Non perforated septum supra-patellaris mimicking a soft tissue tumour

Plicae synoviales of the knee are well known to present as anterior knee pain but can also cause blocking, clicking, and swelling of the knee mimicking pathology such as meniscal tears or other internal derangements, and even mono-arthritis. Embryologically, the knee is divided into a medial and lateral compartment and a supra-patellar pouch by thin membranes of synovium. Incomplete involution of these membranes gives rise to a plica. When there is no involution at all, the membrane is called a septum. There are three frequent types of plicae/septa: a suprapatellar, a medial-patellar, and an infra-patellar type. We discuss on a patient with a non perforated septum supra-patellaris which resulted in a swelling above the knee, mistaken for a soft tissue tumour.