To describe the radiological features of pes anserine bursitis with intramedullary extension and cortical scalloping and to determine the prevalence of these bony changes among patients with pes anserine bursitis.

Reports of knee magnetic resonance imaging (MRI) examinations performed at our institution between July 2007 and June 2017 in patients with pes anserine bursitis were retrospectively reviewed, and a total of 542 cases showing MR evidence of pes anserine bursitis were identified. From these, cases of pes anserine bursitis with intramedullary extension and cortical scalloping were identified. Two experienced musculoskeletal radiologists evaluated the MRI by consensus. The medical records of these patients were also reviewed.

Eight patients were diagnosed with pes anserine bursitis with bony changes (prevalence, 1.47% [8 out of 542]), over the study period. All of these patients had a history of chronic knee pain. Seven patients also underwent radiography at the time of diagnosis; these images demonstrated variable appearances depending on the depth of the cortical scalloping and intramedullary extension. On MRI, all patients demonstrated a mass-like fluid extension around the pes anserine bursa and into the bone. None of the patients underwent biopsy; diagnosis was based on MRI features alone.

Pes anserine bursitis with intramedullary extension is an unusual presentation of bursitis that may simulate a neoplasm clinically and radiologically. To avoid misdiagnosis, radiologists should be aware of the occurrence of osseous changes in the tibia confluent with pes anserine bursitis.