Can Cartilage Be Replaced?

Surgery for the repair of knee cartilage is commonplace today. It involves removing loose cartilage and smoothing the surface of existing cartilage. But what if cartilage is missing—can a defect in the surface of the cartilage be repaired in such a case? In fact, surgeons in Sweden began performing this type of surgery more than a decade ago. They removed small, matchstick-shaped pieces of cartilage from areas of the knee that did not bear weight and inserted them into areas on the weight-bearing surface where cartilage was missing. This technique worked reasonably well—but what if you did not have enough cartilage tissue to graft?

To address this issue, scientists developed a process for harvesting cartilage cells, growing them outside the body, and then replacing them into a damaged joint. The company that initially developed this laboratory technique, Genzyme Biosurgery, calls its product Carticel. More generally, the surgical procedure of harvesting, culturing, and replacing the cells is called autologous chondrocyte implantation (ACI) or autologous chondrocyte transfer (ACT). The word “autologous” means that the person uses his or her own cartilage cells to grow new cells; that is, the cells are not donated by another person.

The ACI technique involves a two-step surgical procedure. In the first surgery, the surgeon removes a small amount of cartilage tissue. The cells in this tissue (chondrocytes) are then brought to a lab, where they can be grown and the population of cells can be multiplied. When there are a sufficient number of cells (usually after a few weeks), a second surgery is performed, in which the cells are placed in the cartilage defect in the joint. Over time, these cells develop into cartilage and significantly improve the function of the joint.

The disadvantages of this procedure include its high cost and the length of the rehabilitation following the surgery, which can take months of exercise and crutch walking. This technique has been approved by the FDA for people who have experienced cartilage-related injuries from sports or other accidents, but it is not an approved therapy for people with OA.

Research continues on ways to employ ACI to help patients with OA. For example, tissue engineers are working on techniques to grow larger amounts of chondrocytes in a customized mold that has the same size and shape as the damaged joint surface. It is hoped that these cells will be able to replace all of the cartilage in a joint and that their implantation will not require the prolonged rehabilitation period needed today.