2Objectives & Methods
3Test Sample & CNC Milling
4Polishing & Sintering
The purpose of this comparative test is to show the translucency level (total light transmittance: TLT) of 9 commercially available brands of zirconia discs that are most widely used in U.S.A. The level of translucency is one of the most important components in optical characteristics of crown and bridge restorative bio-material
1. Sample Preparation
Total of 9 commercially branded zirconia discs were chosen for a test comparison of translucency levels.
2. Test Sample Size and CNC Milling
Multiple small disc samples were created as STL files using Solidworks (Dassault Systems SolidWorks Corporation, MA, USA) 3D design software. Then the samples were milled using each manufacturers enlargement factor so that the dimensions of each sample would measure 17.5 mm (diameter) X 1.00 mm (thickness) after final sintering and polishing (the samples were slightly thicker than 1.00 mm after milling to allow an exact thickness of 1.00 mm after polishing). An enlargement factor of 1.2500, for example, is equivalent to a shrinkage rate (1/1.2500=0.200) of 20%. This clarification is given since these concepts of enlargement factor and shrinkage rate are often used interchangeably by mistake in the dental industry.
The sample thickness of 1.00 mm was chosen, instead of 0.6mm or 0.5mm, since these zirconia materials being tested are primarily and preferably used for full contour crown and bridge applications of which the thickness of the body portion, mesial, ligual and distal area would be normally 1-2 mm in thickness. Test samples of 0.5 - 0.6 mm thick that are found in publications of other manufacturers would be more appropriate for the application of crown and bridge substructures (copings or frameworks).
3. Polishing & Sintering
After precision CNC milling, both sides of each of the samples were mirror polished using 3M Imperial Wetordry 401 Q (2500A grit) and then surface cleansed with Isopropanol alcohol ((CH3)2CHOH). Each surface was polished to an extent to clearly reflect the light to minimize surface deflection of the light source in the spectrophotometer.
Each different brand of zirconia was sintered according to each manufacturer's recommendation as appears in their Instructions For Use manual or published website information. The sintering schedule for each material type is below. To ensure accuracy and consistency in temperature of the sintering chamber, Process Temperature Control Rings (PTCR from Ferro, Netherlands) were used each time multiple samples were sintered. After going through this sintering process, the samples are understood to be fully sintered and no post HIP processing was done.
Sintering Schedule of Test Samples (Manufacturer’s recommendation)
Doceram Nacera ZT
SageMax NexxZr T
Pentron Zirlux FC
3M Lava Plus
Origin Live Plus
2 - 5
Ramp up time
1510 - 1530
4. Measuring the Translucency
One method of measuring translucency is by determining total transmission, including scattering, using a spectrophotometer with an integrating sphere as shown below. Translucency of a material can be expressed as a transmission coefficient or total (direct and diffused) light transmittance(%) as the relative amount of light passing through the unit thickness of the material. Total light transmittance was measured by a double beam-system spectrophotometer (LAMBDA 35, UV/Vis Spectrophotometer manufactured by Perkin Elmer, USA) based on “Standard test method for transmittance and color by spectrophotometer using hemispherical geometry” of ASTM E1348-11 and “Materials and articles in contact with foodstuffs – Test methods for translucency of ceramic articles” of Dansk Standard/EN 1184.
All the samples were cleaned with isopropyl alcohol before light transmittance measurement. Light emitted from a light source (deuterium lamp and halogen lamp) was passed through a sample and scattered, and the light transmission amount was measured using an integrating sphere.
5. Equipment Used
Perkin Elmer LAMBDA 35 UV/VIS Spectrophotometer, USA
6. Measure of Total Light Transmittance (TLT)
Visible light which had passed through the sample was collected with an integrating sphere to determine the intensity of the visible light (I) as it passed through the sample. On the other hand, the intensity of visible light (I.sub.0) was measured without placing the sample. The total light transmittance was calculated in terms of the proportion of the former to the latter intensity (=I/I.sub.0). A transmission spectrum and digital data record were obtained for each measurement with the light beam entering the samples.
YTZP Zirconia Translucency Test Results
Comparison of Commercially Available High Translucent Zirconia Brands in USA