Degradation, fatigue, and failure of resin dental composite materials.

The intent of this article is to review the numerous factors that affect the mechanical properties of particle- or fiber-filler-containing indirect dental resin composite materials. The focus will be on the effects of degradation due to aging in different media, mainly water and water and ethanol, cyclic loading, and mixed-mode loading on flexure strength and fracture toughness. Several selected papers will be examined in detail with respect to mixed and cyclic loading, and 3D tomography with multi-axial compression specimens. The main cause of failure, for most dental resin composites, is the breakdown of the resin matrix and/or the interface between the filler and the resin matrix. In clinical studies, it appears that failure in the first 5 years is a restoration issue (technique or material selection); after that time period, failure most often results from secondary decay.

Analysis of the degradation of a model dental composite.

UNLABELLED: Dental composites undergo material property changes during exposure to the oral environment and may release compounds of potential toxicity, such as bisphenol A. Degradation of dental composites was studied in a simplified overlayer model in which bisphenol A diglycidyl methacrylate (BisGMA) was covalently bound to a porous silicon oxide surface. It was hypothesized that the chemical structure of this overlayer would allow release of bisphenol A, BisGMA, and the decomposition products thereof, upon exposure to water for an extended period. Liquid chromatography mass spectrometry found leaching of intact BisGMA and several degradation products that contained the bisphenol A moiety from the overlayer into distilled water after 2 wks of aging. The absence of bisphenol A release from the overlayer reduces concerns regarding its potential health risk in dental composites. Nevertheless, health concerns might arise with respect to BisGMA and the leached degradation products, since they all contain the bisphenol A moiety. ABBREVIATIONS: BisGMA, bisphenol A diglycidyl methacrylate; HPLC, high-performance liquid chromatography; LCMS, liquid chromatography mass spectrometry; MA, methacrylic acid; MPS, 3-(trimethoxysilyl) propyl methacrylate; m/z, mass-to-charge ratio; and TIC, total ion chromatogram.

Regional myocyte size in normotensive and spontaneously hypertensive rats.

Regional differences in cell size in the hearts of rats with and without cardiac hypertrophy were studied using isolated muscle cells. Isolated cardiac myocytes were prepared from left ventricular free wall inner and outer halves and the right ventricle of six male 12-week-old spontaneously hypertensive (SHR), Wistar-Kyoto (WKY) and Fischer-344 rats. In SHR, blood pressure was increased to 188 +/- 4 (SEM) mm Hg versus 143 +/- 2 and 133 +/- 10 for WKY and Fischer rats, respectively (p less than 0.001). Total heart weight was increased to 1103 +/- 29 mg in SHR compared to 824 +/- 21 in WKY and 951 +/- 23 in Fischer rats (p less than 0.001. Isolated cardiac myocytes were prepared by perfusion of isolated hearts with Ca++ free Hanks' solution containing EGTA followed by collagenase-containing media. Mean length, width and volume of 150 cells stained with hematoxylin and eosin from each site were measured with a sonic digitizer. Two nuclei were present in 85 to 87% of isolated cells from all strains and regions. There was no difference among strains in right ventricular cell length, width, or volume, nor between left ventricular inner and outer halves within each strain. Left ventricular cells were larger than right ventricular cells (p less than 0.05) in all strains. Left ventricular cells of SHR were larger than left ventricular cells of WKY of Fischer rats in proportion to the increase in total heart weight, indicating that cardiac enlargement in SHR is due to increased cell size rather than increased cell number.

Shear testing of the porcelain-metal bond.

A simple shear test was employed to determine the bonding strength of porcelain to a gold metal substrate and to a non-precious metal substrate after they had been aged in double-distilled water at 37 degrees C for four and 12 months. A 7.4% and 18.1% decrease in bond strength after four months and 12 months, respectively, was observed for the porcelain-gold system, and a decrease of 21.2% and 21.4% after four and 12 months, respectively, for the non-precious-porcelain system. A significant difference was observed between the bond strengths of the porcelain-metal composites: 51.17 +/- 11.29 MPa for the porcelain-gold system and 31.83 +/- 3.65 MPa for the porcelain-non-precious system.

In-vivo comparisons of clot formation on titanium and hydroxyapatite-coated titanium.

We investigated in vivo clot formation on the surface of hydroxyapatite (HA)-coated titanium (Ti) implants and on non-coated Ti implants. Immediately after tooth extraction implant samples were inserted into the blood clot, in the same extraction site, for 1, 30, 60, or 120 seconds. Samples were processed for scanning electron microscopy (SEM). Qualitative observations of clotting topography were made by direct SEM viewing. Neither of the implant surfaces appeared to differ markedly in the degree of clotting during the 120 seconds of implantation; they revealed very early clot formation and limited clot attachment. These results were compared to the findings obtained in a previous study using identical methods with an intact periodontal ligament (PDL), root planed roots, and roots planed and treated with pH 1 citric acid. The PDL surface had the most rapid clot formation at all time periods. By 120 seconds, all root surfaces had completed clot formation.

Potentiometric determination of plutonium by argentic oxidation, ferrous reduction and dichromate titration.

A simple and rapid method is described for the routine determination of plutonium with a coefficient of variation of better than 0.2%. It is directly applicable to nitrate solutions containing a large amount of uranium; moderate amounts of iron, molybdenum, fluoride and phosphate do not interfere. Chromium, cerium and manganese interfere quantitatively, and the procedure may also prove convenient for the determination of these elements. The plutonium is oxidised to the sexivalent state with argentic oxide in nitric acid solution, and the excess of oxidant is destroyed by reaction with sulphamic acid. A weighed small excess of iron(II) solution is then added, and the excess is titrated potentiometrically with standard potassium dichromate solution using polarised gold indicator electrodes. The whole determination is performed in one vessel at room temperature, and takes about 20 min.

Weibull models for the statistical analysis of dental composite data: aged in physiologic media and cyclic-fatigued.

The modulus of rupture or flexural strength of dental composites aged in distilled water and saline solution or cyclic-fatigued to fracture in distilled water was analyzed by use of Weibull statistics. Two- and three-parameter Weibull models were applied to the data. For the case of the dental composites aged for five distinct time periods, the most appropriate model was a two-parameter Weibull model for each separate aging time. For the cyclic-fatigue data, a three-parameter accelerated failure time model with a Weibull baseline distribution and scale parameter, depending on the variable cycle, was appropriate.

Shear strength and filler particle characterization of Maryland (acid etch) bridge resin cements.

The shear strengths of Maryland (acid etch) bridge cements were investigated and ranged from 44 to 70 MPa. The bulk shear strength of the resin cements is sufficient; the weak link in the system (other than the technique sensitivity) is the interface at or adjacent to the resin and metal or the resin and enamel. The resin cements were polished and etched, and scanning electron micrographs were taken and digitized to illustrate the filler particle sizes and distribution.

In vitro aging of a heat/pressure-cured composite.

Specimens of a heat/pressure-cured microfill composite were aged in saline or distilled water at 37 degrees C up to 12 mon to determine if the flexure strength was affected by the aging media. No significant difference in the flexure strength was observed between the controls and the 12 mon aged specimens. Cyclic testing in distilled water resulted in a lower flexure strength at fracture than that observed for the aged specimens. This lower strength is attributed to fatigue of the silane bond, the resin matrix, and/or the filler particles.

Physiological aging of an all-ceramic restorative material.

Ceramic materials exposed to a liquid environment may be subject to stress corrosion and/or time-delayed failure. The intent of this project was to evaluate the susceptibility of a magnesia alumina spinel (Cerestore) to stress corrosion and degradation. Bars 2.5 x 2.5 x 30.0 mm were prepared according to manufacturer's instructions. Specimens were aged in distilled water or air at 37 degrees C. The modulus of rupture was evaluated at zero, six, and 12 months in four-point loading at loading rates of 0.05, 0.5, and 5.0 mm/min. The modulus of rupture of each specimen was tested in its respective aging medium. The data were analyzed by one-way analysis of variance with a multiple-means comparison test, linear regression analysis, and Weibull statistics. The pooled data sets of specimens aged in water vs. those aged in air showed a significant difference in the respective modulus of rupture (air, 108.50 +/- 16.11; water 96.94 +/- 15.04 MPa). The one-way analysis of variance showed no significant difference between the aging times zero, six, and 12 months in each respective aging medium. The Weibull analysis also showed no difference between the Weibull constants, 7.66 air vs. 7.64 water, but a significant difference between the characteristic strengths, 115.22 air vs. 103.02 water. This study indicates that distilled water has a significant degradative effect on a magnesia alumina spinel, more likely affecting the mode of fracture rather than the stress corrosion characteristics.

Fracture studies of selected dental restorative composites.

OBJECTIVES: The purpose of this study was to evaluate the flexure strength, elastic modulus, and fracture toughness (mode I, mode II, and mixed mode) of resin and four specially made dental restorative composite materials. METHODS: Testing was done on prismatic bars in flexure and disk specimens in diametral compression. Fracture strengths were analyzed using Weibull statistics. Statistical analysis consisted of a one-way analysis of variance followed by a Tukey multiple means analysis for each of the materials. In addition, the fracture strengths were analyzed using Weibull statistics due to the brittle behavior exhibited by these materials. RESULTS: The experimental results showed that the addition of fillers resulted in a significant three-fold increase in flexure modulus and a significant 30-50% increase in fracture toughness from the resin. As was indicated by the different Weibull modulus values, strength data obtained from four-point bending were not related with strength data from three-point bending. A straight notch vs. a relatively sharp V-notch gave higher fracture toughness values. Fracture toughness was dependent on the depth of a straight notch and was practically independent of the V-notch depth. Mode I and II fracture toughness in two composites (75Sr and 75Sr10) were carried out on precracked disk specimens in diametral compression. The results of mode I toughness were close to those obtained from the flexure testing. The mode II toughness values were greater than the mode I values by more than 30%. The data fit an equation of the form KI/KIC + (KII/KIIC)2 = 1(where KI, KII are the mode I and II stress intensity factors and KIC, KIIC are the respective critical values). SIGNIFICANCE: Notching technique, testing configuration (three-point vs. four-point loading), and method of testing (bar vs. disk) have significant effect on the fracture properties.

Particle size analysis of amalgam powder and handpiece generated specimens.

OBJECTIVES: The increasing interest in the elimination of amalgam particles from the dental waste (DW) stream, requires efficient devices to remove these particles. The major objective of this project was to perform a comparative evaluation of five basic methods of particle size analysis in terms of the instrument's ability to quantify the size distribution of the various components within the DW stream. METHODS: The analytical techniques chosen were image analysis via scanning electron microscopy, standard wire mesh sieves, X-ray sedigraphy, laser diffraction, and electrozone analysis. The DW particle stream components were represented by amalgam powders and handpiece/diamond bur generated specimens of enamel; dentin, whole tooth, and condensed amalgam. RESULTS: Each analytical method quantified the examined DW particle stream components. However, X-ray sedigraphy, electrozone, and laser diffraction particle analyses provided similar results for determining particle distributions of DW samples. These three methods were able to more clearly quantify the properties of the examined powder and condensed amalgam samples. Furthermore, these methods indicated that a significant fraction of the DW stream contains particles less than 20 microm. SIGNIFICANCE: The findings of this study indicated that the electrozone method is likely to be the most effective technique for quantifying the particle size distribution in the DW particle stream. This method required a relative small volume of sample, was not affected by density, shape factors or optical properties, and measured a sufficient number of particles to provide a reliable representation of the particle size distribution curve.

Thermal conductivity of cement base materials.

This study was designed to determine the most efficient dentin-cement-restorative material system to minimize temperature changes from either a hot (58 degrees C) or cold (4 degrees C) source. Extracted human molar teeth were used to conduct the experiment utilizing a standard Class I occlusal preparation 3.5 mm in diameter with a remaining dentin thickness of 0.5 or 1.0 mm and a control dentin thickness of 2.0 mm. A bi-electrode thermocouple was utilized with one thermocouple mounted on the roof of the pulp chamber and the other on the occlusal surface of the specimen. The cements investigated were zinc oxide and eugenol (ZOE), zinc phosphate (ZP), glass ionomer (GI), and zinc polycarboxylate (ZPC) with a thickness of either 0.5 mm or 1.5 mm. Statistical analysis consisted of a one-way analysis of variance followed by a multiple means comparison test. For heated specimens consisting of cement and dentin only, ZOE was the least effective in limiting temperature change. When cold was applied, ZOE was the most effective. Statistically for cold application, the other three cements, ZP, GI and ZPC, and 2.0 mm of dentin were the same for limiting temperature change. When amalgam was used to restore the tooth, ZOE and GI were most effective in limiting temperature change for cooled teeth. For heated teeth, all the cements were identical except for ZOE, which was the least effective. For composite restored teeth, all cements and dentin were identical in limiting temperature changes for either heating or cooling.

Mechanical property evaluation of pressable restorative ceramics.

OBJECTIVE: The aim of this study was to evaluate the flexure strength under static and cyclic loading and the fracture toughness under static loading of six restorative ceramic materials. Specifically the intent was to compare four leucite (K2O.Al2O(3).4SiO2) strengthened feldspathic (pressable) porcelains (OA, OI, E, and FP) to a low fusing feldspathic porcelain (F) and an experimental (EC) lithium disilicate containing ceramic. METHODS: All materials were tested as a control in air and distilled water (without aging) and after three months aging in air or distilled water to determine flexure strength and fracture toughness. A staircase approach was used to determine the cyclic flexure strength. EDS and SEM analysis were conducted on the polished and fracture surfaces. RESULTS: The mean flexure strength for the controls in air and water (without aging or cyclic loading) ranged from 67 to 99 MPa, except the experimental ceramic that was twice as strong with a mean flexure strength of 191-205 MPa. For the mean fracture toughness, the range was 1.1-1.9 MPa/m0.5 with the experimental ceramic being 2.7 MPa/m0.5. The effect of testing in water and aging for three months caused a moderate reduction in the mean flexure strength (6-17%), and a moderate to severe reduction in the mean fracture toughness (5-39%). The largest decrease (15-60%) in mean flexure strength was observed when the samples were subjected to cyclic loading. SIGNIFICANCE: The lithium disilicate containing ceramic had a significantly higher flexure strength and fracture toughness when compared to the four pressable leucite strengthened ceramics and the low fusing conventional porcelain. All of the leucite containing pressable ceramics did provide an increase in mean flexure strength (17-19%) and mean fracture toughness (3-64%) over the conventional feldspathic porcelain. Further, the influence of testing environment and loading conditions implies that these ceramic materials in the oral cavity might be susceptible to cyclic fatigue, resulting in a significant decrease in the survival time of all-ceramic restorations.

Surface roughness of polished amalgams.

The purpose of this project was to determine the amalgam surface roughness following finishing and polishing procedures. Nine high-copper amalgams were tested. They were hand-condensed in a 3 x 3 x 30 mm mold. Each sample was divided into four sections, and each section was finished by one of four methods: carving only; carving and then burnishing 15 minutes after carving; carving and then polishing at the prescribed times; or carving, burnishing 15 minutes after carving, and then polishing at the prescribed times. Each bar of amalgam had these surface treatments done at either 1, 3, 6, or 24 hours. The results indicated surface roughness may be more dependent on particle size, shape, and distribution of the individual high-copper amalgams than the polishing time or the surface treatment performed.

In vitro evaluation of endodontic posts.

PURPOSE: To compare stainless steel posts and three different fibrous posts with respect to pullout (shear) strength from extracted third molars embedded in denture acrylic. MATERIALS AND METHODS: Post space was prepared and the posts cemented with a resin cement according to manufacturer's instructions. Single step and multi-step dentin bonding systems were also evaluated. The testing was in tension at a loading rate of 2 mm/min. RESULTS: The statistical analysis indicated no significant difference in the pullout (shear) strength between any of the post groups tested. Also evaluated was the flexure strength of the fibrous posts before and after thermal cycling. Statistical analysis indicated a significant decrease in flexure strength for the respective fibrous posts following thermal cycling.

Geriatric dentistry in the predoctoral curriculum.

The elderly component of the general population has been increasing in the past decades and will continue to increase well into the future. This group of people presents to the dentist with a unique set of characteristics, both dental and physical. To treat the elderly more competently and to provide clinical training to undergraduates simultaneously, a geriatric dental program was initiated in 1976 at the University of Illinois College of Dentistry. The program's current status and operation are described.

Type of physical activity, variables describing participation in physical activity, and self-perceived fitness.

This study examined the association between self-perceived fitness and the type of physical activity and variables describing participation in physical activity by 91 university students. Type of physical activity and variables describing participation were significantly related to self-perceived fitness in the total group (r = .43) and women (r = .52), but nonsignificant for men. Self-reported anaerobic activity was a significant independent predictor of self-perceived fitness in the total group (r = .29) and for women (r = .42) but not for men as was frequency of physical activity in the total group (r = .33) and for women (r = .35) but not for men.

The Fitness Facility Membership Questionnaire: a measure of reasons for joining.

This study describes the development of the Fitness Facility Membership Questionnaire of 43 items, designed to identify reasons for joining a fitness facility. Items were generated from responses to an open-ended questionnaire. A 50-item version of the questionnaire was completed by 152 members of five community-based fitness facilities. Principal components analysis with varimax rotation yielded 8 factors, accounting for 63.8% of the variance. The factors were labeled Socialization, Aquatic-related Facilities, Extrinsic Motivation, Recreational Facilities, Intrinsic Motivation, Resistance Equipment, Aerobic Equipment, and Amenities. The internal consistency of the eight factors was acceptable with Cronbach coefficients alpha ranging from .72 to .89. Discriminant analysis of responses is also presented.

Leisure-time physical activity and self-perceived fitness of hospital employees.

For 58 hospital employees, an r of .40 was obtained for self-reported time spent engaged in regular physical activity and was the only significant independent predictor of self-perceived fitness.