ORIGINAL RESEARCH

https://doi.org/10.5005/jp-journals-10019-1413
International Journal of Prosthodontics and Restorative Dentistry
Volume 13 | Issue 2 | Year 2023

Fracture Resistance of Functionally Graded Three-unit Fixed Partial Denture with Titanium Dioxide and Silica Nanoparticles: An In Vitro Study

Aditi A Kanitkar¹ https://orcid.org/0000-0002-6661-0931, Paresh V Gandhi² https://orcid.org/0000-0003-1566-2608, Ajay V Sabane³ https://orcid.org/0000-0003-1433-3802, Vijaysingh More⁴, Aneesh S Kanitkar⁵ https://orcid.org/0009-0004-3341-8603, Rajashree Jadhav⁶

^1-4,6Department of Prosthodontics and Crown and Bridge, Bharati Vidyapeeth Dental College and Hospital, Pune, Maharashtra, India

⁵Department of Prosthodontics and Crown and Bridge, Yogita Dental College, Khed, Maharashtra, India

Corresponding Author: Aditi A Kanitkar, Department of Prosthodontics and Crown and Bridge, Bharati Vidyapeeth Dental College and Hospital, Sangli, Maharashtra, India, Phone: +91 8007039904, e-mail:
dradisam@gmail.com; aditi.samant@bharatividyapeeth.edu.

Received on: 30 May 2023; Accepted on: 20 June 2023; Published on: 28 June 2023

ABSTRACT

Purpose: The objective of this study was to assess the fracture resistance of functionally graded monolithic zirconia with different nanoparticles in three-unit fixed dental prostheses (FDPs) after undergoing thermal and mechanical aging.

Materials and methods: A total of 32 three-unit monolithic zirconia prostheses were machined and randomly assigned to four groups (n = 8 each) as Group A—control group (without any nanoparticle), Group B—titania sol group, Group C—silica sol group, and Group D—silica and titania nano-sol group. Grading with nanoparticles was carried out on presintered monolithic zirconia and then was sintered. Fixed prostheses were exposed to thermocycling for 5–55°C for 10,000 cycles. The long-term clinical performance of monolithic zirconia was assessed by quasi-static fracture strength of 0–300 N for 1,00,000 cycles. After following loading conditions, prostheses were loaded until fracture. Fracture mode and evaluation of nanoparticles were seen under a field-emission scanning electron microscope (FE-SEM). Energy dispersive spectroscopy (EDS) was done to find an elemental composition of nanoparticles in zirconia. Weibull’s modulus implies the reliability of material for each of the four materials. Kruskal–Wallis analysis of variance (ANOVA) followed by a post hoc test done for the between-group differences in the maximum load-bearing capacity of the four groups.

Results: Significant variance (p = 0.001) in the fracture resistance of three-unit FDPs after mechanical and thermal cycling was observed. The fracture resistance of the control group A (703.60 N) was significantly lesser than that of the titania sol group B (1031.35 N) and silica and titania nano-sol group D (1094.74 N). Weibull moduli values of all four groups are as follows in descending order groups D > A > B > C.

Conclusion: Functional grading of monolithic zirconia with silica and titanium dioxide nanoparticles can increase the fracture resistance of three-unit FDPs after aging. The addition of titanium to zirconia has been shown to increase the Weibull modulus, which corresponds to a higher level of homogeneity of the material and more excellent reliability as a structural material.

How to cite this article: Kanitkar AA, Gandhi PV, Sabane AV, et al. Fracture Resistance of Functionally Graded Three-unit Fixed Partial Denture with Titanium Dioxide and Silica Nanoparticles: An In Vitro Study. Int J Prosthodont Restor Dent 2023;13(2):94-103.

Source of support: Nil

Conflict of interest: None

Keywords: Aging, Nanotechnology, Reliability, Silica nanoparticle, Three-unit monolithic zirconia prostheses, Titanium dioxide nanoparticle

INTRODUCTION

Due to their outstanding biocompatibility and pleasing esthetics, zirconia-based all-ceramic dental restorations have grown in dentistry. Fixed dental prostheses (FPDs) have a wider variety of uses thanks to the development of monolithic zirconia, which addresses the problem of porcelain chipping in bilayered zirconia restorations.¹^,² Bilayered zirconia restorations with veneer chipping can be minimized using monolithic zirconia restorations. Due to its remarkable biocompatibility and mechanical properties, three mol% of yttria-stabilized tetragonal polycrystalline zirconia is the selected dental material.³ Moreover, the construction process is more efficient as it eliminates the need for an additional veneering step.⁴^,⁵

The focused problem of monolithic zirconia is low-temperature degradation (LTD), also known as hydrothermal degradation. Water presence can lead to both the occurrence of gradual crack growth and the spontaneous formation of a monoclinic phase on the surface, known as LTD.⁶^-¹⁰ During LTD, moisture infiltrates oxygen vacancies and advances flaws in the zirconia material, ultimately impacting its mechanical properties by affecting the zirconia grains.¹¹^-¹⁴ When the internal stress exceeds the tensile strength of a material at a specific temperature, fatigue fractures related to aging can occur.¹⁵^,¹⁶ Thermal cycling and repetitive chewing forces facilitate the subcritical propagation of cracks in zirconia ceramics. This phenomenon can result in an excessive transformation from the tetragonal phase to the monoclinic phase.¹⁷ As a result, this transformation can contribute to clinical failures, even though zirconia-based restorations have high mechanical properties, especially in areas near the pontic of FDPs. Compressive layers can enhance strength and reduce the formation of microcracks. The literature documents various methods for creating compressive layers, and one commonly used approach is mechanical pretreatment methods such as airborne particle abrasion.¹⁸ However, it is important to note that this technique can sometimes lead to the formation of deeper layers than intended, which may impact the surface integrity of the zirconia material.¹⁹

Another approach to introducing a compressive layer is through the utilization of the “functional grading technique.” This method involves the creation of functionally graded materials (FGMs) that exhibit variations in material properties. Compared to ungraded structures, FGMs exhibit enhanced resistance to contact, improved flexural strength, and increased resistance to fatigue damage.²⁰^-²⁵

Recently, there has been growing interest in utilizing sol–gel-derived titania coatings to enhance the bioactivity of zirconia surfaces. In a study by Dos Santos et al.,²⁶ studies in the literature have demonstrated the successful incorporation of titania nanotubes onto the surface of zirconia²⁵^,²⁷ have not provided any updates regarding investigations on the fracture resistance of monolithic zirconia prostheses with graded nanoparticles. In their study, Mezarina-Kanashiro et al.,²⁸ investigated the shear bond strength between resin cement and zirconia using various methods of incorporating titanium dioxide nanotubes. Their findings shed light on the potential influence of these nanotubes on the bond strength between zirconia and resin cement.

There is a lack of understanding regarding the fatigue resistance of three-unit FDPs constructed from this novel functionally graded monolithic zirconia with varying nanoparticle gradients. This knowledge gap emphasizes the significance and urgency of further research in this particular area. By conducting more studies, we can gain valuable insights into the potential benefits and limitations of utilizing these nanoparticles in functionally graded monolithic zirconia and assess their ability to withstand fatigue damage in FDPs. The aim of this study was to examine the impact of functional grading with two distinct nanoparticles on the fracture resistance of three-unit monolithic zirconia prostheses following both thermal and mechanical aging. The null hypothesis posited that the functional grading of monolithic zirconia with different nanoparticles would not have any effect on the fracture resistance of three-unit FDPs after undergoing mechanical and thermal cycling.

MATERIALS AND METHODS

This in vitro study was conducted at the Department of Prosthodontics of the Dental College. The sample size for this study was determined using “GPower” software (version 3.1.9.2, Heinrich Heine University, Düsseldorf, Germany). The effect size for sample size estimation was calculated based on the reference article by Villefort et al.,²³ considering the mean and standard deviation values of the fatigue limit. The study aimed for a 99% confidence interval and a power of 90%. Consequently, a total of 32 three-unit prostheses made from monolithic zirconia were included in the sample size.

Experimental Procedures

For this study, a total of 32 mandibular three-unit FDPs were fabricated. The prostheses were randomly divided into four groups, with each group containing an equal number of specimens (n = 8). Except for the control group, all four groups were functionally graded using silica and titanium nanoparticles, as depicted in Figure 1.

Fig. 1: Study design

Preparation of nano-sol for functional grading—to prepare the titanium nano-sol, two solutions were combined. Solution A was prepared by mixing 0.5 wt% titanium dioxide nanoparticles, with a size range of 50–80 nm (ISO 9001: 2015 NRL Research Lab, Susnigerya, India), in 95% ethanol to create a homogeneous solution. Solution B was prepared by mixing ultrapure water, ethanol, and nitric acid. This solution was then added drop by drop to solution A while continuously stirring. The resulting solution was vigorously stirred and left to age at room temperature for 24 hours. The entire synthesis process was conducted at ambient atmospheric pressure at the National Chemical Laboratory, Pune, Maharashtra, India.²⁹

For the preparation of the silica nano-sol, a similar procedure as described by Campos et al.,²² was followed. The silicic sol used in the study was prepared by adding a 10% m/m aqueous solution of sodium metasilicate (Na₂SiO₃·5H₂O) to 0.5 wt% silica nanoparticles. NRL Research Lab, Susnigerya, India.

Manufacturing of tooth analogs—in the study, a patient’s cast was chosen to replicate oral anatomy. Tooth preparation was performed on the selected cast using a surveyor, and the prepared cast was then scanned using a four-axis optical scanner (Medit Identica Blue Scanner from Seoul, Korea). Exocad software (Exocad GMBH, Darmstadt, Germany) was used to design tooth analogs and milled with Ruthenium (Ruthinium Dental Products Pvt. Ltd., New Delhi, India) to simulate three-unit FPD with the second premolar and the second molar as the abutment teeth and mandibular left first molar as pontic (Fig. 2).

Figs 2A to I: Sequential flow of experimental procedures

Manufacturing of monolithic zirconia prostheses—the three-unit tooth analogs were scanned (Medit Identica Blue, Seoul, Korea). The design specifications included a material breadth of 1 mm and a 25 μm space for the luting agent. The connector dimensions were optimized to be 3 × 3 mm.²^,³⁰ In this study, a total of 32 identical three-unit prostheses were fabricated using four blanks of 12 mm thickness (Ivoclar Zenostar, manufactured by Ivoclar Vivadent AG in Liechtenstein, Germany). The prostheses were milled in a full contour design using a milling unit equipped with rotary tools (Zenotec Mini, Ivoclar Vivadent, Liechtenstein, Germany).

Grading and sintering procedure—the brush infiltration technique was utilized to incorporate silica and titania sol into a presintered monolithic zirconia prosthesis.³¹ For the specimens in group B, the brush (Camelin No. 9, Mumbai, Maharashtra, India)was used to apply the nano titania sol twice, directed from the mesial to the distal surface of the prosthesis. Similarly, for the specimens in group C, the brush was used to apply the silica sol twice, directed from the mesial to the distal surface of the prosthesis. In group D, a specific application sequence was followed for the functional grading process. Firstly, a nano-sol containing titanium dioxide was applied to the surface. Subsequently, a nano-sol containing silica was applied on top of the titanium dioxide layer. This sequential application of titanium dioxide and silica nano-sols allowed for the functional grading of the material in group D. After grading; all the prostheses were sintered following the manufacturer’s instructions at 1450°C. The margins of the prostheses were manually adjusted using a dental micromotor (Marathon 3, Saeyeng Microtek, Daegu, Korea).

Thermal aging—prostheses were stored in distilled water at 37°C for 24 hours. Individual groups were subjected to thermocycling (1 × 10 cycles between 5°C and 55°C, 30-second dwell time at each temperature) an approximate equivalent of one year of clinical service.³²^,³³

Luting procedure—both three-unit prostheses and tooth analogs were ultrasonically cleaned in pure water. After cleaning, the inner surface of the monolithic zirconia prosthesis was treated with 5% hydrofluoric acid (CeraEtch™, Prevest DenPro Ltd, Jammu, India) and then washed and cleaned with water. Further treated with silane coupling agent (Silane-X^™ Prevest DenPro Ltd, Jammu, India). Rendering to the manufacturer’s instructions, resin cement was used for cementation (Wonder Universal, Wizdent, Mumbai, Maharashtra, India). Cementation of monolithic prostheses was done under a static load of 20 N, and applied to the tooth analog assembly in a universal testing machine (Unitest 10, ACME, Pune, Maharashtra, India).

Mechanical aging—quasi-static fracture strength was accomplished using a universal testing machine (Unitest 10, ACME, Pune, Maharashtra, India) having a load cell capacity of 0.001N–1kN. All the tooth analogs with cemented prostheses were attached to the platform of the machine. The prostheses were loaded occlusal 0 to 300 N for 1,00,000 cycles. The prostheses were kept parallel to the floor. A 5 mm diameter ball indenter was loaded at a speed of 10 mm/minute crosshead. Even if the samples did not fracture completely but if a visible crack was seen, it was considered fractured.³⁴^,³⁵

Load-to-failure test—almost all the prostheses survived after fatigue testing from each group. The load-to-failure test was conducted on the groups using a universal testing machine. The tooth analogs assembly was loaded occlusal along the long axis. An audible crack was associated with the load-to-failure test. For statistical analysis, the maximum load to cause fracture was recorded in newtons.³⁶

Fractography—to determine fracture origin and discontinuity in the load-displacement curve, fractography was performed. Isopropyl alcohol was used to clean for 10 minutes fractured three-unit prostheses. The fractured monolithic zirconia prostheses were repaired using carbon tape, and a portion of their surface was coated with gold using a gold sputtering technique (Emitech Gold Sputter, East Sussex, England) to ensure surface conductivity. Quanta 200 (Hillsboro, Oregon, United States of America) Scanning electron (SEM) was used to capture images at various magnifications. FEI Nova NanoSEM 450 microscope (Hillsboro, Oregon, United States of America) was used to estimate the grain size of the nanoparticles.³⁷

Statistical Analysis

The collected data were entered into an Excel, and statistical analysis was conducted using Statistical Package for the Social Sciences (SPSS) software (IBM Corp. Released 2017. IBM SPSS Statistics for Windows, version 25.0. Armonk, New York: IBM Corp.). Kruskal–Wallis analysis of variance (ANOVA) test was performed to analyze the between-group differences in the maximum load-bearing capacity of the four groups. Post hoc tests were conducted to further examine specific group differences. Weibull’s modulus analysis was employed to determine the load-bearing capacity of the prostheses.

RESULTS

Fractography

All the prostheses could sustain the mechanical loading and fractured at the load-to-failure test showing a similar pattern of fracture. Figure 3A displays an SEM image of a sagittal section of a fractured prosthesis from group D, providing a detailed view of the fractured surface. Figure 3B illustrates the fractured prostheses, demonstrating a consistent pattern of fracture with an oblique orientation of the fracture path. The fracture line extends from the gingival embrasure (GE) toward the occlusal contact area. This indicates a common mode of failure across the prostheses.

Figs 3A and B: (A) Scanning electron micrographs of the fractured prosthesis of group D at the connector area; (B) orientation of fracture path on the fractured monolithic zirconia three-unit prosthesis

In Figure 4, SEM examinations of fractured surfaces are presented for representative specimens from each group. Figures 4B to D confirmed the presence of different nanoparticles in the respective group even after thermal and mechanical aging.

Figs 4A to D: Scanning electron micrographs of the fractured surfaces of the different groups. A, Group A; B, Group B, C, Group C; D, Group D

Figure 5 displays the average particle size of the titanium dioxide and silica nanoparticles infiltrated in the monolithic zirconia of group D at various magnifications (20000, 40000, and 80000×). The field-emission scanning electron microscopy (FE-SEM) images were captured using an FEI Nova NanoSEM 450 microscope under high vacuum conditions at 20.00 kV. FE-SEM analysis of the different groups gave the average particle size of 69.56 nm for both nanoparticles.

Figs 5A to D: FE-SEM images. (A) titanium dioxide and silica nanoparticles at 20000× magnification; (B) titanium dioxide and silica nanoparticles at 40000× magnification; (C) titanium dioxide and silica nanoparticles at 80,000× magnification; (D) average size of titanium dioxide and silica nanoparticles at 80000× magnification

Figure 6A and B shows SEM-EDS images as a supplement to SEM analysis for elemental composition analysis of the prosthesis. The elemental analysis depicts the different percentages of nanoparticles as 0.28, 1.23, and 1.90 weight percentages with groups B, C, and D, respectively. Group D showed the highest percentage of titanium dioxide nanoparticles, which corresponds with the highest fracture resistance values depicted in Tables 1 and 2.

Figs 6A and B: EDS analysis of fracture surfaces after fracture resistance testing of all groups. (A) a percentage of the nanoparticle; (B) SEM-EDX analysis

**Table 1:** Comparison of the mean fracture resistance (in Newton) of the four materials using Kruskal–Wallis ANOVA
Group	N	Minimum	Maximum	Mean	Weibull modulus	Standard deviation	F statistic	p-value
A	8	638.45	778.14	703.60	16.49	49.89	24.54	0.001*
B	8	868.24	1128.48	1031.35	13.90	84.73
C	8	638.45	820.46	743.84	12.49	66.49
D	8	968.54	1200.30	1094.74	18.39	69.37

*p < 0.05 is considered significant

**Table 2:** *Post hoc* comparison between each of the four groups
Groups compared	Z statistic	p-value
A * B	−3.361	0.001*
A * C	−1.314	0.189
A * D	−3.361	0.001*
B * C	−3.361	0.001*
B * D	−1.785	0.074
C * D	−3.361	0.001*

*p < 0.05 is considered significant

Statistical Analysis

To examine the differences in maximum fracture resistance values among the four groups, a Kruskal–Wallis ANOVA test was performed, which showed significant differences among the groups (p = 0.001) (Table 1). The fracture resistance data showed parametric distribution. Post hoc test showed significantly (p = 0.001) lesser mean fracture resistance of group A (703.60 N) than that of groups B (1031.35 N) and D (1094.74 N). The mean fracture resistance of group D was found to be significantly higher (p = 0.001) than that of group C (743.84 N) (Table 2). The sequence of mean fracture resistance, from highest to lowest, was observed as follows—groups D > B >C > A (p = 0.001).

Weibull Analysis

Figure 7 illustrates the Weibull modulus values of the four groups in descending order—group D > A > B > C. This ordering indicates the relative strength and reliability of the prostheses in each group, with group D having the highest Weibull modulus value and group C having the lowest. The Weibull modulus is an indicator of material reliability, and the higher the value, the greater the reliability of the material. As per Weibull analysis in the present study, group D with titanium and silica nanoparticles was found to be the most reliable monolithic zirconia material among the other groups.

Fig. 7: Weibull plots of Weibull modulus for all groups

DISCUSSION

The null hypothesis in the study was rejected, indicating that functional grading with nanoparticles had a significant (p < 001) impact on the variance of fracture resistance in three-unit FDPs after undergoing mechanical and thermal cycling. The results of the study further supported the promising potential of utilizing titanium dioxide and silica nanoparticle-graded monolithic zirconia as a material for prostheses.

Zirconia is a chemically inert and polycrystalline material that is resistant to easy etching. However, to improve the bonding between zirconia and teeth, various surface pretreatments have been developed.³⁸ Zirconia ceramics are renowned for their unique “transformation toughening mechanism,” which enhances their strength and ability to withstand fractures. This means they can become tougher during processes like grinding, machining, and aging.³⁹^,⁴⁰ However, this mechanism may not improve strength in all applications, particularly when there is excessive transformation caused by the presence of water. To address the challenges related to surface flaws and enhance the durability of zirconia material, the functional grading technique is utilized. This technique involves introducing compressive stresses into the zirconia material, making it more resistant to surface flaws.⁴¹ By employing functionally graded zirconia, the risk of fatigue failure can be reduced. Previous studies have provided support to this belief, with findings indicating the effectiveness of functionally graded zirconia in improving material resilience and reducing the likelihood of fatigue failure.⁴²^,⁴⁴

Nanotechnology is a field that concentrates on studying and manipulating objects composed of particles at the nanometer scale. Titanium oxide nanoparticles, in particular, are characterized by a high elastic modulus and serve as cost-effective, nontoxic semiconductors with a relatively high melting temperature of around 1870°C.⁴⁵^,⁴⁶ The incorporation of nanoparticles into presintered zirconia is considered feasible due to the porous nature of zirconia. However, some studies have indicated that the addition of titania to zirconia may result in a reduction in its overall mechanical properties.⁴⁷^,⁴⁸ It is important to consider the potential impact on the mechanical performance of zirconia when incorporating titania nanoparticles, as this may affect the material’s ultimate mechanical properties. Some studies⁴⁹^,⁵⁰ have focused on graded zirconia-titania-silica composites to improve the bond strength of prostheses or enhance the bioactivity of zirconia implants. The present study is among the few investigations to propose assessing the fracture resistance of three-unit graded monolithic zirconia prostheses incorporating titania and silica nanoparticles after an aging process.

Previous studies have demonstrated the effectiveness of titania coatings on zirconia substrates in promoting the attachment of soft tissues.⁴⁸^,⁵¹^,⁵² The literature presents several methodologies for infiltrating titania into monolithic zirconia. One common approach is the sol–gel route,⁴⁹ which involves a multistep process but typically results in a more homogeneous distribution of titania nanoparticles within the zirconia matrix.²⁹ Another method involves mixing commercial powders of zirconia and titania.⁴⁸ However, it is important to note that this method may potentially alter the mechanical properties of the existing material due to the introduction of external powders. Each approach has its advantages and considerations, and the choice of method should be based on the specific requirements and desired outcomes of the study or application. In the current study, a nano-sol was employed to successfully functionally grade the monolithic zirconia with nanoparticles during the presintered stage. This approach has proven to be effective and can be seamlessly integrated into routine computer-aided design and computer-aided machining processes in the laboratory. Compared to other methodologies mentioned in the literature, incorporating nanoparticles in the presintered stage offers a practical and efficient technique. It allows for the controlled and precise introduction of nanoparticles into the zirconia material, leading to improved properties and enhanced performance. The grading on the zirconia prosthesis is achieved by using a brush, which is considered a less technique-sensitive method compared to the dip coating processing method, which involves multiple steps. Applying nano titania and silica sol on the monolithic presintered fixed partial denture helps to maintain a homogeneous layer, which is different from the use of titania nanotubes on zirconia.²⁶

The examination of the prosthesis under an FE-SEM revealed the presence of titania nanoparticles, providing evidence of their sustained presence even after undergoing thermal and mechanical aging processes. This observation aligns with the findings of the present study, which suggest that the functional grading technique using titania nanoparticles effectively enhances the durability and integrity of the prosthesis. The EDS results provide valuable information about the elemental composition of the prosthesis and help confirm the presence of titania nanoparticles.

The incorporation of nano-sized ductile particles into oxide ceramics, such as the titania nanoparticles used in this study, has been shown to have beneficial effects. These particles are capable of deflecting the crack and facilitating crack bridging. By acting as barriers to crack propagation, the nano-sized ductile particles enhance the fracture resistance and mechanical reliability of the prosthesis.

The observed improvement in fracture resistance can be attributed to the formation of zirconium titanate in the present study, and these findings are consistent with previous research.⁵³^-⁵⁵ The sintering process of the milled zirconia prosthesis promotes particle attraction, reducing porosity and increasing material density. The smaller grain size of titania compared to zirconia is a contributing factor, similar to the findings of the present study.⁵⁶ Functional grading with nanoparticles helps reduce the presence of pores, which in turn decreases stress concentrators and potential fracture origins. This leads to an overall improvement in the fracture resistance of the graded zirconia material.⁵⁷ These results provide further evidence of the beneficial effects of functional grading with nanoparticles on enhancing the mechanical properties and durability of zirconia-based prostheses.

Given the widespread utilization of monolithic restorations in posterior regions, it is crucial to address the degradation of zirconia caused by cyclic loading during mastication and exposure to moisture in the oral environment. We have simulated a realistic clinical situation that includes both conditions mentioned above in our study on the three-unit prostheses. It is worth noting that previous studies²⁶^,²⁸^,⁴⁷ investigating the addition of titania on zirconia have primarily focused on disk-shaped specimens or flat geometries, which do not fully replicate the complex shape and contour of FDPs. The shape of a fixed prosthesis is diverse, consisting of a combination of convex and concave surfaces that replicate the alignment and contour of natural teeth. Therefore, the present study is unique in that it is the first known study to investigate the functional grading of a three-unit monolithic prosthesis using silica and titania nanoparticles. Considering the mechanical forces exerted during normal mastication, loads typically range from 50 to 200 N. However, parafunctional behaviors such as teeth grinding (bruxism) can exert significantly higher forces, ranging from 500 to 880 N, with extreme cases of bruxism reaching forces as high as 1000 N.⁵⁷

The highest fracture resistance values were determined for a group D with titania and silica nanoparticles (1094 N) and the minimum fracture resistance values for control group A (703.60 N). These values recorded from different groups are within the range reported by other studies.⁵⁰^,⁵⁷

Microcracks within the structure of a material and flaws in the material, which result in asymmetrical strength distribution, can be assessed by a statistical analysis known as the Weibull modulus. It also checks for the homogeneity of strength data values and assessment of lifetime analysis with aging. The asymmetrical strength data values are mostly inclined towards the high strength portion. Variations in fracture resistance values within the same group can be analyzed by Weibull analysis. Data collected from Weibull statistics is a clinically suitable value for the probability of failure; maximum allowable stresses can be calculated.⁵⁸^,⁵⁹ A low Weibull modulus indicates more defects and imperfections within the material and hence a decreased reliability.³⁷ However, a higher Weibull modulus corresponds to lesser flaws and therefore greater structural reliability.¹⁷ It directs the transition between a prosthesis’s success and failure against the applied force to become steeper. In our study, a higher Weibull modulus was found in the group D of monolithic zirconia graded with titania and silica nanoparticles. There is more predictability in mechanical behavior when the Weibull modulus is higher; this is as the earlier studies.³⁷^,⁶⁰ The observed increase in Weibull modulus can be correlated to graded monolithic zirconia for better susceptibility to aging and surface flaw distribution. The probable cause for the increase in the Weibull modulus might be the inclusion of titanium or silica nanoparticles into defects or pores on the surface of monolithic zirconia, which might help in stabilizing crack growth.⁶¹ This was also supported by findings by FE-SEM.

Furthermore, the mechanical properties of the graded zirconia were maintained, suggesting that the addition of nanoparticles did not compromise the overall strength and durability of the material.⁶² Additionally, the rate of phase transformation and the content of the monoclinic phase, which is associated with the susceptibility to fatigue failure, were reduced in the graded monolithic zirconia.⁶² These findings suggest that the nanoparticle-graded prosthesis may exhibit superior performance in terms of fatigue resistance compared to ungraded prostheses, particularly in restoring posterior regions.

These hypotheses are supported by previous studies that have demonstrated the benefits of functional grading and the incorporation of nanoparticles in enhancing the mechanical properties and fatigue resistance of dental prostheses.²¹^,⁶¹ Therefore, it can be speculated that the nanoparticle-graded prosthesis developed in this study may offer improved outcomes and longevity in restoring posterior regions of the oral cavity.

The present study had a few limitations. The present study performed thermal and mechanical loading one after another; clinically, thermal, and mechanical loading occur simultaneously. Also, in the present study only, axial loading was considered. Indeed, clinical trials are necessary to validate the findings of the present study and assess the performance of nanoparticle-graded monolithic zirconia in real-world conditions. Since monolithic prostheses are subjected to dynamic and oblique loading in clinical situations, it is important to evaluate their durability and fracture resistance in such scenarios.

Long-term clinical studies with larger sample sizes and extended follow-up periods would provide valuable insights into the long-term performance of nanoparticle-graded monolithic zirconia prostheses. These studies can help assess the clinical success, survival rates, and any potential complications associated with the graded material.

CONCLUSION

The functional grading of monolithic zirconia using silica and titania nanoparticles has the potential to enhance the fracture resistance of three-unit FDPs compared to using either silica or titanium dioxide nanoparticles alone. The fracture resistance of ungraded monolithic zirconia was affected by mechanical and thermal aging more than the fracture resistance of graded zirconia. The functional grading technique has the potential to improve the clinical longevity of monolithic restorations.

ORCID

Aditi A Kanitkar https://orcid.org/0000-0002-6661-0931

Paresh V Gandhi https://orcid.org/0000-0003-1566-2608

Ajay V Sabane https://orcid.org/0000-0003-1433-3802

Aneesh S Kanitkar https://orcid.org/0009-0004-3341-8603

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