The patellofemoral joint in flexion and extension

Previous blogs have discussed the patella as a sesamoid bone and on the patella as part of the extensor mechanism.

I mentioned there that the patella is a bone that exists within the tendon of the quadriceps muscle, and that the muscle is tethered above to the hip and bits of it to the upper femur, and that it is tethered below to the tibia bone.

One can imagine then that there is no restraint on the patella itself and it is free to move from side to side if one moves the tendon of the quadriceps muscle. Indeed if you sit on a chair with the leg up on a stool one can move the patella quite easily from side to side. But just stand up and try to move it then - the majority of people will find that the patella seems locked in place and no longer free to be wiggled. Why is this?

Let's start by looking at the three main bones from the side with the knee straight. You can see that in this position the patella is actually quite high up in relation to the joint line of the two bigger bones. Imagine in your head the patellar tendon below the patella tethering it to the tibia bone. Because of this tether, which is tendon not muscle and cannot contract, the relationship of patella to tibia stays the same as the knee bends. But the relationship of the patella to the rounded end of the femur does NOT stay the same.

As the knee is bent the patella comes more and more in contact with the rounded end of the femur, and as it does so it slips into a groove that exists there. You can get a better idea of this if you look at the illustration on the right.

In this illustration the bones have been rotated so that you can see the front, and the patella has been stripped back to show the structures below it. For clarity I have drawn in the patellar tendon and left it attached to the patella. Actually this is an old image and the femur is labelled the thighbone and the tibia is labelled the shinbone, but I am sure you can cope with that. What I am wanting to show you is that the rounded part of the femur is actually two rounded parts, and between them is a groove where the patella lies when the knee is bent. This groove is called the trochlear groove. 

As the knee is bent, the patella slips into this groove and it is hard to wiggle it about because of the steeper walls on the side of the groove. In some people the anatomy here is disturbed and this can cause problems with patellar stability. We will get to that in later sections of this keynote. For the moment, though, you have the key anatomical words and concepts for me to be able to discuss more complicated issues.