Joseph B. Dennison

Cytotoxic Interactive Effects of Dentin Bonding Components on Mouse Fibroblasts

Previous studies have shown a wide range of pulpal reactions to dentin bonding systems and a poor correlation between in vitro and in vivo toxicity of dentin bonding agents. Because dentin bonding agents are composed of multiple components which may diffuse through dentin, we hypothesized that these components may cause cytotoxicity through interactive (synergistic) effects. We investigated the cytotoxicities of four dentin bonding components-HEMA, Bis-GMA, TEGDMA, and UDMA-and interactive effects for three binary combinations of the dentin bonding components-HEMA and Bis-GMA, Bis-GMA and TEGDMA, and TEGDMA and UDMA. Cytotoxicities to Balb/c 3T3 mouse fibroblasts were measured by the MTT assay. Concentrations which caused 50% toxicity compared with controls (TC50 values) were compared, and the interactive effects were determined by evaluation of the differences between observed and expected MTT activities of the cells. The ranks of toxicity of the dentin bonding components in terms of TC50 values were as follows: Bis-GMA > UDMA > TEGDMA >>> HEMA (least toxic) after 24- and 72-hour exposures. As binary combinations, the three combinations of dentin bonding components interacted in three ways—synergism, additivism, and antagonism-which were influenced by the concentrations of both components. The longer period of exposure resulted in a significant increase in the cytotoxicity of the dentin bonding components and combinations. The findings indicate that both exposure time and the interactions between the dentin bonding components may be important parameters in determining the cytotoxicity of dentin bonding agents in vivo.

Dentin Bonding: SEM Comparison of the Resin-Dentin Interface in Primary and Permanent Teeth

Previous studies have suggested minor differences between primary and permanent teeth in terms of dentin composition and morphology. Other reports indicated lower bond strengths of resin composites to dentin of primary teeth compared with dentin of permanent teeth; however, no information is available regarding differences in the micromorphology of the resin-dentin interface that may explain these lower bond strengths. Therefore, the purpose of the present study was to compare primary and permanent teeth in terms of the thickness of the hybrid layer developed with two bonding systems. Our hypothesis was that bonding differences previously reported between primary and permanent dentin would be reflected in hybrid layer differences observable in SEM analyses. Twenty human extracted and non-carious teeth were divided into 4 groups: 5 primary and 5 permanent teeth restored with All-Bond 2/Bisfil P system; and 5 primary and 5 permanent teeth restored with Scotchbond Multi-Purpose/ZlOO. The sample area available on each tooth was divided for the two dentin conditioning times (7 and 15 sec). Measurements of hybrid layer thickness were performed by means of SEM at xl3,000. The results of this study indicated that the hybrid layer produced is significantly thicker in primary than in permanent teeth (p = 0.0001), suggesting that primary tooth dentin is more reactive to acid conditioning. No difference was observed in the hybrid layers produced by the two adhesive systems (p = 0.7920). The increased thickness of the hybrid layer in primary teeth (25 to 30%) and the subsequent lack of complete penetration of adhesive resin into previously demineralized dentin may contribute to the lower bond strengths to primary dentin reported in the literature. If a narrower hybrid layer more uniformly infused with resin is the goal of dentin bonding, it is concluded that a differentiated protocol for bonding to primary dentin (with shorter time for dentin conditioning) can be used as a means to reproduce the hybrid layer thickness seen in permanent teeth.

Shear bond strength of a resin cement to densely sintered high-purity alumina with various surface conditions

Procera Sandvik AB is now manufacturing a densely sintered high-purity alumina core for an all-ceramic crown designed for anterior and posterior restorations. Whereas the material holds promise on the basis of in vitro strength tests, the ability to alter the surface and use conventional bonded resin cements has not been reported previously in the literature. Samples of the core were treated by means of one of four methods routinely used for all ceramic restorations, and then a commercially available resin cement was bonded to the surface. A shear bond test of the adhesion showed that the highest shear bond strengths of 11.99 +/- 3.12 MPa were obtained with air abrasion at 80 psi and 50-microm alumina particles.

Color Stability of Restorative Resins Under Accelerated Aging

The color stability of seven commercial composite resins, an unfilled resin, and three glazes was studied under conditions of accelerated aging by reflection spectrophotometry and visually with Munsell color tabs. After aging for 900 hours, most of the resins had lower values of luminous reflectance and excitation purity and higher values of dominant wavelength and contrast ratio compared to values at baseline.

Adhesive Resin Induces Apoptosis and Cell-cycle Arrest of Pulp Cells:

The application of an adhesive resin near or directly over the pulp was shown to induce pulp inflammation and lack of dentin regeneration. We hypothesize that the absence of dentin bridging is due to adhesive-resin-induced apoptosis of cells responsible for pulp healing and dentin regeneration. Mouse odontoblast-like cells (MDPC-23), undifferentiated pulp cells (OD-21), or macrophages (RAW 264.7) were exposed to SingleBond polymerized for 0–40 seconds. Annexin V and propidium iodide assays demonstrated that SingleBond induced apoptosis of MDPC-23, OD-21, and macrophages. The proportion of apoptotic cells was dependent on the degree of adhesive resin polymerization. Adhesive-resin-induced death of pulp cells was associated with activation of the pro-apoptotic cysteine protease Caspase-3. Interestingly, most cells exposed to adhesive resin that did not undergo apoptosis showed cell-cycle arrest. We conclude that an adhesive resin induces apoptosis and cell-cycle arrest of cells involved in the regeneration of the dentin-pulp complex in vitro.

In vitro Depth of Cure of Photo-activated Composites

The in vitro depth of cure of six photo-activated composites was studied by measurement of hardness, transverse strength, and light transmission coefficients. Both Barcol hardness and transverse strength were sensitive techniques for characterizing depth of cure. As a group, the visible light-activated composites had higher values of depth of cure and higher transmission coefficients than did the ultraviolet light-activated composites.

Parameters that Affect the Color of Direct Restorative Resins

The effects of thickness, background color, specular reflection, and surface roughness on the color of five commercial restorative resins were studied by reflection spectrophotometry. As thickness increased, values of luminous reflectance and excitation purity increased for a black background but decreased for a white background. Opacity increased dramatically as thickness increased from 1.3 to 3.9 mm.

Influence of post material and length on endodontically treated incisors: An in vitro and finite element study

STATEMENT OF PROBLEM Cast posts require sufficient length for prosthesis retention and root strength. For prefabricated metal and fiber posts, the effects of different post lengths on the strength and internal stress of the surrounding root need evaluation. PURPOSE The purpose of this study was to examine, using both experimental and finite element (FE) approaches, the influence of post material and length on the mechanical response of endodontically treated teeth. MATERIAL AND METHODS Sixty extracted incisors were endodontically treated and then restored with 1 of 3 prefabricated posts: stainless steel (SS), carbon fiber (CF), and glass fiber (GF), with intraradicular lengths of either 5 or 10 mm (n=10). After composite resin core and crown restorations, these teeth were thermal cycled and then loaded to fracture in an oblique direction. Statistical analysis was performed for the effects of post material and length on failure loads using 2-way ANOVA (α=.05). In addition, corresponding FE models of an incisor restored with a post were developed to examine mechanical responses. The simulated tooth was loaded with a 100-N oblique force to analyze the stress in the root dentin. RESULTS The SS/5 mm and all fiber post groups presented no statistical differences, with mean (SD) fracture loads of 1247 to 1339 (53 to 121) N. The SS/10 mm group exhibited a lower fracture load, 973 (115) N, and a higher incidence of unfavorable root fracture (P<.05). The FE analysis showed high stress around the apical end of the long SS post, while stress was concentrated around the crown margins in the fiber post groups. CONCLUSIONS Both long and short fiber posts provided root fracture resistance comparable to that of SS posts. For metal posts, extending the post length does not effectively prevent root fracture in restored teeth.

Polymerization shrinkage and depth of cure of bulk fill flowable composite resins.

OBJECTIVE To evaluate polymerization shrinkage and depth of cure of two bulk fill flowable composites, one nanohybrid composite modified to a flowable consistency, and one standard flowable composite, comparing the scraping method to the Knoop hardness test. METHODS Two bulk fill flowable composites, SureFil SDR flow (SSF) (Dentsply) and Venus Bulk Fill (VBF) (Heraeus Kulzer), one standard flowable, Filtek Supreme Ultra Flowable (FSUF) (3M/ESPE) (control), and one regular bulk composite that can be made flowable, SonicFill (SF) (Kerr), were used in this study. For polymerization shrinkage (PS), ten 2-mm samples were made for each composite and cured for 20 seconds and shrinkage was measured with a Kaman linometer. For hardness, ten specimens of each composite were made in a 10 × 10-mm mold and cured for 20 seconds; the bottom surface was scraped according to ISO 4049 specification, and the remaining thickness was measured with a micrometer. Hardness samples were prepared at 2-, 3-, 4-, and 5-mm thick ×14-mm diameter, cured for 20 seconds, and polished. After 24 hours of dry storage, a Knoop indenter was applied at 100 g load for 11 seconds. Three readings were made on the top and bottom of each specimen and averaged for each surface to calculate a Knoop hardness value and a bottom/top hardness ratio. One-way analysis of variance and Tukey tests were used to determine significant differences between thicknesses and between test methods for each material. RESULTS PS values were 3.43 ± 0.51%, 3.57 ± 0.63%, 4.4 ± 0.79%, and 1.76 ± 0.53% for FSUF, SSF, VBF, and SF, respectively. VBF showed significantly greater shrinkage (4.4 ± 0.79%), followed by FSUF (3.43 ± 0.51%) and SSF (3.57 ± 0.63%), which were similar, and SF (1.76 ± 0.53%), which had significantly less shrinkage (p<0.05). Values for the scraping method for depth of cure were significantly greater for SSF and VBF (>5.0 mm), followed by SF (3.46 ± 0.16 mm) and FSU (2.98 ± 0.22 mm). Knoop top hardness values (KHN) were: VBF 21.55 ± 2.39, FSUF 44.62 ± 1.93, SSF 29.17 ± 0.76, and SF 72.56 ± 2.4 at 2 mm and were not significantly different at 3-, 4-, and 5-mm thick within each material. Ratios for bottom/top values (depth of cure) for 2, 3, 4, and 5 mm were: VBF 0.80 ± 0.1, 0.78 ± 0.03, 0.67 ± 0.10, and 0.59 ± 0.07, respectively; SSF 0.74 ± 0.08, 0.72 ± 0.08, 0.69 ± 0.18, and 0.62 ± 0.08, respectively; SF 0.82 ± 0.05, 0.68 ± 0.05, 0.47 ± 0.04, and 0.21 ± 0.02, respectively; and FSUF 0.56 ± 0.08 at 2 mm and 0.40 ± 0.08 at 3 mm. The bottom/top ratio was .80 or less at all depths and decreased below 0.70 at 4-mm depth for VBF and SSF, at 3 mm for SF and at 2 mm for FSUF.

Physical properties and repair bond strength of direct and indirect composite resins

Use of composite resins now includes indirect curing methods. Surface chemistry, repair bond strength, plus location of failure and five physical properties of two direct and three indirect composite resins were determined. There were statistically significant differences in flexural strength of materials and in hardness values. Repair bond strength failures were not significantly different; failures occurred primarily at the interface. Multiple internal reflection spectroscopy confirmed the presence of unpolymerized material after cure in all products. Indirect cure of the direct composite resin increased the degree of cure.