Contemporary polymer adhesive systems are used primarily to protect the pulp-dentin complex and bond the restorative composite resin to the tooth structure, providing retention and resistance to microleakage.1 Most of the current procedures in dentin bonding technology involve the acid etching of enamel and dentin. While dentin etching has been performed since the late 1970s,2 this concept has only recently received acceptance. Several total etch one step adhesive materials have been recently developed in response to the demands of clinicians, who have sought more efficacious materials for use with the technique.
Self-etch approaches too have gained popularity among clinicians, purportedly to help simplify the adhesive process and minimize postoperative sensitivity in patients. Selection of the total etch or self-etch technique now depends principally on the preference of the operator.
The total etch technique depends upon the micromechanical retention established by an acidic treatment of the enamel and dentin prepared surfaces and subsequent infiltration of a blend of polymerizable monomers into the etched substrate.3 Due to the wet tubular ultrastructure and organic composition of the dentin substrate, bonding to dentin has been more difficult than bonding to enamel. High vapor pressure organic solvents (eg, acetone or ethanol) are now utilized to carry resin monomers into contact with the dentin collagen fibers exposed by the demineralizing action of the acidic etchant, resulting in a nanostream of resin within the spaces formed by the proximity fibers.
This restorative step requires the application of an acidic agent to the enamel and dentin simultaneously (Figure 1). The acid etching of dentin exposes a rich layer of collagen fibrils with little mineral support, which must be kept slightly moist following the removal of the etchant with water (Figure 2).4 Once the etchant has been rinsed from the preparation and the surface has been gently dried, the adhesive system is applied (Figure 3). Several of the current dentin adhesives rely on the permeation of hydrophilic monomers into acid conditioned dentin,3 resulting in the development of resin tags and an entanglement of resin with collagen and with residual hydroxyl apatite crystals that have been named the “hybrid layer” or “resin dentin interdiffusion zone”3 (Figure 4). This layer has been reported to result in high bond strengths, particularly when acid etched dentin remains in a moist state.4,5 The adhesive system can be applied as a monocomponent system (Figure 5) or with a primer and a fluid resin. Additional systems have integrated the acid and the primer into a single solution without the separated etching step. It is important that an adhesive system with proven efficacy, both in vitro and in vivo (if possible), is selected, and that it be compatible with the base free total etch restorative technique.
The Use of Stress Absorbing Lining Materials or Flowable Resins
It has been suggested to use low viscosity resins, or “stress absorbing lining materials” as intermediate restorative materials between the adhesive system and the restorative composite resin (Figure 6).6 These materials partially compensate for the stresses generated upon composite polymerization. In the authors’ experience, these advantages appear to reduce postoperative sensitivity in direct posterior composite resin restorations.
Low viscosity (or “flowable”) composite resins also have limitations, however, and their use has not been universally accepted. Since these materials have a high organic matrix content, their polymerization shrinkage is greater than the shrinkage of the restorative composite by itself, which could result in increased curing stress.7 In addition, the “elasticity” inherent to the low viscosity resins can introduce a “cushion” effect at the restoration/cavity interface, which can assist in stress absorption, but could also result in adhesion fatigue.8,9
Selection of Restorative Resins
Among the variety of available composite resins, hybrid or “microhybrid” resins10 (Figures 7 and 8) and nanofill resins appear to be suitable for use in the posterior region. Composites are generally classified according to the size of their fillers, and advances in such materials are ongoing as manufacturers work to improve their mechanical properties, wear resistance, radiopacity, handling, and other such characteristics desired by dental professionals.
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Placement and Polymerization of the Composite Resin
Extensive research has been completed on placement and polymerization techniques used for composite resins; horizontal, oblique, vertical, and bulk incremental techniques have all been recommended.11,12-14 Although the incremental layering technique has been widely advocated for the reduction of polymerization shrinkage, reports and recent finite element analyses now dispute this theory.15 Microleakage assessments comparing different restorative techniques are not conclusive,16 and a standard, state-of-the-art sequence has not yet been universally accepted.
For numerous reasons, however, the incremental placement technique is still regarded as the most practical. This procedure provides enhanced control of “condensation” of the individual increments of composite resin, thorough polymerization of the restorative material, and control of overhangs in the lateral margins prior to curing. The incremental process also permits the orientation of the light beam according to the position of each composite layer, the intrinsic characterization of the restoration with tints, and control of sculpture on the occlusal surface of the restoration in the final increments (Figures 9-10-11-12-13).
The number of increments required varies according to the size of the cavity. Conservative cavities can be restored with a single increment. These procedures require the use of an efficient curing unit with a minimum light output of 450 mW/cm2.17 Moderate and large proximal cavities should be restored incrementally, using
layers with a maximum thickness of 2 mm,17 and the distribution of each increment varies according to the geometric shape of the cavity.
The application of the composite resin on the occlusal aspect should be guided by the anatomy of the tooth. If the entire anatomy can be shaped during the insertion of the composite resin, the need for burs during the finishing phase is reduced, minimizing the complications associated with this procedure. This is a considerable advantage of the light cured composites, ie, the clinician can apply and shape the composite resin to the final contour prior to the setting of the material. Uncured composite can be effectively shaped with a variety of metal or plastic instruments and brushes to establish contour and surface smoothness.
The use of ceramic or prepolymerized “inserts” has been suggested as a way to improve the performance of posterior Class II composite resin restorations.18 This procedure attempts to minimize the volume of composite inside the cavity, diminishing polymerization stress, and to add reinforcement to the restoration mass with an optimized material, either by the extraoral polymerization of the resin insert or by the inherent properties of the ceramic insert. Despite these hypothetical advantages, the clinical benefits have not been clearly proven, and this demanding procedure involves the use of additional techniques and apparatus.
Finishing and Polishing
When the composite resin is carefully placed with an anatomical matrix, the need for finishing and polishing with hand or rotary instruments is significantly reduced (Figure 14). The use of cutting instruments on the polymerized resin can induce faults on the tooth/restoration interface and also on the restoration surface,19 which can cause the short term failure of the restoration.
When necessary, excess composite resin can be removed with hand instruments. Reciprocating instruments are also indicated for the removal of marginal and proximal overhangs on Class II cavities. The anatomy of the restoration can also be refined with medium, fine, and super fine diamonds in high speed (with slow velocity). In order to facilitate the visualization of the tooth restoration interface, these instruments should be used in a dry field.
It is difficult to polish posterior composite resins to a high gloss, as is possible with microfill composites for anterior teeth.20 Aesthetic results can be achieved, however, utilizing extra fine polishing pastes applied with nylon brushes and/or silicone points and cups at slow speed.
Surface penetrating sealants have been advocated19 as a method to “seal” the restoration surface and margins following polishing. Once phosphoric acid has been applied on the accessible surface and margins of the restoration, the sealants should be applied on the etched areas. Unfortunately, no longitudinal clinical studies have been completed to validate this procedure.
Maintenance of Direct Restorations With Composite Resins
In addition to meticulous clinical execution, one of the objectives of the treatment should be to increase the longevity of the restoration. The patient should be periodically reevaluated to detect possible failures due to technique, material, or case related factors, and correct these restorations. In addition, the patient should be reoriented regarding the control and prevention of dental caries and periodontal disease. The procedures most frequently performed in the follow up appointments are minor surface and/or marginal repolishing and/or recontouring.21
In order to improve the life expectancy of direct composite resin restorations, ultrasonic scalers, sonic scalers, hand scalers, coarse prophy pastes, and dyes (caries and/or plaque detectors) should not be applied and/or used on these restorations. Aluminum oxide polishing pastes should be utilized to remove superficial stains and polish the restorations. Finishing diamond strips and dental floss can be applied with the polishing paste to remove stains interproximally.
The next phase in the evolution of the composite resin materials will be the introduction of the “condensable” composites and the “shrinkage free” polymers.22,23 The “condensable” composites are materials with improved handling properties. These materials offer greater resistance to condensation, enabling the achievement of interproximal contacts and contour. Their condensability, however, is not comparable to the condensability offered by amalgam. Other specifically reinforced composite resins, such as composites based on the Polymeric Rigid Inorganic Matrix Material, may be available shortly.22 Although no independent clinical and laboratory data have been published to support these new materials, preliminary clinical trials performed with a condensable composite indicate favorable initial results (Dr. Jorge Perdiago, personal communication).
Other researchers are working to produce nonshrinking monomers. 24 “Shrinkage free” polymers will also be a positive development in the use of composite resins as restorative materials, since complications caused by concentration stress will be eliminated.
The use of direct composite resins in posterior teeth has gained acceptance and is becoming a routine procedure to implement. As an understanding of the properties and characteristics becomes increasingly familiar to the clinician, the quality of treatment provided by these healthcare professionals will similarly increase. This article has presented a series of factors related to the application of composite resins for the direct restoration of posterior teeth. It has addressed potential advantages and limitations of this modality, as well as discussed the various stages of treatment (planning, preparation, application, adhesion, and finishing) that comprise the restorative process. In order to further illustrate the procedures involved in this application of composite resin, several cases have been presented to illustrate the techniques described. Proper attention to these techniques allows the attending clinician to provide comprehensive aesthetic care that satisfies not only the objectives of the patient, but also provides enhanced aesthetics.
*Department of Operative Dentistry, School of Dentistry, Universidade Federal de Santa Catarina, Florianopolis, SC, Brazil
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- Neiva IF, Andrada MAC, Baratieri LN, et al. The effect of restorative techniques on marginal leakage in posterior composite resin restorations: An in vitro study. Oper Dent (In press).
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- Coli P, Derhami K, Brännström M. In vitro marginal leakage around Class II resin composite restorations with glass-ceramic inserts. Quint Int 1997;28:755-760.
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