Articular Cartilage Disease
Introduction / Anatomy
The loss of articular (joint) cartilage leads to degenerative arthritis (osteoarthritis). The firm, resilient cartilage tissue covers the ends of the bones as they come together to form mobile, gliding joints. This special tissue is somewhat unique in that it has relatively little ability to repair itself. Thus, a great deal of effort is made to protect, preserve and when necessary restore integrity to the articular cartilage surface. Weight-bearing joints, particularly the knee and hip, are prone to injury and wear of the articular cartilage. Due to its relative lack of joint conformity (therefore less stability), the knee is especially vulnerable. The cartilage surface may be damaged in a specific location due to an injury (figure1) or wear and thin in a diffuse, gradual manner. The meniscus, a different type of cartilage, is the firm, gristly tissue shaped in a “C” (inside aspect) or “O” (outside aspect) configuration which is triangular in cross section. These cartilage rings serve to distribute the weight of the thigh bone (femur) over a larger surface and thus protect the adjacent articular cartilage from stress and wear.
Pain may be fairly specific or diffuse in location, depends on the extent of the damage and is often related to weight-bearing activities, especially running or cutting. Joint swelling (“water on the knee”), stiffness, and catching may also be present. If actual fragments of articular cartilage break free from the bone, mobile “loose bodies” may form, become entrapped between the joint surfaces and cause locking.
A careful history and physical examination often suggests the possibility of articular cartilage damage. X-rays usually appear normal with focal disease, but reveal a loss of space between the bones forming the joint with more diffuse or advanced disease. When there is a high index of suspicion for cartilage injury, an MRI (sometimes with accompanying contrast dye) provides the best non-invasive evaluation of the cartilage and underlying bone integrity. At times, an arthroscopic evaluation is necessary to clearly determine the extent of the damage.
A great deal of research into the repair, restoration, and regeneration of damaged articular cartilage. Advances in technology and biological engineering are offering promising new treatment options when the defect is confined to 1 to 1 1/2 square inches of the joint surface.
When the defect is relatively small, the exposed bone is scraped to create an adherent surface. Then, a small “pick” is employed arthroscopically to perforate the bone in the defect. These punctures in the bone are created approximately two to three millimeters apart. Blood and marrow cells are allowed to escape and adhere to the exposed bone. A layer of scar cartilage then forms and serves to provide a measure of cushioning for the bone. This scar cartilage is not thought to be as durable as articular cartilage, but may suffice for smaller defects.
The cartilage defect site for new grafting is prepared by drilling or coring a hole in the bone, approximately 1/4 inch wide (figure 2). A coring device is then used to harvest an identical sized “plug” of bone (osteo) with an attached cap of cartilage (chondral) from a remote area of the knee less prone to weight-bearing stresses. The plug (bone/cartilage graft) is then delivered into the prepared hole to “resurface” the defect with new cartilage. Several plugs of varying sizes are used to create the new joint surface in a “mosaic” fashion (figure 3). “Scar” cartilage then fills in around the individual plugs.
When the size of the defect in the articular surface is too great to permit harvesting of enough tissue from the patient’s own knee, an osteochondral allograft may be employed. Allograft tissues are harvested in a sterile manner from an individual who is recently deceased. A certified tissue bank then stores the various “grafts” until they are needed. This donor tissue is then available to size and match the defect in the patient’s knee using special instruments.
Articular Cartilage Implantation
When the articular defect exceeds a “thumbnail” size, articular cartilage implantation may be an alternative. Small segments of healthy articular cartilage are harvested from the margins of the patient’s own knee arthroscopically. That cartilage tissue (which does not normally regenerate on its own after damage) is sent to a special lab where millions of the patient’s own cartilage cells are carefully grown in culture. After approximately one month, those cells may be reimplanted into the patients own knee. During a second surgery, performed in an open manner, a small sheet of tissue is sewn around the margin of the defect, sealed with a biologic “glue” and then the cells are implanted into the defect beneath the tissue lid. The cells adhere to the exposed bone and begin to generate new articular cartilage.
Mesenchymal Stem Cell Regeneration
One of the promising new techniques uses mesenchymal stem cells (MSC) which are relatively undifferentiated, embryonic-like cells which have the potential to develop into different tissues, depending on the local environment into which they are placed. These MSCs are found in adult bone marrow and in periosteum which is a thin tissue covering the bones. Work is now underway to place MSC into a gel substance which can then be “injected” into the articular cartilage defect. There is hope that the transplanted cells will begin to produce articular cartilage once implanted.
Motion is begun within a day or two of the resurfacing procedure. Weight-bearing is often delayed for six to eight weeks to permit the fragile tissue to begin to mature. Strengthening exercises are initiated during the second month. Running, pivoting and cutting may be delayed from five to nine months depending on the particular procedure and defect size.