Materials Science and Engineering C

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Evaluating mechanical properties and degradation of YTZP dental implants

Pablo Sevilla

a,

, Clara Sandino

a

, Milena Arciniegas

a

, Jordi Martínez-Gomis

b

,

Maria Peraire

b

, Francisco Javier Gil

a a

Biomaterials and Biomechanics Division, Department of Materials Science and Metallurgy, Technical University of Catalonia, Spain b

Department of Prosthodontics, Faculty of Odontology, University of Barcelona, Spain

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 28 April 2009

Received in revised form 10 July 2009 Accepted 4 August 2009

Available online 13 August 2009

Keywords:

Y-TZP Dental implants Fatigue and fracture Degradation Nanoindentation

Lately new biomedical grade yttria stabilized zirconia (YTZP) dental implants have appeared in the implantology market. This material has better aesthetical properties than conventional titanium used for implants but long term behaviour of these new implants is not yet well known. The aim of this paper is to quantify the mechanical response of YTZP dental implants previously degraded under different time conditions and compare the toughness and fatigue strength with titanium implants. Mechanical response has been studied by means of mechanical testing following the ISO 14801 for Standards for dental implants and byfinite element analysis. Accelerated hydrothermal degradation has been achieved by means of water vapour and studied by X-ray diffraction and nanoindentation tests. The results show that the degradation suffered by YTZP dental implants will not have a significant effect on the mechanical behaviour. Otherwise the fracture toughness of YTZP ceramics is still insufficient in certain implantation conditions.

Published by Elsevier B.V.

1. Introduction

The zirconia ceramics are used in different applications due to their remarkable mechanical and physical properties, such as their very low thermal expansion coefficient and stability under very high temperature [1]. Important advances have been achieved in these materials, such as the increment of fracture toughness produced by the stabilization of the tetragonal phase with different oxides (i.e.yttria, ceria or magnesia) [2]. This material, called Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP), exhibits a very highflexural strength (900 to 1200 MPa), a notable fracture toughness (KIC 7 to 10 MPa·m1/2) and a Young's modulus of 210 GPa[3]which has permitted to expand its applications to biomedicalfield, with special interest in load-transfer applications such as dental implants. The assumed advantages of the Y-TZP in comparison with the widely employed c.p. titanium or Ti6Al4V for dental implants are a better aesthetical outcome due to its shiny white when the implant shows through a thin mucosa or when it becomes visible following a mucosa retraction, and a great biocompatibility due to the absence of metal ions in the surrounding tissue [4]. The experience with the utilization of biomedical grade zirconia femoral heads for orthopaedic hip implants showed that a low temperature hydrothermal degradation, also called ageing, takes place when the material is exposed to physiological conditions. The current knowledge on ageing process and on its effect on the long term performance shows that there is a strong variability of zirconia to in vivo degradation as a consequence of different process related to microstructure[5]. The use

of zirconia for dental implants is in its early phase and the issue of ageing needs further research. Therefore, the purpose of this study was to quantify the mechanical response of YTZP implants previously degraded under different time conditions and to compare the toughness and fatigue strength of YTZP implants with those of titanium implants. 2. Materials and methods

21 commercial biomedical grade yttria stabilized zirconia dental implants (Zlock3411 YTZP A-BIO HIP®, Z-System, Konstanz, Ger-many) were acquired and 43 c.p. grade 3 implants (SK2 Ti Klockner Implant Systems, Barcelona, Spain) were provided by the manufac-turer (Fig. 1). The Zlock3–411 implants have a threaded length of 11 mm with a diameter of 4 mm and a diameter of the crest module of 6 mm. The SK2 implants have a threaded length of 10 mm with an average diameter of 3.8 mm and a diameter of the crest module of 4.2 mm. These two types of implants were chosen because they are the most used of both companies and have similar applications in mouth. The chemical composition of the two kinds of implants is shown inTables 1 and 2, according to manufacturer's data.

The zirconia implants were divided in two groups. Thefirst group of 3 implants was submitted to nanoindentation test after a degradation process. The second group of 18 implants was submitted to mechanical tests.

2.1. Low-temperature autoclave ageing

Each one of the 3 YTZP Zlock3®implants of therst group were cut with diamond disk to obtain 21 slices 1 mm thick and were ⁎ Corresponding author. Tel.: +34 934010711; fax: +34 934016706.

E-mail address:pablo.sevilla@upc.edu(P. Sevilla). 0928-4931/$–see front matter. Published by Elsevier B.V. doi:10.1016/j.msec.2009.08.002

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mechanically polished with diamond paste from a solution with a particle size of 30 up to 1 µm andfinished with colloidal silica to give an average surface roughness of <5 nm. Finally, all samples were washed with acetone and dried at room temperature.

18 slices were submitted to a degradation process conducted by means of an autoclave at 134 °C under 2 Bar of pressure. The degradation times were 1, 2, 3, 4, 5 and 10 h and were applied to 3 slices each one[6]. 3 slices were left not degraded.

2.2. X-ray diffraction

The martensitic transformation from tetragonal to monoclinic phase induced by the degradation was analysed by X-ray diffraction using a Bruker diffractometer with Cu Kα of 1.54. The voltage, intensity and step-size applied were 40 kV, 20 mA and 0.02°, respectively. The counting time per each step was 16 s. In order to quantify the transformed phase fraction the following equations were employed[7]:

Xm=

Im 111ð Þ+Im 111ð Þ

Im 111ð Þ+Im 111ð Þ+It 101ð Þ ð

1Þ

whereItandImrepresent the integrated intensity (area under peaks) of the tetragonal (101) and monoclinic (111) and (−111) peaks. The volume of monoclinic phase is then given by[6]:

Vm=

1:311· Xm

1 + 0:311· Xm ð

2.3. Nanoindentation test

Samples of each degradation time, as well as non degraded samples, were used for instrumented nanoindentation tests on an MTS XP System Nano Indenter. On each sample nine monotonic tests were performed, in which the penetration depth was linearly increased up to the maximum depth of 1600 nm. The tests were conducted with a Berkovich tip with a measured radius of 750 nm. The elastic modulus and contact pressure values were evaluated with Continuous Stiffness Measurement (CSM) module [8]. The CSM module overlaps high frequency oscillations to the P–h curves, evaluating the elastic response from the unloading portion of such

oscillations. All tests were conducted at 22 °C and the indenter was held in contact with the surface until the thermal drift was lower than 0.05 nm s−1.

It is possible to know the thickness of the degraded layer by means of a model of thin layer properties that has in account the contribution of the substrate to the mechanical response of the surface layer submitted to nanoindentation and proposes the next equation[9]:

1 E⁎c = 2a 1 +ð 2t=πaÞ t πa2E⁎ f + 1 2aE⁎s ! ð3Þ where:Ec⁎is the Apparent Young modulus obtained in the test. Ef⁎ is the Young modulus of the degraded layer

Es⁎ is the substrate Young modulus. (210 GPa)

a is the contact radius corresponding to cylindrical indenter with the same contact area and it's defined as:Ac=πa2 t is the thickness of the degraded layer

2.4. Mechanical tests

The mechanical tests were carried out following the ISO 14801 Standard[10]. Five Zlock3–411 andfive SK2 implants were embedded in an acrylic resin Tecnovit 4071, Sulzer®(Switzerland) 3 mm over the nominal depth, simulating 3 mm of bone resorption, and placed in an angle of 30° with respect to the vertical axis of the implant (Fig. 2). Since the Zlock3 implants are implant and abutment in one piece, a special abutment were connected to SK2 implants to equal the test conditions. The testing machine was a Bionix 858, MTS (Minnesota, USA) controlled by MTS Testworks 4 software. The test was donefirstly statically to determine the 30°flexure resistance of the implants. Later Fig. 1.Dental implants used in the study. Left: Zlock3–411 Z-Systems®

(Germany). Right: SK2 Klockner Implant System®

(Spain).

Table 1

Chemical composition of the commercial YTZP A-BIO HIP®

acquired for the studies. Oxides Percentage (in wt.%) ZrO2(+HfO2) >95.5

Y2O3 4.00

Al2O3 0.25

Table 2

Chemical composition of the c.p. grade 3 titanium.

O C N H Fe Ti

Ti c.p. grade 3 0.35% 0.08% 0.05% 0.01% 0.30% Balance Percentages in weight.

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on, fatigue tests were carried out setting a maximum load of 40, 50, 60 and 80% of the failure load detected on the static tests. The minimum load was always 10% of the maximum load, the waveshape was sinusoidal and the frequency was established in 15 Hz. In each case 3 Zlock3–411 and 3 SK2 Implants were tested. A total of 18 SK2 implants and 18 Zlock3411 implants were tested following the ISO 14801 Standard.

Testing condition specified by ISO 14801 Standard are the most unfavourable conditions that a dental implant is supposed to stand. It was found interesting to know the mechanical response of both types of implants under not so demanding conditions. With this purpose, the 25 SK2 remaining implants were tested changing the slope of the long implant axis with respect to the vertical machine axis from 30° to 15 and 0°. Also, at these 3 slopes implants were tested simulating bone resorption (3 mm over the nominal depth) and without simu-lating bone resorption. For each case 5 SK2 implants were tested.

Zlock3–411 implants were not tested at conditions different from the specified by ISO 14801 Standard. Instead of it, afinite element model of Zlock3411 implant was developed as it is explained in the next paragraph.

2.5. Finite element analysis

Afinite element model of the Zlock3–411 implant was created. Initially a 3d model of the implant was created by means of Solid Works 2007®software (USA), next the 3d model was meshed using Patran®, MSC Software (USA) andnally the contour conditions and the calculus were carried out by means of Marc® software, MSC Software (USA). The conditions studied byfinite element study were: 0°, 15° and 30° of slope with respect to the axis of the force and 3 mm over the nominal depth, simulating 3 mm of bone resorption as ISO 14801 standard recommends and the implant immersed until the nominal depth, simulating no bone resorption at all (Fig. 3). To validate the model the results of 30° slope with 3 mm of bone resorption were compared with the results of mechanical testing following the ISO 14801 Standard.

2.6. Degradation studies

To determine how the hydrothermal degradation affects to the fracture resistance of the Zlock3 implants submitted toflexure forces the stress intensity factor was calculated. A mathematical model was established for a theoretical bar submitted to a 3 point bending test to the stress calculated byfinite element on the Zlock 3 implants with a

surface crack of the length of the degraded layer thickness obtained in the nanoindentation test by the following equation[11]:

KI=σ ffiffiffiffiffiffiffiffiffi π· a p ·F að =bÞ ð4Þ F að =bÞ= 1ffiffiffi π p ·1:99−a=bð1−a=bÞ2:15−3:93a=b+ 2:7ða=bÞ 2 h i 1 + 2a=b ð Þð1−a=bÞ3=2 ð5Þ whereKIis the stress intensity factor;a, the crack length andbis the bar thickness, which was established in 4 mm.

3. Results

3.1. X-ray diffraction

X-ray diffraction proles of the degraded slices of zirconia implants are shown inFig. 4. In thisfigure a fully tetragonal pattern is observed in the non degraded sample. In contrast, from the degraded samples a peak at 28.2° is detected, which is small under short time of degradation, but is increased as a function of the degradation time. The presence of this peak and the intensity changes are associated with the martensitic transformation from a tetragonal to monoclinic structure.

The transformed phase fraction and volume values calculated for each sample are shown in Table 3. A notable increment of the transformed phase is produced as a function of the degradation time.

Fig. 3.3-D FEA models of implant and acrylic resin in two different loading cases: Left: 3 mm elevate apex (simulating 3 mm bone resorption like ISO 14801 testing). Right: Immersed apex (without bone resorption).

Fig. 4.X-ray diffraction spectra of the different times hydrothermally aged samples. The monoclinic peak at 2θ= 28.2° is marked with the letter m. The peaks belonging the tetragonal phase are marked with the letter t.

Table 3

Monoclinic fraction of the hydrothermally aged samples calculated by Garvie and Nicholson method.

Degradation time Monoclinic fraction (%) Monoclinic fraction in volume (%)

0 h 0 0

1 h 0.5 0.7

3 h 6.4 8.2

5 h 13.7 17.2

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Itfits to say that the penetration of the X-ray beam is around 1 µm. So this technique is useful in the range of the beam penetration to detect the quantity of phase transformation.

3.2. Nanoindentation tests

Figs. 5 and 6show constant hardness and elastic modulus values as functions of the penetration depth, respectively, obtained by nanoindentation on the non degraded sample, with an average value of 16.9 ± 0.9 GPa and 245.7 ± 11.6 GPa.

Conversely, a significant reduction of these properties is presented by the all degraded samples, measured infirst 1600 nm, with an increment of the values, up to reach similar values to those found from the non degraded sample, as the penetration into surface is raised (Figs. 5 and 6).

The Young modulus of the degraded layer was determined using the results of the most degraded sample. Showing the graphic of the Film Young modulus (Ef⁎) vs indentation deepness for different layer thicknesses (t) and choosing the value ofEf⁎andtthat produces a constant value across the indentation deepness. In this case a value of 165 GPa was chosen asFig. 7shows.

Finally it is possible to determine the degraded layer thickness comparing the curves ofEc⁎vs tip displacement calculated by the Bec

equation for different values of degraded layer thicknesstwith the curves ofEc⁎ vs tip displacement obtained on the nanoindentation test.Fig. 8shows an example for the sample degraded during 10 h obtaining a degraded layer thickness of 0.7 µm. The values of the degraded layer thickness for each time of autoclave aging are shown inTable 4.

3.3. Mechanical testing

Fig. 9shows the mechanical response of the Zlock3–411 implant and the SK2 implant to a monotonic 30°flexure test. It can be seen that the slope (stiffness) of the curve on the YTZP implants is much higher than the slope on the titanium implants (108.8 ± 8.78 N/mm for Zlock3 implant and 40.6 ± 2.26 N/mm for the SK2 implants). Also the figure shows that the behaviour of the Zlock3 implants is completely fragile and it don't tolerate any plastic strain. Otherwise the SK2 implants can be deformed permanently during an overload without breaking. Finally it fits to say the failure force for both implants is very similar although the elastic limit of the SK2 implant is slightly higher than the maximum resistance of Zlock3–411 implants Fig. 5.Hardness as a function of tip displacement in the nanoindentation test for

different times of hydrothermally aging samples.

Fig. 6.Young Modulus as a function of tip displacement in the nanoindentation test for different times of hydrothermally aging samples.

Fig. 7.Degraded layer Young modulus as a function of tip displacement calculated with Bec's equation for different layer thicknesses.

Fig. 8.Apparent Young modulus as a function of tip displacement calculated with Bec's equation for different layer thicknesses and compared with nanoindentation results.

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(Maximum resistance of Zlock3: 736.3 ± 65.3 N, Elastic limit of SK2: 812.2 ± 25.0 N).

For the fatigue testing the results are shown inFig. 10. As it can be seen in the graphic the fatigue limit for both implants is very similar (about 300 N) and higher than the Standard recommendation (225 N). Otherwise the Zlock3 implant presents a ceramic behaviour where no very high overloads can be tolerated by the material while SK2 implants can tolerate many cycles of higher forces than the fatigue limit of the implant.

3.4. Comparison of mechanical response

Table 5 shows the toughness of the implants in different conditions as it was specified in the methodology. The values obtained for SK2 implants are from mechanical testing while the values for Zlock3–411 implants are obtained by means offinite element analysis. It can be seen that in the worst possible conditions (angle of 30° and 3 mm of elevated crest module) the behaviour of Zlock3–411 implants is worse than the behaviour of SK2 implants. Nevertheless in not so demanding conditions (angles of 15° and 0°) the Zlock3 implants have better response than SK2 implants and also when there is no bone resorption at all theflexure resistance of the Zlock3 implants even doubles the maximum resistance of the SK2 implants.

3.5. Degradation studies

The calculus of the stress intensity factor of the Zlock3 implants is shown inTable 6. It can be seen that the stress intensity factor produced by the degraded layer is very low even on the highest times of hydrothermal ageing (10 h where the KIis 1.49 MPa·m1/2). To arrive at a stress intensity factor equal to the fracture toughness of the material (6 MPa·m1/2) it is necessary to make the hydrothermal

ageing treatment on the material during about 170 h which implies a degraded layer thickness of 12 µm.

4. Discussion

A fully tetragonal pattern on non degraded samples demonstrates that the polishing method was correct and it didn't produce stresses on the samples surface[12]. The martensitic transformation from a tetragonal to a monoclinic structure observed with X-ray diffraction during the degradation process is attributed to the hydrothermal degradation that takes place when the material is in contact with a liquid medium, as it was reported[6,13,14]. The notable increment of the phase transformed, as a function of the degradation times is in agreement with the results reported by other authors [6,14]. The significant reduction of hardness and elastic modulus presented by the all degraded samples, measured in first 1600 nm, with an increment of the values, up to reach similar values to those found from the non degraded sample, as the penetration into surface is raised is a phenomenon well reported in literature[15]and it is attributed to a degradation process that is started on the surface due to the autoclave conditions, and continue through the material in a very homogeneous mode.

Although the variation in hardness could be attributed to the appearance of the monoclinic phase, the marked variation in elastic modulus suggest that the mechanism responsible of this variation is the generation of microcracks. When a partial unload is carried out, the cracks open and the contact stiffness decreases on decreasing the effective area of load transfer. These microcracks are generated due to the volumetric expansion of the monoclinic phase and they are more evident when the materials are submitted to indentation[16].

Nevertheless, the problem of hydrothermal degradation that has affected many YTZP Hip prostheses in the past doesn't seem to be so critical on the application of dental implantology. It can be explained because the working mode of this material on a hip prosthesis and on Table 4

Thicknesses of the degraded layer for different hydrothermal aging time. Aging time Degraded layer thickness (µm)

0 h 0 1 h 0.05 2 h 0.10 3 h 0.15 4 h 0.20 5 h 0.30 10 h 0.70

Fig. 9.Force as a function of displacement for both implants studied in the static 30°

flexure test.

Fig. 10.Fatigue curve of both implants studied done with ISO 41801 Standard recommendations.

Table 5

Mechanical resistance of the studied implants in different mechanical conditions. Angle

(°)

Elevated apex 3 mm (ISO 14801 Standard)

Inmersed apex (ideal case)

SK2 Zlock3–411 SK2 Zlock3–411 Maximum resistance (N) Elastic limit (N) Maximum resistance (N) Maximum resistance (N) Elastic limit (N) Maximum resistance (N) 30 967.0 812.1 736.0 1378.7 1052.3 2810.2 15 1894.1 1219.0 2459.4 3046.7 1923.3 8826.1 0 10,203.3 2005.5 11,827.3 11,326.6 2665.4 13,106.2

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a dental implant is completely different. In a hip prosthesis the ball and the cup are wearing each walking cycle. When the water in contact with the YTZP components transforms the surface from tetragonal to monoclinic phase it becomes rougher and the friction force is increased performing grain pull-out leaving a new tetragonal surface to transform and provoking abrasive wear [17]. A dental implant has a structural function. It stays static without friction between parts and without grain pull-out.

Comparing the design of Zlock3–411 and SK2 implants fromFig. 1 some differences can be observed. The most important difference relating to the mechanical properties of the implants is that the Zlock3 implant has a higher diameter (6 mm) on the crest module of the implant. This is a design solution to minimize the stress bore by the threaded zone which, because of the low fracture toughness of the YTZP, is the weakest point of the implant. It is very important for Zlock3 implants that the crest module will be in contact with the bone to be able to shield the stress on the lower zones. It can be seen in Fig. 11from finite element analysis results which shows that an immersed implant has a maximum stress 4 times lower than an implant with a 3 mm exposed implant. This fact has a paramount importance in the case of a bone resorption occurs. The results of monotonic 30°flexure tests demonstrate that the stiffness of the YTZP implants is much higher than the titanium ones. These results suggest that the YTZP implants can provoke stress shielding [18,19]and, consequently, bone resorption more than titanium implants because the low deformation of the implant due to the applied forces will generate low transmission of forces to the cortical bone.

Fatigue tests have shown that although the Zlock3–411 implants have a good fatigue limit they are not as capable to resist one-off overloads as the titanium implants. It is important to take it in account when treating patients with bruxism. For the fatigue testing it is important to remark that the graphic doesn't show any static fatigue effect[2023]given that the tests were too short in time to show this effect.

Comparing new YTZP implants with conventional titanium implants the mechanical test have shown that for Zlock3 implants at 30° angle of inclination the mechanical response will be poorer than the response of SK2 implants. Clinically, in anterior situation or in posterior situation with lateral occlusal contacts, the force axis over the implant can be greater than 30°[24]. Nevertheless with lower angle of inclination the Zlock3 implants have higher resistance than SK2 implants. Itfits to say that the manufacturer recommends never implant the Zlock3 implants with a slope higher than 20° with respect to the force axis.

5. Conclusions

In ideal implantation conditions: minimal inclination of the implant with respect to the force axis, crest module of the implant in intimate contact with bone and absence of overloads, the Zlock implant shows better resistance than SK2 implant. Notwithstanding the low fracture toughness of YTZP implants make them not capable to stand one-off overloads. It is essential that the Zlock implant will be the more parallel to the force axis in order to avoid the stress concentration and the fracture risk. If bone resorption occurs, even minimal, the fracture risk increases noticeably.

The hydrothermal degradation won't play a relevant role on the mechanical response of the YTZP implants.

Acknowledgments

We thank Klockner Implant System for funding this study and providing the test implants.

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Table 6

Stress intensity factors calculated for different times of hydrothermal aging. Aging time Crack length (µm) KI(MPa·m1/2)

0 h 0 0 1 h 0.05 0.40 2 h 0.1 0.56 3 h 0.15 0.69 4 h 0.2 0.79 5 h 0.3 0.97 10 h 0.7 1.49 12.0 6.12

Fig. 11.Stress plots of Zlock3–411 implant in the case of 30°flexion test. Left: 3 mm elevated apex. Right: Immersed apex. The arrows show the direction of the force. Horizontal dark blue lines indicate the position of the implant inside the resin. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

Figure

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