Compressive viscoelastic properties of human dentin: I. Stress-relaxation behavior.

Stress-relaxation measurements were performed on specimens of radicular human dentin. The relaxation modulus showed a linear dependence on the logarithm of time and the approximation to the logarithmic distribution function of relaxation times was used to predict the behavior of other viscoelastic properties. This experimental technique provides significant criteria for the design of polymeric restorative and prosthetic materials.

Wetting effects of surface treatments on inlay wax-investment combinations.

Gypsum-bonded and phosphate-bonded investments were applied to wax surfaces which were untreated, treated by buffing with cotton moistened with a die lubricant containing organic solvent, or treatment with a wax pattern cleaner. Contact angles between the investment material and wax surfaces were measured and compared. The treatment of a wax pattern with a surface tension reducing agent significantly increases the degree of wetting by both gypsum- and phosphate-bonded investments.

The effect of single plane curvature on half-round cast clasps.

The quality of retention produced by a half-round cast clasp is affected by its curvature in the plane of its flat side. Uncurved clasps and those that curve 90 degrees, 180 degrees, and 270 degrees become progressively stiffer. Conversely, as the radius of the curvature increases, so does the flexibility of the clasp.

Compressive stress relaxation measurements of selected dental restorative composites.

Compressive stress relaxation measurements were performed on specimens of selected dental restorative composites. The relaxation modulus showed a linear dependence on the logarithm of time. The values obtained indicated that stress was dissipated more rapidly in dental composites than in enamel, but less rapidly than in human dentin.

Contours of the edentulous palate.

Diagnostic casts were made of the palates of 123 patients who had complete dentures. Categorization of all cross-arch palatal forms yielded a distribution in which 93% showed some variation of a U-shaped palatal form, be it angular or mildly curved. Along the midline, 12% had steep anterior inclines, curved midpalatal regions, and curved posterior palates; 69% had moderate anterior inclinations, curved midpalates, and curved posterior palates; 12% had palates that were moderately inclined anteriorly, flat in the midpalatal section, and flat posteriorly. The remaining palates were predominantly flat in two of the three regions measured.

Nonprecious metal alloys for fixed restorative dentistry.

Non-noble alloys are available for use in the fabrication of cast restorations. These alloys are not necessarily designed for subsequent application of porcelain. Because they are promoted for use in place of the traditional gold alloys, significant changes in technique and armamentarium are necessary. Casting of acceptable clinical quality can be obtained, but not without additional equipment and personnel training. Obtaining casting of gold alloy quality on a routine basis may not be easily achieved. Casting into detail is possible (see Fig. 5), although greater surface roughness of the casting may be observed with the non-noble alloys. A high polish is possible (Fig. 6), however technicians and clinicians accustomed to and expert in polishing single or multiple gold alloy castings will require time and training to adjust to the somewhat more difficult and time-consuming techniques required in the polishing of non-noble alloys. Well fitting castings of the non-noble alloys require investments capable of greater thermal and/or hygroscopic expansion. The solidification shrinkage in non-noble alloys is greater than that of gold alloys. The clinician may find casting of these alloys to be "tight-fitting" on insertion. At present, because additional time and armamentarioum may be required, laboratory fees for the non-noble metal casting may not be any more economical than those of cast gold alloys. Therefore, the clinician and patient may benefit little with regard to reduced fees for restorations constructed of these alloys. In conclusion, the non-noble metals may find increased use in fixed restorative dentistry as expertise develops to the level at which usage of these products becomes routine. These is still considerable clinical information needed as to the durability and corrosion resistance of these alloys. Some work has been work has been done on the corrosion of these alloys, but much data remains to be obtained. Soldering these alloys is difficult and requires training of the technician in the required usage of special solders. The following facts should be considered in casting and finishing non-noble alloys for single and multiple unit casting in dentistry. 1. Silicate or phosphate bonding investments are required. 2. Recognition of proper casting appearance of the molten alloy is essential. 3. Additional armamentarium is required to obtain casting temperatures necessary. 4. Polishing procedures are more laborious and require harder abrasives. 5...

Bond strengths of fluoride-releasing restorative materials.

PURPOSE: To compare the shear bond strengths to dentin of four tooth-colored fluoride-releasing restorative materials that utilize different mechanisms of dentin-bonding. MATERIALS AND METHODS: Four materials were tested: a chemically-cured glass ionomer (Ketac-Fil); a light-cured resin-modified glass ionomer (Photac-Fil); a light-cured resin-modified glass ionomer in conjunction with a resin dentin-bonding agent (Vitremer); and a light-cured resin composite in conjunction with a resin dentin-bonding agent (Tetric). The enamel was removed from four sides of the twenty human noncarious extracted molars which had their roots embedded in acrylic blocks. Cylindrical samples of the materials were prepared in plastic molds and bonded to the dentin surface according to the manufacturers' instructions. After 24 hours of storage in a humidor, the samples were sheared with an Instron testing machine at a crosshead speed of 0.5 mm/minute. RESULTS: Statistical evaluation (ANOVA and Duncan's test) suggests that the fluoride-releasing resin composite material utilizing a resin dentin-bonding agent provided a significantly greater mean shear bond strength (16.5 MPa) after 24 hours storage than any of the selected glass ionomer materials. Of the three glass ionomer materials compared, the light-cured resin-modified material with a resin dentin-bonding agent provided a significantly greater mean shear bond strength (6.7 MPa) than the light-cured resin-modified material without a resin dentin-bonding agent (3.0 MPa) or the chemically-cured material (3.8 MPa).

Effectiveness of four sterilizing procedures on prophylaxis cups.

Though manufactured as a single use, disposable item, prophylaxis (prophy) cups are often cleaned, sterilized or disinfected, and re-used. The purpose of this study was to determine whether 1) sterilization of prophy cups is achieved during routine procedures 2) debris is completely removed after cleaning. Three types of prophy cups (250 of each type) were evaluated. Prophy cups were contaminated with a pumice slurry containing B. stearothermophilus spore. After contamination, cups were distributed equally between ultrasonic or manual cleaning, then further distributed to one of four sterilization procedures: ETO, chemiclave, autoclave, and glutaraldehyde (GLU) immersion (6 3/4 hours). Five cups served as control and were not subjected to sterilization. Additionally, 15 prophy cups were contaminated and immersed in GLU for 10 minutes. Results indicate sterilization may be achieved by all sterilization methods tested. However, recommended disinfection immersion time of 10 minutes for GLU was not effective in achieving adequate disinfection of prophy cups. Photomicrographs indicate debris remains trapped in prophy cups after manual or ultrasonic cleaning. Prophy cup sterilization may be achieved through standard routine sterilization procedures, but remaining debris is likely due to porosity of rubber and configuration of the cups. Time management, cost effectiveness, and sound clinical judgment suggest single use.

Effect of luting media on the compressive strengths of two types of all-ceramic crown.

This study evaluated the effect of selected luting media on the compressive strength of two types of all-ceramic crown. Tooth preparation was standardized; each preparation had a shoulder width of approximately 1.2 mm, and all internal preparation angles were rounded. Hi-Ceram and Dicor all-ceramic crowns were fabricated and cemented into the preparations with zinc phosphate, glass-ionomer, or composite resin cement. Coronal compressive fracture strengths were determined, using a set of unrestored teeth as a control. There were no statistically significant differences among the mean compressive strengths of the three luting media, and there was no statistically significant difference between the mean compressive strength of Dicor and that of the natural tooth control.

In vitro weathering of selected direct esthetic restorative materials.

Reflectance spectroscopy measurements were used to compare the degree of surface color shift in two classes of visible light-cured resin composites (hybrid and small-particle) and a glass-ionomer restorative. The weathering times investigated were 0, 100, 200, 300, and 400 hours. Environmental factors of heat, ultraviolet light, and moisture were used to affect the surface integrity of the restoratives. The parameter of delta E * ab (overall color difference) of the CIELAB system for measuring small color differences was used. Chroma changes [delta C * ab] and hue differences [delta H * ab] were also determined. Color shifts, chroma changes, and hue differences were observed for all restorative materials evaluated, regardless of the weathering conditions or time intervals imposed. Glass-ionomer cement demonstrated a significantly greater degree of color shift than did the resin composites. There was no significant difference in the amount of overall color change between the two resin composites, although the difference in change in lightness was significant.

Interfacial bond strengths of amalgam bonded to amalgam and resin composite bonded to amalgam.

OBJECTIVE: The effects of time, surface preparation, and use of a bonding agent on the ability to bond amalgam or resin composite to amalgam were studied. METHOD AND MATERIALS: Cylindrical amalgam specimens were prepared in a split die (32 groups of 10 each). The 32 groups were divided into two groups of 16 each: amalgam bonded to amalgam and resin composite bonded to amalgam. One half of each group of 16 was bonded within 1 hour after the initial condensation (fresh) and the other half was bonded after 21 days' storage in physiologic saline (old). Four bonding methods (control [no bonding agent], Advance, All-Bond 2, or Amalgambond Plus) were each utilized on one abraded group (air abrasion with 50-microns aluminum oxide) and one unabraded group. After an additional 21 days of storage, bond strengths were measured with a three-point flexure test. Solid amalgam cylinders also tested. RESULTS: Solid amalgam control cylinders exhibited mean bond strengths 10,000 psi greater than any of the bonded specimens. Among specimens of amalgam bonded to amalgam, those that were abraded after 21 days and used no bonding agent showed a significantly greater bond strength than did specimens utilizing any of the other combinations of the selected independent variables. Among specimens of resin composite bonded to amalgam, those bonded after 21 days and utilizing a bonding agent exhibited significantly greater bond strengths than did specimens utilizing any of the other combinations of the selected independent variables. CONCLUSIONS: The results of this in vitro study suggest that the strength of a repaired amalgam is only about 40% of that of an unrepaired amalgam. They also indicate that the strongest repair of amalgam using additional amalgam material may be accomplished without the use of any bonding agents utilized in this study. When adding resin composite material to amalgam, the resin composite should be added after the amalgam has had adequate time to set, and the use of a bonding agent increases the strength of the repair.

Stiffness-deflection behavior of selected orthodontic wires.

Treatment of horizontal and vertical tooth discrepancies requires wires of low stiffness to produce forces as the teeth are leveled and aligned. In this investigation, the stiffness characteristics of several solid and multistrand nickel-titanium and stainless steel orthodontic wires were determined at selected clinically relevant deflections. Twenty specimens of 24 different wires were tested in both three-point and three-bracket bending modes. The unloading force deflection plot of each wire was described by a polynomial regression from which wire stiffnesses were obtained by mathematical differentiation. Graphs of the functional relationship between stiffness and deflection are presented. The results of this investigation show that wire stiffness can be altered not only by changing the size, but also by varying the number of strands and the alloy composition. An equally important finding was the dependence of stiffness on deflection for most of the wires measured. Comparisons were also made between the stiffness values obtained in three-point bending and the three-bracket bending systems.

Load-deflection rate measurements of activated open and closed coil springs.

The purpose of this investigation was to provide load-deflection rate data for a variety of open and closed coil springs. Ten millimeter lengths of open and closed coil stainless steel and Cobalt-Chromium-Nickel (Co-Cr-Ni) alloys in combinations of 0.008, 0.009 and 0.010 inch wire sizes, and 0.030 and 0.032 inch lumen sizes were tested. Other groups included heat treated Co-Cr-Ni springs and springs of 15 and 20 millimeter lengths. Forces and activations were measured by a tension load cell with an Instron universal testing instrument. Stiffness increased dramatically with wire size and pitch angle of the coils. Stiffness decreased slightly with increased lumen size. Co-Cr-Ni closed coil springs were slightly stiffer than stainless steel, whereas stainless steel open coil springs were much stiffer than Co-Cr-Ni. Heat treatment increased the stiffness of Co-Cr-Ni coil springs. The length of the spring had a great effect on the load-deflection rate. A shorter spring is stiffer than a longer spring by an amount directly proportional to the ratio of the length of the longer spring to that of the shorter spring.

Fracture strengths of ceramic brackets subjected to mesial-distal archwire tipping forces.

This study tested the strength of ceramic orthodontic brackets subjected to mesial-distal tipping forces on five types of preadjusted, maxillary right central incisor ceramic twin brackets for both 0.018" and 0.022" slot sizes. Description of each bracket was by manufacturer's abbreviation-crystallinity-slot bracket, eg., AL-P-18, meaning Allure-polycrystalline-0.018" slot bracket. Thirty brackets of each type were used for a total of 300 brackets, each bonded to a porcelain denture tooth. A special apparatus was designed to hold the denture tooth, the wire, and the bracket in a standard position while an Instron machine applied a tipping force to the full size rectangular archwire at a distance of 7.0 mm lateral to the center of the bracket. The tipping force was applied until the bracket fractured. The fracture force, fracture angle, and fracture location were recorded. High fracture force values tended to accompany large fracture angles while low fracture force values tended to be associated with small fracture angles. The clinical significance was that the stronger ceramic brackets can be expected to withstand larger amounts of archwire tipping adjustments prior to bracket fracture. With the literature indicating the optimum force for tipping of maxillary incisors to be from 50 to 125 g, all the brackets are sufficiently strong to consistently withstand the suggested magnitude of archwire tipping forces. However, if excessive tipping forces were required by the clinician, ceramic brackets would be prone to fracture.

Uses and properties of current glass ionomer cements: a review.

Glass ionomer cements have been used for a variety of restorative purposes. Recently, the formulation of these and other restorative materials has changed rapidly, so it is difficult to keep pace with the literature concerning effective use. A major modification to glass ionomer cements has been incorporation of resin components. This and other modifications over the years have changed some handling characteristics and physical properties of glass ionomer cements. A general review of some changes is reported here.