Effect of Polymerization Time and Thickness of Monolithic Zirconia on the Bond Strength of Self-Adhesive Resin Cement

Introduction

During the past decade, the demand for all-ceramic restorations have increased tremendously.^1,2 Even though Yttria stabilized tetragonal zirconia core veneered with glass ceramics is a suitable alternative to metal ceramic restorations, the veneering junction is found to be the weakest part of it. Hence fracture and chipping are commonly seen in such restorations.¹

Recently monolithic zirconia crowns are being widely used in dentistry. Such restorations reduce the possible chipping and fracture seen with veneered restorations. They are manufactured using computer aided design and computer aided manufacturing, can be shaded prior to sintering and have high dimensional stability and esthetics.³

Kim et al., found that yttria stabilized zirconia can be made more translucent by altering the sintering conditions and grain size. Conventional monolithic zirconium oxide crowns are more opaque. The newly developed high translucency zirconia provides improved translucency without compromising the strength. These high translucency zirconia materials show translucency similar to lithium disilicate and exhibit improved strength at clinically recommended thickness similar to lithium disilicate.²

Resin based cements are favorable luting agents for zirconia restorations since they are highly retentive, less soluble and has high wear resistance. They can provide better bond strength and good marginal adaptation.² The use of resin cements can also help to reduce the irregularities on the inner surface of zirconia restoration created due to surface treatments like sandblasting, thereby increasing its mechanical strength. The clinical longevity of the indirect restoration depends upon the adhesiveness of the resin cements to the tooth structure.⁴

Self-adhesive resin cements which do not require any surface pretreatments are one of the recent modifications. In order to obtain optimal polymerization in deeper areas, dual polymerizing systems of resin cements are widely adopted. The bond strength between the resin cement, dentin and the ceramic material depends on the degree of conversion of the cement during polymerization.

Increased thickness of ceramic material may influence the light transmittance, cause a curing light attenuation and thereby affect the quality of light reaching the bottom layers of cement. Curing light attenuation can reduce the degree of conversion and thereby the bond strength, which can be resolved by using monolithic translucent zirconia which has superior aesthetic quality and improved transmission of light.⁵

The purpose of this study was to determine the optimal curing conditions in order to maximize the bond strength by varying the polymerization time of self-adhesive resin cement and thickness of monolithic translucent zirconia.

Materials and Methods

This in vitro study was conducted in the Department of Prosthodontics, The Oxford Dental College, Bangalore. Pre-formed polytetrafluoroethylene moulds with a hole in the center was used to fabricate cylinders made of composite resin. The moulds were of 6 mm diameter and 3 mm height. Composite resin was placed into the moulds and was condensed evenly without contaminating the material (Figure 1). Fifty-two cylinders were made which were cured with multi LED lamp for 20 seconds each. 

After curing, they were carefully removed from the mould and the dimensions were checked with a digital Vernier calipers. Any defective samples were discarded. The samples were stored in a dark box for seven days at 37^o C.

Fifty-two pre-sintered zirconia discs each of diameter 10 mm were milled from commercially available highly translucent monolithic zirconia blocks. Twenty-six discs of 1.5 mm thickness each were grouped into group A. Twenty-six discs of 2 mm thickness each were grouped into group B. Group A was subdivided into A1, A2 of 13 discs each. Group B was subdivided into B1, B2 of 13 discs each. One side of the zirconia specimens were air abraded with 50 micron alumina for 20 seconds at 60 psi pressure, according to the manufacturer’s instructions.

After sandblasting, all the samples were ultrasonically cleaned with distilled water for 15 minutes and air dried for one minute. The dimensions of the disc were checked with a digital Vernier calipers and any defective samples were discarded.

After seven days of storage of composite cylinders, they were cemented on to the zirconia discs using selfadhesive resin cement (Figure 2). Self-adhesive resin cement was placed onto the composite cylinder and was cemented on to the zirconia disc. The samples were then subjected to a force of 40 N for two minutes under a compressive load applied to the luted samples in order to standardize the luting pressure using the Micro Universal Testing machine. The excess cement was removed using an explorer.

Shear bond testing

Shear load was applied at a constant cross-head speed of 0.5 mm/minute until failure. Maximum force until failure was recorded, shear bond strength was calculated and was expressed in Mpa using the following formula:

Shear bond strength (Mpa)= F/ A

where, F = Force in Newton, A = surface area = πr2, π = constant= 3.14, r = r adius of the luted surface

Statistical analysis

Data were entered into Microsoft excel and analyzed using SPSS software 20. Categorical variables were expressed in frequency and percentage and continuous variables were expressed in mean and standard deviation. One way ANOVA (Analysis of variance) test was used to find any difference among groups. P value <0.05 at 95 percentage confidence interval was considered as statistically significant. To do pair wise comparison among study group, Tukey’s Post hoc comparison was applied. Final results were presented in the forms of tables and suitable graphs.

Results

Load at break for each specimen was procured from the load-deflection curve that was obtained from the shear bond strength in an UTM. Using this value, shear bond strengths of all the specimens of groups A1, A2, B1, and B2 were calculated and tabulated (Table 1).

The test results show the mean shear bond strength between the four groups. The mean shear bond strength for Group A1 was 4.86±0.551, Group A2 was 6.29±0.862, Group B1 was 4.38±0.365, Group B2 was 6.03±0.811. This difference in the mean shear bond strength was statistically significant at P <0.001. This infers that highest value of shear bond strength was shown by Group A2, followed by Group B2, then Group A1. The least shear bond strength was shown by Group B1 (Table 2).

Table 3 shows multiple pair wise comparison of mean shear bond strength between the four groups. The mean difference in shear bond strength between Group A2 and Group A1 was 1.4276923, between Group B1 and Group A1 was -0.4753846, Group B2 and Group A1 was 1.1676923, Group B1 and Group A2 was -1.9030769, Group B2 and Group A2 was -0.26, Group B2 and Group B1 was 1.6430769. Groups A1 and A2, B1 and A2, A1 and B2 and B1 and B2 showed statistically significant shear bond strength values as compared to other groups at P <0.001. Multiple pair wise comparison of shear bond strength between Groups A2 and B2, A1 and B1 did not show statistically significant values with P >0.001. This infers that there was a significant increase in shear bond strength values when comparing groups A1 and A2, B1 and A2, A1 and B2 and B1 and B2, but the increase in shear bond strength between groups A2 and B2, A1 and B1 was not much significant.

Discussion

Even though conventional zirconia is an esthetic material of choice, it is opaque due to the typical grain size (approximately 0.4 μm) compared with the wavelength of light (approximately 0.1-0.7 μm) and the resultant refractive index mismatch. It results in the light being scattered rather than being transmitted.

In order to increase the translucency, manufacturers have come up with strategies which are aimed to match the refractive indices of the grain particles. These include increasing the grain size, reducing the impurities, increasing the sintering temperature and yttria content, thereby increasing the translucent cubic phase.

The bond strength of resin cement used to lute the ceramic materials plays a crucial role in the success of the prosthesis. Some of the ways to improve the bond strength includes air abrasion with aluminum oxide particles, plasma processing, silica infiltration by the sol-gel technique, tribochemical silica coating, subsequent use of a silane agent, application of resin cement which contain Methacryloyloxydecyl dihydrogen phosphate (MDP) monomer, selective infiltration-etching method, glaze on technique and heated silane.⁶ Dual cure resin cements are a popular choice for cementation of such restorations.

In the present study, surface treatment of highly translucent monolithic zirconia specimen was carried out by air abrasion with 100-micron aluminum oxide particles at a distance of 10 mm at 60 psi pressure, based on the manufacturer’s recommendations for the cementation of the self-adhesive resin cement used in the study.⁷ Unlike feldspathic porcelain and lithium disilicate, it is difficult to obtain a desirable mechanical retention in this way while using zirconia.⁸ The degree of conversion and rate of polymerization of resin cement also plays a major role in such scenarios.⁹

Recently developed self-adhesive resin cements do not require this pretreatment which helps to reduce the chairside time, technique sensitivity, thereby minimizing the procedural errors. Self-adhesive resin cements contain multifunctional phosphoric acid methacrylates which react with the hydroxyapatite and provides a considerably good bond strength.¹⁰

Cylinders made of composite resin were used as the substrate to cement the zirconia discs in the present study. The composite resin material used in the study is Vivoclar Teconom Plus, which is a hybrid composite material. The elastic modulus of hybrid composites ranges from 15-20 Gpa which is comparable to dentin (18 Gpa). Hence in the present in vitro study, hybrid composite resin was chosen as the substrate for bonding the highly translucent monolithic zirconia blocks using the self-adhesive resin cement.¹¹

Rate of polymerization of resin cement depends on the intensity of light passing through the zirconia. Difference in thickness of zirconia used can affect the intensity of light passing through and thereby reduces the overall energy of incident light in deeper layers.^12,13 By dual polymerization, the portion of resin cement that may receive inadequate irradiance will continue to polymerize with a delayed chemical reaction. If this chemical reaction cannot compensate for the curing light attenuation that occurs, it may result in lower degree of conversion, lower hardness of cement, which can eventually lead to early adhesive failures, microleakage, postoperative sensitivity and recurrent caries.¹⁴

Studies have shown that when increased thickness of zirconia is used, there could be a curing light attenuation.^12-14 Many authors have suggested that curing light attenuation can be related to factors like thickness, shade and type of the material.¹⁵ The opacity of zirconia restoration is one of the factors that adds to this curing light attenuation. In order to overcome this, the use of a highly translucent zirconia can be recommended. Increasing the curing time can also be one of the ways to counteract the curing light attenuation.⁹

In the present study, by increasing the curing time, the shear bond strength of self-adhesive resin cement also increased. There was a significant increase in the shear bond strength of resin cement when the curing time was increased from 10 seconds to 20 seconds. An in vitro study done by Alovisi et al. concluded that increasing the curing time could increase the bond strength of Rely X Ultimate self-adhesive resin cement.⁹

Adequate duration of light exposure will result in higher degree of conversion and thereby adequate bond strength. However, it is not desirable to increase the exposure time beyond limits to compensate for any loss of energy since it may result in generation of excess heat and gingival tissue damage.¹⁶ In the present study, manufacturer’s recommended curing time of 10 seconds was compared with an increased curing time of 20 seconds which was in the acceptable limit of irradiation.

For the shear bond strength testing, a force F is applied to the specimen at a constant distance from the interface parallelly. The load at failure is noted and is divided by the surface area of the bonding to determine the shear bond strength values.

In the present study, One-way ANOVA was used to compare the mean shear bond strength between different thicknesses of highly translucent zirconia cured at two different timings. The highest shear bond strength values were observed in Group A2 and the least shear bond strength was noted in Group B1. Multiple pair wise comparison between the different groups was done using Tukey’s Post hoc test. It was found that by increasing the curing time for the same thickness of highly translucent zirconia specimens, the bond strength increased significantly (P <0.001).

A study done by Lee et al. also showed that by increasing the curing time from 20 seconds to 40 seconds, self-adhesive resin cement showed a statistically higher shear bond strength value. These results support the findings of the current study.⁸

Multiple pair wise comparison of shear bond test values demonstrated increased bond strength with reduced thickness of highly translucent zirconia. Increased bond strength was noted for 1.5 mm thickness, for 10 seconds and 20 seconds curing time, when compared to 2 mm thickness. But the reduction in bond strength values was statistically insignificant when the thickness of highly translucent zirconia was increased. Even though there is a reduction in shear bond strength values, with the increase in thickness of zirconia specimens, this reduction was statistically insignificant (P>0.001).

Highly translucent zirconia when compared to conventional zirconia is more translucent and hence does not cause much attenuation of the incident curing light. Church et al., in their study to assess the translucency of highly translucent monolithic zirconia ceramic materials that are recently introduced concluded that at clinically recommended minimum thicknesses, highly translucent zirconia materials are as translucent as lithium disilicate.² Pick et al., stated that the difference in transmittance among the different restorative materials would significantly influence the degree of conversion of resin cements.¹⁷ The shear bond strength values are dependent on the degree of conversion of the cement; hence the use of highly translucent zirconia can compensate for the curing light attenuation.

The limitations of the study are, being an in vitro study, the study does not simulate the intra oral conditions. This study was conducted at room temperature while the intraoral temperature may affect the resin cement mechanical properties. The effect of aging on shear bond strength was also not considered in the present study.

Conclusion

Within the limitations of the study, the following conclusions were drawn. The polymerization time has a significant effect on the shear bond strength values of self-adhesive resin cement. On increasing the curing time within the limits, the shear bond strength increases. Even though varying the thickness of super highly translucent monolithic zirconia affected the bond strength of self-adhesive resin cement, there is no considerable reduction in shear bond strength values. Highest shear bond strength is obtained by increasing the polymerization time with reduced thickness of monolithic zirconia. By using super highly translucent monolithic zirconia restorations, the possible curing light attenuation can be reduced which results in better shear bond strength even with increased crown thickness. Highly translucent zirconia luted with self-adhesive resin cement produces promising results with long term clinical success.

Acknowledgement

This study is self-financed.

Conflict of interests

None