Сraig. Dental Materials

Maintenance of color (18 mo)

Marginal discoloration (18 mo)

Marginal integrity (18 mo)

Caries-recurrent or marginal (18 mo) Maintenance of interproximal contact (18 mo) Postoperative sensitivity

Failure (18 mo)

Wear between 6 and 18 mo

I

No more than 10%Charlie

No more than 10%Charlie

No more than 5% Charlie

No more than 5% Charlie

95% showing no observable broadening of contacts Thorough history of adverse sensitivity to hot, cold,

and biting stimuli

No more than 5%

No more than 50 ym

critical factor in completeness of cure at the surface and within the material. The tip of the light source must be held within 1 mm of the surface to gain optimum penetration. More-opaque shades reduce light transmission and cure only to minimal depths (1 mm). A standard exposure time using most visible lights is 20 seconds. In general, this is sufficient to cure a light shade of resin to a depth of 2 or 2.5 mm. A 40-second exposure improves the degree of cure at all depths, but it is required to obtain sufficient cure with the darker shades. Application of the light beam through 1 mm or less thickness of tooth structure produces a sufficient cure at shallower depths, but the hardness values obtained are not consistent. Because the light beam does not spread sufficiently beyond the diameter of the tip at the emitting surface,it is necessary to "step" the light across the surface of large restorations so the entire surface receives a complete exposure. Larger tips have been manufactured for placement on most light-curing units. However, as the light beam is distributed over a larger surface area, the intensity at a given point is reduced. Use a longer exposure time of up to 60 seconds when larger emitting tips are used.

To evaluate the effective depth of cure of a specific light-curing unit, cut a small section of 5 to 10 mm from a clear straw and place it on a glass slide. Pack the section with composite. Apply the light directly to the top surface for 20 to 40 seconds according to the recommended technique. Cut off the straw, and scrape uncured composite from the bottom of the specimen with a sharp knife. Measure the length of the apparently cured specimen and divide in half to estimate the effective depth of cure.

Radiopacity Modern composites include glasses having atoms with high atomic numbers, such as barium, strontium, and zirconium. Some fillers, such as quartz, lithium-aluminum glasses, and silica, are not radiopaque and must be blended with other fillers to produce a radiopaque composite. Even at their highest volume fraction of filler, the amount of radiopacity seen

in composites is noticeably less than that exhibited by a metallic restorative like amalgam. The microhybrid composites achieve some radiopacity by incorporating very finely divided heavymetal glass particles.

Aluminum is used as a standard reference for radiopacity. A 2-mm thickness of dentin is equivalent in radiopacity to 2.5 mm of aluminum, and enamel is equivalent to 4 mm of aluminum. To be effective, a composite should exceed the radiopacity of enamel, but international standards accept radiopacity equivalent to 2 mm of aluminum. Amalgam has a radiopacity greater than 10 mm of aluminum, which exceeds all the composite materials available.

Wear Rates Clinical studies have shown that composites are superior materials for anterior restorations in which esthetics are essential and occlusal forces are low. One problem with composites is the loss of surface contour of composite restorations in the mouth, which results from a combination of abrasive wear from chewing and toothbrushing and erosive wear from degradation of the composite in the oral environment (Fig. 9-6).

Fig. 9-6 A posterior composite resin restoration exhibiting excessive occlusal wear and marginal discoloration.

Wear of posterior composite restorations is observed at the contact area where stresses are the highest. Interproximal wear has also been observed. Ditching at the margins within the composite is observed for posterior composites, probably resulting from inadequate bonding and polymerization stresses. Currently accepted composites for posterior applications require clinical studies that demonstrate, over a 5-year period, a loss of surface contour less than 250 pm or an average of 50 pm per year of clinical service. Products developed as packable or laboratory composites usually have better wear resistance than microfilled or flowable composites.

Biocompatibility Details about the biocompatibility of composites are discussed in Chapter 5, but some of the central issues are mentioned here. Nearly all of the major components of composites (Bis-GMA, TEGDMA, and UDMA, among others) have been found cytotoxic in vitro if used in pure form, but the biological liability of a cured composite depends on the release of these components from the composite. Although composites release some levels of components for weeks after curing, there is considerable controversy about the biological effects of these components. The amount of release depends on the type of composite and the method and efficiency of the cure of the composite. A dentin barrier markedly reduces the ability of components to reach pulpal tissues, but these components can traverse dentin barriers, albeit at reduced concentrations. The effects of low-dose, long-term exposures of cells to resin components is not generally known. On the other hand, the use of composite materials as direct pulp-capping agents poses a higher risk for adverse biological responses, because no dentin barrier exists to limit exposure of the pulp to the released components.

The effects of released components from composites on oral or other tissues is not known with certainty, although no studies have documented any adverse biological effects. The tissue at highest risk from this type of release would appear to

be gingiva in close, long-term contact with composites. Components of composites are known allergens, and there has been some documentation of contact allergy to composites. Most of these reactions occur with dentists or dental personnel who regularly handle uncured composite and, therefore, have the greatest exposure. There are no good studies documenting the frequency of allergy to composites in the general population.

Finally, there has been some controversy about the ability of components of composites to act as xenoestrogens. Studies have proven that Bis-phenol A and its dimethacrylate are estrogenic in i n vitro tests that measure this effect using breast cancer cell growth. Trace levels of these components have been identified in some commercial composites; however, estrogencity from cured commercial composites has not been demonstrated. Furthermore, there is considerable controversy about the accuracy and utility of i n vitro tests using breast cancer cells to measure a true estrogenic effect. An early study in this area, which claimed that dental sealants and composites were estrogenic in children, has since been largely discredited.

MANIPULATION OF C

PULPAL PROTECTION

If a deep cavity exists after preparation, protect the pulp with a calcium hydroxide cavity liner or glass ionomer, hybrid ionomer, or compomer base. Liners and bases are described in Chapter 20.

ETCHING AND BONDING

To provide a bond between composite and tooth structure, etch the enamel and dentin of the cavity preparation with acid for 30 seconds with an etchant supplied by the manufacturer, often a 34% to 37% phosphoric acid solution or gel. Flush the acid away with water, and gently dry the surface with a stream of air. The etched

enamel will appear dull. The bonding agent penetrates the etched enamel and dentin surfaces and provides micromechanical retention of the restoration. Recently, self-etching primers have been developed that do not require etching with phosphoric acid or rinsing. Bonding agents and their interactions with tooth structure are discussed in Chapter 10.

DISPENSING

Light-CuredComposites Dispense small increments of composite onto a paper pad and pack into the cavity preparation as described subsequently. A controlled setting time allows for the individual polymerization of small increments of composite, thus permitting the use of multiple shades of composite within a single restoration and accommodating polymerization shrinkage within each increment as opposed to the total shrinkage in a bulk-cure method.

Self- and Dual-Cured Composites An example of a self-cured, core composite supplied as two pastes is shown in Fig. 9-4. One syringe contains the peroxide initiator or catalyst and the other syringe includes the amine accelerator. Mix equal amounts of universal and catalyst pastes thoroughly for 20 to 30 seconds. Use plastic or wooden spatulas, but avoid metal spatulas, because the inorganic filler particles are abrasive and small amounts of the metal can be abraded and discolor the composite.

INSERTION

The composite can be inserted into the cavity preparation by several methods. Place it with a plastic instrument, such as one of those shown in Fig. 9-7, A, which does not stick to the composite during insertion. The composite may also be placed in the plastic tip of a syringe, such as shown in Fig. 9-7, B, and then injected into the cavity preparation. The syringe or compule allows the use of small mixes, reduces the problem of incorporating voids in the composite during

Fig. 9-7A, Instruments for placing composites. B, Sy- ringe for injecting composites.

(From Craig RG, Powers JM, Wataha JC: Dental materials: properties and manipulation, ed 7, St Louis, 2000, Mosby.)

insertion, and facilitates placement of the material in the areas of retention.

POLYMERIZATION

Light-CuredComposites Exposure times for polymerization vary depending on the type of light-curing unit and the type, depth, and shade of the composite. Times may vary from 20 to 60 seconds for a restoration 2 mm thick. Microfilled composites require longer exposure than microhybrid composites because the small filler particles scatter the light more. Darker shades or more opaque composites require longer exposure times (up to 60 seconds longer) than lighter shades or more translucent composites. In deep restorations, add and polymerize the composite in layers. One layer bonds to another without any loss of strength.

The setting time of light-cured composite and the depth of cure within a given mass depend on the intensity and penetration of the light. A material with a low absorption coefficient cures to the greatest depth. The presence of ultraviolet absorbers for color stabilization, fluorescent dyes for esthetics, or excessive initiator concentration has a detrimental effect on completeness of cure.

Light-curing units are discussed in detail later in this chapter.

Selfand Dual-Cured Composites After mixing, the self-cured composite has a working (or insertion) time of 1 to 11/2 minutes. The mix will begin to harden, and the material should not be disturbed until the setting time of about 4 to 5 minutes from the start of the mix.

Dual-cured composites contain chemical accelerators and light activators, so polymerization can be initiated by light and then continued by the self-cured mechanism.

FINISHING AND POLISHING

For gross reduction, use diamonds, carbide finishing burs, finishing disks, or strips of alumina. For final finishing of either microhybrid or microfilled composites, use abrasive-impregnated rubber rotary instruments or a rubber cup with various polishing pastes. Finishing should be done in a wet field with a water-soluble lubricant. Final finishing of light-cured composites can be started immediately after light curing.

Polishing is the final step of finishing and is usually performed with aluminum oxide abrasives with progressively finer grit sizes. Polishing

of composites is important, because a smooth surface is desired to prevent retention of plaque and is needed to lnaintain good oral hygiene.

A measure of the quality of polishing is surface roughness. A comparison of surface roughness of various composites is listed in Table 9-6. The smoothest surface is achieved by use of a Mylar matrix. Carbide burs produce smoother surfaces than diamond burs, but after polishing the surface roughness is similar.

COMPOSITES FOR SPECIAL

APPLICATIONS

MICROFILLED COMPOSITES

These composites are recommended for use in Class 3 and Class 5 restorations, where a high polish and esthetics are most important. One product has been used successf~~llyin posterior restorations. They are composed of lightactivated, dimethacrylate resins with 0.04-ym colloidal silica fillers with a filler loading of 3250% by volume (see Table 9-2).

Typical properties of microfilled composites are listed in Table 9-3. Because they are less highly filled, microfilled composites have higher values of polymerization shrinkage, water sorption and thermal expansion as compared with microhybrid composites.

PACKABLE COMPOSITES

These composites (see Table 9-1) are recommended for use in Classes 1, 2, and 6 (MOD) cavity preparations. They are composed of light-

activated, dimethacrylate resins with fillers (fibers or porous or irregular particles) that have a filler loading of 66% to 70% by volume (see Table 9-21, The interaction of the filler particles and modifications of the resin cause these composites to be packable.

Typical properties of packable composites are listed in Table 9-3. Important properties include high depth of cure, low polymerization shrinkage, radiopacity, and low wear rate (3.5 pm/ year), which is similar to that of amalgam. Several packable composites are packaged in unit-dose compules. A bulk-fill technique is recommended by manufacturers but has not yet been demonstrated effective in clinical studies. Use singlebottle bonding agents with these composites.

FLOWABLE COMPOSITES

These light-cured, low-viscosity composites are recommended for cervical lesions, pediatric restorations, and other small, low stress-bearing restorations (see Table 9-1). They contain dimethacrylate resin and inorganic fillers with a particle size of 0.7 to 3.0 pm and filler loading of 42% to 53% by volume (see Table 9-21.

Typical properties of flowable composites are listed in Table 9-3. Flowable composites have a low modulus of elasticity, which may make them useful in cervical abfraction areas. Because of their lower filler content, they exhibit higher polymerization shrinkage and lower wear resistance than microhybrid composites. The viscosity of these composites allows them to be dispensed by a syringe for easy handling.

LABORATORY COMPOSITES

Crowns, inlays, veneers bonded to metal substructures, and metal-free bridges are prepared indirectly on dies from composites processed in the laboratory (see Table 9-I), using various combinations of light, heat, pressure, and vacuum to increase the degree of polymerization and the wear resistance.

Typical properties of laboratory composites are listed in Table 9-3. For increased strength and rigidity, laboratory composites can be combined

Fig. 9-8 A reconstructed composite resin core pre- ~ a r e dfor a cast metal crown.

with fiber reinforcement. Restorations are usually bonded with composite cements. Cavity preparations for indirect composites must be nonretentive rather than retentive, as typically prepared for direct placement.

CORE COMPOSITES

At times, so much tooth structure is lost from caries that the crown of the tooth must be built up to receive a crown. Amalgam is the most common core material, but composite is becoming popular. Composite core materials are typically two-paste, self-cured composites (see Fig. 9-41, although light-cured and dual-cured products are available. Core composites are usually tinted (blue, white, or opaque) to provide a contrasting color with the tooth structure. Some products release fluoride. An example of a composite core build-up is shown in Fig. 9-8. Typical properties of core composites are listed in Table 9-3.

Composite cores have the following advantages as compared with amalgam: can be bonded to dentin, can be finished immediately, are easy to contour, have high rigidity, and have good color under ceramic restorations. Composite cores are bonded to remaining enamel and dentin using bonding agents. Be careful to use a bonding agent recommended by the manufacturer of the core material, because some selfcured composite core materials are incompatible with some light-cured bonding agents.

Fig. 9-9 A fractured porcelain-fused-to-metal restoration that could be repaired with composite resin.

posite. To achieve the n~aximumbond strength, the remaining ceramic/alloy surface is cleaned and treated with a silane, resin, or silane-resin primer supplied in a liquid form. The primer is supplied separately, and a composite of choice is used with it.

The repair of composites is accomplished by abrading the surface of the remaining composite with 50-pm alumina, then keeping the surface well isolated from saliva and moisture. Treat the surface of the composite with primer, and add the new composite. Repair bond-strength is about 60% to 80% of the cohesive strength of the original composite.

PROVISIONAL COMPOSITES

Temporary inlays, crowns, and long-span bridges are usually fabricated from composite or acrylic resins. The purposes of provisional restorations are to maintain the position of the prepared tooth, seal and insulate the preparation and protect the margins, establish proper vertical dimension, aid in diagnosis and treatment planning, and evaluate esthetic replacements. The properties of acrylic and composite provisional materials are compared in Table 9-7.

REPAIR OF CERAMIC OR COMPOSITE

A fractured porcelain-fused-to-metal restoration, as shown in Fig. 9-9, may be repaired using an all-purpose or flowable composite. These repairs require adequate bond strength between the remaining ceramic and alloy and the added com-

Compomers or poly acid-modified composites are used for restorations in low stress-bearing areas, although a recent product is recommended by the manufacturer for Class 1 and Class 2 restorations in adults (see Table 9-11, Compomers are recommended for patients at medium risk of developing caries.

COMPOSITION AND SETTING REACTION

Compomers contain poly acid-modified monomers with fluoride-releasing silicate glasses and are formulated without water. Some compomers have modified monomers that provide additional fluoride release. The volume percent filler ranges from 42% to 67% and the average filler particle size ranges from 0.8 to 5.0 pm. Compomers are packaged as single-paste formula-

Fig. 9-10 Examples of compomers (packaged in compules and syringes) and a hybrid ionomer (packaged in capsules with applier) for restoring cervically eroded teeth.

(From Craig RG, Powers JM, Wataha JC: Dental materials: properties and manipulation, ed 7, St Louis, 2000, Mosby.)

tions in compules and syringes, as shown in Fig. 9-10.

Setting occurs primarily by light-cured polymerization, but an acid-base reaction also occurs as the compomer absorbs water after placement and upon contact with saliva. Water uptake is also important for fluoride transfer.

PROPERTIES

Typical properties of compomers are listed in Table 9-3. Compomers release fluoride by a mechanism similar to that of glass and hybrid ionomers. Because of the lower amount of glass ionomer present in compomers, the amount of fluoride release and its duration are lower than those of glass and hybrid ionomers. Also, compomers do not recharge from fluoride treatments or brushing with fluoride dentifrices as much as glass and hybrid ionomers.

MANIPULATION

Compomers are formulated as a single-paste packaged in unit-dose compules. Because of their resin content, compomers require a bonding agent to bond to tooth structure. Some compomers are used with single-bottle bonding

agents that contain acidic primers. Acidic primers bond to enamel and dentin without the need for additional etching with phosphoric acid. However, most manufacturers recommend phosphoric acid etching before priming to improve bond strength.

OVERVIEW

The most common light-curing source used in dentistry is the quartz-tungsten-halogen light. In the mid-1990s, high-intensity, plasma-arc lights were introduced. In 2000, blue lightemitting diodes became available. Definitions of terms used to describe light sources used to polymerize dental resins are listed in Table 9-8.

QUARTZ-TUNGSTEN-HALOGEN

LIGHT-CURING UNITS

An example of a quartz-tungsten-halogen (QTH) light-curing unit used to activate polymerization of composites is shown in Fig. 9-11. The peak wavelength varies among units from about 450 to 490 nm. Typically, the intensity (power density) ranges from 400 to 800 mw/cm2, but highintensity QTH units are available. Some units provide energy at two or three different intensities (step cure) or at a continuously increasing (ramp cure) intensity. A typical resin composite requires an energy density of 16 ~ / c m ' (400 mw/cm2 x 40 s = 16,000 mW s/cm" for polymerization.

A decrease in line voltage of 6% shows a corresponding reduction in output of about 25% in intensity in some lamps, but only 10% in lamps with voltage regulators in their circuitry. In general, the output from the various lamps decreases with continuous use and the intensity is not uniform for all areas of the light tip, being greatest at the center. Also, the intensity of the light decreases with distance from the source nearly linearly to the log of the intensity divided by the distance. Although the intensity is important with respect to the depth of cure, it has been shown for some products that a threefold dif-

Power

Power density (intensity)

I

Energy

Energy density

1

I

mw/cm2

J*

~ / c m "

I

I

Number of photons per second emitted by light source Number of photons per second emitted by light source

per unit area of curing tip

Power x time

Power density x time

Fig. 9-1I Visible-light sources for photo-initiation of composition.

(From Craig RG, Powers JM, Wataha JC: Dental materials: properties and manipulation, ed 7, St LOUIS,2000, Mosby.)

ference in intensity had only a 15% difference in the depth of cure. Bulb life ranges from 50 to 75 hours.

Although there is minimal potential for radiation damage to surrounding soft tissue inadvertently exposed to visible light, use caution to prevent retinal damage to the eyes. Because of the high intensity of the light, do not look directly at the tip or the reflected light from the teeth.

A number of devices are marketed to filter the visible-light beam so the operator can directly observe the curing procedure and to protect the patient and staff. These devices are eyeglasses, flat shields that can be held over the field of vision, and curved shields that attach directly to the handpiece delivering the light beam (Fig. 9-12).

Some lamps produce considerable heat at the curing tip, which can produce pulpal irritation. Too much heat is being generated if one cannot hold a finger 2 to 3 mm from the tip for 20 seconds.

Maintenance of QTH lights must be provided on a regular basis, as summarized in Table 9-9.

PLASMA-ARC LIGHT-CURING UNITS

Plasma-arc (PAC) lights are high-intensity lightcuring units. Light is obtained from an electrically conductive gas (plasma) that forms between two tungsten electrodes under pressure. The output is filtered to minimize transmission of infrared and ultraviolet energy. The energy transmitted is in the visible range between 380 and 500 nm, with an output peak near 480 nm. Because of the high intensity of light available at lower wavelengths, PAC lights are able to

Fig. 9-12 Eye protection devices that can be used with a polymerizing visible-light device. Top to bottom: glasses, an instrument shield, and a flat plate.

cure composites with photo-initiators other than camphoroquinone.

PAC lights save time during procedures requiring multiple exposures, such as incremental build-ups, quadrant restorations, veneers, and bonding of orthodontic brackets. Typically, an exposure of 10 seconds from a PAC light is equivalent to 40 seconds from a QTH light. Use of 2-rnm increments is still required.

Properties of composites cured with PAC lights depend on the composite and light-curing unit. In general, PAC lights produce equal or lower degrees of conversion, depths of cure, and flexural moduli, but flexural strengths equal to QTH light-curing units. Wall-to-wall polymerization shrinkages are equal or less with PAC lights.

LIGHT-EMITTING DIODES

Solid state light-emitting diodes (LEDs) use junctions of doped semiconductors (p-n junctions) based on gallium nitride to emit blue light. The spectral output of blue LEDs falls between 450 and 490 nm, so these units are effective for curing materials with camphoroquinone photoinitiators. LED units do not require a filter, have a long life span, and do not emit significant heat. Recently, a hybrid, light-curing unit that combines LED and QTH sources was introduced. With this unit, polymerization is initiated by the LED source and then completed by the combination of light from both sources.

Composites cured with LED units have flexural properties similar to those cured with QTH units. Depth of cure with LED units is higher.