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Table of Contents   
ORIGINAL RESEARCH  
Year : 2022  |  Volume : 33  |  Issue : 2  |  Page : 193-197
The effect of adding nanoparticles to dental porcelain on the fracture resistance and bond strength to zirconia core


1 Department of Prosthetic Dentistry, College of Dentistry, University of Mosul, Mosul, Iraq
2 Department of Conservative Dentistry, College of Dentistry, University of Mosul, Mosul, Iraq

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Date of Submission11-Mar-2022
Date of Decision27-Apr-2022
Date of Acceptance04-Jul-2022
Date of Web Publication13-Oct-2022
 

   Abstract 


Background: Porcelain combined with zirconia core substructure has low fracture toughness. Nanoparticles are incorporated into the porcelain to boost its mechanical properties. Aims: To evaluate the effect of the incorporation of silver oxide and titanium oxide nanoparticles into porcelain powder on the bond strength of porcelain veneer to zirconia core. The flexural strength of nanoparticles-modified porcelain was also evaluated. Materials and Methods: The flexural strength of feldspathic porcelain was measured (according to ISO specifications number 6872) after the incorporation of silver and titanium oxide nanoparticles. For measuring the bond strength at the porcelain-zirconia interface, 70 bars (4 × 4 × 12 mm) of zirconia were cut and fired in a furnace. The control and nanoparticles-modified porcelain powders were built up on the zirconia bars and fired to create veneers of 3 mm height, 4 mm width and 4 mm thickness. The porcelain veneers were de-attached from the zirconia core by the universal testing machine. The failure load was recorded to calculate the bond strength. Statistical Analysis: The data were analysed with one-way analysis of variance followed by Tukey's test. Results: The addition of 0.5–1.5% silver oxide nanoparticles to feldspathic porcelain increased the flexural strength. The addition of 1.0–4.0% titanium oxide nanoparticles to feldspathic porcelain increased the flexural strength. Either 0.5–1.0% silver oxide or 3.0–4.0% titanium oxide nanoparticles in feldspathic porcelain increased the shear bond strength to zirconia core. Conclusion: The flexural strength of porcelain veneer and the bond strength at porcelain-zirconia interface can be improved by adding either 0.5% silver oxide nanoparticles or 4% titanium oxide nanoparticles to porcelain powder.

Keywords: Nanoparticle, porcelain, silver, titanium, zirconia

How to cite this article:
Mohammed AJ, Dawood AE, Saeed MA. The effect of adding nanoparticles to dental porcelain on the fracture resistance and bond strength to zirconia core. Indian J Dent Res 2022;33:193-7

How to cite this URL:
Mohammed AJ, Dawood AE, Saeed MA. The effect of adding nanoparticles to dental porcelain on the fracture resistance and bond strength to zirconia core. Indian J Dent Res [serial online] 2022 [cited 2022 Nov 29];33:193-7. Available from: https://www.ijdr.in/text.asp?2022/33/2/193/358440



   Introduction Top


Because of the superior aesthetic appearance of all-ceramic restorations, they have become a preferable treatment option (compared to metal-supported fixed crowns) for replacement of missing anterior teeth.[1] It has also become possible to use all-ceramic restorations in posterior teeth by the usage of CAD/CAM-fabricated zirconia core as substructure with high mechanical properties.[2] Zirconia cores have superior biocompatibility, high compressive strength and almost natural appearance.[3] The feldspathic porcelain is fragile and has low fracture toughness, and this may limit the mechanical strength of the porcelain veneer combined with zirconia core substructure.[4] Therefore, chipping of porcelain veneer and bond failure (at porcelain-zirconia interface) are possible clinical complications of zirconia-feldspathic veneer restorations.[5]

The advancement of new technology, involving the use of novel nanoparticles, has made a possible and promising improvement of the biological and mechanical properties of dental materials.[6] It was reported that the addition of aluminium oxide microfiber into zirconium dioxide ceramic had improved its mechanical properties.[7] Different amounts of various nanoparticles (such as silver oxide, zirconia, hydroxyapatite, titanium dioxide, alumina and platinum) were incorporated into feldspathic porcelain to boost its mechanical properties.[8],[9]

Although silver and titanium oxide nanoparticles were used to improve the mechanical properties of feldspathic porcelain,[9] the effect of this modification on the bond strength to the underlying zirconia core is not tested yet. Therefore, the present study aimed to evaluate the effect of adding silver and titanium nanoparticles to dental porcelain on the bond strength at porcelain-zirconia interface. This study also aimed to determine the maximum quantity of nanoparticles that could be added to the porcelain without deteriorating the flexural strength.


   Materials and Methods Top


Materials

A feldspathic porcelain powder (Vita VMK master, VITA Zahnfabrik, Bad Säckingen, Germany) for all-ceramic and metalo-ceramic restorations were used in this study. Powders of silver oxide (NANOSHELL LLC, Wilmington, DE, USA) and titanium oxide nanoparticles (Skyspring Nanomaterials, Inc., Houston, TX, USA) were used to modify the powder of feldspathic porcelain.

Measurement of flexural strength

The flexural strength of feldspathic porcelain was measured after the incorporation of silver and titanium oxide nanoparticles. This experiment aimed to determine the maximum quantity of nanoparticles that could be incorporated into feldspathic porcelain without compromising the flexural strength. Silver and titanium oxide nanoparticles were added to the powder of dental porcelain starting with 0.5% (w/w), then the concentration was increased by 0.5% (w/w) increments. Feldspathic porcelain powder and nanoparticles were weighed by digital balance (with four digits precision; PG 503-S MonoBloc inside, Mettler Toledo Ltd, Greifensee, Switzerland) and blended together by a ball-head plastic instrument. The nanoparticles-modified feldspathic porcelain was mixed with the modelling liquid according to the manufacturer's instructions to produce a creamy mixture. The mixture was poured into a metal mold (2 mm depth, 4 mm width and 25 mm length) using a vibrator to remove the air bubbles. Hydraulic press with 10 kg weight was used for 10 min to compact the porcelain mixture to produce a pressed powder compact. The pressed powder was sintered in furnace (Programat x1 Porcelain Furnace, Ivoclar Vivadent, Germany) using the dentine firing program under vacuum pressure at 920°C and heating rate of 65°C/min.

The dimensions of each sample were measured with a digital calliper (Lezaco, Art. 2771, China) and adjusted to produce uniform measurements for all the samples. International Organization for Standardization (ISO) specifications number 6872[10] were followed to measure the flexural strength using the universal testing machine (GESTER International Co., LTD, Quanzhou, China) with a cross head speed of 0.5 mm/min. The flexural strength was calculated using the following equation:

Flexural strength = 3 PL/2 wt2,

where P is the load at failure, L is the distance between the points supporting the sample during the test (which is equal to 20 mm), and w and t are the width and thickness of the specimen, respectively. This experiment was repeated 10 times for each concentration of nanoparticle-modified feldspathic porcelain powders (0.5–2% for silver oxide and 0.5–5% for titanium oxide).

Measurement of shear bond strength

In the present study, the bond strength of zirconia core to nanoparticle-modified feldspathic porcelain was measured. Seventy samples were prepared and divided into three groups: control group of 0% (w/w) nanoparticles (10 samples), silver oxide group (30 samples) and titanium oxide group (30 samples). The concentrations of nanoparticle-modified feldspathic porcelains that produced the highest flexural strength were used to test the bond strength. For silver oxide, the tested concentrations of nanoparticle-modified feldspathic porcelain were 0.5%, 1% and 1.5% (w/w). For titanium oxide, the tested concentrations of nanoparticle-modified feldspathic porcelain were 3%, 3.5% and 4% (w/w). Zirconia blocks (Whitepeaks Dental Solutions GmbH and Co. KG, Lange Heide, Essen, Germany) were cut with electrical saw machine (Carlo De Giorgi, Milano, Italy) using diamond wheels to create 70 bars (4 × 4 × 12 mm). Zirconia bars were fired in a furnace (Sinterofen, MIHM-VOGT GmbH and Co. KG, Baden-Württemberg, Germany) using the normal firing program (according to manufacturer's instructions) at 1500°C for 90 min. The heating rate was 10°C/min until 950°C was reached, followed by heating rate of 6°C/min until 1500°C was reached. The control and nanoparticles-modified feldspathic porcelain powders were built up on the zirconia bars to create veneers of 3 mm height, 4 mm width and 4 mm thickness. After porcelain firing, the zirconia bars were embedded with resin in plastic PVC tubes (12 mm height and 1.2 mm diameter) to match the holder of the bond strength testing device. The interface of the porcelain-zirconia was placed on the same level of the upper plane of the PVC tube. The test was carried out using the universal testing machine with a cross head speed of 0.5 mm/min, the testing machine blade was adjusted as close as possible to the porcelain-zirconia interface. The bond strength was calculated by dividing the failure load (Newton) by the bonded surface area (mm2).

Statistical analysis

The data were analysed using SPSS ver. 11.5.0 (SPSS Inc, Chicago, IL, USA). The data were analysed by Kolmogorov–Smirnov (K–S) test and they were found to be normally distributed. Therefore, the statistical comparison of the data was performed by one-way analysis of variance followed by Tukey's test for multiple comparisons. The level of statistical significance was set at P < 0.05.


   Results Top


The means and standard deviations of the flexural strength of the control and nanoparticles-modified feldspathic porcelain are presented in [Table 1]. The addition of 0.5–1.5% (w/w) silver oxide nanoparticles to feldspathic porcelain significantly increased the flexural strength (P < 0.05). The presence of 2.0% (w/w) silver oxide nanoparticles in the feldspathic porcelain significantly reduced the flexural strength (P < 0.05). The addition of 0.5% (w/w) silver oxide nanoparticles to feldspathic porcelain produced the highest flexural strength in comparison with the control feldspathic porcelain and other groups of silver oxide nanoparticles-modified feldspathic porcelain (P < 0.05). The addition of 1.0–4.0% (w/w) titanium oxide nanoparticles to feldspathic porcelain significantly increased the flexural strength (P < 0.05). The addition of 3.0–4.0% (w/w) titanium oxide nanoparticles to feldspathic porcelain produced the highest flexural strength in comparison with the control feldspathic porcelain and other groups of titanium oxide nanoparticles-modified feldspathic porcelain (P < 0.05). The addition of 4.5–5.0% (w/w) titanium oxide nanoparticles to feldspathic porcelain produced no significant effect on the flexural strength of feldspathic porcelain (P < 0.05).
Table 1: Flexural strengths (MPa) of nanoparticles-containing feldspathic dental porcelain

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The means and standard deviations of the shear bond strength of the control and nanoparticles-modified feldspathic porcelain to zirconia core are presented in [Table 2]. The presence of 0.5–1.0% (w/w) silver oxide nanoparticles in feldspathic porcelain significantly increased the shear bond strength to zirconia core (P < 0.05). The presence of 3.0–4.0% (w/w) titanium oxide nanoparticles in feldspathic porcelain significantly increased the shear bond strength to zirconia core (P < 0.05). The addition of 0.5% (w/w) silver oxide nanoparticles to feldspathic porcelain produced the highest bond strength to zirconia core in comparison to the control feldspathic porcelain and other groups of silver oxide nanoparticles-modified feldspathic porcelain (P < 0.05). The addition of 3.5% (w/w) titanium oxide nanoparticles to feldspathic porcelain produced the highest bond strength to zirconia core in comparison to the control feldspathic porcelain and other groups of titanium oxide nanoparticles-modified feldspathic porcelain (P < 0.05).
Table 2: Shear bond strengths (MPa) of nanoparticles-containing feldspathic dental porcelain to zirconia core

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   Discussion Top


Measuring the bond strength at the interface of porcelain bonded to zirconia core using the shear bond strength test has limitations and challenges related to uneven stress distributions, undetected faults produced during the manufacture of samples, concentration of stress, samples failure before and during the test, elastic modulus discrepancy and complex failure modes lacking clinical relevance.[11],[12],[13] Despite these limitations, shear bond strength test is still useful for the in vitro assessment of the bond strength at porcelain-zirconia interface. The assessment of the strength of porcelain using the three-point flexural strength test is more relevant than the use of the compressive strength test as dental porcelain is brittle and susceptible to fracture by the extension of cracks by tensile stresses.[14]

The findings of the present study indicated that 0.5–1.5% (w/w) silver oxide-containing feldspathic porcelain and 1.0–4.0% (w/w) titanium oxide-containing feldspathic porcelain showed higher flexural strength in comparison with the flexural strength of the control and other test groups. The presence of nanoparticles in the porcelain might have increased the strength by filling up the microcracks within the structure of the porcelain and inhibiting the extensions of these cracks.[15] Another explanation for the improved flexural strength of nanoparticles-modified feldspathic porcelain is that the addition of silver or titanium nanoparticles to dental porcelain might have produced compressive stress within the glass matrix during the cooling down of the porcelain.[9] This could be attributed to the higher coefficient of thermal expansion of silver and titanium nanoparticles in comparison with the coefficient of thermal expansion of the glass matrix.[16] A possible chemical change in the structure of feldspathic porcelain through ions exchange between the nanoparticles and the glass matrix might have also happened and generated compressive stress leading to an increase in flexural strength.[9]

In the present study, the addition of either 0.5–1.0% (w/w) silver oxide or 3.0–4.0% (w/w) titanium oxide to feldspathic porcelain improved the bond strength at the porcelain-zirconia interface. Weak and brittle feldspathic porcelain contributes to fragile feldspathic-zirconia interface which may lead to bond failure and chipping of porcelain veneer[17]; therefore, the improved flexural strength of feldspathic porcelain by the incorporation of nanoparticles might have contributed to higher bond strength of nanoparticles-modified felspathic porcelain to zirconia core in comparison with the control group. The silver and titanium oxide nanoparticles may fill up the air voids generated within the feldspathic porcelain during mixing.[18] The nanoparticles may also fill up the irregularities at the porcelain-zirconia interface and improve the surface adhesion and the bond strength as shown in the present study.

Metal nanoparticles have high surface energy and, in high concentration, they tend to accumulate as clusters which may contribute to reduced toughness of nanoparticles-modified feldspathic porcelain.[19] This may explain the reduced flexural strength of 2.0% (w/w) silver oxide nanoparticles-modified feldspathic porcelain in comparison with the control group and other groups of silver oxide nanoparticles-modified feldspathic porcelain, as reported in the present study. On the other hand, the addition of 0.5% (w/w) silver oxide to feldspathic porcelain produced the highest flexural strength and the highest bond strength to zirconia core in comparison with the control feldspathic porcelain and other groups of silver oxide nanoparticles-modified feldspathic porcelain. Therefore, it is not recommended to add more than 0.5% (w/w) silver oxide to feldspathic porcelain. High percentage of silver oxide nanoparticles may also contribute to colour change due to the dark colour of silver oxide nanoparticles.[20] The addition of more than 4.0% (w/w) titanium oxide is not recommended, because 4.5–5.0% (w/w) titanium oxide did not improve the flexural strength of feldspathic porcelain and therefore it is not expected to improve the bond strength to zirconia core.

This study has some limitations. The specimen's shape does not resemble the shape of the clinical crowns. Furthermore, the mechanical and thermal factors involved within the oral environment have not been considered. Further studies are also required to address the possible colour change of the feldspathic porcelain produced by the incorporation of the metal nanoparticles.


   Conclusion Top


Within the limitations of the present study, it seems possible to recommend the modification of feldspathic porcelain powder either by 0.5% (w/w) silver oxide nanoparticles or 4% (w/w) titanium oxide nanoparticles to improve the flexural strength of porcelain veneer as well as the bond strength at porcelain-zirconia interface.

Acknowledgements

This study was performed at the laboratories of the College of Dentistry at the University of Mosul in Mosul/Iraq.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Correspondence Address:
Dr. Alaa E Dawood
Department of Conservative Dentistry, College of Dentistry, University of Mosul, Mosul
Iraq
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijdr.ijdr_222_22

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