Machinability of ceramic blanks for dental CAD/CAM restorations
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Citation
Abstract
OBJECTIVES: One objective was to evaluate the machinability of a variety of dental ceramic blanks. The other objective focused on evaluating the damage of various CAD/CAM materials arising from machine type, CAM setup, machining parameters, and tool quality.
MATERIALS & METHODS: The tested materials were VITA Mark II (feldspathic block), Ivoclar e.max CAD (un-crystallized lithium disilicate), and VITA Enamic (interpenetrating phase ceramic), Dentsply Sirona InCorsTZI (3Y-TZP Zirconia), and 3M Lava Ultimate (composite resin). A new set of burs were used for each material. The first phase of the experiment was an assessment to determine machining damage utilizing the merlon fracture test. STL files of Merlons with the wall thickness of 0.1, 0.2, 0.3, 0.4, and 0.5 mm were used for the test. Four merlons of different thicknesses were machined from 14 mm blocks on two separate milling machines of Dentsply Sirona MCX5 and MCXL equipped with inLab CAM 20.0.1 software. All merlon specimens were milled under the normal speed milling using wet milled with MCXL and wet/dry milled with MCX5. The unfractured merlon walls were counted. The thickness at which 100% of specimens had unbroken walls was designated as the minimum machined thickness. The edge chipping on the merlons’ wall was characterized. The chip counts, chipping length and chipping factor were compared by the effect of different machining conditions using statistical software JMP Pro 15. The second phase of the experiment aimed to evaluate the damage that occurred from different machining modes of speed, which included surface and subsurface damage, surface roughness as well as the related flexural strength of each material. Rectangular bars with the dimension of 4.0 mm x 2.0 mm x 12.0 mm (width x thickness x length) were machined using the Sirona inLab software system. Eight bar specimens of each material were milled by MCXL and MCX5 Sirona inLab milling Dental Systems using two machining speed modes normal and fast. The effects of different machining conditions on surface roughness and flexural strength were statistically analyzed.
RESULTS: Merlon thickness, material type as well as the interactions between milling unit and material type, and thickness and material had significant effects on the merlon success rate. The merlon test indicated that Incoris Zirconia TZI had less machinability using MCX5 whereas 3M Lava Ultimate had better machinability using MCX5. The edge chipping analysis in this study showed IPS e.max CAD, VITA Mark II, 3M Lava Ultimate, and Enamic had a similar pattern when compared to the total chipping count for MCX5 and MCXL in both sides – inner and outer. The chipping count for MCX5 was lower than MCXL accounting for all these materials. However, for TZI, the trend was opposite as MCX5 had higher chipping count over MCXL in both the inner and outer side of the material. For IPS e.max CAD with MCXL, the chipping factor (CF) result showed a significantly higher chipping factor of 65.94% for inner walls and 55.89% for outer walls compared to those of 3M Lava Ultimate, which showed the lower CF of 12.8% for the inner walls and 21.7% for the outer walls. In this study, surface roughness RMS was found significantly related with the material type and machine used. (4-axis) MCXL-milled bars had a higher surface roughness than (5-axis) MCX5-milled bars. TZI had the lowest surface roughness followed by IPS e.max CAD, VITA Mark II, then 3M Lava Ultimate and the highest surface roughness was for Vita Enamic and MCX5 machine. Similar trend was observed with MCXL in which TZI had the lowest surface roughness, whereas the highest surface roughness was for IPS e.max CAD. The flexural strength of bars machined by MCXL and MCX5 at two different speeds were investigated. The flexural strengths of 3M Lava Ultimate were 171.20 ± 19.05 MPa and 164.73 ± 21.78 MPa for fast and normal speed using MCX5, respectively. Under the normal milling speed, the highest flexural strength of 819.16 ± 97.14 MPa was InCoris TZI using MCX5 while the lowest flexural strength of 93.43 ± 7.55 MPa was Vita Mark II using MCXL. The coefficient of variation (CV) of flexural strength of IPS e.max CAD was 19.60% and 16.64% for fast speed and normal speed, which were the highest for MCX5.
CONCLUSION & CLINICAL IMPLICATIONS: For the Merlon test, minimum machining thickness varied based on material type and milling machine system. The minimum machined thickness of 0.5 mm was recommended for all materials in both machines. The milling unit (MCXL, MCX5), material, and side (inner, outer) had a major impact on the chipping factor. There was no significant difference in surface roughness for the area (edge/middle of each bar), speed, or interaction between material and area. The milled bars by MCXL had a higher surface roughness than the milled bars by MCX5. Under the normal milling speed, all materials milled by MCX5 had greater flexural strength than those milled by MCXL except for 3M Lava Ultimate. The materials with normal milling demonstrated higher flexural strength than those with fast milling.