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What is 8 times more slippery than ice, and acts like a sponge?


Joints need cartilage to function properly. Up to eight times more slippery than ice, and with the ability to soak up and push out water as easily as a sponge, cartilage is perfectly designed to permit seamless motion between bones, while at the same time providing ideal shock-absorbing capacity. In the world of joints, cartilage is a true superhero. Scientists have put men on the moon, eradicated polio, made flying an everyday event, and decoded DNA, but they haven't yet been able to create a substance that is better suited for joints than the body's own healthy cartilage.

A typical synovial joint

Cartilage is made of collagen, proteoglycans (core proteins that are attached to carbohydrate chains), chondrocytes (cells that make cartilage), and up to 80 percent water. When you are at rest and your joints are not bearing weight, cartilage stores synovial fluid and water within it. When a joint is loaded with a force, the fluid stored in the cartilage is redistributed to the joint. In other words, when you stand from a seated position, the weight you put on your knee pushes the synovial fluid and water out of the cartilage in your knee, much as it would push the water out of a wet sponge inside your knee. The fluid pushed into the joint space helps to cushion your weight and also nourishes and lubricates the joint. It moves back into the cartilage when you sit down.

In addition to this sponge-like property, cartilage has another cushioning advantage -- it is filled with negatively charged chondroitin molecules. Negative particles resist touching each other with astounding atomic force. When the joint is made to bear weight, these chondroitin molecules are pushed together, but their negative charges resist. Pushing two negatively charged chondroitin molecules together is like trying to force two negatively charged magnets together. The closer they come to touching, the stronger they repel each other.

Cartilage doesn’t have a direct blood supply, which makes injury healing sluggish. The same goes for ligaments and tendons. When you have a direct blood supply to tissues, nutrients and reparative cells like fibroblasts can get there fast. With no blood supply, the injured tissue has to rely on absorption or other means to get nutrients in.

This is why joints, whose main structures are cartilage, ligaments and tendons, take so long to heal when injured and often require surgery. Once you hurt it, it can take months, even years before it fully heals, if it does at all.

Find out next week about the causes of injury and what we can do to help with pain and recovery.

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