Like all joints, the knee joint is also surrounded by synovial membrane. It keeps the joint supple, so that all movements can be performed without friction. The plica infrapatellaris refers to a fold of this articular mucosa in the knee joint. The word "infrapatellaris" indicates the location of the crease below the kneecap (patella). It is a continuation of Hoffa's fat body, which lies between the tibial bone and the lower edge of the kneecap.


The plica infrapatellaris is a mucosal fold in the knee joint, which consists predominantly of connective tissue. The mucosal fold is sometimes referred to as the mucosal ligament. It is surrounded by several cell layers and is thus separated from the surrounding tissue. It can occur in different variants and usually separates the knee joint into two chambers. But it can not exist at all. When applied, the plica is located in a bony depression below the femur. From this called the Fossa intercondylaris femoris near the anterior cruciate ligament, the mucosal fold pulls to the anterior joint cavity and ends at the Hoffa fat body.

During its course, the plica increases in width towards the front. In some cases, the plica infrapatellaris may be associated with other folds of the articular mucosa. Basically, the different mucosal folds develop during embryonic development and form in the further course. In the embryo, the mucosal fold acts as a sort of septum and divides the knee into two chambers. In adults, the fold is about 65% formed and forms at the rear edge a gap, so that lateral and central joint space are interconnected.


The plica infrapatellaris is located below the kneecap. It runs as an extension of the applied fat body, which is also under the kneecap, through the knee joint. It divides the knee joint into a lateral and middle compartment, creating two chambers. The plica begins in a bony depression and ends at the front fat body. In its course, it is steadily increasing in breadth. Often it is associated with the other mucosal folds, the plica suprapatellaris and mediopatellaris.


The plica infrapatellaris and the other two mucosal folds are created during the embryonic period and often develop back to adulthood. The mucosal folds do not have a direct specific function. When applied, the Plica infrapatellaris forms a reinforcing fibrous cord that extends from the underside of the kneecap over the fat body into the bony cavity. Occasionally it may even come to movement restrictions. It can swell and block the knee joint, so that no more sliding movement takes place.


The plica infrapatellaris may also rupture in rare cases. Often a rupture is caused by overuse of the knee joint. Muscle weakness or unbalance of balance may favor tearing of various ligaments of the joint or mucosal folds. In most cases, however, the plica mediopatellaris is affected. A rupture can be accompanied by pain, swelling of the plica and the entire joint. MRI images often show a thickening of the mucosal fold. Depending on the severity of the injury, surgery may be necessary to remove the mucosal folds.


Magnetic resonance imaging on the knee joint is very well suited to visualize soft tissue such as muscles and organs. This also makes it possible to detect the plica infrapatellaris. As a rule, it is a very narrow, low-signal line. Low-signal means that the structure appears darker compared to the surrounding tissue. Since it is often only recognizable, if there is a simultaneous fluid accumulation in the joint space (effusion), it is difficult to diagnose on MRI.

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The different mucosal folds may be different in the knee. So it can occasionally happen that a fold is thicker than normal. This can lead to friction during movement, as a result of which jamming can also occur. Not infrequently, the result is a painful inflammation that spreads to the entire knee joint, often accompanied by swelling and effusion. Typically, the plica mediopatellaris causes this plica syndrome. If conservative therapy is not enough, surgery is often necessary. The operation often takes place as part of a knee reflex (arthroscopy) in general anesthesia and keyhole technique.

Only small cuts are made, through which the necessary instruments with integrated camera are advanced into the knee joint. The doctor can thus obtain a very good overview of the inflammatory processes in the joint. The mucous folds, as well as the inflammatory tissue are removed, sparing the articular cartilage. By means of this very gentle method, the patient can return his knee to normal weight after one to four weeks. The surgery is very successful in most cases and has low complications. The typical symptoms such as the feeling that the knee joint is blocked are then usually completely gone. After four to six weeks, light sporting activities can be started again.

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