Introduction

Human tooth tissues are comprised of diversified micro nanostructures. For example, carbonated hydroxyapatite, with a range of 10–200 nm in size, is made up of 96% of enamel structure [1]. Dentin nanostructures are also unique, with intertubular dentin being 60 nm long and 2–5 nm thick. Peritubular dentin nanostructures are roughly 25 nm long and 2–5 nm thick, and dentine collagen fibrils range in size from 20 to 75 nm [1, 2].

Nanotechnology is essentially a science and engineering of functional systems at a nanoscale (one-billionth of a meter) [3,4,5,6]. Accordingly, when a material is smaller than 100 nm in one dimension, it is defined as a nanomaterial [3,4,5, 7, 8]. Hence, the aim of nanotechnology in dentistry is to mimic the natural tissue architecture, both soft and hard, by adapting new dental biomaterials to achieve better restoration of lost tissue that occurs due to disease, and to provide antimicrobial activity where necessary [1].

Nanomaterials can be synthesized in a variety of ways depending on a multitude of factors, such as the dimension of the materials being created. Hu and Shaw categorized nanoparticles as zero-dimensional, one-dimensional, two-dimensional, and three-dimensional [9]. Zero-dimensional nanoparticles (NPs) are defined as the nanostructure that has all dimensions in the nano-range, and they are amongst the most commonly employed type of nanomaterials in dentistry [9].

The main characteristic of NPs is that they have a very potent antimicrobial action against bacterial biofilm [10]. As NPs have a high surface area and high charge density, the nanoparticle ions in contact with microorganisms produce a germicidal effect [10,11,12,13]. They can also fill the gaps between the inter-polymeric chains, resulting in augmented mechanical and physical strength [14].

Different types of NPs have different properties. There are two main types of NPs: organic and inorganic. Quaternary ammonium polyethyleneimine (QPEI) [15], quaternary ammonium dimethacrylate [16], dimethylaminohexadecyl methacrylate [17] and chitosan [18, 19] are examples of organic NPs. Metal and metal oxides are examples of inorganic NPs.

Meanwhile, polyhedral oligomeric silsesquioxane (POSS) is an example of a nanosized organic-inorganic hybrid material consisting of a Si-O bond in three-dimensional architecture that is used with dental nanocomposites [20, 21].

Studies have shown that both organic and inorganic NPs have antimicrobial properties. The mechanical and physical properties have been more extensively studied with inorganic NPs, whereas there is limited data on the mechanical and physical properties of organic NPs. Similar to NPs, the large surface to volume ratio of nanomaterials is one of their most notable characteristics [7]. When considering a nanoparticle with a diameter of a few nm, all the atoms are either at the nanoparticle’s surface or inside the particle within a few atomic distances from the surface, depending on the atoms’ size and the nanoparticle’s size [7]. All atoms in a bulk material bind to their neighbors. Surface atoms, on the other hand, have fewer near neighbors, resulting in hanging or unsatisfied bonds, which results in an extra energy called surface energy, surface free energy, or surface tension that has a significant impact on the particle’s physical properties [16]. The above-mentioned physical and chemical phenomena have a number of significant implications for the characteristics of nanoscale materials and their manufacturing. In order to lower their surface energy, NPs try to agglomerate, resulting in the loss of its optical properties [4, 5, 7, 22]. It also loses some of its effective characteristics like the antimicrobial action, or it can leave inter-polymeric chain spaces unfilled, reducing the unique mechanical features. For this reason, good dispersion of NPs in the matrix is considered the main key for an effective nanomaterial. Similarly, the biological safety of nanoparticles is a critical aspect, as it depends on several factors such as their shape, size, method of preparation, and the chemicals used [23].

Recently, NPs are purposefully embedded in dental products to improve the material’s qualities as well as improve its longevity and success rate [3, 24]. Such fillers can be found in a variety of dental materials, including resin-based composites, cements, and impression materials [8, 12, 13] for the treatment of common dental diseases such as caries and periodontal infections [1, 25, 26]. As patient treatment needs, both esthetic and function, continue to expand, today’s scientists and engineers need to diversify nanoscale materials to take advantage of their superior features [1, 27].

Many studies and reviews focus on the antimicrobial properties of NPs-reinforced materials and demonstrated that NPs possess superior antimicrobial activity compared to their regular sized counterparts [10,11,12,13, 16, 28,29,30,31,32,33,34,35,36,37,38,39,40,41]. On the other hand, the studies that evaluate the physical properties of NPs are limited and the results presented are conflicted [42,43,44]. Inorganic nanoparticles incorporated into dental materials have been evaluated for their potential benefits, but the body of literature that focuses on the mechanical properties and their importance specifically to dental materials is lacking. In response to this, we sought to provide a comprehensive analytical review on how inorganic NPs influence the mechanical properties of dental materials in a variety of dental applications including prosthodontics, orthodontics, restorative dentistry, and endodontics.

Methodology

In order to achieve a conclusive result regarding the effectiveness of NPs on the mechanical properties of different dental materials, three databases (PubMed, Web of Science and Scopus) were searched during the time period from January 2021 to December 2021 using the association of the keywords: ‘inorganic nanoparticles,’ ‘dentistry’ and ‘mechanical properties’. The Boolean operators for each database were written as [“inorganic nanoparticles” AND “dentistry” AND “mechanical properties”]. For an article to be considered for inclusion in the review, it had to be written in English, and performed as a laboratory study or on humans (in vivo or in vitro on human cells). Furthermore, the papers had to be done on materials that were reinforced by NPs, and the test must have involved the evaluation of the mechanical properties of these NPs-reinforced materials.

The initial search in PubMed, Web of Science and Scopus databases conducted by the keywords revealed a total of 367 articles (Fig. 1). The screened data resulted in 133 duplicated articles. 234 articles remained to be subjected to title and abstract analysis. The eligibility criteria involved original articles in the English language related to NPs incorporated with conventional dental materials and the measurement of the mechanical properties. Review articles were excluded. This resulted in 63 articles of which 13 were excluded after reading the full text. Finally, full texts of 50 articles were included in the review.

Fig. 1
figure 1

Visual diagram of article filtration and selection from the PubMed, Web of Science, and Scopus databases

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Results

The total number of full-text articles included in this review is 50 articles. All tables show results of different parameters including author and year of the publication, sample size, NPs used in the dental material, size, concentration, the type of dental material, treatment groups, type of mechanical test, and outcome/result findings. Tables 1, 2, 3, 4 and 5, and 6 show results for the review of the materials: polymethyl methacrylate, glass ionomer cement, resin composite, resin adhesive, orthodontic adhesive, and endodontic sealer, respectively.

Table 1 Mechanical properties of PMMA reinforced by different NPs.
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Table 2 Mechanical properties of Glass Ionomer Cement reinforced by different NPs.
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Table 3 Mechanical properties of Resin composite reinforced by different NPs.
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Table 4 Mechanical properties of resin adhesive reinforced by different NPs.
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Table 5 Mechanical properties of orthodontic adhesive reinforced by different NPs.
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Table 6 Mechanical properties of endodontic sealer reinforced by different NPs.
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Fig. 2
figure 2

Diagram of the distribution of articles by the dental material and type of nanoparticle

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The total number of articles included in this review is 50 articles as shown in Fig. 2. The most common material that was investigated by researchers was PMMA with a total of 21 studies, followed by glass ionomer cement (GIC), resin composite, resin adhesive, and orthodontic adhesive with a total of 14, 6, 5 and 2 studies, respectively. The least investigated material was endodontic sealer with two articles. The most commonly used NPs used in all the studies were SiO2, TiO2, and ZrO2.

DISCUSSION

All 50 reviewed studies were in-vitro bench studies that used inorganic NPs as enforcing fillers in different dental materials to improve their mechanical properties. Their main objective was focused on enhancing the mechanical properties of some of the widely used conventional dental materials as those materials possessed some limitations. In order to improve those mechanical properties, the researchers resorted to using inorganic NPs as fillers. The studies then assessed the different mechanical properties of those NPs-incorporated dental materials with different concentrations of the inorganic NPs in an attempt to formulate a hybrid material with superior mechanical properties to the conventional one. However, the most challenging factor that these studies faced when incorporating the NPs with the dental materials is the tendency of NPs to agglomerate. This is a serious problem that can lead to a reduction of the nanoparticles’ surface energy, resulting in a loss and change of their nano-properties [7]. For that reason, and to achieve an appropriate homogeneous dispersion, all researchers used the NPs only with low levels of concentrations as it aided to minimize or eliminate the NPs agglomeration. In addition, some researchers found that using a silane coupling agent with the NPs was highly effective in avoiding that issue [45, 54, 56, 62, 77, 92]. The outcome of the mechanical properties of the modified dental materials reviewed in this study varied from one material to another, and from one mechanical testing to the other, based on the type and concentration of the NPs used, together with the original inherit composition and characteristics of the materials themselves.

Polymethyl methacrylate (PMMA)

PMMA-based resins are widely utilized in dentistry for a variety of applications, including removable base plates, functional appliances, and denture bases [93]. Its benefits stem from its biocompatibility and esthetics [81, 94]. It is also simple to manufacture, inexpensive, and has specific characteristics such as low weight, low water sorption, and low solubility [45, 95]. The material’s weakness involves the limited mechanical strength of PMMA resin denture base materials, with low impact and flexural strengths [94, 96, 97]. As a result, denture failure occurs frequently when eating or falling [98]. For these reasons, PMMA was the top material on the list of most studies (21 studies) that used NPs as a reinforcing filler to strengthen the PMMA’s mechanical properties. The reinforcing agent’s chemical bond with the polymer matrix was strong enough to withstand and transmit occlusal forces from the weaker polymer to the stronger reinforcing agent [51, 52, 55, 56, 59,60,61]. Additionally, the consistent impregnation of the reinforcing agent in the matrix inhibits the formation of stress concentrators [50, 51, 55, 56, 61] which could compromise the resins’ mechanical qualities.

The most commonly used NPs as fillers to reinforce PMMA were SiO2 and ZrO2 (7 studies each). The purpose for selection of those two NPs was due to their inherent properties that can enhance acrylic resins’ mechanical characteristics [50, 52, 54]. They possess a number of desirable features, including high toughness, mechanical strength, abrasion and corrosion resistance, and biocompatibility [99, 100]. Additionally, ZrO2 has excellent mechanical qualities that allow it to resist crack propagation, and it is noted to hold the greatest hardness of any oxide [101, 102]. The flexure strength, tensile strength, impact strength, fracture toughness and the surface hardness of PMMA reinforced by 0.5 to 1 wt% of ZrO2 or SiO2 increased significantly [45, 46, 49, 50, 52]. Surface treated ZrO2 or SiO2 with a silane coupling agent led to a decrease in the surface tension of the particles and influenced the spatial distribution of fillers, resulting in higher mechanical properties than with untreated NPs [51, 56, 92].

Conversely, one study concluded that using SiO2 incorporated with PMMA led to a significant decrease in the flexure strength of the original material. However, while further examining their technique, it was found that they incorporated the NPs in the monomer liquid and that they mixed it manually. That approach can cause uneven distribution and dispersion of the NPs within the matrix, which was confirmed by their SEM analysis that detected porosity in the PMMA matrix [47].

Although there is less published work on using Al2O3, TiO2 and diamond NPs fillers with PMMA compared to ZrO2 and SiO2, the results of those studies showed similar significant increase in the flexure strength, impact strength, and surface hardness [48,49,50, 60, 61]. However, due to the variability of the NPs, more studies with those NPs are needed to verify and validate those results.

Using Ag NPs did not appear to improve the mechanical properties of PMMA [57, 58, 103]. Ag NPs is mainly used for their antimicrobial activity to treat common infections of oral mucosal tissues in complete denture users [104]. They have been demonstrated to be effective against many microorganisms such as E. coli, Staphylococcus aureus, Staphylococcus epidermidis, Candida albicans, and Streptococcus mutans [105,106,107]. However, a significant increase in the flexure strength of PMMA was found when it was modified with graphene-Ag NPs [59] Additionally, adding Au NPs to PMMA showed no significant decrease in the flexure strength of the material; however, a significant increase in the micro hardness of the PMMA was observed when 0.43 wt% Au was added [63]. Similar results regarding the flexure strength was observed when ZnO was incorporated with PMMA [62].

The overall results of those studies indicated that most NPs proved to have a positive significant effect in improving the different mechanical properties of PMMA, especially when used in low concentrations of 0.5%.

Glass ionomer cement (GIC)

Biological compatibility, adhesion to moist tooth structure that allows for little removal of sound tooth structure, and anticariogenic qualities due to fluoride release are just a few of the benefits of GIC. Furthermore, it has a coefficient of thermal expansion that is similar to that of tooth structure. Despite those advantages, conventional glass ionomer cements possess limitations as restorative materials due to their brittleness, low flexural strength, low fracture toughness, low wear resistance, slow setting rate, high solubility and the relatively high sensitivity to water at the initial stage of setting [106, 107]. Numerous modifications had been developed over the years in attempt to overcome these drawbacks and improve the mechanical properties [108]. Most recently, NPs have been incorporated into glass ionomers with the objective of enhancing their mechanical strength. GIC was the second in list of number of studies (14 studies) that used NPs to augment its properties.

TiO2 was the top NPs selection used for GIC. Because of the relatively smaller size of TiO2 NPs supplemented into the glass powders, they can fill in the voids between the bigger GIC glass particles and serve as extra polyacrylic polymer bonding sites [35, 64,65,66,67,68,69]. For this reason, the flexure strength, compressive strength, and hardness value increased significantly by incorporation of 3, 5 and 7 wt% TiO2 [64,65,66,67,68,69]. Moreover, the compressive strength of GIC reinforced with 2 and 10 wt% ZrO2 also increased, while the addition of Al2O3 showed no effect on the mechanical properties [68, 69].

Together with the exceptional antimicrobial properties of Ag NPs, the incorporation of Ag to GIC showed a significant increase in the hardness, flexure strength, compressive strength, and shear bond strength [70,71,72]. The concentrations of 0.1–0.5 wt% were the most optimum concentrations for increasing those mentioned properties. At that very low level of concentration, the voids in the GIC matrix were filled with the small size of Ag nanoparticle fillers [35, 68, 70,71,72, 109]. Filling those voids resulted in the improved packing of particles within the matrix, which ultimately led to the enhancement of those mechanical properties.

Enforcing GIC with hydroxyapatite (HA) and fluorapatite (FA) NPs were evaluated in other studies [73, 74]. HA has a comparable composition and structure to enamel and dentin [109,110,111], which gives it the advantage and the edge to enhance the shear-bond strength with tooth structure. Furthermore, compressive strength, diametral tensile strength, and microhardness had all been significantly improved [73, 74] Alternatively, FA showed better results than HA due to its higher crystallinity [111, 112].

A combination of HA and SiO2 have been successfully used to enhance GICs [113]. Moreover, ZrO2 or a combination of HA and ZrO2 had been incorporated in attempts to strengthen GIC with improved outcomes [114, 115]. Zirconium and its oxide, due to their good dimensional stability and toughness, have been widely used for the toughening and strengthening of brittle HA bio-glasses in biomedical applications [116]. Because of that, using 5 or 7 wt% of ZrO2-SiO2-HA with GIC resulted in a significant improvement in the mechanical properties.

The crystalline structure of forsterite NPs resulted in the production of crystalline phases in the amorphous cement matrix [109, 117]. This could justify the enhancement of the flexure strength, compressive strength, and tensile strength of GIC reinforced with 1 wt% Mg2SiO4 [75].

Incorporation of MgO nanoparticles into different dental products was done to imbue antimicrobial properties [76, 118,119,120]. Regarding its mechanical effect on the GIC, the results showed a significant increase in the compressive strength and diametral tensile strength values when MgO was used at 1 wt% [77].

In general, GIC’s limited mechanical properties were improved when modified with different NPs in all the in-vitro studies reviewed. A low concentration of 0.1-1% of different NPs was found to give the most optimum results. Furthermore, comparable composition and structure of NPs to those of enamel and dentin significantly improved the GIC’s different mechanical properties, particularly the ones with higher crystallinity.

Resin composite

Resin-based composites are currently one of the most popular dental restorative materials due to their superior esthetic features and good adhesive properties [121,122,123]. However, resin composites have a number of mechanical shortcomings, including wear resistance, hardness, and shrinkage tendency [121, 124]. Secondary caries continues to be the most common cause of dental restorations’ service life being shortened, ultimately necessitating restorative material replacement. In terms of improving the features of resin composites, there are a few solutions that could be achieved. Dental resin nanocomposites are one of them; they are made up of a resin matrix, nanofillers, photo-initiator, and other components that are integrated together.

Regarding the benefits and drawbacks of nanocomposites, manufacturers rarely disclose the accurate proportional quantity, geometry, and size of nanofiller, which can be troublesome when these are important factors in determining the outcome of the mechanical properties. Furthermore, mostly all restorative “nanocomposites” are “nano-hybrids” with substantially larger volume ratios of non-nano sub-micron or micron-sized particles [7]. The other way in order to improve the mechanical properties of conventional resin composite is to use NPs [77,78,79,80,81,82,83]. NPs with nano-scale dimensions allow a wider area of interactions with microorganisms, thereby increasing their antibacterial activities, which is a requisite for dental restorative materials to prevent secondary caries [79, 80, 125]. Most of the studies involving the addition of NPs to dental composite resins mainly focused on their anti-bacterial effects, while the information regarding their mechanical properties was limited.

The mechanical properties, radiopacity, and optical properties of conventional composite resins have been improved by addition of inorganic NPs such as ZnO and TiO2 [77,78,79,80,81]. TiO2 offers a wide range of positive features. It is nontoxic, chemically stable, and has high photocatalytic efficiency [77]. In addition, TiO2 NPs are tooth colored and does not stain the restoration [78, 126]. The presumably smaller size of theTiO2 NPs promotes close cross-linking to the resin particles and prevents their degradation [79, 127]. Using 1 wt% TiO2 with dental resin composite leads to a significant increase in the flexure strength and surface hardness [78, 79]. These results increased specifically when the NPs were treated with a silane coupling agent, improving the dispersal and bonding of the filler particles throughout the matrix [78]. Additionally, the supplementation of Fa-HNT based fillers at low concentrations in dental composites was found to greatly improve the mechanical properties [82].

ZnO showed no effect on the flexure strength, but had a significant effect on the compressive strength and flexure modulus at 1 wt% concentration [80]. Combining ZnO with CaF2 showed a significant increase in the flexure strength at 0.5 wt% [81]. The opacity of ZnO NPs against visible light may have had a negative impact on light curing and, as a result, the mechanical characteristics of composites [80, 81, 86]. Because of that, further increase in the concentration of NPs led to decrease in the mechanical properties. It is possible to deduce that the decrease in mechanical characteristics was more likely a result of the effect of the NPs on composite curing rather than the occurrence of structural defects owing to particle agglomeration.

Despite the few number of studies that assessed the effect of enhancing composite resin with NPs (6 studies), their outcome was in favor of the positive added value of NPs in improving the tested mechanical properties of resin composite.

Resin adhesive

Dental adhesives have become commonly employed in restorative dentistry due to their esthetic and conservative characteristics. In reality, the clinical efficacy of resin composite is influenced by the restorative materials’ full adherence to enamel and dentin [95]. The acid-etch process is frequently successful when it comes to enamel [128, 129]. Dentin, on the other hand, is a hydrated biological composite made up of inorganic compounds, organic compounds, and water, with capabilities that vary greatly depending on where it is found. Furthermore, dentin has fluid-filled dentinal tubules, which create a dynamic and wet surface for bonding chemicals, as well as a more difficult situation than enamel [130, 131]. NPs have recently been introduced into dental adhesives with the goal of increasing mechanical qualities [132, 133]. Filler particles are used in dental adhesives to strengthen the bond between the adhesive and the dentin by entering the tubules of the dentin, reducing polymerization shrinkage, and raising the elastic modulus of the adhesive layer. [134]

Adding HA NPs to dental adhesives showed an increase in the micro-shear bond strength, tensile and flexure strengths [86]. Moreover, using Cu NPs demonstrated a better shear bond strength than conventional resin [83,84,85]. When compared to Cu free adhesives, these adhesives formed interfaces capable of reducing the deterioration of resin–dentin bonded surfaces [134]. Furthermore, since the collagen crosslinking enzyme Lysyl oxidase (LOX) is Cu dependent, Cu NPs had an indirect effect as a crosslinking agent, which consequently increased the strength of the collagen network, one of the components of the hybrid layer. Copper’s activity as a cross-linker may help collagen become more resistant [135, 136]. Using Ag NPs did not give any significant results on the mechanical properties of dental resin adhesive [87].

Orthodontic adhesive resin

Failure of the orthodontic bracket bonding method results in frequent debonding of the brackets, delaying treatment results. The bonding mechanisms and the failure rates of orthodontic brackets might be affected by tooth or material-related variables [136]. Previous studies have concentrated on the pre-treatment of resin monomers [137], inorganic fillers, and the development of curing procedures to improve the properties of orthodontic resin adhesives [110, 125, 138,139,140]. In dental adhesives, NPs have been explored as strengthening fillers—adding these NPs will result in an increase of the adhesive’s mechanical properties [88, 89, 137, 141]. Using Cu NPs with the orthodontic adhesive resulted in a significant increase in the shear bond strength [88]. Mixing of ZrO2 with TiO2 showed a significant increase in the shear bond strength, compressive strength, and tensile strength of the orthodontic adhesive [89].

Endodontic sealer

The physicochemical and biological features of the newly proposed root canal filler materials should be investigated. Setting time, flow, film thickness, solubility, radiopacity, dimensional stability, and compressive strength of endodontic sealers are among the criteria evaluated by the American National Institute/American Dental Association and the International Organization for Standardization [142, 143]. Combining ZrO2 with NbO NPs showed a significant increase in the compressive strength in Portland sealer compared to MTA Fillapex and Sealapex. However, the compressive strength of AH Plus sealer was significantly higher than reinforced Portland cement [90]. Using QPEI NPs with AH Plus and Pulp Canal Sealer resulted in no significant difference in the mechanical properties between all experimental groups [91]. Nonetheless, due to lack of sufficient studies on measuring the mechanical properties of sealer materials reinforced with NPs, we cannot give a conclusive statement about using NPs in different endodontic sealers.

Analysis of NPs used

The incorporation of NPs into different dental materials was utilized as a positive means of increasing their mechanical properties. TiO2 NPs offered flexure strength, impact strength, and surface hardness when incorporated into PMMA, higher flexure and compressive strengths with GIC, and enhanced the flexure strength and the surface hardness of the resin composite. ZrO2 and SiO2 offered better flexure strength, tensile strength, impact strength and surface hardness properties when incorporated into PMMA and enhanced the flexure strength and compressive strength of GIC. The shear bond strength of Cu nanoparticle-modified dental adhesive material was significantly higher when compared with the original material. The improvement in the mechanical properties of different dental materials that included NPs occurred due to several reasons: these NPs filled the empty voids within the matrix of the original materials, which improved their strength. Furthermore, the large interfacial area of NPs provided more contact points with the materials, and also interrupted crack propagation by transferring stresses from the weak original material to the strong NPs filler. Additionally, using silane-coupling agent created a strong adhesion between the original material matrix and the NPs. Regarding resin composite materials, the small size of NPs promoted cross-linking to the resin particles. Using NPs with adhesive resin improved the mechanical properties due to an increase in the strength of the collagen network.

All of the reviewed studies dealing with NPs-reinforced dental materials were in-vitro studies. Many of the studies operated on the basis that the use of various inorganic metal oxide NPs has the potential to be an innovative solution to improve those materials’ weaknesses. Despite the great potential that their promising results revealed for the effectiveness of incorporating NPs in different dental materials, unfortunately those bench studies were not followed up yet with clinical trials that could support those findings. Currently, there are very few studies that performed in-vivo testing in that context. Hence, those in-vitro results cannot be generalized to the real clinical settings without the confirmation of many and different in-vivo studies. NPs can be a great contemporary addition that could provide those dental materials tested with superior properties compared to those of the original ones. They can provide those enhanced materials with longevity and a higher success rate, and consequently augment their quality along with enhancing the quality of life of the patients using them. Therefore, well-designed clinical trials are essential to confirm the results of the published in-vitro claims and to encourage manufacturers to include NPs as a standard ingredient in the composition of those materials.

Summary

Several of the in-vitro studies demonstrated that the incorporation of NPs within various dental materials revealed very promising significant results in improving the different mechanical properties of the original material. Nonetheless, due to the variety of the nanoparticles, it is challenging to identify the optimum specifications that suit the spectrum of dental materials. Generally, the studies used NPs in low concentrations less than 1% by weight along with a silane coupling agent to minimize agglomeration issues; however, further clinical trials can validate the claimed positive results and confirm the performance and long-term effectiveness of those new hybrid nano materials in real clinical settings.