Simulated occlusal adjustments and their effects on zirconia and antagonist artificial enamel.

PURPOSE: The aim of this study was to evaluate the effect of occlusal adjustments on the surface roughness of yttria-tetragonal zirconia polycrystal (Y-TZP) and wear of opposing artificial enamel. MATERIALS AND METHODS: Twenty-five Y-TZP slabs from each brand (Lava, 3M and Bruxzir, Glidewell Laboratories) with different surface conditions (Control polished - CPZ; Polished/ground - GRZ; Polished/ground/repolished - RPZ; Glazed - GZ; Porcelain-veneered - PVZ; n=5) were abraded (500,000 cycles, 80 N) against artificial enamel (6 mm diameter steatite). Y-TZP roughness (in µm) before and after chewing simulation (CS) and antagonist steatite volume loss (in mm3) were evaluated using a contact surface profilometer. Y-TZP roughness was analyzed by three-way analysis of variance (ANOVA) and teatite wear by two-way ANOVA and Tukey Honest Difference (HSD) (P=.05). RESULTS: There was no effect of Y-TZP brand on surface roughness (P=.216) and steatite loss (P=.064). A significant interaction effect (P<.001) between surface condition and CS on Y-TZP roughness was observed. GZ specimens showed higher roughness after CS (before CS - 3.7 ± 1.8 µm; after CS - 13.54 ± 3.11 µm), with partial removal of the glaze layer. Indenters abraded against CPZ (0.09 ± 0.03 mm3) were worn more than those abraded against PVZ (0.02 ± 0.01 mm3) and GZ (0.02 ± 0.01 mm3). Higher wear caused by direct abrasion against zirconia was confirmed by SEM. CONCLUSION: Polishing with an intraoral polishing system did not reduce the roughness of zirconia. Wear of the opposing artificial enamel was affected by the material on the surface rather than the finishing technique applied, indicating that polished zirconia is more deleterious to artificial enamel than are glazed and porcelain-veneered restorations.

Effect of salivary esterase on the integrity and fracture toughness of the dentin-resin interface.

Human Salivary Derived Esterases (HSDE) are part of the salivary group of enzymes which show strong degradative activity toward the breakdown of one of the most common monomers used in dental composites and adhesives, 2,2-[4(2-hydroxy 3-methacryloxypropoxy)-phenyl] propane (Bis-GMA), to form the degradation product 2,2-bis [4 (2,3-hydroxy-propoxy)phenyl] propane (Bis-HPPP). This study was aimed to evaluate the effects of HSDE on the biodegradation and fracture toughness of the adhesive resin-dentin interface. Adhesive resin (Scotchbond Multi Purposes), resin composite (Z250) and mini short-rod specimens, were either not incubated; or incubated in phosphate-buffered saline (PBS) or HSDE media for up to 180 days (37 degrees C, pH 7.0). The amount of Bis-HPPP was analyzed by high performance liquid chromatography and mini-SR specimens were tested for fracture toughness using universal testing machine following 30, 90, or 180-day incubation periods. Significantly higher amounts of Bis-HPPP were produced in HSDE than in PBS incubated specimens (p < 0.05). Non-incubated mini-SR specimens had the higher fracture-toughness values, while specimens incubated for 180-days in HSDE had the lowest fracture toughness (p < 0.05). This study suggests that biodegradation is an on-going clinically relevant process that progressively compromises the integrity of the critical resin restoration-adhesive interface, as well as the resin-composite component with time.

Flexural strength and modulus properties of carbamide peroxide-treated bovine dentin.

BACKGROUND: The effects of carbamide peroxide bleach on tooth structure are uncertain. PURPOSE: The purpose of this study was to investigate the effects of direct or indirect carbamide peroxide application on dentin flexural strength and modulus in vitro. MATERIALS AND METHODS: Dentin bars were machined from bovine incisors and treated with 10% carbamide peroxide for 6 h/d for 14 days. Four groups were created, with eight bars in each group: group 1-direct bleach application only; group 2-indirect bleach application only; group 3-direct bleach application followed by a 2-week storage in artificial saliva; group 4-direct bleach application followed by a 2-week storage in artificial saliva and daily topical fluoride treatments. The specimens' flexural strength and modulus were tested after 24 hours, and the results were compared with water controls using independent t-tests (p < .05). RESULTS: Flexural strength results (in megapascals) for bleached versus control dentin (1=205+/-26 vs 215+/-14, 2=257+/-25 vs 261+/-14, 3=180+/-22 vs 193+/-36, 4=157+/-18 vs 184+/-11) were significantly lower in group 4 (p=.005). Modulus results (in gigapascals) for bleached versus control dentin (1=10.7+/-1.4 vs 12.2+/-0.6, 2=14.2+/-1.7 vs 14.4+/-1.6, 3=10.0+/-2.0 vs 10.9+/-1.3, 4=9.0+/-1.2 vs 11.1+/-1.0) were significantly lower in groups 1 and 4 (p=.013 and p=.003, respectively). There were no significant differences in strength and modulus results between the bleached and control dentin in groups 2 and 3. A direct application of carbamide peroxide to bovine dentin significantly decreased some dentin mechanical properties in vitro. An indirect application of carbamide peroxide to dentin did not significantly decrease dentin strength and stiffness. CLINICAL SIGNIFICANCE: This in vitro study suggests that a bleaching treatment, when applied to the enamel of intact teeth, does not significantly affect the mechanical properties of the underlying dentin. However, when applied directly to dentin in clinical situations such as root exposure or occlusal attrition, the bleaching treatment may result in altered mechanical properties of dentin. The clinical consequences of this observation are, however, unknown.