Biomedical subjects

W M Johnston

Publications and source records attributed to W M Johnston.

At least 19 recordsLinked to original sources

PubMed2005

The effect of different adhesives on vinyl polysiloxane bond strength to two tray materials.

STATEMENT OF PROBLEM: Although stock trays often provide mechanical retention for elastomeric impression materials, manufacturers typically recommend the use of an adhesive, whether a stock or custom tray is used. Universal adhesives for vinyl polysiloxane (VPS) are used in practice, but comparative bond strengths have not been reported. PURPOSE: The aim of this study was to evaluate the bond of 3 VPS materials with a methylmethacrylate autopolymerizing and a light-polymerizing tray material, using the adhesive recommended by the manufacturer of the impression material, and 2 universal adhesives (paint-on and spray-on). MATERIAL AND METHODS: Ten specimens per group, for a total of 180 flat 15 x 15 x 20-mm specimens, were made from the 2 tray materials and finished to 320 grit with silicon carbide paper. Four paint-on adhesives (Coltene, Caulk, Kerr VPS, or universal VPS) and 1 spray-on adhesive (Sili Spray) were applied and dried following manufacturers' instructions or for 10 minutes. Three impression materials, Affinis, Aquasil, and Take I, were automixed and injected into a perforated cylinder positioned in a universal testing machine. Tray specimens were positioned against the open cylinder end in contact with the VPS material. Tensile tests (cross-head speed 5 mm/min) were conducted until adhesive separation failure. Mean values and standard errors of the adhesive strength were recorded in MPa for each material combination. Data were first analyzed with Welch analysis of variance (ANOVA) to determine overall significance, and then with a 3-way ANOVA and the Tukey HSD test to make pairwise comparisons, with alpha=.05 for all testing. RESULTS: For all impression materials tested, the universal spray-on adhesive consistently demonstrated significantly lower bond strengths than all other adhesives (P<.05). Equivalent or significantly (P<.05) higher bond strength values were found for the universal paint-on adhesive for the 3 impression materials tested. CONCLUSION: The use of GC paint-on universal adhesive provided significantly higher adhesive values than those obtained with the adhesives supplied by the manufacturers of the impression materials tested, with the exception of the Kerr impression and adhesive material combination where no significant differences were found.

Adhesives↗

PubMed2002

Electrochemical impedance spectroscopy study of high-palladium dental alloys. Part I: behavior at open-circuit potential.

Electrochemical impedance spectroscopy (EIS) was used to study the in vitro corrosion of three representative high-palladium alloys and a gold-palladium alloy for comparison. The corrosion resistances (measured as the charge transfer resistance R(CT) from an equivalent circuit) of the high-palladium alloys and the gold-palladium alloy were comparable in simulated body fluid and oral environments, and under simulated dental plaque. The great similarity in corrosion behavior for the three high-palladium alloys is largely attributed to their substantial palladium content and passivity in the laboratory test media, and possibly to their similar structure at the submicron level. Differences in composition and microstructure at the micron level and greater, including the effects of heat treatment simulating the firing cycles for dental porcelain, do not have noteworthy effects on the in vitro corrosion of the three high-palladium alloys. Good accuracy and convenience of extracting corrosion characteristics from equivalent circuit modeling, along with the capability of providing intrinsic information about the corrosion mechanism, enable EIS to be an excellent alternative method to conventional potentiodynamic polarization for evaluating the corrosion behavior of noble dental alloys.

Journal Article↗

PubMed2002

Electrochemical impedance spectroscopy study of high-palladium dental alloys. Part II: behavior at active and passive potentials.

Electrochemical impedance spectroscopic (EIS) analyses were performed on three high-palladium alloys and a gold-palladium alloy at active and passive potentials in five electrolytes that simulated body fluid and oral environmental conditions. All four alloys were previously found to have excellent corrosion resistance in these in vitro environments. Before performing the EIS analyses, alloy specimens were subjected to a clinically relevant heat treatment that simulated the firing cycles for a dental porcelain. It was found that the EIS spectra varied with test potential and electrolyte. Diffusional effects, related to the dealloying and subsequent surface enrichment in palladium of the high-palladium alloys, along with species adsorption and passivation, were revealed at both active and passive potentials, although these effects were more evident at the passive potentials.

Journal Article↗

PubMed2001

Evaluation of high-temperature distortion of high-palladium metal-ceramic crowns.

STATEMENT OF PROBLEM: Crown fit is a prerequisite for long-term clinical success; however, crown distortion may occur during porcelain firing. The dimensional stability of some high-palladium alloys at high temperatures has been questioned. PURPOSE: The purpose of this study was to use a new method to measure the distortion of copings for metal-ceramic single units of selected high-palladium alloys with compositions representative of commercial alloys. MATERIAL AND METHODS: Four high-palladium alloys containing copper and 3 containing no copper were tested. A palladium-silver alloy was included for comparison, and a gold-palladium alloy served as the control. By using reference points scribed on the margin, the mesiodistal and buccolingual margin diameters of identical copings were measured with a traveling microscope at 4 stages: as-cast, oxidized, after 2 simulated opaque porcelain firings, and after 2 simulated dentin porcelain firings. The margin distortions for the various specimen groups representing combinations of alloys, stages, and measurement diameters were compared with the use of 1-way analysis of variance and a multiple range test. RESULTS: Most of the high-palladium alloys had high-temperature distortions that were not significantly different from those of the control alloy. The distortions occurred principally during the oxidation cycle. The effect of mesiodistal groove reinforcement on preventing distortion was not the same for all alloys. CONCLUSION: The results suggest that small observed distortions of these alloys will not produce clinical problems. Several laboratory techniques are available to counteract the distortions.

Analysis of Variance↗

PubMed2001

Accuracy and pourability of gypsum preweighed packages.

PURPOSE: The purpose of this study was to determine the amount of gypsum provided by the manufacturers in their preweighed packages and to determine the amount of gypsum dispensed from the packages as a function of the method used. MATERIALS AND METHODS: One hundred twenty unopened packages from 8 manufacturers were weighed. Ten packages from each manufacturer were opened with the contents completely removed by tapping the packages, and the contents were weighed (thorough method). The other 5 packages from each manufacturer were opened, quickly poured, and the contents weighed (quick method). All packages were then slit open on all sides and completely cleaned. The cleaned packages were then weighed. Calculations were made to determine (1) the amount of gypsum that was "supplied" by the manufacturer and (2) the amount removed from the packages using the "quick" and "thorough" methods. The raw data was normalized to percentage of stated weight. A Welch analysis of variance was used to evaluate the heteroscedastic data at a 95% confidence level. Thereafter, a post hoc Student t test was used to determine if the percentages of stated weight for (1) supplied, (2) thorough, and (3) quick were significantly different from 100%. RESULTS: A statistically significant difference was found for the amount of gypsum supplied by the manufacturers (p <.001) and the amount of material removed from the packages via the 2 different methods used (p <.001). Seven of the 8 manufacturers' preweighed packages were remarkably close to their stated weight for amounts "supplied" and the "thorough" method. High standard deviations from the mean were found for the "quick" method of gypsum removal. CONCLUSION: Within the limits of this study, results indicate that the use of a "thorough" method to remove gypsum from preweighed packages was important. Ideally, gypsum supplied in preweighed packages should be weighed initially to ensure an optimal water/powder ratio.

Analysis of Variance↗

PubMed2001

Minimum color differences for discriminating mismatch between composite and tooth color.

PURPOSE: The purpose of this study was to determine if there are differences between patients and dental professionals in their ability to identify small color differences (deltaE) in composite resin restorative materials in vitro and to determine the deltaE that would indicate acceptability of color match between a restoration and an adjacent tooth. MATERIALS AND METHODS: Subjects were asked to evaluate composite resin disks to distinguish acceptability of deltaE between disk pairs. Color difference discrimination of dental professionals groups D1 (dentists, n = 12) and D2 (dental auxiliaries, n = 12) was compared to that of dental patient groups P1 (patients, n = 12) and P2 (scientists, n = 12). Each group was pretested for normal color vision. Color differences between a standard and restoration disks of composite were measured in Colour Measurement Committee (CMC) (1:1) color units. Data were analyzed by logistic regression, and results were used to calculate a probability level for minimal acceptance or rejection of AE for all observers. Mean 50:50 deltaE replacement points (RP) for each group were obtained and analyzed by analysis of variance, and the Tukey pairwise comparison test was applied (alpha = 0.05 for all statistical analyses). RESULTS: There were significant differences found between the experimental groups (p = .020). Group D2 (mean 50:50 deltaE RP = 1.78) proved to be more discriminating in accepting differences between tooth and composite resin restorative material color than group P1 (mean 50:50 deltaE RP = 2.69). The mean 50:50 deltaE RP for all subjects was 2.29 CMC (1:1) units. CLINICAL SIGNIFICANCE: The ability to generate an excellent color match between a tooth-colored restoration and the tooth is critical to esthetic success. This study demonstrates that patients are not as discriminating in their ability to identify small color differences between composite restorations and the tooth as are dental professionals. Dental auxiliaries proved to be more discriminating in accepting differences between tooth and composite resin restorative material color than were patients.

Analysis of Variance↗

PubMed2001

Accuracy of Kubelka-Munk reflectance theory applied to human dentin and enamel.

The Kubelka-Munk (K-M) theory provides a reflectance model for translucent materials placed on backings of different colors. We hypothesize that Kubelka-Munk (K-M) theoretical diffuse reflectance spectra of dentin and enamel are in good agreement with observed diffuse reflectance. The aim of this study was to measure the reflectance of enamel and dentin specimens and to compare the measured values of reflectance with K-M theoretical values. Disc-shaped specimens of enamel, dentin, and enamel/dentin were prepared from extracted teeth. Diffuse reflectance spectra were measured on three backings by means of a reflectance spectrophotometer over every wavelength (lambda) from 400 to 700 nm at three thicknesses. The measured reflectance values were fit by non-linear regression to corrected K-M theory. The low value of the reported errors associated with the application of K-M theory illustrated that the theoretical diffuse reflectance spectra of dentin and enamel are in good agreement with the observed diffuse reflectance.

Absorption↗

PubMed2000

Effect of different high-palladium metal-ceramic alloys on the color of opaque porcelain.

PURPOSE: The purpose of this study was to assess the effect of different high-palladium alloys on the resulting color of opaque porcelain. Three Pd-Cu-Ga alloys, Spartan Plus (S; Williams Dental Co/Division of Ivoclar North America, Amhest, NY), Liberty (B; J.F. Jelenko & Co, Armonk, NY), and Freedom Plus (F; J.F. Jelenko & Co); 4 Pd-Ga alloys, Legacy (L; J.F. Jelenko & Co), IS 85 (I; Williams Dental Co), Protocol (P; Williams Dental Co), and Legacy XT (X; J.F. Jelenko & Co); and a Pd-Ag alloy, Super Star (T; J.F. Jelenko & Co), were examined. The Au-Pd alloy Olympia (O; J.F. Jelenko & Co) served as the control. MATERIALS AND METHODS: Three cast 16-mm discs, 1-mm thick, were prepared from each of the alloys. After metallurgically polishing and air-abrading, the specimens were oxidized following the manufacturer's recommendations. Shade B1 opaque porcelain (Vita-Omega; Vident, Baldwin Park, CA) was applied at a final thickness of 0.1 mm using a mold. After 2 opaque porcelain firing cycles, the surfaces were air-abraded. The colors of the specimens were measured using a colorimeter and expressed in Commission International de l'Eclairage (CIE) L*a*b* coordinates. Color differences (delta E) were determined between the control and each experimental group. Analysis of Variance and Tukey-Kramer tests were performed on the delta E data. RESULTS: The 3 Pd-Cu-Ga alloys showed significantly greater (p < .01) delta E values (S = 2.8 +/- 1.1, B = 3.0 +/- 0.6, and F = 2.1 +/- 0.2) than the remaining 5 experimental groups (L = 0.7 +/- 0.5, I = 0.7 +/- 0.4, P = 0.7 +/- 0.2, X = 0.7 +/- 0.4, and T = 0.7 +/- 0.5). The directions of the significant color changes were relatively equally distributed along the L*, a*, and b* axes, and all delta L*, delta a*, and delta b* values were negative (lower value, more green and blue relative to control O). CONCLUSIONS: This work suggests that a 0.1-mm-thick layer of opaque porcelain in the Pd-Cu-Ga alloys studied, did not reliably reproduce the color of porcelain.

Air Abrasion, Dental↗

PubMed2000

Evaluating porcelain color match of different porcelain shade-matching systems.

PURPOSE: This in vitro study was designed to evaluate the new ShadeEye-EX Chroma Meter and a new visual porcelain shade-matching system (Vitapan 3-D Master, Omega 900) in comparison to the traditionally used visual porcelain shade-matching system (Vita Lumin Vacuum, VMK 68). MATERIALS AND METHODS: Shade matches and porcelain fabrications on individualized shade tabs were carried out. The fabricated porcelain tabs were evaluated against the respective natural tooth using the United States Public Health Service (USPHS) criteria to determine clinically acceptable match. The tabs also were ranked according to the final-color match to the corresponding natural tooth. RESULTS: The percentage of clinically acceptable color matches were as follows: Vita Lumin Vacuum (46.67%), ShadeEye-EX (40%), and Vitapan 3-D Master (56.67%). The generalized estimating equation (alpha = 0.5) showed no statistically significant difference between systems in obtaining a clinically acceptable match (p = .5729). The chi-squared test (alpha = 0.5) showed no statistically significant difference between systems in terms of rank order (p = .5017). The two new systems do not produce color match that is better than the conventional visual porcelain shade-matching system.

Chi-Square Distribution↗

PubMed1999

Optical properties of human trabecular meshwork in the visible and near-infrared region.

BACKGROUND AND OBJECTIVES: Despite disparate treatment parameters, similar success in laser trabeculoplasty (LT) is attained using the argon (514.5 nm) and diode (810 nm) laser. However, the mechanism of this success remains unresolved. To further understand LT, this study characterizes the optical properties of trabecular meshwork (TM). STUDY DESIGN/MATERIALS AND METHODS: Reflectance was measured from 10 TM samples over wavelengths of 400-820 nm, using an integrating sphere/spectrophotometer. Corrections were made for reflections at boundaries of refractive index mismatch. Kubelka-Munk coefficients were calculated and converted to linear transport coefficients. RESULTS: Scattering greatly dominated absorption. The scattering and absorption coefficients were, respectively, 141.20 +/- 15.80 cm(-1) and 4.89 +/- 1.95 cm(-1) at 514.5 nm, and 94.44 +/- 15.03 cm(-1) and 0.0874 +/- 0.111 cm(-1) at 810 nm (estimated anisotropy of 0.90). The corresponding penetration depths (1/e) were 69 microm (514.5 nm) and 106 microm (810 nm). CONCLUSION: The absorption coefficient of 514 nm energy is two orders of magnitude greater than 810 nm energy, while scattering coefficients are much closer. The fluence used at 514.5 nm is higher at the surface than that at 810 nm, but falls below it deep within the TM due to the differential absorption. Therefore, similar initial therapeutic effects are obtained with 810 nm using less total absorbed energy. Thermal damage resultant from excess energy deposited at 514.5 nm may be related to the lack of success in repeat argon LT, pointing out the need for studies of repeat diode LT.

Anisotropy↗

PubMed1998

Effects of dental laboratory processing variables and in vitro testing medium on the porcelain adherence of high-palladium casting alloys.

PURPOSE: This study assessed the effects of dental laboratory processing variables and testing medium on porcelain adherence of representative high-palladium alloys. MATERIAL AND METHODS: The processing variables were recasting two and three times, porcelain stripping with hydrofluoric acid and rebuilding, and changing the recommended oxidation procedures. The testing medium was air for these four groups and the nontreatment group. In the last group, the specimens were stored and tested in artificial saliva. The metal ceramic specimens were fractured with biaxial flexure in constant strain. The area fraction of adherent porcelain (%) was calculated with a standardized spectrometric technique. A gold-palladium alloy served as the control. RESULTS: Recasting without adding new alloy negatively affected some high-palladium alloys. Porcelain stripping did not cause a decrease in adherence of the tested alloys. The alternate oxidation treatment reduced significantly the porcelain adherence of the Au-Pd and one Pd-Cu-Ga alloy. Storage and testing in artificial saliva did not significantly affect the porcelain adherence of the alloys. The Au-Pd alloy exhibited the highest porcelain adherence.

Adhesiveness↗

PubMed1998

Handpiece degradation associated with performance testing of diamond rotary cutting instruments.

Reliable standardized testing of dental rotary cutting instruments remains a problem because of the many significant parameters that require identifying and quantifying. In a previous study, handpiece wear or degradation was identified as a potentially significant confounding variable leading to the suggestion that the use of industrial handpieces might be necessary. Therefore, the objective of this investigation was to measure and compare industrial and dental air rotor degradation during a simulated diamond rotary cutting instrument test. Two dental and two industrial products were tested in a random sequence using three replicate samples of each air rotor. The simulated test consisted of 10 diamond instruments each making 90 passes through a float glass substrate. The testing apparatus comprised a computer-interfaced device that controlled the contact force between the cutting diamond and the substrate. Air rotor performance was based on the time required to cut a standardized path through the substrate. The performance change that occurred from diamond instrument to instrument during the simulated test was attributed to air rotor degradation. Multiple regression analyses showed significant differences between replicate samples of the same type and between regression intercepts for different air rotor types. Surprisingly, four air rotors exhibited improving performance during the simulated test. Changing air rotor performance must be considered to achieve reliability for the testing of diamond rotary cutting instruments.

Air↗

PubMed1997

Color and translucency changes during and after curing of esthetic restorative materials.

OBJECTIVES: This study was conducted to evaluate various types of contemporary resin composites and glass ionomer restorative materials for color differences that occur during light curing and after submersion in water. Additionally, translucency was evaluated through the use of a translucency parameter for each of these materials at these times. METHODS: A translucency parameter was calculated from observed colors of 2 mm thick layers of light, medium and dark shades of popular restorative materials on black and white backings. The colors of these materials were also calculated, based on Kubelka-Munk theory, for layers on a B2 shade backing. Repeated measures analyses of variance were used to identify statistically significant factors for color differences and translucency parameters. RESULTS: A statistically significant interaction was found between the products, shades and time intervals or times studied for both color difference and translucency parameter. There is relatively high variability in both lightness and translucency changes among these popular products, with some products increasing and others decreasing in these parameters with curing and subsequent water submersion. SIGNIFICANCE: Kubelka-Munk theory can be used to calculate the color of a layer of material on a backing other than those used to determine the Kubelka-Munk coefficients, thereby increasing the efficiency of research into the optical properties of esthetic restorative materials. The relatively large optical changes which occur as popular esthetic restorative materials are cured and subsequently submersed in water indicate that these changes should be taken into account during any initial clinical appearance match at placement of the unset material.

Analysis of Variance↗

PubMed1997

The psychological impact of negative TV news bulletins: the catastrophizing of personal worries.

This study investigated the effect of the emotional content of television news programmes on mood state and the catastrophizing of personal worries. Three groups were shown 14-min TV news bulletins that were edited to display either positive-, neutral- or negative-valenced material. Participants who watched the negatively valenced bulletin showed increases in both anxious and sad mood, and also showed a significant increase in the tendency to catastrophize a personal worry. The results are consistent with those theories of worry that implicate negative mood as a causal factor in facilitating worrisome thought. They also suggest that negatively valenced TV news programmes can exacerbate a range of personal concerns that are not specifically relevant to the content of the programme.

Adolescent↗

PubMed1996

Smear layer instability caused by hemostatic agents.

The effect of hemostatic agents, other than a 15.5% Fe2(SO4)3 solution, on prepared tooth structure is unknown. The purpose of this study was to (1) compare the effect of six commonly used hemostatic solutions and two nondental astringents on the dentinal smear layer and (2) determine whether different responses caused by product and/or time could be established. Standardized dentinal smear layers were exposed to eight astringent solutions for 30, 120, and 300 seconds (n = 6). A total of 144 SEM photographs at x2400 magnification were ranked according to predetermined criteria for five categories of smear layer removal and etching of underlying tooth structure. There were significant differences (p < 0.001) caused by the solution, exposure time, and their interaction. Greatest smear layer removal was observed with 21.3% AlCl3-6 hydrate, 8% racemic epinephrine HCl, and 15.5% Fe2(SO4)3 solutions at longer exposures. These caused significantly more removal than did almost pH neutral tetrahydrozoline or oxymetazoline (p < 0.05).

Acid Etching, Dental↗

PubMed1996

Three-year effect of unfilled resin dilution on water sorption of a light-cured microfill and hybrid composite resin.

This in vitro study compared water sorption for various intervals over a 3-year period for visible light-cured microfill and hybrid composite resin with and without unfilled resin dilutions. Statistically significant sorption increases were evident for the 960-day interval for certain dilutions. At 3 years the microfill control revealed a 0.72% relative water sorption increase and its various dilutions 1.92% to 2.36%. By comparison, the hybrid control exhibited an increase of 0.4% and its dilutions were 0.72% to 0.98%.

Absorption↗

PubMed1996

Compressive characteristics of an internally threaded post system.

PURPOSE: The effects of oblique compressive forces were evaluated for extracted teeth restored with posts of solid and internally threaded designs. MATERIALS AND METHODS: A group of 25 roots was restored with crowns supported by Para-post dowels and pin-retained composite resin cores, and a second group was restored with gold copings cast to internally threaded stainless steel-post prototypes. Samples were then embedded in epoxy resin and loaded at 45 degrees until failure. Failure force and type were statistically analyzed to determine statistical difference (P < .05) between groups. RESULTS: No statistical difference was shown between groups for either fracture force or type. CONCLUSIONS: The internally threaded post system resisted oblique loading forces similarly to a solid post system.

Analysis of Variance↗

PubMed1996

Analysis of edge-losses in reflectance measurements of pigmented maxillofacial elastomer.

Edge-losses occur during reflectance measurements of pigmented maxillofacial elastomer when light is scattered within a sample beyond that part of the surface exposed to the observation system of the optical device. A custom sample-holder is presented which redirects light that would not be measured during conventional reflectance measurement back into the sample. The amount of edge-loss occurring within thin layers of maxillofacial elastomer with tan pigment on black-and-white backings was found to depend on sample thickness, the backing, the beam size used during conventional reflectance measurement, and the optical term bS = (2KS + K2)1/2. Data analysis revealed a significant interaction among these four factors. Additionally, the edge-loss occurring during the tristimulus reflectance measurement of thick samples of maxillofacial elastomer with various concentrations of tan and black pigment was found to be linearly related to bS up to a limiting value, with no additional edge-loss occurring for bS values above this limiting value. Edge-loss is an important consideration during the matching of the optical characteristics of pigmented maxillofacial material to those of human skin.

Absorption↗