a 39-year - old female presented with a three month history of right knee pain and swelling . tenderness was diffuse and mild in nature with a rubbery hard consistency and the mass was not mobile in any plane . a private clinic made a provisional diagnosis of a ganglion or a lipoma and decided to do an excision biopsy under local anaesthesia . however , it was discovered during surgery that the soft tissue mass was arising from the proximal tibiofibular joint and communicating with the anterior and posterior aspects of the joint . 1 ) . mri showed a well circumscribed lesion with a capsular margin in the posterior aspect of the proximal tibiofibular joint with decreased heterogeneous signal intensity on both t1-weighted and t2-weighted images . the t2-weighted image showed a relatively hyper - intense signal compared to the skeletal muscle ( fig . intraoperatively , the mass was yellowish brown in colour , arising from the proximal tibiofibular joint and circumscribing the joint . pathological findings of the biopsy specimens showed proliferation of round to polygonal cells , with scant cytoplasm forming villous and fingerlike or rounded mass underlying the synovium . hemosiderin - laden macrophages varied in number , giving the synovium a dark - brown appearance . postoperative recovery was uneventful and at the most recent follow - up at 12 months after surgery , the patient was fully asymptomatic with no evidence of recurrence on mri . although pvns is a benign disease , it can be locally destructive causing significant and permanent damage to the joint . the distribution of lesion location depends on the lesion subtypes , either localized or diffuse , and on whether the lesion is intra - articular or extra - articular3,5 ) . localized pvns is an uncommon pathology characterized by the presence of nodular pedunculated mass protruding into the joint cavity7 ) . extra - articular lesions are rare and tend to occur in the same joint locations as intra - articular lesions8 ) . this case report presents a rare site of occurrence of which only one citation is found in the english - language literature . ryan et al.2 ) reported a case of pvns at the proximal tibiofibular joint which was located just anterior to the tibiofibular joint . however , in our case , the mass was arising from the proximal tibiofibular joint extending to the extra - articular lesion and communicated with the anterior and posterior aspects of the joint . the diagnosis of localized pvns is difficult , and the time period between the onset of symptoms and the definitive diagnosis is usually two years4 ) . plain radiographs are usually unremarkable . however , in about 9% to 25% of the total cases , extrinsic erosion , often with well - defined sclerotic margins of the underlying bone may be seen as in our case where the simple radiograph showed bony erosion at the tip of the fibular head . it helps to delineate the location , size , and extent of the lesion4,9 ) . the lesion usually has heterogeneous signal intensity depending on the amount of hemosiderin content and is characterized by decreased intensity on both the t1-weighted and t2-weighted images7,10 ) . albeit sensitive , mri is not specific due to other conditions such as fibroxanthoma , hemangioma , and desmoid tumors giving similar pictures7 ) . histologically , the lesion is characterized by proliferation of polygonal mononuclear cells , histiocytes , scattered multinucleated giant cells , and foam cells with hemosiderin pigment deposits10 ) . the recurrence rate of localized pvns has been reported to range from 0% to 13%4,10 ) , whereas the recurrence rate of diffuse pvns has been reported to range from 8% to 35%7 ) . the location of pvns described in this article , which originated at the proximal tibiofibular joint , is extremely rare . pvns that involves small joints such as the tibiofibular joint can be easily misdiagnosed as other mass lesions . therefore , the clinicians should pay more attention and must consider different diagnosis for mass lesions around the knee joint . complete surgical excision is the treatment of choice and the patient should be followed up for recurrence .
pigmented villonodular synovitis ( pvns ) is a rare disease . 
 it is a benign neoplastic process typically affecting young to middle - aged adults and most commonly involving the knee , hip , and shoulder joints . 
 the symptoms include diffuse pain and swelling with discomfort . 
 we report a rare case of localized pvns originating at the proximal tibiofibular joint in a 39-year - old female patient with radiologic changes for short duration of time . the clinical history , plain radiographs , magnetic resonance imaging , and pathologic findings of the reported patient were reviewed . 
 complete surgical excision was performed and there was no evidence of recurrence after one - year follow - up .