Injectable gel that could speed repair of torn cartilage

Dec 13 2004

In a project that will likely be watched by football players, runners and other athletes, researchers at MIT and Harvard Medical School say they are developing an injectable gel that could speed repair of torn cartilage, a common sports injury, and may help injured athletes return to competition sooner.

The technique uses the patient’s own cartilage-producing cells and has the potential to be more effective and less invasive than conventional cartilage repair techniques, which may include extensive surgery, they say.

When the liquid mixture is injected into areas where cartilage is torn, such as a knee joint, the material hardens into a gel upon exposure to ultraviolet light, leaving the transplanted cells in place so they can grow new cartilage where it is needed. The biodegradable material will be described in the Jan. 10 issue of Biomacromolecules, a peer-reviewed journal of the American Chemical Society, the world’s largest scientific society.

Torn cartilage is an extremely painful, hard-to-heal injury, particularly since cartilage does not regenerate on its own. It most often occurs as a result of traumatic injuries, as during sports, and is most common in the knee joint, but the condition also can occur as a result of normal daily activity. Conventional treatment methods include rest, pain medication and, sometimes, invasive repair surgery. Patients undergoing surgery can face a slow, painful recovery.

“Using a patient’s own cartilage-producing cells, our goal is to place the cells into our new gel and inject them into the injury site so that cartilage grows where it is needed,” says study lead author Jason A. Burdick, Ph.D., a postdoctoral fellow in the Department of Chemical Engineering at the Massachusetts Institute of Technology in Cambridge, Mass. “The gel itself won’t initially replace damaged cartilage, but will provide an optimum growth environment for implanted cartilage-producing cells so that new cartilage can be formed and help restore function.”

The gel material itself is composed of a natural polysaccharide called hyaluronic acid which is modified with photoreactive groups (methacrylates) and a photosensitive molecule. Burdick compares the procedure, in which the injectable liquid is turned into a gel, to “making Jell-O.” But instead of using cold temperature for gelation, this technique uses light, which he says is a much more rapid and controlled process. Don’t look for the gel anytime soon, he warns, as the research is in its early testing stages and could take at least five years before it’s available to consumers. “We would eventually like to make a material that is as strong as cartilage in order to bear the load of the joint immediately after implantation,” Burdick says, “but we’re not quite there yet.”

In a proof-of-concept experiment, the researchers implanted the material, using cartilage-producing cells (chondrocytes) obtained from the ears of pigs, under the skin of a small group of mice, which are commonly used to examine cartilage formation in a physiological environment. The material was gelled with exposure to ultraviolet light and cartilage formation was examined over the course of three months. The material produced progressively higher amounts of healthy new cartilage during the study period, according to the researchers. Although ultraviolet light is used in the current gelling process, the process eventually can be performed using visible (regular) light, the scientists say.

Because the starting material is liquid, it potentially can be used with arthroscopic surgery instruments for a less invasive procedure, Burdick says, adding that the material will likely work best for repairing small, localized cartilage defects and injuries. As torn cartilage often accompanies damaged ligaments, the technique also could be used to improve the outcome of ligament repair surgery, he says.

Burdick predicts that it might one day be possible to use the technique to repair worn cartilage covering the large surface area of joints, as occurs in arthritis. The same material, he says, also can be used as “clay” to custom-mold new cartilage outside the body for eventual use in plastic surgery reconstruction, such as building new ears and noses.

The research team soon plans to test the new material on actual animal models with torn cartilage. If those tests are successful, human studies could eventually follow. Until then, no one knows how fast it will repair damaged cartilage in humans, but researchers are optimistic that it will be much speedier once the technique is optimized. For injured athletes anxious to resume their career, the new material could make a difference.

The National Institute of Dental and Craniofacial Research provided funding for this study. The project is one of several biomaterial innovations being developed in the laboratories of study leader Robert Langer, Sc.D., one of the pioneers in tissue engineering and biomaterials research.

Other researchers involved in this study include Xinqiao Jia, Ph.D., of MIT; Cindy Chung, B.S., currently a Ph.D. candidate at the University of Pennsylvania; and Mark A. Randolph, of the Harvard Medical School.