الفهرس 
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Abstract
This study was carried out to evaluate the effect of load cycling on fracture resistance of a bioactive restorative material and comparing it with a zinc oxide nanoparticle containing bulk fill composite.
A total of 60 upper premolar teeth were used in the study. MOD cavities were prepared and teeth were randomly divided into three groups. In the first group 20 specimens, cavities were kept unfilled (control group) (positive control). The remaining 40 specimens were classified into 2 groups of 20 specimens each according to testing restorative materials (A); group (A1) was restored by Bioactive restorative material, group (A2) was restored by Fill Up bulk fill restorative material.
Each group was subdivided into 2 subgroups of 10 specimens each according to the number of load cycles the specimen was received (T), the first sub group (T1) was subjected to cyclic loading representing 24 hours ( 417 cycles) and the second subgroup (T2) was subjected to cyclic loading representing 6 months (75000 cycles ). Control group also further divided into two subgroups each had 10 specimens according to number of load cycles.
Each tested restorative material was applied and cured in the prepared cavities according to manufacturer’s instructions, after application of selective etching technique using 37% phosphoric acid etching gel and One Coat Universal bonding agent according to manufacturer’s instructions
To mimic the intra-oral conditions the specimens were subjected to mechanical fatigue in a pneumatic fatigue device. Mechanical loading test was conducted using the newly developed four stations multi- modal ROBOTA chewing simulator which has four chambers simulating the vertical and horizontal movements simultaneously in the thermodynamic condition. All samples were individually mounted on a computer controlled materials testing machine with a load cell of 5 kN and data were recorded using computer software. The load at failure manifested by an audible crack and confirmed by a sharp DROP at load-deflection curve recorded using computer software. The load required to fracture was recorded in Newton which changed into MPa according to equation MPa = N/mm2.
The analysis of variance ANOVA test revealed that:
1- Fill Up bulk fill composite had the significantly the highest fracture resistance values.
2- There was a statistically significant difference between groups that loaded equivalent to 24 hours and that equivalent to 6 months. There was significant decrease in the mean values of fracture resistance that loaded for 6 months.
3- Fracture resistance means values significantly influenced by mechanical loading. (p=<0.0001<0.05) where (24h > 6m cyclic loading).
Under the limitation of the current study the following conclusions could be derived:
1. Fill Up resin composite has the ability to withstand the mechanical and thermal loading and restore the weakened tooth structure without fracture.
2. Application of cycling loading for longer periods negatively affects the fracture resistance of tested restoratives.
3. Chewing simulator is a valuable research tool for simulation of the oral environmental condition that helps in evaluating the longevity and success of any tested material.