Find out why the patella bone and its position within the tendon of the quads muscle is central to understanding many problem both of extension and of flexion.
Transcript of video
So far in this course we have looked at how the hinge part of the knee joint, and many important joint structures, are sealed within a capsule which will normally hold a small amount of lubricating fluid, but which can fill up with very much more fluid under abnormal circumstances. We have taken a 'keyhole-surgeon view' of those structures that the surgeon will first see when the joint is entered behind the kneecap with an arthroscope.
Now we are going to take a much closer look at the kneecap itself, and how it forms an integral part of the system that allows one to straighten the knee. The proper name for this bone is the patella, and I will use that term from now on.
The patella is very unusual. And that is one of the reasons that so many people struggle with problems related to the patella. If you were a keyhole surgeon and you looked into the joint from above rather than from the front, you would be able to see the shiny white undersurface of the patella from within the joint. So that part of it is inside the capsule. The main bulk of the patella, however is outside the capsule, so it is like those squeaky things in a toy ball, with part of it inside the cavity and part of it outside.
The strangeness does not end there, however. The part of the patella outside the capsule is contained within the tendon of the quadriceps muscle, that big muscle bulk that makes up your ‘lap’. Very few bones in the body have this arrangement, and what it means in practical terms is that anything affecting the quadriceps, or quads, affects the patella, too. And anything affecting the patella will have an impact on the quads.
So now you can see that there are three bony parts making up the articulation within the joint - the ends of the femur and tibia forming the hinge part, and the undersurface of the patella which has a rather special role. All three of these are covered with shiny white joint cartilage so that they can slip over each other more easily as the knee bends and straightens.
In order to understand the patella, you need to understand the quads, so let's start there.
The quads muscles stretch from the hip and top of the femur, all the way to the tibia. When you sit in a chair and straighten your leg it is the quads contraction that is most responsible for the movement. The reason why it is called the quads or 'quadriceps' is because that word means 'four heads', as the quads is in actuality four separate muscles that come together at the tibia. The two central heads become a common tendon just above the knee joint, continuing down to insert at the tibia. This is the part containing the patella. The two outer heads expand into a supportive fibrous fan at either side of the patella, merging with the capsule, but also attaching to the sides of the patella.
The bit of the tendon above the patella is called the quads tendon and the bit below the patella is called the patellar tendon, or patellar ligament, but in reality it is all one tendon that just happens to have a very unusual bone in the middle of it, the undersurface of which pokes through the capsule to form part of the bony articulation. In this position in the middle of the quads muscle, the patella acts as a ‘fulcrum’ when the quads contract, enhancing the power of the quads in straightening the knee.
Where the lower portion of the outer quads fan out at the sides of the patella, the region is known as the retinaculum. The outer retinaculum is stronger and is known as the lateral retinaculum. The word 'retinaculum' just means a 'network' - so it is a network of stronger fibrous tissue that keeps the patella in a good position. So the retinaculum is an integral part of the capsule.
That is all I want to introduce you to in this lecture. In the next lecture I will focus in greater depth on the two key regions above and below the patella, because these are extremely important in allowing the knee hinge to operate fully. I will also talk about how the capsule at the back of the knee can prevent the quads from achieving full knee extension.