One of the best ways to understand how something, anything, works is to take it apart and put it back together again. For the knee, this is fairly simple. We need a relatively short list of parts: 4 bones, 2 tendons, 4 ligaments, and 2 types of cartilage.
To start, we place the femur, tibia, and fibula bones in their proper positions (Figure 1-1). Next, we need a system of ligaments to hold them together (Figure 1-2) and a coating of articular cartilage on the surface of the femur and tibia, two of the three bones that will articulate against each other (Figure 1-3). Of all the structures used to assemble the knee we are building, this thin layer of glistening articular cartilage tissue is probably the most important and the most interesting (please read the sidebar on articular cartilage). Now we are ready to add the meniscus cartilages, which sit like two rubbery, horseshoe-shaped pads on the surface of the tibia (Figure 1-4). The exact role that the meniscus cartilages play in the function of the knee is poorly understood, but they do not act as a “cushion” between the femur and tibia, as many of us were taught (see sidebar). The last bone we need to add if we are building a knee is the patella. The patella is a link in the chain of structures known as the extensor mechanism (Figure 1-5). These structures—the quadriceps muscle, the quadriceps tendon, the patella, and the patellar tendon—allow us to forcibly straighten (extend) our knees. When it contracts, the quadriceps muscle (via its quadriceps tendon attachment to the patella) pulls the patella proximally. As it is pulled proximally, the patella (via its patella tendon attachment to the tibia) pulls the anterior tibia proximally, which rotates the knee into extension.
Building a knee: the femur, tibia, and fibula bones.
A. The medial and lateral collateral ligaments. B. The anterior and posterior cruciate ligaments.
The articular cartilage coatings on the surfaces of the femur and tibia.
The medial and lateral meniscus cartilages.
THEY ARE NOT CUSHIONS!
I can’t tell you how many times I’ve heard it said that the meniscus cartilages are the “cushions” that reside between the femur and tibia. This isn’t true. They do reside between the tibia and femur bones, and it is ...