CHAPTER 3
The science of dental materials must include a knowledge and appreciation of certain biological considerations that are associated with selection and use of materials designed for the oral cavity.
Strength and resistance to corrosion are unimportant if the material injures the pulp or soft tissue. The biological characteristics of dental materials cannot be isolated from their physical properties.
Biological Requirements of Dental Materials A dental materials should:
1 . Be non-toxic to the body,
2 . Be non-irritant to the oral or other tissues, 3 . Not produce allergic reactions, and
4 . Not be mutagenic or carcinogenic.
Classification of Materials from a Biological Perspective A. Those which contact the soft tissues within the mouth, B. Those which could affect the health of the dental pulp, C. Those which are used as root-canal filling materials, D . Those which affect the hard tissues of the teeth, and
E . Those used in the dental laboratory which though not used in the mouth, are handled and may be accidentally ingested or inhaled.
Examples of Hazards from Chemicals in Dental Materials • Some dental cements are acidic and may cause pulp irritation.
• Polymer based filling materials may contain irritating chemicals such as unreacted monomers, which can irritate the pulp.
• Phosphoric acid is used as an etchant for enamel.
• Mercury is used in dental amalgam, mercury vapor is toxic.
• Dust from alginate impression materials may be inhaled, some products contain lead compounds.
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• Monomer in denture base materials is a potential irritant. • Some people are allergic to alloys containing nickel.
• During grinding of beryllium containing casting alloys, inhalation of beryllium dust can cause berylliosis.
• Some dental porcelain powders contain uranium.
• Metallic compounds (e.g. of lead and tin) are used in elastomeric impression materials.
• Eugenol in impression pastes can cause irritation and burning in some patients. • Laboratory materials have their hazards, such as: cyanide solution for electro- plating, vapors from low fusing metal dies, silicious particles in investment materials, fluxes containing fluorides, and asbestos.
• Some periodontal dressing materials have contained asbestos fibres.
A biomaterial can be defined as any substance, other than a drug, that can be used for any period of time as a part of a system that treats, augments, or replaces any tissue, organ or function of the body.
PHYSICAL FACTORS AFFECTING PULP HEALTH Microleakage
One of the greatest deficiencies of all materials used for restoring teeth is that, they do not adhere to tooth structure and seal the cavity preparation (except those systems based upon polyacrylic acid and certain dentin-bonding agents). Thus, a microscopic space always exists between the restoration and the prepared cavity. The use of radioisotope tracers, dyes, scanning electron microscope, and other techniques have clearly shown that fluids, micro-organisms, and oral debris can penetrate freely along the interface between the restoration and the tooth, and progress down the walls of the cavity preparation. This phenomenon is referred to as microleakage.
Microleakage Can Result In
1 . Secondary caries The seepage of acids and micro-organisms could initiate caries around the margins of the restoration.
2 . Stain or discoloration can also develop.
3 . Sensitivity Sometimes, because of microleakage, the tooth remains sensitive even after placement of the filling. If the leakage is severe, bacterial growth occurs between the restoration and the cavity and even into the dentinal tubules. Toxic products liberated by such micro-organisms produce irritation to the pulp.
Thermal Change
Tooth structure and dental restorations are continually exposed to hot and cold beverages and foods. Instantaneous temperature fluctuation during the course of an average meal may be as great as 85°C.
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The temperature fluctuations can crack the restorative materials or produce undesirable dimensional changes in them because of thermal expansion and contraction.
Many restorative materials are composed of metals. Metals conduct heat and cold rapidly. Patients may often complain of sensitivity in a tooth with a metallic restoration when they are eating hot or cold foods. The problem is more in a very large restoration, where the layer of dentin remaining at the floor of the cavity may be so thin that it is not adequate to insulate the pulp against the temperature shock.
Protection from thermal changes The Dentist must place a layer of insulating cement (called base) under the restoration.
Galvanism
Another cause for sensitivity is the small currents created whenever two different metals are present in the oral cavity (Fig. 24.1, Chapter 24). The presence of metallic restorations in the mouth may cause a phenomenon called galvanic action, or galvanism. This results from a difference in potential between dissimilar fillings in opposing or adjacent teeth. These fillings, in conjunction with saliva as electrolyte, make up an electric cell. When two opposing fillings contact each other, the cell is short circuited and the patient experiences pain. A similar effect may occur when a restoration is touched by the edge of a metal fork.
Studies have shown that relatively large currents can flow. The current rapidly falls off if the fillings are maintained in contact, probably as a result of polarization of the cell. The magnitude of the voltage is not of primary importance, but the sensitivity of the patient to the current has a greater influence on whether he will feel pain. Some patients may feel pain at 10 u amp and other at 110 u amp (average: 20 to 50 u amp). That is why some patients are bothered by galvanic action and others are not despite similar conditions in the mouth.
The galvanic current magnitude depends on the composition and surface area of the metals. Stainless steel develops a higher current density than either gold or cobalt chromium alloys when in contact with an amalgam restoration. As the size of the cathode (e.g. a gold alloy) increases relative to that of the anode (e.g. amalgam), the current density may increase. The larger cathode can enhance the corrosion of the smaller anode. Current densities associated with non Y2-containing amalgams appear to be less than those associated with Y2-containing amalgam. TOXIC EFFECTS OF MATERIALS
A few dental materials may contain a variety of potentially toxic or irritating ingredients, e.g. phosphoric acid in zinc phosphate cements.
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Materials to be utilized in the oral cavity must also be non-irritating to soft tissue. For example, any material used to fabricate an artificial denture should not produce an allergic reaction on the underlying tissue.
Toxicity Evaluation
Toxicity test are classified as:
Level I Tests (Screening Tests)
The material is first checked for acute systemic toxicity and for its cytotoxic, irritational, allergic and carcinogenic potentials.
• Acute systemic toxicity test is conducted by administering the material orally to laboratory animals. If more than 50% of the animals survive, the material is safe.
• Cytotoxic screening may be done in vivo or in vitro. In vitro tests are conducted on cultured cells like mouse L-929 fibroblasts and human Hela cells. There are many in vitro tests. Example Agar overlay technique—Agar is spread over a layer of culture cells in a culture plate. The test material is then placed on it and incubated. A toxic material will show a clear zone of dead cells.
• Irritational properties are checked by placing the material beneath the skin in rats or intramuscularly in rabbits. The animals are killed at different time intervals. The tissue response is then examined and compared.
• Allergic potential The material is first placed inside the skin of guinea pigs. Later, the material is placed on the skin surface. Erythema and swelling at the site show allergic reaction.
• Carcinogenic potential (i) In vivo tests A material is placed beneath the skin (subcutaneously) of mice. They are then killed after 1 and 2 years and examined for tumors, (ii) In vitro tests Include Ames test. Here the material is tested with the help of mutant histidine dependent bacteria.
Level II (Usage Tests)
The material is tested in experimental animals similar to how it is used in humans, e.g. Pulp reaction is studied by placing the material into class V cavities in teeth of primates (apes or monkeys). The teeth are then extracted periodically and compared with negative controls (ZOE cement) and positive controls (silicate cement).
Level III (Human Trials)
Once the material has passed screening and usage tests in animals, it is ready for trials in humans. The reactions and performance under clinical conditions are studied.
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INFECTION CONTROL
There is increased interest in expanding infection control measures to the dental laboratory. Concern over possible cross contamination to dental office personnel by micro-organisms, including hepatitis-B virus and human immunodeficiency virus (HIV), through dental impressions has promoted the study of the effect of disinfecting techniques on dental materials.
Infection Routes
There are many ways by which micro-organisms can spread, e.g. 1 . Contaminated instruments and needles
2 . Direct splashing of saliva and blood into the mouth or on to wounds 3 . Breathing of contaminated aerosol from the air-rotor handpiece 4 . Through contaminated dental materials.
Except for contamination occurring through dental materials, the other routes are beyond the scope of this book.
Disinfection of Dental Materials
Impressions are the main source of spread of infection among the dental materials. However disinfecting impression materials is more complex. The disinfectant must not affect its properties and accuracy. If the impression has not been disinfected, we must disinfect the cast.
Materials may be disinfected by: 1 . Immersion in a disinfectant 2 . Spraying with a disinfectant