Stress Distribution in Peri-implant Bone in the Replacement of Molars with One or Two Implants: A Finite Element Analysis.

Statement of the Problem: In most cases, insertion of single implants with a standard diameter is used to replace a molar tooth but placing two implants with a narrow diameter seems to be a viable treatment modality to withstand functional and biomechanical forces. Purpose: This study aimed to evaluate and compare stress distribution in the bone surrounding a single molar area rehabilitated by a single implant versus two implants with a narrow diameter. Materials and Method: The study was conducted by computer-aided in vitro modeling. The initial model used a single implant, 4.8 mm wide in diameter, inserted with a 3.9-mm distance from both sides and 12.6-mm mesiodistal space. The second model used two 3.3-mm narrow-sized implants with a 3-mm distance from one another, 1.5 mm from both sides, and a 12.6-mm mesiodistal space. Following the completion of these models, a 100-N force was exerted obliquely, once in three locations and once in the mesial aspect of the implant-supported crown. Stress distribution was then measured using finite element analysis (FEA) with ANSYS Workbench software package in both models. Results: The maximum stress in the bone around the single implant was less than that around double implants. The maximum stress of cortical bone in three-point loading was lower than mesial loading either in one (146.7 vs. 126.72 MPa) or two implants model (186.8 vs. 139.24). Conclusion: According to the results, because of more cortical bone contact area, the stress of surrounding bone in wide implant was decreased.

Assessment of peri-implant bone stress distribution with the effect of attachment type and implant location using finite element analysis.

Background: The objective of the current research was to evaluate how stress is distributed in the peri-implant bone of a mandibular overdenture with implants placed asymmetrically to the midline. Methods: A 26-year-old male's mandible, with missing teeth, was examined using computed tomography (CT) scanning. Two implants were inserted at right angles to the occlusal plane, in the positions of the right canine and left lateral incisor of the mandible, with an internal connection. Two types of attachments (bar and ball) were designed. To simulate the clinical condition, anterior (on central incisors) and bilateral posterior (on premolars and molars) loadings were applied. The stress distribution was assessed using finite element analysis (FEA). Results: The lateral incisor level implant was found to have the highest maximum principal stress (about 33 MPa) in both models in the anterior loading condition. However, in both models, the canine-level implant revealed more stress values (about 22 MPa) in the posterior loading condition. Conclusion: In mandibular implant-supported overdentures, when implants were placed asymmetrically to the midline, one acted as a fulcrum and sustained more occlusal load. The bar attachment system did not reveal superior results in terms of stress distribution compared to the ball attachment.

Comparison of the effect of post space preparation time on the apical seal of two different sealers.

BACKGROUND: The present study compared the effect of post space preparation time on the apical seal of two different sealers. METHODS: In the in vitro study, 94 central incisors were used. After the samples' root canal preparation, they were randomly assigned to four experimental groups (n = 21). The samples in groups 1 and 2 were obturated with AH Plus sealer, gutta-percha, and in groups 3 and 4 with Endoseal MTA bioceramic sealer and single cone technique. The post spaces in groups 1 and 3 were prepared immediately and in groups 2 and 4 with a delay. The samples were evaluated at 7-, 30-, and 90-day intervals for apical microleakage using the fluid filtration technique. The data were analyzed with SPSS 25, using three-way ANOVA and independent t-test. RESULTS: The apical microleakage in groups 3 and 4, obturated with Endoseal MTA bioceramic sealer and prepared immediately and after a delay, respectively, was not significantly different between the interval times. In group 2, obturated with AH Plus sealer and prepared for post space with a delay, the apical microleakage was significantly less than all the other groups. Group 1, obturated with AH Plus sealer and prepared for post space immediately, exhibited the least microleakage after seven days, but its microleakage increased over time to reach the level of groups 3 and 4. CONCLUSION: According to the results, the apical microleakage in the AH + sealer group and the delayed post-space preparation method, was significantly less than all the other groups over time.

The effect of attachment types and implant level on the stress distribution in a mandibular overdenture: a 3D Finite Element Analysis / O efeito dos tipos de sistemas de retenção e nível dos implantes na distribuição de tensões em overdentures mandibulares: Análise por Elementos Finitos 3D

Objective: The objective of this study was to evaluate the stress distribution in bone-surrounding implants placed in different levels in a mandibular overdenture. Material and Methods: A Computerized Tomography (CT) scan of an edentulous mandible was used to generate the models. Two implants with an internal connection were placed perpendicular to the occlusal plane in the Canine sites of the mandible. The implant in the left side was placed 1mm higher than the other side. Dolder bar and ball attachments were designed. Loading was performed by clenching the teeth with reconstruction of the muscles. In the anterior loading condition, force was applied to the central incisors and in the posterior loading conditions, it was applied to the molars and premolars. Then the Maximum Principal Stresses in the peri implant bone was evaluated with finite element analysis. Results:In both models, the highest stress values were recorded in the cortical bone surrounding the higher implant except in the ball model with unilateral load application on the right side (64.7 MPa). In almost all loading conditions the stress value differences in models with bar and ball attachments were low. Only in the anterior loading condition, the stress magnitude was higher in two implants of the ball model (60.5 MPa in the left side and 21 MPa in the right side) compared to the bar model (54.5 MPa in the left side and 17.5 MPa in the right side). Conclusion: The stress concentration did not affected considerably by the attachment system. High stress values were found adjacent the implant with a higher level. To reduce the amount of stress, bilateral balance occlusion should be considered. (AU)

Objetivo: O objetivo deste estudo foi avaliar a distribuição de tensões em tecido ósseo ao redor de implantes instalados em diferentes níveis em uma Overdenture mandibular. Material e Métodos: Uma tomografia computadorizada (TC) de uma mandíbula edêntula foi usada para gerar os modelos. Dois implantes de conexão interna foram instalados perpendicularmente ao plano oclusal na região de caninos inferiores. O implante do lado esquerdo foi instalado 1mm acima do que do lado direito. A barra Dolder e o pilar tipo bola foram projetados. A carga foi realizada apertando os dentes com reconstrução dos músculos. Na condição de carga anterior, a força foi aplicada nos incisivos centrais e nas condições de carga posterior, foi aplicada nos molares e pré-molares. Em seguida, as Tensões Máximas Principais no osso periimplantar foram avaliadas com análise de elementos finitos. Resultados: Em ambos os modelos, os maiores valores de tensão foram registrados ao redor do osso cortical ao redor do implante superior, exceto no modelo tipo bola com aplicação de carga unilateral no lado direito (64,7 MPa). Em quase todas as condições de carregamento, as diferenças nos valores de tensão nos modelos com fixações de barra e tipo bola foram baixas. Apenas na condição de carregamento anterior, a magnitude da tensão foi maior em dois implantes do modelo tipo bola (60,5 MPa no lado esquerdo e 21 MPa no lado direito) em relação ao modelo barra (54,5 MPa no lado esquerdo e 17,5 MPa no lado direito). Conclusão:A concentração de tensão não foi afetada considerando o sistema de retenção. Maiores valores de tensão foram encontrados adjacentes ao implante com um nível mais alto. Para reduzir a quantidade de tensão, a oclusão bilateral balanceada deve ser considerada.(AU)

Evaluation of the relationship between religious beliefs and academic achievements of dental students.

INTRODUCTION: Considering the importance of the academic level of university graduates in the advancement of communities, it is necessary to evaluate factors affecting the academic achievements of university students. Therefore, this study investigated the relationship between religious beliefs and academic achievements of students in the Dental School in Qazvin University of Medical Sciences. MATERIALS AND METHODS: In this correlational study, all the dentistry students in the last 4 years of education were selected as the study samples. Of 109 students who had the inclusion criteria, 81 (31 males and 50 females) completed the questionnaires. The data collection tools were three questionnaires, including Gluck-Stark's questionnaire, two researcher-made questionnaires on "sympathy with and responsibility for patients," and the "self-satisfaction" questionnaire. Data were analyzed with descriptive and deductive statistics (t-test, Friedman test, and Pearson's correlation coefficient). RESULTS: According to the analyses, a significant and positive correlation was found between religiosity and students' overall mean grades (r = 0.27, P = 0.016) or "sympathy with and responsibility for patients" (r = 0.21, P = 0.05). No significant correlation was found between self-satisfaction and religious beliefs. CONCLUSION: Based on the results of the present study, the religious beliefs of the students were at a moderate level. On the other hand, this variable had a direct relationship with academic achievement and responsibility. Therefore, attention has to be paid to the promotion of these beliefs among students in colleges. Reinforcement of religious beliefs will improve the students' academic achievement. Based on the findings of this study, it should be considered by university professors and educational planners that the average score of students with higher religiosity was higher, and with the increase of religious beliefs, academic achievement is increased.

In vitro comparison of the effects of microwave irradiation and chemical and mechanical methods on the disinfection of complete dentures contaminated with Candida albicans.

BACKGROUND: Dentures can be a source of infection or cross-contamination with microorganisms like Candida albicans. The aim of this in vitro study was to compare the effects of microwave irradiation, chemical techniques, and a mechanical method (i.e., brushing) on the disinfection of complete dentures contaminated with C. albicans. MATERIALS AND METHODS: In this experimental study, sixty sterilized dentures were divided into six groups of 10 dentures each. The dentures in Groups 1 and 6 served as negative and positive controls, respectively. The dentures (Groups 2-5) were contaminated with C. albicans and subjected to four disinfection procedures: Corega tablets, 2% glutaraldehyde, brushing, and microwave irradiation. Replicate aliquots (25 mL) of the suspension were cultured in Sabouraud dextrose broth. The colonies were counted after 48 h of incubation at 37°C. To confirm long-term disinfection, the Trypticase soy broth (TSB) containers were stored at 37°C for 7 days, and turbidity was visually observed. Data were analyzed with one-way ANOVA and independent-samples t-test on SPSS the level of statistical significance was set at 0.05. RESULTS: The dentures disinfected with microwave irradiation (650 W, 3 min) and glutaraldehyde (2%, 10 min) exhibited no evidence of fungal growth after 48 h of incubation and also no turbidity in the TSB containers after 7 days of incubation. However, the dentures disinfected using the mechanical method and Corega tablets exhibited turbidity after 7 days and fungal growth after 48 h that was significantly more than that in the two other methods (P = 0.000) and less than that in the positive control group (P = 0.000). The differences between mechanical cleaning and cleansing tablet were not significant (P = 0.017). CONCLUSION: Base on the results of this study, microwave irradiation (650 W, 3 min) and 2% glutaraldehyde completely disinfected the dentures contaminated with C. albicans in the short term and long term.

Stress distribution pattern of screw-retained restorations with segmented vs. non-segmented abutments: A finite element analysis.

Background. Screw-retained restorations are favored in some clinical situations such as limited inter-occlusal spaces. This study was designed to compare stresses developed in the peri-implant bone in two different types of screw-retained restorations (segmented vs. non-segmented abutment) using a finite element model. Methods. An implant, 4.1 mm in diameter and 10 mm in length, was placed in the first molar site of a mandibular model with 1 mm of cortical bone on the buccal and lingual sides. Segmented and non-segmented screw abutments with their crowns were placed on the simulated implant in each model. After loading (100 N, axial and 45° non-axial), von Mises stress was recorded using ANSYS software, version 12.0.1. Results. The maximum stresses in the non-segmented abutment screw were less than those of segmented abutment (87 vs. 100, and 375 vs. 430 MPa under axial and non-axial loading, respectively). The maximum stresses in the peri-implant bone for the model with segmented abutment were less than those of non-segmented ones (21 vs. 24 MPa, and 31 vs. 126 MPa under vertical and angular loading, respectively). In addition, the micro-strain of peri-implant bone for the segmented abutment restoration was less than that of non-segmented abutment. Conclusion. Under axial and non-axial loadings, non-segmented abutment showed less stress concentration in the screw, while there was less stress and strain in the peri-implant bone in the segmented abutment.

Effect of Storage Period on Dimensional Stability of Alginplus and Hydrogum 5.

OBJECTIVES: This study aimed to evaluate the effect of storage period on dimensional stability of Alginplus and Hydrogum 5. MATERIALS AND METHODS: In this in vitro experimental study, 60 impressions were taken of an upper jaw typodont, including 10 impressions for each storage period to be tested (12 minutes, 24 and 120 hours) for each type of alginate. Then, the impressions were stored in an incubator with stable temperature and humidity, and poured using a type III dental stone. Subsequently, the mesiodistal dimension, occlusogingival height, and interarch distance were measured using a digital caliper with an accuracy of 0.01mm. The data were analyzed using ANOVA and t-test (P<0.05). RESULTS: Alginplus and Hydrogum 5 impressions were not significantly different from the master model after 12 minutes and 24 hours in terms of dimensions (P>0.05). After 120 hours, all dimensions measured on casts were significantly different from those measured on the master model, except for the mesiodistal dimension of the Hydrogum 5 impressions. CONCLUSIONS: At a consistent temperature and humidity, the Alginplus and Hydrogum 5 impressions were dimensionally stable for at least 24 hours.

Effect of Nickel-Chromium and Non-Precious Gold Color Alloy Cast Posts on Fracture Resistance of Endodontically Treated Teeth.

INTRODUCTION: Root canal-treated teeth are weaker than vital teeth and are more susceptible to fractures. Therefore, special precautions should be adhered to, such as the use of various types of cast or prefabricated posts. Regarding the effect of post material on fracture resistance of teeth, the aim of this in vitro study was to evaluate the effect of non-precious gold color alloy (NPG) and nickel-chrome (Ni-Cr) cast posts on resistance of endodontically treated teeth. METHODS AND MATERIALS: In this study, 30 freshly extracted single-rooted premolar teeth were randomly divided into two groups. After root canal treatment, post patterns were made with Duralay in group 1 and cast with Ni-Cr alloy; in group 2, the patterns were cast with NPG alloy. Zinc phosphate cement was used for cementation in this study. Shear force was applied at 1 mm/min at 45ºC to the buccal cusps until root fracture occurred. Independent sample t test was used for data analysis by using SPSS version 21. The level of significance was set at 0.05. RESULTS: Mean fracture resistance values were 1380±454 N for Ni-Cr versus 1964±640 N for NPG, with significant differences (P=0.007). CONCLUSION: The fracture resistance of endodontically treated teeth with non-precious gold color alloy cast post was higher than that of endodontically treated teeth with Ni-Cr cast post.

A comparison of the efficacy of mechanical, chemical, and microwave radiation methods in disinfecting complete dentures.

BACKGROUND: Poor denture hygiene can be a potential source of pathogens. The aim of this study was to compare the efficacy of microwave radiation with that of chemical and mechanical techniques in disinfecting complete dentures contaminated with Staphylococcus aureus and Pseudomonas aeruginosa. MATERIALS AND METHODS: Seventy-two sterilized mandibular dentures were separately contaminated with S. aureus (n = 32) and P. aeruginosa (n = 32) and then incubated at 37°C for 48 h. The contaminated dentures were disinfected as follows: chemical disinfection with Corega tablets; chemical disinfection with 2% glutaraldehyde; mechanical disinfection by brushing the denture; and physical disinfection by 650-W microwaves irradiation for 3 min with six samples in each subgroup. Six dentures served as negative control group, and six contaminated dentures with no disinfection served as the positive control group. 10-3-10-6 dilutions were cultured in the nutrient agar, and the colonies were counted after incubation at 37°C for 48 h. To evaluate the lasting time of disinfection, the containers with nutrient agar and dentures were stored for 7 days at 37°C to evaluate turbidity. Data were analyzed using Kruskal-Wallis and Mann-Whitney U-test (α = 0.05). RESULTS: There was no evidence of bacterial growth in 48 h and turbidity after 7 days of incubation of dentures disinfected by microwaves, glutaraldehyde, and Corega tablets, which was statistically significant compared to the positive controls (P < 0.001). In mechanically disinfected dentures (brushing), bacterial growth was detected after 48 h which was statistically significant compared to the positive controls (P < 0.001) and turbidity was seen in all the nutrient agar plates. CONCLUSION: Microwave iradiation, 2% glutaraldehyde, and Corega tablets disinfected complete dentures contaminated with S. aureus and P. aeruginosa which lasted for a long and a short terms.

Fabrication of an Implant-Supported Orbital Prosthesis with Bar-Magnetic Attachment: A Clinical Report.

Implant-supported craniofacial prostheses are made to restore defective areas in the face and cranium. This clinical report describes a technique for fabrication of an orbital prosthesis with three adjacent implants in the left lateral orbital rim of a 60-year-old woman. Selection of appropriate attachment system (individual magnetic abutments versus bar-clip attachment) for implant-supported orbital prostheses depends upon the position of implants. Bar-magnetic attachment has been selected as the retention mechanism in the present case.