Knee Pit/The popliteal fossa (sometimes referred to as the hough or Knee Pit in analogy to the armpit) is a shallow depression located at the back of the knee joint. The bones of the popliteal fossa are the femur and the tibia. Like other flexion surfaces of large joints (groin, armpit, cubital fossa, and essentially the anterior part of the neck), it is an area where blood vessels and nerves pass relatively superficially, and with an increased number of lymph nodes.
The popliteal fossa is a shallow depression located posterior to the knee joint. This area is often referred to as the knee “pit” and may develop vascular, nervous, lymphatic, and adipose tissues as well as swelling and masses.[rx] The fossa houses several important structures that are essential for lower extremity function.
Anatomic boundaries of the popliteal fossa include the following
- Superomedially, the semimembranosus, and semitendinosus muscles
- Superolaterally, the biceps femoris muscle (short and long heads)
- Inferomedial and inferolateral, the medial and lateral heads of the gastrocnemius muscle contribute to the lower border, respectively.
- The plantaris muscle also runs deep to the gastrocnemius to form the inferolateral border.
- The roof of the fossa, proceeding from most superficial to deep, consists of the skin, superficial fascia, and deep (popliteal) fascia.
- The popliteal surface of the femur, a capsule of the knee joint, popliteal ligament, and fascia encasing the popliteus muscle form the floor.
Muscles of Knee Pit
The musculoskeletal structures found in the popliteal fossa form its boundaries. The fossa has the shape of a rhombus or diamond and is made up of the following musculoskeletal structures.
- Superomedial – The semimembranosus and semitendinosus muscles from the superomedial aspect of the fossa. The semimembranosus is the most medial of the hamstring muscles and functions to extend the hip and flex the knee as well as assist in medial rotation of the knee. The semitendinosus muscle is superficial to the semimembranosus and is the middle of the three hamstring muscles. Not surprisingly, it gets its name because of the long tendinous nature of its structure and assists in the same mechanism as the semimembranosus muscle.[rx]
- Superolateral – The biceps femoris forms the superolateral portion of the popliteal fossa and has two functioning heads. The long head is part of the hamstring muscles while the short head is not included in this description. Both structures participate in knee flexion, while the long head additionally assists in hip extension.[rx]
- Inferomedial – The medial head of the gastrocnemius forms the anteromedial border of the fossa. It assists the lower leg in plantar flexion at the ankle and knee flexion when contracted. Along with the lateral head of the gastrocnemius and soleus, it forms the entire calf muscle.[rx]
- Inferolateral – The lateral head of the gastrocnemius and the plantaris muscle forms the inferolateral border of the popliteal fossa. Both contribute to plantarflexion of the ankle. The lateral gastrocnemius head also contributes to knee flexion. The plantaris muscle is notably the longest tendon in the body and is not found in every human being.[rx]
Blood Supply and Lymphatics of Knee Pit
The popliteal artery is the primary vascular supply in the region of the knee and lower leg. Muscle aerobic activity improves the function of the artery: its endothelium, its ramifications, its elasticity, and the ability to transport oxygen and nutrients useful to the target tissues.
The popliteal vein runs posterior to the popliteal artery and receives blood from multiple tributaries.
- Superior geniculate arteries – the knee joint, distal femur and muscles of the medial and lateral knee.
- Sural arteries–structures within the popliteal fossa and gastrocnemius, soleus and plantaris
- Middle genicular artery–cruciate ligaments and synovial membrane of the knee
- Inferior geniculate arteries–knee joint, muscles of the medial and lateral knee, and proximal tibia
- Genicular veins
- Sural veins
- Small saphenous vein
- Anterior tibial vein
- Posterior tibial vein
- Peroneal veins
Popliteal lymph nodes can be found throughout the popliteal fossa, closely associated with the popliteal vessels. These nodes drain into the deep inguinal lymph nodes.[rx]
Nerves of Knee Pit
The regulation of the function of the artery is via a proper balance between the sympathetic and parasympathetic fibers.
The integration of afferent information from the popliteal artery (shear stress, compression from external tissues, stretching, etc.,) undergoes interpretation in the central nervous system, which collaborates the autonomic system to manage the arterial function.
The popliteal fossa is 2.5 cm wide and mainly consists of fat tissue. There are many important neurovascular structures, however, which pass through the fossa. The nerves are the most superficial of these structures and include:
- Tibial
- Common fibular
- Sural
- Posterior femoral cutaneous
Tibial nerve
The sciatic nerve bifurcates into the tibial and common fibular nerves at the superior angle of the popliteal fossa. The larger medial branch, the tibial nerve, passes through the fossa inferiorly, before it exits deep to the plantar muscle and enters the posterior compartment of the leg. While in the fossa, the tibial nerve and its branches supply the soleus, gastrocnemius, plantaris, and popliteus muscles. One of its branches, the medial sural cutaneous nerve, joins with the sural communicating branch of the common fibular nerve to form the sural nerve.
Common fibular nerve
The other branch of the sciatic nerve, the common fibular nerve, traverses the fossa close to the medial border of the biceps femoris muscle before it exits the fossa superficial to the lateral head of the gastrocnemius muscle. It then crosses the posterior aspect of the head of the fibula before it winds around the neck of the fibula and divides into its terminal branches.
Borders and Boundary of Knee Pit
The popliteal fossa is diamond-shaped with four borders. These borders are formed by the muscles in the posterior compartment of the leg and thigh:
- Superomedial border – semimembranosus.
- Superolateral border – biceps femoris.
- Inferomedial border – medial head of the gastrocnemius.
- Inferolateral border – lateral head of the gastrocnemius and plantaris.
The floor of the popliteal fossa is formed by the posterior surface of the knee joint capsule, popliteus muscle and posterior femur. The roof is made of up two layers: popliteal fascia and skin. The popliteal fascia is continuous with the fascia lata of the leg.
Contents
- Fat
- Popliteal artery – deepest, gives off genicular branches
- A popliteal vein – in between the artery and tibial nerve
- The tibial nerve – most superficial, giving off the sural nerve which descends to pierce the roof
- Common fibular (peroneal) nerve – runs along the lateral border, giving off a communicating sural nerve which descends to pierce the roof in the lateral fossa and join the sural nerve
- Posterior femoral cutaneous nerve – descends and pierces the roof
- Small saphenous vein – ascends and pierces the roof to enter the popliteal vein in the lower half of the fossa
- Popliteal lymph nodes
