Dental Materials

Rod Salter Australia

We will briefly discuss the categories and uses of some of the products most useful in veterinary dentistry. The presentation will focus on the practical clinical use of the materials and their clinically relevant properties and idiosyncrasies. At first, the use of dental materials seems complicated and confusing, yet each material has its own properties and handling abilities that lend it to certain applications. Veterinary dentistry requires materials that can be used to restore as normal function as possible. The forces of mastication placed on animals and the lack of total control we have over them makes restorative veterinary dentistry much more concerned with function than aesthetics.

The ideal restorative material would form a chemical bond to enamel and dentine, be non irritant to pulp and soft tissues, be bacteriostatic, wear at the same rate as tooth structure, have high compressive and tensile strength, not fatigue, have the same thermal expansion coefficient as dentine, form a perfect seal between restorative material and tooth, not distort upon setting, and have ideal aesthetics. Such a material has yet to be invented. Many materials possess a number of these properties, however, and as such, each can be used for certain procedures.

DENTAL AMALGAM

Amalgam remains the hardest and strongest material available today for direct placement restorations. It is easy to use, helps maintain a seal against leakage by developing corrosion at the amalgam-tooth interface (although modern amalgams do this much less effectively), and has withstood the test of time. Use of amalgam in humans has become controversial due to concerns regarding its mercury content and resultant potential health hazard. As in many controversies, opposing views vary widely and are argued passionately by their proponents. A poor aesthetic result is another reason for a decline in amalgam use since it does not match natural tooth structure and darkens over time.

In veterinary dentistry, amalgam can be a good selection for treatment of occlusal table (Class I) caries lesions on molar teeth. In humans, it wears only 5�10 µ per year. The occlusal surface bears large loads and therefore benefits from a hard material. It is also an area in which the required undercuts can be produced with little effect on tooth strength.

As amalgam does not bond to tooth structure, it requires an �undercut� to provide the retention that holds it in place. A lining of a dentin-bonding agent is placed and cured, and then amalgam is triturated in an amalgamator and placed incrementally into the cavity. It is condensed and carved prior to being burnished. The bonding agent is used primarily for sealing the tooth against leakage. If an auto-cure or dual-cure bonding agent is used, then some bonding of the amalgam to tooth structure occurs, but this is a very weak bond. Using a bonding agent is referred to as �bonded amalgam,� but the poor bond strength cannot be relied on to hold the filling in place�it still needs an undercut cavity preparation.

COMPOSITES

The term composite merely indicates that various components or ingredients have been mixed together to create a new substance. In this paper, we will be considering dental restorative composites. These are composed of an acrylic resin (matrix phase), filler particles (dispersed phase) and a coupling phase (which coats the filler particles allowing them to couple with the matrix phase).

The resin most commonly employed in dental restoratives is known as bis-GMA, which is produced by a reaction between bisphenol A and a glycidal methacrylate. The resin is a dimethacrylate monomer, which is induced to polymerize by the presence of free radicals. These free radicals can be generated either by chemical reaction or the introduction of external energy (heat or light).

Chemically activated resins come as two components (two pastes usually). One paste contains a benzoyl peroxide initiator and the other a tertiary amine activator. When the two pastes are spatulated, the amine reacts with the benzoyl peroxide to form free radicals that initiate polymerization.

Light-activated resins come as a single paste in a syringe or compule. The paste contains a photoinitiator molecule (camphoroquinone) and an amine activator. When exposed to light in the 400 to 500 nm wavelength range, the photoinitiator becomes excited and reacts with the amine to produce the free radicals, thereby initiating the polymerization process.

Chemical-cure resins have the advantage of curing throughout their entire mass, but must be placed quickly before they �set� and then may take several minutes to cure sufficiently to allow finishing. Light-cure resins only �set� when exposed to the curing light and so have a much longer working time and then cure more rapidly when exposed. However, only that portion of the material exposed to sufficient intensity of light energy will cure and so deep restorations often need to be layered. Most references suggest that the maximum thickness of composite that should be cured is 2.0 to 2.5 millimeters (depending on the shade of the composite). Therefore, for a 4 millimetre deep defect, you would place a 2 millimeter thick layer, cure it and then add another 2 millimeter thick layer and cure it.

The dispersed phase of composites is composed of filler particles such as quartz, lithium, aluminium silicate, borosilicate, barium and various other glasses. The classification of the composite and many of its physical properties are dictated by the mean particle size.

Conventional composites have the largest mean particle size and may have particles up to 100 microns. Though these composites offered may advantages over the unfilled resins (relative to compressive strength, tensile strength, hardness, thermal expansion), they did not polish to a smooth surface. Abrasion tends to remove the softer resin matrix, leaving the large filler particles exposed. Microfilled composites use much smaller particles and are therefore more highly polishable. However, they have a higher percentage of resin matrixes than conventional composites and so are as neither hard nor as strong. In humans, they make very nice esthetic restorations for anterior teeth, but in the mouths of the veterinary patient, they have little application. Small particle composites are an attempt to achieve the high polishablility of the microfilled in combination with the strength and hardness of the macrofilled. Hybrid composites constitute a better compromise. Most contain a mixture of colloidal silica (microfiller size) and ground glass particles (0.6 to 1.0 microns). These composites have good strength and can be polished to a smooth surface, making them a good general-purpose restorative.

No composite material bonds to dental tissues, therefore, physical union with the tooth is achieved using a dental bonding agent of some sort. They will not be discussed in this paper.

One of the most important disadvantages of composite restorative materials is polymerization shrinkage. As the resin matrix polymerizes, the organized polymer molecule occupies less space than its disorganized constituent monomers did. Therefore, as the composite cures, it shrinks and so may pull away from the cavity walls. Chemical-activated composites tend to shrink toward the centre and so pull away from all walls equally. Light-activated composites tend to shrink toward the light and so will pull away from the walls furthest from the light. This pulling away from the walls can lead to marginal leakage and eventual failure of the restoration. Dental bonding systems help to overcome this tendency to pull away from the walls but do not always prevent it completely.

Another method advocated for reducing the effect of polymerizarion shrinkage is to place the composite incrementally. That is place a small amount, cure it, and then place another small increment, repeating the process until the defect is filled.

The degree of shrinkage is somewhat dependent on the percentage of filler in the composite (most hybrids are 60 to 65% filler by volume). As the filler does not shrink, more filler means less shrinkage.

GLASS IONOMER CEMENTS

Glass ionomer refers to a group of materials that use silicate glass powder and an aqueous solution of polyacrylic acid. The components must be carefully measured and mixed. They may come in bottles with measuring devices, or in ampules with pre-measured amounts of liquid and powder. The ratio is important, so the most reliable form is as pre-measured ampules. Bonding to dentin is felt to occur both by iononic and micromechanical forces. To place a glass ionomer restoration, the dentin and enamel are acid etched with a �conditioner� (usually polyacrylic acid), then rinsed. The tooth is slightly dried (not bone dry) and the material placed. It must be placed while the surface of the material is still shiny, or it will not bond to the dentin. A varnish or unfilled resin should be applied immediately after placement to protect it from either desiccation or water sorption, both of which can adversely affect its final properties. After the first stage set is complete (minimum of five minutes), the varnish is removed and the restoration can be shaped, sanded, and smoothed. Then a surface coat of varnish, or preferably unfilled resin, should be placed, again to protect against desiccation and moisture. The final set to end hardness takes 24 hours.

Glass ionomers have the advantage of chemically bonding directly to enamel and dentin. There is an ionic bond formed between the calcium of the tooth and the set material. As enamel is richer in calcium, the bond to enamel is stronger than the bond to dentin. The bond strength of glass ionomer is much lower than that of bonding agent/composite systems, but in some situations, huge bond strength is not the top priority.

Glass ionomers have other desirable properties and one is fluoride release. Since westernized humans are susceptible to caries, a restorative that releases fluoride and helps to prevent secondary caries under the restoration is desirable. How important this fluoride release is in veterinary patients might be open to debate. Another desirable feature is relatively good biocompatibility. This allows glass ionomers to be placed close to or even under the gingival margin with minimal reaction (if the margins are finished well).

In general, glass ionomers are weaker in compression and tension than composites and are less abrasion resistant. However, in non-occlusal, low abrasion areas they are quick and easy to place and have done well clinically.� Glass ionomers may be reinforced to become �cermets� (silver or silver amalgam powder mixed into original powder or pure metallic silver fused to glass powder) for increased strength, more radio-opaque and increased abrasion resistance).

Compomers are mixtures of glass ionomers and composites. The goal was to provide some properties (such as fluoride release, autocuring, and dentin bonding) to composite materials, or to provide higher strength, polishability and esthetics to glass ionomers. In implementation, the materials do possess a practical mix of properties, but none are as pronounced as the same properties in the parent material. They tend to have lower wear strength than composites.

SUMMARY

The goal of veterinary dental restoration is to restore health and function to compromised teeth. An additional goal is to preserve as much of the tooth as possible during the restoration procedure�i.e., to remove as little tooth as possible during preparation of the defect. Amalgam is strong, but requires a little extra tooth removal to create mechanical retention. Glass ionomers bond to dentin, allowing less tooth removal for preparation, but they are not as esthetic as composites and wear faster. Composites are strong, and can be bonded with an adhesive to avoid excess tooth removal. Even with glass ionomer and bonded composites, a slight undercut for extra mechanical retention can be helpful.

REFERENCES ARE AVAILABLE ON REQUEST