PROPERTIES OF DENTAL MATERIALS

An understanding of the physical, electrical, and mechanical properties of material used in dentistry is of tremendous importance. First, materials used to replace missing portions of teeth are exposed to attack by the oral environment and subjected to bitting forces. Second, the restorative materials are cleansed and polished by various prophylactic procedures. As a result, their properties are the basis for the selection of materials to be used in particular dental procedure and restorations. Clinical experience and research have related clinical success to certain properties of materials. Third, the establishment of critical physical properties for various type of dental materials has led to the development of minimum standards , or specifications. The American National Standards Institutes (ANSI) and the American Dental Association (ADA), in conjunction with federal and international standards organizations, have established more than 100 standards or spesificationa, for dental material and maintains lists of materials that satisfy the minimum standards of quality. This information is available from the American Dental Association office in Chicago, or on their website (http://www.ada.org) and is helpful in the selection of materials for dental practice and ensure quality control of certified materials.

DIMENSIONAL CHANGE

Maintaining dimensions during dental procedure such as preparing impressions and models is important in the accuracy of dental restorations. Dimensional change may occur during setting as a result of a chemical reaction, such as with rubber impression materials or composite tooth restorative materials, or from the cooling of wax patterns or gold restorations during fabrication. To compare materials easily, the dimensional  change usually is expressed as a percentage of an original length or volume. A typical example is the linear dimensional change of a polysulfide impression material from a time just after setting until 24 hours after setting. An impression is taken of two marks on a metal plate approximately 51 mm apart; then the distance between the two marks transferred to the impression is measure with a measuring microscope just after the impression sets, l₀  and, again, n24 hours later, l₁ .The result of -0.21% indicates that a linear shrinkage yook place within 24 hours after setting. Values for other elastic impression materials can be used to compare their accuracy. Volumetric dimensional change is more difficult to measure and assumed to be three times the linear dimensional change for a specific material.