Content Reviewers:Viviana Popa, MD, Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.)
The popliteal fossa is the space behind your knee; and even though it may not look like much from the outside, it’s actually a complex region, delimited by muscles and fascia, and it contains a lot of vessels and nerves that pass from the thigh to the leg.
So first, let’s look at the popliteal fossa, which, superficially, resembles a diamond-shaped cavity when the knee is slightly flexed. This means the popliteal fossa has two superior borders and two inferior borders.
So, lets go over these borders; The two superior ones are the superolateral border, delimited by the biceps femoris; and the superomedial border, delimited by the semimembranosus and semitendinosus muscles.
The two inferior borders are the inferomedial border, represented by the medial head of the gastrocnemius muscle, and the inferolateral border, delimited by the lateral head of the gastrocnemius muscle. The popliteal fossa also has a floor and a roof. The roof consists of two layers: the popliteal fascia and skin.
However, things get a bit more complicated with the floor, which has three sections: the bony popliteal surface of the femur superiorly, the posterior aspect of the joint capsule of the knee joint centrally, and the popliteus muscle and fascia covering the popliteus muscle inferiorly.
The popliteal fossa contains the small saphenous vein; the posterior cutaneous nerve of the thigh; the sciatic nerve, which divides into the tibial and common fibular nerves at the superior border; the popliteal arteries and veins along with their branches and tributaries; and the popliteal lymph nodes and lymphatic vessels.
Ok, so now let’s dive in with a little bit more detail, layer by layer. First, there’s the subcutaneous or superficial fascia, which contains the small saphenous vein, the terminal branch of the posterior cutaneous nerve of the thigh, and the medial and lateral sural cutaneous nerves.
Then comes the deep fascia, also called the popliteal fascia which covers and protects the neurovascular structures within the popliteal fossa. Next there are the neurovascular structures that traverse the popliteal fossa to pass from the thigh to the leg.
When dissecting from superficial to deep within the fossa, first are the nerves, then the veins and finally the arteries, which lie deep, directly on top of the floor of the fossa.
So let's start with the nerves. You might have already heard about the sciatic nerve, which begins in the lower part of the sacral plexus and ends at the superior angle of the popliteal fossa, where it divides into the tibial and common fibular nerves.
The tibial nerve is the medial and larger terminal branch and it derives from the anterior divisions of the anterior rami of the L4 to S3 spinal nerves.
From its origin at the apex of the popliteal fossa, it divides the fossa into two equal parts as it passes from its superior to its inferior angle. Within the fossa, it gives muscular branches to the soleus, gastrocnemius, plantaris and popliteus muscles.
The tibial nerve eventually passes deep to the plantaris and gastrocnemius muscles at the inferior border of the popliteal fossa, then it enters the foot beneath the flexor retinaculum and finally divides into the medial and lateral plantar nerves.
The tibial nerve also gives off the medial sural cutaneous branch that contributes to the sural nerve. The sural nerve mainly provides cutaneous innervation for the leg.
Now let’s look at the common fibular nerve which is the lateral terminal branch of the sciatic nerve, and derives from the posterior divisions of the anterior rami of the L4 to S2 spinal nerves.
It begins in the same place as the tibial nerve, and it follows closely to the medial border of the biceps femoris and its tendon along the superolateral boundary of the fossa.
It then leaves the fossa by passing superficial to the lateral head of the gastrocnemius, it reaches the posterior aspect of the head of the fibula, where it winds around the fibular neck and divides into its terminal branches: the deep and superficial fibular nerves.
It also gives off a branch called the lateral sural cutaneous nerve, which contributes to forming the sural nerve. After the sural nerve receives branches from both the tibial and common fibular nerve, it descends between the two heads of gastrocnemius and becomes superficial in the middle leg.
It then descends with the small saphenous vein and passes inferior to the lateral malleolus to reach the lateral part of the foot. Ultimately, the sural nerve supplies the posterior and lateral part of the inferior third of the leg and the lateral side of the foot.