The effect of internal roughness and bonding on the fracture resistance and structural reliability of lithium disilicate ceramic
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.
References (45)
- et al.
Mechanical performance of implant-supported posterior crowns
J Prosthet Dent
(2015) - et al.
Slow crack growth and reliability of dental ceramics
Dent Mater
(2011) - et al.
All-ceramic partial coverage restorations—midterm results of a 5-year prospective clinical splitmouth study
J Dent
(2009) - et al.
Clinical outcomes of lithium disilicate single crowns and partial fixed dental prostheses: a systematic review
J Prosthet Dent
(2014) - et al.
Development of a clinically validated bulk failure test for ceramic crowns
J Prosthet Dent
(2010) - et al.
Fracture-surface analysis of dental ceramics
J Prosthet Dent
(1989) - et al.
Survival of Dicor glass-ceramic dental restorations over 16 years. Part III: effect of luting agent and tooth or tooth-substitute core structure
J Prosthet Dent
(2001) - et al.
Fatigue loading and R-curve behavior of a dental glass-ceramic with multiple flaw distributions
Dent Mater
(2013) - et al.
The influence of surface roughness on porcelain strength
Dent Mater
(2000) - et al.
Effect of sandblasting, grinding, polishing and glazing on the flexural strength of two pressable all-ceramic dental materials
J Dent
(2004)
Internal adaptation, marginal accuracy and microleakage of a pressable versus a machinable ceramic laminate veneers
J Dent
Marginal and internal fit of heat pressed versus CAD/CAM fabricated all-ceramic onlays after exposure to thermo-mechanical fatigue
J Dent
The fracture resistance of a CAD/CAM Resin Nano Ceramic (RNC) and a CAD ceramic at different thicknesses
Dent Mater
Fracture frequency of all-ceramic crowns during dynamic loading in a chewing simulator using different loading and luting protocols
Dent Mater
Strength of a dental glass-ceramic after surface coating
Dent Mater
In vitro fatigue resistance of CAD/CAM composite resin and ceramic posterior occlusal veneers
J Prosthet Dent
Statistical failure analysis of adhesive resin cement bonded dental ceramics
Eng Fract Mech
Penetration of 38% hydrogen peroxide into the pulp chamber in bovine and human teeth submitted to office bleach technique
J Endod
A radiographic assessment of enamel thickness in human maxillary incisors
Arch Oral Biol
A practical and systematic review of Weibull statistics for reporting strengths of dental materials
Dent Mater
ADM guidance-ceramics: guidance to the use of fractography in failure analysis of brittle materials
Dent Mater
Maximal bite forces in healthy young adults as predicted by surface electromyography
J Dent
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