 kg, and he worked as a postman. He had undergone unicompartmental knee arthroplasty (UKA) 5 years previously and had been symptom-free and functioning well. A radiograph performed in the emergency department was interpreted as normal (Fig. 1). The patient was advised to return if he had persistent symptoms.
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