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Table of Contents   
ORIGINAL ARTICLE  
Year : 2022  |  Volume : 25  |  Issue : 5  |  Page : 492-497
Zirconia surface infiltration with low-fusing glass: A surface treatment modality to enhance the bond strength between zirconia and veneering ceramic


1 Department of Conservative Dentistry and Endodontics, Government Dental College and Research Institute, Bengaluru, Karnataka, India
2 Department of Prosthodontics and Implantology, Government Dental College and Research Institute, Bengaluru, Karnataka, India

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Date of Submission28-Apr-2022
Date of Decision15-May-2022
Date of Acceptance19-May-2022
Date of Web Publication12-Sep-2022
 

   Abstract 

Background: The pursuit of esthetics and superior mechanical properties prompted the layering of Yttrium stabilized Zirconia with ceramic material. However, the bioinert nature of zirconia causes the chipping off of this ceramic layer. Selective infiltration etching (SIE) of zirconia provides good bond strength between zirconia and veneering ceramic.
Materials and Methods: One hundred and fifty zirconia specimens of dimensions 5 × 5 × 10 mm were divided into 5 groups. Group 1: Air abrasion with 30 μ Al2O3 for 15 s with 0.4 bar pressure. Group 2: SIE and heat-induced maturation (HIM) as demonstrated by Abousheilb. Group 3: Sintered zirconia specimens were taken up for air abrasion followed by SIE/HIM. Group 4: Air abrasion and SIE performed on unsintered specimens followed by heat treatment at 1500°C. Group 5: Air abrasion performed on unsintered zirconia specimens followed by heat treatment at 1500°C followed by SIE/HIM. The samples were then layered with ceramic and subjected to shear bond strength (SBS) analysis.
Results: The mean and standard deviation were calculated for the data. The mean SBS among the groups was compared using ANOVA. The post hoc Bonferroni test was applied to compare between the groups. The mean SBS was highest for Group 5 (47.89 ± 6.53) followed by Group 2 (34.94 ± 3.04), Group 3 (32.56 ± 6.04), Group 1 (29.12 ± 7.37), and Group 4 (27.56 ± 7.54). ANOVA test showed statistically significant differences among the groups (F = 48.86, P = 0.00).
Conclusion: SIE/HIM when combined with sandblasting with appropriate heat treatment demonstrated a significant increase in bond strength. This prolongs the longevity of the restoration, thereby meeting the clinical needs.

Keywords: Air abrasion; ceramic; heat-induced maturation; selective infiltration etching; shear bond strength; zirconia

How to cite this article:
Kumar N K, Nair A, Thomas PM, Hariprasad L, Brigit B, Merwade S, Shylaja V. Zirconia surface infiltration with low-fusing glass: A surface treatment modality to enhance the bond strength between zirconia and veneering ceramic. J Conserv Dent 2022;25:492-7

How to cite this URL:
Kumar N K, Nair A, Thomas PM, Hariprasad L, Brigit B, Merwade S, Shylaja V. Zirconia surface infiltration with low-fusing glass: A surface treatment modality to enhance the bond strength between zirconia and veneering ceramic. J Conserv Dent [serial online] 2022 [cited 2022 Sep 25];25:492-7. Available from: https://www.jcd.org.in/text.asp?2022/25/5/492/355903

   Introduction Top


Partially-stabilized zirconia with its unsurpassed mechanical properties has led to its increased clinical usage ever since its introduction almost two decades ago. Zirconia, essentially a ceramic, performs much better in terms of fracture toughness and is suitable substitute for metal framework in high stress-bearing areas.[1] Even though the clinical community is edging toward all-ceramic systems, the opaque nature of zirconia intercepts its usage in esthetic zones. Due to its opaque nature and characteristic white color, zirconia lack esthetics, compelling for a “double-layered restoration” with a translucent veneering ceramic.[2] Prospective clinical trials of such veneered zirconia restorations have demonstrated delamination of the veneering ceramic over a period.[2] Clinically, zirconia systems have demonstrated compromised survival rates due to chipping off the ceramic. An insight into this material further reveals the fact that zirconia is “chemically stable” or “bioinert.” The bio inertness of zirconia causes a lack of adhesive bonding between the core and veneering material, thereby leading to the weak delamination resistance of the zirconia–ceramic interface.

This interface between zirconia and ceramic has been under scrutiny over a prolonged period. The scientific community is in a constant quest to enhance the bond strength of zirconia to ceramic by means of various surface treatments. Mechanical as well as chemical modifications of the zirconia surface are employed to create micromechanical retention between zirconia and veneering ceramic. Diverse surface treatment protocols are inflicted on zirconia surfaces, but none of the treatments have been labeled as the “gold standard.”

Among the various protocols that are followed in the laboratory, selective infiltration etching/heat-induced maturation (SIE/HIM) stands intricate and requires utmost attention in each and every step. This surface roughening method basically involves the infiltration of low-fusing glass or organic oxides into the zirconia surface with HIM. Aboushelib et al. evaluated the zirconia/resin interface and demonstrated a strong durable bond. Another added advantage of this protocol is that it does not produce gross structural changes to the zirconia surface when compared to other conventional modalities.[3]

The task of formulating the infiltrant agent with the exact concentration of various oxides as demonstrated by Aboushelib is quite arduous. In this current study, instead of following the formulations for the infiltrant agent given by Abousheilb, we employed a commercially available low-fusing ceramic. The study further employed a combination of air abrasion and SIE/HIM protocol and evaluated its effect on the bond strength between zirconia and ceramic.


   Materials and Methods Top


Study design

This in vitro study was carried out in the Department of Conservative Dentistry and Endodontics, Government Dental College and Research Institute, Bengaluru.

Sample preparation and sample size

A standard yttria-stabilized zirconia blank (Amann Girrbach, Ceramill zi white, Singapore, Asia) was selected for this study. To perform the surface treatment protocols, cuboid-shaped zirconia specimens were planned. A cuboid of dimensions, (5 mm × 5 mm × 10 mm) was made from type II inlay wax (GC Inlay Wax, Hard) and was subjected to scanning (computer-aided design/computer-aided manufacturing, Amann Girrbach, Singapore, Asia). This scanned model acted as a template for the fabrication of cuboid zirconia specimens. One hundred and fifty cuboid-shaped zirconia blocks (5 mm × 5 mm × 10 mm) were milled from the zirconia blank (Amann Girrbach, Ceramill zi white, Singapore, Asia) and were subjected to various treatment protocols. The blocks were polished with the diamond paste using an ascending stepwise approach starting with 120 grit and ending with 800-grit silicon carbide paper, (Ascending order of grit size, 120-,240-,320-,400-,600-, and finally 800-). Further, the blocks were divided into five groups, 30 in each group.

Group (1): Air abrasion

Unsintered zirconia specimens were subjected to air abrasion using a sandblast machine with aluminum oxide (Al2O3) particle size of 30 μ for 15 s at a 10 mm distance from the surface and with a pressure of 0.4 bar. The specimens were then cleaned in 96% isopropyl alcohol for 3 min ultrasonically and steam-cleaned for 15 s. The 30 specimens were then subjected to sintering at 1300°C for 8 h (following the manufacturer's instructions). These specimens were then layered with veneering zirconia and shear bond strength (SBS) was assessed.

Group (2): Selective infiltration etching

Thirty zirconia specimens were sintered following the manufacturer's instructions and then processed for SIE/HIM protocol. A low-fusing ceramic powder (Ivoclar, IPS e.max Ceram) was mixed with 70% ethyl alcohol and 30% distilled water to form a slurry. This slurry was coated on the zirconia specimens and was heat treated in open air to 750°C (60°C/min), brought back to 650°C for 1 min (60°C/min), and was then again taken back to 750°C for 1 min (60°C/min), and then finally brought to room temperature. Five percent hydrofluoric acid solution was used to remove any remnant infiltrant agent from the surface of the specimens before veneering of ceramic.

The first two groups received single surface treatment procedures whereas the remaining three groups received combination of air abrasion and SIE protocols.

Group 3: Sandblasting and selective infiltration etching/heat-induced maturation on sintered zirconia specimens

Thirty sintered zirconia specimens were taken and sandblasting performed as mentioned in the above sandblasting protocol, followed by which SIE/HIM procedure was performed on the zirconia surfaces.

Group (4): Sandblasting and selective infiltration etching/heat-induced maturation on unsintered zirconia specimens

The protocols for sandblasting and SIE were performed on unsintered specimens. After the completion of both protocols, the specimens were subjected to sintering.

Group (5): Sandblasting followed by sintering followed by selective infiltration etching/heat-induced maturation

Thirty unsintered specimens were taken up and sandblasting was performed. After the sandblasting procedure, the specimens were ultrasonically cleaned and kept for sintering. Once the sintering was over, these specimens were subjected to SIE/HIM.

All five groups were then ultrasonically cleaned in deionized water for 30 min and gently air-dried. To make sure that no infiltrant agent is left behind or no residues present on the zirconia surface, the specimens were evaluated using Energy-dispersive spectroscopy X-ray microanalysis.

A scanning electron microscope (SEM) was used to assess the surface topography and morphology of the specimens. Images were evaluated at ×1000 (low magnification) and ×30000 (higher magnification).

Shear bond strength analysis

Porcelain application

Low-fusing porcelain (Ivoclar, IPS e.max Ceram) was used for veneering zirconia blocks. The dimensions of the veneering porcelain were kept at 3 mm × 3 mm × 5 mm with the help of a metal template. The manual layering of porcelain was performed. Initially, two layers of porcelain were applied and fired in the porcelain furnace independently. Further layers of dentin porcelain were condensed using the vibration blotting technique and fired according to the manufactures instructions.

Veneered samples were then stored in distilled water at 37°C for 1 week. To simulate the oral environment, these specimens were subjected to a thermocycling process for 5000 cycles, 5°–55°C with a 30-s dwell time.

The samples were then mounted on a universal testing machine for SBS analysis. The shear load was applied at a crosshead speed of 0.5 mm/min until fracture occurred. The ultimate load to failure was recorded in Newton (N). The average SBS (MPa) was calculated by dividing the load (N) at which failure occurred by the bonding area (mm2) as follows:

Shear stress (MPa) = Load (N)/surface area.

The fractured surfaces were visually analyzed with a stereomicroscope to determine the failure modes of specimens. Failure modes were classified as follows: cohesive fracture within the veneer, the adhesive fracture between the core and veneer, or a combination of both.

Data analysis

Data were subjected to a normality test: Shapiro–Wilk test. Data obtained showed normal distribution hence parametric tests were employed for the analysis. The mean and standard deviation were calculated for the data. The mean SBS among the groups was compared using ANOVA. The post hoc Bonferroni test was applied to compare the SBS between the groups.


   Results Top


Among the various groups, the mean SBS was highest for Group 5 (47.89 ± 6.53) followed by Group 2 (34.94 ± 3.04), Group 3 (32.56 ± 6.04), Group 1 (29.12 ± 7.37), and Group 4 (27.56 ± 7.54). ANOVA test showed statistically significant differences among the groups (F = 48.86, P = 0.00). Statistically significant difference was seen between Group 1 and Group 2 (P = 0.00), Group 1 and Group 5 (P = 0.00), Group 2 and Group 4 (P = 0.00), Group 2 and Group 5 (P = 0.00), Group 3 and Group 4 (P = 0.026), Group 3 and Group 5 (P = 0.00), and Group 4 and Group 5 (P = 0.00), whereas there was no statistical significant difference seen between Group 1 and Group 3 (P = 0.367), Group 1 and Group 4 (P = 1.00), and Group 2 and Group 3 (P = 1.00): [Table 1].
Table 1: Comparison of the mean shear bond strength among the groups using ANOVA

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The sandblasted group (Group 1) mainly showed interfacial failure (60%), followed by cohesive and least mixed failures. Group 5 showed only cohesive failure whereas Groups 2 and 3 predominantly showed cohesive failure (90%) and adhesive failure of 10%: [Table 2].
Table 2: Percentage of failure modes

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The SEM images of the sandblasted groups in the lower magnification [Figure 1]a showed larger surface defects with cracks and crevices on the surfaces. These surface damages were uneven and resulted in peaks and valleys on the zirconia surface, this was in contrast to the SIE (Group 1B) treated groups where the surface was relatively undamaged with the creation of three-dimensional (3D) retentive areas ultrastructurally. The sandblasted and SIE-treated groups possessed surface morphology that was more diverse. There were areas of larger structural defects as noted in sandblasted specimens and other areas showed the creation of nano spaces with intergrain widening [Figure 1]c. The SEM images in the higher magnification showed that these surface changes in Groups 3, 4, and 5 up to 1 μ. The image depicts the effects of both sandblasting and SIE on these surfaces [Figure 2]c, [Figure 2]d, [Figure 2]e.
Figure 1: (a) Low magnification (×1000) image of sandblasted specimens showing crevice and cracks. Gross structural defects were noted (Group 1) (b). SIE infiltrated specimens with ultrastructural changes in the nanolevel. (Group 2) (c). Sandblasted and SIE/heat-induced maturation specimens (Group 5). SIE: Selective infiltration etching

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Figure 2: High magnification (×30,000) scanning electron microscope images Cracks and crevices in Groups 1, 4, and 5, where sandblasting was performed before the sintering process noted in Fig 2a, 2d and 2e respectively. (b) (Group 2) reveals selective infiltration etching/heat-induced maturation procedure performed on sintered specimens, architectural changes noted in nanolevel, (c) (Group 3) reveals similar architectural features to that of Group 2

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   Discussion Top


The proposed hypothesis was supported based on the results obtained. Sandblasting when used along with HIM/SIE improved the bond strength between ceramic and zirconia. Sandblasting removes the surface contamination layers and increases the surface roughness and surface energy, thereby facilitating better wetting of the zirconia.[4],[5] Bitencourt et al.[6] observed peaks and valleys on sandblasted zirconia specimens in SEM suggestive of increased surface roughness. Matani et al.[7] reported surface characteristics as microcracks, small voids, and nodules, and the creation of crevices or sharp protrusions that may serve as retentive areas.[8] Sandblasting is often regarded as a successful treatment procedure considering the increased surface roughness and surface energy, however, the scientific community is not concordant with the use of sandblasting procedure.

The HIM/SIE groups showed bond strength values lower than the sandblasted/sintered/HIM/SIE group. This could be attributed to the increased retentive features on sandblasted zirconia specimens that enhanced the penetration of the infiltration agent. HIM/SIE aims to create surface roughness comparable to that of sandblasting with no external stresses applied and grain changes in the microstructural level without any structural deformity.[3] This study combined the benefits of sandblasting and HIM/SIE to enhance the bond between zirconia and ceramic. The temperature changes between 750°C and 650°C during the HIM prompt the grain boundaries of zirconia to contract and expand, essentially mimicking a thermal etching process. The condensed and concentrated infiltrant agent applied on the zirconia surface flows into the prestrained grain boundaries. To avoid any inordinate reactions the infiltrant chosen should have a matching coefficient of thermal expansion to that of low-fusing ceramics. The dissolution of this agent in Hydrofluoric acid (HF) gives the desired architecture for the zirconia surface. The 3D retentive areas created by SIE and the surface roughness by sandblasting helps in the mechanical interlocking of ceramic to zirconia [Figure 1]b. The mode of failure noted in both specimens was cohesive failures [Figure 1]b and [Figure 2]b.

Sandblasting may lead to phase transition from tetragonal to monoclinic that causes building up of tensile stress and further in debonding of zirconia. Matani et al.[7] reported maximum monoclinic content in sandblasted zirconia specimens. Coarser the particle size and increased amount of the monoclinic phase is noted.[9],[10],[11],[12] Size of the Al2O3 particle and pressure used affect the phase transition to a greater extent.[6],[7],[9] Use smaller particles of Al2O3 at low pressure around 0.2–0.4 MPa to decrease the monoclinic phase.[6],[9] Bitencourt et al. reported higher bond strength values for specimens with Al2O3 −27 μ than 110 or 250 μ. Furthermore, smaller the particle size, the possibility of inducing surface damage also decreases. Hence, to covert the undesired effects of sandblasting, Al2O3 of 30 μ with 0.4 MPa pressure was used in the study protocol.

Groups 3, 4, and 5 differed only in the sequence of heat treatment. In Group 3, heat treatment was done before sandblasting and HIM/SIE, whereas Group 5 proceeded with heat treatment after sandblasting, and in Group 4, heat treatment was done after sandblasting and HIM/SIE. As stated earlier, the transformed monoclinic phase in sandblasted specimens creates compressive stresses that lead to bond failure between zirconia and ceramic. Heat treatment subsequent to sandblasting drove the monoclinic phase back to the tetragonal phase, thereby releasing the compressive stresses.[9],[13] The lower bond strength values in Group 3 compared to Group 5 could be ascribed to the absence of heat treatment that resulted in an increased monoclinic phase in these specimens. However compared to Group 1 (sandblasted), Group 3 performed better in terms of bond strength. The failure mode noticed in Group 1 was mainly interfacial, whereas that of Group 3 was cohesive.

SEM of the sandblasted group showed gross surface defects with cracks and crevices on the surface [Figure 2]a. This finding was in accordance with the findings in other in vitro studies which employed sandblasting procedures.[14],[15] These surface defects could act as areas of stress concentration and could contribute to the lower bond strength values between ceramic and zirconia. The specimens with sandblasting and SIE showed surface morphology interspersed between surface defects with ultrastructural changes at a nanoscale [Figure 2]d and [Figure 2]e. Unsintered zirconia specimens demonstrated more surface irregularities when compared to sintered specimens. This observation was in par with previous studies conducted on sintered and unsintered zirconia specimens.[16]

The better performance of Group 3 can be due to the heat-induced maturation process that was done. The heat treatment followed was 1000°C with a holding time of 10 min. The conditions of heat treatment applied correspond to those used to veneer the core material with conventional porcelain, where a maximum temperature of 930°C and holding time of 1 min. The study by Guazzato et al.[17] and Kosmac et al.[18] demonstrated that monoclinic to tetragonal phase transformation may occur instantaneously as a given temperature is reached regardless of the holding time. The HIM procedure employs a temperature of 750°C that helps in crystals getting prestressed and strained. This temperature range employed in HIM may also help in transformation from the monoclinic to tetragonal phase, thereby alleviating the compressional stress created by sandblasting.

In Group 4, where all the surface treatments were carried on unsintered specimens demonstrated the least bond strength values. Generally, surface treatment procedures are carried out after sintering of the zirconia specimens. There has been scientific evidence of both tetragonal and monoclinic phase formations when surface treatments are carried out on sintered specimens. The results obtained in this study are consistent with other studies on flexural strength.[19],[20],[21],[22] Literature supports the lack of monoclinic phase formed in unsintered specimens when surface treated. The decreased bond strength values could be due to the extensive wear and irregularities on the surface of zirconia specimens.


   Conclusion Top


A combination of SIE/HIM and air abrasion with appropriate heat treatment significantly enhanced the bond strength of zirconia and ceramic. This protocol can be followed as a surface treatment protocol for zirconia as there is enhanced bond strength with minimal structural defects. Further studies on these combinations of surface treatments are required to establish it as a standardized protocol.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Sola-Ruiz MF, Agustin-Panadero R, Fons-Font A, Labaig-Rued C. A prospective evaluation of zirconia anterior partial fixed dental prostheses: Clinical results after seven years. J Prosthet Dent 2015;6:578-84.  Back to cited text no. 1
    
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Correspondence Address:
Dr. Priya Mariam Thomas
Department of Conservative Dentistry and Endodontics, Government Dental College and Research Institute, Fort, Bengaluru - 560 002, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcd.jcd_247_22

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