Introduction
Growing awareness and concerns regarding amalgam toxicity with demand for aesthetics from patients have led to the increased use of tooth-coloured restorations. In paediatric dentistry a child patient’s cooperating ability is one of the factors considered for the selection of the restorative technique. Glass ionomer cements with superior handling properties have been the material of choice to date. However, glass ionomer cements are more prone to occlusal wear [
1] and mechanical strength of the same does not reach par with amalgam. Composite resins on the other hand, in spite of being technique sensitive are proven to exhibit superior mechanical and aesthetic properties. A recent systematic review on the survival of restorations in primary teeth showed the lowest annual failure rate for composite restorations (1.7–12.9%) [
2]. In permanent teeth, composite resins have been the first material of choice for direct restorations with an annual failure rate ranging from 1 to 3% [
3,
4]. Further advancements in dental composites in terms of filler size, shape and volume have resulted in the introduction of conventional/traditional, microfilled, small particle filled, and hybrid composites. The main driving force behind such developments was to improve the mechanical properties of dental composites without compromising the esthetic qualities.
Mechanical stability is a critical factor for the long-term success of resin composites used in dentistry [
5,
6]. Restorations involving multiple surfaces, predominantly Class II and Class IV restorations, are prone to fractures due to excessive concentration of stresses at the proximal region and incisal edges [
4,
7‐
9]. According to recent systematic reviews, fracture is one of the common causes of composite restoration failure in the posterior region [
4,
5,
10].
A direct correlation between the filler characteristics and mechanical properties of dental composite resins is well acknowledged. Hence, most strategies aiming to improve the mechanical characteristics of dental composites revolve around the development of novel filler materials and altering their size, shape and concentration in the resin matrix. It has been reported that both filler size and morphology directly influence the mechanical properties of resultant composites [
11‐
13]. Though colloidal silica and glasses with heavy metals are most commonly employed, fibers made of glass and ceramics have also been investigated to reinforce dental restorative materials [
14‐
17]. However, some of these attempts did not fructify owing to the large filler size, uneven filler distribution and weak bonding of filler to the resin matrix [
14,
18].
With the advent of nanotechnology, various nanosized fillers in dental composites have been extensively investigated. Nanofillers exhibit unique physical and chemical properties, including a high surface area to volume ratio for better interaction with the resin matrix [
19]. As the size of nanofillers approaches the wavelength of visible light, it allows easy passage of the light through the resin matrix compared to microfillers that reflect the light, thus providing flexibility to adjust the shade of the composite. Further, their smaller size facilitates easy polishability with a superior surface finish. Therefore, the incorporation of nanofillers offers a promising approach to enhance both the esthetic and mechanical properties of dental restorative materials [
14,
18].
Titanium dioxide has been used as a filler in various dental restorative materials owing to its influence on colour, superior strength, corrosion resistance and biocompatibility. Incorporation of titanium dioxide nanoparticles into the resin composites is known to increase the opalescence and at concentrations between 0.1 and 0.25% mimic the opalescence of human enamel [
20]. In addition, the nanoparticulate form of titanium dioxide has been reported to exhibit antimicrobial activity [
3]. Several studies have demonstrated the ability of nano-titanium dioxide to improve the mechanical properties of dental materials [
21,
22]. A significant increase in fracture toughness, flexural strength and flexural modulus was observed in dental composites reinforced with titanium dioxide [
14]. Similarly, the compressive strength of glass ionomer cements significantly increased with the incorporation of 3% of titanium dioxide nanoparticles [
23].
Despite intense research on the suitability of various materials as fillers in dental composites, to the best of our knowledge, the effect of morphology of fillers, nano-sized fillers in particular, on the mechanical characteristics of dental composites has not been reported widely. Therefore, the present study aims to evaluate the influence of the incorporation of various shapes and concentrations of titanium dioxide nanoparticles on the flexural and shear bond strength of the flowable composite. The null hypothesis of the study was that the shape/morphology and concentration of titanium dioxide nano-fillers does not enhance the flexural strength and shear bond strength of the flowable resin composites.
Discussion
The main aim of the present study was to investigate the effect of variations in filler morphology on their reinforcing ability in dental composites. An ideal resin composite is expected to possess low polymerization shrinkage, superior mechanical properties, excellent handling characteristics, and aesthetics [
26]. According to a meta-analysis, at least 5% of posterior resin restorations failed due to fracture and 10% of them showed excessive wear within ten year period of observation [
9,
27] and the replacement rate was higher after seven to ten years [
28].
Fillers in the resin composites are stronger than the resin matrix, thereby directly influences the mechanical properties and their size has been widely used for their classification in dental applications [
21,
28‐
30]. Filler particle size, size distribution, loading, hardness, index of refraction and radiopacity play an important role in their selection. Composites with large filler particles exhibit superior mechanical strength but poor aesthetics as they are difficult to polish due to the selective wear of the softer resin matrix leading to protrusion or plucking of fillers. Composites with smaller filler particles, in contrast, are easy to polish but exhibit inferior mechanical characteristics as such fillers exhibit large surface area and hence require large amounts of resin to wet their surface [
21,
28‐
30]. Hence, small filler sizes with wide size distribution are generally preferred to improve the mechanical properties of composites without compromising on their polishability.
Recently, a variety of nanomaterials such as titanium dioxide, silica, hydroxyapatite, silver, zinc oxide and graphene oxide have been evaluated as fillers to reinforce resin composites used in dentistry to enhance mechanical properties and impart antibacterial properties [
22,
30‐
33]. However, some of the nanomaterials, such as silver nanoparticles and graphene nanoparticles, are known to cause discoloration and thereby compromise aesthetics [
34]. Among them, titanium dioxide has been widely used in dentistry as an opacifying agent. It exhibits excellent biocompatibility and mechanical properties [
14,
35].
The pure anatase form of TiO
2 nanoparticles is prepared by a solvothermal process employing hydrolysis of titanium alkoxides in the presence of surface stabilizers under non-oxidizing conditions at relatively low temperatures. The pure crystalline anatase phase was obtained in white powder by calcination at 400–500 °C [
24,
36,
37]. Controlling hydrolytic reaction is challenging and can lead to a diverse population of different shapes of nanoparticles. Using long carbon chain hydrophobic surfactants such as oleic acid and oleylamine in optimum ratio, monodispersed rhombic, bar, spherical, truncated rhombic, dog-bone shaped nanoparticles have been synthesized previously [
23]. Adding a trace amount of water with surface stabilizers in a non-aqueous solvent yield shape-controlled and highly crystalline nanoparticles, which may be suitable for use in dental applications [
24].
In the present study, the shapes of nanoparticles were controlled by varying the ratio of TB:OA:OM, as confirmed by HR-TEM. The capping agent OA binds strongly on the surface of TiO
2 via its free carboxyl groups, whereas OM has been shown to support the growth of rhombic-shaped nanoparticles [
24,
38]. Further, the slow hydrolysis rate of TB facilitated the controlled morphological distribution of nanoparticles. Therefore, the ratio of TB:OA:OM was found to be a critical factor in the synthesis of anisotropic TiO
2 nanoparticles. Compared to spherical-shaped nanoparticles, the crystallinity of TiO
2 was more prominent in the rhombic group, suggesting that the optimum ratio of TB:OA:OB to synthesize uniform crystalline TiO
2 nanoparticles is 1:5:5 at 180 °C. SAED and XRD analysis confirmed the pure anatase phase, as indicated by different crystal planes observed in both samples. However, crystalline growth along (103) and (112) plane direction was only observed in nanoparticles causing rhombic anisotropy when the ratio of stabilizing agents was 1:5:5. The crystalline size data further supported the anisotropic nanoparticle formation. FTIR study indicated Ti–O bonding thus justifying TiO
2 nanoparticle formation.
Flowable composites are used widely in pediatric dental practice with indications in areas requiring improved mechanical strength, such as minimally invasive class II lesions, fracture reattachment cases, preventive resin restorations, as cavity liners, splinting of traumatized teeth, for bonding of fiber posts in endodontically treated teeth, etc. Flowable composites are referred to as an inhomogeneous group of materials with a wide range of applicability. Thus modification of this group of the composite provides the clinician a prospect to choose the right material for the indicated clinical situation. Thus the addition of titanium oxide nanofillers can alleviate the mechanical properties of flowable composite to facilitate its use in areas demanding better mechanical properties [
35,
39,
40]. Earlier study by Darfur et al. showed a significant increase in the elastic modulus and fracture toughness of the flowable composites when reinforced with titanium nanotubes. The enhancement of mechanical properties was evident at all concentrations (0–5%), however, a minimal decrease in flowability and radiopacity was observed at concentrations above 3% of filler reinforcement. Therefore, in our study 0.5 wt.% and 1.5 wt.% of TiO
2 were used to reinforce the flowable composite and significant increase in flexural strength was observed [
35].
Addition of nTiO
2 increased the flexural strength of the flowable composite compared to the control group. A similar improvement in strength was observed in earlier investigations [
14,
35]. It indicates that TiO
2 nanoparticles reinforced the matrix of flowable composite leading to better flexural strength. However, a reduction in flexural strength with increasing concentration of nTiO
2 could be attributed to the agglomeration of nanoparticles resulting in poor adhesion between resin matrix and nanoparticles [
14,
41,
42]. Alternatively, these nanofillers can be functionalized to overcome their agglomeration and to facilitate uniform dispersion in the resin matrix [
35,
43‐
46]. The improvement in mechanical characteristics of flowable composites was found to be similar irrespective of morphologies of nTiO
2. Nevertheless, further comparison with other anisotropic nanoparticles is necessary to justify the effect of the anisotropy of nanofillers on the mechanical properties of dental composites.
Shear bond strength test was used to evaluate the adhesion between the teeth and reinforced flowable composites. Ease and speed, with the lack of specimen processing requirements, make it the most common and reliable technique to test bonding [
47]. The results depicted that the nanoparticle shape and amount did not alter the shear bond strength of flowable composites. Though an increase in shear bond strength was evident with increasing concentration of TiO
2 nanoparticles, no significant difference in shear bond strength between the reinforced flowable composite and the tooth was observed. A similar insignificant increase in shear bond strength with glass ionomer reinforced with TiO
2 nanoparticles was reported [
21].
Analysis of failure modes revealed that debonding between the tooth and reinforced composite occurred through mixed failure indicating firm bonding of the composite to the tooth during failure [
20,
29]. However, there was no statistically significant difference in the mode of bond failures between the groups.
On the basis of results obtained in the present study, the null hypothesis could only be accepted partially as the incorporation of TiO2 nanoparticles significantly increased the flexural strength of the flowable composite although no significant changes in the shear bond strength to natural tooth was observed. The limitations are that the study compares only two morphologies of the fillers and silane coupling of the fillers is not performed. Further studies with different anisotropic forms and functionalisation of fillers are recommended to confirm the effect of the shape of fillers on the mechanical properties and to recommend a specific shape of the filler that demonstrates better mechanical properties for dental applications.
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