GENERAL Dental materials used in the dental profession are indeed many, varied, and complex. The dental specialist who prepares and uses many of these materials in assisting the dental officer must know their composition, properties, uses, and manipulation. Restorative materials, miscellaneous dental materials, dental waxes, gypsum products, and impression materials are covered in this subcourse. A thorough knowledge of dental materials and the skill to manipulate these materials is one of the important duties of a dental specialist RESTORATIVE MATERIALS - GENERAL Restorative materials are the metallic or nonmetallic materials used to restore diseased or damaged teeth to health and function. Restorative materials have been greatly improved, although a universally ideal restorative material has not yet been developed. The corrosive nature of saliva and the expansion and contraction of tooth structure with changes in temperature make great demands upon a restorative material. The stress brought to bear on the restoration by masticatory forces also makes great demands. Restorative materials must be compatible with living tissue. If used in the anterior region of the mouth, the materials must also be esthetically pleasing. Restorative materials, used when and where indicated, help to ensure the placement of a successful restoration and preservation of the tooth. PHYSICAL PROPERTIES OF MATERIALS Definite and precise terms are used to describe the physical properties of dental materials. These terms must be clearly defined in order for one to understand the interrelationships between physical properties, structures, and composition. The following definitions apply to metals or alloys used in the Army dental service.
a. Hardness. Hardness is the measure of the resistance of a metal to indentation or scratching. It is an indication of the strength and wearability of an alloy or metal.
b. Ductility. Ductility is the measure of the capacity of a metal to be stretched or drawn by a pulling or tensile force without fracturing. This property permits a metal to be drawn into a thin wire.
c. Malleability. Malleability is the measure of the capacity of a metal to be extended in all directions by a compressive force, such as rolling or hammering. This property permits a metal to be shaped into a thin sheet or plate.
d. Flexibility and Elasticity. These terms differ in their technical definition but they are very closely related. Flexibility is the characteristic of a metal, which allows it to deform temporarily. The elasticity of a metal is used when it returns to its original shape when the load or force is removed.
e. Fatigue. Fatigue is the property of a metal to tire and to fracture after repeated stressing at loads below its proportional limit.
f. Structure (Crystalline or Grain Structure). Metals are crystalline and many of their physical properties depend largely upon the size and arrangement of their minute crystals called grains.
(1) Grain size. The size of the grains in a solidified metal depends upon the number of nuclei of crystallization present and the rate of crystal growth. In the practical sense, the faster a molten is cooled to solidification, the greater will be the number of nuclei and the smaller will be the grain size. Generally speaking, small grains arranged in an orderly fashion give the most desirable properties.
(2) Grain shape. The shape of the grains is also formed at the time of crystallization. If the metal is poured or forced into a mold before cooling, the grains will be in a flattened state. Metal formed by this method is known as cast metal. If the metal is shaped by rolling, bending, or twisting, the grains are elongated and the metal becomes a wrought wire.
g. Crushing Strength. Crushing strength is the amount of resistance of a material to fracture under compression.
h. Thermal Conductivity. Thermal conductivity is defined as the ability of a material to transmit heat or cold. A low thermal conductivity is desired in restorative materials used on the tooth whereas a high thermal conductivity is desirable where the material covers soft tissue. METALLURGICAL TERMS a. Cold Working. This is the process of changing the shape of a metal by rolling, pounding, bending, or twisting at normal room temperature.
b. Strain Hardening. This occurs when a metal becomes stiffer and harder because of continued or repeated application of a load or force. At this point, no further slippage of the atoms of the metal can occur without fracture.
c. Heat Softening Treatment (Annealing). This treatment is necessary in order to continue manipulating a metal after strain hardening to prevent it from fracturing. The process of annealing consists of heating the metal to the proper temperature (as indicated by the manufacturer's instructions) and cooling it rapidly by immersing in cold water. Annealing relieves stresses and strains caused by cold working and restores slipped atoms within the metal to their regular arrangement.
d. Heat Hardening Treatment (Tempering). This treatment is necessary to restore to metals properties that are decreased by annealing and cold working. Metals to be heat hardened should first be heat softened (annealed) so that all strain hardening is relieved and the hardening process can be properly controlled. Heat hardening is accomplished in dental gold alloy by heating to 840o Fahrenheit, allowing it to cool slowly over a 15-minute period to 480o Fahrenheit, and then immersing it in water. |
