Hoffa’s recess in the infrapatellar fat pad of the knee on MR imaging

The infrapatellar fat pad of Hoffa is a structure that is located in the space between the back side of the patellar ligament and the real capsule. It is routinely visualized on magnetic resonance (MR) images of the knee. The purpose of this study was to determine the prevalence and shape of a fluid-like indentation at the inferior posterior margin of the infrapatellar fat pad of the knee (recess) and to look for a relation between the liquid image in the fat pad and joint effusion. Three hundred and fifty consecutive MR imaging examinations of the knee were evaluated for the presence, location, size and shape of a recess in the infrapatellar fat pad and existence of joint effusion. The study population consisted of 145 females and 205 males aged between 5 and 80 years old (mean age 37.5 years). The recess in the infrapatellar fat pad was revealed on MR imaging in 54 of 350 knees (15.43%) and had a variable shape, consisting of 35 linear or ovoid (64.81%), five pipe-shaped (9.26%) and 14 globular (25.92%). Additionally, one ganglion cyst was observed in the infrapatellar fat pad. Globular and pipe-shaped recesses may be confused with cystic infrapatellar fat pad pathologies such as ganglion cyst, meniscal cyst, loose body or nodular synovitis. The existence of an infrapatellar fat pad may not be related to the presence of joint effusion.