Elsevier

Journal of Dentistry

Volume 37, Issue 10, October 2009, Pages 799-806
Journal of Dentistry

Edge strength of indirect restorative materials

https://doi.org/10.1016/j.jdent.2009.06.009Get rights and content

Abstract

Objectives

To evaluate the edge strength and fracture patterns of different all-ceramic and indirect composite materials used in prosthodontic applications.

Methods

Fourteen rectangular-shaped samples (2–2.5 mm thick) were prepared of each of the following materials: BelleGlass NG (Kerr) (BG), fibre-reinforced BelleGlass/EverStick (Kerr/Stick Tech Ltd.) (BGES), Vita Mark II (Vita Zahnfabrik) (VMII), bilayered IPS e.max Ceram/CAD (Ivoclar-Vivadent) (Ceram/CAD), bilayered IPS e.max Ceram/ZirCAD (Ivoclar-Vivadent) (Ceram/ZirCAD) and unilayered IPS e.max CADLT (Ivoclar-Vivadent) (CADLT). Each group was further subdivided into seven subgroups (n = 2) corresponding to different edge distances at which samples were loaded. Samples were tested with an edge strength machine (CK10, Engineering Systems, Nottingham, UK) using a diamond Vickers indenter. The force-to-failure (N) was recorded and four readings were obtained per sample. The mode of failure was analysed microscopically. One-way ANOVA was used to detect differences in edge strength among the groups and correlation and regression analyses were used to detect the correlation between distance of loading and failure force.

Results

The mean values of edge strength (N) were 94.1 (11.2) for BG, 134.4 (10.9) for BGES, 50.5 (8.9) for VMII, 54.4 (7.5) for Ceram/CAD, 53.2 (8.4) for Ceram/ZirCAD and 69.1 (9.1) for CADLT. Significant (p < 0.05) strong positive linear correlations were found between the force-to-failure and edge distance among all groups. Three patterns of failure were identified: surface indentation without visible cracking, surface indentation with visible cracking and chipping.

Conclusions

Indirect composite materials had better edge fracture behaviour than all-ceramic materials. The addition of fibres enhanced the performance of the resin-composite material. Fracture strength increased as the distance from the edge increased. Edge fracture was restricted to the veneer material in bilayared systems and therefore a stronger veneer material is recommended.

Introduction

Intracoronal restorations, such as inlays, onlays, inlay- and onlay-retained bridges are attractive options for dentists and patients as conservative means for restoring defective teeth or replacing missing ones. Increasing patients’ demands for aesthetically pleasing restorations have driven the search for different materials with improved optical properties that could be used for such types of restorations. Ceramic inlays have been commonly used since 1980s with high survival rates,1, 2 chairside ceramic inlays have also become feasible with the introduction of CAD/CAM systems3 and enhanced-strength all-ceramic materials, like zirconia-reinforced ceramics, have been used for the construction of resin-bonded onlay/inlay-retained bridges.4, 5 On the other hand, indirect composite and fibre-reinforced composite materials can be used directly or indirectly for the construction of these prostheses.4

Proper marginal adaptation of such restorations at the time of cementation and the long-term maintenance of this adaptation are among the most important factors determining their success. Although marginal gaps are initially filled with the luting cement, initial marginal deficiencies are expected due to polymerisation shrinkage of the resin-cement. Furthermore, the margins of these restorations are continuously subjected to direct heavy occlusal loading that wears the resin-cement more rapidly, leaving the restoration margins unsupported.3, 6, 7 Marginal deficiencies were commonly found in all-ceramic inlay/onlay restorations especially in extended class I inlays and when non-adhesive luting agents were used.8 Therefore, proper case selection, avoiding placing the margins of the restorations under high occlusal forces and the use of adhesive cements are expected to improve the marginal adaptation and reduce the marginal fractures of these restorations.8, 9, 10

Although small marginal chippings do not always require replacement of the restorations, they might result in significant negative consequences including compromised aesthetics resulting from marginal staining caused by microleakage and plaque accumulation at the broken edges,11 an increased risk of secondary caries, especially with bigger defects, in areas not accessible for proper cleaning and when oral hygiene is generally poor,12 increased tooth sensitivity,2 an increased risk of future bulk failure and catastrophic marginal fracture necessitating replacement of the restoration2, 6, 13, 14 in addition to increasing the risk of fracture of unsupported tooth margins under repetitive occlusal loading.15

Many efforts have been made to evaluate the tendency for marginal fracture of various dental materials using different techniques.12, 15, 16, 17, 18, 19, 20, 21, 22, 23 Although the concept of edge strength was introduced several decades ago, a well-defined methodology for edge strength measurement was only recently proposed. Edge strength is defined as the ability of restorative materials to withstand fracture of a thin edge and it is determined by measuring the force to fracture at a distance of 0.5 mm of the sample edges.11, 24 Edge strength reflects the ability of a restoration of a particular material to maintain its marginal integrity upon loading.20 The load to cause edge fracture depends on many factors including the shape of the object applying the contacting force, the distance from the edge, the angle of the applied force, the angle of the edge and the material's fracture toughness.25

Studies evaluating edge strength based on this recently introduced methodology are scarce in the literature.11, 24, 26 Furthermore, none of these studies evaluated the edge strength of ceramic materials.

Therefore, the aim of this study was to compare the edge strength of different all-ceramic materials and a resin-composite material.

The specific objectives were

  • 1.

    To measure the edge strength of some indirect restorative materials.

  • 2.

    To investigate the relationship between fracture strength and the distance of loading from the edge.

  • 3.

    To analyze the edge fracture pattern microscopically.

The null hypotheses tested were

  • 1.

    There is no difference in edge strength values between the different materials tested.

  • 2.

    Fracture strength is not correlated with the distance of loading from the edge.

  • 3.

    There is no difference in edge fracture patterns among the different materials.

Section snippets

Preparation of the specimens

Six groups were investigated in this study. Table 1 shows the materials used, their basic composition and their manufacturers.

BG samples were prepared using a Teflon mould with a 12.5 mm × 11 mm × 2.5 mm opening. A glass slab was attached underneath the mould and the surface of the slab was coated with vaseline to facilitate the specimen removal from the mould. BG composite was packed into the mould in two increments each cured for 40 s using an Optilux curing unit (OptiLux 501, Danbury, CT, USA) with

Edge strength

The average values of edge strength (force (N) of fracture at a distance of 0.5 mm from the edge) were calculated for each group. These are presented in Table 2 and Fig. 2. The same superscript letters in Table 2 denote the mean values without statistically significant differences (p > 0.05).The results showed that values of edge strength of resin-composite groups were significantly higher than those of all-ceramic groups (p < 0.05). Within the composite groups, BGES had significantly higher edge

Discussion

This study aimed to determine the edge fracture of various aesthetic dental materials used in the fabrication of indirect restorations. The results showed statistically significant differences in the edge strength between the groups and therefore the first null hypothesis was rejected. Furthermore, significant and strong correlations between the distance of loading from the edge and the fracture strength were found among the different groups. Subsequently, the second null hypothesis was

Acknowledgement

The authors wish to thank Ivoclar-Vivadent (Schaan, Liechtenstein) for providing the materials and equipment for the fabrication of IPS e.max samples.

References (30)

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    As the distance from point of loading to the edge was small (0.3 mm), the load until fracture was rather low. This could be expected according to the studies by Ereifej et al 38 and Quinn.23 Pfeilschifter et al25 reported similar findings when they tested various resin-containing CAD-CAM restorative materials.

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    Further, reducing the present F vs d data in terms of fracture toughness (Chai and Lawn, 2007; Chai, 2011), they well fitted by the required power law (power of 1.5). A recent systematic review on edge chipping (Brandeburski et al., 2020) found a high heterogeneity of study methods and reporting parameters, including F vs d (Chai et al., 2011; Zhang et al., 2012), edge toughness, Te (Quinn et al., 2000, 2010, 2013; Chai and Lawn, 2007; Quinn and Quinn, 2010; Whitbeck et al., 2011, Zhang et al., 2013, 2014a, 2014b, 2014c; Quinn, 2015; Argyrou et al., 2016), edge strength, SE (Kim and Watts, 2007; Baroudi et al., 2008; Watts et al., 2008; Ereifej et al., 2009; Quinn et al., 2014c; Pfeilschifter et al., 2018), and edge chip resistance, ReA (Quinn et al., 2013, 2014a; Taufer and Della Bona, 2019), which is recommended by the European Committee for Standardization (CEN/TS 843–9:2010). Studies have also used different indenters, but it has been shown that the indenter type influences on the stress distribution during loading as the edge distance changes (Quinn et al., 2014b).

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    d relation and a linear regression to fit the data. Thus, some studies have shown a linear data behavior [5,13,16–18,20] and others have shown a non-linear behavior mostly because of the great dispersion of data [1,7–9] associated with the indenter type [3,10,11,20], densification of materials below the indenter prior to fracture [6,19], and plastic deformation and residual stresses [10–12], needing to adjust the Fvs. d data using a power law or a quadratic function based on an energy balance between indenter work and fracture and deformation energies in the chipped material [40].

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