Restorative Materials
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MATERIAL SCIENCE REVIEW
Ultimate strength is characterized by compressive, tensile and shear strength. On a stress-strain curve it is the point where fracture/failure occurs.
Compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size, as opposed to which withstands loads tending to elongate.
The tensile strength of a material is the maximum amount of tensile stress that it can take before failure. Dentin has a compressive strength of around 300 MPa (megapascals) and a tensile strength of about 100 MPa. Enamel has a higher compressive strength (400 MPa, increased hardness) but lower tensile strength (10MPa, increased brittleness). Restorative materials vary in their strength, but composites and amalgam are engineered to have compressive and tensile strengths that are close to or even exceed dentin and enamel.
The shear strength of a material is the maximum shear stress that the material can withstand before failure occurs.
Flexural strength is defined as the stress in a material just before it yields in a flexure test. Amalgam and composite have more compressive strength than tensile strength. The material you restore the tooth with should have physical properties close to the tooth’s.
Toughness is the total energy absorbed to the point of fracture. A tough material is difficult to break. Toughness is affected by the percent elongation, modulus of elasticity and yield strength.
Brittleness is the opposite of toughness, a vulnerability to fracture. A brittle material may have a high compressive strength but low tensile strength.
Resilience refers to the energy that a material can absorb before plastic deformation.
Stress is the force per unit of area. Increasing the force or decreasing the area it’s applied to will both increase the stress. Strain is the change in deformation per unit of a material when stress is applied. If the same stress is applied, a rubber band will deform far more (greater strain) than say steel.
The Elastic Modulus (or Modulus of Elasticity) is determined by the ratio of stress to strain, and is a measure of the rigidity of a material. The higher the elastic modulus, the more rigid the material. The proportional limit is the maximum stress a material can take when there is a direct/proportional relationship between stress and strain (straight line on the stress/strain graph). The elastic limit is the greatest stress an object can weather without permanent deformation, it will return to its original dimension. Essentially the proportional limit and the elastic limit are the same. The yield point sits just above the elastic limit, a point where the material can no longer return to its original dimensions. Permanent deformation has occurred.
The coefficient of thermal expansion (CTE) is a measurement of the tendency of a material to change shape when subjected to temperature changes. If the thermal expansion of the dental material matches the tooth’s it will prevent stress buildup and leakage. Enamel and dentine have the lowest CTE, followed by direct gold, then amalgam and composite. Unfilled resin has the highest CTE. Percolation refers to fluid moving to and fro between the tooth and restorative material, which can lead to secondary caries. Glass ionomers and gold have a very similar coefficient of thermal expansion compared to dentin. Amalgam and composite expand more than tooth structure.
Thermal conductivity is a measure of how much heat is transferred through the material, or how insulative the material is. Because of their higher thermal conductivity, amalgam and metal (e.g. gold) restorations often require a liner or base between it and the material to provide thermal insulation.
Galvanic shock – an electric charge can be created when two dissimilar metals come into contact within a wet environment. A patient may experience sharp pain.
AMALGAM
Amalgam has been used in dentistry for 150 years and is used because it’s cheap and (perhaps) less technique sensitive. Amalgam is quickly becoming supplanted by adhesive restorative materials.
Amalgam is a dental alloy commonly used to restore teeth. It has a favorable coefficient of thermal expansion but is a poor thermal insulator. Conventional amalgam alloy commonly contains silver (~65% ), tin (~29%), copper (~8%) and other trace metals; current amalgam alloy (high copper alloys) contains silver (40%), tin (32%), copper (10-30%, termed high copper) and other metals. Varying the composition changes the physical properties. Increasing the silver content decreases the working time and creep, but increases the expansion and strength. Increasing the tin content increases the working time and creep, but decreases the expansion and strength. Increasing the copper content decreases the working time and creep, and increases the expansion and strength. It also decreases corrosion. Increasing mercury content increases working time, creep and expansion, but decreases the strength. Increasing the zinc content greatly increases expansion if contaminated with saliva, and prevents oxidation. The components are mixed with mercury which should be within 45-53% by weight. Increasing the mercury will increase the setting expansion and decrease the strength.
Weak condensation during packing or improper mixing will lead to weakened amalgam. Creep describes the gradual dimensional changes as a result of prolonged stress. It is time dependent. Modern amalgam formulas result in no clinically significant creep or flow. Adding mercury to the silver-tin particles (gamma phase) forms the gamma I phase (silver-mercury) which is the strongest resulting phase of amalgam. Adding copper to the amalgam recipe eliminates the weak and corrosive prone gamma 2 phase (tin-mercury) and decreases creep.
Chemical reactions can damage the amalgam. Tarnish results in discoloration at the surface, corrosion deeper in the body of the amalgam. The corrosion byproducts (tin sulfide) does help to prevent marginal leakage, as the restoration ages marginal leakage decreases due to the buildup of corrosion products. If amalgam is contaminated with saliva the water reacts with zinc to form hydrogen gas, leading to greater (delayed) expansion, decreased compressive strength, and increased corrosion and post op sensitivity.
Trituration refers to the process of mixing the alloy (in an amalgamator) to “rub away” the oxide film coating on each alloy particle, allowing mercury to coat the alloy particles instead. If under-triturated the amalgam will come out dull and crumbly, with reduced strength, accelerated corrosion, and increased creep. Over-triturated amalgam will be wet, runny and sticky, with greatly reduced strength, reduced setting expansion time, increased creep and increased corrosion. Properly triturated amalgam will be shiny, wet, homogenous and smooth. If you had to choose between the two, slightly over triturated would be preferred compared to under-triturated. Amalgamation occurs when the constituents are mixed together. Once mixed, there is no free unbound mercury, but if heated to 80 degrees Celsius or above it can be freed and vaporized. Be careful to avoid heat generation during amalgam removal.
Amalgam has the ability to stain surrounding tooth structure by leaching corrosion products into the dentin tubules. The tooth may also be darker because of the dark filling and the translucent properties of a tooth. Elective replacement of the filling may be warranted for esthetic purposes.
CONCERNS ABOUT MERCURY
You will commonly find patients concerned about the mercury content in amalgam fillings. There is a lack of scientific evidence that amalgam poses health risks except for allergic reactions (very rare, 1 in 100 million). The amount of mercury that leeches out of amalgam restorations is very small. The total daily intake of mercury from a dozen amalgam surfaces is about 1-2 micrograms. Compare this to around 7-14 micrograms from one seafood meal, or 9 micrograms/day from all environmental sources.
From the environmental standpoint, however, amalgam is losing popularity. Care also needs to be taken when removing amalgam. An increase in temperature over 80 degrees celcius can liberate mercury. Water cooling alongside high volume suction is essential. The current American Dental Association’s position on amalgam can be found here.
COMPOSITE RESIN
Dental composites are the most frequently used esthetic material because of its appearance and great physical properties. Modifications to the components improves the mechanical properties making it appropriate for many applications, including complicated Class I and II restorations. The benefits of composites include:
- Fantastic esthetics.
- Insulation, poor thermal conductor.
- Strong bond to tooth structure, strengthening of the remaining tooth structure.
- Conservation of tooth structure compared to amalgam.
- Less mechanical retention form needed.
- Minimal microleakage, staining, and postoperative sensitivity.
- Easily repairable with hydrofluoric acid etching and silane coupling.
Composite placement is more technique sensitive compared to other materials. The thermal conductivity is much lower than amalgam resulting in better thermal insulation for the pulp. Micro mechanical adhesion to the tooth surface can be very strong.
Composites are composed of a resin matrix, filler particles and a coupling agent. There are great variations and combinations of these components changing the physical properties of the materials. Initiator, accelerators and pigments are included to improve handling, control and esthetics.
Etching (30-40% phosphoric acid) prepares the surface by creating 10-25 micrometer micromechanical tags in enamel, allowing resin bond infiltration. The surface energy of the enamel is increased, which promotes wetting and adhesion. Etched enamel has a frosty appearance, dull white and chalky. Beveling enamel will allow for a greater surface area of exposed enamel rods improving the bond. Contaminated enamel needs to be re-etched. You cannot over-etch enamel. After etching the bond to enamel is very strong and stable.
Composite resins consist of three main components: the resin matrix, coupling agent, and filler particles. The resin matrices are usually dimethacrylate resins. The most common monomers are bisphenol A diglycidildimethacrylate (BisGMA), its ethoxylated version (BisEMA), triethylene glycol dimethacrylate (TEGDMA) and urethane dimethacrylate (UDMA). There are also non-methacrylate polymers in production (e.g. Ormocers, polymer systems are prepared from alkoxysilanes containing silicone and oxygen atoms, or semicrystalline polyceram (PEX)). You need to be able to visually identify these components.
The coupling agent is usually a silane. This connects the filler particles and resin monomer/polymers. Silane is also used as an extra step when bonding porcelain (e.g. direct porcelain veneers). Initiators include benzoyl peroxide and diketones (camphorquinones). Organic amines are used as accelerators.
Composite activation can be chemical cure, light cure or dual cure. Light activation is the most common and is initiated by a blue curing light emitting 400-500 nanometer wavelengths of blue light (peak is 470). Output should really be over 700mW/cm2. Curing lights can be halogen bulbs, or more commonly LED. The light is absorbed by a diketone (camphorquinone) which leads to an amine activator initiating polymerization by releasing free radicals. Curing time is usually between 20 and 40 seconds, though some curing lights and composites say you can get away with less. The closer the light is to the composite the more energy is imparted, though with older bulbs heat generation is a problem. It’s recommended to keep the curing light within 2mm of the composite to be the most effective. Eye protection is recommended, the light can cause retinal damage. Darker resin shades will see poorer light penetration and take longer to cure.
A composite with chemical activation (Self cure) will use an organic peroxide initiator (benzoyl peroxide) and a tertiary amine activator. The inorganic filler particles are usually colloidal silica, quartz or glass and vary in size, shape and composition. Lithium, barium and strontium are added to enhance optical properties. Composites can be classified according their filler size:
- Megafill – 1-2mm.
- Macrofill – 10-100 micrometer.
- Midfill – 1-10 micrometer.
- Minifill – 0.1-1 micrometer.
- Microfill – 0.01-0.1 micrometer.
- Nanofill – 0.005-0.01 micrometer.
The last two are the most common. Hybrids combine more than one particle size for added physical properties. The larger the filler particles, the stronger the material but the poorer the esthetics (polishability). Small filler particle size results in better finishing and greater resistance to occlusal wear. The higher the proportion of filler particles the harder and stronger the material (higher compressive and tensile strength), the lower the coefficient of thermal expansion, and lower the percentage of contraction. This affects handling characteristics (viscosity). The monomer TEGMA reduces overall viscosity.
Flowable composites contain low filler content (less than 50%) and are used for low stress bearing areas, like cervical restorations, as fissure sealants, or base layers in composite restorations. Condensable (packable) composites contain more than 65% filler by volume and are used in high load areas like class I and class II posterior restorations.
All composites undergo dimensional change upon polymerization, termed polymerization shrinkage. As it shrinks during curing, composites can generate stresses up to 7 megapascals within the tooth. This is reduced as much as possible by increasing filler content, and by certain operative techniques. Incremental buildup of 2mm or less is suggested. The C-factor is the ratio between bonded and unbonded surfaces. A higher C-factor will result in higher stresses imparted to the tooth structure during curing. A low C factor is optimal.
DENTIN BONDING SYSTEMS
When etching enamel for resin bonding, ~37% phosphoric acid etch is used. The lower mineral composition of dentin results in a lower bond strength compared to etched enamel. Etching dentin removes the smear layer and smear plug, decalcifies the intertubular dentin, and opens up dentin tubules. You can over-etch dentin. The excess loss of the mineral components will cause the collapse of protein fibers resulting in an inferior hybrid layer. A dentin conditioner like 10% polyacrylic acid or 25% tannic acid can be used instead of phosphoric acid. A bipolar primer (hydroxyethyl methacrylate/biphenyl dimethacrylate HEMA/BPDM) is added to allow bonding to dentin. The primer makes no difference to enamel bonding and is not necessary or a hindrance. Next, an unfilled resin adhesive is used after the primer, air thinned and cured. This layer can now bond composite. 9% hydrofluoric acid can be used to establish a retentive etched patterns on porcelain. 17% Ethylenediaminetetraacetic acid (EDTA) is a chelating agents used to remove the smear layer within pulpal canals during endodontic treatment. It has limited antimicrobial activity.
In bonding systems where the etching step is completely separate, the enamel can be etched alone (selective etch) or the enamel and dentin together (total etch). Many bonding systems have the etchant built into the formula and doesn’t require a separate step (self etch). Adhesives have evolved from no-etch to total-etch (4th and 5th generation) to self-etch (6th, 7th and 8th generation) systems. For many multi bottle 4th generation bonding systems are still the gold standard. The generations that lack a separate etching and rinsing procedure would not be removing the smear layer. They also focus on the correct etching and priming of dentin and probably under-etch enamel. You can use a selective etch technique to overcome this. There is a slow shift towards the 8th generation bonding systems.
Matrix metalloproteinases (MMPs) may be partially responsible for hybrid layer degradation. Chlorhexidine inhibits MMPs and has been included in some bonding systems. It can also be incorporated as an additional step in the bonding sequence.
FISSURE SEALANTS
The placement of sealants are highly effective for preventing pit and fissure caries. It is safe, relatively easy and conservative. The properties of sealants are closer to unfilled direct resins. They are weak compared to composite resins, but the strength is traded for improved handling, reducing viscosity to allow flow into the pits and fissures. The tooth is first cleaned with a rubber prophy cup and pumice. Consider roughening up the enamel. 30-50% phosphoric acid etch gel is applied for 1 minute, then washed. A bonding agent is applied before the low viscosity resin sealant. Both are light cured separately. Isolation is important. Glass ionomer sealants are also available and used. As long as the sealant stays in place and keeps the fissures 100% sealed caries protection is 100%. They suffocate aerobic bacteria inhibiting the carious process, and prevent new bacteria or nutrients from reaching the fissure system.
GLASS IONOMERS
Glass ionomers consist of fluoro-aluminosilicate glass powder and liquid solution of polymers and copolymers of acrylic acid. It can be fortified by combining it with a resin matrix based system (Resin Modified Glass Ionomers, or RMGI. e.g Fuji II LC). Glass ionomers can be auto/self cure or light cure. The advantages of glass ionomers include:
- Strong chemical adhesion to tooth structure, no bonding agent required.
- Fluoride release and recharge (fluoride sponge).
- Thermal expansion coefficient similar to dentin.
- Thermal insulation.
- Low solubility.
- High opacity on a radiograph.
- Compatibility with other restorative materials. Bonds to composite.
- Conservative preparations compared to amalgam.
- Acceptable esthetics though not as good as composite. Lower polishability.
Structurally glass ionomers are weaker than composites, with lower compressive strength, tensile strength and hardness. They are often used in non-load bearing areas like Class V restorations, or as bases/bulk fill bases under composite restorations. They would not be appropriate to rebuild marginal ridges, incisal edges, cusp tips or other high load areas. Glass ionomers are used in open sandwich or closed sandwich techniques. RMGI are great for pediatric cases and are the material of choice in someone with high caries risk that requires a root surface restoration. Prior to placing a glass ionomer restoration the tooth surface is prepared by adding a 10% polyacrylic acid or 25% tannic acid to remove the smear layer and to facilitate the chemical bond to dentin and enamel. A phosphoric acid etch will reduce the strength of the bond by reducing the potential for ion exchange.
