الفهرس 
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Abstract
Purpose: An in-vitro study evaluating the retention and fracture resistance of endocrown restorations fabricated from different CAD/CAM materials luted to maxillary premolars. Materials and methods: Forty-two endodontically treated maxillary premolars were prepared in a standardized method to be restored with endocrown restorations using different materials. They were divided into three main groups (n =14) according to the material used to fabricate endocrown restorations: Endocrowns of group Z (Yttria-partially stabilized zirconia ceramic, Zolid FX), group P (Poly-ether-ether-ketone, PEEK), and group E (Lithium disilicate, IPS e. max CAD). Each group was further subdivided into two subgroups (n=7) according to the test: ZF, PF and EF groups to evaluate fracture resistance and ZR, PR and ER groups to evaluate retention. After proper surface treatment for all endocrowns, the restorations were cemented using self-adhesive resin cement (Rely X U200). All specimens were thermocycled for 5000 cycles in a water bath between 5º C and 55º C. Universal testing machine was used to evaluate the retention and fracture resistance of each material, and all possible failure modes were detected. One-way ANOVA and Tukey’s post hoc significant difference tests were used to analyze the data. Result: There was a significant difference in fracture resistance between three different materials (P< 0.05). The highest mean fracture load value (1488±168.92N) was recorded for the ZF group, while the lowest value (593.71±79.99N) was recorded for the PF group. Statistically, there was a significant difference in the retention between three different materials (P< 0.05). The highest mean retention value (73.51±9.89N) was recorded for the ER group, while the lowest value (29.37±3.37N) was recorded for the BR group. Conclusions: Within the limitations of this in-vitro study, it was concluded that: 1) In terms of fracture resistance, zirconia endocrowns are better than e.max and PEEK endocrowns. However, using PEEK for the same purpose recorded a restorable mode of failure that avoided the possibility of tooth fracture. 2) IPS e.max endocrowns recorded the highest mean value of retention, followed by zirconia and PEEK endocrowns, respectively.