Chemical and physical properties of tooth-colored restorative materials

The ideal tooth-colored restorative (ie, directrestorative) would have the capacity to

adhere to enamel and dentin,

maintain a smooth surface,

maintain desired color,

resist water (insolubility),

resist wear,

resist fracture,

resemble tooth structure in stiffness,

react to temperature change like other tooth structures,

resist leakage,

maintain marginal integrity,

not irritate pulpal tissues,

inhibit caries,

place easily, and

repair easily.

No available restorative can meet all of these requirements; however, by matching the characteristics of various materials to the needs of a specific tooth, it is possible to approach this ideal for each restoration undertaken.

Clinicians who understand the chemical nature and physical properties of a material are more likely than those who do not to make good deci-sions concerning its use and application. This book begins with a review of basic concepts in material and restorative science to provide a foundation for improved understanding of dental materials.

CHEMISTRY OF TOOTH-COLORED RESTORATIVES

A direct restorative transforms in the mouth from a fluid or putty-like material into a tooth-like solid. There are three common mechanisms by which direct tooth-colored restoratives undergo transformation: acid–base reactions, polymerization reactions, and precipitation reactions.

Acid–base reactions

Many dental materials undergo an acid–base reaction when they set. The resulting material is chemically referred to as a salt. Examples of this reaction in everyday life are commonplace (eg, concrete). The first tooth-colored restoratives to undergo this type of setting were the silicate cements. All acid–base reactions are similar. Acid molecules (a configuration of atoms) have a shortage of electrons, and base molecules have an excess of elec-trons. When acids and bases react, they transfer electrons between them, creating a more stable compound. This exchange of electrons results in heat generation during the setting reaction. Since the ions required to initiate a setting reaction exist only in water, all acid–base materials contain water. Once an acid–base reaction is complete, however, the resulting salt typically does not include water.

There are two types of acid–base reactions: those involving inorganic components and those involving organic components. Examples of an inorganic acid–base reaction are zinc phosphate cement and silicate cement. Examples of an organic acid–base reaction are glass ionomer cement and polycarboxylate cement. Acid–base reactions that contain inorganic components are generally stable outside the mouth in the absence of moisture, whereas those containing organic components are generally not stable. For example, zinc phosphate cement is stable in a dry environment whereas glass ionomer cement is not.

Polymerization reactions

The most common type of setting reaction for direct tooth-colored restoratives involves the formation of resin polymers. A polymer is a molecule or group of molecules made up of repeating single units that are covalently bonded. The individual units of a polymer are referred to as monomers.  “Poly” means many, and “mono” means one. Hence, polymethylmethacrylate is a polymer made up of multiple methacrylate monomers. Polymers form from one or more types of monomer. The process of convert-ing monomers into a polymer is called polymeriza-tion. If two or more different monomers are poly-merized, the resulting material is a copolymer. Combining different monomers cre-ates materials with unique properties that reflect the characteristics of the individual monomers.

All monomers have at least one carbon–carbon double bond (C=C) that becomes a single carbon–carbon bond when they join to form a polymer. Monomers with two or more carbon–carbon double bonds can transform to cross-linked polymers. Cross-linking usually results in improved physical properties in dental materials. Most dental restorative polymers contain cross-linked copolymer components for durability.

An initiation system starts the transformation of monomers into polymers and copolymers. The initiation reaction creates a molecule with a free radical (an unpaired electron). This unpaired electron makes the radical highly reactive. When a free radical collides with a monomer’s double bond, it pairs with one of the electrons of the double bond, leaving the other member of the pair free. Thus, the monomer itself becomes a free radical that can react with another monomer. Ideally, this process continues until all of the monomers become polymerized. The degree to which monomers convert into a polymer is referred to as the degree of conversion. The most common polymers used in the dental field contain methacrylates. Methacrylates with two double bonds are called dimethacrylates. The advantage of a dimethacrylate is that it allows for cross-linking. Most resins used in dentistry have a conversion of about 40 to 60% when polymerized in the mouth and over 60% to nearly 100% when cured in a laboratory.

A, A linear polymer is made up of multiple units of one type of monomer. B, A copolymer includes more than one type of monomer.

A, Cross-linking of polymer. Monomers with two carbon–carbon double bonds make cross-linking possible. B, Polymers with cross-linked components have better durability.

Polymerization shrinkage patterns

All polymerizing resins shrink during curing. Composite resin shrinkage is about 2 to 5% by volume, depending on the filler loading (filler particles do not shrink) and the percentage of conversion. The less the filler loading and the higher the rate of con-version, the greater the shrinkage. In the laboratory, or when cured outside the mouth, chemically cured materials shrink toward their center, because the initiators are mixed throughout the material. Light-cured materials, on the other hand, shrink toward the source of initiation, which is the curing light.

 In clinical use, shrinkage patterns are much more complex. Active bonding agents placed on a tooth usually start the initiation process when the composite contacts them. Some researchers also believe a tooth’s inherent heat causes curing to occur along the tooth interface sooner than it does along cooler portions of composite away from tooth structure. Thus, composites generally start to shrink toward cured bonding agents, since the polymerization process has already started there. When a composite is placed against a light-cured bonding agent and then light-cured, the composite is initiated from two sides. However, the rate of polymerization is not equal on the two sides in that the composite facing the light polymerizes more quickly and has larger effect on the direction of composite shrinkage.

Autocured resin, polymerization shrinkage pattern. The left sphere represents the volume of an autocured material prior to polymerization. The right sphere represents the volume and shrinkage pattern of the material after polymerization.

The left sphere represents the volume of light-cured composite prior to polymerization. The right sphere represents the volume and shrinkage pattern of the material after polymerization when not attached to any surface. Note how the material moves toward the light of the curing tip at the right.

Precipitation reactions

Precipitation reactions involve the loss of a solvent. In this case, the liquid materials commonly contain resins diluted in an organic solvent. When exposed to air, the solvent evaporates and concentrates the resin into a solid. Examples of these reactions are wall paint, fingernail polish, dental varnish, etc. The setting reaction is referred to as drying. Presently, few dental restoratives set through a precipitation reaction. Precipitation is used, however, in setting bonding agents, cavity varnishes, and some surface coatings. With bonding agents, the solvent enhances the agent’s penetration of the tooth. Evaporation of the solvent concentrates the monomer prior to polymerization and improves durability.

Most materials that set by drying contain a large molecule (resin) that is suspended in a volatile solvent (thinner). During drying, the loss of the solvent brings the component of greater molecular weight out of the solution and turns it into a solid. Precipitation materials used in the mouth must be insoluble in water (at least the resulting resin) to avoid reversal of this process in the oral fluids. Precipitation reactions result in the least durable restoratives and are recommended only for temporary treatment of tooth structure.

  Precipitation reaction. A volume of solvent evaporates and leaves a solid behind.

PHYSICAL PROPERTIES AND DESTRUCTIVE FORCES

Knowledge of the physical properties of restorative materials can help predict their susceptibility to breakage under occlusal function.

Compressive strength

Compressive strength is a measure of the amount of force a material can support in a single impact before breaking. This physical property is one of the easiest to measure and is often cited in advertisements for dental materials. Compressive strength is such a commonly used physical property that it has acquired a greater respectability in the profession than is appropriate to its actual clinical relevance. There is no direct correlation between compressive strength and clin-ical performance. However, compressive strength does measure strength, and it gives an indication of a material’s resistance to creep and plasticity. In conjunction with a sound understanding of the clinical purpose of a dental material, measurement of compressive strength is sometimes used as a screening test in the development of new materials.

 Compressive strength. The amount of force a material can support in a single impact.

Tensile strength

Tensile strength is the amount of force that can be used to stretch a material in a single impact prior to breaking. This physical property is more difficult to measure than compressive strength. The tolerance of the measuring device is critical. Materials must be pulled at an exact 180-degree angle from each other to eliminate the influence of shear forces. The clinical relevance of tensile strength is limited.

 Tensile strength. The amount of stretching force a material can withstand.

Diametrical tensile strength

This is a theoretical tensile strength measurement that is calculated by measuring the compressive strength of a disc of material. This test is easier to perform and is more consistent than the normal tensile strength test.

Shear strength

Shear strength is the maximum shear stress that a material can absorb in one impact before failure ness of the material tested. Shear strength has been used to measure the bond strength between different materials. In this test, a disc of material is bonded to a surface, a chisel instrument is placed above the disc, or a loop of wire is attached. The force required to shear the disc from the bonded surface is the bond strength of the tested adhesive. This test is easier to perform than a tensile test on two bonded materials.

Unfortunately, the punch test has no direct correlation to the clinical performance of a material. Further, there is little agreement in the research community on how to conduct this test, although standards are being developed. Shear strength data from different testing laboratories show extremely large variations are possible even when testing the same materials with the same instruments.

  Shear strength. The maximum shear stress a material can absorb in one impact

Bond strength. The force required to shear a disc of material from the surface to which it is bonded.

Stiffness

Stiffness is also called the modulus of elasticity, elastic modulus, or Young’s modulus. Stiffness determines resistance to flexure and deformation, or the amount of bending when loaded. The measure of stiffness has been related to predicting the potential results of cyclic loading outside the oral environment. Stiffness can be measured by placing a force on a material and measuring the deformation. It can be calculated in a nondestructive way by measuring the harmonics of a material when vibrated.

Stress and strain are related in that the elastic modulus is the ratio of stress over strain. Elastic modulus is expressed in the same units as stress. Most dental composites have an elastic modulus between 5 and 15 GPa (gigapascals). The elastic modulus indicates the amount of stress that needs to be applied to achieve a certain strain, or, if the strain is known, what level of stress is in effect.

Stiffness. The resistance of a material to flexure and deformation when loaded

Stress

Stress is defined as force per unit area, expressed in Newton’s per square millimeter (N/mm2) or pounds per square inch (psi). The unit N/mm2 is properly known as the pascal and abbreviated Pa. The pascal is a small unit; for dental applications, stress forces are usually expressed as megapascals or MPa. For example, adhesive bonds to dentin typically fail with the application of stress in the 20 to 30MPa range.

Strain

Strain is defined as the change in the length of a material after the application of stress divided by its original length—a unit with no dimensions. A material capable of high strain, such as rubber or latex, can tolerate strain values of 0.5 to 50.0% before failure. For most solid materials, strain is expressed as microstrain in parts per million (ppm) or 10–6 strain.

Stress and strain. Strain is measured as the percentage of change in length when a stress is applied in a single application.

Fatigue

Fatigue occurs in all rigid materials undergoing con-tinual stress and strain. Fatigue occurs in a tooth when a functional cusp can no longer support occlusal forces. It is also a common cause of con-ventional restoration failure. Over time, fatigue results in cohesive microcracks and external chip-ping in a restoration. It occurs in direct placement composites under heavy function. The intraoral degradation of restorative materials is a complex process that has not been mimicked to any great extent by simple laboratory tests. Each group of materials—metals, polymers, and cements—seems to fail by mechanisms specific to that group, mak-ing generalization difficult. The phenomenon is called fatigue because, under certain loading condi-tions, a component appears to tire, losing strength over a period of time in service. Two types of load-ing conditions can cause these symptoms: (1) cyclic loading and (2) steady loading. Both are more severe in the presence of a chemically active agent. The progressive loss of strength that accompanies cyclic loading is attributable to the gradual spread of cracks. Cyclic loading is illustrated in bellow.

The progressive and cumulative damage that occurs during cyclic loading. The restoration eventually fails.

The mouth is unique in that it combines cyclic loading with a chemically active environment. The most common chemically active agent in dentistry is saliva, which contains varying amounts of water and other components. Saliva varies from patient to patient, and individual differences can explain some of the atypical results seen in some mouths. A patient’s diet may also contain substances that are chemically reactive to teeth and restorations.

Cyclic loading might appear less harmful to restorative materials than steady loading, because the average deflection (over the cyclic period) is less than the steady deflection. In practice, it is the cyclically loaded materials that break first from occlusal forces; statically loaded materials, such as those maintaining, for example, resting contact points, last considerably longer. Since the growth of a crack requires plastic deformation, cracking occurs more rapidly in ductile materials, such as plastics. Stiffer restoratives are more resistant to fatigue, because they are under less strain when loaded.

The clinical effects of fatigue are important in all dental restoratives, because the force needed to cause failure decreases over time. The rate of weakening is thought to be related to the rate of crack propagation in the material in response to stress absorption over time. Fatigue explains why many dental restorations provide excellent service for a number of years and then suddenly break under a relatively minor load. Most restoration fractures occur in the marginal ridge areas. These areas are the least supported and absorb the most static and cyclic stress and strain; thus, they are the most inclined to fracture.

The weakening of a material over time as a result of cyclic loading. Clinically, this means that materials become more brittle and less durable over time

Fracture toughness

Fracture toughness is an important measure of a material’s susceptibility to fatigue. Stress is the amount of force placed on an object, and strain is the amount of deformation that occurs under that stress. All materials undergo strain (such as a bending force) when stressed. Following figure illustrates how different materials react to stress up to their breaking point. As shown, porcelain bends little, even when placed under considerable stress. Resins (plastics) are different in that they bend a lot even under low stress. Metals can tolerate considerable stress and bending.

The stress–strain curves of a material show the amount of flexure it produces under a given stress. At a critical stress, the material fractures, because its maximum amount of deformation (elastic limit) has been exceeded.

Fracture toughness is defined as the area under the curve when viewing a plot of the stress and strain relation of a restorative material. It is a measure of the total amount of stress a material can take before failing. It is related to the energy needed for flexure to a breaking point, which is called flexural strength. Flexural strength, bending strength, and fracture resistance are terms used interchangeably. Owing to its ease of measurement, flexural strength is the physical property most commonly used to indicate the fracture toughness of a material. However, many re-searchers believe that fracture toughness is the best physical property to measure to predict the wear and fracture resistance of a restorative.

The graphs in following  indicate the differences in fracture toughness among porcelains, resins, and metals. The clinical performance of metal restorations bears out their fracture resistance. Porcelain and resins used alone have a long history of breakage under stress; to extend their longevity, they are often supported with metal. The way in which these materials are used together can profoundly affect the physical properties of the resulting restoration.

Fracture toughness is related to the area under the stress–strain curve. Note that metals are far superior to porcelains and resins.

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