Boundaries of the Popliteal Fossa
The popliteal fossa is a shallow depression located at the back of the knee. It is bounded by the biceps femoris superiorly and laterally. The biceps femoris is inserted into the fibular head, so it is a lateral structure. The superior and medial boundary of the popliteal fossa is formed by the semimembranosus and semitendinosus. These muscles are a part of the pes anserinus that is inserted medial onto the tibia, so these two muscles are medial structures. The inferior and medial boundary of this space is formed by the medial head of the gastrocnemius. The inferior and lateral boundary is formed by the lateral head of the gastrocnemius muscle and the plantaris muscle.
Base of the Popliteal Fossa
The floor of the popliteal fossa is formed by the popliteal surface of the femur, the capsule of the knee joint, the oblique popliteal ligament, and the strong fascia covering the popliteal muscle.
Structures within the Popliteal Fossa
From medial to lateral, the structures of the popliteal fossa include the popliteal artery, the popliteal vein, including the small saphenous vein, and the tibial nerve, including the common peroneal nerve, which runs across the upper lateral border of the popliteal fossa.
Roof of the Popliteal Fossa
Posteriorly, the skin and the deep popliteal fascia form the roof of the popliteal fossa.
Clinical Entity
Baker’s cyst is an interesting clinical entity of the popliteal fossa. A Baker’s cyst lies between the semimembranosus and the medial gastrocnemius muscles. Baker's cysts are commonly caused by knee arthritis or a meniscal tear. The cyst is connected to the knee joint through a valvular opening, and knee effusion from intra-articular pathology allows the fluid to go through the valve to the cyst in one direction.
Another entity that may be of special interest is a posteromedial fracture of the tibial plateau, and this fracture must be recognized. If this fracture is displaced, it must be fixed with two separate incisions. The first is using a lateral plate for the lateral plateau fracture and the second is using a posteromedial plate for the posteromedial fragment. The posteromedial incision is placed between the semimembranosus and the medial head of the gastrocnemius, which is the same location as a Baker’s cyst. During this incision, the medial gastrocnemius is protecting the neurovascular structures, and is easily identifiable by its location and by its large size.
