MRI sequences at different degrees of flexion to investigate knee popping: an unusual way to diagnose an isolated pigmented villonodular synovitis lesion.

Knee popping is a frequent symptom among knee disorders which requires further investigation in case of a recent evolution of the symptom or pathological associated ones. This article reports a rare presentation of pigmented villonodular synovitis (PVNS), identified as the cause of knee popping symptoms, by performing MRI sequences at various degrees of knee flexion for a patient complaining from a gradual onset of knee popping, occurring when bending the knee over 120° of flexion. MRI sequences were performed just before the popping occurs (flexion 90°) and right after it had occurred (flexion 120°). The latter confirmed the origin of the symptom as the lesion moved forward, passing brutally through the interstice between the PCL and the ACL at 120° of flexion, explaining the popping. Treatment decision was to perform an arthroscopic resection of the lesion. Diagnosis of isolated PVNS was confirmed after anatomopathological analysis.