Raghu Srinivasan

Studies on the metastable phase retention and hardness in zirconia ceramics

Abstract In the development of tough ceramic materials, partially stabilized zirconia (PSZ) has been considered as the analogue of steels which exhibit transformation-induced plasticity. The main reason for the high strength of the PSZ system appears to be the existence of a metastable tetragonal phase which is strongly dependent on several factors such as the percentage of stabilizer, the sintering temperature, the soak time and the nature of the ZrO2 raw material. In the present work the influence of the percentage of YO1.5 stabilizer and sintering temperature on the tetragonal phase retention and hardness was explored. ZrO2YO1.5 mixtures containing 0–15 mol.% YO1.5 were prepared, pelletized and fired at various temperatures from 1100 to 1500°C. The tetragonal phase content, density, porosity and hardness of the pellets were evaluated. It was found that the percentage of tetragonal phase increases with YO1.5 content and decreases as the sintering temperature is increased. The effect of partial stabilization in increasing the hardness was clearly demonstrated.

Effect of organic versus inorganic fluoride on enamel microhardness: An in vitro study

Introduction: Dental caries is one of the most prevalent infectious diseases affecting the human dentition. Fluorides are effective anti-carious agents and have been widely used for caries prevention in the form of systemic and topical fluorides. Neutral sodium fluoride (NaF) is commonly used as a topical fluoride agent. A special category of topical fluorides are organic fluorides in the form of amine fluorides (AmF). Researchers have reported that AmF is superior to inorganic fluorides in improving the caries resistance of enamel due to the significant anti-enzyme effect of the organic fragment. Aim: The aim of the present study was to compare the enamel surface micro hardness after topical application of NaF and AmF solutions. Materials and Methods: Twenty fresh samples of sound human enamel were treated with demineralizing solution for 72 h and divided into Group A (treated with NaF) and Group B (treated with AmF) solutions for 3 min twice daily for 7 days. In between treatment, the samples were stored in artificial saliva. The enamel surface hardness was measured with Vickers hardness test at baseline, post-demineralization and post-treatment with two different fluoride solutions (NaF and AmF) and a comparative analysis was made. Results: The increase in mean micro hardness of human enamel after treatment with AmF application was found to be statistically significant (P < 0.01) when compared to the mean micro hardness after treatment with NaF. Conclusion: Fluoride enhances the remineralization process by accelerating the growth of enamel crystals that have been demineralized. It can be concluded from the present study that AmF compounds result in a marked increase in enamel micro hardness when compared to NaF.

Optimizing tooth form with direct posterior composite restorations.

Advances in material sciences and technology have provided todays clinicians the strategies to transform the mechanistic approach of operative dentistry into a biologic philosophy. In the last three decades, composite resins have gone from being just an esthetically pleasing way of restoring Class III and Class IV cavities to become the universal material for both anterior and posterior situations as they closely mimic the natural esthetics while restoring the form of the human dentition. In order to enhance their success, clinicians have to rethink their protocol instead of applying the same restorative concepts and principles practiced with metallic restorations. Paralleling the evolution of posterior composite resin materials, cavity designs, restorative techniques and armamentarium have also developed rapidly to successfully employ composite resins in Class II situations. Most of the earlier problems with posterior composites such as poor wear resistance, polymerization shrinkage, postoperative sensitivity, predictable bonding to dentin, etc., have been overcome to a major extent. However, the clinically relevant aspect of achieving tight contacts in Class II situations has challenged clinicians the most. This paper reviews the evolution of techniques and recent developments in achieving predictable contacts with posterior composites. A Medline search was performed for articles on “direct posterior composite contacts.” The keywords used were “contacts and contours of posterior composites.” The reference list of each article was manually checked for additional articles of relevance.

Disinfection of gutta-percha cones using three reagents and their residual effects

Aims and Objectives: The purpose of this study was to compare the efficacy of 5.25% sodium hypochlorite, 2% chlorehexidine and MTAD in disinfecting gutta-percha cones and to analyze the surface topography of gutta-percha cones after the rapid chemical disinfection procedure. Materials and Methods: Gutta-percha cones were immersed in suspensions of Enterococcus faecalis and Staphylococcus aureus separately. The cones were then immersed in 5.25% NaOCl, 2% CHX and MTAD for 30 seconds, 1 minute and 5 minutes separately. The disinfected cones were then incubated in thioglycollate media for 7 days. The thioglycolate media was sub-cultured and colony forming units were counted. For topographical examination of gutta-percha cones, the cones were immersed in respective solutions for one minute and allowed to air dry for 30 minutes after rinsing or without rinsing the cones with distilled water. The topography of the cones were then analyzed under SEM. Data was statistically analyzed using one way anova. Results: MTAD was found to be the most effective disinfecting solution. The topographical examination of GP cones found some deposits after the disinfection procedure in every group. These deposits were removed when the GP cones were rinsed with distilled water. Conclusions: MTAD possesses superior bactericidal activity when compared with NaOCl and CHX and a final rinse with distilled water is essential after the disinfection procedure.

A comparative evaluation of the effect of 8% and 17% ethylenediaminetetraacetic acid exposure for 1 min and 10 min on the fracture resistance of endodontically treated roots: An in vitro study

Objective: The objective of this study was to evaluate the effect of 8% and 17% ethylenediaminetetraacetic acid (EDTA) exposure for 1 min and 10 min on the fracture resistance of endodontically treated roots. Methodology: Sixty human single-rooted teeth were decoronated and divided into six groups (n = 10). Canal preparation was carried out except in negative control group, using the ProTaper rotary file system. Final irrigation was performed using distilled water, 17% EDTA for 1 min and 10 min, 8% EDTA for 1 min and 10 min. Thereafter, roots were obturated with ProTaper F3 Gutta-percha points and AH Plus sealer using a single-cone technique. The specimens were loaded vertically at 1 mm/min crosshead speed until vertical root fracture occurred. Results were evaluated statistically with one-way analysis of variance and post hoc Tukey test. Results: Analysis of results showed that the unprepared roots showed the highest fracture resistance and roots irrigated with 17% EDTA for 10 min showed the lowest fracture resistance. The mean fracture resistance of unprepared roots, roots irrigated with 8% EDTA for 10 min and 17% EDTA for 1 min was significantly higher than roots irrigated with 8% EDTA for 1 min, 17% EDTA for 10 min and distilled water. Conclusion: From a clinical viewpoint, if EDTA has to be used, it is safer to use higher concentration for a shorter application time or a low concentration with a longer application time. Prolonged use of high concentrations of EDTA might increase the risk of root fracture.

Antimicrobial effect of three disinfecting agents on Resilon cones and their effect on surface topography: An in vitro study

Aim: The objective of this study was to evaluate the effectiveness of 5.25% sodium hypochlorite (NaOCl), 2% chlorhexidine (CHX), and 2% peracetic acid (PAA) in disinfecting Resilon cones and to evaluate topographical changes microscopically under scanning electron microscope (SEM) after rapid chemical disinfection. Materials and Methods: Resilon cones were disinfected in an ultraviolet (UV) light chamber for 20 min and contaminated by immersing in a microbial suspension of Enterococcus faecalis for 30 min. The contaminated cones were then immersed in the 5.25% NaOCl, 2% CHX, and 2% PAA for 1 min, 5 min, and 10 min, separately. The cones were then incubated at 37΀C in thioglycollate broth for 7 days and examined for turbidity. The samples showing turbidity were subcultured on blood agar and incubated at 37΀C for 48 h. Gram staining was done to confirm that the cultured bacteria were E. faecalis. Surface changes of disinfected Resilon cones were evaluated under SEM. Statistical Analysis Used: The data were analyzed statistically using Kruskal-Wallis, analysis of variance (ANOVA), and Mann-Whitney U-test. Result: In eliminating E. faecalis, 5.25% NaOCl was most effective followed by 2% PAA and 2% CHX. Topographic examination of tested Resilon cones revealed some surface deposits after disinfection with 5.25% NaOCl and 2% CHX, whereas 2% PAA caused surface erosion. Conclusion: In disinfecting Resilon cones, 5.25% NaOCl is most effective followed by 2% PAA and 2% CHX.

Evaluation of smear layer removal from ultrasonically prepared retrocavities by three agents.

Objectives: To evaluate the efficacy of 35% orthophosphoric acid, 24% ethylenediaminetetraacetic acid (EDTA) and 10% citric acid in the removal of smear layer from retrocavities prepared with ultrasonic retro-tips using scanning electron microscopy. Materials and Methods: Root canals of fifty single-rooted teeth were cleaned, shaped, and obturated with gutta-percha and AH plus sealer. The apical 3 mm of each root was sectioned with a diamond disc and retrograde cavities were prepared with Kis # 1 ultrasonic retro-tips to a depth of 3 mm. Retro cavities in Group I were treated with a gel of 35% orthophosphoric acid for 15 seconds, Group II were treated with a gel of 24% EDTA at neutral pH for 2 minutes, and Group III were treated with a gel of 10% citric acid for 2 minutes, followed by 1 minute rinsing with distilled water for all groups. The samples were prepared for scanning electron microscopic observation. Scoring was performed for the presence of the smear layer on the walls of the retrocavity. Results: In the orthophosphoric acid group, it was observed that all dentinal tubules were open in 70% of the samples. The majority of analyzed samples in EDTA and citric acid group showed dentinal tubules covered with the smear layer. Conclusions: Application of 35% orthophosphoric acid gel for 15 seconds on retrocavities prepared with ultrasonic retro-tips is the most effective means for removal of smear layer.

An in vitro evaluation of effect of eugenol exposure time on the shear bond strength of two-step and one-step self-etching adhesives to dentin

Objectives: To evaluate the effect of the eugenol exposure time of an eugenol-based provisional restorative material on the shear bond strength of two-step and one-step self-etching adhesives to dentin, at three different time intervals of 24 h, 7 days, and 14 days. Materials and Methods: Forty extracted human posterior teeth were sectioned mesiodistally to obtain two halves and the resulting 80 halves were randomly assigned into four groups of 20 specimens each (Group-I, -II, -III, and -IV). Cavities of specified dimensions were prepared to expose dentin surface. In Group-I, temporarization was carried out with noneugenol cement (Orafil-G) for 24 h (control group). In Group-II, -III, and -IV, temporarization was carried out with eugenol cement (intermediate restorative material (IRM)) for 24 h, 7 days, and 14 days, respectively. Each group was further divided into two subgroups of 10 teeth each for two-step (Adper SE Plus) and one-step (Adper Easy One) self-etch adhesive systems, respectively. A plastic tube loaded with microhybrid composite resin (Filtek Z-350, 3M) was placed over the dentin surface and light cured. The specimens were subjected to shear stress in universal testing machine. Results: Group-II yielded low shear bond strength values compared with Group-III, -IV, and Group-I, which was statistically significant. Conclusions: The prior use of eugenol containing temporary restorative material reduced the bond strength of self-etch adhesive systems at 24-h period. No reduction in bond strength at 7 or 14 days exposure was observed with either two-step or one-step self-etch adhesive.

Interdisciplinary approach for the treatment of multiple adjacent recession type defects

Multiple adjacent recession type defects in the dentition of an individual are routinely encountered in the clinical practice, and hence present a challenge for management. If left untreated or unmonitored, they can be detrimental to periodontal or dental health. Patient esthetic demands have become so stringent that root-coverage procedures carried out should provide soft-tissue anatomy comparable to and indistinguishable from adjacent tissue. A combination of semilunar flap and veneers was used to treat such a kind of defect in a 45-year-old female patient complaining of unsightly and long appearing teeth since 4 years. The present case report demonstrates that the semilunar flap can be an effective treatment for the management of multiple recession defects affecting adjacent teeth. This surgical technique resulted in 100% root coverage of all the anterior teeth except 22 at 10-month post-treatment examination. Further, follow-up is required to evaluate the stability of this treatment. Clinical relevance to interdisciplinary dentistry Biologic width violation is one of the contributing factors for gingival recession. Multiple adjacent recession type defect with altered pink to white ratio in the esthetic zone is a challenge for management. Altered pink to white ratio in such cases may be treated with a combination of semilunar flap and veneers. Team approach needed to decide on the location of the gingival margin and veneer margin to create ideal, stable dentogingival relationships.

Influence of restorative technique, Bevelling and aging on composite bonding to sectioned incisal edges: A comparative in vitro study

Aim: To evaluate the effect of direct and indirect technique, bevel placement, and aging on the fracture resistance of composite restorations bonded to sectioned incisal edges. Materials and Methods: Incisal thirds of 80 human maxillary incisors were sectioned. Four treatment groups of 20 teeth each were formed as follows: Direct composite with bevel and without bevel (Adper single bond 2/Filtek Z350); indirect composite with bevel and without bevel (prepolymerized Filtek Z350 cemented with Adper single bond 2/Relay × ARC). Ten teeth from each group were stored in distilled water for 24 h. The remaining ten teeth were stored in distilled water for 180 days with two thermocycling treatments. The specimens were subjected to shear testing using universal testing machine. Fractured specimens were examined with a stereomicroscope at ×10 magnification to evaluate the failure patterns. The obtained data were statistically analyzed by using ANOVA, Bonferroni test, and Students t-test. Results: Beveled restorations exhibited higher fracture resistance values than nonbeveled restorations. Long-term water storage decreased the fracture resistance. Conclusion: The use of bevels resulted in improved fracture resistance of composite restorations and reduced the impact of aging.