ORIGINAL RESEARCH
Fracture Resistance of Functionally Graded Three-unit
Fixed Partial Denture with Titanium Dioxide and Silica
Nanoparticles: An In Vitro Study
Aditi A Kanitkar1 , Paresh V Gandhi2 , Ajay V Sabane3 , Vijaysingh More4 , Aneesh S Kanitkar5,
Rajashree Jadhav6
Received on: 30 May 2023; Accepted on: 20 June 2023; Published on: 28 June 2023
A B S T R AC T
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 (n = 32) 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 quasistatic 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.
Keywords: Aging, Long-term survival, Nanotechnology, Reliability, Silica nanoparticle, Three-unit monolithic zirconia prostheses, Titanium
dioxide nanoparticle.
International Journal of Prosthodontics and Restorative Dentistry (2023): 10.5005/jp-journals-10019-1413
INTRODUCTION
1–4,6
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 lowtemperature 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
Department of Prosthodontics and Crown and Bridge, Bharati
Vidyapeeth Dental College and Hospital, Pune, Maharashtra, India
5
Department 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.
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
© The Author(s). 2023 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.
org/licenses/by-nc/4.0/), which permits unrestricted use, distribution, and non-commercial reproduction in any medium, provided you give appropriate credit to
the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain
Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Functionally Graded Three-unit Fixed Partial Denture
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–gelderived 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.
M AT E R I A L S
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 “G*Power” 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.
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
Fig. 1: Study design
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Functionally Graded Three-unit Fixed Partial Denture
and the second molar as the abutment teeth and mandibular left
first molar as pontic (Fig. 2).
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 fullcontour design using a milling unit equipped with rotary tools. The
milling unit used in the study was the 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 II, the brush was used to apply the nano titania sol twice,
with a thick brush, directed from the mesial to the distal surface
of the prosthesis. The brush used for this purpose was Camelin
No. 9 (Mumbai, Maharashtra, India). Similarly, for the specimens in
group III, the brush was used to apply the silica sol twice, directed
from the mesial to the distal surface of the prosthesis. In group IV,
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 IV. 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 × 105 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
Figs 2A to I: Sequential flow of experimental procedures
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Functionally Graded Three-unit Fixed Partial Denture
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 threeunit 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
microscope (Hillsboro, Oregon, United States of America) Scanning
electron microscopy (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.
Figs 3A and B: (A) Scanning electron micrographs of the fractured
prosthesis of group IV at the connector area; (B) orientation of fracture
path on the fractured monolithic zirconia three-unit prosthesis
R E S U LTS
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.
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.
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.
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
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
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.
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Functionally Graded Three-unit Fixed Partial Denture
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
III 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.
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
Table 2: Post hoc comparison between each of the four groups
Groups compared
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
Z statistic
p -value
A*B
A*C
A*D
B*C
B*D
−3.361
−1.314
−3.361
−3.361
−1.785
0.001*
0.189
0.001*
0.001*
0.074
C*D
−3.361
0.001*
*p < 0.05 is considered significant
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
A
B
C
8
8
8
638.45
868.24
638.45
778.14
1128.48
820.46
703.60
1031.35
743.84
16.49
13.90
12.49
49.89
84.73
66.49
D
8
968.54
1200.30
1094.74
18.39
69.37
*p < 0.05 is considered significant
98
International Journal of Prosthodontics and Restorative Dentistry, Volume 13 Issue 2 (April–June 2023)
F statistic
p-value
24.54
0.001*
Functionally Graded Three-unit Fixed Partial Denture
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
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
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Functionally Graded Three-unit Fixed Partial Denture
Fig. 7: Weibull plots of Weibull modulus for all groups
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
100
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
International Journal of Prosthodontics and Restorative Dentistry, Volume 13 Issue 2 (April–June 2023)
Functionally Graded Three-unit Fixed Partial Denture
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 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 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 nanoparticlegraded 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.
C O N C LU S I O N
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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