Edward C. Combe

A protocol for determining the surface free energy of dental materials

OBJECTIVE The purpose of this study was to develop a standard methodology for measuring the surface free energy (SFE), and its component parts, of dental biomaterials. METHODS The contact angle of each of four samples of two materials--low density polyethylene and poly(methyl methacrylate)--was measured three times in each of six liquids (1-bromonaphthalene, diiodomethane, ethylene glycol, formamide, glycerol and distilled water). Critical surface tension estimates were obtained from Zisman plots. Data were then analyzed by the least-squares method to estimate the components of SFE. Estimates were also made for each of 12 liquid triplets, and by maximum likelihood and Bayesian analyses. RESULTS The use of liquid triplets could yield misleading estimates of the components of SFE. A testing protocol is suggested in which multiple test liquids are used, and multiple methods of statistical analyses employed. SIGNIFICANCE SFE is important, in that high SFE is desirable when adhesion is required, but undesirable if plaque resistance is needed. Methodology that avoids some of the limitations of existing studies has been proposed.

Calcium hypochlorite as a disinfecting additive for dental stone

STATEMENT OF PROBLEM Dental casts come into direct contact with impression materials and other items that are contaminated by saliva and blood from a patients mouth, leaving the casts susceptible to cross-contamination. Topical methods of disinfecting casts are difficult to control, while immersion methods are potentially destructive. Thus, an additional method to control cross-contamination between patients and laboratory personnel is needed. PURPOSE This study was undertaken in an attempt to develop a dental stone with disinfecting properties and adequate compressive and tensile strengths. MATERIAL AND METHODS Calcium hypochlorite [Ca(OCl)(2)] in aqueous solution in concentrations from 0 to 1.5% was tested as a disinfecting additive to type V dental stone. The compressive and tensile strength properties of the modified stone were measured (MPa) using a universal testing machine at a consistency similar to unmodified stone. Strength data were analyzed by 1-way ANOVA and post hoc Tukey-Kramer procedure (alpha < or =.05). To measure the disinfecting ability, the effect on Bacillis subtilis bacteriophage phi29 was tested in triplicate to find the minimum concentration at which no phage was detected. Additionally, 3 impressions were disinfected with CaviCide, and 3 impressions rinsed in water served as controls. RESULTS In general, the effect of adding the disinfectant to the stone was a decrease in strength. Exceptions were the dry compressive strength, for which there was a significant increase in strength (P=.048) at 0.5%, and the wet compressive and wet tensile strength, which showed no significant difference between the 1.5% and the control. When Ca(OCl)(2) was added at the concentration 0.5% (2765 ppm available chlorine), the gypsum had acceptable mechanical properties; dry compressive strength was 78.86 +/- 4.12 MPa, and dry tensile strength was 10.64 +/- 1.27 MPa, compared to control values of 67.85 +/- 6.28 and 13.41 +/- 1.24 MPa, respectively. At concentrations of 0.3% and higher (36 1650 ppm of available chlorine), calcium hypochlorite was able to completely inactivate phi29. CONCLUSION It is possible to prepare a type V dental stone that contains a disinfectant, has adequate mechanical properties, and will reduce numbers of residual microorganisms. For example, stone mixed with water containing 0.5% Ca(OCl)(2) meets these criteria.

The effect of disinfectants on the properties of dental gypsum: 1. Mechanical properties

PURPOSE This study was conducted to evaluate the effect of selected disinfectants incorporated in the liquid of dental stones on material strength properties with the aim of developing a material with acceptable mechanical properties. MATERIALS AND METHODS Two types of dental stone (types III and V) were mixed with aqueous solutions of 0.525% sodium hypochlorite, 0.1% and 10% povidone-iodine, and 2% glutaraldehyde, and with water as a control. The liquid/powder ratios recommended by the manufacturer were used. These materials were subjected to further modification by adding a mixture of 1.0% gum arabic and 0.132% calcium hydroxide to the hemihydrate powders before mixing with the disinfectant solutions at 2 different liquid/powder ratios for each. Both the regular and the modified materials were tested for compressive and diametral tensile strength after 1 hour and 1 week from the start of the mix. The structure of set materials was determined by scanning electron microscopy examination of fracture surfaces. RESULTS The disinfectants often reduced the strength of both types of dental stone. However, using either 0.1% povidone-iodine or 0.525% sodium hypochlorite resulted in strength values comparable with that of the control. The addition of gum arabic and calcium hydroxide helped reduce the mixing liquid/powder ratios, improving the strength properties of the disinfected materials. CONCLUSIONS Chemical disinfectants reduce the strength of dental gypsum when used as mixing water substitutes. Gum arabic and calcium hydroxide additives permit a lower liquid/powder ratio and can help offset this weakening.

Polymerization of orthodontic adhesives using modern high-intensity visible curing lights.

This study was undertaken to assess the efficacy of 3 visible curing lights: a conventional halogen light and 2 high-intensity halogen lights in the polymerization of a polymer-based and resin-modified glass ionomer orthodontic cement. Degree of polymerization was measured by Fourier transform infrared spectroscopy and the development of mechanical properties by Barcol hardness. The results were analyzed with either 2- or 3-way analysis of variance. It was shown that, for the polymer-based material, there was a significant increase in degree of cure and hardness with time of application of the light for each light source. For chemical conversion, there was no significant difference between the lights. However, there was a difference in hardness: the higher intensity lights produced greater hardness in shorter time. Thus, there was poor correlation between degree of polymerization and hardness. For the resin-modified glass ionomer, similar trends were found, but there was a difference in hardness between the top and the bottom of the specimens. It was concluded that the higher intensity lights could aid in the more rapid development of mechanical properties of the tested adhesives.

New core and sealer materials for root canal obturation and retrofilling.

A new endodontic obturation and retrofilling material has been invented to satisfy Grossmans requirements for an ideal material. Methylene blue dye leakage studies were carried out to compare the new material with conventional products, for both obturation and retrofilling. The experimental material when used in root canal obturation had a mean apical leakage of 0.49+/-0.27 mm, compared with 3.75+/-2.81 mm for the conventional material (p < 0.01, t test). Retrofilling leakage was not significantly different from that of a glass-ionomer (p < 0.01, t test) but was substantially lower than that of MTA and Super EBA (p < 0.01, Chi-square test). It was concluded that leakage in root canal obturation was reduced by an order of magnitude and in retrofilling was significantly better than clinically advocated materials. It was inferred that the adhesion mechanism used has contributed to the reduction in leakage.

Properties of gypsum with the addition of gum arabic and calcium hydroxide

The purpose of this work was to study the effects of adding a mixture of gum arabic and calcium hydroxide to a type III stone and an improved die stone. The effect on the water/powder ratio, compressive and tensile strength, hardness, and setting expansion was determined. Data for mechanical properties were subjected to two-way analysis of variance, followed by the Bonferroni-Dunn procedure to determine any significance (1%) between pairs of means. For the type III material the additives had little effect on setting expansion, but the additives significantly improved surface hardness. The additives did not improve compressive or tensile strength for the type III materials. In contrast, the improved die stones hardness was not enhanced by the additives, and in some instances a reduction in strength was observed. It was concluded that dental cast materials with superior surface properties can be produced by a simple change of formulation.

A study of the viscosity and working time of resin-based root canal sealers

The viscosity and working time of three resin-based root canal cements, namely AH26, silver-free AH26, and Diaket, were studied under in vitro conditions using an indirect extrusion viscometer that was designed to simulate aspects of the clinical situation. The effects of prolonged spatulation and the rate of shearing upon the working time of these materials were also evaluated. All three cements exhibited pseudoplastic behavior but the compaction of the materials during testing caused a thickening effect which reduced their working times. Prolonged spatulation, on the other hand, increased their working time. In view of these findings, we recommed that prolonged spatulation followed by slow extrusion of the cements into the canal be adopted for their clinical usage in order to delay the onset of the setting reaction until after a closely adapted thin interlayer has been formed.

A new single-step technique for apical retrofilling that significantly reduces microleakage

One of the properties of an ideal retrofilling material is little or no microleakage. A new dentin bonding agent based on polyacrylic acid (PAA) is ideally suited to the moist apical environment because of its hydrophilic nature. PAA as a dentin bonding agent for apical retrofillings was tested in an in vivo leakage study using single canal roots of extracted human teeth. Bonding was attained by pretreatment of the dentin in apical retrofilling preparations with a solution of PAA, followed by placement of a nonaqueous restorative material. Using PAA as dentin bonding agent reduced apical leakage at statistically significant levels with Super EBA as a restorative material. In vivo testing is required to validate these findings.

Rheological characteristics of tooth bleaching materials

Tooth bleaching materials need to flow easily on insertion but should have high viscosity at low stresses to stay in place on the teeth. Some degree of elasticity may also aid retention on the teeth thereby maximizing efficacy. The present work was undertaken to study the comparative rheology of three tooth bleaching systems: two gels (Opalescence, Ultradent; Perfecta Trio, American Dental Hygienics) and a paste (Colgate Platinum, Colgate). A dynamic stress rheometer (Rheometrics Scientific) with cone and plate geometry was used, with the materials maintained at 37.0+/-0.1 degrees C with a vapour hood to minimize volatilization. Stress creep and recovery experiments were carried out. Steady shear viscosity for all three systems was high (>10(6) Pa s(-1)) for stresses <20 Pa. Between 100 and 200 Pa stress, all three materials showed a large drop in viscosity and flowed readily. The recovery portion of the data showed a marked difference where the elasticity of the gels was nearly two orders of magnitude higher than that of the paste. It was concluded that all materials would flow readily on insertion into the mouth and all have desirable high viscosity at low stress, but the paste material had the lowest elasticity. The effect of elasticity on performance needs to be determined clinically.