2022, Number 1
Micro tensile strength and surface appearance of a CAD/CAM nanoceramic resin treated by different etching methods
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ABSTRACT
Introduction: CAD/CAM polymer infiltrated ceramic materials need micromechanical and chemical treatments that modify their surface and favor adhesion to dentin of indirect restorations. Objective: To evaluate in vitro the micro tensile strength caused by the combination of micro-sandblasting (MS) plus hydrofluoric acid (HF 10%) applying a universal adhesive (UA) on Lava™ Ultimate (LVU) specimens. Materials and methods: LVU resin nanoceramics (3M ESPE, St. Paul, MN, USA) were used. The samples were divided into 4 groups: Group 1: Control (No treatment), Group 2: MS with 50 µm Al2O3 (Zeta Sand Zhermarck Dental SpA, Italy) + UA (All-Bond Universal® Bisco Inc. Schaumburg, IL). Group 3: Etching with 10% FA Angelus® (Solucoes Odontologicas, Londrina, Brazil) + UA, group 4: MS + FA + UA. Specimens were cemented with Duo-Link™ dual resin (Bisco Inc. Schaumburg, IL) in the middle dentin of extracted third molars. After 24 h, micro tensile testing was performed. The failure mode of the samples was recorded under a stereomicroscope at 20X. Surface roughness and appearance were evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results: surface treatment of LVU with MS +FA + UA showed significant differences in micro-tractional strength (p < 0.011). The predominant failure mode in the groups was adhesive. A significant correlation was identified by observing that the higher the roughness of the treated surface the higher the micro-tractional resistance (r = 0.88). Conclusions: LVU nanoceramic resin etched with MS and FA creates a surface roughness that combined with a UA increases the adhesive strength resistance in dentin.INTRODUCTION
Computer-aided design and manufacture of dental restorations (CAD/CAM) is a technology that over time has diversified the clinical applications of prosthodontics.1 This technology provides high quality and reproducibility of different blocks of ceramic, glass-ceramic, or resinous CAD/CAM materials.2,3 Traditional ceramic materials are biocompatible, esthetic, have good wear resistance, and offer color stability. In recent years, materials with different compositions have emerged that are milled using the CAD/CAM method. The objective of these materials is to obtain properties similar to those of ceramic materials.4 Some of their advantages are: excellent marginal adaptation, modulus of elasticity similar to dentin, and wear similar to that of enamel.5 Currently, the most prominent milling blocks are Vita Enamic® (Vita Zahnfabrik, Bad Sackingen, Germany), this hybrid ceramic is composed of a dominant ceramic matrix (86% by weight) reinforced by a polymeric matrix.6 Also, resin materials with a uniformly dispersed ceramic nanoparticle filler (80-90% by weight) such as Lava Ultimate Restorative (LVU; 3M ESPE) and Cerasmart (GC Dental Products) are also available.7-9 The adhesion of these materials to the tooth substrate in indirect restorations is decisive for long-term success. For this reason, the surface treatment of these materials plays a significant role in adhesion.10
In recent years, the search for a micro retentive surface that favors good adhesion strength has been pursued. To date, the most widely used methods for surface treatment are micro-sandblasting (MS) with aluminum oxide (Al2O3) particles,3,10-12 hydrofluoric acid (HF) etching9,10,13,14 and the tribochemical method (micro-sandblasting with silica-coated particles), which favors a chemical bond with the silane before cementation.14,15 Of these treatments, MS with 50 μm Al2O3 particles removes contaminants retained on the surface of the material to be bonded, increasing surface roughness and micromechanical retention of the adhesive.13,16 On the other hand, HF chemically removes the vitreous phase of the material from the treated surface, leaving it irregular and micro retentive.9,17 It has been reported that the etching of lithium disilicate ceramic blocks with 10% HF for 60 seconds reaches values between 31 to 39 Mpa.18,19 The microtensile bond strength test (μTBS) has been successfully applied to assess adhesive strength in different studies.16,20-23 This test has the advantage that tensile loads generate mainly an adhesive-type failure mode, which favors the identification of the accuracy of the materials' adhesive strength.24 It is possible that the combined application of 2 different surface treatment methods may generate an irregular surface on nano ceramic resins that promotes high bond strength to dentin. Therefore, because the joint effect of MS etching and HF as surface treatments of nanoceramic resins has not been investigated, this study aimed to evaluate in vitro the micro tensile strength caused by the combination of MS (50 μm Al2O3) and HF (10%) surface treatment by applying a UA on LVU nanoceramic resin specimens (3M ESPE, St. Paul, MN, USA).
MATERIAL AND METHODS
An experimental study was performed with a sample of 4 lower third molars with integral crowns extracted for orthodontic indications. Once cleaned, these were stored in containers containing deionized water to later obtain dentin samples.
Obtaining dentin samplesThe molars were seeded in self-curing acrylic resin (Nic-Tone® MDC Dental, Jalisco, Mexico) by sectioning them transversely at the middle of the clinical crown with a disk on a low-speed motor (Ray Foster, Dental Equipment Huntington Beach, CA) thus exposing the coronal medial dentin. The cut surfaces were abraded with a 600-grit silicon carbide blade until a uniform, smooth surface was obtained.
Obtaining LVU specimensFrom LVU resin nanoceramic blocks, 4 specimens with dimensions of 10 × 10 × 4 mm were fabricated using a 0.8 mm diamond disk on a low-speed trimmer (Buelher, IsoMet 5000). The surface samples were smoothed to be bonded using 600-grit silicon carbide (SiC) paper (Fandelli®, Tlalnepantla, Edo. de Mexico) under cooling with potable water for 1 min. The sanded samples were cleaned for 5 min inside an ultrasonic cuvette (BioSonic UC50D Coltene/Whaledent OH, USA) and then stored in distilled water at 37 oC for 24 h in an ambient chamber (Shel-lab Mod. 1510E, Sheldon Manufacturing, OR, USA). The LVU samples were then randomly assigned to 4 groups:
Group 1: control/No surface treatment. Only the LVU specimens were ultrasonically cleaned.
Group 2: LVU specimens were treated with MS with 50 μm Al2O3 particles (Zeta Sand®/Zhermack) with a micro-sandblaster (Essence Dental, Araraquara, Brazil) for 10 s under a pressure of 3 bar at a distance of 10 mm between the nozzle and the specimen surface. Finally, a layer of UA (All-Bond Universal® Bisco) was applied with a micro-applicator (Bisco Inc. Schaumburg, IL), allowed to dry for 1 min, and light cured for 20 seconds.
Group 3: LVU specimens were etched with 10% Angelus® HF for 60 s; washed and rinsed for 60 s, followed by application of a UA layer with a micro applicator, dried for 1 min, and light-cured for 20 seconds.
Group 4: the samples in this group were first treated with MS as in group 2 and then with HF as in group 3. After the surface treatment, a layer of UA was applied.
One specimen from each group was randomly taken for surface appearance evaluation under a scanning electron microscope (JEOL JSM 7800F) and atomic force microscope (NT-MDT Spectrum Instruments) which also determined the surface roughness of the treated samples.
Bonding of LVU to dentinFor the bonding process the sectioned samples were rinsed with deionized water for 20 s, followed by etching with 35% Select HV® Etch phosphoric acid (Bisco, Schaumburg, IL, USA) for 15 s, rinsed and dried, and a layer of UA was applied on the exposed dentin with a regular thickness (2.0 mm) micro applicator. The UA was allowed to dry for 1 min and light cured for 20 s at a distance of approximately 2 mm. Duolink® dual resin was used to bond the LVU specimens to the dentin samples. With a plastic cannula, a single increment was placed in the central part of the dentin sample, and then the LVU specimen was placed, removing the excess cement. Then a metallic weight was placed on the LVU nanoceramic resin and a constant axial load of 1 kg was maintained on the entire surface, light curing the cement in a multidirectional manner for 20 s with a Coltolux Led Curing Light® (Coltene/Whaledent Inc, Cuyahoga Falls, OH, USA) at a power of 650 mW/cm2, corroborated by a Coltolux Light Meter radiometer (Coltene).
Once the specimens were cemented, they were stored in distilled water at room temperature for 24 hours. Subsequently, with a digital vernier (Mitutoyo) each specimen was marked and then vertically sectioned every millimeter with a 0.8 mm thick diamond disc that was fixed to a specimen trimming machine (Buelher, IsoMet 5000)®. The cuts were made first in the buccolingual direction and then in the mesiodistal direction. Once the cuts were finished, they were separated from the LVU block by cutting the base of the grooves made with a diamond disc mounted on a micro motor (Champion M3 Marathon SDE-SH3L) obtaining LVU/dentin beams of 1 mm in diameter by 7 mm in length. The beams were washed with deionized water for 5 minutes and dried with 2 applications of air from the triple syringe. These beams were then taken to a universal testing machine (Alliance RT/30 MTA) for micro tensile testing. The obtained beams were glued at their ends with cyanoacrylate (3M™ Super Glue Gel, St. Paul, MN) to an attachment according to the method proposed by Sano et al.21 This attachment was fixed to the universal testing machine (Alliance RT/30) performing the microtractional test at a speed of 1 mm/min, with a force of 5.4 kg per min. After the test, the specimens were taken to the stereo microscope (Leica Microsystems, Switzerland) at 20X to record the failure mode (adhesive, cohesive and mixed) of each specimen.
The Kappa coefficient between 2 observers blindly and independently (ADI-CAL) yielded a concordance coefficient of 0.89.
Kolmogorov-Smirnov normality and Levene variance tests were performed. Subsequently, Kruskal-Wallis and post-hoc Scheffe comparisons were practiced in the analysis of the microtractional strength obtained with the different LVU surface treatments. Pearson's r-test identified the degree of correlation between surface roughness and microtractional strength. Statistical tests were handled at an alpha value of 0.05 in the IBM SPSS Statistics 23 statistical package.
RESULTS
Samples from the control group were lost because the specimens were fractured during the cuts to obtain LVU/dentin beams. Statistical analysis was performed on 9,10 and 11 beams from groups 4 (MS+FA+UA); 2 (MS+UA) and 3 (FA+UA) respectively. The descriptive results are shown in Table 1. There were no statistically significant differences between the MS+UA and HF+UA groups. The group that combined the MS+HF+UA presented the highest values of micro tractional adhesive strength being statistically superior to that found in the MS+UA and HF+UA groups (p < 0.05).
The highest value for surface roughness (Ra) was found in the group that combined the two LVU surface treatment techniques with MS and HF (Figures 1 and 2). The predominant failure mode in all study groups was adhesive, with 81.4% followed by cohesive (18.6%) in all groups (p > 0.99). In a correlation test, it was identified that the higher the surface roughness (Ra) of LVU the higher the micro-tractional resistance (r = 0.88) thus determining a statistically significant correlation (p < 0.011).
DISCUSSION
Micro tensile tests that seek to assess the adhesive strength of different techniques and resinous and/or ceramic materials to dentin should be characterized by providing accuracy in the measurement of the bond strength at the interface between the material and the substrate to be bonded. In the present investigation, it was observed that the highest percentage of failure in the µTBS tests was the adhesive kind; this supports the reliability of the micro tensile tests by providing data that led us to identify the true adhesive strength.23,24 The obtained results indicate that it is possible to obtain a higher adhesive strength to dentin when LVU is treated with MS with 50 μm Al2O3 particles followed by 10% HF. Other materials such as Enamic and Cerasmart blocks treated only with MS and using a self-adhesive cement (G-CEM LinkAce, GC) have reported values of 40.5 Mpa;10 these results are similar to those observed in our study with LVU etched with MS plus HF (43.4 Mpa). For their part, Strasser et al.25 reported that MS with 50 μm Al2O3 particles at 2 bar air pressure increases surface roughness up to 225%. According to Tekce et al.,26 it is important to take care that the MS time with 50 μm Al2O3 does not exceed 30 s since the μTBS force decreases.
The findings obtained from HF etching coincide with those of other authors, such as the work of Duzyol et al.12 where they reported that HF mainly attacks the leucite phase of feldspathic ceramics forming small holes around the leucite crystals. Similarly, Do Amaral et al.13 reported greater dissolution of the vitreous phase and exposure of crystals in samples of Enamic CAD/CAM blocks (ceramic composite with resin matrix), LVU, IPS Empress CAD (leucite-based vitreous ceramic) and IPS e.max (lithium disilicate ceramic), observing more irregular surfaces in samples treated with 10% HF as was noted in the present investigation. The aforementioned studies help to understand the adhesive strength found with MS-treated samples and the advantage that this method offers when combined with HF etching. The SEM observation at 3000X showed that MS left a surface with cracks and grooves, with elevated and depressed areas on the surface, while the HF created micropores. Therefore, the surface treatment with HF added to the previous effect of the MS conditions to leave irregular retentive reliefs on the surface of the LVU nanoceramic resin. This irregular surface in LVU samples etched with MS and HF was superior to the roughness observed by each etching technique performed independently. The roughness found with the combination of MS and HF etching methods was confirmed with the 3D topographic representation given by the atomic force microscope which revealed a heterogeneous and irregular surface with high and low ridges in LVU samples. Based on the above, it is understandable to find that the correlation analysis established that the higher the roughness the higher the micro tensile strength. Further studies with different study methods and treatment protocols will be necessary to deepen the knowledge of the ideal bonding conditions needed in CAD/CAM blocks with different characteristics and compositions to implement clinical protocols to investigate the long-term results of these materials.
CONCLUSIONS
The nanoceramic resin LVU requires the combination of mechanical and chemical etching methods to create a rough and micro-retentive surface that along with the application of a universal adhesive increases the micro-tractional adhesive strength to dentin. Surface treatment of LVU with MS followed by 10% HF favored higher surface roughness. MS treatment using 50 μm Al2O3 particles followed by etching of specimens with 10% HF and a UA layer conditioned to a higher microtractional force than that observed with LVU etching using MS with 50 μm Al2O3 particles or 10% HF. These methods for etching nanoceramic resin may provide higher bond strength of LVU to dentin, favor an optimal marginal seal and increase the longevity of this kind of indirect restoration.
REFERENCES
AFFILIATIONS
1 Egresada de la Maestría en Prostodoncia de la Facultad de Odontología de la Universidad Autónoma de Tamaulipas.
2 Profesor de tiempo completo, Facultad de Odontología de la Universidad Autónoma de Tamaulipas.
CORRESPONDENCE
Jorge Humberto Luna Domínguez. E-mail: jhluna@docentes.uat.edu.mxReceived: Julio 2020. Accepted: Septiembre 2021.