WHAT IS THE FUNCTIONAL AND ANATOMICAL CLASSIFICATION OF THE KNEE JOINT?

The knee joint is functionally classified as a diarthrosis because of it freedom of movement. Joints that are immovable or virtually immovable are designated synarthroses, whereas joints that are slightly movable are classified as amphiarthroses.

Anatomically, the articulation at the knee is classified as a type of synovial joint. Some authors classify the knee as a hinge joint but this distinction should be strictly reserved for joints that only move in one axis (uniaxial). The knee joint is more complex than a simple hinge joint as the skeletal elements (femur, tibia, and patella) permit rotation, flexion, and extension. Structurally, the femur exhibits two condyles (medial and lateral) that articulate with the corresponding plateaus of the tibial condyles. The patella possesses a saddle-shaped surface that articulates with the femur. Therefore, it is best described as having two condyloid joints and one saddle (sellar) joint.

WHAT ARE THE THREE DAMAGED STRUCTURES THAT CONSTITUTE THE “UNHAPPY TRIAD”?

The “unhappy triad” is characterized by injury to the medial (tibial) collateral ligament, the anterior cruciate ligament, and the medial meniscus (Fig. 5-7).

WHY ARE THESE STRUCTURES VULNERABLE TO INJURY?

The lateral side of the knee is more exposed than its medial surface. Therefore, as in this clinical scenario, the lateral aspect more frequently bears the force of impact. This impact causes the joint angle along the medial border of the knee to increase (valgus strain—forceful abduction of the tibia on the femur), thereby increasing the strain on the medial collateral ligament to the point of being torn. The clipping action caused a twisting motion about the knee joint. Because the deep fibers of the medial collateral ligament are attached to the medial meniscus, this abrupt rotating action tears the medial meniscus. This anatomical arrangement is not duplicated on the lateral side. Instead, the lateral meniscus and the lateral collateral ligament, which is stronger than the medial collateral ligament, are separated by the tendon of the popliteus muscle. The lateral meniscus, therefore, is less vulnerable to injury because it is not adherent to the lateral collateral ligament. The anterior cruciate ligament, which is weaker than the posterior cruciate ligament, is more vulnerable to injury during applied rotational forces that increase the tension on the ligaments. The strain on the anterior cruciate ligament was exacerbated by the anterior displacement of the tibia on the femur caused by the clipping infraction.

WHAT ARE THE FUNCTIONS OF THE COLLATERAL LIGAMENTS, THE CRUCIATE LIGAMENTS, AND THE MENISCI?

The lateral and medial collateral ligaments (Fig. 5-8) function to check extension and lateral rotation of the tibia, as well as limit side-to-side movements. Individually, the medial collateral ligament would limit abduction of the tibia, whereas its fellow would check adduction of the tibia. In the clinical scenario above, it was excessive abduction that caused the medial collateral ligament to rupture.

FIGURE 5-8 Knee joint with joint capsule removed.

(Drake R, Vogl W and Mitchell A: Gray’s Anatomy for Students. Churchill Livingstone, 2004. Fig. 6-68.)

The anterior and posterior cruciate ligaments (Figs. 5-8 and 5-9

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