Oxygen-inhibited surface layers on Microfill Pontic

Effect of Tribochemical Coating on Composite Repair Strength

Clinical Relevance

Tribochemical treatment of existing composite surfaces is highly effective for composite repair. When repairing an old composite restoration, the clinician should try to use the same composite originally used for the restoration. If the information about the original restoration is not known, a composite with strong mechanical properties should be used for the repair restoration.

SUMMARY

This study evaluated the effect of tribochemical coating on composite-to-composite repair interfacial fracture toughness (iFT). Sixty beam-shaped specimens (21×4×3 ± 0.2 mm) were prepared with a nanofill composite (Filtek Supreme Ultra [FSU]) and a nanohybrid composite (Clearfil Majesty ES-2 [CME]) and aged for 50,000 thermocycles (5°C-55°C, 20-second dwell time) and then sectioned in half. The resulting 120 hemispecimens (60 for each composite) were randomly assigned to different repair methods (n=10): universal adhesive (Clearfil Universal Bond Quick [CUB]), sand-blasting followed by CUB, or tribochemical coating (CoJet, CoJet sand, Espe-Sil, and Visio-Bond). The repair surface was prepared with a diamond bur (Midwest #471271), rinsed, and dried. Each aged composite brand (FSU, CME) was repaired with either the same composite or the opposite composite. All adhesives and composites were light cured with a high-irradiance LED curing light (Elipar DeepCure-S). After postrepair storage in 100% humidity and at 37°C for 24 hours, iFT was measured as KIc (MPa m½). Data were analyzed for statistical significance using two-way analysis of variance (ANOVA) and the Tukey honest significant difference post hoc test (α=0.05). Regardless of the substrate composite, ANOVA showed significant differences for surface treatment (p<0.0001) and repair composite (p<0.0001). Mean iFT values (SD) ranged from 0.91 (0.10) MPa·m½ to 2.68 (0.12) MPa·m½. Repairs made with FSU after CoJet resulted in significantly higher iFT (FSU: 2.68 MPa·m½; CME: 2.21 MPa·m½) when compared to the other experimental groups. The repair iFT was higher with CoJet treatment and when the nanofill composite FSU was used as the repair composite.

Effects of surface finish on indentation modulus and hardness of dental composite restoratives

OBJECTIVES

The depth-sensing micro-indentation testing was recently introduced for the characterization of dental composites. One of the critical issues raised was the possible influence of surface finish on material properties. The aim of this study was to investigate the effects of surface finish on the indentation modulus and micro-hardness of resin-based dental composite materials.

METHODS

The materials used included minifill (Z100, 3M ESPE), microfill (A110, 3M ESPE) and poly-acid modified (F2000, 3M ESPE) composites. The specimens were polished successively using SiC grinding papers of different grit size and diamond suspensions to achieve varying surface roughness. The arithmetic mean of the roughness (R(a)) was measured using profilometry. In the depth-sensing micro-indentation test, specimens (n=7) were indented to 10N with Vickers indenter and the load-displacement (P-h) data was obtained using a universal testing system. The indentation modulus (E(in)) and hardness (H) were then computed using the developed analytical solutions. Data was analyzed using ANOVA/post-hoc Scheffe's test at significance level 0.05.

RESULTS

The polished specimens had surface roughness ranging from 0.02 to 0.81 microm. The roughness of F2000 was significantly higher than A110 and Z100. The E(in) and H for Z100 ranged from 14.02 to 14.83GPa and 1.18 to 1.27 GPa, respectively. E(in) for F2000 and A110 ranged from 12.25 to 13.82 GPa and 5.26 to 5.52 GPa and hardness ranged from 0.89 to 0.98 GPa and 0.52 to 0.55 GPa, respectively.

SIGNIFICANCE

The indentation modulus and hardness of dental composite restoratives were independent of the surface finish provided indenter penetration is sufficiently deep (h(max)/R(a)>30).

In vitro cytotoxicity of orthodontic bonding resins on human oral fibroblasts

Polymerization of bonding resins is compromised by atmospheric oxygen, giving rise to a layer of low molecular weight chemical species commonly known as the oxygen inhibited layer. The aim of this study was to evaluate the cytotoxic effect of this layer on primary cultures of human oral fibroblast. The cytotoxic effect related to the modes of polymerization of seven commercially available orthodontic bonding resins was also evaluated statistically. Each material was polymerized into 12 resin disks of standardized dimensions. Half of them were washed with 99% acetone to remove the oxygen inhibited layer. In duplicates, human oral fibroblasts were exposed to the intact and washed resin disks in tissue culture inserts. Cell viability was assessed by tetrazolium bromide reduction assay (MTT) 1, 3, and 6 days after exposure. Glass disks served as controls. ANOVA was used to test for statistical significance. Overall, the presence of an oxygen inhibited layer renders bonding resins 33% more cytotoxic (P <.01, F = 11.83, DF = 1). Light-cured and chemically cured 2-pastes materials had their mean cytotoxicities approximating their inert controls over 6 days. In chemically cured liquid-paste materials, the viability of human oral fibroblasts was only 37% (P <.001, F = 26.4, DF = 2) comparing to the control, 64% on day 1, 30% on day 3 and 14% on day 6. This suggested that the oxygen inhibited layer formed on the surface of bonding resins is an important cytotoxic source in vitro. Chemically cured liquid-paste materials were more cytotoxic than light-cured and chemically cured 2-paste materials. Further investigation into the influence of the modes of polymerization on materials' toxicodynamic effect is warranted to verify its clinical implication.

Cytotoxicity tests of in situ polymerized resins: methodological comparisons and introduction of a tissue culture insert as a testing device

The in vitro cytotoxic potential of six commonly used methacrylate polymers was evaluated using human oral fibroblast cultures with different cell-material contact systems. A tissue culture insert was introduced to test resin-released components. Both acute and delayed cytotoxic effects of resin were quantified by cellular enzymatic and DNA synthetic activities of fibroblasts over a 6-day exposure period. Resin toxicity was material-dependent. Statistical analysis showed that the experimental conditions significantly contributed to the overall toxicity and the cytotoxicity pattern for a given material. DNA synthesis activity of human oral fibroblasts assayed by 3H-thymidine incorporation was more sensitive to resins than cellular enzyme activity, as determined by tetrazolium bromide reduction. However, extended exposure increased the cytotoxicity of all resins, as measured by tetrazolium bromide reduction, which seemed to be a better indicator of the development of resin toxicity than 3H-thymidine incorporation. Removal of the oxygen inhibition layer on resin specimens partially enhanced cell viability, indicating that this surface layer together with other unknown factors contributed to resin toxicity.

Marginal breakdown of fired ceramic inlays cemented with glass polyalkenoate (ionomer) cement or resin composite

The marginal breakdown of fired porcelain inlays in vivo, luted with either a dual-cured resin composite or a glass polyalkenoate(ionomer) cement, was evaluated by scanning electron microscopy. Immediately following polishing (baseline) and after 1 year, replica impressions were made of the occlusal and proximal margins of two inlays, one cemented with the resin composite and the other with the glass ionomer cement, in each of 12 patients. At baseline the best marginal adaptation was observed along the proximal margins of the composite luted inlays, while occlusally glass ionomer luted inlays showed better marginal qualities than the composite luted inlays. At 1 year the composite luted inlays showed significantly better marginal adaptation than the glass ionomer luted ones at all interfaces. The greatest breakdown was observed occlusally, especially in the glass ionomer luted inlays. Gap sizes varied proximally between 83 and 100 microns and occlusally between 107 and 143 microns. Severe ditching was observed occlusally in the glass ionomer luted inlays. The overall marginal quality was significantly better for the inlays luted with the resin composite both at baseline and after 1 year.