Surface modifcation of zirconia or lithium disilicate‑reinforced glass ceramic by laser texturing to increase the adhesion of prosthetic surfaces to resin cements: an integrative review

Abstract

Objective The purpose of this study was to perform an integrative review on laser texturing the inner surface of lithium disilicate-reinforced glass ceramic or zirconia to increase their bond strength to resin-matrix cements.
Materials and method A bibliographic review was performed on PubMed using the following search terms: “zirconia” OR “lithium disilicate” AND “laser” AND “surface” OR “roughness” AND “bond strength” AND “luting agent” OR “resin cement.” Studies published in English language until March 15, 2023, were selected regarding the purpose of this study.
Results A total of ffty-six studies were identifed althoug thirteen studies were selected. The fndings revealed that zirconia surfaces were signifcantly modifed after laser irradiation resulting in macro-scale aligned retentive regions with depth values ranging from 50 to 120 µm. Average roughness values of laser-textured zirconia by Er,Cr:YSGG laser (~0.83 µm) were quite similar when compared to grit-blasted zirconia surfaces (~ 0.9 µm) although roughness increased up to 2.4 µm depending on the laser type and parameters. Lithium disilicate-reinforced glass ceramics textured with Er:YAG revealed an average roughness of around 3.5 µm while surfaces textured using Nd:YAG laser revealed an average roughness of 2.69 µm; that was quite similar to the roughness values recorded for etched surfaces (2.64 µm). The shear bond strength (SBS) values of zirconia surfaces textured on Nd:YVO₄ laser irradiation were slightly higher (~33.5 MPa) than those recorded for grit-blasted zirconia surfaces (28 MPa). Laser-textured zirconia surfaces on CO₂ laser revealed higher SBS values (18.1±0.8 MPa) than those (9.1±0.56 MPa) recorded for untreated zirconia surfaces. On lithium disilicate-reinforced glass ceramics, higher SBS values to resin-matrix cements were recorded for specimens textured with a combination of fractional CO₂ laser irradiation and HF acid etching (~ 22–24 MPa) when compared with grit-blasted specimens (12.2 MPa). Another study revealed SBS values at around 27.5 MPa for Er:YAG-textured lithium disilicate-reinforced glass ceramics to resin-matrix cements.
Conclusions The laser irradiation at high power increases the roughness of the inner surface of lithium disilicate-reinforced glass ceramic or zirconia leading to an enhanced bond strength to resin-matrix cements. Thus, the laser type and irradiation parameters can be adjusted to enhance the macro- and micro-scale retention of zirconia and glass ceramic surfaces to resin-matrix cements.
Clinical relevance Alternative methods for surface modifcation of lithium disilicate-reinforced glass ceramic and zirconia surfaces have been assessed to provide proper morphological aspects for enhanced adhesion to resin-matrix cements. An increase in the bond strength of glass ceramics or zirconia to resin-matrix cements can improve the long-term performance of cemented prosthetic structures in the oral cavity.

Keywords Lithium disilicate · Zirconia · Laser · Bond strength · Resin cement

Introduction

Nowadays, the use of polycrystalline ceramics and glass ceramics has been increasingly preferred by clinicians and patients considering esthetic outcomes ^{[1, 2]}. Lithium disilicate-reinforced glass ceramics and polycrystalline ceramics such as zirconia are the frst choice for manufacturing veneer, crown, inlay, and onlay restorations over teeth or dental implant abutments ^{[1, 2]}. The surface modifcation of dentin or enamel is carried out by acid etching with 37% H₃PO₄ acid followed by conditioning with methacrylate-based adhesives for adhesion to glass ceramics or zirconia. Titanium abutment base can also be modifed by grit blasting with alumina particles (Al₂O₃) followed by acid etching with (5–10%) HF acid and then conditioning with silane and methacrylate-based adhesives ^{[1, 3]} (1,9). In the same way, surfaces of lithium disilicate-reinforced glass ceramics can also be modifed by acid etching with (5–10%) HF acid providing a standard rough surface for adhesion to adhesive systems and resin-matrix cements. Thus, the roughness of enamel, dentin, glass ceramics, and titanium surfaces can be increased by acid etching that increase the adhesion area for mechanical interlocking to adhesives and resin-matrix cements ^[1–4]. However, acid etching with HF or H₃PO₄ acids cannot alter surfaces of polycrystalline ceramics such as zirconia considering a highly chemical stability of the ceramic ^{[5, 6]}. Zirconia has well-reported enhanced mechanical properties although the surface modifcation using traditional methods for further adhesion to resin-matrix cements is a major disadvantage ^{[2, 5–9]}. Therefore, alternative surface modifcation methods must be studied to enhance the zirconia bonding to resin-matrix cements ^{[6, 7, 10]}.

The long-term clinical success of ceramic-based restorations is strongly dependent on their retention to teeth or abutment substrates by using adhesive systems and resin-matrix cements ^{[1, 2, 7]}. Traditional methods of surface modifcation such as grit blasting and acid etching have limited capability to achieve optimum morphological aspects and roughness over zirconia for mechanical interlocking of adhesive systems and resin-matrix cements ^[8–10]. Also, grit blasting with Al₂O₃ or SiO₂ particles shows limitations considering limited morphological aspects, contamination with remnant abrasive particles, and risks of cracks’ propagation ^[5]. The combination of diferent traditional surface modifcation techniques has been studied and associated with novel methods such as laser-assisted approaches ^{[6, 7, 10]}.

The laser-assisted approaches already have many applications in oral pathology, endodontics, operative dentistry, and prosthodontics ^{[5, 6, 10, 11]}. The surface modifcation of implants and prosthetic structures by high intensity laser irradiation has gathered attention to improve roughness and wettability for adhesion to resin-matrix cements ^{[7, 10, 12–15]}. Currently, diferent morphological surface features (i.e., micro-grooves, cross-lines, pits, valleys, and peaks) can be produced on ceramics and glass ceramics by using diferent laser intensity, type, time, and frequency ^{[7, 10, 12–17]}. Previous studies have reported the surface modifcation of zirconia leading to an increased roughness after short (mili- to nano-seconds) or ultra-short (pico- to femtoseconds) periods of time (pulses) using diferent types of lasers such as Nd:YAG, Er:YAG, CO₂, Er,Cr:YSGG, and Nd:YVO₄ ^{[7, 10, 12–19]}. However, zirconia is susceptible to defects such as cracks depending on the mode and intensity of the laser irradiation. Minimal surrounding defects and contaminants have been reported after surface modifcation of zirconia by using ultra-short pulsed lasers such as pico- and femtosecond lasers ^{[7, 10, 13]}. Also, a minimal tetragonal-to-monoclinic phase transformation was detected in zirconia after irradiation with ultra-short laser pulses ^[19]. Thus, further studies are required to validate adequate parameters for surface modifcation of zirconia and glass ceramics by laser-assisted approaches to improve their bond strength to resin-matrix cements.

The main aim of the present study was to carry out an integrative review on laser texturing the inner surface of zirconia in comparison to lithium disilicate-reinforced glass ceramics used for enhanced bonding of veneer and crowns to resin-matrix cements. It was hypothesized that the laser irradiation at high power increases the roughness of the inner surface of zirconia or lithium disilicate-reinforced glass ceramics leading to an enhanced bond strength to resin-matrix cements.

Method

Information sources and search strategy
A bibliographic review was performed on PubMed (via National Library of Medicine) taking into account such database includes the major journals in the feld of dentistry and biomaterials. The present method was performed in accordance with the search strategy applied in previous studies on integrative or systematic reviews ^{[2, 6, 11, 20–25]}. The following search terms were applied: “zirconia” OR “lithium disilicate” AND “laser” AND “surface” OR “roughness” AND “bond strength” AND “luting agent” OR “resin cement.” At frst, combination of three or four key terms was used to fnd relevant studies.

Also, a hand search was performed on the reference lists of all primary sources and eligible studies of this integrative review for further relevant publications. The inclusion criteria encompassed articles published in the English language, from January 2012 until March 15, 2023, reporting the efects of the laser irradiation on the surface modifcation of lithium disilicate-reinforced glass ceramic or zirconia veneers and their adhesion to resin-matrix cements. The eligibility inclusion criteria used for article searches also involved: in vitro studies, meta-analyses, randomized controlled trials, animal assays, and prospective cohort studies. The exclusion criteria were the following: papers without abstract, case report with short follow-up period, and articles assessing only traditional methods of surface modifcation. Studies based on publication date were not restricted during the search process.

Study selection and data collection process
At frst, studies were examined for relevance by title, and the abstracts were assessed for those studies which were not excluded at this step. Two of the authors (JCMS, ARE) individually analyzed the titles and abstracts of the retrieved potentially relevant articles meeting the inclusion criteria. The total of the studies was compiled for each combination of key terms, and therefore, the duplicates were removed using Mendeley citation manager. The second step comprised the evaluation of the abstracts following the eligibility criteria. Selected studies were independently read and analyzed concerning the purpose of this study. At last, the eligible articles received a study nomenclature label, combining frst author names and year of publication. The following variables were collected for this review: authors’ names, journal, publication year, aims, materials, laser parameters, roughness, and adhesion. PICO question was adjusted to the issue where “P” was related to the patients or specimens while “I” referred to the methods of analyses while “C” was related to comparison of fndings and “O” to the main outcomes. Data of the reports were harvested directly into a specifc data collection form to avoid multiple data recording considering multiple reports within the same study (e.g., reports with diferent set-ups). This evaluation was individually carried out by two researchers, followed by a joint discussion to select the relevant studies.

Results

The search of articles on PubMed identifed a total of 56 studies and duplicates were removed considering the initial combination of key terms, as seen in Fig. 1. A total of 22 studies was discarded considering that they did not meet the inclusion criteria. The remnant 10 studies were full read and selected. Three studies were added by hand search since they were considered as relevant regarding information on laser parameters, methods, and main outcomes.

Fig. 1 Study selection fowchart

Of the 13 selected studies, yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) were assessed by 9 studies ^{[8, 9, 26–32]} although ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP) were also assessed by one study ^[27]. The efects of laser irradiation on lithium disilicate-reinforced glass ceramic were investigated by four studies ^[33–36]. The following types of laser were assessed by the selected studies: Er,Cr:YSGG ^{[9, 30]}, Er:YAG ^{[33, 34, 36]}, Nd:YAG ^[34], CO₂ ^[8,32,35,], and Nd:YVO₄[27]. The laser irradiation parameters varied among the studies as seen in Table 1. For instance, the power of the Er,Cr:YSGG laser irradiation ranged from 1 up to 6 W ^{[9, 30]} while Er:YAG laser was assessed at power intensity range between 4 and 10 W ^[33–35]. The power assessed for CO₂ laser ranged from 3 up to 20 W ^{[8, 32, 35]} while the power of Nd:YAG laser was assessed at 2.5 W ^[34]. After laser irradiation, roughness values were recorded by four studies ^{[26, 29, 32, 34]} although the shear bond strength (SBS) of laser-textured zirconia to resin-matrix cements was recorded by each one of the selected studies.

The main fndings of the selected studies can be drawn as follows:

– The surfaces of the zirconia were signifcantly modifed after laser irradiation with Yb-doped fber or Nd:YVO₄ laser resulting in aligned retentive regions with macroscale depth values ranging from 50 up to 120 µm ^[27].
– The average roughness of laser-textured surfaces using Yb-doped fiber laser system was recorded at around 2.4 µm that was signifcantly higher when compared to the untreated zirconia surfaces (~0.2 µm). Nevertheless, the average roughness recorded for laser-textured surfaces using Er,Cr:YSGG (~0.83 µm) was quite similar to those 
recorded for grit-blasted zirconia surfaces (~0.9 µm) ^[26].
– Surfaces of lithium disilicate-reinforced glass ceramics were also modifed by laser irradiation although the etching with HF acid resulted in high mean values of roughness ^[34]. Lithium disilicate-reinforced glass ceramics textured by Er:YAG laser revealed an average roughness of around 3.5 µm while surfaces textured with Nd:YAG revealed an average roughness of 2.69 µm that was quite similar to the roughness values recorded for etched surfaces (2.64 µm) ^[34].
– Laser-textured zirconia surfaces showed high values of shear bond strength (SBS) to resin-matrix cements ^{[8, 27]}. The SBS values of zirconia surfaces textured on Nd:YVO₄ laser were slightly higher (~ 33.5 MPa) than those recorded for grit-blasted zirconia surfaces (28 MPa) ^[27]. Laser-textured zirconia surfaces on CO₂ laser revealed the highest SBS values (18±0.8 MPa) while lowest values (9.1±0.56 MPa) were recorded for untreated zirconia surfaces ^[8].
– On lithium disilicate-reinforced glass ceramics, higher values of SBS to resin-matrix cements were recorded for specimens textured with a combination of fractional CO₂ laser irradiation and HF acid etching (~22–24 MPa) when compared with grit-blasted specimens (12.2 MPa) ^[35]. Another study revealed SBS values at around 27.5 MPa for Er:YAG-textured lithium disilicate-reinforced glass ceramics to resin-matrix cements ^{[33, 34]}. Surfaces gritblasted with Al₂O₃ particles showed the lowest mean bond strength to the resin composite (15.62 MPa) ^[33].

Discussion

This review reported relevant fndings on laser-textured lithium disilicate-reinforced glass ceramic or zirconia surfaces and their adhesion to resin-matrix cements. Results showed an increased roughness of the glass ceramics and zirconia when compared to untreated surfaces. Thus, high values of roughness reveal an increase in the retentive area to silane agents, adhesive systems, and resin-matrix cements. Indeed, those fndings validate the hypothesis of the present review. Nevertheless, diferent types of laser and irradiation parameters are reported in literature. A detailed discussion on laser parameters as well as on zirconia and glass ceramics and their adhesion to resin-matrix cements is given as follows.

Previous studies revealed the surface modifcation of zirconia and glass ceramics by using diferent laser types (Er,Cr:YSGG, Er:YAG, Nd:YAG, CO₂, and Nd:YVO₄) and irradiation parameters (Table 1). A previous study reported adhesive efects of Y-TZP textured by CO₂ laser irradiation (wavelength of 10.6 mm) at a pulse repetition rate of 1000 Hz and pulse duration of 160 ms at an average power setting of 3 W and at 1 mm away from the surface ^[8]. Specimens were bonded to a resin-matrix cement and submitted to SBS testing. The highest shear bond strength (SBS) values (1.±0.8 MPa) were recorded for laser-textured zirconia surfaces while the lowest values (9±0.5 MPa) were recorded for untreated zirconia (control group) surfaces ^[8].

In another study, conical spots were shaped onto zirconia structures by CO₂ laser texturing and compared to machined, grit-blasted, and polished zirconia surfaces ^[29].The morphological aspects of the surfaces were inspected by scanning electron microscopy (SEM) while the zirconia phases were identifed by X-ray difraction (XRD). Surfaces were bonded to a resin-matrix cements and assessed by shear bond strength assays. The shear bond strength of grit-blasted or laser-textured zirconia to resin cement was 2.3 times higher than that for untreated zirconia. Surface modifcation by laser spotting improved the SBS to the resin cement. However, no signifcant diference was detected between grit-blasted and laser-textured surfaces ^[32].

In a previous study, a texturing on Y-TZP resin at diferent laser beam incidence angles (90, 75, 60, and 45 degree) was assessed using a femtosecond laser, and the SBS values of Y-TZP to resin cement (Rely X™, 3 M ESPE, USA) were measured ^[31]. The SBS values of each specimen was measured using a universal testing machine at crosshead speed of 1 mm/min. SEM images of the zirconia to resin cement interfaces showed the profle of the orifces shaped by the femtosecond laser. A higher contact area between Y-TZP and resin-matrix cement increased as function of the decrease in the laser beam incidence, as seen in Table 1. The results showed that the laser beam was applied to a surface at an angle of 45 degree which resulted in signifcantly higher SBS (~18 MPa) than that recorded for other groups: at 90 degree (~10. MPa), at 75 degree (~13 MPa), and at 60 degree (~15 MPa). Thus, fndings revealed that the decrease of the angle between the ceramic surface and the laser beam resulted in an increase in the contact area of Y-TZP and resin-matrix cement leading to higher SBS values ^[31]. A femtosecond laser irradiation was also assessed on highly translucent zirconia or lithium disilicate-reinforced glass ceramics producing dot patterns with a line distance of 20 or 40 µm over the surfaces. Dot patterns were produced with peak fuence of 4 or 8 J/cm² and irradiation number of 10 or 20 shots. Roughness parameters were measured by 3D confocal laser microscopy, and the microstructure was analyzed by SEM. The fndings revealed similar morphological aspects for both glass ceramic and zirconia considering the surface modifcation parameters. The laser-textured surfaces with crossed-line patterns at a distance of 20 or 40 µm revealed a great potential for further studies ^[17].

A previous study also assessed the efects of thermal cycling on the SBS values of laser-textured zirconia surfaces to resin-matrix cements. Y-TZP was textured on Yb-doped laser irradiation at a pulse repetition rate of 60 Hz and pulse duration of 6.6 s and at an average power setting of 24 W ^[26]. Then, laser-textured surfaces were conditioned with a primer adhesive and bonded to a resin-matrix cement (Panavia V5™, Kuraray, Japan) ^[26]. Half of the specimens were submitted to thermal cycling, and then, the specimens were assessed by SBS testing. After thermal cycling, laser-textured Y-TZP surfaces to the resin-matrix cement were slightly higher (10 MPa, respectively) than those recorded for gritblasted zirconia ones (2 MPa) ^[23]. There were no signifcant differences in SBS values when comparing laser-textured (12 MPa) and grit-blasted (14.5 MPa) surfaces without the efect of the thermal cycling ^[26]. Even though SBS values were not statistically diferent without thermal cycling, the adhesion of the laser-textured zirconia surfaces reached noticeable high adhesion to resin-matrix cements. In another study, zirconia surfaces were textured on Yb-doped pulsed fber laser irradiation (wavelength of 1070 nm) with a maximum average output power of 20 W, a pulse duration of 100 ns, and a repetition rate in the range 20–100 kHz ^[28]. Surfaces were conditioned with a primer adhesive following by bonding to a fowable resin-matrix composite (Tetric Evo Flow™, Ivoclar Vivadent, Liechtenstein) and then assessed by SBS tests ^[28]. Laser-textured surfaces revealed higher fracture forces to resin-matrix cements (132.5±61.35 N) when compared with non-textured ones (105.7±56.1 N). However, the diference was not considered statistically signifcant (p=0.2155).

In a previous selected study, Y-TZP or Ce-TZP surfaces were modifed on Nd:YVO4 laser irradiation (wavelength of 1065 nm) at 70 kHz from 60 mm away to the surface ^[27]. Morphological aspects of surfaces showed aligned retentive regions with macroscale depth values ranging from 50 to 120 µm ^[27]. The rough profle of the zirconia to the resin-matrix cement interface can be noticed in Fig. 2 while non-textured zirconia surfaces provide a smooth interface profle. Surfaces were conditioned with a primer adhesive (Clearfl Ceramic Primer Plus™, Kuraray, Japan) and bonded to resin-matrix cements (Gradia™, GC, Corp, Japan). Half of the specimens were submitted to thermal cycling, and then, all the specimens were mechanically tested by SBS tests. After thermal cycling, SBS values of laser-textured Y-TZP or Ce-TZP surfaces to the resin-matrix cement were slightly higher (33.5 and 24.5 MPa, respectively) than those recorded for gritblasted zirconia ones (15.5 and 16.5 MPa, respectively) ^[27]. There were no signifcant diferences in SBS values when comparing laser-textured Y-TZP or Ce-TZP (28.6 and 27.6 MPa, respectively) and grit-blasted surfaces (28.9 and 25.6 MPa, respectively) without the efect of the thermal cycling ^[27].

Fig. 2 Schematics of the A CAD-CAM and B laser approaches for C crown and D implant abutment surfaces. SEM images of the interfaces of E non-textured and F laser-textured zirconia to a resin-matrix cement. Adapted from Henriques et al. ^[7]

Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) are produced by introducing 2–5 mol% Y₂O₃ into ZrO₂ formulations ^{[37, 38]} for stabilization of the tetragonal phase that results in Y-TZP with a signifcant increase in the following properties: elastic modulus at 230–270 GPa, fracture toughness of approximately 9–10 MPa.m^1/2, fexural strength values at around 1200 MPa, and hardness at 1.1 GPa ^{[39, 40]}. Also, mechanical properties of zirconia are improved when the tetragonal phase is stabilized by the incorporation of small contents of oxides such as cerium oxide (ceria, CeO₂), magnesium oxide (magnesia, MgO), and calcium oxide (calcia, CaO) ^{[37, 38]}. The abovementioned mechanical property values are signifcantly higher when compared with those recorded for glass ceramics. Additionally, oxide-stabilized zirconia has an inherent and remarkable pathway to inhibit the propagation of cracks when the material is submitted to high stresses (i.e., from surface modifcation method). High stresses result in a phase transformation from tetragonal to the monoclinic with an increase in the surrounding volume leading to compressive stresses over the crack. For instance, tetragonal-to-monoclinic phase conversion can take place under cyclic stresses from thermal fuctuations or mastication loading as well as can occur over the surface modifcation by grit blasting or laser texturing ^[37–42]. Nevertheless, Y-TZP is susceptible to the propagation of cracks depending on the pressure, medium, and surface modifcation parameters ^[41–43]. Thus, zirconia surface modifcation is still a contemporary challenge considering a balance among chemical stability, physical properties, and degradation behavior. It should be emphasized that the integrity of the zirconia or glass ceramic prosthetic structures can be readily compromised by improper treatment of inner surfaces for cementation and/or failures in the cementation procedures.

Lithium disilicate-reinforced glass ceramic contains approximately 65% volume fraction of lithium disilicate crystals 34% volume fraction of residual glass, and 1% volume fraction of porosity after heat treatment ^[44]. The glass matrix is derived from a multi-component system, formulated from SiO₂-Li₂O-K₂O-ZnO-Al₂O₃-La₂O₃-P₂O₅ glass system ^{[44, 46]}. Lithium disilicate glass ceramics have shown elastic modulus of around 60–105 GPa, fracture toughness at 2–3 MPa.m^1/2, bend strength values around 320±30 MPa, and hardness at 5.5±0.3 GPa ^[44–47]. Such properties are adequate for their application on anterior teeth region although depending on the patient conditions. Thus, low mean values of strength and fracture toughness of the glass ceramics can induce mechanical failures over the surface modifcation method as well as under cyclic loading in the oral cavity.

In a previous study, lithium disilicate-reinforced glass ceramics were textured on CO₂ laser irradiation (wavelength of 10.6 µm) at two diferent irradiation intensities (10W and 28 J or 20 W and 28 J) at a pulse repetition rate of 200 Hz and pulse duration of 1.7 ms or 0.58 ms ^[35]. Surfaces were conditioned with a silane compound (Silane Bond Enhancer™, Pulpdent Corp., USA) followed by bonding to a resin-matrix cement (Clearfll SA Luting™, Kuraray, Japan), and then, the specimens were submitted to SBS tests ^[35]. The highest SBS values to resin-matrix cement were recorded for specimens textured with a combination of fractional CO₂ laser irradiation and HF acid etching (22.4 and 24.3 MPa, respectively). Grit-blasted specimens exhibited the lowest shear bond strength (12.2 MPa) to resin-matrix cements ^[35]. Considering the glass matrix of lithium disilicate-reinforced glass ceramics, roughness can be increased by etching with 5–10% HF for further adhesion to resin-matrix cements. Additionally, a combination with laser texturing approach becomes benefcial for increasing the retentive area leading to a high bonding to the resin-matrix cement.

Another previous study reported the infuence of grit blasting, acid etching, and laser irradiation on the shear bond strength of lithium disilicate-reinforced glass ceramic (IPS Empress 2™, Ivovlar Vivadent, Liechcheinstein) and a feldspar-based porcelain (VITA VM 9™, VITA Zahnfabrik, Germany) to a resin-matrix cement (Panavia 2.0, Kuraray, Japan) ^[36]. Each of the two ceramic groups was further divided into fve groups regarding the surface treatment method and parameters. The laser-textured surfaces were irradiated with Er:YAG laser at 1 mm distance, 500 mJ energy, 100 μs pulse length, 20 Hz, and 10 W power. After bonding to the resin-matrix cement, specimens were stored in water for 24 h and then assessed by SBS test using a universal testing machine at a crosshead speed of 0.5 mm/min. Statistical analyses indicated that the SBS was signifcantly afected by the surface modifcation methods (p<0.05), although there was signifcant correlation between the ceramic systems. Grit-blasted plus laser-textured surfaces showed the highest mean SBS values for both ceramics groups. Atomic force microscopy images revealed a rough morphological aspect for grit-blasted surfaces ^[36].

In another study, lithium disilicate-reinforced glass ceramic was also modifed by Er:YAG (200 mJ, pulse repletion at 10 pps) or Nd:YAG (120 mJ, pulse repletion at 15 pps) pulsed laser irradiation ^[34]. Groups of specimens were etched with HF, and all of the surfaces were conditioned with silane coating (Monobond Plus™, Ivoclar Vivadent, Liechtenstein) and then bonded to a dual-cure resin-matrix cement (Variolink II™, Ivoclar Vivadent, Liechtenstein). Lithium disilicate-reinforced glass ceramics textured by Er:YAG laser revealed an average roughness of around 3.5 µm while surfaces textured by Nd:YAG laser revealed an average roughness of 2.69 µm that was quite similar to the roughness values recorded for etched surfaces (2.64 µm) ^[31]. Er:YAG-textured surfaces revealed the highest SBS values to resin-matrix cements (27.5 MPa). Thus, lithium disilicate-reinforced glass ceramics etched with HF acid or laser-textured resulted in surfaces with roughness values higher than those recorded for untreated surfaces ^[34].

Nowadays, ceramics and glass ceramics are used to synthesize metal free (e.g., zirconia or zirconia-to-porcelain systems) and metal ceramic (e.g., feldspar-based ceramic fused on metallic materials) for implant- or teeth-supported prostheses (Fig. 2). On teeth, crowns or multi-unit prostheses are retained by using resin-matrix cements. Screw or cement-retained crowns consist of two ways of retaining the implant abutment prosthetic ^{[1, 48, 49]}. Thus, ceramics and glass ceramics have shown optical properties to significantly improve esthetic appearances, although higher failure rates are associated with fractures, crack propagation, and poor cement and/or bonded retention ^{[1, 50–52]}. Thus, the brittle mechanical behavior of glass ceramics and zirconia remains a concern regarding catastrophic fracture or abrupt stress distribution across the structural materials and interfaces. High concentration of stresses at prosthetic structural materials can increase the risks of brittle fractures at the zirconia or glass ceramics to adhesive and resin-matrix interfaces ^{[1, 7, 41–44]}. Then, surface modifcation of ceramics using traditional physical methods and laser-texturing approaches can be optimized regarding novel approaches to avoid clinical failures.

Even though a limited number of studies in literature, the selected studies reveal noteworthy data on laser-textured lithium disilicate-reinforced glass ceramic or zirconia surfaces and their adhesion to resin-matrix cements. In fact, several laser protocols have been assessed to enhance the retentive morphological aspects and increase the roughness for adhesion of ceramics and glass ceramics to resin-matrix cements. Indeed, the variability of laser types and irradiation parameters brings a broad data on the efciency of the laser-assisted methods for surface modifcation of zirconia and glass ceramics. Also, further studies are performed to validate the efects of diferent laser parameters, restorative materials, and types of resin-matrix cements. Laser-assisted methods have been increasingly gathering attention in the technological feld although most of laser apparatus are still quite expensive for clinical or laboratory application.

Conclusions

Within the limitations of the selected studies, the main outcomes can be drawn as follows:

• On lithium disilicate-reinforced glass ceramics, higher shear bond strength values to resin-matrix cements were recorded for specimens textured with a combination of fractional CO₂ laser irradiation and HF acid etching when compared with grit-blasted specimens. In fact, the association of traditional surface modifcation methods and laser-texturing provide benefcial efects for adhesion of glass-ceramics to resin-matrix cements.
• Laser-textured zirconia surfaces on CO2 or Nd:YVO₄ laser revealed higher shear bond strength to resin-matrix cements than those recorded for untreated zirconia surfaces. Thus, laser texturing approaches can become an alternative method for the modifcation of zirconia surfaces.
• The laser irradiation at high power increases the roughness of the inner surface of lithium disilicate-reinforced glass ceramic or zirconia veneers and crowns leading to an enhanced bond strength to resin-matrix cements. Thus, the laser type and irradiation parameters can be adjusted to enhance the macro- and micro-scale retention of zirconia and glass ceramics surfaces. Further studies should consider other parameters related to the power output, frequency, and mode as well as the fuence of the laser irradiation to establish a standard guidelines for laser-texturing.

References: Omitted