Micro-shear bond strengths of resin-matrix ceramics subjected to different surface conditioning strategies with or without coupling agent application

Abstract

PURPOSE

This study aimed to assess the influence of various micromechanical surface conditioning (MSC) strategies with or without coupling agent (silane) application on the micro-shear bond strength (µSBS) of resin- matrix ceramics (RMCs).

MATERIALS AND METHODS

GC Cerasmart (GC), Lava Ultimate (LU), Vita Enamic (VE), Voco Grandio (VG), and Brilliant Crios (BC) were cut into 1.0-mm-thick slices (n = 32 per RMC) and separated into four groups according to the MSC strategy applied: control-no conditioning (C), air-borne particle abrasion with aluminum oxide particles (APA), 2W- and 3W-Er,Cr:YSGG group coding is missing. The specimens in each group were further separated into silane-applied and silane-free subgroups. Each specimen received two resin cement microtubules (n = 8 per subgroup). A shear force was applied to the adhesive interface through a universal test machine and µSBS values were measured. Data were statistically analyzed by using 3-way ANOVA and Tukey HSD test. Failure patterns were scrutinized under stereomicroscope.

RESULTS

RMC material type, MSC strategy, and silanization influenced the µSBS values (P<.05). In comparison to the control group, µSBS values increased after all other MSC strategies (P<.05) while the differences among these strategies were insignificant (P>.05). For control and APA, there were insignificant differences between RMCs (P>.05). The silanization decreased µSBS values of RMCs except for VE. Considerable declines were observed in GC and BC (P<.05).

CONCLUSION

MSC strategies can enhance bond strength values at the RMC - cement interface. However, the choice of MSC strategy is dependent on RMC material type and each RMC can require a dedicated way of conditioning.

INTRODUCTION

The procurement of a durable bond between machine-milled restorations and luting cement by the genesis of chemical bonds and mechanical interlocking is one of the major climacterics influencing the long-term viability of prosthetic treatment as poor bonding may lead to low fracture strength, low retention, inferior marginal fit, and micro-leakage.¹,²,³,⁴,⁵,⁶ As attempts to achieve a durable bond, manifold micromechanical (mechanical, chemical, and laser irradiation) and chemical (silicon coatings, use of coupling agents) surface conditioning strategies are in use.¹,²,³,⁴,⁷,⁸,⁹,¹⁰

Micromechanical surface conditioning (MSC) strategies function by removing the loose contaminated surface, forming micro-retentive grooves, increasing roughness, and enhancing wettability for the genesis of micromechanical interlock between substrate and adherent.¹¹,¹² Supportively, the Wenzel equation emphasized that micro-surface roughness is essential to enhance hydrophilicity.¹³ Previously, a number of MSC strategies have been proposed.¹⁴ Of these; air-borne particle abrasion (APA) and laser irradiation (LI) are common.⁵,¹⁵,¹⁶ In APA, the substrate surface is roughened by throwing abrasive particles.¹⁷,¹⁸ Although being a well-established strategy;⁶,⁹,¹⁸ it has a propensity for surface-damage¹,⁹ and microcrack creation.¹¹,¹⁷,¹⁸ Changes in particle type, particle size, propulsion pressure, distance from the nozzle to the substrate surface, and abrasion time may lead to differences in the indentation pattern of the substrate surface.¹⁴ In LI, the inorganic content of the very superficial layer is removed with the aid of micro-explosions and vaporization.²,¹¹ Alterations in laser type, power output, distance from the nozzle to the substrate surface, irradiation time may differently influence the surface topography of the substrate.¹⁴ Although several types have emerged to roughen ceramics;⁴,¹¹,¹⁴ the use of erbium, chromium-doped yttrium, scandium, gallium, and garnet (Er,Cr:YSGG) laser has recently been widespread.¹,²,⁴,⁵,¹⁶ Due to its hydrokinetic nature, there is less risk of forming a heat-damaged layer.⁴,¹⁵

Chemical surface conditioning strategies function by chemically bonding inorganic and organic materials.¹⁰ With this regard, coupling agents have gained popularity and are recommended to promote luting cement hydrophilicity⁹ subsequent to the MSC strategies.⁶,¹² However, their benefits are still controversial.¹⁹,²⁰,²¹ These mediators are synthetic organic-inorganic silicon compounds that can be either monofunctional (non-functional), bifunctional, or bis-functional (cross-linking / dipodal). Monofunctional agents contain only one reactive (alkoxy) group that can react with inorganic materials and are used as primers in surface modification. Bifunctional agents contain two reactive groups and thereby, function as chemical adhesion promoters between dissimilar matrices through dual reactivity.⁸ Bis-functional silanes possess two silicon atoms with three hydrolysable alkoxy groups and are incorporated with bifunctional silanes to increase bonding and hydrolytic stability.²²,²³,²⁴ The chemical formula for a bifunctional silane is Z-(CH2) n-Si-(OR)3; where Z is organo-functional end connecting with organic materials such as resin composites, (CH2) n is linker group, and (OR)3 is alkoxy end connecting with inorganic materials such as ceramics. Organo-functional end reacts with the methacrylate groups of the resins via a free radical polymerization process. After hydrolysis to silanol groups, the alkoxy groups react to the surface hydroxyl groups of inorganic substrates.⁶,⁹,¹¹,¹⁷,²²,²³,²⁴,²⁵,²⁶

Lately introduced resin-matrix ceramics (RMCs) amalgamate the advantageous characteristics of dental ceramics and composite resins,¹,⁴,⁷,⁸,⁹,¹¹,²⁷,²⁸,²⁹,³⁰ and thereby, provide the followings: enhanced and fast machinability, superior fatigue resistance, acceptable wear resistance, low abrasive effect to opposing teeth, promising bond strength, polishability, no requirement for firing and intraoral reparability.¹,³,⁷,¹⁸,³¹,³²,³³,³⁴,³⁵,³⁶ It is of clinical significance to have advanced knowledge about the restorative material from which the indirect restoration is produced, to promote a strong bonding since the surface conditioning strategy varies according to the chemical composition of the restorative material used.¹⁷,²⁵ The RMCs consist of organic and inorganic portions in different proportions and are polymerized under high pressure and temperature leading to the increased monomer to polymer conversion rates (up to 96%).⁹,²¹,³⁷ Few free monomers stay available for copolymerization with monomers of the luting cement. Therefore, prior to adhesive cementation, the use of a customized surface conditioning strategy is essential.²¹,³⁸

The preference for luting cement can be considered as another critical factor in the establishment of a durable bond.⁹,¹⁷,²⁶ Since self-adhesive luting cements do not have any preprocessing of the tooth surface, they have commonly been used.⁴,⁸,⁹ Moreover, the bonding workflow in self-adhesive cements was made very simple in contrast to that of total-etch cements.⁵,⁸ Thereby, ease of handling property overcame the technique sensitivity of luting cements. Although total-etch adhesive systems provide better service in terms of bond strength when compared to self-adhesive systems,¹ a study by Bellan et al.⁸ demonstrated that the self-adhesive resin cement did not differ from the total-etch adhesive and self-etch adhesive resin cement. Additionally, despite conflicting results, the curing mode of luting cements can be influential on the durability of the bond. Dual-cured cements are found more reliable.⁴,⁹ From this perspective, it is plausible to combine the customized surface conditioning strategy with appropriate luting cement.³⁹ For instance, Alp et al.⁷ recommended APA with dual-polymerizing resin cements that contain 10-metacryloxydecyl dihydrogen phosphate (MDP) monomer for nanohybrid-composite resin materials.

To the best knowledge of authors, data regarding the influence of APA, LI, and silane application on the micro-shear bond strength (µSBS) of RMCs are scarce. Therefore, this study aimed to investigate the micro-shear bond strength of dual-cured resin cement to 5 different RMCs surface-conditioned with 8 different strategies. The null hypothesis was that there would be no statistically significant effect of MSC protocols and silanization on the data of micro-shear bond strength of different RMCs.

MATERIALS AND METHODS

In the current study, 5 different RMCs (GC Cerasmart [GC], Lava Ultimate [LU], Vita Enamic [VE], Voco Grandio [VG], and Brilliant Crios [BC]) were used. The brands, manufacturers, chemical compositions, and batch numbers of the materials used are depicted in Table 1. The schematic setup of the study is shown in Fig. 1.

Table 1. Brands, manufacturers, chemical compositions, and batch numbers of the materials used in the present study.

Material	Brand	Manufacturer	Chemical composition	Batch number
CAD-CAM resin-matrix ceramic block	GC Cerasmart	GC Dental Products, Leuven, Belgium	Organic part: Bis-MEPP, UDMA, DMA	1509052
			Inorganic part: 71 wt% silica and barium glass nanoparticles
	Lava Ultimate	3M ESPE, St. Paul, MN, USA	Organic part: Bis-GMA, Bis-EMA, UDMA, TEGDMA	N644403
			Inorganic part: 80 wt% silica and zirconia nanoparticles and zirconia/silica nanoclusters
	VITA Enamic	VITA Zahnfabrik, Bad Säckingen, Germany	Organic part: UDMA, TEGDMA	43230
			norganic part: 86 wt% glass ceramic (SiO2, Al2O3, Na2O, K2O, and other oxides)
	Voco Grandio	VOCO GmbH, Cuxhaven, Germany	Organic part: methacrylates	1925249
			Inorganic part: 86 wt% filler
	Brilliant Crios	COLTENE, Altstätten, Switzerland	Organic part: cross-linked methacrylates	I24143
			Inorganic part: 70.7 wt% barium glass and amorphous silica
Dual-cured resin luting cement	Bifix SE	VOCO GmbH, Cuxhaven, Germany	Base: UDMA, GDMA, catalysts, initiators.	1924379
			Catalyst: GDMA, acidic adhesive monomer, UDMA, Bis-GMA, Hydroxypropyl methacrylate, benzoyl peroxide
Silane coupling agent	Ultradent Silane	Ultradent Products, South Jordan, UT, USA	Methacryloxypropyltrimethoxy silane < 10%, isopropyl alcohol < 95%	BJ5XL

Al₂O₃, alumina; Bis-EMA, ethoxylated bisphenol dimethacrylate; bis-GMA, bisphenol-A-glycidyl methacrylate; Bis-MEPP, 2,2-Bis(4-methacryloxypolyethoxyphenyl) propane; DMA, dimethacrylate; GDMA, glycerol dimethacrylate; K₂O, potassium oxide; Na₂O, sodium oxide; SiO₂, silica; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate

Fig. 1. Schematic setup of the experiment.

From each RMC block, 32 specimens (in total 160) were cut into slices (12 × 14 × 1.0 mm) by using a low-speed (150 rpm) diamond saw (Microcut 201, Metkon Instruments Ltd, Bursa, Turkey) under water cooling. Subsequently, the specimens were fixed with a cyanoacrylate luting cement (Zapit, Dental Ventures of America, Corona, CA, USA) onto auto-polymerized acrylic resin cylinders (Ortho-jet, Lang Dental, Wheeling, IL, USA).

The bonding surfaces of the specimens were ground with 600-grit silicon carbide paper under constant water flow with a 100 rpm/min polishing device (Gripo 2V, Metkon Instruments Ltd, Bursa, Turkey) for 15 seconds. All specimens were ultrasonically-cleaned (Biosonic Ultrasonic Cleaner UC1-110, Coltene Whaledent, Cuyahoga Falls, OH, USA) in distilled water for 24 hours, and dried with oil-free air to remove surface-waste. The bonding surfaces of the specimens were then subjected to one of the following MSC strategies:

• · Group 1 (Control - C): Neither silane nor MSC was applied.

• · Group 2 (S): Silane was applied for 60 seconds by using a disposable micro-tipped applicator. The surface was then air-dried.

• · Group 3 (APA): The surfaces were roughened with 50 µm aluminum-oxide (Al₂O₃) particles (Korox 50, Bego, Bremen, Germany) from a distance of 10 mm for 20 seconds under 2 bar propulsion pressure (Rotaks-Dent, Istanbul, Turkey) by positioning nozzle perpendicular to the surface. The specimens were then cleaned in distilled water and air-dried.

• · Group 4 (APA + S): After abrading the surface with the same procedure performed in Group 3, silane was applied as described in Group 2.

• · Group 5 (LI^2W): The surfaces were irradiated with Er,Cr:YSGG laser (Waterlase MD, Biolase, Irvine, CA, USA) on hard tissue mode with an MG6 sapphire tip by using a non-contact mode at an energy level of 2W, a repetition rate of 10 Hz, and 140 ms pulse duration with 55% water and 65% air for 20 sec.

• · Group 6 (LI^2W + S): After irradiating the surface with the same procedure performed in Group 5, silane was applied as described in Group 2.

• · Group 7 (LI^3W): The surfaces were irradiated by using Er, Cr: YSGG laser with the same parameters that were used in Group 5. However, the output power was increased to 3.0 W.

• · Group 8 (LI^3W + S): After irradiating the surface with the same procedure performed in Group 7, silane was applied as described in Group 2.

Following the above-mentioned MSC strategies, two polyvinyl microtubules with an inner diameter of 0.75 mm and thickness of 1.0 mm were fixed on the bonding surfaces with the aid of flowable composite resin (Clearfil Majesty Flow, Kuraray Noritake Dental Inc., Okayama, Japan). A dual-cured self-adhesive resin cement (Bifix SE, VOCO, Cuxhaven, Germany) was syringed into these polyvinyl microtubules and subsequently light-cured according to the manufacturers' instructions for 20 seconds by using a quartz-tungsten halogen light-curing unit (Hilux UltraPlus, Benlioglu Dental, Ankara, Turkey) in standard mode with an intensity setting of 800 mW/cm². Accordingly, two resin microtubules were generated on each specimen (n=8 per subgroup). Polymerized specimens were kept in distilled water at 37℃ for 24 hours and then the polyvinyl microtubules were carefully removed with a sharp scalpel.

The test was conducted by using a universal testing machine (EZtest-500 N Shimadzu; Kyoto, Japan). A metal wire with a diameter of 0.2 mm was wrapped around the resin microtubules and a tensile force of 0.5 mm/min was applied until breaking/rupture occurred. Failure load was recorded in Newtons (N) and µSBS values were calculated in megapascals (MPa) by using the following equation:

(Load at failure (N)) / (Surface area (mm²))

The bonding interfaces were scrutinized under a stereomicroscope (Leica S8 APO; Leica Microsystems GmbH, Germany) at ×40 magnification in order to determine the failure patterns of the specimens and categorized as 3 types: (1) adhesive failure at the RMC surface with no residues of resin cement; (2) cohesive failure in the inner portion of resin cement or RMC; (3) mixed failure consisting of both cohesive and adhesive failures (Fig. 2).

Fig. 2. Different failure patterns. (A) adhesive, (B) mixed failure, (C) cohesive failure.

The data were statistically analyzed using a software program (IBM SPSS Statistics v23, IBM Corp., Chicago, IL, USA). Conformity to normal distribution was done by using the Shapiro-Wilk test. Three-way analysis of variance (3-Way ANOVA) was used to test the influence of 3 variables (RMC material type, MSC strategy, and silane application) on the µSBS values. Tukey HSD test was used for multiple comparisons. The significance level was taken as P < .05.

RESULTS

In accordance with the results of 3-way ANOVA, µSBS values were significantly affected by all variables and their interaction terms (P < .05), except RMC material type × MSC strategy × silane application (P = .172) (Table 2). The mean µSBS values and standard deviations with Tukey post hoc comparisons are presented in Table 3.

Table 2. Three-way ANOVA results of µSBS values.

Source	Type III sum of squares	Df	Mean square	F	P
Resin-matrix ceramic material type (A)	434.068	4	108.517	7.776	< .001
Surface conditioning strategy (B)	862.929	3	287.643	20.612	< .001
Silane application (C)	325.043	1	325.043	23.292	< .001
A * B	445.442	12	37.120	2.660	.002
A * C	632.906	4	158.227	11.338	< .001
B * C	195.645	3	65.215	4.673	.003
A * B * C	231.999	12	19.333	1.385	.172

Df, degree of freedom; F, variance analysis test statistics. P < .05 indicates a significant difference.

Table 3. Mean ± standard deviation of µSBS values (MPa) of CAD-CAM RMC materials treated with different surface conditioning strategies to resin luting cement.

MSC strategy	Silane application	RMC Type					Total
		GC	LU	VE	VG	BC
Control	No	13.93 ± 4.37	13.31 ± 3.97	11.40 ± 3.09	14.47 ± 4.25	14.32 ± 2.06	13.48 ± 3.64D
	Yes	13.54 ± 4.37	13.35 ± 3.01	11.92 ± 2.74	14.32 ± 3.73	13.72 ± 3.84	13.37 ± 3.49D
	Total	13.74 ± 4.23CD	13.33 ± 3.40CD	11.66 ± 2.83D	14.40 ± 3.87CD	14.02 ± 3.00CD	13.43 ± 3.55a
APA	No	17.98 ± 4.83	17.89 ± 4.05	14.10 ± 2.46	19.86 ± 3.11	18.88 ± 1.35	17.74 ± 3.77AB
	Yes	13.69 ± 3.45	16.11 ± 3.48	20.44 ± 3.65	19.27 ± 3.39	12.91 ± 3.18	16.48 ± 4.43BC
	Total	15.84 ± 4.62BCD	17.00 ± 3.76ABC	17.27 ± 4.44ABC	19.57 ± 3.16AB	15.89 ± 3.88BCD	17.11 ± 4.14b
LI2W	No	18.96 ± 4.22	21.82 ± 4.30	15.76 ± 5.23	17.43 ± 3.74	18.61 ± 4.69	18.51 ± 4.69AB
	Yes	12.83 ± 3.52	19.81 ± 3.66	18.08 ± 4.29	18.03 ± 2.76	12.14 ± 2.60	16.18 ± 4.51BC
	Total	15.89 ± 4.91BCD	20.82 ± 3.99A	16.92 ± 4.77ABC	17.73 ± 3.19ABC	15.37 ± 4.96BCD	17.35 ± 4.72b
LI3W	No	19.10 ± 4.12	18.96 ± 3.24	16.63 ± 5.43	23.86 ± 6.05	18.29 ± 2.09	19.37 ± 4.85A
	Yes	12.73 ± 2.25	14.88 ± 2.37	19.03 ± 3.42	17.79 ± 4.09	10.60 ± 3.94	15.01 ± 4.45CD
	Total	15.92 ± 4.59BCD	16.92 ± 3.46ABC	17.83 ± 4.56ABC	20.83 ± 5.89A	14.44 ± 5.00CD	17.19 ± 5.12b
Total	No	17.49 ± 4.69A	17.99 ± 4.85A	14.47 ± 4.53BCD	18.90 ± 5.47A	17.52 ± 3.30A	17.28 ± 4.80
	Yes	13.20 ± 3.34CD	16.04 ± 3.87ABC	17.37 ± 4.74AB	17.36 ± 3.84AB	12.34 ± 3.47D	15.26 ± 4.38
	Total	15.34 ± 4.58ab	17.02 ± 4.46bc	15.92 ± 4.82ab	18.13 ± 4.75c	14.93 ± 4.25a	16.27 ± 4.70b

a–c: No difference between the same superscript lowercase letter, A–D: No difference between the same superscript uppercase letter

SC, surface conditioning; RMC, resin-matrix ceramic; GC, GC Cerasmart; LU, Lava Ultimate; VE, Vita Enamic; VG, Voco Grandio; BC, Brilliant Crios; APA, air-borne particle abrasion; LI^2W, laser irradiation with 2.0 W power; LI^3W, laser irradiation with 3.0 W power

The RMC material type had a statistically significant effect on µSBS values (P < .001). The highest and lowest mean µSBS values were obtained at VG (18.13 ± 4.75) and BC (14.93 ± 4.25). Except for the differences between the mean µSBS values of GC-VG, LU-BC, VE-VG, and VG-BC, the comparisons among restorative materials were detected as statistically insignificant (P > .05). The MSC strategy significantly influenced µSBS values (P < .001). The highest and lowest mean µSBS values were observed at LI^2W (17.35 ± 4.72) and C (13.43 ± 3.55). The surface-conditioned groups showed significantly higher µSBS values than the C group (P < .05). No statistically significant differences among the SC groups were detected (P > .05). The silanization also significantly affected µSBS values (P < .001). The highest and lowest mean µSBS values were obtained at silane-free (17.28 ± 4.80) and silane-applied (15.26 ± 4.38) groups.

Considering RMC material type and MSC strategy interaction, the highest and lowest mean µSBS values were exhibited by VG conditioned with LI^3W (20.83 ± 5.89) and by unconditioned VE (11.66 ± 2.83). While comparing control and conditioned groups of RMCs, all MSC strategies in VE, APA and LI^3W in VG, and only LI^2W in LU were found to be significantly higher than their control groups (P < .05). However, no significant differences were observed in all other groups (P > .05). The differences between the SC groups in all RMCs did not show significance (P > .05). The difference among RMCs in the C and APA groups was also statistically insignificant (P > .05). The differences only between GC-LU and LU-BC materials in the LI^2W group and the differences only between GC-VG and VG-BC materials in the LI^3W group were significant (P < .05).

Considering RMC material type and silanization interaction, the highest and lowest mean µSBS values were found at silane-free VG (18.90 ± 5.47) and silane-applied BC (12.34 ± 3.47). The µSBS values of silane-applied materials except VE were lower. However, the differences between silane-applied and silane-free groups showed statistical significance only for GC and BC (P < .05). Among the silane-free restorative materials, the VE group exhibited a significantly lower µSBS value compared to other RMCs (P < .05), and no significant difference was found among the other 4 RMCs (P > .05). While silane-applied restorative materials were compared, differences between GC-VE, GC-VG, LU-BC, VE-BC, and VG-BC groups were statistically significant (P < .05).

Considering MSC strategy and silanization interaction, the highest and lowest mean µSBS values were detected at silane-free LI^3W (19.37 ± 4.85) and silane-applied C (13.37 ± 3.49) groups. The µSBS values of the surface-conditioned specimens were higher in both the silane-applied and silane-free groups compared to the C group. In silane-free groups; while a significant difference was observed among each MSC strategy and the control group (P < .05), there was no significant difference between the MSC strategies themselves (P > .05). In silane-applied groups, while APA and LI^2W differed significantly from the C group (P < .05), there was no significant difference between LI^3W and the C group (P > .05). Again, there was no significant difference between MSC strategies.

Considering RMC material type, MSC strategy, and silanization interaction, silane-free VG conditioned with LI^3W (23.86 ± 6.05) and silane-applied BC conditioned with LI^3W (10.60 ± 3.94) indicated the highest and lowest mean µSBS values, respectively.

The failure patterns of RMC specimens after SCM and silane application are presented in Table 4.

Table 4. Failure patterns of RMC bloks subjected to different MSC stratiegies and silane application.

MSC strategy	Silane application	RMC type	Failure patterns
			Adhesive	Cohesive	Mixed
Control	No	GC	7	-	1
		LU	8	-	-
		VE	8	-	-
		VG	6	-	2
		BG	7	-	1
	Yes	GC	8	-	-
		LU	8	-	-
		VE	8	-	-
		VG	7	-	1
		BG	7	-	1
APA	No	GC	6	-	2
		LU	6	-	2
		VE	7	-	1
		VG	5	1	2
		BG	6	-	2
	Yes	GC	7	-	1
		LU	7	-	1
		VE	4	2	2
		VG	6	1	1
		BG	8	-	-
LI 2W	No	GC	5	1	2
		LU	4	2	2
		VE	7	-	1
		VG	6	-	2
		BG	6	1	1
	Yes	GC	8	-	-
		LU	5	1	2
		VE	6	1	1
		VG	6	1	1
		BG	8	-	-
LI 3W	No	GC	5	1	2
		LU	5	-	3
		VE	6	-	2
		VG	4	2	2
		BG	6	-	2
	Yes	GC	8	-	-
		LU	7	-	1
		VE	5	1	2
		VG	6	-	2
		BG	8	-	-

DISCUSSION

Since reliable bonding is essential for the long-term clinical success of indirect restorations,⁴,⁵,¹⁷ this comparative in-vitro study evaluated the influences of different MSC strategies and the coupling agent implementation on the adhesive features of dual-cured resin cement to recently introduced different RMCs. The results of 3-way ANOVA proved that RMC material type, MSC strategy, and silane implementation had significant effects on µSBS values. Therefore, the null hypothesis was rejected.

Various surface conditioning strategies are recommended to create micro-porosities (transformed zone) on the bonding surface of the ceramics for better infiltration of the luting cement.⁹,¹⁷ Supportively, Şişmanoğlu et al.³⁹ and Çelik et al.⁴⁰ reported favorable outcomes for the RMCs conditioned with different strategies. This is in accordance with the results of the current study as all surface-conditioned silane-free specimens demonstrated significantly higher µSBS values. Conversely, Barutcigil et al.¹ reported that the SC-strategies (tribo-chemical silica coating, APA, 10% hydrofluoric acid etching, universal adhesive, and LI^2W) had insignificantly increased the bond strength, except for universal adhesive group.

For GC and VE, the highest increase was observed in LI^3W followed by LI^2W, and APA. This finding may be related to the transformed zone formed on the bonding surface after conditioning. For these RMCs, the depth of this zone might be much greater in LI. Moreover, the increased power output creates a deeper transformed zone.² Even so, it cannot be deduced that the relationship between power-output and bond strength is directly proportional as the irradiated material type is another dependent-variable.²,⁴¹ This also provides consistency with the current study: (i) while comparing LI^2W and LI^3W RMCs, a decline in µSBS values was detected only in LU and BC. Gökçe et al.⁴² associated inferior bond strength values with high power settings and mentioned from a heat-damaged layer. The bonding behavior of LU and BC can be attributed to this layer which might be poorly attached to the infra-layers; (ii) while comparing APA and LI^2W conditioned VG groups, a decline in µSBS values was also detected. However, the µSBS values increased in VG conditioned with LI^3W. From this perspective, it can be understood that LI^2W is not strong enough to provide sufficient contribution for surface activation in VG.

LU in LI^2W laser and VG in LI^3W laser exhibited significantly higher µSBS values than other RMCs. This bonding behavior of LU can be attributed to a number of factors: First, it contains ZrO₂ in high proportion.³⁷ In accordance with Vickers Hardness Scale, Al₂O₃ presents superior hardness (2000) than that of ZrO₂ (1200). Accordingly, during abrasion, harder Al₂O₃ micro-abrasive particles become more effective on the intaglio surface and successfully form an active surface (deeper transformed zone) for strong adhesion. Second, LU has manifold chemicals (Bis GMA, UDMA, Bis-EMA, and TEGDMA) in its organic part.³¹,³⁷ Different chemicals offer different hardness and roughness at different levels. This may create a uniformly activated surface for adhesion. Third, LU has a lower micro-hardness than VE and VG.⁴³,⁴⁴ Thereby, it can be roughened easily with LI^2W and this conditioning strategy can be strongly recommended to the clinicians. However, since LI^3W detrimentally influences the inorganic structure, lower µSBS values than those of LI^2W were found. Higher µSBS values of VG may be correlated with its micro-chemical structure as it contains high amount of nanohybrid fillers (86 wt%).⁴³

In silane-applied groups, all MSC strategies caused less increase (APA > LI^2W > LI^3W) in µSBS values. In the LI process, inorganic content is removed from the surface of the ceramic.²,¹¹ This situation may weaken the connection of silane with ceramic. The highest increase was seen in APA. In this process, Al₂O₃ particles with a diameter of 50 µm were sprayed towards the surface. Only chemicals with less hardness than the hardness of the abrasive agent were eliminated from the ceramic surface. As with the LI, the whole inorganic part was not affected.

The specimens in Group 4 exhibited lower µSBS values. This can be attributed to the nature of APA. During APA, some of the Al₂O₃ particles sprayed remain embedded in the ceramic surface. Also a layer coated with Al₂O₃ may form on the surface. Increasing abrasion pressure also increases the amount of Al₂O₃. After the silane application, unstable =AL-O-Si ≡ bonds are formed, which in turn negatively affects the µSBS values.²²

Silane application only increased the values of the VE and can be linked to the robust microstructural geometry of polymer-infiltrated-ceramic network:³³ First, this inorganic part (86 wt%) consists of feldspar glass-ceramic reinforced with Al₂O₃.³⁷ It microstructurally differs from other RMCs including dispersed-fillers.²⁸ Second, the silane content of this RMC system may account for increasing the bond strength.¹⁷ Third, 58 – 63% of the inorganic structure is SiO₂. The amorphous ceramic structure allows selective dissolution when exposed to MSC strategies.¹⁷ It is thought that this may positively affect the silane-substrate connection.

Silane application mostly decreased the values of GC and BC groups because the ratio of the inorganic filler in these materials (GC: 71 wt% and BC: 70.7 wt%) is relatively lower than others (LU: 80 wt%, VE: 86 wt%, VU: 86 wt%).³³ In other words, the polymeric structure is more. Also, both groups showed a similar pattern of decline. This situation was associated with the presence of similar inorganic content (silica and barium glass). The silane application generally lowered the µSBS. The organo-functional group of the silane normally links with the organic resin monomer as mentioned above.²²,²³ However, RMCs have both organic and inorganic parts. Thus, the organo-functional group of the silane has to be shared between the resin cement and RMC. Since it was previously occupied by RMC, the number of C-C connections between the resin cement is reduced. A much greater decline in GC and BC is associated with this. The polymeric phases present approximately similar proportions (30%) in both materials.

The results of this study confirm that the RMC material type had a significant influence on bond strength. However, this does not provide consistency with the results of a study by Çelik et al.⁴⁰ reporting that RMC material type did not significantly cause alterations in bond strength values. They emphasized that only MSC strategy can become influential on bond strength.

For the assessment of bond strength, the micro-shear (µSBS), the micro-tensile (µTBS),¹ and 4-point flexural²⁹ tests are available. Although µTBS test permits a more homogeneous and uniform distribution of stress during loading; µSBS test is more common as: (i) it is easy and rapid; (ii) specimen preparation stage involving cutting sticks that have small bonded areas in µTBS test is very difficult.¹ Therefore, the µSBS test was preferred in this study.

In a number of studies,⁴⁵,⁴⁶ cohesive and mixed failures at higher bond strength values and adhesive failures at lower bond strength values were reported. This provides consistency with the results of the current study.

The current study has several limitations. The longpulsed laser was used. However, ultra-short pulsed femtosecond lasers limit temperature distribution, reduce energy loss on the surface, and thereby minimize thermal destruction. Surface topographies of the conditioned-specimens were not examined. Only one size of Al₂O₃ particles for APA protocol was used. For investigating surface topographies after conditioning, a profilometer and scanning electron microscopy were not used. Further studies need to be performed with thermal aging for better understanding the hydrolytic stability.

CONCLUSION

Within the limitations of the present study, although MSC strategy is dependent on RMC material type and each RMC can require a dedicated way of conditioning, all micromechanical surface conditioning strategies enhanced the micro-shear bond strength values of all resin-matrix ceramics to dual-cured resin cement.

Silane application adversely affected the bond strength of dispersed filler resin-matrix ceramics. However, silane application can improve the bonding efficiency of resin-matrix ceramic with polymer infiltrated ceramic network.