Structure and properties of methacrylate based dental restorative materials

The blood-brain barrier in aging and neurodegeneration

The blood-brain barrier (BBB) is vital for maintaining brain homeostasis by enabling an exquisite control of exchange of compounds between the blood and the brain parenchyma. Moreover, the BBB prevents unwanted toxins and pathogens from entering the brain. This barrier, however, breaks down with age and further disruption is a hallmark of many age-related disorders. Several drugs have been explored, thus far, to protect or restore BBB function. With the recent connection between the BBB and gut microbiota, microbial-derived metabolites have been explored for their capabilities to protect and restore BBB physiology. This review, will focus on the vital components that make up the BBB, dissect levels of disruption of the barrier, and discuss current drugs and therapeutics that maintain barrier integrity and the recent discoveries of effects microbial-derived metabolites have on BBB physiology.

Shear Bond Strength and Optical Properties of Resin Composite - Layered Provisional Poly(methyl methacrylate) (PMMA)

Aims:

To compare the shear bond strength between resin composite and Poly(methyl methacrylate) (PMMA) with various bonding protocols and to evaluate the optical properties of resin composite - layered provisional Poly(methyl methacrylate) (PMMA).

Materials and Methods:

Eighty cylindrical shape specimens were fabricated from self-polymerized provisional Poly(methyl methacrylate) (PMMA) and they were randomly divided into eight groups. Poly(methyl methacrylate) (PMMA) was mixed and bonded onto the specimens as a positive control group. Resin composite was bonded to MMA-wetted surface without bonding agent as a negative control group. All remaining groups were bonded to resin composite using different bonding agents (Scothbond Universal, Luxatemp glaze&bond, and HC Primer) with and without MMA wetting. Shear bond strength testing was performed using a universal testing machine. Various shades of 0.5 mm-thick resin composites were layered onto 1.5 mm-thick PMMA both light and dark shade, with the most effective bonding protocol. Color differences between resin composite and – layered provisional Poly(methyl methacrylate) (PMMA) were measured using Spectrophotometer.

Results:

Bonding resin composite onto Poly(methyl methacrylate) (PMMA) using luxatemp glaze, bond and HC Primer without methyl methacrylate wetting provided statistically significantly lower bond strength than those of the MMA-wetted Poly(methyl methacrylate) (PMMA) surface. The highest shear bond strength was achieved with the application of Scothbond Universal Adhesive regardless of MMA wetting. The colors of resin composite - layered provisional Poly(methyl methacrylate) (PMMA) were different from the original resin composite color with ΔE results greater than the acceptable threshold (>3.7).

Conclusion:

Resin composites were able to effectively bond to the MMA-wetted Poly(methyl methacrylate) (PMMA) surface with the application of a tested bonding agent. Layering Poly(methyl methacrylate) (PMMA) with 0.5 mm-thick resin composite could not modify the Poly(methyl methacrylate) (PMMA) shade to the original resin composite color.

A New Antibacterial Agent-Releasing Polydimethylsiloxane Coating for Polymethyl Methacrylate Dental Restorations

Chlorhexidine (CHX) has been incorporated into the composition of polymethyl methacrylate (PMMA) dental restorations to enhance their antimicrobial performance. However, the controlled delivery of CHX remains a challenge. Although previous findings with pure silica or polymer coatings demonstrated the resistance to bacterial adhesion, they did not provide antibacterial activity beyond the coated surface. Polydimethylsiloxane (PDMS) and mesoporous silica nanoparticles (MSNs) are widely used in biomedical science as a transfer medium in drug delivery systems. Here, the MSNs are used to encapsulate CHX, and the combination is added to PDMS. A thin coating film is formed on the PMMA, using oxygen plasma and thermal treatment. The liquid chromatography analysis shows that the coating film has high encapsulation efficiency and loading capacity, with a slow and stable release rate of CHX. The cytotoxicity tests also show that the coating does not affect the proinflammatory cytokines, cellular mitotic activity, or apoptotic cell death. The ability of the coating to release CHX indicates that the coating may even be effective against bacteria that are not directly in contact with the surface. This antibacterial protective film is expected to be a novel method to inhibit bacterial activity distal to the coated surfaces of PMMA restorations.

Ester-free thiol-ene dental restoratives--Part B: Composite development

OBJECTIVES

To assess the performance of thiol-ene dental composites based on selected ester-free thiol-ene formulations. Further, to point out the benefits/drawback of having a hydrolytically stable thiol-ene matrix within a glass filled composite.

METHODS

Composite samples containing 50-65wt% of functionalized glass microparticles were prepared and photopolymerized in the presence of a suitable visible light photoinitiator. Shrinkage stress measurements were conducted as a function of the irradiation time. Degrees of conversion were measured by FT-IR analysis by comparing the double bond signals before and after photopolymerization. Mechanical tests were carried out on specimens after curing as well as after extended aging in water. Dynamic mechanical analysis was employed to track the changes in storage modulus near body temperature. The properties of the thiol-ene composites were compared with those of the BisGMA/TEGDMA control.

RESULTS

Depending on the resin type, similar or higher conversions were achieved in thiol-ene composites when compared to the dimethacrylate controls. At comparable conversions, lower shrinkage stress values were achieved. Although exhibiting lower initial elastic moduli, the thiol-ene composites' flexural strengths were found to be comparable with the controls. Contrary to the control, the mechanical properties of the ester-free thiol-ene composites were shown to be unaffected by extensive aging in water and at least equaled that of the control after aging in water for just five weeks.

SIGNIFICANCE

Employing non-degradable step-growth networks as organic matrices in dental composites will provide structurally uniform, tough materials with extended service time.

Rubber-toughening of dimethacrylate dental composite resin

Dimethacrylate dental composite resins exhibit inherently low toughness. Toughening of these materials may reduce the incidence of marginal and bulk fracture of composite restorations.

OBJECTIVE

To determine if dimethacrylate dental restorative materials can be rubber-toughened, and if so, to identify a possible mechanism.

METHODS

A filler composed of aggregates of polybutadiene/silica as well as irregularly-shaped silica slabs was produced by mixing silica with polybutadiene in dichloromethane. The dried filler was subsequently ground and sieved to < 25 microm. Polybutadiene/silica ratios were varied from 0:1 (control) to 0.5:1. EDAX analysis verified the composition of the complex filler. Filler was added to a bis-GMA/bis-EMA/TEGDMA resin system and fractured in three-point bend test mode at a crosshead speed of 1 mm/min. In addition, 1 bar was fractured at a crosshead speed of 0.001 mm/min to identify a possible mechanism for toughening.

RESULTS

In specimens fractured at 1 mm/min, flexural modulus is increased or maintained and flexural strength and energy to break increase as the amount of polybutadiene in the aggregates increases. Cavitation of high-rubber-containing aggregates is demonstrated. In the one specimen fractured at 0.001 mm/min, a marked increase in size of high-rubber-containing aggregates along with severe shear damage in the surrounding matrix is shown, suggesting that cavitation with subsequent absorption of energy during shear yielding is the likely mechanism behind the increase in energy to break in bars fractured at 1 mm/min.

SIGNIFICANCE

These results indicate that dimethacrylate dental composite materials can be rubber toughened, which may potentially reduce marginal and bulk fractures of composite restorations, and consequently extend their service lifetime.

Relationships between conversion, temperature and optical properties during composite photopolymerization

Optical properties of composite restoratives, both cured and uncured, are of obvious importance in a procedure reliant on photoactivation, since they may affect light transmission and therefore materials conversion upon which mechanical properties and ultimate clinical performance are dependent. The objective of the present study was to evaluate simultaneous, real-time conversion, and the development of the temperature and optical properties. The dimethacrylate resin (Bis-GMA/TEGDMA 70/30mass%) was prepared at three filler loading (0, 35 or 70mass%: no fill, low and high fill, respectively) combined with three initiator concentrations (CQ/EDMAB: 0/0, 0.2/0.8 or 1.0/1.6mass%). Specimens were exposed to either low (50mWcm(-2)) or high (500mWcm(-2)) irradiance. Simultaneous conversion (near-IR peak area), temperature (thermocouple) and visible light transmission (UV-vis spectroscopy) measurements were conducted throughout the polymerization process. The refractive index of the resin rises linearly with conversion (r(2)=0.976), producing a refractive index match between resin/filler at approximately 58% conversion in these materials. The percentage increase in light transmission during conversion was greater for increasing filler levels. Higher CQ content led to maximum light transmission at slightly higher levels of conversion (60-65% and 50-55% for the high and low filled materials, respectively). The broad distribution of filler concentrations allows for the clinically relevant generalization that highly filled composites not only jeopardize absolute light transmission, conversion and depth of cure, but also demonstrate the complex interrelationship that exists between materials, processing conditions and the optical properties of dental composites.

Heat induced changes to dental resin composites: a reference in forensic investigations?

The objective was to investigate color change and surface damage in dental resin composites exposed to high temperatures over different time intervals for comparative purposes. Samples were prepared using two resins - Z100(R) (R1) and Charisma (R2), heated at the following temperatures: 200 degrees C, 400 degrees C, 600 degrees C, 1000 degrees C, for 15, 30 and 45 min (n = 104 for each resin sample). Color (DeltaE) and brightness (DeltaL) changes were analyzed by spectrophotometry using the CIE Lab system and surface changes by Scanning Electron Microscopy (SEM). R1 showed more intense color changes after heat exposure than R2. DeltaL values were found to be the best parameter for evaluation of light and color change. A biphasic pattern after thermal exposure was detected, from dark brown to light white. SEM showed more intense alterations in R2 than in R1. These results indicate that the parameters observed in both resins are useful as a guide in forensic analyses.

Comparative bond strengths of autopolymerising denture resin and light cured composite resin to denture teeth

AIM

To investigate the shear bond strengths between light cured composite and autopolymerised acrylic resin bonded to acrylic resin denture teeth.

METHODS

Surface treatments were used for the denture teeth included wetting with methylmethacrylate (MMA) monomer, composite bonding agent and acid etching. The samples were divided into seven groups. Two groups of specimens were immersed in distilled water for 30 days to hydrate the teeth. Five other groups of embedded denture teeth were stored in air at room temperature for 30 days.

RESULTS

Analysis of variance indicated statistically significant differences between groups (P < 0.001).

CONCLUSION

The greatest bond strengths to denture teeth were in acrylic resins polymerised with pressure and wetted with monomer. The bond strengths of hydrated and unhydrated samples gave similar results both with acrylic resins and composites.

Patient preference for light-cured composite bite splint compared to heat-cured acrylic bite splint

BACKGROUND

Heat-cured acrylic has been the most commonly used material for construction of bite splints. Although effective, its processing involves several steps and is time consuming. Furthermore, acrylic splints distort easily if not kept in water when not worn for long periods of time. A newly developed light-cured composite material is now being used for bite splint fabrication. The composite material offers benefits in ease and speed of construction, has minimal warping and distortion, and has proven patient acceptance. The aim of this study was to determine if patient satisfaction with the composite splint was as good as, or preferential to, the acrylic splint.

METHODS

Both a composite and an acrylic bite splint were fabricated for each of 10 patients. The splints were worn alternately on a nightly basis and were adjusted as needed after the first week. After 3 weeks, each patient completed a questionnaire regarding the properties of each splint and any preferences they had in reference to fit, comfort, and other parameters of satisfaction.

RESULTS

All of the patients were able to wear at least one of the splints comfortably. All 10 preferred the composite splint over the acrylic splint, agreeing that it felt more natural and was more comfortable to wear.

CONCLUSIONS

The light-cured composite bite splint is preferable from the patient's perspective to the heat-cured acrylic bite splint. The composite splint is rapidly constructed on the original model, easily seated, and comfortably worn. Other properties of composite material also make it preferable for long-term use. Future studies are necessary to evaluate the functional differences between the composite and acrylic splint.

Shear bond strengths for composite and autopolymerized acrylic resins bonded to acrylic resin denture teeth

Statement of problem. Composite has been used to modify acrylic resin denture teeth. Purpose. This in vitro investigation examined the shear bond strengths between composite and autopolymerized acrylic resin bonded to acrylic resin denture teeth. Material and methods. The surface treatments used for the denture teeth included wetting with methyl methacrylate (MMA), vinylethyl methacrylate monomer (VEMA), unfilled liquid resin, composite bonding agent, and composite color modifier. Nonhydrated and hydrated denture tooth groups were included. A commercial composite was bonded to the denture teeth. The control group consisted of autopolymerized PMMA resin bonded to the acrylic resin denture teeth, and another group consisted of polyvinylethyl methacrylate bonded to acrylic resin denture teeth. The samples were thermocycled and tested in shear. Results. Acrylic resin denture teeth prewetted with MMA and treated with unfilled resin or a bonding agent had bond strength values comparable to the control group. VEMA was not as effective in promoting the bond. Composite color modifier did not produce a significantly weaker bond between the acrylic resin teeth and added composite. No prewetting of the teeth with MMA resulted in the lowest bond strength. Mean shear bond strengths for corresponding hydrated and non-hydrated groups were not significantly different. Conclusion. Bond strength of composite-to-acrylic resin denture teeth was comparable to the bond strength of autopolymerized acrylic resin.

A simple method for the measurement of polymerization shrinkage in dental composites

OBJECTIVE

In this study a simple non-contact method was developed to measure the polymerization shrinkage of dental composites.

METHODS

A gas pycnometer was used to determine the volumes of specimens prior to and after photopolymerization and from which the total volumetric shrinkage could be determined.

RESULTS

Four commercial composites were studied and were found to have polymerization shrinkages varying from 1.6 to 2.5%. The method was found to be labour efficient and produced reproducible results with a standard deviation of approximately 10%.

SIGNIFICANCE

This method is appropriate for shrinkage measurements where only the total amount shrinkage is required and in particular for the measurement of shrinkage of photocured materials which are sensitive to water absorption.

Synolite as a base resin for dental composites and related biomaterials

Synolite resin was analysed using FTIR, NMR and HPLC to determine its components and purity. The resin was found to be predominantly BIS-GMA containing traces of its two isomers. The analysis indicated that there was no unreacted methacrylic acid or other impurities.

Colour stability of composite resins: a clinical comparison

Ninety three Class III cavities were prepared in 66 patients, and restored with one of three resin-based restorative materials; a hybrid self-cure material, a microfine self-cure material, or a microfine light-cure material. Standard sets of clinical colour transparencies were used to evaluate colour match with the surrounding tooth and marginal discoloration over five years. After five years, all materials had become slightly darker, the microfine self-cure material significantly more so than the light-cure material. There was no significant development of marginal discoloration.

Laser photopolymerization of dental materials with potential endodontic applications

Photopolymerizing resins were exposed to three different wavelengths of light emanating from the argon laser. It was determined that the most efficient wavelengths for photopolymerization of camphorquinone-activated resins were at 477 and 488 nm. The 514.5-nm wavelength was relatively ineffective in activating polymerization. Four camphorquinone-activated resins were placed in the root canals of teeth and tested for polymerization depth using a 488-nm wavelength laser beam coupled to an optical fiber 200 microns in diameter. In regard to polymerization depth, these materials ranked as follows: Genesis greater than Prisma-Fil greater than Prisma Microfine greater than Prisma VLC Dycal. Alterations in the positions of the optical fiber and the surface of the resin in the canal made only minor differences in polymerization depth of the samples. The results indicate that an argon laser coupled to an optical fiber could become a useful modality in endodontic therapy.

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.

Stiffness increase during the setting of dental composite resins

Changes in the dynamic Young's modulus under flexure of self-cured and light-cured composite resins during setting were demonstrated to reflect accurately the condition of the curing composite resins. The rate of stiffness increase during setting varied considerably with the product, and the light-cured composite resins generally showed a faster rate of increase of stiffness. In the initial stage, the Young's moduli were very low, especially for the self-cured composite resins. This accounts for the damage (adhesion disruption and cross-linking interruption) that can be inflicted on a freshly placed composite resin filling that is not yet mechanically stabilized. Therefore, such fillings are vulnerable to aggressive distortion for at least 10 to 15 minutes after placement.