الفهرس 
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Abstract
Aim: The aim of this in-vitro study was to measure the fracture resistance and relative stiffness of root canal treated premolars restored with two different preparation designs of ceramic restorations (ceramic partial coverage restorations with palatal cuspal coverage and ceramic partial coverage restoration with palatal and buccal cuspal coverage).
Methodology: The present study was conducted using 16 extracted human maxillary premolars. Teeth were divided into two equal groups (n=8): group (A) received an onlay with palatal cusp coverage and group (B) received an overlay with total cuspal coverage. The strain was measured by using strain gauges under axial, compressive, non- destructive force (150 N with cross-head speed 0.5 mm/min) by using universal testing machine; in three instances: M1: sound tooth sample, M2 Prepared tooth sample and M3 restored tooth sample. After M1, root canal treatment and preparation for the two groups were done. M2 was measured. Afterwards, the restorations were designed from exocad Dental CAD software. Lithium di-silicate CAD blocks were milled using MCX5 milling machine and cementation was done. M3 was performed. All Specimens were then subjected to fracture with cross-head speed 1.00 mm/min.
Results: Within fracture resistance results; there was no statistically significance between the two preparation designs. While the relative stiffness results showed that the restored overlay (group B) is significantly higher than the restored onlay (group A).
Conclusion: Within the limitation of this study, it was concluded that:
Both Partial coverage restorations designs (overlay and onlay) restored the relative stiffness of the tooth structure and could work as an alternative line of treatment for restoring endodontically treated maxillary bicuspids