A 37-year-old man presented to the orthopaedic oncology clinic reporting a mobile right knee mass over the anteromedial knee. Per the patient, the mass had been present for at least 2 decades. He noted occasional swelling and pain in the knee and associated popping and clicking in the knee. He had a remote history of 2 mild hyperextension injuries to the knee, but otherwise noted no severe trauma. On physical examination, he had a full arc of motion of the knee. He had a 1+ effusion and a palpable, mobile, 3-cm mass medial to the patellar tendon. He was noted to have substantial tenderness to palpation along the lateral border of the patella as well. The knee was stable on ligamentous stress testing. Radiographs taken at that time demonstrated no osseous abnormality but did show a joint effusion.
Magnetic resonance imaging (MRI) was obtained, which did demonstrate proliferative synovitis involving the infrapatellar region and femoral notch (Fig. 1). The patient was offered surgical treatment in the form of arthroscopic resection. Informed consent was obtained. Intraoperatively, there were at least 11 discrete lesions encountered in the lateral gutter, femoral notch, and Hoffa fat pad (Fig. 2). These were able to be debrided using the standard inferomedial and inferolateral portals.
What is the diagnosis?
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