ANATOMY AND CLINICAL-SIGNIFICANCE OF THE HORIZONTAL CLEFT IN THE INFRAPATELLAR FAT PAD OF THE KNEE - MR-IMAGING

OBJECTIVE. The purpose of this study was to determine the prevalence a nd nature of a horizontal cleft in the posterior aspect of Hoffa's inf rapatellar fat pad and to show pathologic processes involving this cle ft. MATERIALS AND METHODS. Fifty consecutive MR imaging examinations o f the knee were evaluated for the presence and appearance of a cleft i n the infrapatellar fat pad. Examples of abnormalities involving the c left were collected from additional MR studies. MR imaging, gross diss ection, and histologic examination of a cadaveric knee were also perfo rmed to evaluate the anatomy and histology of the cleft. RESULTS. The cleft in the infrapatellar fat pad was revealed on MR imaging in 45 of 50 knees and had a variable shape, either linear (82%), pipe-shaped ( 7%), or globular-shaped (11%). Joint effusion or anterior cruciate lig ament tear did not affect the appearance of the cleft. The cleft was l ocated anterior to the distal insertion of the anterior cruciate ligam ent on the tibia. At gross dissection of the cadaveric knee, the roof of the cleft was formed by the ligamentum mucosum (infrapatellar plica ), and the cleft was lined with synovium. The prospective evaluation o f additional MR imaging examinations of the knee revealed pathologic e ntities of the cleft such as ganglion cysts, loose bodies, nodular syn ovitis, and amyloid. CONCLUSION. A horizontal cleft located in the pos terior aspect of the infrapatellar fat pad is a common and normal MR i maging finding with a prevalence of 90%. The horizontal cleft is lined with synovium and its roof is formed by the ligamentum mucosum (infra patellar plica). This cleft communicates with the knee joint. A disten ded cleft can form a prominent recess mimicking pathologic processes; conversely, disorders can arise in the cleft.