Effect of Photo-polymerization Delay on the Bond Strength and Microhardness of Dual-polymerizing Resin Cements.

STATEMENT OF PROBLEM: To fully maximize the potential of dual-polymerizing resin cements, a thorough understanding of how the light- and chemical-polymerizing components interact in a resin system is required. Disorder in the polymerization process between the two components may hurt one of the components versus the other, affecting the overall properties and performance of the resin cements. PURPOSE: Evaluate photo-polymerization delay time on dentin shear-bond strength and Vickers microhardness of dual-polymerizing resin cements. METHODS AND MATERIALS: Shear bond strength (SBS) of self-adhesive (RelyX Unicem 2, 3M ESPE) and adhesive (RelyX Ultimate, 3M ESPE) dual-polymerizing resin cements were evaluated. Dentin specimens (n=80) were prepared for the SBS test according to ISO standard 29022:2013. Teeth were randomly allocated into eight groups based on the type of cement, and photo-polymerization delay times (0, 2, 5, and 10 minutes). Vickers microhardness test (HV) was performed following ASTM E384-17 (n=32) prepared based on cement type and photo-polymerization delay times; specimens were tested after 24 hours of storage. Statistical analysis was performed using two-way ANOVA to determine the individual and combined effects of resin cement type and photo-polymerization delay time on SBS and HV. RESULTS: Resin cement and photo-polymerization delay times for the adhesive cement at 0- and 2-minute pairings had significantly higher SBS means than all other combinations (p<0.0001). Resin cement type was also statistically significant (p<0.0001). Resin cement type and photo-polymerization delay times were not significant (p=0.3550) for HV. CONCLUSIONS: Photo-polymerization delay time affected dentin SBS with higher bond strength when photo-polymerization delay time was performed between 2 and 5 minutes with a self-adhesive resin cement, and between 0 and 2 minutes with an adhesive resin cement. Delaying photo-polymerization time to 10 minutes led to inferior dentin SBS and HV for both self-adhesive and adhesive dual-polymerizing resin cements.

Wear and Color Stability of Preheated Bulk-fill and Conventional Resin Composites.

OBJECTIVES: Bulk-fill resin composite is commonly used in direct restorations. It is recommended that the high-viscosity version of these materials be preheated to improve flowability and adaptability. It is unknown what effects preheating (PH) might have on the wear resistance and color stability of these resin composites. This study compared the wear and color stability of high-viscosity bulk-fill and conventional resin composites with and without PH, using the CIEDE2000 formula. METHODS AND MATERIALS: Thirty-two disc-shaped specimens (n=16, 10×3 mm) were prepared with Filtek One Bulk Fill Restorative (FOBFR; 3M ESPE) and Filtek Supreme Ultra (FSU, 3M ESPE) to determine wear. Each group was divided into subgroups based on preparation conditions, that is, PH and room temperature (RT) (n=8). Thermomechanical fatigue was applied to specimens, and volumetric material loss was calculated using a laser scanner (LAS-20, SD Mechatronik, Feldkirchen-Westerham, Germany) and three-dimensional imaging software (Geomagic, 3D Systems, Rock Hill, SC, USA). An additional 124 disc-shaped specimens (n=64, 10×1 mm) were prepared with FOBFR and FSU, and subgroups were formed based on preparation conditions (PH and RT, n=32). Specimens were aged for 14 days in coffee, grape juice, distilled water, and thermocycling (20,000 cycles, 5°C-55°C). To determine color change, the lightness (L), chroma (C), and hue (H) values were measured at baseline and after 14 days using the CIEDE2000 formula. All data were statistically analyzed, and a 95% significance level was set. RESULTS: In both the PH and RT groups, FSU exhibited greater wear than FOBFR (p<0.05). Both materials were subjected to increased wear as a result of PH. After aging, a significant difference in color change was observed (p<0.05) when controlling for time and resin composite type, except for deionized water and thermocycling (p>0.05). PH had no effect on the color stability of the resin composite at any time (p<0.05). CONCLUSION: The high-viscosity bulk-fill resin composite displayed superior wear resistance and similar color stability compared with its conventional counterpart. PH resin composite may increase its susceptibility to wear and has no effect on its color stability.

Mechanical Properties of Bisacryl-, Composite-, and Ceramic-resin Restorative Materials.

OBJECTIVE: Resin-based materials used in restorative dentistry are introduced at a fast pace with limited knowledge about their properties. Comparing properties of these materials from different restorative categories is lacking but can help the clinician in material selection. This study aimed to compare mechanical properties and wear resistance of bis-acryl-, composite-, and ceramic-resin restorative materials. METHODS AND MATERIALS: Bisacryl-resin (Bis-R, LuxaCrown, DMG), composite-resin (Com-R, Filtek Supreme Ultra, 3M Oral Care), and ceramic-resin (Cer-R, Enamic, VITA Zahnfabrik) specimens were prepared for mechanical tests: fracture toughness (FT) with and without initial thermomechanical loading using a mastication simulator, flexural strength (FS), and flexural modulus (FM), compressive strength (CS), and volumetric wear loss measurement. The datasets for FT and wear resistance were each analyzed using two-way ANOVA followed by pairwise comparisons or Tukey testing as appropriate. The datasets for FS, FM, and CS were analyzed using one-way ANOVA followed by the Tukey test. RESULTS: Analysis of FS, FM, and CS showed significant differences between materials, with all pairwise comparisons between materials showing significance. Analysis of FT resulted in a significant interaction between the material and treatment, with analysis of wear loss showing a significant interaction between the material and the number of cycles. CONCLUSIONS: Cer-R demonstrated superior FT, CS, and wear resistance compared to Bis-R and Comp-R materials. Fracture toughness of Bis-R increased after thermomechanical loading.

Why is there a paucity of clinical trials in Africa?

Disproportionately few clinical trials are undertaken on the African continent, in part due to lingering neocolonial attitudes in the Global North which keep research activity primarily in developing countries, while being skeptical of the abilities of those in the Global South to undertake organized clinical studies. In the era of the COVID-19 pandemic, applicable research and clinical trials should be undertaken in relevant populations in order to extrapolate to a population level. This is all the more important in Africa, which has a rich genetic diversity. We suggest that a lack of organized research ethics committees across the continent and a deficiency of appropriate training are responsible in part for the reluctance of clinical trial organizers in the developed countries of the Global North to engage with medical leadership in Africa. We consider ways of alleviating this problem, including suggesting a pan-continental surveillance of ethics committee agendas and of training, either through the auspices of the African Union or the World Health Organization. In addition, medical leadership in African nations must be encouraged to take ownership of their medical ethics agendas to facilitate decent international clinical trial participation for the good of the continent as a whole.

Interferometric measurements of cusp deformation of teeth restored with composites.

A Michelson interferometry apparatus was used for measurement of the displacement of the buccal cusps of premolars after restoration of MOD preparations with composites. The effects of composite type, cavity size, and hydration conditions were examined. Interferometry permitted real-time measurement of cusp movement as it occurred. Contraction occurred very rapidly, about 1/3 of the 60-minute amount within the two-minute period of exposure to the curing light. Cusp movement was smooth rather than interrupted, indicating lack of microfracturing at deformations of 11-46 microns. Contraction, 0.94% for Heliomolar and 1.2% for P-50, was similar to the linear polymerization shrinkage of the resins. Less cusp movement occurred in small cavities than in large cavities. Hydrated teeth had less cusp movement than dehydrated teeth.

Cusp movement in premolars resulting from composite polymerization shrinkage.

This study examined the effect of cavity size, hydration conditions and type of composite on cusp movement following placement of MOD composite restorations in vitro. A microscope with a micrometer stage was used to measure deflection of cusps for 14 days after initiation of curing. The intercuspal width decreased for the first few minutes after the composites were placed, then increased toward the original dimensions. Teeth with small cavities contracted less than those with large cavities, and recovery was complete or nearly complete only in teeth with small cavities. Hydrated teeth and restorations recovered more rapidly and more completely than those that were dry. Differences in deflection of the cusps were observed with two different restorative materials.

Microleakage between different types of base materials.

This study investigated the sealing properties of various combinations of base materials. Three chemically cured and three light-activated base materials were combinationally used to make 15 sample groups. The samples were thermocycled and immersed in 0.05% crystal violet solution for 1 hour. They were then embedded in clear casting resin, sectioned, and photographed. The interfaces of the samples were digitized. The ratio of the total interface length to the penetration of leakage was calculated and compared. The results indicated that the combination of Cavalite and VLC Dycal materials had the least microleakage and was significantly different from the other 14 groups.

Effects of etching and rinsing times and salivary contamination on etched glass-ionomer cement bonded to resin composites.

This study investigated the effect of salivary contamination on glass-ionomer/resin-composite bond strength after establishment of an appropriate etching and rinsing time for the glass-ionomer cement. Three hundred and thirty samples of glass-ionomer cement (Ketac-Fil) were injected into cavities prepared in acrylic molds. Groups containing 15 samples each were etched for 0, 15, 30, or 60 sec, after which Scotchbond and either Silux or P-30 were applied and polymerized. The samples were stored under water for 24 hr at 37 degrees C and were then tested in shear. A 30-second etching time was found to be optimal when Silux and P-30 were considered together. In a similar manner, a 20-second optimal rinsing time was determined. Samples of glass-ionomer cement were then contaminated with saliva, dried, and bonded to composite resins and tested in shear as before. Significantly lower bond strengths resulted than with the uncontaminated controls. Efforts to clean contaminated samples by rinsing or re-etching failed to increase the bond strength obtained with the contaminated samples.