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  1. 1. BIOLOGICAL CONSIDERATION OF DENTAL MATERIALS AND CUTTING MECHANISMS Introduction Dental materials share with other of biotechnology the problem of biocompatibility; that is the interaction of manufactured compounds with body tissue fluids, biocompatible ace to the Dolland’s Medical Dictionary id defined “as being harmonious with life and not having toxic or injurious effects on biologic function”. Manufacturer and biologic and materials scientists are endeavoring to 1. Develop testing methodology for biocompatibility. 2. Survey a variety of appropriate materials for usefulness in particular biologic context. 3. Evaluate the materials and devices in a clinical setting. The first efforts of the ADA to establish guidelines for dental materials came in 1926 when scientists developed specification for Dental Amalgam. But it was only in the early 1960’s that a committee develop the testing procedure for generalized use. The documents of these tests “Recommended standard practice for biological evaluation of dental material” was published in 1972 and republished in 1979 as document no. 41. 1
  2. 2. A similar document was published by the FDI in 1984. The draft was entitled “Preclinical evaluation of Biocompatibility of Medical devices used in Dentistry” test methods. In general biocompatibility is measured on basis of localized cytotoxicity, systemic responses, allergenialy and carcenogenicity. Based on these criteria, the requirements for dental material biocompatibility include. • It should not be harmful to the pulp and soft tissue. • It should not contain toxic diffusable sabs tat can be released and absorbed into the circulatory system to cause systemic toxic response. • It should be free of potentially sensitizing agents that are likely to cause an allergic response. • It should have no carcinogenic potential a broad sense. Biomaterial can be defined as any substance, other that a drug, that can be used for any period as a part of a system that treats, augments or replaces any tissues, organ or function of the body”. The host environment for dental biomaterial is complicated because of the presence of bacteria and debris in the oral cavity and the corrosive properties of saliva and other fluids. Tests for evaluation of Biocompatibility The purpose of biocompatibility tests is to recognize and eliminate any potential product or component of a product that can cause harm to oral or maxillofacial tissues. 2
  3. 3. Usually these tests are classified on three levels with the most rapid and economical occurring at the primary level. A product with promising attributes is subjected to more expensive secondary tests and finally to expensive tests in animals and humans. Group I: Primary tests Primary consists of cytotoxic evaluation in which dental materials in a flesh or curved state are placed directly on tissue culture cells or on membrane overlying tissue culture cells that leach to effects of products or components that leach through the barriers. Products that are quite cytotoxic can be modified by the manufactures. Genetoxicity tests Mammalian or nonmammalian cells, bacteria, yeast or fungi are used to determine whether gene mutations, changes in chromosomal structure or other deoxyribonucleic acid or generic changes are caused by the test materials, devices and extracts from materials. Group II: Secondary tests The product is evaluated for its potential to create systemic toxially, inhalation toxicity, skin irritation and sensitization and implantation responses. Systemic toxicity test such as the oral medium lethal dose (LD50) test, the test sample is administered daily to rats for 14 days either by oral gauge or by dritary inclusion. If 50% of the animals survive the product has passed the test. Dermal toxicity test is important because of the great number of chemical substances not only dental products, that we contact daily. To stimulate dermal 3
  4. 4. toxicity the test material is held in contact with the shaved skin of albino rats for a period ranging from 24 to 90 days. A primary irritant id capable of producing a infl response in most susceptible pts after the first exposure. Once a toxic material, product or component is identified it can be replaced, diluted, neutralized and chelated to reduce the risk for toxicity. “Irritation is defined as an inflammation brought about without the intervention of an antibody or immune system”. Sensitization is an inflammatory response requiring the participation of an antibody system specific for material allergen in question. The guinea pig is the laboratory material used to establish allergic contact sensitization. The test material is introduced intradermally on the shaved intrascapular region. After 24 hrs the resulting dermal reaction is assessed for the main test, the highest concentration of the test material that causes no more than slight erythema and edema are selected. After an interval of 7 days, the test material of same concentration is placed on gauze patches and applied to cover the previously injected sites fourteen days late the test material is applied to the shaved flank of the animals. After removal of the dressing at 24, 48 and 72 hrs, the skin reactions at the challenged skin sites are evaluated and graded. The inhalation toxicity tests are performed on rats, rabbits or guinea pigs in an exposure chamber with aerosol preparations by releasing the spray material around the upper trunks of the animals. The animals are subjected to 30 sec of continuous spray released at 30min intervals. After 10 consecutive exposures, the animals are observed once a 4 day if any animal dies with 4
  5. 5. 2/3mins the agent is considered very toxic, if none of the animals die, the agent is not likely to be hazardous to humans. Implantation tests For any in vivo implantation technique animal species are selected according to the size of implant test specimen and the intended duration of the test in relation to the life span of the animal. For short term tests (less than 12 weeks) in subcutaneous tissue of muscle animals such as mice, rats, hamsters, guinea pigs and rabbits are commonly used for long term tests (more than 12 weeks) in muscle or bone animals such as rabbits, dogs, sheep, goats with a relatively long life expectancy are used.  For subcutaneous and muscle implementation the test materials is packed into various types of plastic tubes.  For bone implants, holes are drilled using low speed intermittent cutting under profuse irrigation, the cylinders of the test implant material are inserted into the drilled holes using finger pressure. The diameter of the implant and drilled hole should correspond to prevent the ingrowth of fibrous tissue. Histopathologically, the formation of new bone on to the surface of the test implant material without intervening CT is evaluated. Group III: Preclinical usage test Pulp and dentin usage test The test is designed to assess the biocompatibility of dental materials placed in dentin adjacent to the dental pulp. Dogs, miniature pigs with dentition recently erupted and intact permanent teeth are selected. 5
  6. 6. • Class V cavity on labial / buccal surfaces of teeth using sharp burs with adequate air water spray to leave 1mm of tabular dentin between the floor of the cavity preparation and pulp. • The cavities are restored some are retained as control specimens. • As a negative control, some form of 20E is used. • For a positive control a restorative material that consistently induces moderate to secure pulp response is selected. The animals are scarified after 7 days, 28 days, and 70 days. The specimens are graded for a degree of inflammatory response, prevalence of reparative dentin formation and the number of microorganisms entrapped in the surrounding cavity walls and cut dentinal tubules. Test materials that induced least inflammatory response in pulp were considered promising. Less reparative dentin better the material, because more bulk of the dental vital pulp is available to deal with future episodes of caries and dental treatment. Pulp capping and pulpotomy usage tests Similar to those described earlier except that the pulp is mere exposed for pulp capping evaluation and is partially removed for the pulpotomy assessment. A calcium hydroxide product is used as a negative control. Animals are sacrificed at 7 and 70 days. 6
  7. 7. Observations are made of dentinal bridge formation adjacent to or subadjacent to the applied capping material. A bridge directly against the capping material implies minimal destruction of pulp tissue at the time the pulp-capping agent was applied. Endodontic usage test The pulp of the teeth of the test specimen are completely removed and the root canals are replaced with obturating test material and control material. 20E is used as a control material. The animals are sacrificed after 28 days. The teeth are removed together with their surrounding apical periodontal tissues in a single block. The degree of inflammation is evaluated in the percapical tissue for a biocompatible material there should be no response or the shortest resolution time if a response is detected. This is affected by resistance of the material to degradation and dissolution. When dissolution occurs tissue fluid accumulates in the porous areas of the obtrusion material and it may contribute to the growth of microorganism recurrent infection and clinical failure. Pulp response to specific agents and techniques Amalgam Conventional amalgam restoration are considered either inert or illicit a very mild pulpal response. Mercury itself does not seen to contribute to any pulp response. 7
  8. 8. But there has been increasing concern regarding the physical insertion of an amalgam being a major contributing factor responsible for greater responses rather than chemical, toxic or thermal properties of amalgam. • It has been reported that a common histophathologic feature of amalgam restored teeth is a dense accumulator of nutrophils between the predentin and odontoblastic lager. • Some other authors have shown that radioactive mercury reached the pulp in humans after 6 days if no cavity liner was used – they also found that the rate of diffusion of mercury into enamel and dentin was inversely to the degree of mineralization. This implies that in old pts the penetration of mercury ions in less. (Owing to the formation of sclerotic dentin). The penetration was less in toothless water control (non intal teeth). To summarize in a nutshell little inflammatory response is elicited when a cavity preparation is cut using a high speed air water spray technique. However if the practitioner places a conventional amalgam restoration after cutting a cavity preparation at high speed, the pressure of condensation will intensify the initial minimal inflammatory response and it will subsequently increase the formation of reparative dentin to level comparable with that formed after use of low speed air water technique . 8
  9. 9. Visible light cure composite resins The level of pulp response to resin composite restoration depends on the degree of curing. Because an incomplete curing of the resin permits an even higher concentration or residual unpolymerized monomer to reach the pulp. No matter what types of lamps (UV or visible light) are developed for dental profession, insufficient energy is available to cure a large volume or thickness in one application hence curing should be done in incremental layers. Generally an increase in the size of a tooth preparation and the mass of the restoration are associated with greater shrinkage of the restoration. Volumetric shrinkage + polymerization shrinkage is still the overwhelming obstacle in maintaining adhesion and minimizing microleakage. Hence a more conservative cavity preparation incremental placement of resin composite is highly recommended for posterior restorations. 9
  10. 10. Ace to BDJ composite resins have been found to produce an inflammatory response in the pulp when placed in test cavity in animals, strongly allergenic inhibit RNA cele synthesis. Peroxides used to generate free radicals in the polymerization of composite resins are known to be promoters of skin tumors. Free radicals may be involved in development of some cancers and it is known that these chemicals leach out of set composites from both the resin and filler. Bisphenol a has been reported to leach out and been claimed that this compound may mimic the effects of natural estrogens and may attach to oestrogen receptors on cells and their possible contributory role in breast and testicular cancer has also been claimed. Zinc Phosphate Cement Zinc phosphate has been widely used to cement all sorts of castings and applications from crown to orthodontic bands. They have been used in deep cavities to build up dentin since the thermal consecutively of the cement is approximately equal to that of enamel. The fluid component of zinc phosphate cement consists of orthophosphoric acid and small amounts of aluminium + zinc. The powder contains zinc oxide. The set product is integrated amorphous zinc phosphate containing unreacted zinc oxide particles. 10
  11. 11. The usage tests with deep cavities suggest that there could be a moderate to sense localized pulpal damage produced within the first 3 days since it has a very low ph (4.2 at 3min) initially and gradually neutralizes at 48 hrs. any free orthophosphoric acid functions to irritate the pulp particularly in deep cavities with open tubules. There is a marked pulp tissue response when zinc phosphate cement is used for luting purpose. This may be, because the orthophosphoric acid is forced into the dentinal tubules when the pt bites on a tongue blade to seat the restoration. Glass Ionomer Cements The pulp responses to GIC are considered moderate and less irritating. This is attributed to the absence of strong acids and toxic monomers. The polymers used possess higher molecular weighs that limit their diffusion through the dentinal tubules of the pulp. Zinc Polyacrycate Cements The cement was developed to combine the properties of strength of Zn phosphate with the adhesiveness and biocompatibility of zinc oxide eugenol. The acrylic acid ions bind to the metallic ions so tightly that they are not easily reached from the set cement. The freshly set and completely set cements show a low degree of cytotoxicity. One hypothesis is that it is an artifact of tissue culture, because phosphate buffers of tissue culture media reach zinc ions from the cement. Also free zinc ions at a concentration approximately 200mg/ml reduces the rate of cell growth in culture. 11
  12. 12. Toxicity can be reversed by cherating zinc with EDTA (Zn polyacrylate are not bacteriostatic) subcutaneous, bone implants have not indicated toxicity of the cement. Zinc Oxide Eugenol Cement Zinc oxide eugenol cements are least injurious to the dental pulp. Not only there is no irritation produced by the material but actually it exerts a mild palliative and sedative effect on the pulp. It seems to be much a bland substances that it may even lack necessary irritating products to stimulated the formation of secondary dentin formation. Silicate Cement Silicate in set form consist of glass particles covered with a layer of alumino-silica get and a matrix of amorphous insoluble phosphates and fluorides. Most in intro screening tests have shown that silicate cements are markedly cytotoxic, due to the retention of phosphoric acid even after 24 hrs and to some extent fluoride ions in concentrations of 15/25mg / ml reduce cell growth. Silicates implanted into subcutaneous tissue inhibit cell enzyme activity, elicit severe inflammatory response and cause necrosis of tissue with fibrous capsule formation. There has been noticeable degree of acute inflammatory cell infiltration with disruption of odontoblastic cell layer within 1 to 3 days of placement. Response shifts from moderate to severe after 5 – 8 weeks. 12
  13. 13. Recent studies have demonstrated that microleakage around the silicate may be important in promoting pulp response as is reaching of the material. Conditioning Agents Etching agents are used with both resin composite systems and GICs. Etching agents remove the surface contaminants to permit the micro mechanical attachment or the ionic exchange of dental material with the tooth. Beannstrom showed that conditioning of dentin and removal of the smear unit allows the ingress of bacteria and the outward flow of dentinal fluid within the tooth material interfacial region and possibly contributes to formation of a biofilm that interfaces with adhesion. Consequently it is recommended that the smear layer should remain, but in a modified form. So only the surface layer of dentin (10mm depth) needs to be modified and not it’s deeper layers. Hence conditioning techniques that are associated with weaker acids, shorter periods of application and the elimination of rubbing and scrubbing procedure produce a minimal pulp response and satisfactory bonding. Bonding Agents Bonding agents are used to reduce the expected pulp responses induced by the subsequent placement of more toxic resin based composite materials. Bonding agents do not appear to be toxic. Specimens from subhuman primates revealed low to average inflammatory cellular response values at all time intervals. 13
  14. 14. ALLERGIC RESPONSES TO DENTAL MATERIALS Allergic contact Dermatitis Dermatitis usually occurs where the body surface makes contact with an allergen. “Allergen is defined as a substance that is not primarily irritating on the first exposure but produces reaction more rapidly in animals of appropriate genetic constitution on subsequent exposure to similar concentrations”. An allergic contact dermatitis associated with monomers of bonding agents frequently involves the distal parts of the fingers and palmas aspects of the fingertips. Allergy to Latex Products Hypersensitivity to latex may represent a true latex allergy or a reaction to accelerators and oxidants used in latex processing. Theorem – a chemical used in latex processing may cause allergic response. Ammonia used to preserve the rubber sap hydrolyzes and degrades the sap to produce allergens. Leaching of the rubber products by soaking then in hot water brings the allergens the surface and places the highest concentration of allergens next to the skin of the wearer. Reactions range from localized rashes and swelling to more serious types such as wheezing and anaphylaxis. Eczema is the most common adverse reaction. 14
  15. 15. It is also important to know from a detailed history if the pt who is being treated is allergic to latex because the operators glove often makes contact with the skin of the pt. Besides the rubber dam sheet is made from latex. To avoid these adverse responses to latex products of vinyl gloves or gloves made from other synthetic polymers may be used. Allergic Contact Stomatitis It is by far the most common adverse reaction to dental materials. The adverse reactions may be obtained as a local or contact type lesions, but the reaction may be far away from the material site (itching on the palms of the hands or soles). The most definitive diagnostic test for allergic contact dermatitis / stomatitis is the patch test. The suspected allergen is applied to the skin with the intent to produce as small area of allergic contact dermatitis. The test takes about 48 – 96 hrs. reaction may cause hyperemia, edema, vesicle formation and itching. Dental materials contain many components known to be common allergens such as chromium, cobalt mercury eugenol, colophonium, formaldehyde. Resin Base The allergic reactions associated with resin based materials effect not only pts but also the dental personnel working with these materials. The polymerization of composite materials is never complete. The incomplete polymerization of resin restorative material may predispose to material 15
  16. 16. degradation. Degradation and wear of the materials release components of the resin based materials, and these may cause reactions locally and systemically. A postmortem study in dental staff and controls using radiochemical neutron activation analysis. The study stated that a high level of mercury was found in two hyroid specimens. Accumulation of mercury in several organs particularly kidney pituitary gland occipital lobe of brain was seen. Sheep experiments showed the some mercury from amalgam fillings can pass from the mother to the fetus. However human experiments appear to indicate that there is no correlation between the mercury levels in the fetus and number of amalgam fillings in the mother. Allergy to Nickel About 10% of female population is allergic to nickel. This is attributed to greater exposure of females to nickel because all the gold plated jewellary is made with a nickel undercoat beneath the gold plating only 30% of those pts with a known nickel allergy develop a reaction to an inteaoral nickel chromium dental alloy. Toxicity and Allergenicity of Beryllium Berrylliosis is an inflammatory lung disease resulting from inhalation of beryllium dust or fumes. Beryllium controls cast ability and surface oxidation. 16
  17. 17. MICROLEAKAGE A microscopic space always exists between the restoration and the prepared cavity, with the exception of those systems based on polyacrylic acids and possibly certain dentin bonding agents. If leakage is more bacterial growth occurs between the restoration and the cavity wall and extends upto the dentinal tubules. It has been concluded that the toxic products liberated by such microorganisms might produce continuing irritation to the pulp. Thermal Changes Tooth structure and dental restorations are continually exposed to hot and cold beverages and food. Instantaneous temperature changes during the course of a meal may be as high as 65°C. the thermal conductivity and coefficient of thermal expansion of restorative materials are important properties to be considered in pressuring the health of the pulp and in minimizing the increased microleakage that may occur as a result of temperature cycling. 17
  18. 18. Galvanism The presence of dissimilar metals in the oral cavity creates small currents. Thus producing irritation and on many occasions sensitivity to the pulp. When any two dissimilar restorations are wet in saliva, an electric couple exists with a difference in potential between the restorations, when these restorations are brought in contact, the potential is suddenly short circuited through the two alloys. The result is sharp pain. A similar effect may be observed by touching the time of a silver fork to a gold foil or inlay restoration and at the same time allowing some part of the fork to come in contact the oral soft tissue. When the tooth all not in contact a circuit stills exists. Saliva forms the electrolyte and the hard and soft tissues can constitute the external circuit. Role of Dentin Dentin is a dynamic structure with constant fluid exchange throughout the structure. Any changes in the hydrodynamics can result in pulpal reaction. (Ace to Branstrom’s hydrodynamic theory). This can occur by undue desiccation of the surface or by pressure exerted in the placement of a restoration. Instrumentation involved in cutting a cavity preparation produces a tenacious layer of debris / particularly on the dentin. This thin layer or the smear layer provides additional protection to the dentin and the pulp from a potential irritant. 18
  19. 19. BIOLOGICAL CONSIDERATIONS TO CUTTING INSTRUMENTS Pulp reactions to rotary cutting instruments. The most important single factor in determining pulpal response to a given stimulus e.g. Cutting instruments is the Remaining Dentin Thickness (RDT) between the floor of the cavity preparation and the pulp chamber. The heat generated by the cutting instruments during the removal of tooth structure has be generally accepted as the cause of pulp injury RPM, Pressure and Surface area of control. The factors to be considered when pulp reactions are evaluated are. a. Type of Cutting instruments Steel burs produce more heat than carbide burs because of inefficient cutting. Burs and diamond instruments that are dull do not cut efficiently and results in heat production. Spiral fluted burs produce more heat than straight fluted burs. Diamond instruments generate more heat than carbide burs when cutting at high speeds without coolants. 19
  20. 20. b. Method of Application In low speed, high speed era intermittent cutting is the best method of controlling frictional heat pressure more than and ounces contributed to the intensity of pulp injury. c. Coolants Proper use of efficient instruments coolant are required to control frictional heat at the cutting site. The three most important coolants are Air, Water and Air-Water. Air alone was not found to be a consistently protective when less than 2mm of dentin was remaining between cavity floor and the pulp. The use of air coolant can cause desiccation of the dentinal tubules which intensity pulp damage. Air coolants can be used only when visibility is a problem in cavity preparation. E.g. ‘During removal of deep caries or finishing procedures of cavity preparations’. d. Water Copious flow of warm water is effective in controlling temperature increases. It is logical to use water to minimize trauma by keeping warm tissues warm and wet tissue wet. e. Air-Water Coolant Most popular and effective method and has several advantages over a water coolant. Visibility is not as great a problem since less water is required (6 to 8cc/min). The lubricating and cleaning action of the air water spray increases. The cutting efficiency and instrument life. 20
  21. 21. Air water spray is superior to the water stream of subgingival margination because the spray helps to keep the gingival crevice open for better vision. In areas of difficult vision hand pieces with fiber optics are an excellent alternative. The aerosol created by dry cutting with a air coolant is a health hazard at dentist, pt and the personnel present. The odour is disturbing to pts because they think that the tooth is being burned. To Conclude It is imperative for a dentist purchasing a material to know if the material is safe and if it is safe, how safe it is relative to other materials. Dentists, Dental students should know the most likely side effects of materials, whether they affect dental pts or the auxillary personnel and laboratory techniques. They should also invariably recognize mechanisms through which these effects are produced and efforts should be made to minimize it. 21