Articular Cartilage Injury
Reviewed by Dr. Jack Farr
Three bones meet within the knee joint: the femur, or thighbone, the tibia, or shinbone, and the patella, or kneecap. Like many other joints or "articulations," the bones in the knee are covered with articular, or hyaline, cartilage. Along with articular cartilage, two other types of cartilage may be found in the knee: meniscal cartilage (in the menisci, which act as shock absorbers at the top of the tibia), and fibrocartilage, or scar cartilage. Articular cartilage, however, has unique chemical and physical qualities that makes it very functional as a joint surface. When functioning properly with articular cartilage surrounding the bones and joint fluid as a lubricant, there is less friction in the joint than water on ice. Articular cartilage helps the bones glide easily past each other during knee motion, and serves as a thin protective layer to the bones it covers.
Articular cartilage resists wear quite well, but time and wear can take their toll. When it does break down, the condition is known as degenerative arthritis or arthrosis. If extreme wear occurs, the bones may actually grind on each other, resulting in pain and stiffness. Since hyaline cartilage has such poor healing qualities, arthrosis is rarely left to "heal itself." Researchers continue to examine ways to protect the cartilage from wear and induce healing when it is injured. At the present time, however, the healing response is most often stimulated by a "microfracture" procedure, in which small breaks are made in the underlying bone. Unfortunately, however, this results in fibrocartilage (scar cartilage), which is not as smooth as hyaline cartilage.
Articular cartilage problems are common conditions affecting the knee. These often fall into or more of the following categories:
- Chondral Fracture
An articular cartilage (chondral) fracture is occurs when there is trauma to the joint surface without breaking the underlying bone. This most often occurs when the bones are forced to slide across one another with marked force. This can occur, for example, where the tibia and femur meet, and may be caused by a twisting force, as happens when the Anterior Cruciate Ligament tears. A chondral fracture may also occur where the kneecap and the femur meet, and may be caused by a subluxation or dislocation. Direct impact usually does not cause this type of injury.
Softening (malacia) of the articular cartilage (chondral) may be termed Chondromalacia. Before more modern understanding of the knee came about, the term "chondromalacia" was used to describe all general knee pain. Although this use is errant, patients may still see it in use today. The more general term is "chondrosis," or disease of the chondral tissue. Scientists and surgeons divide chondrosis into four categories, ranging from "softening" to "fissured" to a "crab meat" appearance, and, finally, exposed bone, when all the chondral tissue is absent. Any process that injuries the articular cartilage whether chemical or mechanical may result in chondrosis.
Osteochondral fractureA stiff blow to the knee may cause a piece of articular cartilage to break off and pull with it a piece of the underlying bone. This is known as an osteochondral fracture, and creates a "loose body," a piece of tissue that floats inside the joint. A loose body may rub against the tissues within the knee, causing pain and locking. A surgeon can often place the piece back into its original position and secure it in place. Since they alter the joint surface, osteochondral fractures can increase the force on the remaining tissues.
Degenerative Joint Disease (Osteoarthritis) At one time, doctors believed osteoarthritis to involve only older patients. However, a more modern understanding describes it as multifactorial, involving age, alignment of the joint, and the weight of the patient. The knee can be compared to an automobile's tires. If the car is out of alignment or overloaded, the tires may wear out quickly. In addition, some tires are also poorly made at the factory, and may be destined to fail. Alignment abnormalities can often be corrected surgically to slow the progression of joint degeneration. Advanced bone degeneration causes exposed bone, which leads to pain and dysfunction.
Professional athletes, such as football, basketball, or hockey players, are at risk for developing articular cartilage injuries, due to the high forces their actions generate on the knee joint. However, one need not be a professional to elevate your risk. Any factor that increases the load on the articular cartilage may be a risk factor. Absolute loading such a body weight has been shown to cause articular cartilage wear. Relative load increase occurs with abnormal alignment (for example, very bowed knees), or alteration of the contact area one bone has on another such as when a meniscal cartilage is lost through tearing. There may be certain biologic and genetic risk factors that will be able to be detected in the near future.