[Investigating the effect of bone radius, thickness and strength on stress distribution of bone tissue around a dental implant using finite element method]

The goal of this study was to investigate the stability of a dental implant which is implanted in clinical models of the mandible and to evaluate the effectiveness of strength, radius and thickness of the bone around a dental implant by using finite element method. A 3D wire frame model of the mandible was developed based on computer tomography data of a 25-year-old subject in MIMICS. This model was smoothed in CATIA. Finally, a 3D model of the dental implant was assembled in the mandible and finite element analysis was done on the model with various material properties, loading condition and constraints in ABAQUS. In the clinical models, the highest stress was found in the mandible with a thin layer of cortical bone around the core of the cancellous bone with the lowest density. It was shown that increase of the cortical bone radius resulted in the decrease of stress in the mandible. The 2 mm thickness of the cortical bone around a dental implant was evaluated as the critical thickness of the bone. Increase of stiffness of the cortical or cancellous bones resulted in increase of the stress in that specific part of the bone. In addition, the increase of the bone thickness was more considerable in less than 2 mm thicknesses

[Comparison of compressive strength of glass-ionomer and zinc phosphate cements]

Insufficient compressive strength of dental cements leads to crack formation and further complications such as unrestrained restorations, secondary caries and pulpal infections. The aim of this study was to determine the compressive strength of two kinds of glass ionomer [GI] and zinc phosphate cements [ZPH]. In this study, the compressive strength of two kinds of zinc phosphate cements manufactured by GC [Elite cement 100] and Ariadent company was evaluated. Furthermore, two kinds of glass inomoer cements made by GC [Fuji I] and Ariadent company were examined. Five cylindrical samples were prepared according to ISO 9917 from each cement. After processing the samples in incubator and distilled water for 23 +/- 0.5 hours, they were examined in Instron mechanical tester. The data were analyzed by one way ANOVA and Tukey HSD methods. The compressive strength of GI cements made by GC and Ariadent were 187=/-40 Mpa and 129 +/- 10 Mpa, respectively. Moreover, compressive strengths of ZPH cements made by GC and Ariadent were 83 +/- 21 and 123 +/- 21 Mpa. According to the results of one way ANOVA, GI cement made by GC had significantly higher compressive strength as compared to other cements p<0.05]. Tukey HSD test showed no significant difference between GI and ZPH cements manufactured by Ariadent [p>0.05]. Glass ionomer cement manufactured by GC company had a significantly higher compressive strength compared to other cements. However, GI and ZPH cements made by Ariadent were acceptable according to ISO 9917 standards

[Fracture resistance of two different designs in fiber-reinforced composite restorations in mandibular anterior bridges: an in-vitro study]

The purpose of this study was to assess the fracture resistance of two different designs of tooth preparation and fiber placement in fiber-reinforced composite restorations replacing a missing mandibular lateral incisor. Forty newly extracted human mandibular intact teeth [20 centrals and 20 canines] were selected for fabrication of twenty FRC inlay bridges. A box preparation design with straight fibers, and a slot design preparation with curved fibers were implemented. After preparation, the teeth were mounted in self-cured acrylic resin with 6.5 mm distance from each other. PDL was simulated with polyether material. After taking impressions with polyether material, the impressions were poured with dental stone. The two groups of bridges were fabricated and bounded to teeth with panavia F[2]. The cyclic load of 1.2 x 10[6] x 20N x 1.66HZ was applied with 130° angle. The samples were stored in 37°C water for seven days and then thermo cycled [2000 cyles, 5-55°C]. The fracture strength was tested by a universal testing machine [Instron 1195] at a speed of 1 mm/min. The mode of fracture was observed under stereomicroscope. The data was analyzed by using independent sample T-test. The mean fracture resistance for the box design and direct fiber group was 1411.07 N and in the slot and curved fibers group was 377.33 N. The group difference was statistically significant [P=0.012]. It could be concluded that in fiber-reinforced composite restorations, box design with straight fiber has more fracture resistance than slot design with curved fiber

[Reliability of six methods of measuring convergence angle in posterior teeth for single PFM crown]

There are several methods of measuring Convergence Angle in posterior single PFM crown. The aim of the present study was to compare the reliability of these methods. This experimental study was conducted with twenty selected stone dies, numbered 1 to 20. The convergence angles were measured and recorded by five last year dental students using manual and computer assisted methods. The manual methods used were: 1. Photocopy machine, 2. Digital camera, 3. Analogue camera [photography], 4. Overhead projector; the computer assisted methods were: 5. Scanner machine and 6. Microscope. Data were analyzed using One Way Random Effect, and SPSS version 11 statistical software. Coefficient of reliability for methods 1 to 6 was 0.9981, 0.9988, 0.9986, 0.9991, 0.9957, and 0.9994 respectively. No significant difference was detected among these methods and all were found to be reliable. There were no significant differences among the methods evaluated. Moreover, all methods were found to be significantly reliable and can be used for measuring convergence angles

Evaluation of different materials used for post construction and stress distribution in the radicular dentin using finite element method

This study was designed to evaluate the stress distributions of different post materials in radicular dentine for providing a useful clinical guideline. In this experimental study, 5 three - dimensional models of maxillary central incisors were simulated by using ANSYS software and Finite Element Method. The models included: 1] Stainless Steel post 2] Titanium post 3] Carbon fiber post 4] Glass fiber post, and 5] Quartz fiber post. A composite core and PFM crown was constructed for all samples. Each model contained gingival, cortical and spongy bone, PDL and gutta-percha as well. All specimens were subjected to a compressive load at 45-degree angle to its long axis with a constant intensity of 100N. In all models there were two stress concentration sites: 1- a point between middle third and cervical third of the root and 2 - the cervical edge of the root. The amount of stresses between middle and the cervical third of fiber reinforced posts were nearly 1 MPa greater than those of the metallic ones. Compared to other materials, the metallic posts caused less stress in the radicular dentin and therefore are recommended for clinical use. Among fiber reinforced posts, Quartz fiber post showed the lowest stress peak