The articular cartilage is a special type of connective tissue that lines the three bones that form the knee joint—the patella, the femur, and the tibia—and allows the joint to move in a fluid motion. Without this protective layer of articular cartilage, the bones forming the joint would rub together, which could cause a great deal of pain and discomfort, as occurs in arthritis. Also known as hyaline cartilage, articular cartilage consists of a white elastic material, similar to the gristle found inside the joint of a turkey leg, and is anywhere from 1/8 to ? inch in thickness.
Articular cartilage consists primarily of water, collagen, and other molecules known as proteoglycans. Articular cartilage is extremely resilient and is designed so that it can withstand both shear (side) and compression force. This special cartilage can sustain the enormous force exerted through the knee when you are running on pavement (which could be up to seven times your body weight!) or when you are twisting or pivoting in a sideways motion.
Articular cartilage may be a biomechanical wonder, but it is not immune to injury from overuse or abuse. In fact, injuries to the articular cartilage are the most common knee injury. There is some evidence that continual stress on the articular cartilage may make it more prone to injury. In addition, over time, the articular cartilage can simply wear out. In fact, by middle age, nearly everyone will show some degree of damage to their articular cartilage, and if this damage becomes severe enough, it may develop into arthritis. In fact, all forms of arthritis—from the inflammatory variety such as rheumatoid arthritis to “wear-and-tear” osteoarthritis—involve destruction of the articular cartilage.
Problems involving the articular cartilage are one of the most frustrating in all of medicine. Unlike ligament or meniscal injuries, once the articular cartilage is damaged, there is little that can be done. Unfortunately, articular cartilage cannot be replaced or made synthetically. Although not from lack of trying, researchers have been stymied in their attempts to stimulate the body to make it on its own. There are some treatments, however, that I will discuss later in this chapter that may make patients more comfortable and some promising new ones that may actually help the body to better heal itself.
There are four basic types of injuries involving articular cartilage.
Chondral fracture. If you fall directly on your knee or bang your knee very hard, it could result in a fracture of the articular cartilage that does not involve bone. The knee may feel painful and swollen. In this case, the articular cartilage may not be permanently damaged, rather it may simply have become compressed—similar to the way plastic can bend under pressure—and will return to its normal shape immediately. However, there might still be microscopic parts of the articular cartilage that are permanently damaged. The pain is variable and often does not reflect the extent of the injury. Because articular cartilage has no nerve endings, the pain is secondary to abnormal stress placed on the bone that the articular cartilage covers—the subchondral bone. The normal forces of daily activity become abnormal because the shock absorber is damaged. The bone is subsequently stressed, and the patient might experience pain. Fortunately, the bone will eventually remodel itself to withstand the new stress, and the pain will subside, but it could take up to 6 months in some patients.
An MRI cannot “see” articular cartilage, but it will show a tremendous amount of hyperemia, or blood flow, to the injured area, which suggests a chondral fracture.
Chondromalacia. Chondromalacia refers to the softening of the articular cartilage. Articular cartilage is arbitrarily graded from 0 to 4, 0 being normal and 4 being the most damaged. A healthy, smooth surface—the kind that would be found in an uninjured surface—is graded 0. Grade 1 means that there is some blistering or disturbance on the surface; grade 2, the surface is scratched or fissured; grade 3, the Assuring is deeper, almost down to bone; grade 4, the surface is worn away to the bone and the bone is also worn out.
Arbitrarily, if there is destruction to the articular cartilage surface and its appearance to the naked eye is indistinguishable, the terminology is different depending on the patient’s age. In those patients under thirty, damage is termed chondromalacia grade 0 to 4, whereas over thirty years of age, it is called osteoarthritis.
We still have a lot to learn about articular cartilage, but it appears that once the surface is damaged, it is more vulnerable to repetitive wear and tear, destruction, and overuse phenomena. Professional hockey and basketball players invariably have abrasive wearing of the articular cartilage due to the magnitude of the forces and the repetitive nature of the activities involved in their careers. Amateur athletes similarly might suffer from overuse wear and tear, which is technically arthritis.
Traumatic chondromalacia. A significant blow to the knee, such as one that might occur in a football injury, could tear off either a small piece of articular cartilage or a large fragment containing a piece of bone directly under the subchondral surface. This is called an osteochondral fracture. Such a severe injury would cause much discomfort, and if the bone fragment is visible on X ray, it might require surgery.
Osteoarthritis. In older patients, pain and perhaps swelling of the knee in the absence of other injuries could suggest osteoarthritis, a condition that is caused by the gradual wearing down of the articular cartilage.
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