Comparison of Zirconia to Dentin Bonding Using Resin-Based Luting Cements and Resin-Modified Glass-Ionomer Cement: In vitro.

Introduction: The aim of this study was to compare the shear bond strengths of zirconia to dentin using two resin-based luting cements and a resin-modified glass-ionomer cement (RMGIC). Materials and
Methods: Thirty six zirconia blocks of 2 mm × 3 mm × 5 mm were milled and luted to the exposed dentin surfaces and grouped into three according to the cement used for luting: Group I - luted with Panavia F2.0, Group II - luted with RelyX U200, and Group III - luted with FujiCEM. After thermocycling, specimens were subjected to shear bond testing in an Universal Testing Machine (UTM). Data analysis using Kruskal-Wallis ANOVA and post hoc Mann-Whitney U-test with P < 0.05 was done.
Results: Mean bond strengths were of the order Panavia F2.0 (5.99 MPa)>RelyX U200 (4.79MPa)>FujiCEM (1.59 MPa). Maximum failures were at the zirconia-cement interface and were adhesive in nature. Conclusions: It can be concluded with the study that there is a better bonding of zirconia to dentin with resin-based luting cements than a RMGIC. The single-step resin-luting cement RelyX U200 produced comparable bond strength to that of a multistep Panavia F2.0. Copyright:

INTRODUCTION

Restorative dentistry is aimed to restore the teeth to proper form, function, and esthetics and achieve a conductive stomatognathic environment.[1] In spite of multiple visits and expenses, indirect restorations gained popularity due to their better proximal contacts, accurate fit, better esthetics, and clinical longevity.[23456] Zirconia (ZrO2) stabilized with yttrium oxide (Y2O3) has its mechanical properties better than other combinations. Yttrium-stabilized zirconia, also known as yttrium tetragonal zirconia polycrystal (Y-TZP), among all others is the the most preferred combination.[6789] There are clinical situations, such as an endodontic treated tooth, wherein excessive tooth reduction can compromise on the tooth strength and even retention severely. In such situations, a monolithic zirconia onlay with minimal tooth preparation can effectively protect the tooth, with the bonding tooth surface almost entirely as dentin substrate.[78910]

With the advancements in adhesive cements, preparations with even lesser tooth structure removal (minimally invasive) are advocated for enhancing the strength and survivability of the teeth. These types of restorations are called adhesively retained indirect restorations. The fracture resistance of such adhesively restored teeth was found to be comparable to natural teeth (intact teeth). However, withstanding shear forces in oral cavity are also as important as fracture resistance.[111213] In case of adhesively retained restorations where the tooth material available to bond will be minimal and the mechanical retention will be next to null, the retention of a zirconia restoration will be entirely dependent on the strength of dentin-cement-zirconia interfaces. Absence of any reactive glass phase or bondable layer in the intaglio surface makes the zirconia restorations more or less inert to any chemical bonding.[1415] With advancements in adhesive dentistry, resin cements claim better bond strength due to the presence of methacryloyloxydecyl phosphoric acid (methacryloyloxydecyl dihydrogen phosphate [10 MDP]) monomers or ester-phosphoric methacrylate in their composition. This, in addition to the surface treatments of zirconia restorations during manufacturing, was postulated to better its bond strength to dentin. Glass-ionomer cement (GIC) has been the material of choice in most of the other crowns cemented in recent times. The aim of this in vitro study was to compare the shear bond strength (SBS) between zirconia and dentin when luted using two resin-based luting cements and a traditional resin-modified GIC (RMGIC). The null hypothesis was there is no difference in SBS of zirconia to dentin when using these cements.

MATERIALS AND METHODS

Thirty-six extracted maxillary premolars were chosen for the study. The teeth were cleaned and stored as per CDC guidelines.

Tooth preparation

Each tooth was mounted on self-cure acrylic molds for better handling and testing purposes. The premolars were prepared so as to expose dentin occlusally by removing the enamel completely in the occlusal third, using high-speed air turbine handpiece (NSK, Japan) with wheel-shaped burs (Mani, Japan, number 110).

Zirconia blocks preparation

Thirty-six zirconia blocks of dimension 2 mm × 3 mm × 5 mm dimensions were milled from Sagemax zirconia blocks (Sagemax Bioceramics Inc, USA). They were sintered at 1500°C followed by sandblasting for 15 s with 50 μm aluminum oxide particles at 0.25 MPa (as done by manufacturers of zirconia crowns, to their intaglio surfaces).

Groups: n = 12

I-Luted with Panavia F2.0

Equal amounts of ED primer A and B were mixed, then applied to the prepared tooth surface. After 30 s it was air-dried. Equal amounts of paste A and B were dispensed and mixed for 20 s. This was applied to the sandblasted zirconia block which was then pressed on to the prepared tooth surface while light curing of margins for 20 s per surface (Bluephase light-emitting diodes light curing) was done.

II-luted with RelyX U200

Cement was mixed and applied to the prepared zirconia block and was placed under finger pressure. The specimen was then light cured for 20 s.

III-Luted with FujiCEM (resin-modified glass-ionomer cement)

GC FujiCEM, a resin-reinforced GIC, was used. GC FujiCEM dispenser was used to dispense material from cartridge to mixing pad and then hand mixed. Mixed paste was applied o the zirconia block and it was placed on the dentin surface with continued finger pressure till setting happens (5 min).

Zirconia blocks were under a constant finger pressure during setting and excess cement got removed. For aging, the specimens were subjected to thermocycling of 2000 thermal cycles, 5°C and 55°C in water with a dwelling time of 30 s. This was followed by shear bond testing in a universal testing machine. A mono-beveled chisel (crosshead speed of 1.0 mm/min) was applied at the adhesive interface until failure occurred. The load was recorded in Newton. It was converted to MPa by dividing with area of bonding. This calculation was done by the computerized universal testing machine and was recorded for each sample.

Following this, the specimens were observed under stereomicroscope and modes of failures were recorded. Failure modes were classified as cohesive (within the cement), adhesive at zirconia-cement interface, adhesive at zirconia-dentin interface, and mixed adhesive failures. If cement covered less than 1/8th of the zirconia or dentin surface, it was considered to be a pure adhesive failure at the respective interface.

Statistical analysis

The probability of committing type I error (α) was fixed at 5% and that of type II error (β) was fixed at 20%. Data collected were subjected to statistical analysis with P value set at <0.05 using SPSS (Statistical Package for the Social Sciences, IBM, Chicago, USA), version 19. Intergroup comparison on the data obtained was done using Kruskal–Wallis ANOVA followed by groupwise comparison using Mann–Whitney U-test.

RESULTS

Intergroup comparison using Kruskal–Wallis ANOVA showed that there is a statistically significant difference between the groups with P < 0.001. Groupwise comparison using post hoc Mann–Whitney U-test showed that there is no statistically significant difference between Group I and Group II for force or SBS. However, there is a statistically highly significant difference when comparing Group III with Group I and Group II with P < 0.000 and < 0.001, respectively [Table 1]. All the failures were adhesive in nature for all the groups [Table 2].

Table 1

Groupwise comparison of force and shear bond strength

Dependent variable	(I) groups	(J) groups	Mean difference (I−J)	Significance
Force (N)	Group I	Group II	7.11250	0.08
		Group III	26.28000	<0.000
	Group II	Group III	19.16750	<0.001
Shear bond strength (MPa)	Group I	Group II	1.20833	0.07
		Group III	4.40108	<0.000
	Group II	Group III	3.19275	<0.001

*Mann-Whitney U-test

Table 2

Modes of failure

Modes of failure	Cohesive	Adhesive at dentin	Adhesive at zirconia (%)	Adhesive mixed (%)
Group I	0	0	5 (41.66)	7 (58.33)
Group II	0	0	4 (33.33)	8 (66.67)
Group III	0	0	10 (83.33)	2 (16.67)

DISCUSSION

The present study has shown that both the resin cements bond better to zirconia surfaces. There has been overwhelming literature support on this finding.[14151617] The resin-modified glass ionomer showed lesser bond strength than both resin cements. Hence, the null hypothesis was rejected. It has been known that the shear forces are crucial in the oral cavity and can vary from 0.530N to a maximum of 39.8N. Group III (FujiCEM, RMGIC group) showed the least bond strength with a mean value of 1.5981 MPa and the maximum mean force to debond at 9.5808 N. There was a highly significant difference between this group and Group I (Panavia F2.0) and Group II (RelyX U200) in terms of mean force and mean SBS with P < 0.000, < 0.001, respectively. The analysis of the fracture modes showed that the maximum failures of this group are of adhesive nature at the zirconia-cement interface. The inability of the RMGIC to create a significant bond with the inert zirconia surface can be the reason for its poor performance. Hence, the use of an RMGIC for bonding minimally invasive adhesive zirconia restorations may not be recommended. The good clinical success reported with RMGIC was for the conventional zirconia restorations which had additional mechanical retention. This mechanical retention may have produced an enhanced result even with the weaker bonding of cement to zirconia surfaces.[11]

The two resin cements showed a better bond strength. Among them, Group I (Panavia F2.0) showed a higher mean force for debonding (35.86N) and mean SBS (5.99MPa) than Group II (RelyX U200) 24.75N and SBS of 4.79MPa [Table 1]. However, this was not statistically significant [Table 2]. The increased bonding of Panavia cement could be due to the functional monomer, 10-MDP, which is phosphate based. It reacts with collagen and hydroxylapatite crystals giving a durable bond to dentin. It has been postulated that the monomer has some affinity and ability to bond to the zirconia surfaces as well, leading to a chemical bonding.[2345678] The dentin hybridization by a separate primer application step also could have enhanced the bonding to dentin.[9] In the present study, 41.66% for Group I and 33.33% for Group II are adhesive failures at zirconia-cement interface. The adhesive mixed failures were 58.34% and 66.67% for Group I and II, respectively. There is no correlation to the mode of failures and mean bond strengths. Bottino et al. had reported 60% bonding failure to occur at cement-dentin interface and 40% as a cohesive failure, that is within the cement. In spite of reporting high bond strengths, their study had specimens failing due to aging. In the present study, there were neither cohesive failures within cement nor any failures during aging procedures. Bottino et al.[1417] defined the adhesive surface with a tape of 3.4 mm diameter hole, and a uniform load of 750 g was applied during cementation. It may have also caused the cement to have a higher film thickness than the present study, hence leading to cohesive failures. RelyX U200 (Group II) immediately after mixing is acidic and has hydrophilic properties. Acidic methacrylate monomers have many phosphoric acid groups and carbon double bonds per molecule, and a high degree of cross-linking of the resin matrix is seen. This could have enabled the cement to achieve a bond with the zirconia surface as well as to the dentin surface without any pretreatments. The earlier generation cements, RelyX Unicem Aplicap and RelyX U100 from the same manufacturer, were used in many studies. Different studies have shown these cements to be effective in bonding to zirconia as well as dentin.[714151617] The manufacturers claim RelyX U200 to be better than its previous generations in terms of bond strength, long-term stability, and esthetics. With the mean force to debond similar to that of the Panavia F2.0, this cement with reduced clinical steps showed promising bonding to both the interfaces. It is worth to note that despite the questionable bonding procedures, the zirconia crowns – conventional and adhesively bonded – have shown acceptable survival rates, similar to the bondable lithium disilicate crowns. The various limitations of the study include using finger pressure during bonding, size of the blocks used, and no occlusal contouring compared to restorations used in clinical situations. Hence, the results of this study cannot be extrapolated to clinical situations. This is an in vitro study, and the actual clinical longevity, marginal fit, occurrence of secondary caries, and other criteria need to be evaluated with a long-term follow-up of adhesive zirconia restorations cemented with these resin cements. With no statistically significant difference in bond strength and force to debond Panavia F2.0 and RelyX U200, both can be recommended for adhesive cementation of minimally invasive zirconia restorations. RelyX U200 with lesser clinical steps may be more clinician-friendly compared to Panavia F2.0. Further clinical trials with long-term follow-ups are recommended.

CONCLUSIONS

Within these limitations:

Shear bond strength of zirconia to dentin using resin cements Panavia F2.0 and RelyX U200 cements is higher than the resin-modified GIC Fuji CEM

All the groups showed adhesive failures only

Group I (Panavia F2.0) and Group II (RelyX U200) had higher mixed adhesive failures than adhesive failures at cement-zirconia interface

Group III (FujiCEM) had higher adhesive failures at cement-zirconia interface than mixed adhesive failures.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.