[Evaluation of some physical properties of the Acropars soft liner and Molloplast-B]

To date no product is available that have all the desirable properties of an ideal resilient material. Therefore, it is necessary to determine certain physical and mechanical properties of new soft liner materials that are introduced in the market. The aim of this study was to investigate several properties of Acropars compared to Molloplast B. Two groups of permanent resilient liner materials, a heat cured acrylic copolymer [Acropars; Marlik Medical Inc., Iran] and silicone based rubber [Molloplast B; Detax Ettlingern, Germany], were tested for hardness, elongation percentage, tensile and tear strengths.Ten dumbbell-shaped specimens were fabricated for each test group according to ASTM-D412 [Die-c] and ASTM-D638 [type 4] standards. The universal testing machine was used to test all specimens for tensile strength, elongation percentage. Ten cylinder shaped specimens of 31x10 mm [in diameter and height] were prepared for hardness test. This test was conducted, using a shore-A hardness instrument, according to ASTM-D2240 standards. Also the standard test method of ASTM-D624 for tear resistance of rubber material was performed. Ten bar shaped specimens of 60x10x2 mm, with a 45 degrees notch 6 mm in width on lower side were prepared. All specimens were forced to failure using a universal testing machine. All specimens were stored at room temperature for 40 hours before testing process started. The mean group differences and student-t test at alpha= 0.05 were used for statistical data analysis. Acropars showed statistically significant greater mean tensile strength [P<0.0001], tear strength [P<0.0001] and hardness [P<0. 0001] compared to Molloplast B. In addition Molloplast B showed a significant higher elongation percentage. [Pv<0.001] Within the limits of the present study, it was demonstrated that Acropars had higher tensile and tear strength, but harder than Molloplast- B

Effect of Abutment Taper on the fracture resistance of all- ceramic three-unit bridges

The connector area is the weakest zone of an all-ceramic fixed partial denture [FPD], where most catastrophic failures of the prostheses tend to occur. The aim of the present study was to assess the effect of the convergence angle of abutments on the fracture resistance of three-unit fixed partial dentures made of IPSEmpress2. Forty extracted human premolars and molars were used to reproduce twenty, 3-unit fixed partial dentures, for the replacement of second premolars. All teeth were prepared according to the guidelines outlined for all-ceramic crowns and bridges, except for the convergence angles of the abutments. The specimens were randomly divided into two groups of 10, with total occlusal convergence angles of 12° and 22°. Fixed partial dentures with a uniform thickness of 0.8 mm were fabricated using IPS-Empress2 and were bonded to the orresponding models. Connector dimensions were set to 4 mm height and 4mm width. The radius of curvature at the gingival embrasure was carved to 0.9 mm. All specimens were exposed to 10,000 preloading cycles and a load of 40 N at a frequency of 1.3 Hz in a standardized testing machine at a cross head speed of 1mm/min. Student t-test was performed to detect any difference in the mean fracture resistance between the two groups [á = 0.05]. Mean failure loads [and standard deviations] of the 12° and 22° groups were 1009.12 N [208.05] and 1182.72 N [144.67], respectively. Statistical analysis revealed a significant difference [P <0.04] between the mean failure loads of the two groups. Most fractures occurred through the connectors. The mean failure loads of the investigated fixed partial dentures were higher in the abutments with 22° taper as compared to those with a taper of 12°