Pigmented villonodular bursitis (PVNB) in the medial gastrocnemius bursa:report of an unusual case.

INTRODUCTION: Pigmented villonodular synovitis (PVNS) represents an uncommon benign proliferative disorder that may involve the synovium of the joint diffusely or focally, or that may occur extraarticularly in a bursa (pigmented villonodular bursitis [PVNB]) or tendon sheath (pigmented villonodular tenosynovitis [PVNTS]). The knee, followed by the hip, is the most common location for PVNS or PVNB, whereas PVNTS occurs most often in the hand and foot. It primarily involves young adults, the peak age being between the second and fourth decade of life and appears with non-specific symptoms which mimic other pathologies, including traumatic knee injuries, thus often delaying the correct diagnosis. REPORT/DISCUSSION: A 17 year old female presented with swelling and mild degree of pain in her left knee for three months. No history of trauma or any other significant illness was present. Routine laboratory investigations were normal. X-ray was normal. MRI of the knee was performed and it showed a joint effusion with non-specific synovial proliferation and two focal nodular formations at the insertion of medial head of gastrocnemius muscle in the medial gastrocnemius bursa. Those two nodular formations had low signal intensity on all pulse sequences and bloom artifacts on the FGRE-sequence. T1 weighted fat-saturated gadolinium enhanced MR image showed a low marginal contrast-enhancement of the two nodular formations, and high contrast-enhancement of the synovial proliferation. Biopsy from the two lobulated nodules revealed synovial nodules which contained haemosiderin pigment. Imaging characteristics of PVNS and PVNB are almost the same: radiographically visible calcifications are rare; CT scans show the lesions as high attenuating because of the haemosiderin content; MRI findings are prominent low signal intensity (seen with T2-weighting) and "blooming" artifacts from the hemosiderin deposits (seen with gradient-echo sequences) that are nearly pathognomonic. In addition, MR imaging is optimal for evaluating lesion extent. This information is crucial to guide treatment and to achieve complete surgical resection.