We frequently observed a fluid-like indentation at the inferior posterior margin of Hoffa's fat pad of the knee and sought to establish the incidence and differential diagnostic criteria of this cleft. In total, 133 MRI studies and 35 cadaver specimens were analyzed for the location, size, and shape of clefts at the inferior posterior margin of Hoffa's fat pad. The incidence of a fluid-like ovoid cleft on MR images was 13.5% and in cadavers 14.3%. The cleft was located just below the insertion of the infrapatellar synovial fold (plica synovialis infrapatellaris, ligamentum mucosum). More linear-shaped indentations at the posterior margin were visible in all patients and cadavers due to the horizontal course of the alar folds. A fluid-filled indentation within the inferior posterior margin of Hoffa's fat pad has to be expected in more than 10% of knee studies and should not be confused with tumors like ganglion cysts. We term this cleft the infrahoffatic recess. One hypothesis of its origin concerns the embryological regression process of the infrapatellar membrane into the infrapatellar synovial fold. It should not be confused with linear clefts due to the alar folds.

Bursae around the knee are frequently visualized as asymptomatic fluid like structures. Knowledge of the typical localization, size and contour is important for identification and differentiation diagnosis against ganglia, cysts or joint recesses.