Knee plica. Arthroscope (endoscope) view of a plica or synovial fold (running diagonally from upper left) projecting into the cavity of the knee joint. This can hamper movement. The plica is the remnant of one of synovial membranes that divided the foetus' knee into three compartments. The membranes normally break down before the foetus is six months old, to create the knee cavity.