Takeshi Muguruma

Torsional properties and microstructures of miniscrew implants

INTRODUCTION Titanium miniscrew implants are popular in orthodontics, but there is little information about their torsional performance. METHODS Four brands of miniscrew implants (A-D) with 1.6-mm diameters were compared, with miniscrew A implants also having diameters of 1.2 to 2.0 mm. Nominal compositions of the implants were determined by x-ray fluorescence (n = 8). The miniscrews were loaded to failure in torsion, and the mean moment and twist angle were determined for each group (n = 8). Data were compared by ANOVA and the Tukey multiple range tests. Micro x-ray diffraction (n = 3) was used to identify phases in the implants, and the phases were also examined in etched cross-sections with a scanning electron microscope. RESULTS Miniscrew A and C implants were pure titanium, whereas miniscrew B and D implants contained small amounts of vanadium, aluminum, iron, and manganese. Only alpha-titanium peaks were detected for all implants by micro x-ray diffraction, but beta titanium was observed in the microstructures of miniscrew B and D implants, which had significantly higher torsional moments at failure. CONCLUSIONS Addition of small amounts of other elements to titanium yielded significantly improved torsional performance for miniscrew implants. Research to develop optimum compositions for mechanical properties and biocompatibility is needed.

Effects of long-term storage and thermocycling on bond strength of two self-etching primer adhesive systems

The effects of 2 years of storage and 6000 thermocycles on the shear bond strength (SBS) of two self-etching adhesive systems were studied. Two self-etching primer (SEP) systems (Transbond Plus and Beauty Ortho Bond) and one etch and rinse system (Transbond XT) were used to bond brackets to 126 human premolars that were then stored in artificial saliva for 24 hours or 2 years and thermocycled in distilled water before SBS testing with a universal testing machine. The adhesive remnant index (ARI) scores were calculated. Data were compared by two-way analysis of variance and chi-square analysis. Enamel/adhesive interfaces were examined by scanning electron microscopy. There was no significant difference in the mean SBS for the bonding materials among the three conditions. ARI scores showed that Transbond XT and Beauty Ortho Bond had less adhesive remaining on the teeth after ageing compared with storage for 24 hours. Specimens bonded with Beauty Ortho Bond showed leakage between the resin adhesive and enamel after ageing. Both SEP systems produced adequate SBS even after 2 years or 6000 times thermocycling. Thermocycling is an appropriate technique for determining the durability of orthodontic bracket bonding materials.

Comparisons of nanoindentation, 3-point bending, and tension tests for orthodontic wires

INTRODUCTION The purposes of this study were to obtain information about mechanical properties with the nanoindentation test for representative wire alloys and compare the results with conventional mechanical tests. METHODS Archwires having 0.016 × 0.022-in cross sections were obtained of 1 stainless steel, 1 cobalt-chromium-nickel, 1 beta-titanium alloy, and 2 nickel-titanium products. Specimens of as-received wires were subjected to nanoindentation testing along the external surfaces and over polished cross sections to obtain values of hardness and elastic modulus. Other specimens of as-received wires were subjected to Vickers hardness, 3-point bending, and tension tests. All testing was performed at 25°C. RESULTS Differences were found in hardness and elastic modulus obtained with the nanoindentation test at the external and cross-sectioned surfaces and with the conventional mechanical-property tests. Mechanical properties obtained with the nanoindentation test generally varied with indentation depth. CONCLUSIONS The 3 testing methods did not yield identical values of hardness and elastic modulus, although the order among the 5 wire products was the same. Variations in results for the nanoindentation and conventional mechanical property tests can be attributed to the different material volumes sampled, different work-hardening levels, and an oxide layer on the wire surface.

Effects of a diamond-like carbon coating on the frictional properties of orthodontic wires

OBJECTIVE To test the hypothesis that a diamond-like carbon coating does not affect the frictional properties of orthodontic wires. MATERIALS AND METHODS Two types of wires (nickel-titanium and stainless steel) were used, and diamond-like carbon (DLC) films were deposited on the wires. Three types of brackets, a conventional stainless steel bracket and two self-ligating brackets, were used for measuring static friction. DLC layers were observed by three-dimensional scanning electron microscopy (3D-SEM), and the surface roughness was measured. Hardness and elastic modulus were obtained by nanoindentation testing. Frictional forces and surface roughness were compared by the Kruskal-Wallis and Mann-Whitney U-tests. The hardness and elastic modulus of the wires were compared using Students t-test. RESULTS When angulation was increased, the DLC-coated wires showed significantly less frictional force than the as-received wires, except for some wire/bracket combinations. Thin DLC layers were observed on the wire surfaces by SEM. As-received and DLC-coated wires had similar surface morphologies, and the DLC-coating process did not affect the surface roughness. The hardness of the surface layer of the DLC-coated wires was much higher than for the as-received wires. The elastic modulus of the surface layer of the DLC-coated stainless steel wire was less than that of the as-received stainless steel wire, whereas similar values were found for the nickel-titanium wires. CONCLUSIONS The hypothesis is rejected. A DLC-coating process does reduce the frictional force.

Effect of coating on properties of esthetic orthodontic nickel-titanium wires

OBJECTIVE To determine the effect of coating on the properties of two esthetic orthodontic nickel-titanium wires. MATERIALS AND METHODS Woowa (polymer coating; Dany Harvest) and BioForce High Aesthetic Archwire (metal coating; Dentsply GAC) with cross-section dimensions of 0.016 × 0.022 inches were selected. Noncoated posterior regions of the anterior-coated Woowa and uncoated Sentalloy were used for comparison. Nominal coating compositions were determined by x-ray fluorescence (JSX-3200, JOEL). Cross-sectioned and external surfaces were observed with a scanning electron microscope (SEM; SSX-550, Shimadzu) and an atomic force microscope (SPM-9500J2, Shimadzu). A three-point bending test (12-mm span) was carried out using a universal testing machine (EZ Test, Shimadzu). Hardness and elastic modulus of external and cross-sectioned surfaces were obtained by nanoindentation (ENT-1100a, Elionix; n  =  10). RESULTS Coatings on Woowa and BioForce High Aesthetic Archwire contained 41% silver and 14% gold, respectively. The coating thickness on Woowa was approximately 10 µm, and the coating thickness on BioForce High Aesthetic Archwire was much smaller. The surfaces of both coated wires were rougher than the noncoated wires. Woowa showed a higher mean unloading force than the noncoated Woowa, although BioForce High Aesthetic Archwire showed a lower mean unloading force than Sentalloy. While cross-sectional surfaces of all wires had similar hardness and elastic modulus, values for the external surface of Woowa were smaller than for the other wires. CONCLUSIONS The coating processes for Woowa and BioForce High Aesthetic Archwire influence bending behavior and surface morphology.

Effects of mechanical properties of thermoplastic materials on the initial force of thermoplastic appliances

OBJECTIVE To measure the forces delivered by thermoplastic appliances made from three materials and investigate effects of mechanical properties, material thickness, and amount of activation on orthodontic forces. MATERIALS AND METHODS Three thermoplastic materials, Duran (Scheu Dental), Erkodur (Erkodent Erich Kopp GmbH), and Hardcast (Scheu Dental), with two different thicknesses were selected. Values of elastic modulus and hardness were obtained from nanoindentation measurements at 28°C. A custom-fabricated system with a force sensor was employed to obtain measurements of in vitro force delivered by the thermoplastic appliances for 0.5-mm and 1.0-mm activation for bodily tooth movement. Experimental results were subjected to several statistical analyses. RESULTS Hardcast had significantly lower elastic modulus and hardness than Duran and Erkodur, whose properties were not significantly different. Appliances fabricated from thicker material (0.75 mm or 0.8 mm) always produced significantly greater force than those fabricated from thinner material (0.4 mm or 0.5 mm). Appliances with 1.0-mm activation produced significantly lower force than those with 0.5-mm activation, except for 0.4-mm thick Hardcast appliances. A strong correlation was found between mechanical properties of the thermoplastic materials and force produced by the appliances. CONCLUSIONS Orthodontic forces delivered by thermoplastic appliances depend on the material, thickness, and amount of activation. Mechanical properties of the polymers obtained by nanoindentation testing are predictive of force delivery by these appliances.

Effect of bracket bonding on nanomechanical properties of enamel

INTRODUCTION In this study, we investigated the nanohardness and elastic modulus of enamel after debonding metal brackets. METHODS The surfaces of 3 maxillary premolars were subdivided into 3 regions. Two regions were exposed to a conventional etching system (Transbond XT, 3M Unitek, Monrovia, Calif) and a self-etching system (Transbond Plus primer, 3M Unitek); the third region was not etched. Metal brackets were bonded with Transbond XT composite to the 2 etched regions. After storage for 24 hours in distilled water, the brackets and residual adhesive were removed, and the teeth were sectioned transversely. Seven nanoindentations (2 mN load) were placed 1 to 25 μm from the surface in each region. Mean nanohardness and elastic modulus were compared with analysis of variance (ANOVA) and the Scheffé test. RESULTS Locations 1 and 5 μm from the enamel surface had significantly (P < 0.05) lower nanohardness and elastic modulus values for the conventional system compared with the self-etching system and the unetched region. All other locations for the conventional system and all locations for the self-etching system and unetched area had no significant differences. The nanohardness was much higher than the Vickers hardness for enamel. CONCLUSIONS The minimal effect of the self-etching system on the nanomechanical properties of enamel arises from much lower chemical attack. The much greater effects of the conventional system require further study.

Effects of bonding materials on the mechanical properties of enamel around orthodontic brackets

OBJECTIVE To determine if the enamel around orthodontic brackets is significantly altered after demineralization followed by application of adhesives with and without fluoride-releasing ability. MATERIALS AND METHODS One hundred eight noncarious human premolars were divided into six groups of 18 each and exposed to a demineralization solution. Stainless steel brackets were bonded using two conventional composite resin etch-and-rinse systems, three self-etching primer (SEP) composite resin systems, and one resin-modified glass ionomer cement (RMGIC) system. One conventional and one SEP composite resin adhesive did not have fluoride-releasing ability, which was claimed for the other four adhesives. The elastic modulus and hardness of the enamel were determined with a nanoindenter at 10 equidistant depths ranging from 1-46 µm and at four regions: control (not exposed) enamel surface, under the adhesive, and at 50 µm and 100 µm from the bracket edges. Using the Kruskal-Wallis and Mann-Whitney U-tests (P < .0125 for statistical significance), these properties were compared at different regions. RESULTS The same behavior was observed for values of elastic modulus and hardness. Significant differences were found within approximately 21 µm of the enamel surface for etching with 35% phosphoric acid or priming with SEP, but only minimal changes occurred for the SEP adhesive. Increases in near-surface elastic modulus and hardness of enamel were found with the SEP adhesive and RMGIC with fluoride-releasing ability. CONCLUSIONS Clinical use of the fluoride-releasing adhesives investigated may prevent demineralization of enamel around brackets during orthodontic treatment.

Effect of mechanical properties of fillers on the grindability of composite resin adhesives

INTRODUCTION The purpose of this study was to investigate the effect of filler properties on the grindability of composite resin adhesives. METHODS Six composite resin products were selected: Transbond XT (3M Unitek, Monrovia, Calif), Transbond Plus (3M Unitek), Enlight (Ormco, Glendora, Calif), Kurasper F (Kuraray Medical, Tokyo, Japan), Beauty Ortho Bond (Shofu, Kyoto, Japan), and Beauty Ortho Bond Salivatect (Shofu). Compositions and weight fractions of fillers were determined by x-ray fluorescence analysis and ash test, respectively. The polished surface of each resin specimen was examined with a scanning electron microscope. Vickers hardness of plate specimens (15 × 10 × 3 mm) was measured, and nano-indentation was performed on large filler particles (>10 μm). Grindability for a low-speed tungsten-carbide bur was estimated. Data were compared with anlaysis of variance (ANOVA) and the Tukey multiple range test. Relationships among grindability, filler content, filler nano-indentation hardness (nano-hardness), filler elastic modulus, and Vickers hardness of the composite resins were investigated with the Pearson correlation coefficient test. RESULTS Morphology and filler size of these adhesives showed great variations. The products could be divided into 2 groups, based on composition, which affected grindability. Vickers hardness of the adhesives did not correlate (r = 0.140) with filler nano-hardness, which showed a significant negative correlation (r = -0.664) with grindability. CONCLUSIONS Filler nano-hardness greatly influences the grindability of composite resin adhesives.

Effects of CO2 laser debonding of a ceramic bracket on the mechanical properties of enamel

OBJECTIVE To investigate the effects of CO(2) laser debonding of a ceramic bracket on the mechanical properties of tooth enamel. MATERIALS AND METHODS Fifty-three human premolars were used in this study. The temperature changes of cross-sectioned specimens during laser irradiation were monitored with an infrared thermographic microscope system. Different laser output settings (3, 4, 5, and 6 W) were compared. The shear bond strength of brackets after laser irradiation was measured for specimens bonded with a conventional etch and rinse adhesive or with a self-etching adhesive, and the adhesive remnant index score was calculated. The hardness and elastic modulus of cross-sectioned enamel after laser irradiation were investigated by the nanoindentation test. Data were compared by one-way and two-way analysis of variance, followed by the Scheffé test. RESULTS The temperature of enamel increased by about 200 degrees C under CO(2) laser irradiation with a relatively high output (5 and 6 W), and a temperature increase of about 100 degrees C to 150 degrees C was seen under laser irradiation with a low output (3 and 4 W). The bracket shear bond strength decreased under all laser irradiation conditions. The hardness and elastic modulus of enamel were not affected by CO(2) laser debonding. CONCLUSION CO(2) laser debonding may not cause iatrogenic damage to enamel.