Effects of Laser Treatment on the Bond Strength of Differently Sintered Zirconia Ceramics

Abstract

Objective: The purpose of this study was to investigate the effects of carbon dioxide (CO₂) and Erbium-doped yttrium aluminum garnet (Er:YAG) laser irradiations on the shear bond strength (SBS) of differently sintered zirconia ceramics to resin cement. Materials and methods: Eighty zirconia specimens were prepared, sintered in two different periods (short = Ss, long = Ls), and divided into four treatment groups (n = 10 each). These groups were (a) untreated (control), (b) Er:YAG laser irradiated with 6 W power for 5 sec, (c) CO₂ laser with 2 W power for 10 sec, (d) CO₂ laser with 4 W power for 10 sec. Scanning electron microscope (SEM) images were recorded for each of the eight groups. Eighty composite resin discs (3 × 3 mm) were fabricated and cemented with an adhesive resin cement to ceramic specimens. The SBS test was performed after specimens were stored in water for 24 h by an universal testing machine at a crosshead speed of 1 mm/min. Data were statistically analyzed with two way analysis of variance (ANOVA) and Tukey honest significant difference (HSD) test (α = 0.05). Results: According to the ANOVA, the sintering time, surface treatments and their interaction were statistically significant (p < 0.05). Although each of the laser-irradiated groups were significantly higher than the control groups, there was no statistically significant difference among them (p > 0.05). Conclusions: Variation in sintering time from 2.5 to 5.0 h may have influenced the SBS of Yttrium-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics. Although CO₂ and Er:YAG laser irradiation techniques may increase the SBS values of both tested zirconia ceramics, they are recommended for clinicians as an alternative pretreatment method.

Introduction

Yttrium-stabilized tetragonal zirconia polycrystalline (Y-TZP) is a recently introduced alternative ceramic material for fixed partial dentures (FPD). Its popularity has been significantly increased because of relatively good aesthetic results and biocompatibility and excellent mechanical properties such as high flexural strength (700–1200 MPa) and fracture toughness (7–10 MPa m^1/2).^1

–5 Zirconia is a polymorphic material and has three allotropes: monoclinic, tetragonal, and cubic. The monoclinic phase is stable up to 1170^°C, and the tetragonal phase remains stable in the range of 1170^°–2370^°C. The cubic phase exists above 2370^°C up to the melting point of 2680^°C as well.^3,6

–9 Although the tetragonal form of zirconia, which is optimum in dental usage, may remain at room temperature by adding stabilizing oxides such as Y₂O_3, some external stress factors, such as sandblasting, grinding, impact, or thermal aging, can initiate the tetragonal to monoclinic phase transformation.^6

–10

Zirconia ceramic restorations have usually been produced with computer-aided design and computer-aided manufacturing (CAD/CAM) technologies that enable the formation of more stable and strong substructures even in complex morphologies.^6,8,9 Zirconia restorations are fabricated by using fully sintered (hard-milling) or partially sintered (soft-milling/green stage milled) blocks. Although hard milling is difficult to machine, as a result of its hardness, soft-milling is easier and, therefore, makes it more popular in dentistry.^9,10 After the milling process, pre-sintered frameworks are subsequently sintered to full density, and sintering conditions, such as final sintering temperature or heating method may influence the physical or mechanical behavior of the material.^3,9 The final temperature and the holding time were shown as direct determinants of translucency, density, porosity, and grain size of zirconia ceramics. Mechanical properties such as modulus of elasticity and fracture toughness were also affected by the sintering process.^5,9,11 It was indicated in recent studies that increasing the sintering temperatures as well as a long sintering time yielded the larger grain size of zirconia ceramics. The increased grain size of zirconia may trigger the spontaneous t-m phase transformation, and thus reduce the stability and strength of material. Reduction in the grain size may also adversely affect the phase stability and strength of the material.^3
–5,9 Therefore, the effects of sintering conditions on the mechanical properties of partially sintered zirconia ceramics should be further investigated.

It was shown in a number of studies that the long-term efficiency of indirect ceramic restorations mainly depended upon a durable and stable micromechanical interlocking and/or chemical bonding between cement and ceramic.^12

–17 The surface conditioning methods have been usually performed for ceramic materials, and significantly influenced their bond strength values.^5,12,18 However, the composition and physical properties of high crystalline content zirconia ceramics varied to silica-based ceramics and, therefore, the conventional surface conditioning methods did not produce significant topographic alterations for them.^12,15,18 The absence of a silica or glass phase makes zirconia resistant to both hydrofluoric/phosphoric acid etching and silane coupling. For this reason, different surface conditioning techniques, such as abrasion via diamond burs, airborne-particle abrasion, and tribochemical silica coating, have been suggested to increase the bond strength of high-strength ceramics with cement materials.^{7,12,16

–19} However, recent studies showed that excessive airborne-particle abrasion may cause crack formation and strength degradation on the surface of zirconia ceramics.^7,8,10 Further, it was claimed that airborne-particle abrasion as well as stress and temperature could initiate the tetragonal to monoclinic phase transformation, thus weakening the mechanical properties and enhanced fracture tendency.^{3,5,7

–10,20} These unfavorable potential results led to improved new alternative surface treatment techniques to alter the mechanical or chemical bonding of zirconia ceramics.

Laser technology has been widely used in medical science and dentistry, since its first trial in 1960. In dental practice, laser irradiation is also preferred to roughening surface dental materials such as zirconia ceramics.^16

–20 Although many types of lasers are available now, the Erbium-doped yttrium aluminum garnet (Er:YAG), carbon dioxide (CO₂) and Neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers are suggested for modifying the surface of zirconia ceramics, because of improved bond strength values.^{13,14,16,20

–23} The Er:YAG laser has the ability to remove particles by micro-explosions and by vaporization, a process called “ablation.”¹⁴ Additionally, the wavelength of Er:YAG laser coincides with the main absorption peak of water, and it is well absorbed by –OH groups in hydroxyapatite.²⁰ Because its emission wavelength was highly absorbed by the ceramic, the CO₂ laser was also shown to be suitable for the treatment of zirconia ceramics.^16,19 It was believed that CO₂ laser irradiation is created for conchoidal slot areas on the surface of zirconia ceramic by the rapid rise of heat, and that slots provided the micromechanical retention of resin cement.²¹

Although the effects of laser irradiation techniques on the bond strength of zirconia ceramics have been fairly investigated, there was no common consensus. Although the Er:YAG or CO₂ laser treatments were indicated as successful methods in some of these studies,^19

–23 others showed opposite conclusions.^17,18 Therefore, the purpose of this study was to evaluate the effects of Er:YAG and CO₂ laser treatments on the shear bond strength (SBS) of differently sintered zirconia ceramics. The null hypothesis of this study was that the neither laser surface treatment would increase the SBS of zirconia ceramics, and that the sintering time variable would not be effective on SBS values either.

Materials and Methods

Specimen preparation

Eighty cubic form (6 × 6 × 2 mm) pre-sintered zirconia specimens were prepared by Y-TZP ceramic block (65 × 30 × 20 mm) (Ceramill Zi; Amann Girrbach AG, Koblach, Austria) in a cutting machine (Isomet 1000; Buehler, IL) under water cooling and then divided into two main sintering groups. Specimens of the first group (set as long sinter group: Ls) were sintered in a sintering oven (Ceramill therm; Amann Girrbach) at a temperature of 20°–1500°C using a rise time of 3 h, and kept at 1500°C for 2 h (manufacturer's recommendation). Specimens of the second group (short sinter group: Ss) were sintered at 20°–1500°C using a rise time of 1 h 30 min and kept at 1500°C for 1 h.³ A volumetric sintering shrinkage of 20% occurred, and the specimen's dimensions (4.8 × 4.8 × 1.6 mm) were checked with a digital micrometer (Digimatic Caliper; Mitutoyo MC, Kawasaki, Japan). Then, a silicone mold was prepared and specimens were embedded into autopolymerizing acrylic resin blocks (Meliodent; Heraeus Kulzer, NY). Prior to surface treatments, all specimens were ground-finished with 600–1000 sanding discs (3M ESPE, MN) respectively on a sanding machine (Phoenix Beta; Buehler, IL) under water cooling, and ultrasonically cleaned for 4 min in distillated water and air dried.

Surface treatments

Both Ls and Ss specimens were divided into four surface treatment groups, randomly (n = 10 each).

GC: No extra surface treatment was applied to the specimens of control groups.

GEr: A light delivery Er:YAG laser (Asclepion MCL 30; Asclepion Laser Tech, Jena, Germany) was applied to the ceramic surface at a power output of 6 W (wavelength of 2.94 μm, pulse penetration rate of 20 Hz, energy of 300 mJ, pulse duration of 700 μs, and energy density of 37.68 J/cm²) for 5 sec. Laser irradiation was applied using a noncontact tip (Vario; 4.5 mm spot size) at an approximate distance of 10 mm with the water spray at a rate of 5 mL/min.^14,17.22

GCo-2W: A light delivery CO₂ laser (Smart US20D; Deka Laser, Firenze, Italy) was applied to the ceramic surface at a power output of 2 W (wavelength of 10.6 μm, pulse penetration rate of 100 Hz, energy of 20 mJ, pulse duration of 160 ms, and energy density of 124.40 J/cm²) for 10 sec. Laser irradiation was applied using a contact tip (R14) at a diameter of 4 mm with air coaling.

GCo-4W: The CO₂ laser was also applied to these specimens at 4 W (pulse penetration rate increased to 200 Hz and energy density to 159.22 J/cm²) power output level for 10 sec. Both lasers optical tips were placed perpendicularly to specimen's surface.^14,19,22,23

Scanning electron microscope (SEM) analysis

Following the surface treatments, one sample from each group of Ls and Ss specimens was taken to examine in an SEM (Nova Nano-SEM 450; FEI Europe, Eindhoven, The Netherlands) (×5000 magnification). The acceleration voltage of cathode was set to 3 kV and the working distance was set to 9–10 mm. All specimens were coupled using a gold alloy conductive layer of ∼30 nm.

Bonding procedures

Seventy-two composite resin (Filtek Z250; 3M Espe, Seefeld, Germany) discs (3 × 2 mm) were fabricated by compressing the material in a brass mold with 10 cylindrical holes, and light cured for 20 sec (Hilux LED 550; Benlioglu Dent, Ankara, Turkey) with a light intensity of 600 mW/cm².^20,22 Then, composite resin discs were cemented to the zirconia surfaces with a dual-cured resin cement (Panavia F 2.0; Kuraray Co Ltd, Osaka, Japan) that contained an adhesive 10-Methacryloyloxydecyl dihydrogen phosphate (MDP) monomer. The application of resin cement material was performed according to the manufacturer's instructions. A composite resin disc was placed over the center of the resin cement applied ceramic surface under a load of 1000 g in a press. Then, excess cement material was removed with a brush and the entire object was light cured for 20 sec. The samples were rinsed with an air-water syringe and stored in distilled water at 37°C for 24 h.^20

–23

SBS test and statistical analyses

A universal test machine (Lloyd-LRX; Lloyd Inst., Fareham, England) was used to performed an SBS test at the crosshead speed of 1 mm/min. The SBS values were calculated in mega pascals (MPa) by dividing failure load (N) with the bonding area of the composite resin (α = P/A). Then data were statistically analyzed (SPSS 20.0 V; SPSS Inc., Chicago, IL). First, a normal distribution was obtained for variables by Kolmogorov–Simirnov test. Then, the SBS results were analyzed by two way ANOVA for evaluating the effects of surface treatment, sintering time, and their interactions. The mean SBS values were compared by Tukey's multiple comparison test and independent sample t test for pairwise comparisons among sinter groups (α = 0.05).

Results

The SBS values of untreated (control) groups were compared with the values of other surface treatment (test) groups. According to the two way ANOVA, the sintering time (p < 0.001) and surface treatment (p < 0.001) were statistically significant on SBS values (Table 1). The mean SBS values, standard deviations (SD), and multiple comparisons are shown in Table 2.

Table 1.

Results of Two Way ANOVA Test

Variable (source)	df	Sum of squares	Mean squares	F	p^*
Sintering time	1	28.238	28.238	16.268	0.000
Surface treatment	3	166.565	55.522	31.987	0.000
Interaction	3	4.074	1.358	0.782	0.508
Error	64	111.090	1.736
Total	72	17753.663

Significantly different at p < 0.05.

Table 2.

Mean and SD of Shear Bond Strength Values and Differences Among Sinter/Surface Treatment Groups

	Long sinter (Ls)		Short sinter (Ss)
Group	Mean/SD	Differences^*	Mean/SD	Differences^*
GC	13.54 ± 0.70	Aa	12.33 ± 0.96	Ba
GEr	17.17 ± 1.22	Ab	16.02 ± 1.51	Ab
GCo-2W	17.47 ± 1.60	Ab	15.49 ± 1.28	Bb
GCo-4W	16.57 ± 1.31	Ab	15.91 ± 1.68	Ab

The multiple comparisons of surface treatment groups by Tukey honest significant difference (HSD) test are shown as small letters and pairwise comparisons (independent sample t test) among sinter groups are shown as capital letters. The values having same letters are not significantly different (p > 0.05).

The smallest SBS values were observed in GC for both Ls (13.54 MPa) and Ss (12.33 MPa) and the highest SBS value was determined in GCo-2W for Ls (17.47) and in GEr for Ss (16.02) groups. The SBS values of each laser treated (test) group were significantly higher than those of control groups (p < 0.001), whereas there was no significant difference between test groups for both Ls (p = 0.824–0.999) and Ss (p = 0.989–0.999) sinter. Independent sample t test showed that there were significant differences between Ls and Ss sintered GC (p = 0.008) and GCo-2W (p = 0.010) surface treatment groups (Fig. 1).

FIG. 1.

The mean/SD of shear bond strength (SBS) values and statistical summaries in a box-pilot graphic.

SEM images (×5000 magnification) of each surface treatment applied to Ls and Ss specimens are shown in Figs. 2 and 3, respectively. Whereas the control groups showed smoother surfaces, the test specimens had rougher surface with prominent irregularities. However, on the image of the Ss GCo-2W group, profound cavities with ruptured ceramic particles were observed (Fig. 3c).

FIG. 2.

Scanning electron microscopic (SEM) images (×5000 magnification) of long-sintered specimens. (a) GC, no extra surface treatment applied; with relatively smooth surface. (b) GEr. (c) GCo-2W. (d) GCo-4W. The images of the test groups show prominent changes on topographic textures.

FIG. 3.

Scanning electron microscopic (SEM) images (×5000 magnification) images of short-sintered specimens. (a) GC-no extra surface treatment applied; with relatively smooth surface. (b) GEr. (c) GCo-2W. (d) GCo-4W. The images of the test groups show prominent changes, and profound cavities can also be seen in the GCo-2W group.

Modes of failure percentages (%) are also shown in Fig. 4. The analyses of failure modes after the SBS test revealed that almost all failures in both control groups were adhesive. Mixed and less cohesive failure modes were observed for laser groups.

FIG. 4.

Failure type distributions of groups for both long- and short- sintered specimens.

Discussion

From the results of this in vitro study, the null hypothesis was rejected. Not only the surface treatment but also the sintering time influenced the SBS values.

Sintering conditions significantly influenced the physical or mechanical properties of pre-sintered zirconia frameworks.^3,4,10 In present study, the variations on sintering time, which has been shown to be an effective factor on the SBS of zirconia ceramic, may also change the grain size, homogeneity, and mechanical properties of Ls and Ss specimens. It was indicated in a related study that increase in sintering temperature above 1300°–1500°C was not only in line with enlarged grain size but also raised flexural strength of zirconia ceramics. The flexural strength and contrast ratio of zirconia were also significantly decreased, and a number of micropores appeared when the specimens sintered above 1600°C.^5,9 This result was in accordance with another study, and both of them stated that increased grain size would propagate the number of micropores, and cavitation thus reduced the mechanical properties of zirconia ceramics.^9,24 In the present study, the different SBS values or topographical variations on the SEM images of same surface treatments applied to Ls and Ss groups may be attributed to their probable microstructural differences. In contrast, Hjerppe et al.³ reported that different sintering times (short, 20°–1500°C temperature at rise time of 1 h 40 min and kept at 1500°C for 1 h, and long, 20°–1500°C using rise time of 3 h and kept at 1500°C for 2 h) had no influence on the mechanical properties (biaxial flexural strength, surface microhardness) of zirconia ceramics. Therefore, the effects of sintering time on the physical or mechanical properties of partially sintered zirconia ceramics should be further investigated.

Laser irradiation is an alternative and spreading technique in dental practice to achieve reliable bond strength between resin cement and zirconia, because it improves the surface roughness.^13,14 However, the effective laser type and its application modes are still unclear and, therefore, give rise to the performance of new investigations. Therefore, the effect of Er:YAG and CO₂ laser treatments on the SBS of resin cement to differently sintered zirconia was investigated in the present study. Er:YAG and CO₂ laser irradiation can significantly increase the SBS of both Ss and Ls zirconia ceramics. This result is in accordance with the study of Akın et al.,²⁵ who concluded that Er:YAG laser treatment on zirconia ceramic resulted in higher SBS than that of untreated specimens. In another study, although Er:YAG laser irradiation was declared as a suitable technique for the surface characterization of zirconia ceramics, Nd:YAG and CO₂ lasers were found to be very destructive of the mechanical properties and adhesion of zirconia because of a large amount of heat and microcracks.²³ In parallel to these results, CO₂ laser irradiation at a power output setting of 2 W was found to be possibly destructive on short-sintered zirconia specimens in present study. However, there was no significant difference between each laser-irradiated group. Contrary to these results, Akın et al.¹⁴ advocated that CO₂ laser irradiation of zirconia showed lower SBS than when it was untreated or sandblasted or treated with Er:YAG or Nd:YAG laser. The CO₂ laser irradiation caused heat induction and rapid expansion on the surface of the zirconia ceramics, and the subsequent contraction during solidification caused shell-form porosities,^14,22,23 and these porosities provided micromechanical retention areas for resin cement material.²⁰ Local temperature changes may also involve microcrack formation and melted-worn areas, and thus damage the mechanical properties of zirconium oxide ceramics.^14,22,23 The range of these mechanical deteriorations largely depends upon the energy intensity of laser irradiation and the type of ceramic.²³ Therefore, it was recommended that CO₂ laser beam with low energy settings such as 80, 150, or 200 J/cm² for zirconia ceramics be used.¹⁴ In addition, the pulse penetration rate was indicated as being another important factor in the energy density of the CO₂ laser, which influences the thermal effects on the surface of ceramic.¹⁶ The discrepancies of the SBS values of the CO₂ laser groups may be explained by the variations in the sintering conditions of Ls and Ss zirconia specimens and the different energy intensities of CO₂ laser beam executions in the present study.

It was indicated in some related studies that despite that the CO₂ laser beam may be completely absorbed by the surface of the zirconia ceramics, Er:YAG laser beam is not absorbed likewise.^13,16,22,23 Additionally, Er:YAG laser was irradiated with a noncontact hand piece at a distance of 10 mm, which could reduce its efficacy. Therefore, the surface of the zirconia ceramics was covered with graphite powder, and the power output setting of Er:YAG laser was altered to 6 W, in order to increase the absorption ratio of energy by zirconia.¹⁶ The Er:YAG laser irradiation on zirconia ceramics was tested at different energy levels.^{14,16,18,22,23,25} The high power output of the Er:YAG laser in present study was obtained by the increase in pulse penetration rate from 10 Hz^14,22,13 to 20 Hz. Although the Er:YAG laser energy intensities between 100 and 300 mJ had no significant impact on the surface of zirconia ceramics,^16,18 higher power settings (400–600 mJ) can cause excessive material deterioration and can induce phase transformation.²⁵ Therefore, the energy intensity of the present study was kept at the level of 300 mJ with constant water cooling. Additionally, the pulse duration of present study was decreased to 5 sec from 10 to 20 sec^14,22,23 at the same wavelength settings (10.6 μm), in order to prevent microcrack formation on the zirconia ceramic surface resulting from the high power output of 6 W. Kırmali et al.²⁶ evaluated the effects of sandblasting, linear application, and Nd:YAG and Er:YAG laser treatments on the pre-sintered surface of the zirconia to prevent microcrack formation and phase transformation. They found that Nd:YAG or Er:YAG laser irradiation alone was not effective in improving the zirconia surface morphology and SBS between zirconia and veneering ceramic. However, the SEM images in the present study were shown to be regular irregularities on the surface of Er:YAG laser-irradiated Ls and Ss zirconia specimens without microcrack formation (Figs. 2b and 3b), and SBS values determined significant increases as high as in the CO₂ laser groups (Fig. 1).

In the present study, composite resin discs were cemented to the zirconia specimens of both the test and control groups with a resin cement containing 10-MDP monomer, because there has been significant evidence showing the superiority of this monomer in improving the resin bond strength to Y-TZP ceramics.^{5,12,13,16,22} The 10-MDP monomers not only improve the surface wettability of ceramic, but also form cross-linkage with methacrylate groups of resin cement and hydroxyl groups of ceramic surface.¹³ It has been stated that using phosphate monomer-containing cement could not be beneficial without any pretreatment, which roughens the surface of zirconia ceramics.^12,13,27 The failure modes in present study after SBS testing also promoted this result. Although predominantly adhesive failures were observed in control groups (90–100%), cohesive (10%) and mixed (10–35%) failures were observed in laser-applied groups (Fig. 4). Therefore, using a resin cement with a phosphate monomer may have influenced the success rates of CO₂ and Er:YAG laser irradiation techniques in the present study of the SBS of zirconia ceramics.

This in vitro study has some limitations. The effects of sintering conditions on the physical or mechanical properties of zirconia ceramics, such as sintering temperature, time, or heating method, should be evaluated in future studies. In this study, the effects of Er:YAG and CO₂ laser irradiations on zirconia ceramic specimens were stored in distilled water at 37°C for 24 h and then evaluated by SBS test, which is easy to perform and can produce results quickly. The effects of aging procedures, such as thermos-cycling and long-term storage, may be simulated for in vivo conditions. The results of present study are important for evaluating the effects of some laser irradiation techniques, and can lead to performing long-term clinical studies.

Conclusions

Within the limitations of this study, it can be concluded that variations in sintering time from 2.5 to 5.0 h may have influenced the SBS of zirconia ceramics. The CO₂ and Er:YAG laser irradiation techniques were effective in achieving sufficient SBS between zirconia and resin cement. Both CO₂ and Er:YAG laser irradiation techniques are recommended as a surface pretreatment method for differently sintered zirconia ceramic.

Footnotes

Author Disclosure Statement

No conflicting financial interests exist.

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