The influence of clinically induced variability on the bi-axial fracture strength of cemented aluminous core porcelain discs

Objectives: Zinc phosphate cements are supplied in powder/liquid form so that a range of mixing ratios are naturally generated in clinical practice. However, these cements are known to have a deleterious effect on dental ceramics and the impact of variations in cement mixing on the strength of dental porcelain was investigated in the present study. Methods: Vitadur-N core porcelain was fabricated into discs (15 mm diameter and 2 mm thickness), zinc phosphate cements of different mixing ratios were applied to the discs to produce a luting cement thickness and the samples were stored for 24 h prior to testing in a water-bath at 37±1°C. The mean fracture strength, standard deviation and associated Weibull moduli (m) of the coated porcelain discs were determined as a function of mixing ratio using bi-axial flexure (ball-on-ring) by fracturing sets of 30 specimens coated with cements manipulated at different mixing ratios. Results: The strength data for the coated porcelain discs showed little variation in magnitude and consistency ranging from 108±8.3 MPa (m=14.0±2.6) at 1.7 g/ml to 112±8.5 MPa (m=14.6±2.7) at 2.6 g/ml and 108±8.1 MPa (m=14.4±2.6) at 3.2 g/ml. However, the plots of survival probability against strength for coated specimens prepared using mixing ratios below that recommended by the manufacturers’ for luting purposes (2.6 g/ml), appeared to develop an asymmetry at the lower values of strength. Significance: It was proposed that the increased corrosive acidic environment for cements prepared at mixing ratios below 2.6 g/ml may have extended pre-existing flaws in the porcelain discs thereby possibly producing the asymmetry in the survival distributions.