Effect of Hydroxyapatite Coating in Combination with Physical Modifications on Microshear Bond Strength of Zirconia to Resin Cement

Background Zirconia has been used as a reliable core material in dental restorations for years; however, its bonding to resin cement is a matter of challenge. Physical, chemical, and combinations of these techniques have been investigated to boost the properties of zirconia surface bonding. The objective of this work was to evaluate the effect of hydroxyapatite coating as a chemical therapy in combination with physical modifications on the microshear bond strength of the resin cement over zirconia. Methods In the present research, 60 sintered zirconia blocks (4 × 4 × 4 mm) were randomized into four groups of 15, including Al2O3 particle abrasion (group 1), HA coating (group 2), Al2O3 particle abrasion + HA coating (group 3), and Er, Cr: YSGG laser irradiation + HA coating (group 4). The microshear bond strength was determined by bonding the blocks to the resin cement. Results The bond strengths (mean ± standard deviation) of modified zirconia surfaces were 16.93 ± 4.94 MPa, 16.14 ± 5.4 MPa, 19.4 ± 5.27 MPa, and 16.21 ± 3.7 MPa in groups 1–4, respectively. Test results of the ANOVA test revealed no significant difference regarding the bond strength values of zirconia surfaces to the resin cement between the studied preparation modalities (p > 0.05). Conclusion Observations from the present study showed that HA coating can be as effective as the air-borne particle abrasion technique in improving bond strength to zirconia surfaces. Moreover, sandblasting by an aluminum oxide or Er, Cr: YSGG laser irradiation prior to HA coating of zirconia showed no significant effect on the reinforcement of bond strength values when compared to HA coating alone. The clinic hydroxyapatite coating alone or in combination with physical treatments improves the bond strength of zirconia to resin cement.


Background
All-ceramic materials optimized for mechanical properties have recently attracted special attention for use in the manufacture of fxed metal-free restorations. Among these, zirconium oxide (ZrO 2 ) is one of the most popular ceramics [1]. Tis ceramic has desirable characteristics such as biocompatibility, good appearance, and chemical stability. Tus, it becomes a favorable material for various applications in dentistry, such as dental braces, posts, implant restoration abutments, and the framework of fxed partial prostheses. Likewise, it is applicable in conservative restorations, including veneers, inlays, onlays, and Maryland bridges. Te nonretentive preparation of these restorations has given special importance to the bond strength between teeth and restorations [2]. More conservative tooth preparation, improved marginal adaptation, increased restoration retention, and reduced microleakage are among the advantages of a chemical bond of the restoration with the resin cement [3]. Hence, it is of clinical importance to achieve a chemical bond on the zirconia surface, especially when the macromechanical retention is compromised. Moreover, it has been shown that the coupling agents were not successful in the attainment of an acceptable chemical bond between the resin cement and zirconia disparate silica-based ceramics due to their polycrystalline glass-free structure that cannot be etched by common acids [4]. Resin-bond strengths are associated with their potential to penetrate into the surface irregularities of the underlayer [5]. Te methods to achieve improved bond strength of zirconia to resin cement can be classifed into 3 groups, including physical, chemical, and physicochemical techniques. Tribochemical treatment by RocatecTM is the available physicochemical technique for ceramics like Y-TZP. In this technique, silica-coated aluminum particles are subjected to low-pressure sandblasting, followed by silica-mediated chemical modifcation of the ceramic surface [6]. Te application of primers containing organophosphate monomers and carboxylic acid is among the chemical techniques for improving adhesion [6]. Resin cement containing 4-methacryloxyethyl trimellitate anhydride (4-META), methacryloxy decyl phosphoric acid (MDP), or 3-trimethoxysilylpropyl methacrylate (MPS) monomers could have a reaction with oxide groups in the structure of Y-TZP crystalline. Tis process resembles the interaction between the silane coupling agent and silicabased ceramics [7]. Air-borne particle abrasion with aluminum oxide and laser etching are examples of creating surface roughness as a physical technique [8]. Alumina sandblasting can successfully enhance the surface area and create an active surface with increased wettability in dental materials [9]. Nonetheless, this method has been criticized for its adverse efects on the mechanical profles of zirconia like fexural strength as well as the possibility to induce subcritical crack growth within the material [9]. Lasers have diferent applications in dental treatments. Te neodymiumdoped: yttrium aluminum garnet (Nd: YAG) laser can be used for reducing tooth sensitivity, caries removal, bleaching, and producing surface roughness in high-strength core ceramics [10]. Liu et al. [11] demonstrated a rougher surface of zirconia ceramics with a higher output power of the Nd: YAG laser. However, the shear bond strength of the laser group was not clearly increased. Te pulsed erbium lasers (i.e., Er, Cr: YSGG, and Er: YAG) have been applied for the treatment of surfaces. Kasraei et al. [12] revealed that the bond strength of zirconia to resin cement was greater in groups treated with the Er: YAG laser in comparison to CO 2 laser-treated samples. Referring to the fndings, the Er, Cr: YSGG lasers can generate comparable surface roughness rather than acid etching of dentin or enamel surfaces [13]. Likewise, it could efectively change the surface roughness of the lithium disilicate ceramics and increase the shear bond strength of this ceramic to resin cement [14]. Contradictory fndings are available regarding the preference for diferent surface conditioning approaches to improve the adhesion of resin cement to zirconia. Te inorganic matrix of bone and teeth of human beings can be found in hydroxyapatite or HA [Ca10 (PO4)6·(OH)2], as phosphocalcic hydroxyapatite [15]. It exhibits admirable biocompatibility, with a crystal structure and composition like apatite in the skeletal and dental architectures of humans. Seo et al. [16] fabricated a coating of HA zirconia substrate by a room-temperature spray process. Tey exhibited no severe dissolution as observed during in vitro experimentation [17]. Signifcant advances in nanotechnology have suggested practical applications for nano-HA in dentistry, with crystals ranging from 50 to 1000 nm. Nano-HA possesses bioactive and biocompatible properties. Moreover, they are similar to the tooth enamel apatite crystal morphologically, and in terms of crystallinity and crystal architecture [18]. Furthermore, nano-HA has a fller function because it repairs tiny depressions and holes on the surface of enamel, an action that increases with the small size of the constituent particles [15]. Nanoparticles produce a thin and consistent coating layer with adequate strength [19]. Nanosized HAp increases the level of crystallinity in glass ionomer cement and enhances mechanical features such as microshear bond strength [20]. Panavia F 2.0 is an extensively applied resin cement in dentistry that contains a bifunctional monomer, 10methacryloyloxydecyl dihydrogen phosphate (MDP). According to the favorable adhesion of the resin cement to the tooth structure, which is mainly composed of HA, it is hypothesized that a coated layer of HA on the zirconia could result in a suitable bond strength. Te present study investigated the infuence of HA coating on microshear bond strength (μSBS) of zirconia to resin cement (Panavia F 2.0). Tere were two null hypotheses: (1) there is no diference in the μSBS among the study groups. (2) Tere is no diference in the patterns of failure.

Materials and Methods
Tis research is in line with the previous work that was also conducted by these researchers [21]. In the current in vitro work, 60 pre-sintered commercial dental zirconia blocks (Cercon, Dentsply, Amherst, N.Y.) with dimensions of 4 × 4 × 4 mm were ground for 60 s via 600 and 800 Struers RotoPol 11 silicon carbide grit abrasive (Struers A/S, Rodovre, Denmark), followed by cleaning in an i600B steam cleaner (Italy ELT) for 10 minutes using ethanol (96%) and subsequently air-drying for 30 s. In accordance with earlier identical work [21] and regarding the Bonferroni formula for the determination of the sample size, the number of specimens in each group was determined as n � 15. Te sampling method was nonrandomized in this study. However, the allocation of specimens to each group was randomized.
Group 1 consisted of zirconia blocks sandblasted with Al 2 O 3 particles (50 μm) under a pressure of 4 kg/cm 2 and a distance of 10 mm for 15 s in a MESTRA sandblast apparatus (TALLERES MESTRAITUA S. L., Espana), followed by cleaning with ethanol (96%) for 10 minutes in an ultrasonic cleaner. Group 2 consisted of zirconia blocks whose surfaces were coated via the HA thermal protocol so that a slurry solution was prepared by adding 10 gr of nanoparticle HA powder (less than 100 nm) (Merck, Germany) to distilled water (50 cc). Ten, 1 gr of polyvinyl alcohol (Merck, Germany) was also added as the binder of the suspension, followed by heating on a magnet stirrer with 1000 rpm at 100°C for 60 min to achieve a uniform suspension. At last, the zirconia blocks were placed in this slurry at an angle of 45°for 5 s. Group 3 consisted of zirconia blocks sandblasted with the same condition as described in group 1 and then layered with a coating of HA following the same procedure as described in group 2.
Group 4 consisted of zirconia blocks whose surfaces were frst exposed to Er, Cr: YSGG laser irradiation with the following details: a wavelength of 2.78 μm, 140 μs pulse duration, a 20 Hz repetition rate, and 4 W output power. Te laser optical fber (with a diameter of 600 μm and a length of 6 mm) with the gold handpiece was positioned to the surface at a distance of 10 mm perpendicularly and moved in a sweeping manner manually within a 30 s exposure time over the whole area. Continuous water (55%) and air (65%) fows were applied while performing irradiation. After laser treatment, the surface was coated with HA, as explained for group 2.
Subsequent to the surface treatment, all blocks in groups 2, 3, and 4 were sintered following this protocol: ambient temperature to 300°C for 10 minutes, 300-600°C for 10 minutes, 600-900°C for 30 minutes, 900-1200°C for 40 minutes, maintenance at 1200°C for 120 minutes, and cooling for annealing, employing a CWF furnace (Keison Products, UK). Following the surface treatment in all study groups, the mold was selected to be Tygon Norton Performance Plastic tubes with an inner diameter of 0.8 mm and a height of 1 mm (Cleveland, OH, USA) for the Panavia F 2.0 cement bonding (Kuraray Medical Inc.) to the prepared surfaces. Ten, the tubes were removed with a heated, sharp scalpel. All samples were placed in distilled water for 24 hours, incubated (incubator, model PL-455G, PecoPooya Electronic Co.) at 37°C, and positioned in a microtensile tester (Bisco Inc., USA) for the determination of microshear bond strength. A metal loop (0.2 mm thick; Ligature, Dentsply GAC_SOF) was put around the cement cylinder in the bonding site for the exerted tensile load conversion to shear load through vertical soldering of casting molds to the jig, as shown in Figure 1. Te load extent at failure (crosshead speed � 0.5 mm/min) was determined, followed by computing the values of microshear bond strength in accordance with the equation S (MPa) � F (N)/A (mm). Following the shear experiment, an SEM was used to determine the fractured surfaces for achieving the failure variants that included cohesive failure, adhesive failure, and mixed failure.

Statistical Analysis Method.
To analyze the data, the SPSS software version 21.0 was used. Te statistical comparison of the bond strength was conducted with a one-sided analysis of variance (ANOVA) among the 4 groups. In the present study, the type 1 error rate (α or p value) was considered equal to 0.05.

Results
Te mean μSBS in groups 1, 2, 3, and 4 were calculated as 16.93 ± 4.94 MPa, 16.14 ± 5.4 MPa, 19.4 ± 5.27 MPa, and 16.21 ± 3.7 MPa, respectively (Table 1). According to a onesided analysis of variance results, the diferences in the amount of μSBS were not signifcant among the study groups (p � 0.23). Since there were no signifcant diferences as determined by the ANOVA analysis, no pairwise comparison was performed between the groups.
Te most common failure modes in all study groups were the adhesive types. All cohesive fractures occurred in the cement cylinder. Te mixed type of failure was the less common type of fracture in groups 1 and 3. Table 2 describes the number of diferent types of fractures in the study groups ( Figure 2).

Discussion
According to the mineral tooth structure, which is mainly composed of hydroxyapatite crystals, and the favorable resin cement bond strength to such a structure, it is hypothesized that coating the zirconia surface with the nanosized HA might boost the bond strength with resin cement. Te current efort investigates the infuence of hydroxyapatite coatings solely and in combination with physical surface modifcation via Er, Cr: YSGG laser irradiation, and airborne particle abrasion. In our research line, Azari et al. [21] were just the pilot study for the surface treatment of zirconia with hydroxyapatite. In the following, Azari et al. [19] compared the bonding strength of zirconia to cement resin in two techniques: HA coating and air abrasion. In the present study, we tested the efect of HA coating in combination with physical surface treatments such as air abrasion and laser on the bond strength of resin cement. Te results indicated that the null hypothesis should be accepted, as the μSBS was not signifcantly diferent among the study groups. While Y-TZP ofers many advantages, such as high strength, fracture toughness, and good wear resistance, there are some disadvantages, such as the nonpolar nature, which leads to negligible bonding capacity with dental structure and/or overlaying ceramics [22]. Multiple chemical and mechanical methods have been reportedly combined for the enhancement of the bonding activity of dental zirconia [1]. International Journal of Dentistry Te surface of zirconia can be modifed by a HA coating, which has been previously used as a promising method to modify bioinert metallic surfaces in implant dentistry [16]. As reported by Sagsoz et al., the highest bond strength within resin ceramics and resin cement was among the HAp group. According to their conclusions, HAp coating can be used instead of hydrofuoric acid etching and sandblasting in CAD/CAM materials and resin cement to improve the bond strength [23].
Over the years, using digital technologies in dentistry has developed. In a study by Pagano et al. [24], CAD/CAM prostheses had improved mechanical characteristics in frequency scopes. Besides, they are less cytotoxic than conventional prostheses. Also, a novelty in prosthetics is the use of milled discs of polymethylmethacrylate (PMMA) with CAD/CAM machinery, which exhibit adequate mechanical features and greater biocompatibility especially for removable prostheses. In addition, Brillouin spectroscopy and dynamic mechanical analysis (DMA) are pioneering procedures for mechanical analysis [24].
Tere are diverse coating approaches available for exploiting the merits of HA. No standard instructions have been issued so far for the various dimensions of a particular approach, such as surface profle, porosity, texture, coating thickness, and crystallinity or crystal size efects. Several methods of coating a HA layer onto bioinert metallic implant surfaces have been previously reported, which include the plasma spray and sol-gel methods. Te present work benefted from the thermal coating approach because of its fne mechanical profles, practical feasibility, and admirable phase composition [21]. Te thermal coating technique has possibilities for modifying the surface features of zirconia and might be capable of shifting the bonding pattern of zirconia ceramics [21]. As concluded by Okada et al., the thermal coating of zirconia after applying silane coupling agents has the capacity to boost the shear bond strength within the zirconia and composite resin cement [25]. Previous research has indicated that, in comparison to the control group, air-borne particle abrasion signifcantly boosted the bond strength of the zirconia surface to Panavia F 2.0 resin cement [21]. In addition, one of the best surface treatment approaches is sandblasting for surface modifcation of zirconia ceramics [26]. Tus, it was employed as the control group in our work. Controversies and limitations of the sandblasting method can be addressed by coating the zirconia surface via HA regarding its merits, some of which are great surface tension, radiopacity, optimal hardness for natural teeth, a potent wear profle, and admirable bonding and wettability. Te present study revealed that the HAdecorated zirconia surface boosted the bond strength to resin cement similar to the sandblasted group. However, the combination of other surface treatments such as Er, Cr: YSGG laser irradiation, or air abrasion has shown no additional beneft in the improvement of bond strength. Te lowest range for acceptable clinical bonding was suggested to be between 10 and 13 MPa [27]. Te μSBS values of all study groups were beyond the acceptable range, and their differences were not statistically signifcant. As stated by Ranjbar Omidi et al., silica coating through Cojet sand presented substantially greater μ-shear bond strength compared to Er: YAG laser and sandblasted groups. Moreover, the μSBS values of the sandblasted group were higher than those of the Er: YAG laser [28]. Sandblasting increases the bond strength between zirconia and resin cement [21]. Tanış and Akçaboy [29] indicated that the values of shear bond strength between Y-TZP and Panavia F 2.0 were signifcantly greater in the group modifed with sandblasting + tribochemical silica coating + silane in comparison to the sandblasted specimens. Consistent with this, Zandparsa et al. [26] reported that the sandblasted zirconia surface showed inferior shear bond strength to enamel in comparison to the Al 2 O 3 air abraded + Z-PRIME Plus group. According to the fndings of these two studies and other similar studies [1], it might be assumed that an increase in surface roughness via laser irradiation or air-borne particle abrasion in combination with modifying the surface with a layer of HA might result in higher values of μSBS. Although the μSBS value was higher in group 3 in comparison to group 1, such a diference was not statistically signifcant. Tis might be due to the replacement of the surface irregularities with nanosized HA, which would leave no efcient indentation for micromechanical retention of the cement layer. We coated the surface of zirconia with nano-HA on the basis of a thermal protocol. Nanoparticles provided a thin and uniform controllable coating layer with adequate strength. In addition, Abdulkader and Aljubori reported that the addition of nanohydroxyapatite (nHAp) to self-adhesive resin cement reinforced the mechanical features and bond strength of zirconia [30,31]. Tere is limited scientifc evidence compared to the amount of surface roughness produced via Er, Cr: YSGG laser irradiation, and Al 2 O 3 particle   abrasion. An SEM observation revealed more surface irregularities with 50 μm Al 2 O 3 abrasion than the Er, Cr: YSGG laser (2 W and 3 W) treated Y-TZP surface [27]. Although specifc surface roughness tests are required for more accurate conclusions on diferences in surface roughness, the higher values of μSBS in group 3 of the present study, in comparison to group 4, could be the result of better micromechanical bonds. While several studies have investigated the impact of Al 2 O 3 particle abrasion and its infuencing factors on the sintered zirconia surface roughness [32]. In the present study, particle size was utilized. Te data of the previous studies reported that the 50 μm Al 2 O 3 particle abrasion could result in elevated fexural strength because of the formation of compressive felds owing to the induced tetragonal-monoclinic transformation of the crystals present in the surface [33]. However, such transformation and the formation of small faws might compromise the fatigue strength of Y-TZP restoration in the long term [34]. In a work by Kern and Wegner [35], the phosphate ester monomer of MDP exhibits a water-resistant durable chemical bonding. Te MDP monomer, with bonding agents, reportedly could boost the bond strength of sandblasted zirconia with resin cement [36]. Hence, we recruited Panavia F 2.0 in our experiments. Te SEM images captured to analyze the failure mode revealed that the majority of fractures were adhesive in the solely air-abraded group (group 1), and other groups coated with HA showed more mixed fractures than group 1. It can be related to the high quality of the chemical bond of resin cement with coated HAp zirconia [19]. Te present paper is a continuation of previous work on the improved bonding characteristics of zirconia ceramics [27,29]. Similarly, Azari et al. concluded that failure patterns in the HAp-coated group were mixed and adhesive in the sandblasting and control [19]. SEM observations also revealed some of the adhesive failures in the Er, Cr: YSGG groups. While it seems that HA coatings can be considered for the preparation of zirconia surfaces before using luting cement, the efect of temperature, moisture environment, and cyclic loads is still unclear for this mode of surface modifcation. In addition to more in vitro evidence on the applicability of this technique, further clinical investigations are required in this regard. In line with the benefcial adhesion of the resin cement to the tooth structure, the infuence of the HA coating on the bond strength of zirconia to resin cement (Panavia F 2.0) was evaluated in this study. Lately, a new material named Activa ™ Bio-Active Restorative has been presented. As highlighted by the producer, a feature of bioactive substances is the probable appearance of HA at the point of contact within the material and dental tissue. Tis material has the characteristics and benefts of both glass ionomer cement and resin composites. It also has the chemical and physical features of natural teeth. In future studies, the use of this material as a frontier in the science of dental materials is recommended [37]. Te main limitation of this research was achieving a uniform HA coating on the zirconia. It was a techniquesensitive procedure. Te most common test methods used to evaluate adhesive bonding between two materials are tensile, microtensile, and shear bond strength tests [38]. Te shear bond strength test has an easy application procedure because no additional process is required once the bonding procedure is complete. But this test method has the disadvantage of inhomogenous stress distribution [29,38]. One limitation of this study is that the shear bond strength test was used because of the easy application procedure. However, the microshear test shows a lower standard deviation than the shear test because the small adhesive interface used in the microshear test contains fewer defects compared to the larger specimens used in the shear test [29].

Conclusion
Te following conclusions were obtained in the present work: (i) Te μSBS of zirconia were improved by HA coating surface treatment (ii) Improvement of μSBS with HA surface treatment is comparable to air-borne particle abrasion (iii) To improve the bond strength, there is no signifcant additional beneft from combining air-borne particle abrasion or laser irradiation with HA coating (iv) HA coating might be a promising alternative method for air-borne particle abrasion on zirconia surfaces

Data Availability
All the data generated or analyzed during this study are included within the article.

Consent
Consent was not applicable for this study.