GLOSSARY
Adhesion - bonding of dissimilar materials by the attraction of atoms or molecules. There is always some attraction between atoms, so adhesive strength is a matter of magnitude. Adhesion can be mechanical or chemical. Chemical adhesion is bonding at atomic or molecular levels and mechanical adhesion is based on interlocking of one surface with another. Chemical and mechanical adhesion can exist together. Differences in the thermal coefficient of expansion or dimensional changes during the setting of an adhesive can weaken adhesive bonds. Adhesion is a principal concern in solving the problem of leakage around restorative materials.
Brittleness - opposite of fracture toughness. A brittle material, however, does not necessarily lack strength.
Cavity classification - Standardized methods of recording the need for restoration exist to facilitate communication among clinicians, researchers, and dental educators. The most commonly accepted means of classifying cavities is by the names of the surfaces involved. Cavity type is classified further based on the type of treatment and anatomical area involved This classification, developed by Dr. G. V. Black in 1908, is designated by Roman numerals as Class I, Class II, Class III, Class IV, Class V, and Class VI.
It is important to remember that the classification relates to location and not size of the cavity. Generally, the more surfaces of the tooth that are involved, the more extensive the destruction; however, a Class I lesion may be a tiny pit in the occlusal (biting) surface of a posterior tooth, barely penetrating the dentin, or it may involve the entire occlusal surface and encroach on the pulp. The size of a restoration has major implications for its life-expectancy, or longevity, especially for the direct restorative materials. The ability of a material to withstand the stresses of the oral environment will be enhanced by conservative preparation size.
Coefficient of thermal expansion - change in length per unit length of a material for a 1°C change in temperature. This coefficient is important in that tooth structure and restorative materials expand when warmed by hot foods or liquids and contract with cold. Such expansion and contraction may result in the breaking of the bond between the tooth and restoration, resulting in leakage.
Color stability - ability to maintain original shade. Can be influenced by wear, water sorption, chemical corrosion, and diet habits.
Corrosion - chemical reaction of a material with its surrounding medium. May result in the release of substances that can be harmful. The oral environment and dental structures present a complex environment that can promote corrosion. The variables of diet, bacterial activity, pharmaceuticals, rinses, smoking, oral hygiene habits, chewing patterns, and chewing strength unquestionably account for a great portion of the differences in corrosion and overall material performance often noted in different patients in whom the same dental alloy, handled in the same manner, was employed.
Creep or flow - a stressed material may sustain immediate plastic deformation and, in the first short period of time, make additional plastic adjustments along flaws within its internal structure. After these initial changes, a slow steady-state rate of strain occurs, which is called creep. This property is associated with the deformation and marginal failure of amalgam restorations under masticatory stresses.
Crown - a restoration of a major part or entire clinical crown of a tooth. It may be made of cast metal, cast metal with a veneer of tooth-colored porcelain baked onto its surface, or a "jacket crown" composed entirely of porcelain or ceramic.
Fatigue failure - cyclic loading causes critical flaws in a material where stress is highest and will eventually cause the material to fail.
Finishability - the ability to be polished to a smooth finish for acceptable feel, appearance, and reduced plaque adherence.
Flexure strength (modulus of rupture) - resistance to bending forces and stresses.
Fracture toughness - ability of a material to absorb and dissipate energy around flaws without allowing cracks to propagate and cause failure or breakage. A tough material generally is strong.
Galvanic action - results from a difference in electric potentials between dissimilar metal filings in opposing or adjacent teeth. Fillings, in conjunction with saliva, make up an electric cell. When opposing fillings touch, they short-circuit the cell and, if the flow of current takes place through the pulp, the patient will experience a sensation or, sometimes, pain. It has been shown that small galvanic currents associated with electrogalvanism are continually present in the oral cavity (Schreiver, 1952). Although postoperative pain due to galvanism is not a common occurrence, it can be a source of real discomfort to the occasional patient. Such pain usually occurs immediately after insertion of a new metallic restoration and generally subsides or disappears, often within a few days.
Hardness - ability to resist wear or indentation.
Inlay - a restoration that fits into a prepared cavity in a tooth and is secured in place by cementation. The inlay usually is cast in gold alloy, but newer techniques allow for fabrication of composite and ceramic inlays.
Onlay - a restoration that restores one or more complete cusps of a tooth and is cemented onto the prepared surface of the tooth. It can be constructed of the same materials as the inlay. It is used generally for coverage of extensively damaged posterior teeth.
Polymerization shrinkage- shrinkage that occurs in the mass as a result of reaction of the polymer and monomer, which results in the hardened restoration. Polymerization shrinkage results in stresses in tooth structure, in composite restorations, and at the interface between the two. It may lead to postoperative pain, open margins resulting in bacterial leakage, and recurrent caries. The magnitude of shrinkage depends on the material, the size of restoration, and the placement technique.
Radiopacity - tendency of a material to be seen on a radiograph. The restoration-tooth interface should be distinguishable on the radiograph in order to detect caries beneath or around a restoration. The radiopacity of the restoration should be greater than the dentin or enamel.
Stiffness (elastic modulus) - measure of the resistance to elastic deformation (reversible) of a material under an applied force. The higher the modulus, the less the elastic deformation for a given amount of stress.
Stress—compressive - internal forces, per unit area, that resist a load which tends to shorten or compress the material. Dental materials rarely fail due to pure compression.
Stress—shear - a stress that tends to resist a twisting motion or a sliding of one portion of a body over another. It is very difficult to induce a pure stress of a single type in a dental material. Most stresses are complex.
Stress—tensile - any induced force, per unit area, which resists a deformation caused by a load which tends to stretch or elongate a material.
Thermal conductivity - relative quantity of heat passing through a specific area of a body when there is a temperature difference. Metals are better temperature conductors than non-metals. Thermal transfer can damage the pulp.
Undercut - internal area of a cavity preparation beveled inward from the occlusal surface. Slight undercuts are necessary for non-adhesive materials, such as amalgam, to provide a retention lock. This necessitates greater removal of tooth structure.
Water sorption - represents the amount of water taken up within the structure of a material during fabrication or while the restoration is in service. Warpage and dimensional change are often associated with a high percentage of water sorption.
Wear resistance - resistance to the loss of substance from the surface under mechanical or chemical forces such as mastication, tooth brushing, acid attack, and corrosion.