Effect of Grinding and Fluoride-gel Exposure on Strength of Ion-exchanged Porcelain

Abstract

Strengthening of dental porcelain through a diffusion heat treatment at 450°C of a potassium-enriched, ion-exchange surface coating has been demonstrated in several recent studies. However, little attention has been focused on the potential strength reduction of these materials when the treated surfaces are ground or etched under clinically simulated conditions. The objective of this study was to test the hypothesis that partial removal of the surface layers of ion-exchanged porcelains by grinding or exposure to acidulated fluoride gel will significantly reduce their flexure strength. Nine groups of body porcelain disks were ion-exchanged at 450'C for 30 min. One of these groups was subjected to ion exchange and no further surface treatment. Eight specimen groups were subjected to the following procedures after ion exchange: grinding to depths of 50 μm, 100 μm, 150 μm, 200 μm, and 250 μm, and exposure to acidulated fluoride for 30 min, 60 min, and 300 min. A tenth group (FC) was fired at 960°C and fast-cooled in air, but the disks were not subjected to the ion-exchange treatment. Surface stress was calculated from measured values of cracks induced in the treated surfaces. Fluoride exposure for up to 60 min resulted in a significant decrease in surface compression (P ≤ 0.05), although this treatment had no effect on strength. Grinding to a depth of from 100 μm to 250 μm caused a significant decrease in strength, while removal of a 50-μm layer caused no significant change (P > 0.05).