The knee is a hinged joint allowing flexion and extension movements. The three bone segments making up the joint are the femur, tibia and patella. The distal* portion of the femur has two distinct structures, the medial and lateral condyles, with convex surfaces free to slide and roll on the two tibial plateaus. The surface of the tibia, called the tibial plateau, is also formed by two symmetrical parts, the medial and lateraltibial plateaus, having concave surfaces. The flexion-extension mechanism is performed by the rolling and sliding of the convex surface of the femur on the concave surface of the tibial plateaus.
The patella is in the anterior part of the knee, having an anterior surface and a posterior surface that slides during flexion-extension inside the intercondylar notch in the anterior wall of the femur between the femoral and medial condyles. The patella plays a fundamental role in the knee’s extension function since the quadricipital muscle is inserted into its top end and the patellar tendon stems from its bottom end. When quadricipital contraction occurs, the patella is “pulled” proximally, anteriorly stretching the patellar tendon and thence the tibia into which it is inserted, and the knee extends.
Functionally speaking, there are three compartments in the knee joint:
- the medial compartment, formed by the articulation between the medial femoral condyle and the medial tibial plateau;
- the lateral compartment, formed by the articulation between the lateral femoral condyle and the lateral tibial plateau;
- the femoropatellar articulation, formed by the anterior surface of the femur (intercondylar notch) and the posterior surface of the patella.
The joint as a whole is covered by the articular capsule, a robust membrane containing the synovial liquid. In addition to the muscles that reinforce the joint and enable its movement, the knee’s “scaffolding” is provided by the ligaments, which counter the forces of stress and render it stable.
The cruciate ligaments are in the middle of the knee and stabilize the femur and tibia in the anterior-posterior plane.
The collateral ligaments are at the sides of the knee and stabilize the femur and tibia against inward and outward bending forces (varus and valgus).
The menisci perform a very important function within the structure of the knee. They are two fibrous cartilage disks between the two femoral condyles and the two tibial plateaus. The medial and lateral menisci, in the form of an inward facing C, have a triangular section and increase the congruence between the articular surfaces of the femur and tibia, thus acting as protective “bearings” for the cartilage.
* Distal: in a system of anatomical coordinates, this defines a structure or part thereof situated at the far end with respect to a given reference point in the body, which is usually the head in the human body. The “distal portion of the femur” therefore indicates the end of the bone furthest from the head. The term “proximal” indicates the part closer to the head.
Delfino G, Lanciotti E, Liguri G, Stefani M, Dizionario Enciclopedico di scienze mediche e biotecnologiche e di biotecnologie, 2° edizione. Zanichelli. 2003.