PROPERTIES OF DENTAL MATERIALS 3

ELECTRICAL PROPERTIES

                Two electrical properties of interest are galvanism and corrosion. Galvanism results from the presence of dissimilar metals in the mouth. Metals placed in an electrolyte (a liquid that contains ions) have various tendencies to go into solution.

Aluminium, alloys of whichare sometimes used as temporary crows, has a strong tendency to go to into solution an has an electrode potential 0f + 1.33 volts. Gold on the other hand, has a little tendency to go into solution, as indicated by an electrode potential of - 1.36 volts. The oral fluids function as the electrolyte, and the system is similar to that of an electrical cell. When the two restoration touch, current flows because the potential difference is 2.69 volts, and the patient experiences pain and fraquently complains of a metallic taste. The some effect can be experienced if a piece of aluminium foil from a baked potato becomes wedged between two teeth and contacts a gold restoration. Temporary plastic crown are used to prevent this problem because they are poor electrical conductors.

Corrosion also can result from this same condition when adjacent  restorations are of dissimilar metals. As a result of the galvanic action, material goes into solution, and roughness and pitting occur. This effect also may occur if a gold alloy is contaminated with a metal such as iron during handling in the dental laboratory or because of variations in concentaration of elements from one part of the restoration to another. Corrosion also may result from chemical attack of metals by components in food or saliva. Dental amalgam, for exapmle, reacts with sulfides and chlorides in the mouth, as shown by polished amalgams becoming dull and discolored with time. This effect sometimes is reffered to as tarnish.

Taken from : Dental Material, Robert G. Craig

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

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.

THE ECONOMICS ROLE OF FINANCIAL INSTITUTIONS

In 1982 the United States insurance industry, including orioerty and liability as well as life and health insurers, invested $52.6 billion of new funds in the United States economy. This made the insurance industry the third largest institutional investor in te United States in 1982, with 10.4% of all new investments. Only commercial banks and federal loan agencies accounted for a larger percentage of investments funds. Life and health insurers alone accounted for 8.2% of all new investment in the United States economy, and Canadian life and health insurers held a similar position in the Canadian economy.

Since a primary role of insurers is to provide protection against risk, many people are unaware of the important part which insurers play in providing funds for a stable economy. Insurance companies receive vastsums of money, largery in the form of premium payments. Successful investment of such money and reinvestment of the proceeds of maturing investments are necessary to guarantee that insurers can fulfill all contractual commitments to policyowners. This continuous infusion of large qualities of money into the economy places insurance companies among the most important financial institutions in the United States and Canada.

Financial institutions are organizations which help to channel funds through an economy by accepting the surplus money of savers and supplying that money to borrowers, who pay to use the money. Payment for the use of money is called interest. This interest is paid to the financial institutions, and the financial institutions transfer a portion of the interest to the savers. Financial institutions take many different forms and have many different characteristics. They include, for example, commercial banks, saving and loan associations, mutual funds, and life and health insurance companies. In a manner of speaking, financial institutions process and trade money just as other industries process and trade soy beans, oil, and iron ore. Money is regarded by financial institutions as a commodity which is borrowed and loaned to satisfy the needs of gaverments, business, and individuals and to encourage the development of an economy.

In their role as financial institutions, insurance companies provide funds for building office complexes, shopping centres, new homes, and apartements. They have even provided funding for major construction projects, such as the Alaskan pipeline. They invest in oil exploration and the development of new sources of energy. Through their different types of investments, insurance companies not only provide financial support for the economy but also provide policyowners with a means of investing their own money and making it grow.

…….taken from Life and Health Insurance Companies as Financial Institutions…….