Cementation of all-ceramic restorations with resin-cements has been demonstrated to reduce the incidence of fracture in service. The aim was to investigate the influence of loading force and loading duration applied during cementation on the reinforcement conferred by a resin-cement on a leucite reinforced glass–ceramic.
210 glass–ceramic discs (12 mm diameter, 0.8 mm thickness) were allocated to seven groups ( n = 30). The bi-axial flexure strength (BFS) was determined for ‘as-fired’ specimens (group A), following HF-acid etching (group B) and following etching and centrally loading on a semi-compliant substrate with 30 N for 300 s (group C). Further etched specimens were coated with Rely-X Unicem resin-cement under cementation loading of 5 or 30 N, for 60 or 300 s (groups D–F) prior to BFS determination using multilayered analytical solutions. BFS data was compared using factorial or one-way analyses of variance and the Weibull analysis.
HF-acid etching of the ‘as-fired’ surface resulted in a significant reduction in mean BFS ( P < 0.001) whereas resin-cementation subsequently conferred significant strengthening ( P < 0.001) ranging from 24 to 38% and dependent on the loading parameters employed. A factorial significance ( P = 0.004) of seating load (5 or 30 N) and significant factorial interaction ( P < 0.001) of seating load and load duration (60 or 300 s) on the mean BFS were identified.
The magnitude of resin-reinforcement conferred was sensitive to the loading parameters investigated although the mechanisms underlying the variation appear complex. Similar patterns of variability can be expected clinically impacting on the clinical longevity of resin-cemented glass–ceramic restorations.
The premature failure of all-ceramic dental restorations due to fracture will result in inconvenience and expense for both the patient and the dental operator. Failure may be manifested as the development of unaesthetic fracture lines that acquire staining over time and act as a potential route for microleakage. Alternatively, fracture may be associated with the partial loss of ceramic fragments or the total loss of the restoration from the supporting tooth structure . Both scenarios can lead to unacceptable aesthetics, dentin sensitivity and the impairment of masticatory function. It has been widely reported that the use of adhesive cementation techniques employing methacrylate resin-based composite cements enhances clinical performance and increases the resistance to fracture of many classes of dental ceramic . A prerequisite of the mechanisms that have been proposed to account for the observed reinforcement of the dental ceramic conferred by the resin-cement, is an interaction between the cement and the defects present on the ‘fit’ surfaces of the ceramic restoration and from which fracture has been demonstrated fractographically to propagate . In clinical practice the interaction is determined by the ability of the resin-cement to wet the ceramic surface which is itself a function of the ceramic surface chemistry and roughness , the viscosity and chemistry of the resin-cement and the forces applied during cementation loading . In vitro studies of the reinforcement of all-ceramic crown materials by resin-cements have to date excluded much of the variability introduced by the dental operator during the cementation procedure . Inevitably differences will exist between practitioners not only in the choice of resin-cement but also in the mode of application of the cement to the ceramic and the subsequent seating of the ceramic restoration onto the prepared tooth. The magnitude, duration, and distribution of the applied seating forces will impact on the stresses generated within the ceramic restoration and within the setting cement . The loading parameters are likely to be particularly significant when the setting reaction of a dual-cured resin-cement is dominated by the chemical initiated reaction which frequently occurs when the thickness and opacity of the overlying ceramic restoration attenuates the intensity of the curing light resulting in a reduction of photo-initiation . The nature and distribution of the stresses induced within the setting resin-cement mass determines the rheological characteristics and the ultimate resin-cement thickness. In addition a modification of the mechanical characteristics of the resin-cement is possible due to viscoelastic deformation throughout the loading period or due to alteration of the constraint of the associated setting shrinkage . It is anticipated that the operative variability will be reflected as inconsistencies in the interaction of the resin-cement with the ceramic surface defect population. Qualitative assessments of resin–ceramic interfaces have demonstrated that resin-cements can fully, partially or fail to infiltrate ceramic surface defects when applied using clinically representative techniques . Previously investigators have demonstrated that the integrity of adhesive interfaces between dentin and resin-cements and dental restorative materials, determined by bond strength measurements, are influenced by the magnitude and duration of the loading force applied during cementation . In the context of dental ceramic materials, examinations of the impact of cementation loading parameters have focused on the assessment of marginal adaptation and cement film thickness and consequently the interaction of the cement with surface defects is poorly understood . Therefore the aim of the current study was to test the hypothesis that the magnitude of reinforcement achieved when an all-ceramic material was cemented with a resin-cement was dependent on magnitude and/or duration of the applied seating force.