So What Exactly Is Knee Cartilage Anyway?

In my last blog post, I dropped a rather long, important-sounding word: glycosaminoglycans.

These polysaccharides help to keep cartilage elastic and resilient. And, according to a Swedish study, their content increased in the knee joints of people who exercised.

Now I want to take a whirlwind tour through the make-up of cartilage, which I studied when I was battling knee pain, to put that fifty-cent word in some meaningful context.

First, hyaline articular cartilage (that’s the kind we're looking at, because it pads the ends of the bones that meet in the knee joint) is like a tough, rubbery, wet sponge. It’s four-fifths water (showing you why it’s important to stay hydrated).

Take out the water and what do you have left? There’s a tough, ropy protein called collagen that is found in higher concentrations on the surface than deeper in the tissue (to keep your cartilage tear-resistant where it matters most). Then you have molecules that weave around that collagen skeleton that are called proteoglycans.

Let’s look up close at those proteoglycans. They’re large molecules that consist of a protein core and many long-chain sugar molecules (if you want to visualize a proteoglycan, the easiest way is to imagine a bottle brush -- the spine of the brush is the protein core, and the many bristles are the sugar chains).

What are the sugar chains called? Go to the head of the class if you've already figured this out: glycosaminoglycans. (A quick aside: if you were to tear apart a glycosaminoglycan in your molecular toolshed, you would find it's composed of sugars such as glucosamine. Ah, so you now you know where that glucosamine you’re swallowing for your joint pain is supposed to be going! But unfortunately it never gets there, making glucosamine supplements useless -- more on that some other time).

So why should you give two hoots if your cartilage has a low content of glycosaminoglycans or a high content?

Well, when you subject your knee to load, that tough, rubbery tissue that is cartilage gets compressed. Remember, it’s not like a plate of metal, but rather a tough sponge with a high proportion of water. It needs some way to protect itself from intense forces being transmitted through the knee joint during everyday activities. Otherwise, your cartilage could get chewed up pretty fast.

Of course, there is that tough collagen, which helps. But the glycosaminoglycans also play a key role. On the atomic level, they carry a negative charge. So they repel each other when pushed closer together.

So let’s say you jump in the air to grab a Frisbee. When you land, knee cartilage has to absorb that load of your body hitting the ground. The pressure will expel water and synovial fluid from the tissue and push the glycosaminoglycans closer together. But as they’re pushed closer, that negative charge causes them to repel each other more strongly (you know how intense this pushback can be if you've ever tried to place the like poles on a pair of magnets in contact with each other).

So having plenty of glycosaminoglycans is critical for keeping the tissue resilient.

Knee cartilage 101! Important stuff to know.