Indirect resin composites

Aesthetic dentistry continues to evolve through innovations in bonding agents, restorative materials, and conservative preparation techniques. The use of direct composite restoration in posterior teeth is limited to relatively small cavities due to polymerization stresses. Indirect composites offer an esthetic alternative to ceramics for posterior teeth. This review article focuses on the material aspect of the newer generation of composites. This review was based on a PubMed database search which we limited to peer­reviewed articles in English that were published between 1990 and 2010 in dental journals. The key words used were ‘indirect resin composites,’ composite inlays,’ and ‘fiber­reinforced composites.’


INTRODUCTION
Dental composite formulations have been continuously evolving ever since BisGMA was introduced to dentistry by Bowen in 1962. Recent developments in material science technology have considerably improved the physical properties of resinbased composites and expanded their clinical applications. Dental restorative composite materials can be divided into direct and indirect resin composites (IRC). IRCs are also referred to as prosthetic composites or laboratory composites. These materials offer an esthetic alternative for large posterior restorations. There are a plethora of materials available nowadays.

THE NEED FOR IRC
Dental resin composites were introduced initially for use as anterior restorative materials. Later, with technological improvements, the prospect of restoring posterior teeth with composite was introduced. Though there are numerous causes for failure of clinical restorations made of direct composites, the major cause with the earlier posterior composites was poor wear resistance. [1,2] While the newest direct composite resins offer excellent optical and mechanical properties, their use in larger posterior restorations is still a challenge since polymerization shrinkage remains a concern in cavities with high Cfactor. Though there have been numerous advances in adhesive systems, it is observed that the adhesive interface is unable to resist the polymerization stresses in enamelfree cavity margins. [3,4] This leads to improper sealing, which results in microleakage, postoperative sensitivity, and recurrent caries. The achievement of a proper interproximal contact and the complete cure of composite resins in the deepest regions of a cavity are other challenges related to direct composite restorations. Various approaches have been developed to improve some of the deficiencies of directplacement composites. [5,6] However, no method has completely eliminated the problem of marginal microleakage associated with direct composite. [4,7] IRCs were introduced to reduce polymerization shrinkage and improve the properties of material.
Though the mechanical properties of the IRCs are much inferior to that of ceramics, in some clinical situations, IRCs can supplement and complement (rather than replace) ceramic restorations: for example, in coronal restoration of dental implants. As ceramics exhibit a high modulus of elasticity and absorb little of the masticatory energy, considerable amount of the masticatory force is transmitted to the implant and the periosseous structure, reducing the longevity of the restoration. Polymers become the materials of choice in this situation because they absorb relatively more of the occlusal stress. For patients with poor periodontal structures who require occlusal coverage, stressabsorbing materials like IRCs are indicated [8] This review article focuses on the material aspect of this newer generation of composites. This review was based on a PubMed database search that we limited to peerreviewed Englishlanguage articles published between 1990 and 2010 in dental journals. For the literature search the key words used were 'indirect resin composites,' 'composite inlays,' and 'fiberreinforced composites.'

TYPES OF IRCS
Touati and Mörmann introduced the first generation of IRCs for posterior inlays and onlays in the 1980s. [9] Direct resin composites were composed mostly of organic resin matrix, inorganic filler, and coupling agent. The firstgeneration IRCs had a composition identical to that of the direct resin composite marketed by the same manufacturer and the materials also bore names similar to that of the direct materials.
Upon light initiation, camphoroquinone decomposes to form free radicals and initiates polymerization, resulting in the formation of a highly crosslinked polymer. It is observed that 25%-50% of the methacrylate group remains unpolymerized. [10] For inlay composites, an additional or secondary cure is given extraorally, which improves the degree of conversion and also reduces the side effects of polymerization shrinkage. The only shrinkage that is unavoidable is that of the luting cement. [11] It was observed that the firstgeneration IRCs showed improved properties only in lab studies but had failures in clinical studies. With the firstgeneration composites either a direct-indirect /semiindirect method or an indirect method was used to fabricate the restoration. [12] Directindirect/semiindirect method The composite material is condensed into the cavity after the separating medium is applied to the cavity. This separating medium helps in easy removal of the inlay after the initial intraoral curing. The restoration is then subjected to extraoral light or heat tempering in an oven. DI500 Oven (Coltene Whaledent) or a Cerinate Oven (DenMat Corp) can be used at 110°C for 7 min. This technique eliminates the need for an impression of the cavity and the procedure can be completed in a single sitting. [12] Brilliant DI (Coltene Whaledent) and True Vitality (DenMat Corp) are examples of material that uses both light and heat for this technique.

Indirect
The inlay is fabricated in a die. After the separating medium is applied to the die, composite material is condensed in increments into the cavity and light cured for 40 sec for each surface. The inlay is then removed and heat cured in an oven at 100°C for 15 min (CRC100 Curing Oven , Kuraray). The advantage of this technique is that the proximal contours can be achieved appropriately. One of the first materials introduced by Ivoclar was SRIsosit , which was marketed as Concept in the US. This system uses a hydropneumatic heat cure in the Ivomat apparatus. The polymerization takes place in water at 120°C and a pressure of 6 bar for 10 min. [13] Another example of indirect material is Clearfil CR Inlay (Kuraray),which uses light and heat for the indirect technique. Conquest (Jeneric/Pentron), EOS (Vivadent), and Dentacolor (Kulzer) use only heat for additional curing, whereas VisioGem (ESPEPremiere) uses heat and vaccum for additional curing. [12] It is possible to use any posterior composite for indirect techniques with additional curing.

PROPERTIES OF FIRSTGENERATION COMPOSITES
Various studies have demonstrated the properties of the firstgeneration composites. It was observed that the degree of conversion increased by 6%-44%. Flexural strength ranges from 10-60 MPa and elasticity  [13][14][15][16] The effect of additional cure may vary among the different studies because certain materials respond better to additional cure and because different methodologies may have been employed for determining these parameters. Postcure temperature had a much higher influence on the degree of conversion than postcure duration. Wendt [17] demonstrated that a 5min postlightheat treatment at 123°C (253°F) increased the hardness and wear resistance by as much as 60%-70%. But, clinically, heat treatment did not influence the wear resistance of the clinical restorations. Regardless of time, the wear rates for the heattreated and nonheattreated resin restorations were exactly the same: around 60 µ in 3 years. Clinical studies of other compositions given the same heat treatment generated similar results. [18,19] It was observed that supplementing conventional photocure with additional cure increased the monomer conversion but did not necessarily improve the physical properties. [13]

DISADVANTAGES OF FIRSTGENERATION COMPOSITES
Firstgeneration composites showed poor In vitro and clinical performance. Deficient bonding between organic matrix and inorganic fillers was the main problem leading to unsatisfactory wear resistance, high incidence of bulk fracture, marginal gap, microleakage, and adhesive failure in the first attempts to restore posterior teeth. Measures to solve these problems included increasing of inorganic filler content, reduction of filler size, and modification of the polymerization system.

SECONDGENERATION IRC
The clinical failures endured with firstgeneration composites and the limitations faced with ceramic restorations led to the development of improved secondgeneration composites. The improvements occurred mainly in three areas: structure and composition, polymerization technique, and fiber reinforcement. [9] Structure and composition The secondgeneration composites have a 'microhybrid' filler with a diameter of 0.04-1 µ, which is in contrast to that of the firstgeneration composites that were microfilled. The filler content was also twice that of the organic matrix in the latter composites. By increasing the filler load, the mechanical properties and wear resistance is improved, and by reducing the organic resin matrix, the polymerization shrinkage is reduced. [9] The new composite resins like Artglass and belleGlass HP contain high amounts of filler content, which make them adequate for restoring posterior teeth. Others, such as Solidex (Shofu Inc.), have intermediate filler loading, which enables better esthetics and are preferred for anterior tooth. [20,21] Polymerization techniques Even additional light curing extraorally did not efficiently improve the degree of conversion. Thus, specific conditions like heat, vacuum, pressure, and oxygenfree environment are utilized for polymerization of secondgeneration IRCs. [22] The various techniques used for additional cure are desribed below.
The temperature usually used for IRC ranges from 120-140°C. Ideally, the temperature applied in this treatment must be above the composite's glass transition temperature (Tg). [23] This allows a significant increase in polymer chain mobility, favoring additional crosslinking and stress relief. [24] Nevertheless, it is noteworthy that overheating may cause degradation of the composite. The heat can be applied in autoclaves, cast furnaces, or special ovens. [25] Postcure heating of resin composite materials decreases the levels of unreacted monomer after the initial lightcuring stage. Basically, two mechanisms can be involved in this phenomenon. First, the residual monomer would be covalently bonded to the polymer network as a result of the heat treatment, leading to increase in the conversion itself. Second, unreacted monomers would be volatilized during the heating process. [16] The combination of heat and light increases the thermal energy sufficiently to allow better doublebond conversion. This concept was first used by HeraeusKulzer in the development of Charisma . It was observed that the wear resistance increased by 35% on curing with both light and heat when compared to curing with light only. [26] Air, because it contains oxygen, tends to inhibit polymerization and also plays an important role in the apparent translucency or opacity of the cured resin restoration. Oxygen entrapment in the restoration tends to break up or diffract natural light as it reflects from the surface of the restoration. Removing all of the encased air causes the restoration to become considerably more translucent. Entrapped oxygen increases the wear rate by weakening the wall around it. Nitrogen pressure eliminates internal oxygen before the material begins to cure. This influences the degree of conversion, esthetics, wear, and abrasion. [8] BelleGlass and Sculpture Plus employ this method of curing in a nitrogen bell.
The concept of slow curing described by Mehl [26] is based upon the concept that a slower rate of curing will allow a greater level of polymerization. Faster rates of polymerization tend to prematurely rigidify the newly formed polymerized branches. Such a condition will increase their stiffness, disallowing further propagation of the molecule. Such a concept is incorporated in the curing process for both belleGlass and Cristobal .
Electron beam irradiation is another method described for improving the composite's properties. [27] This methodology is used with polymers like polyethylene, polycarbonate, or polysulfone. [28] The two main reactions that occur when a polymer is subjected to electron beam irradiation are chain breakage and chain linkage. When breakage of chains occurs at the region of entanglement, there is induction of dense packing. This influences the bond between the filler and matrix, thus improving the mechanical properties and increasing success rates. The possible disadvantage of this method is polymer degradation and discoloration of the resin. The radiation dosage usually given is 200 KGy, but lower dosage like 1 KGy also has been shown to improve the properties. [29] Due to economic reasons it is impossible to irradiate single crowns or FPDs. Behr and Rosentritt demonstrated that irradiated raw materials of composites can be mixed with new material to improve properties.
Fiberreinforced composites were introduced by Smith in the 1960s. Polyethylene fibers, [30] carbon/graphite fibers, Kevlar , and glass fibers [31][32][33] were tested. Glass and polyethylene are the commonly used fibers in dentistry. Fibers act as crack stoppers and enhance the proprety of composite. The resin matrix acts to protect the fiber and fix their geometrical orientation. [34,35] Boron oxide, a glass forming agent is present at 6-9 wt% in Efibers and <1 wt% in Sfibers. E and Sfibers are the ones most commonly used in dentistry.[36,37] The details of the FRC are shown in Table 1a and 1b.[38,39] The fibers can be arranged in one direction (unidirectional), with the fibers running from one end to other in a parallel fashion. Alternatively, the fibers can be arranged in different directions to one another, resulting either in a weave or meshtype architecture. [34] When the directional orientation of the fiber long axis is perpendicular to the applied forces, it will result in strength reinforcement.
[40] Forces that are parallel to the fiber orientation will produce matrixdominated failures and consequently yield little reinforcement. Multidirectional reinforcement is accompanied by a decrease in strength in any one direction when compared with unidirectional fiber.
In high stress-bearing areas, a material with high flexural strength, high elastic modulus, low deformation, and high impact and fatigue resistance is required.

Mechanical properties
The additional cure and the increased volume of inorganicfillers has improved flexural strength to 120 160 MPa and elastic modulus to 8.5-12 GPa. An improvement in the degree of conversion itself does not necessarily result in better mechanical properties, because there are other factors involved, such as resin composition, filler content, and particle size and distribution. Filler content could be an important factor in deciding the physical and mechanical properties of different composite materials. Chung et al. [64] observed a positive relation between the volume fraction of filler and diametral tensile strength and hardness. But no correlation was observed between the degree of conversion and the mechanical properties evaluated. Neves et al. [65] also concluded that the filler content directly affects the hardness values. Other studies also investigated the association between the mechanical properties of composites and the filler volume. The authors reported that materials with higher filler volumes showed better mechanical properties. [66,67] Borba et al. observed that the hardness and flexural strength of direct resin composites were better than that of the IRCs. This was attributed to the high filler content of 7884 wt% of D250 and D350 than Sinfony and Vita . Thus, IRCs with lower percentage of inorganic content (e.g., Sinfony , Vita Zeta , with 50 wt% and 45-48 wt%, respectively) and lower values for the mechanical properties evaluated than expected for secondgeneration systems could be classified as intermediate laboratory composite resins [68]. Miranda et al. observed that Targis had the highest microhardness among the IRCs even though its filler content was less than in the others. This may be because there is a correlation between the method of polymerization and the microhardness. Tanoue et al. [69] pointed out that the best mechanical and physical properties are achieved by using a combination of composite material and curing unit from the same manufacturer. Yamaga et al. [70] reported that heat might facilitate monomer conversion by breaking the double bonds on the polymer network into single bonds, thus optimizing the polymerization of the residual monomers. IRCs polymerized under light activation only may have intermediate mean microhardness values (e.g., Artglass and Solidex ). On the other hand, Sinfony presents inferior mechanical properties, even though it is polymerized with light and vacuum. This suggests that the composition of the material influences the degree of conversion during polymerization resulting into lower resistance to indentation. [71] Wear of composite resin materials has been evaluated in terms of two main clinical components: occlusal contact/attrition wear and contact free/abrasive wear. Filler size, volume, shape, and bonding to matrix affects wear. The chemical treatment of filler to increase bonding to matrix decreases wear. [72] Bayne et al. studied the wear rates and proved that the wear of Concept was less than that of belleGlass . This could be due to the use of microfillers and the small particle size and the interparticle spacing, which resists wear. Belleglass showed less wear than Artglass and Targis , which may be attributable to the volume of filler. [73] Krecji and colleagues demonstrated that Artglass was considerably more wearresistant than conventional lightcured composite resins. Charisma , a conventional composite resin, exhibits an average annual wear rate of only 8 µ, while the Artglass formulation exhibits only 50%-60% this amount. The substantial increase in wear resistance of the indirect material can be attributed in part to the incorporation of multifunctional monomers, which permits better control over the positions along the carbon chain where the crosslinking does occur. Consequently, this can aid in improving the wear resistance and the other physical and mechanical properties of the resin matrix.
[45] A change in concentration of BisGMA can also improve the wear resistance.
Faria et al. observed that the wear resistance and hardness of Artglass detroriates on immersion in water, whereas that of Solidex does not. [61] Freund and Munksgaard have found that there is a hydrolytic action of the esterase enzyme on resin restorations in the oral environment.

Optical properties
One of the problems associated with composite materials is the unpredictable color stability. The mode of curing and the remaining double bonds may influence the color stability of the material. Nakazawa et al. observed that Sinfony , when cured with the manufacturerprescribed curing unit, did not discolor when immersed in water but showed color deterioration when immersed in tea. This was because of the number of remaining double bonds. On the other hand, when Sinfony was cured with the Hyper LII unit, the mechanical properties increased but it showed yellowish discoloration even on immersion in water. This is because degradation of the material may have occurred due to the heat generated by the high level of light energy. [74] Kim et al. also observed that there is a net color change of belleGlass during curing that should be taken into consideration when shade matching. The curing of uncured material on the tooth with a hand held curing unit has to be done for enhanced shade matching of IRCs. [75] Papadopoulos et al. observed that there was an increase in lightness and a greenyellow or greenblue shift in color in IRCs on curing as well as after aging in various environments, but the changes were found to be within the clinically acceptable range. [76] Marginal adaptation and microleakage Leinfelder et al. observed that heattreated inlays showed less microleakage than direct restorations. Similar observations were found in other studies. [77,78] However, a few other studies found no significant differences in microleakage after thermocycling of direct and indirect resin restorations. [79,80] Aggarwal et al. observed that marginal adaptation and bond strength of an indirect resin system after thermocycling was better than that after direct restoration. [81] IRCs shows better marginal adaptation than ceramics because of lower polymerization contraction. The refractory die is fractured to remove the ceramic inlays and this may result in marginal microfracture, thus increasing the marginal gap. [82] Although ceramic inlays perform poorly in lab analysis, composite inlays tend to degrade in the oral environment, which can result in similar clinical behavior of both the materials

Surface properties
One of the main failures of IRC restoration is the formation of secondary caries due to plaque accumulation, which is aggravated by the surface roughness of the material. The biofilm accumulation is based on the filler size and matrix monomer. Smaller filler size with more weight% produces a smooth surface and, consequently, less biofilm adhesion. The surface roughness ranges from 6-8 µ. Polishing with diamond pastes also renders a smooth surface. Another possible factor for bacterial adherence is the presence of remaining uncured monomers. [56] Surface treatment of IRCs The treatment of the intaglio surface of indirect restorations determines the bonding of the restoration to the tooth. The use of hydrofluoric acid for surface treatment causes microstructural alteration of the composite because of the dissolution of the inorganic particles. [83] The best alternative method to raise the surface energy is by sandblasting with aluminium oxide particles for 10 sec. [20] This causes a nonselective degradation of the resin and promotes better adhesion. According to Soares, application of silane after sand blasting resulted in higher bond strength. Since the compositions of the IRCs are similar, the surface treatment for all materials can be the same. [84] The various clinical studies comparing the materials are tabulated in Table 4. [85][86][87][88][89][90][91][92][93][94][95][96][97][98] Clinical advantages of IRCs A properly fabricated indirect restoration is wear resistant, esthetic, and relatively less prone to postoperative sensitivity. Since, the only polymerization that occurs is that associated with a thin liner of luting agent, the potential for tensile stresses on the odontoblastic processes is considerably less, which translates into less potential for postoperative sensitivity. Indirect laboratoryprocessed composite resin systems provide an esthetic alternative for intracoronal posterior restorations and may also reinforce tooth structure. IRC restorations offer some benefits as compared to direct restorations, such as better mechanical performance and a significant reduction in polymerization shrinkage (i.e., limited to the dualcured luting cement). [2,22] Additional clinical benefits include precise marginal integrity, ideal proximal contacts, excellent anatomic morphology, and optimal esthetics. [21] When compared to porcelain and porcelainfusedtometal restorations, the transfer of masticatory forces is considerably less. Composite materials have shown a greater capacity to absorb compressive loading forces and reduce the impact forces by 57% more than porcelain. Thus, a polymer of the abovementioned materials is considered when restoring the coronal aspect of a dental implant. It has been shown that the edge strength ® ® of belleGlass , either alone or with fiber reinforcement, is more than that of ceramics. This reflects the ability of the material to maintain the marginal integrity to occlusal loading.
[99] Tsitrou found that resin composites have a lower tendency for marginal chipping than ceramics.
[100] Due to the similar composition of the luting cement and composites, the marginal adaptation of composites is better than that of ceramics

CONCLUSION
Our literature review shows that there are numerous IRCs available nowadays. These materials perform well in In vitro and shortterm In vivo studies. It is also apparent that IRCs can effectively supplement the use of ceramics in certain clinical conditions. The improvement in properties due to the additional polymerization, which was observed in these studies, needs to be assessed with longterm clinical trials. In the absence of multiple longterm studies, the survival rate of IRC restorations cannot be assessed. Further clinical research is needed to evaluate the success rates with these newer IRCs.    FillerThe dentin material is a highly filled hybrid (85% by weight, 73% by volume).

Footnotes
The body and the incisal material consist of a reinforced microfill (70% by weight).
Added to the nanoparticles is a "reinforcement" particle that averages 1 µm in size, compared to the main filler, which is 0.04µm in size.