Kickoff means more than just the start of football season. It also means the start of football injuries, when heavily padded players tackle each other and crash to the ground. The result is twisted ankles, bruised bodies and battered backbones. But most of all, football means damaged knees -- an injury that can end a player's season, sometimes his career, and can make life in retirement a living nightmare of pain and disability.

When a badly broken leg brought Redskins quarterback Joe Theismann's career to a crashing halt in 1985, team surgeon Charles Jackson called the wound that made the crowd wince "less severe than some knee injuries" he had seen -- and less severe than some common knee injuries of ordinary people who don't play football.

Even spectators sitting on the sidelines can suffer the same sore knees as football players. It's only a matter of time before the repetitious, jolting thuds of running and walking, the twisting and turning of weekend tennis, the bouncing of jump rope and even the day-in, day-out chores of climbing steps and carrying groceries will grind down the knee. Experts estimate that some 50 million Americans have suffered knee pain sometime during their life.

All this has prompted a range of new treatments for knee problems. In severe cases, physicians used to cut out deteriorated knee joints and directly fused the thigh bone to the shin bone -- which helped stop the pain but left a stiff leg that was hard to walk on. Today, new techniques allow surgeons to repair knees without cutting them open. This preserves flexibility and enables a person to remain active.

When the knee is completely destroyed, it can be replaced with an artificial joint. Every year, more than 100,000 Americans get new knees -- most of them women.

The need for treatment will increase because Americans are outliving their knees, said Stuart I. Springer, an orthopedic surgeon at the Hospital for Joint Diseases in New York City. Nature designed the joints to last 40 to 50 years, he estimated, but people now live twice that long.

The knee wears out because all the force of the body's weight passes through that 3- to 5-inch joint, which is composed of four bones, four ligaments and a couple of cartilage pads that cushion the blows of daily living. For most people, bad knees are just another consequence of the wear and tear of aging. Exercises such as running can pound the knees with force exceeding five times the body weight, accelerating damage. For an overweight person, just walking down the street can have a similar destructive effect.

"Many grossly overweight people end up damaging their knees," said Peter Kenmore, vice chairman of orthopedic surgery at Georgetown University Medical Center.

Some people inherit a recently discovered genetic defect that accelerates the most common type of osteoarthritis, a disease that strikes the joints of 16 million Americans. An additional 2 million suffer rheumatoid arthritis, a disease in which the immune system mistakenly attacks and destroys the joints.

And then there are those who play sports. From the pros in the major leagues to the weekend warriors on neighborhood gridirons, athletes are at risk of injury, and sports doctors agree that the knee is the body's most vulnerable part.

In football, clips -- a type of block -- and blind-sided tackles are especially devastating because they bend the knees in directions they were not meant to go.

Overall, knee injuries significantly contribute to the short average length of a professional football career: 4 1/2 years in the National Football League, and retirement forced by injury at the average age of 26. The other most frequent career-ending injuries occur to the neck and back.

Players fight back with exercises and braces that help stabilize the knee. Rule changes -- such as prohibitions against blocking below the knees on kickoffs -- have helped, but the onslaught continues.The Normal Knee

The biggest problem is the design and construction of the joint itself. Unlike the hip or shoulder, where a ball on the end of the bone fits securely into a socket, the knee can be thought of as a place where a rubber pad separates the ends of two poles held together by strong rubber bands.

The poles are the thigh bone (the femur, the longest bone in the body) and shin (the tibia, the main support bone in the lower leg, with help from the smaller side bone, or fibula). The kneecap, or patella, protects the joint and prevents the knee from bending the wrong way.

The ends of the bones are covered with tough cartilage, more slippery than ice, that allows them to move freely against each other. Also, they ride on pillows of fibrous cartilage that forms the meniscus -- a crescent-shaped pad positioned on top of the shin bone. Although resilient, the meniscus can be ruptured or frayed by excessive or repetitive forces, or pushed out of position and painfully pinched between the outer edge of the knee bones.

The entire joint is held together by four major ligaments, tough bands of fibrous tissue that stretch between the ends of the thigh and shin bones and pull them toward one another. Two ligaments -- the collaterals -- run along the outside of the knee, one on the left side and one on the right. Two others -- the cruciates -- crisscross inside the knee, from front to back, limiting how far the knee bends. Despite these constraints, the knee bends and straightens, much like a door hinge, and also rotates to a limited extent. The whole assembly is moved by the thigh muscles.

Bumps and Bruises

For the knee to work properly, each part must remain in balance. If one element is misaligned, the whole joint begins to wear out faster, just as the edge of a car tire wears down rapidly when the wheel is off center.

Injuries that can knock a knee out of whack occur in one of three ways: trauma, chronic overuse and the effects of diseases such as arthritis. Depending on the force, the effects of trauma range from a bruise to torn ligaments to broken bones. If the injury is severe, the knee quickly swells with fluid because muscles or tendons have been torn and are bleeding into the sac that surrounds the knee. This is called "water on the knee." In many cases, three to five ounces of bloody fluid can be drained out of the joint with a needle.

The more catastrophic trauma, however, affects the ligaments. While the knee can withstand tremendous downward pressure, it cannot handle the lateral or sideward thrusts that occur when a football player tackles another around the legs from the side.

In the past, repairing traumatic knee damage meant cutting the knee open. Today, orthopedic surgeons rely on a fiberoptic instrument called an arthroscope. A steel tube that can be inserted through a quarter-inch slit in the skin, this device is used to illuminate the injury inside the knee, display it on a TV screen and then manipulate surgical instruments from outside the body to repair the damage. Said Georgetown's Kenmore, surgeons now use arthroscopy to reconnect severed ligaments, shave frayed edges off a damaged meniscus and remove debris such as bone chips before they cause extensive damage.

This technique is not just reserved for trauma. The wear and tear of chronic overuse injuries and even some effects of arthritis can be repaired or reduced with arthroscopy. For example, the arthroscope can shave away a damaged side of the knee so the portion left intact bears most of the weight, in essence rebalancing a knee just as a mechanic might balance a tire. This can relieve pressure within an arthritic knee.

Because the knee is not surgically opened, recovery is much quicker than from older forms of surgery. Usually, it is done as an outpatient procedure, and it usually only takes people a day or two to get back on their feet, though it might take up to a week to be ready for heavy, manual labor, Kenmore said.

The operation can be repeated with each new injury. Some pro football players have multiple arthroscopies, at times as many as 10. Arthroscropy now accounts for 90 percent of all knee surgery performed in the U.S.

Artificial Joints

Terrible traumas, the grinding wear of long-distance running and the destructive inflammation of arthritis can damage the joint so badly that arthroscopic surgery isn't enough. That leaves one alternative: an artificial knee.

Although there has been an ongoing debate about which patients will best be helped by knee replacement, most physicians rely on practical criteria. "I use pain that limits the patient to less than reasonable activity," said Randall J. Lewis, a orthopedic surgeon in the District. "If you have pain at rest or during sleep, then you need surgery."

"Most of our patients are very impaired by arthritis pain," said orthopedic surgeon Anthony S. Unger, who also practices in the District. "If they can't walk a few blocks, then we do it. We only use surgery as a last resort." For people with rheumatoid arthritis, he added, there are no good alternatives to a total knee replacement once the disease has caused severe joint deterioration.

Age is also a factor. Individuals under age 40 or 50 who suffer some pain after actively exercising are usually treated with rest and anti-inflammatory drugs. "Someone simply walking two to three miles with some discomfort doesn't need a new knee," Unger said.

Although some knee implants are given to ex-jocks, most go to older men and women in their sixties or above in whom arthritis has essentially destroyed their knees.

The two-hour procedure itself looks more like sculpting than surgery. It begins with a foot-long incision along the top of the knee. The surgeon uses electric saws, a hammer and chisel, and different-shaped guides or dies that direct the blades, to shape the bone ends to attach the new knee.

The lower half of the new knee is cemented into the shin bone and the upper half to the thigh bone. But the two halves are not actually attached to each other like a hinge. Older rigid, hinge-like designs caused too much stress, breaking the cement. Instead, the ends of these newer devices float on top of each other, relying on natural ligaments running along either side of the knee to hold the joint together.

Pioneered more than 30 years ago, the plastic and metal joints have undergone extensive changes that improved their function to the point that they work much like natural joints. "All designs in use today are similar," Lewis said. They replicate the knee's function, including the role of the kneecap and the job of the ligaments inside the joint. Current models are considered safe and long-lasting, several surgeons said. The National Center for Health Statistics said that in 1987, approximately 105,000 knee replacements were performed, 71,000 of them on women. Some experts now estimate that 120,000 knee implants -- at an average cost between $25,000 and $35,000 -- are performed annually.

Fewer than one in 100 patients develops serious complications, Kenmore said. The most common risks are the same as those for any major surgery. They include infections and blood clots that can break off and travel to the lungs or brain. Moverover, modern joints seem very durable: more than 90 percent still work after a decade, several experts said.

At the same time, the success of an artificial knee implant depends on how well the patient tolerates physical therapy and how soon he or she can resume activity.

"We keep patients in bed 48 hours," Lewis said. But by the third day, they are standing, and by the fourth, they are starting physical therapy. It can take weeks or months for a person to stop walking with a cane and resume normal activity, Lewis said. The level of activity also determines how long the knee will last. The patients who do best are older and place relatively few physical demands on their new knees, Lewis said. But many patients do feel better after the joint is installed and return to playing golf and other forms of low-stress exercise.

But, "you cannot get a total knee and then ski on it or play basketball," Unger said. The big problem is wear. Sure, he said, a young person could run or play football with an artificial knee, but it won't last long. "It's like buying a new car," Unger said. "You can drive it at 150 miles per hour every time you get in it, but it will only last about 15,000 miles before you blow the crankcase. Or, you can get in it and drive 50 {mph} and it will last 200,000 miles. Knees are the same way."

Durability problems have begun to increase as some knee implants begin passing the 10-to-15-year mark. The plastic begins to fray after the knee has bent a few hundred thousand times, sometimes causing painful inflammation. The metal, too, repeatedly stressed like a paper clip bent back and forth, can fatigue and break.

When the devices fail, surgeons can replace the artificial joint with a new one. But the second surgery is technically more demanding, since the patients are older, frequently sicker and already have had their bones altered by the first operation. Roughly 85 percent of 40- to 50-year-olds who have had total replacements will be reoperated on within a decade, according to James M. Fox, an orthopedic surgeon in Van Nuys, Calif., in his book "Save Your Knees."

A growing number of Americans receive second artificial knees every year, and, in general, surgeons have reported good results for the second operations.

As knee replacement technology continues to improve, doctors expect artificial joints to provide a second chance for aging football players and those with arthritis. As Unger said: "We make the lame walk."