Resin matrix ceramics – mechanical , aesthetic and biological properties

Resin matrix ceramics consist in a polymeric matrix with predominantly inorganic refractory compounds which may include porcelain, glass, ceramics, and glass ceramics, and are divided into three subgroups: Nanoceramics, Vitroceramics, and Zirconia-silica. The aim of this study was to compare, through a literature review, the mechanical and biological properties of resin matrix ceramics, with glass matrix ceramics and polycrystalline ceramics. After reviewing 44 articles found in the US National Library of Medicine (PubMed) database (studies published in English, human clinical studies, in vitro or in vivo studies) that evaluated some properties of this material, such as elasticity modulus, wear resistance, adhesiveness, stain resistance and hardness, this article concluded that, although they belong to the same group, resin matrix ceramics are different from each other due to their microstructures. Moreover, when compared to other ceramic groups, it showed some superior properties, such as flexural strength, fatigue strength and internal adaptation. Indexing terms: Ceramics. Composite resins. Computer-aided design. Resins.


INTRODUCTION
Launched in the market around 1962, dental composite resins have undergone several changes to improve their properties for having more space in the market. At the same time, scientific investigations on these materials have also expanded [1].
The mechanical properties of different types of composite resins, such as hardness, tensile strength, compression and shear, increase the clinical success of the restorative material [2,3]. These properties are directly related to the polymeric matrix (monomer composition), inorganic filler (type, size and distribution of filler) and bonding agent.
The incorporation of nanoparticles helped to obtain a restorative material capable of being used in all areas, with high initial polishing, superior gloss retention (typical of microparticles), as well as excellent mechanical properties such as wear resistance in areas with high chewing stress (typical of hybrid composites) [4].
On the other hand, dental ceramics are known for their satisfactory and similar properties to natural teeth: translucency, fluorescence, chemical stability, coefficient of linear thermal expansion close to the dental structure, biological compatibility, as well as increased compressive and abrasion resistance [5].
There are three large groups of ceramics divided according to their microstructure. The criteria used to differentiate them are based on the phase or phases present in their chemical composition: glass matrix ceramics (nonmetallic inorganic ceramic materials containing a glass phase); polycrystalline ceramics (non-metallic inorganic ceramic materials without any glass phase); or resin matrix ceramics (polymeric matrix containing predominantly inorganic refractory compounds which may include porcelain, glass, ceramics, and glass ceramics). Resin matrix ceramics are subdivided into three subgroups: Nanoceramics, Vitroceramics, and Zirconia-silica [6].
Compared to ceramics, indirect restorations of resin composites are more flexible, less rigid, have a better finishing and polishing, and are less abrasive to the antagonistic dental element. They are easier to adjust when necessary, but their aesthetic properties are inferior [7]. In addition, ceramics are more wear-resistant, more biocompatible and more resistant to discoloration; however, they are more friable and susceptible to fracture. The purpose of resin matrix ceramics is to combine the positive properties of these two materials [7] in compact blocks ready to be milled, without any physical-chemical alteration that could change their molecular structure and without affecting their mechanical properties.
Thus, the objective of this study was to evaluate, through a literature review, the mechanical and biological properties of resin matrix ceramics and to compare them with glass matrix ceramics and polycrystalline ceramics.

METHODS
The integrative literature review was performed by a single researcher who started the activities with an electronic search in the US National Library of Medicine (PuBMeD) database.
The term "resin matrix ceramic" were used to select the articles: a total of 162 studies. After reading their titles and abstracts, only 71 studies were selected. Of this total, 27 studies were excluded from the review for not being related to the theme, according to the following inclusion and exclusion criteria:

Inclusion criteria:
• Studies published in English; • Human clinical studies; • In vitro or in vivo studies.

Exclusion criteria:
• Studies published in a language other than English; • Case reports; • Clinical studies in animals.

DISCUSSION
Resin matrix ceramics are materials with an organic matrix highly filled with ceramic particles.
The presence of an organic matrix would theoretically exclude resin matrix ceramic materials from the materials classification before 2013. However, the 2013 version of the ADA Code in Dental Procedures and Nomenclature defines the term "porcelain/ceramics" as "pressed, polished, milled or fired materials containing predominantly inorganic refractory compounds including porcelains, glasses, ceramics and glass-ceramics".
Despite the controversies, resin ceramics aim to obtain a material that simulates the elasticity modulus of the dentin and that is more easily adjustable. Currently, these resin matrix ceramic materials can be divided into several subfamilies according to their inorganic composition: resin nanoceramic (Lava Ultimate, 3M ESPE; Cerasmart, GC America); glass ceramic in a resin interpenetrating matrix (Enamic, Vita); zirconia-silica ceramic in a resin interpenetrating matrix (e.g., Paradigm MZ100, 3M ESPE; Shofu Block HC, Shofu) [8].

Mechanical properties
Several studies comparing the mechanical properties of resin matrix ceramics show different results due to their microstructures. For example [9], the difference between the type of filler in cerasmart (zirconia-silica) gives this material greater hardness and toughness than Lava Ultimate. On the other hand, Lava Ultimate, because of its crystalline content (32% ZrO 2 ) -compared to the crystalline content (20.6% Al 2 O 3 ) in Enamic -, has higher flexural strength [10][11][12][13]. On the other hand, Enamic has the highest elasticity modulus when compared to Lava Ultimate, Cerasmart and Paradigm MZ100 [14]. These materials, when compared to polycrystalline or glass-matrix ceramics, show significantly superior results for flexural strength, fatigue and reliability than restorative feldspar-based and leucite-reinforced CAD/CAM restorative materials [15][16][17] due to the absorption of chewing forces by the organic content in the resin matrix, greatly increasing the flexural strength of this type of material [10,18,19]. However, when compared to lithium disilicate, they show significantly lower mechanical properties [14,20] and lower elasticity modulus [14].
The Weibull modulus of resin matrix ceramic materials were similar [10,[21][22][23] or superior [24] to other types of ceramics, which proves their homogeneity. On the other hand, a lower surface hardness and fracture resistance of resin matrix ceramics were observed, caused by the lower concentration of inorganic load in its microstructure [20]. This material's characteristic causes less damage to the CAD CAM milling burs during restorations [14,25], with smoother margins due to the ease of milling [16], as well as less enamel wear of the antagonist element [18,26]. However, the resilience of these materials in relation to the restorations margins could cause microinfiltrations and damage the cementation line [16].
Even in studies in which resin matrix ceramics have lower mechanical properties, this material group was in accordance with the ISO standard for ceramics (ISO 6872: 2008), revealing considerably higher fracture resistance than the average occlusal force on the posterior dentition, and may show a long term treatment to restore the occlusal surfaces of the posterior teeth [27,28]. They are, however, contraindicated in fixed partial dentures [12].

Roughness
Surface roughness, material thickness and type, and polishing are factors directly related to translucency. Translucency, associated with stain resistance and gloss, may interfere with the aesthetic result of a restoration. Promising results related to the translucency of resin matrix ceramics have been shown in some studies, except for Vita Enamic due to the relatively high amount of Al 2 O 3 [29]. Samples of this material show significant differences in gloss and stain resistance when polished or unpolished. However, they still have inferior results in relation to lithium disilicate in these aspects.

Wear Resistance
In addition to mechanical, physical and chemical properties partially detailed by manufacturers, wear resistance is another important property studied by researchers with simulations of contact with human enamel. Although being part of the same ceramic group, the results of Shofu Block HC and Katana Avancia, due to their smaller amount of filler particles, have lower wear resistance when subjected to abrasion and fatigue in comparison to Cerasmart and Brilliant Crios. However, when subjected to hydroabrasive erosion, the size of the filler particles have more influence on the result, which leads Katana Avancia to obtain more promising results [30]. The complex mechanisms of wear in vivo are difficult to reproduce in vitro. When compared to human enamel, Lava Ultimate and Vita Enamic had similar characteristics of ceramics and composites, not differing from the enamel behavior in relation to the wear of the material itself and the antagonist [31]. Vita Enamic, for having higher elasticity modulus in comparison to resin matrix ceramic materials, causes greater wear to the antagonist enamel when compared to Lava Ultimate, Paradigm MZ100 and Cerasmart, behaving similarly to lithium disilicate. However, considering the wear of the material itself, there is less wear in Vita Enamic than in lithium disilicate, behaving similarly to other resin ceramics. Feldspar ceramic, due to its vitreous microstructure, has higher wear resistance than Vita Enamic, 3M and Paradigm MZ100 resin matrix ceramics [32]. Nevertheless, in a study on the subject, when compared to cerasmart and Block HC, they showed better results [12]. This same Cerasmart, when compared to lithium disilicate, showed greater wear [30]. Degradation When exposed to the oral environment, resin materials show increased roughness [9] which, when greater than 2µm, increases the possibility of bacterial deposition, leading to a higher incidence of periodontal disease or caries [33]. Regarding in vitro studies, Cerasmart has greater flexural strength after aging than other resin matrix ceramics, such as Vita Enamic and Lava Ultimate [21,34]. In addition to surface roughness [35], the mechanical properties of materials may change due to the degradation of the organic matrix, loss of charge particles [9] and destruction of silane adhesion to charge particles [36], causing a reduction in the flexural strength of resin matrix ceramics [21,34]. The effect of brushing and artificial storage on surface roughness depends on the material and polishing system, with a difference in the behavior of resin matrix ceramics related to their microstructure. While materials such as Amberino High-Class, Lava Ultimate and Paradigm MZ100 have poorer micromechanical properties and behave analogously to composite resins, Vita Enamic acts like feldspar ceramics and are less susceptible to storage and brushing degradation [37]. There is still no consensus in literature regarding the influence of aging on resin matrix ceramics when compared to other materials and to each other concerning color stability. Some studies concluded that resin matrix ceramics have similar [20,21] or clinically similar [35] color stability, whereas other studies found the opposite [10]. When comparing resin matrix ceramics with each other, Lauvauthanon et al. concluded Enamic (86.4% filler content) is less sensitive to thermocycling than Lava Ultimate (73.1%) [12], whereas Sonmez et al. -through EDS microanalysis and similar load contents -concluded there is no significant difference when materials are exposed to thermocycling [10]. The lower the amount of inorganic filler of a material, the higher its resin organic matrix responsible for "shock absorption", i.e., greater resistance to cracks and better adhesion, which justifies the better results found for Katana Avencia Blocks and Shofu Block HC when compared to Cerasmart. However, the lower amount of fillers creates a higher risk of lixiviation when exposed to the oral environment [17].
Restorations are subject to load in the presence of moisture, thermal and chemical variation, with average cycles of 500,000 per year [38], and fractures are more likely to occur under subcritical loads when restorations are subject to fatigue. Laboratory tests show that resin matrix ceramics, when compared to polycrystalline and resin matrix ceramics, are less sensitive to fatigue [39].

Adaptation and adhesion
For a better restoration, it is important to choose a proper prosthesis, with correct surface treatment, and high quality resin cements.
Regarding the adjustment of materials, Alexis et al. [27] described the internal adaptation of polymer-based blocks, comparing them with a lithium disilicate glass-ceramic block. IPS e.max CAD and Cerasmart were considered the best concerning flexural strength and had better internal fit.
Adhesive cementation systems are generally the choice for cementing resin matrix ceramics [12,40]. For these materials, adhesiveness is directly related to chemical reactions or mechanical retention, depending on the resin cement composition and the pretreatment of the material surface.
In the surface treatment, glass ball abrasion is more effective to increase durability between resin cements and CAD/CAM materials than alumina blasting, ceramic primer [41] or hydrofluoric acid [51]. Resin matrix ceramics differ in their bond strength. The worst results found in Lava Ultimate and Cerasmart are due to the penetration of water into the resin matrix, a fact minimized by the interpenetrating ceramic matrix of Vita Enamic [51].

CONCLUSION
Although belonging to the same group, resin matrix ceramics show different characteristics due to their microstructures. These nanoceramic resins have greater flexural and wear resistance; glass ceramics embedded in resin matrix have higher elasticity modulus, lower translucency, generate more wear on the antagonist enamel and are less subject to degradation by storage; and silica zirconia ceramics are better in terms of hardness and toughness.
When compared to other types of ceramics, resin matrix ceramics were less sensitive to fatigue, had higher flexural strength, greater or similar reliability, lower elasticity modulus, lower gloss and stain resistance, lower surface hardness, higher internal adaptation, and were more susceptible to fracture.
To achieve better adhesive resistance, glass ball abrasion is the gold standard for conditioning.