MR imaging of the infrapatellar fat pad of Hoffa

The infrapatellar fat pad of Hoffa is an intracapsular structure that is routinely visualized on magnetic resonance images of the knee. Because disease in this region is not uncommon, it is important to be familiar with the various pathologic entities that may occur here. Abnormalities that are intrinsic to this fat pad include Hoffa disease, intracapsular chondroma, localized nodular synovitis, postarthroscopy and postsurgery fibrosis, and shear injury. In addition, the infrapatellar fat pad may be involved secondarily from extrinsic processes, including articular disorders (eg, joint effusion, intraarticular bodies, meniscal cyst, ganglion cyst, cyclops lesion), synovial abnormalities (eg, pigmented villonodular synovitis; hemophilia; synovial hemangioma; primary synovial chondromatosis; chondrosarcoma; lipoma arborescens; rheumatoid, seronegative, and septic arthritis; arthritis associated with inflammatory intestinal disorders; synovitis associated with primary osteoarthritis), and anterior extracapsular abnormalities. The approach to pathologic processes involving the infrapatellar fat pad of Hoffa is simplified when one is familiar with regional anatomy and possible differential diagnostic considerations.