What's What in the Knee?
By Steven A. Marciniak, M.D.

It is common to hear someone say that he or she has torn a cartilage in the knee. Sometimes it is said that there is a torn meniscus. So just what are knee cartilages all about?

There are actually two different types of cartilage in the knee. The first is called articular cartilage. It is a white, hard, smooth surface that covers the ends of the bones within the joint. It's a lot like a hard plastic that gets very slippery (to decrease friction) when it is coated by the oil-like fluid within the joint. The second type of cartilage is called meniscus. Rather than being hard and smooth like plastic, a meniscus is more like a rubbery cushion situated between the bones. A meniscus is shaped like a quarter moon and is thicker toward its outer rim and tapers to paper thin toward the middle to create disk-like effect to help the rounded contour of the lower femur (thigh bone) conform to the flat configuration of the upper tibia (shin bone). There is one meniscus on the inner side of the knee (medial) and another on the outer side (lateral), and together they are called menisci. They serve to help distribute pressure evenly over as large an area as possible to minimize wear on the articular cartilage.

When we talk about a torn cartilage we are referring to a tear of one or both menisci. Tears can result directly from an injury but can also be the result of simple wear and tear on the knee. Symptoms may range from a minor ache to severe pain and may be associated with swelling, locking or giving away. Often symptoms are worse with twisting motions or full flexion of the knee such as squatting. Quite often the symptoms are intermittent. If the torn piece of cartilage sits where it belongs in the joint it may cause no problems. But if it moves or flips and gets caught between the bones, any or all of the symptoms may occur.

Torn cartilages are diagnosed by a combination of history and physical examination and often an MRI is performed to confirm the diagnosis as well as to assure the nothing else (torn ligaments, etc.) is wrong within the joint. Most commonly a torn cartilage will require surgery to remove or repair the tear. If left untreated the tear is likely to become larger and actually cause damage to the articular cartilage that the meniscus was designed to protect. Surgery for a torn cartilage is arthroscopic surgery done on an outpatient basis through three small incisions using a small camera to look into the joint and small instruments to work on the cartilage. In a small number of cases the cartilage can actually be repaired but in most cases the torn portion needs to be removed. That determination is made at the time of surgery and depends on the location, size and severity of the tear. If nothing else is wrong inside the knee most people having surgery for a torn cartilage will be able to be back on their normal levels of activity in four to six weeks. Whenever a meniscus it torn, however, some of its protective cushioning effect is lost so that anyone who has had a torn cartilage is more likely that normal to develop arthritis in that knee. Treating the tear early or while it is small reduces that risk.

Articular cartilage, too, can be damaged by either injury or by wear and tear. When the damage is the result of wear and tear it is called arthritis, and that is not the subject of this article. Traumatic injury to the articular cartilage can result in anything from simple cruising to actually knocking off pieces of the cartilage surface. If the surface is bruised or roughened and symptoms persist despite conservative treatment, then arthroscopic surgery can be used to smooth the surface in a procedure like scraping peeling paint to make the surface smooth again. If the full thickness of the articular cartilage is lost then the bone is exposed and the articular cartilage needs to be replaced to cover the bone. Depending on the size and location of the defect as well as the age of the patient and the demands that the patient expects to put on the knee, there are a variety of ways to repair such defects.

Abrasion is an arthroscopic procedure in which the exposed end of the bone is scarped to make it bleed and to stimulate a scar cartilage to grow in and cover the defect. It is useful for small defects or for patients with low demands on the knee. Larger defects or those in more active patients can be treated by actually grafting new articular cartilage is removed from the knee in an initial arthroscopic procedure. The sample is then sent to a laboratory (Genzyme) where the cartilage cells are cultured and grown until they have grown enough cartilage to fill the defect. At a second surgery (not arthroscopic), the cultured cells are placed in the defect, filling it with new articular cartilage. In a different kind of procedure, plugs of articular cartilage can be removed from certain noncritical parts of the knee and transplanted to the area of the defect. Finally, for very large defects, portions of cartilage taken from people who have died (just as kidneys are removed for transplantation) can be transplanted into damaged knees. All these procedures are intended for individuals with localized defects in articular cartilage and are not appropriate for individuals with diffuse arthritis in the knee. Additionally, all these procedures require at least three to four months of restricted weight bearing using crutches or a walker in addition to extensive rehabilitation thereafter.

Not every knee cartilage problem can be repaired but advances as noted above have gone a long way toward restoring years of use to some knees the would of previously have been damaged beyond repair.

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