Features August 2003 Issue

Cartilage: A Cushion For Your Bones

Your body’ s shock absorbers, just like a car’s, can fail. When the miles pile up, here’s how to keep them in tune.

Cartilage is amazing stuff. This slick, rubbery material cushions and protects the ends of bones, allowing them to roll, rotate and glide smoothly against and over each other. Cartilage is 85 percent water. It has no blood supply or nerves, yet it absorbs enormous forces as we walk, run, bend, or flex. Cartilage also has a “memory”; it can change form when under stress, then spring back to its original shape.

Cartilage is actually a framework of fibers called collagen. Interlaced with molecules called aggrecan, this framework holds water and makes cartilage an elastic, compressible, self-lubricating shock absorber.

When cartilage is under pressure, stored water leaches onto its surface in a lubricating, gel-like form. At the same time, the cartilage itself compresses slightly, transferring force outward. By lubricating the joint surfaces and absorbing force, cartilage protects underlying bone, which would otherwise wear away from friction.

When Cartilage Goes Bad
By the time doctors see signs of osteoarthritis, the supporting structure of cartilage has long since begun to break down. For reasons that aren’t yet clear, the water content of cartilage increases in early osteoarthritis, while the collagen framework becomes thinner and weaker.

Healthy knee cartilage (left) provides a smooth slidiug surface to protect the ends of bones. As arthritic cartilage erodes (right), bone spurs, swelling and pain develop, limiting joint movement.

The tissue loses many of the large aggrecan molecules that provide elasticity and absorb pressure. Over time, the cartilage softens, becoming less resilient and more prone to damage. Fissures, like tiny crevasses on a glacier, develop on the cartilage surface, and bits begin to break off. Cartilage-forming cells called chondrocytes, scattered throughout the framework, can no longer keep up with necessary repairs. To make matters worse, the levels of enzymes that normally break down the aggrecan molecules and collagen framework become higher than the levels of repair enzymes.

Identifying how natural triggers and environmental factors conspire to set in motion this degenerative process is a key area of osteoarthritis research.

Repair And Replacement
Current efforts to repair injured cartilage have met with some success and may some day lead to successful treatments for osteoarthritis. Although cartilage normally is able to renew itself, it does not effectively heal injuries such as tears, which sometimes cannot be repaired satisfactorily even with surgery. But thanks to recent advances in tissue engineering (see “Experimental Therapies May Hold Key To Future Pain Treatment,” July AA), some patients may be able to grow their own replacement cartilage. Called “cartilage implantation,” this technique involves harvesting healthy cartilage cells, which can be duplicated and grown in a laboratory, then implanted at the injured site.

Work is also being carried out on other techniques that may some day allow the replacement of cartilage. In some studies, gels and other collagen-framework substitutes are being infused with stem cells that can be used to rebuild various components of cartilage. Still other studies are experimenting with gene therapy to create new cartilage.

Self Fixes
In the meantime, exercise and maintaining proper weight are the two best things you can do to protect your cartilage. Until a cure for cartilage breakdown is discovered, these—in addition to medication and a positive mental outlook—are the best ways to ease pain and preserve quality of life.