TISSUE ENGINEERING TAKES MANY FORMS
Clinical problems relating to the loss and/or failure of tissues extend beyond dentistry to all fields of medicine, and are estimated to account for approximately one-half of all medical-related problems in the United States each year.10 Currently, the replacement of lost or deficient tissues involves prosthetic materials, drug therapies, and tissue and organ transplantation. However, all of these have limitations, including the inability of synthetic prostheses to replace any but the simplest structural functions of a tissue. An extreme shortage of organs and tissues for transplantation exists. Fewer than 10,000 organs are available for transplantation each year in the United States, while more than 50,000 patients are registered on transplantation waiting lists.11 Such problems have motivated the development of tissue engineering, which can be defined as a "combination of the principles and methods of the life sciences with those of engineering to develop materials and methods to repair damaged or diseased tissues, and to create entire tissue replacements."12
Many strategies have evolved to engineer new tissues and organs, but virtually all combine a material with either bioactive molecules that induce tissue formation or cells grown in the laboratory. The bioactive molecules are frequently growth factor proteins that are involved in natural tissue formation and remodeling. The basic hypothesis underlying this approach is that the local delivery of an appropriate factor at a correct dose for a defined period of time can lead to the recruitment, proliferation and differentiation of a patient’s cells from adjacent sites. These cells can then participate in tissue repair and/or regeneration at the required anatomic locale.
The second general strategy uses cells grown in the laboratory and placed in a matrix at the site where new tissue or organ formation is desired. These transplanted cells usually are derived from a small tissue biopsy specimen and have been expanded in the laboratory to allow a large organ or tissue mass to be engineered. Typically, the new tissue will be formed in part from these transplanted cells.
With both approaches, specific materials deliver the molecules or cells to the appropriate anatomic site and provide mechanical support to the forming tissue by acting as a scaffold to guide new tissue formation.13 Currently, most tissue engineering efforts use biomaterials already approved for medical indications by the U.S. Food and Drug Administration, or FDA. The most widely used synthetic materials are polymers of lactide and glycolide (Figure 114; see page 310), since these are commonly used for biodegradable sutures. Both polymers have a long track record for human use and are considered biocompatible, and their physical properties (for example, degradation rate, mechanical strength) can be readily manipulated. A natural polymer—type 1 collagen—is often used because of its relative biocompatibility and ability to be remodeled by cells. Other polymers familiar to dentistry, including alginate, are also being used.