표면조도가 치과수복용 CAD/CAM 세라믹-고분자 블록의 기계적 물성에 미치는 영향

Abstract

ABSTRACT

Effect of Surface Roughness on the Mechanical Properties of Ceramic-Polymer Blocks for CAD/CAM Dental Restoration

Seong Ki Son D.D.S., Ms.D. Advisor : Prof. Ko Yeong-Mu D.D.S., Ph.D. Department of Dentistry, Graduate School of Chosun University

The ideal dental restoration is replacing lost tooth by a restorative material whose properties are similar to a natural tooth. As the aesthetic demands increase, in addition, ceramics are widely used as dental restorative materials because of their natural appearance, color stability, chemical stability, good mechanical properties and excellent biocompatibility. CAD/CAM technology has been introduced in dentistry 3 decades ago and is rapidly becoming popular. It reduces the number of clinical sessions and increased quality in a shorter period of time. Furthermore, allows to extend the use of ceramics with various composition. Ceramic dental CAD/CAM blocks, however, still experience a relatively high failure chipping due to the inherent brittleness. Excessive wear of the opposing teeth is potential hazard due to the high hardness. Ceramic/polymer composite blocks, namely hybrid ceramics or resin nanoceramics have been introduced to the market recently. They improved the machinability and edge stability. They are easier to operate and repair if necessary. When occlusal adjustment of restoration materials is required at the time of insertion, it is not easy to achieve a surface smoothness by polishing with hand piece as compared to the glazed surface. However, some studies have reported that mechanical polishing can produce equally smooth or smoother surfaces, as compared to glazing. When a ceramic restoration having a surface flaw is placed in the mouth, fracture can occur even it is not subjected to excessive occlusal loading through slow crack growth. Moisture may hasten this process. Therefore, polishing the restoration after occlusal adjustment may be an important factor of the mechanical properties of the ceramic restorations. Composite resin restorations have been extensively reported to degrade over time by pH change and salivary enzymes. Water diffusion through the polymer chains and boundaries with fillers can occur the hydrolytic deterioration. Initially, this process would affect surface properties, however, as time goes by, it also interferes with the bulk mechanical properties, compromising the long-term durability of the restoration. In this study, the effect of surface roughness on the mechanical properties for two different microstructured dental ceramic/polymer CAD/CAM blocks, commercially available Polyglass (Vericom, Korea) and Enamic (Vita, Germany). They were cut into (4.0×1.2×17.0) mm to measure the flexural strength and into (3.0×4.0×17.0) mm to determine the fracture toughness according to ISO 6872. Weibull characteristics and elastic moduli were calculated. Surface roughness was determined using atomic force microscope. When polished with 6 μm diamond paste after grinding, the arithmetic mean roughness decreased to 46.64~48.62%, ten point height decreased to 67.80~69.02%, and the arithmetic mean roughness after polishing with 1 μm diamond paste was 12.51~21.61 %, ten point height decreased to 15.94~18.52%. The stress-strain curves under the flexural load showed that the strain was linearly increased and then destroyed regardless of the surface roughness of both Polyglass and Enamel as the stress was increased. Polyglass showed higher flexural strength than Enamic in all surface roughness, and there was a significant difference between groups 1 and 3 (p<0.05). Polyglass showed the highest flexural strength at 136.31±7.23 MPa at P3, which was significantly higher than P1 and P2 (p <0.05) as the surface roughness decreased. Regarding Enamic, E2 showed a significantly higher bending strength (117.68±7.35 MPa) than E1 (p <0.05), but E3 showed 117.15±11.18 MPa similar to E2. Weibull characteristic strength representing 63.21% probability of breakage was increased as the surface roughness decreased. Polyglass showed higher value than Enamic. The maxima values of Polyglass and Enamic were 139.60 MPa for P3 and 121.96 MPa for E3. Enamic exhibited higher Weibull moduli (12.72±0.52 ~ 19.01±0.67) than those of Polyglass (19.52±0.64 ~ 22.46 ±0.89). The elastic modulus of Enamic was significantly higher than that of polyglass (p<0.05). As the surface roughness decreased, the elastic modulus of both enamel and polyglass was increased to 7.0±0.4 GPa at E3, P3 showed a maximum modulus of elasticity of 5.1±0.8 GPa. The fracture toughness evaluated by single-edged V notch beam method was higher than that of the enamel at the same surface roughness, but there was no significant difference (p>0.05). As the surface roughness decreased, the enamics showed a negative correlation with increasing fracture toughness at both Polyglass and Enamic. The maximum toughness of Enamic was 1.51±0.21 MPa√m at E3 and the maximum at 1.84±0.6 MPa√m at P3.

---------------------------------------------------------------------------- Keywords : Dental restoration, CAD/CAM, Block, Ceramic, Composite resin, Surface roughness, Strength, Weibull analysis, Fracture toughness