In vitro fracture resistance of composite-resin-veneered zirconia crowns.

AIMS: The aim of this study is to investigate the fracture load to failure and damage mode of the composite resin-veneered zirconia crowns preparing with two different zirconia surface treatments compared conventional porcelain-veneered zirconia crowns.
MATERIALS AND METHODS: Metallic molar-shape dies prepared with 10° convergence angle a 1.5 mm deep chamfer finish line were used. Two groups of composite-resin-veneered zirconia crowns were prepared using different surface treatment (Group A - sandblasting and Group B - glaze-on technique). Group C (conventional porcelain-veneered zirconia crowns) was served as control. Load to failure test was performed to evaluate the fracture resistance of the crowns using a universal testing machine. One-way ANOVA was used to evaluate the differences of mean values (P < 0.05) followed by Tukey's honest significance test multiple comparisons.
RESULTS: The mean fracture load to failure of Group A was 1078.45 ± 72.3, Group B was 1215.68 ± 100.76, and Group C (control) was 1203.67 ± 88.05. Modes of failure are 100% bulk fracture of the core through the veneering materials for Group B and C. However, Group A showed 40% delamination of composite veneering leaving zirconia coping exposed.
CONCLUSIONS: Group B and C showed significant higher load to failure than Group A. Four specimens of Group A revealed the delamination of composite resin veneering.

INTRODUCTION

Yttrium-oxide partially stabilized zirconia (Y-TZP) has been widely used in dentistry due to its superior mechanical properties, and it is feasible to be used in the esthetic area. Manufacturing of Y-TZP coping should be performed with a sophisticated computer-aided designing/computer-aided manufacturing (CAD/CAM) system.[12] Y-TZP is commercially available in a partial sintered blank to reduce milling time and machine damage. A partially sintered blank is milled to form a coping structure and fully sintered to achieve the final form. A zirconia coping has the highest opacity among other all-ceramics; therefore, it requires veneering materials such as feldspathic porcelain to cover its opacity.[3] A 3-year study by Ortorp et al.[4] showed a different result in the survival rate of zirconia coping compared to alumina coping. There was no zirconia coping fractures after 3 years of service. However, four cases of veneering porcelain chipping were observed. Fracture of the veneering porcelain is the most commonly reported complication in Y-TZP-based restorations.[5] Veneer fracture rates were reported at 2%–9% for single crowns after 2–3 years and 3%–36% for FPDs after 1–5 years.[6] Another limitation of using porcelain veneering on zirconia is the excessive wear of the opposing dentition. A previous study found that veneering feldspathic ceramic group showed significantly more antagonistic tooth wear than other groups.[7]

Recently, veneering zirconia substructure with high strength indirect composite resin has been proposed as an alternative veneering method to the conventional porcelain veneering system.[8] A finite element analysis investigating the effect of various veneering material on stress distribution in implant-supported FPDs found that the reduction in stresses associated with the use of the composite resin material was 15% greater than the of porcelain or gold alloy.[9] A previous experimental study reported that the fracture loads of monolithic composite resin crowns were not significantly different to the fracture loads to the failure of monolithic all-ceramic crowns; however, the study was carried out with monolithic composite crown without zirconia substructure.[10] To veneer zirconia with composite resins, the bonding between zirconia and the composite resin is a crucial factor. Superior bond strength between veneering composite resin and zirconia framework was found in an in vitro study using a zirconia primer containing methacryloxydecyl dihydrogen phosphate monomers (MDP).[11] Another study showed the superior bond strength between zirconia coping and the adhesive composite resin cement using “glaze-on” technique. The investigators recommended using etchable feldspathic porcelain to coat the surface of zirconia.[12] Feldspathic coating was etched and silanized before applying the composite resins. The result showed a significant increase in shear bond strength between veneering composite resin and zirconia framework. Even though, many in vitro studies suggested that indirect composite application is a promising alternative to the porcelain layering technique for tooth-supported restorations, those studies were investigated in bar-shaped or disc-shaped specimens, which were not simulated to the clinical realistic. The study of the mechanical failure of composite veneered zirconia has not yet been investigated. To test the mechanical properties of the material for fabricating a crown restoration, the fabrication of specimens into crown shape can simulate more realistic because, in practice, crown failure usually occurs under complex types of stresses. Therefore, the objective of this study was to investigate the fracture load to failure and damage mode of the composite veneered zirconia crowns preparing with two different zirconia surface treatments.

MATERIALS AND METHODS

Fabrication of a metal die

An ivorine mandibular first molar was prepared for a zirconia crown with 10° convergence angle a 1.5 mm deep chamfer finish line. The prepared tooth was duplicated using polyvinyl siloxane duplicating silicone to create a mold for waxing. The heated wax was poured into the prepared silicone mold to form a wax pattern of a die. Die wax pattern was casted into a cobalt-chromium die and total 30 metal dies were fabricated.

Fabrication of a zirconia crown

Each metal die was subjected to a surface scanning process using (Q700 Scanners, 3Shape, Copenhagen, Denmark) for CAD/CAM processing. A zirconia coping with 5 mm thickness was designed to fit on each digital die using CAD software (Dental System, 3Shape, Copenhagen, Denmark). Yttria-stabilized zirconia blanks (VITA YZ, VITA Zahnfabrik, Bad Säckingen, Germany) were milled to a coping shape and fired at 1530°C temperature using a high-temperature furnace (ZYRCOMAT 600 MS, VITA Zahnfabrik, Bad Säckingen, Germany) for 2 h to achieve the final dimensions. Total 30 zirconia copings were divided into 3 experimental groups as follows: Group A, Group B, and Group C (control). For Group A, 10 zirconia copings were sandblasted 50 μm aluminum oxide particle and surface treated with a universal adhesive containing MDP-monomers (Single Bond Universal, 3M ESPE, St. Paul, MN, USA). The treated copings were hand-layered with an indirect composite resin (ENA HRi (Micerium), Synca, Le Gardeur, Canada) and light-polymerized for 120 s (Spektra LED, Schütz Dental GmbH, Rosbach vor der Höhe, Germany). The thickness of veneering composites was controlled using silicone index to achieve a consistent thickness. The complete polymerized crowns were finished and polished according to manufacturer's instruction using an ENA-HRi (Micerium) polishing kit (ENA HRi (Micerium), Synca, Le Gardeur, Canada). For Group B, the occlusal surfaces of 10 zirconia copings was fused with a very thin layer of glazing porcelain (VITA Akzent, Vita Zhanfabrik, Germany) or called “glaze-on” technique and sintered with glaze firing protocol at 850°C for 1 min to achieve a shiny surface according to the manufacturer's guidelines. A glazed surface was etched away with 5% hydrofluoric acid gel for etching ceramic restorations (CEREC® Ceramic Etch, Vita Zhanfabrik, Germany) for 60 s and silanized with ceramic primer (Clearfil Ceramic primer, Kuraray, Japan) for 10 s before veneered with indirect composite resin. The surface treated of porcelain-coated zirconia framework was hand-layered with indirect composite resin with the same protocol as mentioned in Group A. For Group C, zirconia copings were veneered with conventional feldspathic porcelain (VM9, Vita Zhanfabrik, Germany) to be served as control group. All specimens were polished and cleaned with ultrasonic cleaner before the cementation.

Zirconia crowns were cemented with self-adhesive luting cement (RelyX Unicem, 3M ESPE, St Paul, MN, USA) with the application of seating forces for 30 N. All cemented crowns were stored in distilled water at room temperature for 24 h before the mechanical testing.

Load to failure test

All cemented crowns were positioned in the center of the custom-made testing apparatus. Water was filled in the cup to the level above the top of the specimen. The specimens were subjected to load to failure test by compressive loading in a universal testing machine (Lloyd LRX-Plus, Lloyd Instruments Ltd., Fareham Hants, UK) with the 10 KN load cell at a crosshead speed of 0.5 mm/min. The load was applied to the center of the crown by a 6.5 mm diameter stainless steel ball. The load required to cause failure was recorded for each specimen.

Statistical analysis

Statistical analysis was performed using statistical software SPSS version 14 (SPSS INC, Chicago, IL, USA). One-way ANOVA and Tukey's honest significance test (HSD) multiple comparisons were used to compare the differences.

Failure mode

Failure modes of the fractured specimens were investigated using visual inspection. Two failure modes were categorized as follows: (a) chipping of veneering materials and (b) fracture of core materials and veneering materials together or total fracture.

RESULTS

Load to failure

Table 1 illustrates the descriptive statistics of the recorded load to failure for each experimental group and statistical significance among groups. Distribution of load to failure in each experimental group was shown in a box plot chart [Figure 1]. One-way ANOVA showed a significant difference of load to failure among all experimental groups (P < 0.05). Tukey's HSD detected no significant difference in mean load to failure between Group B and Group C. However, the results of Group B and Group C were statistically significantly different from Group A (P < 0.05).

Table 1

Means and standard deviations of each experimental group

Figure 1

A box plot graph represents distribution of load to failure of all experimental groups

For Group A, chipping and delamination of composite resin [Figure 2a] was observed in 4 specimens, whereas total specimens in Group B and C showed total fractures [Figure 2b]. Relative frequency of bulk fracture (%) was calculated by dividing the numbers of bulk fracture specimens by total numbers of the specimen in each group. Numbers of specimens with different failure mode are shown in Table 2.

Figure 2

Failure modes found in this study. (a) Fracture of both zirconia coping and veneering composite resin (total fracture), (b) delamination of composite-resin veneering

Table 2

Relative frequency of failure mode of all experimental groups

DISCUSSION

This in vitro study compared load to failure of zirconia-based restorations with three different veneering methods: conventional feldspathic veneering, composite resin veneering using sandblasting as a surface treatment method and composite resin veneering using the glaze-on technique. The null hypothesis was rejected due to the significant difference in load to failure of three experimental groups (P < 0.05). The means load to failure of all experimental groups were exceeded 1 KN which is higher than the reported average maximum bite forces of 450 N in the first molar area.[1314] Some study reported maximum posterior masticatory force was approximately 900 N in healthy subjects.[1516] Zirconia coping in this study were designed to have even thickness because it is easy to control standardization of the specimen. The load to failure results in this study were in the same range of load to failure from the previous study using the same design of zirconia coping and veneering layer.[17]

This study aimed to veneer zirconia coping with indirect composite resin. There were attempts to use high strength composite resin as a veneering material for zirconia substructure since it could provide functional advantages.[78] However, the proper fabrication protocol for composite veneering on zirconia coping has not been concluded. Bonding resin composite to zirconia surfaces requires proper surface conditioning. Previous studies reported that an MDP-containing primer can improve bonding of resin-based materials to the zirconia surface.[18] Sandblasting the zirconia surface followed by applying MDP primer has been reported as a successful method to improve bonding of composite repair to exposed zirconia surface.[19] In this study, sandblasting the zirconia surface followed by MDP primer was used as surface treatment of zirconia surface before applying veneering composite resin.[12] However, the result showed that this method revealed significantly lower fracture resistance compared to the glaze-on technique and conventional feldspathic veneering. The failure mode of the sandblasting group (Group A) showed 40% of adhesive failure between zirconia coping and composite veneering. This could be an explanation that inadequate bonding between two layers and poor adaptation of composite to zirconia substrate could occur resulting in low fracture resistance of the crown system.

Specimens preparing from glaze-on technique showed comparable fracture resistance to the conventional porcelain layering zirconia crown. Glaze-on technique provides clear advantages including simple laboratory technique, no required special equipment and less cost. In addition, as glaze material is silica-based, etchable and silanable surface enhances the bond strength to composite resin veneering.[12] The previous study has shown a promising result of using the glaze-on technique to enhance the shear bond strength of zirconia surface to resin composites. Furthermore, the study showed that using the powder form of glazing material exhibit higher shear bond strength than spray type of glazing material.[20] In the current study, Vita Akzent glazing power was used in Group B. As a result, mean load to failure of glaze-on group (Group B) was significantly higher than Group A. In addition, the failure mode found in Group B revealed 100% bulk fracture meaning that the bond strength between composite resin veneering and zirconia coping is relatively high enough to withstand the fracture load.

The limitation of this study was the small sample size of 10 specimens in each group. Increasing sample size could increase more reliable results. However, the standard deviations of all experimental groups were not larger than the mean value, and hence, the results were statistically acceptable. The further study requires thermocycling and cyclic fatigue to simulate aging condition to the specimens. Moreover, controlled clinical trials are necessary to evaluate the material performance in the long term.

CONCLUSIONS

Within the limitation of this study, the following conclusions can be drawn:

Group B and C showed significant higher load to failure than Group A

Delamination of composite veneering was found 40% of specimens in Group A.

Financial support and sponsorship

This study was financially supported by Thailand Research Fund and the Commission on Higher Education, Thailand.

Conflicts of interest

There are no conflicts of interest.