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.
Identifying the Signs and Symptoms of Osteoarthritis
How do you know if the joint pain you’re suffering from is due to osteoarthritis? Most of those with the disease have at least one of the following symptoms:
Joint pain: Most people experience joint pain as a deep-seated ache radiating from the inner core of the joint. The feeling is distinctly different from a muscular ache and may come and go according to changes in the weather. (“I can feel it in my bones that it’s going to rain.”) The pain typically increases as the joint is used and eases off with joint rest. As the disease worsens, though, the pain can become fairly steady. If joint pain occurs during the night, poor sleep and next-day fatigue may be two unpleasant side effects.© John Wiley & Sons, Inc.FIGURE 2-2: The neck, lower back, knees, hips, ends of the fingers, and the base of the thumbs are the sites most commonly affected by osteoarthritis.Some people really can feel it in their joints when it’s going to rain, because as the barometric pressure falls, the lining of an arthritic joint can become inflamed, causing pain and the release of excess fluid (swelling).
Stiffness and loss of movement: Stiff joints, limited range of motion, and, in later stages, joints that freeze into a bent position are all signs of osteoarthritis.
Tenderness, warmth, and swelling around the joint: Although swelling is not usually a big problem with osteoarthritis, some joints do swell in response to cartilage damage and irritation, especially if they’ve been overused. The finger joints and the knees are most often affected.
“Cracking” joints: If you hear popping or crunching sounds when you move a joint, you may have osteoarthritis. These cracking sounds (doctors call them crepitus) can be created by roughened cartilage. (This isn’t the same thing as “cracking” your joints by applying pressure to them, which causes a harmless release of nitrogen bubbles and isn’t associated with OA.)
Bony growths on the fingers: Bony lumps, either at the ends of the fingers (called Heberden’s nodes) or on the middle joint of the fingers (called Bouchard’s nodes) are signs of osteoarthritis. These types of bony growths may be hereditary.
Discovering What Causes Cartilage Breakdown
Sometimes, we really don’t know why the cartilage disintegrates. In that case, we designate the problem as primary osteoarthritis, or osteoarthritis of unknown cause. Other times we know that the osteoarthritis has been triggered by another problem, in which case we call it secondary osteoarthritis.
Considering causes of primary osteoarthritis
The ultimate cause of primary osteoarthritis remains a mystery. Although scientists aren’t sure why, the collagen mesh of the cartilage becomes scrambled; it weakens and can’t hold its structure. The proteoglycans, once so cozily intertwined in the collagen mesh, suddenly find themselves evicted from their secure homes. As they float off into the joint fluid, they take their water-retaining abilities with them. The cartilage is left high and dry; it thins and may even crack. At the same time, the newly freed proteoglycans draw excess fluid into the joint capsule, causing swelling. (Unfortunately, this fluid can’t get back into the cartilage, where it’s desperately needed. It’s something like dying of thirst in the middle of the ocean.)
Although no one is absolutely certain what causes primary osteoarthritis, here are a few theories:
The chondrocytes become too efficient at breaking down the collagen and proteoglycan molecules. In healthy cartilage, the amount of breaking down enzymes is equal to the amount of building up enzymes. An overabundance of destructive enzymes leads to weakened collagen and a lack of proteoglycans.
The chondrocytes go wild and start making too many proteoglycan and collagen molecules. The opposite of the previous condition, these chondrocytes are too good at making new cartilage components. The excess proteoglycan and collagen molecules, in turn, pull extra fluid into the joint, flooding it and washing away most of the chondrocytes. The cartilage, then, is left bereft of cartilage-producing molecules.
Sorting out the sources of secondary osteoarthritis
Although the origins of primary