Primary stability and longevity of zirconia and titanium implants submitted to thermomechanical cycling

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INTRODUCTION
Titanium is the material most
successfully used in the fabrication
of dental implants, and has become
the gold standard for rehabilitation
in implant dentistry, due to excellent
biocompatibility and mechanical
properties.1, 9, 28 However, treatment
with titanium must be seen in a crit-
ical manner, since there are popula-
tion groups that present innumerable
diseases related to the use of metals,
such as sensitivity and allergies.2, 22
Considering the increased expec-
tations for esthetic treatments and
the need for metal free implants for
titanium allergic patients, new mate-
rials have been proposed in Implant
Dentistry, such as Zirconia implants
(yttrium-stabilized tetragonal zir-
conia polycrystals (Y-TZP) which
have shown excellent results when
submitted to simulated masticatory
forces.14,15
In comparison with traditional
titanium implants, the zirconia im-
plants showed encouraging biologic
results, with a low level of plaque ac-
cumulation,23, 30 good bone/implant
contact values10, 24 and direct bone
apposition with good osteoblastic
Renata Espíndola Silveira1;
Renata Costa Morais1,
Carla Cecilia Alandia-Román1;
Fernanda de Carvalho Panzeri
Pires-de-Souza1.
Primary stability and
longevity of zirconia and
titanium implants submitted
to thermomechanical cycling
1 Department of Dental Materials and Pros-
thodontics, Ribeirão Preto School of Dentistry
– University of São Paulo, Ribeirão Preto, SP,
Brazil.
Corresponding author:
Fernanda de Carvalho Panzeri Pires-de-Souza
Department of Dental Materials and Prost-
hodontics. Ribeirão Preto School of Dentistry–
University of São Paulo, Brazil.
Av. do Café s/n, 14040-904 Ribeirão Preto (SP),
Brazil.
Tel.: +55 16 3602 3973
E-mail address: ferpanzeri@usp.br
Espíndola, E. et al, Primary stability and longevity of zirconia and titanium implants submitted to thermomechanical cycling.
(Rev. odontol. dominic. 12 (1): p12-17, Enero / Junio, 2016).
Abstract Purpose: The aim of this study was to compare the primary stability and longevity of zirconia and titanium implants when
submitted to thermomechanical cycling, analyzing the maximum pullout strength and removal torque.
Material and Methods: A total of 42 implants were fabricated of each type of the material studied (10mm length of active part x 8mm
length of coronal portion and 4mm in diameter with thread pitch of 0.5 mm). The implants were inserted perpendicularly in the center of
artificial bone cylinders and then divided into 8 groups (n=10) according to the material (zirconia or titanium), treatment (thermomecha-
nical cycling) and tests (Removal Torque or Pullout Test) they would be submitted. The interface between the implant/artificial bone set
was also analyzed (n=2) using high resolution photographs. The maximum pullout strength and removal torque values obtained were
submitted to statistical analysis using 2-way ANOVA and Bonferroni tests at a 95% level of significance.
Results: The highest torque removal and maximum pullout strength values were found for the the titanium implants, statistically di-
ferent (p<0.05) from the zirconia implants, irrespective of submission to thermomechanical cycling. When analyzing the efect of ther-
momechanical cycling on the same material, titanium implants showed a reduction in pullout strength (p<0.05), and no diference was
found between the zirconia groups (p>0.05) submitted to this treatment.
Conclusions: The primary stability of titanium implants is higher than that of zirconia implants, proved by the higher pullout strength
and maximum removal torque values presented. In addition, it was concluded that thermomechanical cycling is a significant factor only
for the longevity of titanium implant stability.
Keywords: dental implants, material resistance, torque.
12

cell proliferation on the zirconia surface.12, 15 In spite of
these advantages, when submitted to stresses and humid-
ity, conditions present in the oral cavity, destabilization
of the tetragonal phase of zirconia could occur, slowly
transforming it into a monoclinic phase,2,16, 30 a process
known as low temperature degradation. This process may
lead to the formation of micro and macrocracks, followed
by supercial roughness and reduction in strength, hard-
ness and density; so that, it is important to evaluate the
longevity of these implants in the oral cavity.17, 30
Although zirconia seems to be a suitable material for
the fabrication of oral implants at present, there are an in-
sufcient number of studies about its physical properties.
Considering that the evaluation of primary stability, a
property related to the absence of micromovements of the
implant during the surgical act and longevity, dened as
the implant capacity to remain in function over the years,
are essential parameters for predicting the success of reha-
bilitative treatment,18, 20, 29 the aim of this study was
to compare the primary stability and longevity of zirconia
(Y-TZP) and titanium (Grade 4) implants when submit-
ted to thermomechanical cycling (TC), analyzing the
maximum pullout strength and removal torque. The null
hypothesis tested was that there would be no difference
in the primary stability of these implants, irrespective of
submission to TC.
MATERIAL AND METHODS
A total of 42 implants were fabricated of each type of
material studied: zirconia and Grade 4 titanium (10mm
length of active part x 8mm length of coronal portion and
4mm in diameter) with thread pitch of 0.5 mm. The ther-
momechanical test was performed in accordance with ISO
14081.
To obtain the titanium implants, Grade 4 bars with
4.76 mm in diameter, (Realum Ind. Com., Metais Puros e
Ligas Ltda., São Paulo, SP, Brazil) were cut and machined
on a CNC lathe (Veker – Model FEL-1860 ENC – Bener,
Vinhedo, SP, Brazil), at a speed of 600 rpm and depth
of cut of 0.1mm (Figure 1). Zirconia implants were ob-
tained by machining pre-sinterized zirconia blocks (VIPI
BLOCK ZIRCONN, VIPI, SP, Brazil) on a convention-
al mechanical lathe (Romi ID-20, Indústrias Romi S.A.,
Santa Bárbara d’Oeste, SP, Brazil).
Two zirconia proto-
types were obtained: the
rst one (Z-01) repro-
duced the exact mac-
rostructure of titanium
implants. However, this
design was not capable
to resist the tensile stress
resulting from the pullout
test. This may have oc-
curred because zirconia
is a friable material 25 and
the stress concentrated
in the acute angle of the
coronal portion of the im-
plant generated fracture
of all the samples tested.
Therefore, a second zirco-
nia prototype, with a new
design for the coronal part,
was prepared, sintered in
a furnace (Fornos Jung,
Model 0916, Blumenau,
SC, Brazil) at a nal tem-
perature of 1530ºC and
tested, and the nal mac-
rostructure determined
may be observed in Figure
2. Although its coronal
portion is different from
that of titanium implants, the primary stability - main
purpose of the study - varies according to the quantity and
quality of the local bone, implant geometry (length, di-
ameter, thread distribution), surgical technique used and
diameter of the last bur used.19 These parameters were
maintained during the study for both materials.
The implants were inserted perpendicularly in the cen-
ter of articial bone (Nacional Ossos, Bauru, SP, Brazil)
cylinders (25 mm in diameter x 22mm high), with a thick-
ness of 2 mm simulating cortical bone (40 pcf = 0.64 g/
cm3) and 20 mm thick simulating spongy bone (20 pcf=
0.32 g/cm3). The bone bed was progressively prepared,
using cutters of different diameters and height markings
(NEODENT, Curitiba, PR, Brazil). Each implant was
screw-retained to its bone bed by means of a hex key,
Figure 1. Machined
titanium implant
Figure 2. Final macrostructure
of zirconia implant
13
V.12 ENERO-JUNIO 2016 Revista Odontológica Dominicana

specically for each material, and digital torque meter TQ
680 (Instrutherm, São Paulo, SP, Brazil). One hour after
insertion,5 the bone/implant sets were randomly divided
into 8 Groups (n=10), according to the implant material,
thermomechanical cycling (TC) and tests to which they
would be submitted (Removal Torque or Pullout Test), as
shown in Table 1.
Table 1. Studied Groups
Material Group Thermomechanical cycling (TC) /Tests
Titanium
G1 Removal Torque
G2 Pullout
G3 TC + Removal Torque
G4 TC + Pullout
Zirconia
G5 Removal Torque
G6 Pullout
G7 TC + Removal Torque
G8 TC + Pullout
The pullout test was performed using an axial traction
force (N) toward the long axis of the implant (1.0 mm/
min) through a device, which allowed the implants to re-
main parallel to the long axis of the mechanical test ma-
chine with a load cell of 100 Kgf (EMIC DL 2000, São
José dos Pinhais, PR, Brazil ). The removal torque (Ncm)
was analyzed by means of a digital torque meter TQ 680
(Instrutherm, São Paulo, SP, Brazil), with the implant/
articial bone set adapted to a parallelometer. The ther-
momechanical cycling (TC) was performed (Sistema de
Desgaste Termomecânico ER 37000 – ERIOS Ltda., São
Paulo, SP, Brazil) with an axial load of 133 N and tem-
peratures ranging between 5ºC, 37ºC and 55ºC (± 2ºC)
for 1,200,000 cycles, at a frequency of 2 Hz, simulating
chewing for 5 years.14 One hour after concluding the TC
test, samples were submitted to the pullout or removal
torque test. The interface between the implant/arti-
cial bone set was analyzed (n=2) using high resolution
photographs (Canon MP-E 65mm, Canon Inc., Japan),
immediately after implant placement and after TC. For
this purpose, the bones of each implant/articial bone
set were sectioned longitudinally with a cutting machine
(SYJ - 150 Digital Diamond Low Speed Saw 4, MTI
Crystal, Richmond, CA, USA), and then, images were
obtained.
The maximum pullout strength and removal torque
values obtained were submitted to statistical analysis
using 2-way ANOVA and Bonferroni tests at a 95% level
of signicance.
RESULTS
Comparison of means of torque removal and maxi-
mum pullout values can be observed in Tables 2 and 3,
respectively.
The highest torque removal value was found for the
titanium implants, statistically different (p<0.05) from
the zirconia implants, irrespective of submission to TC.
This factor was not signicant (p>0.05) for any type of
implant tested (Table 2). Table 3 shows that the mean max-
imum pullout strength values were higher for the titanium
implants, different (p<0.05) from the zirconia implants,
irrespective of the treatments used. When titanium im-
plants were submitted to TC, there was a reduction in
pullout strength (p<0.05), signicant in comparison with
the group without TC. There was no difference (p>0.05)
for the zirconia implants.
Table 2. Comparison of means (standard deviation)
of torque removal (Ncm) for titanium and zirconia
implants (2-way ANOVA, Bonferroni, p<0.05).
Material No treatment Thermomechanical Cycling (TC)
Titanium 24.2 (±6.21) aA 22.4 (±9.43) aA
Zirconia 10.07 (±6.96) bA 6.76 (±5.76) bA
Diferent letters, lower case in the column and upper case
in the line, indicate statistically significant diference.
Table 3: Comparison of means (standard deviation) of
maximum pullout strength (N) for titanium and zirconia
implants (2-way ANOVA, Bonferroni, p<0.05).
Material No treatment Thermomechanical Cycling (TC)
Titanium 575.7 (±55.34) aA 454.8 (±83.60) aB
Zirconia 252 (±92.52) bA 215.2 (±140) bA
Diferent letters, lower case in the column and upper case in the line,
indicate statistically significant diference.
14
ESPÍNDOLA, E. et al

Qualitative analysis of the implant/articial bone in-
terface, immediately after implant insertion into bone,
may be observed in Figures 3A and B. The images show
an integrated implant/articial bone interface for both
types of material. After TC, (Figures 3B and 4B), it was ver-
ied that the interface remained faultless, demonstrating
that TC did not produce loss of implant insertion in ar-
ticial bone.
The analysis of the effect of the pullout test of implants
on the articial bone and their interface sectioned lon-
gitudinally (Figure 5) showed the presence of articial
bone stuck between the threads of both types of implants,
in larger quantity for the titanium implant (Figure 5- A1).
When the remaining bone was analyzed, it was veried
that the the shape of the spirals was lost in bone that re-
ceived the zirconia implant (Figure 5 - B2), whereas a larger
number of spiral remained in the bone that received the
titanium implant (Figure 5 - A2).
DISCUSSION
In this study, the primary stability and longevity of
experimental implants of zirconia stabilized with yttrium
(ZrO2 –3%Y2O3) and Grade 4 titanium of the same model
were compared, by means of evaluating the maximum
pullout strength and removal torque. The results indicat-
ed that there were differences between the titanium and
zirconia implants, for both removal torque and maximum
pullout strength. Thermomechanical cycling was signi-
cant only with regard to maximum pullout force in titani-
um implants, and therefore, it was not possible to accept
the hypothesis of the study.
The implant resistance to pullout suggests the correla-
tion between shape, physical and chemical properties of
the screw surface13 and implant stability in the axial di-
rection.21 The removal torque test measures the bone/im-
plant strength when the implant is removed from bone4,
26 thereby quantitatively obtaining the tensile strength
necessary to remove it from the bone bed.18
Since the parameters of bone quantity and quality, im-
plant geometry (length, diameter, and thread distribution)
and diameter of the last bur used were maintained for
the two materials tested, the primary stability was com-
pared considering only the material used for fabricating
Figure 3. Qualitative analysis of titanium implant/artificial
bone interface. A) Immediately ater implant insertion
into bone. B) Ater thermomechanical cycling.
Figure 4. Qualitative analysis of zirconia implant/artificial
bone interface. A) Immediately ater implant insertion
into bone. B) Ater thermomechanical cycling.
Figure 5. Analysis of the efect of the pullout test of implants
on the artificial bone. A1-A2) Tinium. B1-B2) Zirconia.
15
V.12 ENERO-JUNIO 2016 Revista Odontológica Dominicana

the implants and its mechanical properties. Thus, any
differences in the results would be related to the intrinsic
properties of the materials.
Titanium and zirconia present physical, chemical
and mechanical differences that characterize them. It is
known that zirconia partially stabilized with 3% mol of
Y2O3 presents high compressive strength (2000 MPa),
density of approximately 6.00 g/cm3, porosity lower than
0.1%, mean grain size of 0.2-0.4 µm, exural strength
of 900-1200 MPa, Vickers hardness 1200 MPa, Weibull
modulus 10-12, fracture strength from 7 to 10 MPa, and
modulus of elasticity of 210 GPa, double that of titani-
um.3, 6, 8, 27, 30 Whereas, Grade 4 titanium presents a mean
density of 4.51 g/cm3 , modulus of elasticity of 103-107
GPa, Young modulus of approximately 120 GPa, ow
limit of 500MPa, tensile strength limit of 550Mpa and
elongation of 15%.8
The pullout maximum force and the removal torque of
zirconia implants were not inuenced by TC (p>0.05).
However, TC had an inuence on the results of maximum
pullout force for the titanium implants (p<0.05), demon-
strating that the alveolar bone adjacent to the implant
responded differently to the load applied. Furthermore,
due to the thermal conductivity shown by this material,
the temperatures used during thermal cycling could have
damaged the properties of the surrounding articial bone,7
which would also explain the statistically signicant effect
of the TC on the results of the pullout test of titanium
implants.
The lower results obtained for zirconia, differing
(p<0.05) from those of titanium implants, can be ex-
plained by the process of machining zirconia, which
could have introduced supercial cracks in the material,
causing a reduction in its strength.11 Although TC had no
inuence on zirconia implants, the mean maximum pull-
out and maximum removal torque values, when compared
with the gold standard titanium implants, were still far
below the values expected in order for them to be a feasi-
ble alternative for dental implants.
The images obtained of the implant/articial bone set
showed greater articial bone destruction after pullout of
the zirconia implant. These results may be explained be-
cause zirconia presents double the modulus of elasticity of
titanium, which guarantees the greater rigidity of this ma-
terial.8 Therefore, the stress on the material will be greater
in the surrounding bone.8
Titanium, however, is more ductile and has a lower
modulus of elasticity. Therefore, when tensile load is ap-
plied to pullout the implant, titanium undergoes plastic
deformation (elongation) with a reduction in its diameter,
which allows some of the spirals of the adjacent bone still
to remain without destruction.
The results obtained in this study showed that the
mean pullout strength of zirconia implants represents less
than half the strength of that of titanium implants. In
spite of this, zirconia with a lower pullout strength caused
greater destruction of the articial bone, due to its inher-
ent mechanical properties.
CONCLUSION
It was concluded that the primary stability of titanium
implants was higher than that of zirconia implants, proved
by the higher pullout strength and maximum removal
torque values presented. In addition, it was concluded
that thermomechanical cycling is a signicant factor only
for the longevity of titanium implant stability.
Espíndola, E. et al, Estabilidad primaria y longevidad de implantes de titanio y zirconio sometidos a fatiga termomecánica.
(Rev. odontol. dominic. 12 (1): p12-17 Enero / Junio, 2016).
Resumen Objetivo: El objetivo de este estudio fue comparar la estabilidad primaria y la longevidad de implantes de zirconio y de
titanio sometidos a fatiga termomecánica, analizando la máxima resistencia a la tracción y torque de remoción.
Material y métodos: Un total de 42 implantes (10mm de parte activa x 8 mm de porción coronal y 4 mm de diámetro con paso
de rosca de 0,5 mm) fueron fabricados con cada material estudiado. Los implantes fueron insertados perpendicularmente en el
centro de cilindros óseos artificiales y luego fueron divididos en 8 grupos (n = 10) de acuerdo con el material utilizado (zirconio o ti-
tanio), el tratamiento (fatiga termomecánica) y ensayos (resistencia a la tracción o torque de remoción) a los que serían sometidos.
También se analizó la interfase entre el implante/hueso artificial (n = 2), utilizando fotografías de alta resolución. Los datos fueron
16
ESPÍNDOLA, E. et al

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analizados estadísticamente mediante el Análisis de Varianza (ANOVA) de dos vías y la prueba de Bonferroni considerando una
p<0,05 como estadísticamente significativa.
Resultados: Los valores más elevados de resistencia a la tracción y torque de remoción se encontraron para los implantes de tita-
nio, estadísticamente diferentes (p <0,05) de los implantes de zirconio, independiente de la fatiga termomecánica. Al analizar el
efecto de la fatiga termomecánica en el mismo material, los implantes de titanio mostraron una reducción de la resistencia a la
tracción (p <0,05), y no se encontraron diferencias entre los grupos de zirconio (p> 0,05) sometidos a este tratamiento.
Conclusiones: La estabilidad primaria de los implantes de titanio fue mayor que la de los implantes de zirconio, demostrado por
la mayor resistencia a la tracción y torque de remoción presentados por este material. Además, se concluyó que la fatiga termome-
cánica es un factor significativo sólo para la longevidad y estabilidad primaria en implantes de titanio.
Palabras clave: implantes dentales, resistencia de materiales, torque.
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V.12 ENERO-JUNIO 2016 Revista Odontológica Dominicana