Moustafa N. Aboushelib

Selective infiltration-etching technique for a strong and durable bond of resin cements to zirconia-based materials

STATEMENT OF PROBLEM Establishing a strong and a stable adhesive bond between yttrium, partially stabilized, tetragonal zirconia, polycrystal materials (Y-TZP) and resin luting agents has proven to be difficult using conventional surface roughening and coating methods. PURPOSE The purpose of this study was to evaluate the zirconia-resin bond strength and durability using a selective infiltration-etching technique. MATERIAL AND METHODS Seventy-two Y-TZP discs (19.5 x 3 mm) were airborne-particle abraded with 110-mum aluminum oxide particles and divided into 4 groups (n=18). One test group received the selective infiltration-etching surface treatment. Three commercial adhesive systems (Panavia F 2.0, RelyX ARC, and Bistite II DC) were used to bond the airborne-particle-abraded zirconia specimens to preaged restorative composite resin discs (Filtek Z250). Panavia was used to bond the selective infiltration-etched specimens. The bonded specimens were cut into microbars (6 x 1 x 1 mm), and a microtensile bond strength test (MTBS measured in MPa) was conducted immediately, after 1 week, 2 weeks, 3 weeks, and after 1 month of water storage (5 microbars/disc/time interval/group, n = 450 microbars/group). Scanning electron microscopy was used to examine the fractured microbars. The density (g/cm(3)) and the 4-point flexure strength (MPa) of the selective infiltration-etched and airborne-particle-abraded specimens were measured to evaluate the effect of selective infiltration etching on the structural integrity of the Y-TZP specimens. A repeated measures ANOVA with 1 within-subjects factor (time, 5 levels) and 1 between-subjects factor (technique, 4 levels) was used to analyze the data (alpha=.05). Pairwise comparisons were made using the Bonferroni post hoc test. RESULTS There were significant differences in the initial MTBS values (MPa) between the 4 bonding techniques (P<.001). Airborne-particle-abraded specimens bonded with either Panavia F 2.0, RelyX ARC, or Bistite II DC resulted in a mean (SD) bond strength of 23.3 (2.4), 33.4 (2.1), 31.3 (2.8) MPa, respectively, while the highest bond strength of 49.8 (2.7) MPa was achieved for the selective infiltration-etched specimens bonded with Panavia F 2.0. There was a significant interaction between water storage time and the bonding technique (P<.001) as reduction in MTBS values was observed with time, except for the specimens bonded with Panavia (selective infiltration-etched and airborne-particle-abraded specimens). Additionally, the observed failure mode was primarily cohesive for the selective infiltration-etched specimens, in contrast to the other groups, which showed primarily an interfacial failure. CONCLUSIONS For the materials used in this study and under the same testing conditions, selective infiltration etching is a reliable method for establishing a strong and durable bond with zirconia-based materials.

Effect of zirconia type on its bond strength with different veneer ceramics

PURPOSE The bond strength between veneer ceramic and the zirconia framework is the weakest component in the layered structure. This bond was proven to be sensitive to the surface finish of the framework material and to the type of the veneer ceramic and its method of application. New colored zirconia frameworks were introduced to enhance the final esthetics of the layered all-ceramic restoration. The aim of this study was to investigate the effect of zirconia type, white or colored, and its surface finish on the bond strength to two veneer ceramics. MATERIALS AND METHODS Five commercial zirconia framework materials (Cercon white and yellow, Lava white and yellow, Procera zirconia) received either of the following surface treatments: CAD/CAM milled surface, airborne-particle abrasion, and liner application. Two veneering ceramics were used to veneer the specimens: Noble Rondo and Ceram Express. The disc-shaped layered specimens were cut into microbars, and microtensile bond strength (MTBS) test was conducted. Structural and chemical differences between the white and colored frameworks were evaluated using scanning electron microscopy (SEM) and energy dispersive analysis. Two-way ANOVA and Tukey post hoc tests were used to analyze the data (p < 0.05 was considered significant). RESULTS The type of zirconia framework had a significant effect on the core-veneer bond strength, which was material dependent. The bond strength to colored zirconia was significantly weaker compared to white zirconia frameworks. Different surface treatments had different effects on the core-veneer bond strength according to the zirconia material used. Although no marked chemical differences between the examined zirconia materials could be found, there were structural differences, especially between white and colored zirconia and for different zirconia frameworks of different manufacturers, which significantly affected core-veneer bond strength values. CONCLUSION The addition of coloring pigments to zirconia frameworks resulted in structural changes that require different surface treatment before veneering. To prevent delamination and chipping failures of zirconia veneered restorations, careful selection of both framework and veneer ceramic materials, in addition to proper surface treatment, are essential for maintaining good bond strength.

Bridging the gap between clinical failure and laboratory fracture strength tests using a fractographic approach

OBJECTIVE The aim of this study was to analyze and to compare the fracture type and the stress at failure of clinically fractured zirconia-based all ceramic restorations with that of morphologically similar replicas tested in a laboratory setup. METHODS Replicas of the same shape and dimensions were made for 19 crowns and 17 fixed partial dentures, all made of veneered zirconia frameworks, which fractured during intra-oral service. The replicas were statically loaded by applying axial load in a universal testing machine. The principles of fractography were used to identify the location and the dimensions of the critical crack and to estimate the stress at failure. Failure was classified according to origin and type (P<0.05 was considered significant). RESULTS Clinically fractured restorations failed due to either: delamination of the veneer ceramic (28.2+/-9MPa), defects at core veneer interface (27.7+/-6MPa), the generation of Hoop stresses (884.3+/-266MPa), radial cracking (831MPa), or fracture of the connector (971+/-343MPa). The replicas failed by mainly by cone cracking of the veneer ceramic (52.4+/-34.8MPa) or by fracture of the connector (1098.9+/-259MPa). The estimated stress at failure was significantly higher for the replicas compared to the clinically fractured restorations (F=6.8, P<0.01). SIGNIFICANCE Within limitations of this study, careful design of fracture strength test would lead to more clinically relevant data. The performance of zirconia veneered restorations could be further improved with careful design considerations.

Innovations in bonding to zirconia-based materials. Part II: Focusing on chemical interactions

OBJECTIVES The zirconia-resin bond strength was enhanced using novel engineered zirconia primers in combination with selective infiltration etching as a surface pre-treatment. The aim of this study was to evaluate the effect of artificial aging on the chemical stability of the established bond and to understand the activation mechanism of the used primers. METHODS Selective infiltration etched zirconia discs (Procera; NobelBiocare) were coated with one of four novel engineered zirconia primers containing reactive monomers and were bonded to resin-composite discs (Panavia F2.0). Fourier transform infrared spectroscopy (FT-IR) was carried out to examine the chemical activation of zirconia primers from mixing time and up to 60min. The bilayered specimens were cut into microbars (1mm(2) in cross-section area) and zirconia-resin microtensile bond strength (MTBS) was evaluated immediately and after 90 days of water storage at 37 degrees C. Scanning electron microscopy (SEM) was used to analyze the fracture surface. RESULTS There was a significant drop in MTBS values after 90 days of water storage for all tested zirconia primers from ca. 28-41MPa to ca. 15-18MPa after completion of artificial aging. SEM revealed increase in percentage of interfacial failure after water storage. FTIR spectra suggested adequate activation of the experimental zirconia primers within 1h of mixing time. SIGNIFICANCE The novel engineered zirconia primers produced initially high bond strength values which were significantly reduced after water storage. Long-term bond stability requires developing more stable primers.

Bonding to zirconia using a new surface treatment

PURPOSE Selective infiltration etching (SIE) is a newly developed surface treatment used to modify the surface of zirconia-based materials, rendering them ready for bonding to resin cements. The aim of this study was to evaluate the zirconia/resin bond strength and durability using the proposed technique. MATERIALS AND METHODS Fifty-four zirconia discs were fabricated and divided into three groups (n = 18) according to their surface treatment: as-sintered surface (control group), airborne-particle abrasion (50-mum aluminum oxide), and SIE group. The zirconia discs were bonded to preaged composite resin discs using a light-polymerized adhesive resin (Panavia F 2.0). The zirconia/resin bond strength was evaluated using microtensile bond strength test (MTBS), and the test was repeated after each of the following intervals of accelerated artificial aging (AA): thermocycling (10,000 cycles between 5 and 55 degrees C), 4 weeks of water storage (37 degrees C), and finally 26 weeks of water storage (37 degrees C). Silver nitrate nanoleakage analysis was used to assess the quality of zirconia/resin interface. A repeated measures ANOVA and Bonferroni post hoc test were used to analyze the data (n = 18, alpha= 0.05) RESULTS There were significant differences in the MTBS values between the three test groups at each of the test intervals (p < 0.001). AA resulted in reduction in the bond strength of the as-sintered and the particle-abraded groups (5.9 MPa and 27.4, MPa, respectively). Reduction in the bond strength of these groups was explained by the observed nanoleakage across the zirconia/resin interface. The bond strength of the SIE specimens was stable after completion of AA (51.9 MPa), which also demonstrated a good seal against silver nitrate penetration across the zirconia/resin interface. CONCLUSION SIE established a strong, stable, and durable bond to zirconia substrates. Conservative resin-bonded zirconia restorations are now possible using this new technique.

Prestresses in Bilayered All-Ceramic Restorations

INTRODUCTION A general trend in all ceramic systems is to use veneering ceramics of slightly lower thermal expansion coefficients compared with that of the framework resulting in a positive mismatch in thermal expansion coefficient (+DeltaTEC). The concept behind this TEC mismatch is to generate compressive stresses in the weaker veneering ceramic and thus enhance the overall strength of the restoration. This technique had excellent results with porcelain fused to metal restorations (PFM). However, there are concerns to apply this concept to all-ceramic restorations. The aim of this research was to determine the stresses in bilayered all-ceramic restorations due to the mismatch in TEC. MATERIALS AND METHODS Two commercial veneering ceramics with a TEC lower than that of zirconia (+DeltaTEC); NobelRondo zirconiatrade mark and Lava Ceramtrade mark, plus one experimental veneering ceramic with an identical TEC that matches that of zirconia (DeltaTEC = 0) were used to veneer zirconia discs. The specimens were loaded in biaxial flexure test setup with the veneer ceramic in tension. The stresses due to load application and TEC mismatch were calculated using fractography, engineering mathematics, and finite element analysis (FEA). RESULTS In this study, the highest load at failure (64 N) was obtained with the experimental veneer where the thermal mismatch between zirconia and veneering ceramic was minimal. For the two commercial veneer ceramics the magnitude of the thermal mismatch localized at the zirconia veneer interface (42 MPa) exceeded the bond strength between the two materials and resulted in delamination failure during testing (ca. 50 MPa). SIGNIFICANCE For all-ceramic zirconia veneered restorations it is recommended to minimize the thermal mismatch as much as possible.

Evaluation of a high fracture toughness composite ceramic for dental applications

PURPOSE The introduction of yttrium partially stabilized zirconia polycrystals (Y-TZP) has pushed the application limits of all-ceramic restorations. The mechanical properties of these materials can be further improved by the addition of a secondary dopant phase. The aim of this work was to evaluate the properties of a new nano-composite ceramic used as a dental framework material. MATERIALS AND METHODS The properties of a new ceria-stabilized tetragonal zirconia polycrystal co-doped with alumina (Ce-TZP-Al) were investigated. Y-TZP was used as control. Sixty bars (20 x 2.5 x 1.5 mm(3)) from each material were prepared by cutting CAD/CAM milling blocks. Twenty specimens were used to measure the 4-point flexural strength and the modulus of elasticity of the tested materials. The remaining specimens were used to measure the fracture toughness using indentation strength (IS), single edge notched beam (SENB), and fractography (FR). The thermal expansion coefficient (TEC) was measured using temperature expansion diagrams. The bond strength of the two framework materials to two esthetic veneer ceramics was tested using the microtensile bond strength test (MTBS). Finally, scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX) were used to analyze the internal structure of the materials. One- and two-way analysis of variance (ANOVA) and Bonferroni post hoc tests were used to analyze the data (alpha= 0.5). RESULTS The flexural strength and modulus of elasticity of Ce-TZP-Al (856 MPa, 170 GPa) were significantly weaker (p < 0.001) than those of Y-TZP (1003 MPa, 215 GPa). The (IS) fracture toughness of the former (19.02 MPa m(1/2)) was significantly higher (p < 0.001) than SENB (12.6 MPa m(1/2)) or FR (12.8 MPa m(1/2)) values. These values were significantly higher (p < 0.001) than the fracture toughness of Y-TZP (7.4 MPa m(1/2)), which showed statistically similar values using the same three techniques. The measured TEC for the two materials was relatively similar, 10.1 microm/ degrees C and 10.4 mum/ degrees C, respectively. Regarding MTBS values, Ce-TZP-Al had significantly lower bond strength values (p < 0.001) and a higher percentage of interfacial failure than Y-TZP, which failed completely cohesively with the two used veneer ceramics. SEM analysis revealed zirconia grains pull out and structural defects at the core-veneer interface for Ce-TZP-Al material, which explained its weak bond to the two used veneers. CONCLUSION Despite the promising mechanical properties of Ce-TZP-Al nano-composite ceramic, its very low bond strength to esthetic veneers leaves such layered restorations highly susceptible to delamination and chipping under function. Further studies are needed to enhance the surface stability of this high fracture toughness ceramic.

Influence of surface nano-roughness on osseointegration of zirconia implants in rabbit femur heads using selective infiltration etching technique.

This study evaluates osseous healing of selective infiltration-etched (SIE) zirconia implants compared to as-sintered zirconia and titanium implants. Twenty implants of each group were inserted in 40 adult New Zealand white male rabbits. After 4 and 6 weeks, bone blocks containing the implants were retrieved, sectioned, and processed to evaluate bone-implant contact (BIC) and peri-implant bone density. SIE zirconia implants had significantly higher BIC and marginally higher bone density. The results suggest that selective infiltration-etched zirconia implant surface may improve implant osseointegration.

Effect of surface treatment on flexural strength of zirconia bars

STATEMENT OF PROBLEM Clinical and laboratory processing techniques induce damage to the surface of zirconia frameworks, which significantly lessens their strength. PURPOSE The purpose of this study was to investigate the influence of 3 surface restoration methods on the flexural strength of zirconia bars. MATERIAL AND METHODS Bar-shaped specimens were polished and received 1 of 2 surface treatments (n=20): airborne-particle abrasion with 110-microm aluminum oxide particles at a pressure of 0.2 MPa, or grinding with a diamond point under water cooling and a load of 2 N using an air turbine. Polished specimens served as the control (n=20). The induced surface damage was restored using one of the following restoration methods: polishing of the specimens using an aluminum oxide polishing point (2-microm grit) coated with 0.5-microm diamond polishing paste, application of a thin coat of glazing porcelain according to the manufacturers instructions, or application of a phosphate ester monomer (MDP) containing an adhesive resin. The specimens were subjected to a 4-point flexural strength test and then examined using a scanning electron microscope (SEM). Two-way ANOVA and the Bonferroni post hoc test were used to analyze the data (alpha=.05). RESULTS There was a significant interaction between the type of surface damage and the restoration method (P<.001). For the ground specimens, all restoration methods resulted in a statistically significant regain in strength, with the polishing procedure being the most effective (1027 MPa). For airborne-particle-abraded specimens, application of the bonding agent was the only effective restoration method. SEM analysis of the fractured surfaces of specimens indicated that application of the bonding agent resulted in sealing of the surface damage produced by airborne-particle abrasion, while polishing was successful in removing the grinding lines produced by the diamond point. CONCLUSIONS Within the limitations of this in vitro study, restoration of surface damage improved the flexural strength of zirconia specimens.

Fracture resistance and failure patterns of endodontically treated mandibular molars with and without glass fiber post in combination with a zirconia-ceramic crown.

OBJECTIVE The aim of this study was to investigate the influence of a fiber post on the fracture mechanics of zirconia crowns inserted over endodontically treated teeth with different extent of coronal damage. METHODS Endodontically treated human molars with three types of coronal damage received fiber posts before cementation of zirconia-veneered crowns. Controls received composite resin cores without fiber posts. The specimens were loaded to failure and fractographically examined using a scanning electron microscope (SEM). RESULTS Statistical analysis revealed that specimens with fiber posts demonstrated significantly higher failure loads and favorable fracture pattern compared to the controls. At fractographic analysis, specimens with fiber posts demonstrated delamination of the veneer ceramic from intact zirconia under structure. Meanwhile, the specimens that were restored without a fiber post demonstrated micro-cracking of the composite core build-up resulting in loss of the support under the zirconia crowns which was responsible for the initiation of radial crack and catastrophic damage. CONCLUSIONS Within the limitation of this study, the insertion of fiber post improved the support under zirconia crowns which resulted in higher fracture loads and favorable failure type compared to composite core build-up.