Many online images, and most 3D models that patients are shown in the consulting room, make it appear that the menisci simply 'float' in the space between the femur and the tibia, attached only at the horns.
This is not the case at all.
Let's imagine you were playing football and twisting this way and that as you run and dribble the ball. Each twist results in some movement of the body of the menisci, as the one meniscus slides forwards and the other one backwards to cushion the movement. The menisci are seated each on its own little 'platform' on the top of the tibial plateau and it is on this platform that the body of the meniscus slides. I stress 'body' because each of the crescentic menisci has two horns, and these horns are attached to the tibia via a deep ligamentous attachment called a meniscus root - the roots are firmly fixed, so it is only the body that can slide, and so when the meniscus slides it tends to pivot from the root.
If you could look down at your menisci from above, you would notice that the body of the O-shaped lateral meniscus is able to pivot much more than the C-shaped medial one. This difference in mobility is due to differences in the capsular ligaments that constrain the two structures around their outer edges.
Let's take a better look at the outer rim of the meniscus.
This is an illustration of the knee bones from the front, with the knee bent. You can see the crescent-shaped meniscus sandwiched between the rounded femur bone and the flattened tibia bone. What I want to show you here is that the outer rim of the meniscus is tethered to the capsule - the waterproof sac that encloses the joint - via ligaments that are called the 'menisco-capsular ligaments'. You can appreciate that this would still allow some pivoting or sliding of the body of the meniscus.
However, these menisco- capsular connections vary in strength and distribution around the edge, and are stronger and more extensive on the medial meniscus than on the lateral one. You would think that this would mean that the medial meniscus is less commonly injured, but actually the opposite applies. Because the medial meniscus cannot slide to accommodate high stresses, it more commonly suffers tears of its substance or tears of the ligamentous rim itself.
The lateral meniscus and popliteus tendon
Now let's consider the lateral meniscus.
Not only is the lateral meniscus very mobile because of being less constrained around the edges, but it also has a defect in its capsular attachment at the back where an odd structure runs between the outer meniscus rim and the capsule. This is the tendon of the popliteus muscle, which you can best appreciate if you look at a picture of the back of the knee.
The popliteus is an unusual muscle, that has a special function that we don't need to know about here. You can see its tendon passing into the joint through the capsule and along the back of the lateral meniscus. This strange anatomy renders this corner of the knee vulnerable if any of the structures there are torn.
So although the lateral meniscus is more mobile and less commonly torn, the consequences of a tear, particularly at the back of the meniscus, tend to be worse than a tear in the same region of the medial meniscus.
Then the lateral meniscus also has two additional ligamentous bands that pass to the other side of the femur, and anchor the meniscus to the femur this way. These are the meniscofemoral ligaments.
It is not shown in the illustration but one of these ligament bands passes in front of the posterior cruciate ligament and the other passes behind it. We have also removed the capsule and the popliteus to make things easier for you to see.
So this further adds to the complexity of the lateral meniscus. These ligaments can become torn, but also they can be confused by the unwary MRI observer with the posterior cruciate ligament itself.
Anterior intermeniscal ligament
Finally, the last stabilising ligament we are going to consider is the anterior intermeniscal ligament, which holds the two anterior horns stable in relation to one another. Although it is thin, it is quite important, but I won't go into more detail here about ligaments (except a few words about the meniscal roots). So you can see that there is some rather complex anatomy for the knee surgeon to consider in cases of meniscus damage and instability!
The roots are situated in the horns of the menisci, and project downwards into the bone of the tibia like little fat anchors. They are really important. If the meniscus tears adjacent to a root, or the root itself is pulled out from the bone (like a carrot!) then the meniscus can become very unstable.