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By Hai Lua

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Significant environmental concerns are surfacing regarding the disposal of certain materials used in dentistry, especially mercury. Efforts are also being made to develop an anticaries vaccine. However, even if developed, the widespread use of such an agent may not occur in the foreseeable future in the United States because of the already documented caries reduction from multiple fluoride use, limitations imposed by regulatory agencies, and concern about possible side effects . Whereas the use of such an agent in developing countries may provide greater immediate benefits, its use in the United States may be confined to high-risk patients.

The remaining constituents of tooth enamel are an organic content of about 1% to 2% and a water content of about 4% by weight; these total approximately 6% by volume. Structurally, enamel is composed of millions of enamel rods or prisms, which are the largest structural components, as well as rod sheaths and a cementing inter-rod substance in some areas. Inter-rod substance, or sheath, may be the increased spacing between crystallites oriented differently to where the "tail" portion of one rod meets the "head" portion of another.

These craze lines usually are not clinically significant unless associated with cracks in the underlying dentin. Dentin is not as prone to cleavage as is the enamel rod structure. The tensile strength of dentin is approximately 40 MPa (6000 PSI), which is less than cortical bone and approximately one half that of enamel. The compressive strength of dentin is much higher-266 MPa (40,000 PSI). During tooth preparation, dentin is usually distinguished from enamel by: (1) color, (2) reflectance, (3) hardness, and (4) sound.

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