Development of a Processed Composite Restoration: Adhesive and Finishing Protocol—Part II
Douglas A. Terry, DDS, and Karl F. Leinfelder, DDS, MS
The first part of this discussion on developing a processed composite resin restoration provided an overview of the general properties of the next-generation laboratory-processed composite resins and described the preparation and laboratory fabrication of an onlay restoration. The second part of this discussion will describe the principles that should be followed to achieve long-term success with these restorations and will explain the use of these principles by demonstrating adhesive bonding and finishing protocols in a clinical presentation.
Optimizing the adhesion of restorative biomaterials to the mineralized hard tissues of the tooth is a decisive factor for enhancing the mechanical strength, marginal adaptation, and seal while improving the reliability and longevity of the adhesive restoration. The search for a tooth-restorative interface to mimic the natural tooth condition has resulted in establishing an effective micromechanical bond between composite and mineralized tooth structure. Surface alterations of dental tissue substrates (enamel and dentin) for adhesive application of composite material have provided the following clinical benefits for bonded restorations:
-
reduces the need for mechanical retention and resistance form, thus conserving sound tooth structure
- restores tooth integrity and fracture resistance
- improves marginal seal1,2
- reduces sensitivity
- reinforces remaining enamel and dentin
- cusp reinforcement after tooth preparation in posterior teeth1,3,4
- retention for composite restorations1,5
- reduces or eliminates marginal micro-leakage.1,2,6-10
Adhesive Concepts for Indirect Composite Resins
Indirect laboratory-processed composite resin restorations provide an alternative solution to compete with the adverse polymerization shrinkage forces of a polymerizing composite resin mass that is directly placed in a cavity. Adhesive bonding of laboratory-processed composite resins increase their resistance to fracture and the potential for tooth strengthening.
A principal determinate in the long-term success of these restorations is the strength and durability of the interface.11 This adhesive joint consists of a complex bond, which includes three interfaces:tooth-resin interface, resin-luting resin interface, and the luting resin-composite interface. Several factors that influence this adhesive joint should be considered: the thickness of the resin cement, the restoration fit, stabilization of the hybrid layer, and the wear of the luting cement.
A well-adapted restoration should have a marginal fit of 100 µm; however, the use of dye spacers may increase that dimension to 300 µm.12,13 With thin layers of resin cement, the polymerization shrinkage is primarily directed uniaxially.12,14 Under normal clinical conditions, the resulting “wall-to-wall contraction” of the composite is proportional to the thickness of the cement. Consequently, well-adapted restorations will reduce the polymerization strains applied on the adhesive interfaces and provide improved internal adaptation and seal.12 Tight-fitting restorations can lock inside the cavity and prevent micromovements of the restoration or the tooth; however, larger cementing spaces can partially compensate for the polymerization forces by allowing the restoration and tooth to slightly move during the luting procedure. Marginal integrity with a passive insertion is the ideal clinical situation.
Furthermore, the vertical loss of the luting agent is related in part to the interfacial width between the wall of the preparation and the restoration itself. A clinical study at the University of Alabama has determined that the vertical loss of cement closely approximates the width by 50%.15,16 A marginal gap that is 200 µm at the time of cementation will result in a wear of the cement by 100 µm.
Precuring the bonding resin after acid-etching during the cementation appointment can prevent correct positioning of the restoration. Prehybridization, dual bonding, or resin bonding at the first appointment before the impression will stabilize the hybrid layer while preventing ingress of microorganisms through the dentinal tubules and prevent tooth sensitivity during the provisional phase. Additional curing of the adhesive through a layer of glycerin is suggested to remove the oxygen inhibition layer and prevent interaction of the dentin adhesive with the impression material (particularly polyethers).17,18 The wear pattern of resin-luting cements on the occlusal surfaces is greater than with restorative composites15 and, since the occlusal wear is directly proportional to the interfacial gap,19 the cementing gap should be reduced occlusally (Figure 1 and Figure 2).12 The proper sequence in the luting procedure may balance this complex interplay between polymerization shrinkage and adhesion and improve marginal adaptation and seal while improving bond strengths and further reducing the polymerization stress at the interface.
Clinical Considerations for Improving Adhesion and Finishing of Laboratory-Processed Composite Restorations
The clinical success of a composite restoration requires accomplishing function, esthetics, biocompatibility, and longevity.20 The attainment of these four criteria begins at the adhesive interface. A restorative material properly bonded to the enamel and dentin substrate will reduce marginal contraction gaps, microleakage, marginal staining, and caries recurrence; improve retention; reinforce tooth structure; and dissipate and reduce functional stresses across its interface throughout the entire tooth while improving the natural esthetics and wear resistance.21-25 The following clinical considerations provide the prerequisites for achieving these criteria.
A fundamental requirement for successful bonding requires the use of the dental dam. Contamination of the enamel and dentin with saliva, moisture from intraoral humidity, blood, and crevicular fluid can compromise the longevity of the adhesive restoration by affecting the adhesion at the interface and reducing bond strengths.26,27 Numerous studies have reported microleakage, reduced adhesion, and bond strength reduction from contamination of enamel with saliva,28-31 moisture,32-37 and contamination from crevicular fluid.37
In saliva contamination, the saliva acts as a film barrier at the contact level between the resin and the enamel; therefore, the surface energy of the enamel is lowered, which prevents optimal adhesion.31 Also, Hormati and others have reported that saliva contamination of the etched surface of the enamel affects the morphological characteristics of the surface and the glyco-proteins in the saliva block the micropores which form during the etching procedure.27,30 Moisture contamination from intraoral humidity can form a thin layer of moisture on the etched enamel, which could prevent maximum adaptation between the resin composite and the enamel surfaces. This phenomenon has been reported by Jorgensen32,33 and Fuji and colleagues.32,34 Moisture contamination from crevicular contamination has been reported to reduce bond strengths of composite to etched enamel by 70%.37,38 An in vivo study demonstrated that composite resin bonded to etched enamel surfaces using rubber dam isolation had significantly less microleakage than those with isolation using cotton rolls.31,32 Therefore, for optimal bonded composite restorations, moisture control should be performed throughout the adhesive procedure which requires adequate insulation and isolation of the operating field by rubber dam.39
The tooth-restorative interface is constantly subjected to polymerization shrinkage stress. Before a restoration is even subjected to functional loads and thermal strains, there is an initial interfacial stress developing during the polymerization of restorative materials and adhesion to tooth structure.40 Therefore, a comprehensive understanding of the complex interplay between polymerization shrinkage and adhesion is necessary. The cross-linking of resin monomers into polymers is responsible for an unconstrained volumetric shrinkage of 2% to 5%.41 The uncompensated forces may exceed the bond strength of the tooth-restoration interface, resulting in a gap formation from a loss of adhesion.42 Bacterial and fluid penetration through the marginal gaps may occur, causing colonization of microorganisms,43 recurrent caries, and postoperative sensitivity with possible subsequent irritation of the pulp,44 all of which effectuate clinical failure.45,46 However, this phenomena is more specific to direct restorations with certain cavity configurations (C-factors). Since indirect restorations are polymerized outside of the oral cavity, shrinkage stress is less of a concern because of the limited volume of the resin luting cement.
Changes in the chemical composition of the primary dentin as a result of response to stimuli can compromise an adhesive procedure. Alterations in the mineral content and structure of the dentin can result from age, caries, or external stimuli from the oral cavity (ie, abrasion, abfraction, or erosion lesions ).47,48 The peritubular cuff increases in thickness toward the center of the tubule, which can result in a calcified blockage of the tubular lumina.49 Dentin that has experienced such microstructural modifications is called sclerotic dentin.7,50 These sclerotic regions are hard, dense, and calcified to protect the pulp from subsequent stimuli. When there is significant sclerosis, the dentin commonly becomes dark yellow or discolored with a glassy or translucent appearance. These calcified regions are resistant to acidic conditioning solutions and, therefore, resin penetration is limited and the hybrid formation is very thin,51,52 which presents a challenge to consistent and reliable bonding. Therefore, during the diagnostic and treatment planning phase, the clinician should focus attention to the differing dentin compositions in preparation for the proper restorative modality.
The proper finishing and polishing protocol can influence the longevity of indirect restorations by affecting wear resistance53,54 and marginal integrity. Delayed finishing and polishing with various techniques result in a surface of similar hardness to or harder than that obtained with immediate finishing. Incidentally, the effects of delayed finishing appear to be bonding system- and tissue-specific.55 The bond strength of resin to enamel and dentin is higher at 24 hours than immediately after placement.56,57 Since some of this improvement occurs within several minutes after placement of the restoration, it is suggested that a brief delay in the finishing procedure may help to preserve the marginal integrity.56
Next-generation formulations of microhybrids have altered filler components with finer filler size, shape, orientation, and concentration, and, in combination with a higher degree of conversion through postcuring, their physical and mechanical characteristics are improved,58-62 thereby allowing the resin composite to be polished to a higher degree.59 The variation in hardness between the inorganic filler and the matrix can result in surface roughness because these two components do not abrade uniformly.59,60 Accordingly, it is imperative that the surface gloss between the restorative material and tooth interface is similar because the gloss can influence color perception and shade matching of the restoration and tooth surface.61,62
The esthetic appearance of the surface of a composite resin restoration is a direct reflection of the instrument system used.63 The surface of the composite can be finished and polished with a variety of techniques. Diamonds, multi-fluted burs, discs, polishing points, and cups have all been used to reproduce the shape, color, and luster of the natural dentition. As Pratten and Johnson have indicated, there is no statistical difference between finishing and polishing anterior and posterior restorative materials.64 The consideration factors for finishing and polishing any restoration are dependent on the instrument shape, the surface shape and texture of the tooth and restorations, and the surface of the finishing and polishing instruments as well as the sequence of the restorative treatment.64
The margins of indirect composite restorations should be re-etched and sealed with low-viscosity resins (eg, Fortify™ Plus, Bisco, Inc, Schaumberg, IL; PermaSeal®, Ultradent Products, Inc, South Jordan, UT). Application of a composite surface sealant after the initial finishing procedure may help to seal microcracks or microscopic porosities that may have been formed during the procedure. In addition, this application has been shown to reduce the wear rate of posterior composite restorations.65,66
Clinical Procedure: Adhesive and Finishing Protocols
Adhesive Tooth Surface Preparation
At the time of final bonding of the restoration, a throat pack of gauze was placed before removal of the provisional and during try-in of the composite inlay to protect the patient from aspirating the restoration.67 The provisional was removed and the restoration was tried in for the evaluation of color and marginal adaptation. The interproximal contact was inspected and necessary equilibrations were made. The teeth were isolated with a dental dam to protect against contamination. This process involved the creation of an elongated hole that allowed placement of the dental dam over the retainers to achieve adequate field control.68,69 The cavity preparation was cleaned with a slurry mixture of disinfectant and pumice (Consepsis®, Ultradent Products, Inc) to remove the pre-hybridization layer and rinsed thoroughly to eliminate all of the abrasive particles (Figure 3). The total-etch technique was used because of its ability to minimize the potential of microleakage and enhance bond strength to dentin and enamel.70-72 A soft metal strip was placed interproximally to isolate the prepared tooth from the adjacent dentition. The preparation was etched for 15 seconds with 32% phosphoric acid semigel with benzalkonium chloride (Uni-Etch® with BAC, Bisco, Inc; Ultra-Etch® 35%, Ultradent Products, Inc; Scotchbond™ Etchant, 3M™ ESPE™, St. Paul, MN), rinsed for 5 seconds, and lightly air-dried to avoid dessication. Once the dentin and the enamel were remoistened with water or a rewetting agent (Aqua-Prep™ F, Bisco, Inc; Tubulicid Red, Global Dental Products, North Bellmore, NY) on an applicator, a hydrophilic adhesive system was used. A dual-cure composite resin (Duo-Link™, Bisco, Inc; RelyX™, 3M™ ESPE™; Variolink® II, Ivoclar Vivadent, Inc, Amherst, NY) was selected as a cementation material. A single component adhesive (One-Step® Plus, Bisco, Inc; G-Bond, GC America, Alsip, IL; Prime & Bond® NT™, Dentsply Caulk, Milford, DE) was applied in two to three coats with an applicator, air-dried for 10 seconds, and light-cured for 10 seconds (Figure 4A and Figure 4B).
Adhesive Surface Preparation of the Restoration
The surface of laboratory-processed composite resins is highly polymerized with minimal unreacted free-end radicals for bonding to the resin cement. While microleakage has been reported to occur at this interface between the internal surface of the inlay/onlay and the resin cement in the absence of composite softening agents,73 several surface treatments have been advocated to promote adhesion between the resin cement and the indirect composite restoration. Mechanical roughening of the internal surface of the inlay can be accomplished with small particle diamond burs or micro-etching with either 50-µm aluminum oxide particles or 30-µm silanized silica-coated aluminum oxide particles, which creates a micromechanical retention bond at a microscopic level between the restorative material and the resin cement. In addition to mechanical roughening, an application of proprietary softening agents, wetting agents, or silane has been reported to enhance the bond strength between the restoration and the resin cement.74
The manufacturers of indirect resin systems have recommended various pre-cementation protocols. The authors’ standard cementation protocol for laboratory-processed composite resins includes micro-etching with a silicate ceramic sand (CoJet™ Sand, 3M™ ESPE™) and subsequent application of silane to restore any coating on the original fillers that may have been removed by sandblasting. As a bifunctional molecule, the silane acts as a coupling agent between the filler particles on the indirect resin surface and the resin cement. Newer formulations of silane that include a monomer (ie, unfilled resin) further simplify the bonding process. Micro-etching of aged composite resin with silica-coated aluminum oxide particles results in higher bond strengths compared with other surface treatments for intraoral repair of composites.75 The mechanism of action allows the silicate particles to become embedded in the surface of the restoration during sandblasting, which then reacts with the silane to improve bond strengths.76 Reports indicate, however, that etching or rinsing after such surface treatment can significantly decrease shear bond strengths (Figure 5A and Figure 5B).74,77
Adhesive Bonding of the Onlay Restoration
After the surface treatment, the restoration was bonded with a dual-cure composite cement (Duo-Link, RelyX, Variolink II). The cement was mixed and loaded into a needle-tube syringe tip (Needle Tube, Centrix, Inc, Sheldon, CT) and injected into the entire preparation. A blunt tip instrument was used to seat and hold the restoration firmly in place and the excess cement was removed with a sable brush (Figure 6). It is imperative to leave a residual amount of cement to prevent voids and to compensate for the polymerization shrinkage of the cement. Initial polymerization was for 20 seconds while the restoration was held in place with the blunt tip instrument. The residual cement was removed with a sable brush and the interproximal was flossed, leaving only a small increment at the margin to counteract any polymerization shrinkage of the cement. A thin application of glycerin was applied to all of the margins to prevent the formation of an oxygen inhibition layer on the resin cement.71 The restoration was polymerized from all aspects (facial, occlusal, lingual, and proximal surfaces) for 60 seconds, respectively. After the resin cement had been polymerized, any excess at the gingival margin was removed with a scalpel (#12, Bard Parker, Franklin Lakes, NJ) (Figure 7).
Finishing and Polishing Protocol
The fabrication of indirect composite resin restorations requires careful development and shaping of the composite resin in accordance to the confines of the preoperative occlusal registration before curing of the material, which facilitates the establishment of anatomic morphology and minimizes the finishing protocol.78 A meticulous finishing protocol may provide the benefit of increased longevity of the restoration.79,80 The smoothness of the composite surface depends on the curing system, the components within the restorative material, and the finishing instruments.58 The gingival and interproximal regions were finished with #30 fluted needle-shaped finishing burs (Composite Finishing Preparation Kit, Bisco, Inc; ET® 6, Brasseler USA, Savannah, GA) and the occlusal anatomy was refined with #30 fluted egg-shaped finishing burs (9406 Midwest®, Dentsply Professional, York, PA; R.A.P.T.O.R.® posterior finishing bur, Bisco, Inc) (Figure 8). After the initial finishing procedure, the margins and surface defects were sealed. All accessible margins were etched with a 32% phosphoric acid semi-gel, rinsed, and dried. A composite surface sealant (Fortify™, Bisco, Inc) was applied and cured to seal any cracks or microscopic porosities that may have formed during the finishing procedures (Figure 9). The restoration was final polished with rubber points and cups and composite resin polishing paste (Figure 10A, Figure 10B and Figure 10C). The proximal surface was smoothed with polishing paste and plastic finishing strips. The dental dam was removed and the patient was asked to first perform closure without force and then centric, protrusive, and lateral excursions. Any necessary equilibration was accomplished with a #12 and #30 egg-shaped finishing bur and the final polishing was repeated. The contact was tested with unwaxed floss and the margins inspected. The final result illustrates the harmonious integration of composite resin with existing tooth structure (Figure 11).
Conclusion
As the restorative philosophy for the modern dentist changes, the mindset of the clinician must be transformed to continue to explore and develop ideas, techniques, and protocol. However, the knowledge and desire to create are limited by the products clinicians have available to them for restorative procedures, and knowledge must be integrated with the proper technique for each clinical situation. Considerable progress in adhesive technology and indirect restorative systems has allowed the clinician to create esthetic restorations that not only preserve, but reinforce, tooth structure.
Manufacturers and scientists are leading the way in the new advances in restorative materials and adhesive technology. These techniques, concepts, and ideas from clinicians, scientists, and technicians worldwide are the spark that ignites the reaction. However, it is the experience and judgment of the clinician and technician that is the “true catalyst” of the reaction that creates form, function, esthetics, and longevity.
Therefore, by working together in proper sequence, the clinician and ceramist team can develop restorations that are biologically and mechanically sound. Only through continued education, commitment to excellence, and communication (both quantitative and qualitative) between clinical and laboratory colleagues can restorations that reflect the continuous progress in esthetic and restorative dentistry be fabricated and delivered.
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About the Authors
Douglas A. Terry, DDS
Assistant Professor, Department of Restorative Dentistry and Biomaterials
University of Texas Health Science Center, Dental Branch
Houston, Texas
Private Practice, Esthetic and Restorative Dentistry
Houston, Texas
Karl F. Leinfelder, DDS, MS
Adjunct Professor, Biomaterials Clinical Research
University of North Carolina
Chapel Hill, North Carolina
Professor Emeritus, University of Alabama School of Dentistry
Birmingham, Alabama
