Case Reports in Medicine 3 PVNS of pes bursa Bursa (opened) Soleus m Knee PVNS nodules Lat gast m (a) PVNS pes bursa Nodules ST tendon Bursa wall (b) Figure 2: (a) Intraoperative photograph of the lesion and (b) the gross specimen showed multiple yellow to brown nodules inside the pes anserine bursa (ST: semitendinosus tendon). (a) (b) Figure 3: (a) Histopathological slide showed villous proliferation covered by reactive appearing synovial cells (H&E, ×4). The cores of the villi had a cellular inﬁltrate consisting of mononuclear cells and foamy histiocytes with scattered giant cells ((b); H&E, ×20). cytometry point towards a neoplastic process [2, 12–15]. The incidence of PVNS is rare with an estimation of 1.8 patients per million population/per year [16]. PVNS is almost exclu- sively an intra-articular process. Extra-articular lesions are extremely rare and tend to occur in the same joint locations as the intra-articular PVNS, and are usually extensions from intraarticular lesions [17]. Rarer is the involvement of a true bursa (PVNB) with no articular communication. Although bursa around the knee (suprapatellar and popliteal) are more commonly reported, PVNB has also been sporadically described in the bursae around acromium, olecranon, iliop- soas, ﬁngers, toes, temporomandibular region, and sacro- iliac areas [1, 14, 18–23]. A review of literature review revealed only twelve prior reports of exclusively extra- articular PVNB of the pes anserinus (Table 1) [1–11]. PVNB has shown a tendency to occur in young persons. Most of the cases are diagnosed prior to 40 years, with a mean age similar to PVNS [14]. Typically, symptoms are of relatively long duration and include pain and swelling, but limitation of knee range of motion is not common. The age range of PVNB of pes anserine has varied from 11–63 years with a mean of 31 years with no predilection for either sex or side [1–11]. Only one case was associated with a history of trauma 13 years previously [4]. Direct mechanical pressure to the bone and direct vascular foramina involvement was suggested as possible cause of bone involvement in one case [9]. PVNB, like PVNS, is usually monofocal, although Kay et al. reported a metachronous lesion in the wrist [7]. MRI is the imaging modality of choice and is helpful for diagnosis, surgical planning, and followup [5, 6, 8, 24]. The diﬀuse and nodular thickening of the synovium shows low to intermediate signal intensity, as compared to muscle signal, on all sequences. This low signal is due to the deposits of hemosiderin. Heterogeneous or septal enhancement may be observed after contrast administration. Apart from localization, MRI may also show the extension of abnormal synovial tissue in the periarticular region, in adjacent joints or bursae. Eckhardt and Hernandez [24]