Written by Leendert Boksman, DDS, BSc, and Tony Pensak, DDS Saturday, 31 December 2005 19:00
Stress produced during polymerization of dental composite materials during light-curing is a leading cause of adhesive restoration failure, resulting in postoperative sensitivity, marginal staining, and recurrent caries.1 This polymerization contraction shrinkage creates stresses as high as 13 MPa between the composite material and tooth interface2 (1 MPa = 145 psi). This stress can exceed the tensile strength of the enamel and result in stress cracking and enamel fractures along the interface.2 When a composite resin is cured, the surrounding tooth structure deforms due to polymerization shrinkage.3 This deflection can range from 4 to 6 µm, depending on the filling technique used.4 It has been well established that the higher the intensity of the light source, the greater the contraction force at the composite-tooth interface, and use of intermittent or lower light intensities results in improved marginal integrity.5
To achieve clinically successful posterior composite resin restorations, it is vital to maintain the integrity of the bond and the marginal adaptation to tooth enamel and dentin.6 Due to inherent shortcomings of the composite restorative material, it is imperative to evaluate the restorative protocol to minimize technique-related problems and thereby maximize long-term results. This article will discuss how the choice of bonding agent, the use of flowable liners, and the choice of composite material affect the long-term viability of a posterior composite restoration by reducing polymerization stress.
ETCHING OF TOOTH STRUCTURE
The bonding of composite to tooth structure can be achieved with one of 4 different etching systems: total-etch 3 step, total-etch 2 step, self-etch 2 step, and self-etch 1 step.
The self-etch adhesives have increased in popularity due to their simplified technique and lower incidence of postoperative sensitivity.7 However, the acidic nature and permeability of simplified, self-etch adhesives are characteristics that make them incompatible with self-cured and dual-cured composites.8 One-step adhesives have a lower bonding effectiveness; this is attributed in part to the dissolution of hydrophilic and hydrophobic monomers in a highly concentrated solvent, which jeopardizes bond durability.9 Tay has demonstrated that single-bottle adhesives, because of the lack of a more hydrophobic bonding resin layer, behave as permeable membranes after polymerization. They permit the continuous transudation of dentinal fluid and do not provide a hermetic seal for vital deep dentin. This may interfere with optimal polymerization of composites and resin cements in both direct and indirect restorations.10
These findings preclude the use of self-etching adhesives if a self-cured composite is used for a posterior composite base. The shear bond strengths of self-etching primer/adhesive systems and total-etch, 1-bottle systems to enamel have been shown to be much lower than the other systems, resulting in increased leakage and staining at the enamel-composite interface. The bonding of these self-etching primers to enamel may depend on their specific composition (pH).11 The more acidic primers etch the enamel with a more distinct etch pattern, decreasing marginal staining and leakage.
The conventional, 3-step, etch-and-rinse adhesives still perform favorably and are the most reliable in the long-term.12 Multiple studies comparing contemporary adhesives reveal that these adhesives remain the gold standard in terms of durability; simplifying the clinical procedure results in loss of bonding effectiveness.13 Even though the 2-step total-etch systems exhibit slightly lower bond strengths than the 3-step systems after aging,14 the recent introduction of filled, 2-step total-etch systems shows promise. These filled adhesives provide better coverage of the dentinal substrate. When evaluating microleakage of packable composites, the filled adhesives were associated with a reduction in microleakage when compared to unfilled adhesives.15,16
According to Feilzer, et al17 restriction of flow is affected by the configuration of the restoration, known as the C-factor. The C-factor is the ratio of bonded (flow-inactive) to unbonded or free (flow-active) surfaces. An increase in the number of bonded surfaces results in a higher C-factor and greater contraction stress on the adhesive bond.17 When examining C-factor effects on microtensile bond strengths, Armstrong cited data suggesting that the durability of the bonded joint is threatened by hydrolysis and that the most susceptible region is the bottom half of the hybrid layer. In low C-factor cavity designs, a more flexible, filled adhesive resin was more durable than unfilled adhesive resins.18
FLOWABLE COMPOSITES AS LINERS
The use of flowable composites as low-modulus cavity liners under large composite resin restorations, particularly in conjunction with packable and universal composite resins, has been extensively studied.19,20 Since these materials effectively wet the surfaces to which they are applied, they exhibit excellent adaptation to the prepared walls of the cavity.5 The application of a thin layer of a flowable composite at the cervical margin as a liner underneath packable composites enhances the marginal adaptation of the restoration.21 When used in cavity preparations associated with removal of occlusal enamel caries that results in undermined cusps, the flowable composites can be placed to buttress the undermined cusps, minimizing deflection.
In a study by Leevailoj and colleagues, 3 packable composites tested showed higher leakage than the microhybrid control. Flowable liners were not used.22 When used in class II, light-cured composite resin restorations with margins below the cemento-enamel junction, flowable composite reduced marginal leakage and voids at the interface as well as the total number of voids in the restoration.23,24 In a study by Yazici, et al25 the combination of a flowable resin composite and a hybrid composite light-cured separately was associated with the least amount of microleakage.25
Other beneficial effects of the use of a flowable composite as a liner under resin-based composite restoratives are an increase in flexural strength26 and creation of a stress-absorbing layer that improves the integrity of the bonded interface area.27 However, when the flowable resin composite is cured simultaneously with the adhesive, increased microleakage is the result.28
The cross-linking of resin monomers into polymers is associated with an unconstrained volume shrinkage of 2% to 5%.29 Newer posterior hybrid composites (eg, Aelite LS Posterior and Aelite LS Packable; Bisco) demonstrate greatly reduced shrinkage and offer a significant advantage by reducing the stress that polymerization contraction causes. Another posterior restorative technique, known as the directed contraction shrinkage technique, utilizes a self-cured base composite to the level of the dentoenamel junction. The self-cured reaction, being a slower set than what occurs with light-curing, places less stress on the bond-tooth interface.30 However, it must be remembered that some 1-bottle adhesives may not provide acceptable bonding to tooth structure when they are used with self-cure composites. Using self-cure composites, mean bond strengths to dentin ranged from zero for light-cured Prime and Bond NT (DENTSPLY/Caulk) to 21.4 MPa for One-Step (Bisco).31
The recent introduction of nanofil and nanohybrid composites, which have an average particle size smaller than that of microfilled composites, has altered the restorative landscape. These composites are highly polishable, have good aesthetic and mechanical properties, and have good handling characteristics.32 They offer an excellent option for the final enamel replacement layer in posterior composite restorations.
APPLYING THE EVIDENCE—RESTORING POSTERIOR TEETH WITH COMPOSITE
Based on the supporting evidence cited previously, there are 2 viable approaches for composite restoration of the posterior dentition. One approach involves the use of a total-etch filled adhesive, which is compatible with all other composites, a flowable liner to reduce stress, a low-shrinkage, light-cured composite as the dentin replacement, a nanofil composite as the enamel replacement, and a sealing resin as the last step after finishing the restoration.33 The other approach is to use a total-etch filled adhesive; a self-cure, base posterior composite to replace the dentin; a nanofil composite to replace enamel; and a sealing resin to create a smooth surface after finishing.34,35
CLINICAL PROCEDURES—OPTION 1
|Figure 1. Class II preparation with wide lingual extension.||Figure 2. Post rinse glossy surface of preparation.|
|Figure 3. Application of 2 to 3 coats of One-Step Plus to moisten dentin and enamel, followed by gentle air evaporation of the solvent and 10-second light-cure.|
Figure 1 shows an MO preparation of a maxillary right first molar with a wide lingual extension due to caries. The tooth has been isolated with a rubber dam. After placement of a standard ComposiTight Gold band (CLINICIAN’S CHOICE) with a standard G-ring to create separation and anatomic contour, the preparation is acid-etched for 15 seconds and rinsed, leaving a glossy wet surface (Figure 2). The acid-etching step is accomplished after matrix placement to avoid inadvertently etching the adjacent tooth and to control the flow of gingival fluid. One-Step Plus (Bisco), a filled, single-bottle adhesive, is applied in 2 to 3 coats to moisten the dentin and enamel (Figure 3). The solvent is gently evaporated with air to remove the acetone and then light-cured for 10 to 20 seconds. A dedicated, air-only syringe is recommended for use in all bonding procedures.
|Figure 4. Aeliteflo LV is placed directly on the pulpal floor, axial wall, and gingival floor in a thin layer and light-cured for 10 seconds.||Figure 5. REJ No. 20 double-ended condenser is used to adapt Aelite LS Packable to the proximal box and cured in 2-mm increments.|
|Figure 6. Final sculpting of the dentin replacement layer reflecting the normal anatomic contour of the DEJ.||Figure 7. Creation of anatomical detail and burnishing of composite-to-cavosurface margins with REJ No. 22. Light-cure for 20 seconds.|
|Figure 8. Final anatomy created with multifluted finishing burs.||Figure 9. Application of BisCover, air-thinned and light-cured for 30 seconds after acid-etching.|
|Figure 10. Final restoration with high-gloss surface.|
Aeliteflo LV (Bisco), a radiopaque, low flexural modulus (3.6 giga pascals), flowable composite, is placed in a thin layer on the pulpal floor, axial wall, and gingival floor (Figure 4). After light-curing the flowable composite for 10 seconds, the dentin replacement material Aelite LS Packable (Bisco) is placed in 2-mm increments (Figure 5), and each increment is cured for 20 seconds. This is continued until the buildup reflects the normal anatomic contour of the DEJ and includes the contact point (Figure 6). The creation of the normal anatomic detail of the enamel is achieved with Aelite Aesthetic Enamel (Bisco), a nanofil composite, and a Ronald E. Jordan (REJ) No. 22 (CLINICIAN’S CHOICE) burnishing instrument (Figure 7). After light-curing, final anatomy and marginal finishing are accomplished with multifluted finishing burs (Figure 8) to create verified centric stops. This is followed by re-etching the surface and application of BisCover (Bisco), an aesthetic sealant, and liquid polish that is light-cured for 30 seconds with a broadband light (Figure 9). The result is a glossy finished surface. Further, any enamel cracks caused by polymerization contraction are sealed. The final restoration is shown in Figure 10.
CLINICAL PROCEDURES—OPTION 2
|Figure 11. Preoperative photograph of amalgam restorations in premolar and molar teeth that require replacement.|
Figure 11 shows 2 premolars and a molar with failing amalgam restorations. The teeth have been isolated with a rubber dam. After removal of the existing restorations, caries, and weak tooth structure, Tofflemire matrix bands and retainers were placed with wedges to control the gingival floor and create separation prior to placement of the restoration. The use of a regular, Tofflemire-type band and retainer can translocate the contact point occlusally. This contact point is often lost when the marginal ridges are shaped to create both the correct height and the proper occlusal embrasure form. Therefore, it is critical to burnish a convex interproximal contour into the bands and ensure that the interproximal wedge is not placed too high in the occlusal direction. Alternatively, a precontoured band such as the Dixieland Band (Getz), which has the correct preburnished height of contour, can be used.
|Figure 12. Acid-etching after placement of matrix band and wedge to create separation.||Figure 13. Desired glossy surface after 2 to 3 coats of One-Step Plus and evaporation of the solvent with gentle air-drying.|
After acid-etching for 15 seconds and gently rinsing for 5 to 15 seconds, excess water is evacuated, and the cavity preparations are gently air-dried for 1 or 2 seconds, leaving a glossy surface (Figure 12). If the dentin surface has been dehydrated, apply Aqua Prep F (Bisco), a dentin/enamel wetting agent that functions as a desensitizer and contains hydroxyethylmethacrylate (HEMA), water, and fluoride. This solution will rehydrate the collagen and precoat the dentin with HEMA. One-Step Plus (Bisco) is applied in 2 to 3 coats to saturate the dentin and enamel fully. After 10 to 15 seconds (Figure 13), the solvent is carefully evaporated without thinning the adhesive on the dentin and is cured for 10 seconds.
|Figure 14. Bisfil II is placed halfway up the contact point and 2 mm from the occlusal surface.||Figure 15. Aelite Esthetic Enamel has replaced the missing enamel and then is contoured, sculpted, and light-cured.|
Since a self-cure composite does not place the same stress on the bond as a light-cured material, the placement of a flowable composite is optional here. Bisfil II (Bisco), a self-cure hybrid composite, is mixed and placed with a high-viscosity tip to a point halfway through the contact point and across the pulpal floor to within 2 mm of the occlusal surface (Figure 14). Gently tap the material into place until it starts to gel, and then with gradually increasing pressure, condense into the proximal box, contact area, and across the pulpal floor, continuing until the material is fully solidified. Place Aelite Esthetic Enamel (Bisco) to fill the cavity preparation completely, and sculpt occlusal anatomy (Figure 15).
|Figure 16. The occlusal surface and margins are finished with fine diamonds and multifluted finishing burs prior to removal of the rubber dam.||Figure 17.Verification of occlusion and final anatomic contouring.|
|Figure 18. Final postoperative result after placement of 1 layer of BisCover to impart a final polish and seal the restoration.|
After light-curing, finish with fine diamonds and multifluted fine finishing burs to remove excess and create final anatomy (Figure 16). Next, check the occlusion and refine the final anatomy and interproximal contour (Figure 17). After acid-etching, apply 1 layer of BisCover, thin with an air stream, and light-cure the entire restoration for 30 seconds. Figure 18 shows the final clinical restorations.
The polymerization stress produced by dental composite material during light-curing is a leading reason for bond failure of adhesive restorations. In order to achieve clinically successful posterior composite resin restorations, the clinician must maintain the integrity of the bond and the marginal adaptation of the composite to tooth enamel and dentin. This article has discussed the materials and restorative protocol for minimizing technique sensitivity and improving long-term results when placing direct posterior composite resin restorations.
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Disclosure: Dr. Boksman holds a paid, part-time consulting position with CLINICIAN’S CHOICE and Clinical Research Dental, with the title of director of clinical affairs.
Disclosure: Dr. Pensak is a clinical evaluator for several major dental manufacturers, including DENTSPLY, Bisco, and 3M ESPE.
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