Improving fracture toughness of dental nanocomposites by interface engineering and micromechanics.

The fracture toughness of dental nanocomposites fabricated by various methods of mixing, silanization, and loadings of nanoparticles had been characterized using fatigue-precracked compact-tension specimens. The fracture mechanisms near the crack tip were characterized using atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The near-tip fracture processes in the nanocomposties were identified to involve several sequences of fracture events, including: (1) particle bridging, (2) debonding at the poles of particle/matrix interface, and (3) crack deflection around the particles. Analytical and finite-element methods were utilized to model the observed sequences of fracture events to identify the source of fracture toughness in the dental nanocomposites. Theoretical results indicated that silanization and nanoparticle loadings improved the fracture toughness of dental nanocomposites by a factor of 2 to 3 through a combination of enhanced interface toughness by silanization, crack deflection, as well as crack bridging. A further increase in the fracture toughness of the nanocomposites can be achieved by increasing the fracture toughness of the matrix, nano-filled particles, or the interface.

Effect of poling conditions on growth of calcium phosphate crystal in ferroelectric BaTiO3 ceramics.

Recently, ceramic materials have been given a lot of attention as candidates for implant materials, since they possess biologically favorable characteristics for osseointegration. Among them, BaTiO3 (BTO) ceramics are ferroelectric and piezoelectric after poling treatments. However, little or no information is available on the poling condition of BTO and their effect on calcium phosphate (CaP) formation. In this study, the effect of poling conditions on the formation of CaP layer was investigated. It was observed from this study that CaP was formed on negatively charged BTO surfaces. An increase in Ca/P ratio to 1.67 was observed when the poling temperature was increased above the Curie temperature. On positively charged BTO, no CaP layer was observed.

Structural basis for the binding affinity of xanthines with the DNA intercalator acridine orange.

Caffeine (CAF), a methyl-substituted xanthine, interacts with polyaromatic DNA intercalators and has been hypothesized to interfere with their intercalation into DNA. Optical absorption spectroscopy was used to determine the binding affinities (K(assoc)) and structural effects of a series of methyl-substituted xanthines and a series of methyl-substituted uric acids (8-oxoxanthine) with the known DNA intercalator acridine orange (AO). There is evidence that complexation occurred (K(assoc) > or = 150 M(-1); binding curve saturation approximately > or =50%) between AO and 1,7-dimethylxanthine (155 M(-1)), 1,3-dimethylxanthine (theophylline, 157 M(-1)), 1,3,7-trimethylxanthine (CAF, 256 M(-1)), 1,3-dimethyl-8-chloroxanthine (413 M(-1)), 1,3,7,9-tetramethyl-8-oxyxanthine (tetramethyl uric acid or TMU, 552 M(-1)), and theophylline ethylenediamine (aminophylline, 596 M(-1)). No definitive evidence of complexation occurred between AO and 16 other substituted xanthines or purines, although there was some evidence of weak complexation (K(assoc) < 150 M(-1)) between AO and eight of the sixteen. Three common structural similarities were identified among those compounds found to form significant bonding with AO: (i) the N(1) or N(3) on the xanthine structure must be substituted with a methyl group; (ii) oxygen or chlorine substitution at C(8) increases binding affinity to AO when resonate states remain unchanged; and (iii) K(assoc) increases with an increase in number of methyl group substitutions on the 1- or 3-methylxanthine core structure. These results are explained on the basis of complex stabilization due predominately to hydrophobic attraction, with a contribution from charge transfer between donor and acceptor components. This information can be used in the manipulation of the physical or chemical characteristics of biologically active polyaromatic molecules.

The use of dynamic mechanical analysis to assess the viscoelastic properties of human cortical bone.

The purpose of this study was to examine the use of a dynamic mechanical analyzer (DMA) system to study the viscoelastic nature of bone. Cortical bone specimens from human femora were tested isothermally for 150 min at 37 degrees C and the loss factor (tan delta) and storage modulus (E') were measured. To explore the effects of test conditions on tan delta and E', different levels of applied stress, two specimen sizes, and two hydration conditions (wet and vacuum-dried) were evaluated. Finally, nonisothermal tests were performed, wherein specimens were heated up to 70 degrees C at different heating rates: 1 degrees C/min, 3 degrees C/min, and 5 degrees C/min. The results indicated that a threshold level of minimum applied stress was required to obtain repeatable and relatively constant values of tan delta. Specimen size did not significantly affect tan delta although it influenced E'. Moisture content had a significant effect on tan delta; vacuum-dried specimens exhibited a lower tan delta compared to wet specimens. Lastly, heating rates influenced tan delta values with lower rates producing more consistent results. The study demonstrated that DMA can be used as an effective tool to test bone.

Metal-oxide nanoparticles for the reinforcement of dental restorative resins.

Metal oxide nanoparticles were synthesized from tantalum ethoxide and zirconium isopropoxide and subsequently surface grafted with vinyl silane and silyl methacrylate coupling agents. The nanoparticles were then dispersed into a commercial dental resin, and the composite was photocured into rigid three-point bend and fracture toughness specimens. The optically transparent/translucent cured composites demonstrated strength, toughness, and elastic modulus inferior to the unfilled material. Therefore, modifications in surface functionalization are being made to improve coupling and reduce interparticle associations.

Radiopacity of tantalum oxide nanoparticle filled resins.

OBJECTIVES: Radiopacity of composite resins allows radiographic distinction of existing restorations and recurrent caries. Current composites must be supplemented with heavy metal-containing glasses or minerals to achieve a desired radiopacity. The purpose of this study was to evaluate the radiopacity of Tantalum oxide (Ta2O5) filled resins at varying percentage loadings. METHODS: Methacrylate functionalized Ta2O5 nanoparticles (< 50 nm) in methanol-dissolved or powder forms were mixed into either glycerol dimethacrylate (GDMA) or a bisGMA, TEGDMA, bisEMA mixture (GTE). Specimens were made in a split brass mold (2 x 2 x 15 mm) and compared with an aluminum stepwedge (99.5% pure Al) and a dentin slice of the same thickness. Kodak Ultraspeed periapical X-ray film on a lead plate at a target distance of 45 cm was exposed at 70 kVp and 10 mA, for 0.5 s and processed automatically. Optical density was measured (n = 3) with an RMI Processor Control Densitometer. Radiopacity was calculated as percent relative linear attenuation coefficient (Alpha). ANOVA and Student-Newman-Keuls comparisons were used to determine significance at the 95% confidence level. RESULTS: Radiopacity increased significantly with Ta2O5 loading (p = 0.001). Ta2O5 nanoparticle filled resins enter the optimal range of diagnostic detectability (alpha = 150-250) at 50 wt.% and approach equivalence with enamel at approximately 70 wt.%. SIGNIFICANCE: The introduction of tantalum oxide nanoparticle filler has potential as a miscible component of a resin composite to provide radiopacity for microfiller-type restorative materials and to circumvent the need for hydrolysis-prone glass reinforcing fillers.

Development of a new photoinitiation system for dental light-cure composite resins.

OBJECTIVES: The purpose of this study is to explore the synergistic effect of combining camphorquinone (CQ) with 1-phenyl-1,2-propanedione (PPD) as a new photoinitiator. METHODS: A BisGMA, UDMA, TEGDMA monomer mixture was made light-curing with CQ and/or PPD plus 0.2 wt.% N,N-cyanoethyl-methylaniline (CEMA). Seventeen groups, three specimens each, were tested in which the concentrations of PPD and CQ were varied. The effect of photosensitizer type (CQ or PPD) and ratio (PPD/CQ) on degree of conversion (DC) was investigated using FTIR spectrophotometry. The results were analyzed by ANOVA and Student-Newman-Keuls' multiple range comparison. The absorption spectra of PPD and CQ were recorded by UV-Vis spectrophotometry. A blind color comparison of specimens containing combinations of the two photosensitizers was also undertaken. RESULTS: Alone, PPD induces a DC which is not significantly different from that of CQ alone. In combination, CQ + PPD produces a DC that generally exceeds that produced by the same concentration of either used alone. The maximum DC occurs between PPD/CQ = 1:1 and 1:4. At total photosensitizer concentrations above 1.8 wt.%, DC is increased by PPD but depressed by CQ, which is evidence that different mechanisms are involved. PPD (lambda max approximately 410 nm) and CQ (lambda max approximately 468 nm) have different wavelength absorption ranges, resulting in a perceptibly lighter shade of yellow for PPD. SIGNIFICANCE: 1-phenyl-1,2-propanedione is a photosensitizer of potential value in reducing color problems associated with visible light cured dental resins. In combination with camphorquinone, it acts synergistically to produce a more efficient photoinitiation reaction.

Directly placed esthetic restorative materials--the continuum.

Because new restorative materials have had little clinical testing, it is difficult to make specific material recommendations for the esthetic restoration of carious teeth. Although fluoride-releasing materials have long been used successfully to restore carious teeth, little clinical documentation has been presented to support their use to inhibit recurrent caries, and their use as an effective restorative material may be questioned. Glass ionomers, compomers, and resin-modified glass ionomers are esthetic fluoride-releasing materials designed to restore teeth by bonding to tooth structure. This article describes the continuum of directly placed esthetic dental restorative materials, the efficacy of amalgam replacement restorative materials, and the role that fluoride-releasing materials may play in the inhibition of recurrent caries in vitro and in vivo.

Thermomechanical investigation of poly(methylmethacrylate) containing an organobismuth radiopacifying additive.

Previously we demonstrated the feasibility of using up to 24% triphenylbismuth (TPB) as a radiopaque, monomer-miscible additive for dental acrylic resins. In this study we examined the influence of TPB on thermomechanical properties of a representative polymethylmethacrylate (PMMA) ambient-cured resin used for temporary dental crowns and bridges. TPB (0%, 5%, 15% or 30% w/w) was dissolved in the monomer component, added to the powder component, and allowed to cure in rectangular molds. After 1 h they were either stored at 23 degrees C for 23 h, or heated for 5 min at either 40 degrees C or 50 degrees C, and then stored for 23 h. They were then scanned from -10 degrees to 125 degrees C in a dynamic mechanical thermal analyzer using the three-point bending mode of deformation at 1-Hz frequency. The onset to the glass-transition temperature (Tg) is decreased by 13 degrees to 32 degrees C by addition of TPB, while the storage modulus (E') at 25 degrees C is either unchanged or is slightly increased. TPB did not interfere with the curing reaction, and postcure heating at 40 degrees C had no effect on either E' or Tg. However, heating at 50 degrees C generally increased Tg but had very little effect on E' throughout the 0-50 degree C operating temperature range. TPB crystals were observed to have precipitated at TPB levels above 8%. These crystals, dispersed throughout the PMMA, act as reinforcing fillers. This reinforcement can account for the lack of a decrease in E', as would be expected if TPB had a plasticizing effect below Tg. However, even at 5%, a concentration at which all the TPB remains dissolved in the solid polymer, no decrease in E' was observed. This implies that TPB exerts an antiplasticizing effect at temperatures below 50 degrees C, possibly by occupying free volume among the polymer chains. It is concluded that TPB, in amounts adequate to impart diagnostic levels of radiopacity, is unlikely to adversely affect the clinical utility of PMMA-based dental acrylic resins.

Development of a radiopaque, autopolymerizing dental acrylic resin.

PURPOSE: Current prosthetic acrylic resins are radiolucent and cannot be imaged using standard radiographic techniques. If accidentally impacted or ingested, delays in localizing or removing the foreign body may be life-threatening. The purpose of this study was to evaluate the influence of an experimental radiopaque additive, triphenyl bismuth (TPB), on polymethyl methacrylate resins formulated for dental use. We also investigated methods to improve TPB-containing resin microbeads and optimize processing variables for specimen fabrication in autopolymerizing resin systems. MATERIALS AND METHODS: Selfcured samples of experimental resins were prepared containing 0% to 27% TPB and were tested according to American National Standards Institute/American Dental Association and International Organization for Standardization performance standards. A control group and two commercial provisional crown and bridge resins were used for comparison. RESULTS: The standard of radiopacity (> or = aluminum radiopacity) is met at TPB levels of > or = 14.5%. The control resin had a greater transverse deflection compared with the TPB-resin groups, but deflection was within standard limits for all resins. Polishability, color stability, and solubility were unaffected by TPB, whereas sorption decreased, although not significantly, at higher TPB levels. Translucency decreased at 27% TPB, and specimens containing 0% to 20% TPB were transparent. A tendency to entrain air bubbles, because of the hydrophobicity of TPB, resulted in increased susceptibility to brittle failure at the higher TPB levels. Solubility slightly exceeded American Dental Association standards for all TPB-resins and the control. All other performance standards were acceptable for resins containing 0% to 20% TPB. CONCLUSIONS: At concentrations that provide a diagnostic level of radiopacity, TPB does not significantly alter required performance and processing properties. Thus, TPB is capable of commercially acceptable performance as a radiopacifying additive for dental acrylics.

Effect of environmental stress and surface treatment on resin-to-metal bonding.

This study compares the bond strength and durability of three metal surface treatments subjected to two types of environmental stress for both short- and long-term exposures. The luting resins Panavia and Comspan were applied to alumina-blasted, non-beryllium, nickel-chromium alloy coupons. Metal surface treatments consisted of either microscopic roughening by electrochemical etching, or one of two types of adhesives: a silanated silica coating (Silicoating) or a phosphate ester monomer (a component in the Panavia liquid). Shear bond strength was determined following short- or long-term exposure to either thermocycling in 6-60 degrees C water (2,672 cycles/7 days or 10,584 cycles/42 days) or storage in 37 degrees C water (7 or 42 days). Three-way ANOVA showed that both the type of environmental stress and the exposure time affected the bond strength of electroetched surfaces, but that only exposure time affected the two chemical adhesives (P < 0.05), regardless of the environmental stress used. In the short-term, the silica/silane coated surfaces produced and maintained the higher shear bond strengths (15.9 +/- 2.3 MPa). However, after 42 days the silica/silane bonds decreased 30% (to 11.3 +/- 2.2 MPa), while the phosphate ester bonds were essentially unchanged (11.4 +/- 3.0 at 4 days, 10.4 +/- 2.2 MPa at 42 days). Electroetched bonds were the weakest and decreased by 18% between 7 and 42 days in water (8.8 +/- 1.2 to 7.2 +/- 3.0 MPa) and 27% after 42 days of thermocycling (7.2 +/- 2.8 to 5.3 +/- 1.8 MPa).

An in vitro and in vivo study of toothbrush bristle splaying.

The extent to which a toothbrush is worn out, as evidenced by bristle splaying, is an important determinant of cleaning efficacy. In order for this aspect of oral hygiene to be studied, an accelerated means of producing a life-like pattern of bristle splaying was investigated and compared with splaying induced by a human test-panel. Porcelain denture teeth were brushed under a static applied load in a slurry of dentifrice, by means of a revolving "figure 8" motion, for up to 12,000 revolutions. This was found to induce an increase in the degree of splaying (as measured by "wear index") that was well-fitted (R2 = 0.96) by a second-order expression of the form: WI = WIi + Ritj - Ctj2 where WI = wear index, WIi = initial wear index, Ri = initial rate of increase in WI, tj = brushing time measured in either machine revolutions (tr) or weeks of human use (tw), and C = a separate constant for each brushing mode (machine or human panel). The rate of splaying was found to be strongly influenced by the quality of the bristle filament, but not by small differences in toothbrush design. The splaying pattern induced by the machine could not be distinguished visually from that in a set of brushes used at home by human volunteers. A strong correlation was found between the splaying produced by machine-brushing and by a panel who brushed free-style at home for 13 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo biocompatibility of an acrylic, fluoride-releasing, anion-exchange resin.

This study evaluated the biocompatibility of an unfilled, fluoride-releasing acrylic resin by subcutaneous implantation in guinea pigs. The experimental fluoride resin was compared to a nonfluoride, dental pit and fissure sealant (DELTON) of similar composition. Thirty-four male albino guinea pigs received four Teflon tubes each, implanted in the dorsal area. The tubes were open at both ends, three contained the experimental fluoride resin and one held the nonfluoride resin. The tubes and surrounding tissue were excised in 1-2 cm blocks, by necropsy, at 14 and 84 days. Histological evaluation showed that inflammatory response was none-to-slight at 14 days for 95% of the fluoride and 100% of the nonfluoride specimens. Five percent of the fluoride specimens produced a moderate tissue response. At 84 days, inflammatory response was none-to-slight for 82.5% of the fluoride and 61.5% of the commercial nonfluoride specimens, while 17.5% of the fluoride and 38.5% of the nonfluoride specimens produced moderate tissue responses. Chi-squared analysis and Fisher's Exact test revealed no statistically significant difference (p less than or equal to 0.05) in tissue response between the two resins at either 14 or 84 days. Hence it is concluded that the experimental, fluoride-releasing resin produces a very mild subcutaneous tissue response and that its biocompatibility is comparable to that of a widely used nonfluoride dental resin. It can, therefore, be considered as having a high potential for biological safety as a dental restorative resin or adhesive, or for other biomedical applications.

Bristle end-rounding in children's toothbrushes: a comparative study.

Animal and clinical studies have shown that sharp, unpolished toothbrush bristles can injure gingival tissues and that substantial variation exists in the degree of end-roundness of commercially available toothbrushes. In this study, eight brands of children's toothbrushes were assessed for their relative potential to cause oral tissue injury based upon the degree of end-roundness of their bristles. The brands studied were: Blend-a-dent Jr., Butler GUM Jr., Colgate Plus Jr., Johnson & Johnson Prevent Jr., Johnson & Johnson Reach Child, Lever Bros. Disney, Oral-B P20, and Sensodyne Jr. Brushes were ranked on the basis of three criteria: the percentage of bristles with sharp ends (89% roundness or less), the percentage of bristles with smooth edges (96% roundness or greater), and the average roundness of bristle ends. The Oral-B P20 brush had significantly more (p < 0.05) bristles with a roundness of 96% or greater; it also was in the group having the lowest number of bristles with sharp edges and had the highest average roundness. Based on these criteria, the Oral-B P20 showed the least potential for oral tissue damage, whereas the Johnson & Johnson Reach Child and Butler GUM Jr. showed the most potential for tissue injury. The other brands investigated were intermediate in end-rounding values and damage potential.

An electron microscopic comparison of bristle end-rounding of three commercial toothbrushes.

The results from three separate examinations using SEM photomicrographs and shape factor (SF) measurements show that Oral-B P-35 brushes have a highly significant difference in bristle end-rounding when compared to Colgate Precision and Crest Complete toothbrushes. The proportion of Oral-B P-35 bristles with highly rounded ends was substantially greater than that for Colgate Precision and Crest Complete toothbrushes. Oral-B also had fewer bristles in the very sharp-edged category than both Complete and Precision toothbrushes. Each toothbrush sample in these studies was purchased from geographically dispersed retail outlets throughout the United States. Scanning electron micrographs were made from a representative random sample of bristles, then digitized and evaluated using image analysis software. The comparisons were made on the basis of the number and distribution of bristles having varying degrees of roundness as determined by shape-factor analysis. Previous studies asserted that low shape-factor values indicate sharp bristle edges that are less conducive to good oral hygiene than end-rounded bristles with high SF values. In separate studies, Oral-B P-35 had a significantly higher number of highly end-rounded bristles than did either the Colgate Precision or Crest Complete toothbrushes. The averages for end-roundness fall in the order of Oral-B P-35 > Complete > Precision. Thus we conclude that the potential for harming dental tissues is less for the Oral-B P-35 toothbrush than for either the Colgate Precision or Crest Complete toothbrushes.

Cytotoxicity evaluation of a new radiopaque resin additive--triphenyl bismuth.

Triphenyl bismuth (Ph3Bi) is a promising new additive for making biomedical resins visible on x-ray images. We evaluated the cytotoxicity of Ph3Bi, both alone and as a component of a denture resin, as an initial step in determining its biocompatibility. These experimental materials were compared with several types of dental materials that are in current clinical use (PMMA denture acrylic resin, two photo-cured sealants, and two glass-ionomer cements). Human embryonic lung fibroblast tissue cultures (WI-38 cells) were exposed to 24-hour aqueous extracts of the materials. Changes in cell growth, cell viability, and the visual appearance of cells were used for the assessment of toxic response. Only a slight degree of cytotoxicity was observed for Ph3Bi, both alone and in combination with self-cured PMMA. All clinical materials showed a higher level of cytotoxicity than did Ph3Bi. The sealants and cements exhibited the most cytotoxicity and PMMA acrylic the least. The cytotoxicity of PMMA was elevated slightly by inclusion of Ph3Bi, probably due to decreased monomer conversion. When stored in water, the already low levels of cytotoxicity of both PMMA and PMMA with added Ph3Bi were reduced even further. From these results, we can predict a high level of safety for Ph3Bi as a radiopaque additive for biomedical resins. Any toxicity associated with Ph3Bi-containing resins can be reduced or avoided by prior extraction. Alternatively, curing conditions can be selected that would drive the polymerization reaction to a higher level of conversion.

Preventive dental materials: sustained delivery of fluoride and other therapeutic agents.

In the 1940's, it was observed that silicate restorative cements were seldom associated with secondary caries. Fissure sealants, designed to protect caries-prone surfaces, appeared in the 1960's. The 1970's and 1980's have seen an increasing emphasis on the use of dental materials for preventive purposes. Glass ionomers, the modern version of silicates, release fluoride and have been available for some time. Polymeric materials that release various therapeutic agents are currently under development for use in topical anesthetic, endodontic, prosthodontic, and periodontal applications. An attachable membrane-reservoir device for oral fluoride delivery will soon be available for general clinical use. Several fluoride-releasing filling and adhesive resins have recently been marketed. These materials are the leading edge of a new class of preventive materials that serve as controlled-release and/or site-specific sources of therapeutic agents. This trend is expected to continue, and the near future should see the introduction of additional oral sustained-delivery systems for use in periodontal and other applications.

Radiopaque acrylic resins containing miscible heavy-metal compounds.

Radiopacity is needed in order to facilitate diagnosis of polymeric appliances, which may be dislodged and become impacted in the upper respiratory or digestive tracts. In order for a stable, optically transparent, radiopaque material to be provided, heavy-metal compounds were investigated which we had previously shown to form homogeneous structures with methyl methacrylate-based systems. It was found that, when present in PMMA at 11 to 14%, several compounds of either bismuth or uranium or 35% of an organo-zirconium compound impart radiopacity equivalent to that of aluminum. A low level of cytotoxicity and lack of mutagenicity indicated that a high level of biocompatibility can be expected. Processing characteristics are somewhat altered, but formulations satisfactory for use in various dental devices were found.

A mathematical model for predicting toothbrush stiffness.

Both cleaning efficiency and gingival damage depend on the access of toothbrush bristles to sheltered areas and their ability to deliver sufficient force to remove plaque as they travel over tooth surfaces. A mathematical expression was therefore developed which relates bristle properties and features of brush construction to overall brush stiffness, in order to provide a framework for the prediction and systematic investigation of brush performance: Brush stiffness = 0.125E(DBDT)2NTPf/L3. This shows that brush stiffness is predominantly affected by bristle modulus (E), bristle and tuft diameter (DB and DT), the number of tufts (NT), the number of bristles per unit area packed into a tuft hole (Pf = packing factor), and the trim length of bristles (L). Bristle composition and shape had no measurable effect. The 0.125 factor is empirically derived and probably depends on visco-elastic, frictional, and other dynamic effects which were not examined. Thus, this is a first-order approximation, and further work must be done to account for bristle interactions and brushing rate, and to correlate stiffness with a measure of cleaning efficiency.

The measurement of toothbrush wear.

Toothbrushes wear out and progressively lose their ability to clean as the bristles abrade and become curled and matted. To study the factors that influence this process, we developed a quantitative measure of bristle splaying (wear index) and a method for judging and ranking the overall state of brush deterioration (wear rating) and used these to determine the effects of the individual user, brush design, time in use, and bristle material. Wear index is the average increase in brush-head dimensions normalized for maximum bristle length, and is dimensionless. Wear rating is a subjective means of classifying the increasing severity of deterioration on a scale of 0 to 3. The wear characteristics of a variety of commercial and experimental brushes with different design features were determined. Test variables were time in use, brush design (e.g., geometry and size of the brush head), and bristle composition. Time of use, the individual user, and bristle composition were found to have the strongest influences on splaying, and brush design was found to have the least influence. The wear index offers a quantitative means of comparing brushes of different dimensions at various stages of splaying. The wear rating, although qualitative, is a quick means of ranking brushes in various stages of deterioration. The two methods correlate (R2 = 0.87), and both are sensitive to several factors that affect brush durability. Therefore, these methods appear to be suitable not only for research, but also for quality control, the setting of standards, and for substantiation of advertising claims.