Now that we’ve seen the bones of the knee joint, let’s see how the joint looks in the living body.
In building up our picture of this quite complicated joint, there are several structures that we need to understand: first the two joint cartilages or menisci, then the ligaments, the two cruciate ligaments and the two collateral ligaments, then the patella and the quadriceps tendon on the front, and lastly the capsule which encloses the joint.
Here are the two articular surfaces of the tibia. The two menisci sit on top of them. Here are the menisci. They’re made of flexible fibrocartilage. They’re shaped a little differently, the lateral one is almost a circle, the medial one is more C-shaped. In cross section, each meniscus is thick at the outer edge and thin at the inner edge. The two ends of each meniscus are attached to the inter-articular area of the tibia, the medial ones far apart, the lateral ones close together.
In addition each meniscus is attached all the way round its edge, both above and below, to the joint capsule. Here’s part of the joint capsule. We’ll see more of it later.
The lateral meniscus is much more mobile than the medial one, partly because its two ends are attached close together, partly because of a big difference in the mobility of the joint capsule around the edge.
By filling in the spaces between the femoral and tibial condyles, the menisci produce an even distibution of synovial fluid, to nourish and lubricate the articular cartilage of the femur and tibia. Now lets look at the two pairs of ligaments which hold the bones together at the knee joint - the two cruciate ligaments on the inside, and the two collateral ligaments on the outside.
We’ll look at the cruciate ligaments first. They’re the important structures which prevent forward and backward movement of the femur on the tibia. Their name comes from the fact that they form a cross like this.
Here’s the anterior cruciate ligament, seen from in front. Here’s the posterior cruciate ligament, seen from behind. To get a better look at them, we’ll remove the lateral condyle of the femur.
Now we can see the whole of the anterior cruciate ligament. The anterior cruciate ligament goes from here on the tibia, to here on the femur, on the inner aspect of the lateral condyle. The anterior cruciate ligament prevents the femur from moving backward in relation to the tibia.
Now we’ll look at the posterior cruciate ligament. We’ll remove the anterior cruciate ligament to see it better. The posterior cruciate ligament goes from here on the femur, to here on the back of the tibia. The posterior cruciate ligament stops the femur from moving forward on the tibia.
By preventing backward and forward movement, the cruciate ligaments ensure that the condyles of the femur stay in one place, as they roll on the condyles of the tibia. Without them, the femur would roll off the back of the tibia in flexion, and would roll off the front of it in extension.