A crystalline mimic of patellar tendon tumor

A 41-year-old male presented to hospital with acute sudden onset atraumatic left knee and bilateral ankle pain for 3 days. This occurred on a background of inflammatory bowel disease (IBD) diagnosed 3 months earlier and on treatment with mesalazine for 1 month. There was no history of arthritis, psoriasis, inflammatory eye disease, diabetes, chronic kidney disease or diuretic use. His uric acid level was 0.33 mmol/L (Reference range 0.2–0.42 mmol/L). The clinical pattern included tenosynovitis of the flexor tendons of his feet as well as enthesitis in the left Achilles tendon and left patellar tendon. The left Achilles tendon changes continued for more than 2 weeks and were slow to settle despite high-dose prednisone. Clinically, the presentation seemed consistent with IBD-related arthritis, and the patient was subsequently treated with methotrexate. His symptoms improved; however, several months after his initial treatment, he was noted to have a small left knee effusion associated with mechanical symptoms. The left knee effusion persisted over several months. His inflammatory markers remained normal.