Dental gypsum is routinely used as an investing and molding material for prosthodontics restoration with some acceptable linear dimensional changes in the finished restorations. This study aimed to evaluate linear dimensional stability and surface hardness of dental stone type Ⅳ when adding silica nanoparticles to it. 40 type Ⅳ stone specimens were prepared from stainless steel molds for linear dimensional stability and plastic mold for hardness, 20 of which contained silica nanoparticles (test group) while the other 20 were without any addition (control group). Each group was divided into subgroups containing 10 specimens for each one in order to compare the dimensional stability and hardness of them. The linear dimensional changes were measured using a digital caliper, while the hardness was tested with Vickers' hardness test. The data were analyzed with t-test (two-sample assuming unequal variances) and p-value. The mean value of linear dimensional changes for the control group was -0.396 and -0.386 for specimens of the test group. Mean hardness was 50.638 for the control group and 53.295 for the test group. The difference was significant for linear dimensional changes and highly significant for hardness. Adding of silica nanoparticles improved the hardness and reduced the linear dimensional changes of type Ⅳ stone.

The chemical modification of conventional acrylic resin by butadiene-styrene rubber to get high-impact acrylic resin has not been completely effective due to its potential bad influence on the resistance of bending or flexural and also the toughness of acrylic resin substance for the base of denture. So, to overcome this detrimental effect of rubber modification of acrylic resin, we tried to reinforce it in this research by the addition of titanium oxide and aluminum oxide nanoparticle mixture to the liquid (monomer) of acrylic resin (high-impact) with sonication at 3 wt% (TiO₂: Al₂O₃ ratio1:1) which was then blended with polymer (powder) of acrylic resin using conventional procedure to form nanocomposite. 40 specimen of two groups (20 for each group) according to the performed property, 20 specimens for flexural strength group and 20 specimens for impact strength group were prepared. Each group was divided into two subgroups, control group A: 10 samples of pure high-impact acrylic, and study group B: 10 samples of modified high-impact acrylic resin by 3wt% nanoparticle mixture. Testing of the flexural strength was examined by flexural testing universal machine (Instron), while testing of the impact strength was examined by impact testing Charpy's machine. The results were gathered from the tests and translated by statistical program (SPSS version 20) for analysis. The flexural strength and the impact strength increased with high significance upon the addition of 3 wt% nanoparticle mixture, according to t-test (Two-Sample Assuming Unequal Variances) and p-value.