Release of formaldehyde from denture base polymers

Environmental and biological monitoring of formaldehyde inside a hospital setting: a combined approach to manage chemical risk in workplaces

Background: The safety of healthcare workers exposed to formaldehyde remains a great matter of concern for healthcare management units. This work aimed at describing the results of a combined monitoring approach (environmental and biological) to manage occupational exposure to formaldehyde in a hospital setting.

Design and Methods: Environmental monitoring of working spaces and biological monitoring of urinary formaldehyde in 16 exposed healthcare workers of the Anatomic Pathology Unit of a University Hospital in Southern Italy was performed on a fouryear timescale (2016-2019).

Results: Values of aero-dispersed formaldehyde identified were on average low; although workers’ urinary formaldehyde levels were also minimal, the statistical analysis highlighted a slight weekly accumulation.

Conclusions: Our data confirm that both environmental and biological monitoring are important to identify risk situations, in particular when values of hazardous compounds are below the accepted occupational exposure levels.

Histocompatibility of Novel Cycloaliphatic Comonomer in Heat-cured Denture Base Acrylic Resin: Histomorphometric Analysis in Rats

Background:

Prosthodontics is impossible without denture base resins. Allergic reactions to these resins are not uncommon, albeit favorable properties. Monomeric modifications are being done to improve the properties of the material. Tricyclodecane dimethanol diacrylate (TCDDMDA) monomer has been recently identified and experimented as a comonomer with methyl methacrylate (MMA).

Aim:

This study aimed to investigate the histocompatibility of TCDDMDA comonomer in polymerized resin at 10% and 20% (vol/vol) concentrations in rats by histomorphometric analysis.

Materials and Methods:

Twenty-four male Wistar rats were randomly divided into the following four groups: NP group (control; n = 6), with no palatal appliance, Groups P0, P10, and P20 were fixed with palatal appliances fabricated of 100% MMA, 10% TCDDMDA + 90% MMA, and 20% TCDDMDA + 80% MMA, respectively. Weights of the animals were recorded just before the appliance placement and after 14 days. The animals were sacrificed, and the palatal tissues were processed for histopathological analysis. Histomorphometric parameters assessed were total epithelial (ET), connective tissue (CT), and keratin layer (KT) thicknesses.

Results:

No significant difference was observed regarding body weight. Group P0 showed increased ET, CT, and KT when compared to other groups. Bonferroni multiple comparison tests showed a statistically significant difference between all the groups except between P10 and P20 for all the three morphometric parameters.

Conclusion:

Palatal appliances with TCDDMDA comonomer showed good histocompatibility in rats up to 20% (vol/vol) concentration.

Effect of Incorporation of Nanoclay on the Properties of Heat Cure Denture Base Material: An In vitro Study

Purpose:

The aim of the study was to evaluate the effect of incorporation of organically modified nanoclay in 1%, 3%, and 5% by weight on the flexural strength, surface hardness, and linear polymerization shrinkage of heat cure denture base material.

Materials and Methods:

One hundred and twenty specimens of heat-polymerized acrylic resin were fabricated. The specimens were divided into four groups (n = 10) coded I to IV. Group I was the control group (unmodified acrylic resin specimens). The specimens of the remaining three groups were reinforced with nanoclay (organically modified montmorillonite) nanoparticles to achieve loadings of 1%, 3%, and 5% by weight. The resulting nanocomposites were subjected to mechanical testing and were characterized using X-ray diffraction, scanning electron microscope, and transmission electron microscope.

Results:

The statistical analysis showed that there was no significant increase in flexural strength within and between the groups. The most significant increase in surface hardness was observed between Group I (control) and Group II (1% nanoclay). Linear polymerization shrinkage of the specimens showed a significant decrease in the control and all the experimental groups.

Conclusion:

Addition of 1 wt% nanoclay to polymethyl methacrylate heat cure denture base material could enhance the surface hardness and reduce the linear polymerization shrinkage of the resin. However, there was no significant increase in flexural strength of the resulting nanocomposite.

Contact Sensitization to Formaldehyde in Veterinary Medicine - An Unexplored Field in Occupational Health

Background:

Veterinary staff and students could be exposed to formaldehyde – a ubiquitous agent, common cause of contact allergy.

Aim:

To evaluate the incidence of contact sensitization to formaldehyde in exposures in veterinary medicine.

Settings and Design:

A cross-sectional study was conducted during July–December 2017.

Materials and Methods:

A total of 206 individuals were included, patch tested with formaldehyde 1.0%/aq – 36 veterinary medicine students, 20 veterinarians, 47 students and 28 trainees of dental medicine, 41 dental professionals, and 35 non-occupationally exposed individuals.

Results:

The incidence of contact sensitization to formaldehyde among the whole studied population was 48.1%, highest being among the students of veterinary medicine (94.4%) and the veterinarians (85%). With very high significance, the sensitization incidence was higher in the groups of students of veterinary medicine and veterinarians, if compared to the control group (P < 0.001); (P = 0.004), dental professionals (P < 0.001); (P = 0.001), trainees of dental medicine (P < 0.001); (P = 0.005), and students of dental medicine three-fourth year of education (P < 0.001); (P = 0.001). Significantly, higher was the incidence of contact allergy in the control group if compared to those of dental professionals (P = 0.033) and dental students three-fourth year of education (P = 0.028).

Conclusions:

The exposure to formaldehyde during the education in veterinary medicine and practice could be an important risk factor for the onset of contact sensitization. Stricter preventive measures are needed to reduce veterinary student's and lecturer's exposures. Equipment of dissection tables with local exhaust ventilation system could reduce the concentration of formaldehyde in the gross anatomy laboratory.

Physical and Chemical Aspects Related to Substances Released from Polymer Materials in an Aqueous Environment

Exposure to an aqueous environment will change the physical properties of polymer materials with time. Due to water absorption, surface hardness is reduced and mechanical properties are adversely affected. The composition of polymer dental materials varies greatly. The organic substances are represented by monomers, oligomers, polymers, initiators, activators, inhibitors, anti-oxidants, UV stabilizers, plasticizers, fluorescent compounds, and other additives. The inorganic substances are represented by oxides and glasses (fillers and pigments) of various composition. Both organic and inorganic substances are released into water from composite materials. Combined-HPLC/UVspectroscopy analysis of an aqueous extract from a composite filling material demonstrates that the eluate contains several components. Several inorganic metal-ions released into aqueous solutions have been identified. Only a few of the released organic substances have been identified. Several investigators have determined the quantity of residual monomers in the polymerized materials, and the decrease of such residual monomers with time. Exposure to water, or aqueous salivary enzyme solutions, results in release of degradation products, such as formaldehyde, methacrylic acid, and others.

A review of the effects of formaldehyde release from endodontic materials

Formaldehyde is present in most living cells and the environment. In dentistry, patients may be exposed to formaldehyde through the use of several endodontic materials (e.g. AH 26) and during formocresol pulpotomies. This review outlines how the human body reacts to formaldehyde exposure, how recent data has relooked at the issue of carcinogenicity and leukaemia associated with formaldehyde, and whether it is possible to quantify the amount of formaldehyde produced by endodontic cements. The review analyses the way formaldehyde is produced from epoxy resins and addresses the question of whether the amount of formaldehyde from endodontic cements is large enough to override the body's ability to deal with its own endogenous levels of formaldehyde and should the amount of formaldehyde produced be a concern.

The bacterial adhesion on and the cytotoxicity of various dental cements used for implant-supported fixed restorations

OBJECTIVE

Bacterial adhesion on and cytotoxicity of eight luting agents used for implant-supported restorations were investigated.

MATERIALS AND METHOD

Surface roughness (Ra), surface free energy (SFE) values and three-dimensional images by atomic-force microscopy of circular specimens were determined. Bacterial suspensions of Streptococcus sanguinis and Streptococcus epidermidis were incubated at 37°C for 2 h. Adhering bacteria were examined with fluorescence dye CytoX-Violet, stained with 4',6-diamidino-2-phenylindole (DAPI) and visualized by fluorescence-microscopy. Cytotoxicity-testing was done with WST-1-tests (water soluble tetrazolium). No significant differences, neither with regard to Ra nor regarding SFE were determined.

RESULTS

Adherence of S. sanguinis was less on titanium, TempBondNE and TempBond. TempBond, TempBondNE, RelyX Unicem and Implantlink Semi Classic presented low amounts of S. epidermidis. WST-testing showed high cytotoxic potential of Harvard, Aqualox, TempBondNE and TempBond. No combination of low adherent bacteria with low cytotoxicity was found.

CONCLUSION

From a biological in-vitro perspective, none of the cements may be recommended for implant-supported restorations.

In vitro evaluation of long-term cytotoxic response of injection-molded polyamide and polymethyle metacrylate denture base materials on primary fibroblast cell culture

OBJECTIVE

This study investigated the long-term cytotoxic response of thermoplastic polyamide and conventional polymethyle metacrylate (PMMA) denture base materials.

MATERIALS AND METHODS

Twenty discs were prepared for each polyamide, heat and cold cured PMMA denture base resins (totally 60) and divided into four sub-groups (n = 5). Cytotoxicity was assessed with the direct cell contact method using cell viability and neutral red (NR) uptake assay. Each sub-group was tested at initial and after being aged for 24 h, 1 week and 8 weeks with artificial saliva according to ISO 10993 standards.

RESULTS

There were no significantly difference among the materials and control groups after initial, 24 h and 1 week testing. In 24 h testing, only Deflex was more toxic according to the Control group (p < 0.05). After 8 weeks of aging with artificial saliva, all materials were significantly cytotoxic when compared to the control group. QC20 was more toxic than Deflex and SC Cold Cure (p < 0.05). There were significant differences between the 8 week aging group and the initial, 24 h and 1 week testing for all materials (p < 0.05).

CONCLUSIONS

Cytotoxicity of all tested denture base materials increased significantly after the long-term aging. Therefore, long-term aging may be useful to determine a dental material's toxicity. Polyamide denture base material had a similar toxicity profile with conventional heat- and cold-cured PMMA.

Synthesis and biological evaluation of PMMA/MMT nanocomposite as denture base material

Inorganic-polymer nanocomposites are of significant interest for emerging materials due to their improved properties and unique combination of properties. Poly (methylmethacrylate) (PMMA)/montmorillonite (MMT) nanocomposites were prepared by in situ suspension polymerization with dodecylamine used as MMT-modifier. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to characterize the structures of the nanocomposites. Cytotoxicity test, hemolysis test, acute systemic toxicity test, oral mucous membrane irritation test, guinea-pig maximization test and mouse bone-marrow micronucleus test were used to evaluate the biocompatibility of PMMA/MMT nanocomposites. The results indicated that an exfoliated nanocomposite was achieved, and the resulting nanocomposites exhibited excellent biocompatibility as denture base material and had potential application in dental materials.

Glutaraldehyde release from heat-polymerized acrylic resins after disinfection and chemical and mechanical polishing

This study evaluated the release of glutaraldehyde from heat-polymerized acrylic resins subjected to disinfection followed by chemical and mechanical polishing. Ninety disc-shaped specimens (15 x 4 mm), 30 per resin (Lucitone 550, QC-20 and Classico), were made and assigned to 2 groups according to the type of polishing. One side of each specimen was not polished and the other was either mechanically (n=45) or chemically (n=45) polished, and immersed in water at 50°C for 1 h to allow the release of intrinsic substances and then kept in distilled water for 7 days. The specimens were disinfected by immersion in 2% glutaraldehyde for 10 min. After this period, 3 specimens from each group were immersed in water for 15, 30, 60, 120 and 240 min. For the 15-, 30-, 60-min immersions, 4 water exchanges were done at the end of period. High performance liquid chromatography (HPLC) was used to detect and quantify the glutaraldehyde released after each period. Data were analyzed statistically by two-way ANOVA and multiple comparisons were done by Tukey’s and Scheffé’s tests (α=0.05). No glutaraldehyde release was observed from the specimens with chemical polishing at any of the immersion periods, while the mechanically polished specimens released glutaraldehyde. In the groups with water exchanges, Lucitone released more disinfectant in the 15-min period (0.040 μg/mL), Classico in the 30-min (0.021 μg/mL) and 60-min (0.018 μg/mL) periods, and QC-20 the same amount (-1.760 μg/mL) in all periods. In the groups without water exchanges, Lucitone released the highest amount of disinfectant (-1.370 μg/mL), differing significantly from QC-20 (0022 g/mL) and Classico (0019 g/mL), which were similar. The findings of this showed that chemically polished specimens from the 3 resin brands did not release glutaraldehyde after different periods of immersion, while glutaraldehyde release was observed from the mechanically polished specimens, especially from those made of Lucitone resin.

Identification and quantification of leachable substances from polymer-based orthodontic base-plate materials

The aim of this study was to analyse leachable monomers, additives, and degradation products from polymer-based orthodontic base-plate materials. One heat-cured resin (Orthocryl), one light-cured (Triad VLC), and three thermoplastic materials (Biocryl C, Essix A+, and Essix Embrace) were investigated. Elution was performed in water at 37°C for 10 days. The extract medium was changed and analysed daily. Chromatographic methods were used to identify and quantify the leachables. In addition, the content of residual methyl methacrylate (MMA) was quantified in the poly(methyl methacrylate) (PMMA)-based materials. Statistical analysis of the quantitative results was performed using a t-test for comparison of two independent samples. Monomers and additives leached from the materials polymerized in situ and from the thermoplastic PMMA-based material. No leachable substances were found in the extracts from the other thermoplastic materials. Accumulated over 10 days, a larger amount of MMA leached from the powder-and-liquid material, Orthocryl (42 μg/cm(2)), than from the thermoplastic material, Biocryl C (0.49 μg/cm(2)). The accumulated amounts of monomers leached from Triad VLC were 91 μg/cm(2) of urethane dimethacrylate and 2.2 μg/cm(2) of 2-hydroxyethyl methacrylate. Formaldehyde was found to leach from methacrylate-based materials: 3.2 μg/cm(2) from Orthocryl and 0.16 μg/cm(2) from Triad VLC. However, formaldehyde was not detectable in extracts from Biocryl C. Residual MMA was 5.4 wt % in Orthocryl and 0.4 wt % in Biocryl C. No phthalates were detected in the tested materials. In this in vitro study, minimal leaching was found from the thermoplastic materials, while leaching of methacrylates and formaldehyde was observed from the powder-and-liquid type and the paste material. Within the limitations of this study, the results suggest that prefabricated thermoplastic plates should be preferred for patients with an allergy to methacrylates.

Effect of post-polymerization heat treatments on the cytotoxicity of two denture base acrylic resins

INTRODUCTION

Most denture base acrylic resins have polymethylmethacrylate in their composition. Several authors have discussed the polymerization process involved in converting monomer into polymer because adequate polymerization is a crucial factor in optimizing the physical properties and biocompatibility of denture base acrylic resins. To ensure the safety of these materials, in vitro cytotoxicity assays have been developed as preliminary screening tests to evaluate material biocompatibility. (3)H-thymidine incorporation test, which measures the number of cells synthesizing DNA, is one of the biological assays suggested for cytotoxicity testing.

AIM

The purpose of this study was to investigate, using (3)H-thymidine incorporation test, the effect of microwave and water-bath post-polymerization heat treatments on the cytotoxicity of two denture base acrylic resins.

MATERIALS AND METHODS

Nine disc-shaped specimens (10 x 1 mm) of each denture base resin (Lucitone 550 and QC 20) were prepared according to the manufacturers' recommendations and stored in distilled water at 37 degrees C for 48 h. The specimens were assigned to 3 groups: 1) post-polymerization in a microwave oven for 3 min at 500 W; 2) post-polymerization in water-bath at 55 degrees C for 60 min; and 3) without post-polymerization. For preparation of eluates, 3 discs were placed into a sterile glass vial with 9 mL of Eagle's medium and incubated at 37 degrees C for 24 h. The cytotoxic effect of the eluates was evaluated by (3)H-thymidine incorporation.

RESULTS

The results showed that the components leached from the resins were cytotoxic to L929 cells, except for the specimens heat treated in water bath (p<0.05). Compared to the group with no heat treatment, water-bath decreased the cytotoxicity of the denture base acrylic resins.

CONCLUSION

The in vitro cytotoxicity of the tested denture base materials was not influenced by microwave post-polymerization heat treatment.

Streptococcal adhesion to various luting systems and the role of mixing errors

OBJECTIVE

This study aims at ranking various luting systems according to their susceptibility to adhering Streptococcus mutans and at evaluating the influence of incongruent mixing ratios on adhesion quantities.

MATERIAL AND METHODS

Circular specimens measuring 8 mm in diameter were made of nine widely used dental cements -- three of them mixed in different ratios -- and then incubated with S. mutans. Adhering streptococci were quantified using a biofluorometric assay in combination with an automated plate reader for cell quantification. Surface roughness (R(a)) was determined by perthometer measurements.

RESULTS

Meron plus revealed the highest R(a) (0.90 microm) and glass the lowest R(a) (<0.01 microm). In regular cement mixtures, the highest mean fluorescence intensities indicated the presence of many viable bacteria [Meron Plus (35,533 relative fluorescence units (rfu)), Maxcem (13,374 rfu), and Panavia F 2.0 (11,701 rfu)]. Moderate fluorescence intensities were found in Harvard (4,171 rfu), Ketac cem (3,766 rfu), Durelon (3,276 rfu), Calibra (3,259), Rely X Unicem (4,358 rfu), and Bifix SE (3,102 rfu). A medium correlation between R(a) and S. mutans adhesion was found. Changes in regular cement proportions (powder/liquid and base/catalyst, respectively) had a significant influence on relative fluorescence intensities, which linearly increased with a higher proportion of liquid in Harvard and with a higher proportion of catalyst in Calibra and Maxcem.

CONCLUSIONS

Various luting systems revealed considerable differences in their potential to adhere S. mutans. Variations from recommended cement proportions led to significant changes in the amount of adhering streptococci.

Biocompatibility of denture base acrylic resins evaluated in culture of L929 cells. Effect of polymerisation cycle and post-polymerisation treatments

OBJECTIVE

The purpose of this study was to evaluate the effect of two post-polymerisation treatments and different cycles of polymerisation on the cytotoxicity of two denture base resins.

MATERIALS AND METHODS

The resins tested were Lucitone 550 and QC 20. Discs of resins were fabricated following the manufacturer's instructions. Lucitone 550 was processed by long cycle or short cycle. The resin QC 20 was processed by reverse cycle or normal cycle. The specimens were divided into groups: (i) post-polymerised in microwave for 3 min at 500 W; (ii) post-polymerised in water-bath at 55 degrees C for 60 min and (iii) without post-polymerisation. Eluates were prepared by placing three discs into a sterile glass vial with 9 ml of Eagle's medium and incubated at 37 degrees C for 24 hours. L929 cells were seeded into 96 well culture plates and DNA synthesis was assessed by (3)H-thymidine incorporation assay.

RESULTS

The results were submitted to two-way anova and Tukey HSD test. QC 20 specimens polymerised by the normal cycle and submitted to microwave post-polymerisation were graded as moderately cytotoxic. Similar results were observed for Lucitone 550 processed by long cycle without post-polymerisation. The other experimental groups were graded as not cytotoxic. After water-bath post-polymerisation, specimens of Lucitone 550 processed by long cycle produced significantly lower inhibition of DNA synthesis than the other groups.

CONCLUSION

The long cycle increased the cytotoxicity of Lucitone 550 and water-bath post-polymerisation reduced the cytotoxicity of Lucitone 550 processed by long cycle.

Hygroscopic and hydrolytic effects in dental polymer networks

OBJECTIVES

The objective of this manuscript is to outline the factors associated with hygroscopic and hydrolytic effects in dental polymer networks, and to review the literature generated over the past thirty years or more in this area.

METHODS

Information was gathered from nearly 90 published articles or abstracts appearing in the dental and polymer literature. Studies were predominantly identified through a search of the PubMED database.

RESULTS

Studies were included that provided direct evidence for the uptake of solvent by a polymer network and its subsequent physical or chemical effect, or the loss of molecular species into solvents. An attempt was made to select articles that spanned the timeframe from approximately 1970 to today to ensure that most of the classic literature as well as the latest information was included.

CONCLUSIONS

Dental polymer networks have been shown to be susceptible to hygroscopic and hydrolytic effects to varying extents dependent upon their chemistry and structure. The importance of these effects on the clinical performance of polymer restoratives is largely unknown, though numerous investigators have alluded to the potential for reduced service lives.

SIGNIFICANCE

While the physical and mechanical properties of these materials may be significantly altered by the effects of solvent uptake and component elution, what may constitute the greatest concern is the short-term release of unreacted components and the long-term elution of degradation products in the oral cavity, both of which should be strongly considered during restorative material development.

The effect of N-acetyl-l-cysteine and ascorbic acid on visible-light-irradiated camphorquinone/N,N-dimethyl-p-toluidine-induced oxidative stress in two immortalized cell lines

Recent studies have revealed that visible-light (VL)-irradiated camphorquinone (CQ), in the presence of a tertiary amine (e.g., N,N-dimethyl-p-toluidine, DMT), generates initiating radicals that may indiscriminately react with molecular oxygen forming reactive oxygen species (ROS). In this study, the ability of the antioxidants N-acetyl-l-cysteine (NAC) and ascorbic acid (AA) to reduce intracellular oxidative stress induced by VL-irradiated CQ/DMT or VL-irradiated hydrogen peroxide (H(2)O(2)) was assessed in an immortalized Murine cementoblast cell line (OCCM.30) and an immortalized Murine fibroblast cell line, 3T3-Swiss albino (3T3). Intracellular oxidative stress was measured with the membrane permeable dye, 2',7'-dichlorodihydrofluorescein diacetate (H(2)DCF-DA). VL-irradiated CQ/DMT and VL-irradiated H(2)O(2) each produced significantly (p<0.001) elevated intracellular oxidative levels in both cell types compared to intracellular ROS levels in VL-irradiated untreated cells. OCCM.30 cementoblasts were found to be almost twice as sensitive to VL-irradiated CQ/DMT and VL-irradiated H(2)O(2) treatment compared to 3T3 fibroblasts. Furthermore, 10mm NAC and 10mm AA each eliminated oxidative stress induced by VL-irradiated CQ/DMT and VL-irradiated H(2)O(2) in both cell types. Our results suggest that NAC and AA may effectively reduce or eliminate oxidative stress in cells exposed to VL-irradiated CQ/DMT following polymerization.

HPLC determination of residual monomers released from heat-cured acrylic resins

HPLC was used to examine the leachability of three non-phthalic and four phthalic post-polymerized residual monomers from three commercially available heat-cured acrylic resins. Specimens of equal dimensions were constructed from each brand of material by following the standardized procedure and were stored under three different conditions, namely, distilled water, artificial saliva, and a binary mixture of ethanol/water. The resulting liquids provided samples for analysis by HPLC. Three different experiments were performed for each brand of acrylic and each storage condition in order to examine the effects of parameters, particularly time and temperature. The results obtained from this study suggest that a wide spectrum of residues diffuse out of the three examined acrylic resin materials. The non-phthalic compounds were leached at high concentrations, whereas all the phthalates examined exhibited different degrees of elusion commensurate with the storage condition, brand of material, and type of experiment. It seems that a significant quantity of non-phthalic and phthalic residues diffuse out of the acrylic resin materials examined. The main component extracted was methyl methacrylate, the level of which seems to be time-dependent and decreases for a period of up to 5 days when resins are stored in distilled water at room temperature.

Assessment of genetic damage by methyl methacrylate employing in vitro mammalian test system

Methyl methacrylate (MMA) is a volatile liquid widely used in the manufacture of acrylic polymers. In modern dentistry, MMA is the mainstream material in denture bases. MMA has been implicated as primary irritant and sensitizer, which can cause allergic eczematous reaction on the oral mucosa as well as skin. To date, there is growing concern that MMA may produce genetic damage by inducing mutation. In this study, colony forming efficiency, DNA synthesis, and cytogenetic assays were performed to investigate the adverse effects of MMA in cultured CHO cells. MMA was found to decrease colony formation in a dose- and time-dependent manner (P<0.05). MMA also inhibited DNA synthesis in a dose-dependent manner (P<0.05). The chromosome aberrations induced by MMA were the chromatid-type aberrations in the treated cultures. Moreover, the gaps and breaks were the most common type of aberrations observed. The sister-chromatid exchange frequencies were found to increase in the concentration of MMA. In this study, MMA was found to be not only a cytotoxic agent but also a genotoxic agent. The effects observed following treatment with low dose for longer duration is of relevance to the condition of the oral mucosa of the denture wears. Denture base resin could constantly release MMA extended periods, possibly causing moderate toxic reactions and possibly contributing to adverse effects on the mucosa.

In vitro cytotoxicity evaluation of endodontic sealers exposed to heat before assay

During warm-vertical condensation of gutta-percha, endodontic sealers are exposed to temperatures that potentially alter their cytotoxicity profiles. This study investigated the effects of heat on sealer toxicity by using the CyQuant assays for cell adhesion and proliferation of human gingival fibroblasts. Seven sealers were exposed to one of the following temperatures for 10 min before assay: 23 degrees C, 43 degrees C, or 100 degrees C. Adhesion data were used to determine percentage of cells bound, and proliferation data were used to determine percentage of change in cell number from 24 to 72 h, for each sealer at each temperature. The temperatures of 23 degrees C, 43 degrees C, and 100 degrees C were significantly different in both the adhesion and proliferation assays. Sealer type and temperature were significant variables in both assays. The changes in toxicity measured by adhesion and proliferation for each sealer were not always similar. Roth 801 and Roth 811, after exposure to 100 degrees C, showed enhanced adhesion but not proliferation, whereas Kerr PCS and Dentalis, after exposure to 100 degrees C, showed decreased adhesion but enhanced proliferation. AH Plus and AH26 showed an increase in adhesion and proliferation after heat exposure, whereas Sealapex remained virtually unaffected.

Influence of polymerization conditions on monomer elution and microhardness of autopolymerized polymethyl methacrylate resin

Residual monomer contents and surface hardness are important factors in determining the serviceability of provisional restorations. The intent of this study was to systemically evaluate the effects of curing conditions on provisional polymethyl methacrylate (PMMA) resins which utilize a free-radical polymerization reaction. Combinations of the three curing factors of temperature, pressure, curing environment (water/air) were adjusted during the fabrication of autopolymerized specimen disks. The initial hardness of tested materials was measured with a microhardness tester 1 h after disc fabrication, and the amounts of residual methyl methacrylate (MMA) released into water were analyzed by reverse-phase HPLC after 7 d of water immersion. Results from multiple regressions showed that curing temperature was the dominant factor in improving resin surface hardness, whereas curing in water was the key factor for reducing the quantity of residual monomer. The pressure factor, which was thought to be critical for managing autopolymerized resins, showed no significant influences on the properties tested. ANOVA results showed that provisional PMMA resins cured in hot water, with or without pressure, significantly reduced the amount of residual MMA elution (up to 80%) and increased the microhardness values (up to 50%).

From vulcanite to vinyl, a history of resins in restorative dentistry

This article provides historical background on the development of resin-based dental restorative materials. With an understanding of the evolution of these materials, clinicians can better appreciate both the complexity of and similarities among the wide variety of resins and polymerization techniques available today. Common problems associated with the use of resin-based materials are explained, and more advanced resin-based systems currently under development are briefly reviewed.

Prosthodontic biomaterials and adverse reactions: a critical review of the clinical and research literature

Prosthodontic biomaterials include impression materials, luting cements, and restorative materials. They consist of metals and alloys ceramics, and polymer materials and are retained in patients for <60 min or for decades. Oral release of compounds from biomaterials occurs, and adverse reactions may follow dental treatment. Especially in allergically vulnerable patients contact allergy may occur. There are reports from many different countries on contact allergy from gold/palladium alloys, components from polymer-based materials, chromium/cobalt alloys, and nickel. Notifications on adverse reactions in Norway, Sweden, and England are handled by a registry in which patient reactions and occupational exposure are recorded. Data from The Adverse Reaction Unit in Bergen and Umeå have been a most valuable basis in extending knowledge in a field of current interest in dentistry. A review of the clinical and research literature relating to prosthodontic biomaterials and adverse reactions shows that reliable methods seem necessary to expose the frequency of adverse reactions in general dentistry, including prosthodontic treatment.

Formaldehyde release from ground root canal sealer in vitro

The formaldehyde release from three different ground root canal sealer materials was examined. Ten specimens each of AH26, Amubarut, and N2 were stored under dry conditions for 6 months. An amount of approximately 100 to 200 mg ground material was obtained from each sample by using a round bur and stored for 10 min in distilled water. The formaldehyde concentration of the immersion water was determined by high-performance liquid chromatography. The mean formaldehyde release per mg material was 6.6 (+/-2.5) microg for AH26 and 8.3 (+/-1.0) microg for Amubarut. A lower formaldehyde release was detectable by our method from the N2 samples (0.3 +/- 0.1 microg/g; p < 0.0001). In conclusion formaldehyde release from ground root canal material is low, although a risk of an allergic reaction in susceptible patients cannot be excluded.

Measurement of formaldehyde as dimedone adduct and potential formaldehyde precursors in hard tissues of human teeth by overpressured layer chromatography

Simple, automatic overpressured layer chromatographic methods for the parallel determination of endogenous formaldehyde in the form of dimedone adduct and potential formaldehyde generators (betaines) in hard tissues of human teeth are described together with an efficient densitometric evaluation. These simple procedures involve the special preparation of teeth and extraction of formaldehyde, of different binding force in teeth, with methanol containing dimedone, and the isolation of betaines with an aqueous solution of methanol.

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.

Biologic testing of leachable aromatic compounds from denture base materials

The aromatic compounds phenyl benzoate (PB), phenyl salicylate (PS), and biphenyl (BP), which have previously been found to leach from poly(methyl methacrylate) denture base materials, were tested for cytotoxicity and biologic effects by L929 cells in culture. The octanol-water partition coefficient (log P(ow), a descriptor for the lipophilicity, was determined for the compounds. Cytotoxicity was evaluated by total cell growth and the plating efficiency test, and biologic effects by the total fatty acid composition of L929 cells. The commonly used tests, total cell growth and plating efficiency, did not show any significant changes of the cells due to the compounds. On the other hand, BP and PS, in particular, induced changes in the total fatty acid composition of L929 cells. The problem of bioavailability of aromatic compounds in cell culture assays and the relation to lipophilicity was addressed.

Residual monomer content and its release into water from denture base materials

OBJECTIVES

The aim of this study was to determine the content and amount of residual monomer released from heat-cured and chemical-cured polymethyl methacrylate (PMMA) during storage in water.

METHODS

Residual monomer was extracted from ground PMMA with tetrahydrofuran and from the storage water with n-hexane. Samples were analyzed by using HPLC-chromatography. Duration of the storage period in water was up to 8 wk at temperatures of +22 degrees C and +37 degrees C.

RESULTS

Residual monomer content of chemical-cured PMMA was higher than that of heat-cured PMMA. The effect of storage temperature was significant (p<0.001). During the first day of storage at +37 degrees C, the monomer release from chemical-cured PMMA was 1906.7 ppm and from the heat-cured PMMA 34.5 ppm and that release was higher at +37 degrees C than at +22 degrees C (p<0.001).

SIGNIFICANCE

This study suggests that the amount of released monomer can be diminished by storing the PMMA product in water for at least 1 d before use, preferably at +37 degrees C.

Leaching from denture base materials in vitro

Specimens made from denture base materials were leached in Ringer solution and in ethanol. The specimens comprised a heat-cured product processed in two different ways and two cold-cured materials. The organic compounds leaching from the specimens to the solutions were separated, identified, and quantified by a combined gas-chromatography and gas-chromatography/mass-spectrometry technique. Additives and degradation products, possibly made by free radical reactions, were released from the denture base materials. In Ringer solution only phthalates could be quantified. In ethanol solvent, biphenyl, dibutyl phthalate, dicyclohexyl phthalate, phenyl benzoate, and phenyl salicylate were quantified. In addition, copper was found in the ethanol solvent from one of the denture base materials. The amount of leachable organic compounds varies among different materials. Processing temperature influences the initial amount of leachable compounds.

Cytotoxicity of eluates from light-polymerized denture base resins

This study examined the metabolic effects of eluates from four light-polymerized denture base resins and one heat-polymerized denture base resin on oral epithelial cells in vitro. The eluate was cell culture medium that contained either or both of apparently nonpolymerized components and reaction products that diffused out of the resin samples. Eluates were prepared by daily transfer of sample disks in a cell culture medium over 10 days. Toxicity of eluates was tested immediately after transfer (fresh) and after storage for 30 days (aged) by use of radioisotope incorporation and cell viability studies. The fresh eluates inhibited cell metabolism, whereas the aged eluates stimulated then inhibited the responses. Results suggest that the components that leach out of the tested materials do so at different rates and have prolonged toxic effects on cells. Thus soaking prosthesis in water before insertion may be beneficial.

Leaching and cytotoxicity of formaldehyde and methyl methacrylate from acrylic resin denture base materials

Acrylic resin dentures have the potential to elicit irritation, inflammation, and an allergic response of the oral mucosa. Studies of substances leachable from acrylic resins, their cytotoxicity to cultured cells, and means of reducing their leaching were systematically conducted. Under in vivo and in vitro conditions, formaldehyde and methyl methacrylate were significantly leached into human saliva and saliva-substitute buffer, especially from autopolymerized resins. Both leachable substances showed cytotoxic potentials in the range of their leaching concentrations. Formaldehyde was cytotoxic at lower concentrations than methyl methacrylate. Preleaching in water reduced subsequent leaching of both formaldehyde and methyl methacrylate, and the amount of reduction depended on an increase in the preleaching temperatures. Immersion of acrylic resin dentures in hot water (50 degrees C) before insertion is recommended, especially for autopolymerized resins used either for rebasing or as denture base materials, to minimize the risk of adverse reactions in patients who wear acrylic resin dentures.

Biocompatibility of visible light-cured resin systems in prosthodontics

Frequently dental products are introduced that have had little or no biologic testing. Cell culture systems that traditionally have been used for the study of cellular responses have recently been used to assess biocompatibility. This article reviews various cellular toxicity assays and their application to the resin systems used in clinical prosthodontics.

Flow injection analysis of formaldehyde leached from denture-base acrylic resins

Formaldehyde is responsible for allergic inflammation in acrylic denture wearers and the quantitation of formaldehyde is necessary to study its leaching from denture-base materials. Flow injection analysis was developed to quantify the formaldehyde leached from acrylic resins. Different resins were immersed in aqueous solvents at 37 degrees C and the immersion solutions were directly injected into the flow system, in which formaldehyde was converted on-line to a fluorescent derivative and its fluorescence was detected. Under the optimized conditions, the leached formaldehyde could be quantified in a short time (within 4 min) with high sensitivity (pmol levels per injection) and high specificity (no fluorescent response to the other leachables). In leaching experiments, significant amounts of formaldehyde were leached from autopolymerized resins, but not from heat- and microwave-polymerized resins.

Denture adhesives: cytotoxicity, microbial contamination, and formaldehyde content

Denture adhesives are extensively used to enhance retention of dentures. Denture adhesives and their leachable components are ingested, but reports on the biologic aspects of denture adhesives are scarce. This study investigated some biological properties of 19 commercially available denture adhesives. The adhesives were assessed by the agar overlay technique and analyzed for microbial contamination and formaldehyde content. In the agar overlay test, all of the materials caused severe cytotoxic effects. Sabouraud dextrose agar was used for cultivation of fungi and tryptone soya broth for cultivation of aerobes, including fungi. Most of the samples tested showed microbial growth. The formaldehyde test revealed the presence of substantial amounts in four products and minor amounts in two products.

Leachability of denture-base acrylic resins in artificial saliva

We studied the influence of salivary acidity on leachability of denture-base acrylic resins with etiological interest in denture stomatitis because denture surfaces are frequently exposed to acidic conditions in the oral cavities. Auto-, heat-, and microwave-polymerized resins were immersed in artificial saliva with pH ranging from 4.0 to 6.8 at 37 degrees C, and leachables were pursued quantitatively with time. Methyl methacrylate, methacrylic acid, and benzoic acid leached from all resins. Their concentrations in the saliva were markedly high for auto-polymerized resins, while leachability of heat- and microwave-polymerized resins was so low that quantitative analysis of leachables was impossible. Lower pH showed higher concentrations of methyl methacrylate, although no apparent association was confirmed between salivary acidity and its own leachability. The concentrations of methacrylic acid increased remarkably with an increase in pH, which was probably due to hydrolysis of methyl methacrylate. These results suggest that chemotoxic actions of auto-polymerized resins are potentially ascribable to methyl methacrylate under more acidic conditions and to methacrylic acid under less acidic conditions.

Release of formaldehyde from dental composites

Polymeric composite materials may contain releasable degradation products or unreacted constituents. Release of formaldehyde from nine different composites was investigated by means of HCHO-hydrazone derivative analyzed with high-performance liquid chromatography. Formation of formaldehyde was found in all the investigated materials. The highest concentrations were observed in specimens polymerized in contact with air. A correlation coefficient, r = 0.83, was found between released formaldehyde and the thickness of the unpolymerized surface inhibition layer. The formaldehyde concentrations were reduced when the inhibition layer was removed prior to testing. A continuous release of formaldehyde was evident during the first ten days. The release decreased with time, but was still detectable after 115 days.

The release of residual monomeric methyl methacrylate from acrylic appliances in the human mouth: an assay for monomer in saliva

A gas-liquid chromatography assay has been developed for the estimation of methyl methacrylate monomer (MMA) in whole saliva, with a lower limit of detection in the order of 1 microgram/mL. Healthy human dentate subjects wore recently made autopolymerized or heat-polymerized polymethyl methacrylate (PMMA) palatal appliances. MMA released into saliva was detected for up to one week after insertion of autopolymerized appliances, with a maximum concentration of 45 micrograms/mL in whole saliva or 180 micrograms/mL in the salivary film on the fitting surface. The MMA was not detected in blood or urine. MMA was also present in the saliva of volunteers wearing appliances which had been heat-polymerized at 70 degrees C for one hr but not cured at 70 degrees C for three hr. The maximum amount of monomer released by an autopolymerized base plate was 29.5 micrograms in the first hour, which, while not a toxic or primary irritant dose, could possibly sensitize patients or elicit an allergic reaction. For minimization of monomer release, autopolymerized appliances should be immersed for 24 hr in water before being worn.

Some aspects of the chemistry and physics of dental resins

The status of denture-base poly(methylmethacrylate) (PMMA) has been reviewed, and in spite of the availability of mechanically superior injection-molded resins, it remains the material of choice. PMMA can be reinforced with fibers, the most recent being ultra-high-modulus polyethylene fibers. Various elastomer reinforced materials are also available. Alternatives to room-temperature polymerizing PMMA resins include higher methacrylates and epimine resins. High-modulus resins have been produced with Bisphenol A-glycidyl methacrylate (BisGMA) and tetrahydrofurfuryl methacrylate, involving an anti-plasticizer function. Also reviewed are room-temperature polymerizing initiators, radio-opaque resins, and studies on polymerization shrinkage.

Composites--characterization of composite filling materials: reactor response

Dental composite restorative materials consist of an organic matrix, ceramic fillers, and the interface between the inorganic fillers and the matrix. Marked variations in the composition of the composite materials, as well as different degrees of conversion after polymerization, have been observed. These circumstances lead to substantial differences in the properties of polymerized composite materials. The variations in clinical behavior of the different composite materials, e.g., discoloration or lack of wear resistance, may be explained on the basis of differences in composition. This review paper describes the composition, conversion, and properties of presently available composite materials.

Resins: reactor response

A commentary is presented on the review paper by Dr. Braden on the state of the art of the "science of resins" in dentistry. The subject area is defined as principally including those materials involved with the denture-base applications. Recent studies involving investigations of structure and properties are noted. Most reports of properties have concentrated on physical, chemical, and mechanical properties. The limited amount of information on biological properties is noted. The absence of clinical investigations of materials performance is emphasized. Future research strategies should concentrate on areas which involve clinical research information, other product properties, and the adaptation of new technology to these applications. CAD/CAM is suggested as an example of this new technology. Finally, investigators in the area are admonished to take advantage of information management systems, to collaborate more with each other, to collaborate more with industry, and to focus more on a few promising systems in order to produce significantly better denture-base materials for clinical practice.