Indirect Restorative Polymeric Dental Materials

Abstract

The current chapter, entitled “Indirect restorative polymeric dental materials,” is dedicated to one class of dental materials with wide indications in dental medicine. The chapter is an overview of polymeric resin composites dedicated to indirect restauration currently used and latest avail resin composite and polymers in dentistry. This chapter includes chemical composition, indications, light-curing mechanism and physical and mechanical properties of indirect conventional and new polymeric materials. Important aspects about resin composites are related to clinical and physical properties such as light-curing, aesthetics, wear rates and biocompatibility. The accuracy and longevity of indirect composite restorations depend on choosing the right material according to clinical requirements, material’s properties, and behaviour in the oral cavity and clinical indications. The chemical composition of resin composite, curing mechanism and advantages of these materials are presented in Introduction. The second section is dedicated to structure and composition of indirect polymers, indirect resin composites, first and second generation of indirect resin composites and their indications. Section 3 describes the structure and composition of high-performance hybrid polymers, hybrid ceramic and high-performance polymers, and poly ether ketone ketone dedicated to different processing technologies.

1. Introduction

Resin composites, combined with best adhesive protocols, are elective dental materials for varied treatment plans and protocols offering a predictable, conservative and safe-working protocol. Composite resins are also designed for dental laboratory or for indirect composite restorations, being a multifunctional material without limitations in terms of esthetic or costs.

Resin composites were first introduced in 1958 along with the perfecting of a high-molecular-weight monomer, bisphenol A-glycidyl methacrylate (Bis-GMA). Michael Buonocore noticed that some industries like automobile industry use phosphoric acid to chemically treat metal surfaces and obtain better adhesion of the primer to the metal. Three years before, in 1955, Buonocore observed that application of an inorganic acid can alter the enamel’s surface and the bonding resin can be made. Buonocore applied this technique on enamel to enhance the adhesion of resin to enamel surface. He also experimented with acrylic resins but the occlusal stresses exerted over the indirect restorations in the oral cavity were too high. Rafael Bowen improved the system by developing a 25% by weight a polymerizing monomer and 75% by weight a vitreous filler-resin composite- and acid-etching technique, which became a protocol in restorative dental medicine [1, 2, 3, 4, 5, 6].

First resin composites were self-curing resins obtained from a base and catalyst for which the polymerization was initiated chemically. The procedure was difficult but the material had superior esthetics in comparison with silicate cements, the elective dental material for direct restorations at that time. In the 1970s was introduced the light-curable resin composite which provided good physical properties, longer working time and better color stability. In the late 1970s were designed micro-fill resins with medium submicron particle size and the result was improvement of wear and finishing. In the following decades, the tendency in decrease in size of inorganic particles and increase in filler load continued and the result was the improvement of the physical properties of light-cure resin composites. Other factors that improve the physical properties are represented by free of voids resin composite and maximally polymerized organic matrix. These polymeric materials were continuously improved (physical and chemical properties) and are now validated as universal restorative dental materials [7].

Resin composites are also referred to as resin-based composites or filled resins and are defined as restorative dental materials made of synthetic resins and a matrix of organic monomer. Resin composite composition is formed by monomers Bis-GMA (bisphenol A-glycidyl methacrylate), TEGMA (triethylene glycol methyl ether methacrylate), UDMA (urethane dimethacrylate), HDDMA (1,6-hexanediol dimethacrylate), inorganic filler and a coupling agent, a silane which has the property to enhance the bond between the two components—monomer and filler. Other chemical constituents are added in different concentrations to tailor and adjust the mechanical and optical properties of resin composite [8].

These dental materials are classified after different considerations like handling characteristics, composition and size of filler particles, curing mechanism and direct or indirect indication of use. The direct composite resins are manipulated in the dental office, are placed directly in the oral cavity, modeled, light-cured by the dentist and are indicated for filling cavity preparation, diastema closure, minor reshaping of the teeth, built-ups and even partial crowns. Indirect resin composite restorations are designed in the dental laboratory and cemented in the oral cavity.

Conventional indirect resin composites have higher filler content and are cured for longer times and the deficiencies of the material can be better handled and controlled due to extra-oral design and curing methods. Indirect restorations made from indirect resin composites, after being designed by dental technician through different layering protocols, are cured at higher intensities and more accurately finished, polished and adapted to the natural or artificial abutment, abutment’s limit preparation and proximal and antagonist teeth. The advantage of indirect resin composites designed for dental laboratory is that take benefit by curing under pressure, vacuum, inert gas, intense light, heat or a combination of these technologies, all generating a dense and completely restauration with no voids, less prone to marginal gaps and shrinkage. In the same time, indirect restauration emphasizes excellent and functional morphology, and ideal proximal and occlusal contacts. The clinical recommendations for indirect composite restorations depend on the remaining tooth structure, cost and intra-oral relations of teeth and dental arched. Indirect resin composites are indicated for inlays, onlays, veneers, full or partial crowns on single teeth and even bridges [9].

One of the main issues of direct resin composites is light-curing shrinkage and degree of conversion but indirect resin composites can overcome these shortcomings because of modern materials and laboratory technological process. Indirect composites overcome some shortcomings of direct resin composite restorations because the material is manipulated out of the oral cavity. Clinical indications for indirect resin composite restorations always need to consider remaining tooth structure and possibility to properly isolate the abutment. Advances in chemistry, physics and field of adhesive biomaterials in the latest decade provide increased retention, reduced microleakage, improved marginal adaptation and good sealing for teeth or natural abutments and these successes of sciences completely changed the dentistry and clinical practice in dental offices. Modern polymeric restorative materials provide more conservatory dental treatments, which preserve tooth structure, and improve the longevity and esthetics.

Indirect resin composite restorations are also an esthetic alternative for intra-coronal posterior restorations such as inlays, onlays and overlays that reinforce the tooth structure and strengthen the cusps. The need for a conservative tooth reduction and in the same time good marginal fit and esthetics are recommending indirect resin composites as restorative dental materials. Additionally, clinical benefits of indirect composite resins include acceptable wear resistance and wear compatibility, ideal proximal contact esthetic and functional morphology [10]. The technological development from the last 20 years represented by digital impression, computer-aided design/computer-aided manufacturing (CAD-CAM) system and continuous improvements of dental ceramics, composite resins and adhesive systems are now sustainable for conservative restorative dentistry with excellent esthetics and mechanical properties of hybrid dental materials.

Until now, there has not been an ideal material for prosthodontic restorations. Currently, dental medicine has several classes of polymers used for the manufacture of fix partial restorations: light-curing composite resins, thermo-polymerizing resins, dual cure resins (light and chemically curing) and self-cure resins (chemical curing). Polymers represent a large series of synthetic resins, and composite materials are part of these classes. The very important stage in restorative dental treatments is the one in which the material is chosen as elective material depending on several factors: the extent of the dentition, the technical-material endowment, the manufacturing technique and time of use.

Composite resins in recent years have diversified a lot, so manufacturing prosthetic restoration is using these materials either through direct or indirect technique. Indirect composite materials processed through direct technique eliminate the laboratory phases though may face problems such as marginal and proximal adaptation.

Polycarbonate resins belong to the polymeric materials and are indirectly processed through plastering and injection. Because are thermal-plastic polymers, a special injection system is required. Glass fiber-reinforced resin composites are indirect materials that fulfil all the requirements for long-lasting FPP (fixed partial prostheses), with a maintenance period of time in the oral cavity between 2 and 5 years. Acetyl resins offer very good biocompatibility, this material is thermoplastic, and it is processed through thermo-injection and has some special mechanical characteristics. All the physical properties of these materials have fulfil the clinical requirements and do not change different working conditions (chemical agents, humidity and isolation control in the oral cavity). Milling from polymeric blocks with CAD/CAM technology or plastering and injection of polymers are indirect techniques performed outside the oral cavity with new materials and technologies [11, 12, 13, 14].

2. Structure and composition of conventional indirect resin composites

2.1 Polymers as indirect restorative materials

Polymers are one of the four materials that belong to composites and are commonly used as sealants, tooth restorative dental materials, cements, obturators for palatal clefts, impressions, provisional restorations and denture bases. The basic nature of polymers and the term polymer emphasize a molecule that is made up of many (poly) parts (mer). Mer ending shows the simplest repeating chemical structural unit that generates the polymer material. Polymer (methyl methacrylate) has a chemical structural units derived from methyl methacrylate. The molecules that form the polymers are called monomers (one part). These polymer molecules can be obtained from a mixture of two or more different monomers and are called copolymers (methyl methacrylate-ethyl methacrylate copolymer) and terpolymers when containing three different unis (methyl-, ethyl- and propyl methacrylate copolymer or terpolymer). In normal polymers, mer units are spaced and random orientated on the chain polymer. It is possible to obtain copolymers with arranged unit mer so that a larger number of another mers are connected and the polymer is called block polymer.

The mers of the polymers are linked through covalent bonds C∙C and during the polymerization process, C∙C double bonds are converting into C∙C single bonds and the mer will be attached to one of the carbon atoms that belonged to C∙C double bond.

The molecular weight of polymer molecule equals the molecular weight of mers, depending on preparation conditions. If the molecular weight of the polymer made from single monomer is higher, the degree of polymerization is higher. The polymers with low-, medium- or high-molecular weight molecules in a material or more precisely the molecular weight distribution have a pronounced impact on the physical properties of the polymer. For this reason, two poly (methyl methacrylate) specimen can have the same chemical composition but very different physical properties from a specimen with high-molecular-weight molecules. The variations in molecular weight distribution are obtained by altering the polymerization procedure. The spatial structure of the polymer molecules can be linear, branched and cross-linked and is important in defining the properties of the polymer. Polymers can be classified after setting mechanism, and these two mechanisms are thermoplastic in which the polymer can be softened by heat and solidified under cooling, and thermosetting; polymers that are solidified during fabrication cannot be re-softened through reheating. The variations in chemical composition, molecular weight, arrangements of mer unis, added fillers and initiators make polymer a versatile material with different physical properties [15]. In addition to being used as veneering materials, polymers are also used to make all-polymeric crowns or bridges. The most modern technologies are using CAD/CAM and the polymers used show remarkable physical and chemical properties, and can be inserted both as definitive restorations and as long-lasting provisional restoration. These restorations can be carried out either with the participation of the laboratory or only in the office. Currently, restorations based on polymethyl methacrylate (PMMA), composite materials and hybrid ceramics can be made using CAD/CAM technology. The PMMA blocks are made industrially and may contain inorganic fillers (which influence, among other things, the degree of opacity of the material) or prepolymerized organic particles. Some manufacturers also provide practitioners with pink blocks, intended for the bases of prostheses. PMMA indications in association with CAD/CAM technologies are as follows: single crowns, temporary occlusal veneers, provisional fixed prosthetic restorations (maximum 2 bridge bodies), for a maximum duration of 2 years and bases of prostheses. The basic structure of indirect composite resins does not differ from that of composites for direct techniques. Both are being represented by the organic phase, coupling agents and the inorganic phase (silicate macro- and micro-filling particles, made of glass ceramics or silica). Some products contain individual nano-filling particles or agglomerated in nanoclusters. With some exceptions related to the manufacturer, these materials are only suitable for single-identity restorations such as crowns, inlays, onlays and veneers. The resistance of industrially produced polymers for CAD/CAM technologies is higher compared to products with direct use, polymerized by a doctor or technician, which is also found in prosthetic restorations.

The modulus of elasticity of PMMA-based materials (2.7–3.2 GPa) is lower compared to composites (8–15 GPa) or hybrid ceramics (30 GPa), which gives the former a higher capacity cushioning of occlusal stresses and a lower susceptibility to fracture.

The resistance to spontaneous fracture is dependent on the Weibull modulus. The higher it is, the lower the fracture susceptibility of a material. CAD/CAM composites have a Weibul modulus (10–17) lower than hybrid ceramics.

The wear of CAD/CAM polymer materials is dependent on their composition. Thus, PMMA has the highest occlusal wear, followed by composites. Hybrid ceramic shows the least wear. Regarding the wear of antagonistic enamel, the highest rate is induced by hybrid ceramics, followed by composites and PMMA.

Water absorption is also dependent on the chemical composition of the material, being all the greater the higher the content of the organic phase. Thus, composites absorb more water than hybrid ceramics. Water absorption has a direct influence on the superficial dyschromia of CAD/CAM polymer materials. These differences are more common than in the case of vitreous ceramics.

2.2 Indirect resin composites

The first generation of indirect resin composites (IRCs) for inlays and onlays was introduced in the 1980s by Touati and Mφrmann. Direct resin composites were composed of organic resin matrix, inorganic filler, and coupling agent. The first-generation IRCs had an identical composition to that of the direct resin composite marketed by the same manufacturer and the material bore names similar to that of the direct resin composites.

The term resin composite generally refers to a reinforced polymer system used for restoring dental hard tissues. The proper scientific term is polymer matrix composite or for those composites with filler particles often used as direct-placed restorative composites or particulate-reinforced polymer matrix composite. Indirect resin composites are very similar to direct resin composite. Resin composites are composed of two phases, an organic resin matrix phase made from a mixture of multifunctional monomers and inorganic filler used to reinforce the matrix. Beyond these two phases, another two fundamental components are present in the chemical structure: the coupling agent (silane) that links the organic and inorganic phase and the different initiator accelerator systems. Additional chemical substances added in the chemical composition have influence over shade and other specific features of the materials. Organic resin matrix is consisted by monomers, the most common organic polymer matrix is a cross-linked matrix of dimethacrylate monomer and its double bonds at each end of the molecules undergo the addition of polymerization by free radical initiation with high-molecular weight and low contraction. These monomers provide optimal optical and mechanical properties. Because are viscous polymers, to be able to incorporate inorganic filler, need to be blended with low-molecular weight diluent monomer. The shrinkage of resin composites is limited by introducing monomers with epoxy functional groups at ends (oxirane). The conditions imposed to monomers from the resin composites are as follows: biocompatibility, physical properties similar with the dental hard tissues, chemical stability, chromatic stability and high reactivity. Conventional monomers, Bis-GMA, UDMA, TEGDMA and bisphenol A ethoxylate dimethacrylate (Bis-EMA) have been widely used because of their double bond polymerization mechanism that displayed satisfying mechanical properties for clinical practice. Bis-GMA was the first monomer for resin composites and is still in use because of low shrinkage but in the same time has a high viscosity and increased water sorption. TEGDMA monomer is attributed to cytotoxicity, low mechanical properties and high polymerization shrinkage, and in this regard, several dimethacrylate monomers including acetyloxypropylenea (acet-GDMA), bio-based ethoxylated isosorbide dimethacrylate (ISETMDA) and poly (propylene glycol) dimethacrylate (PPDGMA) were proposed to substitute the TEGDMA from resins because these monomers exhibit a close viscosity with TEGDMA, reduced volume shrinkage comparable mechanical properties and lower toxicity. The new monomers used for resin composites are as follows: methacrylate-based monomers, vinyl monomers, click chemistry monomers and ring-opening polymerization monomers. Methacrylate-based monomers are widely used for dental composites because of the double bonds of the polymerization mechanism. Highly cross-linked methacrylate-based monomer mixture displayed satisfying mechanical and aesthetical properties but their high viscosity imposed some limitation on conversion degree and incorporation of filler phase [16, 17, 18, 19]. Research from the recent years introduces new multi-functional monomers; the use of vinyl, diallyl monomers instead of TEGDMA offers higher degree of conversion (DC), lower volume shrinkage (VS), optimal mechanical resistance and cytocompatibility [20, 21].

The appearance of click chemistry in 2001, thiol-X reaction has been introduced to formulas of dental resins and several kinetic advantages were provided by these monomer systems such as thiol-ene, thio-urethane oligomers and thiol-Michael’s binary. The main characteristic is that the thiol-ene binary monomer system converts into polymer through a radical step-growth polymerization mechanism. Thiol-enes demonstrated several advantages when compared to methacrylate such as low amounts of unreacted functional groups, insignificant polymerization inhibition by oxygen and a uniform polymeric material. The ring-opening polymerization mechanisms, present at monomers such as spiro orthocarbonates (SOCs), vinylcyclopropanes epoxies and silorane, have remarkably attenuated the VS and shrinkage stress (SS-S) of dental materials. Polymerizing antibacterial monomers are introduced into resin matrix based on copolymerization among the resin monomers to overcome the short-lasting release of the antibacterial agents. The antibacterial effects occur through the contact of bacteria with the composite surface. The antibacterial fillers and following cationic groups such as quaternary ammonium, pyridinium and phosphonium are commonly found in the functional groups of polymerizing antibacterial monomers. Basic agents for polymerizing antibacterial monomers are quaternary ammonium dimethacrylate and quaternary ammonium polyethylenimine. Antibacterial filler particles can be part of inorganic matrix and are represented by silver nanoparticle, zinc oxide, titanium dioxide and bioactive glass. Antibacterial filler particles are water-insoluble and can release a small number of ions into the surrounding environment. Leachable agents are antibacterial soluble agents incorporated into the resin matrix and are released in the oral environment. The most common leachable agents are benzalkonium and chlorhexidine but their effect is for only few days [22, 23, 24, 25, 26].

Filler phase consists of various inorganic particles such as silica, silicate glass, quartz, alumina, zirconia, barium glass, strontium glass, hydroxyapatite, titania and ceramics and are being employed in resin matrix as reinforcing filler and tailor different mechanical and optical properties such as translucency and control of volume shrinkage. Beyond composition of filler, other characteristics such as size, shape, morphology, distribution of size and surface properties are modeling the behavior of IRC and the mechanical reinforcement is developed by a high content of inorganic filler (Table 1). Composite resins, studied with regard to the three filler contented, can be described as follow:

• low filler concentration—particles are in a non-aggregated form and only a small fraction of them form aggregated;

• intermediate filler concentration in which the particles are in close proximities to form a particle gel-like system and are called percolate particle network, known as percolating threshold;

• maximum possible filler concentration (specific to IRC) for which the network does not allow the incorporation of additional filler [27, 28, 29, 30].

Composition	Size	Distribution	Interfacial properties and porosity	Shape and morphology	Content
Barium glass; Hydroxyapatite; Ceramics; Zirconia; Silica; Alumina	Nano; Micron	Narrow/wide size; Monomodal; Multimodal	Surface silane modification; Mesoporous structure	Spherical; Irregular; Fiber; Nanotube	Non-aggregated Aggregated (percolation network)

Table 1.

Classification of filler phase-influencing factors on the properties of dental composite resins.

Size of filler particle and its distribution play an important role in the indirect resin composite performance. The early composites were macro-fills and contained spherical or irregular shaped particles with a diameter of 20–30 μm. The currently used resin composites are incorporating nano-size range and micro-size range inorganic particle size or a mixture of both and are named hybride, micro-hybride and nano-composites. The hybrid composites are blending two types of fillers: fine particles with a 2–4 μm in size and 5–15% micro-fine particles of 0.2 μm. The fine particles can be obtained by grinding glasses such as silicate glass, strontium and borosilicate glass. Quartz or ceramic materials have irregular surface shape. Hybrids and micro-hybrids have good clinical wear resistance and mechanical properties and are suitable for stress-bearing applications but lose their surface polish with time and become rough.

Nanotechnology is produced by various physical and chemical methods, functional materials and structures at a range of 1 to 100 nanometers (nm). Materials with novel properties and functions can be obtained because of the very small particle size. There are two types of dental resin composites that are using the nano-filler particles: nano-fills contain nanometer filler particles with a range from 1 to 100 nm and nano-hybrids that contain large particles of 0.4 to 5 microns with added nanometer-sized particles and are considered hybrid composites not true nano-filled composites. Nano-filled composites combine the advantage of mechanical strength of micro-hybrid resins and service like micro-fill. The initial gloss of hybrid composites (micro-hybrid and nano-hybrid) is fading because of the difference in size of filler content. The nano-filled composited have similar matrix abrasion and the polish surface is kept on long term [16, 31].

Coupling agents from resin composites bond the organic resin matrix and inorganic filler during setting or polymerization with the use of compounds. These compounds are called coupling agents and are organic silicon compounds called silane coupling agents (is 3-methacryloxypropyltrimethoxysilane). Filler surfaces are treated with a coupling agent during the manufacturing process with following functions: to form an interfacial bridge that binds the filler to the resin matrix, enhance the mechanical properties; to manage stress distribution between polymer matrix and adjacent filler and to minimize water absorption.

Initiators and accelerator for polymerization are triggered by either light or chemical reaction. IRC requires specific conditions for curing such as oxygen-free environment, heat, pressure and vacuum. Heat curing is done at a temperature of 120–140°C and there is an increase in polymer chain mobility, which supports the cross-linking and stress relief, though prolonged heating can cause the degradation of the composite. Autoclaves, cast furnaces and special ovens can generate the heat source. An amount of unreacted monomer decreases during post-curing heating of the resin composite. In this technological process, two mechanisms are involved; firstly, due to heat treatment, the unreacted monomer bonds lead to an increased conversion itself and secondly, the heating is volatilizing the residual monomer. The combination of light-curing and heat increases wear resistance by 35% in contract with light-curing alone [32, 33, 34].

Nitrogen atmosphere is an isolating environment for resin composites toward oxygen, which inhibits the light-curing and thermal-curing. Once oxygen is entrapped in the resin from the surrounding air, it will weaken the restoration and increase wear and influence the translucency. The air removal makes the restoration more translucent and so, before curing the IRC is treated with nitrogen under pressure and the internal oxygen eliminated. Another method is the slow or soft curing, which allows greater level of polymerization; fast curing makes resin composite rigid and stiff. Electron beam irradiation at a usual radiation dosage of 200KGy improves the mechanical properties and positively influences the bond between la matrix and filler [35].

2.3 First generation of indirect resin composites

In 1980 was introduced the first generation of IRC and such examples are SR-Isosit Inlay system, Brilliant (Coltene, Switzerland), Visio-gem (ESPE, Germany), Dentocolo (Heraeus Kulzer GmbH &, Germany) and Concept (Ivoclar Vivadent, USA). First generation of IRC had the same chemical composition to that of direct resin composite. Under light initiation, camphorquinone is decomposing and forms free radical and triggers the polymerization. The result is a highly cross-linked polymer but with 25–50% methacrylate groups non-polymerized monomer. For composite inlays, a secondary cure improves the degree of conversion but the only shrinkage that cannot be avoided is that of the luting cement. The effect of a secondary cure may vary among different material, post-cure though the post-light-heat treatment at 123°C increases the hardness, and wear resistance with 60–70%. The first generation has micro-filled inorganic phase and monomers with high shrinkage contraction for both phases and the developed properties lead to unsatisfactory results. The wear rates for heat-treated and non-heat treated resins are almost the same, around 60 μm in 3 years [30, 33].

The drawbacks of first generation were poor clinical performance; poor wear resistance; high incidence of bulk fracture and inadequate bond between organic matrix and inorganic filler, marginal gap and micro-leakage. At that time, resin composites were competing with ceramic and due to poor clinical results, the resin composites were abandoned when second generation of IRC was developed.

2.4 Second generation of indirect resin composites

The second generation of resin composites was mainly improved in structure, composition, polymerization technique and fiber reinforcement. The filler phase of the second generation is a micro-hybrid, 0.04–1 μm. Second generation of IRC had increased filler load twice that of organic matrix and as a consequence the mechanical properties, wear resistance and polymerization shrinkage significantly decrease. The resin was adequate for restoring the posterior teeth. Such composites are Artglass, belle Glass HP and Solidex (Schofu Dental GmbH, Germany.) having an intermediate filler loading with direct impact over esthetics and anterior teeth. Polymerization techniques for these resin composites are heat polymerization at 120–140°C and combination of heat with light increase wear resistance by 35%. Nitrogen atmosphere, electron beam irradiation and soft start or slow curing improve the quality of indirect restoration [31].

Artglass was launched in 1996 by Heraeus-Kulzer. The matrix composition is formed by Bis-GMA and TEGDMA (24–39 Wt%) UDMA-0.3 wt% pre-impregnated glass 60% for pontics and 45–50% for other materials. Artglass has a composition of 70 wt% filler of barium silicate glass of 0.7 μ and organic matrix of 30 wt% organic resin and four to six functional groups, which provides the more double-bond conversions. The resin composite can be light cured in a special unit using a xenon stroboscopic light to increases polymerization potential. The short excitation time followed by a longer period of non-exposure partially relaxes the already cured resin molecules and more of nonreactive double-bond carbon groups are made available for reaction. The indication is for inlay, onlays and crowns with metal substrate range from nickel-chromium to gold-based metals but in the same time can be used without metal substrate. Belleglass HP (Belle de St. Claire, US) introduced by Belle de St. Claire in 1996 has a chemical composition formed by silanated micro-hybrid fillers of 0.6 μ. This composite is available as base and surface composite for dentin and enamel with different compositions and five shades for enamel. The base has barium glass fillers as inorganic phase and surface material has borosilicate fillers that provide enhanced optical characteristics. For polymerization, Belleglass HP uses two different curing units that give the advantage of incremental build-up. The result is a natural tooth with the hard, translucent, enamel covering the more opaque and softer dentin, able to absorb the stresses. The base composite is light cure with conventional light-curing units to stabilize the restoration. The surface composite is heat cured in an oven at 140°C for 20 minutes. The atmosphere is oxygen free and under nitrogen gas pressure. The reduction in size of the filler improves the polishing and smoothness of the material. Newer composites have a filler diameter of 30 μ in the base composite, which will allow for further reduction in polymerization shrinkage. Other resin composite systems contain inorganic ceramic as micro-fillers. These materials have the advantages of both composite resins and porcelains without being confined by the inherent limitations of either ones. The filler particles are silanated for suitable adhesion to the organic matrix. The presence of these micrometric reinforcement particles acts as a “crack arrester,” while the increase in particle concentration of the micro-fill particles provides improved clinical performance [32, 33, 34, 35, 36].

The design of fiber composite restorations is strongly related to the architecture of the restoration, which can be a single, frame or pontic, and the single and frame are glass-fiber-woven E fibers. Initial polymerization is for 1 minute with light-curing unit and final polymerization is made with light- and heat-curing units for 25 minutes. FiberKor (Jeneric/Pentron, US) contains glass fibers 60% in 100% bis-GMA matrix and the architecture is strips-like that contains unidirectional individual fibers. The initial polymerization is light curing for 1 minute with light-curing units and 15 minutes in alpha light. Other resin composites like Ribbond (Ribbond, US) are directly processed in dental office and need chair side impregnation. Connect (Kerr, Germany), Splint It (Jeneric/Pentron, US) and Everstick (Stick Tech Ltd., GC Japan) have braid or unidirectional fiber architecture and are pre-impregnated. The flexural strength and modulus of resin composites influence fiber volume, architecture and aging. The effective reinforcement is achieved when fibers are placed in the sides where tensile stresses are present. New hybrid composite resin restorative materials have replaced these materials [37].

2.5 Indication of indirect resin composites

There are differences between direct and indirect resin composite related to chemical composition, indications and manipulation. Dental practice is embracing new materials and processing technology, and now the growing demands for esthetics and predictable clinical results are achieved. Resin composites respond to many demands and can compensate many demerits associated with ceramic like high cost, risk of fracture, brittleness and wear of natural teeth. Literature reports many differences between direct and indirect resin composites according to laboratory, in vitro and clinical research and many studies have reported that the clinical efficacy of the indirect resin composites is superior to the direct ones. Other studies reported that the two materials are similar, and few reported that direct resin composites are superior to indirect ones. One of the main issues for resin composites along its history is polymerization shrinkage, which though was continuously improved. The studies demonstrated the polymerization shrinkage is less present for indirect composites because these require the application of curing with heat, light and pressure done outside oral cavity and even more the new composites are designed for CAD-CAM system or can be plasticized and injected. So, their physical properties are improved and the polymerization shrinkage is better controlled.

There are some important differences between direct and indirect resin composite restorations. Indirect resin composite restorations are relatively smaller in size as a result of extra-oral preparation and polymerization shrinkage and marginal fit is compensated by the luting cement. Indirect resin composites report higher resistance to occlusal wear than the direct composite with an estimated value of <1.5 μm/year. Indirect resin composites can be properly light activated under vacuum or pressure and as consequence exhibit greater conversion of monomers to polymers. The optimized conversion is found in improved hardness, polymerization shrinkage control, increased wear resistance, shade stability and biocompatibility. The morphology outcome, interproximal contacts and occlusal outcomes provide a good control and result. The filler contents are in higher quantities and this enhances the physical properties such as strength, hardness, marginal integrity and wear resistance. Regarding esthetics of indirect resin composite, this can be very easily individualized through a combination of shades and excellently polished though color and shade stability, according to the literature, may depend on the product, chemical degradation, stain retention, oxidized carbon double bonds, water sorption, dehydration, the presence of rough surface and poor bonding, increased filler to resin ration, decreased particle hardness and size. The cost of indirect composite restoration includes the laboratory work and additionally, there is a needed to increase tooth reduction to assure the insertion path of the restauration. Also, the luting layer of the resin cement may be subjected to shrinkage [16, 17, 18].

Indirect resin composites (IRC) are now having a wide indication in the restorative dentistry due to clinical performance and esthetics but with specific limitation that needs to be considered in the elaboration of the treatment plan. Indirect resin composites are used as esthetic laminating material for metallic crowns and replaced successfully the acrylic resin. The new trends in dental medicine try to completely remove the metallic cores, an achievable goal with the new resin composites, ceramics and laboratory technology.

Indirect prosthodontic restorations are designed in the dental laboratory based on the registration of dental arch and the natural or artificial abutments with a silicone impression dental material or with the digital scanning technology.

IRC restorations have indications for frontal and lateral teeth:

• inlays and onlays emphasize excellent morphology, marginal fit and contact areas but the reduction of the tooth needs to be more invasive for indirect restoration just to assure the insertion pathway of the restoration;

• laminate veneers are indicated for improving or masking tooth position, shape, size, diastema, color or congenital malformed teeth; enamel hypoplasia, fluorosis, abrasions, previously non-satisfactory esthetic direct restoration and diastemas;

• Jacket crowns;

• full coverage crowns, restauration based on implant-supported restorations; in cases where occlusal coverage is required as in patients suffering from periodontal conditions or bone and poor periodontal support requiring occlusal coverage;

• fiber-reinforced bridges/retainers.

The contraindication of IRC is teeth with heavy wear due to temporomandibular joint and occlusal disharmony, patients with parafunctional habits and the clinical situation in which the working cannot be properly isolated [18].

3. Structure and composition of high-performance polymer

3.1 Hybrid ceramics

Hybrid ceramics are materials designed for CAD/CAM technology, which combine the reduced fragility and increased fracture resistance of composites with the ceramic esthetics. The CAD/CAM technology led to the revolution of materials and their use in many dental applications and is using two categories of restorative dental materials: glass-ceramic/ceramic blocks, hybrid ceramics and resin-composite blocs with shades of natural shade (RCBs).

Conventional resins are cured with high-intensity light sources but RCBs are already pre-polymerized by the manufacturer being ready to be milled and have superior homogeneity, mechanical resistance and no polymerization shrinkage. CAD/CAM technology involves three steps: conventional or digital impression with an intra-oral scanner of the dental units, digital data processing with a program that delimits the dental preparation, restoring contacts and occlusion and creates in the same time the design of the restoration and milling from a block made from ceramic or polymeric material using a subtractive technique.

Hybrid ceramics are divided into two classes: first class—nano-ceramic resin blocks, industrially obtained through high temperature and pressure of composite resin coupled with ceramic filler (80% by weight) and second class represented by polymer-infiltrated-ceramic network (PICN) blocks with 86% by weight ceramic structure infiltrated by composite resin 14% by weight [38].

Resin composites and ceramics are the most common restorative dental materials combined in one material. The result is a hybrid indirect composite named PICN with indications for minimally invasive restoration like crowns, veneers, inlays and onlays and implant supported crowns. PICN is formed from sintered ceramic matrix, 86% in weight infiltrated with a polymer matrix (Bis-GMA, UDMA, UTMA, Bis-EMA or TEGDMA) 14% in weight [39]. This material is combining the ceramic and resin composites properties, though has superior properties toward resin composites and inferior to ceramics. The Young’s modulus of resin composite is the same with the dentin’s, while the ceramic particles assure the high esthetic. This hybrid materials are trying to compensate the polymerization shrinkage of the monomer matrix and increase the mechanical resistance through ceramic particles and is presented as a material for CAD/CAM milling systems.

The ceramic microstructure is depending on the sintering process, which significantly influences the translucency, chemical solubility, thermal expansion and optical appearance. For PICN there is a dominant ceramic network with leucite phase, zirconia as minor phase both interconnected with the polymer-based network. The microstructural characterization, in particular the size and shape of ceramic particles, plays an important role in the physical and mechanical behavior. Chipping of restoration made from PICN dental materials is due to ceramic’s brittle character and CAD/CAM milling can induce micro-cracks, which can be repaired easily without the need for additional thermal cycles. PICN exhibits high bending resistance and can be designed at a reduced thickness. The first nano-ceramic material was Lava Ultimate (3 M Ultimate, 3 M Oral Care, US) and its compositions contains silica particles (20 nm) and zirconia (4–11 nm) up to 80% in weight. The force resistance reported by the manufacturer is about 200MPs and has indication for occlusal coverage, inlays and onlays but are not indicated for crowns [40]. Cerasmart (GC, Japan) is another nanoceramic material with silica filled and barium glass up to 71% by weight and a resistance force of 230 MPa. The material is indicated for single tooth restorations like crowns, cuspidal coating, onlays and inlays. Many materials are avail on the market with different filler and monomer combination and physical average properties around: 170–230 MPa bending resistance, 7.8 GPa elastic modulus and compression force resistance of about 680 MPa. The survival rate for 3-year clinical work is 97.4% for inlays and 95.6% for inlays. The characteristics of hybrid ceramics and their indications are presented in Table 2 [41].

Material characteristics	Indirect resin composite	Hybrid ceramic
Microstructure	Inorganic filler in resin matrix	Ceramic nanoparticles in resin matrix and ceramic network infiltrate of polymer
Optical properties	Medium	Good
Bonding properties	Excellent	Excellent
Advantages	Rapid milling, mechanical properties, possibility to be directly repaired	Rapid milling, mechanical properties, possibility to be directly repaired
Disadvantages	Optical properties	Optical properties
Indications for use	Veneers, inlays, onlays, overlays, crown, bridges with reduced number of elements	Veneers, inlays, onlays, overlays

Table 2.

Features and indication of hybrid ceramics.

PICN as implant-supported crowns needs to have high biological properties considering the contact with marginal gingiva and even bone. The PICN must promote proliferation and spreading of human gingival fibroblasts and keratinocytes. The materials used for the abutments and prostheses have to promote cell adhesion that is a critical property for long-term stability of the implant’s supportive tissues. PICN shows intermediate results between titanium and zirconia dental materials [42].

Paradigm MZ100 (3 M Oral Care, US) was the first resin composite marketed in 2000 as CAD/CAM blocks with different filler percentage: silica zirconia 0.6 μm up to 85% by weight with a bending strength of 157 MPa, similar to feldspathic ceramic.

Since then, other materials like starting from 2016, the Brilliant Crios (Coltene, Switzerland) developed different formulas for reinforced composites with amorphous silica particles <20 nm, glassy ceramic barium particles (<1.0 μm) embedded in cross-linked methacrylate matrix with a filling content of 70.7% by weight and 51.5% filling by volume. These are translated in bending resistance of 198 MPa and elastic modulus of 10.3 GPa. The elastic modulus is close to the dentin’s and this imprints a favorable behavior regarding the concentration of stress in the restoration and minimization of fracture. The main indications are for onlays and overlays.

Tetric CAD (Ivoclar Vivadent, US) has a matrix composed of Bis-GMA, Bis-EMA, TEGDMA and UDMA nano-filled 70% with silicon dioxide and barium glass. The bending resistance is 273.8 MPa and elastic modulus has values of 10.2 GPa similar with dentine.

LuxaCam Composite (DMG, US) has a composite matrix in which silicate glass inorganic filler is embedded. The ration develops a flexural strength of 164 MPa and 10.1GPa close to the natural hard tissue. The material’s physical properties are recommending it for table tops, inlays, onlays, veneers, partial crowns, crowns and bridges up to three elements.

Gradio Blocks (Voco GmbH, Germany) is a highly filled (86%) resin composite based on nanoceramic technology dedicated to CAD/CAM milling. The value of flexural strength is 250–290 MPa and 15.5 GPa elastic modulus and a coefficient of thermal expansion similar to enamel and dentin [39, 40, 41, 42].

3.2 High-performance PEEK polymers for CAD/CAM and heat and pressed technology

A recently new material is polyetheretherketone (PEEK), a high-performance polymer for definitive esthetic restoration, and is a colorless organic thermoplastic polymer from polyaryletherketone (PEAK) family. The polymer was first developed in 1978 and later introduced in engineering industry and medical field. PAEK (Polyaryletherketone) is a blend of high-quality thermoplastic and semi-crystalline resins. The members of this family differ according to the ratio of ether and ketone categories. The increased ratio, but also the sequence of the ketone groups, determines the rigidity of the polymer chain and increases the melting point.

PEEK polymers are obtained by step-growth polymerization by the dialkylation of bisphenolate salts. Regarding its advantages, PEEK has low water solubility (0.5%), and minimizes bio-corrosion avoiding in the same time the release of metal ions which can trigger cytotoxic phenomena, allergic reaction and inflammation of gingival margin. These features recommend PEEK for protecting abutment teeth and adjacent tissues. The aromatic chemical structure makes PEEK resistant to electron and gamma beam used for sterilization and this feature opens new applications and indications of the material in the surgical field. The main disadvantage is the poorly adhesive hydrophobic surface, and the surface contact angle is 0 at 65^o that is not favorable in wide applications of fix prosthodontic but can provide a balanced stress distribution, and has excellent mechanical properties compared to metal alloys. The mechanical strength of PEEK can be enhanced by adding glass fibers (GFR-PEEK) or carbon fibers (CFR-PEEK) in its composition and become a reinforced material with even closer of flexural strength (170 MPa), Young modulus (12 and 18 GPa) values to human bone and dentin and better color stability.

PEEK is nonmutagenic and nontoxic to fibroblast and osteoblast, and exhibits lower susceptibility to biofilm development and no evidence was found about allergic responses induced by PEEK and in the same time shows favorable response to osteointegration and antibacterial properties. Though is a metal free material, PEEK has a gray color and esthetic properties can only be discussed in case of veneering with composite resins, which is difficult to achieve and various modification methods for improving the bonding are needed.

From its class of dental materials, PEEK has varied indications and applicability in dental medicine such as: long-term fixed dental prostheses, crown, fix partial dentures, post-and core, other fixed-dental-prostheses, orthodontics, oral implantology as dental implant abutments, abutment crown and abutment screw and as material for removable prostheses. Related to fixed dental prostheses, PEEK is a promising alternative to zirconia being less abrasive and properties more closer to natural dental tissues. Related to crowns, when PEEK is compared to zirconia, it shows more balanced distribution because of the elastic modulus and regarding wear, PEEK exhibits increased material loss, but superior flexural strength that protects the restoration from bulk fractures. Compared with PMMA, PEEK has the lowest marginal and internal gap values. Precise margins are essential for a successful restoration and PEEK has better marginal fit and internal adaptation than crowns with zirconia though both are clinically acceptable.

PEEK is an aromatic semi-crystalline linear thermoplastic polymer and was obtained by step-growing polymerization. PEEK consists of aromatic nucleus linked by ether and ketone groups and was developed from bisphenol salts and aromatic dihalides, and the typical reaction is the reaction between 4,4′-difluorobenzophenone with the disodium salt of hydroquinone. During melting, chemical properties do not undergo any change. The different resins of the PAEK family present similar characteristics such as: good dimensional stability at high temperature, resistance to bending and traction, mechanical and chemical resistance against wear. According to the manufacturer, these resins are compatible with reinforcing materials such as carbon and glass fibers. Its chemical structure provides material stability at temperatures exceeding 300°C, has a melting point of 343°C and is reinforced with glass and carbon fibers [43].

Depending on the possibility to modify PEEK at the nanometric scale and to overcome its limits, the material is classified as follows: PEKK polyetherketoneketone (PEKK), ceramic-reinforced PEEK (Bio-HPP), carbon fiber-reinforced PEEK (CFR-PEEK), PEEK reinforced with glass fiber (GFR-PEEK), PEEK reinforced with hydroxyapatite, nano-TiO₂/PEEK (n-TiO₂/PEEK) and nano-fluorapatite PEEK (n-FA/PEEK) [44].

The external structure of this material can be changed through different chemical processes, such as that of wet surfaces, which allows the formation of functionalized layers at this level. Thus, hydroxylated polymers (PEEK-OH) are obtained by reduction, carboxylated polymers (PEEK-NCO) by coupling a dissociated reagent to PEEK-OH, aminated polymers (PEEK-NH₂) obtained by hydrolysis of PEEK-NCO and aminocarboxylated polymers (PEEK-GABA and PEEK-Lysine) resulting from the coupling of amino acids to PEEK-NCO. Due to these chemical changes, larger amounts of covalent fibronectin are fixed to the structure of this material, and thus, it is allowed to apply higher pressures on the surface of restorations made of modified PEEK compared to those made of untreated PEEK [45].

Pekkton (PEKK) is considered a high-performance thermoplastic polymer, being easy to use and prepare, becoming the perfect alternative to ceramic and precious metal restorations. Being a member of the PAEK (polyaryletherketone) family, it was produced specifically for dental applications with a chemical composition that ensures the best qualities of all the materials in the same family.

According to the technological process, pressed PEEK has larger marginal gaps than milled CAD/CAM restorations. Related to properties and fix partial dentures, the material absorbs stress and protects the abutment, especially the connectors provide greater stress distribution than the other elements of the prostheses; however, the occlusal area supported the highest stress. As post-core material, fulfil the accurate matching to the root morphology and has similar Young’s modulus to the dentin. PEEK requires composite veneering to be integrated into the class of esthetic dental materials. About 98% sulfuric acid for 30 seconds for printed PEEK and 120 seconds for milled PEEK were considered the ideal concentration and time of action for promoting surface modification of PEEK. Plasma treatment (He, Ar, O, H, N) is a quick, safe and effective conditioning method that shown excellent surface modification. Sandblasting with 110-μm particles for 15 s at 0.2 MPa generates better bonding strength vs. untreated PEEK. This method can be associated with plasma and acid condition. CO₂ laser treatments of PEEK did not exceed good results [46, 47, 48].

Compared to the rest of the materials resulting from this class of polymers, Pekkton (PEKK) describes both an amorphous and a crystalline phase and is the best choice from its family. In the primary phase, the macromolecules in the chemical composition are disorganized, in this form the polymer has specific elasticity. From the second phase, the macromolecules are presented in the form of linear carbon chains, knotted under the process of weak physical bonds. Thanks to its crystalline form, the material describes a very high persistence and rigidity. The difference between the amorphous and the crystalline phase appears at the moment of melting. At the moment of cooling, the amorphous material presents a low shrinkage compared to the crystalline form. High-performance polymers are clearly the way to go: flexibility of use, dimensional stability, hardness, tenacity and abrasion resistance are just a few of their attributes.

Compared to PEEK (poly ether ether ketone), PEKK (poly ether ketone ketone) both belonging to the same family of PEAK, exhibits an additional ketone group resulting in improved mechanical properties. The different organization of the polymer leads to different results regarding strength, stiffness, melting temperature and melting behavior. Compared to the rest of the materials resulting from the same family of polymers, PEKK has both an amorphous and a crystalline phase. From a chemical point of view, the crystalline composition is much more rigid and resistant. PEEK has only a crystalline phase.

Pekkton should not be seen as an ordinary material, but as a therapeutic solution. In order to increase the mechanical properties of the polymer, carbon and/or glass fiber can be added to its composition. Its remarkable qualities, the specific resistance of human bone tissue, as well as the biocompatibility and esthetic appearance of prosthetic works show that Pekkton is the most high-performance polymer today and is perfect for fixed prosthetic restorations. Both the flexibility and the typically relatively low weight represent clinically important characteristics when addressing implant-supported restorations.

Currently, several revolutionary materials have appeared, namely high-performance polymers that allow the production of very light and very resistant superstructures. Among these, we mention the two groups and they are PEEK AND PEKK (poly ether ether ketone and poly ether ketone ketone). The differences between the two materials are relevant:

• PEKK is at least 80% stronger than PEEK;

• PEKK has elasticity similar to that of natural dentin compared to PEEK which is much more elastic;

• PEKK is harder and easier to polish, while PEEK is softer and hard to polish;

• PEKK is recommended in definitive restorations, instead PEEK only for restorations of 180 days.

Compared to the rest of the materials resulting from this class of polymers, PEKK (poly ether ether ketone) describes both an amorphous and a crystalline phase. In the primary phase, the macromolecules in the chemical composition are disorganized, in this form the polymer has specific elasticity. From the second phase, the macromolecules are presented in the form of linear carbon chains, knotted under the process of weak physical bonds. Thanks to its crystalline form, the material describes a very high persistence and rigidity. The difference between the amorphous and the crystalline phase appears at the moment of melting. At the moment of cooling, the amorphous material presents a low contraction compared to the crystalline form. PEKK (Pekkton) should not be seen as an ordinary material, but as a therapeutic solution. In order to increase the mechanical properties of the polymer, carbon and/or glass fiber can be added to its composition. Its remarkable qualities, the specific resistance of human bone tissue, as well as the biocompatibility and esthetic appearance of prosthetic works show that PEEK is the most high-performance polymer and is perfect for fixed prosthetic restorations.

Thanks to its chemical and physical properties, PEKK is among the top high-performance polymers used in the dental field. The manufacturer (Cendres + Métaux) reports up to 80% higher compressive strength compared to PEEK. The identical characteristics of human bone allow better bio-mechanical integration than typical non-precious materials. While PEEK is the most characteristic member of the material family, it may not be the right choice for dental applications where esthetic considerations and long-term structural properties are of utmost importance. Products made from polyetherketoneketone (PEKK) are better option. The characteristic properties of polyaryl polymers are basic qualities of the material, qualities identical to all polymers that are part of the PAEK family. Thus, both PEKK and PEEK share surprising mechanical, chemical and physical qualities. High-performance polymers are clearly the way to go: flexibility of use, dimensional stability, hardness, tenacity and abrasion resistance are just a few of their attributes. Currently, several revolutionary materials have appeared, namely high-performance polymers that allow the production of very light and very resistant superstructures. Among these, we mention the two groups and they are PEEK and PEKK (poly ether ether ketone and poly ether ketone ketone). The differences between the two materials are relevant:

Due to its incomparable chemical, mechanical and physical properties, Pekkton offers a wide range of applications compared to other polymers (Table 3). As for mechanical properties, Pekkton represents the material that has the most similar biological characteristics to the human body. From these properties, we mention: density 1.4 g/cm3, solubility 0.2 μg/mm3, hardness 252 MPa, elasticity 5.1 Gpa, water absorption 8.7 μg/mm3, compressive strength: 246 MPa, bending strength 200 MPa, tensile strength 115 MPa, melting point 363°C. The advantages and disadvantages of PEKK (Pekkton) are described in Table 3.

Advantages	Disadvantages
Very good weight-to-strength ratio	High cost
Increased comfort for patients thanks to the naturalness of the work, then the bite but also the sensation of the oral cavity	Laboratory technology
Fast processing;
Elastic and resistant like natural bone, reduces occlusal stress;
Resistance to abrasion and dental plaque;
Compatible with all sterilization methods
It has perfect dimensional stability
Radiotransparent
No thermal or electrical conductivity
100% biocompatible, suitable for long-term restorations
The density of the material is similar to cortical bone and dentine;
Low specific gravity, less than 20 grams.

Table 3.

Advantages and disadvantages of PEKK polymer in dental medicine.

The indication for Pekkton is as follows: bridges and crowns over dental implants having two intermediates retained with a screw type system, which can be plated with composite bond press crowns or prefabricated acrylic teeth, mobile restorations such as bars and telescopic crowns, transverse connectors, the base of the dental prosthesis, hybrid variants that have special attachments, crowns and bridges that have a maximum of one intermediary located between the pillar teeth; and unplated parts such as marginal collarette.

The contraindication for Pekkton is patients who are allergic to the material, crowns and bridges that do not have sufficient occlusal space—less than 1.3 mm, patients with bruxism and inadequate oral hygiene [49, 50, 51].

PEKK presents the same characteristics of the human bone, due to its chemical and physical properties. This is why it covers a high spectrum of indications.

4. Conclusions

Mixed metal polymeric crown (PMMC) were an alternative and the first attempt to improve the esthetic for fix prosthodontics that involved the metallic cores. For the esthetic component of the mix metal polymeric crowns, the metallic core is designed, which is veneered with a polymeric material. Both polymers without inorganic loading can be used, as well as composite materials, including photopolymerizable ones. To achieve the adhesion between the physiognomic veneer and the metallic component, macro-retentions such as pearl retentions, nets and loops need to be designed. Micro-retentions are made by sandblasting or chemically conditioning and increase the adhesion of the polymer to the metallic core. To improve the marginal closure, which, only in the case of the use of macro-retentions, can be deficient, chemical adhesion was also associated and obtained with the help of compounds that chemically condition the metal component. Among the chemical conditioning processes, systems based on 4-Meta, Silicoater and Silicoater MD or Rocatec gave remarkable results in terms of improving adhesion. Despite the progress made in the field of metal polymeric crowns, clinical studies show chromatic changes, superficial wear or even detachment of the physiognomic component. And for these reasons, it is recommended to avoid the use of polymers in areas subject to occlusal stress. The use of fixed metal-polymeric prosthetic restorations is recommended only as temporary long-term restorations or the plating of removable components in combined restorations (crowns associated with removable dentures) [15].

Modern veneering materials, regardless of their nature, based on poly-methyl-methacrylate (PMMA) or composite resins, obtain superior physiognomic effects, especially due to the possibilities of layering different types of masses (opaque layer for masking the metallic core, dentine, enamel and shades for individualization). The polymerization can take place at temperature and pressure or by introducing it into a photo-polymerization device [16].

Hybrid ceramic materials consist of a three-dimensional ceramic network (approximately 86% by weight) that is then infiltrated with a polymer based on UDMA (urethane dimethacrylate) and TEGDMA (triethyleneglycol dimethacrylate). Due to this structure, the modulus of elasticity is superior to the other two categories of materials. Indications for hybrid ceramic materials are strictly limited to single restorations: crowns in the frontal and lateral areas, inlays, onlays and veneers.

In terms of translucency, PMMA has the highest value, followed by composites and hybrid ceramics. The translucency of the material is one of the factors that influence the choice of the restorative material in the case of dyschromic preparations.

Biocompatibility is positively influenced by the industrial process of block production. The well-controlled production conditions determine a denser structure and a lower proportion of residual monomer, by increasing the internal conversion rate, which minimizes the occurrence of allergic manifestations [17].

PEEK and PEKK belong to the PAEK family of high-performance polymers. PEEK has a crystalline phase, compared to the PEKK, which has both amorphous and a crystalline phase. Due to its physical and chemical properties, the latter is among the top high-performance polymers in the dental field.

Conflict of interest

The authors declare no conflict of interest.

Appendices and nomenclature

Bis-GMA	bisphenol A-glycidyl methacrylate
TEGMA	triethylene glycol methyl ether methacrylate
UDMA	urethane dimethacrylate
HDDMA	1,6-hexanediol dimethacrylate
CAD-CAM	computer-aided design/computer-aided manufacturing
FPP	fixed partial prostheses
IRCs	indirect resin composites
UDME	urethane dimethacrylate
TEGDMA	triethylene glycol dimethacrylate
Bis-EMA	bisphenol A ethoxylate dimethacrylate
acet-GDMA	acetyloxypropylenea
ISETMDA	ethoxylated isosorbide dimethacrylate
PPDGMA	poly (propylene glycol) dimethacrylate
VS	volume shrinkage
SOCs	spiro orthocarbonates
SS-S	shrinkage stress
PICN	polymer-infiltrated-ceramic network
UTMA	urethane tetramethacrylate
PEEK	polyetheretherketone
PEAK	polyaryletherketone
CFR-PEEK	Carbon fibre reinforced PEEK
PEKK	polyetherketoneketone
PMMA	poly-methyl-methacrylate
PAEK	polyaryletherketone