1. Introduction

Dentists and other dental practitioners have been challenged with new difficulties due to recent advancements in digital dentistry. The prevalence of computer-aided design and manufacturing (CAD/CAM) technology has increased in general dentistry operations [1] due to its many advantages, such as its efficacy, user-friendliness, and therapeutic excellence. The use of this technology has unveiled several applications in the dental office and laboratory, including the production of indirect prosthodontic restorations such as inlays, veneers, crowns, fixed partial dentures, and implant abutments [2].

The advancement of CAD/CAM technology is marked by the need for durable and esthetically effective prosthodontic restorations, which may be achieved through precise and uncomplicated technical procedures [3]. Ceramic materials are widely used in the fabrication of permanent dental prostheses in contemporary dentistry. CAD/CAM systems have been developed due to their enhanced process stability, cost-effectiveness, and significant minimize of working time [4].

The principal classifications of ceramic materials include those dependent on their composition (e.g., silica-based ceramics, oxide ceramics, and resin-matrix ceramics), as well as those produced by layering, pressing, and CAD/CAM machining [5].

Under optimal conditions, clinicians may use a material that has both acceptable esthetic qualities and strong mechanical properties, capable of resisting high occlusal pressures and impervious to fracture propagation [6]. It is inherent that the microstructure of ceramics significantly influences the mechanical characteristics of the material.

Prior to bonding, it is necessary to acid-etch ceramic restorations that include a glass phase in order to achieve the desired surface structure and optimize the adhesion of resin cement [7]. It is essential to enhance resin cement adhesion. To get the maximum bond strength when utilizing adhesive-resin cement and glass ceramics, dentists may use a process that involves etching the tooth structure with 37% phosphoric acid, treating the porcelain’s surface with 5–9.5% hydrofluoric acid, and using a silane coupling agent. The outcomes of the acid treatment exhibit variability since they are contingent upon the specific ceramic material being treated, the concentration of the conditioning agent, and the length of the etching process [8]. The resin exhibits a fluidic behavior and forms a strong bond inside the intricate recessed areas, hence increasing its adhesion to the etched surfaces.

After the etching process, the restoration is submerged in water and subjected to ultrasonic cleaning for a duration of five minutes. It is then dried with air and a layer of silane is applied to the intaglio surface [7]. In order to enhance the resistance to fractures of glass ceramics, it is recommended to inquire final adhesive cementation with composite resin due to their fragility and poor flexural strength [9]. Studies have shown that adhesive cementation improves fracture resistance and prolongs the longevity of the restoration [10]. When dealing with glass ceramics, it is recommended to use composite resin materials that can be polymerized using either light, dual, or chemical methods [11].

The adhesive cement’s bonding strength significantly affects the retention and durability of indirect ceramic restorations. The most often used methods for studying cement types and adhesives are shear and micro tensile bond strength tests [12].

The testing technique involves analyzing the adhesion between the tooth or ceramic and the cement material to assess its capacity to withstand the stress induced by occlusal pressures. Shear bond strength values are not considered material properties since they are influenced by the substrate material and surface form, and their values vary based on the test design [13]. Prior research investigations have examined surface conditioning, the cementation process, and the expected failure of all-ceramic prosthodontic restorations at various interfaces [14]. Despite the current research on adhesive cementation methods used by dentists, recent investigations in the scientific literature suggest that the cautious application of adhesives is necessary for all-ceramic restorations. In order to decrease the incidence of clinical failure, the dentist should prioritize the dental preparation design and the expected thickness of the restoration. Previous to applying the adhesive cement, it is necessary to clean and condition the bond surfaces in order to provide strong resistance to masticatory forces and maximize durability [14].

The development and distribution of user-friendly self-etching adhesive cements by manufacturers encourage the issue of whether dentists should choose for these products instead of the traditional adhesive preparation that involves etching and priming [15].

This study aimed to investigate the impact of the application method and temperature of preheated hydrofluoric acid pretreatment on the surface morphology of leucite-reinforced glass-ceramic materials (IPS Empress CAD, Ivoclar Vivadent). Understanding this is significant for mastering the adhesive cementation process.

2. The interpretation of research data

2.1 The preparation of specimens and surface conditioning

Fifty specimens of leucite-reinforced glass-ceramic (IPS Empress, Ivoclar Vivadent, Schaan, Liechtenstein) were polished using silicon carbide papers with grit levels of #1200, #1500, and #2500 in a grinding machine (ECOMET Grinder/Polisher, Buehler). Afterward, the samples were immersed in distilled water in an ultrasonic bath for 5 minutes. An incubator (Ivoclar Vivadent Cultura) was used to warm 9.5% HF gel (Yellow Porcelain Etch, Cerkamed) for 20 minutes at 50°C.

The ceramic blocks were divided into five groups (n = 10) based on random allocation, with each group receiving a different surface treatment.

Group 1: NT (control group)—no surface conditioning;
Group 2: DH—a dynamic application of preheated HF-gel on the ceramic surface using continuous movements of a micro brush for 60 seconds;
Group 3: SH—a static application of preheated HF-gel on the ceramic surface without brushing for 60 seconds;
Group 4: DNH—a dynamic application of nonheated HF-gel (at room temperature) on the surface using active movements with a micro brush for 60 seconds;
Group 5: SNH—a static application of nonheated HF-gel (at room temperature) on the surface without brushing for 60 seconds.

Following the HF treatment, all specimens were subjected to a 20-second cleansing using an air-water spray, followed by a 10-second session of air-drying.

2.2 Scanning electron microscopy (SEM) for surface morphology analysis

The specimens’ topographic patterns were examined using a scanning electron microscope (SEM Quanta FEG 250, FEI, Hillsboro, OR, USA) and a secondary electron detector (SE). This analysis aimed to assess the impact of treatments on the surface morphology of the etched surfaces of specimens in each experimental group. The scanning electron microscope (SEM) was used in a low vacuum mode to avoid the occurrence of charge.

SEM micrographs were captured at magnifications of 1000× and 5000× to visually analyze the morphological changes on the ceramic surfaces. The objective was to detect any impacts on the ceramic area resulting from the treatments applied to the specimens. Each specimen was observed at the center of the ceramic-conditioned surface.

Figure 1(a–j) provide SEM images illustrating the notable ceramic surface morphologies of each experimental group.

Figure 1.
(a–j) SEM evaluation images with significant ceramic surface morphologies of all experimental groups. (a) 1000× group 1 NT, (b) 5000× group 1 NT, (c) 1000× group 2 DH, (d) 5000× group 2 DH, (e) 1000× group 3 SH, (f) 5000× group 3 SH, (g) 1000× group 4 DNH, (h) 5000× group 4 DNH, (i) 1000× group 5 SNH, (j) 5000× group 5 SNH.

Observable patterns might vary depending on the temperature and application method of hydrofluoric acid. The most suitable design for each specimen was selected for analysis. Without hydrofluoric acid (HF) treatment, the sample exhibited a consistent surface shape without noticeable ceramic microstructural characteristics.

Unlike the porous surface reported in all the analyzed etched groups, the NT group exhibited a much less retentive pattern. The study revealed that each of the HF treatments lead to significant porosities on the ceramic material’s surface. After receiving surface treatments, the surface morphology of each CAD/CAM block underwent substantial alterations. The alterations in surface roughness were readily discernible on the SEM micrographs.

The microscopic examination demonstrates that the ceramic conditioning procedure has an impact on the surface micro retentions of ceramics.

2.3 Assessment of surface roughness

The surface roughness was evaluated using a contact profilometer (Surftest SJ-201, Mitutoyo, Kanagawa, Japan). Prior to completing measurements, the profilometer was calibrated using a reference sample given by the manufacturer.

Two perpendicular measurements were obtained along certain orientations after surface conditioning, starting from the center point of each sample (with a cutoff length of 0.25 mm).

The analysis documented the Ra values, which indicate the average roughness of the treated surfaces. These values are calculated by measuring the absolute roughness of the peaks and valleys from a reference plane. The measurements were denoted in micrometers (μm) and the average of the measurements was recorded.

Ten roughness measures were conducted for each group, and the results are shown in Table 1.

Roughness Ra (μm)
Group specimen	NT	DH	SH	DNH	SNH
1	0.27	0.56	0.87	1.05	1.02
2	0.25	0.51	0.83	0.85	1.18
3	0.26	0.52	0.78	0.86	0.99
4	0.24	0.62	0.75	0.94	0.87
5	0.27	0.56	0.87	0.91	0.95
6	0.28	0.72	0.77	1.02	0.88
7	0.30	0.64	0.91	0.98	1.16
8	0.29	0.61	0.92	0.96	1.07
9	0.27	0.74	0.94	0.84	1.22
10	0.30	0.47	0.82	0.93	1.23
Mean	0.273	0.595	0.846	0.934	1057
SD	0.020	0.088	0.067	0.071	0.128

Table 1.

The roughness measurements results. Mean—average value, SD—standard deviation.

2.4 Shear bond tests are conducted to evaluate the strength of adhesive bonds

All specimens were treated with a silane coupling agent (Clearfil Ceramic Primer Plus; Kuraray Noritake Dental Inc., Tokyo, Japan) following instructions provided by the manufacturer.

Precisely designed, translucent cylindrical items with parallel ends were methodically shaped out from a polyvinyl tube that had an inner diameter of 3 mm and a height of 5 mm. For each specimen, a single cylinder was used to adhere the adhesive cement to the prepared surfaces. Once placed on the surface of the treated samples, each cylindrical support made of polyvinyl was carefully filled with cement (Panavia V5, Kuraray Noritake Dental Inc., Tokyo, Japan). Subsequently, adhesive cement cylinders were distributed onto the treated surfaces and subjected to light-curing for a duration of 20 seconds utilizing a DTE LUX-E Plus Curing Light (Woodpecker, 1000 mW/cm²) LED curing device. The LED curing device was activated in contact with the tube due to its 1 mm thickness. The polyvinyl tubes were removed after the adhesive got fully cured. Before conducting the bond strength tests, all specimens were immersed in distilled water for a period of 7 days.

Requirements for conducting shear tests:

The Zwick/Roell Z005 universal machine was used to conduct the testing, possessing the following technical specifications:

the Zwick/Roell Z005 universal machine is equipped with a force cell of 5 (kN) in uniaxial stress, in the accuracy class 0.5 on the force measurement range 1–130 (%) according to ISO 7500-1;
the Zwick/Roell Z005 machine owns the TestXpert data processing software and an incremental extensometer with a maximum error of ±1 (μm) for the differential measurement of the displacement between two measuring points in the range of 20–200 (μm) (class of precision 0.5).

The machine is prepared with equipment for conducting tensile, compression, and three-point bending tests.

The tests were carried out in controlled movement mode, at an ambient temperature of 23°C, as follows:

pre-load at 2 (N), with a traverse speed of 2 (mm/min);
shear test with a crossbar travel speed of 0.5 (mm/min);
real-time recording of the force F and the displacement of the crossbar.

The specimens were fixed using a precision vice, and the tester blade was positioned perpendicular to the adhesive cement cylinder at a 90° angle. The specimens underwent shear loading along the interface until complete failure, which was indicated by the highest recorded force F_max. The force was measured using the TestXpert II software [16].

The shear strength τ_max, expressed in MPa, was determined from the conventional formula (1):

τmax=Fmax/A.E1

Before performing the tests, the diameter of the specimens was measured using a caliper with an accuracy of 0.01 (mm), in three different directions.

F_max represents the maximum force recorded at failure, expressed in newton (N), and A the shear area, i.e., the area of a circle having diameter d, expressed in (mm²) from the formula (2):

A=π·d2/4E2

Figure 2(a–e) show the force-displacement curves for each treatment type, with three curves per group, selected randomly.

Figure 2.
(a–e). Force-displacement curves for the specimens tested in shear bond tests. (a) Group NT. (b) Group DH. (c) Group SH. (d) Group DNH. (e) Group SNH.

2.5 Statistical analysis

The data were subjected to statistical analysis using SPSS Statistics 29.0 software (IBM, New York, USA, 2022) at a significance level of α = 0.05.

The SBS data underwent preliminary testing for normality and homogeneity using the Shapiro-Wilk and Levene tests. The initial null hypothesis, asserting that the variable SBS follows a normal distribution, was not found to be statistically significant. The second null hypothesis posits that the variances of the five groups are the same, hence lacking statistical significance in their differences.

In addition, a one-way analysis of variance (ANOVA) and the post hoc Tukey HSD test were used to see whether there were any significant differences in terms of SBS across the five groups. The null hypothesis of equal mean SBS levels across all five groups was rejected.

A two-way analysis of variance (ANOVA) using a general linear model was conducted to investigate the differential impact of temperature (heated or nonheated HF-gel) and application regime (static or dynamic application) on the dependent variable SBS. The groups DH, SH, DNH, and SNH were included in the analysis [16].

2.6 Digital microscopy for fracture surfaces

The failure mechanisms of the ceramic and adhesive cement interfaces were analyzed at a 50× magnification using a digital microscope installed at the CMDTCA (Research Centre in Dental Medicine Using Conventional and Alternative Technologies) at the university research center. The failure types were categorized as shown in Figure 3(a and b) into three distinct groups: adhesive (A), which refers to failure occurring at the bond surfaces between the ceramic and the resin cement substrate; cohesive (C), which refers to failure of either the ceramic or the adhesive cement substrate; and mixed (M), which combines both adhesive and cohesive failures [16].

Figure 3.
(a and b) Images showing the typical fractographic characteristics after the impact of the shear test: (a) ceramic interfaces after SBS forces; (b) resin cement interfaces after SBS tests. Adhesive (failure at the bond interfaces where the ceramic and the resin cement substrate were connected), cohesive (failure of at least one of the substrates—the ceramic or the composite resin), and mixed failures can be observed.

The use of static surface treatment procedures enhanced the adhesive strength between the ceramic material and resin cement. The SBS was significantly affected by two contributing factors: the temperature of the hydrofluoric acid (whether heated or not) and the application method (static or dynamic). The temperature at which HF is delivered has a significant impact compared to the technique of application.

The final findings are not statistically substantially affected by the interaction between these two factors in terms of SBS shear strength.

There is no statistically significant variation in the mean SBS values across the NT, DH, and SH preparations. The surface treatments applied to the DNH and SNH groups resulted in average SBS values that were 56.32% and 74.88% higher, respectively, compared to the NT control group. The SBS values for DNH and SNH conditioning increased by 65% and 84.59%, respectively, compared to the DH group, indicating the impact of these two parameters. The preparations of the SH and DNH groups do not exhibit statistically significant results in terms of SBS shear strength due to the conflicting effects of the two influencing factors. However, the SNH group shows a notable 35.73% enhancement in SBS compared to the SH group. The only difference in the shear strengths of the DNH and SNH groups is in the manner in which HF was applied, and this discrepancy is deemed negligible [16].

Resin cement is essential for ensuring the long-term effectiveness of ceramic-based dental restorations. A fracture in the restorative material or inadequate adherence at the cement contact might cause the restoration to fail [17]. The intaglio surfaces of all ceramic restorations have been specifically roughened by etching. This roughening process is said to enhance the surface area, allowing for better adhesion between the ceramic surface and resin-based components [18].

Acid selectively dissolves the crystalline or amorphous phases of the ceramic, resulting in unsaturated oxygen connections that may interact with phosphate monomers with dual capacities [19]. Hydrofluoric acid creates porous and uneven surfaces, as well as micro retention sites, in ceramic materials by precisely eliminating the glassy or crystalline matrix. The microporous ceramic surfaces undergo expansion, hence increasing their surface area and facilitating resin infiltration [20].

Chemical conditioning, such as acid etching, improves the roughness and energy of the ceramic surface. This process increases micromechanical retention and the ability of the primer to adhere, leading to higher bond strength [21]. The primary factors influencing bond strength values are the surface energy of the material and the interfacial tension between the material itself and the adhesive [22]. Previous literature studies show how while there is no direct relationship between surface roughness and surface energy, an increase in surface energy leads to higher bond strength values [23]. The use of HF treatment and silanization resulted in the highest bond strength values for feldspathic ceramic [24]. The most effective surface treatment for leucite-based ceramics is etching by using a combination of hydrofluoric acid and silane [25].

With the introduction of glass-based ceramics and a greater understanding of the benefits of adhesive cementation in dentistry, hydrofluoric acid has become a common surface conditioner for restorative materials [26]. Universal adhesives facilitate clinical application processes for practitioners while strengthening them [27].

Though a specific silane treatment is necessary, particularly for feldspathic ceramics, previous studies have shown that the primary factor for achieving a strong bond is the mechanical interlocking that occurs due to the roughness of the ceramic surface [28]. Roughness is a major surface property of restorative materials that affect the ability of abrasive and mechanically organized substances to interact with their external environment. Material adhesion is influenced by other parameters apart from surface roughness, such as porosity, residual microstructural tension, composition, and internal defects [29].

The bond strength between two materials may be assessed in vitro using tensile, microshear, and shear techniques. The basic concept of these tests is to subject the specimen to stresses that exert stress on the adhesive contact until the failure of the specimen becomes apparent. Each of these tests has both benefits and drawbacks, although none is universally acknowledged as the optimal approach [30]. Furthermore, the results may be influenced by several factors such as the shape of the specimen, the brittleness of the substrate, and the speed at which the cross-head moves [31]. The critical load measured during the shear bond tests did not accurately represent the bond strengths attained by the various surface treatments at the adhesive interface.

Shorter processing times, fewer costs, and better patient results have resulted from the development of CAD/CAM technology. Moreover, digital dental technology enables the use of same-day restorations for patients, hence eliminating the need for many sessions and interim restorations. Consequently, novel ceramic materials have been developed explicitly for use with CAD/CAM systems. The purpose of developing these biomaterials was to meet the specific demands of dental restorations, such as the need for strength, durability, and biocompatibility. In general, the use of CAD/CAM technology in dentistry has substantially improved the quality of treatment that dentists can deliver to their patients, while also streamlining processes in laboratories and clinics.

The limitations of the SBS test in imitating clinical loading forces and aging within the oral environment should be noted as they may have an influence on this in vitro investigation. It is certainly suggested to conduct additional research to investigate the effects of ceramic surface conditioning on bond strengths in a clinical setting, focusing on the most optimal methods of ceramic surface conditioning.

The success of the prosthodontic treatment relies significantly on the dentist’s capacity to choose the appropriate restoration material, manufacturing process, and cementation or bonding processes, taking into account both the circumstances within the oral cavity and the esthetic goal. The clinical efficacy of dental restorations’ cementation procedures will be influenced by the surface conditioning and its continuous enhancement [16].

3. Conclusions

The limitations of this study permit the following conclusion to be drawn:

Both the temperature of the HF and the technique of its application significantly affect SBS values, although the temperature has a more pronounced influence. The shear bond strength values show an increase in comparison to the control group when subjected to the other four types of ceramic treatment.
The ceramic surface patterns are determined by the application technique and temperature of hydrofluoric acid.
Following the removal of the bond, the interfaces were inspected, revealing the presence of all three types of bonding failures: adhesive, cohesive, and mixed. The occurrence of cohesive failure was limited to the ceramic material, whereas no cohesive fracture was seen in the adhesive cement.

Conflict of interest

The authors declare no conflict of interest.

Kunihiko Hiraiwa

Chapters authored

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