Quick onset of pigmented villonodular synovitis: atypical onset or infective involvement?

A 43 years old woman, was sent to our attention from the first aid station as she suffered from a sudden involvement of the left knee joint blockade. Non rheumatic disease, trauma or infective events in the last months. The knee looked very swollen, painful and there was a serious reduction of its extension corresponding to 45 degrees. MRI revealed a big synovial nodule consistent with a Pigmented Villonodular Synovitis (PVNS) diagnosis. The arthrocentesis evidenced a synovial fluid slightly haematic with 23,600 leucocytes/mm3 and the cultural examination was negative. Arthroscopy revealed hyperplastic areas of very hyperaemic and bleeding synovial membrane alternate to areas of fibrotic tissue. Hystological examination proved the diagnosis of PVNS, but it showed also neutrophils infiltration, that suggested a possible infection. The cultural examination of synovial membrane was positive for an atypical form of Mycobacteria (Mycobacterium gordonae). In spite of a prompt start of the rehabilitation, the recovery of the joint mobility after the operation resulted very difficult. About 6 months after the first operation, the patient was subjected again to a knee arthroscopy with removal of the arisen fibrotic adherences. The different synovial membrane biopsies carried out for the cultural examination, were negative. After the second operation a good recovery of the articular function was observed. Pigmented villonodular synovitis (PVNS) is considered to be a neoplastic-like disorder, that may involve the synovial membrane of joints, bursa or tendon sheaths. The pathogenesis of the disease is still unknown. When the joints are involved, the present terminology differentiates between a localized (LPVNS) and a diffuse form (DPVNS). Atypical mycobacteria are ubiquitary environmental microrganisms that only seldom cause infections at the muscle-skeleton system level. Primitive joints diseases, trauma, joints infiltration with corticosteroid, and immunosuppression can act as susceptibility factors. This case proves a concomitant presence of LPVNS and intra-joint infection by atypical mycobacteria, not previously described in literature. We deem that the delay in the full recovery, due to a significant development of intra-joint fibrotic tissue, might be caused by the presence of the Mycobacterium