Presentation
Intermittent locking of the right knee. History of tibial and fibular shaft fracture 10 years prior treated with external fixation. X-rays of the knee (not available).
Patient Data
Well-circumscribed intra-articular mass within the Hoffa's fat pad, displaying an isosignal to the muscles on T1 and T2, intermediate to high signal on PD fat sat and relatively isointense to the articular cartilage on all sequences with foci of low signal on GE sequence (most likely calcifications). A peripheral rim of low signal is seen on PD fat sat, T2 and GE sequences. No enhancement noted on postcontrast sequences.
Mild joint effusion.
Normal appearance of both menisci, cruciate and collateral ligaments.
Healed tibial and fibular shaft fracture.
Case Discussion
MRI features oof a well-circumscribed intra-articular mass within the Hoffa's fat pad relatively isointense to the articular cartilage on all sequences suggestive of soft tissue chondroma of the Hoffa fat pad.
Soft tissue chondroma of the Hoffa (infrapatellar) fat pad is a rare form of chondroma. It is considered by some to be the end-stage of Hoffa disease of the infrapatellar fat pad. It may occur as a result of metaplasia from mesenchymal cells due to acute or repetitive trauma related to hyperextension of the knee and subsequent inflammation and hemorrhage.
The main differential diagnoses include:
soft tissue chondrosarcoma