The quantitative effect of diamond grit size on the subsurface damage induced in dental adjustment of porcelain surfaces.

Diamond burs with different grit sizes are often applied to adjust ceramic prostheses in restorative dentistry. However, the quantitative influence of diamond grit size on subsurface damage in adjusting ceramic prostheses is unknown. The aim of this study was to investigate and visualize the quantitative effect of diamond bur grit size on subsurface damage in dental adjusting of a feldspar prosthetic porcelain. Diamond burs with coarse (106-125 rm), medium (53-60 microm), and fine (10-20 microm) grit sizes were selected. Dental adjusting-induced subsurface damage was quantitatively investigated with the aid of finite element analysis (FEA) and scanning electron microscopy (SEM). Significant differences in subsurface damage depth were found among the coarse, medium, and fine diamond burs (ANOVA, p < 0.05). Coarse diamond burs induced approximately 6-8 times deeper subsurface damage than fine burs. Diamond grit size is confirmed to be a controlling factor in determining the degree of subsurface damage. Subsurface damage depths also significantly increased with removal rate (ANOVA, p< 0.05). The correlation of the SEM-measured subsurface damage depths and the diamond grit sizes supports the FEA predictions. From a practical standpoint, dental porcelains should be adjusted using smaller diamond grit sizes with lower removal rates to minimize subsurface damage.

Stress and damage at the bur-prosthesis interface in dental adjustments of a leucite-reinforced glass ceramic.

This article reports on the effects of dental adjustment parameters on stress and damage induced in a leucite-reinforced glass ceramic using a high-speed dental handpiece and coarse diamond burs. As one of machinable dental ceramics for prosthetic restorations, a leucite-reinforced glass ceramic has higher fracture toughness than feldspar porcelains. However, the extent of subsurface damage and stress induced in clinical dental adjustments is unknown. Tensile, shear, compressive and von Mises stresses at the bur-ceramic interface were investigated as functions of dental adjustment parameters using finite element analysis (FEA). The depths of subsurface damage were predicted using FEA according to the maximum principal stress criterion and experimentally measured using scanning electron microscopy (SEM). The resulting predicted subsurface damage depths agree well with the experimentally measured data. Both adjustment parameters, depth of cut and feed rate, were found to have significant influences on adjustment-induced stresses (P < 0.01) and subsurface damage (P < 0.01). It is also found that the predicted and measured subsurface damage depths increased linearly with the diamond grit depth of cut.

In vitro study on high rotation deep removal of ceramic prostheses in dental surgery.

In vitro study on high rotation (up to over 300,000 rpm) deep removal (up to 150 microm) of ceramic prostheses, made of a machinable ceramic, Vita Mark II, was performed in dental surgery using a high-speed dental handpiece. Dental clinical removal relevance, including tangential and normal grinding forces, specific grinding energy, and surface roughness, was investigated to establish the relationships among the surgery parameters, chip geometry, and fracture mechanism. The results show that both the tangential and normal grinding forces increased with increases in both depth of cut and maximum undeformed chip thickness, but decreased with an increase in grinding speed. Specific grinding energy decreased with increase in the depth of cut and the maximum undeformed chip thickness, but increased with an increase in grinding speed. Surface roughness and morphology appeared to be independent of the increases in depth of cut, grinding speed, and maximum chip thickness. The limitation for deep removal using the dental handpiece was found that the operation at the depth of cut of 150 microm or beyond resulted in a huge normal force exertion of 3 N with a great variation. The microfracture, the lateral fracture, and the ductile microcutting were found to occur simultaneously in dental surgery to remove the ceramic prostheses.

Surface integrity and removal mechanism in simulated dental finishing of a feldspathic porcelain.

Dental abrasive finishing of a fine-grained feldspathic porcelain was performed on a computer-assisted apparatus for simulation of a 2-degrees-of-freedom restorative operation with a dental handpiece and a coarse diamond bur of grit size of 106-125 mum. Finishing forces, surface roughness, and morphology were investigated as functions of finishing conditions. The tangential and normal forces were measured using a piezoelectric dynamometer and a data processing system. The results indicated that these forces increased with either the depth of cut or with the feed rate, in the ranges of 0.12-0.31 N and 0.45-1.09 N, respectively. However, an increase in either depth of cut or feed rate affected neither the surface roughness measured using a stylus profilometer nor the morphology observed under a scanning electron microscope. The finished porcelain surfaces were found to consist of the microfracture and chipping areas, ductile removal areas, smeared areas, and debris. Irregular fracture and chipping resulted from the extension of lateral/median cracks; ductile micromachining was attributed to the plastic deformation accompanied by distributed microcracks. It was determined that a combination of the microfracture and ductile micromachining was the primary mechanism for material removal.

Performance evaluation of a dental handpiece in simulation of clinical finishing using a novel 2DOF in vitro apparatus.

This paper reports on the performance evaluation of a dental handpiece in simulation of clinical finishing using a novel two-degrees-of-freedom (2DOF) in vitro apparatus. The instrumented apparatus consisted of a two-dimensional computer-controlled coordinate worktable carrying a dental handpiece, a piezoelectric force dynamometer, and a high-speed data acquisition and signal conditioning system for simulating the clinical operations and monitoring the dental finishing processes. The performance of the dental handpiece was experimentally evaluated with respect to rotational speed, torque, and specific finishing energy under the applied clinical finishing conditions. The results show that the rotational speeds of the dental handpiece decreased by increasing either the depth of cut or the feed rate at a constant clinically applied air pressure and water flowrate. They also decreased when increasing both the tangential and normal finishing forces. The specific finishing energy decreased with an increase in either depth of cut or feed rate, while the finishing torque increased as either the depth of cut or the feed rate was increased. Implications of these results were to provide guidance for proper applications of dental handpieces in clinical practice.

[Study on chitosan-gelatin/hydroxyapatite composite scaffolds--preparation and morphology].

OBJECTIVE: To prepare chitosan-gelatin/hydroxyapatite (CS-Gel/HA) composite scaffolds, and to investigate the influence of components and preparing conditions to their micromorphology. METHODS: The CS-Gel/HA composite scaffolds were prepared by phase-separation method. Micromorphology and porosity were detected by using scanning electron microscope and liquid displacement method respectively. RESULTS: Porous CS-Gel/HA composite scaffolds could be prepared by phase-separation method, and their density and porosity could be controlled by adjusting components and quenching temperature. CONCLUSION: The study suggests the feasibility of using CS-Gel/HA composite scaffolds for the transplantation of autogenous osteoblasts to regenerate bone tissue.