influence of surface treatments on bonding cad/cam...

* Lecturer, Fixed Prosthodontics Department, Faculty of Oral and Dental medicine, Cairo University, Egypt .

INTRODUCTION

In recent years, dentists and patients have expressed growing interest in tooth-colored, metal-free fixed partial dentures because of their excellent biocompatibility and enhanced aesthetic outcomes(1). This increasing demand led to the development of all-ceramic materials with optimized mechanical properties, such as densely sintered aluminum oxide and zirconium oxide ceramics (2). Zirconia is a crystalline dioxide of zirconium. Its

mechanical properties are very similar to those of metals and its color is similar to tooth color (3). With the advance of computer-aided design/ computer-aided manufacturing (CAD/CAM) technologies, dental zirconia is currently applied to frameworks for fixed dental prostheses, implant abutments as well as single all-ceramic restorations. (4-6). Zirconia restorations are made either by milling enlarged restorations out of homogenous ceramic green-body blanks of zirconia, which are then sintered and

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INFLUENCE OF SURFACE TREATMENTS ON BONDING CAD/CAM ZIRCONIA TO RESIN CEMENT

Mohamed Fattouh Abdullah*

ABSTRACT

Objectives: The objective of this study was to evaluate the effect of different surface treatments on the bond strength of an adhesive luting agent to CAD–CAM zirconia ceramic (Cercon).

Methods: thirty Ceramic cylinders (2 mm x20mmx10 mm) were fabricated from Cercon ceramic blocks. Cylinders were divided into three groups and submitted to the following surface treatments: group 1: sandblasting with 110µm alumina;group 2: silica coating (cojet system) and group 3: laser irradiation (CO2laser). After surface treatments, The ceramic cylinders were cemented to composite cylinders by resin cement (Panavia F 2.0). Scanning electron microscope was also performed to evaluate the surface morphology changes. Shear bond strength was recorded using a universal testing machine at a cross-head speed of 0.5 mm/min and expressed in megapascals (MPa). Shear bond strength values of group 1(4 ± 0.26 MPa); group 2(3.74 ± 0.89 MPa); group 3(2.48± 0.38 MPa).

Conclusions: The results show that Sandblasting obtained highest shear bond strengths followed by CoJet while Co2 laser treatment gave the lowest bond strength values

KEYWORDS: zirconia, sandblasting, cojet, laser surface treatment, resin cement.

(2) Mohamed Fattouh AbdullahE.D.J. Vol. 60, No. 1

shrunk to the desired final dimensions,or by milling the restorations directly with the final dimensions out of highly dense sintered prefabricated blanks (7,8). The use of different types of cements (conventional, glass ionomer, self-adhesive and resin) have been proposed for luting zirconia. Among these, adhesive resin cements are preferred owing to its better retention and marginal fit (9, 10). A major weakness of dental zirconia is its inferior ability to adhere to resin cement (11). As zirconia has a polycrystalline structure and limited vitreous phase, neither hydrofluoric acid etching nor silanization can achieve durable zirconia- resin bonding (12-14).

A strong resin bond depends on micromechanical interlocking and chemical bonding to the ceramic surface(15), this is achieved by different surface treatment methods, common options are grinding, airborne particle abrasion with aluminum oxide, acid etching, silica coating, and a combination of any of these methods (16-18). Also, it was reported that CO2 laser produces distinct surface alterations to zirconia surface at various laser parameters (19).Thus; the purpose of this study was to evaluate the shear bond strength between zirconia and resin cement cylinders after different surface treatments.

MATERIALS AND METHODS

Preparation of the specimens

Thirty zirconia specimens were made of Cercon base (Degudent, Hanau, Germany ) measuring 2 mm in thickness, 20 mm in length, and 10 mm in width .The specimens were constructed by cutting cylinders from Cercon base blanks (color white). Specimens were then placed on a Cercon heat sintering furnace tray and sintered for 7 hours at the maximum temperature of 1350˚C. The bonding surfaces of the zirconia blocks were polished with 600-grit silicon carbide paper on a rotating device under water-cooling. Then specimens were equally divided into three groups (n = 10) according to surface treatment of cylinders,

Group 1: specimens were airborne particle abraded with 110-μm aluminum oxide particles (S-UAlustral, Schuler-Dental, Eberhard-Finckh, Germany) for 10 seconds at working distance of 5mm perpendicular to the surface at 0.2 MPa pressure (P-G 400, Hornisch + Rieth, Winterback, Germany).

Group 2: specimens were treated with 30 μm Al2O3 particles modified by silica (CoJet System, #171456; 3M ESPE AG, Seefeld, Germany) using an airborne- particle–abrasion device (CoJet System; 3M ESPE AG, Seefeld, Germany) filled with 30μm silicon-dioxide particles (CoJet Sand, #172562; 3MESPE AG, Seefeld, Germany). The abrasive was applied perpendicular to the surface at 0.28 MPa for 15 seconds at a distance of 10 mm.

Group 3: specimens were laser treated using CO2 laser machine (CO2 medical laser System ATL-150, Advanced Technology Laser Co., Ltd. ,china ). A beam of 10.6 um wavelength was delivered in a continuous mode through an articulating arm. The laser parameters used were (20 watt Power, 400 J Energy and 10 seconds application time repeated three times with a resting period of 10 seconds between each application). After the surface treatments, samples were ultrasonically cleaned in 99 % acetone for 5 minutes and then in distilled water for another 5 minutes to remove excess sand particles from the surface, in a manner similar to the process given by Yang et al (20) . Thirty composite cylinders 2 mm in thickness, 20 mm in length, and 10 mm in width) were made by layering increments of a microhybrid resin composite (Tetric EvoCeram, Ivoclar-Vivadent, Schaan, Liechtenstein) in a silicone mold. Each increment was condensed with a clean plastic filling instrument to avoid contamination, and light-cured for 40 seconds (BluePhase, Ivoclar-Vivadent, Schaan, Liechtenstein, with output of 600mmW/cm2). The last increment was compressed with a glass microscope slide in order to obtain a flat surface. After removing the specimen from

INFLUENCE OF SURFACE TREATMENTS ON BONDING CAD/CAM ZIRCONIA (3)

the mold, an additional 40 seconds irradiation was performed on the portions that were previously in contact with the silicone mold.

Cementation technique

Silanate the adherend surfaces using Clearfil ceramic primer (Kuraray Co., LTD, Osaka, Japan). After silanization, the composite resin cylinders were bonded to the treated ceramic surfaces using Panavia F 2.0(Kuraray Co., LTD, Osaka, Japan), the composite cylinder was seated on top with a fixed pressure (50Newtons for 1 minute), and excess cement was wiped off. A specific loading device and a vertical micrometer ensured even cement

thickness (30 μm) for all specimens. Finally, the adhesive cement was light-polymerized at four locations on the specimens for 60 seconds each, then oxygen-blocking gel (Oxyguard II,; Kuraray Co., LTD, Osaka, Japan) was applied for 10 minutes before removing the specimens from the press(21,22) .The specimens were washed with air-water spray and stored in distilled water at 37°C for 24 hours. The cementation procedure was conducted immediately after the required surface preparation for every test group to prevent possible contamination of the specimens. The chemical composition and operation mode of the investigated materials are detailed in Table 1.

TABLE (1) Chemical composition and application of materials used in the study

Material Composition(acc.to manufacturer)

Application

Cercon Zirconia base Batch No. 51247

Zirconium-oxide (92%), yttriumoxide(5%), hafnium-oxide (<2%), aluminium-oxide + silicon-oxide (<1%)

Sinter the ceramic cylinders in a special oven (Cercon Heat, Dentsply) keeping thetemperature at 1350 8C for 6 hrs.

Panavia F 2.0Batch No. 41244

Clearfil Ceramic Primer: 3-MPS,10-MDP, ethanolPanavia F 2.0 Cement Paste AU: 10-MDP, hydrophobic and hydrophilic aliphatic dimethacrylate, silanated silica, silanated barium, silanated colloidal silica, dlcamphorquinone, benzoil peroxide, poliethoxy dimethacrylate, *catalysts, initiatorsPanavia F 2.0 Cement Paste BU: hydrophobic and hydrophilic aromatic dimethacrylate, silanated barium silanated titanium oxide, sodium fluoride, bisphenol A, poliethoxy dimethacrylate, colloidal silica, diethanol-p-toluidine, sodium 2,4,6-triisopropyl benzene sulfinate, *catalysts, accelerators, pigments*Catalysts: Bis-GMA, TEGDMA; fillers: 76.9 _ 0.23 wt%)

Apply the Clearfil Ceramic Primer on the zirconia bonding surface for 40 s. Gently air dry. Mix equallengths of Paste A and B for 10 s until a uniform colour is achieved and apply the mixture on the ceramic surface. Remove excessafter luting the composite disk under pressure. Self-cure for 5 min and light-cure each axial surface for 40 s

Tetric Evo CeramBatch No. H34328

Matrix: dimethacrylates (17– 18 wt%). Fillers: barium glass, ytterbium trifluoride, mixed oxide and prepolymer (82– 83 wt%), additives, catalysts,stabilizers and pigments (<1 wt%)

Condense 2 mm-thick layers.Light-cure each increment for 20 sec.

Abbreviations: Bis-GMA: bisphenol A glycidyl methacrylate; UDMA: urethane dimethacrylate; 3-MPS: 3-methacryloxypropyl trimethoxysilane; 10-MDP: 10-methacryloxydecyl dihydrogen phosphate; TEGDMA: triethylene glycol-dimethacrylate.


Testing of samples

A scanning electron microscope (SEM; VEGA II LSH, TESCAN, Brno, Czech Republic) at 2000x magnification and 20 kV accelerating voltage was used to show the effect of the surface treatment methods on zirconia surface topography.

The shear bond strength was tested for ceramic-composite bonded samples with a universal testing machine (Lloyd-LRX; Lloyd Instruments, Fareham, UK) at a crosshead speed of 0.5 mm./min. until failure occurred . Sample was secured to the lower fixed compartment of testing machine by tightening screws. Shear bond strength test was done by compressive mode of load applied at ceramic-composite resin interface using a monobeveled chisel shaped metallic rod attached to the upper movable compartment of the testing machine, The maximum force (MPa) to produce fracture was recorded using a corresponding software.

RESULTS

Results of the Scanning Electron Microscope

SEM examination was made for all the samples under magnification of 2000x .

In group 1, Sandblasting created a rougher surface, the image showed a change in surface texture with the formation of micro-retentive grooves. The entire surface was roughened but in an irregular pattern.

In group 2, SEM showed deposition of silica particles on the zirconia surface after treated with CoJet system.

In group 3, SEM observations showed that the laser energy promoted surface changes in the form of little material removal related to formation of pores, elevations and depressions over the surface. Also, debris that adheres to the melted surfaces could be seen.

Fig (1): Scanning electron micrographs of zirconia specimen after sandblasting

Fig. (2): Scanning electron micrographs of zirconia specimen after treatment with CoJet system.

Fig. (3): Scanning electron micrographs of zirconia specimen after Laser surface treatment.


Results of the Shear bond strength

Statistical analysis was carried out using SPSS statistical software package 11.5 (SPSS Inc., Chicago, IL, USA). Analyses of variance were performed with the bond strength as the dependant variable. Descriptive statistics of shear bond strength results for all tested groups including mean, standard deviation (SD) values are summarized in table (2).

TABLE (2) Shear bond strength mean values in (Mpa) for all tested groups.

Mean SD NK rankGroup 1 4.00 0.26 AGroup 2 3.74 0.89 AGroup 3 2.48 0.38 B

Different letter in the same column indicating statistically

significant difference

Specimens treated with laser exhibited the lowest bond strength and those treated with air abrasion exhibited higher bond strengths while there are no statistically significant differences between sandblast group and CoJet group

DISCUSSION

The composition and the physical properties of the zirconium oxide based ceramics differ from those of the silica-based ceramics and need alternative bonding techniques to achieve a strong, long term, durable resin bond. The silica phase is the only phase able to be etched by hydrofluoric acid; the etching was therefore inefficient for high strength zirconia ceramics (23, 24).

This study evaluated the surface changes caused in zirconia after different surface treatments and the influence of the surface treatment on shear bond strength to zirconia. All the treatments were used to achieve micromechanical retention on the ceramic surface. The particle sizes used for the air abrasion

and silica coating were selected based on previous studies (25, 26).Selection of CO2 laser type to be used in our current study based on the previous findings (27) ,they reported that CO2 laser revealed distinct surface alterations to zirconia surface at various laser parameters. This could be attributed to that the CO2 laser beam does not penetrate the ceramic surface or cause surface preservation as other laser types. And since the principal effect of laser energy is the conversion of light energy into heat and the most important interaction between laser and substrate is the absorption of the laser energy by the substrate (28).

For the evaluation of the resin cement and ceramic interface, pretreated zirconia surfaces were cemented on standard resin composite cylinders instead of tooth structures owing to the uniform structure of the composite resin. This prevented the results from being misinterpreted owing to the microstructure variables of the tooth structure (29, 30).

Microscopic analyses revealed changes in the surface topography in the air abrasion, silica and air abrasion and laser groups with the formation of microretentive grooves and pits.

The highest shear bond strength mean value recorded for the sandblasting could be attributed to the increase in the surface area created by sandblasting allowing acceptable roughness facilitating resin/ceramic micromechanical interlocks formation. The abrasive process removes the loose contaminated layers, increases the surface area available for bonding and improves the wettability of the luting material. It was also suggested that air abrasion reduces inherent surface defects or those generated as a result of the manufacturing process(31) .Also sandblasting is associated with less material removal from the surface and low temperature developing. The decrease in shear Bond strength of silica coated zirconia could be attributed to the slight modification it produced on zirconia surface, although silica coating deposited a silica layer on


zirconia surface promoting a chemical bond at the ceramic- resin cement interface but it did not result in a frank surface modification and did not produce a stable uniform silica layer (32). The silica coverage originating from the coating particles appeared not to be really firmly attached to the hard zirconia surface. Therefore, the weakly attached silica layer seems to be the weak link during bonding (33).They attributed this to the increased silica content resulting from silica coating and formation of a chemical bond with the resin cement through the silane coupling agent. However, most of these studies were working with In-Ceram Zirconia and since In-Ceram zirconia is an aluminous ceramic composed of 62% by volume of AL2O3 and 20% by volume of ZrO2mixed with glass, it is safe to suggest that the application of cojet sand on In-Ceram Zirconia present greater facility for silica coating than ceramics without the vitreous phases such as YTZP (34) . It provides better fixation of silica particles in the vitreous phases of the glass infiltrated ceramics than in the compact surface of Y-TZP. Zirconia framework materials may have different surface and bulk structural characteristics because of differences in grain size, shape, composition, density and hardness this may explain the difference seen in Bond strength results (35). The shear bond strength values recorded with Y-TZP cylinders treated with CO2 laser showed the lowest values bonding values. This may be attributed to less absorption of laser energy by zirconia ceramics owing to the fact that they are water-free materials. Also, the sharp, accentuated and differently oriented and distributed peaks created by laser radiation, may have promoted stress concentration at the interface causing a decrease in the ceramic cement bond strength (31).

CONCLUSIONS

Within the limitations of this in vitro study, the following conclusions were drawn:

1- Sandblasting obtained highest bond strengths

followed by CoJet while CO2 laser treatment gave the lowest bond strength values.

2- Sandblasting remains the most suitable and easier method for enhancing the bond strength between zirconia ceramics and resin cement.

3- Future work is needed to verify the results in oral simulated media e.g. artificial saliva, long-term storage and thermal cycling.

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