Chemical and Ultrastructural Characterization of Dentin Treated with Remineralizing Dentifrices.

The aim of this study is to investigate dentin chemical and ultrastructural changes upon exposure to remineralizing dentifrices. Dentin disks were obtained from permanent human molars and treated for 7 days with the dentifrices: (1) C group-control (no dentifrice); (2) S group-Sensodyne Repair & Protect; (3) D group-Dentalclean Daily Regenerating Gel; and (4) DB group-D group + Dentalclean regenerating booster. Afterwards, samples were submitted to an additional 7 days of toothbrushing associated with daily acidic challenge. Samples were imaged and analyzed (days 1, 7, and 14) for Young's modulus by atomic force microscopy (AFM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). SEM and AFM revealed precipitate deposition on dentin surfaces in groups S, D, and DB, formed as early as day 1. Surface elemental analysis showed a Si increase on all brushed surfaces. Similar surface morphology was maintained after the acidic challenge period. Bright-field TEM/SAED revealed the formation of nanocrystalline hydroxyapatite inside the dentin tubules of groups S, D, and DB after day 7. Group C presented a gradual reduction of Young's modulus from days-1-14, whereas all remaining groups had increased values. All evaluated dentifrices led to successful formation of hydroxyapatite and increased dentin stiffness.

Effect of artificial aging on optical properties and crystalline structure of high-translucency zirconia.

PURPOSE: To investigate the effect of different in vitro aging protocols on the optical properties and crystalline structure of high-translucency (HT) zirconia. MATERIALS AND METHODS: Thirty-six specimens of HT and extra-high translucency (XT) zirconia were divided into three groups: control (CO)-no treatment; hydrothermal aging (HA)-autoclave aging for 12.5 h at 134°C, 2 bar; clinically related aging (CRA)-aging in the chewing simulator for 1.2 million cycles, followed by 50,000 thermocycles (5-55°C) and immersion in HCl (pH 1.2) for 15 h. Optical properties, crystalline structure, and surface roughness were analyzed and compared using analysis of variance (5% significance level). RESULTS: There was no statistically significant effect of aging on translucency (p = 0.10), but CRA promoted the development of a high contrast ratio (p = 0.03). Aging did not cause significant color changes for HT (p = 0.65) or XT (p = 0.36). The proportion of monoclinic crystals increased to 40% for HT-zirconia after HA and 5% after CRA. No monoclinic crystals were detected for XT groups. There was no effect of aging on surface roughness (p = 0.77). CONCLUSIONS: Although hydrothermal aging has been widely used to verify zirconia crystalline stability, it did not generate an effect similar to clinically related aging on the optical properties and crystalline structure of zirconia. HA affected the crystalline structure of HT-zirconia, and CRA compromised the optical properties of XT zirconia.

Postinsertion adjustment appointments and their association with cast removable partial denture survival.

STATEMENT OF PROBLEM: Successful outcomes with cast removable partial dentures (RPDs) are dependent on adequate fabrication and adjustments at the delivery appointment. Evaluation of the number and frequency of postinsertion follow-up appointments helps determine whether the prosthesis continues to fit comfortably and whether function and esthetics continue to be satisfactory. Reports on the number of appointments and frequency and type of adjustments required for RPDs following insertion are sparse. PURPOSE: The purpose of this university-based population study was to determine the number of appointments and type of adjustments following RPD insertion and their association with patient demographics, type of RPD, and denture survival. MATERIAL AND METHODS: This retrospective clinical study examined the records of 257 patients at the University of Toronto, Faculty Dentistry, wearing 308 RPDs inserted between 2013 and 2014 with a 5-year follow-up. The outcome measures investigated included postinsertion appointments, type of adjustments, and denture survival. RESULTS: A total of 48.1% of the dentures (19.5% tissue supported; and 28.6% tooth supported) were maxillary, and 51.9% (34.7% tissue supported; and 17.2% tooth supported) mandibular. Most patients (68.9%) had 1 to 3 postinsertion appointments, with 78.6% having no major adjustments. Twenty-six dentures failed (failure rate 8.4%), with the failure-free time estimated at 4.58 years (95% confidence interval (CI) 4.42-4.73 years, Kaplan-Meier survival analysis). Failed dentures were significantly associated with more minor adjustments (Mean (M)=4.12, SD=3.90, Kruskal-Wallis (K-W) P=.027; OR=1.18; 95% CI 1.05-1.32, P=.006). More minor adjustments were needed for mandibular dentures (multivariable Poisson regression (MPR) P=.003) compared with maxillary dentures. More major adjustments were needed for maxillary dentures (MPR P=.030) compared with mandibular dentures. More minor and major adjustments were needed for dentures that were remade from within 5 years to beyond 10 years compared with first time denture wearers (MPR P<.001). Patients with musculoskeletal disorders required a significantly higher number of minor adjustments (M=3.67, MPR P<.001) and appointments (M=3.87, MPR P<.001) than those without these disorders. CONCLUSIONS: The 5-year survival of RPDs following insertion was estimated at 91.6%. Most patients required 1 to 3 appointments after insertion. Mandibular RPDs required significantly more minor adjustments, and maxillary RPDs more major adjustments. More minor and major adjustments were needed for dentures that were remade at any time previously compared with first time denture wearers.

Effect of Simulated Gastric Acid on the Interface Between Zirconia and Resin Cement

PURPOSE: To evaluate the effect of simulated gastric acid solution (SGAS) and resin cement composition on the shear bond strength (SBS) of zirconia-based materials with different levels of translucency to composite resin. MATERIALS AND METHODS: A total of 40 medium-opacity (MO; 3Y-TZP) and 40 medium-translucency (MT; 4Y-PSZ) zirconia slabs were distributed into four groups according to the composition of the resin luting system (MDP free or with MDP [primer + Panavia V5]) and storage method (distilled water or SGAS [5% hydrochloric acid]). Composite resin cylinders were cemented on the zirconia surface and stored for 91 hours. SBS, failure mode, and surface characterization analyses via scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were performed. SBS data were analyzed using three-way ANOVA and Tukey test, and failure mode was assessed using one-way ANOVA (P < .05). RESULTS: Storage media (P = .180), resin cement (P = .110), zirconia (P = .404), and their interactions did not affect SBS values. Bond strength ranged from 21.41 to 26.11 MPa. SEM images showed that SGAS modified the surface topography of zirconia and resin cement. The presence of chlorine and silicon (wt%) were higher after SGAS storage than after water storage in both cements used, while barium was higher only for the MDP cement. There was a prevalence of mixed failures for most of the groups. CONCLUSION: The SBS between both types of zirconia and resin cement was not affected by SGAS, although changes in zirconia topography were observed after SGAS exposure. The presence of MDP in the cement layer had no effect on the SBS challenged by SGAS.

The effect of accelerated aging on crystalline structures and optical properties of different monolithic zirconia: A qualitative systematic review.

OBJECTIVE: This study aimed to systematically review the literature related to the impact of low temperature degradation (LTD) on the crystalline structures and optical properties of different types of dental monolithic zirconia materials. METHODS: The systemic review was performed based on the PRISMA statement. In vitro studies investigating the effect of accelerated aging in autoclave (2 bar pressure 134°C - ISO standard 13356-2008) on the crystalline structure and/or optical properties of Yttria-partially stabilized zirconia (Y-PSZ) were included. Specific search terms were used for peer-reviewed articles published in PubMed, Ovid MEDLINE, and EMBASE databases. RESULTS: From 286 eligible articles, 51 articles were selected for full-text analysis, 10 failed to meet the inclusion criteria, and 41 articles were included in this review. Autoclave aging (30 min - 300 h) results in an increase in monoclinic phase (m) content up to 80% for tetragonal zirconia and reaching saturation after 35 h of autoclave aging. All included articles reported less than 1% of monoclinic phase for cubic zirconia after autoclave aging. Translucency parameter was reported between 2.34 and 19.7 after autoclave aging (4-100 h). For same aging time, contrast ratio ranged between 0.48 and 0.95. SIGNIFICANCE: An increase in monoclinic phase was reported for tetragonal zirconia, while cubic zirconia demonstrates resistance to LTD. The optical properties for all zirconia materials investigated seem more compromised with increasing aging time.

Improving effectiveness of dental students' feedback and course evaluation.

Dental students providing feedback about a course they take, in a timely manner, benefits not only teachers, but also indirectly the students themselves, especially if given with confidence in a constructive manner. Therefore, the aim of this study was to train students on how to give feedback, to ask them to provide feedback before and after the instructions were given, and analyze the change in their responses. Participants were students who attended the second-year preclinical course in prosthodontics. They were asked to provide feedback anonymously with online surveys after completing modules of the course during the academic year. There was no intervention prior to the first feedback; however, before providing the second feedback, students were asked to read a 1-page handout related to feedback modalities. Following this, an interactive workshop in feedback was provided prior to the third survey. The received responses were ranked as either: neutral, positive, negative, or constructive and were analyzed using a mixed repeated measures test with Bonferroni correction at a 0.05 significance level. The results showed a higher number of constructive and positive responses than both neutral and negative feedback (P ≤ 0.05) within the same surveys, but no interaction effect was found between the surveys (P = 0.076). Our data showed an increase in constructive feedback provided by students after the 2 different training methods, but the modality of delivery did not seem to significantly influence the results. In summary, training students on how to provide constructive feedback may be beneficial for teachers to improve their courses.

Effect of ionizing radiation and chewing simulation on human enamel and zirconia.

PURPOSE: To evaluate the effect of ionizing irradiation on human enamel and zirconia after chewing simulation. METHODS: Twenty enamel and twenty translucent Yttria-stabilized zirconia (Y-PSZ) specimens were divided in 4 groups: Co (control) - no irradiation on enamel cusps/opposing zirconia slabs; E70 - irradiated (70 Gray) enamel cusps/opposing irradiated enamel slabs; Z70 - irradiated zirconia cusps/opposing irradiated zirconia slabs; EZ70 - irradiated enamel cusps/opposing irradiated zirconia slabs. Cusps were abraded against slabs in a chewing simulator (CS - one million cycles, 80 N, artificial saliva, 37˚C). Wear and roughness of zirconia and enamel were analyzed using a stylus profilometer. The abraded enamel was analyzed by Electron probe micro-analyzer (EPMA) and zirconia was characterized by nanoindentation and X-ray diffraction. One-way analysis of variance (ANOVA) and Tukey test were used for analysis of wear, Repeated Measures and Bonferroni test for roughness, and hardness and modulus values were compared using Wilcoxan Mann Whitney rank sum test (overall 5% significance). RESULTS: Significantly higher volume loss was presented by cusps in the E70 group (p<0.001). Wear was similar between Co and EZ70 groups. There was no significant effect of irradiation on roughness of enamel or zirconia slabs (p=0.072). Irradiated Y-PSZ slabs had significantly higher hardness and modulus than non-irradiated ones and a 7% increase in m phase content was detected after irradiation. CONCLUSIONS: The opposing surface characteristics played a more significant role on enamel wear than did ionizing radiation. However, radiation affects Y-PSZ crystalline composition, hardness and modulus of elasticity.

Irradiation therapy and chewing simulation: effect on zirconia and human enamel.

PURPOSE: Ionizing radiation therapy (RT) is the main option for head and neck cancer treatment, but it is associated with multiple side effects. This study aimed to evaluate the effect of RT associated with chewing simulation on the surface of human enamel and Yttria-partially stabilized zirconia (Y-TZP). METHODS: Maxillary premolar cusps and Y-TZP slabs were divided in 7 experimental groups: CO: no RT (control); EZ groups had irradiation applied to both, enamel and zirconia samples (simulating restoration prior to RT); E groups had irradiation applied to enamel only (simulating restoration after RT). RT doses were either 30, 50 or 70 Gray (Gy). Enamel cusps were abraded against zirconia slabs in a chewing simulator (CS - one million cycles/ 80 N/ 60 mm/min, 2 mm horizontal path, artificial saliva, 37˚ C). Zirconia hardness was evaluated before CS; zirconia roughness and enamel volume (wear) were evaluated before and after CS. Hardness and wear data were analyzed by one-way Analysis of Variance and Tukey post hoc test. Roughness was analyzed by Repeated Measures test and Bonferroni test (p=0.05). RESULTS: There was no significant effect of enamel or zirconia irradiation on enamel cusp wear (p=0.226), regardless of the irradiation dose used - up to 70 Gy. Irradiation also did not affect Y-TZP surface roughness (p=0.127) and hardness (p=0.964). CONCLUSIONS: RT does not promote significant changes to the surface characteristics of zirconia. Irradiated enamel abraded against zirconia does not show higher wear volume when compared to non-irradiated enamel.

Effect of cleaning protocol on silica deposition and silica-mediated bonding to Y-TZP.

OBJECTIVES: To evaluate the effect of cleaning methods on the deposition of silica on yttria-stabilized zirconium dioxide (Y-TZP) surface and on the silane-silica mediated bond strength between Y-TZP and resin cement. METHODS: Y-TZP slabs were air-abraded with 30µm silica-coated alumina particles and distributed in three groups: no cleaning, cleaning with a stream of oil-free air/water spray for 5s and cleaning with an ultrasonic bath in water for 10min. The distribution of Si on the Y-TZP surfaces was recorded using energy dispersive spectroscopy (EDS). After the treatment was applied, Y-TZP slabs (n=20) received a primer application and resin cement cylinders were built on the surface. After storage (24h) in water storage or 3 months plus thermocycling; n=10), microshear bond strength test (µSBS) was performed. X- ray Photoelectron Spectroscopy (XPS) characterized the chemical bonds between the silica layer and the silane-containing primer. Data were analyzed using ANOVA and Tukey test, as well as Weibull analysis (α=0.05). RESULTS: Cleaning method had a significant effect on the amount of Si deposited on zirconia surface (p<0.001) and, consequently, on bond strength (p<0.001). Storage/aging also had a significant effect on bond strength (p<0.001). Low values of Weibull moduli for bond strength were observed for all groups after aging. XPS showed silane-silica chemical interaction for all groups. SIGNIFICANCE: The silica deposited by tribochemical coating to Y-TZP was removed by the cleaning methods evaluated, compromising bond strength. Stability of the bonding is also a concern when no cleaning method is applied.