الفهرس 
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Abstract
This study aimed to assess the influence of new light curing lab composite, lithium-disilicate glass-ceramic and yttrium-stabilized zirconia-based ceramic on the fracture resistance of maxillary premolars with class II inlay and onlay preparations. Seventy sound maxillary premolars were divided randomly into seven main groups of 10 teeth each. Teeth in the first group were left intact (control group). Teeth in the remaining six groups were prepared with inlay and onlay cavities and restored with new light curing lab composite (SR Nexco), lithium-disilicate glass-ceramic (IPS e.max Press) and yttrium-stabilized zirconia-based ceramic (ICE Zirkon). The inlay and onlay restorations were cemented with a total-etch, dual-curing luting resin composite (Variolink N). All specimens were thermocycled 5000 cycles between 5°C ± 2°C and 55°C ± 2°C. The specimens were subjected to a compressive load in a universal testing machine using a metal sphere that contacted the cusp slopes until fracture occurred. The results were analyzed by 2-way ANOVA and Tukey HSD post-hoc multiple comparison tests. The results showed that there were no statistically significant differences among the means of control group and the groups restored with zirconia ceramic inlays and onlays (P > 0.05). However, statistically significant differences were found among the means of control group and the groups restored with lab composite inlays, lab composite onlays, pressable glass ceramic inlays and pressable glass ceramic onlays (P < 0.05). The resistance of teeth to fracture is significantly influenced by the preparation design and the restorative material. The fracture resistance of prepared teeth for inlay and onlay restorations is inferior to the intact teeth when a lab composite is used. Conversely, when a ceramic material being used, the prepared teeth for inlay and onlay restorations showed a comparable strength to the intact teeth especially zirconia ceramic.