The axial image demonstrates increased signal (arrow) within fat interposed between the lateral aspect of the patellar tendon and the lateral trochlear ridge. Mild lateral patellar subluxation is present as indicated by the off-midline positioning of the patellar tendon (arrowhead).
The sagittal image demonstrates increased signal (arrow) within the fat below the caudal margin of the patella as well as moderate patella alta (arrowhead). Note the high riding appearance of the patella.

Three normal anterior knee fat pads are present at the knee; the quadriceps (anterior suprapatellar) (red), the prefemoral (posterior suprapatellar or supratrochelar) (blue), and Hoffa (infrapatellar) (yellow) fat pads.


The infrapatellar fat pad is bordered by the inferior pole of the patella superiorly, the joint capsule and patellar tendon anteriorly, the proximal tibia and deep infrapatellar bursa inferiorly, and the synovium-lined joint cavity posteriorly. Thus, it is intracapsular but extrasynovial. It is tethered to the intercondylar notch superiorly by the infrapatellar synovial fold or infrapatellar plica. It also is attached directly to the anterior horns of the menisci inferiorly and to the periosteum of the tibia.


On MRI, a focal area of high signal or edema on STIR or T2 weighted sequences is present at the inferolateral aspect of the patellofemoral joint, specifically the lateral portion of the infrapatellar fat pad (D,E). Specifically, this is just below the inferior margin of the patella and anterior to the lateral trochlear ridge. A focal lobulated mass with signal characteristics of fluid or cystic change can sometimes be present in the lateral soft tissues of the knee between the lateral femoral condyle and the lateral retinaculum. Associated findings of lateral patellar subluxation (F) and/or patella alta are frequently present.



http://www.radsource.us/clinic/0809