The impact of modifying alumina air abrasion parameters on the fracture strength of a porcelain laminate restorative material.

OBJECTIVES: The modification of the "fit" surface of porcelain laminate veneer restorations in order to improve adhesion prior to cementation is often indiscriminate. As a consequence, the surface flaw distribution which is implicated in the probability of failure of the restoration is likely to be dramatically modified. The purpose of the current study was to examine the impact of different air abrasion surface treatments on the bi-axial flexure strength and surface roughness of a porcelain restorative material. METHODS: Sets of 30 Vitadur-Alpha dentin porcelain discs (15 mm diameter, 0.9 mm thickness) were alumina abraded with three different grades of alumina particle (25, 50 and 110 microm), utilizing two different air stream pressures (35 and 70 psi) and two distinct angles of incidence of particle delivery (45 degrees and 90 degrees ). Mean bi-axial flexure strengths, standard deviations, the associated Weibull moduli (m) and characteristic stress were determined using bi-axial flexure (ball on ring). RESULTS: A univariate general linear analysis of means revealed a significant difference between the mean bi-axial flexure strength values of the control group and those of groups subjected to alumina particle air abrasion. Further significance (P<0.05) was discovered with the impact of alumina particle size and the interaction between particle size and angle of incidence of particle delivery. The reliability of the fracture strength data generally improved when 50 microm alumina particles were used whereas discontinuities existed at lower strength values when 25 and 110 microm alumina particles were employed. CONCLUSIONS: Alumina particle air abrasion has a significant degradative effect on the bi-axial flexure strength of the porcelain disc-shaped specimens. Variation of alumina size, delivery pressure and angle of particle delivery all impacted on the degree of strength reduction and the shape of the survival probability distributions. It is suggested that alumina particle air abrasion acts to remove/modify the initial flaw distribution replacing it with flaws of differing geometry and stability. The premature failure of porcelain laminate restorations may be markedly influenced by alumina particle air abrasion depending upon the size and distribution of the crystalline phase present in different dentine porcelains materials used in construction of the restoration.