Biological Properties Of Dental Materials Notes

Biological Properties Of Dental Materials

The science of dental materials must include a knowledge and appreciation of the 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 irritates or injures the pulp or soft tissue.

The biological characteristics of dental materials cannot be isolated from their physical properties. In the early days of dentistry, the patient’s mouth was often the testing ground of dental materials. Modern dentistry, however, involves extensive testing before the material is certified for human use.

Biomaterials

• Many materials used in the mouth are classed as ‘biomaterials’.
• 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.

Read And Learn More: Basic Dental Materials Notes

Biological Requirements of Dental Materials A dental material should

• Be nontoxic to the body
• Be nonirritant to the oral or other tissues
• Not produce allergic reactions
• Not be mutagenic or carcinogenic

Classification of Materials from a Biological Perspective

• Those that contact the soft tissues within the mouth
• Those which could affect the health of the dental pulp, e.g. restorative materials and luting cements.
• Those which could affect the periapical areas of the tooth such as root-canal medicaments, filling materials, etc.
• Those that affect the hard tissues of the teeth.
• Those used in dental clinics and laboratories when handled may be accidentally ingested or inhaled, e.g. alginate dust, mercury vapors, alloy dust containing beryllium formed while cutting metal.

Biohazards related to the 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.
• Monomer in denture base materials is a potential irritant.
• Some people are allergic to alloys containing nickel. Dental applications of nickel alloys include orthodontic wires, fixed and removable partial dentures, etc. Allergies are confirmed using the patch test.
• The frequency of titanium allergy seems to be very rare. Titanium allergies are similar to other metal allergies. They show symptoms adjacent to the area where it is placed. Some patients report worsening health after the placement of titanium implants.
• During the 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, tin, etc.) are used in elastomeric materials.
• Eugenol in materials like restorations and impressions can cause irritation and burning in some patients.
• Laboratory materials have their hazards, such as cyanide solution for electroplating, vapors from low fusing metal dies, siliceous particles in investment materials, fluxes containing fluorides asbestos, etc.
• Some periodontal dressing materials have contained asbestos fibers.

Biological considerations of restoration Design Besides material considerations, the design of the restoration plays an important part in biological response and function. Faulty design is a major cause of recurrent caries, gingival inflammation, periodontal disease, and tooth loss or damage. Every year countless restorations and teeth are lost due to faulty design.

Biological requirements of restoration design The restoration should be designed such that

• It should not impede the natural cleansing mechanisms of the mouth (crevicular fluid and saliva).
• It does not provide a habitat for bacterial colonization.
• It should not trap food.
• It should not trap defoliating epithelial cells lining the gingival sulcus.

In short, a restoration design should avoid ‘biotraps’ and allow natural cleansing mechanisms.

Biotraps in the oral environment Many restorations serve as traps because of improper design or poor clinical skills.

• Amalgams overhang in the proximal regions.
• Overhanging crown margins especially those which are subgingival orin close proximity to the biological width.

• Microleakage due to improper restoration adaptation or bonding.
• Cracked teeth or roots.
• Cracked restorations in the interproximal or subgingival zone.
• Porosity within restorations contacting tissue.
• Uneven or rough surfaces in restorations or prostheses contacting tissue.
• Bridge pontics with saddle or ridge lap designs.
• Biotrapsin the root canal system resulting from incomplete obturation of the root canal system.

Physical Factors Affecting Pulp Health

Microleakage A great deficiency of materials used for restoring teeth is that, they do not adhere to tooth structure and preexisting restorative materials already on the tooth (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, microorganisms, 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

• Recurrent or secondary caries The seepage of acids and microorganisms could initiate caries around the margins of the restoration. Recurrent caries if left unchecked can lead to loss of the restoration and destruction of tooth structure.
• Stain or discoloration can also develop.
• 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 microorganisms produce irritation to the pulp.
• Pulpitis from continued thermal and bacterial irritation.
• Foul smell from trapped and decaying organic matter.
• Inflammation of adjacent tissue caused by microorganisms or their byproducts.

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. 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 on 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. 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 an 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 drops if the fillings are maintained in contact, probably as a result of the 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 µ amp and others at 110 µ amp (average: 20 to 50 µ 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 nonγ2 -containing amalgams appear to be less than those associated with γ2 —containing amalgam.

Classification Of Adverse Reactions From Dental Materials A number of biological responses are possible from materials. However, they may be broadly grouped into

• Toxic
• Inflammatory
• Allergic
• Mutagenic

Fortunately, most materials are screened very early on for toxicity and mutagenicity, therefore, most of the possible responses if any to dental materials usually fall in the inflammatory or allergic category.

Adverse effects may also be classified as

• Local
• Systemic

• A local effect is a result of the direct contact of the material to the regions immediately adjacent to the material. An example of a local reaction is the allergic response of the oral mucosa to the denture seen in some individuals.
• A systemic reaction is caused by the absorption of the material into the body through local absorption, ingestion, or inhalation.

Toxicity Evaluation Toxicity tests are classified as

• Level 1 tests (screening tests)
• Level 2 tests (usage tests)
• Level 3 tests (human trials)

Level 1 Tests (Screening Tests) The material is first checked for acute systemic toxicity and for its cytotoxic, irritational, allergic, and carcinogenic potentials.

• An 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. For example, in the 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 reactions.
• Carcinogenic potential (1) 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 (2) In vitro tests Include the Ames test. Here the material is tested with the help of mutant histidine-dependent bacteria.

Level 2 (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 5 cavities in the 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 3 (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.

Therapeutic effects of dental materials Certain dental materials are utilized for their beneficial biological effects. For example, zinc oxide eugenol cement has a pain-relieving effect on irritated pulp. Calcium hydroxide pulp capping agent promotes the formation of secondary dentin and helps repair dentinal tissue.

Osseointegration

• The osseointegration potential of titanium has been well-documented in the literature. It is this property which allowed the successful use of materials like titanium as an implant material. The surface of titanium forms a very thin layer of oxide which promotes osseointegration.
• Materials that allow osseointegration have a very low degradation rate.
• Osseointegration with intervening connective tissue is called fibrous osseointegration and is generally considered a failure. When the bone closely approximates the implant without intervening connective tissue, it is called osseointegration. If the bone actually fuses with the implant, it is called bio integration.

Effect of pressure on tissues

• It was once believed that pressure from the pontic of a fixed dental prosthesis on the ridge resulted in permanent inflammation based on an article by Stein in 1966 (Pontic-residual ridge relationship: A research report, JPD 1966).
• With the increasing popularity of the ovate pontic, it is now known that pressure results in the re-modelling of the tissues and does not invariably lead to permanent inflammation. Pressure from the pontic may result in short-term injury and inflammation as a result of tissue response to trauma.
• The tissues usually recover in 10 to 12 days. Porcelain restorations were relatively uncommon prior to the 1960’s. Acrylic was commonly used for masking the metal for bridges and saddle pontics were widely used.
• Both saddle pontics and acrylic have been implicated in ridge inflammation and thus the inflammation reported by Stein may have been related to the pontic designor material.

Effect of Pontic design Saddle or ridge lap pontics designs usually result in inflammation as they act as traps. Ovate pontics on the other hand present a convex interface with the tissues with little or no biotrap. It has a bio-cleansing effect on the tissues. Relatively little inflammation is seen underneath a porcelain ovate pontic even when the patient has not performed sub-pontic flossing.

Effect of material – Porcelain versus resin In a fixed dental prosthesis, the pontic is often in continuous contact with the tissue. Among the various materials polished or glazed, porcelain is the most inert. However, the pontic design must be taken into consideration.

Except for saddle design, long-term contact with porcelain shows little or no inflammation. On the other hand long-term contact with resin, and pontics can result in inflammation of tissues regardless of the design.

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 microorganisms, 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 microorganisms can spread.

• Contaminated instruments and needles.
• Direct splashing of saliva and blood into the mouth or onto wounds.
• Breathing of contaminated aerosol from the air-rotor handpiece.
• 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

• Impressions are the main source of the spread of infection among 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
  • Immersion in a disinfectant.
  • Spraying with a disinfectant.
  • Incorporating the disinfectant into the material as part of its composition.
• Immersion in a disinfectant is the most common method of disinfection in the dental office as well as the laboratory. However, certain materials like alginates may be affected if immersed beyond the recommended period. Alginates imbibe water and swell thereby affecting its accuracy.

Contaminated restorations and prostheses

• Crowns, dentures, and other prostheses that have been tried in the mouth are also a possible source of contamination. After the try-in, many of these are returned to the laboratory without disinfection. These contaminate the polishing lathe and the pumice powder used for polishing which in turn cross-contaminate other restorations and prostheses.
• Studies have shown complete dentures are massively contaminated with microorganisms and can serve as the primary source in the cycle of cross-infection within dental laboratories. The polishing of dentures without previous disinfection leads to a high level of transfer of microorganisms.
• Infection control measures such as the use of barriers during polishing, the disinfection of dentures before being sent to the laboratory and upon return to the dental clinic, the disposal or sterilization of the cone after each use, as well as the addition of disinfectant to pumice and unit doses of pumice should be adopted with the objective of reducing the risk of cross-infection.