Hoffa's recess: incidence, morphology and differential diagnosis of the globular-shaped cleft in the infrapatellar fat pad of the knee on MRI and cadaver dissections.

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.

[Magnetic resonance imaging (MRI) of the bursa around the knee joint]. / Magnetresonanztomographie (MRT) der Bursae um das Kniegelenk.

INTRODUCTION: Since fluid filled bursae lead to visible structures on MR images it is important to establish criteria to recognize them and to differentiate them from tumorous fluid-like structures. With this study the bursae around the knee joint were analyzed for potential differential diagnostic criteria such as localisation, size and shape. MATERIAL AND METHODS: In a retrospective study of 133 MR exams the frequency, localization and morphology were registered. The frequency of occurrence was correlated with the frequency of an effusion by means of the chi 2 test. RESULTS: The most frequently recognized bursae were bursa subtendinea musculi gastrocnemii medialis (54.9%), bursa musculi semimembranosi (21%), and bursa infrapatellaris profunda (18.8%). 8 different bursae were identified in complete. Size roanged from 2 to 18 mm. The shape was round and ovoid. The bursa subtendinea musculi gastrocnemii medialis and bursa musculi semimembranosi correlated with the occureance of an effusion. CONCLUSION: 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.