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Resin Elasticity and the Strengthening of All-ceramic Restorations
1 Biomaterials Unit, School of Dentistry, University of Birmingham, St. Chad’s Queensway, Birmingham B4 6NN, UK; and Correspondence: * corresponding author, addisono{at}bham.ac.uk
Resin luting of all-ceramic restorations results in increased performance; however, the strengthening mechanism and the role of the mechanical properties of the resin are not fully understood. The hypothesis tested is that ceramic strength enhancement is dependent on the elastic modulus of the resin. Three-point flexural moduli of a flowable, luting, and hybrid composite resin were characterized. Two hundred forty porcelain discs were air-abraded. One group acted as a control, and 3 additional groups were coated with 120 ± 20 µm of each resin prior to bi-axial flexure testing. All resins significantly increased in mean strength, and the associated strength increase was related to the elastic modulus of the resin (R2 = 0.9885), so the hypothesis was accepted. The combination of Poisson constraint and the creation of a resin-inter-penetrating layer sensitive to the elastic modulus of the resin may provide an explanation of the strengthening mechanism.
Key Words: low-fusing feldspathic porcelain • bi-axial flexure strength • elastic modulus
The cementation of all-ceramic restorations with resin cements has been shown, in laboratory (Marquis, 1992; Rosenstiel et al., 1993; Pagniano et al., 2005; Fleming et al., 2006) and clinical (Malament and Socransky, 1999a,b) studies, to improve performance. Numerous strengthening theories—including crack closure stresses (Roach et al., 1988; Nathanson, 1994), full- or partial-crack healing (Fabes and Uhlmann, 1990; Marquis, 1992), and Poisson constraint effects (Wang et al., 1995)—have been proposed. Investigations (Roach et al., 1988; Fabes and Uhlmann, 1990; Wang et al., 1995) regarding the coating of glass, reported in the ceramic literature, previously involved the use of Vickers indentation as a uniform defect, since glasses contain a surface defect population that accounts for the discrepancy between the theoretical and the actual strengths of ceramics (Griffith, 1921). Vickers indentation was also used on a resin-coated dental porcelain, and the strength enhancement was identified as being independent of defect severity (Fleming et al., 2006). Therefore, the previously proposed theories of Marquis (1992) and Nathanson (1994), both of which implied a sensitivity of resin strengthening to defect size, are not entirely correct, although the exact strengthening mechanisms are not fully understood.
The mechanical properties of resin cements have been dictated by favorable working characteristics and adhesion between the restoration and the tooth substrate. The role of the resin in transferring stress from the loaded restoration to the underlying tooth structure has not been studied in detail, although it has been proposed that luting cements require an intermediary elastic modulus ( The aim of the current investigation was to examine the impact of resins with markedly different elastic behavior on the strengthening mechanisms operative in a simulated resin-bonded all-ceramic restoration. We hypothesized that ceramic strength enhancement is dependent upon the elastic modulus of the resin cement chosen.
Resin Modulus Determination The elastic moduli of Flowline (lot no. 010104, Heraeus Kulzer GmbH & Co., Hanau, Germany), Rely-XTM Veneer Cement (lot no. 5BP, 3M ESPE, St Paul, MN, USA), and Clearfil AP-X (lot no. 1076AB, Kuraray Medical Inc., Okayama, Japan) were determined. Five bar-shaped specimens of each resin were condensed into a split plastic mould (25 mm length, 2 mm width, 2 mm height). Specimens were covered with Mylar and a glass coverslip, and were light-activated according to the manufacturers’ instructions with 3 overlapping cure cycles, in an Optilux 501 (SDS Kerr, Danbury, CT, USA) curing unit at an output intensity of 740 mW/cm2 and an 11-mm tip diameter. Specimens were tested at 1 mm/min, three-point flexural strengths were calculated (Eq. 1), and flexural modulus within the elastic segment of stress-stain curves was determined (Eq. 2) (ISO 4049, 2000):
Specimen Condensation and Preparation
Strength Determination
and
The bi-axial flexure stress can be calculated at axial positions (z) at the center of the disc-shaped specimen, where the bonded interface is located, at z = 0, the porcelain surface at z = t1, and the resin surface at z = –t2, as:
(0
(–t2
P was the load at fracture, v1 (0.25; Zeng et al., 1998) and v2 (0.27; De Jager et al., 2004) the Poisson’s ratios of ceramic and resin, and a, b, and R the radii of the knife-edge support, loaded region, and specimen, respectively.
Profilometry
Statistical Analysis
The mean three-point flexural strengths and standard deviations of the Flowline, Rely-XTM, and Clearfil AP-X bar-shaped specimens were 73.3 (10.1), 118.2 (0.5), and 145.1 (4.9) MPa, respectively, and the paired Tukey test comparisons identified a significant difference (P < 0.001) among groups (Table 1  ). The mean elastic moduli and standard deviations were 4.9 (0.1), 8.2 (0.4), and 16.8 (0.5) GPa, respectively (Table 1). One-way ANOVA and paired Tukey test comparisons identified significant differences (P < 0.001) among the mean elastic moduli for the 3 materials tested.
The one-way ANOVA and paired Tukey test comparisons identified a significant difference (P < 0.01) between the mean bi-axial flexure strength of the control (Group A) 58.0 (4.7) MPa (Fig. 1a ) and that of the cement-coated groups B–D [78.5 (6.6), 94.1 (7.8), and 120.2 (8.0) MPa, respectively]. Interestingly, following analysis, there was reduced confidence in the linearization of the Weibull distribution by regression analysis (P > 0.05) for Groups A, C, and D, with separate distributions at low- and high-stress levels identified (Fig. 1b). We determined the number of specimens in each distribution by optimizing the percentage variation of the data explained by the fitted lines with increasing sample size (Table 2). However, the 95% confidence intervals of the Weibull moduli of the low-stress failure distribution of Groups A, C, and D overlapped, indicating that the reliability of the fracture strength data was not significant (Table 2).
The surface texture of the alumina-abraded disc-shaped specimens utilized in the current investigation was investigated by contact surface profilometry (Fig. 2a ). The surface consisted of regularly distributed small flaws of an amplitude of < 10 µm, with flaws exceeding 15 µm encountered no more than once per 5-mm trace.
We used bi-axial flexure testing to replicate the failure mechanism identified by axisymmetric finite-element analysis of molar crowns (Anusavice and Hojjatie, 1992) and quantitative fractography of failed restorations (Kelly et al., 1996, 1989; Kelly, 1999; Quinn et al., 2005), namely, tensile failure on the inner surface of the restoration in service. In this study, we used n = 60 to enhance the Weibull analysis (McCabe and Carrick, 1986). The mean porcelain strength [58.0 (4.7) MPa] was significantly increased by 35, 62, and 107% following coating with resins with elastic moduli of 4.9 (0.1), 8.2 (0.4), and 16.8 (0.5) GPa, to produce associated mean strengths of 78.5 (6.6), 94.1 (7.8), and 120.2 (8.0) MPa, respectively. The elastic moduli of the resins related directly to the filler loading, namely, 60, 66, and 82 wt% for Flowline (Group B), Rely-XTM Veneer Cement (Group C), and Clearfil AP-X (Group D), respectively. In addition, a linear relationship (R2 = 0.9885) was observed between the mean strength at the ceramic/resin interface and the elastic modulus of the individual resins, highlighting a dependence of the strengthening processes on the elastic properties of the resin. The observations were consistent with the findings of Fleming et al.(2006), who reported that the strengthening of ceramics following resin coating was independent of a controlled defect population, although the strengthening observed was not attributed to the elastic behavior of the resin. The mathematical methods described by Hsueh et al.(2005) enabled bi-axial flexure stress calculations in this study to be made at axial positions throughout the ceramic/resin bilayer during failure. The maximum flexural stresses generated in each resin coating were considerably below the ultimate flexural strength and elastic limits of each respective resin. Therefore, it is unlikely that failure initiated at the resin surface or within the resin bulk, as previously proposed (Fleming et al., 2006), but strengthening may be dependent on the behavior of the resin inter-penetrating the ceramic surface. Roach et al.(1988) investigated bridging as a result of layers of deposits of environmental species behind a crack, and demonstrated that the layers can bridge the interface, exert a closure stress on the crack, and thereby increase the apparent strength. Hand et al.(2003) likened closure tractions to thermal expansion mismatch between the resin and ceramic. Following irradiation, the contraction of resin within a crack is restricted compared with an unconstrained state, producing an associated strain in the resin (Hand et al., 2003). Where a strong bond exists between resin and ceramic, closure stresses may arise as a result of polymerization shrinkage of the resin (Nathanson, 1994). The degree of strengthening will be dependent on defect geometry; however, it has been demonstrated that strength enhancement is independent of defect severity (Fleming et al., 2006). Furthermore, the degree of polymerization shrinkage for resin-based composites increases with decreasing filler volume (Kleverlaan and Feilzer, 2005), and the greatest strengthening would be expected for Group B and the least for Group D, which contradicted the observations in the current study and does not support the closure traction theories. Fabes and Uhlmann (1990) suggested a glass-strengthening process, by sol-gel coatings based on the ’filling in or partial healing’ of surface flaws, thereby reducing the crack length, blunting the crack tip, crack traction, or a combination of the above. Marquis (1992) proposed a similar theory for crack shortening, whereby the resin partially or totally heals flaws; however, the less-filled resin (Group B), which would have been expected to have the greatest defect penetrance (Fabes and Uhlmann, 1990), was associated with the least strengthening. In addition, the ’filling in or partial healing’ strengthening of surface flaw theory implies a defect sensitivity, inconsistent with the current observations. The Weibull distribution for Group A distinguishes two separate distributions related to failure at low- and high-stress levels. The Weibull distributions for Groups C and D show a similar pattern, implying consistent sensitivity to the defect population. In contrast, Group B showed a uniform Weibull distribution at all stress levels, suggesting a reduced impact of the original surface defect population. Modification of the Weibull distributions may be explained by the increased penetrance of smaller flaws compared with heavily filled resins (Fabes and Uhlmann, 1990). When resins are restricted to thin layers in cracks, the mechanical response can be significantly altered (Wang et al., 1995). During bi-axial flexure, the resin within a crack extends perpendicular to the crack face, producing a compensating Poisson contraction parallel to the crack surface (Wang et al., 1995). This contraction is restricted by the resin thickness compared with the stiffer bulk ceramic, which also has a lower Poisson’s ratio, effectively increasing the stiffness of the resin. The resin in the crack therefore behaves more closely to the bulk ceramic (Wang et al., 1995). When bilayered structures with smooth interfaces are subjected to loading under bi-axial flexure conditions, shear stresses are generated, parallel to the material interface, resulting in de-adhesion or resin deformation. The roughened surface used in the current study consisted of a multiplicity of defect sizes, and resin interpenetration of this surface can be considered to introduce a hybrid ceramic-composite layer. The stressing patterns of the resin within the region of surface defects are more complex, but the elastic properties of this hybrid layer will inevitably increase with increasing resin elastic modulus. The system becomes sensitive to the characteristics of the hybrid layer, mediated by complex interactions of various elements, and would follow the pattern observed. Therefore, the combination of Poisson constraint and the creation of a resin inter-penetrated layer sensitive to the elastic modulus of the resin may provide an explanation of the observations in the current investigation. Significant porcelain strengthening following the application of resins with increasing elastic moduli has been shown. The proposed hypothesis that strengthening is a function of the resin elastic modulus was accepted. Consequently, the strength and performance of resin-cemented all-ceramic restorations can be enhanced by the use of higher elastic modulus cements.
This work was carried out as partial fulfillment of the requirements for a self-funded PhD by Owen Addison. Received for publication April 27, 2006. Revision received October 9, 2006. Accepted for publication February 6, 2007.
Journal of Dental Research, Vol. 86, No. 6,
519-523 (2007) This article has been cited by other articles:
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ABSTRACT
INTRODUCTION
) between dentin and the ceramic. The elastic moduli of commercially available resin cements range from 5 to 12 GPa (
max is the maximum flexural stress, P the load, L the support span (20 mm), b the width, d the specimen thickness, EB the flexural modulus, and D the deflection. 
z 
