MR features of a giant cyamella in a patient with osteoarthritis

History

A 66-year-old man presented with chronic left knee pain accompanied by instability and locking. A magnetic resonance imaging study was performed, which demonstrated extensive degenerative changes with osteophytosis, cartilage loss, and meniscal damage as well as disruption of the anterior cruciate ligament (Fig. 1). At the posterior aspect of the knee, adjacent to the posterior lateral tibial margin, and medial to the fibular head, a structure was identified surrounded by fibers of the musculotendinous junction of the popliteus muscle. What is your diagnosis?

Fig. 1

a Sagittal pelvic diaphragm magnetic resonance image demonstrating an ossific corticated body at the popliteus musculotendinous junction consistent with a cyamella. b Coronal STIR and c axial FSET2 images demonstrating suppression of the fatty marrow signal from within the cyamella