Читать книгу Arthritis For Dummies - Barry Fox - Страница 40

Healthy cartilage is absolutely essential for joints to function properly and painlessly. Slick as polished marble and tough as galvanized rubber, cartilage protects the ends of your bones from wearing each other away where they meet inside a joint. It also provides a smooth, slick surface so bone ends can glide easily across each other. And cartilage is an excellent shock absorber, cushioning the bones and soaking up the impact created by movement and physical stresses. Without intact cartilage, bones grind away at each other and bear the brunt of the impact of movement. Eventually, the joint itself can be damaged or even destroyed.

Four elements help cartilage do its all-important job:

 Water: Sixty-five to 80 percent of cartilage is water — a crucial substance that lubricates the joints, cushions bones, and absorbs shock.

 Collagen: Elasticity and a superb capability to absorb shock make collagen an integral part of healthy cartilage. A connective tissue that helps hold bones, muscles, and other bodily structures together, collagen is the mesh-like framework that provides a home for the proteoglycans.

 Proteoglycans: These large, oblong molecules are covered with centipede-like “arms” that weave themselves securely into the collagen mesh and soak up water like a sponge. Then, when pressured, they release water. Thanks in part to the proteoglycans, cartilage can mold itself to the shape of the joint and respond to the ever-changing amount of pressure within the joint capsule.

 Chondrocytes: These cells follow the principle “out with the old and in with the new” as they break down and get rid of old proteoglycan and collagen molecules, forming new ones to take their place.

Water, collagen, proteoglycans, and chondrocytes all work together to keep your joints moving like well-oiled machinery. When the pressure is released from a joint, say your knee when you lift your leg to take a step, water rushes into the cartilage, nourishing, bathing, and plumping it up. The water-loving proteoglycans, woven securely into the collagen web, soak up water and hold on to it until pressure is applied to the joint (that is, you take another step). Then the water and wastes rush out of the cartilage. But as soon as the pressure is off, the proteoglycans thirstily soak up the water again. The resilient collagen stretches and shrinks to accommodate joint pressure and water content, so your cartilage can bounce back after being flattened out.

But if your cartilage loses its ability to attract and hold water, it becomes thin, dry, cracked, and unable to provide a slippery surface (see Figure 2-1). No longer plump and resilient, it makes a poor shock absorber and cushion for the bones, particularly affecting the weight-bearing joints.

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FIGURE 2-1: Osteoarthritic joints have narrowed joint space, and thin, rough, broken-down, or completely missing cartilage.

You can visualize the action of the cartilage by thinking of two cans of soup facing each other end-to-end with an almost-filled water balloon in between them. As you press the soup cans together, the water balloon changes shape to accommodate the pressure, but never lets the cans actually touch. When you release the pressure, the water balloon (like your cartilage) resumes its old shape.