M. L. Pretterklieber, F. Kainberger, and N. Leneva

1    Functional Anatomy of the Knee Joint

The knee joint (articulatio genus) is the largest synovial joint in the human body. Joint articulation is formed by the pairs of femoral and tibial condyles together with the menisci between them. In terms of arthrokinematics, this creates a joint with two axes that may be divided functionally into a meniscotibial and a meniscofemoral compartment. The saddle joint located between the patellar surface of the femur and the patella completes the knee joint (Fig. 1.1).

Joint Mechanics

Flexion, Extension, Rotation

The knee joint can be flexed or extended around a transversal axis passing through the femoral condyles. These movements occur primarily in the meniscofemoral compartment of the joint.

At flexion of about 30° and greater, the knee joint can be rotated inwardly or outwardly. Compared to the range of motion possible in flexion and extension, however, this degree of arbitrary rotation, occurring chiefly in the meniscotibial compartment, is low. The rotational axis connects the centers of the medial femoral condyle and medial tibial condyle.

The “Screw-home Mechanism”

There is a second, fundamental rotation that occurs automatically during the last 30° of extension. The so-called “screw-home mechanism” (knee-locking mechanism), produced by the varying diameters of the femoral condyles, causes an inward rotation of the femur of the standing leg of around 5° against the tibia, which is held in place by the weight of the body. This holds the knee joint in a locked position, i.e., the articular surfaces obtain a position of almost complete congruence. The fully extended knee is thus held in place only by the action of the joint, i.e., without any muscular activity, forming an integral component in the so-called amuscular standing posture.

“Unlocking” the knee from this position and resuming flexion requires the action of the popliteus muscle (see Figs. 1.6, 1.7, 1.9), a small and almost trivial muscle. Located deep in the hollow behind the knee (popliteal fossa), this muscle arises from the medial two-thirds of the dorsal aspect of the tibia, proximal to the soleal line. It attaches to the lateral and dorsal rim of the lateral meniscus and to the lateral femoral epicondyle. When the popliteus muscle contracts at the beginning of knee flexion, it pulls the lateral meniscus slightly dorsally while rotating the femur outwardly. This motion eliminates the congruence between the joint surfaces, freeing the femoral and tibial condyles to move against one another and the menisci.

Femur (Fig. 1.1)

  joint surfaces on the medial and lateral condyles, as well as on the patellar surface of the femur, are covered with hyaline cartilage

  larger diameter of medial condyle (anteroposteriorly and proximodistally) than lateral condyle, the former serving as the center of voluntary rotation in the flexed knee

  the nonarticular condylar surfaces facing the intercondylar fossa serve as attachment sites for the cruciate ligaments (see below)

  medial and lateral epicondyles as attachment sites for the collateral ligaments and the lateral epicondyle as insertion site of the popliteus muscle

Tibia (Figs. 1.1, 1.2)

  articular surfaces on medial and lateral condyles covered with hyaline cartilage

  anterior intercondylar area as attachment site for the anterior cruciate ligament (ACL) as well as for the infra-patellar fat pad (Hoffa’s fat pad)

  intercondylar eminence as site of attachment for the anterior and posterior meniscal horns: medial meniscus at the medial intercondylar tubercle and lateral meniscus at the lateral intercondylar tubercle

  posterior intercondylar area and upper central posterior surface of tibia as attachment sites of the posterior cruciate ligament (PCL)

  tibial tuberosity as an insertion site for the patellar ligament, the distal prolongation of the quadriceps aponeurosis

  Gerdy tubercle on the anterior side of the lateral condyle anchoring the iliotibial tract, the reinforced part of the fascia lata

  “tuberculum tendinis” on the posterior aspect of the medial condyle: main attachment of the semimembranosus muscle (see Figs. 1.7, 1.8)

Patella (Figs. 1.1, 1.3)

  largest sesamoid bone in the human body, embedded in the quadriceps aponeurosis

  anterior (nonarticular) surface is grooved from the pulling forces exerted by the superficial fibers of the quadriceps tendon, which pass directly into the patellar ligament; not infrequently, chamberlike bony processes guiding the tendon fibers may occur as an anatomic variant

  posterior (articular) surface of the patella covered by a thick layer of hyaline cartilage

  the base of the patella is directed proximally, while the apex, which also serves as attachment for the deep fibers of the patellar ligament, is pointed distally

Fabella (Fig. 1.4)

  generally comprised of fibrocartilage, only seldom of purely bony material, forms additional sesamoid “bone” in the lateral head of the gastrocnemius muscle

  articulates with the posterior surface of the lateral femoral condyle

Fibula (Figs. 1.1, 1.2)

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