Radiopacity and mechanical properties of dental adhesives with strontium hydroxyapatite nanofillers

https://doi.org/10.1016/j.jmbbm.2019.103447Get rights and content

Abstract

Objectives

Dental resins filled with hydroxyapatite (HAp) nanoparticles have significantly revolutionized restorative dentistry offering advantages such as remineralization potential and increase of polymerization. However, these materials have limited radiopacity hindering the diagnosis of secondary caries. The present study investigated the development of a new radiopaque dental adhesive by incorporating novel pure strontium hydroxyapatite nanoparticles [Sr10(PO4)6(OH)2, SrHAp] synthesized by a simple hydrothermal method.

Methods

Strontium phosphates were prepared using co-precipitation (SrHAp0h) and hydrothermal treatment for 2 and 5h (SrHAp2h and SrHAp5h). The crystallinity, crystallite size, textural and morphology features of the nanoparticles were characterized by XRD, FT-IR, micro-Raman and TEM. Strontium hydroxyapatite (SrHAp) or calcium hydroxyapatite (HAp) nanoparticles were then incorporated (10 wt%) into an adhesive resin of a commercial three-step etch-and-rinse adhesive to evaluate the radiopacity of disk-shaped specimens, degree of conversion (DC) assessed by FT-IR and mechanical properties by three-point bending test. The unfilled adhesive was used as negative control.

Results

While SrHAp0h and SrHAp2h resulted a phase mixing, the pure and highly crystalline phase of strontium hydroxyapatite free of calcium was obtained with 5h hydrothermal treatment (SrHAp5h). The TEM images revealed nanorods morphology for SrHAp5h. SrHAps incorporated into adhesive showed higher radiopacity, no alteration on DC despite slightly reducing the mechanical properties.

Significance

Although the mechanical properties are slightly impaired, incorporation of SrHAp nanoparticles offers potential method to improve radiopacity of restorative dental resins, easing the tracking of actual remineralization effects and enabling diagnosis of recurrent caries.

Introduction

Resin composites have become the primary material for restorative dentistry. However, several studies indicate that resin composite has less durability than amalgam (Orrego et al., 2017; Elkassas et al., 2017). Therefore, adhesive compositions and bonding procedures have been improved by addition of different inorganic nanofillers. In dental biomaterials, hydroxyapatite (HAp) nanoparticles have shown to be an adequate filler for adhesive resins to improve their adhesion to dental hard tissues and preserve mechanical properties after water aging (Andrade Neto et al., 2016).

HAp crystallites represent the main constituent of the mineralized dental structures (Vassal et al., 2019). The apatite forms have beneficial dental applications due to its biocompatibility in addition to biological and chemical similarity to the dental structures that comprises hexagonal prisms of Ca10(PO4)6(OH)2. This similarity of structures facilitates integration with the mineral components of teeth and can be used to induce tooth remineralization (Elkassas et al., 2017; Mazumder et al., 2019; Melnikov and Gonçalves, 2015). In addition, control of crystallite size and morphology may result in increased biocompatibility (Andrade Neto et al., 2016). Nevertheless, the clinical diagnosis of teeth restored with resin composites fillings is still difficult due to the low radiopacity of fillers, what difficult to verify the occurrence of secondary caries by radiographs. Restoration is often replaced without necessity thanks to the difficulty of evaluating this material (Zanatta et al., 2019).

Therefore, an alternative strategy to overcome the radiopacity drawback of HAp and restorative composites is to dope bioactive trace elements with higher atomic weight into HAp by ion substitution. There are many reports of calcium ion substitution in apatites by bivalent cations (Gopi et al., 2014; Huang et al., 2016; Aina et al., 2012; Krishnan et al., 2016). Among the bioactive metal ions, strontium ions (Sr2+) has better performance in biomaterials for bone regeneration (Huang et al., 2016; Aina et al., 2012). Comparative studies have demonstrated the superiority of strontium ions incorporated in the structure of HAp in stimulating osseointegration and biomechanical properties with respect to the Zn and Mg ions (Tao et al., 2016). Strontium is, like calcium, a group II element and, from a chemical point of view, they behave similarly (Shahid et al., 2014; Habibovic and Barralet, 2011) in the human body. It is reported that Sr2+ can reduce bone resorption by inhibiting osteoclast activity and improve bone formation by stimulating osteoblast activity (Tao et al., 2016; Masala et al., 2014). Sr-substituted apatite structure shows increased radiopacity due to its high atomic number and weight compared to Ca leading to an increased absorption of X-ray energy (Wang et al., 2007). Strontium is also known to favors cellular proliferation (Sarkar et al., 2019), exhibit weak antibacterial activity (Liu et al., 2016; Guida et al., 2003; Brauer et al., 2012; Jayasree et al., 2017) and cariostatic properties in most animal caries and epidemiological studies reported (Jayasree et al., 2017; Curzon, 1985).

The addition of Sr to glasses for dental restoratives is designed to enhance radiopacity. Shahid and collaborators report the effect on esthetics (translucency) and radiopacity of substitution of Ca2+ ion by Sr2+ on production of radiopaque glass ionomer cements. They concluded that the level of radiopacity is found to increase linearly with Sr, without adverse effects on visual properties of the cement (Shahid et al., 2014; Meininger et al., 2019).

In particular, strontium hydroxyapatite (SrHAp) would serve as a viable alternative to overcome the disadvantages of HAp such as radiopacity, and track the sustained release of Sr2+ ions from SrHAp for proving the true remineralization, which is difficult with Ca-containing materials (Lei et al., 2017). In addition, there are reports that the presence of nanometric apatites significantly increases the bioactivity and biocompatibility of the biomaterials (Sadat-Shojai et al., 2013), besides being able to improve the mechanical properties when associated with resins (Andrade Neto et al., 2016).

Thus, the aim of this investigation was demonstrate a simple method to synthesize pure SrHAp, and study its potential as filler to dental adhesives, in order to obtain a dental material improved in terms of radiopacity and physicochemical properties. The first hypothesis is that the addition of SrHAp nanoparticles will effectively improve the radiopacity, and the second one is that the presence of the nanoparticles in the adhesive does not alter significantly the mechanical properties.

Section snippets

Synthesis of SrHAp nanoparticles

The preparation of SrHAp was adapted from the method previously described by Andrade Neto and collaborators (Andrade Neto et al., 2016). A solution of H3PO4 0.3 mol.L−1 (Quimex) was added to Sr(NO3)2 0.5 mol.L−1 solution (Dinamica, 99.67%) (molar ratio Sr/P = 1.67) under vigorous stirring. The pH of solution was adjusted to pH = 9.0 by adding NH4OH (Dinamica, 30%). A white precipitate was formed, and the suspension was stirred for 2 h. Thereafter, the precipitate was washed with distilled water

XRD

SrHAp0h and SrHAp2h specimens showed Sr10(PO4)6(OH)2 phase (Hexagonal system) (Fig. 2A), according with reported data (JPCDS n° 00-033-1348). Nevertheless, it observed peaks attributed to a secondary phase of strontium hydrogen phosphate, SrHPO4 (JPCDS n° 00-012-0359). The structural parameters results, refined by the Rietveld method (Table 1), indicate that the hydrothermal treatment promotes a phase transformation in which SrHAp is produced from SrHPO4 phase. Consequently, the specimen with a

Discussion

The present study showed a simple and inexpensive method for synthesis of pure SrHAp nanoparticles with high crystallinity and defined morphology. Dental resins were doped with these nanoparticles to evaluate the radiopacity and mechanical properties of this new material. The first hypothesis was accepted due to the resin doped with SrHAp exhibit a higher radiopacity compared to other adhesives. The second hypothesis was partially accepted because the DC was not changed as a function of the

Conclusions

The results of this study confirm that the hydrothermal method is a simple and efficient chemical synthesis route for the preparation of pure SrHAp nanoparticles. XRD patterns, Raman and FT-IR spectra indicated the crystalline phase of SrHAp5h, without secondary phases. Increased duration of hydrothermal treatment promoted the rearrangement of atoms in the crystalline lattice results in higher purity producing nanorod shape confirmed by TEM. Therefore, the dental adhesive incorporated with

Conflicts of interest

No conflict of interest.

Acknowledgments

This work was supported by CAPES (Finance Code 001 and grant 23038.006958/2014-96), FUNCAP (PNE-0112-00048.01.00/16) and CNPq (408790/2016-4) (Brazilian agencies). We thank the X-ray laboratory of Federal University of Ceará (UFC) for XRD analysis and Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for Research in Energy and Materials (CNPEM) for providing the equipment and technical support for the experiments involving TEM.

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