ABSTRACT A 34-year-old male patient presented severe pain in his left knee in association with functional incapacitation, with no apparent triggering factor. He sought medical attention in December 2006, at which time he was prescribed NSAIDs. After a year, he reported increased swelling and pain at the site. He was referred to a knee specialist with a suspected meniscal injury. Upon examination, severe swelling of the joint, with movement limitation, severe pain and negative joint aspiration, was found. Since the simple radiographic results were normal, an MRI of the knee was requested. The MRI revealed a large accumulation of fluid inside the joint, together with marked synovial proliferation, especially focal thickening in clumps with an intermediate signal in T1 and T2, and a discrete hyposignal in T2 that was suggestive of pigmented villonodular synovitis with intact meniscus and ligaments. The patient underwent arthroscopy on the left knee, which revealed whitish irregular fragments, and then underwent arthrotomy with removal of the lesion and extensive synovectomy. The material was sent for anatomopathological examination, which showed the presence of synovial chondromatosis. Eight months after the surgery, the patient does not have any complaints, with a range of motion of 130° in the left knee without joint effusion or signs of inflammation. Synovial chondromatosis is a rare benign type of metaplasia of the synovial membrane that leads to the formation of cartilaginous free bodies in the joint space. It is difficult to diagnose because 95% of the nodules, when not calcified, can be overlooked radiologically.
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