General information
A plica is a fold in the mucous membrane that originates from the inner joint skin. It is formed from collagen fibers and a very thin mucosa with a smooth surface (synovial skin) that lines the inner surface of the joint capsule. The synovial skin secretes a liquid mass, the so-called synovial fluid (synovia). This reduces friction within the joint cavity and provides nutrients to supply the joint cartilage.
Anatomy of the mucosal fold in the knee
During human embryonic development, the synovial skin forms a layer (membrane) that divides the knee into two separate areas. This membrane often recedes in the course of further development, which increases the freedom of movement within the joint. However, in about 70% of adults a mucous membrane fold (plica) remains.
This mucosal fold in the knee does not take over important functions. In most cases, the plica moves from the inside of the central area of the knee joint towards the middle. Since it is located either above, below or to the side of the kneecap, it can be divided into plica suprapatellaris, plica infrapatellaris or plica mediopatellaris.
The suprapatellar plica is a fold of the inner joint mucosa that extends above the patella in the knee joint. It begins at the lower end of the thigh bone and moves over to the inner wall of the knee joint. It occurs rather rarely.
The plica infrapatellaris is a fold that is located below the kneecap in the knee joint. It extends from a depression in the bone at the lower end of the thigh (Fossa intercondylaris femoris) to the anterior joint cavity of the knee, where it is attached to a fat body (Hoffa fat body). The mediopatellar plica is the most common.
It is located between the medial articular process of the femur and the kneecap. The plica extends from the inner knee joint compartment towards the middle. Due to its location, it is often compared to a tensed bowstring. It is the most common cause of a plica syndrome.
