الفهرس 
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Abstract
Several conditions may be attributed to erosive wear and destruction of the occlusal enamel and even exposure of the underlying dentin. As a result, over eruption occurs to allow for occlusal contact and compensate for such loss.
In such clinical situations a minimally invasive treatment should be adopted to conserve the remaining intact tooth structure and preserve tooth vitality.
With the advent of CAD/CAM, restorative materials and bondodontics in dentistry, stronger restorative materials were introduced that enable the use of high strength restorations in a very thin thickness that could be truly bonded to the underlying tooth structure encouraging the use of minimally invasive preparation designs in the posterior area rather than onlays or complete coverage
Therefore in our present study, the use of occlusal veneers as a new treatment modality utilizing various CAD/CAM materials and various preparation designs restoring worn molars teeth was investigated by applying cyclic loading then fracture resistance testing to all survived restorations under laboratory conditions.
Three different CAD/CAM materials with three different preparation designs were utilized in the present study.
Forty five intact caries free archived extracted human mandibular second molar were selected with approximate similarity in size, shape and root morphology. All teeth were randomly divided into three main groups of 15 teeth each according to the preparation design.
In the first group, a beveled finish line (no axial preparation was adopted. In the second group, Chamfer finish line of 0.5 mm thickness was adopted. While in the third group a rounded shoulder finish line of 1 mm thickness was adopted.
Each group was subdivided into three subgroups of 5 teeth each according to the type of the material used for restoring such teeth. In The first subgroup lithium disilicte ceramic was used, while in the second subgroup, translucent YTZP was used and for the third subgroup nano ceramic composite will be utilized.
CAD/CAM was used for indirect fabrication of all restorations using BruxZir blank and IPS e.max CAD and CERASMART blocks.
The fabricated occlusal veneers were then cemented to their corresponding teeth using resin cement after proper and strict surface treatment. Samples were then fatigue loaded in a masticatory simulator with a weight of 5 kg, which is comparable to 49 N of chewing force at 1.5Hz 120,000 cycles then visualized under magnification for any cracks or marginal chipping.
Afterwards teeth were statically compressed until fracture. All teeth were visualized under magnification to determine the mode of failure.
All data was collected, tabulated and statistically analyzed to detect any significant values among the different groups.
The following results were obtained:
All teeth survived the 120,000 cycle without cracks or marginal chipping.
BruxZir groups recorded the highest fracture resistance mean value (2253.232±277.316) followed by IPS e.max CAD groups (2134.773±189.308) while CERASMART ones recorded the lowest values (1562.775±77.093). Fracture resistance for ceramic posterior veneer was statistically significantly affected by the veneer material although BruxZir and IPS e.max CAD showed non-significant difference among both materials with only few teeth showing catastrophic failure.
Rounded Shoulder design showed the highest fracture resistance mean value (2249.185±246.809) followed by Bevel design (no axial preparation) preparation (1951.788±246.809) while Chamfer one recorded the lowest values (1749.807±201.781). Fracture resistance for ceramic posterior veneer was statistically significantly affected by the finish line configuration although the results were non-significant between (Bevel and Chamfer) and (Bevel and Rounded Shoulder) preparation designs with only few teeth showing catastrophic failure.