The MR appearance of the infrapatellar plica

Our purpose is to describe the MR appearance and clinical relevance of the infrapatellar plica of the knee and to assess possible imaging pitfalls resulting from lack of recognition of this normal structure. Seven patients--four from our institution and three from our teaching file from outside institutions--were selected because they displayed the imaging features of the infrapatellar plica.

The infrapatellar plica is readily appreciated on sagittal MR images of the knee as a low-signal-intensity structure in the intercondylar notch just anterior to the anterior cruciate ligament (ACL). The infrapatellar plica is important to recognize for the following reasons: It may be confused with an intact ACL because of its proximity and similar signal characteristics; it may pose difficulty to the arthroscopist when attempting to move instruments from the medial to lateral compartments of the knee; it can block clear visualization of the ACL during arthroscopy; and it can impair retrieval of loose bodies in the intercondylar notch. Familiarity with the infrapatellar plica should allow one to differentiate this normal structure from other structures in the knee and aid in preoperative planning.