Among the most important advances in modern medicine is the requirement that all disease processes should be converted into a flow chart. Each disease must first have a cause which in computer language is its beginning. It then follows a set of steps which form a pathway to its end. In computer language this pathway is known as an algorithm or a program.

“The process of going from an art to a science involves learning to construct an algorithm.” Encyclopedia of Science and Technology, 4th Edition, McGraw-Hill 1998.

This standard computer flow chart illustrates some of the steps that create a pathway from its beginning to its end. Each disease or type of arthritis has a beginning or cause. In mechanical arthritis the steps are mechanical like in the levers and wheels inside an old wind-up clock. In the chemical types of arthritis the steps are usually due to different enzymes. In sports related arthritis the steps are a mixture of injury and chemical attempts at repair.

The beginning of mechanical arthritis in the knee varies depending upon the type. A bow leg or knock knee deformity starts in infancy. Because of the mechanical overload on one side of the knee it eventually wears down the joint on that side to bone on bone.

The beginning of the chemical forms of arthritis are known in some cases and not in others. For instance the chemical error responsible for gout is inherited from before birth although it may not produce symptoms until later on in life. The beginning of rheumatoid arthritis is unclear although researchers are getting closer to its cause.

The sports related arthritis of the knee may or may not have a clear beginning as for instance from one single injury. A torn anterior cruciate ligament or the removal of a torn meniscus can begin the characteristic chronic wear and repair pathway to arthritis of the knee. Or there may be no specific beginning as in the cumulative damage for repeated small injuries that occurs in soccer players.

Each of these three types of arthritis of the knee has their own distinct pathway to joint destruction. Each has in common that their end- stage can require a Total Knee Replacement.

Mechanical based arthritis of bow legs and knock knees is the easiest of the three types to prevent or to intervene in its early stage. It is also the best understood and easiest to explain with computer graphics. For these reasons it is described in more detail.

The pathways of chemical and sports related arthritis of the knee are briefly described because of their complexity.

Mechanical OA of the knee.

Among the more important advances made in our understanding of who gets arthritis of the knee and why comes from investigations in the alignment of the lower extremity and how the load of the body is distributed on the knee surfaces when we walk.

When a person walks the load on the knee is several times the body weight. The mechanism causing these high loads is explained with the following model.

If a person squeezes a hand gripper to 10 pounds then the muscle contraction cause a compression of 10 pounds at the wrist joint (arrow).

When the fist is then pressed down up to 20 pounds on a bathroom scale then the total compression on the wrist joint is now 30 pounds (arrow).

Because the muscles across the knee have to contract during walking the total compression on the knee can go up to several times body weight in activities such as walking up stairs.

The knee is composed of three smaller joints all of which move in different directions while walking. Its complexity makes it easy for something to go wrong. We know that the key to longevity of a joint is a balanced distribution of the load on its surfaces when it moves. The analogy to the relationship between longevity and load distribution is the accelerated wear of a tire when the front wheel is out of alignment.

When errors of load distribution occur in the knee several times the body weight may be loaded on a much smaller area than the knee is designed to withstand. The result is that there is accelerated wearing down on the overloaded sites resulting in arthritis. Fig. X below:

The X-ray in Fig.x shows that the bow leg angulation results in overload and destruction of just one side of the knee. (arrow).

Let us reconstruct the pathway that the legs and knees of the patient in Fig.1. must have traveled from toddler to older candidate for Total Knee Replacement.

This two year old toddler (ABOVE) has been bow legged since he began to stand. This is not rickets it is a condition known as physiological bowing. He had been carried on the side of his mothers hip which keeps his knees apart. This type of carrying of infants helps the hip develop normally in its socket. When he stands his feet and knees are apart. The soft bones in his legs begin to bow.

This bowing is normally only temporary and the leg usually straightens out normally by a system known as the "pendulum system." It is so-called because the bow leg often changes into a mild knock knee before eventually straightening out before the age of 5 years. This system defies gravity since any object such as a bent nail can only bend more when a load is put on it from above. This pendulum system was first described in 1988 and the riddle of how it works has only recently solved. Unfortunately sometimes the system fails so that at the age of five the child may develop a persistent bow legged or knock kneed deformity.

In this 5 year old (ABOVE) the pendulum system failed to correct the left leg. Because of the maturity of his skeleton he will now need staples in the left knee to restore the leg to proper alignment.

The knee grows in length by adding one layer of bone on top of the other. See Fig.x a below. In a growing bow legged or knock kneed child, one side of the knee is growing faster than the other side (Fig.x b).

Unless this stapling is carried out before growth stops the knee will be permanently bowed. Eventually he will develop arthritis on the inner side of the knee in later life.

The X-ray of a left knee from the front, Fig.a, shows the long term effect of a bowed leg. The overloaded medial side (arrow) has worn away the meniscus and the articular cartilage. Bone is rubbing on bone.

Fig.b. is an X-ray of a cross/section of a cadaver knee with a similar condition to the patient in Fig.a. Note that the lateral side of the knee has no arthritis and the meniscus is normal (arrow).

The meniscus shares some load but their other functions are to act as an attractor to keep the load centered in the proper part of the tibia during weight bearing.

This is a normal left knee seen from above. There are two somewhat donut shaped menisci which keep the lower femur positioned in the center of the donut during walking.

In this side view of the knee the arrows point to the wedge shape of the cut meniscus. They keep the load coming from the two ends of the femur centered in each compartment when weight is put on the knee.

Their donut shape from above and their wedge shape in cross section allows them to function as what is called an attractor. An attractor is a concave structure that steers a convex structure toward its center.

Imagine a golf course where the greens were the usual size but instead of being flat were cup-shaped like the sand trap and the hole that is in the lowest part of the green. In that situation the design of the green would attract the golf ball into the hole. The meniscus attracts the femoral condyle into a special position in the center of the upper tibia where it can withstand sudden heavy loads.

The effect of this instability is often seen in Total Knee Replacements. When the femoral condyle comes in contact with a weak area in the upper tibia it begins to wear down a hole in that site. Specimens of the upper tibia that have been removed at surgery can be placed on a flat surface and a ball bearing placed in the medial compartment. The ball bearing will settle in various sites.

In a normal knee specimen a ball bearing rolls down (is attracted to) and comes to rest in the lowest point which is in the center of the compartment. In this specimen from a person with a long standing bow leg, the medial meniscus had been surgically removed several years previously. After this surgery the pain became worse and the bow leg deformity slowly increased. The convex femoral condyle was then attracted to the edge of the joint and quickly wore down this area. When the ball bearing was placed in the medial compartment it rolled toward the edge where the loss of bone was greatest. The lateral meniscus is badly frayed (arrow).

Absence of a meniscus means that the convex surface of the femoral condyle must now be in contact with the flat surface of the upper tibia. This promotes rapid wear at the contact site and causes flattening of the joint and localized OA.

Three years after removal of the lateral meniscus the lateral side of the knee was worn down to bone on bone and very painful. A decade ago the whole knee would have been replaced. In this case only the lateral compartment was replaced with a partial joint replacement. The patient had an excellent result and kept the healthy remaining two-thirds of the knee.

Removal of a meniscus makes the knee more unstable because of loss of its attractor function. When the medial meniscus is gone the femoral condyle is free to wander around to different sites in the compartment. This instability is worsened if there is also laxity in some of the ligaments.

Alignment

An important variation of a bow leg called the straight leg deformity has recently been identified.
In this variation the femur is in its normal alignment but the tibia is angled so that when the person stands with the feet together, the knees are apart equal to the width of the hand.

The importance of the “straight leg deformity” is that it puts an extra load on the meniscus in the medial side of the knee. The first sign that something is wrong is in midlife when the patient begins to get pain from a worn down meniscus. In many cases the meniscus is removed which eventually results in greater pain in the knee and increased bow leg deformity. The evidence is that the first treatment for these cases should be early intervention to realign the knee.

Fig.a is an X-ray of the knee in a middle aged tennis player with the “straight leg” deformity. The X-ray is normal. She had developed pain in the medial meniscus, which was removed.

Gradually she developed more pain and deformity in the knee. Fig.b is an X-ray of her knee taken several years later. The medial compartment is worn down to bone on bone and the angulation of the leg has significantly increased. A realignment procedure was necessary. 

The most common errors are in alignment of the lower extremity are bow legs or knock knees.
These deformities overload one side of the knee during walking. The result is increased frictional wear and OA on either the medial or lateral compartment of the knee. The mechanism is similar to that seen when a fracture of the tibia heals with angulation.

The case on the left was slightly bow-legged since skeletal maturity. Thirty years later there was OA of the medial compartment of the knee.

The case on the right had normal alignment of the lower extremity at skeletal maturity but later sustained a fracture of the upper tibia (arrow) which healed with angulation. Thirty years later there is OA of the medial compartment of the knee. In both cases the arthritis could have been prevented.

These cases illustrate that “keep your knee” means keep your meniscus, your ligaments, and keep your alignment and the muscles that keep it in position.

Chemical Arthritis

Rapid advances in the understanding and treatments for the group of chemical based arthritis are only briefly described.

For many years researchers had been trying to discover the initial cause of chemical arthritis and the biochemical steps that are involved in their progression to late stage destruction of the joint.

Since inflammation of the joint is the key mechanism of its destruction it was natural that the research was directed to identifying an early step in the pathway of the disease and then blocking its progression. A product was discovered called a COX-2 inhibitor that appeared to stop a critical enzyme.

Unfortunately the use of nonsteroidal anti-inflammatory drugs (NSAIDS) in both chemical and mechanical arthritis had a long history of fatal and non-fatal complications. (? Use. In the US in1998 the number of people dying from NSAIDS, 16,000, was the same as the number of people dying from AIDS.

New improved drugs are coming on the market for the treatment of chemical arthritis. Effective early intervention does not seem to be far away.

Sports Related Arthritis of the Knee

There has been a considerable amount of time and money spent on developing new techniques that will prevent progression of Sports Related Injuries progressing into a Total Knee replacement.

To date most procedures have not been proved to accomplish this goal. Most severe injuries to the ligaments and meniscus seem to go onto arthritis despite attempts to prevent it.

The X-ray of the patient in Fig.x had a severe football injury as a young adult. He had many procedures on his ligaments and medial meniscus. He required a Total Knee Replacement at a young age.

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