During embryonic development, the knee is divided initially by synovial membranes into three separate compartments. By the third or fourth month of fetal life, the membranes are resorbed, and the knee becomes a single chamber. If the membranes resorb incompletely, various degrees of septation may persist. These embryonic remnants are known as synovial plicae. Four types of synovial plicae of the knee have been described in the literature.
The suprapatellar plica (plica synovialis suprapatellaris) divides the suprapatellar pouch from the remainder of the knee. Rarely, it may initiate a suprapatellar bursitis or perhaps chondromalacia. Anatomically, this plica can be complete or in the form of a porta, which only partially separates the compartments. It courses from the anterior femoral metaphysis or the posterior quadriceps tendon to the medial wall of the joint. It usually begins proximal to the superior pole of the patella but may begin anywhere.
The mediopatellar plica (referred to by some as the Aoki ledge or Iino band) is the most frequently cited cause of plica syndrome. It lies on the medial wall of the joint, coursing from a suprapatellar origin obliquely down to insert on the infrapatellar (ie, Hoffa) fat pad. This plica, sometimes known as a shelf, lies in the coronal plane.
The rare and poorly documented lateral synovial plica is a wider and thicker band than the medial plica. It is located along the lateral parapatellar synovium, inserting on the lateral patellar facet and extending distally toward the infrapatellar region. It has been argued that the lateral plica, rather than being a vestigial septum, is derived from the parapatellar adipose synovial fringe.
The plica that is the least symptomatic of all, the infrapatellar plica (ligamentum mucosum) is, ironically, the one most commonly encountered. Some authors even claim that the infrapatellar plica is never responsible for plica syndrome. This bell-shaped remnant originates in the intercondylar notch, widens as it sweeps through the anterior joint space, and attaches to the infrapatellar fat pad. This plica's ability to obscure portal entry sites or interfere with visualization during arthroscopy is touted as its only significance.