Over the last two decades, the decrease in caries frequency, increasing aesthetic expectations, and the controversy over the toxicity of amalgam have contributed to the introduction of novel materials and the development of innovative techniques for providing aesthetic posterior restorations.1 Composite resins can be used in the posterior with different techniques (eg, direct, semidirect, indirect),3 and selection among them is dictated by different factors that include general parameters (eg, caries susceptibility, aesthetic requirements) and local parameters (eg, cavity dimension, tooth position in arch).4 This article will discuss a variety of means for achieving success in the posterior regions and provide indications for the clinician’s selection of each technique and material.
The direct technique is indicated for preventive purposes, such as Class I or II restorations of small to medium size. While composite resin can be used with relative ease to obtain excellent results in Class I defects (Figures 1 and 2), Class II restorations present greater challenges (Figures 3-4-5-6). When performing direct Class II restorations, clinicians have two fundamental requirements: 1) compensate for the inevitable polymerization contraction; and 2) recreate adequate morphology (ie, an effective proximal contact point) that is functionally correct and aesthetically valid. The effect of polymerization stress is evident at two levels: proportional to the cavity with possible distortion or microfracture of the actual walls and at the level of the adhesive interface at the margin or internally.5,6
Polymerization stress is correlated to the physicochemical characteristics of the material and the adhesive bond between the restorative material. Feilzer et al have demonstrated a relationship between the cavity configuration and the development of contraction stress, introducing the concept of the C-factor that consists of the relationship between the adhesive surface of the dental substrate and the free surface to deform and absorb the tension during contraction.7 To oppose the polymerization contraction and improve the marginal adaptation of Class II restorations, various procedures have been suggested.
The placement of a material with a low modulus of elasticity (ie, Young’s modulus, 4 Gpa) is recommended at the base of the cavity in contact with the dental adhesive. The use of a flowable composite provides increased elasticity than dentin (ie, 18 Gpa), with a higher capacity to flow internally; this material is able to partially compensate for polymerization stress with notable advantages at both the level of adaptation between adhesive and composite as well as reduction of postoperative sensitivity.15-17 A “natural” stratification construction of the core of the restoration should be performed using dentin materials, while the marginal ridge and superficial layers are created with enamel materials and stains. Such anatomic stratification allows the development of a homogenous material thickness, a more natural and aesthetic reconstruction, and aids in occlusal adjustment and finishing.
The use of a rubber dam to ensure isolation of the field is an integral phase of the operative protocol.18 Tooth preparation for an adhesive restoration must respect the principle of maximum conservation of healthy residual tissue,3 preserving certain structures (ie, proximal ridges, enamel ridges, healthy superficial occlusal surfaces) even if the enamel is not completely supported by dentin. As far as the classification of adhesive cavities is concerned, one can distinguish three types of preparations for direct restorations: preventive preparations, adhesive preparations,19 and traditional bevel preparations.
When performing the direct technique, a butt joint can be placed at the gingival margin level.23 The metallic matrix and the wedge are positioned in such a way as to have an optimal adaptation to the cervical margin and contact with the adjacent tooth. The next step is application of the adhesive agent. Among the most recent adhesives, the authors prefer a three-stage material with the total-etch technique: 1) etching with 35% phosphoric acid gel; 2) application of a primer in two or three layers; and 3) application of a bonding agent.24,25
Stratification of the composite with a first increment is then begun with a 0.5-mm layer of flowable composite. Next, the proximal wall is reconstructed with a thin, vertical layer of enamel composite stretching between the walls of the box, which can be further reinforced with a second layer. After the matrix is removed, one has transformed a Class II cavity into a Class I cavity (Figures 7-8-9-10), which aids in the achievement of proper occlusal and proximal anatomies. The third increment is an oblique stratification performed with the use of opaque dentin masses. The fissure patterns anticipated in the dentin are highlighted using resin with intense coloration, which gives an increased depth and natural appearance to the restoration. The fourth increment consists of anatomic stratification and modelling with opalescent enamel masses having different luminosity and translucency. The photopolymerization of the final layer of composite is performed under glycerine gel so as to inhibit the surface oxygen and enable a complete conversion of the free radicals.
A box carver is used to remove any excess resin from the external axial walls. After preparing the convex part of the marginal ridge above the contact point, round carbide burs are used to reduce the marginal ridge and carry out minor occlusal adjustments. The interproximal space at the level of the gingival margin is finished with thin abrasive strips; a flame-shaped fine-grit diamond bur is used on the occlusal surfaces to level the cavity borders. These structures are subsequently polished using rubber points mounted in the high speed contrangle handpiece with copious water spray.
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Semidirect and Indirect Restorations
Restorations cemented adhesively using semidirect (ie, intra- and extraoral) techniques are indicated in clinical cases that involve large Class I and Class II cavities, or when cuspal coverage, treatment of cervical enamel with low thickness (less than 1 mm), and accentuated concavities of the cervical border require treatment. The fundamental advantage of these cemented techniques lies in the possibility of reducing the polymerization contraction of the resin material, which occurs externally during fabrication. These techniques offer the possibility of having an ideal anatomic buildup and occlusal rehabilitation.
The semidirect technique utilizes a single visit to complete the entire restoration. With the semidirect technique, the prepared tooth is used as a model to produce the inlay. The author finds that it is the most fastidious technique, particularly during the removal of the restoration from the cavity prior to cementation, and the most precise because the inlay is produced directly on the tooth. It is indicated in medium to large cavities of one or two surfaces in areas with excellent operative access. The preparation should also have divergent cavity walls (ie, 15° to 18°), a uniform cavity design with smooth walls, and no undercuts.27 Following cavity preparation, careful isolation with a water soluble glycerine-based separator, is necessary and precedes composite stratification with the classic dentin and enamel masses and their polymerization. The inlay is then removed and finished, tried in the cavity, postpolymerized in an oven, and then adhesively cemented.
The extraoral semidirect technique provides for the production of an inlay chairside (ie, in rigid quick-set addition-cured silicone) from an impression of the prepared tooth and cemented at the same session (Figures 11-12-13-14).28 The impression material recommended should be either a condensation silicone or polyether that can be easily separated upon removal from the model. Indications include large Class I and II cavities with up to three surfaces. The difference from the intraoral technique is that a large divergence of the cavity walls is not required (ie, 6° to 10° is sufficient) and small correctable undercuts are acceptable. Once the composite stratification is completed, the inlay is easily removed, tried in, finished, tempered, and cemented.
The indirect technique takes place over two visits: the first for the preparation and impression taking, the second for the cementation of the restoration and application of provisional restorations. Indications include numerous and large Class I and II cavities with eventual cusp coverage, complete crown coverage (ie, overlay), and quadrant or full-arch rehabilitation (Figures 15-16-17-18).
The first step of the cavity preparation is the removal of carious tissue and tissue that is not adequately supported before evaluating the residual thickness. The clinician then proceeds with the internal reconstruction of the cavity to seal the dentin, block out any undercuts, determine the correct form, and reduce the thickness of the inlay via the composite material and adhesive technique. The modified dual-bonding technique can be used for this purpose,29 which involves the hybridization and complete sealing of dentin, the application of a thin layer of flowable composite, and stratification of a composite base using an incremental technique that anticipates and follows the anatomy of the tooth. The principles of tooth preparation for adhesive restorations require a cavity design with divergent walls, rounded internal angles, and bevel-free butt finishing of enamel. The oblique sectioning of the enamel prisms, which are not ideal for adhesion, is compensated for by the reduced thickness of composite on cementation. It is also important to evaluate the occlusal preparation depth to ensure adequate thickness of the inlay.
Inlays require smooth and well-defined walls with a uniform general cavity design and divergence of walls 6° to 10° (ie, semidirect extraoral) or 15° to 18° (ie, semidirect intraoral). The restoration margins must not be coincident with occlusal contacts, and the thickness of the residual walls can be greater than 2 mm to 2.5 mm, the size of occlusal isthmus that is greater than 2 mm for composite, the thickness of material for cusp coverage that is greater than 1.5 mm for composite (greater than 2 mm for ceramic),4 and overjet of material proximally less than 2 mm. Following preparation, an impression is taken and the cavity is temporarily filled with soft photopolymerizable resin.
For semidirect techniques, the composite inlay is directly manufactured and no separator is required. For the direct technique, the plaster model is isolated with a wax application on the internal angles of the cavity and separating agent on the plaster. For each technique, a microhybrid composite is stratified using a central dentin nucleus that is chromatically saturated, followed by opalescent and transparent enamel masses, prior to creation of the final anatomic definition. The inlays thus obtained are subjected to postpolymerization with photothermic treatment (ie, 130°C for 7 minutes), which confers anexcellent conversion rate, a greater dimensional stability, and improved physical and mechanical properties.30 Prior to cementation, the fit, precision, and color are verified, and corrected intraorally with composite.
When using an adhesive cementation protocol, it is extremely important to correctly isolate the field by applying a rubber dam. Cavity treatment consists of cleaning, disinfecting, etching, and application of dentin-enamel adhesive. No photopolymerization of resin is required. Treatment of the inlay involves a sandblast-fitting surface, silanization, and application of composite resin (ie, no polymerization).Aphotopolymerizable restorative composite is used for cementing the inlay and is applied in the cavity and/or on the restoration and distended over all of the cavity walls. The composite cement is isolated with glycerine gel and each surface is cured for two minutes, for a total of 6 minutes, if necessary using two curing lamps. Finishing and polishing procedures for indirect restorations are similar to those for direct restorations. After the removal of the dam, an occlusal examination and a bitewing radiograph are necessary. For the maintenance of the restorations, the margins under the rubber dam may be sealed with highly filled resin after etching.
Recent advancements in dentin-enamel bonding and contemporary composite materials have completely revolutionized the therapeutic approach to the restoration of the posterior region. These advancements have allowed improved maintenance of healthy dental tissue during tooth preparation--in fact, bonding techniques have entirely eliminated the need for destructive mechanical retention. Contemporary restorative materials have also resulted in increased material reliability and improved aesthetics.
In order to ensure restorative success, it is important to know the indications for posterior composite resins and if a direct, indirect, or semidirect technique is required. In order to attain a predictable result, an in-depth knowledge of the materials, techniques, and their applications, as well as a rigorous respect of clinical procedures are, therefore, of fundamental importance.
*L.M.D. University of Geneva Switzerland; private practice, Parma and Piacenza, Italy.
†Visiting professor of the Accademia Italiana di Conservativa; private practice, Piacenza, Italy.
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