Effect of Various Veneering Techniques on Mechanical Strength of Computer-Controlled Zirconia Framework Designs ¨ ˘ ¨ ¨ ˘ Burcu Kanat, DDS, PhD,1 Erhan M. C¸omleko glu, DDS, PhD,1 Mine Dundar-C ¸ omleko glu, DDS, PhD,1 2 3 ¨ ¨ or, ¨ DDS, PhD1 Bilge Hakan Sen, DDS, PhD, Mutlu Ozcan, DDS, DMD, PhD, & Mehmet Ali Gung 1
Department of Prosthodontics, Ege University, School of Dentistry, Izmir, Turkey Department of Endodontics, Izmir Katip C¸elebi University, School of Dentistry, Izmir, Turkey 3 University of Zurich, Dental Materials Unit, Center for Dental and Oral Medicine, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, Zurich, Switzerland 2
Keywords CAD/CAM; file splitting; fracture resistance; flexural strength; shear bond strength; finite element analysis. Correspondence Burcu Kanat, Ege University School of Dentistry, Department of Prosthodontics, Bornova, Izmir, Turkey. E-mail: [email protected] This research was supported in part by the Ege University, Scientific Research Project ˙ ¸ -006). Department (Project no. 2011-DIS The authors deny any conflicts of interest. Accepted September 3, 2013 doi: 10.1111/jopr.12130
Abstract Purpose: The objectives of this study were to evaluate the fracture resistance (FR), flexural strength (FS), and shear bond strength (SBS) of zirconia framework material veneered with different methods and to assess the stress distributions using finite element analysis (FEA). Materials and Methods: Zirconia frameworks fabricated in the forms of crowns for FR, bars for FS, and disks for SBS (N = 90, n = 10) were veneered with either (a) file splitting (CAD-on) (CD), (b) layering (L), or (c) overpressing (P) methods. For crown specimens, stainless steel dies (N = 30; 1 mm chamfer) were scanned using the labside contrast spray. A bilayered design was produced for CD, whereas a reduced design (1 mm) was used for L and P to support the veneer by computer-aided design and manufacturing. For bar (1.5 × 5 × 25 mm3 ) and disk (2.5 mm diameter, 2.5 mm height) specimens, zirconia blocks were sectioned under water cooling with a lowspeed diamond saw and sintered. To prepare the suprastructures in the appropriate shapes for the three mechanical tests, nano-fluorapatite ceramic was layered and fired for L, fluorapatite-ceramic was pressed for P, and the milled lithium-disilicate ceramics were fused with zirconia by a thixotropic glass ceramic for CD and then sintered for crystallization of veneering ceramic. Crowns were then cemented to the metal dies. All specimens were stored at 37◦ C, 100% humidity for 48 hours. Mechanical tests were performed, and data were statistically analyzed (ANOVA, Tukey’s, α = 0.05). Stereomicroscopy and scanning electron microscopy (SEM) were used to evaluate the failure modes and surface structure. FEA modeling of the crowns was obtained. Results: Mean FR values (N ± SD) of CD (4408 ± 608) and L (4323 ± 462) were higher than P (2507 ± 594) (p < 0.05). Mean FS values (MPa ± SD) of CD (583 ± 63) and P (566 ± 54) were higher than L (428 ± 41) (p < 0.05). Mean SBS values (MPa ± SD) of CD (49 ± 6) (p < 0.05) were higher than L (28 ± 5) and P (30 ± 8). For crown restorations, while cohesive failures within ceramic and zirconia were seen in CD, cohesive failures within ceramic were found in both L and P. Results were verified by FEA. Conclusion: The file splitting technique showed higher bonding values in all mechanical tests, whereas a layering technique increased the FR when an anatomical core design was employed. Clinical significance: File splitting (CAD-on) or layering veneering ceramic on zirconia with a reduced framework design may reduce ceramic chipping.
Presintered yttrium-stabilized zirconium oxide blocks fabricated to be milled with computer-aided design/computeraided manufacturing (CAD/CAM) systems as a framework, have been increasingly used in esthetic dentistry.1 CAD/CAM
technology has been reported to provide homogenous zirconia frameworks with no imperfections and/or porosities that may lead to veneering ceramic delamination and clinical failures.2,3
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
1
Mechanical Performance of Veneering on Zirconia
Kanat et al
Different methods can be applied for veneering ceramic on zirconia frameworks. In the traditional layering technique, the mixed ceramic powder and its liquid are built on the sintered zirconia core being larger than the final dimension to compensate for the shrinkage of ceramic,4,5 whereas the overpressing technique requires a final contour wax-up model on the sintered zirconia framework to be invested under heat-pressed vacuum with pressable ceramics.4 The subtractive type of file-splitting method involves the fabrication of metal-ceramic restorations through scanning by a digital probe for data aquisition.6 In addition to these commonly used veneering techniques for zirconia, a new technique with the generic term “CAD-on” has been introduced.3,7 In this technique, a presintered zirconia framework and a high-strength ceramic veneering material are designed and milled through CAD/CAM, and then sintered zirconia and ceramic suprastructure are combined by a fusion glass ceramic.3,7 Because of the thixotropic feature of the fusion glass ceramic, mixing it with a special vibration device is recommended by the manufacturer in the CAD-on technique.7 The capsule fabricated for single use is put onto a flat surface of a special vibrator for 10 seconds to liquefy and be inserted into superstructures for fusing procedures. The mixer allows the material to be more homogenous by getting the components together between the cores and suprastructures.8 Anatomical cusp support design of the CAD/CAMfabricated zirconia core with layering ceramic under mechanical loading was reported to significantly reduce the chipping failures of zirconia-supported crowns,9 which has been attributed to optimal preservation of the uniform thickness of the veneering ceramic through controlled CAD.9,10 By decreasing the chipping of the veneering ceramic, removal and renewal of the restorations can be prevented clinically. Also, by this route, time and cost for both the patient and the dentist, as well as excessive material consumption in the laboratory, can be eliminated.7,11 Studies have emphasized the significance of form and thicknesses of the frameworks, veneering techniques, quality, and homogeneity of the veneering ceramic as well as the physical properties such as the coefficient of thermal expansion and elastic modulus of veneering ceramic and zirconia for durable bond strengths between the two materials.10,11 Chipping of the veneering ceramic in zirconia-supported restorations has been reported to be one of the most common clinical failure types12 with a 25% failure rate in a 3-year follow-up randomized controlled clinical trial.13 Since the nature of the clinical problem determines the systematic approach for mechanical/physical and/or clinical testing, the main question to be answered when incorporating a novel veneering method on a zirconia framework depends on the above-mentioned major failure type: ceramic chipping. Such a failure can be identified by analytical and experimental procedures validated by clinical studies. Recommended mechanical tests for monolithic and/or bilayered specimens might be three- or four-point flexure tests, biaxial flexure tests, or shear bond tests coupled with finite element analysis (FEA) and/or fractographic analyses.14 Thus, mechanical tests may allow for identification of the failure initiation site of zirconia framework and veneering ceramic combinations fabricated with different layering techniques.14,15 2
The aim of this study was to investigate the effect of different veneering procedures (layering, overpressing, and CAD-on) on the fracture resistance (FR) of single molar crown restorations with zirconia framework. Three complementary tests were performed to interpret the findings: (a) the effect of flexural strength (FS) of the veneering ceramic on the FR of the restoration by performing a three-point bending test on the bar-shaped specimens; (b) the effect of interfacial bonding on the FR of the restoration by a shear bond strength (SBS) test using diskshaped specimens; and (c) finite element modeling of the crown system to observe the location of the accumulated stresses by simulating loading conditions. The null hypothesis of this study was that the veneering technique would not affect the FR of the bilayered zirconia crowns.
Materials and methods Crown, bar, and disk-shaped specimens for FR, three-point bending (FS), and SBS tests, respectively, were prepared with CAD/CAM-fabricated (InLab; Sirona Dental Systems GmbH, Bensheim, Germany) zirconia frameworks and veneered with three techniques on the substructures as: layering (L), overpressing (P), and CAD-on (CD) (N = 90) (n = 10/subgroup). The brands, manufacturers, chemical compositions, some mechanical properties, and batch numbers of the materials used in this study are listed in Table 1. Preparation of zirconia frameworks
For crown-shaped specimens, thirty stainless steel dies (n = 30) with a 1 mm standard circumferential chamfer, which had a groove on the axial surface to prevent the rotation of the restorations during the mechanical test, were scanned (InEos Blue; Sirona Dental Systems) using a labside contrast spray (IPS Contrast Spray Labside; Ivoclar Vivadent). Digital impressions were obtained. The restoration form was chosen for the mandibular left first molar in the CAD/CAM software (CEREC 3D, Sirona InLab V3.88; Sirona Dental Systems). Multilayered design was performed for CD, whereas the reduced form with 1 mm was preferred for both L and P, producing a uniform veneering ceramic thickness and support for the cusps.16 The thickness of the die spacer was selected as 10 μm, since the die material was not a tooth substance, but made of metal. The adaptation of the framework to the metal die was aimed to decrease the rotation of the substructure during fracture tests.3 Circumferential margin and insertion path of the metal dies were determined as the groove was positioned buccally. The zirconia frameworks were milled (InLab MC XL; Sirona Dental Systems) out of presintered zirconia blocks (IPS e.max ZirCAD) with 20% to 25% enlarged volume to compensate for shrinkage after the sintering process. Then, the frameworks were sintered (InFire HTC speed; Sirona Dental Systems), and their fit to the dies was controlled. Bar- (length: 25 mm, width: 5 mm, height: 1.5 mm) and square- (4 × 4 mm2 ) shaped specimens (n = 30/per group), which would be used in three-point bending and SBS tests, respectively, were sectioned in three axes under water cooling using a low-speed diamond saw (Isomet 1000; Buehler Ltd., Lake Bluff, IL) from presintered yttrium-stabilized
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
Kanat et al
Mechanical Performance of Veneering on Zirconia
Table 1 Materials used
Brand IPS e.max ZirCAD IPS e.max Ceram
Manufacturer Ivoclar Vivadent, Schaan, Liechtenstein Ivoclar Vivadent, Schaan, Liechtenstein
IPS e.max ZirPress
Ivoclar Vivadent, Schaan, Liechtenstein
IPS e.max CAD
Ivoclar Vivadent, Schaan, Liechtenstein
IPS e.max CAD Crystall/Connect RelyX luting cement IPS e.max Ceram Glaze
Ivoclar Vivadent, Schaan, Liechtenstein 3M ESPE, Seefeld, Germany Ivoclar Vivadent, Schaan, Liechtenstein
Chemical composition (%) ZrO2 (87–95) Y2 O3 (4–6) HfO2 (1–5) Al2 O3 (0–1) SiO2 (60–65), Al2 O3 (9–11), K2 O (7–8), Na2 O (7–8), ZnO2 (2–3), CaO, P2 O5 , and F (2,5–7,5) SiO2 (57–62), Na2 O (6–8), K2 O (2–4), CaO (2–4), Al2 O3 (12–16), ZrO2 (1,5–2,5), P2 O5 (1–2), and F (0,5–1) SiO2 (57–80), Li2 O (11–19), K2 O (0–13), P2 O5 (0–11), ZrO2 (0–8), ZnO (0–8), Other and coloring oxides (0–12) Oxides, water, butandiol, and chloride Sılane-treated glass (> 98), potassıum persulfate ( 0.05) (Table 3). For FS (MPa ± SD), the L group exhibited the lowest values (p < 0.05), and there were no significant differences between P and CD groups (p ≥ 0.05) (Table 3). SBS tests (MPa ± SD) resulted in significantly higher values in the CD group (p < 0.05), but there were no significant differences between P and L groups (p ≥ 0.05). Estimates of the parameters of the Weibull distribution for the FR, FS, and SBS mechanical tests are presented in Table 4. The test to estimate shape parameters (p ≥ 0.05) indicated no statistical differences in Weibull shape parameters among the veneering groups for any of the mechanical tests (Table 4). The test for equal scale parameters (p < 0.05), together with the Bonferroni post hoc confidence interval, indicated that all of the veneering method groups had different scale parameters for FR, FS, and SBS mechanical tests (Table 4). According to the analysis with the Weibull distribution, probability plot and
The exact reason for veneer chipping observed in zirconia core restorations is unclear, but it is thought that generally three factors play an important role in the chipping of the ceramic for zirconia-supported restorations, namely interfacial bonding, match of the core-veneer materials, and strength of the veneering ceramic.2,20,21 Veneering techniques also have a potential effect on the chipping of ceramic due to the processing methods of ceramic.2,4 Accordingly, in the present study, the FR of single molar zirconia crowns with zirconia frameworks and veneering ceramics prepared with different techniques (CAD-on, layering, overpressing) and the effect on chipping were investigated. Three complementary tests were conducted, in addition to the FR test, to interpret the findings, namely three-point bending test to investigate the effect of FS of the veneering ceramic on the restoration; SBS testing of the veneering ceramic and zirconia framework bilayered system to find out whether the bond strength affected the FR of the crowns; and finite element modeling simulating the loading conditions of the crown system to evaluate the location of the accumulated stresses. Also, to eliminate the mismatch of the framework and suprastructures, the materials were chosen according to the
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
5
Mechanical Performance of Veneering on Zirconia
Kanat et al
Figure 1 Survival and probability plots of FR, FS, and SBS test groups.
manufacturers’ instructions. The results of this study demonstrated that the CAD-on technique resulted in significantly higher FR values, although anatomical zirconia frameworks were used in the L and P groups. There were no significant differences between CD and L groups with anatomical cores according to ANOVA analysis, whereas the CD group had significantly higher values with respect to the L group according to Weibull analysis. Thus, the null hypothesis was rejected. Currently used veneering techniques on zirconia frameworks are layering, overpressing, or recently improved file-splitting (CAD-on). The layering technique is a very delicate method, as several factors such as the experience of the dental technician, homogeneity of the mixed ceramic slurry, duration of the firing and cooling, the number of firings, and shrinkage
6
of the ceramic play an important role in the success of the veneering technique.4,5,14,17 The veneering ceramic used in this technique has been reported to have an FS of approximately 90 MPa. In the overpressing technique, the modeling Table 5 Failure-type distributions of mechanical test groups. A: Adhesive, CC: Cohesive within veneering ceramic, CZ: Cohesive within zirconia, M: Mixed FR
CD P L
FS
SBS
A
CC
CZ
M
A
CC
CZ
M
A
CC
CZ
M
-
2 10 10
8 -
-
-
10 8 9
-
2 1
7 5 7
1 -
-
3 4 3
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
Kanat et al
Mechanical Performance of Veneering on Zirconia
Figure 2 Representative failure types and FEA models of the crown restorations. (a1) Cohesive failures within ceramic and zirconia in CD group. (a2-a3) von Mises stress distributions of CD can be seen. (b1) Cohesive failures within ceramic in L group. (b2-b3) von Mises stresses
distributed over the entire surfaces can be observed in L. (c1) Cohesive failure within ceramic and lateral cracks due to the stress peaks as indicated with an arrow in the P group. (c2-c3) von Mises stress distribution of P.
wax is placed on the sintered zirconia, and this combination is attached to a flask by a sprue. The flask is filled with a special investment material according to the manufacturer’s instructions. After the wax is burnt out, the pressable ceramics, which have about 110 MPa FS, are pressed into the spaces, and the final anatomic restorations are obtained.4 This technique allows the restoration to be produced faster and easier, compared with the conventional layering technique. Moreover, the firing shrinkage observed in the layering technique can be minimized, and a good marginal adaptation of the restoration can be obtained.17 However, preliminary processes such as accurate attachment to the flask, cleanness of the modeling wax, quality of the pressing material, temperature of the preheating
furnace, and controlled air-abrasion during separation of the restorations from the investment material might affect the success of this method. Particle size, morphology, and pressure during air-abrasion may damage the ceramic surface. Since the surfaces of the zirconia in the present study were already veneered, they were not subjected to additional air-abrasion during the airborne-particle abrasion procedure for divesting. With the newly developed file-splitting (generic term: “CADon”) technique, time-consuming and operator-sensitive laboratory procedures such as impression making, model obtaining, investing, and finishing can be eliminated with the aid of computer-controlled design and fabrication.22 Furthermore, the file-splitting (CAD-on) technique is introduced as a more
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
7
Mechanical Performance of Veneering on Zirconia
Kanat et al
Figure 3 Representative SEM images of the fractured bar-shaped specimens. (C: ceramic, Z: zirconia, A: adhesive material). (a1-a2) (left column) Homogeneous ceramic and crack origin in the adhesive material as indicated with an arrow can be seen for CD. (b1-b2) (center column) Crack
propagation arose from lateral cracks and porosities due to layering technique observed in L. (c1-c2) (right column) Porosities along the interface and lateral cracks in P.
Figure 4 Representative SEM images of the debonded surfaces of disk-shaped specimens. (C: ceramic, Z: zirconia). (a1-a2) (left column) Mixed failures in the CD group. (b1-b2) (center column) Mixed failures in the L group. (c1-c2) (right column) Mixed failures in the P group.
reliable method because of the advantages of prefabricated blocks manufactured by industrial pressing without any porosities, perfect adaptation of the restorations by CAD/CAM system, and fast processing.3 For in vitro studies, preparation of bilayered specimens has been recommended since it has been reported that the super8
structure, framework, and the interfacial material act mutually under force.20 Using analysis methods such as SEM and/or FEA of fractured surfaces for assessment of the failed surfaces together with the mechanical tests, a connection between in vivo and in vitro studies can be established.14 In the literature, the chipping problem was investigated using only one mechanical
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
Kanat et al
test, and no studies about the recently introduced CAD-on technique could be derived with 3-point bending and SBS tests. In this study, the crown-shaped restorations were used to imitate the clinical situation for FR tests. To evaluate the effect of FS of the ceramic on the FR, bar-shaped specimens were prepared for 3-point bending tests. Also, disk-shaped specimens were obtained for the SBS test to investigate the interfacial bonding effect on the FR of crown restorations. In addition to mechanical tests, stereomicroscopy and SEM were used to evaluate the failure modes and debonded surface structures. The results of the present study may provide clinically relevant data for wider application of these materials. It was previously stated that the rate and area of ceramic chipping in zirconia-supported crowns might be significantly reduced when an anatomically reduced core design was used because of the cusp support and homogenous thickness of the ceramic.9,16 It has also been reported that support of the anatomical zirconia framework prevented chipping compared to the results of FS of the ceramic.9 Therefore, anatomical framework design was used for the L and P groups in the present study. Eisenburger et al17 found that layering of the veneering ceramic group (IPS e.max ZirCAD-IPS e.max Ceram) showed better FR values (919 ± 265 N) than the overpressing group (IPS e.max ZirCAD-IPS e.max ZirPress) (798 ± 226 N). Although the results of the present study were compatible with these results, the reason for higher FR values (L: 4323 ± 462 N; P: 2507 ± 594 N) obtained in the present study could be attributed to the differences in geometry and the anatomically shaped framework used in this study.15 In a previous study, fracture strengths of crown specimens made by CAD/CAM fabricated nonanatomic formed zirconia framework (IPS e.max ZirCAD) and veneering ceramics (IPS e.max CAD for S, IPS e.max Ceram for L, IPS e.max ZirPress for P) were found to be 6265 ± 2257 N with sintering, 3700 ± 1239 N with layering, and 3524 ± 1181 N with overpressing methods.3 In the present study, the FR of CAD/CAM-fabricated zirconia frameworks veneered with different veneering techniques tested on crown specimens were 4408 ± 608 N for CD and 4323 ± 462 and 2507 ± 594 N for L and P, respectively. The results of the present study showed that higher FR values could be obtained in the layering technique when an anatomical zirconia framework design is employed. In the P group, lower FR values could be attributed to chipping of the veneering ceramic into smaller pieces, causing lateral cracks. Also, the fracture types observed in S and L in the literature3 were compatible with our study, since failure within both veneer and zirconia was observed more frequently in CD. On the other hand, only ceramic chipping was observed in L. The overpressing veneering technique (IPS e.max ZirPress) (1514 ± 332 N) was reported to show higher load-bearing capacity than the layering technique (IPS e.max Ceram) (894 ± 160 N) on the zirconia core (ZENO TEC; Wieland Dental, Pforzheim, Germany) according to the Voss test of canine-form restorations.5 Tsalouchou et al23 showed that the differences between the layering (IPS e.max Ceram) (2189.9 ± 317.6 N) and overpressing (IPS e.max ZirPress) (2135.6 ± 330.1 N) veneering materials on nonanatomically designed zirconia frameworks (Kavo Everest, Biberach, Germany) following cyclic loading were insignificant. The differences between the results obtained in the above-mentioned
Mechanical Performance of Veneering on Zirconia
studies and in our study could be attributed to different framework and veneering ceramic combinations as well as differing mechanical testing modalities that may affect the bond strength. The anatomical framework design in the present study might have added to the increased bond strength values. Liu et al24 reported that zirconia-supported specimens under 3-point bending test were broken into two pieces as in the present study. It has been indicated that the FS of the ceramic plays an important role on the fracture pattern. A ceramic with 300 MPa FS can lead to less delamination.24 In the CD group in the present study,3 a lower delamination rate was observed in the lithium disilicate group, because the FS was 360 MPa. According to SEM analysis of fractured bar specimens in the present study, the ceramic structure without any imperfections in CD can be due to the homogeneity of the ceramic block. Lateral cracks and porosities observed in both the L and P groups can be explained with the sensitivity of the veneering techniques. When the effect of FS of the ceramic according to FS test on the FR values were examined in the present study, higher FS of the ceramic for the CD group played an important role on the FR. For the L group, higher FR values could not be supported with FS values, which indicated the effect of the anatomical framework design on the FR rather than the FS of the layering ceramic. For the P group, the authors attributed the differences found between lower FR and higher FS values to contributing factors such as different fabrication procedures, experience in handling of overpressing technology for the dental technician, and recently suggested slower cooling parameters.16,25-27 Ishibe et al4 investigated the SBS of zirconia cylinders (Lava; 3M ESPE, St. Paul, MN) (diameter: 5 mm) to layered or pressed ceramics (diameter: 3 mm). While the overpressing group (e.max ZirPress) showed 40.41 ± 10.28 MPa SBS, in the layering group (e.max Ceram), 30.03 ± 9.49 MPa was obtained. The differences in the framework-superstructure combination and thermocycling application could be reasons for the discrepancy between the results. L´opez-Moll´a et al28 also reported SBS of zirconia disks (IPS e.max ZirCAD) (length: 15 mm; diameter: 8 mm) to layered (IPS e.max Ceram) and pressable ceramics (IPS max ZirPress) (length: 2 mm; diameter: 8 mm) as 7.85 ± 2.50 MPa and 12.69 ± 2.14 MPa, respectively. Higher SBS results in the present study, which were similar to the veneering techniques can be explained by different geometries and loading conditions of the specimens. Although various opinions about the failure types after SBS test were found in the literature, no correlations between the SBS values and types of failures were observed in the present study. Ereifej et al29 reported that layering veneering ceramic (IPS e.max Ceram) on zirconia cores (IPS e.max ZirCAD) led to lower SBS values (28.8 ± 9.5 MPa), adhesive failures because of insufficient wettability of the liner material applied on the zirconia, and imperfections that occurred at the interfacial surface. SBS values (28 ± 5 MPa) and adhesive fracture types compatible with the literature were also observed in the present study. Failure types after SBS test, observed in both stereomicroscopy and SEM, could be different.30 SEM analysis revealed a thin ceramic layer on the zirconia framework. Hence, careful investigation was recommended when stereomicroscopy was used, since it provides less detailed topographic information.
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
9
Mechanical Performance of Veneering on Zirconia
Kanat et al
In the present study, no correlations were found between the failure types following SBS tests and different veneering techniques, since stereomicroscopy was conducted for overall surface inspection, and only representative specimens were observed under SEM. When the effect of interfacial bonding strength on the FR values according to SBS tests were examined in the present study, higher interfacial bonding in the CD group was noted. That eventually played an important role on the FR. This can be attributed to the durable bonding provided by the glass interfacial ceramic. For the L group, higher FR values were also not replicated with the SBS values, and an explanation for this result might be that the anatomical framework design plays an important role on the higher FR rather than interfacial bonding. For the P group, lower FR and SBS values can be attributed to low strength of the ceramic. According to the overall results of the three mechanical tests, FEA of the crowns and failure assessment, higher FR, bond strength, and FSs of the ceramic in the CD technique, as well as cohesive failure observed within the ceramic and zirconia for most specimens can be attributed to durable interfacial bonding. The stress propagation to the framework meant that a threelayered system behaved as a monolithic structure. The FSs of the interfacial fusion glass-ceramic material and veneering ceramic play an important role in the success of the veneering technique. For the L group, higher FR, low strength of the interfacial bonding and veneering ceramic, cohesive failures within the ceramic, porosities in SEM, and the stress distribution along the interfacial surface indicated that the success of the crown restoration under the FR test arose from anatomical framework design. For the P group, low FR, low interfacial bonding, cohesive failures within the ceramic, lateral cracks observed in crown restorations, porosities along the interface in the SEM images, and localized stresses revealed insufficient interfacial bonding and weak ceramic strength as well as possible lack of wettability. The cutting procedure for ceramic materials by a diamond saw at a slow speed under water cooling is used as a routine process.31,32 Either some polishing stages can be followed after sawing procedures,31 or the sectioned specimens can be subjected to the application of veneering ceramic directly.32 The influence on FS of surface roughness obtained after grinding showed a decrease in surface roughness, but caused an increase in FS.33 It was not in the scope of this study to investigate the effect of surface alterations arising from standard sawing but could be the subject of future studies. Also, future studies are required to investigate the effect of wettability properties of the ceramic on the bond strength. Long-term in vivo studies would also demonstrate the clinical performance of the CAD-on technique on the veneering ceramic chipping complications.
Conclusion From this in vitro study, the following conclusions were drawn: 10
1. The file-splitting (CAD-on) technique could decrease ceramic chipping due to higher strength of the ceramic and the interfacial bonding. 2. Anatomical framework design increased the FR when a layering veneering technique was employed.
Acknowledgments The authors would like to thank Prof. Dr. Tijen Pamir at the Department of Restorative Dentistry, Ege University School of Dentistry, Izmir, Turkey and Assist. Prof. Dr. Osman C ¸ ulha at the Department of Materials Engineering, Celal Bayar University, Manisa, Turkey for the mechanical tests and Canderim ¨ Onder at Netform, Izmir, Turkey for FEA analyses. The authors extend their gratitude to Ivoclar Vivadent, Schaan, Liechtenstein for providing the veneering ceramic and fusion glassceramic materials.
References 1. Abduo J, Lyons K, Swain M: Fit of zirconia fixed partial denture: a systematic review. J Oral Rehabil 2010;37:866-876 2. Denry I, Kelly JR: State of the art of zirconia for dental applications. Dent Mater 2008;24:299-307 3. Beuer F, Schweiger J, Eichberger M, et al: High-strength CAD/CAM-fabricated veneering material sintered to zirconia copings—a new fabrication mode for all-ceramic restorations. Dent Mater 2009;25:121-128 4. Ishibe M, Raigrodski AJ, Flinn BD, et al: Shear bond strengths of pressed and layered veneering ceramics to high-noble alloy and zirconia cores. J Prosthet Dent 2011;105:29-37 ¨ 5. Stawarczyk B, Ozcan M, Roos M, et al: Load-bearing capacity and failure types of anterior zirconia crowns veneered with overpressing and layering techniques. Dent Mater 2011;27:1045-1053 6. Goiato MC, Santos MR, Moreno A, et al: Prototyping for surgical and prosthetic treatment. J Craniofac Surg 2011;22:914-917 7. Schmitter M, Mueller D, Rues S: Chipping behaviour of all-ceramic crowns with zirconia framework and CAD/CAM manufactured veneer. J Dent 2012;40:154-162 8. Wiedhahn K: The impression-free Cerec multilayer bridge with the CAD-on method. Int J Comput Dent 2011;14:33-45 9. Rosentritt M, Steiger D, Behr M, et al: Influence of substructure design and spacer settings on the in vitro performance of molar zirconia crowns. J Dent 2009;37:978-983 10. Komine F, Blatz MB, Matsumura H: Current status of zirconia-based fixed restorations. J Oral Sci 2010;52:531-539 11. Rekow ED, Silva NR, Coelho PG, et al: Performance of dental ceramics: challenges for improvements. J Dent Res 2011;90:937-952 12. Raigrodski AJ, Hillstead MB, Meng GK, et al: Survival and complications of zirconia-based fixed dental prostheses: a systematic review. J Prosthet Dent 2012;107:170-177 13. Sailer I, Gottner J, Kanel S, et al: Randomized controlled clinical trial of zirconia-ceramic and metal-ceramic posterior fixed dental prostheses: a 3-year follow-up. Int J Prosthodont 2009;22: 553-560 14. Anusavice 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;18:218-231
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
Kanat et al
¨ 15. D¨undar M, Ozcan M, G¨okc¸e B, et al: Comparison of two bond strength testing methodologies for bilayered all-ceramics. Dent Mater 2007;23:630-636 16. Guess PC, Bonfante EA, Silva NR, et al: Effect of core design and veneering technique on damage and reliability of Y-TZP-supported crowns. Dent Mater 2013;29: 307-316 17. Eisenburger M, Mache T, Borchers L, et al: Fracture stability of anterior zirconia crowns with different core designs and veneered using the layering or the press-over technique. Eur J Oral Sci 2011;119:253-257 ¨ 18. Comlekoglu M, Dundar M, Ozcan M, et al: Influence of cervical finish line type on the marginal adaptation of zirconia ceramic crowns. Oper Dent 2009;34:586-592 19. Al-Dohan HM, Yaman P, Dennison JB, et al: Shear strength of core-veneer interface in bi-layered ceramics. J Prosthet Dent 2004;91:349-355 20. Studart AR, Filser F, Kocher P, et al: Mechanical and fracture behavior of veneer-framework composites for all-ceramic dental bridges. Dent Mater 2007;23:115-123 21. Swain MV: Unstable cracking (chipping) of veneering porcelain on all-ceramic dental crowns and fixed partial dentures. Acta Biomater 2009;5:1668-1677 22. van Noort R: The future of dental devices is digital. Dent Mater 2012;28:3-12 23. Tsalouchou E, Cattell MJ, Knowles JC, et al: Fatigue and fracture properties of yttria partially stabilized zirconia crown systems. Dent Mater 2008;24:308-318 24. Liu Y, Feng H, Bao Y, et al: Fracture and interfacial delamination origins of bilayer ceramic composites for dental restorations. J Eur Ceram Soc 2010;30:1297-1305
Mechanical Performance of Veneering on Zirconia
25. Christensen RP, Ploeger BJ: A clinical comparison of zirconia, metal and alumina fixed-prosthesis frameworks veneered with layered or pressed ceramic: a three-year report. J Am Dent Assoc 2010;141:1317-1329 26. Rosentritt M, Kolbeck C, Handel G, et al: Influence of the fabrication process on the in vitro performance of fixed dental prostheses with zirconia substructures. Clin Oral Investig 2011;15:1007-1012 27. Guazzato M, Walton TR, Franklin W, et al: Influence of thickness and cooling rate on development of spontaneous cracks in porcelain/zirconia structures. Aust Dent J 2010;55:306-310 28. Lopez-Molla MV, Martinez-Gonzalez MA, Manes-Ferrer JF, et al: Bond strength evaluation of the veneering-core ceramics bonds. Med Oral Patol Oral Cir Bucal 2010;15:19-23 29. Ereifej N, Rodrigues FP, Silikas N: Experimental and FE shear-bonding strength at core/veneer interfaces in bilayered ceramics. Dent Mater 2011;27:590-597 30. Ashkanani HM, Raigrodski AJ, Flinn BF, et al: Flexural and shear strengths of ZrO2 and a high-noble alloy bonded to their corresponding porcelains. J Prosthet Dent 2008;100:274-284 31. Kim HJ, Lim HP, Park YJ, et al: Effect of zirconia surface treatments on the shear bond strength of veneering ceramic. J Prosthet Dent 2011;105:315-322 32. Aboushelib MN, Kleverlaan CJ, Feilzer AJ: Microtensile bond strength of different components of core veneered all-ceramic restorations. Part II: zirconia veneering ceramics. Dent Mater 2006;22:857-863 33. Flury S, Peutzfeldt A, Lussi A: Influence of surface roughness on mechanical properties of two computer-aided design/computer-aided manufacturing (CAD/CAM) ceramic materials. Oper Dent 2012;37:617-624
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
11
Department of Prosthodontics, Ege University, School of Dentistry, Izmir, Turkey Department of Endodontics, Izmir Katip C¸elebi University, School of Dentistry, Izmir, Turkey 3 University of Zurich, Dental Materials Unit, Center for Dental and Oral Medicine, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, Zurich, Switzerland 2
Keywords CAD/CAM; file splitting; fracture resistance; flexural strength; shear bond strength; finite element analysis. Correspondence Burcu Kanat, Ege University School of Dentistry, Department of Prosthodontics, Bornova, Izmir, Turkey. E-mail: [email protected] This research was supported in part by the Ege University, Scientific Research Project ˙ ¸ -006). Department (Project no. 2011-DIS The authors deny any conflicts of interest. Accepted September 3, 2013 doi: 10.1111/jopr.12130
Abstract Purpose: The objectives of this study were to evaluate the fracture resistance (FR), flexural strength (FS), and shear bond strength (SBS) of zirconia framework material veneered with different methods and to assess the stress distributions using finite element analysis (FEA). Materials and Methods: Zirconia frameworks fabricated in the forms of crowns for FR, bars for FS, and disks for SBS (N = 90, n = 10) were veneered with either (a) file splitting (CAD-on) (CD), (b) layering (L), or (c) overpressing (P) methods. For crown specimens, stainless steel dies (N = 30; 1 mm chamfer) were scanned using the labside contrast spray. A bilayered design was produced for CD, whereas a reduced design (1 mm) was used for L and P to support the veneer by computer-aided design and manufacturing. For bar (1.5 × 5 × 25 mm3 ) and disk (2.5 mm diameter, 2.5 mm height) specimens, zirconia blocks were sectioned under water cooling with a lowspeed diamond saw and sintered. To prepare the suprastructures in the appropriate shapes for the three mechanical tests, nano-fluorapatite ceramic was layered and fired for L, fluorapatite-ceramic was pressed for P, and the milled lithium-disilicate ceramics were fused with zirconia by a thixotropic glass ceramic for CD and then sintered for crystallization of veneering ceramic. Crowns were then cemented to the metal dies. All specimens were stored at 37◦ C, 100% humidity for 48 hours. Mechanical tests were performed, and data were statistically analyzed (ANOVA, Tukey’s, α = 0.05). Stereomicroscopy and scanning electron microscopy (SEM) were used to evaluate the failure modes and surface structure. FEA modeling of the crowns was obtained. Results: Mean FR values (N ± SD) of CD (4408 ± 608) and L (4323 ± 462) were higher than P (2507 ± 594) (p < 0.05). Mean FS values (MPa ± SD) of CD (583 ± 63) and P (566 ± 54) were higher than L (428 ± 41) (p < 0.05). Mean SBS values (MPa ± SD) of CD (49 ± 6) (p < 0.05) were higher than L (28 ± 5) and P (30 ± 8). For crown restorations, while cohesive failures within ceramic and zirconia were seen in CD, cohesive failures within ceramic were found in both L and P. Results were verified by FEA. Conclusion: The file splitting technique showed higher bonding values in all mechanical tests, whereas a layering technique increased the FR when an anatomical core design was employed. Clinical significance: File splitting (CAD-on) or layering veneering ceramic on zirconia with a reduced framework design may reduce ceramic chipping.
Presintered yttrium-stabilized zirconium oxide blocks fabricated to be milled with computer-aided design/computeraided manufacturing (CAD/CAM) systems as a framework, have been increasingly used in esthetic dentistry.1 CAD/CAM
technology has been reported to provide homogenous zirconia frameworks with no imperfections and/or porosities that may lead to veneering ceramic delamination and clinical failures.2,3
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
1
Mechanical Performance of Veneering on Zirconia
Kanat et al
Different methods can be applied for veneering ceramic on zirconia frameworks. In the traditional layering technique, the mixed ceramic powder and its liquid are built on the sintered zirconia core being larger than the final dimension to compensate for the shrinkage of ceramic,4,5 whereas the overpressing technique requires a final contour wax-up model on the sintered zirconia framework to be invested under heat-pressed vacuum with pressable ceramics.4 The subtractive type of file-splitting method involves the fabrication of metal-ceramic restorations through scanning by a digital probe for data aquisition.6 In addition to these commonly used veneering techniques for zirconia, a new technique with the generic term “CAD-on” has been introduced.3,7 In this technique, a presintered zirconia framework and a high-strength ceramic veneering material are designed and milled through CAD/CAM, and then sintered zirconia and ceramic suprastructure are combined by a fusion glass ceramic.3,7 Because of the thixotropic feature of the fusion glass ceramic, mixing it with a special vibration device is recommended by the manufacturer in the CAD-on technique.7 The capsule fabricated for single use is put onto a flat surface of a special vibrator for 10 seconds to liquefy and be inserted into superstructures for fusing procedures. The mixer allows the material to be more homogenous by getting the components together between the cores and suprastructures.8 Anatomical cusp support design of the CAD/CAMfabricated zirconia core with layering ceramic under mechanical loading was reported to significantly reduce the chipping failures of zirconia-supported crowns,9 which has been attributed to optimal preservation of the uniform thickness of the veneering ceramic through controlled CAD.9,10 By decreasing the chipping of the veneering ceramic, removal and renewal of the restorations can be prevented clinically. Also, by this route, time and cost for both the patient and the dentist, as well as excessive material consumption in the laboratory, can be eliminated.7,11 Studies have emphasized the significance of form and thicknesses of the frameworks, veneering techniques, quality, and homogeneity of the veneering ceramic as well as the physical properties such as the coefficient of thermal expansion and elastic modulus of veneering ceramic and zirconia for durable bond strengths between the two materials.10,11 Chipping of the veneering ceramic in zirconia-supported restorations has been reported to be one of the most common clinical failure types12 with a 25% failure rate in a 3-year follow-up randomized controlled clinical trial.13 Since the nature of the clinical problem determines the systematic approach for mechanical/physical and/or clinical testing, the main question to be answered when incorporating a novel veneering method on a zirconia framework depends on the above-mentioned major failure type: ceramic chipping. Such a failure can be identified by analytical and experimental procedures validated by clinical studies. Recommended mechanical tests for monolithic and/or bilayered specimens might be three- or four-point flexure tests, biaxial flexure tests, or shear bond tests coupled with finite element analysis (FEA) and/or fractographic analyses.14 Thus, mechanical tests may allow for identification of the failure initiation site of zirconia framework and veneering ceramic combinations fabricated with different layering techniques.14,15 2
The aim of this study was to investigate the effect of different veneering procedures (layering, overpressing, and CAD-on) on the fracture resistance (FR) of single molar crown restorations with zirconia framework. Three complementary tests were performed to interpret the findings: (a) the effect of flexural strength (FS) of the veneering ceramic on the FR of the restoration by performing a three-point bending test on the bar-shaped specimens; (b) the effect of interfacial bonding on the FR of the restoration by a shear bond strength (SBS) test using diskshaped specimens; and (c) finite element modeling of the crown system to observe the location of the accumulated stresses by simulating loading conditions. The null hypothesis of this study was that the veneering technique would not affect the FR of the bilayered zirconia crowns.
Materials and methods Crown, bar, and disk-shaped specimens for FR, three-point bending (FS), and SBS tests, respectively, were prepared with CAD/CAM-fabricated (InLab; Sirona Dental Systems GmbH, Bensheim, Germany) zirconia frameworks and veneered with three techniques on the substructures as: layering (L), overpressing (P), and CAD-on (CD) (N = 90) (n = 10/subgroup). The brands, manufacturers, chemical compositions, some mechanical properties, and batch numbers of the materials used in this study are listed in Table 1. Preparation of zirconia frameworks
For crown-shaped specimens, thirty stainless steel dies (n = 30) with a 1 mm standard circumferential chamfer, which had a groove on the axial surface to prevent the rotation of the restorations during the mechanical test, were scanned (InEos Blue; Sirona Dental Systems) using a labside contrast spray (IPS Contrast Spray Labside; Ivoclar Vivadent). Digital impressions were obtained. The restoration form was chosen for the mandibular left first molar in the CAD/CAM software (CEREC 3D, Sirona InLab V3.88; Sirona Dental Systems). Multilayered design was performed for CD, whereas the reduced form with 1 mm was preferred for both L and P, producing a uniform veneering ceramic thickness and support for the cusps.16 The thickness of the die spacer was selected as 10 μm, since the die material was not a tooth substance, but made of metal. The adaptation of the framework to the metal die was aimed to decrease the rotation of the substructure during fracture tests.3 Circumferential margin and insertion path of the metal dies were determined as the groove was positioned buccally. The zirconia frameworks were milled (InLab MC XL; Sirona Dental Systems) out of presintered zirconia blocks (IPS e.max ZirCAD) with 20% to 25% enlarged volume to compensate for shrinkage after the sintering process. Then, the frameworks were sintered (InFire HTC speed; Sirona Dental Systems), and their fit to the dies was controlled. Bar- (length: 25 mm, width: 5 mm, height: 1.5 mm) and square- (4 × 4 mm2 ) shaped specimens (n = 30/per group), which would be used in three-point bending and SBS tests, respectively, were sectioned in three axes under water cooling using a low-speed diamond saw (Isomet 1000; Buehler Ltd., Lake Bluff, IL) from presintered yttrium-stabilized
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
Kanat et al
Mechanical Performance of Veneering on Zirconia
Table 1 Materials used
Brand IPS e.max ZirCAD IPS e.max Ceram
Manufacturer Ivoclar Vivadent, Schaan, Liechtenstein Ivoclar Vivadent, Schaan, Liechtenstein
IPS e.max ZirPress
Ivoclar Vivadent, Schaan, Liechtenstein
IPS e.max CAD
Ivoclar Vivadent, Schaan, Liechtenstein
IPS e.max CAD Crystall/Connect RelyX luting cement IPS e.max Ceram Glaze
Ivoclar Vivadent, Schaan, Liechtenstein 3M ESPE, Seefeld, Germany Ivoclar Vivadent, Schaan, Liechtenstein
Chemical composition (%) ZrO2 (87–95) Y2 O3 (4–6) HfO2 (1–5) Al2 O3 (0–1) SiO2 (60–65), Al2 O3 (9–11), K2 O (7–8), Na2 O (7–8), ZnO2 (2–3), CaO, P2 O5 , and F (2,5–7,5) SiO2 (57–62), Na2 O (6–8), K2 O (2–4), CaO (2–4), Al2 O3 (12–16), ZrO2 (1,5–2,5), P2 O5 (1–2), and F (0,5–1) SiO2 (57–80), Li2 O (11–19), K2 O (0–13), P2 O5 (0–11), ZrO2 (0–8), ZnO (0–8), Other and coloring oxides (0–12) Oxides, water, butandiol, and chloride Sılane-treated glass (> 98), potassıum persulfate ( 0.05) (Table 3). For FS (MPa ± SD), the L group exhibited the lowest values (p < 0.05), and there were no significant differences between P and CD groups (p ≥ 0.05) (Table 3). SBS tests (MPa ± SD) resulted in significantly higher values in the CD group (p < 0.05), but there were no significant differences between P and L groups (p ≥ 0.05). Estimates of the parameters of the Weibull distribution for the FR, FS, and SBS mechanical tests are presented in Table 4. The test to estimate shape parameters (p ≥ 0.05) indicated no statistical differences in Weibull shape parameters among the veneering groups for any of the mechanical tests (Table 4). The test for equal scale parameters (p < 0.05), together with the Bonferroni post hoc confidence interval, indicated that all of the veneering method groups had different scale parameters for FR, FS, and SBS mechanical tests (Table 4). According to the analysis with the Weibull distribution, probability plot and
The exact reason for veneer chipping observed in zirconia core restorations is unclear, but it is thought that generally three factors play an important role in the chipping of the ceramic for zirconia-supported restorations, namely interfacial bonding, match of the core-veneer materials, and strength of the veneering ceramic.2,20,21 Veneering techniques also have a potential effect on the chipping of ceramic due to the processing methods of ceramic.2,4 Accordingly, in the present study, the FR of single molar zirconia crowns with zirconia frameworks and veneering ceramics prepared with different techniques (CAD-on, layering, overpressing) and the effect on chipping were investigated. Three complementary tests were conducted, in addition to the FR test, to interpret the findings, namely three-point bending test to investigate the effect of FS of the veneering ceramic on the restoration; SBS testing of the veneering ceramic and zirconia framework bilayered system to find out whether the bond strength affected the FR of the crowns; and finite element modeling simulating the loading conditions of the crown system to evaluate the location of the accumulated stresses. Also, to eliminate the mismatch of the framework and suprastructures, the materials were chosen according to the
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
5
Mechanical Performance of Veneering on Zirconia
Kanat et al
Figure 1 Survival and probability plots of FR, FS, and SBS test groups.
manufacturers’ instructions. The results of this study demonstrated that the CAD-on technique resulted in significantly higher FR values, although anatomical zirconia frameworks were used in the L and P groups. There were no significant differences between CD and L groups with anatomical cores according to ANOVA analysis, whereas the CD group had significantly higher values with respect to the L group according to Weibull analysis. Thus, the null hypothesis was rejected. Currently used veneering techniques on zirconia frameworks are layering, overpressing, or recently improved file-splitting (CAD-on). The layering technique is a very delicate method, as several factors such as the experience of the dental technician, homogeneity of the mixed ceramic slurry, duration of the firing and cooling, the number of firings, and shrinkage
6
of the ceramic play an important role in the success of the veneering technique.4,5,14,17 The veneering ceramic used in this technique has been reported to have an FS of approximately 90 MPa. In the overpressing technique, the modeling Table 5 Failure-type distributions of mechanical test groups. A: Adhesive, CC: Cohesive within veneering ceramic, CZ: Cohesive within zirconia, M: Mixed FR
CD P L
FS
SBS
A
CC
CZ
M
A
CC
CZ
M
A
CC
CZ
M
-
2 10 10
8 -
-
-
10 8 9
-
2 1
7 5 7
1 -
-
3 4 3
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
Kanat et al
Mechanical Performance of Veneering on Zirconia
Figure 2 Representative failure types and FEA models of the crown restorations. (a1) Cohesive failures within ceramic and zirconia in CD group. (a2-a3) von Mises stress distributions of CD can be seen. (b1) Cohesive failures within ceramic in L group. (b2-b3) von Mises stresses
distributed over the entire surfaces can be observed in L. (c1) Cohesive failure within ceramic and lateral cracks due to the stress peaks as indicated with an arrow in the P group. (c2-c3) von Mises stress distribution of P.
wax is placed on the sintered zirconia, and this combination is attached to a flask by a sprue. The flask is filled with a special investment material according to the manufacturer’s instructions. After the wax is burnt out, the pressable ceramics, which have about 110 MPa FS, are pressed into the spaces, and the final anatomic restorations are obtained.4 This technique allows the restoration to be produced faster and easier, compared with the conventional layering technique. Moreover, the firing shrinkage observed in the layering technique can be minimized, and a good marginal adaptation of the restoration can be obtained.17 However, preliminary processes such as accurate attachment to the flask, cleanness of the modeling wax, quality of the pressing material, temperature of the preheating
furnace, and controlled air-abrasion during separation of the restorations from the investment material might affect the success of this method. Particle size, morphology, and pressure during air-abrasion may damage the ceramic surface. Since the surfaces of the zirconia in the present study were already veneered, they were not subjected to additional air-abrasion during the airborne-particle abrasion procedure for divesting. With the newly developed file-splitting (generic term: “CADon”) technique, time-consuming and operator-sensitive laboratory procedures such as impression making, model obtaining, investing, and finishing can be eliminated with the aid of computer-controlled design and fabrication.22 Furthermore, the file-splitting (CAD-on) technique is introduced as a more
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
7
Mechanical Performance of Veneering on Zirconia
Kanat et al
Figure 3 Representative SEM images of the fractured bar-shaped specimens. (C: ceramic, Z: zirconia, A: adhesive material). (a1-a2) (left column) Homogeneous ceramic and crack origin in the adhesive material as indicated with an arrow can be seen for CD. (b1-b2) (center column) Crack
propagation arose from lateral cracks and porosities due to layering technique observed in L. (c1-c2) (right column) Porosities along the interface and lateral cracks in P.
Figure 4 Representative SEM images of the debonded surfaces of disk-shaped specimens. (C: ceramic, Z: zirconia). (a1-a2) (left column) Mixed failures in the CD group. (b1-b2) (center column) Mixed failures in the L group. (c1-c2) (right column) Mixed failures in the P group.
reliable method because of the advantages of prefabricated blocks manufactured by industrial pressing without any porosities, perfect adaptation of the restorations by CAD/CAM system, and fast processing.3 For in vitro studies, preparation of bilayered specimens has been recommended since it has been reported that the super8
structure, framework, and the interfacial material act mutually under force.20 Using analysis methods such as SEM and/or FEA of fractured surfaces for assessment of the failed surfaces together with the mechanical tests, a connection between in vivo and in vitro studies can be established.14 In the literature, the chipping problem was investigated using only one mechanical
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
Kanat et al
test, and no studies about the recently introduced CAD-on technique could be derived with 3-point bending and SBS tests. In this study, the crown-shaped restorations were used to imitate the clinical situation for FR tests. To evaluate the effect of FS of the ceramic on the FR, bar-shaped specimens were prepared for 3-point bending tests. Also, disk-shaped specimens were obtained for the SBS test to investigate the interfacial bonding effect on the FR of crown restorations. In addition to mechanical tests, stereomicroscopy and SEM were used to evaluate the failure modes and debonded surface structures. The results of the present study may provide clinically relevant data for wider application of these materials. It was previously stated that the rate and area of ceramic chipping in zirconia-supported crowns might be significantly reduced when an anatomically reduced core design was used because of the cusp support and homogenous thickness of the ceramic.9,16 It has also been reported that support of the anatomical zirconia framework prevented chipping compared to the results of FS of the ceramic.9 Therefore, anatomical framework design was used for the L and P groups in the present study. Eisenburger et al17 found that layering of the veneering ceramic group (IPS e.max ZirCAD-IPS e.max Ceram) showed better FR values (919 ± 265 N) than the overpressing group (IPS e.max ZirCAD-IPS e.max ZirPress) (798 ± 226 N). Although the results of the present study were compatible with these results, the reason for higher FR values (L: 4323 ± 462 N; P: 2507 ± 594 N) obtained in the present study could be attributed to the differences in geometry and the anatomically shaped framework used in this study.15 In a previous study, fracture strengths of crown specimens made by CAD/CAM fabricated nonanatomic formed zirconia framework (IPS e.max ZirCAD) and veneering ceramics (IPS e.max CAD for S, IPS e.max Ceram for L, IPS e.max ZirPress for P) were found to be 6265 ± 2257 N with sintering, 3700 ± 1239 N with layering, and 3524 ± 1181 N with overpressing methods.3 In the present study, the FR of CAD/CAM-fabricated zirconia frameworks veneered with different veneering techniques tested on crown specimens were 4408 ± 608 N for CD and 4323 ± 462 and 2507 ± 594 N for L and P, respectively. The results of the present study showed that higher FR values could be obtained in the layering technique when an anatomical zirconia framework design is employed. In the P group, lower FR values could be attributed to chipping of the veneering ceramic into smaller pieces, causing lateral cracks. Also, the fracture types observed in S and L in the literature3 were compatible with our study, since failure within both veneer and zirconia was observed more frequently in CD. On the other hand, only ceramic chipping was observed in L. The overpressing veneering technique (IPS e.max ZirPress) (1514 ± 332 N) was reported to show higher load-bearing capacity than the layering technique (IPS e.max Ceram) (894 ± 160 N) on the zirconia core (ZENO TEC; Wieland Dental, Pforzheim, Germany) according to the Voss test of canine-form restorations.5 Tsalouchou et al23 showed that the differences between the layering (IPS e.max Ceram) (2189.9 ± 317.6 N) and overpressing (IPS e.max ZirPress) (2135.6 ± 330.1 N) veneering materials on nonanatomically designed zirconia frameworks (Kavo Everest, Biberach, Germany) following cyclic loading were insignificant. The differences between the results obtained in the above-mentioned
Mechanical Performance of Veneering on Zirconia
studies and in our study could be attributed to different framework and veneering ceramic combinations as well as differing mechanical testing modalities that may affect the bond strength. The anatomical framework design in the present study might have added to the increased bond strength values. Liu et al24 reported that zirconia-supported specimens under 3-point bending test were broken into two pieces as in the present study. It has been indicated that the FS of the ceramic plays an important role on the fracture pattern. A ceramic with 300 MPa FS can lead to less delamination.24 In the CD group in the present study,3 a lower delamination rate was observed in the lithium disilicate group, because the FS was 360 MPa. According to SEM analysis of fractured bar specimens in the present study, the ceramic structure without any imperfections in CD can be due to the homogeneity of the ceramic block. Lateral cracks and porosities observed in both the L and P groups can be explained with the sensitivity of the veneering techniques. When the effect of FS of the ceramic according to FS test on the FR values were examined in the present study, higher FS of the ceramic for the CD group played an important role on the FR. For the L group, higher FR values could not be supported with FS values, which indicated the effect of the anatomical framework design on the FR rather than the FS of the layering ceramic. For the P group, the authors attributed the differences found between lower FR and higher FS values to contributing factors such as different fabrication procedures, experience in handling of overpressing technology for the dental technician, and recently suggested slower cooling parameters.16,25-27 Ishibe et al4 investigated the SBS of zirconia cylinders (Lava; 3M ESPE, St. Paul, MN) (diameter: 5 mm) to layered or pressed ceramics (diameter: 3 mm). While the overpressing group (e.max ZirPress) showed 40.41 ± 10.28 MPa SBS, in the layering group (e.max Ceram), 30.03 ± 9.49 MPa was obtained. The differences in the framework-superstructure combination and thermocycling application could be reasons for the discrepancy between the results. L´opez-Moll´a et al28 also reported SBS of zirconia disks (IPS e.max ZirCAD) (length: 15 mm; diameter: 8 mm) to layered (IPS e.max Ceram) and pressable ceramics (IPS max ZirPress) (length: 2 mm; diameter: 8 mm) as 7.85 ± 2.50 MPa and 12.69 ± 2.14 MPa, respectively. Higher SBS results in the present study, which were similar to the veneering techniques can be explained by different geometries and loading conditions of the specimens. Although various opinions about the failure types after SBS test were found in the literature, no correlations between the SBS values and types of failures were observed in the present study. Ereifej et al29 reported that layering veneering ceramic (IPS e.max Ceram) on zirconia cores (IPS e.max ZirCAD) led to lower SBS values (28.8 ± 9.5 MPa), adhesive failures because of insufficient wettability of the liner material applied on the zirconia, and imperfections that occurred at the interfacial surface. SBS values (28 ± 5 MPa) and adhesive fracture types compatible with the literature were also observed in the present study. Failure types after SBS test, observed in both stereomicroscopy and SEM, could be different.30 SEM analysis revealed a thin ceramic layer on the zirconia framework. Hence, careful investigation was recommended when stereomicroscopy was used, since it provides less detailed topographic information.
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
9
Mechanical Performance of Veneering on Zirconia
Kanat et al
In the present study, no correlations were found between the failure types following SBS tests and different veneering techniques, since stereomicroscopy was conducted for overall surface inspection, and only representative specimens were observed under SEM. When the effect of interfacial bonding strength on the FR values according to SBS tests were examined in the present study, higher interfacial bonding in the CD group was noted. That eventually played an important role on the FR. This can be attributed to the durable bonding provided by the glass interfacial ceramic. For the L group, higher FR values were also not replicated with the SBS values, and an explanation for this result might be that the anatomical framework design plays an important role on the higher FR rather than interfacial bonding. For the P group, lower FR and SBS values can be attributed to low strength of the ceramic. According to the overall results of the three mechanical tests, FEA of the crowns and failure assessment, higher FR, bond strength, and FSs of the ceramic in the CD technique, as well as cohesive failure observed within the ceramic and zirconia for most specimens can be attributed to durable interfacial bonding. The stress propagation to the framework meant that a threelayered system behaved as a monolithic structure. The FSs of the interfacial fusion glass-ceramic material and veneering ceramic play an important role in the success of the veneering technique. For the L group, higher FR, low strength of the interfacial bonding and veneering ceramic, cohesive failures within the ceramic, porosities in SEM, and the stress distribution along the interfacial surface indicated that the success of the crown restoration under the FR test arose from anatomical framework design. For the P group, low FR, low interfacial bonding, cohesive failures within the ceramic, lateral cracks observed in crown restorations, porosities along the interface in the SEM images, and localized stresses revealed insufficient interfacial bonding and weak ceramic strength as well as possible lack of wettability. The cutting procedure for ceramic materials by a diamond saw at a slow speed under water cooling is used as a routine process.31,32 Either some polishing stages can be followed after sawing procedures,31 or the sectioned specimens can be subjected to the application of veneering ceramic directly.32 The influence on FS of surface roughness obtained after grinding showed a decrease in surface roughness, but caused an increase in FS.33 It was not in the scope of this study to investigate the effect of surface alterations arising from standard sawing but could be the subject of future studies. Also, future studies are required to investigate the effect of wettability properties of the ceramic on the bond strength. Long-term in vivo studies would also demonstrate the clinical performance of the CAD-on technique on the veneering ceramic chipping complications.
Conclusion From this in vitro study, the following conclusions were drawn: 10
1. The file-splitting (CAD-on) technique could decrease ceramic chipping due to higher strength of the ceramic and the interfacial bonding. 2. Anatomical framework design increased the FR when a layering veneering technique was employed.
Acknowledgments The authors would like to thank Prof. Dr. Tijen Pamir at the Department of Restorative Dentistry, Ege University School of Dentistry, Izmir, Turkey and Assist. Prof. Dr. Osman C ¸ ulha at the Department of Materials Engineering, Celal Bayar University, Manisa, Turkey for the mechanical tests and Canderim ¨ Onder at Netform, Izmir, Turkey for FEA analyses. The authors extend their gratitude to Ivoclar Vivadent, Schaan, Liechtenstein for providing the veneering ceramic and fusion glassceramic materials.
References 1. Abduo J, Lyons K, Swain M: Fit of zirconia fixed partial denture: a systematic review. J Oral Rehabil 2010;37:866-876 2. Denry I, Kelly JR: State of the art of zirconia for dental applications. Dent Mater 2008;24:299-307 3. Beuer F, Schweiger J, Eichberger M, et al: High-strength CAD/CAM-fabricated veneering material sintered to zirconia copings—a new fabrication mode for all-ceramic restorations. Dent Mater 2009;25:121-128 4. Ishibe M, Raigrodski AJ, Flinn BD, et al: Shear bond strengths of pressed and layered veneering ceramics to high-noble alloy and zirconia cores. J Prosthet Dent 2011;105:29-37 ¨ 5. Stawarczyk B, Ozcan M, Roos M, et al: Load-bearing capacity and failure types of anterior zirconia crowns veneered with overpressing and layering techniques. Dent Mater 2011;27:1045-1053 6. Goiato MC, Santos MR, Moreno A, et al: Prototyping for surgical and prosthetic treatment. J Craniofac Surg 2011;22:914-917 7. Schmitter M, Mueller D, Rues S: Chipping behaviour of all-ceramic crowns with zirconia framework and CAD/CAM manufactured veneer. J Dent 2012;40:154-162 8. Wiedhahn K: The impression-free Cerec multilayer bridge with the CAD-on method. Int J Comput Dent 2011;14:33-45 9. Rosentritt M, Steiger D, Behr M, et al: Influence of substructure design and spacer settings on the in vitro performance of molar zirconia crowns. J Dent 2009;37:978-983 10. Komine F, Blatz MB, Matsumura H: Current status of zirconia-based fixed restorations. J Oral Sci 2010;52:531-539 11. Rekow ED, Silva NR, Coelho PG, et al: Performance of dental ceramics: challenges for improvements. J Dent Res 2011;90:937-952 12. Raigrodski AJ, Hillstead MB, Meng GK, et al: Survival and complications of zirconia-based fixed dental prostheses: a systematic review. J Prosthet Dent 2012;107:170-177 13. Sailer I, Gottner J, Kanel S, et al: Randomized controlled clinical trial of zirconia-ceramic and metal-ceramic posterior fixed dental prostheses: a 3-year follow-up. Int J Prosthodont 2009;22: 553-560 14. Anusavice 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;18:218-231
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
Kanat et al
¨ 15. D¨undar M, Ozcan M, G¨okc¸e B, et al: Comparison of two bond strength testing methodologies for bilayered all-ceramics. Dent Mater 2007;23:630-636 16. Guess PC, Bonfante EA, Silva NR, et al: Effect of core design and veneering technique on damage and reliability of Y-TZP-supported crowns. Dent Mater 2013;29: 307-316 17. Eisenburger M, Mache T, Borchers L, et al: Fracture stability of anterior zirconia crowns with different core designs and veneered using the layering or the press-over technique. Eur J Oral Sci 2011;119:253-257 ¨ 18. Comlekoglu M, Dundar M, Ozcan M, et al: Influence of cervical finish line type on the marginal adaptation of zirconia ceramic crowns. Oper Dent 2009;34:586-592 19. Al-Dohan HM, Yaman P, Dennison JB, et al: Shear strength of core-veneer interface in bi-layered ceramics. J Prosthet Dent 2004;91:349-355 20. Studart AR, Filser F, Kocher P, et al: Mechanical and fracture behavior of veneer-framework composites for all-ceramic dental bridges. Dent Mater 2007;23:115-123 21. Swain MV: Unstable cracking (chipping) of veneering porcelain on all-ceramic dental crowns and fixed partial dentures. Acta Biomater 2009;5:1668-1677 22. van Noort R: The future of dental devices is digital. Dent Mater 2012;28:3-12 23. Tsalouchou E, Cattell MJ, Knowles JC, et al: Fatigue and fracture properties of yttria partially stabilized zirconia crown systems. Dent Mater 2008;24:308-318 24. Liu Y, Feng H, Bao Y, et al: Fracture and interfacial delamination origins of bilayer ceramic composites for dental restorations. J Eur Ceram Soc 2010;30:1297-1305
Mechanical Performance of Veneering on Zirconia
25. Christensen RP, Ploeger BJ: A clinical comparison of zirconia, metal and alumina fixed-prosthesis frameworks veneered with layered or pressed ceramic: a three-year report. J Am Dent Assoc 2010;141:1317-1329 26. Rosentritt M, Kolbeck C, Handel G, et al: Influence of the fabrication process on the in vitro performance of fixed dental prostheses with zirconia substructures. Clin Oral Investig 2011;15:1007-1012 27. Guazzato M, Walton TR, Franklin W, et al: Influence of thickness and cooling rate on development of spontaneous cracks in porcelain/zirconia structures. Aust Dent J 2010;55:306-310 28. Lopez-Molla MV, Martinez-Gonzalez MA, Manes-Ferrer JF, et al: Bond strength evaluation of the veneering-core ceramics bonds. Med Oral Patol Oral Cir Bucal 2010;15:19-23 29. Ereifej N, Rodrigues FP, Silikas N: Experimental and FE shear-bonding strength at core/veneer interfaces in bilayered ceramics. Dent Mater 2011;27:590-597 30. Ashkanani HM, Raigrodski AJ, Flinn BF, et al: Flexural and shear strengths of ZrO2 and a high-noble alloy bonded to their corresponding porcelains. J Prosthet Dent 2008;100:274-284 31. Kim HJ, Lim HP, Park YJ, et al: Effect of zirconia surface treatments on the shear bond strength of veneering ceramic. J Prosthet Dent 2011;105:315-322 32. Aboushelib MN, Kleverlaan CJ, Feilzer AJ: Microtensile bond strength of different components of core veneered all-ceramic restorations. Part II: zirconia veneering ceramics. Dent Mater 2006;22:857-863 33. Flury S, Peutzfeldt A, Lussi A: Influence of surface roughness on mechanical properties of two computer-aided design/computer-aided manufacturing (CAD/CAM) ceramic materials. Oper Dent 2012;37:617-624
C 2014 by the American College of Prosthodontists Journal of Prosthodontics 00 (2014) 1–11
11
