In vitro rapid intraoral adjustment of porcelain prostheses using a high-speed dental handpiece

In vitro rapid intraoral adjustment of porcelain prostheses was conducted using a high-speed dental handpiece and diamond bur. The adjustment process was characterized by measurement of removal forces and energy, with scanning electron microscopic (SEM) observation of porcelain debris, surfaces and subsurface damage produced as a function of operational feed rate. Finite element analysis (FEA) was applied to evaluate subsurface stress distributions and degrees of subsurface damage. The results show that an increase in feed rate resulted in increases in both tangential and normal forces (analysis of variance (ANOVA), P < 0.01). When the feed rate approached the highest rate of 60 mm min^-1 at a fixed depth of cut of 100 μm, the tangential force was nearly seven times that at the lowest feed rate of 15 mm min^-1. Consequently, the specific removal energy increased significantly (ANOVA, P < 0.01), and the maximum depth of subsurface damage obtained was approximately 110 and 120 μm at the highest feed rate of 60 mm min^-1 using SEM and FEA, respectively. The topographies of both the adjusted porcelain surfaces and the debris demonstrate microscopically that porcelain was removed via brittle fracture and plastic deformation. Clinicians must be cautious when pursuing rapid dental adjustments, because high operational energy, larger forces and severe surface and subsurface damage can be induced.