The influence of resin chemistry on a dental composite's biodegradation

Computational analysis of 3D printing: Selecting the better among newly released materials

Resin-based three-dimensional (3D) printing finds extensive application in the field of dentistry. Although studies of cytotoxicity, mechanical and physical properties have been conducted for newly released 3D printing resins such as Crowntec (Saremco), Temporary Crown Resin (Formlabs) and Crown & Bridge (Nextdent), the resistance of these materials to esterases in saliva has not been demonstrated at the molecular level. Therefore, in this study, the binding affinities and stability of these new 3D printing resins to the catalytic sites of esterases were investigated using molecular docking and molecular mechanics with Poisson-Bolzmann and surface area solvation (MM/PBSA) methods after active pocket screening. Toxicity predictions of the materials were also performed using ProTox-II and Toxtree servers. The materials were analyzed for mutagenicity, cytotoxicity, and carcinogenicity, and LD50 values were predicted from their molecular structures. The results indicated that out of the three novel 3D printing materials, Nexdent exhibited reduced binding affinity to esterases, indicating enhanced resistance to enzymatic degradation and possessing a superior toxicity profile.

Effect of Three Different Liquid Medias in the Sorption and Solubility of Luting Cements: An In Vitro Study

Background Among the various mechanical and biological properties of luting cement, the most important are its resistance to disintegration, degradation, and stability in the oral cavity. The sorption and solubility of cement alter the mechanical properties by impeding the half-life of the filling. It also leads to variations in dimensions, discoloration, and margin breakage. It is, therefore, essential to choose a low-solubility cement since there is always an interaction between teeth and restorative margins. The aim of this study is to assess and compare the solubility and sorption values of three different luting cements in three liquid media. Materials and methods Three luting cements were used for the investigation. Disc-shaped specimens of the cement, which were of 10 mm diameter and 2 mm height, were prepared. The sample included a total of 126 disc-shaped specimens made up of three materials, glass ionomer cement (GIC), resin cement, and resin-modified GIC, which were used in three liquid media (14 of each material in each medium). Fourteen specimens of each material were placed in glass vials containing 20 ml of each medium: distilled water, artificial saliva, and carbonated water. The samples were then put in an incubator at 37 °C. The measurements and masses of the samples were documented on days one, three, seven, 14, 21, 28, and 35. The samples were taken out of the solution after five weeks and stored in a desiccator with calcium sulphate for another five weeks. The weight and dimensional changes were estimated on days one, three, seven, 14, 21, 28, and 35. The values of water sorption (WSO) and solubility (WSL) were estimated. To determine the mean and standard deviation of each cohort, descriptive statistics were employed. Utilizing the Shapiro-Wilkinson test, the normality was determined. An independent test was used to determine the difference between all pairs of groups, while one-way ANOVA, Dunn test, and post hoc analysis were used to establish the distinction between the three groups. Results One-way ANOVA showed that significant differences existed among the groups: resin cement showed the least sorption and solubility, resin-modified GIC showed the highest solubility in distilled water (0.40 ± 0.03), and GIC showed the highest solubility in both artificial saliva (0.36 ± 0.03) and carbonated water (0.04 ± 0.05). Conclusion Considering the experimental outcomes and the limitations of an in vitro investigation, it was concluded that in the complex setting of the oral environment, this selection procedure is crucial for maintaining mechanical strength and for the long lifespan of dental restorations.

Light Transmission Characteristics and Cytotoxicity within A Dental Composite Color Palette

Modern light-cured, resin-based composites are offered in a wide range of shades and translucencies. This large variation, created by varying the amount and type of pigmentation and opacifiers, is essential to enable an esthetic restoration in each patient situation, but may affect light transmission in the deeper layers during curing. We quantified optical parameters and their real-time variation during curing for a 13-shade composite palette of identical chemical composition and microstructure. Incident irradiance and real-time light transmission through 2 mm thick samples were recorded to calculate absorbance, transmittance, and the kinetic of transmitted irradiance. Data were supplemented by the characterization of cellular toxicity to human gingival fibroblasts up to 3 months. The study highlights a strong dependence of light transmission and its kinetic as a function of shade, with the largest changes occurring within the first second of exposure; the faster changes, the darker and more opaque the material. Transmission differences within progressively darker shades of a pigmentation type (hue) followed a hue-specific, non-linear relationship. Shades with similar transmittance but belonging to different hues were identified, while the corresponding kinetic was identical only up to a transmittance threshold. A slight drop in absorbance was registered with increasing wavelength. None of the shades were cytotoxic.

Degree of conversion and residual monomer elution of 3D-printed, milled and self-cured resin-based composite materials for temporary dental crowns and bridges

The aim of this work was to investigate the elution of residual monomers as a function of the manufacturing process, which are CAD/CAM manufacturing, self-curing and 3D printing. The experimental materials used consisted of the base monomers TEGDMA, Bis-GMA and Bis-EMA and 50 wt.% fillers. Additionally, a 3D printing resin without fillers was tested. The elution of the base monomers into the different media (water, ethanol and ethanol/water (75/25 vol. %)) at 37 °C over a period of up to 120 d as well as the degree of conversion (DC) by FTIR were investigated. No monomer elution could be detected in water. Most residual monomers in both other media were released from the self-curing material whereas the 3D printing composite released relatively little. The CAD/CAM blanks released hardly any quantitatively detectable amounts of monomers. Relative to the base composition, TEGDMA eluted less than Bis-GMA and Bis-EMA. DC did not correlate with residual monomer release; thus, leaching was determined not only by the amount of residual monomers present but by further factors as possibly network density and structure. The CAD/CAM blanks and the 3D printing composite showed similar high DC but lower residual monomer release from the CAD/CAM blank, likewise the self-curing composite and the 3D printing resin exhibited similar DC but different monomer elution. In terms of residual monomer elution and DC, the 3D printing composite seems promising as a new material class for the use as temporary dental crowns and bridges.

Degradation and Failure Phenomena at the Dentin Bonding Interface

Damage in the bonding interface is a significant factor that leads to premature failure of dental bonded restorations. The imperfectly bonded dentin-adhesive interface is susceptible to hydrolytic degradation and bacterial and enzyme attack, severely jeopardizing restorations' longevity. Developing caries around previously made restorations, also called "recurrent or secondary caries," is a significant health problem. The replacement of restorations is the most prevailing treatment in dental clinics, leading to the so-called "tooth death spiral". In other words, every time a restoration is replaced, more tooth tissue is removed, increasing the size of the restorations until the tooth is eventually lost. This process leads to high financial costs and detriment to patients' quality of life. Since the complexity of the oral cavity makes prevention a challenging task, novel strategies in Dental Materials and Operative fields are required. This article briefly overviews the physiological dentin substrate, features of dentin bonding, challenges and clinical relevance. We discussed the anatomy of the dental bonding interface, aspects of the degradation at the resin-dentin interface, extrinsic and intrinsic factors affecting dental bonding longevity, perspectives on resin and collagen degradation and how these subjects are connected. In this narrative review, we also outlined the recent progress in overcoming dental bonding challenges through bioinspiration, nanotechnology and advanced techniques to reduce degradation and improve dental bonding longevity.

Logistic Regression Analysis of LC-MS/MS Data of Monomers Eluted from Aged Dental Composites: A Supervised Machine-Learning Approach

Compound identification by database searching that matches experimental with library mass spectra is commonly used in mass spectrometric (MS) data analysis. Vendor software often outputs scores that represent the quality of each spectral match for the identified compounds. However, software-generated identification results can differ drastically depending on the initial search parameters. Machine learning is applied here to provide a statistical evaluation of software-generated compound identification results from experimental tandem MS data. This task was accomplished using the logistic regression algorithm to assign an identification probability value to each identified compound. Logistic regression is usually used for classification, but here it is used to generate identification probabilities without setting a threshold for classification. Liquid chromatography coupled with quadrupole-time-of-flight tandem MS was used to analyze the organic monomers leached from resin-based dental composites in a simulated oral environment. The collected tandem MS data were processed with vendor software, followed by statistical evaluation of these results using logistic regression. The assigned identification probability to each compound provides more confidence in identification beyond solely by database matching. A total of 21 distinct monomers were identified among all samples, including five intact monomers and chemical degradation products of bisphenol A glycidyl methacrylate (BisGMA), oligomers of bisphenol-A ethoxylate methacrylate (BisEMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA). The logistic regression model can be used to evaluate any database-matched liquid chromatography-tandem MS result by training a new model using analytical standards of compounds present in a chosen database and then generating identification probabilities for candidates from unknown data using the new model.

Effect of Hydrothermal Factors on the Microhardness of Bulk-Fill and Nanohybrid Composites

This study evaluates the effect of aging in artificial saliva and thermal shocks on the microhardness of the bulk-fill composite compared to the nanohybrid composite. Two commercial composites, Filtek Z550 (3M ESPE) (Z550) and Filtek Bulk-Fill (3M ESPE) (B-F), were tested. The samples were exposed to artificial saliva (AS) for one month (control group). Then, 50% of the samples from each composite were subjected to thermal cycling (temperature range: 5-55 °C, cycle time: 30 s, number of cycles: 10,000) and another 50% were put back into the laboratory incubator for another 25 months of aging in artificial saliva. The samples' microhardness was measured using the Knoop method after each stage of conditioning (after 1 month, after 10,000 thermocycles, after another 25 months of aging). The two composites in the control group differed considerably in hardness (HK = 89 for Z550, HK = 61 for B-F). After thermocycling, the microhardness decrease was for Z550 approximately 22-24% and for B-F approximately 12-15%. Hardness after 26 months of aging decreased for Z550 (approximately 3-5%) and B-F (15-17%). B-F had a significantly lower initial hardness than Z550, but it showed an approximately 10% lower relative reduction in hardness.

Cytotoxic, Elastic-Plastic and Viscoelastic Behavior of Aged, Modern Resin-Based Dental Composites

The development of resin-based composites (RBCs) is a delicate balance of antagonistic properties with direct clinical implications. The clear trend toward reducing filler size in modern RBCs solves esthetic deficiencies but reduces mechanical properties due to lower filler content and increases susceptibility to degradation due to larger filler-matrix interface. We evaluated a range of nano- and nano-hybrid RBCs, along with materials attempting to address shrinkage stress issues by implementing an Ormocer matrix or pre-polymerized fillers, and materials aiming to provide caries-protective benefit by incorporating bioactive fillers. The cytotoxic response of human gingival fibroblast (HGF) cells after exposure to the RBC eluates, which were collected for up to six months, was analyzed using a WST-1 assay. The microstructural features were characterized using a scanning electron microscopy and were related to the macroscopic and microscopic mechanical behaviors. The elastic-plastic and viscoelastic material behaviors were evaluated at the macroscopic and microscopic levels. The data were supplemented with fractography, Weibull analysis, and aging behavioral analysis. The results indicate that all RBCs are non-cytotoxic at adequate exposure. The amount of inorganic filler affects the elastic modulus, while only to a limited extent the flexural strength, and is well below the theoretical estimates. The nanoparticles and the agglomeration of nanoparticles in the RBCs help generate good mechanical properties and excellent reliability, but they are more prone to deterioration with aging. The pre-polymerized fillers lower the initial mechanical properties but are less sensitive to aging. Only the Ormocer retains its damping ability after aging. The strength and modulus of elasticity on the one hand and the damping capacity on the other are mutually exclusive and indicate the direction in which the RBCs should be further developed.

Dentin primer based on a highly functionalized gelatin-methacryloyl hydrogel

Gelatin-methacryloyl hydrogels (GelMA) have demonstrated their utility as scaffolds in a variety of tissue engineering applications.

OBJECTIVES

In this study, a highly functionalized GelMA hydrogel was synthesized and assessed for degree of functionalization. As the proposed GelMA hydrogel was coupled to a visible-light photoinitiator, we hypothesized it might serve as base to formulate a model dentin primer for application in restorative dentistry.

METHODS

GelMA was mixed with photoinitiator lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP), photopolymerized for 0-40 s using a dental light-curing device and tested for extrudability, degree of photo-crosslinking (DPxlink), water sorption/solubility/swelling (WS/SL/SW) and apparent modulus of elasticity (AE). Model dentin primer was prepared by mixing GelMA+LAP with a primer of a commercial three-step etch-and-rinse adhesive. After application of GelMA-based primer to acid-etched dentin, samples were bonded with correspondent adhesive agent, photopolymerized and had their immediate bond strength compared to control samples primed and bonded with the same commercial material.

RESULTS

Extrudability of hydrogel was confirmed using a microsyringe to write the acronym "CDMI". DPxlink of GelMA+LAP changed significantly as a function of photopolymerization time (20 s < 30 s ≤ 40 s). WS, SL and SW were significantly reduced in hydrogels polymerized for 30 and 40 s. AE of hydrogels varied significantly as a function of photopolymerization time (20 s < 30 s ≤ 40 s; 20 s ‡ 40 s). Bond strength of dentin primed with GelMA-based primer was lower (∼29.3 MPa) but not significantly of that of control (∼34.6 MPa).

CONCLUSIONS

Optimization of a GelMA-based dentin primers can lead to the development of promising biomimetic adhesives for dentin rehabilitation.

Design and tribological performance of short S-Glass fibre reinforced biocomposites on the influence of fibre length and concentration

Fibre-reinforced biocomposites usage has gained prominence over the past decade. Although higher fracture toughness was observed when fibres were added to biocomposites, material degradation could occur due to filler and fibre content intolerance in the biocomposite matrix. Optimisation of resin-fibre-filler ratios helps in increasing the tribological performance of high load-bearing applications. However, the tribological performance is less understood due to limited in-vitro studies on the effect of fibre microstructures. A comprehensive investigation of the reciprocating and rotary wear behaviour of different compositions was carried out by varying fibre (0%, 5%, 10% and 15%) to particulate filler (40%, 45%, 50%, and 55%) weight fractions. The investigation aimed to identify the optimal composition of fibre-reinforced biocomposites based on the in-vitro ball-on-disc reciprocating and rotary wear tests in the presence of modified Fusayama solution. The cross-sectional areas of wear tracks were analysed using laser microscopy and scanning electron microscopy techniques to assess the surface morphology and subsurface damage of the wear tracks on biocomposites and the antagonist. The numerical results were statistically analysed using two-way ANOVA followed by a posthoc Tukey's test (p = 0.05). The results showed a combination of adhesive, abrasive and fatigue wear for all the tested Groups. The friction coefficient had a longer transient period for the 5 wt% and 10 wt% Groups. Based on the surface roughness, coefficient of friction, SEMs, specific wear rate, and ease of manufacturing, the threshold limit for fibre loading was found to be 10 wt%. The rotary test had a considerably lower specific wear rate compared to the reciprocating test. Fibre weight fraction was found to be the influencing factor of the abrasive wear behaviour compared to fibre length for the tested Groups.

Biodegradation of Dental Resin-Based Composite—A Potential Factor Affecting the Bonding Effect: A Narrative Review

In recent years, although resin composite has played an important role in the restoration of tooth defects, it still has several disadvantages, including being biodegraded by saliva, bacteria and other enzymes in the oral cavity, which may result in repair failure. This factor is not conducive to the long-term survival of the prosthesis in the mouth. In this article, we review the causes, influencing factors and prevention methods of resin biodegradation. Biodegradation is mainly caused by esterase in saliva and bacteria, which breaks the ester bond in resin and causes the release of monomers. The mechanical properties of the prosthesis can then be affected. Meanwhile, cathepsin and MMPs are activated on the bonding surface, which may decompose the dentin collagen. In addition, neutrophils and residual water on the bonding surface can also aggravate biodegradation. Currently, the primary methods to prevent biodegradation involve adding antibacterial agents to resin, inhibiting the activity of MMPs and enhancing the crosslinking of collagen fibers. All of the above indicates that in the preparation and adhesion of resin materials, attention should be paid to the influence of biodegradation to improve the prosthesis’s service life in the complex environment of the oral cavity.

Meta-analytical analysis on components released from resin-based dental materials

OBJECTIVES

Resin-based materials are applied in every branch of dentistry. Due to their tendency to release substances in the oral environment, doubts have been raised about their actual safety. This review aims to provide a comprehensive analysis of the last decade literature regarding the concentrations of elutable substances released from dental resin-based materials in different type of solvents.

MATERIALS AND METHODS

All the literature published on dental journals between January 2010 and April 2022 was searched using international databases (PubMed, Scopus, Web of Science). Due to strict inclusion criteria, only 23 papers out of 877 were considered eligible. The concentration of eluted substances related to surface and volume of the sample was analyzed, considering data at 24 h as a reference. The total cumulative release was examined as well.

RESULTS

The most eluted substances were HEMA, TEGDMA, and BPA, while the less eluted were Bis-GMA and UDMA. Organic solvents caused significantly higher release of substances than water-based ones. A statistically significant inverse correlation between the release of molecules and their molecular mass was observed. A statistically significant positive correlation between the amount of released molecule and the specimen surface area was detected, as well as a weak positive correlation between the release and the specimen volume.

CONCLUSIONS

Type of solvent, molecular mass of eluates, and specimen surface and volume affect substances release from materials.

CLINICAL RELEVANCE

It could be advisable to rely on materials based on monomers with a reduced elution tendency for clinical procedures.

Release of Bisphenol A from Pit and Fissure Sealants According to Different pH Conditions

Changes in intraoral pH can cause changes in the chemical decomposition and surface properties of treated resin-based pits and fissure sealants (sealant). The purpose of this study is to evaluate the release of bisphenol A (BPA) from sealants under three different pH conditions over time. The test specimen was applied with 6 sealants 5 mg each on a glass plate (10 × 10 mm) and photopolymerized. The samples were immersed for 10 min, 1 h, and 24 h in solutions of pH 3.0, 6.5, and 10.0 at 37 °C. BPA release was measured using a gas chromatography-mass spectrometer. A statistical analysis was performed by two-way ANOVA and one-way ANOVA to verify the effect of pH conditions and time on BPA release. The BPA concentration in the pH 3.0 group was higher at all points than with pH 6.5 and pH 10.0 (p < 0.05), and gradually increased over time (p < 0.05). As a result, it was confirmed that low pH negatively influences BPA release. Therefore, frequent exposure to low pH due to the consumption of various beverages after sealant treatment can negatively affect the sealant’s chemical stability in the oral cavity.

Antimicrobial agents in dental restorative materials: a on polymerization, short-term drug release and biological impact

With the aim to design bioactive dental restorative material, the present study investigated the influence of the antimicrobial agents chlorhexidine diacetate (CHX) and octinidine (di)hydrochloride (ODH) when incorporated in two different materials. Selected parameters were polymerization enthalpy, short-term drug release, and the effect on Streptococcus mutans as well as human gingival fibroblasts. Samples were made by mixing a nano-hybrid ormocer (O) and a methacrylate-based nano-hybrid composite (C), each with a mass fraction of 2% CHX or ODH. Release profiles and concentrations of active agents from the resins were assessed, and the cell proliferation of human gingival fibroblasts as well as Streptococcus mutans cultured with the eluates were evaluated. The influence on polymerization was assessed by means of differential scanning calorimetry. Both drugs, especially ODH, showed a decreasing effect on polymerization enthalpies associated with a lowered crosslinking degree. At the same time ODH appeared to be released more persistently than CHX. Moreover, ODH was more efficient with regard to bacteria growth inhibition but also more cytotoxic in terms of reduction of cell viability. ODH is deemed more appropriate for application in a dental resin-based drug delivery system, because of the more persistent drug release than seen for CHX.

Effect of Streptococcus mutans on surface-topography, microhardness, and mechanical properties of contemporary resin composites

OBJECTIVE

Dental caries is the most prevalent disease globally, and Streptococcus mutans (S. mutans) is a common associated oral bacteria. Additionally, S. mutans possess esterase activity capable of degrading resin composites (RC). However, the effect of degradation on the physical-mechanical properties of the RC has not been extensively studied. We evaluated the flexure strength (FS), the diametral tensile strength (DTS), the modulus of elasticity (ME), and the microhardness of three contemporary RC to establish if S. mutans could affect them.

METHODS

One hundred thirty-eight bar-shaped and 276 disc-shaped specimens were fabricated with Enamel Plus HRi, IPS Empress Direct, and Clearfil AP-X, and physical-mechanical testing was done after been incubated during 30 and 60 days in culture media with or without S. mutans. Also, a scanning electron microscope was used to identify surface changes.

RESULTS

None of the tested RC were affected in their mechanical properties (FS, ME, and DTS). However, Clearfil AP-X and Enamel Plus HRI showed eroded surfaces and a decreased microhardness after 30 and 60 days S. mutans incubation. IPS Empress Direct presented the lowest values in all the tests, but its physical-mechanical features and surface were not affected by bacteria's exposure.

CONCLUSIONS

Exposure to S. mutans could affect some contemporary RC; however, the effect seems superficial since its mechanical features were not affected.

Effect of Artificial Aging on Mechanical and Tribological Properties of CAD/CAM Composite Materials Used in Dentistry

With easy-to-process 3D printing materials and fast production, the quality of dental services can be improved. In the conventional procedure, the dentist makes temporary crowns directly in the patient’s mouth, e.g., from the most commonly used bis-acrylic composites. Temporary crowns made directly in the office without the use of CAD/CAM are often of inferior quality, which directly results in impaired hygiene, poorer masticatory mechanics, greater deposition of plaque, calculus and sediment, and may adversely affect periodontal and gum health. The mechanical strength, resistance to aging and abrasion of 3D printing materials are higher than those of the soft materials used in conventional methods. This translates into durability. The patient leaves the surgery with a restoration of higher utility quality compared to the conventional method. The objective of the paper was to determine the influence of aging in artificial saliva of AM (additive manufacturing) orthodontic composites on their functional properties. For the purpose of the study, fillings well-known worldwide were selected. These were traditional UV-curable resins (M I, M II, M III, M V) and a hybrid material based on a UV-curable resin (M VI). Samples were stored in artificial saliva at 37 ± 1 °C in a thermal chamber for 6 months. Indentation hardness, frictional tests and sliding wear measurements were conducted. A comparison between various materials was made. Descriptive statistics, degradation coefficients, H²E, Archard wear and specific wear rate were calculated. The Weibull statistical test for indentation hardness was performed and Hertzian contact stresses for the frictional association were calculated for unaged (M I, M II, M III, M V, M VI) and aged (M I AS, M II AS, M III AS, M V AS, M VI AS) samples. M I exhibited the lowest average hardness among the unaged materials, while M III AS had the lowest average hardness among the aged materials. Comparably low hardness was demonstrated by the M I AS material. The coefficient of friction values for the aged samples were found to be higher. The lowest wear value was demonstrated by the M I material. The wear resistance of most of the tested materials deteriorated after aging. The M VI AS material had the highest increase in wear. According to the results provided, not only the chemical composition and structure, but also aging have a great impact on the indentation hardness and wear resistance of the tested orthodontic materials.

Characteristics of biofilm-induced degradation at resin-dentin interfaces using multiple combinations of adhesives and resins

OBJECTIVE

We aimed to evaluate morphological, mechanical and chemical characteristics at resin-dentin interfaces using multiple combinations of adhesives and resins after a short-term biofilm-induced degradation.

METHODS

Cervical cavities were prepared in bovine incisors, treated by Clearfil SE Bond 2 (SE) or FL-Bond II (FL), restored by Clearfil Majesty ES Flow (ES) or Beautifil Flow Plus (BFP) and grouped into SE-ES, SE-BFP, FL-ES and FL-BFP. After biofilm challenge, interfacial gaps and dentin wall lesions were examined by optical coherence tomography (OCT). Gap depth (GD), gap pattern scale (GPS) and dentin wall lesion depth (WLD) were evaluated from confocal laser scanning microscope. Microhardness of dentin lesions was measured with a Vickers microhardness tester. Chemical elements in resins and dentin wall lesions were analyzed by scanning electron microscopy and energy dispersive X-ray spectrometry (SEM/EDS). Morphological structures of interfacial gaps were observed by SEM.

RESULTS

OCT could detect adhesive-dentin-bonded and adhesive-dentin-debonded gaps. SE-containing groups showed significantly lower GPS than FL-containing groups. FL-BFP showed significantly lower WLD than FL-ES. Microhardness of dentin wall lesions was higher than that of outer lesions and they showed significant differences in FL-BFP. SE-BFP showed a lower GPS curve and higher intensities of Ca and P in the upper half of dentin wall lesions than other groups. From SEM, microgaps between filler and matrix, break and loss of matrix, separation of adhesive matrix with hybrid layer occurred at interfacial gaps.

SIGNIFICANCE

The morphological, mechanical and chemical characteristics of resin-dentin interfacial degradation depend on the component and chemistry of restorative materials.

Durable Oral Biofilm Resistance of 3D-Printed Dental Base Polymers Containing Zwitterionic Materials

Poly(methyl methacralyate) (PMMA) has long been used in dentistry as a base polymer for dentures, and it is recently being used for the 3D printing of dental materials. Despite its many advantages, its susceptibility to microbial colonization remains to be overcome. In this study, the interface between 3D-printed PMMA specimens and oral salivary biofilm was studied following the addition of zwitterionic materials, 2-methacryloyloxyethyl phosphorylcholine (MPC) or sulfobetaine methacrylate (SB). A significant reduction in bacterial and biofilm adhesions was observed following the addition of MPC or SB, owing to their protein-repellent properties, and there were no significant differences between the two test materials. Although the mechanical properties of the tested materials were degraded, the statistical value of the reduction was minimal and all the properties fulfilled the requirements set by the International Standard, ISO 20795-2. Additionally, both the test materials maintained their resistance to biofilm when subjected to hydrothermal fatigue, with no further deterioration of the mechanical properties. Thus, novel 3D-printable PMMA incorporated with MPC or SB shows durable oral salivary biofilm resistance with maintenance of the physical and mechanical properties.

Bioinspired multifunctional adhesive system for next generation bio-additively designed dental restorations

Resin-based composite has overtaken dental amalgam as the most popular material for the repair of lost or damaged tooth structure. In spite of the popularity, the average composite lifetime is about half that of amalgam restorations. The leading cause of composite-restoration failure is decay at the margin where the adhesive is applied. The adhesive is intended to seal the composite/tooth interface, but the adhesive seal to dentin is fragile and readily degraded by acids, enzymes and other oral fluids. The inherent weakness of this material system is attributable to several factors including the lack of antimicrobial properties, remineralization capabilities and durable mechanical performance - elements that are central to the integrity of the adhesive/dentin (a/d) interfacial seal. Our approach to this problem offers a transition from a hybrid to a biohybrid structure. Discrete peptides are tethered to polymers to provide multi-bio-functional adhesive formulations that simultaneously achieve antimicrobial and remineralization properties. The bio-additive materials design combines several functional properties with the goal of providing an adhesive that will serve as a durable barrier to recurrent decay at the composite/tooth interface. This article provides an overview of our multi-faceted approach which uses peptides tethered to polymers and new polymer chemistries to achieve the next generation adhesive system - an adhesive that provides antimicrobial properties, repair of defective dentin and enhanced mechanical performance.

Biodegradation of orthodontic composites by Streptococcus mutans: An in vitro qualitative and quantitative assessment

Objectives:

The purpose of this study was to evaluate the degradation products of orthodontic composites (Grengloo, Blugloo, Transbond XT, and Transbond LR) by Streptococcus mutans and then to quantify the levels of released bisphenol A (BPA) using gas-phase chromatography and mass spectrometry (GC–MS).

Materials and Methods:

Orthodontic light-cured composite discs were incubated at 37°C in brain heart infusion (BHI) (control group) and in a culture of S. mutans with BHI (test group). Incubation solutions were collected every 48 h in each group and replaced with fresh solutions. These incubation solutions were accumulated and grouped. The assessment of degradation products from composites was done at 1 and 30 days. Detected BPA was then quantified. The limit of quantification was 0.01 μg/mL.

Results:

Degradation products were present at day 30. For the test group, BPA was detected in Blugloo at day 1 (0.38 μg/mL) and triethylene glycol dimethacrylate (TEGDMA) was detected in Grengloo and Transbond LR at day 1.

Conclusion:

S. mutans can hydrolyze long-term orthodontic composites. Monomers such as BPA and TEGDMA may be present in degradation products. It is possible to separate and identify leaching compounds by GC–MS technique.

Antimicrobial antidegradative dental adhesive preserves restoration-tooth bond

OBJECTIVE

Assess the ability of an antimicrobial drug-releasing resin adhesive, containing octenidine dihydrochloride (OCT)-silica co-assembled particles (DSPs), to enhance the biostability and preserve the interfacial fracture toughness (FT) of composite restorations bonded to dentin. Enzyme-catalyzed degradation compromises the dental restoration-tooth interface, increasing cariogenic bacterial infiltration. In addition to bacterial ingress inhibition, antimicrobial-releasing adhesives may exhibit direct interfacial biodegradation inhibition as an additional benefit.

METHODS

Mini short-rod restoration bonding specimens with total-etch adhesive with/without 10% wt. DSPs were made. Interfacial fracture toughness (FT) was measured as-manufactured or post-incubation in simulated human salivary esterase (SHSE) for up to 6-months. Effect of OCT on SHSE and whole saliva/bacterial enzyme activity was assessed. Release of OCT outside the restoration interface was assessed.

RESULTS

No deleterious effect of DSPs on initial bonding capacity was observed. Aging specimens in SHSE reduced FT of control but not DSP-adhesive-bonded specimens. OCT inhibited SHSE degradation of adhesive monomer and may inhibit endogenous proteases. OCT inhibited bacterial esterase and collagenase. No endogenous collagen breakdown was detected in the present study. OCT increased human saliva degradative esterase activity below its minimum inhibitory concentration towards S. mutans (MIC), but inhibited degradation above MIC. OCT release outside restoration margins was below detection.

SIGNIFICANCE

DSP-adhesive preserves the restoration bond through a secondary enzyme-inhibitory effect of released OCT, which is virtually confined to the restoration interface microgap. Enzyme activity modulation may produce a positive-to-negative feedback switch, by increasing OCT concentration via biodegradation-triggered release to an effective dose, then subsequently slowing degradation and degradation-triggered release.

Effect of amylase activity on mechanical properties of dental composite resin

The purpose of this study was to investigate the effect of amylase activity on the mechanical properties of three dental composite resin (Filtek Z350; Filtek Z250 and Polofil Supra). The specimens were immersed in amylase solution at four different activities (25, 50, 100 and 200 KIU/L) with an artificial saliva solution (AS) as a control. The flexural strength (FS) and elastic modulus (EM) were determined in a three-point bending test after immersion for periods up to 12 months. In addition, surface roughness and surface morphology also determined. The FS of Filtek Z350 reaching the lowest level of 67.86 MPa after AS immersion for one year. There was a general trend for FS to decrease following immersion in AS for all the tested materials. However, the amylase groups did not undergo a significant decrease in FS and EM, and there was a slight increase in FS and EM for Polofil Supra.

A Feasibility Study for 3D-printed Poly(methyl methacrylate)-resin Tracheostomy Tube Using a Hamster Cheek Pouch Model

BACKGROUND/AIM

A three-dimensional (3D) printed tracheostomy tube has potential application for patients who require a specialized tube. The aim of this study was to evaluate the characteristics of various 3D printing materials and determine their use in producing 3D-printed tracheostomy tube.

MATERIALS AND METHODS

Mechanical, chemical, and microbiological in vivo changes in the scaffolds were analyzed using a hamster cheek pouch (HCP) model.

RESULTS

The poly methyl methacylate (PMMA)-resin showed superior pre- and post-insertion mechanical properties and a relatively consistent lower biofilm formation compared with other scaffolds. PMMA-resin was successfully 3D-printed with dimensional accuracy without a support system. The use of a 3D-printed PMMA tracheostomy tube in a rabbit trachea showed no definite signs of infection, allergy or foreign body reaction.

CONCLUSION

PMMA-resin can be proposed as an alternative for a 3D-printed tracheostomy tube material. In addition, we suggest HCPs as an in vivo model for evaluating indwelling medical devices.

Incorporating Aminated Nanodiamonds to Improve the Mechanical Properties of 3D-Printed Resin-Based Biomedical Appliances

The creation of clinically patient-specific 3D-printed biomedical appliances that can withstand the physical stresses of the complex biological environment is an important objective. To that end, this study aimed to evaluate the efficacy of aminated nanodiamonds (A-NDs) as nanofillers in biological-grade acrylate-based 3D-printed materials. Solution-based mixing was used to incorporate 0.1 wt% purified nanodiamond (NDs) and A-NDs into UV-polymerized poly(methyl methacrylate) (PMMA). The ND and A-ND nanocomposites showed significantly lower water contact angles (p < 0.001) and solubilities (p < 0.05) compared to those of the control. Both nanocomposites showed markedly improved mechanical properties, with the A-ND-containing nanocomposite showing a statistically significant increase in the flexural strength (p < 0.001), elastic modulus (p < 0.01), and impact strength (p < 0.001) compared to the control and ND-containing groups. The Vickers hardness and wear-resistance values of the A-ND-incorporated material were significantly higher (p < 0.001) than those of the control and were comparable to the values observed for the ND-containing group. In addition, trueness analysis was used to verify that 3D-printed orthodontic brackets prepared with the A-ND- and ND-nanocomposites exhibited no significant differences in accuracy. Hence, we conclude that the successful incorporation of 0.1 wt% A-ND in UV-polymerized PMMA resin significantly improves the mechanical properties of the resin for the additive manufacturing of precisive 3D-printed biomedical appliances.

Hydro-Thermal Fatigue of Polymer Matrix Composite Biomaterials

This study discusses a quantitative fatigue evaluation of polymer-ceramic composites for dental restorations, i.e., commercial (Filtek Z550) and experimental Ex-nano (G), Ex-flow (G). Their evaluation is based on the following descriptors: mechanical strength, elastic modulus and strain work to fracture. Supposed to reflect factors of environmental degradation conditions, thermal fatigue was simulated with a special computer-controlled device performing algorithms of thermocycling. The specimens intended for the strength test underwent 10⁴ hydro-thermal fatigue cycles. This procedure of thermocycling was preceded by aging, which meant immersing the specimens in artificial saliva at 37 °C for 30 days. The strength tests after aging only and after aging and thermocycles were performed in line with the three-point flexural strength (TFS) test, specified in ISO 4049, and the biaxial flexural strength (BFS) test, specifically piston-on-three-ball in accordance with ISO 6872. Based on the results, it can be stated that composites with higher volume content of inorganic particles after aging only show higher strength than materials with lower filler particle content. For example, the average flexural bending strength of the Ex-flow (G) composite was about 45% lower than the value obtained for the Ex-nano (G) material. The residual strength after thermocycles is significantly lower for the experimental composites, whereas a smaller decrease in strength is recorded for the commercial composites. Decreases in strength were about 4% (Filtek Z550), 43% (Ex-nano (G)), and 29% (Ex-flow (G)) for the BFS test; and about 17% (Filtek Z550), 55% (Ex-nano (G)), 60% (Ex-flow (G)) for the TFS test. The elastic modulus of the experimental composites after only aging is higher (about 42%) than that of the commercial composite, but the elastic modulus of the commercial composite increases significantly after thermocycling. A descriptor known as strain work to fracture turns out to be a good descriptor for evaluating the hydro-thermal fatigue of the tested polymer-ceramic composites.

Threats to adhesive/dentin interfacial integrity and next generation bio-enabled multifunctional adhesives

Nearly 100 million of the 170 million composite and amalgam restorations placed annually in the United States are replacements for failed restorations. The primary reason both composite and amalgam restorations fail is recurrent decay, for which composite restorations experience a 2.0-3.5-fold increase compared to amalgam. Recurrent decay is a pernicious problem-the standard treatment is replacement of defective composites with larger restorations that will also fail, initiating a cycle of ever-larger restorations that can lead to root canals, and eventually, to tooth loss. Unlike amalgam, composite lacks the inherent capability to seal discrepancies at the restorative material/tooth interface. The low-viscosity adhesive that bonds the composite to the tooth is intended to seal the interface, but the adhesive degrades, which can breach the composite/tooth margin. Bacteria and bacterial by-products such as acids and enzymes infiltrate the marginal gaps and the composite's inability to increase the interfacial pH facilitates cariogenic and aciduric bacterial outgrowth. Together, these characteristics encourage recurrent decay, pulpal damage, and composite failure. This review article examines key biological and physicochemical interactions involved in the failure of composite restorations and discusses innovative strategies to mitigate the negative effects of pathogens at the adhesive/dentin interface. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B:2466-2475, 2019.

Influence of silica nanocoating on stain susceptibility and superficial integrity of dimethacrylate-based composites

The aim of this study was to investigate the ability of a novel silica (SiO₂ )-based nanocoating approach to extend the superficial integrity of current composites. Cylindrical discs (7 × 2 mm) were produced from nanohybrid and nanofilled composites. Specimens in control groups were not coated, but SiO₂ nanocoating was performed on specimens in experimental groups (n = 8). Specimens were stored for 24 h in distilled water at 37°C (baseline) and then artificially aged for 15, 90, or 180 d in a low-pH staining solution. Surface roughness (Ra) was measured using a profilometer, and a goniometer was used to determine surface free energy (SFE). Color change was evaluated by a reflectance spectrophotometer, applying the color distance metric, ΔE₀₀ , according to the Commission Internationale de l'Eclairage (CIE) L*a*b* coordinates. Data were subjected to repeated-measures anova and the Tukey post-hoc test. Composites presented visually perceptible color changes (ΔE₀₀  > 0.81) as early as 15 d of aging, with significantly higher ΔE₀₀ values recorded over time. Nanocoating with SiO₂ significantly reduced the SFE of composites at all storage times, and significantly lower Ra values were identified after aging. Nanohybrid and nanofilled composites were susceptible to substantial hydrolytic superficial degradation and staining, which was dramatically attenuated by the proposed SiO₂ nanocoating approach. Nanocoating effectively lowered the SFE of composites, thus minimizing water-composite interactions, which contributed to reduced superficial deterioration and lower stain susceptibility over time.

Evaluation of biofilm formation on novel copper-catalyzed azide-alkyne cycloaddition (CuAAC)-based resins for dental restoratives

OBJECTIVE

For the past several decades, the resins used in dental restorations have been plagued with numerous problems, including their implication in biofilm formation and secondary caries. The need for alternative resins is critical, and evaluation of biofilm formation on these resins is essential. The aim of this study was to evaluate in vitro biofilm formation on the surface of novel copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC)-based resins and composites.

METHODS

CuAAC-based resins/composites made from varying azide monomers and different copper concentrations were compared with BisGMA-TEGDMA resins/composites that served as the control. Biofilms were formed using a mono-species model containing a luciferase-expressing strain of Streptococcus mutans. Luciferase activity was measured and the number of viable bacteria was enumerated on biofilms associated with each resin and composite.

RESULTS

A significant reduction (p<0.05) in luciferase activity, and the number of viable bacteria recovered from biofilms on CuAAC-based resins and composites was observed in comparison to biofilms associated with the BisGMA-TEGDMA controls.

SIGNIFICANCE

CuAAC-based resins do still allow for the formation of biofilms; however, the statistically significant reduction of growth that was associated with the CuAAC resin may enhance the longevity of restorations that incorporate CuAAC-based materials.

Synthesis and characterization of Ciprofloxacin-containing divinyl oligomers and assessment of their biodegradation in simulated salivary esterase

OBJECTIVE

Two leading causes contributing to dental restoration replacement are the marginal breakdown at the composite/dentin interface and secondary caries mediated by bacteria. The objective of the present study was to synthesize oligomers which incorporated enhanced bio-stability but would also be able to generate antimicrobial function if they underwent degradation.

METHODS

Stability was incorporated into the oligomers by generating structural features that would physically hinder the availability of hydrolytically sensitive groups in the oligomers. As a proof-of concept for the antibacterial feature, antimicrobial function was achieved by covalently incorporating Ciprofloxacin (CF) into the backbone of cross-linking divinyl oligomers (referred to as EDV and HLH-CFPEG). The hydrolytic stability of the oligomers was studied in simulated human salivary esterase and compared to the commercial monomer 2,2-bis[4(2-hydroxy-3-methacryloxypropoxy)-phenyl]propane (BisGMA).

RESULTS

Both drug oligomers were found to be significantly more stable than BisGMA. Upon degradation, both drug oligomers released CF differentially in free form. Polymer synthesis from resin formulations containing 15wt% HLH-CFPEG showed a high degree of vinyl group conversion and gel content, and under hydrolytic conditions showed the release of CF during a 28-day monitoring study period.

SIGNIFICANCE

HLH-CFPEG can be used in dental resin adhesive systems for local delivery of CF to the marginal interface. Minimizing the growth of Streptococcus mutans at the marginal site can improve longevity by reducing esterase activity derived specifically from S. mutans.

Novel rechargeable calcium phosphate nanoparticle-filled dental cement

The objectives were to develop a novel rechargeable cement containing amorphous calcium-phosphate nanoparticles (nanoACP) to suppress tooth decay. Five cements were made with: (1) 60% glass particles (experimental control); (2) 40% glass+20% nanoACP; (3) 30% glass+30% nanoACP; (4) 20% glass+40% nanoACP; (5) 10% glass+50% nanoACP. Groups 1-4 had enamel bond strengths similar to Transbond XT (3M) and Vitremer (3M) (p>0.1). The nanoACP cement had calcium and phosphate ion release which increased with increasing nanoACP fillers. The recharged cement had substantial ion re-release continuously for 14 days after a single recharge. Ion re-release did not decrease with increasing recharge/re-release cycles. Groups 3-5 maintained a safe pH of medium (>5.5); however, control cements had cariogenic pH of medium (<4.5) due to biofilm acid. Therefore, nanoACP cement (1) had good bond strength to enamel, (2) possessed calcium and phosphate ion recharge/re-release capability, and (3) raised biofilm pH to a safe level to inhibit caries.

Physical properties and cytotoxicity of antimicrobial dental resin adhesives containing dimethacrylate oligomers of Ciprofloxacin and Metronidazole

OBJECTIVE

Antimicrobial oligomers synthesized from ciprofloxacin (CF) and metronidazole (MN) were investigated for their potential use in dental adhesives.

METHODS

Susceptibility of the cariogenic bacterium Streptococcus mutans UA159 to CF, MN, and CF/MN combination was evaluated. Hydrolytic stability and drug release from the oligomers was studied in buffer and simulated human salivary esterase conditions. Cytotoxicity of films with 15wt% drug oligomers co-polymerized with commercial monomers were assessed using human gingival fibroblasts (HGFs). In-house adhesives were prepared and characterized for viscosity. Polymerized films were analysed for gel content and water swelling. Interfacial fracture toughness (K_IC) of composites bonded to dentin by either a 2 or 3-step etch-and-rinse approach using the in-house formulated adhesives was measured.

RESULTS

The respective minimum inhibitory concentration for CF and MN against S. mutans was 0.7 and 2400μg/mL, with the combination having an additive effect (0.35μg/mL CF with 1200μg/mL MN). Antibiotics were released upon hydrolysis of the oligomers. Films containing the drug oligomers were not cytotoxic against HGFs. Replacing 2-hydroxyethyl methacrylate with the drug oligomers increased the viscosity of the experimental adhesives, reduced gel content, and decreased swelling of films in water. Antimicrobial adhesives demonstrated bonding to dentin with interfacial K_IC values comparable to the in-house control in the 2-step application, and with slightly lower K_IC values in the 3-step approach.

SIGNIFICANCE

The antimicrobial oligomers can be incorporated into dental adhesive systems using formulations that show comparable fracture toughness to commercial materials, and may provide a means to deliver local antimicrobial drug release at the marginal interface.

Biostable, antidegradative and antimicrobial restorative systems based on host-biomaterials and microbial interactions

OBJECTIVES

Despite decades of development and their status as the restorative material of choice for dentists, resin composite restoratives and adhesives exhibit a number of shortcomings that limit their long-term survival in the oral cavity. Herein we review past and current work to understand these challenges and approaches to improve dental materials and extend restoration service life.

METHODS

Peer-reviewed work from a number of researchers as well as our own are summarized and analyzed. We also include yet-unpublished work of our own. Challenges to dental materials, methods to assess new materials, and recent material improvements and research directions are presented.

RESULTS

Mechanical stress, host- and bacterial-biodegradation, and secondary caries formation all contribute to restoration failure. In particular, several host- and bacterial-derived enzymes degrade the resin and collagen components of the hybrid layer, expanding the marginal gap and increasing access to bacteria and saliva. Furthermore, the virulence of cariogenic bacteria is up-regulated by resin biodegradation by-products, creating a positive feedback loop that increases biodegradation. These factors work synergistically to degrade the restoration margin, leading to secondary caries and restoration failure. Significant progress has been made to produce hydrolytically stable resins to resist biodegradation, as well as antimicrobial materials to reduce bacterial load around the restoration. Ideally, these two approaches should be combined in a holistic approach to restoration preservation.

SIGNIFICANCE

The oral cavity is a complex environment that poses an array of challenges to long-term material success; materials testing conditions should be comprehensive and closely mimic pathogenic oral conditions.

Responsive antimicrobial dental adhesive based on drug-silica co-assembled particles

Most dental resin composite restorations are replacements for failing restorations. Degradation of the restoration-tooth margins by cariogenic bacteria results in recurrent caries, a leading cause for restoration failure. Incorporating antimicrobial agents in dental adhesives could reduce interfacial bacterial count and reduce recurrent caries rates, inhibit interfacial degradation, and prolong restoration service life, while minimizing systemic exposure. Direct addition of antimicrobial compounds into restorative materials have limited release periods and could affect the integrity of the material. Attempts to incorporate antimicrobial within mesoporous silica nanoparticles showed theoretical promise due to their physical robustness and large available internal volume, yet yielded short-term burst release and limited therapeutic payload. We have developed novel broad-spectrum antimicrobial drug-silica particles co-assembled for long-term release and high payload incorporated into dental adhesives. The release of the drug, octenidine dihydrochloride, is modulated by the oral degradative environment and mathematically modeled to predict effective service life. Steady-state release kills cariogenic bacteria, preventing biofilm formation over the adhesive surface, with no toxicity. This novel material could extend dental restoration service life and may be applied to other long-term medical device-tissue interfaces for responsive drug release upon bacterial infection.

STATEMENT OF SIGNIFICANCE

This study describes a novel dental adhesive that includes a broad-spectrum antimicrobial drug-silica co-assembled particles for long-term antimicrobial effect. The release of the drug, octenidine dihydrochloride, is modulated by the oral degradative environment and mathematically modeled to predict effective release throughout the service life of the restoration. Steady-state drug-release kills caries-forming bacteria, preventing biofilm formation over the adhesive surface, without toxicity. This novel material could extend dental restoration service life and may be applied to other long-term medical device-tissue interfaces for responsive drug release upon bacterial infection. Since recurrent cavities (caries) caused by bacteria are the major reason for dental filling failure, this development represents a significant contribution to the biomaterials field in methodology and material performance.

Short-time dental resin biostability and kinetics of enzymatic degradation

Resin biostability is of critical importance to the durability of methacrylate-based dental resin restorations. Current methods for evaluating biostability take considerable time, from weeks to months, and provide no short-time kinetics of resin degradation. The objective of this study is to develop a more sensitive method to assess resin biostability over short-time spans (hours to days) that will enhance our understanding of biostability and its resin chemistry. Ultra-flat resin films of equimolar urethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA) are produced through photo-curing between two flat surfaces. Next, metal-covered enclaves and bare-resin channels are generated using stencil lithography to create both degradable and protected (internal reference) regions simultaneously in a single specimen. Resins having three different degrees of vinyl conversion (DC) are compared, and changes of surface roughness and step height in the two regions are monitored by atomic force microscopy (AFM) before and after incubated in enzyme solutions and saline controls. Specimen biostability is ranked based on the topological profile changes when viewed in cross-section before and after enzymatic challenges. In addition, a model is proposed to quantify specimen enzymatic degradation. Based on this model, enzymatic degradation is detected as early as 4 h, and a surge of enzymatic degradation is detected between 4 h and 8 h. The correlation between the DC of resin network and the surge in degradation is discussed. In summary, this new method is effective in ranking biostability and quantifying enzymatic degradation while also reducing labor, time and cost, which lends itself well to materials development and evaluation of dental resins.

STATEMENT OF SIGNIFICANCE

We report, for the first time, the short-time kinetics of enzymatic degradation of methacrylate dental resins. A nanotechnology based method is developed to accelerate the evaluation of resin biostability. This new method reduces experimental time from weeks to one or two days, which will significantly reduce the costs of labor and enzymes. It also introduces a corresponding parameter (ΔH) and a three-cause model for ranking biostability, which confirms the correlation of chemical structure (DC) and material performance and opens new opportunities for studying the resin biostability and its impact on dental applications. Overall, this is a new tool for evaluating resin biostability and developing new materials.

Esterase from a cariogenic bacterium hydrolyzes dental resins

OBJECTIVES

To identify and characterize specific esterases from S. mutans with degradative activity toward methacrylate-based resin monomers.

METHODS

Out of several putative esterases, an esterase encoded in an Open Reading Frame as SMU_118c (The National Center for Biotechnology Information, NCBI), was found to have true hydrolase activities. SMU_118c was cloned, expressed, purified and further characterized for its respective hydrolytic activity towards ester-containing nitrophenyl substrates and the universal resin monomers bis-phenyl-glycidyl-dimethacrylate (bisGMA) and triethyleneglycol dimethacrylate (TEGDMA) at neutral (7.0) or cariogenic (5.5) pH. Mass spectrometry (MS) was used to verify the expression of SMU_118c protein in S. mutans UA159.

RESULTS

Similar to the whole cell activity of S. mutans, SMU_118c showed the highest affinity toward para-nitrophenyl acetate (pNPA) and para-nitrophenyl butyrate (pNPB) vs. ortho-nitrophenyl butyrate (oNPB) and butyrylthiocholine iodide (BTC) (p < 0.05). The esterase retained 60% of its activity after 21 days and hydrolyzed bisGMA at a higher rate than TEGDMA at both neutral and cariogenic pH (p < 0.001), similarly to the predominant human salivary esterase degradative activity. MS confirmed that SMU_118c is an intracellular protein in S. mutans UA159 and expressed under pathogenic (pH 5.5) growth conditions.

SIGNIFICANCE

The similarity in the activity profile to the whole S. mutans bacterial cell, the stability over time at cariogenic pH, the preference to hydrolyze bisGMA and confirmed expression profile suggest that SMU_118c could be a significant contributor to the whole bacterial degradative activity of S. mutans toward the degradation of resin composites, adhesives and the restoration-tooth interface, potentially accelerating restoration's failure.

STATEMENT OF SIGNIFICANCE

The current study builds upon our highly-cited previous study by Bourbia et al., (JDR, 2013) that reported on that the cariogenic bacterium, S. mutans has esterase-like activities that enable the bacterium to degrade dental composites and adhesives. The current submission is the first to report on the isolation and characterization of the specific esterase activity (SMU_118c) from S. mutans that is a significant contributor to the whole bacterial degradative activity toward the hydrolysis of dental resins. This activity compromises the restoration-tooth interface, increases interfacial bacterial microleakage (Kermanshahi et al., JDR 2010), potentially contributing to the pathogenesis of recurrent caries around resin composite restorations. This represent a significant contribution to the field of biomaterials and their clinical performance.

Enterococcus faecalis Hydrolyzes Dental Resin Composites and Adhesives

INTRODUCTION

After root canal treatment, the dentin-sealer interface undergoes degradation, allowing for interfacial microbial biofilm proliferation and treatment failure. Saliva and cariogenic bacteria showed esterase-like activities (ie, cholesterol esterase [CE]-like and/or pseudocholinesterase [PCE]-like) that degrade methacrylate-based resin materials and/or the restoration-tooth interface, increasing microbial interfacial proliferation. Enterococcus faecalis is a gram-positive bacterium that is commonly detected in persistent endodontic infections. The aim of this study was to measure E. faecalis esterase-like, CE-like, and PCE-like activities and to assess the ability of the bacterium to degrade methacrylate-based resin composite (RC) and total-etch (TE) and self-etch (SE) adhesives.

METHODS

CE-like and PCE-like activities from E. faecalis were measured using nitrophenyl and butyrylthiocholine substrates, respectively. The ability of E. faecalis to degrade resin composite, total-etch and self-etch adhesives was examined by quantifying the release of a universal resin degradation by-product (ie, Bis[hydroxypropoxy]-phenyl propane [BisHPPP]) using high-performance liquid chromatography.

RESULTS

E. faecalis showed CE-like (1.23 ± 0.13 U/μg dry bacteria) but no PCE-like activity. After 30 days and/or 14 days of incubation, the amount of BisHPPP released was significantly higher in the presence of bacteria versus media for TE and RC but not SE (P < .05). The amount of BisHPPP released after 30 days of incubation with bacteria was highest for TE (23.69 ± 1.72 μg/cm²) followed by RC (3.43 ± 1.20 μg/cm²) and lowest for SE (0.86 ± 0.44 μg/cm²) (P < .05).

CONCLUSIONS

E. faecalis possesses esterase-like degradative activity toward dental methacrylate resin restoration materials, which could accelerate the degradation of the dentin-methacrylate resin interface, increasing bacterial biofilm proliferation and penetration into the root canal system.

Biodegradation Studies of Novel Fluorinated Di-Vinyl Urethane Monomers and Interaction of Biological Elements with Their Polymerized Films

The monomeric components of resin composites in dental restorative materials are susceptible to hydrolysis in the oral cavity. The main objective of this study was to assess the bio-stability of fluorinated urethane dimethacrylates and determine the nature of fluoro-chemistry interactions with protein and bacterial adhesion (both sources of hydrolytic activity) onto cured resin. Degradation studies were performed in the presence of either albumin (in a mildly alkaline pH) or cholesterol esterase (CE). The surface chemistry of the polymers was assessed by water contact angle measurements, pre- and post- incubation with albumin. Adhesion of Streptococcus mutans to cured resin was investigated. The fluorinated monomers were more stable against degradation when compared to the commercial monomer bisphenol A-diglycidyl methacrylate (BisGMA). While fluorinated monomers showed hydrolytic stability with respect to CE, all fluorinated monomers underwent some degree of degradation with albumin. The fluoro-chemistry did not reduce protein and/or bacterial adhesion onto the surface, however post incubation with albumin, the fluorinated surfaces still presented hydrophobic character as determined by the high contact angle values ranging from 79° to 86°. These monomers could potentially be used to increase the hydrophobicity of polymeric composites and provide a means to moderate esterolytic degradation associated with the monomeric component of the polymers within the oral cavity.

Degradation of Resin Restorative Materials by Streptococcus Mutans: A Pilot Study

OBJECTIVE

To evaluate the degradation of three resin based restorative materials by S Mutans.

STUDY DESIGN

Class I cavity was prepared in extracted premolars and were randomly divided into 3 groups (Group I - Conventional composite (CC), Group II - Resin Modified GIC and Group III-Giomer). Teeth were then restored by respective restorative material and equally divided in two subgroups (Control and Experimental). Experiment subgroup samples were then incubated in 2 ml of BHI with 1:10 dilution of SM (MTCC-497) grown overnight in BHI whereas control subgroup samples were incubated in BHI without SM. The incubation solution was collected at 2,14 and 30 days interval, and the analysis for identification and quantification of Bis-HPPP was done by High performance Liquid Chromatography.

RESULTS

Statistical analysis of the collected data revealed a statistically increased Bis HPPP production in the presence of SM in all the tested materials, with minimum in Resin Modified GIC and a maximum in Conventional Composite (CC).

CONCLUSION

SM degrades the resin based restorative materials & among the tested materials Resin Modified GIC appears to be most Biostable.

Biofilm-induced changes to the composite surface

OBJECTIVES

Composites may undergo biodegradation in the oral cavity. The objective was to investigate the effect of single- and multi-species biofilms on the surface roughness and topography of two composites.

METHODS

Disk-shaped specimens of a paste-like, Bis-GMA-free (Gradia Direct Anterior, GC), and a flowable, Bis-GMA-based composite (Tetric EvoFlow, Ivoclar-Vivadent) were prepared. After ethylene-oxide sterilization (38°C), specimens (n=3) were incubated with Streptococcus mutans or mixed bacterial culture (Streptococcus mutans, Streptococcus sanguinis, Actinomyces naeslundii and Fusobacterium nucleatum). As negative controls, unexposed specimens and specimens exposed to sterile medium (BHI) were used. Specimens exposed to acidified BHI medium (pH=5) and enzymatic solution of cholesterol esterase served as positive control. Following 6-week incubation, the attached biofilms were collected for real-time PCR assessment, after which the surface roughness and topography of the specimens were analyzed with atomic force microscopy. Surface hydrophilicity/hydrophobicity was determined by contact angle measurements. Biofilm structure was analyzed with scanning electron microscopy.

RESULTS

Even though multi-species biofilms were thicker, with more cells attached, they did not significantly affect the surface roughness of the composites. On the other hand, S. mutans alone significantly increased the roughness of Tetric by 40.3%, while its effect on Gradia was lower (12%). The total amount of attached bacteria, however, did not differ between the composites.

CONCLUSIONS

S. mutans can increase the surface roughness of composites, depending on their composition. This ability of S. mutans is, however, mitigated in co-culture with other species. In particular, bacterial esterases seem to be responsible for the increased composite surface roughness upon biofilms exposure.

CLINICAL SIGNIFICANCE

Cariogenic bacteria can degrade composites, thereby increasing the surface roughness. Increased roughness and subsequent improved bacterial accumulation may facilitate the development of secondary caries around composites, which is the most common reason for the restoration failure.

Ultrastructural Correlates of in vivo/in vitro Bond Degradation in Self-etch Adhesives

The morphologic correlates of bond degradation in self-etching primers have not been fully elucidated. We hypothesized that there is no difference between the mechanism of degradation of self-etching primers in vivo and in vitro. Class I cavities prepared in vivo in 24 caries-free human molars were bonded with Clearfil SE Bond or Clearfil Protect Bond, and restored with resin composites. Eight teeth were extracted after 24 hrs, and the rest after 1 yr. The same protocol was repeated in vitro with extracted molars. Degradation of resin-dentin bonds was assessed by microtensile bond testing and TEM of interfaces after tracer immersion. Both in vivo and in vitro bond strengths decreased with time for SE Bond but not for Protect Bond, with more pronounced water treeing observed in the former adhesive under both aging conditions. There is no difference between the mechanism of degradation of self-etch adhesives in vivo or in vitro.

Mechanistic, genomic and proteomic study on the effects of BisGMA-derived biodegradation product on cariogenic bacteria

OBJECTIVES

Investigate the effects of a Bis-phenyl-glycidyl-dimethacrylate (BisGMA) biodegradation product, bishydroxypropoxyphenyl-propane (BisHPPP), on gene expression and protein synthesis of cariogenic bacteria.

METHODS

Quantitative real-time polymerase chain reaction was used to investigate the effects of BisHPPP on the expression of specific virulence-associated genes, i.e. gtfB, gtfC, gbpB, comC, comD, comE and atpH in Streptococcus mutans UA159. Possible mechanisms for bacterial response to BisHPPP were explored using gene knock-out and associated complemented strains of the signal peptide encoding gene, comC. The effects of BisHPPP on global gene and protein expression was analyzed using microarray and quantitative proteomics. The role of BisHPPP in glucosyltransferase (GTF) enzyme activity of S. mutans biofilms was also measured.

RESULTS

BisHPPP (0.01, 0.1mM) up-regulated gtfB/C, gbpB, comCDE, and atpH most pronounced in biofilms at cariogenic pH (5.5). The effects of BisHPPP on the constructed knock-out and complemented strains of comC from quorum-sensing system, implicated this signaling pathway in up-regulation of the virulence-associated genes. Microarray and proteomics identified BisHPPP-regulated genes and proteins involved in biofilm formation, carbohydrate transport, acid tolerance and stress-response. GTF activity was higher in BisHPPP-exposed biofilms when compared to no-BisHPPP conditions.

SIGNIFICANCE

These findings provide insight into the genetic and physiological pathways and mechanisms that help explain S. mutans adaptation to restorative conditions that are conducive to increased secondary caries around resin composite restorations and may provide guidance to clinicians' decision on the selection of dental materials when considering the long term oral health of patients and the interactions of composite resins with oral bacteria.

Triethylene Glycol Up-Regulates Virulence-Associated Genes and Proteins in Streptococcus mutans

Triethylene glycol dimethacrylate (TEGDMA) is a diluent monomer used pervasively in dental composite resins. Through hydrolytic degradation of the composites in the oral cavity it yields a hydrophilic biodegradation product, triethylene glycol (TEG), which has been shown to promote the growth of Streptococcus mutans, a dominant cariogenic bacterium. Previously it was shown that TEG up-regulated gtfB, an important gene contributing to polysaccharide synthesis function in biofilms. However, molecular mechanisms related to TEG’s effect on bacterial function remained poorly understood. In the present study, S. mutans UA159 was incubated with clinically relevant concentrations of TEG at pH 5.5 and 7.0. Quantitative real-time PCR, proteomics analysis, and glucosyltransferase enzyme (GTF) activity measurements were employed to identify the bacterial phenotypic response to TEG. A S. mutans vicK isogenic mutant (SMΔvicK1) and its associated complemented strain (SMΔvicK1C), an important regulatory gene for biofilm-associated genes, were used to determine if this signaling pathway was involved in modulation of the S. mutans virulence-associated genes. Extracted proteins from S. mutans biofilms grown in the presence and absence of TEG were subjected to mass spectrometry for protein identification, characterization and quantification. TEG up-regulated gtfB/C, gbpB, comC, comD and comE more significantly in biofilms at cariogenic pH (5.5) and defined concentrations. Differential response of the vicK knock-out (SMΔvicK1) and complemented strains (SMΔvicK1C) implicated this signalling pathway in TEG-modulated cellular responses. TEG resulted in increased GTF enzyme activity, responsible for synthesizing insoluble glucans involved in the formation of cariogenic biofilms. As well, TEG increased protein abundance related to biofilm formation, carbohydrate transport, acid tolerance, and stress-response. Proteomics data was consistent with gene expression findings for the selected genes. These findings demonstrate a mechanistic pathway by which TEG derived from commercial resin materials in the oral cavity promote S. mutans pathogenicity, which is typically associated with secondary caries.