Editorial Type:
Article Category: Research Article
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Online Publication Date: 01 Sept 2017

Retentive Strength of Y-TZP Crowns: Comparison of Different Silica Coating Methods on the Intaglio Surfaces

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Page Range: E121 – E133
DOI: 10.2341/16-090-L
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SUMMARY

Objective:

To evaluate the effect of different methods of silica deposition on the intaglio surface of yttrium oxide stabilized zirconia polycrystal (Y-TZP) crowns on the retentive strength of the crowns.

Methods:

One hundred simplified full-crown preparations produced from fiber-reinforced polymer material were scanned, and 100 Y-TZP crowns with occlusal retentions were milled. Crown/preparation assemblies were randomly allocated into five groups (n=20) according to the treatment of the intaglio surfaces: TBS = tribochemical silica coating via air-abrasion with 30-μm silica-coated alumina particles; GHF1 = application of thin glaze layer + hydrofluoric acid (HF) etching for 1 minute; GHF5 = glaze application + HF for 5 minutes; GHF15 = glaze application + HF for 15 minutes; NANO = silica nanofilm deposition (5 nm) via magnetron sputtering. All groups received a silane application. The surfaces of the preparations (polymer) were conditioned with 10% HF for 30 seconds and silanized. The crowns were cemented with resin cement, thermocycled (12,000 cycles; 5°C/55°C), stored for 60 days, and subjected to a retentive strength test (0.5 mm/min until failure). The retention data (MPa) were analyzed using one-way analysis of variance, Tukey tests, and Weibull analysis. Failures were classified as 50C (above 50% of cement in the crown) and 50S (above 50% of cement on the substrate).

Results:

The TBS (5.6±1.7 MPa) and NANO groups (5.5±1 MPa) had higher retentive strength than the other groups (p<0.0001) and had the highest values of characteristic strength. There was no difference in Weibull modulus, except for the GHF1 group (lower values). The TBS and GHF15 groups, respectively, had 60% and 70% of their failures classified as 50C, while most of the other groups had 50S failures.

Conclusion:

Tribochemical silica coating and silica nanofilm deposition on the inner surface of zirconia crowns promoted a higher retentive strength.

INTRODUCTION

Yttrium oxide stabilized zirconia polycrystal (Y-TZP) ceramics have been widely used in dentistry owing to their superior mechanical strength (high fracture toughness)1,2 and biocompatibility.3 However, due to their physical, chemical, and microstructural features, the adhesion of Y-TZP ceramics with resin cements is difficult.4-7 Hydrofluoric acid (HF) etching and silane application can increase the bond strength between vitreous/glass ceramics (based on silica) and composite resins by increasing the surface free energy and wettability8-13; in addition, the ceramic primer heat treatment can promote bond improvements.14-16 However, as zirconia ceramics are dense polycrystalline materials and lack a silicon dioxide (silica) phase, HF treatment fails to produce a microporous surface for bonding.9,17,18 Thus, surface pretreatment is necessary to modify the topography to increase the mechanical retention and chemical adhesion, thereby enhancing the retention of the crowns to the prosthetic preparation.19,20 Recently, systematic reviews showed that loss of retention is significantly higher for densely sintered zirconia, compared with all others types of ceramics and metal-ceramics, for both single and multiple crowns.21,22

The most commonly used treatment method is air abrasion with silicon oxide particles. This method involves the inclusion/incrustation of silica-coated alumina particles on the cementation surface by air abrasion, followed by application of a silane primer bonding agent.23-27 These particles increase the roughness of the zirconia surface, while the silane bonding agent promotes adhesion between the abraded surface and the resin matrix of the cement.28-30 However, studies have shown that air abrasion might create superficial defects and cracks that may initiate fractures.31,32 Nonetheless, other studies did not find any negative effects when this method was used on Y-TZP ceramics.33,34 Additionally, clinical failures in Y-TZP crowns (chippings) seem to have no association with the roughness created by the air abrasion of the intaglio surface.35-38

Another recently developed technique involves the application of a thin layer of vitreous porcelain (low-fusing porcelain glaze, vitrification or glaze-on) on the ceramic surface. Basically, the glaze is composed by vitreous porcelain (high silica content, amorphous matrix or SiO2) and pigments (metallic oxides), making a zirconia glazed surface etchable by HF,39,40 and that can be subjected to silanization or air-particle abrasion.41 Vitrification is very effective in promoting the adhesion between the Y-TZP ceramic surface and resin cements.39,42-46 For glass ceramics,47-49 HF reacts with the silica phase of the porcelain, creating retentive microchannels. Therefore, the effect of longer etching times on the creation of surface irregularities for bonding depends on the microstructure of the ceramic.8,50,51 Etching increases the contact area between the adhesive agent and the ceramic,52,53 and the number and size of the irregularities created are associated with the duration of the etching process.47,54 However, when low-fusing porcelain glazes on zirconia surfaces are treated with HF for too long, the acid etching might completely remove the glaze from the surface. The impact of etching time of porcelain glaze applied on zirconia influences resin bonding strength and crown retention rate has yet to be investigated.

Recently, the deposition of silica nanofilms on the zirconia surface has been studied.26,55 For this method, a silica nanofilm is deposited on the zirconia surface by plasma processing (reactive magnetron sputtering), making it more chemically reactive. Following up the nanofilm deposition with silanization results in an increase in adhesive strength without damaging the Y-TZP surface.26,55,56 Moreover, this technique forms a homogeneous film, improves chemical adhesion to the substratum,57 and does not promote m-phase transformation after the film is applied.26

Considering the aforementioned silica deposition methods (low-fusing porcelain glaze application, or silica nanofilm deposition) for improving bonding to zirconia, a question not yet addressed is this: Do these treatments of the intaglio surface of zirconia crowns improve crown retention compared with tribochemical silica coating via air abrasion? Thus, the objectives of this in vitro study were: 1) to evaluate the effects of different silica-based coatings of the intaglio surface of zirconia crowns on retentive strength, 2) to compare three different HF etching times for the groups undergoing the vitrification technique, and 3) to evaluate the reliability of the different treatment methods by Weibull modulus. The null hypothesis tested was that there would be no difference among the zirconia surface conditionings.

METHODS AND MATERIALS

Sample-Size Calculation

To determine the number of specimens for group, a sample-size calculation was performed, based on a pilot study, with software from the site “Java applets for power and sample size” (www.stat.uiowa.edu/∼rlenth/Power/). With a statistical power of 80%, a standard deviation of the mean of 1.15 MPa, and a detectable difference of 1.38 MPa, it was established that the number of specimens per group should be 20, for a total of 100 specimens divided among five experimental groups (see Table 1).

Table 1 Testing Groups and Respective Descriptions of the Surface Conditioning Protocols on the Intaglio Surface
Table 1

Thus, the experimental design was based on one factor (zirconia surface treatment) divided into five levels (testing groups, n=20) according to the treatment of the intaglio surface of the Y-TZP crowns (Table 1).

This study was blinded for: crown cementation, aging procedure, retention test, and failure analysis.

Prosthetic Preparation and Crown Production

G10 rods (G10, FR4 Laminate Round Rods Epoxyglass; NEMA grade FR4, Accurate Plastics, Inc, New York, NY, USA) measuring 11 mm in diameter and 1.2 m in length were sectioned in small cylinders of 16 mm height used to obtain 100 identical, simplified, full-crown preparations by computer-aided machining (6 mm in height and a total occlusal convergence angle of 12° with rounded corners; Figure 1a,b).37,38 Taking into account the fact that all the preparations in G10 had identical dimensions/geometry, just one preparation was impressed using a vinyl polysiloxane impression material (Elite HD + Putty and Light Body Normal Setting, Zhermack, Badia Polesine, Italy), followed by obtaining a model in special plaster (CAM-base, type 4, Dentona AG, Dortmund, Germany).

Figure 1. . (a): Schematic drawing of the G10 prosthetic preparations (ø = diameter and R = radius) as a trunked cone. (b): Lateral and frontal views of the G10 preparation in Solid Works software. The green area corresponds to the adhesive area.Figure 1. . (a): Schematic drawing of the G10 prosthetic preparations (ø = diameter and R = radius) as a trunked cone. (b): Lateral and frontal views of the G10 preparation in Solid Works software. The green area corresponds to the adhesive area.Figure 1. . (a): Schematic drawing of the G10 prosthetic preparations (ø = diameter and R = radius) as a trunked cone. (b): Lateral and frontal views of the G10 preparation in Solid Works software. The green area corresponds to the adhesive area.
Figure 1 (a): Schematic drawing of the G10 prosthetic preparations (ø = diameter and R = radius) as a trunked cone. (b): Lateral and frontal views of the G10 preparation in Solid Works software. The green area corresponds to the adhesive area.

Citation: Operative Dentistry 42, 5; 10.2341/16-090-L

The master die was then scanned (inEos Blue, Sirona, Bensheim, Germany), and the image was transferred to the inLab software (version 3.60, Sirona). Equal crowns (N=100) with occlusal retentions were designed considering a resin cement space of 30 μm, followed by milling the Y-TZP crowns (Cerec InLab MC XL, Sirona; Figure 2a; VITA In-Ceram YZ, YZ-40/19 cubes with dimension of 15.5 × 19 × 40 mm3, VITA, Bad Säckingen, Germany).

Figure 2.  . (a): G10 preparation after milling, zirconia crown with occlusal retention and cemented crown on the G10 preparation. (b): Specimen embedded for the tensile test. (c): Specimen attached on the universal machine test.Figure 2.  . (a): G10 preparation after milling, zirconia crown with occlusal retention and cemented crown on the G10 preparation. (b): Specimen embedded for the tensile test. (c): Specimen attached on the universal machine test.Figure 2.  . (a): G10 preparation after milling, zirconia crown with occlusal retention and cemented crown on the G10 preparation. (b): Specimen embedded for the tensile test. (c): Specimen attached on the universal machine test.
Figure 2 (a): G10 preparation after milling, zirconia crown with occlusal retention and cemented crown on the G10 preparation. (b): Specimen embedded for the tensile test. (c): Specimen attached on the universal machine test.

Citation: Operative Dentistry 42, 5; 10.2341/16-090-L

Sintering was performed according to the manufacturer's instructions (Zircomat oven, VITA). The crowns were checked on their preparations for adaptation (Carbono Arti-Spray, Bausch, Bausch Articulating Papers, Inc, Nashua, NH, USA) and cleaned with an ultrasonic device (1440 D – Odontrobras, Ribeirao Preto, Brazil) with distilled water for 10 minutes.

Crown Cementation

After each treatment of the inner surfaces of the Y-TZP crowns (Table 1), methacryloxypropyltrimethoxy-silane (ESPE-Sil Silane, 3M ESPE, Seefeld, Germany) was applied with a microbrush, followed by a 5-minute wait for solvent evaporation.

The preparation surfaces (G10) were conditioned with 10% HF for 30 seconds, cleaned for the same amount of time, and then received an application of silane-based primer (ESPE-Sil, 3M-ESPE), with a 5-minute wait time.59 The resin cement (RelyX ARC, 3M ESPE) was manipulated according to the manufacturer's instructions and applied to the intaglio surface of the crowns, which were positioned on the preparation. With an adapted surveyor (B2, BioArt, Sao Carlos, Brazil), a load of 750 g was applied to the crowns, the cement excess was removed, and photo activation was performed for 20 seconds on each surface (1200 mW/cm2, Radii-Cal, SDI, Bayswater, Australia). The specimens were stored in distilled water (37°C) for 24 hours before aging.

Aging: Thermocycling + Storage

All specimens were subjected to thermocycling (12,000 cycles; 5°C to 55°C; 30 seconds per bath and 5 seconds between baths; Ethik Technology, Vargem Grande Paulista, Brazil)19,20,60,61 followed by storage for 60 days in a wet environment at 37°C.

Retentive Strength Test

After thermocycling, part of the cemented crown was embedded in self-curing acrylic resin (VIPI Flash, VIPI, Pirassununga, Brazil) until total coverage of the retentive part of the crowns was achieved. This process was carried out with the aid of an adapted surveyor (B2, BioArt), which maintained a vertical embedding axis. Retention areas were made apical to the preparations, and the same embedding procedure was performed, keeping the adhesive interface free to the test. The embedding was necessary to make possible the retention test (Figure 2b).

For the retentive strength test, the superior part of the assembly (the crown) was fixed to the movable axle of a universal testing machine (DL-1000, Emic, São José dos Pinhais, Brazil), which was attached to the load cell (1000 N), while the inferior part (the preparation) was positioned at the fixed base of the testing machine (Figure 2c). The retentive strength test was performed until fracture (decementation) at a speed of 0.5 mm/min.19,20

Adhesive Area Calculation

The cementation area was calculated by the SolidWorks software (DS SolidWorks Corporation, Waltham, MA, USA) according to the dimensions presented in Figure 1a, resulting in an adhesive area of 130 mm2 (Figure 1b). The retentive strength was calculated using the formula: R = Fmax/A, where R = retentive strength, Fmax = maximum force for failure (decementation), and A = adhesive area.

Failure Analysis

The tested assemblies were analyzed under a stereomicroscope (Discovery V20, Carl-Zeiss, Gottingen, Germany) to evaluate the type of fracture. The fractures were classified according to the localization of the largest portion of cement: 50C (more than 50% of the cement on the crown), or 50S (more than 50% of the cement on the preparation). Representative images were taken with a scanning electronic microscope (SEM; JSM 5400, Jeol Ltd, Tokyo, Japan). This classification was adapted from Amaral and others19 and Rippe and others.20

Micromorphologic Analysis (Zirconia Blocks and G10 Polymer Surfaces)

Extra zirconia samples (small blocks) were produced from VITA In-Ceram YZ blocks (VITA) in a cutting machine (IsoMet 1000; Buehler, Lake Bluff, IL, USA). The standardization of the analysis surface was performed with Sof-Lex disks (3M ESPE) and polishing with 1200-grit sandpaper. After sintering, the blocks presented an analysis area of 5 × 5 mm, which was conditioned with the aforementioned surface methods.

Etched and non-etched axial surfaces of the G10 preparations (10% HF for 30 seconds) were also analysed.

Surface analyses of both tested strategies and G10 surfaces were performed under an SEM (JSM 5400, Jeol Ltd) to verify the topography and superficial alterations of the Y-TZP ceramic and G10 preparations.

Data Analysis

The nominal values of retentive strength were tabulated and statistical analysis was performed with the SPSS software (version 21, IBM, Chicago, IL, USA). The normality and homoscedasticity were verified and the data were subjected to one-way analysis of variance (ANOVA) and post hoc Tukey test. A Weibull analysis was also performed (Weibull modulus [m]: reliability of retention values represents the variation of strength data and expresses the size distribution of the flaw population in a structure; characteristic strength [σ0] indicates the strength value at which 63.2% of the specimens survive).

RESULTS

One-way ANOVA (Table 2) showed a statistical difference between the retention values of the groups (p<0.0001). Tukey's test showed that TBS and NANO groups presented the highest strength values in relation to the other groups. The GHF15 and GHF5 groups presented intermediate values, and the GHF1 group had the lowest values (Table 3). The Weibull modulus was lower for the GHF1 group, and the characteristic strength was increased for the TBS and NANO groups.

Table 2 One-Way Analysis of Variance Results for Retentive Strength Data
Table 2
Table 3 Mean ± Standard Deviation Retentive Strength (MPa), Weibull Parameters and Failure Modes of the Zirconia Crownsa
Table 3

Figure 3a through d shows the micromorphologic differences between nonetched and etched surfaces of the polymeric material (G10).

Figure 3. . Representative micrographics of the G10 surfaces with and without HF etching. (a and b): G10 axial surface nonetched and etched, respectively (×500). (c and d): G10 axial surface nonetched and etched (×1000), respectively. G10 analog material presents glass fiber in different senses; it is possible to observe in the nonetched surfaces (a and c) little exposed fiber in only one horizontal direction (black arrow), while the etched surface presents more exposed glass fiber in different directions (black and white arrows).Figure 3. . Representative micrographics of the G10 surfaces with and without HF etching. (a and b): G10 axial surface nonetched and etched, respectively (×500). (c and d): G10 axial surface nonetched and etched (×1000), respectively. G10 analog material presents glass fiber in different senses; it is possible to observe in the nonetched surfaces (a and c) little exposed fiber in only one horizontal direction (black arrow), while the etched surface presents more exposed glass fiber in different directions (black and white arrows).Figure 3. . Representative micrographics of the G10 surfaces with and without HF etching. (a and b): G10 axial surface nonetched and etched, respectively (×500). (c and d): G10 axial surface nonetched and etched (×1000), respectively. G10 analog material presents glass fiber in different senses; it is possible to observe in the nonetched surfaces (a and c) little exposed fiber in only one horizontal direction (black arrow), while the etched surface presents more exposed glass fiber in different directions (black and white arrows).
Figure 3 Representative micrographics of the G10 surfaces with and without HF etching. (a and b): G10 axial surface nonetched and etched, respectively (×500). (c and d): G10 axial surface nonetched and etched (×1000), respectively. G10 analog material presents glass fiber in different senses; it is possible to observe in the nonetched surfaces (a and c) little exposed fiber in only one horizontal direction (black arrow), while the etched surface presents more exposed glass fiber in different directions (black and white arrows).

Citation: Operative Dentistry 42, 5; 10.2341/16-090-L

We observed the differences between the surface morphologic patterns of the zirconia blocks treated with the different strategies (Figure 4A through M). Compared with the untreated surface (Figure 4L,M), all the groups presented topographic alterations, except the NANO group (Figure 4I,J). The etching time affected the topographic pattern of the glaze groups (the longer the etching time, the greater the presence of pores) (GHF1: Figure 4A,B; GHF5: Figure 4C,D; and GHF15: Figure 4E,F). The TBS group presented irregularities promoted by air abrasion (Figure 4G,H).

Figure 4. . Representative scanning electron micrographs of the zirconia blocks treated with the same treatments utilized in the intaglio surface of zirconia crowns. (A): GHF1 ×500. (B): GHF1 ×5000). (C): GHF5 ×500. (D): GHF5 ×5000; (E): GHF15 ×500. (F): GHF15 ×5000. (G): TBS ×500 (H): TBS ×5000. (I): NANO ×500. (J): NANO ×5000. (L): Zirconia surface without treatment ×500. (M): Zirconia surface without treatment ×5000. It is possible to observe in glaze groups the greater etching time and the greater quantity and size of pores compared with the air-abraded surface, which presents irregularities promoted by impact of silica-coated alumina particles, and the NANO group, which presents characteristic of the untreated surface (Sof-Lex disks and 1200-grit sandpaper), possibly because the nanofilm presents a thickness of 5 nm.Figure 4. . Representative scanning electron micrographs of the zirconia blocks treated with the same treatments utilized in the intaglio surface of zirconia crowns. (A): GHF1 ×500. (B): GHF1 ×5000). (C): GHF5 ×500. (D): GHF5 ×5000; (E): GHF15 ×500. (F): GHF15 ×5000. (G): TBS ×500 (H): TBS ×5000. (I): NANO ×500. (J): NANO ×5000. (L): Zirconia surface without treatment ×500. (M): Zirconia surface without treatment ×5000. It is possible to observe in glaze groups the greater etching time and the greater quantity and size of pores compared with the air-abraded surface, which presents irregularities promoted by impact of silica-coated alumina particles, and the NANO group, which presents characteristic of the untreated surface (Sof-Lex disks and 1200-grit sandpaper), possibly because the nanofilm presents a thickness of 5 nm.Figure 4. . Representative scanning electron micrographs of the zirconia blocks treated with the same treatments utilized in the intaglio surface of zirconia crowns. (A): GHF1 ×500. (B): GHF1 ×5000). (C): GHF5 ×500. (D): GHF5 ×5000; (E): GHF15 ×500. (F): GHF15 ×5000. (G): TBS ×500 (H): TBS ×5000. (I): NANO ×500. (J): NANO ×5000. (L): Zirconia surface without treatment ×500. (M): Zirconia surface without treatment ×5000. It is possible to observe in glaze groups the greater etching time and the greater quantity and size of pores compared with the air-abraded surface, which presents irregularities promoted by impact of silica-coated alumina particles, and the NANO group, which presents characteristic of the untreated surface (Sof-Lex disks and 1200-grit sandpaper), possibly because the nanofilm presents a thickness of 5 nm.
Figure 4 Representative scanning electron micrographs of the zirconia blocks treated with the same treatments utilized in the intaglio surface of zirconia crowns. (A): GHF1 ×500. (B): GHF1 ×5000). (C): GHF5 ×500. (D): GHF5 ×5000; (E): GHF15 ×500. (F): GHF15 ×5000. (G): TBS ×500 (H): TBS ×5000. (I): NANO ×500. (J): NANO ×5000. (L): Zirconia surface without treatment ×500. (M): Zirconia surface without treatment ×5000. It is possible to observe in glaze groups the greater etching time and the greater quantity and size of pores compared with the air-abraded surface, which presents irregularities promoted by impact of silica-coated alumina particles, and the NANO group, which presents characteristic of the untreated surface (Sof-Lex disks and 1200-grit sandpaper), possibly because the nanofilm presents a thickness of 5 nm.

Citation: Operative Dentistry 42, 5; 10.2341/16-090-L

In terms of fracture type, the TBS and GHF15 groups presented a higher percentage of 50C failures compared with the other groups (Table 3, Figure 5a, 5b through e).

Figure 5. . (A, B, and C): 50C failure of the GH15 group specimen (A and B correspond to G10 preparation and C to intaglio surface of zirconia crown). (D and E): 50S failure of the GH1 group specimen (D corresponds to G10 peparation, and E corresponds to the intaglio surface of the zirconia crown). G10, G10 analog material; RC, resin cement; YZ, zirconia crown.Figure 5. . (A, B, and C): 50C failure of the GH15 group specimen (A and B correspond to G10 preparation and C to intaglio surface of zirconia crown). (D and E): 50S failure of the GH1 group specimen (D corresponds to G10 peparation, and E corresponds to the intaglio surface of the zirconia crown). G10, G10 analog material; RC, resin cement; YZ, zirconia crown.Figure 5. . (A, B, and C): 50C failure of the GH15 group specimen (A and B correspond to G10 preparation and C to intaglio surface of zirconia crown). (D and E): 50S failure of the GH1 group specimen (D corresponds to G10 peparation, and E corresponds to the intaglio surface of the zirconia crown). G10, G10 analog material; RC, resin cement; YZ, zirconia crown.
Figure 5 (A, B, and C): 50C failure of the GH15 group specimen (A and B correspond to G10 preparation and C to intaglio surface of zirconia crown). (D and E): 50S failure of the GH1 group specimen (D corresponds to G10 peparation, and E corresponds to the intaglio surface of the zirconia crown). G10, G10 analog material; RC, resin cement; YZ, zirconia crown.

Citation: Operative Dentistry 42, 5; 10.2341/16-090-L

DISCUSSION

The null hypothesis was rejected: the TBS and NANO groups presented the highest retentive strength values, followed by the GHF15, GHF5, and GHF1 groups.

The silica nanofilm application and tribochemical silica coating presented the highest retention and characteristic strength values (Table 3) compared with other silica deposition methods. It has been shown that silica coating via air abrasion followed by silanization improves the bond strength for silica-based,9,62 glass-infiltrated alumina,9,63 and zirconium ceramics.9,23,61,64-66 Using a shear bond strength test, some authors showed that silica nanofilms deposited by magnetron sputtering reached values of adhesion similar to air abrasion with silica26 or alumina particles.56 Despite a similar Weibull modulus (m), the NANO group had a 40% higher value of m than the TBS group, which suggests a high reliability of the silica deposition technique (Table 3). Compared with the vitrification method, deposition of the silica nanofilm via sputtering is rapid, and the thickness and chemical composition of the film can be controlled. Also, it does not subject the ceramic to high temperatures, and avoids the damage associated with sandblasting. However, it requires costly, specialized equipment, and specific training for use.26

The NANO group generated mostly 50S (more than 50% of the cement on the G10 substratum surface) failures (80%) compared with the TBS group, which presented 60% 50C (more than 50% of the cement in the crown) failures. According to studies conducted on the magnetron sputtering methods of silica deposition, the coating layer is homogeneous and shows a controlled thickness.57,58 While the magnetron sputtering creates a 5-nm thick homogeneous silica deposition film (NANO group), the 30-μm diameter silica-coated alumina particles (TBS group) penetrate approximately 15 μm into the Y-TZP crowns.67 Therefore, despite the fact that both methods promote silica deposition and make the surface more reactive, air abrasion caused more irregularities, thus favoring micromechanical retention between the zirconia and cement. This can be seen on the micrographs (Figure 4G through J)28-30 and it is the cause of the 50C failure mode. In terms of damage effect of the particle air abrasion on zirconia materials, the literature is controversial.31-34,68

Among the studied silica deposition methods, the vitrification method (or glaze-on)42 seems to be a promising technique. In vitro shear bond strength studies showed that the application of a thin layer of low-fusing porcelain glaze on the zirconia surface followed by HF etching generated similar or higher adhesion values compared with conventional surface-treatment methods. However, these studies conditioned the glazed surface for different durations and used different acid concentrations.39,42-46

In the current study, we aimed to determine the best acid-etching time of the glazed zirconia surface by means of a crown pull-out test, which evaluates not only the bond strength but also all the complex forces involved, including shear and friction. Our results partially agree with other studies that used a similar methodology (vitrification technique + pull-out crown test).19,20 Rippe and others20 verified the effect of the surface treatment and type of cement on the retention of Y-TZP crowns on composite cores. They also showed that the vitrification technique presented higher retention values than the group without treatment but similar to those of a tribosilicatization group using a 2-hydroxylethyl methacrylate (HEMA)-based resin cement. However, when a bisphenol A-glycidyl methacrylate (BIS-GMA)-based cement was used, there was no difference between the groups. In addition, Amaral and others19 evaluated the retention of Y-TZP crowns with different inner treatments and different substrate types and found no difference among the treatments when a dentin substrate was used; however, when the crowns were cemented on composite cores, tribosilicatization and vitrification significantly increased the retention force in comparison with the control group (no treatment). In the current study, the GHF1 group (Figure 4A,B) presented lower values of retention than the silicatized group (TBS) (Figure 4G,H), possibly because the conditioning time (1 minute) was not enough to generate significant morphologic changes on the glazed surface compared with the air-abraded surface.

Among the studied strategies, the GHF5 and GHF15 groups presented statistically similar and intermediate values of retentive strength (Table 3), as well as characteristic strength. An increase in acid-etching time improved the retention values (GHF5 and GHF15 groups) compared with the GHF1 group. However, the GHF15 group showed a higher number of 50C failures, indicating a greater adhesive interaction with the crown, similar only to the TBS group. This could have been caused by the topographic alterations (Figure 4E,F) promoted by the increased etching time49 of the glazed surface and consequently a greater surface area for bonding, which facilitates resin cement penetration into the microretentive ceramic surface.69 The glazing procedure is quick, is performed after the clinical verification of the fit of the crown, being applied by the dental technician. Thus, the clinician receives a zirconia crown with an etchable intaglio surface. But, standardization of the thickness of the low-fusing porcelain glaze is difficult, thus it might increase the marginal gap.45 At the same time, space for the glaze and cement layer could be produced by computer-aided design/computer-sided manufacturing systems, thus ensuring an adequate fit.39 Since the thickness of the glass film was approximately 31.8 μm for Ntala and others39 and 12 μm for Bottino and others,25 the effect on the marginal fit could be not negligible, considering that the clinically recommended maximum misfit is around 120 μm.45,70,71 Even though the vitrification/glaze-on technique has its advantages, we believe technical improvements are needed, primarily in the standardization of the thickness of the glaze layer, in order to prevent any negative effects on the marginal fit.

It is important to highlight that the glaze composition (monophase) may have affected the retention values for glaze groups. If a multiphasic glaze material had been used, maybe better results of crown retention could have been obtained.39,40 In this sense, the development of multiphasic glaze materials might achieve improvements.

In the current study, we utilized a woven glass-fiber-filled epoxy material (G10) as a dentin analog, in order to standardize the substrate and the preparation. This material showed excellent mechanical and adhesive properties,59,72,73 and we understand that standardization of the substrate is crucial to verify the pretreatment factor alone in zirconia crowns. Nevertheless, Kelly and others59 showed that resin cement bond strength to dentin was slightly lower than to the dry and wet analog material (40%-50%), which shows that caution is needed when using the G10 in adhesion tests. Up to now, we know of no other studies that compared the bond strength of dentin and G10 to demonstrate adhesion similarity of these substrates. In this sense, when adhesion studies using G10 are planned, it is very important to evaluate if the G10 as a dentin analog influences the results and interpretation. In the current study, we evaluated the zirconia/resin cement interface and most of the failures were 50S; thus, we believe that it was possible to observe the effect of the zirconia surface treatments. Figure 3a through d shows that etched G10 surfaces present more glass fiber exposed for bonding with the silane couple agent. Additionally, the use of copings in the complex adhesion/retention trial (retentive strength of crowns) reflects the clinical reality more properly, especially due to the axial loads exerted during the test.

The present study had some limitations. It is difficult to compare our results with the current literature because most studies did not present similar geometries of the preparation.19,20,60,61,70 Furthermore, those studies that utilized zirconia crowns had a large number of variables: the substrate, preparation angle, cement type (with or without monomer phosphate), and application of primers. Mechanical cycling or fatigue experiments were not carried out in these studies either, and such test conditions should be employed in the future.72,74-76 These new silica deposition methods should be included in prospective clinical trials to evaluate the bond effectiveness and the effect on retention of zirconia crowns under clinical situations.

CONCLUSIONS

  • 1. 

    Tribochemical silica coating (via particle air abrasion) and silica nanofilm deposition (via magnetron sputtering) as pretreatments of zirconia crowns in combination with RelyX ARC luting cement promote higher crown retention compared with the low-fusing porcelain glaze applications (apart from acid etching time).

  • 2. 

    Application of a thin layer of low-fusing porcelain glaze showed variable retention results depending on the etching time; thus, further studies about glaze coated polycrystalline zirconium oxide ceramics should be performed.

Acknowledgments

This article is based on a doctoral thesis submitted to the Graduate Program in Oral Sciences (Prosthetic Dentistry Unit), Faculty of Odontology, Federal University of Santa Maria (Santa Maria, Brazil) as part of the requirements for the PhD degree (Dr Vinicius F. Wandscher). We thank CAPES (Agency for the High-Standard Promotion of Graduate Courses, Brazil; scholarship process number 99999.005177/2014-06) for supporting this study. We also thank VIPI dental products for the acrylic resin donation.

Conflict of Interest

The authors of this manuscript certify that they have no proprietary, financial, or other personal interest of any nature or kind in any product, service, and/or company that is presented in this article.

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Copyright: ©Operative Dentistry, 2017 2017
Figure 1
Figure 1

(a): Schematic drawing of the G10 prosthetic preparations (ø = diameter and R = radius) as a trunked cone. (b): Lateral and frontal views of the G10 preparation in Solid Works software. The green area corresponds to the adhesive area.

Figure 2
Figure 2

(a): G10 preparation after milling, zirconia crown with occlusal retention and cemented crown on the G10 preparation. (b): Specimen embedded for the tensile test. (c): Specimen attached on the universal machine test.

Figure 3
Figure 3

Representative micrographics of the G10 surfaces with and without HF etching. (a and b): G10 axial surface nonetched and etched, respectively (×500). (c and d): G10 axial surface nonetched and etched (×1000), respectively. G10 analog material presents glass fiber in different senses; it is possible to observe in the nonetched surfaces (a and c) little exposed fiber in only one horizontal direction (black arrow), while the etched surface presents more exposed glass fiber in different directions (black and white arrows).

Figure 4
Figure 4

Representative scanning electron micrographs of the zirconia blocks treated with the same treatments utilized in the intaglio surface of zirconia crowns. (A): GHF1 ×500. (B): GHF1 ×5000). (C): GHF5 ×500. (D): GHF5 ×5000; (E): GHF15 ×500. (F): GHF15 ×5000. (G): TBS ×500 (H): TBS ×5000. (I): NANO ×500. (J): NANO ×5000. (L): Zirconia surface without treatment ×500. (M): Zirconia surface without treatment ×5000. It is possible to observe in glaze groups the greater etching time and the greater quantity and size of pores compared with the air-abraded surface, which presents irregularities promoted by impact of silica-coated alumina particles, and the NANO group, which presents characteristic of the untreated surface (Sof-Lex disks and 1200-grit sandpaper), possibly because the nanofilm presents a thickness of 5 nm.

Figure 5
Figure 5

(A, B, and C): 50C failure of the GH15 group specimen (A and B correspond to G10 preparation and C to intaglio surface of zirconia crown). (D and E): 50S failure of the GH1 group specimen (D corresponds to G10 peparation, and E corresponds to the intaglio surface of the zirconia crown). G10, G10 analog material; RC, resin cement; YZ, zirconia crown.

Contributor Notes

*Corresponding author: Luiz Felipe Valandro, PhD, R. Floriano Peixoto 1184 Santa Maria, RS 97015-372, Brazill e-mail: lfvalandro@hotmail.com
Accepted: 25 Feb 2017
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