The Knee

WHEN IT COMES TO THE KNEE, “instability”is the word. Over 25 percent of all sports injuries involve the knee (75 percent when it comes to surgery), and many of these involve instability of some kind. But it’s not that the knee is necessarily ill designed. It’s just that it has to put up with all manner of stresses and strains. In fact, when you think of it, the knee is very well designed—ingeniously so. Imagine you’re a mechanical engineer, and you’ve just been given a commission: Create a hinge. But not just any old hinge. This hinge must be flexible enough to bend a good 150 degrees front to back, another 3 or 4 degrees side to side, and it must be able to rotate 60degrees as well—all at the same time if need be. It must be able to withstand anywhere from 100 to 2,000pounds of pressure. It must be self-lubricating—no periodic grease jobs allowed. And it must be decidedly hightech: This hinge must be able to adapt to the changing demands made upon it. If it undergoes great stress onone side, it must be able to strengthen the other side, in order not to break.And that’s not all. The hinge must be durable, able to last sixty, seventy, eighty years. And it can’t betoo big. It may be the largest hinge in the machine of which it’s a part, but if it extends more than threeinches across in any direction, that’s just too much. And, by the way, leave some extra room inside becausethe power train for the rest of the machine has to go right through this hinge.So, you begin. The first problem is flexibility.Since this hinge (which we might as well start callinga joint) must bend and rotate at the same time, there’sno possibility of a rigid linkage like a door hinge. And you can’t get too exotic in design, because the moreflexible you make this joint the less stable it becomes—and it has to hold together for a long time. So youdecide simply to have the two parts of the joint abut,one against the other, so that they can bend and twistand do all sorts of maneuvers. You mold them so thetop part ends in two convex mounds, much like twoscoops of ice cream side by side in a single cone.The bottom part you hollow out, making two shallowcups. When you place one part against the other, the ends—the mounds and cups—fit. Next youprotect the ends by coating them with some tough, white, rubbery stuff called cartilage; between them youplace rings of thicker cartilage called a meniscus to further cushion the joint and provide shock absorptionas well. So far so good.Now, how to hold the two ends together? With stays and guy wires called ligaments and tendons. Youhook them up in the middle of the joint and along the outside (the collateral and cruciate ligaments) so thatthey’re able to loosen and tighten their grip as needed, as though there were a minicomputer controllingeverything. Now your joint can bend, twist, push and shove, and withstand immense pressure. It maygive, but it won’t break. And you enclose the whole business in a tough, flexible sac that not only protects thejoint from the outside world but produces its own lubricant as well.One problem remains: the business of the power train (muscles). Finally it comes to you: why not letthe joint function as a pulley? From the power source above, run a cable across the joint and tether it on theother side. As the power source contracts, it reels in the cable along the joint, lifting the weight (whichhappens to be your lower leg) on the other side—just like a prototype giant crane. And to increaseleverage, thereby increasing lifting power and making the joint more efficient, simply make the pulley larger.But, of course, this joint has no turning wheels. Its pulley uses a sliding mechanism. To increase efficiencyyou must increase the angle over which the cable slides. So, a bold stroke: you place a small, lens-shapedpiece of bone on top of the joint. The cable now must run over the piece of bone, which itself slides along theupper side of the joint. As the cable moves, the bone increases the angle between the power source and theweight to be lifted. The result: more strength.Finally, make sure you leave some slack. If the tolerances are too tight, the fit too precise, this jointof yours will degenerate too quickly—just as your car’s transmission will grind itself to death if its gears aretoo tightly aligned.The result? Voilà: the knee. The lens-shaped piece of bone is, of course, the kneecap. The sides ofthe hinge, above and below, are the femur—the large bone in your thigh—and the tibia—the shinbone. Thatingenious power train begins in your thigh with the quadriceps, the body’s largest and most powerful muscle,and turns into tough tendon that extends all the way to the lower leg. This artful conglomeration of bone,muscle, soft tissue, and gristle makes up the largest and most complicated joint in the body and the one mostfrequently injured in sports (although ankle sprains are the most common single injury, the variety of kneeproblems outrank those of any other joint).As you can see by now, the knee isn’t ill designed. It has changed less in our long evolution than anyother joint. It’s just that we subject it to so much abuse. And because it’s so large a joint, and so busy, thereare a lot of things that can go wrong. There’s lots of bone in the knee; seven distinct ligaments; more andthicker cartilage than anywhere else in the body; one huge tendon, as well as smaller ones; and a fullcomplement of bursas that can develop bursitis at any time. But by far the greatest number of knee injurieshave to do with what is called the extensor mechanism.

+ WARNING +

If You Experience Any of TheseConditions, Seek Medical Help

+ Your knee feels unstable or bends the wrong way

+ Your knee is locked or won’t straighten out;

+ Within a few hours your knee becomes swollen;

+ Your knee won’t bear your weight.

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