Effect of niobophosphate and brushite nanoparticles in composite resins

Introduction

Restorative Dentistry materials have remarkably evolved in the last two decades. The search for aesthetics, biocompatibility and minimum requirements for a restorative material has guided researchers in changing biological concepts and chemical formulations. A better understanding of the microbiological processes and the stages of dental decay development provided improvements in the preventive stages.

Dental amalgam used for direct restorations is the only material that minimizes microleakage over time due to the presence of oxides, particularly sulfides, at the tooth restoration interface. However, it lacks aesthetics and does not have adhesion to dental hard tissues, which compromises its clinical longevity (1).

The composite resin consists of monomers that, when joined and polymerized, are responsible for the formation of a polymeric network. However, adhesion is only provided to the tooth by the interposition of a dentin bonding agent, also based on resin, which can be considered as the true restorative material. Over time, the composite resin and even the adhesive system suffer from hydrolysis and permeation of oral fluids. This leads to periodic changes with greater wear on the dental structure (2).

“Smart materials” that release ions and alkalize the environment have been employed to reestablish the jeopardized enamel and dentin after demineralization. Some of these materials have antibacterial properties preventing colonization and biofilm formation. This prevents microleakage and formation of secondary caries. Some of these materials under study are known as bioglass. There is a very large variety of bioglasses, among which we can mention: niobophosphate nanoparticles and dicalcium phosphate dihydrate particles also known as brushite. The purpose of these materials is to enhance the ionic release characteristics, pH increase (hydrogen potential), bactericidal features and degree of conversion,  among others.

The aim of this work was to review the literature on brushite and niobophosphate nanoparticles when used in the formulation of composite resins used in direct restorations.

Methods

The papers were chosen through electronic search in the following databases: PubMed, MEDLINE, LILACS, BBO, SciELO and Google Scholar, with the terms: composite resins, dental materials, surface properties; and, biocompatible materials.

Eligibility criteria were: review articles, longitudinal follow-ups and laboratory researches, preferably in English language, published from 2010 to 2019 with online access.

Classical and enlightening studies concomitant with the proposed topic were added.

As an exclusion criterion, articles with other types of bioglass or other resin formulations were removed.

Of the 104 articles searched, 62 were excluded because they were not relevant to the review. We added 2 book chapters, 1 patent and 39 scientific articles. Some references were added by the rhetorical relevance to the subject of composite resins, totaling in the end 42 references.

Results

The development of restorative materials and techniques culminated in the current advances in dentistry.

Initially, chemically activated acrylic resins appeared in Germany (1) in 1934. The acrylic resins based on PMMA (polymethyl methacrylate) had direct use in the oral cavity, since they were already used for denture bases, were relatively easily manipulated, allowed pigment addition and characterization for aesthetic areas, had properties of insolubility in oral fluids, which can be easily and cheaply acquired (2). In contrast, they presented low resistance to wear, excessive thermal expansion and contraction, stresses in the cavity margins along the cavosurface angle, with drawbacks such as cracks in the interface between the tooth and the restoration, staining, as well as fluid and bacterial permeation (2). Paffenbarger et al. (1953) (3) suggested inorganic filler particles of aluminum silicate or quartz in PMMA, in order to improve the properties of acrylic resins. However, these particles occupied space, but did not participate in the polymerization reaction, having a very low linear thermal expansion coefficient, reducing thermal expansion and contraction (2). Thus, there was a reduction in the volume of the organic matrix, without a close union relationship between this matrix and inorganic particles (2).

In the 1950s, Ralph Lee Bowen (4) attempted to improve the inorganic particle epoxy resins used as binder. They polymerized at room temperature, with low contraction and sufficient adhesion to remain in the teeth (2). However, they had a long polymerization time and suffered rapid discoloration (1,2). In 1956, Bowen (4) successfully combined epoxy resins with methacrylate to form a hybrid resin, bisphenol A ether with glycidyl (2,2`-bis- [4- (methacryl-oxypropoxy) -phenyl] - propane), developing (Bis-GMA) (1,2), a high density monomer with numerous cross-links, with a silane-type bonding agent, linking the resinous matrix of monomers with inorganic filler particles (1,2).

Nowadays, composite resins bear such a name because they have in their formulation an organic matrix (monomers that come together to form a network of polymers), bonding agent (an organofunctional molecule to bond inorganic and organic silane-like parts), inorganic particles (glass filler particles, quartz, among others), dye pigments, radiopacifiers and polymerization inhibitors. The physical activation is performed by halogen light, light emitting diodes (LEDs), argon laser of 454-514 nm (nanometers), plasma arc and modern polywave devices with short wavelength emission (violet), which activates the primers of type I, even the long ones (blue), more suitable for camphorquinone (5). Two types of photoinitiators are used in dentistry: I and II. Type I has a high quantum field and requires few photons to generate free radicals, much more than type II. Type II is more reactive and has a large quantum efficiency. Dibenzoyl germanium is a Type I photoinitiator used in modern patented bulk-fill resins.

The ideal requirements for a dental composite are (6) as follows: it must be nontoxic, tasteless, odorless, have dimensional stability, low static and dynamic creep, impermeability and low leakage of oral fluids, have linear thermal coefficient of expansion and contraction close to that of dental tissues, allow repair, prevent secondary decays, be biocompatible with dentin-pulp complex, have transparency, translucency, opacity, opalescence, be radiopaque for contrast with dental hard tissues on radiographs, have mechanical strength for posterior teeth, have universal use (in both anterior and posterior teeth), have low susceptibility to abrasion, adhesivity, low cost, possibility of polishing; and finally, it can be easily manipulated, allowing sculpture without early polymerization. No material so far has all these desirable characteristics. Therefore, the clinician and the researcher must have knowledge and familiarity with the new materials available in the market, in terms of indications and long-term postoperative control (6).

Discussion

Several modifications have been proposed in the composites formulation to improve their characteristics and minimize problems inherent to resinous materials such as polymerization shrinkage and stress. Polymerization shrinkage stress can lead to numerous clinical problems, such as: microleakage, gap formation in the tooth / restoration interface, pulp irritation and eventually tooth loss (15). The low degree of conversion also compromises the physical properties of the composite resin and increases the leaching of unbound monomers in the form of polymers (residual monomers), greatly influencing biocompatibility. This fact also causes inconveniences, such as: postoperative sensitivity and possible restoration failure (7,8).

Ancient civilizations such as the Chinese, Egyptian and Indian ones used biomaterials to reconstruct body parts, correct aesthetic defects and harmonize segregation forces (9). In 1969, Larry Hench (10) from the University of Florida, developed a bioactive glass for clinical use. Hench has since patented the Biovidro^® 45S5 with 46.1 mol. % SiO₂, 24.4 mol. % Na₂O, 26.9 mol. % CaO and 2.6 mol. % P₂O₅, which stimulates the formation of hydroxyapatite [Ca₁₀(PO₄)⁶(OH)₂]. Moreover, bioactive glasses have been used in achieving osseointegration by inducing the deposition of hydroxyl carbonate apatite, enhancing the natural mineralization of hard tissues. Antibacterial properties (bactericidal and bacteriostatic) and neutralizing properties that increase the pH (hydrogen potential) due to the alkali released from such biomaterials have been extensively studied (11). Thus, several studies have incorporated bioactive particles in dental materials: composite resins (12), pastes used for the treatment of dentinal hyperesthesia (13), glass ionomers (14), adhesive systems (15), cements for protection of dentin-pulpar complex (16), endodontic obturation cements (17), in gutta-percha (18), in dental bleaching agents for enamel remineralization (48), as a cleaning agent for dentin pretreatment prior to the bonding procedure (19), for the removal of decayed tissue and adhesive cavity preparation in the abrasive blasting technique (20); and in a commercial orthodontic adhesive cement (21).

Therefore, when these bioactive glasses are dissolved in organic fluids they are capable of inducing hydroxyapatite formation. This process involves some stages, with the dissolution of calcium ions into the fluids, making them supersaturated, and the formation of a silica rich gel layer on the surface of these glass particles (22,23). Consequently, this leaching process occurs by exchanging H⁺ and H₃O⁺ cations, metal ions such as Na⁺ and Ca²⁺, which increases the local pH to levels close to 7.4 (24). Hydrolysis of the silicate network occurs on the glass surface. These silica ions are condensed and precipitated, crystallization occurs, with formation of amorphous calcium phosphate. Oxygen-silica bridges break down by OH^- hydroxyl ions. Further crystallization of this amorphous calcium phosphate phase leads to the formation of hydroxyapatite structure (11).

Three types of bioactive glass or bioglass have been approved for use in clinical applications by the Food and Drug Administration (FDA) and are widely used as bone grafting materials: 45S5, 45S5F and S53P4, according to Massera et al. (2012) (25) and Yang et al. (2016) (22). The increase in pH provided by bioglass, in addition to the release of cations may provide antibacterial effects. Thus, studies (26) have shown that the S53P4 bioglass, for example, can kill pathogens involved in enamel caries (Streptococcus mutans), root caries (Actinomyces naeslundii, Streptococcus mutans), as well as those involved in periodontal diseases (Aggregatibacter actinomycetemcomitans). It is postulated that the bioglasses must have bactericidal elements that prevent infections and reduce postoperative sensitivity, eliminating the biofilm around the cavosurface margins, greatly delaying their penetration into the tooth / restoration interface. Such components may be metals with bioactivity against microorganisms, which would solve problems due to the low stability of other organic components present in the resinous biomaterial processing. However, there is still no consensus on the best formulation for these bioactive glasses (27).

Bioglass generally has a low chemical stability formulation with short release or leaching of ions that can be resolved with the addition of silica and niobium. The presence of the niobium chemical element increases biocompatibility (28), radiopacity (29) and chemical stability of bioactive glasses (30). Moreover, the addition of niobium to the bioglass composition can be advantageous, resulting in high chemical durability of phosphate glasses, increasing biocompatibility and improving the mechanical properties of resin-based materials (18). According to Bauer et al. (15) (2016), adhesive systems with niobium incorporation act as a stress absorbing layer because of their low elastic modulus, allowing a deflection between dentin and composite and increasing marginal seal. In addition to providing remineralization in areas where collagen is unprotected, mainly at the bottom of the hybrid layer, compensating for the degradation of the polymer matrix. Therefore, the incorporation of niobophosphate particles into composite resins is perfectly plausible, in agreement with the findings of other studies (15,20,27,31).

Recently, brushite nanoparticles (CaHPO₄.2H₂O, dicalcium phosphate dihydrate), functionalized with triethylene glycol dimethacrylate (TEGDMA) have been synthesized (32). The TEGDMA monomer is capable of binding with the calcium ions of the growing crystals through di-pole bonds with the oxygen atoms in the ethylene glycol groups (-O-CH₂-CH₂-O-). TEGDMA in the functionalized layer can co-polymerize with other dimethacrylate monomers in the resin matrix, increasing the interaction between the organic matrix and the particles. A greater addition of TEGDMA also increases the degree of conversion because of its size and flexibility within the monomeric structure. However, the high hydrophilia of TEGDMA may lead to greater water uptake, increasing the hydrolytic degradation of the composite over time (33). The incorporation of organic components in the synthesis of CaP nanoparticles can change their characteristics, such as size, surface area and agglomeration (which may form clusters). Therefore, TEGDMA around bioactive particles is an important factor for the quality of the interface between the resin matrix and the particles, with consequences for the mechanical and ion release properties of composites (33). Thus, it is important to detail the particle fraction and the challenging conditions of the oral fluids. Calcium orthophosphate particles have been used as bioactive particles in composites demonstrating remineralization of white spot lesions (34) and demineralized dentin, delaying the development of carious lesions even in the presence of biofilm (35). In addition, functionalization does not promote an obstacle for calcium ion leaching with TEGDMA (8% by weight) with dicalcium phosphate dihydrate particles (36).

The elastic modulus describes the relative stiffness or rigidity of a material and it is measured by the maximum deformation at the elastic region considering the stress-strain diagram (37). The flexural strength represents the maximum strength of the material during the flexural test prior fracture. The magnitude of the elastic modulus affects the stress generated during composite resin cure. Thus, flexural modulus and adhesivity are important properties in inhibiting microleakage and the development of secondary decay. The flexural modulus of the occlusal-proximal cavities must be high to resist cusp deformation and fracture (37). These tests are important to predict the clinical behavior of these materials in the oral environment (37). The degradation of composites in oral challenges is due to the presence of ester groups (-COO-) in the monomeric structure, subjected to oxidation and hydrolysis when in contact with saliva and other fluids. The degradation of the resin matrix and the interface between this matrix and the particles reduces the elastic modulus of the composite, as well as its flexural strength and fracture resistance (38). Thus, the incorporation of CaP particles, unlike silanized filler particles, does not reinforce the material, in contrast, in some cases the fracture resistance is greatly reduced. The weak interaction between CaP particles and resin matrix, both chemically and mechanically, facilitates crack propagation under low stress levels. To increase the interaction between these phases, surface modifications of particles and resinous (monomeric) components have been proposed, such as: silanization, carboxylic radicals and TEGDMA (36).

Orthophosphates are derived from the traditional phosphoric acid, which has been used for enamel and dentin conditioning. However, this specific orthophosphate is triprotic, presents three dissociation constants (pK_a). The first atom of hydrogen is dissociated at pK_a1 2.12 (H₂PO₄^- : dihydrogen phosphate anion). There are other two constants at 25^oC, like: pK_a2 with 7.21 (HPO₄^2-), called hydrogen phosphate and with pk_a3 with 12.67 (PO₄^3-), which is chemically recognized like simple phosphate. So, calcium orthophosphates like brushite could be defined as salts of calcium derived from orthophosphoric acid (39).

The presence of hydroxyl groups is responsible for different calcium-to-phosphorous (Ca/P) molar ratios. Lower Ca/P ratios promote more acid and relatively water-soluble CaP phases. Amorphous calcium phosphate in supersaturated solution can be precipitated in crystalline apatite (39).

Calcium and phosphate ions penetrate through the pores of the lesion surface, like the demineralization process in tooth structures, by precipitating into the hydroxyapatite crystal voids in the body of the lesion (39).

The literature on enamel and dentin remineralization process using different polymer materials containing CaP particles like brushite is not very extensive. However, It is known that calcium orthophosphate particles do not reinforce the resin matrix. On the contrary, the fracture strength of unfilled resins is significantly reduced by the incorporation of CaP particles. There is not a strong chemical and micromechanical interaction between the nanoparticles and the resin matrix. Consequently, a uniform distribution and effective stress distribution between organic and inorganic composite resin or dentin bonding agent formulation have not been expected (39).

There is a tendency of CaP particles to agglomerate due to surface interactions of Ca-OH and P-OH groups with the formation of Ca-O-Ca and P-O-P bonds. A similar process adopted for silanization of glass particles in composite resins is made by stirring the particles in a solution containing the surface-active agent (particle functionalization). In this context, the functionalization with organic molecules or phases is an important way to improve the chemical compatibility between inorganic particles and organic matrix of current composites from the market (39).

Few studies have evaluated the degradation of CaP-containing resins. Composites containing 10 or 20% (mass) amorphous calcium phosphate in an inorganic content of 75% (amorphous calcium phosphate + reinforcing glass) do not differ from the fracture resistance control after 2 years of storage in water (40). On the other hand, resins containing 10% or 20% (by volume) of non-functionalized dicalcium phosphate dihydrate particles and an organic content containing 60% by volume showed more severe reductions in flexural strength, flexural modulus and fracture strength after 28 days in water compared to the control (without dicalcium phosphate dihydrate particles). Natale et al. (2018) (41) reported a decrease in the size of functionalized dicalcium phosphate dihydrate particles. They hypothesized that TEGDMA-derived ethylene glycol acts as co-solvent in water-stored systems, reducing surface tension and favoring crystal nucleation; this consequently decreased particle size, which is in agreement with Metha et al. (2009) (42). Previous studies testing composites containing 10 and 20% (by volume) dicalcium phosphate dihydrate showed an inverse statistical correlation between mechanical properties and ion release, which is also in line with other studies. On the contrary, it is believed that the dicalcium phosphate dihydrate fraction with 15% by volume achieves a balance of these desirable characteristics (mechanical resistance and ionic release), which is also in agreement with a previous article (41). It is difficult to estimate whether ion release is effective in inhibiting mineral losses, however it is known that low phosphate ion release is due to the structure of the crystalline phases of CaP. Oxygen atoms form hydrogen bridges with the middle water, inhibiting leaching (43). Also, the prediction of this ionic release in the acid environment is more difficult, although carious inhibition around restorations has been demonstrated in situ with a resin containing dicalcium phosphate dihydrate particles with a cumulative release of 3 ppm (parts per million) calcium ions at neutral pH over 28 days (35). As already mentioned, dicalcium phosphate dihydrate particles cannot be considered as reinforcing fillers. However, during crack propagation, fracture of dicalcium phosphate dihydrate particles reduces the intensity of stress and postpones its unstable growth, contributing in some way to an increase in toughness.

Conclusion

Given the above, it is necessary to predict the behavior of resins with “smart particles” of dicalcium phosphate dihydrate(brushite), as well as niobophosphate, in relation to the degree of conversion, to biaxial flexural testing, fracture toughness, bactericidal and bacteriostatic properties and the release of ions. These tests try to simulate the use of such biomaterials in the face of the adversities of the oral environment, seeking what would be the most appropriate intrinsic formulation.

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