PROPERTIES OF DENTAL MATERIAL 2

THERMAL DIMENSIONAL CHANGE

Restorative dental materials are subjected to temperature changes in the mouth. These change result in dimensional changes  in the materials and to the neighbouring tooth structure. Because the thermal expansion of the restorative material usually does not match that of the tooth structure, a differential expansion occurs that may result in leakage of oral fluids between the restoration and the tooth.

The thermal coefficient of expansion is not uniform throughout the entire temperature range and is usually higer for liquids than for solids. The thermal coefficient of expansion for a solid, such a dental wax, generally increase at some point as the temperature is increased. The linear rather than the volumetric of thermal expansion usually is reported.

The relationship between the coefficients of thermal expansion of human teeth and restorative materials is important. The values for amalgam and composites are about two to five times those human teeth. The values for unfilled plastics, however, are five tp seven times those teeth, with porcelain being one half to one third and gold alloys being approximately the same as for human teeth.

A clinical effect of this difference is as follows. If a tooth that contains a nonadhesive plastic restoration is cooled by the drinking of a cold liquid, the restoration contracts substantially more than the tooth, and a small space results at the junction between the two materials. Oral fluids can penetrate this space. When the temperature returns to normal, this fluid is forced out of the space. The phenomenon is called percolation and occurs with a number of restorative dental materials, depending on the relationship of the thermal coefficient of expansion of the material and human teeth and the extent of bonding. Percolation is thought to be undesirable because of possible irritation to the dental pulp and recurrent decay. Dental amalgam is unusual in that percolation decreases with time after insertion, presumably as aresult of the space being filled with corrotion products from amalgam.

THERMAL CONDUCTIVITY                           

                Qualitatively, materials have different rates of conducting heat; metals have higer values than plastics and ceramics. When a portion of a tooth is replaced by a metal restoration such as amalgam or gold alloy; the tooth may be temporarily sensitive to temperature changes in the mouth. Individuals who wear orthodontic appliances or complete denture also notice temperature effects different from those experienced without these appliances.

                Thermal conductivity has been used as a measure of the heat transferred and is defined as the numer of calories per second flowing through an area of 1 cm² in which temperature drop along the length of the specimen is 1⁰C/cm. This is rather complicated quantitative term, but qualitatively it is related simply to the rate of heat flow.

                Human enamel and dentin are poor thermal conductors compared with gold alloys and dental amalgam, although amalgam is substantially lower than gold. Zinc oxide-eugenol and zinc phosphate cements approximately replace lost tooth structure with respect to thermal conductivity. The reason for using cements as thermal isulating base in deep cavity preparations necause although dentin is a poor thermal conductor, a thin layer of it does not provide enough thermal insulation for the pulp unless a cement base is used under the metal restoration. Composite restorations have thermal conductivities comparable to tooth structure and do not present a problem with this property. Cavity varnishes have low thermal conductivities, as do unfilled acrylics, but are used in layers so thin that they are ineffective as thermal insulators.

Taken from : Dental Material, Robert G. Craig