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Biomedical subjects

K J Anusavice

Publications and source records attributed to K J Anusavice.

At least 37 records · Page 2Linked to original sources
PubMed1997

Chemical durability of Dicor and lithia-based glass-ceramics.

OBJECTIVES: The aim of this study was to analyze the effect of a nucleation agent (P2O5) and a colorant/nucleation agent (AgNO3) on the chemical durability of Li2O-Al2O3-CaO-SiO2(LACS) glass-ceramics in 4% HAc solution, deionized-distilled water, and in pH buffer solutions of pH 1, pH 9, and pH 11. METHODS: Glass powder [27.8 mol% Li2O, 2.5% Al2O3, 5.9% CaO, and 63.8% SiO2(LACS)] was melted, poured into a cylindrical graphite mold (16 mm diameter), cooled, cut into 2.2 mm thick disks, polished through 1200 grit SiC, nucleated at 510 degrees C for 3 h, and crystallized at 650 degrees C for 6 h. Dynamic corrosion tests of LACS glass-ceramic, LACS glass-ceramic containing 1.0 mol% P2O5 (LACSP), LACS glass-ceramic containing 0.78 mmol% AgNO3(LACSAg), and Dicor control specimens were performed in a shaker-bath unit at 80 degrees C at a shaker speed of 30 cycles/min for periods of up to 15 d. Differences in mean weight loss and ionic concentration were analyzed for statistical significance (p = 0.05) using ANOVA and the Tukey's Studentized Range Test. RESULTS: The mean weight loss over 15 d in 4% HAc increased in the following order: LACS (0.21 +/- 0.02 mg/cm2), LACSAg (0.25 +/- 0.05 mg/ cm2), and Dicor (0.27 +/- 0.05 mg/cm2). The differences in mean values were not statistically significant (p > 0.05). The amounts of Li+ leached in 32 mL of pH1 and pH11 buffer solutions were 3.1 +/- 0.3 microgram/cm2/mL and 243 +/- 49.0 micrograms/cm2/mL, respectively, for the LACS group, and 3.0 +/- 0.6 microgram/cm2/mL and 166 +/- 28.0 micrograms/cm2/mL for the LACSAg group. The differences in mean values are not statistically significant (p > 0.05). The high chemical durability in acidic environments of LACS glass-ceramics without P2O5 and their decreased durability at pH values of 9 and above were confirmed by SEM observations of the exposed surfaces. SIGNIFICANCE: The weight loss for the three glass-ceramic systems was highest in pH 11 buffer solution, which represents an unlikely in vivo environment. From a toxicological viewpoint, the maximum amount of Li+ released from 28 fully dissolved crowns of LACS glass-ceramic at a temperature of 80 degrees C (which is greater than any temperature experienced in the oral cavity) is approximately 1.2 mg; this amount appears to be below the daily limit of 2 mg allowable from food sources.

Aluminum Silicates↗
PubMed1996

Analysis of tempering stresses in metal-ceramic disks.

Previous studies showed that residual compressive stresses induced by thermal tempering retarded the growth of surface cracks in bilayered porcelain disks. The objectives of the present study were: (1) to determine whether thermal tempering by air blasting reduces the length of cracks induced by microhardness indentation in metal-ceramic disks, and (2) to use visco-elastic finite element analyses to calculate transient and residual stresses in metal-ceramic disks. Ni-Cr-Be disks, 16 mm in diameter and 0.3 mm in thickness, were prepared with a 0.5-mm-thick layer of opaque porcelain and a 1.5-mm-thick layer of body porcelain. Metal-porcelain combinations were selected to provide a range of thermal contraction mismatch values. The disks were fired to the maturing temperature of body porcelain and then were subjected to three cooling procedures: (1) slow cooling in a furnace (SC), (2) cooling in air (FC), and (3) air tempering (T) by blasting the surface of the body porcelain with compressed air. The lengths of cracks induced in the surface of the body porcelain by a microhardness indenter were measured immediately after indentation at 20 points along diametral lines. The results of Tukey's multiple-contrast analyses indicated that the mean crack lengths of air-tempered specimens were significantly smaller (p < or = 0.05) than the crack lengths of the fast-cooled and slow-cooled groups. Except for one case, there were no statistically significant differences in the mean crack lengths between FC and SC specimens independent of thermal contraction mismatch. Residual tensile stresses were calculated for SC and FC specimens for all thermal contraction mismatch cases, with the largest values being associated with combinations containing the body porcelain with the smaller contraction coefficient. Calculations by use of the model confirmed that tempering induces large residual compressive stresses in the surface of body porcelain for all of the thermal contraction mismatch cases included in this study.

Air↗
PubMed1996

Effects of flaw size and auto-glaze treatment on porcelain strength.

The functional surfaces of porcelain restorations are often ground to adjust occlusion. This removes the surface glaze and introduces flaws. Re-firing the restoration before final placement produces a self-glaze layer on the surface that may reduce the extent of damage caused by grinding. The objectives of this study were to determine whether re-firing after grinding increases the strength of dental porcelain and to determine whether the effectiveness of this treatment was dependent on the initial flaw size. Six groups, each containing 12 disk specimens, were prepared from experimental body porcelain (No. 36, J.F. Jelenko & Co., Armonk, NY). The specimens were fired under vacuum, ground to a thickness of 1 mm with 240-grit SiC abrasive, and polished through 600-grit on the surface to be subjected to tensile stress. Control Group A consisted of 12 non-indented specimens. Flaws were induced in the remaining groups by means of a Vickers indenter under the following loads: 3.9 N (Group B), 7.8 N (Group C), 11.8 N (Group D), 15.7 N (Group E), and 19.6 N (Group F). Following indentation, half of the specimens from each group were re-fired. The flexure strength of each specimen was determined by means of a piston-on-three-ball biaxial fixture. The TTEST procedure of SAS (1985) indicated a significant difference between the mean strength values of glazed and non-glazed specimens in Group E only (p = 0.55, 0.24, 0.13, 0.07, 0.01, and 0.69 for Groups A to F, respectively). ANOVA with the GLM procedure of SAS (1985) revealed that the mean strength values of groups subjected to five indentation loads were not significantly different (p = 0.45 for glazed specimens and p = 1.00 for non-glazed specimens). These findings support those of Fairhurst et al. (1992) for specimens with smaller flaws that were induced by grinding with 1 micron abrasive grit. The results of the current study indicate that re-firing of porcelain with large surface flaws does not significantly increase the flexural strength.

Analysis of Variance↗
PubMed1995

Three-dimensional finite element analysis of the shear bond test.

OBJECTIVES: The purpose of this study was to use finite element analyses to model the planar shear bond test and to evaluate the effects of modulus values, bonding agent thickness, and loading conditions on the stress distribution in the dentin adjacent to the bonding agent-dentin interface. METHODS: All calculations were performed with the ANSYS finite element program. The planar shear bond test was modeled as a cylinder of resin-based composite bonded to a cylindrical dentin substrate. The effects of material, geometry and loading variables were determined primarily by use of a three-dimensional structural element. Several runs were also made using an axisymmetric element with harmonic loading and a plane strain element to determine whether two-dimensional analyses yield valid results. RESULTS: Stress calculations using three-dimensional finite element analyses confirmed the presence of large stress concentration effects for all stress components at the bonding agent-dentin interface near the application of the load. The maximum vertical shear stress generally occurs approximately 0.3 mm below the loading site and then decreases sharply in all directions. The stresses reach relatively uniform conditions within about 0.5 mm of the loading site and then increase again as the lower region of the interface is approached. Calculations using various loading conditions indicated that a wire-loop method of loading leads to smaller stress concentration effects, but a shear bond strength determined by dividing a failure load by the cross-sectional area grossly underestimates the true interfacial bond strength. SIGNIFICANCE: Most dental researchers are using tensile and shear bond tests to predict the effects of process and material variables on the clinical performance of bonding systems but no evidence has yet shown that bond strength is relevant to clinical performance. A critical factor in assessing the usefulness of bond tests is a thorough understanding of the stress states that cause failure in the bond test and then to assess whether these stress states also exist in the clinical situation. Finite element analyses can help to answer this question but much additional work is needed to identify the failure modes in service and to relate these failures to particular loading conditions. The present study represents only a first step in understanding the stress states in the planar shear bond test.

Dental Bonding↗
PubMed1995

Treatment regimens in preventive and restorative dentistry.

Due in part to a lack of appropriate training and the incentive of adequate compensation, preventive dentistry in the United States has focused on prophylaxis and fluoride application. Dentistry must shift its attention to developing standardized protocols for "preservative dentistry"--diagnosing caries, assessing and monitoring caries risk, arresting active caries and remineralizing non-cavitated lesions. This article addresses shortcomings in preventive dentistry and proposes a plan for treatment standardization that can ensure optimum treatment and, ideally, lead to adequate compensation.

Adolescent↗
PubMed1994

Guidelines for experimental studies.

OBJECTIVES: The purpose of this document is to present guidelines for the training of researchers and for the planning of experiments that ensure optimum interaction and communication among the investigators and statisticians of the research team. METHODS: The protocol described in this document is designed to systematically guide a researcher through a sequence of planning steps and checkpoints to ensure proper experimental design, data management, statistical analyses, and interpretation. RESULTS: The investigator and the statistician, as a collaborative team, will be able to determine the most effective and appropriate procedures and tools necessary for the successful completion of a research project. SIGNIFICANCE: Planning an experimental study necessitates much thought. Lack of essential communication with a statistician in the initial stages of the experimental design can doom the experiment to failure. Flawed research studies may be associated with one or more of the following: failure of the established testable hypothesis, uncontrolled biases, insufficient sample size, lack of appropriate controls, variability in specimen preparation, improper calibration of instruments or standardization of measurements, incorrect data entries, inappropriate statistical analyses, errors in interpretation of statistical analyses, and erroneously drawn conclusions. Standard guidelines will eliminate many of these potential problems.

Data Collection↗
PubMed1994

Optimization of glass-ceramic crystallization based on DTA exotherm analysis.

OBJECTIVES: Crystallization of glass-ceramics is traditionally achieved through a two-stage heat treatment consisting of an isothermal nucleation stage followed by an isothermal growth stage. A method for determining a more efficient heat treatment schedule for a glass-ceramic material using a thermal analysis technique is proposed. The goal of an optimized heat treatment schedule is the production of a glass-ceramic with a desired microstructure (number of crystals per volume) in the shortest amount of time. METHODS: The proposed method involves differential thermal analysis (DTA) to measure glass crystallization exotherm characteristics which are correlated with the population density of growing crystals, and therefore, to the effectiveness of any prior heat treatment. Traditional thermal processing parameters were investigated and optimized. A method for generating a more efficient heat treatment schedule composed of a series of increasing heating rates was also demonstrated. RESULTS: The thermal analysis method measured a significant effect upon the number density of crystals generated as a function of several experimental variables. Micrographs from samples crystallized with a more time-efficient heat treatment schedule were shown to have equivalent crystal number densities compared to those crystallized with a more time consuming, traditional schedule. SIGNIFICANCE: This work demonstrated that a rapid thermal analysis method was capable of measuring the relative effectiveness of heat treatment schedules to generate crystalline populations. A novel heat treatment schedule was developed based on progressive adjustment of processing heating rates to generate the maximum crystal population in the shortest amount of time.

Ceramics↗
PubMed1994

Stress relaxation behavior of dental porcelains at high temperatures.

OBJECTIVES: The purpose of this study was to measure the stress relaxation behavior at elevated temperatures of three experimental opaque porcelains and three experimental body porcelains. METHODS: Feldspathic porcelain formulations covering a range of thermal contraction coefficients were supplied by a dental ceramics manufacturer. Six specimens, 11 mm in diameter by 22 mm long, were fabricated for each porcelain. The specimens were tested in compression at five temperatures controlled to +/- 1 degree C in a hot stage furnace attached to a screw-type uni-axial testing machine. RESULTS: Mean values of relaxation time, tau u, and the b function were determined by a regression fit to the relation: psi (t) = exp [-(t/tau u)b]. Values of b ranged from 0.23 to 0.53 for opaque porcelain and 0.47 to 0.64 for body porcelain. Relaxation times ranged from 2.6 s to 4 x 10(4) s for the opaque porcelains and 1.5 s to 5.5 x 10(2) s for the body porcelains. A statistically significant variation of b with temperature for three of the experimental porcelains is an indication that these porcelains do not satisfy the theoretical requirements for the porcelains to be classified as thermorheologically simple. SIGNIFICANCE: A knowledge of the relaxation behavior of dental porcelains is necessary so that dental researchers can identify metal/porcelain combinations that will result in low stress values and, therefore, reduce the potential for failure from thermally induced stresses. These properties can be used in the optimization of prosthesis design to reduce the destruction of healthy tissue to accommodate the placement of the dental prosthesis.

Dental Porcelain↗
PubMed1994

Sensitivity of catalyst/base ratio on curing of resin luting agents: polymerization exotherm analysis.

OBJECTIVES: Currently, the proposed test of the International Standardization Organization (ISO) for measuring working and setting times of resin luting agents is based on measurement of times to reach specified stages on the polymerization exotherm. The objective of this study was to use this test to investigate the influence of variations in the mass ratios of catalyst paste to base paste on the working and setting times of three dual-cured dental resin luting agents. METHODS: The materials used were Dicor Light Activated Cement (Dentsply International Inc.), Palfique Inlay Resin Cement (Tokuyama Soda Co.), and Vivadent Dual Cement (Vivadent). Fifteen specimens of each material were tested for working time by spatulating mass ratios from 0.7 to 1.3 for 30s at 23 degrees C and recording the time from beginning of spatulation to the time at which a temperature increase occurs. Ten specimens of each material were tested for setting time by spatulating in a similar manner at 37 degrees C and recording the time at which the temperature reaches a maximum value. RESULTS: The data were fitted to the relation, In t = In A + Bm, where t is the time in seconds, m is the mass ratio, and A and B are regression coefficients. The results suggest that working and setting times of the specimens were independent of variations in mass ratio. A comparison among the materials was made by using a multiple linear regression with the relation, In t = In C + Dm + E gamma + Fm gamma, where gamma is a dummy variable to help distinguish between materials, and C, D, E, and F are regression coefficients. The results suggest that differences in materials influence the working time but not the setting time. SIGNIFICANCE: These results infer that variations in mass ratio (+/- 20%) often observed in the clinical setting should not have a significant influence on the working and setting times of resin luting agents.

Acrylic Resins↗
PubMed1994

Influence of colorants on crystallization and mechanical properties of lithia-based glass-ceramics.

OBJECTIVES: The objective of the present study was to test the hypothesis that colorants such as AgNO3 and FeCl3 act as conucleating agents with P2O5 in the Li2O-Al2O3-CaO-SiO2 system and that the addition of either colorant and P2O5 produces a greater effect on crystallization and selected mechanical properties than the use of P2O5 alone. METHODS: Microstructural effects were observed by SEM and optical microscopy. Mechanical properties were determined to monitor the effects of structural changes after crystallization. These include controlled-flaw flexure strength, fracture toughness (KIC), and Vickers hardness (VHN). RESULTS: Based on a glass composition of 27.84 mol% Li2O, 2.45 mol% Al2O3, 5.88 mol% CaO, and 63.84 mol% SiO2 (LACS), the mechanical properties of LACS glass-ceramics were influenced by P2O5, the colorant type, and the colorant concentration. The mean strength of the glass-ceramic disks without P2O5 increased with AgNO3 concentration to a peak value of 188 MPa at a concentration of 0.78 mmol%. The maximum value of controlled-flaw flexure strength increased from 120 MPa for one of the FeCl3 groups to 188 MPa for one of the AgNO3 groups. The maximum fracture toughness of glass-ceramic disks without P2O5 (2.45 MPa.m1/2) was associated with a AgNO3 concentration of 0.58 mmol%. This value was significantly greater (p < 0.05) than that of the corresponding group (1.90 MPa.m1/2) which also contained P2O5. There was no significant change in KIC of glass-ceramic specimens containing P2O5 as the AgNO3 concentration increased. The increase in controlled-flaw flexure strength and fracture toughness of specimen groups containing 0.58 to 0.78 mmol% AgNO3 support its use as a colorant and as a nucleating agent in LACS glass-ceramics. SIGNIFICANCE: The development of tougher, higher strength glass-ceramics can be controlled by the use of colorants that are also effective as nucleating agents. Although certain colorants are believed to act synergistically when used in combination with known nucleating agents to enhance the fracture toughness of glass-ceramics, this effect was not observed in this study for the combined use of AgNO3 and the classical nucleating agent, P2O5. In fact, the colorant used in this study (AgNO3) was more effective than P2O5 as a nucleating agent for lithia-based glass-ceramics.

Ceramics↗
PubMed1994

Influence of P205, AgNO3, and FeCl3 on color and translucency of lithia-based glass-ceramics.

OBJECTIVES: The objective of this study was to characterize the influence of various metals, metal compounds, and P2O5 as a nucleating agent on the color and translucency of a Li2O-Al2O3-CaO-SiO2 glass-ceramic. METHODS: Glass frits of Li2O-Al2O3-CaO-SiO2 (LACS), LACS with 1 mol% P2O5 (LACSP), and/or LACS with one of 16 colorants were melted, poured into a cylindrical graphite mold, cut into disks, annealed, nucleated, crystallized, and annealed again. Ten translucency measurements of each of five disks were made using a tristimulus colorimeter and a D65 standard CIE illuminant. The color of each disk was analyzed using the CIE L*a*b* color space system (1976) as a function of colorant, colorant concentration, and P2O5. RESULTS: Mean L* values of glass-ceramic disks ranged from 63.5 for LACS containing 6.2 mmol% FeCl3 (LACSP-6.2Fe) to 84.1 for LACS. No significant difference (p > 0.05) was found between the mean L* values for LACS, LACSP, and LACS with 0.19 mmol% AgNO3 (LACS-0.19Ag). The mean contrast ratio of glass-ceramic specimens ranged from 0.42 (LACS and LACS-1.0Fe) to 0.98 (LACS-0.78Ag). Mean color difference values varied from 5.8 (LACSP-1.0Fe vs. LACS) to 36.3 (LACSP-0.78Ag vs. LACSP). SIGNIFICANCE: The results of this study indicate that, because certain colorants in glass-ceramics affect opacity as well as hue and chroma, the development of glass-ceramics should be simplified by: 1) employing a nucleating agent that does not affect hue or chroma significantly, 2) controlling fixed levels of translucency consistent with mechanical and physical property requirements, and 3) varying the hue and chroma by means of colorants that do not affect the crystallization process. This implies that the volume fraction and mean size of crystals must be controlled, since the translucency or opacity of glass-ceramics is associated with scattering of light at the interfaces between adjacent crystals, and between crystals and the glass phase because of differences in refractive indices (McMillan, 1979a).

Analysis of Variance↗
PubMed1994

Effect of surface etching on the flexure strength and fracture toughness of Dicor disks containing controlled flaws.

Previous studies have shown an increase in the failure load of Dicor glass-ceramic crowns of up to 97% when they were etched with NH4HF2 and luted with a resin-based cement compared with the use of unetched crowns luted with zinc phosphate cement. The objective of this study was to evaluate the effect of the acid-etch treatment on the flexure strength and fracture toughness of Dicor glass-ceramic specimens containing controlled flaws. Annealed disks were divided into two groups of 14 each, and microcracks were induced in the surface with a Vickers indenter. Seven specimens from each sample set were etched with 10% NH4HF2 (Dicor etching gel) and were fractured in air using a piston-on-three-ball biaxial fixture. Fracture toughness values were determined from the flexure strengths and fractographic measurements of fracture-initiating flaws. Analysis of data by ANOVA and the Tukey's Studentized Range Test revealed no significant difference (p > 0.05) in the mean flexure strength or fracture toughness of the etched and unetched samples within group A or group B. For the sample groups tested, flaw modification by etching had no effect on the flexure strength or on the fracture toughness of Dicor specimens containing controlled flaws.

Acid Etching, Dental↗
PubMed1994

Effect of grinding and fluoride-gel exposure on strength of ion-exchanged porcelain.

Strengthening of dental porcelain through a diffusion heat treatment at 450 degrees C of a potassium-enriched, ion-exchange surface coating has been demonstrated in several recent studies. However, little attention has been focused on the potential strength reduction of these materials when the treated surfaces are ground or etched under clinically simulated conditions. The objective of this study was to test the hypothesis that partial removal of the surface layers of ion-exchanged porcelains by grinding or exposure to acidulated fluoride gel will significantly reduce their flexure strength. Nine groups of body porcelain disks were ion-exchanged at 450 degrees C for 30 min. One of these groups was subjected to ion exchange and no further surface treatment. Eight specimen groups were subjected to the following procedures after ion exchange: grinding to depths of 50 microns, 100 microns, 150 microns, 200 microns, and 250 microns, and exposure to acidulated fluoride for 30 min, 60 min, and 300 min. A tenth group (FC) was fired at 960 degrees C and fast-cooled in air, but the disks were not subjected to the ion-exchange treatment. Surface stress was calculated from measured values of cracks induced in the treated surfaces. Fluoride exposure for up to 60 min resulted in a significant decrease in surface compression (P < or = 0.05), although this treatment had no effect on strength. Grinding to a depth of from 100 microns to 250 microns caused a significant decrease in strength, while removal of a 50-microns layer caused no significant change (P > 0.05).

Acidulated Phosphate Fluoride↗
PubMed1994

Fracture surface characterization of clinically failed all-ceramic crowns.

The goal of this study was to establish a protocol for the retrieval and fractographic analysis of failed restorations, and to compare the fracture surface features of clinically failed ceramic restorations and with those of controlled laboratory test specimens fabricated from the same materials. Ten fractured Dicor crowns and 12 fractured Cerestore crows were retrieved and analyzed. Optical microscopy of the failed crowns revealed that the critical segments of nine of the 10 (90%) Dicor crowns and nine of the 12 (75%) Cerestore crowns were acceptable for fractographic analysis. Twelve disks of each material were fabricated as controls and fractured by bi-axial flexure for analysis of the similarities and/or differences between the fractographic features of fractured clinical crowns and the disks. Each of the 10 Dicor crowns was observed to fail along the internal surface. For 78% of the Cerestore crowns, failure initiation occurred at the porcelain/core interface or inside the core material. Critical flaw sizes of the failed Dicor crowns ranged from 127 to 272 microns. Failure stresses of the Dicor crowns, estimated by fractographic techniques and fracture mechanics relationships, ranged from 65 to 94 MPa. Estimated failure stresses for two of the Cerestore crowns which had failure initiation sites in the porcelain layer were 15 and 68 MPa. It is concluded that the fracture initiation sites of dental ceramics are controlled primarily by the location and size of the critical flaw, and not by specimen thickness.

Aluminum Oxide↗