Huan Lu

Surface hardness of resin composites after staining and bleaching.

This study investigated the effect of 3 staining solutions and 3 over-the-counter tooth-bleaching systems on the microhardness of 2 dental resin composites. Forty-five specimens of Filtek Supreme and Esthet-X were randomly assigned to 3 groups. Over a 40-day test period, the specimens in each group (n=15) were immersed in 1 of the 2 staining solutions (coffee and red wine) or distilled water as the control for 3 hours a day at room temperature. The 15 specimens in each staining group were further randomly divided into 3 subgroups, and the specimens in each subgroup (n=5) were bleached using one of the bleaching agents (Night Effects, Simply White Night and Opalescence Quick). Surface hardness was measured at 24 hours after polymerization (baseline), after staining and after bleaching. Means and standard deviations were calculated, and the data were analyzed using repeated-measures analysis of variance and Duncans Test. The microhardness of Esthet-X was significantly higher than Filtek Supreme at baseline (p<0.01). All specimens of both materials immersed in coffee and wine revealed a significant hardness decrease compared to baseline values (p<0.05). In the control group, microhardness was increased, and this increase was statistically significant for Filtek Supreme (p<0.05). After bleaching, there was a significant decrease in mean microhardness for all groups tested (p<0.05). No significant difference was found among bleaching agents.

In vitro bond strengths of amalgam added to existing amalgams.

This study determined if a standardized condensation force and dwell time per condensation pressure point could reliably bond new amalgam to older amalgam without applying extrinsic Hg. A stabilization jig was created to hold 15 friction-fit 1-inch diameter (25 mm) cylindrical resin specimen blocks face up with cavities drilled to contain the condensed primary amalgam (Valiant PhD-XT). Freshly mixed secondary amalgam (Valiant PhD-XT) was condensed against the primary amalgam surfaces through the 3.5-mm-diameter central holes of specially fabricated split-ring molds. The 15 disks fit snugly within the holes of the stabilization jig tray. Condensation was with a consistent, calibrated force of 22.5 MPa (4 lbs/0.79 mm2) applied with a spring-loaded amalgam carrier custom adapted with a 1-mm-diameter stainless steel condenser tip. Secondary amalgam additions were built up in three 1-mm thick increments with a pattern of eight 22.5 MPa two-second condenser strokes per incremental layer. Shear-bond testing with a 1-mm/minute crosshead speed occurred 24-hours post-condensation. One-way analysis of variance statistical analysis was conducted to analyze the results. The mean shear-bond forces (MPa, N = 15) found were: Control 28.1 +/- 5, 15 minutes 31 +/-5, one hour 10.7 +/-4 (N = 30), one day 25.5 +/- 4, one week 25.2 +/- 4, two months 25.1 +/- 5 and seven years 24.7 +/- 4. Under the condensation pressures used in the current study, the addition of new amalgam to smooth previously set amalgam surfaces, not including the one-hour group, up to seven years old, resulted in shear-bond forces not statistically different (p = 0.05) from the intact control. Virtually all (94%) of the bonds tested, except for the one-hour sample, resulted in cohesive rather than adhesive failures except those of the one-hour sample. Forty percent bond strengths of the controls were achieved when only 5.6 MPa (1 lb/0.79 mm2) and 14 MPa (2.5 lb/0.79 mm2) condensation pressures were used.