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Loading capacity of zirconia implant supported hybrid ceramic crowns
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Loading capacity of zirconia implant supported hybrid ceramic crowns Nadja Rohr a,∗ , Andrea Coldea b , Nicola U. Zitzmann c , Jens Fischer a,b a
Institute for Dental Materials and Engineering, University Hospital for Dental Medicine, University of Basel, Hebelstrasse 3, CH-4056 Basel, Switzerland b VITA Zahnfabrik, 79713 Bad Säckingen, Germany c Department for Periodontology, Endodontology and Cariology, University of Basel, Hebelstrasse 3, 4056 Basel, Switzerland
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. Recently a polymer infiltrated hybrid ceramic was developed, which is character-
Received 12 October 2014
ized by a low elastic modulus and therefore may be considered as potential material for
Received in revised form
implant supported single crowns. The purpose of the study was to evaluate the loading
9 April 2015
capacity of hybrid ceramic single crowns on one-piece zirconia implants with respect to the
Accepted 21 September 2015
cement type.
Available online xxx
Methods. Fracture load tests were performed on standardized molar crowns milled from hybrid ceramic or feldspar ceramic, cemented to zirconia implants with either machined
Keywords:
or etched intaglio surface using four different resin composite cements. Flexure strength,
Zirconia implant
elastic modulus, indirect tensile strength and compressive strength of the cements were
Resin composite cement
measured. Statistical analysis was performed using two-way ANOVA (p = 0.05).
Hybrid ceramic
Results. The hybrid ceramic exhibited statistically significant higher fracture load values
CAD/CAM crown
than the feldspar ceramic. Fracture load values and compressive strength values of the
Fracture load
respective cements were correlated. Highest fracture load values were achieved with an
Compressive strength
adhesive cement (1253 ± 148 N). Etching of the intaglio surface did not improve the fracture load. Significance. Loading capacity of hybrid ceramic single crowns on one-piece zirconia implants is superior to that of feldspar ceramic. To achieve maximal loading capacity for permanent cementation of full-ceramic restorations on zirconia implants, self-adhesive or adhesive cements with a high compressive strength should be used. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
As an alternative to the well-established dental implants made from titanium, zirconium dioxide has been introduced.
∗
Zirconia is an inert, non-resorbable and biocompatible metal oxide, which facilitates osseointegration in the form of 3–5 mol% yttria stabilized polycrystalline tetragonal zirconia (Y-TZP) [1,2]. Esthetic considerations or potential allergies indicate the use of zirconium dioxide instead of titanium, and
Corresponding author. Tel.: +41 61 267 26 37. E-mail address: [email protected] (N. Rohr).
http://dx.doi.org/10.1016/j.dental.2015.09.012 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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clinical short-term success rates are promising [3–5]. However, due to the lack of long-term data the use of zirconia implants in routine clinical practice is not yet recommended [3]. Implant and suprastructure consist of different materials functioning together as a complex system in withstanding strong intraoral bite forces. Maximum biting forces are reported to be in the range of 286–847 N in the anterior and molar region, respectively [6,7]. Due to its excellent characteristics such as esthetical properties, chemical stability, biocompatibility, and a coefficient of thermal expansion similar to the natural tooth [8] ceramic might be the material of choice for implant restorations. Unfortunately, under tensile stress ceramic is susceptible to fracture as a result of its brittleness, surface and bulk defects and crack propagation under oral function [9]. In order to improve the reliability of ceramics a novel polymer infiltrated ceramic was developed [10–12]. For this material a fracture toughness of 1.21 MPa m1/2 [12] is reported, which is higher than the one of a typical dental feldspar ceramic (0.92–1.12 MPa m1/2 ) [13]. In parallel the hybrid ceramic showed a three times lower hardness value (2.92 ± 1.92 GPa) compared to the feldspar ceramic (10.64 ± 0.46 GPa) [14,15]. In a three point bending test a flexure strength of 144.44 ± 9.61 MPa was measured [12]. Due to its low modulus of elasticity of 31.72 ± 1.43 GPa [12] the hybrid material may work as a buffer area to counterbalance the stiffness of zirconia implants, which is owed to the high elastic modulus of zirconia in the range of 200 GPa [16] and the ankylotic connection to the bone as a result of osseointegration. When investigating the performance of hybrid ceramic restorations the influence of the cement as an intermediate layer has to be considered. The impact of the cement type on fracture load values of tooth supported restorations has been analyzed in several investigations [17–19]. The use of a conventional zinc phosphate cement resulted in lower fracture load values for feldspar and resin composite crowns than a cementation with adhesive cement [18]. Glass-infiltrated alumina as well as lithium disilicate and leucite reinforced ceramic crowns luted with a resin composite cement showed higher fracture load values than those luted with a resin modified glass ionomer cement [19]. On a steel analogue of a prepared upper canine, fracture load values of zirconia, lithium disilicate or ceramic fused to metal crowns were not influenced by the type of cement [20]. This was explained by the fact that the intrinsic strength of these materials was so high that cementing with an adhesive cement could not contribute to the fracture strength. In contrast the fracture load of leucite reinforced glass-ceramic crowns significantly increased by the use of adhesive cement when being compared to glass ionomer cement [20]. The test design has a strong impact on fracture load test results. For instance feldspar ceramic as one of the weaker materials among ceramic systems has been tested with fracture load values of 300–1279 N while using different fracture load test designs [18,21,22]. Fracture load values of 833.4 ± 147.5 N [22] and 1272 ± 109 N [21], respectively were found in two different studies where machined feldspar ceramic crowns were cemented with an adhesive resin composite cement on human teeth or epoxy duplicates of a prepared tooth. These observations indicate that test results
Fig. 1 – Uncemented crown specimens.
cannot easily be matched and a control group is essential in every investigation. The objective of this study was to compare the fracture load values of a new hybrid ceramic material with a feldspar ceramic on zirconia implants while using different luting cements and to detect any correlation between the fracture load values of the ceramics and mechanical properties of the cements. The hypotheses are that (1) the fracture load values of hybrid ceramic crowns are higher than those of feldspar ceramic crowns and (2) fracture load values of feldspar and hybrid ceramic crowns are influenced by mechanical properties of the cement.
2.
Materials and methods
2.1.
Implant preparation
Ten one-piece zirconia implants (ceramic implant, VITA Zahnfabrik, Bad Säckingen, Germany) with a diameter of 4.0 mm, a length of 10 mm in the endosseous part and a machined abutment surface (Ra = 0.42 ± 0.06 m) were used for this study. All implants were embedded according to ISO 14801:2008 in epoxy (RenCast CW 20/Ren HY 49, Huntsman Advanced Materials, Duxford, UK) in order to simulate the elasticity of human bone. The implants were inserted with a 3 mm clearance between implant neck and resin surface as required by the standard.
2.2.
Crown preparation and cementation
One implant was scanned with an optical scanner (inEos Blue, Sirona, Bensheim, Germany). A standardized molar crown (46) was designed by CAD-software (inLab SW4.0, Sirona) and milled (inLab MCXL, Sirona) (Fig. 1). Hundred crowns of a feldspar ceramic (Vitablocs Mark II, VITA) and 100 crowns of a hybrid ceramic (VITA Enamic, VITA) with polished occlusal surfaces were produced following the manufacturer’s recommendations. All crowns were milled with the same design and equipment. Surface polishing was performed manually as it is common practice in a dental laboratory. Trimming and smoothing was done with white polishers, silky luster achieved with pink polishers (porcelain polishers white medium and porcelain polishers pink fine, Hager & Meisinger, Neuss, Germany). All crowns were finally polished with a goat hair buffing wheel and polishing paste (Wetzler Dental, Bielefeld, Deutschland). Prior to cementation crowns and implants were properly cleaned in an ultrasonic
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Fig. 3 – Cement gap of a cemented crown on the zirconia implant visualized with light microscopy at a magnification of 25× (longitudinal section).
distilled water for 24 h under permanent temperature control (CTS T-4025, Hechingen, Germany) at 37 ◦ C.
Fig. 2 – Microscopic Image of a longitudinal section of a crown cemented on the implant at a magnification of 6×.
bath (TPC-15, Telsonic, Bronschhofen, Switzerland) with 96% ethanol for 5 min. After drying, the crowns were labeled and cemented to the implants with different luting cements under a static load of 20 N for 6 min. Fig. 2 provides the proportions of crown and implant, the cement gap is shown in detail in Fig. 3. For 50 crowns the intaglio surface was etched for 60 s with hydrofluoric acid prior to cementation (VITA Ceramics Etch, VITA) whereas the other 50 crowns of the respective material were cemented in the machined state. The 200 crowns were cemented to the implants according to the matrix given in Fig. 4. The materials used are listed in Tables 1 and 2. The cement excess was eliminated using foam pellets. After the cementation process, the specimens were stored in
2.3.
Fracture load test
All specimens underwent fracture load testing which was performed at a cross-head speed of 1 mm/min. The specimens were placed consecutively in a universal testing machine (Z020, Zwick/Roell, Ulm, Germany) and loaded axially to the implant until fracture (Fig. 5). A steel ball (4.75 mm diameter) was adjusted in the center of the occlusal surface with a 0.2 mm thick tin foil (Dentaurum, Pforzheim, Germany) placed between the ball and the occlusal surface to attain a homogenous stress distribution. The fracture load values were recorded (testXpert V 2.2, Zwick/Roell). After the fracture load test the 10 zirconia implants were carefully cleaned and re-used for the subsequent test series. Remaining cement on the implant abutment was completely removed using a blow-dryer at 200 ◦ C (MZ 6 HL, miolectric, Switzerland). After the removal of the cement the implants were cleaned in an ultrasonic bath (TPC-15, Telsonic) with 96% ethanol for 5 min.
Fig. 4 – Fracture load test matrix. No cem, uncemented specimens; HIS, Harvard implant semi-permanent; PSA, Panavia SA; MCE, Maxcem Elite; MLA, Multilink Automix. Please cite this article in press as: Rohr N, et al. Loading capacity of zirconia implant supported hybrid ceramic crowns. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.09.012
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Table 1 – Summary of products used. Name
Code
VITABLOCS Mark II
VM
Feldspar ceramic
VITA Enamic
VE
Hybrid ceramic
Harvard Implant semi-permanent
HIS
Panavia SA
PSA
Semi-permanent cement, dual curing Selfadhesive cement, dual curing
Maxcem Elite Multilink Automix
MCE MLA
Selfadhesive cement, dual curing Adhesive cement, dual curing
Type
Manufacturers VITA Zahnfabrik, Bad Säckingen, Germany VITA Zahnfabrik, Bad Säckingen, Germany Harvard Dental International, Hoppegarten, Germany Kuraray-Noritake Dental Inc., Kurashiki, Japan Kerr, Scafaty, Italy Ivoclar Vivadent, Schaan, Liechtenstein
Lot. no. 27291 100001 7206489 8V0015 4870103 S53599
Table 2 – Composition of resin cement materials used. Product name Harvard implant semi-permanent Panavia SA
Maxcem Elite Multilink Automix
2.4.
Composition Methacrylates, zinc oxide Bisphenol A diglycidylmethacrylate, sodium fluoride, triethylene glycol dimethacrylate, 10-methacryloyloxydecyl dihydrogen phosphate (MDP), hydrophobic aromatic dimethacrylate, hydrophobic aliphatic dimethacrylate, silanated barium glass filler, silanated colloidal silica di-camphorquinone, initiators, accelerators, catalysts, pigments Glyceroldimethacrylate dihydrogen phosphate (GPDM) hydroxyethylmethacrylat (HEMA), 4-methoxyphenol (MEHQ), cumolhydroperoxid (CHPO), methacrylate ester monomers, titanium dioxide, pigments Bis-GMA, 2-hydroxyethylmethacrylat, 2-dimethylaminoethylmethacrylate, urethandimethacrylate, dibenzoylperoxide, barium glass filler, Ba-Al-fluoro-silicate-glass filler, ytterbiumtrifluoride siliciumdioxidefiller, catalysts, stabilizers, pigments
Fracture analyses
Two specimens of each group were randomly selected for a fracture analysis with a stereomicroscope (Wild M7A, Leica,
Heerbrugg, Switzerland) and a scanning electron microscope (Philips XL30 FEG ESEM, Philips Electron Optics, Eindhoven, Netherlands).
2.5. Elastic modulus and flexure strength of the cement materials For the flexure strength and static elastic modulus measurement, bending bars (n = 5) with dimensions of 25 mm × 2 mm × 2 mm were prepared in a Teflon mold (Distrelec, Nänikon, CH) according to ISO 4049:2009. The bars were loaded until fracture in a universal testing machine (Z010, Zwick/Roell) in three-point flexure with a crosshead speed of 0.75 mm/min. The fracture stress, f , was calculated by the formula (according to ISO 4049:2009): f =
3Fl 2wh2
F is the fracture load, l is the roller span (here 20 mm), w is the width and h is the height of the bar. The static elastic modulus was calculated from the threepoint bending results by the formula: Ef =
Fl3 4wh3 d
d is the deflection corresponding to load F.
2.6.
Fig. 5 – Fracture load test set up. The crowns were loaded axially to the implant using a tin foil for stress distribution and a steel ball.
Indirect tensile strength and compressive strength
Indirect tensile strength and compressive strength of the cements were measured with cylindrical test specimens 3 mm in height and diameter. A Teflon mold (Distrelec) was used to produce the test specimens. The cement was filled into the respective cavities of the Teflon mold and kept in place with a
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Fig. 6 – Fracture load values of as-machined and etched feldspar ceramic and hybrid ceramic molar crowns cemented to zirconia implants with varying cements.
plastic foil and a glass plate on each side. After 1 h storage in distilled water at 37 ◦ C the specimens were carefully removed from the mold and stored in water at 37 ◦ C for another 24 h. Specimens with visible bubbles or surface defects were eliminated. Specimens of each cement were produced and loaded until fracture (Z020, Zwick/Roell). Ten specimens were produced to measure the indirect tensile strength and loaded perpendicular whereas another 10 specimens of the respective material were loaded parallel to the cylinder axis to determine compressive strength. Cross-head speed was set to 1 mm/min. The indirect tensile strength was calculated using the following equation: t =
2F dh
t is the indirect tensile strength; F is the fracture load; d is the diameter; h is the height. For calculating the compressive strength the following equation was used: c =
F A
c is the compressive strength; F is the fracture load; A is the cross section.
2.7.
Statistical analysis
The interaction between crown material, intaglio surface treatment and cement type was tested with two-way ANOVA (p = 0.05) (Minitab 16, Minitab, Coventry, UK).
3.
Results
3.1.
Fracture load
All crowns fractured during the test under a load ranging from 249 to 1239 N, forming 2–5 fragments. No damage was detected at the implant or the epoxy resin block. The hybrid material (VE) exhibited significantly higher fracture loads than the feldspar ceramic (VM) for the respective cement (p = 0.037). The highest values were achieved with VE machined and MLA (1253 ± 148 N) and the lowest values for VM etched without cement (249 ± 58 N) (Fig. 6 and Table 3). In no group etching
Table 3 – Means and standard deviations of fracture loads for different crown materials, intaglio surface treatments (m = machined, e = etched) and respective cements. (N)
No cement
HIS
Mean
SD
Mean
PSA
MCE
MLA
SD
Mean
SD
Mean
SD
Mean
SD
VM
m e
259.6 248.7
51.9 58.2
682.8 673.0
117.0 127.8
974.6 956.7
161.2 127.5
1034.6 1009.1
108.0 90.1
1090.9 1097.0
116.7 109.6
VE
m e
505.8 516.3
153.6 75.3
871.3 860.4
40.2 66.0
1133.4 1130.9
102.4 134.5
1161.0 1151.0
138.0 71.0
1253.0 1162.0
147.9 105.0
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Fig. 7 – Correlation between fracture load of machined and etched crowns.
Fig. 8 – Correlation between fracture load of feldspar ceramic crowns and hybrid ceramic crowns.
3.3. improved the fracture load, irrespective of crown material (p = 0.915) or cement type (p = 0.973) used. Machined and etched specimens showed similar results within the respective material combination and therefore, the values were strongly correlated (Fig. 7). For that reason, both machined and etched specimens were summarized to one group when assessing the impact of the cements on the results. Compared to the non-cemented specimens, the fracture load for both VE and VM increased significantly when the semi-permanent cement (HIS) was used (p = 0.000). A further significant gain in fracture load was achieved, when HIS was substituted by selfadhesive (PSA, MCE) or adhesive cement (MLA) (p = 0.000). No significant difference was found between PSA, MCE and MLA (p = 0.005) even though a slight gain in fracture loads from PSA to MCE to MLA was observed (Fig. 6 and Table 3). The fracture load values of feldspar and hybrid ceramic crowns were highly correlated (Fig. 8).
3.2. load
Cement characteristics and correlations to fracture
Flexure strength, elastic modulus, indirect tensile strength and compressive strength values of the cements are listed in Table 4. A correlation was found between compressive strength of the cements and the fracture load of cemented crowns (Fig. 9). The non-cemented fracture load results are included in Fig. 9 at 0 MPa compressive strength.
Fractography
The specimens (VM and VE) failed by fracturing into a different number of fragments. No correlation was found between the number of fragments and the material combinations. The cement was sticking either to the implant, to the crown, or both, regardless of the crown material and cement that was used. The fracture initiation for un-cemented crowns (both VE and VM, Fig. 10a and b) starts at the occlusal surface in contact with the steel ball. Fig. 10a demonstrates second fracturing events occurring during fracture test progress, starting from the contact of the intaglio crown surface with the implant. Further, second fracturing events occur from the margins of the crowns (Fig. 10a and b, right side). The VE crown cemented with HIS (Fig. 10c) exhibits, similar to the un-cemented crown (Fig. 10a), a multiple fracture event pattern. When using self-adhesive or adhesive cements, the fracture for VE (Fig. 10e) was initiated by the steel ball and started from the loading point. Cemented VM crowns (Fig. 10d and f) fractured from the occlusal contact point.
4.
Discussion
The present study was designed to show how the new hybrid ceramic material performs within an implant system and in what way cements influence the fracture load of crowns. The first hypothesis that the fracture load values of hybrid ceramic
Table 4 – Flexure strength, elastic modulus, compressive strength and indirect tensile strength of the cements. Material HIS PSA MCE MLA
Flexure strength (MPa) 4.6 ± 0.4 93.2 ± 14.7 64.4 ± 16.0 119.3 ± 15.2
Elastic modulus (GPa) 0.04 ± 0.00 6.09 ± 0.83 6.15 ± 1.27 8.53 ± 0.60
Compressive strength (MPa) 37.1 ± 7.0 254.2 ± 6.4 288.6 ± 10.5 321.0 ± 9.3
Indirect tensile strength (MPa) 5.2 ± 0.8 35.8 ± 5.2 56.5 ± 4.5 51.3 ± 1.7
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Fig. 9 – Correlation between fracture load of the crowns and compressive strength of the cements.
crowns are higher than those of feldspar ceramic crowns was confirmed. A correlation between mechanical properties of the cements and fracture load of the crowns was only found for the compressive strength. Hence, the compressive strength of the cement has an impact on the strength of a feldspar or hybrid ceramic crown cemented on a zirconia implant. In this respect the second hypothesis that mechanical properties of the cement influence fracture load values of feldspar and hybrid ceramic crowns was confirmed for the compressive strength.
4.1.
Test design
Since only static but no dynamic load was applied and no longterm humid environment or thermal stress was simulated the test design of this study may only to a certain extent be considered as a correct simulation of a clinical situation. Also the use of a steel ball to apply the load to the crown instead of a more anatomy related material and shape might have been a limiting factor. Furthermore, the cements were not submitted to light curing which could also influence the fracture load since the curing mode has an effect on the mechanical properties of the cements [23]. Chemical curing of the cements was chosen in this study to eliminate variations in polymerization due to light curing. Unfortunately, currently no standard method exists for testing the fracture load of ceramic crowns [24]. The test method used in the current study showed reasonable and reproducible results with a low variance in values. The strong correlation between fracture load values of feldspar crowns and hybrid ceramic crowns suggest that the test design yields robust data. Therefore, it may be concluded that the test design can be used as a screening test to simulate the clinical behavior of new dental materials under mechanical forces and wet conditions in a reasonable amount of time and cost.
4.2.
Effect of cementation on fracture load
For ceramics with low intrinsic strength, cementation type can have an influence on the fracture load [20]. In the present
study it was found that for both, feldspar ceramic and hybrid ceramic a significantly relevant increase of fracture load can be achieved with the use of self-adhesive or adhesive cements, compared to the results obtained after cementation with the semi-permanent cement. Even though there was no significant difference between the fracture load values of the self-adhesive and adhesive cements an increase of the fracture load due to a higher compressive strength of the cement can be observed as a general trend. Cements with a high compressive strength seem to work as stress buffer between crown and implant and therefore result in higher stability of the whole system. That may be explained by the fact that the cement fills the gap between implant abutment surface and intaglio surface of the crown thus eliminating any premature contacts between both surfaces which might lead to stress concentrations. Based on these considerations it is selfevident that cements with a higher compressive strength lead to higher fracture load values because the buffering effect is more pronounced. In the literature it is reported that MDP containing cement revealed higher microtensile bond strength to zirconia than cement without MDP [25]. Compared within the same crown material group, the cement used in this study containing MDP (PSA) showed lower fracture load values than the cements without MDP (MLA or MCE). Hence the bond strength did not seem to affect fracture load values in the same degree as the compressive strength of the cements did. Neither silanization of the intaglio surface of the crowns nor application of a ceramic primer on the implant abutment was involved in order to minimize the test parameters. Silane or ceramic primer might have an influence on the obtained fracture load values. Etching of the intaglio surface did not increase the fracture load. That observation might be explained in the way that obviously the compressive strength of the cements is crucial for the fracture resistance of the crowns and for the formation of compressive stress at the interface of crown and cement. The micro-morphology of the interface is not relevant in this test design.
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Fig. 10 – SEM micrograph of fracture origin of machined crowns (18×). (a) VE without cement; (b) VM without cement; (c) VE HIS; (d) VM HIS; (e) VE MLA; (f) VM MLA.
Due to different experimental variables used in other studies, the fracture load values found in this study cannot be compared directly to failure loads reported in the literature. Nevertheless, the values found in the present study for the feldspar ceramic with adhesive cement (1091 ± 117 N) are in the range of those presented in other studies [21,22].
4.3.
Fractography
Central occlusal contact point loading was employed in the present study to cause fractures by inducing stress on the occlusal surface which leads to several possible cracks (e.g. cone, radial, lateral cracks). Previous studies [26,27] addressed the topic of such a test configuration and the correlation to clinical failures. In contrast to the findings by Kelly [27] where
clinical failure origins are located in the bonded interface region, Fig. 10 indicates that fracture of cemented crowns started from the occlusal contact points. In clinical service, tensile stress concentration at the cementation surface of the ceramic crown seems to have a higher potential to cause damage to restorations than the load at the loading point [28–30]. In the present study this was also confirmed when no cement or semi-permanent cement was used for feldspar and hybrid ceramic. The use of a self-adhesive or adhesive cement seemed to have cushioned some tensile stress at the interface, resulting in a fracture origin at the loading point and no second event fractures. The bonded hybrid ceramic crowns revealed the separation of a thin VE plate (not visible in Fig. 10) within the fractured
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pieces. This fracture patterns of the hybrid ceramic compared to feldspar ceramic can be explained by the microstructure and resulting higher fracture toughness of VE. As presented in a previous study [11], the interconnected networks of polymer and ceramic within VE suggest a more tortuous fracture path originating from crack deflection and bridging at the interface of the ceramic and polymer interconnected phases [11]. These toughening mechanisms absorb the energy of the advancing crack tip upon loading, resulting in a higher fracture toughness of 1.21 MPa m1/2 [12] compared to the one of a typical dental feldspar ceramic (0.92–1.12 MPa m1/2 ) [13].
4.4.
Clinical implications
The hybrid ceramic showed fracture load values higher than the ones of the feldspar ceramic, which is a reliable material in the clinic since decades [31,32]. The fracture load values for the cemented hybrid ceramic crowns were higher than maximum biting forces of 847 N for natural teeth in the molar region [6] even when using the semi-permanent cement. Self-adhesive or adhesive cements with a high compressive strength may be considered as the first choice for permanent cementation of hybrid ceramic material on zirconia implants. Most manufacturers recommend a pre-treatment of ceramic crowns with hydrofluoric acid prior to adhesive bonding. In respect of the loading capacity, the present study has proven that etching of the ceramic crowns does not lead to significantly higher fracture load values. However, it has to be considered that in the present study only static loading was applied and no silane was applied. Significant differences were found between the failure behavior created during static fracture load tests and that observed to have occurred during clinical failure of allceramic restorations [27]. In order to demonstrate a clear interaction between mechanical properties and clinical crown fracture events, a combination of tests for the individual material components and their interaction will be required [33]. The effect of the use of silane, dynamic loading, variations in load transmission, or aging might lead to differing results and for that reason will be subject of further investigations. In addition, finite element analysis might be helpful to show the stress distribution and demonstrate the crack initiation and propagation.
5.
Conclusions
In the present test design hybrid ceramic provides higher loading capacity than feldspar ceramic. The fracture load of hybrid ceramic and feldspar ceramic crowns is correlated to the compressive strength of the cement. In the test design used, etching of the intaglio surface of hybrid or feldspar ceramic crowns had no effect on the fracture load of the restoration.
Acknowledgements The authors are grateful to VITA Zahnfabrik for supporting this study with materials and to Fredy Schmidli (University
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Hospital for Dental Medicine) for the lab support. The statistical analysis was kindly performed by Marek Mrzyk, VITA Zahnfabrik.
references
[1] Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1–25. [2] Schultze-Mosgau S, Schliephake H, Radespiel-Tröger M, Neukam FW. Osseointegration of endodontic endosseous cones: zirconium oxide vs titanium. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89:91. [3] Andreiotelli M, Wenz HJ, Kohal RJ. Are ceramic implants a viable alternative to titanium implants? A systematic literature review. Clin Oral Implants Res 2009;4:32–47. [4] Borgonovo AE, Fabbri A, Vavassori V, Censi R, Maiorana C. Multiple teeth replacement with endosseous one-piece yttrium-stabilized zirconia dental implants. Med Oral Patol Oral Cir Bucal 2012;17:981–7. [5] Payer M, Arnetzl V, Kirmeier R, Koller M, Arnetzl G, Jakse N. Immediate provisional restoration of single-piece zirconia implants: a prospective case series – results after 24 months of clinical function. Clin Oral Implants Res 2013;24:569–75. [6] Waltimo A, Könönen M. A novel bite force recorder and maximal isometric bite force values for healthy young adults. Scand J Dent Res 1993;101:171–5. [7] Tortopidis D, Lyons MF, Baxendale RH, Gilmour WH. The variability of bite force measurement between sessions, in different positions within the dental arch. J Oral Rehabil 1998;25:681–6. [8] Anusavice KJ. Recent developments in restorative dental ceramics. J Am Dent Assoc 1993;124, 72–4, 76–8, 80–4. [9] McLean JW. Ceramics in clinical dentistry. Br Dent J 1988;164:310. [10] Coldea A, Swain MV, Thiel N. Mechanical properties of polymer-infiltrated-ceramic-network materials. Dent Mater 2013;29:419–26. [11] Coldea A, Swain MV, Thiel N. In-vitro strength degradation of dental ceramics and novel PICN material by sharp indentation. J Mech Behav Biomed Mater 2013;26:34–42. [12] Coldea A, Swain MV, Thiel N. Hertzian contact response and damage tolerance of dental ceramics. J Mech Behav Biomed Mater 2014;34:124–33. [13] Yu HY, Cai ZB, Ren PD, Zhu MH, Zhou ZR. Friction and wear behavior of dental feldspathic porcelain. Wear 2006;261:611–21. [14] He LH, Swain M. A novel polymer infiltrated ceramic dental material. Dent Mater 2011;27:527–34. [15] He LH, Purton D, Swain M. A novel polymer infiltrated ceramic for dental simulation. J Mater Sci Mater Med 2011;22:1639–43. [16] Christel P, Meunier A, Heller M, Torre JP, Peille CN. Mechanical properties and short-term in-vivo evaluation of yttrium-oxide-partially-stabilized zirconia. J Biomed Mater Res 1989;23:45–61. [17] Burke FJ, Watts DC. Fracture resistance of teeth restored with dentin-bonded crowns. Quintessence Int 1994;25:335–40. [18] Attia A, Abdelaziz KM, Freitag S, Kern M. Fracture load of composite resin and feldspathic all-ceramic CAD/CAM crowns. J Prosthet Dent 2006;95:117–23. [19] Borges GA, Caldas D, Taskonak B, Yan J, Sobrinho LC, de Oliveira WJ. Fracture loads of all-ceramic crowns under wet and dry fatigue conditions. J Prosthodont 2009;18:649–55. [20] Stawarczyk B, Beuer F, Ender A, Roos M, Edelhoff D, Wimmer T. Influence of cementation and cement type on the fracture
Please cite this article in press as: Rohr N, et al. Loading capacity of zirconia implant supported hybrid ceramic crowns. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.09.012
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ARTICLE IN PRESS
xxx.e10
d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx.e1–xxx.e10
[21]
[22]
[23]
[24]
[25]
[26]
[27]
load testing methodology of anterior crowns made of different materials. Dent Mater J 2013;30:888–95. Zahran M, El-Mowafy O, Tam L, Watson PA, Finer Y. Fracture strength and fatigue resistance of all-ceramic molar crowns manufactured with CAD/CAM technology. J Prosthodont 2008;17:370–7. Shahrbaf S, van Noort R, Mirzakouchaki B, Ghassemieh E, Martin N. Fracture strength of machined ceramic crowns as a function of tooth preparation design and the elastic modulus of the cement. Dent Mater 2014;30:234–41. Ilie N, Simon A. Effect of curing mode on the micro-mechanical properties of dual-cured self-adhesive resin cements. Clin Oral Invest 2011;16:505–12. Potiket N, Chiche G, Finger IM. In vitro fracture strength of teeth restored with different all-ceramic crown systems. J Prosthet Dent 2004;92:491–5. de Souza G, Hennig D, Aggarwal A, Tam LE. The use of MDP-based materials for bonding to zirconia. J Prosthet Dent 2014;112:895–902. Lawn BR, Deng Y, Thompson VP. Use of contact testing in the characterization and design of all-ceramic crownlike layer structures: a review. J Prosthet Dent 2001;86:495–510. Kelly JR. Clinically relevant approach to failure testing of all-ceramic restorations. J Prosthet Dent 1999;81:652–61.
[28] Campos RE, Soares CJ, Quagliatto PS, Soares PV, de Oliveira Jr OB, Santos-Filho PC, Salazar-Marocho SM. In vitro study of fracture load and fracture pattern of ceramic crowns: a finite element and fractography analysis. J Prosthodont 2011;20:447–55. [29] Dong XD, Darvell BW. Stress distribution and failure mode of dental ceramic structures under Hertzian indentation. Dent Mater 2003;19:542–51. [30] Scherrer SS, Quinn GD, Quinn JB. Fractographic failure analysis of a Procera AllCeram crown using stereo and scanning electron microscopy. Dent Mater 2008;24: 1107–13. [31] Thompson JY, Bayne SC, Heymann HO. Mechanical properties of a new mica-based machinable glass ceramic for CAD/CAM restorations. J Prosthet Dent 1996;76: 619–23. [32] Federlin M, Hiller KA, Schmalz G. Controlled, prospective clinical split-mouth study of cast gold vs. ceramic partial crowns: 5.5 year results. Am J Dent 2010;23:161–7. [33] Annusavice KJ, Kakar K, Ferree N. Which mechanical and physical testing methods are relevant for predicting the clinical performance of ceramic-based dental prostheses? Clin Oral Impl Res 2007;18:218–31.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Loading capacity of zirconia implant supported hybrid ceramic crowns Nadja Rohr a,∗ , Andrea Coldea b , Nicola U. Zitzmann c , Jens Fischer a,b a
Institute for Dental Materials and Engineering, University Hospital for Dental Medicine, University of Basel, Hebelstrasse 3, CH-4056 Basel, Switzerland b VITA Zahnfabrik, 79713 Bad Säckingen, Germany c Department for Periodontology, Endodontology and Cariology, University of Basel, Hebelstrasse 3, 4056 Basel, Switzerland
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. Recently a polymer infiltrated hybrid ceramic was developed, which is character-
Received 12 October 2014
ized by a low elastic modulus and therefore may be considered as potential material for
Received in revised form
implant supported single crowns. The purpose of the study was to evaluate the loading
9 April 2015
capacity of hybrid ceramic single crowns on one-piece zirconia implants with respect to the
Accepted 21 September 2015
cement type.
Available online xxx
Methods. Fracture load tests were performed on standardized molar crowns milled from hybrid ceramic or feldspar ceramic, cemented to zirconia implants with either machined
Keywords:
or etched intaglio surface using four different resin composite cements. Flexure strength,
Zirconia implant
elastic modulus, indirect tensile strength and compressive strength of the cements were
Resin composite cement
measured. Statistical analysis was performed using two-way ANOVA (p = 0.05).
Hybrid ceramic
Results. The hybrid ceramic exhibited statistically significant higher fracture load values
CAD/CAM crown
than the feldspar ceramic. Fracture load values and compressive strength values of the
Fracture load
respective cements were correlated. Highest fracture load values were achieved with an
Compressive strength
adhesive cement (1253 ± 148 N). Etching of the intaglio surface did not improve the fracture load. Significance. Loading capacity of hybrid ceramic single crowns on one-piece zirconia implants is superior to that of feldspar ceramic. To achieve maximal loading capacity for permanent cementation of full-ceramic restorations on zirconia implants, self-adhesive or adhesive cements with a high compressive strength should be used. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
As an alternative to the well-established dental implants made from titanium, zirconium dioxide has been introduced.
∗
Zirconia is an inert, non-resorbable and biocompatible metal oxide, which facilitates osseointegration in the form of 3–5 mol% yttria stabilized polycrystalline tetragonal zirconia (Y-TZP) [1,2]. Esthetic considerations or potential allergies indicate the use of zirconium dioxide instead of titanium, and
Corresponding author. Tel.: +41 61 267 26 37. E-mail address: [email protected] (N. Rohr).
http://dx.doi.org/10.1016/j.dental.2015.09.012 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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clinical short-term success rates are promising [3–5]. However, due to the lack of long-term data the use of zirconia implants in routine clinical practice is not yet recommended [3]. Implant and suprastructure consist of different materials functioning together as a complex system in withstanding strong intraoral bite forces. Maximum biting forces are reported to be in the range of 286–847 N in the anterior and molar region, respectively [6,7]. Due to its excellent characteristics such as esthetical properties, chemical stability, biocompatibility, and a coefficient of thermal expansion similar to the natural tooth [8] ceramic might be the material of choice for implant restorations. Unfortunately, under tensile stress ceramic is susceptible to fracture as a result of its brittleness, surface and bulk defects and crack propagation under oral function [9]. In order to improve the reliability of ceramics a novel polymer infiltrated ceramic was developed [10–12]. For this material a fracture toughness of 1.21 MPa m1/2 [12] is reported, which is higher than the one of a typical dental feldspar ceramic (0.92–1.12 MPa m1/2 ) [13]. In parallel the hybrid ceramic showed a three times lower hardness value (2.92 ± 1.92 GPa) compared to the feldspar ceramic (10.64 ± 0.46 GPa) [14,15]. In a three point bending test a flexure strength of 144.44 ± 9.61 MPa was measured [12]. Due to its low modulus of elasticity of 31.72 ± 1.43 GPa [12] the hybrid material may work as a buffer area to counterbalance the stiffness of zirconia implants, which is owed to the high elastic modulus of zirconia in the range of 200 GPa [16] and the ankylotic connection to the bone as a result of osseointegration. When investigating the performance of hybrid ceramic restorations the influence of the cement as an intermediate layer has to be considered. The impact of the cement type on fracture load values of tooth supported restorations has been analyzed in several investigations [17–19]. The use of a conventional zinc phosphate cement resulted in lower fracture load values for feldspar and resin composite crowns than a cementation with adhesive cement [18]. Glass-infiltrated alumina as well as lithium disilicate and leucite reinforced ceramic crowns luted with a resin composite cement showed higher fracture load values than those luted with a resin modified glass ionomer cement [19]. On a steel analogue of a prepared upper canine, fracture load values of zirconia, lithium disilicate or ceramic fused to metal crowns were not influenced by the type of cement [20]. This was explained by the fact that the intrinsic strength of these materials was so high that cementing with an adhesive cement could not contribute to the fracture strength. In contrast the fracture load of leucite reinforced glass-ceramic crowns significantly increased by the use of adhesive cement when being compared to glass ionomer cement [20]. The test design has a strong impact on fracture load test results. For instance feldspar ceramic as one of the weaker materials among ceramic systems has been tested with fracture load values of 300–1279 N while using different fracture load test designs [18,21,22]. Fracture load values of 833.4 ± 147.5 N [22] and 1272 ± 109 N [21], respectively were found in two different studies where machined feldspar ceramic crowns were cemented with an adhesive resin composite cement on human teeth or epoxy duplicates of a prepared tooth. These observations indicate that test results
Fig. 1 – Uncemented crown specimens.
cannot easily be matched and a control group is essential in every investigation. The objective of this study was to compare the fracture load values of a new hybrid ceramic material with a feldspar ceramic on zirconia implants while using different luting cements and to detect any correlation between the fracture load values of the ceramics and mechanical properties of the cements. The hypotheses are that (1) the fracture load values of hybrid ceramic crowns are higher than those of feldspar ceramic crowns and (2) fracture load values of feldspar and hybrid ceramic crowns are influenced by mechanical properties of the cement.
2.
Materials and methods
2.1.
Implant preparation
Ten one-piece zirconia implants (ceramic implant, VITA Zahnfabrik, Bad Säckingen, Germany) with a diameter of 4.0 mm, a length of 10 mm in the endosseous part and a machined abutment surface (Ra = 0.42 ± 0.06 m) were used for this study. All implants were embedded according to ISO 14801:2008 in epoxy (RenCast CW 20/Ren HY 49, Huntsman Advanced Materials, Duxford, UK) in order to simulate the elasticity of human bone. The implants were inserted with a 3 mm clearance between implant neck and resin surface as required by the standard.
2.2.
Crown preparation and cementation
One implant was scanned with an optical scanner (inEos Blue, Sirona, Bensheim, Germany). A standardized molar crown (46) was designed by CAD-software (inLab SW4.0, Sirona) and milled (inLab MCXL, Sirona) (Fig. 1). Hundred crowns of a feldspar ceramic (Vitablocs Mark II, VITA) and 100 crowns of a hybrid ceramic (VITA Enamic, VITA) with polished occlusal surfaces were produced following the manufacturer’s recommendations. All crowns were milled with the same design and equipment. Surface polishing was performed manually as it is common practice in a dental laboratory. Trimming and smoothing was done with white polishers, silky luster achieved with pink polishers (porcelain polishers white medium and porcelain polishers pink fine, Hager & Meisinger, Neuss, Germany). All crowns were finally polished with a goat hair buffing wheel and polishing paste (Wetzler Dental, Bielefeld, Deutschland). Prior to cementation crowns and implants were properly cleaned in an ultrasonic
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Fig. 3 – Cement gap of a cemented crown on the zirconia implant visualized with light microscopy at a magnification of 25× (longitudinal section).
distilled water for 24 h under permanent temperature control (CTS T-4025, Hechingen, Germany) at 37 ◦ C.
Fig. 2 – Microscopic Image of a longitudinal section of a crown cemented on the implant at a magnification of 6×.
bath (TPC-15, Telsonic, Bronschhofen, Switzerland) with 96% ethanol for 5 min. After drying, the crowns were labeled and cemented to the implants with different luting cements under a static load of 20 N for 6 min. Fig. 2 provides the proportions of crown and implant, the cement gap is shown in detail in Fig. 3. For 50 crowns the intaglio surface was etched for 60 s with hydrofluoric acid prior to cementation (VITA Ceramics Etch, VITA) whereas the other 50 crowns of the respective material were cemented in the machined state. The 200 crowns were cemented to the implants according to the matrix given in Fig. 4. The materials used are listed in Tables 1 and 2. The cement excess was eliminated using foam pellets. After the cementation process, the specimens were stored in
2.3.
Fracture load test
All specimens underwent fracture load testing which was performed at a cross-head speed of 1 mm/min. The specimens were placed consecutively in a universal testing machine (Z020, Zwick/Roell, Ulm, Germany) and loaded axially to the implant until fracture (Fig. 5). A steel ball (4.75 mm diameter) was adjusted in the center of the occlusal surface with a 0.2 mm thick tin foil (Dentaurum, Pforzheim, Germany) placed between the ball and the occlusal surface to attain a homogenous stress distribution. The fracture load values were recorded (testXpert V 2.2, Zwick/Roell). After the fracture load test the 10 zirconia implants were carefully cleaned and re-used for the subsequent test series. Remaining cement on the implant abutment was completely removed using a blow-dryer at 200 ◦ C (MZ 6 HL, miolectric, Switzerland). After the removal of the cement the implants were cleaned in an ultrasonic bath (TPC-15, Telsonic) with 96% ethanol for 5 min.
Fig. 4 – Fracture load test matrix. No cem, uncemented specimens; HIS, Harvard implant semi-permanent; PSA, Panavia SA; MCE, Maxcem Elite; MLA, Multilink Automix. Please cite this article in press as: Rohr N, et al. Loading capacity of zirconia implant supported hybrid ceramic crowns. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.09.012
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Table 1 – Summary of products used. Name
Code
VITABLOCS Mark II
VM
Feldspar ceramic
VITA Enamic
VE
Hybrid ceramic
Harvard Implant semi-permanent
HIS
Panavia SA
PSA
Semi-permanent cement, dual curing Selfadhesive cement, dual curing
Maxcem Elite Multilink Automix
MCE MLA
Selfadhesive cement, dual curing Adhesive cement, dual curing
Type
Manufacturers VITA Zahnfabrik, Bad Säckingen, Germany VITA Zahnfabrik, Bad Säckingen, Germany Harvard Dental International, Hoppegarten, Germany Kuraray-Noritake Dental Inc., Kurashiki, Japan Kerr, Scafaty, Italy Ivoclar Vivadent, Schaan, Liechtenstein
Lot. no. 27291 100001 7206489 8V0015 4870103 S53599
Table 2 – Composition of resin cement materials used. Product name Harvard implant semi-permanent Panavia SA
Maxcem Elite Multilink Automix
2.4.
Composition Methacrylates, zinc oxide Bisphenol A diglycidylmethacrylate, sodium fluoride, triethylene glycol dimethacrylate, 10-methacryloyloxydecyl dihydrogen phosphate (MDP), hydrophobic aromatic dimethacrylate, hydrophobic aliphatic dimethacrylate, silanated barium glass filler, silanated colloidal silica di-camphorquinone, initiators, accelerators, catalysts, pigments Glyceroldimethacrylate dihydrogen phosphate (GPDM) hydroxyethylmethacrylat (HEMA), 4-methoxyphenol (MEHQ), cumolhydroperoxid (CHPO), methacrylate ester monomers, titanium dioxide, pigments Bis-GMA, 2-hydroxyethylmethacrylat, 2-dimethylaminoethylmethacrylate, urethandimethacrylate, dibenzoylperoxide, barium glass filler, Ba-Al-fluoro-silicate-glass filler, ytterbiumtrifluoride siliciumdioxidefiller, catalysts, stabilizers, pigments
Fracture analyses
Two specimens of each group were randomly selected for a fracture analysis with a stereomicroscope (Wild M7A, Leica,
Heerbrugg, Switzerland) and a scanning electron microscope (Philips XL30 FEG ESEM, Philips Electron Optics, Eindhoven, Netherlands).
2.5. Elastic modulus and flexure strength of the cement materials For the flexure strength and static elastic modulus measurement, bending bars (n = 5) with dimensions of 25 mm × 2 mm × 2 mm were prepared in a Teflon mold (Distrelec, Nänikon, CH) according to ISO 4049:2009. The bars were loaded until fracture in a universal testing machine (Z010, Zwick/Roell) in three-point flexure with a crosshead speed of 0.75 mm/min. The fracture stress, f , was calculated by the formula (according to ISO 4049:2009): f =
3Fl 2wh2
F is the fracture load, l is the roller span (here 20 mm), w is the width and h is the height of the bar. The static elastic modulus was calculated from the threepoint bending results by the formula: Ef =
Fl3 4wh3 d
d is the deflection corresponding to load F.
2.6.
Fig. 5 – Fracture load test set up. The crowns were loaded axially to the implant using a tin foil for stress distribution and a steel ball.
Indirect tensile strength and compressive strength
Indirect tensile strength and compressive strength of the cements were measured with cylindrical test specimens 3 mm in height and diameter. A Teflon mold (Distrelec) was used to produce the test specimens. The cement was filled into the respective cavities of the Teflon mold and kept in place with a
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Fig. 6 – Fracture load values of as-machined and etched feldspar ceramic and hybrid ceramic molar crowns cemented to zirconia implants with varying cements.
plastic foil and a glass plate on each side. After 1 h storage in distilled water at 37 ◦ C the specimens were carefully removed from the mold and stored in water at 37 ◦ C for another 24 h. Specimens with visible bubbles or surface defects were eliminated. Specimens of each cement were produced and loaded until fracture (Z020, Zwick/Roell). Ten specimens were produced to measure the indirect tensile strength and loaded perpendicular whereas another 10 specimens of the respective material were loaded parallel to the cylinder axis to determine compressive strength. Cross-head speed was set to 1 mm/min. The indirect tensile strength was calculated using the following equation: t =
2F dh
t is the indirect tensile strength; F is the fracture load; d is the diameter; h is the height. For calculating the compressive strength the following equation was used: c =
F A
c is the compressive strength; F is the fracture load; A is the cross section.
2.7.
Statistical analysis
The interaction between crown material, intaglio surface treatment and cement type was tested with two-way ANOVA (p = 0.05) (Minitab 16, Minitab, Coventry, UK).
3.
Results
3.1.
Fracture load
All crowns fractured during the test under a load ranging from 249 to 1239 N, forming 2–5 fragments. No damage was detected at the implant or the epoxy resin block. The hybrid material (VE) exhibited significantly higher fracture loads than the feldspar ceramic (VM) for the respective cement (p = 0.037). The highest values were achieved with VE machined and MLA (1253 ± 148 N) and the lowest values for VM etched without cement (249 ± 58 N) (Fig. 6 and Table 3). In no group etching
Table 3 – Means and standard deviations of fracture loads for different crown materials, intaglio surface treatments (m = machined, e = etched) and respective cements. (N)
No cement
HIS
Mean
SD
Mean
PSA
MCE
MLA
SD
Mean
SD
Mean
SD
Mean
SD
VM
m e
259.6 248.7
51.9 58.2
682.8 673.0
117.0 127.8
974.6 956.7
161.2 127.5
1034.6 1009.1
108.0 90.1
1090.9 1097.0
116.7 109.6
VE
m e
505.8 516.3
153.6 75.3
871.3 860.4
40.2 66.0
1133.4 1130.9
102.4 134.5
1161.0 1151.0
138.0 71.0
1253.0 1162.0
147.9 105.0
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Fig. 7 – Correlation between fracture load of machined and etched crowns.
Fig. 8 – Correlation between fracture load of feldspar ceramic crowns and hybrid ceramic crowns.
3.3. improved the fracture load, irrespective of crown material (p = 0.915) or cement type (p = 0.973) used. Machined and etched specimens showed similar results within the respective material combination and therefore, the values were strongly correlated (Fig. 7). For that reason, both machined and etched specimens were summarized to one group when assessing the impact of the cements on the results. Compared to the non-cemented specimens, the fracture load for both VE and VM increased significantly when the semi-permanent cement (HIS) was used (p = 0.000). A further significant gain in fracture load was achieved, when HIS was substituted by selfadhesive (PSA, MCE) or adhesive cement (MLA) (p = 0.000). No significant difference was found between PSA, MCE and MLA (p = 0.005) even though a slight gain in fracture loads from PSA to MCE to MLA was observed (Fig. 6 and Table 3). The fracture load values of feldspar and hybrid ceramic crowns were highly correlated (Fig. 8).
3.2. load
Cement characteristics and correlations to fracture
Flexure strength, elastic modulus, indirect tensile strength and compressive strength values of the cements are listed in Table 4. A correlation was found between compressive strength of the cements and the fracture load of cemented crowns (Fig. 9). The non-cemented fracture load results are included in Fig. 9 at 0 MPa compressive strength.
Fractography
The specimens (VM and VE) failed by fracturing into a different number of fragments. No correlation was found between the number of fragments and the material combinations. The cement was sticking either to the implant, to the crown, or both, regardless of the crown material and cement that was used. The fracture initiation for un-cemented crowns (both VE and VM, Fig. 10a and b) starts at the occlusal surface in contact with the steel ball. Fig. 10a demonstrates second fracturing events occurring during fracture test progress, starting from the contact of the intaglio crown surface with the implant. Further, second fracturing events occur from the margins of the crowns (Fig. 10a and b, right side). The VE crown cemented with HIS (Fig. 10c) exhibits, similar to the un-cemented crown (Fig. 10a), a multiple fracture event pattern. When using self-adhesive or adhesive cements, the fracture for VE (Fig. 10e) was initiated by the steel ball and started from the loading point. Cemented VM crowns (Fig. 10d and f) fractured from the occlusal contact point.
4.
Discussion
The present study was designed to show how the new hybrid ceramic material performs within an implant system and in what way cements influence the fracture load of crowns. The first hypothesis that the fracture load values of hybrid ceramic
Table 4 – Flexure strength, elastic modulus, compressive strength and indirect tensile strength of the cements. Material HIS PSA MCE MLA
Flexure strength (MPa) 4.6 ± 0.4 93.2 ± 14.7 64.4 ± 16.0 119.3 ± 15.2
Elastic modulus (GPa) 0.04 ± 0.00 6.09 ± 0.83 6.15 ± 1.27 8.53 ± 0.60
Compressive strength (MPa) 37.1 ± 7.0 254.2 ± 6.4 288.6 ± 10.5 321.0 ± 9.3
Indirect tensile strength (MPa) 5.2 ± 0.8 35.8 ± 5.2 56.5 ± 4.5 51.3 ± 1.7
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Fig. 9 – Correlation between fracture load of the crowns and compressive strength of the cements.
crowns are higher than those of feldspar ceramic crowns was confirmed. A correlation between mechanical properties of the cements and fracture load of the crowns was only found for the compressive strength. Hence, the compressive strength of the cement has an impact on the strength of a feldspar or hybrid ceramic crown cemented on a zirconia implant. In this respect the second hypothesis that mechanical properties of the cement influence fracture load values of feldspar and hybrid ceramic crowns was confirmed for the compressive strength.
4.1.
Test design
Since only static but no dynamic load was applied and no longterm humid environment or thermal stress was simulated the test design of this study may only to a certain extent be considered as a correct simulation of a clinical situation. Also the use of a steel ball to apply the load to the crown instead of a more anatomy related material and shape might have been a limiting factor. Furthermore, the cements were not submitted to light curing which could also influence the fracture load since the curing mode has an effect on the mechanical properties of the cements [23]. Chemical curing of the cements was chosen in this study to eliminate variations in polymerization due to light curing. Unfortunately, currently no standard method exists for testing the fracture load of ceramic crowns [24]. The test method used in the current study showed reasonable and reproducible results with a low variance in values. The strong correlation between fracture load values of feldspar crowns and hybrid ceramic crowns suggest that the test design yields robust data. Therefore, it may be concluded that the test design can be used as a screening test to simulate the clinical behavior of new dental materials under mechanical forces and wet conditions in a reasonable amount of time and cost.
4.2.
Effect of cementation on fracture load
For ceramics with low intrinsic strength, cementation type can have an influence on the fracture load [20]. In the present
study it was found that for both, feldspar ceramic and hybrid ceramic a significantly relevant increase of fracture load can be achieved with the use of self-adhesive or adhesive cements, compared to the results obtained after cementation with the semi-permanent cement. Even though there was no significant difference between the fracture load values of the self-adhesive and adhesive cements an increase of the fracture load due to a higher compressive strength of the cement can be observed as a general trend. Cements with a high compressive strength seem to work as stress buffer between crown and implant and therefore result in higher stability of the whole system. That may be explained by the fact that the cement fills the gap between implant abutment surface and intaglio surface of the crown thus eliminating any premature contacts between both surfaces which might lead to stress concentrations. Based on these considerations it is selfevident that cements with a higher compressive strength lead to higher fracture load values because the buffering effect is more pronounced. In the literature it is reported that MDP containing cement revealed higher microtensile bond strength to zirconia than cement without MDP [25]. Compared within the same crown material group, the cement used in this study containing MDP (PSA) showed lower fracture load values than the cements without MDP (MLA or MCE). Hence the bond strength did not seem to affect fracture load values in the same degree as the compressive strength of the cements did. Neither silanization of the intaglio surface of the crowns nor application of a ceramic primer on the implant abutment was involved in order to minimize the test parameters. Silane or ceramic primer might have an influence on the obtained fracture load values. Etching of the intaglio surface did not increase the fracture load. That observation might be explained in the way that obviously the compressive strength of the cements is crucial for the fracture resistance of the crowns and for the formation of compressive stress at the interface of crown and cement. The micro-morphology of the interface is not relevant in this test design.
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Fig. 10 – SEM micrograph of fracture origin of machined crowns (18×). (a) VE without cement; (b) VM without cement; (c) VE HIS; (d) VM HIS; (e) VE MLA; (f) VM MLA.
Due to different experimental variables used in other studies, the fracture load values found in this study cannot be compared directly to failure loads reported in the literature. Nevertheless, the values found in the present study for the feldspar ceramic with adhesive cement (1091 ± 117 N) are in the range of those presented in other studies [21,22].
4.3.
Fractography
Central occlusal contact point loading was employed in the present study to cause fractures by inducing stress on the occlusal surface which leads to several possible cracks (e.g. cone, radial, lateral cracks). Previous studies [26,27] addressed the topic of such a test configuration and the correlation to clinical failures. In contrast to the findings by Kelly [27] where
clinical failure origins are located in the bonded interface region, Fig. 10 indicates that fracture of cemented crowns started from the occlusal contact points. In clinical service, tensile stress concentration at the cementation surface of the ceramic crown seems to have a higher potential to cause damage to restorations than the load at the loading point [28–30]. In the present study this was also confirmed when no cement or semi-permanent cement was used for feldspar and hybrid ceramic. The use of a self-adhesive or adhesive cement seemed to have cushioned some tensile stress at the interface, resulting in a fracture origin at the loading point and no second event fractures. The bonded hybrid ceramic crowns revealed the separation of a thin VE plate (not visible in Fig. 10) within the fractured
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pieces. This fracture patterns of the hybrid ceramic compared to feldspar ceramic can be explained by the microstructure and resulting higher fracture toughness of VE. As presented in a previous study [11], the interconnected networks of polymer and ceramic within VE suggest a more tortuous fracture path originating from crack deflection and bridging at the interface of the ceramic and polymer interconnected phases [11]. These toughening mechanisms absorb the energy of the advancing crack tip upon loading, resulting in a higher fracture toughness of 1.21 MPa m1/2 [12] compared to the one of a typical dental feldspar ceramic (0.92–1.12 MPa m1/2 ) [13].
4.4.
Clinical implications
The hybrid ceramic showed fracture load values higher than the ones of the feldspar ceramic, which is a reliable material in the clinic since decades [31,32]. The fracture load values for the cemented hybrid ceramic crowns were higher than maximum biting forces of 847 N for natural teeth in the molar region [6] even when using the semi-permanent cement. Self-adhesive or adhesive cements with a high compressive strength may be considered as the first choice for permanent cementation of hybrid ceramic material on zirconia implants. Most manufacturers recommend a pre-treatment of ceramic crowns with hydrofluoric acid prior to adhesive bonding. In respect of the loading capacity, the present study has proven that etching of the ceramic crowns does not lead to significantly higher fracture load values. However, it has to be considered that in the present study only static loading was applied and no silane was applied. Significant differences were found between the failure behavior created during static fracture load tests and that observed to have occurred during clinical failure of allceramic restorations [27]. In order to demonstrate a clear interaction between mechanical properties and clinical crown fracture events, a combination of tests for the individual material components and their interaction will be required [33]. The effect of the use of silane, dynamic loading, variations in load transmission, or aging might lead to differing results and for that reason will be subject of further investigations. In addition, finite element analysis might be helpful to show the stress distribution and demonstrate the crack initiation and propagation.
5.
Conclusions
In the present test design hybrid ceramic provides higher loading capacity than feldspar ceramic. The fracture load of hybrid ceramic and feldspar ceramic crowns is correlated to the compressive strength of the cement. In the test design used, etching of the intaglio surface of hybrid or feldspar ceramic crowns had no effect on the fracture load of the restoration.
Acknowledgements The authors are grateful to VITA Zahnfabrik for supporting this study with materials and to Fredy Schmidli (University
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Hospital for Dental Medicine) for the lab support. The statistical analysis was kindly performed by Marek Mrzyk, VITA Zahnfabrik.
references
[1] Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1–25. [2] Schultze-Mosgau S, Schliephake H, Radespiel-Tröger M, Neukam FW. Osseointegration of endodontic endosseous cones: zirconium oxide vs titanium. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89:91. [3] Andreiotelli M, Wenz HJ, Kohal RJ. Are ceramic implants a viable alternative to titanium implants? A systematic literature review. Clin Oral Implants Res 2009;4:32–47. [4] Borgonovo AE, Fabbri A, Vavassori V, Censi R, Maiorana C. Multiple teeth replacement with endosseous one-piece yttrium-stabilized zirconia dental implants. Med Oral Patol Oral Cir Bucal 2012;17:981–7. [5] Payer M, Arnetzl V, Kirmeier R, Koller M, Arnetzl G, Jakse N. Immediate provisional restoration of single-piece zirconia implants: a prospective case series – results after 24 months of clinical function. Clin Oral Implants Res 2013;24:569–75. [6] Waltimo A, Könönen M. A novel bite force recorder and maximal isometric bite force values for healthy young adults. Scand J Dent Res 1993;101:171–5. [7] Tortopidis D, Lyons MF, Baxendale RH, Gilmour WH. The variability of bite force measurement between sessions, in different positions within the dental arch. J Oral Rehabil 1998;25:681–6. [8] Anusavice KJ. Recent developments in restorative dental ceramics. J Am Dent Assoc 1993;124, 72–4, 76–8, 80–4. [9] McLean JW. Ceramics in clinical dentistry. Br Dent J 1988;164:310. [10] Coldea A, Swain MV, Thiel N. Mechanical properties of polymer-infiltrated-ceramic-network materials. Dent Mater 2013;29:419–26. [11] Coldea A, Swain MV, Thiel N. In-vitro strength degradation of dental ceramics and novel PICN material by sharp indentation. J Mech Behav Biomed Mater 2013;26:34–42. [12] Coldea A, Swain MV, Thiel N. Hertzian contact response and damage tolerance of dental ceramics. J Mech Behav Biomed Mater 2014;34:124–33. [13] Yu HY, Cai ZB, Ren PD, Zhu MH, Zhou ZR. Friction and wear behavior of dental feldspathic porcelain. Wear 2006;261:611–21. [14] He LH, Swain M. A novel polymer infiltrated ceramic dental material. Dent Mater 2011;27:527–34. [15] He LH, Purton D, Swain M. A novel polymer infiltrated ceramic for dental simulation. J Mater Sci Mater Med 2011;22:1639–43. [16] Christel P, Meunier A, Heller M, Torre JP, Peille CN. Mechanical properties and short-term in-vivo evaluation of yttrium-oxide-partially-stabilized zirconia. J Biomed Mater Res 1989;23:45–61. [17] Burke FJ, Watts DC. Fracture resistance of teeth restored with dentin-bonded crowns. Quintessence Int 1994;25:335–40. [18] Attia A, Abdelaziz KM, Freitag S, Kern M. Fracture load of composite resin and feldspathic all-ceramic CAD/CAM crowns. J Prosthet Dent 2006;95:117–23. [19] Borges GA, Caldas D, Taskonak B, Yan J, Sobrinho LC, de Oliveira WJ. Fracture loads of all-ceramic crowns under wet and dry fatigue conditions. J Prosthodont 2009;18:649–55. [20] Stawarczyk B, Beuer F, Ender A, Roos M, Edelhoff D, Wimmer T. Influence of cementation and cement type on the fracture
Please cite this article in press as: Rohr N, et al. Loading capacity of zirconia implant supported hybrid ceramic crowns. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.09.012
DENTAL-2628; No. of Pages 10
ARTICLE IN PRESS
xxx.e10
d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx.e1–xxx.e10
[21]
[22]
[23]
[24]
[25]
[26]
[27]
load testing methodology of anterior crowns made of different materials. Dent Mater J 2013;30:888–95. Zahran M, El-Mowafy O, Tam L, Watson PA, Finer Y. Fracture strength and fatigue resistance of all-ceramic molar crowns manufactured with CAD/CAM technology. J Prosthodont 2008;17:370–7. Shahrbaf S, van Noort R, Mirzakouchaki B, Ghassemieh E, Martin N. Fracture strength of machined ceramic crowns as a function of tooth preparation design and the elastic modulus of the cement. Dent Mater 2014;30:234–41. Ilie N, Simon A. Effect of curing mode on the micro-mechanical properties of dual-cured self-adhesive resin cements. Clin Oral Invest 2011;16:505–12. Potiket N, Chiche G, Finger IM. In vitro fracture strength of teeth restored with different all-ceramic crown systems. J Prosthet Dent 2004;92:491–5. de Souza G, Hennig D, Aggarwal A, Tam LE. The use of MDP-based materials for bonding to zirconia. J Prosthet Dent 2014;112:895–902. Lawn BR, Deng Y, Thompson VP. Use of contact testing in the characterization and design of all-ceramic crownlike layer structures: a review. J Prosthet Dent 2001;86:495–510. Kelly JR. Clinically relevant approach to failure testing of all-ceramic restorations. J Prosthet Dent 1999;81:652–61.
[28] Campos RE, Soares CJ, Quagliatto PS, Soares PV, de Oliveira Jr OB, Santos-Filho PC, Salazar-Marocho SM. In vitro study of fracture load and fracture pattern of ceramic crowns: a finite element and fractography analysis. J Prosthodont 2011;20:447–55. [29] Dong XD, Darvell BW. Stress distribution and failure mode of dental ceramic structures under Hertzian indentation. Dent Mater 2003;19:542–51. [30] Scherrer SS, Quinn GD, Quinn JB. Fractographic failure analysis of a Procera AllCeram crown using stereo and scanning electron microscopy. Dent Mater 2008;24: 1107–13. [31] Thompson JY, Bayne SC, Heymann HO. Mechanical properties of a new mica-based machinable glass ceramic for CAD/CAM restorations. J Prosthet Dent 1996;76: 619–23. [32] Federlin M, Hiller KA, Schmalz G. Controlled, prospective clinical split-mouth study of cast gold vs. ceramic partial crowns: 5.5 year results. Am J Dent 2010;23:161–7. [33] Annusavice KJ, Kakar K, Ferree N. Which mechanical and physical testing methods are relevant for predicting the clinical performance of ceramic-based dental prostheses? Clin Oral Impl Res 2007;18:218–31.
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