Articular cartilage is a tough, elastic tissue that covers the ends of bones in joints and enables the bones to move smoothly over one another. However, when articular cartilage is damaged through injury or a lifetime of use, it does not heal as rapidly or effectively as other tissues in the body. Instead, the damage tends to spread, allowing the bones to rub directly against each other and resulting in pain and reduced mobility.

Advances in technology and biological engineering are giving new hope to the thousands of Americans who annually experience injuries to the articular cartilage of the knee. Several techniques are now using the patient''s own cells and tissues to restore cartilage to weight-bearing sections of bone. These new techniques include:

• Osteochondral Grafting - A plug of bone and healthy cartilage is harvested from one area and transplanted to the injury site.
• Autologous Chondrocyte Implantation - Healthy cartilage cells are harvested, cultivated, and implanted over the lesion.
• Mesenchymal Stem Cell (MSC) Regeneration - MSCs are "undifferentiated," which means they have not yet developed into a particular type of cell (such as bone or muscle). Research suggests that MSCs can be withdrawn from the 'bone marrow, placed in a gel matrix, and implanted at the defect, where they develop into new cartilage.

An osteochondral graft can use either the individual's own tissue (autograft) or a matched graft from another source (allograft). If an autograft is planned, the plug of bone and cartilage must come from a non-weight-bearing area that has little contact with other bones, which limits its application to treating smaller lesions. For larger injuries, an allograft is more appropriate, provided that a tissue match can be found or the graft is processed to modify the genetic differences and help prevent rejection.

Chondrocytes are mature cartilage cells. In this two-stage surgical procedure, surgeons first use arthroscopic techniques to harvest the cells from a healthy, non-weight-bearing area of the knee joint. The chondrocytes are then treated so they will multiply over several days. During the second surgery, the surgeon cleans the injury site and removes a piece of the soft tissue (periosteum) that covers the tibia. The periosteal tissue is sutured and secured over the injury, and the cultured chondrocytes are then injected beneath the patch. There, the chondrocytes will eventually produce a form of cartilage that is very much like the original articular cartilage.

Because autologous chondrocyte implantation uses the patient's own cells, there is no danger of rejection by the immune system. Complications are rare and, in most cases, the procedure results in a restoration of joint movement without pain.

Autologous chondrocyte implantation is not appropriate for every patient. Several factors must be considered in decision making, including the size of the defect, the number and type of previous surgeries, the patient's demands and expectations, the location of the injury, and the presence of coexisting lesions. The patient's age and the reason for cartilage deterioration must also be considered. An older person with advanced osteoarthritis is not a candidate for autologous chondrocyte implantation, but a younger person with a traumatic injury to the knee may be a appropriate candidate.

The newest technique being developed uses mesenchymal stem cells (MSCs). MSCs are relatively undifferentiated, embryonic-like cells with the potential to develop into various types of cells. They are found in adult bone marrow and in the periosteum, a tissue layer over the areas of bone not covered by articular cartilage.

Doctors anticipate harvesting MSCs by a simple bone marrow aspiration and biopsy. Research is being done on the possibility of placing MSCs in a gel, then inserting the gel into the cartilage defect. Because MSCs appear to be capable of organizing in the same way that cartilage is structured, it is hoped that they will be able to regenerate articular cartilage.

Regardless of the technique used, simply restoring articular cartilage to a damaged joint is not enough to ensure symptom relief and full functioning. Other problems such as malalignment or ligament instability must also be addressed, usually at the time of the surgery.

The rehabilitation protocol generally calls for early active and passive motion, including continuous passive motion, and protected weightbearing for several months. Gradually, strengthening exercises, including isometric and quadriceps sets, are added. The progression to full weightbearing may take up to 6 months. Because high-impact activities such as running or basketball can damage the developing cartilage, these activities should not be resumed for 8 months to 1 year.

Currently, osteochondral grafting and autologous chondrocyte implantation are used primarily to treat knee injuries. Articular cartilage, however, is also found in other joints, such as the shoulder and ankle. As ongoing research continues and long-term studies verify the efficacy of these new techniques, they may be applied to other joints to help prevent progressive loss of articular cartilage, reduce symptoms, and restore function.