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

Objective: The proportions of the constituents required to produce a porcelain slurry used in crown construction are assessed by experience so that variability is inevitable. The present study investigated the strength dependence of porcelain specimens on slurry consistency. Methods: Vitadur-N core porcelain was formed into discs and the mean fracture strength, standard deviation and associated Weibull Moduli (m) determined as a function of slurry consistency using bi-axial flexure (ball-on-ring) by fracturing sets of 30 specimens prepared to different powder contents for a constant 0.4 ml of liquid. Results: The strength data for the porcelain discs showed variation in magnitude and consistency ranging from 145±16.5 MPa (m=9.7±1.8) at 1.2 g to 155±13.8 MPa (m=12.3±2.2) at 1.4 g and 142±13.7 MPa (m=11.7±2.1) at 1.6 g of powder. Increasing or decreasing the powder content of the slurry (related to specimens condensed using 1.4 g of powder) resulted in an increase in the apparent porosity and a decrease in the apparent solid density. Conclusions: There was no significant decrease in the mean bi-axial flexure strengths for the slurry consistencies investigated. However, the reliability of the fracture strength data did vary significantly. The results suggested that an optimum consistency exists (wherein consistent reproducibility of test results was achieved) and appears to have implications for laboratory testing of materials since a comparison between materials can only be achieved if specimen preparation occurs consistently between test centres for all the materials under investigation.