br Statement of the problem

Statement of the problem
Materials and methods Sixty CAD-on restorations were constructed and classified into 3 groups (n = 20) of different core/veneer thickness ratios (Group A 0.5:1 mm, Group B 0.7:0.8 mm, Group C 1:0.5 mm). Each group was subdivided into 2 sub-groups (n = 10) according to the CAD-On veneer translucency (High Translucency HT, Low Translucency LT) (Table 1). A stainless die was constructed to represent the master die for molar preparation (Fig. 1). Instead of using the “multi-layer” design technique of the software for construction of the CAD-on restoration samples, every part of the restoration was designed separately. This was done because the “multi-layer” design technique produces only restorations in an anatomical form while in this ibotenic acid study flat-surface samples with standard thickness were needed to allow accurate measurement of translucency. First, zirconia copings were constructed over the master stainless steel die, from InCoris ZI for InLab blocks. They were designed in the software and milled to high precision in the milling unit, using CEREC Inlab system, with different thicknesses. After milling, zirconia copings were sintered in high-temperature furnace Infire HTC. Following sintering and before the veneering process, the different coping thicknesses were checked using an electronic digital caliper. Zirconia copings were then placed on the prepared stainless steel die and scanned for the construction of the veneering caps. The veneering caps were milled out of IPS e. max CAD of different translucencies, and with different thicknesses that were pre-set in the CAD/CAM software. After the milling process was completed, the thickness of each veneering cap was checked using an electronic digital caliper and its fit was checked over the corresponding zirconia coping. A joining gap for the fusing glass ceramic used for fusion of the coping and veneer together was created through setting the spacer parameter in the InLab software during the step of designing the veneering cap. Homogenous fusion between both the InCoris ZI framework and the IPS e. max CAD took place through specially developed fusion glass-ceramic (IPS e. max CAD Crystall./Connect). Some IPS e. max Crystall./Connect was placed on the occlusal aspect of the zirconia coping and another small amount of the IPS e. max Crystall./Connect was placed in the inner aspect of the IPS e. max CAD veneering structure. The InCoris framework was inserted in the correct position into the veneering structure and the occlusal aspect of the restoration was held against the vibrating plate of the Ivomix. A slight pressure was applied against the Incoris framework so that the IPS e. max Crystall./Connect is evenly squeezed out of the entire circular fusion joint indicating a proper joint with sufficient amount of material (Fig. 2) The crystallization of the IPS e. max CAD material took place during the same firing cycle of IPS e. max Crystall./Connect. Contrast ratio of each sample was calculated according to the following equation; where; CR is the contrast ratio.Lb is the lightness measured over a black background.Lw is the lightness measured over a white background. Unicam spectrophotometer Helios was used to measure the light transmittance % through the restorations. It\'s an UV- Visible spectrophotometer designed for research work. It operates by emitting a light beam that passes through a sample placed in a special cuvete and then measuring the intensity of light reaching the detector (Fig. 3). Before each sample measurement, light intensity passing through a blank (air) (I°) was measured. Each sample was placed in the especially designed holder and placed in the spectrophotometer cell in such a way that light beam passed through it in a cervico-occlusal direction. Light intensity (I) reaching the detector was calculated and for each sample transmittance percentage was calculated where; T = I/ I°
Statistical analysis