Biological aspects of modern dental composites.

Biological evaluation of resin-based dental composites has traditionally been based on in vitro endpoint tests with different methods to determine loss of cell viability and cell morphology changes after exposure to the material or monomer constituents. The data reveals a potential for biological effects, but clinical relevance of such data is limited. Positive allergy tests and allergic clinical reactions to dental monomers are observed in dental personnel and patients. The aim of this review is to address newer research on molecular events caused by exposure to resin-based composites to have a better understanding of the potential for clinical adverse effects. A more accurate understanding of the biological aspects of dental composite materials has been found after studying parameters like glutathione depletion, oxidative stress, genotoxicity, and immunomodulatory key effects in various cell culture models. Using omics-based approaches allow for a broader and non-specified search of changes caused by methacrylate exposure. Defense mechanisms and adaption are observed in cells exposed to monomer concentrations relevant to clinical exposure. The above-mentioned methods are the foundations for modified testing strategies. The clinical relevance of most available in vitro endpoint tests is of limited relevance for the patient. Research focusing on molecular mechanisms has given new insight into methacrylate toxicity in exposed cells. Using this knowledge from mechanistic studies to develop standardized in vitro biocompatibility tests will likely improve their clinical relevance.

Shear bond strength of resin bonded zirconia and lithium disilicate - effect of surface treatment of ceramics and dentin.

Objectives:The purpose of the study was to investigate the effect of ceramic surface pretreatment, effect of resin cement and dentin surface roughness on shear bond strength. Methodology: Zirconia rods (n = 140) were randomly assigned to air born particle abrasion with aluminum oxide (Al2O3) or hot etching with potassium hydrogen difluoride (KHF2). Lithium disilicate rods (LDS; n = 50) etched with hydrofluoric acid served as reference material. In Part 1 of the study, ceramic rods were cemented to bovine dentin using 5 dual-polymerizing resin cements (Variolink Esthetic, Multilink Automix (Ivoclar Vivadent), Duo-Link (BISCO Dental), Panavia F2.0 (Kuraray Dental), RelyX Unicem (3 M)). Shear bond strength was tested and fracture morphology determined. In Part 2 of the study, test groups with the highest frequency of adhesive fractures between cement and dentin were selected for further bond strength testing with different surface roughness of dentin; ground with P1200 or P80 silicon carbide paper. Dentin samples were fractured vertically to the cemented surface and the adherence between cement and dentin was studied. Results: The results of Part 1 showed that hot etching of zirconia significantly improved bond strength to Duo-Link cement. In Part 2, RelyX Unicem showed significantly higher bond strength to P1200 compared to P80 ground dentin. For Variolink Esthetic, bond strengths to P1200 and P80 ground dentin were similar. Adhesive fracture between cement and dentin dominated. Conclusions: A smooth dentin surface (P1200) improved bond strength to RelyX Unicem. Surface roughness was not important for Variolink Esthetic.

Dentin to dentin adhesion using combinations of resin cements and adhesives from different manufacturers - a novel approach.

AIMS: The aims of this study were to present a novel method to analyse dentin bond strength and to evaluate the bond strength of combining adhesive systems and resin cement from different manufacturers. METHODS: Human wisdom teeth were ground flat to the dentin on parallel surfaces and axially cut into two parts. Dentin cylinders (Ø 3 mm) were drilled from one half of each tooth. The other half from each tooth was embedded in epoxy resin with the dentin surface exposed. The specimens were ground with silicone carbide paper and the dentin cylinders were cemented onto the dentin surface of the other half of the same tooth.Materials: Resin cement and adhesive systems from three different manufacturers were used in various combinations (n = 8 per group). Cement and adhesive from the same manufacturer served as control. Shear bond strength (SBS) was measured and fracture modes were registered.Results and conclusions: The highest median SBS value was found in a bonding combination between cement and a non-corresponding adhesive (33.1 MPa) and one of the lowest values was found in one of the controls (15.3 MPa). Cohesive fractures were most frequent. The results indicated that combining adhesive and cement from different manufacturers did not compromise the dentin bonding. The novel test method is recommended for evaluating dentin bonding.

The dental monomer HEMA causes proteome changes in human THP-1 monocytes.

Resin-based biomaterials are widely used in medical and dental treatment, and both clinicians and patients are exposed to the materials. The knowledge of toxicity is mainly based on in vitro studies at exposure concentrations that induce cell death. However, severe cell damage and cell death signaling may overshadow essential cellular events caused by a possible toxicant. For dental resins, the knowledge of interaction with living cells at more clinical relevant exposure doses is sparse. 2-Hydroxyethylmethacrylate (HEMA) is a commonly used monomer in dental resins. Measuring cellular adaptation to HEMA at concentrations that did not reduce cell viability was the main focus of this study. Stable isotope labeling with amino acids in cell culture was used to measure proteome changes in cultured THP-1 cells exposed to HEMA. Western blotting verified the results. Cells exposed to HEMA increased their level of several cytoprotective proteins. The observed adaptation is compatible with increased oxidative burden caused by GSH depletion and the electrophilic characteristic of HEMA. The present approach to analyzing the toxic potential of HEMA yielded information on interactions with living cells is not previously reported. This detailed information is of great value to make better predictions of possible side effects in the clinic. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 851-859, 2019.

Digital evaluation of marginal and internal fit of single-crown fixed dental prostheses.

The present study used a new, digitized version of the impression replica technique, namely the dual-scan technique, to evaluate the adaptation of single-crown fixed dental prostheses (FDPs). Scans of the bare master model and of the master model with a silicone layer representing the cement layer were superimposed and analyzed using designated software. Single crowns produced using the lost-wax metal casting technique were included. The cement space of the band width, 0.5-1.0 mm from the preparation margin (marginal fit), was smallest for crowns made from laser-sintered cobalt-chromium. The internal fit in both mesial-distal and buccal-palatal directions was statistically significantly better for crowns made using the conventional lost-wax metal casting technique than for crowns produced using computer-aided design/computer-aided manufacturing (CAD/CAM). Fixed dental prostheses produced by milled cobalt-chromium had the loosest internal fit. The results agree with those of our previous study of the same test specimens, in which the triple-scan method was used, and imply that the dual-scan method is well suited for adaptation studies.

Optimizing quality and safety of dental materials.

This paper discusses aspects of quality and safety improvement of the most commonly used dental restorative materials, the resin-based composites. From a patient's perspective, long-lasting resin-based restorations without complications are important. Recurrent caries and fracture are the most common causes for restoration failures. Proper handling and curing of the composites improve the mechanical properties of the restorations and increase safety by reducing exposure to residual methacrylate monomers. A number of compounds have been introduced in restorative materials to reduce the risk of recurrent caries, even though a real breakthrough has not yet been achieved. It is concluded that simple measures may improve the quality and safety of resin-based composite restorations.

Internal fit of three-unit fixed dental prostheses produced by computer-aided design/computer-aided manufacturing and the lost-wax metal casting technique assessed using the triple-scan protocol.

Suboptimal adaptation of fixed dental prostheses (FDPs) can lead to technical and biological complications. It is unclear if the computer-aided design/computer-aided manufacturing (CAD/CAM) technique improves adaptation of FDPs compared with FDPs made using the lost-wax and metal casting technique. Three-unit FDPs were manufactured by CAD/CAM based on digital impression of a typodont model. The FDPs were made from one of five materials: pre-sintered zirconium dioxide; hot isostatic pressed zirconium dioxide; lithium disilicate glass-ceramic; milled cobalt-chromium; and laser-sintered cobalt-chromium. The FDPs made using the lost-wax and metal casting technique were used as reference. The fit of the FDPs was analysed using the triple-scan method. The fit was evaluated for both single abutments and three-unit FDPs. The average cement space varied between 50 µm and 300 µm. Insignificant differences in internal fit were observed between the CAD/CAM-manufactured FDPs, and none of the FPDs had cement spaces that were statistically significantly different from those of the reference FDP. For all FDPs, the cement space at a marginal band 0.5-1.0 mm from the preparation margin was less than 100 µm. The milled cobalt-chromium FDP had the closest fit. The cement space of FDPs produced using the CAD/CAM technique was similar to that of FDPs produced using the conventional lost-wax and metal casting technique.

Effect of curing and silanizing on composite repair bond strength using an improved micro-tensile test method.

Objectives: To evaluate the micro-tensile repair bond strength between aged and new composite, using silane and adhesives that were cured or left uncured when new composite was placed. Methods: Eighty Filtek Supreme XLT composite blocks and four control blocks were stored in water for two weeks and thermo-cycled. Sandpaper ground, etched and rinsed specimens were divided into two experimental groups: A, no further treatment and B, the surface was coated with bis-silane. Each group was divided into subgroups: (1) Adper Scotchbond Multi-Purpose, (2) Adper Scotchbond Multi-Purpose adhesive, (3) Adper Scotchbond Universal, (4) Clearfil SE Bond and (5) One Step Plus. For each adhesive group, the adhesive was (a) cured according to manufacturer's instructions or (b) not cured before repair. The substrate blocks were repaired with Filtek Supreme XLT. After aging, they were serially sectioned, producing 1.1 × 1.1 mm square test rods. The rods were prepared for tensile testing and tensile strength calculated at fracture. Type of fracture was examined under microscope. Results: Leaving the adhesive uncured prior to composite repair placement increased the mean tensile values statistically significant for all adhesives tested, with or without silane pretreatment. Silane surface treatment improved significantly (p < 0.001) tensile strength values for all adhesives, both for the cured and uncured groups. The mean strength of the control composite was higher than the strongest repair strength (p < 0.001). Conclusions: Application of freshly made silane and a thin bonding layer, rendered higher tensile bond strength. Not curing the adhesive before composite placement increased the tensile bond strength.

Internal fit of single crowns produced by CAD-CAM and lost-wax metal casting technique assessed by the triple-scan protocol.

STATEMENT OF PROBLEM: Whether single crowns produced by computer-aided design and computer-aided manufacturing (CAD-CAM) have an internal fit comparable to crowns made by lost-wax metal casting technique is unknown. PURPOSE: The purpose of this in vitro study was to compare the internal fit of single crowns produced with the lost-wax and metal casting technique with that of single crowns produced with the CAD-CAM technique. MATERIAL AND METHODS: The internal fit of 5 groups of single crowns produced with the CAD-CAM technique was compared with that of single crowns produced in cobalt-chromium with the conventional lost-wax and metal casting technique. Comparison was performed using the triple-scan protocol; scans of the master model, the crown on the master model, and the intaglio of the crown were superimposed and analyzed with computer software. The 5 groups were milled presintered zirconia, milled hot isostatic pressed zirconia, milled lithium disilicate, milled cobalt-chromium, and laser-sintered cobalt-chromium. RESULTS: The cement space in both the mesiodistal and buccopalatal directions was statistically smaller (P<.05) for crowns made by the conventional lost-wax and metal casting technique compared with that of crowns produced by the CAD-CAM technique. CONCLUSIONS: Single crowns made using the conventional lost-wax and metal casting technique have better internal fit than crowns produced using the CAD-CAM technique.

Silanising agents promote resin-composite repair.

OBJECTIVE: The aim of this study was to investigate the effect of silane in the repair of old and new resin-composite restorations. METHOD: Part 1: repair of old composite was performed on 60 resin-composite substrates that were 6 years old and were made of six different brands of composite. Three experiments were performed. In the first experiment, the substrates were ground flat and composite was fixed to the surface with bonding agent without silane (i.e. Clearfil Bond SE only, the control). Shear bond strength (SBS) was tested according to ISO/TS 11405 after thermocycling. In the second experiment, the same 60 substrates were ground again and treated with bis-silane a 2-part silane mixed shortly before application before applying bonding agent (Clearfil Bond SE plus silane) and repair composite before SBS testing. In the third experiment, the same substrates were ground again and a one-step bonding product containing silane (Scotchbond Universal bond containing silane) was used for the repair procedure before SBS testing. Part 2: to evaluate the repair of newly made composite restorations, 66 composite substrates were made and stored in water for 2 months. The specimens were divided into three groups and were tested using the same protocols as used to evaluate repair of old composite. RESULTS: Mean SBS (± standard deviation), in MPa, for repair of old composite was 6.2 ± 4.0 (Clearfil Bond SE only, control), 14.8 ± 7.8 (Clearfil Bond SE plus silane) and 15.3 ± 5.6 (Scotchbond Universal bond with silane), whereas for new composite mean SBS was 15.4 ± 8.6 (Clearfil Bond SE only, control), 23.4 ± 8.3 (Clearfil Bond SE with silane) and 23.7 ± 5.8 (Scotchbond Universal containing silane). A significant difference was observed between the control and the test groups with silanising agents, both in Part 1 (P < 0.001) and in Part 2 (P < 0.005). CONCLUSION: Silanising agents increase the bond strength of the resin composite repair.

In vitro element release and biological aspects of base-metal alloys for metal-ceramic applications.

Objective: The aims of this study were to investigate the release of element from, and the biological response in vitro to, cobalt-chromium alloys and other base-metal alloys used for the fabrication of metal-ceramic restorations. Material and methods: Eighteen different alloys were investigated. Nine cobalt-chromium alloys, three nickel-chromium alloys, two cobalt-chromium-iron alloys, one palladium-silver alloy, one high-noble gold alloy, titanium grade II and one type III copper-aluminium alloy. Pure copper served as positive control. The specimens were prepared according to the ISO standards for biological and corrosion testing. Passive leaching of elements was measured by using Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) after incubation in cell culture media, MEM, for 3 days. Corrosion testing was carried out in 0.9% sodium chloride (NaCl) and 1% lactic acid for 7 days, and the element release was measured by Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES). The biological response from the extract solutions was measured though MTT cytotoxicity testing and the Hen's egg test-chorio-allantoic membrane (HET-CAM) technique for irritationt. Results: The corrosion test showed similar element release from base-metal alloys compared to noble alloys such as gold. Apart from the high-copper alloy, all alloys expressed low element release in the immersion test, no cytotoxic effect in the MTT test, and were rated non-irritant in the HET-CAM test. Conclusions: Minimal biological response was observed for all the alloys tested, with the exception of the high-copper alloy.

TEGDMA and filler particles from dental composites additively attenuate LPS-induced cytokine release from the macrophage cell line RAW 264.7.

OBJECTIVES: Due to incomplete curing and material degradation, cells in the oral cavity may be exposed to monomers and filler particles from dental composite fillings. The objective of the present study was to investigate if combined exposures to particles and a methacrylate monomer from composite fillings resulted in additive effects on the macrophage immune response. MATERIAL AND METHODS: Two filler particles, Nanosilica (12 nm) and Quartz (1 µm), were studied at concentrations 0.5-4 µg/cm(2), while the methacrylate monomer triethyleneglycol dimethacrylate (TEGDMA) was applied at 5 and 50 µM. RAW 264.7 macrophages were exposed to monomers and/or particles for 24 h, with a subsequent 24 h combined exposure to monomers and/or particles and the bacterial factor lipopolysaccharide (LPS) to stimulate an immune response. Release of the pro-inflammatory cytokines interleukin-1ß (IL-1ß) and tumor necrosis factor-α (TNF-α) were measured as well as the cellular viability. RESULTS: Co-exposure to Nanosilica and Quartz resulted in an additive attenuation of the LPS-induced IL-1ß release. Moreover, co-exposure to TEGDMA and both types of filler particles also resulted in an additive attenuation, although with a weak synergistic trend. The cellular viability and TNF-α release were not significantly affected by the exposures. CONCLUSION: The present findings emphasize the necessity of considering effects of combined exposure to dental degradation products in future risk assessments. CLINICAL RELEVANCE: Attenuated cytokine release could have implications for the macrophage immune response and result in impaired bacterial clearance. Further studies are necessary to determine implications for formation of dental biofilms and caries development.

Role of bonding agents in the repair of composite resin restorations.

Six commonly used composite resin materials and recommended bonding systems were tested to assess shear bond strength at the interface between aged and new composites with and without bonding. Test specimens were aged in water for 60 d before new composite was placed. Shear bond strength was assessed after 22 ± 2 h (Test 1) and after additional ageing by thermocycling (5-55°C/5,000 cycles) (Test 2). After an additional 180 d in water, the aged specimens were randomly divided into three groups to blind the test with respect to the aged composite. New composites were placed on aged specimens (two groups with and one without bonding agent) and thermocycled (Test 3). After 24 h (Test 1), the mean shear bond strength of the test specimens was 21-26 MPa when bonding agents were used, as opposed to 10-15 MPa without bonding agents. After thermocycling (Test 2), the mean shear bond strength was 16-23 MPa with a bonding agent and 17 MPa without a bonding agent. After 180 d in water and subsequent thermocycling (Test 3), the mean shear bond strength was 9-13 MPa with bonding agent and 2-3 MPa when no bonding agent was used. The results of this study therefore indicate that the use of bonding agents significantly improves the quality of composite repair.

In vivo and in vitro irritation testing of low concentrations of hydrofluoric acid.

OBJECTIVE: Acidic fluorides are proposed for the treatment of dental erosion. The aim of this study was to examine the irritation properties of dilute hydrofluoric acid (HF) solutions for potential use in the oral cavity. MATERIAL AND METHODS: Hen's egg test-chorioallantoic membrane (HET-CAM): The CAM was accessed by careful dissection through the egg shell (n=36, 6 eggs/test solution) and exposed to 300 µl of the HF solutions (0.05%, 0.10%, 0.20%, and 1.0%) under macroscope examination over the course of 5 min. Mean time-to-coagulation and average irritation score were recorded based on appearance of hemorrhage, coagulation, and lysis of the blood vessels in the membrane. Mouse skin test: 60 male mice were randomly divided into 10 groups of 6 animals each (control, 0.05%, 0.10%, 0.20%, and 1.0% HF), shaved on the back, exposed to test solution, and euthanized after 2 h or 24 h. Skin samples were evaluated by light microscopy, scoring epithelial leukocyte infiltration, vascular congestion, and edema. RESULTS: HET-CAM: 0.05% HF was slightly irritant, 0.1% HF moderately irritant, 0.2% and 1% HF strongly irritant. 0.1-1% HF solutions were severely irritating on the eye. Mouse skin test: HF concentration was significantly correlated with tissue response, and 24-h exposure to 1% HF solution showed focal erosion of the epithelium and marked localized subepithelial leukocyte infiltration. CONCLUSION: The results of the studies suggest that accidental exposure of soft tissues to solutions containing more than 0.2% HF may be harmful.

In vitro efficacy and risk for adverse effects of light-assisted tooth bleaching.

The use of optical radiation in the so-called light-assisted tooth bleaching procedures has been suggested to enhance the oxidizing effect of the bleaching agent, hydrogen peroxide. Documentation is scarce on the potential adverse effects of bleaching products and on optical exposure risks to eyes and skin. The efficacy of seven bleaching products with or without simultaneous use of seven different bleaching lamps was investigated using extracted human teeth. The bleaching effect was determined immediately after treatment and one week later. Tooth surfaces were examined for adverse alterations after bleaching using a scanning electron microscope. Source characteristics of eight lamps intended for tooth bleaching were determined. International guidelines on optical radiation were used to assess eye and skin exposure hazards due to UV and visible light emission from the lamps. Inspection of teeth one week after bleaching showed no difference in efficacy between teeth bleached with or without irradiation for any of the products. Scratches, probably from the cleaning procedure were frequently seen on bleached enamel irrespective of irradiation. Maximum permissible exposure time (t(max)) and threshold limit values were exceeded for about half the bleaching lamps investigated. One lamp exceeded t(max) even for reflected blue light within the treatment time. This lamp also exceeded t(max) values for UV exposure. The lamps were classified as "low risk" and as borderline to "moderate risk" according to a relevant lamp standard.

Corrosion of dental nickel-aluminum bronze with a minor gold content-mechanism and biological impact.

OBJECTIVES: To study corrosion and to evaluate biological effects in vitro of corrosion products of a copper-aluminum-nickel alloy with 2% gold. METHODS: The alloy NPGtrade mark+2 with the nominal composition Cu:77.3; Al:7.8; Ni:4.3; Fe:3.0; Zn:2.7; Au:2.0; and Mn:1.7 was characterized. Static immersion in acidic saline, pH 2.2-2.4, was used to determine release of metallic elements in a milieu simulating the condition of plaque build-up in interproximal areas of the tooth. Corrosion and surface reactions in saline and artificial saliva were studied by electrochemical techniques including registration of open-circuit-potentials, polarization curves and impedance spectra. Extracts were made in cell culture media and acidic saline and used for MTT test for cytotoxicity and HET-CAM method for irritation. RESULTS: The mean amount of elements released in the acidic saline were in microg cm(-2) : Cu:632; Al:210; Ni:144; Fe:122; Zn:48; Mn:52. No protective film was formed on the surface of the alloy, as extensive corrosion was observed in both saline and artificial saliva. The corrosion rate was higher in saline than in artificial saliva. Acidic extracts of the alloy diluted up to 64 times reduced cell viability with 80% or more. The extract induced coagulation of the blood vessels of the CAM and was rated as moderate irritant solution. SIGNIFICANCE: The nickel-aluminum bronze showed high corrosion rate caused by an inability to create a protective surface layer. High levels of toxic elements were found after static immersion testing, and the corrosion products had a distinct adverse effect on the biological activity.

Filler particles used in dental biomaterials induce production and release of inflammatory mediators in vitro.

Although dental composites are in extensive use today, little is known about the biological effects of the filler particles. As composite materials are gradually broken down in the aggressive environment of the oral cavity, the filler particles may leak and induce toxic effects on the surrounding tissue and cells. The aim of this study was to elucidate possible adverse biological effects of commonly used dental filler particles; bariumaluminiumsilica (BaAlSi) and bariumaluminiumfluorosilica (BaAlFSi) with mean size of 1 microm. BEAS-2B cells were used as a model system. Particle morphology, mean particle size in solution, and particle surface charge were determined by scanning electron microscopy and Malvern zetasizer technology, respectively. Enzyme-linked immunosorbent assay was used to detect secretion of cytokine and chemokine (IL-8 and IL-6) and quantitative PCR for detection of gene activity. Both types of particle increased the release of IL-6 and IL-8 in a dose-dependent manner. BaAlFSi particles induced a more marked IL-8 response compared to BaAlSi particles, whereas no significant difference was observed for the IL-6 response. Mechanistic studies using specific inhibitors and activators indicated that cyclic AMP-dependent protein kinase A is partly involved in the observed IL-8 response. In conclusion, we consider dental filler particles to have potential to induce adverse biological response in cell cultures.