Kinetic and quantitative evaluation of material cytotoxicity in vitro

A critical analysis of research methods and experimental models to study biocompatibility of endodontic materials

Materials used for endodontics and with direct contact to tissues have a wide range of indications, from vital pulpal treatments to root filling materials and those used in endodontic surgery. In principle, interaction with dental materials may result in damage to tissues locally or systemically. Thus, a great variety of test methods are applied to evaluate a materials' potential risk of adverse biological effects to ensure their biocompatibility before commercialization. However, the results of biocompatibility evaluations are dependent on not only the tested materials but also the test methods due to the diversity of these effects and numerous variables involved. In addition, diverse biological effects require equally diverse assessments on a structured and planned approach. Such a structured assessment of the materials consists of four phases: general toxicity, local tissue irritation, pre‐clinical tests and clinical evaluations. Various types of screening assays are available; it is imperative to understand their advantages and limitations to recognize their appropriateness and for an accurate interpretation of their results. Recent scientific advances are rapidly introducing new materials to endodontics including nanomaterials, gene therapy and tissue engineering biomaterials. These new modalities open a new era to restore and regenerate dental tissues; however, all these new technologies can also present new hazards to patients. Before any clinical usage, new materials must be proven to be safe and not hazardous to health. Certain international standards exist for safety evaluation of dental materials (ISO 10993 series, ISO 7405 and ISO 14155‐1), but researchers often fail to follow these standards due to lack of access to standards, limitation of the guidelines and complexity of new experimental methods, which may cause technical errors. Moreover, many laboratories have developed their testing strategy for biocompatibility, which makes any comparison between findings more difficult. The purpose of this review was to discuss the concept of biocompatibility, structured test programmes and international standards for testing the biocompatibility of endodontic material biocompatibility. The text will further detail current test methods for evaluating the biocompatibility of endodontic materials, and their advantages and limitations.

Cell Culture in the Biological Evaluation of Dental Materials: A Review

Systematic research on dental materials began after World War I. For a long time the research was focused on the physical properties of the materials, and papers dealing with biological aspects were scarce. By the late 1950s a growing interest in biological responses to dental materials developed, and from the 1970s biological and physical evaluations were deemed equally important (1).

Mammalian cells have been maintained in vitro since the early years of this century, but the use of cultured cells to evaluate the effects of chemicals and drugs is a more recent occurrence. The first practical application of this technique was in pharmacological investigations (2), but applications in other fields soon followed, and in 1955 the first studies were reported where a cell culture technique had been applied to the biological evaluation of dental materials (3,4). Since then the use of cell culture systems in dental materials research has grown rapidly. The main application has been for the assessment of cytotoxic effects, and the purpose of this paper is to review different test methods and discuss some facets of the problems posed by the cytotoxicity testing of dental materials.

Predicting clinical biological responses to dental materials

OBJECTIVES

Methods used to measure and predict clinical biological responses to dental materials remain controversial, confusing, and to some extent, unsuccessful. The current paper reviews significant issues surrounding how we assess the biological safety of materials, with a historical summary and critical look at the biocompatibility literature. The review frames these issues from a U.S. perspective to some degree, but emphasizes their global nature and universal importance.

METHODS

The PubMed database and information from the U.S. Food and Drug Administration, International Standards Organization, and American National Standards Institute were searched for prominent literature addressing the definition of biocompatibility, types of biological tests employed, regulatory and standardization issues, and how biological tests are used together to establish the biological safety of materials. The search encompassed articles published in English from approximately 1965-2011. The review does not comprehensively review the literature, but highlights significant issues that confront the field.

RESULTS

Years ago, tests for biological safety sought to establish material inertness as the measure of safety, a criterion that is now deemed naive; the definition of biocompatibility has broadened along with the roles for materials in patient oral health care. Controversies persist about how in vitro or animal tests should be used to evaluate the biological safety of materials for clinical use. Controlled clinical trials remain the single best measure of the clinical response to materials, but even these tests have significant limitations and are less useful to identify mechanisms that shape material performance. Practice-based research networks and practitioner databases are emerging as important supplements to controlled clinical trials, but their final utility remains to be determined.

SIGNIFICANCE

Today we ask materials to play increasingly sophisticated structural and therapeutic roles in patient treatment. To accommodate these roles, strategies to assess, predict, and monitor material safety need to evolve. This evolution will be driven not only by researchers and manufacturers, but also by patients and practitioners, who want to use novel materials in new ways to treat oral disease.

Fast Curing Epoxy and Episulfide Resins for Biomedical Applications

Epoxy resins have physical properties that make them suitable for dental and orthopaedic applications such as adhesives and cements. However, it has been observed that epoxy resins harden too slowly for clinical use when mixed with conventional curing agents, e.g. amines and polyamides. A new epoxy - diepisulfide - polyamide system has been developed which gels in 5 to 20 minutes at room temperature. The system consists of two parts: a polyamide curing agent, and a blend of the diepisulfide analog of diglycidyl ether of bisphenol A [DGEBA] dissolved in a mixture of epoxies of the DGEBA type.

Cytotoxicity of endodontic irrigants containing calcium hydroxide and sodium lauryl sulphate on fibroblasts derived from mouse L929 cell line

The aim of this study was to evaluate the cytotoxicity of root canal irrigating solutions containing calcium hydroxide and sodium lauryl sulphate on fibroblasts derived from L929 cell line. Saturated calcium hydroxide aqueous solution (CH), sodium lauryl sulphate (SLS) and SLS associated with calcium hydroxide (HCT20) were diluted with sterile distilled water at 50%, 20%, 10% and 5% concentrations. Minimum essential medium (MEM) served as the control group. The cytotoxicity of the solutions was evaluated on L929 mouse fibroblast cell line, at 4 and 24 h of contact time by the 51Cr radiotracer method. Data were compared and statistical inferences were made with the chi-square test. In all analysis, significance level was set at 5%. CH and HCT20 showed toxicity at 50% concentration, while at concentrations lower than 50% these solutions showed cell tolerance. SLS was cytotoxic at all concentrations. In conclusion, the association of calcium hydroxide and SLS (HCT20) combines the beneficial properties of these solutions and was not harmful to the fibroblast cell line, seeming to be a suitable endodontic irrigating solution.

Biocompatibility of Resin-based Dental Materials

Oral and mucosal adverse reactions to resin-based dental materials have been reported. Numerous studies have examined the biocompatibility of restorative dental materials and their components, and a wide range of test systems for the evaluation of the biological effects of these materials have been developed. This article reviews the biological aspects of resin-based dental materials and discusses the conventional as well as the new techniques used for biocompatibility assessment of dental materials.

Biocompatibility of furcal perforation repair material using cell culture technique: Ketac Molar versus ProRoot MTA

OBJECTIVE

The objective of this study was to evaluate the cytotoxicity of furcal perforation repair materials, GI and MTA, using cell culture technique.

STUDY DESIGN

The extract of ProRoot MTA and Ketac Molar were treated on PDL cells in a 96-well tissue-culture plate. Cell proliferation after an incubation period of 3 days was determined by using MTT assay.

RESULTS

The growth of cultured human periodontal fibroblast cells were suppressed by both perforation repair materials. The percent of cell viability in the Ketac Molar group was lower than in the ProRoot MTA group (P = .000).

CONCLUSIONS

Although Ketac Molar has the advantage of adhering to dentine, it is more cytotoxic to the PDL cells than MTA. In selecting the perforation repair material, it is recommended not only to consider the sealing ability of the material with dentine but also the biocompatibility of material to the underlying tissue.

Cytotoxicity of precious and nonprecious alloys--experimental comparison of in vitro data from two laboratories

The aim of this investigation was to evaluate and compare the reproducibility of cytotoxicity data generated in two different laboratories using the same testing protocols. A series of dental alloys that are widely used in both countries were chosen. These alloys (five precious, two nonprecious) were wet ground up to 1200 grit SiC, sterilized in 70% ethanol, and extracted in sterile culture medium for 7 days. Pure copper was used as a positive control and Teflonreg and media only were used as negative controls. Test and control samples were randomized and blinded to each laboratory. Cells, primary human gingival fibroblasts, and immortalized 3T3 fibroblasts, were exposed to the extracts for 24 h. Extract cytotoxicity was evaluated spectrophotometrically with the use of a mitochondrial enzyme activity assay. Data were collected from both laboratories, combined, and subjected to a mixed-model analysis of variance. No statistical difference was obtained for the immortalized 3T3 cells, except for two extracts in which differences between the two labs were significant but were still not cytotoxic. Furthermore, no statistical differences were found for the primary cells. These data strongly suggest that cytotoxicity tests performed in different laboratories with the use of the same test materials may lead to comparable results if sample preparation, cells, test procedures, and data analyses are carefully considered.

Materials science: biological aspects

The author discusses the biological aspects of dental materials and their applications as integral parts of both dentistry and medicine.

Cytotoxic evaluation of root-end filling materials in cultures of human osteoblast-like cells and periodontal ligament cells

The cytotoxicity of three root-end filling materials (amalgam, IRM, and Super-EBA) was evaluated in cultures of human periodontal ligament cells and human osteoblast-like cells. Ten-millimeter-long plastic test tubes were filled with 3 mm of freshly mixed root-end filling materials at one end (1.5 mm diameter). The opposite end was sealed and attached by heat to a 35-mm cell culture dish. Human periodontal ligament cells and human osteoblast-like cells were seeded in the dishes. The size of cell-free zones around the root-end filling materials and the total cell number per dish were calculated after 3 and 7 days. Empty test tubes used as controls did not influence the growth and distribution of the cultured cells. Cell density increased in all groups in the test period. Amalgam had a larger cell-free zone, compared with IRM and Super-EBA and showed a reduction in total cell number per dish for both tested cell types. IRM and Super-EBA also had a cell-free inhibition zone for both cell types, but no significant reduction in total cell number per dish. This study showed that amalgam had a higher cell toxicity to human periodontal ligament cells and human osteoblast-like cells than IRM and Super-EBA.

In vitro evaluation of biocompatibility of experimental titanium alloys for dental restorations

STATEMENT OF PROBLEM

Applications of titanium casting to fixed and removable prostheses have not been satisfactory because of the high melting point of titanium and its great reactivity with mold materials. Low-melting titanium alloys would alleviate many casting problems. At the present time, low-melting titanium alloys are not available for clinical dental use.

PURPOSE

The study evaluated the safety of 2 prototype low-melting titanium casting alloys for their future development for dental restorations. The 2 experimental titanium alloys were titanium-cobalt(Ti 81.4% wt, Co 18.6% wt) and titanium-silver (Ti 75% wt, Ag 25% wt). Commercially pure titanium and a commonly used nickel-chromium-based dental alloy were also included for comparison.

MATERIAL AND METHOD

Assays for evaluating mutagenicity and cytotoxicity were Ames salmonella/microsome mutagenicity spot test, agar diffusion method, and cell attachment assay.

RESULTS

The results of Ames test showed that none of the experimental titanium alloys, pure titanium, or nickel-chromium samples were mutagenic. There were no leachable components to cause cellular lysis or decolorization in the agar diffusion assay from the 4 group metals. The results of the cell attachment assay demonstrated that there was no significant difference in the numbers of cells attached to the 4 group metals. However, the mean number of cells attached to nickel-chromium samples was significantly lower than that of the control group.

CONCLUSIONS

On the basis of the data obtained from this study, it is concluded that the prototype Ti alloys are not mutagenic and imposes minimal risks associated with cytotoxicity.

The protective effect of zinc on rosin and resin acid toxicity in human polymorphonuclear leukocytes and human gingival fibroblasts in vitro

Combinations of rosin and zinc are used in dentistry as components of periodontal dressings and cements and as root canal sealers. The composition and properties of rosins differ largely depending on source and refinement processes. Rosin (colophony) is composed of approximately 70% resin acids. In order to study the toxic effects of different natural rosins and purified resin acids and the detoxifying effects of zinc, these compounds were analyzed and tested on human polymorphonuclear leukocytes (PMN cells) and human gingival fibroblasts using the radiochromium release method. The rosins and the pure resin acids showed a strong dose-related cytotoxicity, which was inhibited by increased zinc concentrations. The purified resin acids (isopimaric, levopimaric, and neoabietic acid) were more toxic than the natural rosins. The contents of these resin acids might explain the difference in toxicity of the rosins tested. It is concluded that rosin and zinc are not to be considered inert compounds and that the cytoprotective effects of zinc and its role in dentistry products merit further investigations.

Comparison between Sulphorhodamine-B dye staining and 51Cr-release method in cytotoxicity assay of endodontic sealers

The aim of this study was to evaluate Sulphorhodamine-B (SRB) staining against 51Cr-release in cytotoxicity tests of six endodontic sealers, namely, MU sealer (Mahidol University) ROCANAL 2, ROCANAL 3, Apexit, Endomethasone, and AH-26. Monolayers (5 x 10(5) cells/ml) of the mouse cell line Mu-mu-1 were used as test cells. Following incubation at 37 degrees C in 5% CO2 for 24 h in the presence of each sealer, cells were stained with 0.4% SRB and the absorbance at 540 nm determined as measure of cell viability. For 51Cr-release assay, cells were labelled with 51Cr before testing with sealers, and radioactivity in the supernatant was measured in a liquid scintillation counter. Both techniques indicated that Apexit was the least toxic sealer. In view of the ease of conducting SRB staining for tests of cell viability, this may be the method of choice over 51Cr-release assay in the evaluation of endodontic sealer cytotoxicity.

Inflammatory response to calcium hydroxide based root canal sealers

This study evaluated the inflammatory response to Sealapex, CRCS, Apexit, and Sealer 26 in the subcutaneous tissue and in peritoneal cavity of Balb/c mice. The inflammatory response of subcutaneous tissue was analyzed after 2, 4, 8, and 16 days. Intense neutrophilia was seen in response to all sealers during the initial periods. Differences among them related to the presence of necrosis and the number of inflammatory cells. In the intermediate phase marked differentiation of cells of the mononucleate phagocytic system into macrophages, epithelioid cells and multinucleate giant cells were observed with Sealapex. This response was less intense with CRCS and Apexit. Tissue necrosis was observed only at tissue sealer interfaces and only during the initial period with Sealapex but was seen throughout the experiment with all other sealers. The animals were injected in the peritoneal cavity with solutions containing the sealers and five mice from each group were killed 6 and 24 h, and 5 and 15 days later. During the initial periods (6 and 24 h) there was an intense migration of polymorphonuclear leukocytes to the peritoneal cavity in response to all sealers compared to the control. This migration was more intense for Sealer 26 and Apexit. An increase in mononucleate cell number was observed after 6 and 24 h and 5 days for all sealers and no differences were observed in relation to the control after 15 days.

Toxicity parameters for cytotoxicity testing of dental materials in two different mammalian cell lines

The present study compares three specific toxicity parameters for cytotoxicity testing of chemically different dental materials. Two glass ionomer cements, a zinc phosphate cement, and a composite material were used to evaluate the sensitivity of three assays: two viability assays, the MTT assay and the quantifiable neutral red assay, and a proliferation assay based on the determination of the total protein content of a cell culture. The colorimetric assays were carried out using transformed mouse fibroblasts (L-929 cells) and fibroblasts derived from biopsies of normal human gingiva. In most cases, all colorimetric assays detected much weaker cytotoxic responses, if any, in gingival fibroblasts than in L-929 cells. The viability assays indicated cytotoxicity of the extracts to two glass ionomer cements in L-929 cells when the materials were set at 0% relatively humidity for 24 h. The severe cytotoxicity of the zinc-phosphate cement in both viability assays was less influenced by the setting conditions. The cytotoxicity of the composite material was most pronounced in the neutral red assay. In general, both the MTT assay and the neutral red assay were more sensitive than the colorimetric proliferation assay. These assays can be performed very effectively; only few cells are needed for rapid, reliable and inexpensive screening purposes of a large number of samples in a short time. Automated processing with a microplate reader after non-radioactive labeling of the cells and subsequent automated analyses of original data, with no need for sophisticated and expensive equipment, are additional advantages of the systems.

In vitro models of biocompatibility: a review

The objectives of this paper were to define in vitro biocompatibility of materials, to discuss some of the issues concerning why conclusions from tissue culture are sometimes different from in vivo biocompatibility, to give highlights of the sequence of the development of these in vitro assays from the early 1950s to their present state of development, and to discuss possible future trends for in vitro testing. In vitro biocompatibility tests were developed to simulate and predict biological reactions to materials when placed into or on tissues in the body. Traditional assays have measured cytotoxicity by means of either an end-stage event, (i.e., permeability of cytoplasmic membranes of dead and dying cells, or some metabolic parameter such as cell division or an enzymatic reaction). In vitro assays for initiation of inflammatory and immune reactions to materials have also begun to appear in the literature. More recently, the concept of dentin barrier tests has been introduced for dental restorative materials. Four models which measure both permeability and biological effects of materials are compared and discussed. Future efforts may be directed toward development of materials which will allow or promote function and differentiation of tissues associated with materials. New analytical procedures and understanding of optimal characteristics of materials should improve our ability to develop more biocompatible materials. Both molecular biology techniques, and altered design of material surfaces may make the materials either more or less reactive to the biological milieu. These trends suggest a greater future role of the biological sciences in the development of biomaterials.

Comparison of tetrazolium colorimetric and 51Cr release assays for cytotoxicity determination of dental biomaterials

OBJECTIVES

The purpose of this study was to compare a methylthiazole tetrazolium (MTT) dye colorimetric method with the standard 51Cr assay as methods of assessing cytotoxicity of dental materials.

METHODS

Two MTT-based colorimetric formats, test tube and 96-well microplate methods, were compared to the 51Cr release assay. A series of eight dental materials were evaluated. Cytotoxicity profiles were determined for each test material. A TC50 value (Toxic Concentration required to kill 50% of the cells) was determined for each biomaterial, and these results were used to make statistical comparisons between the methods.

RESULTS

The three methods were statistically correlated (p<0.005) by comparison of the eight samples tested. That is, the same rank in toxicity was given by the two tetrazolium sample formats and the 51Cr method.

SIGNIFICANCE

The MTT assay was found to have several advantages in comparison to the current standard 51Cr release assay. Optimized in the 96-well format, complete dose response curves and greater sample comparisons can be made rapidly, making the MTT method more economical in time and cost. Furthermore, the MTT method is based on intracellular biochemical changes, measuring cell viability rather than cell morbidity, and has lower detectable limits than the 51Cr release method. There is also less detector chemical binding interference than encountered in the 51Cr release method.

Application of flow cytometry to determine the cytotoxicity of urethane dimethacrylate in human cells

The effects of an oligomer, urethane dimethacrylate (UDMA), on two human cell lines were studied using flow cytometry (FCM). Untreated and treated cultures of propidium iodine-stained KB (epidermal oral carcinoma cells) and human foreskin fibroblast (HFF) cells were analyzed for cellular DNA content. Concentrations of 10 and 25 microM of UDMA slightly perturbed the KB cell cycle progression at 24 and 48 h of incubation. However, the effect of 50 microM was more pronounced at the latter incubation time period. In cell growth experiments, the sublethal concentrations (10 and 25 microM) produced inhibition of KB cell growth rate at a moderate level, which resulted in the prolongation of cell population doubling time. Significant inhibition of cell growth occurred when 50 microM (lethal concentration) was used. Data obtained from the cell cycle perturbation analysis, evidenced by FCM, correlated with the extent of inhibition in KB cell growth rates. The effects of sublethal concentrations were reversible during a 24 h period of oligomer withdrawal from culture medium. In contrast, the effects of 50 microM were not reversible. In HFF cells the depletion of S phase in the cell cycle was the major effect of 50 microM of UDMA. It was concluded that FCM technology is an ideal and practical approach for studying the cytotoxicity of components of dental composites.

Indirect longitudinal cytotoxicity of root canal sealers on L929 cells and human periodontal ligament fibroblasts

The cytotoxicity of two root canal sealers was evaluated in vitro. The powder components of both sealers, mainly zinc, were the same. The liquid for one sealer, Canals, was clove oil (included eugenol in more than 80%) and other materials. For the other, Canals-N, the liquid was composed of higher fatty acids and glycol. The experiments included two cell lines, heteroploid L929 mouse fibroblasts and diploid human periodontal ligament fibroblasts. Cytotoxicity was assessed using the radiochromium release method with 4-h exposure time. The assay involved using insert chambers in multiwell arrays to produce indirect contact of materials with the cell monolayer at a controlled distance of approximately 1 mm. This model also allowed for the longitudinal study of the same material sample to assess time-dependent changes in toxicity. Freshly mixed Canals was highly toxic (p < 0.01) to both cell lines. On and after 24 h of setting no toxicity was detected. At no time could cytotoxicity be observed when experimenting with Canals-N. These results indicate that both materials have a low content of water diffusible toxic components. Substituting eugenol can further decrease the toxicity of the sealer.

In vitro evaluation of dental materials

Biocompatibility has been described as the ability of a material to perform with an appropriate host response in a specific application. Appropriate host response means no (or a tolerable) adverse reaction of a living system to the presence of such a material. An adverse reaction may be due to the toxicity of a dental material. Therefore toxicity may be regarded as one reason for nonbiocompatibility of a dental material. The toxicity of a dental material can be evaluated by in vitro tests, animal experiments and clinical trials. There exists a variety of different in vitro test methods. The most widely used biological systems for toxicity screening of dental materials are cell cultures. Cell cultures for toxicity screening of dental materials are valuable tools for understanding their biological behavior, if the limitations of the methods are taken into consideration, especially concerning the interpretation of the results. Further research should concentrate on better simulations of the in vivo situation in cell cultures. In this review the applications of various cell culture methods to evaluate the cytotoxicity of a wide range of dental materials, e.g. metals, alloys, polymers and cements, are described.

Reduced cytotoxicity of a root canal sealer through eugenol substitution

The cytotoxicity of two zinc oxide root canal sealers was investigated in vitro. The sealers were freshly mixed and set for 24 and 168 h. The sealers had identical powders but different liquid components. One (Canals) used eugenol, while the other (Canals-N) used fatty acids. L929 cells were incubated for 4 and 24 h in direct contact with the materials or with an eluate of the materials. The toxicity was evaluated using the radiochromium release assay. In the direct exposure assay, both sealers were cytotoxic when freshly prepared or after 24 h of setting. After 1 wk of setting, Canals was still toxic, while Canals-N was not significantly different from the control in the 4-h assay. In the elution assay the materials showed very low cytotoxicity. Only the eluate from freshly prepared Canals was clearly cytotoxic after 24 h. The liquid of Canals-N was clearly less cytotoxic than liquid from Canals. The results showed that the cytotoxicity of a root canal sealer can be reduced by replacing eugenol with fatty acids.

The importance of the culture medium on human dental pulp cell attachment

Radiochromium-labelled pulpal cells were cultured with nine commercially available media and conditioned minimal essential (MEM) medium. Radiochromium release from the 10 cell cultures was recorded after 4 h of incubation. Cells cultured with MEM and conditioned MEM were examined by scanning electron microscopy (SEM) after 2, 4, 8 and 24 h of incubation to study possible differences in attachment. Radiochromium release from cells cultured with most of the media was very high, indicating damage to the external cell membrane. When cells were cultured with conditioned medium, the radiochromium release was within acceptable limits. SEM showed faster attachment when cells were cultured with conditioned MEM. The data indicate that pulpal cells should be cultured with conditioned medium for better results in short-term cytotoxicity experiments with the radiochromium release method.

Indirect cytotoxic evaluation of dental materials

The standard for cellular biocompatibility in vitro testing is American National Standards Institute/American Dental Association Document No. 41a (1982). The standard allows for the contact testing of solid dental materials for cytotoxicity with HeLa or mouse fibroblasts (L929). The purpose of this study was to evaluate a new method of cytotoxicity testing of biomaterials that allows for an economic standardized indirect cytotoxic screening of the originally dispensed material over a period of time at intervals decided by the investigator. The results show a significant difference of cytotoxicity on the basis of the material between the direct contact method and the indirect or diffusion method described.

Cytotoxicity of some modified root canal sealers and their leachable components

The toxic effects of modified endodontic sealers, Fillcanal, N-Rickert, FS, and Sealer 26, were assessed. The material toxicity was measured through radiochromium release method that uses direct and indirect contact methods. The sealers were evaluated freshly mixed and after various setting times. L929 mouse and periodontal ligament fibroblast cells were used as target cells. Fillcanal, N-Rickert, and FS showed high toxicity. Fillcanal leaches toxic material when fresh and after setting time. Sealer 26 was the least toxic.

A novel cytotoxicity screening assay using a multiwell fluorescence scanner

A new assay using a multiwell fluorescence scanner was developed for screening cytotoxicity to cells cultured in 96-well microtiter plates. The assay is based on binding of propidium iodide to nuclei of cells whose plasma membranes have become permeable due to cell death. Fluorescence of propidium iodide measured with a multiwell fluorescence scanner increased in proportion to the number of permeabilized cells. After ATP depletion of hepatocytes and neonatal cardiac myocytes with metabolic inhibitors ("chemical hypoxia"), and exposure of Madine Darby canine kidney cells to the toxic chemical, HgCl2, propidium iodide fluorescence progressively increased. Increases of fluorescence were linearly proportional with release of lactate dehydrogenase into the culture medium. Employing this cytotoxicity screening assay, protection by various agents against lethal injury was evaluated in cultured hepatocytes during chemical hypoxia. Inhibitors of cysteine proteases (i.e., antipain, leupeptin, E-64), serine proteases (i.e., PMSF), and aspartic acid proteases (i.e., pepstatin A) did not protect against chemical hypoxia. In contrast, 1,10-phenanthroline, an inhibitor of metalloprotease, markedly protected against the onset of cell death during chemical hypoxia. Half-maximal protection after 60 min occurred at 0.5 microM. Phospholipase inhibitors, chlorpromazine (50 microM) and mepacrine (50 microM), also substantially retarded cell killing. U74006F, an inhibitor of lipid peroxidation, slowed cell killing to a lesser extent during chemical hypoxia and after oxidative stress with t-butyl hydroperoxide. Calciphor, a dimer of prostaglandin B1, did not protect against cell killing during chemical hypoxia or t-butyl hydroperoxide toxicity. In conclusion, this high capacity cytotoxicity assay for cells cultured in 96-well microtiter plates is suitable for rapid screening of potential cytoprotective agents in a variety of cell types.

Toxicity of Pulpispad using four different cell types

The cytotoxic effect of a zinc oxide-eugenol-based paste (Pulpispad) was evaluated in vitro after setting for 1 day and 1 week. Target cells were L929 cells, gingival, periodontal ligament and pulpal fibroblasts. The material was incubated with the cells for 4 and 24 hours, and its toxicity was evaluated with the 51Cr-release method. Pulpispad was highly cytotoxic to all cell lines even after setting for 1 week. The use of Pulpispad is not recommended for future clinical application. The various responses among the four cell lines indicated that diploid cell lines can, under certain circumstances, be less sensitive than aneuploid cell lines. It is therefore suggested that in the evaluation of biomaterials the choice of cell lines should be carefully considered, as they can display varying sensitivities.

Silver glass ionomer cement as a retrograde filling material: a study in vitro

Comparison was made of the apical microleakage of retrograde fillings with amalgam and with silver glass ionomer cements using a modified dye penetration method. Forty instrumented human teeth were divided into four groups. Each group was characterized by a different retrograde filling material or technique: Group 1--silver glass ionomer (SGI); group 2--SGI with previous acid wash of the cavity; group 3--SGI in a previously acid washed cavity, protected with varnish; and group 4--zinc-free amalgam. A paper cone for dye adsorption analysis was placed in contact with retrofill and the main canal was obturated with thermoplasticized gutta-percha. All teeth were coated, immersed in methylene blue dye, and evacuated to assure complete penetration of the dye in any possible void. Later the teeth were cleared and evaluated under stereomicroscope. Statistical analysis showed that group 1 had the least microleakage of all. In addition, the biocompatibility of the SGI and varnish was compared with that of amalgam using the 51Cr release evaluation method. The results showed that SGI and varnish were less cytotoxic than amalgam. It was concluded that SGI cement can be considered an alternative retrograde filling material.

Comparison of antimicrobial and cytotoxic effects of glutaraldehyde and formocresol

The in vitro antimicrobial and cytotoxic concentrations of glutaraldehyde and formocresol were determined. Minimal antimicrobial concentrations of these two agents against selected microbial flora reported in carious primary teeth were 3.125% for glutaraldehyde and 0.75% for formocresol. At a doubling of these concentrations, most organisms, except Candida albicans, Staphylococcus epidermidis, and Streptococcus mutans, were killed by both substances in 30 seconds. Cytotoxicity was evaluated on tissue cultures of pulp fibroblasts and HeLa cells at minimal cidal concentrations and at 10- and 100-fold dilutions. Exposure of pulp fibroblasts and HeLa cells to formocresol and indirect exposure to vapors caused the cells to become atrophic and to form a less dense tissue pattern. Cells directly exposed to glutaraldehyde retained their normal cell shape and tissue pattern, whereas cells indirectly exposed to vapors continued to proliferate. These data showed effective antimicrobial activity at concentrations of 3.125% glutaraldehyde and 0.75% formocresol and suggested that glutaraldehyde may exert a less cytotoxic effect on the immediate and surrounding tissues when used as a pulpotomy agent.

In vitro cytotoxicity of root canal filling materials: 1. Gutta-percha

Gutta-percha (GP) has been the most widely used root canal filling material because of its well-known low toxicity. The inertness of GP, however, was challenged recently. The purpose of this study was to evaluate the toxicity of marketed endodontic GP using the radiochromium release test. Fourteen commercially available and three experimental GP brands were tested. Raw GP, zinc oxide, and barium sulfate, which were considered major components of GP points, and zinc ions were also evaluated. The material was spread to cover the bottom of testing wells after being dissolved in chloroform or warmed. A labeled suspension of L929 cells was added to the wells. After incubation at 37 degrees C for 4 and 24 h, extracellular radiochromium in the culture medium was measured and calculated in percentage of the total intracellular label. Spontaneous release of radiochromium was used as control and the results were considered to be within normal limits either at 4 or 24 h. All chloroform-dissolved GP showed low toxicity at 4 h, whereas warmed GP showed statistically significant differences at 4 h. Both dissolved and warmed GP were toxic at 24 h. The raw materials and barium sulfate were not toxic, whereas zinc oxide and zinc ions showed marked toxicity. All GP points tested were toxic at longer observation periods, and the toxicity was attributed to leakage of zinc ions into the fluids.

Morphological cell changes due to chemical toxicity of a dental material: an electron microscopic study on human periodontal ligament fibroblasts and L929 cells

New endodontic materials with polymer bases may be more difficult to evaluate in cell cultures in vitro than conventional zinc oxide-eugenol cements. In order to study the morphological changes taking place in cells exposed to such materials, L929 cells and human periodontal fibroblasts were observed using scanning electron microscopic and transmission electron microscopic techniques. The morphological changes of the cells were correlated to the quantitative results observed simultaneously in cytotoxicity studies using the radiochromium release method. Results showed there was a relationship between the chromium release and the degree of individual cell damage. The periodontal ligament fibroblasts were more resistant to this kind of chemical injury than the L929 cells. Consequently, it may be proper to use periodontally derived cells for the study of cytotoxic mechanisms of polymer endodontic filling materials.

Human nasal epithelium: characterization and effects of in vitro exposure to sulfur dioxide

Human nasal turbinate tissue from surgical specimens was dissected free of connective tissue, and primary epithelial cultures were established by explant techniques. Transmission electron microscopy revealed that cultured cells retained homogeneous cytoplasmic granules, tonofilaments, and desmosomes and formed a homogeneous monolayer. The epithelial cells stained positively with cytokeratin antibodies AE1, AE3, and 35BH11 but failed to stain with two other cytokeratin antibodies, AE2 and 34BE12. Staining was also positive with anti-desmoplakin I and II but negative with antivimentin (43BE8), anti-desmin, and anti-human factor VIII antibodies. Cultured cells were exposed to filtered air or sulfur dioxide at 1-5 ppm for 30-60 min. Although there was no increase in cell lysis as measured by chromium-51 release, SO2 exposure significantly inhibited [3H]leucine incorporation compared to air exposure. This effect was dependent on both SO2 concentration and exposure duration. Control experiments revealed that these SO2 effects were not caused by the [H+] load produced by SO2 exposure. Electron microscopy of cells exposed to air or SO2 did not show any significant morphological differences.

A simple model for evaluating relative toxicity of root filling materials in cultures of human oral fibroblasts

Standardized test tubes filled with freshly mixed root filling materials (AH26, CRCS, N2, Kloroperka NO, ZOE cement and 2 experimental cements, ECI and ECII) were transferred into tissue culture flasks. Normal human oral fibroblasts were seeded in the flasks. Morphological cell changes were studied up to 15 days after seeding. The size of cell-free zones around the test tubes and the total cell number per culture flask were calculated after 5, 10 and 15 days. The findings showed N2 cement to be by far the most toxic material at all observation periods, whereas no toxic reactions could be seen in relation to tubes filled with Kloroperka NO. Compared with the 5-day observation period, some cell recovery was observed around test tubes with AH26 and ECII, whereas almost full cell recovery was found around test tubes with CRCS, ZOE and ECI. It was concluded that the present model, which allows long-term observations of human cellular reactions to dental materials, can be used as a simple and relatively cheap screening test for initial toxicity testing of dental materials.

An in vitro method for longitudinal evaluation of toxicity of endodontic sealers

An in vitro method for longitudinal evaluation of root canal sealers was developed and applied. A newly introduced cell culture chamber was used to evaluate the cytotoxic effects of test samples immediately after mixing and for an extended period of time thereafter. A ranking of the test materials, based on their cytotoxicity, was allowed by the method.

Effects of two gutta-percha formulations and one zinc oxide-eugenol and Canada balsam mixture on human blood monocytes and lymphocytes

In this study we performed an in vitro assay of the effects of two gutta-percha formulations and one zinc oxide-eugenol and Canada balsam-based endodontic material on the behavior of a mixed cell population of human monocytes and lymphocytes. Cells were cultured either in direct contact with or near experimental samples of Ultrafil, standard gutta-percha cones or Endoseal and the inhibitory effects of these materials upon cell attachment and spreading on test surfaces were analyzed. Ultrafil and the standard gutta-percha cones showed little or no adverse effects, whereas the inhibitory effects of Endoseal appeared to be severe. There were statistically significant differences between the results obtained from both Ultrafil and gutta-percha cones and those obtained from Endoseal. Differences between all tested materials and their respective controls were also statistically significant.

Characterization of the "in vitro pulp chamber" using the cytotoxicity of phenol

Phenol was found to be an acceptable positive control substance for cytotoxicity assays in a new device, the "in vitro pulp chamber" (IVPC). Various concentrations of phenol were used to study its biologic effects on Balb/c 3T3 cells as well as its diffusion characteristics through various thicknesses of dentin disks. The cells showed a linear cytotoxic response to direct contact with phenol between the concentrations of 5 x 10(-4) and 5 x 10(-2) M. When a dentin barrier was used in the IVPC, phenol concentrations in the lower chamber were reduced below that in the reservoir and were inversely proportional to the thickness of the dentin. In the IVPC with 0.5 mm dentin disks, Balb/c 3T3 cells growing on the bottom of the dentin disks still maintained new protein synthesis at 60% of control levels when the phenol concentration in the reservoir was 0.05 M. When in direct contact with the cells this phenol concentration depressed protein synthesis to basal levels.

Cytotoxicity of dental composites and other materials in a new in vitro device

Several in vitro methods have been developed over the years to improve correlation between screening tests and usage tests of dental restorative materials. Pulpal responses to restorative materials in usage tests are usually less severe than are responses of cells either in vitro screening tests or in implantation tests where there is direct contact between experimental materials and vital cells. In the "in vitro pulp chamber" device tested in the present study, dentin disks were interspersed between composite resins (as well as other dental materials) and the medium which served as the nutrient source for the Balb/c 3T3 test cell system. The dentin restricted the diffusion of materials into the "pulp chamber" to clinically relevant levels. The light-cured composite resins (Fulfil and P30), caused depression of protein synthesis only during the first 24 h postpolymerization, when placed on 0.5 mm but not 1.5 mm thick dentin dishes. The chemically-set composites (SILAR and P10) caused no significant inhibition of protein synthesis (as compared to negative controls) at any postpolymerization time on either 0.5 mm or 1.5 mm dentin disks.

Cytotoxicity of amalgams

The purpose of this study was to compare the relative cytotoxicity of amalgams and to determine whether their toxicity depends upon composition and aging time, by means of a rapid and sensitive in vitro cell culture test. Zinc-containing amalgams showed higher cytotoxicity than did any other amalgams. High-copper amalgams had the same cytotoxicity as did the low-copper amalgam. The addition of selenium did not reduce the cytotoxicity of amalgam. Moreover, excessive additions of selenium increased the cytotoxicity of amalgam compared with that of a similar selenium-free material. The cytotoxicity of amalgam was decreased with aging time, possibly due to the combined effects of surface oxidation and further amalgamation.