Attempted evaluation of three porcelain repair systems: What are we really testing?

Porcelain restorations are widely used in restorative dentistry with general success. On occasion however, fracture of the porcelain occurs. Sometimes a repair may be attempted using resin composite and one of the many bonding systems available. This in vitro investigation sought to determine and compare the shear bond strengths between a visible light-cured resin composite (Prisma TPH®) and dental porcelain (Matchmaker Porcelain®) for three different bonding systems (Scotchbond Multipurpose Plus® (SMP); One-Step® (OS); DenTASTIC® (D)) with a view to conducting a larger study to assist in material selection. All measurements were performed one week following the preparation and storage of specimens in distilled water, at 37°C, using a Universal Testing Machine at a crosshead speed of 0.5 mm/min. Although the mean shear bond strengths differed significantly (P < 0.05) (OS > SMP) and the Weibull modulus value of SMP was significantly (P < 0.05) lower than both OS and D the almost exclusive cohesive mode of failure observed, within the porcelain itself, suggested that the test was more a reflection of the quality of the underlying porcelain than the union under investigation. It was thus concluded that the shear bond strength test employed here, and in other studies did not truly reflect the shear bond strength at the porcelain/composite interface and that a more meaningful test should be developed. The observed cohesive failure may account for repeated clinical failures of porcelain repairs where bonding to an inherently flawed porcelain structure inevitably results in further failure.