Shear bond strength of computer-aided design and computer-aided manufacturing feldspathic and nano resin ceramics blocks cemented with three different generations of resin cement.

AIM: To evaluate the shear bond strength between the dentin substrate and computer-aided design and computer-aided manufacturing feldspathic ceramic and nano resin ceramics blocks cemented with resin cement.
MATERIALS AND METHODS: Sixty cuboidal blocks (5 mm × 5 mm × 5 mm) were fabricated in equal numbers from feldspathic ceramic CEREC(®) Blocs PC and nano resin ceramic Lava™ Ultimate, and randomly divided into six groups (n = 10). Each block was cemented to the dentin of 60 extracted human premolar using Variolink(®) II/Syntac Classic (multi-steps etch-and-rinse adhesive bonding), NX3 Nexus(®) (two-steps etch-and-rinse adhesive bonding) and RelyX™ U200 self-adhesive cement. All specimens were thermocycled, and shear bond strength testing was done using the universal testing machine at a crosshead speed of 1.0 mm/min. Data were analyzed using one-way ANOVA.
RESULTS: Combination of CEREC(®) Blocs PC and Variolink(®) II showed the highest mean shear bond strength (8.71 Mpa), while the lowest of 2.06 Mpa were observed in Lava™ Ultimate and RelyX™ U200. There was no significant difference in the mean shear bond strength between different blocks.
CONCLUSION: Variolink(®) II cement using multi-steps etch-and-rinse adhesive bonding provided a higher shear bond strength than the self-adhesive cement RelyX U200. The shear bond strength was not affected by the type of blocks used.

INTRODUCTION

Good cement should provide sufficient bond strength to maintain the restoration in position even at high masticatory load.[1] Various types of cements are available which include total-etch resin cement that are a multi-step adhesive system. They are technique sensitive and time-consuming.[23] In the attempt to simplify the application procedures, the self-etching adhesive system was developed in which the smear layer is preserved and altered.[4] With this adhesive system, primer and bonding agent can be applied to the tooth without prior etching and rinsing the tooth.[15] According to the manufacturer, NX3 Nexus® cement is a dual-cure resin cement that can be used with both as a total-etch and self-etch adhesive.

New generations of self-adhesives cement, such as RelyX™ U200 is capable of self-etching and bonding to dentine, without the need for separate etching or priming, are easier to handle and less technique sensitive.[67] However, a study reported that self-adhesive cement were not able to demineralize/dissolve the smear layer completely; hence the hybrid layer was not fully formed.[8]

Advances in computer-aided design and computer-aided manufacturing system (CAD/CAM) and increased use of tooth-colored restorations are also driving researchers to develop new restorative materials. CAD/CAM crowns can be generated from ceramic blocks such as leucite-reinforced glass ceramic, lithium disilicate glass ceramic, and conventional feldspathic blocks. Indirect resin composite restoration can also be produced with this system using the prefabricated composite blocks (paradigm MZ100, 3M ESPE, St. Paul, MN, USA). Resin nano ceramic (Lava™ Ultimate, 3M ESPE, St. Paul, MN, USA) is a recently introduced unique CAD/CAM block based on the integration of nanotechnology and ceramics. This nano resin ceramic material is purported to offer the ease of handling of a composite material with the surface gloss and finish retention similar to porcelain. Additionally, crowns made out of these materials are expected to be less abrasive compared to ceramics to their opposing teeth. It contains a blend of three fillers: Zirconia and silica nanoparticles agglomerated into clusters, individually bonded silica nanoparticles and individually bonded zirconia nanoparticles.

Ceramics are biocompatible, esthetic, have high compressive strength, good shade stability and also have high abrasion resistance.[29] However, they have low tensile strength and unable to withstand functional force in the oral cavity.[9] Recently developed nano resin ceramic block is believed to be the product with upgraded characteristics.

However, the literature is unclear on which cements produce the highest bond strength. Furthermore, nano resin ceramic blocks, and self-adhesive resin cement RelyX™ U200 are new in the market, and study on them are scarce. The objective of the study was to compare the shear bond strength value of three resin cements when used to cement CAD/CAM feldspathic ceramic and nano resin ceramic to human premolar dentine structure.

MATERIALS AND METHODS

Sixty extracted, noncarious permanent human premolars were collected following ethical approval of the institution. The teeth were cleaned with chlorhexidine, scaled using ultrasonic scaler and stored in normal saline solution at room temperature. Dentin was exposed by sectioning the crown perpendicular to the long axis of the tooth using Diamond disc (Exakt hard material cutter, Germany) with water cooling. All dentin specimens were mounted in cold-cure acrylic.

Feldspathic ceramic CEREC® Blocs PC, shade S2, size 12, (VITA Zahnfabrik Germany) were cut into 30 blocks (5 mm × 5 mm × 5 mm) Batch Number 18060 and nano resin ceramic blocks, Lava™ Ultimate CAD/CAM Restorative for CEREC Blocs, size 12, shade A2 (3M ESPE Dental, St. Paul, MN, USA) Batch Number 34870783270 were also cut into 30 blocks (5 mm × 5 mm × 5 mm) using Exakt hard material cutter (Germany), and were polished using soflex discs (3M ESPE, St. Paul, MN, USA). All 60 blocks surfaces were treated with 9% Buttered Hydrofluoric Acid (Porcelain Etch, Ultradent Products, Inc.) and Silane (Ultradent Products, Inc.) before cementation.

Cementation procedure

Block randomization was used to divide the prepared teeth into six groups (n = 10). For teeth to be cemented with RelyX™ U200 (3M ESPE Dental, St. Paul, MN, USA) Batch Number 499698, no treatment was done on the dentine surface (Groups A and D). For teeth to be cemented with NX3 Nexus® (Kerr Corporation, USA) Batch Number 4529260 (Groups B and E), dentine surface was etched with 37% phosphoric acid (Batch Number S04037, Ivoclar Vivadent, Liechtenstein) for 15 s, rinsed with water spray for 15 s and gently air-dried. Then, Optibond Solo Plus (Batch Number 4702096, Kerr Corporation, USA) was applied on the tooth surface and light-cured for 15 s with an light-emitting diode (LED) light curing unit, Elipar Free Light 2 (3M ESPE, Germany). For teeth to be cemented with Variolink® II/Syntac Classic (Ivoclar Vivadent, Liechtenstein), Batch numbers for base and catalyst S02602, R71808 (Groups C and F) dentine surface was etched with 37% phosphoric acid for 15 s, rinsed with water spray for 15 s, and gently air-dried. A layer of Syntac primer (Batch Number S00833, Ivoclar Vivadent, Liechtenstein) was applied and again gently air-dried. Syntac adhesive (Batch Number S07235, Ivoclar Vivadent, Liechtenstein) was then applied and gently air-dried. Heliobond (Batch Number S04976, Ivoclar Vivadent, Liechtenstein) was finally applied and light-cured for 15 s with the same LED light curing unit). All blocks were cemented to the teeth according to manufacturer's recommendation and light-cured for 60 s from different directions.

All specimens were thermocycled for 500 cycles at 5°C and 55°C, dwell time 30 s. All cemented specimens were stored in normal saline solution at room temperature until testing. Specimens were mounted on the jig of a universal testing machine (Instron 8874, UK) and shear force at a crosshead speed of 1.0 mm/min was applied to the adhesive interface until fracture occurred. The calculated shear bond strength were determined by dividing the strength at which bond failure occurred by the bonding area.

The data were analyzed using one-way ANOVA using PASW statistical software package, version 20 (IBM, SPSS Inc., USA) for Windows. Differences were considered statistically significant when the P value was <0.05.

RESULTS

Table 1 shows that there is a statistically significant difference in the mean shear bond strength values between CAD/CAM feldspathic ceramic and nano resin ceramics blocks cemented with three different generations of resin cement (P = 0.000).

Table 1

Comparison of mean (SD) of shear bond strength (Mpa) between CEREC® Blocs PC and Lava™ Ultimate blocks cemented with RelyX™ U200, NX3 Nexus® and Variolink® II

Group C shows the highest shear bond strength of 8.71 Mpa, followed by Group F with the value of 7.28 Mpa. The lowest shear bond strength values were observed in Group D with the value of 2.07 Mpa, followed by Group A with the value of 3.18 Mpa.

Table 2 shows that there is a significant difference in mean shear bond strength between Group A and Group C (P = 0.002), Group A and Group F (P = 0.045), Group C and Group D (P = 0.000), Group C and Group E (P = 0.001), Group D and Group F (P = 0.004), as well as between Group E and Group F (P = 0.034).

Table 2

Multiple comparison test (Scheffe) between groups

DISCUSSION

The techniques of application of three different resin cements in the current study represent the different adhesive system in term of the number of steps of application. Variolink® II cement uses a multi-step etch-and-rinse technique in which etching and rinsing, application of primer and application of bonding agent applied step by step. Nexus® NX3 used a two-step etch-and-rinse technique in which etching and rinsing were done on the teeth surface prior to the application of primer and bonding agent that are combined in one step. While RelyX™ U200 is a self-adhesive resin cement that does not require pretreatment of the tooth surface. According to the manufacturer, this self-adhesive cement consist of phosphoric acid modified methacrylate monomers that can mineralize the dentine due to its acidity and at the same time the resin infiltrates the dentine matrix forming hybrid layer with resin tags without prior removal of the smear layer. In other words, it modifies the smear layer to facilitate the formation of bond instead of removing it.[4]

In the current study, there is a statistically significant difference between groups as determined by one-way ANOVA F ( P = 0.000). According to the post-hoc test, there are significant differences observed between the groups of teeth that were cemented with Variolink® II and RelyX™ U200 cements regardless of the type of blocks used. The teeth cemented with RelyX™ U200 self-adhesive resin cement show a significantly lower shear bond strength compared to teeth cemented with Variolink® II cement. The reason of lower shear bond strength demonstrated in the groups cemented with self-adhesive resin RelyX™ U200 might be due to an inadequate dentine demineralization which affected the formation of resin tags in the hybrid layer.[1] The resin tags that formed in the etch-and-rinse adhesive were more frequent and longer than those in the samples bonded with self-etching adhesive.[4] The resin tags formed in both etch-and-rinse adhesive and two-step self-etching adhesives showed conical swelling at their base and numerous lateral branch of microtags that extend from the main resins tags that contribute to proper dentine infiltration.[4] However, the resin tags formed in the all-in-one adhesive were wide, short and funnel shaped with no lateral branches.[4] From this discovery, it shows that the etch-and-rinse adhesive system is still better in terms of the formation of hybrid layer compared to the self-adhesive all-in-one resin cement. The result of the current study was supported by a previous study.[11] Another study also stated that the etch-and-rinse adhesive system provided the best overall bonding performance compared to the all-in-one adhesive.[12] According to the manufacturer, the adhesive mechanism of self-adhesive resin cement depends on both micromechanical retention and chemical interaction between the acidic monomers and residual hydroxyapatite. Functional monomers interact with calcium ion from the residual apatite crystallites within the partially demineralized hybrid layer to form an insoluble calcium salt that may contribute in bond formation between resin and dentine.[13] However, there is no extension of resin tags from resin cement and formation of the interlocking structure were observed in self-adhesive resin cement used in the previous study.[11]

The idea of the total-etch adhesive system is that a proper resin-dentine interdiffusion zone which is also known as hybrid layer should be formed to achieve a good bonding system.[1415] In this system, the smear layer is removed by etching and rinsing to expose the collagen fibers that are deprived of hydroxyapatite and dentine tubule to enhance penetration of the resin.[14] The primer with its hydrophilic end will facilitate the penetration of resin into the moist dentine surface while its hydrophobic end will form a bond with a bonding agent. Unlike the etch-and-rinse adhesives system, the smear layer in the recent generation of self-etching adhesive or cement which are self-adhesive will be preserved and altered. There will be formation of a hybridized complex which is made up of a modified smear layer above the true hybrid layer.[4] Despite the attempts to develop a dental resin adhesive system with the simpler application procedure, the bonding performance by the multi-step etch-and-rinse adhesive system is still superior to another type of adhesive system.

Another variable considered in this study was the use of different type of ceramic blocks. In the current study, both type of blocks which are CEREC® Blocs PC and Lava™ Ultimate were treated by hydrofluoric acid followed by application of silane. However, there was no significant difference in mean shear bond strength between groups that used CEREC® Blocs PC or Lava™ Ultimate regardless of the cements used. These results suggest that shear bond strength of the resin cements were not affected by the type of block used.

In the present study, all resin cements exhibited shear bond strength values less than the suggested minimum for lithium disilicate-reinforced CAD/CAM ceramic materials, which should not be <10–12 MPa for clinical service in the oral cavity.[16] These low bond strength values may be explained by the formation of siloxane bonds between the silanol and hydroxyl groups in the ceramic, a process that is accelerated by acid catalysis.[1718] Therefore, the acidic functional components of the resin cements may not provide enhanced siloxane bonds, resulting in decreased bond strength values.[18]

Previous study shows that not only cements, other factors including substrates which are the restorative materials, and setting reaction as well as all their interactions had a significant effect on the bond strength.[19] To simulate the clinical situation, artificial/accelerated ageing by water storage and thermal cycling are commonly used ageing procedures. The thermal changes may induce a reduction of bond strength.[20] However, the results are most likely not directly comparable to the clinical situation, and, therefore, further studies with clinical geometries are needed.

CONCLUSIONS

Variolink II cement using multi-step etch-and-rinse adhesive bonding that require separate etching, priming and bonding, provided a higher shear bond strength than the self-adhesive cement RelyX U200. The shear bond strength was not affected by the type of blocks used.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.