We present a case of a 44-year-old male, with a history of right knee swelling from insidious onset and two years of progressive evolution. There was not history of trauma or other associated symptoms (fever, pain, asthenia, weight loss, and other swollen joints).

He was referred to the orthopedic consultation by the primary health care specialist, after unspecific knee magnetic resonance imaging (MRI) and needle arthrocentesis, followed by unsuccessful physical therapy sessions.

At physical exploration it was clear a tender and voluminous swelling of the right knee, especially at the sub-quadricipital bursa, without other clear inflammatory signs (Figure 1). The patient denied other symptoms or even knee pain at exploration, although it as present on daily activities.

The X-ray images were considered normal, without bony lesions. At the MRI, there was a signal increase with heterogeneous shape, especially at sub-quadriceps and suprapatellar bursa, without bony erosion (Figure 2). However, small areas of hippo-signal were observed which determine some intra-sac septations and vegetations, aspects which may be compatible with pigmented villonodular synovitis (PVNS).

He has proposed to knee arthroscopy and needle arthrocentesis for final diagnosis and further treatment. It was performed under general anesthesia, the initial arthrocentesis revealed a pathognomonic xanthochromic synovial fluid (Figure 3) >120 cc and the arthroscopy images (Figure 4) were clear of vascularized polypus formations spread through the entire synovial cavity, with higher density in sub-quadricipital area. Tissue and liquid samples were acquired for cytochemical, histologic, and microbiological identification.

The histologic analysis revealed characteristic of chronic inflammation with, reactive hyperplasia of synovial tissue and presence of high hemosiderin deposits in the synovial stroma (Figure 5A).

At a second surgical time, after confirmation of diagnosis, the patient was submitted to total synovectomy with necessity of conversion from arthroscopic to open synovectomy (Figure 5B).

Postoperative period and rehabilitation showed with no complications. At two years of follow-up there is still no clinical or imagiological sign of recurrence, and maintains annual consultation.

Pigmented villonodular synovitis is a rare, benign but potentially locally aggressive and recurrent disease, occurring 1.8 cases per million of inhabitants/year worldwide [1]. It occurs mainly in young adults, predominantly in females, between 30 and 40 years of age [2]. It is a chronic proliferative tumor that may involve joints, tendons, and bursa, characterized by World Health Organization (WHO) as giant cell tumor [3]. The diffuse intra-articular form corresponds only to 23% of these tumors and 0.9% of all benign soft tissue tumors [1].

The occurrence of malignant transformation and metastasis, although rare, is described in the literature [4]. It is typically monoarticular and affects large joints, most often in the knee (80% of cases), hip (15% of cases), and ankle (5% of cases) [2]. The etiology of PVNS is unknown, although numerous causes are pointed out potential, such as the occurrence of a chronic inflammatory process, recurrent trauma, hemarthrosis, inflammatory reaction to an unidentified agent or a local disorder of the lipid metabolism in the synovial membrane [1].

Xanthochromic or dark red joint arthrocentesis may help in diagnosis but its present in only 64% of patients. X-ray evaluation has a minor role in diagnosis workup, reserved for exclusion of other pathology. Magnetic resonance imaging findings are diagnostic in more than 95% of patients and histopathology is still recognized as the gold standard for the final diagnosis of PVNS [5],[6].

The treatment is surgical, with arthroscopic or open resection. In localized PVNS, local lesion excision by arthroscopy is advocated. In diffuse PVNS, synovectomy (total or subtotal) is recommended, but the role of arthroscopy is debatable as it affects a larger area of knee and predominantly affects the posterior compartment, the most difficult to access by arthroscopy [7].

Although benign, pigmented villonodular synovitis can result in significant morbidity if left untreated. Pain, loss of function, and joint destruction may occur. In primary treatment it is surgical, with recession via synovectomy or/and radiotherapy. Recurrence is reduced with complete resection, which is best achieved with localized form, with relapse rates from 10% to 30% to the diffuse form. It is mandatory to maintain patients with diffuse form of PVNS in follow-up for several years after surgery.