Static-dynamic friction transition of FRP esthetic orthodontic wires on various brackets by suspension-type friction test.

A new testing apparatus for the measurement of frictional properties was designed and the frictional coefficients were obtained and compared with each other in various combinations of brackets and orthodontic wires, including esthetic fiber-reinforced plastic (FRP) wire that was especially designed and manufactured. Three kinds of wires (stainless steel, nickel-titanium, and FRP) and four brackets (single-crystal alumina, polycrystalline alumina, polycarbonate, and stainless steel) were used. The testing was done under dry and wet conditions. The friction testing equipment was designed to attach the bracket to a C-shaped bar suspended with a variable mass, and sliding along a fixed wire. The transition between static and dynamic friction was measured as a breakaway force, with the use of a universal test machine. In addition to material properties, this testing fixture eliminates geometrical factors, such as the rotational moment at the edge of the bracket slot, deflection of the orthodontic wire, and tension of the ligature wire. Nearly ideal frictional properties between materials are obtained. The frictional properties of FRP wire were similar to those of metal wires on all brackets, except the polycrystalline alumina bracket. The frictional coefficient between the polycrystalline ceramic bracket and FRP wire was larger than that of other combinations. There was little difference in frictional coefficients between dry and wet conditions.

Visualization and detectability of elements rarely contained in soft tissue by X-ray scanning analytical microscopy and electron-probe micro analysis.

The detectability of elements rarely contained in soft tissue was compared using X-ray scanning analytical microscope (XSAM) and electron-probe micro analysis (EPMA). Mapping images of Ca, S and P in normal soft tissue of the rat and dissolved Ni in Ni implanted soft tissue could be obtained by XSAM and EPMA. EPMA was more sensitive in detection of P, while XSAM was superior for Ca, S and Ni mapping. The high detectability for heavier elements by XSAM was explained by the large volume of characteristic X-ray generation in XSAM and low attenuation of the characteristic X-rays from heavier elements. XSAM could provide clearer mapping images for heavier elements whose concentration was low without radiation damage to specimens.

Biocompatibility and osteogenesis of refractory metal implants, titanium, hafnium, niobium, tantalum and rhenium.

To evaluate the biocompatibility of refractory metals, titanium, hafnium, niobium, tantalum and rhenium were implanted in rats, and histological observation and elemental mapping were performed by X-ray scanning analytical microscope (XSAM) and electron probe microanalyzer (EPMA). The titanium, hafnium, niobium, tantalum and rhenium wires were implanted in the subcutaneous tissue of the abdominal region and in femoral bone marrow of rats for either 2 or 4 weeks. No inflammatory response was observed around the implants, and all the implants were encapsulated with thin fibrous connective tissue. No dissolution of these metals was detected by XSAM in the soft tissue. Histological examination of the hard tissue showed that the amount of new bone formation decreased slightly from the second to the fourth week after implantation, and that the percentage of bone in contact with the implant increased markedly over the same period. No dissolution of these metals was detected by EPMA in the hard tissue. The Ca and P intensities in the mapping images of newly formed bone were higher after 4 weeks than those after 2 weeks, which suggests that the newly formed bone continued to mature from 2 to 4 weeks after implantation. These results indicate that titanium, hafnium, niobium, tantalum and rhenium have good biocompatibility and osteoconductivity.

Tissue reaction around metal implants observed by X-ray scanning analytical microscopy.

The soft tissues implanted with Cu, Ni, Fe, Ag, Ti, Ni-Ti, SUS304 and SUS316 wires were investigated with XSAM and compared with histological observation. The relationship between the distribution of dissolved metal elements and the tissue response was evaluated. Of the metals whose dissolution was clearly observed by XSAM, severe tissue damage was observed around Ni and Cu implants, while fibrous connective tissue was formed around the Fe implant. The concentration in surrounding tissue was estimated by XSAM using the newly prepared standard specimens. The dissolved concentration was approximately 10-20 mm for Ni and Cu and was considered to be in the order of ten times higher in Fe. The results indicated that the toxicity at the same concentration was from greater Ni > Cu > Fe. For Ag, Ti, Ni-Ti, SUS304 and SUS316 implants, significant dissolution and severe tissue damage were not observed. The XSAM was especially useful to obtain the information of dissolution and distribution behavior of rare content of toxic and chemically unstable metals in the soft tissue.

Compositional and morphological imaging of CO2 laser irradiated human teeth by low vacuum SEM, confocal laser scanning microscopy and atomic force microscopy.

Enamel and dentin of human teeth irradiated by CO2 laser were investigated by confocal laser scanning microscopy (CLSM), low vacuum scanning electron microscopy (WET-SEM) and atomic force microscopy (AFM). Optical tomographic imaging by CLSM, compositional imaging based on atomic number effect of reflected electrons by WET-SEM, high resolution observation of surface morphology by AFM were done for both the irradiated and non-irradiated area of the same specimen throughout. The crystals of about 50 microm length and the bright spots were observed by CLSM at the bottom of the cavity induced by laser irradiation. They turned out from the observation by WET-SEM as the acicular crystals with the cross section of an irregularly hexagonal shape situated parallel and perpendicular, respectively, to the inner surface of the cavity. The thickness of the thermally deteriorated zone of the cavity was about 25 microm. The crystals unidirectionally grown up to the size of several hundreds nm were also observed by AFM, while the apatite crystallites of 50-150 nm were recognized all over in non-irradiated area. All the results suggest that after instantaneous melting at the surface of teeth by CO2 laser shot the crystals of calcium phosphate were recrystalized and grown to a large size. The compositional imaging in addition to morphological observation was useful to obtain the information of the change in materials induced by laser irradiation.

Fabrication of a functionally graded dental composite resin post and core by laser lithography and finite element analysis of its stress relaxation effect on tooth root.

Laser lithography was applied for Computer Aided Design and Computer Aided Manufacturing (CAD/CAM) fabrication of dental prostheses made of composite resin. First, the conditions to obtain the optimum resolution for photo-curing were determined, and then a composite resin full crown was fabricated by laser lithography. Second, a functionally graded composite resin post and core which had gradient elasticity in the post, was manufactured by the gradual change in the filler contents of the composite resin. Finally, stress analysis of the functionally graded post and core was performed by a two-dimensional finite element method. This demonstrated the effects of reducing the stress concentration around the apex of the post.

Radiation effects of carbon ions and gamma ray on UDMA based dental resin.

The radiation effects on the mechanical and physical properties of photo-polymerized UDMA resin without filler was investigated by various mechanical tests and spectroscopic measurements. The radiation sources were carbon ion (12C ion) and gamma ray (gamma-ray). With 640 Gy of 12C ion radiation, Vickers hardness increased by about 40%, the degree of abrasion decreased by 30%, and the flexural strength increased by 20%. With the same dose of gamma-ray radiation, only Vickers hardness increased by 19%. The spectra taken by the Fourier Transform Infrared Spectrometer (FT-IR), Raman spectroscopy, and Fluorescence spectrophotometer showed little change in the peak configuration and background intensities. The relative degree of conversion (DC) of carbon double bonds by radiation to the state of non-radiated samples were estimated by FT-IR. Spectroscopic results were indicative for the formation of cross-linking between carbon chains. Cross-linking of carbon molecules induced by radiation might be one of the reasons for the improved mechanical properties of UDMA resin.

In-situ etching observation of human teeth in acid agent by atomic force microscopy.

Atomic force microscopy was applied to the in-situ observation of the etching process of human teeth by acid agents. The change of surface morphology was observed consecutively before and during etching for the same area in the same specimen. The course of the etching process in enamel from dissolution of smear layer just after injection of acid agent, appearance of enamel prisms and progress of demineralization were quantitatively analysed for three fundamental acid agents of 2% phosphoric acid, 10% citric acid and 10% polyacrylic acid. Then the depth profile, etching amount, etching rate and thickness of smear layer were evaluated. Observation by scanning electron microscopy was also done and compared with the results by atomic force microscopy.

Effects of water immersion on mechanical properties of new esthetic orthodontic wire.

New fiber-reinforced plastic orthodontic wire (FRP wire) was fabricated with polymethyl methacrylate (PMMA) for the matrix and biocompatible CaO-P(2)O(5)-SiO(2)-Al(2)O(3) (CPSA) glass fibers for fibers that have not only high esthetics but also mechanical properties similar to those of metal wires. The purpose of this study was to evaluate the effects of water immersion on the mechanical properties of this new wire. The fiber-reinforced plastic orthodontic wire specimens were 0.5 mm in diameter with 29.1% to 60. 4% volume fraction of fibers that were 20 microm in diameter. A three-point flexural test was performed to obtain the elastic modulus and flexural load at the deflection of 1 mm under dry and wet conditions. Stress relaxation of the wires was tested under dry and wet conditions, and the wire diameters were measured before and at 20 days after immersion. The results showed that there were changes in the elastic modulus up until 10 days and in flexural load up until 20 days after immersion. The values of these two at 30 days after immersion were 93% and 87%, respectively, of those before immersion. Stress relaxation occurred rapidly from the start of immersion until about 60 minutes under dry conditions and about 120 minutes under wet conditions and then approached saturation. The swelling of hydrated fiber-reinforced plastic orthodontic wires affected the wire diameter, although this effect was not significant. The results of this study therefore suggest that the mechanical properties of fiber-reinforced plastic orthodontic wires are reduced by water immersion in the initial stage.

Dissolution of nickel and tissue response observed by X-ray scanning analytical microscopy.

The recently developed X-ray scanning analytical microscope (XSAM) was applied for the analysis of the soft tissue of rat in which Ni was implanted and serious inflammation occurred. Our aim is to investigate the relationship between tissue response and elemental distributions. The XSAM observation was conducted in air without pretreatment and staining of samples. Mapping images of P, S, Ca, Fe and Ni, which were the elements with low concentration in soft tissue, were obtained from the soft tissue blocks of rat where Ni was implanted. S mapping image showed the localization of S in muscle and in hairs. Ca mapping image showed the localization of Ca in muscle and hair roots. Fe distribution was also observed and the localized area was consistent with the hemorrhagic area. The Ni dissolution area was clearly detected around the Ni implant. Comparison with the histological observation showed that the Ni dispersed area was consistent with the inflammatory area and the degree of tissue damage was closely related to the dissolved Ni concentration.

Temperature-dependence of the mechanical properties of FRP orthodontic wire.

The temperature-dependence of the mechanical properties of a new esthetic orthodontic wire with fiber-reinforced plastic (FRP) structure was investigated. The new FRP wire, fabricated by a hot drawing method, is 0.5 mm in diameter and has a multiple fiber structure composed of biocompatible CaO-P2O5-SiO2-Al2O3 glass fibers of 20 microns in diameter and a polymethyl methacrylate matrix. The flexural load at a deflection of 1 mm and Young's modulus at 24, 37, and 50 degrees C under wet conditions showed similar fiber fraction dependence to those under dry conditions for a fiber fraction of 40-51%. The flexural load and Young's modulus tended to decrease slightly with increases in temperature. This tendency was larger for the lower fiber fraction. However, the difference in flexural load for a temperature difference of between 24 degrees C and 50 degrees C was at most 10 gf. This is negligibly small, and a constant orthodontic force regarding temperature change would be advantageous from a clinical point of view.

Firing shrinkage of porcelain-resin composites prepared by laser lithography.

Using porcelain and resin-mixed composites as experimental materials, cubic polymerized composites were prepared by the accumulation of thin slices cured by laser scanning. The composites were then fired, and bulk ceramic bodies were made. The optimal firing conditions of polymerized composites and firing shrinkage were investigated. The results showed that cubic ceramic bodies in a form homologous to that before firing could be reproduced. The volume shrinkage of fired ceramic bodies consisting of 1 g of ceramic powders and 0.3 g of epoxy resin was about 30% under all firing conditions, and there were no significant differences between specimens. It was suggested that with further research and development, three-dimensional forms for clinical use in dentistry could be manufactured by the proposed method.

Properties and cytotoxicity of water soluble Na2O-CaO-P2O5 glasses.

Various compositions of Na2OCaO-P2O5 glasses are prepared to estimate glass formation, dissolution properties and cytotoxicity. In the wide composition range of 40 mol% of P2O5 or more, clear glass samples were obtained. The estimated glass forming region was consistent with other ternary phosphate glass systems. The glass transition temperatures and crystallization temperatures decreased with increasing P2O5 content and increased with CaO content. Dissolution properties in distilled water and simulated body fluid (SBF) were measured. In distilled water, CaO free glasses showed extremely fast dissolution. The dissolution rate decreased with increasing CaO content and decreasing P2O5 content. This composition effect results from cross-link formation between the non-bridging oxygens of two different chains by Ca2+ ions which improves the phosphate network strength. In SBF, the dissolution rate followed a similar trend, but glass dissolution was suppressed. This suppression occurred due to the existence of soluble species of glass such as Na+, Ca2+ and HPO(2-)4. The cytotoxicity decreased with increasing CaO content and with decreasing PO2.5 content. This was the result of a change in pH and ion concentration in the medium.

Mechanical properties and aesthetics of FRP orthodontic wire fabricated by hot drawing.

The FRP wires 0.5 mm in diameter with a multiple fiber structure were fabricated by drawing the fiber polymer complex at 250 degrees C for an esthetic, transparent orthodontic wire. Biocompatible CaO-P2O5-SiO2-Al2O3 (CPSA) glass fibers of 8-20 microm in diameter were oriented unidirectionally in the longitudinal direction in PMMA matrix. The mechanical properties were investigated by 3-point flexural test. The FRP wire showed sufficient strength and a very good elastic recovery after deformation. Young's modulus and the flexural load at deflection 1 mm were nearly independent of the fiber diameter and linearly increased with the fiber fraction. The dependence on fiber fraction obeys well the rule of mixture. This FRP wire could cover the range of strength corresponding to the conventional metal orthodontic wires from Ni-Ti used in the initial stage of orthodontic treatments to Co-Cr used in the final stage by changing the volume ratio of glass fibers with the same external diameter. The estheticity in external appearance was excellent. Thus the new FRP wire can satisfy both mechanical properties necessary for an orthodontic wire and enough estheticity, which was not possible for the conventional metal wire.

Compositions of surface layers formed on amalgams in air, water, and saline.

The surface layers formed on both a zinc-free and a zinc-containing dental amalgam after polishing and aging in air, water, or saline, were characterized using x-ray photoelectron spectroscopy (XPS) to determine the compositions of the surface layers which might govern the release of mercury from amalgam. The XPS data revealed that the formation of the surface layer on the zinc-containing amalgam was affected by the environment in which the amalgam was polished and aged, whereas that on the zinc-free amalgam was not affected. In addition, among the elements contained in amalgam, zinc was the most reactive with the environment, and was preferentially dissolved from amalgam into water or saline. Mercury atoms existed in the metallic state in the surface layer.

A new procedure of the tensile fatigue test for dental materials.

A new tensile fatigue test using a stainless steel screw was introduced. A transparent PMMA rod was selected as the test material. A hole was drilled and tapped into the rod; a screw was put into the rod with dental cement or without cementation. Three types of cyclic loads were applied to the test piece; the number of times the load was applied when the initial crack was observed and when the final fracture occurred were recorded. Surface observation showed that actual fatigue fractures occurred in the test piece. The preferred applied load waveform was the cyclic haver-triangles load because of the small coefficient of variance. Fatigue strengths of test pieces with and without cementation were different. Cementation improved the fatigue properties, and the type of cement had an effect on the fatigue properties. Although the number of final fractures minus the number of initial cracks was the constant regardless of the type of cement used.

[Casting accuracy and shrinkage of titanium castings made with zirconia investments].

To obtain inlay, crown and bridge types of titanium castings with sufficient size accuracy, zirconia investments with 10 mol % calcia were studied for their setting expansion, thermal expansion, total expansion, casting shrinkage compensation deficiency on fabrication method, particle size of zirconia, calcia and mixing liquid. Addition of calcia caused the change from contraction to expansion in setting and the additional thermal expansion at 900-1,200 degrees C. The choice of calcia and mixing liquid makes great influence on setting expansion, which is grouped into saturation type and delayed expansion type. By the use of the saturation type, total expansion could be controlled in the range of -2-(+)3%. Titanium castings of inlay, crown, bridge types with the values -2-(+)2% of casting shrinkage compensation deficiency could be successfully obtained. The casting shrinkage of titanium was estimated as 1.8-2%. The castings have the surface with a metallic brilliance as cast.

[Effect of addition of calcia on the properties of zirconia investments].

The zirconia investments with calcia were studied of the compressive strength, setting expansion, thermal expansion, total expansion, thermogravitometry, differential thermal analysis on fabrication method, concentration, particle size of zirconia, calcia and mixing liquid. Addition of calcia caused the change from contraction to expansion in setting expansion. The optimum concentration of calcia was determined to be 10 mol% as judged from total expansion, strength and operability.

[Setting characteristics of calcia-bonded investments by hydration].

Calcium oxide (CaO) slurry mixed with water hardens and expands by hydration. The authors used this setting characteristic as a binder of zirconia-refractory investments. If a mixed slurry was placed in air, unreacted residual calcia powders absorbed water and reverted to calcium hydroxide (Ca (OH)2). This resulted in a large setting expansion that continued for several days. A carbon dioxide (CO2) gas atmosphere controlled this expansion of calcia. When mixed slurry was placed in a carbon dioxide gas atmosphere, calcium carbonate (CaCO3) was formed and prevented the further hydration of unreacted calcium oxide. Thus setting expansion values were controlled. The combined expansion of setting and thermal expansion, in CaO-CO2-bonded zirconia investments, was about 1.5-2.5%. On the whole, calcium oxide is expected to be a good binder for high temperature dental casting refractory.

[High temperature reactivity between titanium and refractory oxides in dental casting procedure. Fundamental study on refractoriness of investments and ceramo-metal bonding by analytical SEM and oxide single crystals].

The reactivity of various refractory oxides (SiO2, Al2O3, MgO, CaO, ZrO2) with molten titanium was investigated by energy dispersive X-ray spectroscopy (EDS), using single crystals of oxide as specimen. The region of the reaction boundary between titanium and single crystal oxide was observed by electron microscopy (SEM). Elemental analysis across the Ti/oxide boundary was done by EDS and the reaction layer thickness was quantitatively determined. Of these refractory oxides, SiO2 is the most reactive with titanium followed by Al2O3. The 250 microns thick Si-rich region and 100 microns thick Al-rich region were formed respectively in Ti around the remnant oxides. The diffusion of Si as a result of reaction is greater than that of Al. MgO, CaO, ZrO2 show very little reaction with Ti and the reaction layer thickness was below the spatial resolution of EDS (approximately 1 micron). The thickness and composition of the reaction layer and reactivity were determined. These results were in good agreement with those obtained by ESCA.