Influence of veneer application on fracture behavior of lithium-disilicate-based ceramic crowns

Abstract

Objectives

To assess the influence of veneer application on fracture behavior, namely failure load and failure mode, of standardized lithium-disilicate-based crowns.

Methods

Forty molar crowns (IPS e.max Press, IvoclarVivadent) were fabricated in full anatomic (without veneer, 1.5–2.0 mm at occlusal surface) and bi-layer (the occlusal surface is 0.7 mm of veneer and 0.8–1.3 mm core) contour representing two groups. Crown specimens were seated and adhered on composite resin dies. All specimens were loaded with a 6 mm diameter steatite sphere over the central fissure to failure. Failure modes and fractographic patterns were analyzed by optical stereo and scanning electron microscopy (SEM). Fracture loads of the two groups were compared by the t -test, while the failure modes were analyzed by Pearson Chi-square test.

Results

There was a statistically significant difference in mean fracture load values (N ± S.D.) between full anatomic [(2665.4 ± 759.2) N] and veneered crowns [(1431.1 ± 404.3) N] ( p < 0.001) and also in failure modes ( χ 2 = 6.465, p = 0.011). Full anatomic crowns mainly showed bulk fracture, whereas veneered specimens predominately showed cohesive veneer and ceramic interface failure (75%); solely cohesive veneer failure (20%); and bulk fracture (5%).

Significance

Within the limitations of this study, veneer application resulted in significant lower fracture load values compared to full anatomic crowns. Fracture initiated from occlusal fissures near the load application site. A combination of cohesive veneer and ceramic interface failure represents the main failure mode of lithium-disilicate-based bi-layered crowns, whereas full anatomic crowns failed mainly from ceramic bulk fracture at the occlusal fissures.

Introduction

All-ceramic restorations offer considerable advantages because of their biocompatibility and esthetic aspects. Current ceramic core materials are considered sufficiently strong to produce reliable monolithic full anatomic all-ceramic restorations . Although several opacities and translucencies have been developed for pressable glass ceramic systems, ceramic cores are generally veneered with weaker porcelain to achieve optimized esthetics. The lower intrinsic strength of veneering porcelain may still determine the longevity in spite of a strong substrate, as the flexural resistance of a bi-layered structure is dependent upon the veneering external layer of the structure. Delamination and chipping of veneering porcelain from the underlying ceramic substrates have been reported for bi-layered all-ceramic restorations. According to long-term clinical studies, the observed chipping related fracture rates of bi-layered zirconia fixed dental prostheses (FDPs) varied from 15% after 24 months , 25% after 31 months to 8% and 13% after 36 and 38 months . Sailer et al. reported that minor chipping of the veneering ceramic was found in 25% of the zirconia–ceramic and 19.4% of the metal–ceramic FDPs in a 3-year follow-up randomized controlled clinical trial. On the contrary, according to a 10-year clinical study, lithium-disilicate-based bi-layered crowns revealed comparatively low chipping and core fracture rates at 1.5% and 0.8%, respectively . Lithium-disilicate-based bi-layered three-unit FDPs showed similar results in an 8-year clinical study . However, conventional porcelain-fused-to-metal FDPs exhibited substantially lower veneer fracture rates ranging from 2.7% up to 5.5% for observation periods from 10 to 15 years .

As esthetic veneering material is weaker compared to the core material, it fails at lower loads under tension. Cracks may also originate from the interface between the core and the veneer, as well as from the free surface of the veneer. Aboushelib et al. analyzed 19 clinically fractured all-ceramic crowns, in which 10 specimens failed by chipping of the veneering ceramic from an intact core structure. For the other 6 crowns, the crack origins were located at the veneer–core interface. According to ISO 6872 and 9693 standards, a minimum flexural strength of 50 MPa for veneering ceramic is required . In previous work, the fracture strength of a variety of widely used veneering ceramics was evaluated and was found to be approximately 90 MPa. However, from a fracture mechanic perspective of material failure, flaw size is a critical structural variable that determines the strength . As a consequence the conventional layering and sintering process with veneering materials is more technically sensitive. The variability due to the individual build-up and firing procedures could cause structural flaws. There are various types of structural flaws which may be located at the surface, in the bulk of the material, or at the veneer–core interface, and lead to stress concentration as well as act as fracture initiation sites . Strength is influenced by flaws such as gaps, voids and bubbles, and therefore it is not an intrinsic material property .

In addition, the bond strength between core and veneering material has been evaluated in previous studies. One interesting finding was, despite a substantially lower fracture strength (300 MPa) of a glass ceramic material (e.g. IPS Empress II system) the shear bond strength with veneering ceramic (44 MPa) was superior to that of zirconia ceramic (17–41 MPa ). A study by Al-Dohan et al. showed that IPS Eris veneering porcelain applied to IPS Empress II exhibited the highest shear strength value for different bi-layered all-ceramic systems, which was not significantly different from the metal–ceramic used as a control group. Consequently, the initial strength of veneering ceramic and the reliability of veneer–core interface are key factors for the successful performance of bi-layered restorations . But whether this phenomenon is merely an inherent weakness of the veneering porcelain and the interface, or whether a fracture is initiated by structural flaws within the bi-layer composite, needs to be addressed under standardized in vitro conditions.

The aim of this study was to investigate the fracture behavior of lithium-disilicate-based ceramic crowns using laboratory load-to-failure test of standardized specimens and fractographic morphology analysis. The study involved ball loading of molar crowns with or without a veneer glaze firing layer. The null-hypothesis is: there is no significant difference of failure load and failure mode between full anatomic and bi-layer lithium-disilicate-based ceramic crowns, observed in the laboratory.

Materials and methods

Sample preparation

A maxilla first molar model tooth (A5A-500-#16, Nissin, Japan) with average dimension, chosen as a master tooth, was prepared ( Table 1 ). All undercuts were eliminated by axial reduction of 1.5 mm, while the palatal occlusal surface was reduced by 2.0 mm, the buccal occlusal surface was reduced by 1.5 mm, both with functional cusp bevels, and 2.0 mm in the deepest points of the main fissures. All sharp angles were rounded after a 1.0 mm deep chamfer finish line and a 6° convergence angle between tooth axis and lateral wall was prepared. An impression of the finished master die was taken using a polyvinylsiloxane impression material (3M ESPE, USA). Then 2.0 mm layers of a fine hybrid composite resin (Z100 A3, 3M ESPE, USA) were filled into the impression and subsequently light-cured for 40 s (EliparFreelight2, 3M ESPE, USA). Before cementing the crowns, surfaces of the prepared samples were roughened by sandblasting from a distance of approximately 5.0 mm, at 2 bar with 120 μm glass beads. All resin dies were stored in 37 °C distilled water for 30 days.

Table 1
Basic dimension of the crown specimens.
Basic dimension Full anatomic crown Bi-layered crown
Occlusal surface preparation (mm) 2.0 2.0
Axial wall preparation (mm) 1.5 1.5
Chamfer depth (mm) 1.0 1.0
convergence angle (°) 6 6
Core thickness (mm) 2.0 Buccal cusp: 0.8
Palatal cusp: 1.3
Veneer thickness (mm) 0 0.7

Fabrication of crowns

The anatomically correct crowns were fabricated using the lost wax casting technique generated from impressions of the standardized model tooth. Veneered crowns were fabricated with the cut-back technique of the standardized wax crowns. The full contour of the wax crown was reduced by 0.7 mm from the occlusal surface, as well as the coronal 1/3 of the axial walls, providing enough space for veneering material. Subsequently, wax crowns of the two groups were invested (SpeedVest, IvoclarVivadent, Liechtenstein). Wax was removed by heating and the resultant void was filled with pressable material (IPS e.max Press, LT A1, IvoclarVivadent, Liechtenstein). After the heat pressing procedure, the crowns were divested and sandblasted with 120 μm glass beads at a pressure of 2 bar.

All 20 full anatomic crowns were double glazed with a universal glaze (IPS e.max Ceram Glaze Paste, IvoclarVivadent, Liechtenstein), which were fired in a furnace (P60, IvoclarVivadent, Liechtenstein) at a temperature of 725 °C. Specimens in the other group were veneered with low-fusing dental porcelain (IPS e.max Ceram, IvoclarVivadent, Liechtenstein) using conventional sintering procedures according to the manufacturer’s instruction. One major goal of the study was to fabricate exact duplicates of the veneered crowns. Consequently, three impressions of the master mold of intact maxilla first molar were made using a putty silicone material. One (working mold) was used to compact the veneering material in order to form the standardized geometry. The other two (checking mold) were cut from mesial side to distal side and labial side to palatal side respectively, serving as a guide for substructure veneering. Following sintering, the space left by shrinkage was filled with a second layer of veneering material. Each specimen was placed in the checking mold to ensure correct dimension of the crown shape. After the second firing cycle, the thickness of the veneering porcelain was evaluated, at 7 defined points (4 points on cusps, the other 3 on fissures), to be 0.7 mm at the occlusal surface. The surface of the veneered crowns were then polished and glazed.

Luting of the crowns

The inner surfaces of all crown specimens were etched with 4.5% hydrofluoric acid (Ceramic Etch Gel, IvoclarVivadent, Liechtenstein) for 20 s and silanized (SE Bond, Clearfil Repair, Kuraray, Japan) for 60 s. A luting agent (Panavia F, Kuraray Medical, Japan) was applied to the inner surfaces of the crowns, which were seated to the resin dies under firm finger pressure. Excess of luting material was subsequently removed. The cemented specimens were immediately subjected to a constant static load of 50 N for 10 min. The excess bonding agent was removed by a brush and a thin layer of anti-oxygen seal (Panavia F, Kuraray Medical, Japan) applied to the margin of the crown. The luting agent was light-cured from five directions (EliparFreeligh2, 3M ESPE, USA), the anti-oxygen layer was removed. Finally, all specimens were stored in distilled water at 37 °C for 24 h prior to testing.

Fracture testing

All specimens were mounted in a universal testing machine (Z010, TN2S, Zwick, Ulm, Germany) and subjected to single load-to-failure test. Failure was defined by bulk fracture of the all-ceramic core material for the full anatomic crowns, while in the veneered group, failure was defined by delamination and/or cohesive fracture within the veneering material. A 6 mm diameter steatite ball (Höchst Ceram Tec, Wunsiedel, Germany) was placed over the central fissure of the specimen to establish a three-point contact. A tin foil (0.2 mm thick, DT Dental Trading, Bad Kissingen, Germany) was inserted between the loading sphere and the crown in order to reduce peak stresses at the contact points. The loading speed was 0.5 mm/min and fracture load recorded.

Fractographic morphological observation

All specimens were examined under a stereomicroscopy (Stemi 2000 CS, Carl Zeiss, Jena, Germany) with low-power (50×) stereo-magnification. After sputter coating with a thin film of gold, fractographic analysis using a scanning electron microscopy (Quanta 200, FEI, Holland). Failure modes of the two kinds of specimens were compared, and the location, origin and propagation of the cracks analyzed. Wake hackle markings from pores were selected as reference points for determination of fracture origins and propagation path of critical cracks in the glass veneer .

Statistical analysis

Statistical analysis was performed utilizing SPSS for windows, version 16.0. Normal distribution of data and homogeneity of variance were checked by Kolmogorov–Smirnov and Levene tests, respectively. Fracture loads of the two groups were compared by the t -test, while the failure modes were analyzed by Pearson Chi-square test, both with the level of significance set at 0.05.

Materials and methods

Sample preparation

A maxilla first molar model tooth (A5A-500-#16, Nissin, Japan) with average dimension, chosen as a master tooth, was prepared ( Table 1 ). All undercuts were eliminated by axial reduction of 1.5 mm, while the palatal occlusal surface was reduced by 2.0 mm, the buccal occlusal surface was reduced by 1.5 mm, both with functional cusp bevels, and 2.0 mm in the deepest points of the main fissures. All sharp angles were rounded after a 1.0 mm deep chamfer finish line and a 6° convergence angle between tooth axis and lateral wall was prepared. An impression of the finished master die was taken using a polyvinylsiloxane impression material (3M ESPE, USA). Then 2.0 mm layers of a fine hybrid composite resin (Z100 A3, 3M ESPE, USA) were filled into the impression and subsequently light-cured for 40 s (EliparFreelight2, 3M ESPE, USA). Before cementing the crowns, surfaces of the prepared samples were roughened by sandblasting from a distance of approximately 5.0 mm, at 2 bar with 120 μm glass beads. All resin dies were stored in 37 °C distilled water for 30 days.

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Influence of veneer application on fracture behavior of lithium-disilicate-based ceramic crowns
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