In view of haemosiderin staining PVNS is suggested as first differential.    From Wordnik.com.  [Sumer's Radiology Site] Reference

IMPRESSION: 32 yrs old female with synovial collection with synovial thickening & collection with septae in relation to lateral femoral condyle with haemosiderin staining, contrast enhancement and erosive process in patella, medial femoral condylar & tibial plateau, early degenerative changes.    From Wordnik.com.  [Sumer's Radiology Site] Reference