Elsevier

Dental Materials

Volume 33, Issue 12, December 2017, Pages 1416-1425
Dental Materials

The effect of internal roughness and bonding on the fracture resistance and structural reliability of lithium disilicate ceramic

https://doi.org/10.1016/j.dental.2017.09.018Get rights and content

Highlights

  • A rough surface reduces the load to failure of non-bonded lithium disilicate discs.

  • Adhesive luting improves the load to failure of lithium disilicate restorations.

  • The load to failure is not different between rough and polished adhesively cemented discs.

  • According to FEA, the stress concentration around the bond interface is higher in a non-bonded scenario.

Abstract

Objective

To evaluate the effect of internal roughness and bonding on the load to failure and structural reliability (Weibull analysis) of a lithium disilicate-based glass ceramic under different testing scenarios.

Methods

IPS e.max CAD blocks (Ivoclar Vivadent AG) were shaped into cylinders (N = 100), crystalized according to the manufacturer’s instructions, and randomly assigned into two surface conditions: (1) polished surface (600-grit SiC polish papers), and (2) a roughened surface (air-abrasion with 50 μm Al2O3). Two assemblies were investigated: a ceramic disc isolated (to isolate the effect of roughness); and a simplified tri-layer setup simulating the restoration of a posterior tooth (ceramic + cement + epoxy resin) to evaluated the influence of bonding isolated and the associated effect of both factors. Four different scenarios were tested: (1) isolated disc under static load (n = 10); (2) disc bonded to an epoxy resin substrate and tested under a static load (n = 10); (3) disc bonded and tested under fatigue (n = 20); and (4) simulated-bonding tested statically (n = 10). The data of load to failure were submitted to One-way ANOVA and Weibull analysis.

Results

At a non-bonded scenario (isolated disc and simulated-bonding) a polished internal surface presented a higher characteristic strength. However, when bonding was present this difference became inexistent. No difference was found in terms of structural reliability (Weibull moduli) among the groups. FEA analysis shows that with bonding the tensile stress is better distributed, while in a non-bonded scenario higher tensile stresses occur at the bonding interface.

Significance

A rough internal surface impacted deleteriously the mechanical properties of lithium disilicate ceramic when it was not properly bonded to the substrate. However, bonding to the substrate appeared to play a more significant role in the fracture resistance than internal roughness.

Introduction

Lithium disilicate is a synthetic glass ceramic that is being used in dentistry for many years to restore decayed or worn down teeth. It is a popular restoration material as it results in good esthetic and mechanical properties [1], [2]. Additionally it present two major advantages: (1) allow the manufacturing of monolithic restorations, that do not require a veneering layer which is known to be susceptible to chipping [3], [4], and (2) that a good adhesion to dental tissues can be established [5], [6]. In comparison to earlier glass ceramics like feltspathic and leucite reinforced it has a higher flexural strength [7], [8], and compared to oxide ceramics like zirconia it has better optical properties. Clinical studies show good medium-term results for single restorations in the anterior and posterior region [9], [10], [11], [12], [13].

A general disadvantage of glass ceramics is their brittleness [14], [15], which makes it susceptible to fractures when submitted to tensile forces [1]. Finite element analysis showed that tensile forces occur in deep fissures and at the internal surface of the restoration, especially at sharp edges [1]. A clinical study with Dicor glass ceramic material also showed that most restorations failed from fractures originating at the internal surface of the crowns [16]. This is explained by the formation of radial cracks, deriving from this surface of the restoration [17], [18], [19], [20], [21], especially when repetitive forces are applied, like during mastication in the oral cavity, or by fatigue testing in the laboratory [18], [22].

Another important factor in the longevity of glass ceramics is surface roughness. Under pressure, small superficial flaws can become initial cracks that are propagated to a fatal fracture [23]. Several laboratory studies showed a significant decrease of the flexural strength when lithium disilicate had a rough surface [1], [14], [24], [25]. Although this can be controlled on the outside of a restoration by polishing or glazing, it is not possible to modify the internal surface because this would affect the adaptation to the abutment tooth. Therefore, as finite element study suggested that most probably the failure of crowns derived from the inside [1], internal roughness might be a common reason for failure of lithium disilicate restorations.

The condition of the internal surface of a lithium disilicate restoration is dependent on the method of manufacturing. It can be made by heat pressing according to the lost-wax technique and by computer-aided design and computer-aided manufacturing (CAD/CAM) in a water-cooled milling device. Heat-pressed restorations have a better internal fit to the abutment tooth [26], [27] and a higher fracture toughness [28]. On the other hand, heat-pressed lithium disilicate has a tactile and optically rougher surface structure and might therefore be more susceptible to fractures deriving from the internal surface.

The effect of roughness on the fracture resistance might be reduced by using adhesive cementation to its substrate. By using hydrofluoric acid and a silane coupling agent for creating micromechanical and chemical retention to a composite resin luting agent, lithium disilicate can be adequately bonded to dentin and enamel [5], [6], [29], [30]. On this sense, laboratory studies showed that a better adhesion to its substrate resulted in a higher fracture resistance [31], [32], [33]. Additionally, the use of a composite resin cement might potentially fill up the irregularities of a rough surface. Thus, it is possible that these factors also influence the effect of internal roughness on the fracture resistance.

This study aimed to evaluate the effect of internal roughness and bonding on the load to failure and structural reliability of rough versus smooth lithium disilicate specimens under different testing scenarios (ceramic discs isolated, ceramic discs bonded to a material simulating dentin, and simulated bonding — using cement but not bonded) to evaluate each factor and the interaction of them. In addition to static tests, a fatigue test was performed to investigate the effect of roughness on the fracture resistance during repetitive increasing forces, because fatigue testing offers the opportunity for slow crack growth and therefore resembles more the clinical situation [26]. The null hypothesis was that there is no effect of internal roughness and bonding on the fracture resistance of lithium disilicate restorations.

Section snippets

Materials and methods

All materials used are listed in Table 1, and the experimental design is illustrated in Fig. 1.

Results

The mean load to failure and surface roughness, with its standard deviations and statistical analysis are summarized in Table 2, together with the Weibull analysis. This Weibull analysis showed that for scenarios with absence of bonding to the substrate (only ceramic; and simulated bonding) significant higher characteristic strength (σ0) values were noticed for polished condition, compared to the roughened condition. However, when bonding was present, the σ0 was statistically equal in polished

Discussion

This laboratory study showed a significant effect of internal roughness on the load to failure of lithium disilicate specimens. The null-hypothesis of this study can therefore be partially rejected. However not entirely, because the internal roughness had no effect when it was adhesively bonded to a substrate with dentin-like properties. Hence, this finding also rejects the second part of the null-hypothesis, that bonding has no effect on the fracture resistance. Especially important for

Conclusion

A rough internal surface impacts deleteriously the mechanical properties of lithium disilicate ceramic when it is not properly bonded to the substrate. When there is a good bonding to the substrate, the fracture resistance increases significantly and the effect of internal roughness disappears. Therefore, bonding to the substrate appeared to play a more significant role in the fracture resistance than internal roughness.

Acknowledgement

The authors thank Arie Werner for delivering the Scanning Electron Microscopy images of the specimens. They would also like to thank Bart van Etten, Sam van Galen and Dr. Ruud Kuijs for delivering the pilot study that lead to this manuscript.

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