The knee joint is a synovial hinge joint. Articulation is between the femur (specifically the femoral condyles and tibial condyles. The patella, also part of the joint, articulates with the front of the femur. The knee joint allows flexion and extension and a small amount of rotation.
Between the articular surfaces of the femur and tibia are two C-shaped cartilages, the medial meniscus and the lateral meniscus.
Stability of the knee relies on the medial and lateral collateral ligaments and the anterior and posterior cruciate ligaments inside the knee, as well as strong functioning muscles.
Innervation of the knee joint is from branches of the obturator, femoral, tibial and common fibular nerves.
The Popliteal Fossa
The popliteal fossa is found at the back of the knee joint. It is diamond-shaped and is bounded by the following muscles:
- Superior medial border – semitendinosus (hip extensor, knee flexor)
- Superior lateral border – semimembranosus (hip extensor, knee flexor)
- Inferior medial and lateral borders – the two heads of gastrocnemius (foot plantar flexor, knee flexor)
- Within the popliteal fossa lie (from medial to lateral) the popliteal artery, the popliteal vein and the tibial nerve.
When performing a popliteal nerve block, the patient is positioned prone with the knee flexed. From the surface, the popliteal fossa can be identified as a triangle between the semitendinosus muscle and long head of biceps femoris, with the base of the triangle being the skin crease running between the medial and lateral femoral epicondyles. Being deeper structures, it is not easy to locate the two heads of gastrocnemius and visualise the full diamond shape.