The joint capsule – why it is key to understanding so much about the knee

The knee patient needs to understand that it is not just the bones and the muscles that are important in regaining knee movement after injury or surgery. The capsule of the knee plays a critical role in allowing normal knee movement.

First published in 2017, and reviewed August 2023 by Dr Sheila Strover (Clinical Editor)

(Narration: George Gibb)

Transcript of this video

In this section we will explain about how the various bony and soft tissue parts of the knee work together to allow a full range of movement.

Anatomy, of course, is three dimensional, and in this course we will largely be using two dimensional illustration, which by its nature will always leave something out. But we think that we will be able to put things into sufficient context for you.

Some people on this course will have some background in anatomy, but others may not, so we’ve used some slightly simplistic concepts, and ask you to bear with us, while we bring everyone up to the same level.

Let’s launch in now, and Dr Sheila Strover will show you around the knee.

Let's begin with the concept of the knee as a hinge joint. It is much more than a hinge, and is not even a true hinge, but let us keep that concept for now because it makes explanations easier. The hinge part involves the two long bones that move in relation to one another to bend and straighten the knee. The upper long bone is the thighbone or femur and the lower one is the shinbone or tibia. I'll get back to them later, but now I want to explain how the hinge part of these two bones is contained within a capsule.

Most of the images online and in anatomy books just ignore this capsule, but it is really important. Imagine a waterproof bladder with the hinge part of the femur and tibia bones inside and the far ends of the bones outside of it.

Here I have cut away the capsule, just leaving the adherent edges so that you can see how it is sealed onto the bone. Note how it extends upwards above the joint into a deep pouch, and relaxes at the bottom into folds or gutters on the sides, down the back and down the front of the joint.

Normally there is only a little bit of lubricating fluid in the joint space and the capsule is deflated, but when the joint is filled up with fluid it can be appreciated that the space inside is really quite big. People who have had a knee injury often exclaim that their knee "swelled up like a balloon" - and they are shocked as they didn't realise that there is all this potential space in there!

So, the important principle that I want to stress here is that the natural slack allowed by the normal capsular folds allows the joint considerable movement which can be reduced early on in injury or surgery if the knee is swollen with fluid inside it. If the fluid is resorbed and any sticky residue allowed to turn into adhesions, then these can gum the walls of the capsular folds together and reduce movement even further. Let's move on now to the next lecture, and take a look at some of the important structures inside the capsule.

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