The impact of cementation and storage variables on ceramic strengthening

Objectives: Resin cementation of all-ceramic restorations increases clinical performance however, the strengthening mechanisms are not fully understood. The authors have previously proposed the existence of a resin-ceramic hybrid layer. The hypothesis tested was that ceramic strength enhancement was dependent upon the characteristics of the resin-ceramic hybrid layer which is dependent on cementation variables and storage environment. Methods: The ‘fit' surfaces of 480 nominally identical porcelain disc-shaped specimens were air abraded introducing a clinically relevant surface texture and consistent surface defect population. 30 specimens were randomly allocated to each of 16 groups. Specimens were coated with silane or unfilled resin or filled resin luting cement or a combination. Eight groups were stored dry and eight were stored in water for 24h before bi-axial flexure testing (ball-on-ring). Improved bilayered solutions provided failure stresses at axial positions throughout the ceramic-resin specimens. Rank regression enabled determination of the Weibull parameters m and σ₀ and a probability estimator was utilised to determine the 95% confidence intervals. Results: σ₀ was significantly increased following coating from 58.8 and 53.8 MPa for the uncoated dry and wet controls, respectively, to a range dependent on coating type of 90.7-114.3 and 66.7-105.8 MPa, respectively. Silane priming conferred no strengthening in isolation or when apposing unfilled resin. 24h water immersion resulted in 9% and 26% reductions in σ₀ for the uncoated and unfilled resin coated specimens, respectively, but no significant reduction in σ₀ was observed following water storage of the filled resin luting cement coated specimens. Conclusions: The magnitude of resin strengthening is likely to impact on clinical performance but is sensitive to the characteristics of the resin-ceramic hybrid layer. The findings support previous observations that commercial resin cements may have less than ideal material properties and fail to maximise the potential of resin strengthening thereby contributing to premature clinical failures.