Essay: ceramics as restorative materials in dental restorations

Ceramics are used as restorative materials in dental restorations to restore absent or damaged dental structures (1). As people are retaining their teeth for a longer period of time than before, the demand for aesthetically pleasing restorations is increasing. There is an expanding use of ceramics in restorations as ceramics have superior aesthetics than metals. Therefore, ceramics will continue to be important restorative materials in the future for many years (2). This increasing demand for metal-free restorations by patients and clinicians has also resulted in the extensive development of all-ceramic restorations. The absence of a metallic substructure in all-ceramic systems results in superior aesthetics as even the underlying tooth structure imitates the shade and optical effects of the natural teeth, rather than having a metal substructure present (3). This results in a more natural and pleasing appearance. All-ceramic restorations also have superior biocompatibility than ceramic-metal and all-metal restorations because the possibility of metallic ions being released into the gingival tissue and fluid is eliminated thereby maintaining and improving the health of the soft tissues (1-3).

1.1-Different types of ceramics
Dental ceramics used in restorations are normally divided into three main groups based on their crystalline content: (1) polycrystalline ceramics, (2) glass ceramics, and (3) particle-filled glasses. These crystalline phases reinforce the ceramic matrix thereby improve the properties of the material. The nature, quantity and distribution of the particles in the crystalline phases affects the physical and mechanical properties of the ceramics (1-4).
Glass ceramics are the ideal ceramics to use when wanting to imitate the optical properties of both dentine and enamel. Glasses are 3D networks of atoms with a random arrangement and an amorphous structure. This trait makes crack propagation easier through the material. Glasses in ceramics are derived from feldspar and are based on alumina and silica. Feldspar glass-ceramics have the ability to withstand slumping if the firing temperatures increase above optimal temperatures because they are resistant to crystallisation (devitrification) and also have a long firing range (1-4).

Polycrystalline ceramics have an increased volume of crystalline material with no glass components. They are considered to be tougher and stronger than the other two types because the atoms are densely packed into a regular arrangement, which makes crack propagation through the ceramic more difficult. However, the high strength and toughness of these ceramics makes it more challenging to manufacture into complex shapes (1-4).

Particle filled glass ceramics are a subset of ceramics generally classified by the type and quantity of filler particles they contain. The addition of filler particles to the base glass composition aids in providing superior mechanical properties and helps control optical effects like opacity, colour as well as opalescence (1-4). The fillers could either be crystalline or particles of a higher melting glass. The first filler to be incorporated into a glass ceramic was leucite. Leucite was chosen because the refraction index of leucite was similar to that of feldspathic glass. The refractive indices of the glassy matrix and the crystalline phase must be similar to control the translucency of the ceramic. Another reason for using leucite was that leucite etches more rapidly and readily than the base glass, and this produces a myriad of tiny pores through which RBCs can enter, resultantly generating a stronger micro-mechanical bond (1-4).

1.2-The use of dental ceramics in all-ceramic restorations
High-strength ceramics are generally polycrystalline whereas ceramic restorations that require superior aesthetics predominantly consist of glass (5).
High strength ceramics are frequently used as core materials for crowns and fixed dental prosthesis as they can tolerate the forces witnessed in high-load bearing areas. Glass ceramics are commonly used as a veneer for metal or all-ceramic cores due to their superior aesthetics which is owed to their greater translucency compared to the other two ceramic groups (5).

1.3-Disadvantages of ceramics.
The advantageous aesthetic and biological properties of all-ceramic systems has directed a great deal of research into improving the mechanical properties as clinicians are still facing problems regarding the longevity and clinical performance of all-ceramic restorations because of their brittleness, poor fracture toughness, low tensile strength and limited flexural strength (6). The brittleness and low tensile strength results in ceramics being susceptible to fracturing under very low strain, which is normally between 0.1-0.2% (1). These traits render these restorations unsuitable for use in posterior regions where there are high masticatory forces. Most all-ceramic restorations placed in the oral cavity fail due to ceramic fracture and the most common failure mode is bulk fracture in the ceramic material. Stress concentrations acting on any defects present results in the fracture of the ceramic at either the surfaces or the interfaces. The defects could pre-exist within the ceramic microstructure or could develop during service (1, 6).

Clinical studies have identified that the survival rates for all-ceramic restorations ranges from 88-100% after 2-5 years of service and then decreases to about 84-97% after 6-14 years of service (7). The most common reason for failure of all-ceramic restorations was fracture and debonding. Due to these traits, a luting agent is utilised as a means of adhesive cementation to enhance the fracture resistance of these restorations (8).

1.4-An ideal luting agent
The luting agent used to cement all-ceramic restorations should attach the restoration to the tooth structure and act as a physical shield to conceal the weak traits of the ceramic. Accordingly, the luting agent should have good compressive and tensile strengths and appropriate fracture toughness. The ideal luting agent must seal the interface between the restoration and the tooth to act as a barrier against microleakage (8). The luting agent should not be prone to degradation in the oral cavity. To fulfill these functions, the ideal luting agent must deliver a durable bond between the restoration and the tooth. Three mechanisms could be used to explain the attachment that occurs between the ceramic restoration, the luting agent and the tooth structure (9, 10). It could be mechanical, through the micro-irregularities present on both the surfaces of the restoration and the tooth. The attachment could be chemical using a bonding technique or a combination of both mechanical and chemical (8-15).

RBCs are frequently used as luting agents for cementing all-ceramic restorations to the prepared tooth structures (9). Various clinical studies have demonstrated that all-ceramic restorations have an increased fracture resistance when cemented to the tooth structure using RBCs (10). The cement is thought to penetrate the defects or irregularities present on the ceramic surface thereby preventing crack propagation and increasing the flexural strength of the otherwise brittle ceramic (11-13). The RBC has a relatively high compressive strength (320MPa) which increases the otherwise poor fracture resistance of the all-ceramic restoration. The RBC is believed to enable more effective stress transfer from the restoration to the supporting tooth structure compared to zinc phosphate and glass ionomer cements (14). Bonding a ceramic restoration to the supporting tooth structure using a RBC is also thought to increase the marginal adaptation and retention of both the restoration and the supporting tooth structure (15). RBCs are considered to be virtually insoluble in the oral environment. They can adhesively bind to many restorative materials like resin composites and ceramics in addition to being available in various shades which offers excellent shade-matching potential to natural teeth (16).

RBCs have a composition similar to that of resin composites used in direct restorations as it is composed of inorganic fillers embedded in an organic matrix. It is usually Bis-GMA, TEGDMA or polyurethane matrix in which quartz is used as a filler. The particle size of quartz is between 0.04-0.2??m (15). A durable and strong bond should be formed between the organic matrix and the inorganic filler for the RBC to have optimal properties. Such a bond could be obtained by coating the filler particles with a silane coupling agent during the manufacturing process. This allows a chemical bond to form between the filler and the resin matrix (17). Furthermore, heavy metals like yttrium, zinc, barium or strontium could be included into the glass to acquire radio-opacity (17).

More viscous RBCs contain a larger volume fraction of fillers which increases the elastic modulus and strength of the RBC. The increased viscosity of these fillers reduces flow and increases the film thickness of the RBC (15-18). Lower viscosity RBCs, or flowable resins, contain lower filler loading which enables them to flow better and achieve thinner film thicknesses. Consequently, this decreases the polymerisation shrinkage generated which reduces the likelihood of gap formation and premature marginal leakage (15-18).

1.7-Types of RBCs
RBCs could be self-cured, light cured or dual-cured.

Dual-cured cements are compatible with silane agents and most other adhesive systems therefore provide good adhesion to multiple substrates like ceramics. Alongside this, they offer easy handling properties, appropriate working time and low solubility (14). They also provide both reduced setting and inhibition times. These cements are ideal for areas such as the bottom of deep cavities because they can be cured by both chemical and light polymerising components so the chemical curing can promote polymerisation when light is absent or restricted (9). As a result, these cements are preferably used in applications where the ceramic restoration is either too thick or too opaque to allow adequate LT through to the RBC. These cements can also be used for metal-free restorations where adequate curing may be required to seal the margins efficiently (19). This type of cement normally involves the mixing of two pastes. In one paste, components such as a reducing agent and a photoinitiator are present whereas the other paste contains peroxide which is usually benzoyl peroxide (BP) (20).

Light cured RBCs are most suitable to use when sufficient light can penetrate through the restoration to the cement because these cements contain photoinitiators, which are activated by light. In this cement, it is pivotal that the light must be able to reach all areas of the cement to activate as many photoinitiators as possible and obtain an adequate polymerisation of the RBC (19). Hence, these cements are generally used under thin and translucent restorations where there would be adequate LT reaching the RBC layer (20). Light curing RBCs can be more advantageous to use clinically than the other types because of their extended working time, their ability to set on demand and the improved colour stability (20). Furthermore, the clinician can remove the excess cement prior to curing which results in a decreased finishing time. These cements are suitable for highly aesthetic restorations and metal-free restorations because of their colour stability in comparison to the other two types of RBCs (19).

The third type of RBC is self- cured or chemical-cured RBCs because these cements polymerise via a chemical reaction and do not require light curing. This cement involves mixing two components together (base and catalyst) to initiate the reaction and are normally used in areas where light curing is challenging. Examples would include metallic restorations and ceramic restorations that prevent the light curing unit from polymerising the RBC (19, 20).

1.8-Polymerisation reaction
RBCs can only achieve optimal properties and strengthen the otherwise brittle all-ceramic restoration if a suitable DC is obtained through the polymerisation reaction. RBC polymerisation occurs via a free radical reaction (21-24).

During the polymerisation of RBC, the terminal aliphatic C=C bonds between the monomers are broken and transformed to C-C covalent bonds to form a polymer (24). In light cured RBCs, the initiation system consists of photoinitiators and a tertiary amine (N,N-dimethylaminoethyl methacrylate). RBCs that require photoactivation like dual- and light-cured systems most commonly have camphorquinone (CQ) as the photoiniator (21-24). Light irradiation with wavelengths between 470 and 480nm allows CQ to enter a ???triplet state???, where in this state; CQ has the ability to combine with two amine molecules to form a photoexcitated complex, better known as an exciplex. Once CQ has removed a proton from each of the two amine molecules, this exciplex complex breaks down to generate free radicals. The free radicals then react with the C=C bonds present within the monomer and start the polymerisation reaction which results in the conversion of the monomer to a polymer (21-24). If insufficient light reaches the RBC due to decreased LT through the all-ceramic restoration, then a lower number of CQ molecules reach the triplet state and consequently, the RBC will not be adequately cured (21-24).

Inadequate LT is more of an issue in light-cured RBC systems rather than dual-cured systems because in dual-cured systems, the cement could be cured by light- and chemical-activation. In dual-cured systems, there are two pastes; a catalyst paste contains a chemical activator, most commonly BP, which can be mixed with the paste containing the light-cured RBC (21-27). This is advantageous because the free radical concentration could still be increased even in situations where there is insufficient light. Mixing the two pastes together and exposing the mixture to light allows the generation of free-radicals by both chemical and light-activation (21-27). This is because, in the paste containing the light cured cement, free radicals would be generated from the process given above which involves the interaction between amine and CQ. However, in areas where light is restricted, free radicals could still be generated via interactions between the two amine molecules and the BP and these would compensate for the reduced number of radicals produced by light curing means (21-27).

Preferably, the RBC would have all their monomers converted to polymers through the polymerisation process. However, due to steric hindrance, monomer conversion is never complete and all resin cements display some residual unsaturation in the polymer and this is in the form of remaining methacrylate monomer groups (25). As the polymerisation process proceeds, the rate at which the propagating free radicals diffuse decreases greatly and this results in some monomers that remain unreacted in the polymeric matrix. The DC can be defined as the percentage of reacted C=C bonds and normally ranges between 35 to 88% depending on the resin composite material used (24-26).

An adequate DC value is vital to obtain optimal RBC properties such as mechanical properties, solubility, biocompatibility and dimensional stability of the RBC (25). To obtain optimal properties of the RBCs, the light should reach the cement in an intensity that can activate the light activation components, which are the photoinitiators (26). The main problem with indirect adhesive restorations is obtaining an adequate DC of the RBC beneath the restoration, especially when light-curing cements are used due to the reasons given in section 1.7 (14). Inadequate polymerisation of the RBC would result in inferior properties, especially mechanical properties like bond strength, elastic modulus, hardness, wear resistance and fracture toughness (10). Therefore, it is vital to achieve adequate LT through to the RBC to achieve an adequate DC and allow the RBC to fulfill its function which is to strengthen the all-ceramic restoration.

1.9-Resin-to-ceramic bonding
The clinical performance of the restoration depends on the durability of the bond between the ceramic-resin cement-tooth structure interfaces. The RBC can only increase the fracture toughness and improve the brittleness of the ceramic if there is an adequate bond between the ceramic and the RBC and between the RBC and the tooth structure. This bonding depends on sufficient DC to allow the resin cement to function optimally and strengthen the restoration (28-30).

The most successful and commonly used method for achieving an adequate bond is by adhesively cementing the restoration to the tooth structure. Here, the brittle ceramic material is adhesively bonded to the surrounding tooth structure using a RBC and by doing this, the tooth and the RBC provides support for the weak ceramic restoration (2). Adhesive cementation is achieved in two stages: firstly by acid etching and then application of a silane coupling agent. The first stage involves etching the internal fit surface of ceramic to roughen the surface. The aim of this stage is to increase both the surface area and the micromechanical retention available for adhesive cementation (21, 22). The second stage involves applying a silane over the etched ceramic surface to enhance the chemical bond to the ceramic. The silane coupling agents are organic compounds that consist of methacrylates at one side and on the other, silane alkoxy groups. The methacrylate groups have the ability to polymerise with the organic matrix of RBCs while the alkoxy group reacts with a hydroxylated surface, in this case the ceramic, to form a siloxane covalent bond (21). A stronger and durable adhesion between the cement and the ceramic would decrease the possibility of crack propagation and therefore contribute to a better clinical life span of the restoration due to an enhanced strength of the all-ceramic restoration (21, 22).

1.10-Resin-strengthening mechanisms
It is not yet understood as to how the resin strengthening mechanism actually works but two different theories have been proposed for the apparent improvement in strength observed (11-13). Marquis has proposed that the RBC modifies the surface flaws present within the ceramic via a process of full or partial crack healing and this consequently increases fracture resistance (11-13). Conversely, Nathanson suggested that the resin shrinkage during polymerisation stresses the ceramic molecules together, causing the molecules to come closer together rather than away from each other which is thought to strengthen the ceramic restoration. However, none of these proposed theories have yet been proven (11-13). The resin is also believed to increase the stress of the ceramics by adhesive bonding by shifting stress efficiently from the loaded ceramic restoration to the supporting tooth structure and to fulfill this function; the luting cement must have an elastic modulus that is between that of dentin and the ceramic (11-1, 23). The elastic modulus of RBCs is generally between 5-12GPa whereas dentine has a modulus of 18GPa making it relatively elastic, and ceramics have a modulus between 55-236GPa meaning that ceramics are considered to be inelastic (11-13). It is essential that the RBC transfers stress effectively from the restoration to the underlying tooth structure because it would enable the creation of strong bonding forces at the enamel/cement/ceramic interfaces which would result in increased strength, stability, clinical performance and longevity of the restoration (6, 11-13, 23).

1.11-Relationship between DC and resin-strengthening mechanisms
The strength of the restoration depends on a strong and durable adhesive bond between the ceramic and the cement. Adhesive bonding depends on adequate polymerisation of the RBC which is dependent on adequate LT to the RBC layer. LT depends upon parameters like the thickness and shade of the ceramic restorative, which interferes with light transmittance and resultantly, decreases the light energy that reaches the RBC and ultimately, the DC of the RBC (28-30). The polymerisation of the RBC would also be affected by the thickness of the RBC layer. All of these factors would affect DC as well as the bond produced between the ceramic and the cement and consequently, the strength of the restoration.

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