Arthritis For Dummies. Barry Fox
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wrong with your joints, you need to understand what a joint is and how it works. Any place in the body where two bones meet is called a joint such as the ball and socket hip joint, or the hinge joint at the elbow or knee. Sometimes the bones actually fuse together; your skull is an example of an area with fused bones. But in the joints that can develop arthritis, the bones don’t actually touch. As you can see in Figure 1-1, a small amount of space exists between the two bone ends. The space between the ends of the bones keeps them from grinding against each other and wearing each other down.
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FIGURE 1-1: Anatomy of a healthy synovial joint.
Bones are living tissue — hard, porous structures with a blood supply and nerves — that constantly rebuild themselves. Bones protect our vital organs and provide the supporting framework for the body. Without bones, we would be nothing more than blobs of tissue — like tents without supporting poles!
But bones are more than broomsticks that prop us up; fortunately, they don’t leave us rigid and awkward. The 200-plus bones that reside in our bodies are connected together in some 150 joints, giving us remarkable flexibility and range of motion. If you don’t believe it, just watch a gymnast, ballet dancer, or figure skater execute a handspring, arabesque, or triple axel. But you don’t have to be an athlete or contortionist to enjoy the benefits of joint flexibility. Just think about some of the things you do regularly — such as bending a knee or an elbow. Now imagine how limiting it would be if you had fewer joints, or if they didn’t move the way they do! (For a few fascinating facts about your joints, see the sidebar “Strange-but-true joint points” on the next page.)
Cartilage: The human shock absorber
Cartilage is extremely important for the healthy functioning of a joint, especially if that joint bears weight, like your knee. Imagine for a moment that you’re looking into the inner workings of your left knee as you walk down the street. When you shift your weight from your left leg to your right, the pressure on your left knee is released. The cartilage in your left knee then “drinks in” synovial fluid, in much the same way that a sponge soaks up liquid when immersed in water. When you take another step and transfer the weight back onto your left leg, much of the fluid squeezes out of the cartilage. This squeezing of joint fluid into and out of the cartilage helps it respond to the off-and-on pressure of walking without shattering under the strain.
Can you imagine the results if we didn’t have this watery cushion within our joints? With the rough, porous surfaces of the bone ends pitted against each other, bones would grind each other down in no time. One thing is certain: Nobody would be getting around too easily without joint fluid and cartilage.
Types of joints
To accommodate the bends, twists, and turns that we all perform without even thinking, the skeletal system is made up of different shapes and sizes of bones, which connect to form different kinds of joints. The joints are categorized according to how much motion they allow:
Synarthrodial joints allow no movement at all. You can find these in the skull, where the bones meet to form tough, fibrous joints called sutures. Because they don’t move, arthritis doesn’t affect them.
Amphiarthrodial joints, such as those in the spine or the pelvis, allow limited movement. Generally, these joints aren’t attacked by arthritic conditions as often as others. (A slipped disc is not arthritis.)
Synovial joints allow a wide range of movement; most of our joints fall into this class. Synovial joints come in all kinds of interesting variations including those that glide, hinge, pivot, look like saddles, or have a ball-and-socket type structure. (For more on these joints, take a look at the section “Looking at the types of synovial joints” later in this chapter.) Because of the synovial joints, you can bend over and pick a flower, kick up your heels while swing dancing, reach for a glass on a high shelf, and turn around to see what’s going on behind you. Unfortunately, these joints are also the ones most likely to be hit with arthritis, precisely because they do move!
STRANGE-BUT-TRUE JOINT POINTS
Here are a couple of things you may not know about your joints:
By the time a fetus is four months old, its joints and limbs are in working order and ready to move.
A newborn baby has 350 bones, many of which fuse to form the 206 bones of the adult body.
Cartilage is 65 percent to 85 percent water. (The amount of water in your cartilage generally decreases as you get older.)
When you run, the pressure on your knees can increase to ten times that of your body weight.
Not a single man-made substance is more resilient, a better shock absorber, or lower in friction than cartilage.
Looking at the types of synovial joints
Gliding joints
A gliding joint contains two bones with somewhat flat surfaces that can slide over each other. The vertebrae in your spine are connected by gliding joints, allowing you to bend forward to touch your toes and backward to do a backbend (well, maybe!). See Figure 1-2 for an example of a gliding joint.
Hinge joints
You can find hinge joints in your elbows, knees, and fingers. These joints open and close like a door. But just like a door, hinge joints only go one way — you can’t bend your knee up toward your face, only back toward your rear. See Figure 1-3 for an example of a hinge joint.
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FIGURE 1-2: A gliding joint. The gliding joint helps keep your vertebrae aligned when you bend and stretch.
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