Journal of Oral Implantology Fracture strength of zirconia and alumina ceramic crowns supported by implants. --Manuscript Draft-Manuscript Number:
AAID-JOI-D-13-00142R3
Full Title:
Fracture strength of zirconia and alumina ceramic crowns supported by implants.
Short Title:
Fracture Strength of zirconia and alumina crowns.
Article Type:
Dental Implant Science Research
Keywords:
fracture, strength, alumina, zirconia, implant-supported prostheses
Corresponding Author:
Tonino Traini, Ph.D.,D.D.S. University of Chieti-Pescara Chieti, Chieti ITALY
Corresponding Author Secondary Information: Corresponding Author's Institution:
University of Chieti-Pescara
Corresponding Author's Secondary Institution: First Author:
Tonino Traini, Ph.D.,D.D.S.
First Author Secondary Information: Order of Authors:
Tonino Traini, Ph.D.,D.D.S. Roberto Sorrentino, Ph.D., D.D.S. Enrico Gherlone, M.D.,D.D.S. Federico Perfetti, D.D.S. Patrizio Bollero, M.D.,D.D.S. Ferdinando Zarone, M.D.,D.D.S.
Order of Authors Secondary Information: Abstract:
Due to the brittleness and limited tensile strength of the veneering glass-ceramic materials, the methods that combine strong core materials, as zirconia or alumina, is still under debate. The present study aims to evaluate the fracture strength and the mechanism of failure through fractographic analysis of single all-ceramic crowns supported by implants. Forty premolar cores were fabricated with CAD/CAM technology using alumina (n=20) and zirconia (n=20). The specimens were veneered with glass-ceramic, cemented on titanium abutments and subjected to loading test until fracture. SEM fractographic analysis was also performed. The fracture load was 1165 (±509) N for alumina and 1638 (±662) N for zirconia with a statistically significant difference between the two groups (p=0.026). Fractographic analysis of alumina-glassceramic crowns, showed the presence of catastrophic cracks of the entire thickness of the alumina core while, for the zirconia-glass-ceramic crowns, the cracks involved mainly the thickness of the ceramic veneering layer. The sandblast procedure of the zirconia core showed an influence on crack path deflection. Only few samples (n=3) showed limited micro-cracks of the zirconia core. Zirconia showed a significant higher fracture strength value in implant-supported restorations, indicating the role-played by the high resistant cores for premolar crowns.
Response to Reviewers:
Ref.: Ms. No. AAID-JOI-D-13-00142R2
Reviewer #5: Authors have revised the manuscript well. However there are some minor corrections to be done: Please revise the following sentence: Fractographic analysis showed catastrophic cracks involving the whole thickness of the Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation
13 core for alumina-glass-ceramic crowns while for the zirconia-glass-ceramic crowns it was noted an 14 extent of fracture paths involving mainly the veneering thickness. R. THE SENTENCE WAS REVISED authors did not explain how did they applied standardized load of 10 N? Please explain the following sentence "A standardized load of 10 N for 7 min was 102 used during the cement setting reaction." R. AS ALREADY REPORTED ……R: we used a universal loading machine connected to a computer, equipped with a loading-cell of maximum 500N ± 0.5% (serial 015072, LLOYD Instruments LTD) as reported in details in page 5 lines 100-112. Nevertheless, this information was added at the end of the sentence.
The title is fracture resistance but in the Results section the following sentence was written: The fracture toughness value??? (mean ± SD) was 1165±509 N for alumina and 1638±662 N 162 for zirconia, the difference of the means was 473 N. Did the authors investigated fracture resistance, strength or toughness?? R: Strength of course. The corrections were done. Please revise the following sentence: Mostly of the specimens was noted a cohesive fractures of the glass-ceramic (Fig. 34). R. The correction was done
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Article File
Fracture strength of zirconia and alumina ceramic crowns supported by implants.
Tonino Traini DDS; PhDa,b, Roberto Sorrentino DDS; PhDc, Enrico Gherlone MD; DDSa, Federico Perfetti DDSd; Patrizio Bollero MD, DDSe; Ferdinando Zarone MD; DDSb
a: Department of Dentistry, Vita Salute University, San Raffaele Hospital, Milan, Italy.; b: Department of Medical, Oral and Biotechnological Sciences, University of Chieti-Pescara, Italy. c: Department of Fixed Prosthodontics, School of Dentistry, University “Federico II”, Naples, Italy; d: Post-Graduate School of oral surgery, Università Sapienza, Rome e: Department of Oral Pathology, Universita Tor Vergata, Rome
Short title: Fracture Strength of zirconia and alumina crowns.
Corresponding author: ToninoTraini DDS; PhD Via Olgettina, 58 20132, Milano (Italy) Tel: +39-0226432921 Fax :+39-0226432953 E-mail: [email protected]
1
Abstract Due to the brittleness and limited tensile strength of the veneering glass-ceramic materials, the methods that combine strong core materials, as zirconia or alumina, is still under debate. The present study aims to evaluate the fracture strength and the mechanism of failure through fractographic analysis of single all-ceramic crowns supported by implants. Forty premolar cores were fabricated with CAD/CAM technology using alumina (n=20) and zirconia (n=20). The specimens were veneered with glass-ceramic, cemented on titanium abutments and subjected to loading test until fracture. SEM fractographic analysis was also performed. The fracture load was 1165 (±509) N for alumina and 1638 (±662) N for zirconia with a statistically significant difference between the two groups (p=0.026). Fractographic analysis of alumina-glass-ceramic crowns, showed the presence of catastrophic cracks of the entire thickness of the alumina core while, for the zirconia-glass-ceramic crowns, the cracks involved mainly the thickness of the ceramic veneering layer.
The sandblast procedure of
the zirconia core showed an influence on crack path deflection. Only few samples (n=3) showed limited micro-cracks of the zirconia core. Zirconia showed a significant higher fracture strength value in implant-supported restorations, indicating the role-played by the high resistant cores for premolar crowns.
Key Words: fracture, strength, alumina, zirconia, implant-supported prostheses
2
Introduction The increased demand for aesthetic dentistry by patients has led to a wide request for allceramic restorations in either posterior or anterior regions.
1
As consequence, metal-free
crowns have become popular, also for implant-supported restorations, due to their excellent biocompatibility reported especially for single missing tooth. 2 The long-term success of allceramic fixed partial dentures (FPDs) is well established in natural dentition, however the risk of failure for the implant-supported prosthesis is less certain. Pjetursson et al 3 reported a significantly lower risk after 5-year of ceramic fracture for tooth-supported versus implant supported FPDs (2.9% versus 8.8%). Kinsel and Lin 4 reported a fivefold increase of failure in implant-supported as compared to tooth-supported ceramic restorations. The absence around osseointegrated implants of periodontal tissue and of neurological reflex mechanism, as protective mechanism against the masticatory force, offers possible explanations for differences in failure rate among restorations involving natural dentition and implants. 5-11 In implant dentistry, the key factor for long-term success of the restorations is the stress distribution and the load transferring to the surrounding bone.
12
With this in mind, the
implant’s abutment plays an important role on transferring occlusal forces to the implant, and mechanical properties of the material of the abutment may influence the stress of implantsupported restorations. The maximum tensile stress within the all-ceramic crown was higher when titanium abutment was used. 13 Adell et al 14 indicated that an all-ceramic crown placed on titanium abutment, went more likely to a fracture than that placed on ceramic abutment. While the resistance to fracture of all-ceramic restorations implant-supported has been investigated mainly on anterior teeth 15,16 little is known about the performance of all-ceramic restorations
prepared
through
computer-aided
design/computer-aided
manufacturing
(CAD/CAM) technology for premolar teeth. On the other hand, the premolar regions belong to highly aesthetic locations in the dental arch, especially for patients with a high lip line. 3
The implant-supported single-tooth restorations of these regions are subject to higher stress than that of anterior region. The occlusal force in adults is in mean of about 100 N higher in the premolar region than in the anterior ones.
17
It has been shown that under loading
conditions ceramic restorations accumulate damage; the accumulated damage weakens the ceramic restoration and can cause clinical failures.
18-20
The ceramics are brittle materials
because of atomic bonds that do not allow the atomic planes, to slide when subjected to loading, thus, the ceramics cannot withstand a deformation of >0.1% without fractures.
21
The formation of microscopic flaws during the laboratory processing or the clinical function, such as voids or cracks, can lead to the failure of dental ceramics; moreover, the load's failure of all-ceramic systems are influenced by prosthesis's geometry, size, and location of flaws. 21 New ceramic materials cores and manufacturing process (CAD/CAM) were developed to improve the resistance of cracks propagation, essentially introducing crystalline structure materials. Fifteen years ago, the introduction of CAD/CAM techniques, allowed the use in dentistry of high purity industrially manufactured alumina.
22,23
CAD/CAM also makes it possible to
store all production steps electronically, and attain good reproducibility, accuracy, and precision.
Subsequent to the CAM step, the alumina cores were densely sintered and
veneered with glass-ceramic, to create the appearance of a natural tooth.
24
Clinical studies
have indicated that such alumina crowns may be used for restorations in all locations of the oral cavity.
25,26
Yet, the best mechanical properties of all the dental ceramics are attained
with yttrium-stabilized zirconium dioxide (zirconia).
27,28
Zirconia is an intermetallic
compound, with a high mechanical resistance due to its tetragonal crystalline structure that allows the material, to undergo locally a transformation under critical load, limiting the propagation of the micro cracks.
29,30
The transformation toughening leads to a high initial
strength and resistance to cracks, results in an excellent durability for zirconia frameworks. 31 4
Zirconia or alumina used as cores for the tooth restorations forms a “laminate system”, with several layers: the abutment material; the cement; the core and the glass-ceramic veneering material. When a complex multi-layered system is subjected to occlusal load, it undergoes to a fatigue stress inside the material's bulk, and theirs interfaces. As response to such stress, the clinical performance differs as function of the material's system adopted. The aims of the present study were of to compare the fracture strength of the premolar's crowns of zirconia and alumina cemented over implant abutments and to evaluate the mechanism of failure through fractographic analysis. Therefore, we tested the hypothesis that there is no difference between the fracture strength of the premolar crowns with alumina or zirconia cores. Materials and methods Specimens’ preparation Forty identical single premolar-crown copings were prepared through CAD / CAM technology on titanium standard ITI abutments (Institut Straumann AG, Zurigo, Svizzera) (Fig. 1). Twenty crowns were made in pure alumina using Nobel ProceraTM Alumina (Nobel Biocare AB, Göteborg, Sweden) whereas the other twenty crowns were made of three-mole % Y2O3 doped zirconia, yielding a predominantly tetragonal fine-grained microstructure. YTZP, Lava™, (3M ESPE, Seefeld, Germany). Prior glass-ceramic firing, alumina and zirconia cores were sandblasted with 110 μm of Al2O3 under two bars of pressure at 2.5 cm of distance from the coping surface. All the copings were veneered by an experienced master dental technician in a dental laboratory using Vita VM 7 (VITA Zahnfabrik H. Rauter GmbH & Co.KG, Bad Säckingen, Germany) over alumina copings and
Lava Ceram Overlay
Porcelain (3M ESPE, Seefeld, Germany) over the zirconia copings. All firing cycles were made according to the manufacturer’s recommendations in a calibrated porcelain furnace (Flagship VPF Jelenko, New York, NY, USA). 5
The furnace was
automatically calibrated using the appropriate device as recommended by the manufacturer. In the last step, the crowns were autoglazed. Cementation The crowns were cemented by means of dual cure self-adhesive resin cement (Relyx Unicem, 3M ESPE, St. Paul, MN, USA) according to the manufacturer’s instructions on the standard ITI abutments (Institut Straumann AG, Zurigo, Svizzera). A standardized load of 10 N for 7 min was used during the cement setting reaction by means of a universal loading machine (Lloyd 30K, Lloyd Instruments Ltd. Segensworth, UK). Excess cement was removed, and the crowns were stored in distilled water with a temperature of 37° C until they were subjected to mechanical test. Compressive loading tests All the samples were ensured into the holding device of a universal loading machine (Lloyd 30K, Lloyd Instruments Ltd. Segensworth, UK) to perform compressive loading tests under static condition until the fracture of the specimens occurs in order to assess the maximum load resistance and the fracture mechanisms. A controlled load at a crosshead speed of 1mm/min was applied by means of a stainless steel rod with a spherical tip of 7 mm of diameter, in order to simulate an occlusal load. The applied force was parallel to the longitudinal axis of the specimens and acted at level of the central fossa, 2 mm from the tip of the supporting cusp (Fig. 2). All samples were loaded from 0 Newton (N) till fracture. The load fracture was recorded in N by means of a computer connected to the loading machine, using a specific measurement software (Nexigen vers. 4.0 issue 23 Lloyd Instruments Ltd. Segensworth, UK ). Both maximum load and work at maximum load were evaluated. Scanning electron fractographic analysis 6
To highlight the influence of the sandblasting procedure, on the fracture strength at the interface between glass-ceramic and zirconia some additional zirconia copings (n=3) were prepared. The copings were sandblasted as reported in specimen’s preparation section except for the following: only one side of the vestibular cusp (occlusal wall) was sandblasted, while the buccal wall of the vestibular cusp was left as delivered by milling center. The additional specimens were veneered with glass-ceramic as reported in the specimen preparation section. The fracture test was made applying the load on the tip of vestibular cusps. All the experimentally fractured specimens were coated with a very thin layer of gold by vacuum evaporation using a Techniques Hummer II-Au-sputtering (Techniques inc, Virginia, USA) and observed with a scanning electron microscope (SEM) (Cambridge Stereoscan 200, Cambridge Instrument Company Ltd. , Cambridge, England.) equipped with tetra solid-state detector for back scattered electrons (BSE ). The fracture surfaces were analyzed starting from the starting point of the fracture, when well identified, until to the arrest point of the cleavage plane.. Type and location of initiating flaws, such as porosity, surface defects, were also evaluated if present. The fractographic patterns commonly observed in brittle fractures were evaluated as follows: the mirror area, that appears as a flat shiny area; the mist area, that appears to be slightly rougher; the hackle area, ridges and grooves radiating from the starting point as well as layers resulting from branching of the fracture path. Different magnifications were used to analyze the fractures because some patterns were easily recognized at higher magnifications and others were more apparent at lower magnifications. To assess the accuracy of the method the same operator performed the analyses twice. Statistical Analysis All the data were analyzed by means of the computerized statistical package (Sigma Stat 3.5, SPSS inc. Ekrath, Germany). The mean fracture loads of alumina and zirconia crowns were 7
compared using the Student t-test with a confidence interval of 95%. Statistical differences were considered significant for a P value of < 0.05. Results Fracture strength The fracture strength value (mean ± SD) was 1165±509 N for alumina and 1638±662 N for zirconia; the difference of the means was 473 N. The statistical analysis showed a significant difference between the two groups (p = 0.026) (table 1). Scanning electron fractographic analysis Most of the specimens were noted a cohesive fractures of the glass-ceramic (Fig. 3). The alumina and zirconia glass-ceramic bond was always stronger than the cohesive forces of the glass-ceramics itself (Fig. 4), in fact the main fracture path propagated through the glassceramic body. In both the experimental groups, the core-glass-ceramics interface was able to transfer most of all the energy accumulated by the system under load to the bulk of the glassceramic, that annihilate it by fracture. In details, for alumina glass-ceramic restorations, mostly of micro cracks were generated on the occlusal surface at the level of the supporting cusp, where the arbitrary load was applied; then, they easily propagated into the bulk of the veneering material and reached the prosthetic copings (the cores), causing a catastrophic fracture of the core (Fig. 3 a and a1). On the contrary, in the zirconia glass-ceramic restorations the fractures were concentrated at the level of the supporting cusp with a limited extent of the fracture path. Since the cleavage fracture appears stopped by the prosthetic copings (the cores) surface with only small partial micro cracks that involves the zirconia core (Fig. 3 b and b1).
8
Analyzing the facture's pattern, from the energy point of view, in the alumina specimens emerges that when the cleavage plane (plane of fracture) reaches the surface of the core (extended mirror area) it conserves an high energy level as much as it is sufficient for core's fracture, without any deflection (hackle area) of the fracture's path (Fig. 4 a and a1). On the other hand, the zirconia core seems to be able to best dissipate the occlusal load (energy) since, a catastrophic fracture of the core was not present, vice versa only few micro crack on the occlusal area of the copings were noted. The energy of the cleavage plane, in this case, appears to be mostly annihilated by the sequence of mirror, mist and hackle areas of fractures inside the glass-ceramic layer so when it reaches the zirconia core surface the residual energy is able to produce just micro cracks in the core (Fig. 4 b and b1). Only limited areas of zirconia framework appeared uncovered by glassceramic (Fig. 4 b and b1). The vitreous phase of the ceramics satisfactorily imbibed the zirconia structure, confirming the effectiveness of the core-glass-ceramics bond also under high load condition. Moreover, the micro texture (sandblasting) of the zirconia core appeared to influences the behavior of the cleavage plane on zirconia specimens (Fig. 5). The evaluation of additional specimens with double surface treatments (rectangle 1 sandblasted and rectangle 2 as received by milling center in Fig. 5 a) showed different fractographic patterns after fracture. The sandblasted surface (Fig. 5 b) appeared to be able to influence the cleavage plane development with arrest lines until to 10 μm from the core surface while outside this frontier wake hackle area was evident. Any detachment line was noted at zirconia/glass-ceramic interface. The zirconia core surface as received by milling centre (Fig. 5 c) showed
arrest
lines at the zirconia/glass-ceramic interface with wake hackle area in proximity of the interface. A detachment line was seen at the interface.
9
As for the microstructure of alumina, high magnification SEM images showed a polycrystalline structure, made up of aluminum oxide sintered grains; a very few postsinterization voids were evident, whose dimension reached a maximum of about 330 nm. The dimension of the grains (mean ± SD) was of 1.77 ± 0.54 μm with a polygonal appearance (Fig. 6 a and a1). The microstructure of 3 mol%Y-TZP zirconia sintered at 1450° C for 2 h showed a mean (± SD) grain sizes of 0.45 ± 0.15 μm with round appearance (Fig. 6 b and b1). Discussion The results of the present study sustain the rejection of the null hypothesis because the fracture strength, between the two groups, showed a difference of 473 N that was statistically significant (p = 0.026). The results revealed also that the breaking strength was higher for glass-ceramic/zirconia. A possible explanation was supported by the fractographic analysis. In implant-supported prosthetic restorations, the most stressful regions are the interfaces, such as the contact area between core and porcelain. The effectiveness of the alumina/ and zirconia/glass-ceramics bond is influenced by the different physical characteristics of both veneering glass-matrix and crystalline core ceramics.
22,32,33
As reported in the scientific literature, a good mechanical
resistance to compression characterizes porcelain but it is negatively affected by shearflexural stresses.
21,33
Such stresses might arise in the occlusal third and at the level of the
cervical margin of the crown, sometimes causing a local failure of the core/glass-ceramic bond and the detachment of porcelain. The present findings on fractographic analysis are in accordance with previously reported observation
34
in which the cleavage planes pass always through areas with low fracture
10
strength, presenting different fracture patterns in the veneering ceramics, alumina and zirconia. The deflection of the crack growth path was evident in both groups since the crack lines changed their direction following the profile of the copings, in order to annihilate the energy as much as possible (Fig. 3 a and b). Moreover, it was associated to the change of the force direction vector from the compressive stress in the occlusal area, to the shearing stress of the axial regions. The explanation involves the local stress level that exceeded the cohesive resistance of the glass-ceramics in all the specimens, breaking the atomic bonds. Yet, due to surface tension, ceramic structures tend to fail catastrophically, where cracks propagate by slow growth from a point where the applied load exceeds the material resistance. If a die made of a high modulus material supports a crown, the fracture strength will increase dramatically compared with that of crowns supported by a low modulus material. In fracture mode, however, the zirconia cores seemed to be more resistant compared with alumina ones, as significantly more of the fractured alumina crowns were totally fractured. This could imply that the zirconia core resists higher loads than alumina ones but that the veneer porcelain fractures at a lower load. A ceramic laminate will always form a constant strain system because of a mismatch of the modulus of elasticity across the core–veneer interface. an important source of structural flaws
33
33-36
The interface is
because of wettability factors such as
microstructural roughness (or sandblasting treatment), which can influence the coupling degree of glass-ceramic during both the ceramic buildup and the subsequent ceramic firing procedures. In this study, different surface properties of the two core materials – causing difficulties in building up a dense and homogenous layer of green porcelain, without trapping air bubbles, over the core surface prior to firing.
11
Veneer fractures often occur during interfacial stresses
35,36
or because microstructural
regions in the porcelain are mechanically defective. Such microstructural flaws include porosities, agglomerates, inclusions and large grained zones. 37 Conclusions Within the limits of this study, basing on the rejection of the null hypothesis, it was possible to state that the glass-ceramic/zirconia had a fracture strength value significantly higher than that of the glass-ceramic/alumina. In addition, both systems of restoration when supported by implants achieved higher values of breaking strength, that the functionally load of the premolar region. Finally, the sandblasting procedure of zirconia core showed to be effective for improving adhesion at the interface between zirconia and ceramic. Acknowledgements We wish to thank Mr. Giuseppe Zuppardi CDT-MDT for his dental laboratory support. We thank Michela Marroni PhD for her assistance with the English revision Conflict of interest All the authors involved in the present research declare any possible conflict of interest.
12
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Table Table 1 – Unpaired t-test: fracture load (Newtons) (SD=standard deviation).
n
Mean (SD)
SEM
Alumina
20
1165 (±509)
169
Zirconia
20
1638 (±662)
160
95% confidence interval for difference: -95.22 to 847.2 t = 1.615 with 38 degrees of freedom; p = 0.026
17
Captions to Figures Fig. 1 (a) the standard ITI abutment analogs. (a1) alumina and zirconia CAD/CAM core copings . (a2) alumina and zirconia cores positioned on the ITI abutments. Fig. 2 specimens under compressive loading tests. Fig. 3 SEM images of copings after fracture. Alumina-ceramic specimen (a) and (a1). Zirconia-ceramic specimen (b) and (b1). White arrows indicate the core microcrack. Fig.4 SEM images of the fracture plane. Alumina specimens (a) with secondary electrons and (a1) with back scattered electrons. Zirconia (b) with secondary electrons and (b1) with back scattered electrons. Fig. 5 SEM images for different surface treatments. (a) sagittal section of coping treated with sandblasting in the area of rectangle (1) and as received by milling center in the area of rectangle (2). (b) fractographic evaluation of the rectangle(1) white arrows indicate the absence of a detachment line at the ceramic-zirconia interface. (c)
fractographic
evaluation of the rectangle (2) white arrows indicate the presence of a detachment line at the ceramic-zirconia interface. Black arrows indicate wake hackle reaching the zirconia-ceramic interface. While an arrest line was just associated to a roughness area of the zirconia surface. Fig. 6. (a) Scanning electron microscopy images of alumina. The mean grain size was of 1.77 (± 0.54) μm with a polygonal shape. (b) Scanning electron microscopy images of 3 mol%Y-TZP ceramics: (b1) The mean grain size was of 0.45 (± 0.15)
μm
approximated with the radius of the circle, which has the same section area as the area of the grains. sintered at 1450° C for 2 h,
18
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AAID-JOI-D-13-00142R3
Full Title:
Fracture strength of zirconia and alumina ceramic crowns supported by implants.
Short Title:
Fracture Strength of zirconia and alumina crowns.
Article Type:
Dental Implant Science Research
Keywords:
fracture, strength, alumina, zirconia, implant-supported prostheses
Corresponding Author:
Tonino Traini, Ph.D.,D.D.S. University of Chieti-Pescara Chieti, Chieti ITALY
Corresponding Author Secondary Information: Corresponding Author's Institution:
University of Chieti-Pescara
Corresponding Author's Secondary Institution: First Author:
Tonino Traini, Ph.D.,D.D.S.
First Author Secondary Information: Order of Authors:
Tonino Traini, Ph.D.,D.D.S. Roberto Sorrentino, Ph.D., D.D.S. Enrico Gherlone, M.D.,D.D.S. Federico Perfetti, D.D.S. Patrizio Bollero, M.D.,D.D.S. Ferdinando Zarone, M.D.,D.D.S.
Order of Authors Secondary Information: Abstract:
Due to the brittleness and limited tensile strength of the veneering glass-ceramic materials, the methods that combine strong core materials, as zirconia or alumina, is still under debate. The present study aims to evaluate the fracture strength and the mechanism of failure through fractographic analysis of single all-ceramic crowns supported by implants. Forty premolar cores were fabricated with CAD/CAM technology using alumina (n=20) and zirconia (n=20). The specimens were veneered with glass-ceramic, cemented on titanium abutments and subjected to loading test until fracture. SEM fractographic analysis was also performed. The fracture load was 1165 (±509) N for alumina and 1638 (±662) N for zirconia with a statistically significant difference between the two groups (p=0.026). Fractographic analysis of alumina-glassceramic crowns, showed the presence of catastrophic cracks of the entire thickness of the alumina core while, for the zirconia-glass-ceramic crowns, the cracks involved mainly the thickness of the ceramic veneering layer. The sandblast procedure of the zirconia core showed an influence on crack path deflection. Only few samples (n=3) showed limited micro-cracks of the zirconia core. Zirconia showed a significant higher fracture strength value in implant-supported restorations, indicating the role-played by the high resistant cores for premolar crowns.
Response to Reviewers:
Ref.: Ms. No. AAID-JOI-D-13-00142R2
Reviewer #5: Authors have revised the manuscript well. However there are some minor corrections to be done: Please revise the following sentence: Fractographic analysis showed catastrophic cracks involving the whole thickness of the Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation
13 core for alumina-glass-ceramic crowns while for the zirconia-glass-ceramic crowns it was noted an 14 extent of fracture paths involving mainly the veneering thickness. R. THE SENTENCE WAS REVISED authors did not explain how did they applied standardized load of 10 N? Please explain the following sentence "A standardized load of 10 N for 7 min was 102 used during the cement setting reaction." R. AS ALREADY REPORTED ……R: we used a universal loading machine connected to a computer, equipped with a loading-cell of maximum 500N ± 0.5% (serial 015072, LLOYD Instruments LTD) as reported in details in page 5 lines 100-112. Nevertheless, this information was added at the end of the sentence.
The title is fracture resistance but in the Results section the following sentence was written: The fracture toughness value??? (mean ± SD) was 1165±509 N for alumina and 1638±662 N 162 for zirconia, the difference of the means was 473 N. Did the authors investigated fracture resistance, strength or toughness?? R: Strength of course. The corrections were done. Please revise the following sentence: Mostly of the specimens was noted a cohesive fractures of the glass-ceramic (Fig. 34). R. The correction was done
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Article File
Fracture strength of zirconia and alumina ceramic crowns supported by implants.
Tonino Traini DDS; PhDa,b, Roberto Sorrentino DDS; PhDc, Enrico Gherlone MD; DDSa, Federico Perfetti DDSd; Patrizio Bollero MD, DDSe; Ferdinando Zarone MD; DDSb
a: Department of Dentistry, Vita Salute University, San Raffaele Hospital, Milan, Italy.; b: Department of Medical, Oral and Biotechnological Sciences, University of Chieti-Pescara, Italy. c: Department of Fixed Prosthodontics, School of Dentistry, University “Federico II”, Naples, Italy; d: Post-Graduate School of oral surgery, Università Sapienza, Rome e: Department of Oral Pathology, Universita Tor Vergata, Rome
Short title: Fracture Strength of zirconia and alumina crowns.
Corresponding author: ToninoTraini DDS; PhD Via Olgettina, 58 20132, Milano (Italy) Tel: +39-0226432921 Fax :+39-0226432953 E-mail: [email protected]
1
Abstract Due to the brittleness and limited tensile strength of the veneering glass-ceramic materials, the methods that combine strong core materials, as zirconia or alumina, is still under debate. The present study aims to evaluate the fracture strength and the mechanism of failure through fractographic analysis of single all-ceramic crowns supported by implants. Forty premolar cores were fabricated with CAD/CAM technology using alumina (n=20) and zirconia (n=20). The specimens were veneered with glass-ceramic, cemented on titanium abutments and subjected to loading test until fracture. SEM fractographic analysis was also performed. The fracture load was 1165 (±509) N for alumina and 1638 (±662) N for zirconia with a statistically significant difference between the two groups (p=0.026). Fractographic analysis of alumina-glass-ceramic crowns, showed the presence of catastrophic cracks of the entire thickness of the alumina core while, for the zirconia-glass-ceramic crowns, the cracks involved mainly the thickness of the ceramic veneering layer.
The sandblast procedure of
the zirconia core showed an influence on crack path deflection. Only few samples (n=3) showed limited micro-cracks of the zirconia core. Zirconia showed a significant higher fracture strength value in implant-supported restorations, indicating the role-played by the high resistant cores for premolar crowns.
Key Words: fracture, strength, alumina, zirconia, implant-supported prostheses
2
Introduction The increased demand for aesthetic dentistry by patients has led to a wide request for allceramic restorations in either posterior or anterior regions.
1
As consequence, metal-free
crowns have become popular, also for implant-supported restorations, due to their excellent biocompatibility reported especially for single missing tooth. 2 The long-term success of allceramic fixed partial dentures (FPDs) is well established in natural dentition, however the risk of failure for the implant-supported prosthesis is less certain. Pjetursson et al 3 reported a significantly lower risk after 5-year of ceramic fracture for tooth-supported versus implant supported FPDs (2.9% versus 8.8%). Kinsel and Lin 4 reported a fivefold increase of failure in implant-supported as compared to tooth-supported ceramic restorations. The absence around osseointegrated implants of periodontal tissue and of neurological reflex mechanism, as protective mechanism against the masticatory force, offers possible explanations for differences in failure rate among restorations involving natural dentition and implants. 5-11 In implant dentistry, the key factor for long-term success of the restorations is the stress distribution and the load transferring to the surrounding bone.
12
With this in mind, the
implant’s abutment plays an important role on transferring occlusal forces to the implant, and mechanical properties of the material of the abutment may influence the stress of implantsupported restorations. The maximum tensile stress within the all-ceramic crown was higher when titanium abutment was used. 13 Adell et al 14 indicated that an all-ceramic crown placed on titanium abutment, went more likely to a fracture than that placed on ceramic abutment. While the resistance to fracture of all-ceramic restorations implant-supported has been investigated mainly on anterior teeth 15,16 little is known about the performance of all-ceramic restorations
prepared
through
computer-aided
design/computer-aided
manufacturing
(CAD/CAM) technology for premolar teeth. On the other hand, the premolar regions belong to highly aesthetic locations in the dental arch, especially for patients with a high lip line. 3
The implant-supported single-tooth restorations of these regions are subject to higher stress than that of anterior region. The occlusal force in adults is in mean of about 100 N higher in the premolar region than in the anterior ones.
17
It has been shown that under loading
conditions ceramic restorations accumulate damage; the accumulated damage weakens the ceramic restoration and can cause clinical failures.
18-20
The ceramics are brittle materials
because of atomic bonds that do not allow the atomic planes, to slide when subjected to loading, thus, the ceramics cannot withstand a deformation of >0.1% without fractures.
21
The formation of microscopic flaws during the laboratory processing or the clinical function, such as voids or cracks, can lead to the failure of dental ceramics; moreover, the load's failure of all-ceramic systems are influenced by prosthesis's geometry, size, and location of flaws. 21 New ceramic materials cores and manufacturing process (CAD/CAM) were developed to improve the resistance of cracks propagation, essentially introducing crystalline structure materials. Fifteen years ago, the introduction of CAD/CAM techniques, allowed the use in dentistry of high purity industrially manufactured alumina.
22,23
CAD/CAM also makes it possible to
store all production steps electronically, and attain good reproducibility, accuracy, and precision.
Subsequent to the CAM step, the alumina cores were densely sintered and
veneered with glass-ceramic, to create the appearance of a natural tooth.
24
Clinical studies
have indicated that such alumina crowns may be used for restorations in all locations of the oral cavity.
25,26
Yet, the best mechanical properties of all the dental ceramics are attained
with yttrium-stabilized zirconium dioxide (zirconia).
27,28
Zirconia is an intermetallic
compound, with a high mechanical resistance due to its tetragonal crystalline structure that allows the material, to undergo locally a transformation under critical load, limiting the propagation of the micro cracks.
29,30
The transformation toughening leads to a high initial
strength and resistance to cracks, results in an excellent durability for zirconia frameworks. 31 4
Zirconia or alumina used as cores for the tooth restorations forms a “laminate system”, with several layers: the abutment material; the cement; the core and the glass-ceramic veneering material. When a complex multi-layered system is subjected to occlusal load, it undergoes to a fatigue stress inside the material's bulk, and theirs interfaces. As response to such stress, the clinical performance differs as function of the material's system adopted. The aims of the present study were of to compare the fracture strength of the premolar's crowns of zirconia and alumina cemented over implant abutments and to evaluate the mechanism of failure through fractographic analysis. Therefore, we tested the hypothesis that there is no difference between the fracture strength of the premolar crowns with alumina or zirconia cores. Materials and methods Specimens’ preparation Forty identical single premolar-crown copings were prepared through CAD / CAM technology on titanium standard ITI abutments (Institut Straumann AG, Zurigo, Svizzera) (Fig. 1). Twenty crowns were made in pure alumina using Nobel ProceraTM Alumina (Nobel Biocare AB, Göteborg, Sweden) whereas the other twenty crowns were made of three-mole % Y2O3 doped zirconia, yielding a predominantly tetragonal fine-grained microstructure. YTZP, Lava™, (3M ESPE, Seefeld, Germany). Prior glass-ceramic firing, alumina and zirconia cores were sandblasted with 110 μm of Al2O3 under two bars of pressure at 2.5 cm of distance from the coping surface. All the copings were veneered by an experienced master dental technician in a dental laboratory using Vita VM 7 (VITA Zahnfabrik H. Rauter GmbH & Co.KG, Bad Säckingen, Germany) over alumina copings and
Lava Ceram Overlay
Porcelain (3M ESPE, Seefeld, Germany) over the zirconia copings. All firing cycles were made according to the manufacturer’s recommendations in a calibrated porcelain furnace (Flagship VPF Jelenko, New York, NY, USA). 5
The furnace was
automatically calibrated using the appropriate device as recommended by the manufacturer. In the last step, the crowns were autoglazed. Cementation The crowns were cemented by means of dual cure self-adhesive resin cement (Relyx Unicem, 3M ESPE, St. Paul, MN, USA) according to the manufacturer’s instructions on the standard ITI abutments (Institut Straumann AG, Zurigo, Svizzera). A standardized load of 10 N for 7 min was used during the cement setting reaction by means of a universal loading machine (Lloyd 30K, Lloyd Instruments Ltd. Segensworth, UK). Excess cement was removed, and the crowns were stored in distilled water with a temperature of 37° C until they were subjected to mechanical test. Compressive loading tests All the samples were ensured into the holding device of a universal loading machine (Lloyd 30K, Lloyd Instruments Ltd. Segensworth, UK) to perform compressive loading tests under static condition until the fracture of the specimens occurs in order to assess the maximum load resistance and the fracture mechanisms. A controlled load at a crosshead speed of 1mm/min was applied by means of a stainless steel rod with a spherical tip of 7 mm of diameter, in order to simulate an occlusal load. The applied force was parallel to the longitudinal axis of the specimens and acted at level of the central fossa, 2 mm from the tip of the supporting cusp (Fig. 2). All samples were loaded from 0 Newton (N) till fracture. The load fracture was recorded in N by means of a computer connected to the loading machine, using a specific measurement software (Nexigen vers. 4.0 issue 23 Lloyd Instruments Ltd. Segensworth, UK ). Both maximum load and work at maximum load were evaluated. Scanning electron fractographic analysis 6
To highlight the influence of the sandblasting procedure, on the fracture strength at the interface between glass-ceramic and zirconia some additional zirconia copings (n=3) were prepared. The copings were sandblasted as reported in specimen’s preparation section except for the following: only one side of the vestibular cusp (occlusal wall) was sandblasted, while the buccal wall of the vestibular cusp was left as delivered by milling center. The additional specimens were veneered with glass-ceramic as reported in the specimen preparation section. The fracture test was made applying the load on the tip of vestibular cusps. All the experimentally fractured specimens were coated with a very thin layer of gold by vacuum evaporation using a Techniques Hummer II-Au-sputtering (Techniques inc, Virginia, USA) and observed with a scanning electron microscope (SEM) (Cambridge Stereoscan 200, Cambridge Instrument Company Ltd. , Cambridge, England.) equipped with tetra solid-state detector for back scattered electrons (BSE ). The fracture surfaces were analyzed starting from the starting point of the fracture, when well identified, until to the arrest point of the cleavage plane.. Type and location of initiating flaws, such as porosity, surface defects, were also evaluated if present. The fractographic patterns commonly observed in brittle fractures were evaluated as follows: the mirror area, that appears as a flat shiny area; the mist area, that appears to be slightly rougher; the hackle area, ridges and grooves radiating from the starting point as well as layers resulting from branching of the fracture path. Different magnifications were used to analyze the fractures because some patterns were easily recognized at higher magnifications and others were more apparent at lower magnifications. To assess the accuracy of the method the same operator performed the analyses twice. Statistical Analysis All the data were analyzed by means of the computerized statistical package (Sigma Stat 3.5, SPSS inc. Ekrath, Germany). The mean fracture loads of alumina and zirconia crowns were 7
compared using the Student t-test with a confidence interval of 95%. Statistical differences were considered significant for a P value of < 0.05. Results Fracture strength The fracture strength value (mean ± SD) was 1165±509 N for alumina and 1638±662 N for zirconia; the difference of the means was 473 N. The statistical analysis showed a significant difference between the two groups (p = 0.026) (table 1). Scanning electron fractographic analysis Most of the specimens were noted a cohesive fractures of the glass-ceramic (Fig. 3). The alumina and zirconia glass-ceramic bond was always stronger than the cohesive forces of the glass-ceramics itself (Fig. 4), in fact the main fracture path propagated through the glassceramic body. In both the experimental groups, the core-glass-ceramics interface was able to transfer most of all the energy accumulated by the system under load to the bulk of the glassceramic, that annihilate it by fracture. In details, for alumina glass-ceramic restorations, mostly of micro cracks were generated on the occlusal surface at the level of the supporting cusp, where the arbitrary load was applied; then, they easily propagated into the bulk of the veneering material and reached the prosthetic copings (the cores), causing a catastrophic fracture of the core (Fig. 3 a and a1). On the contrary, in the zirconia glass-ceramic restorations the fractures were concentrated at the level of the supporting cusp with a limited extent of the fracture path. Since the cleavage fracture appears stopped by the prosthetic copings (the cores) surface with only small partial micro cracks that involves the zirconia core (Fig. 3 b and b1).
8
Analyzing the facture's pattern, from the energy point of view, in the alumina specimens emerges that when the cleavage plane (plane of fracture) reaches the surface of the core (extended mirror area) it conserves an high energy level as much as it is sufficient for core's fracture, without any deflection (hackle area) of the fracture's path (Fig. 4 a and a1). On the other hand, the zirconia core seems to be able to best dissipate the occlusal load (energy) since, a catastrophic fracture of the core was not present, vice versa only few micro crack on the occlusal area of the copings were noted. The energy of the cleavage plane, in this case, appears to be mostly annihilated by the sequence of mirror, mist and hackle areas of fractures inside the glass-ceramic layer so when it reaches the zirconia core surface the residual energy is able to produce just micro cracks in the core (Fig. 4 b and b1). Only limited areas of zirconia framework appeared uncovered by glassceramic (Fig. 4 b and b1). The vitreous phase of the ceramics satisfactorily imbibed the zirconia structure, confirming the effectiveness of the core-glass-ceramics bond also under high load condition. Moreover, the micro texture (sandblasting) of the zirconia core appeared to influences the behavior of the cleavage plane on zirconia specimens (Fig. 5). The evaluation of additional specimens with double surface treatments (rectangle 1 sandblasted and rectangle 2 as received by milling center in Fig. 5 a) showed different fractographic patterns after fracture. The sandblasted surface (Fig. 5 b) appeared to be able to influence the cleavage plane development with arrest lines until to 10 μm from the core surface while outside this frontier wake hackle area was evident. Any detachment line was noted at zirconia/glass-ceramic interface. The zirconia core surface as received by milling centre (Fig. 5 c) showed
arrest
lines at the zirconia/glass-ceramic interface with wake hackle area in proximity of the interface. A detachment line was seen at the interface.
9
As for the microstructure of alumina, high magnification SEM images showed a polycrystalline structure, made up of aluminum oxide sintered grains; a very few postsinterization voids were evident, whose dimension reached a maximum of about 330 nm. The dimension of the grains (mean ± SD) was of 1.77 ± 0.54 μm with a polygonal appearance (Fig. 6 a and a1). The microstructure of 3 mol%Y-TZP zirconia sintered at 1450° C for 2 h showed a mean (± SD) grain sizes of 0.45 ± 0.15 μm with round appearance (Fig. 6 b and b1). Discussion The results of the present study sustain the rejection of the null hypothesis because the fracture strength, between the two groups, showed a difference of 473 N that was statistically significant (p = 0.026). The results revealed also that the breaking strength was higher for glass-ceramic/zirconia. A possible explanation was supported by the fractographic analysis. In implant-supported prosthetic restorations, the most stressful regions are the interfaces, such as the contact area between core and porcelain. The effectiveness of the alumina/ and zirconia/glass-ceramics bond is influenced by the different physical characteristics of both veneering glass-matrix and crystalline core ceramics.
22,32,33
As reported in the scientific literature, a good mechanical
resistance to compression characterizes porcelain but it is negatively affected by shearflexural stresses.
21,33
Such stresses might arise in the occlusal third and at the level of the
cervical margin of the crown, sometimes causing a local failure of the core/glass-ceramic bond and the detachment of porcelain. The present findings on fractographic analysis are in accordance with previously reported observation
34
in which the cleavage planes pass always through areas with low fracture
10
strength, presenting different fracture patterns in the veneering ceramics, alumina and zirconia. The deflection of the crack growth path was evident in both groups since the crack lines changed their direction following the profile of the copings, in order to annihilate the energy as much as possible (Fig. 3 a and b). Moreover, it was associated to the change of the force direction vector from the compressive stress in the occlusal area, to the shearing stress of the axial regions. The explanation involves the local stress level that exceeded the cohesive resistance of the glass-ceramics in all the specimens, breaking the atomic bonds. Yet, due to surface tension, ceramic structures tend to fail catastrophically, where cracks propagate by slow growth from a point where the applied load exceeds the material resistance. If a die made of a high modulus material supports a crown, the fracture strength will increase dramatically compared with that of crowns supported by a low modulus material. In fracture mode, however, the zirconia cores seemed to be more resistant compared with alumina ones, as significantly more of the fractured alumina crowns were totally fractured. This could imply that the zirconia core resists higher loads than alumina ones but that the veneer porcelain fractures at a lower load. A ceramic laminate will always form a constant strain system because of a mismatch of the modulus of elasticity across the core–veneer interface. an important source of structural flaws
33
33-36
The interface is
because of wettability factors such as
microstructural roughness (or sandblasting treatment), which can influence the coupling degree of glass-ceramic during both the ceramic buildup and the subsequent ceramic firing procedures. In this study, different surface properties of the two core materials – causing difficulties in building up a dense and homogenous layer of green porcelain, without trapping air bubbles, over the core surface prior to firing.
11
Veneer fractures often occur during interfacial stresses
35,36
or because microstructural
regions in the porcelain are mechanically defective. Such microstructural flaws include porosities, agglomerates, inclusions and large grained zones. 37 Conclusions Within the limits of this study, basing on the rejection of the null hypothesis, it was possible to state that the glass-ceramic/zirconia had a fracture strength value significantly higher than that of the glass-ceramic/alumina. In addition, both systems of restoration when supported by implants achieved higher values of breaking strength, that the functionally load of the premolar region. Finally, the sandblasting procedure of zirconia core showed to be effective for improving adhesion at the interface between zirconia and ceramic. Acknowledgements We wish to thank Mr. Giuseppe Zuppardi CDT-MDT for his dental laboratory support. We thank Michela Marroni PhD for her assistance with the English revision Conflict of interest All the authors involved in the present research declare any possible conflict of interest.
12
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Table Table 1 – Unpaired t-test: fracture load (Newtons) (SD=standard deviation).
n
Mean (SD)
SEM
Alumina
20
1165 (±509)
169
Zirconia
20
1638 (±662)
160
95% confidence interval for difference: -95.22 to 847.2 t = 1.615 with 38 degrees of freedom; p = 0.026
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Captions to Figures Fig. 1 (a) the standard ITI abutment analogs. (a1) alumina and zirconia CAD/CAM core copings . (a2) alumina and zirconia cores positioned on the ITI abutments. Fig. 2 specimens under compressive loading tests. Fig. 3 SEM images of copings after fracture. Alumina-ceramic specimen (a) and (a1). Zirconia-ceramic specimen (b) and (b1). White arrows indicate the core microcrack. Fig.4 SEM images of the fracture plane. Alumina specimens (a) with secondary electrons and (a1) with back scattered electrons. Zirconia (b) with secondary electrons and (b1) with back scattered electrons. Fig. 5 SEM images for different surface treatments. (a) sagittal section of coping treated with sandblasting in the area of rectangle (1) and as received by milling center in the area of rectangle (2). (b) fractographic evaluation of the rectangle(1) white arrows indicate the absence of a detachment line at the ceramic-zirconia interface. (c)
fractographic
evaluation of the rectangle (2) white arrows indicate the presence of a detachment line at the ceramic-zirconia interface. Black arrows indicate wake hackle reaching the zirconia-ceramic interface. While an arrest line was just associated to a roughness area of the zirconia surface. Fig. 6. (a) Scanning electron microscopy images of alumina. The mean grain size was of 1.77 (± 0.54) μm with a polygonal shape. (b) Scanning electron microscopy images of 3 mol%Y-TZP ceramics: (b1) The mean grain size was of 0.45 (± 0.15)
μm
approximated with the radius of the circle, which has the same section area as the area of the grains. sintered at 1450° C for 2 h,
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