Opacity of glass-ionomer cements

The translucency of the first generation of glass-ionomer cements to appear on the market was too low to be cosmetically satisfactory. In the present study the opacity of newer editions of glass-ionomer cements was evaluated and compared with that of a well-known composite resin. Disc-shaped specimens were placed on black and on white backings, the luminous reflectances measured and corrected to a specimen thickness of exactly 1 mm, whereafter the contrast ratio was calculated. The newer glass-ionomer cements were found to be more translucent than the first generation of glass-ionomer cements on the market. However, the opacity still needs to be reduced to reach the level of a composite resin. It was further established that early water contact results in a considerable increase in opacity.

Bonding of restorative resins to dentin by means of methacryloylchloride and methacryloyl-R-isocyanate

The bonding of restorative resins to dentin by means of an intermediary monomer containing a carboxylic acid chloride or an isocyanate group was investigated. Such intermediary monomers are supposed to react with the organic constituent of dentin. Bond strengths were enhanced by the use of the intermediary monomers, resulting in mean bond strengths of 0.13 kg/mm 2. Bond strengths of this order of size are probably too low to be of clinical interest.

Factors affecting the color stability of restorative resins

The color change resulting from storage in water was measured on a number of experimental and proprietary complex resins. The experimental resins contained varying amounts of amine (DEPT or DEBA), benzoyl-peroxide (BPO) and inhibitor (MHQ). The proprietary resins had earlier been analyzed as regards the composition of the monomer and the content of amine and peroxide. The color change of DEPT-containing resins was positively correlated with the DEPT/BPO ratio, and negatively correlated with the monomer content of BISGMA. DEBA-containing resins were more color stable than resins containing equimolar concentrations of DEPT. The color change was independent of MHQ up to a certain concentration; above this the color change increased with the amount of MHQ. In general, the light activated materials were more color stable than the chemically activated materials. The color change was not affected by pH, but decreased when the oxygen had been removed from the storage water. The test for color stability using UV-light irradiation gave results for proprietary resins that were not correlated with the results obtained by water storage.

Factors affecting the quantity of remaining double bonds in restorative resin polymers

The quantity of remaining unreacted double bonds may be a significant factor pertaining to the mechanical, physical and chemical properties of restorative resins. The said quantity was determined from the transmission IR-spectrum of the materials before and after polymerization. For proprietary resins the quantity of remaining double bonds was found to vary between 23 and 43%. For experimental, chemically cured resins the quantity in question was found to decrease with increasing resin content of amine and peroxide, and to increase with the content of inhibitor. The content of peroxide was the more influential of the three factors. For constant content of amine, peroxide, and inhibitor the number of remaining double bonds decreased with increasing content of diluting monomer in BISGMA-based monomer mixtures. The use of a trifunctional monomer as diluting monomer did not give rise to a higher degree of conversion of the double bonds in comparison to a bifunctional diluting monomer. The quantity of remaining double bonds of a light curing material was relatively small when compared to the quantity present in chemically cured materials of adequate setting time.

Restorative resins: hardness and strength vs. quantity of remaining double bonds

It has been hypothesized that the Wallace indentation hardness of smooth surface resins is a factor of prime importance for the abrasion by food of Class 1 restorations. In the present work factors affecting the hardness of polymers were investigated. In addition the tensile strength of composite resins was measured and related to the catalytic system of the polymer. It was found that for a given composition of the monomer the Wallace hardness number increased with increasing content of inhibitor, decreased with increasing content of peroxide, and was unaffected by changes in the content of amine. The hardness was well correlated with the quantity of double bonds remaining in the polymer. BISGMA-based polymers showed no variation in hardness when the originating monomer varied with respect to content of a bi- or a trifunctional diluting monomer. Light-polymerized polymers were relatively hard as compared to chemically cured materials of adequate setting time. The tensile strength of composite resins was predominantly determined by the monomer content of peroxide and increased herewith. The tensile strength was well correlated with the quantity of remaining double bonds in the constituting polymer.

Microscopical analysis of fractured restorative resin/etched enamel bonds

The purpose of the present work was to analyze the boundary structures of fractured restorative resin/etched enamel bonds. The fracture patterns were studied by scanning electron and optical microscopy. The three main types of failure observed were classified as: apparent interfacial failure, failure in the bulk of the restorative resin and failure in the bulk of the enamel. The fracture of the resin/etched enamel bond in the case of the apparent interfacial failure was located in the resin very near the interface. No true interfacial debonding occurred in any of the bonds studied.

Fatigue strength of some resinous materials

The fatigue properties of resinous materials may be interesting from several points of view. A good resistance to fatigue failure is desirable in a denture base material. The possible relationship between fatigue strength and wear constitutes another aspect of interest. The purpose of the present work was to investigate the fatigue strength of various types of resinous materials used in dentistry. It was found that heat-cured denture base materials do not necessarily have higher fatigue strength than cold-cured materials. At low levels of stress a tendency was demonstrated for microfilled resins to show higher fatigue strength than other types of resinous materials. On this basis a connection between wear and fatigue properties may be surmised.

Heart rate and ventilatory frequency as dimension-dependent variables

Heart rate (HR) and ventilatory frequency (f) were determined at rest in 79 boys and 91 girls aged 0.07-20 years and with a range in heights from 54-198 cm to establish the relationship between decrease in HR and f during early life with increase in body dimensions. From theoretical considerations it is assumed that frequencies such as HR and f are proportional to a characteristic linear dimension of the subjects in minus first power (approximately equal to rho-1). In the present material resting HR and f decreased with increasing height of the subjects according to the equations HR = 6,408 . height-0.9015 (r = 0.8983) and f = 5,969 . height-1.1691 (r = 0.8471). These results suggest that the decrease in resting HR and f during childhood and adolescence corresponds well with increase in body dimensions. A similar equation was developed for HRmax within the range of heights from 149-198 cm. Only a slight dependency on height was observed (HRmax = rho-0.1642, r = 0.3796).

Setting time of composite restorative resins vs. content of amine, peroxide, and inhibitor

The setting time was measured for a number of experimental composite resin formulations with varying content of amine, peroxide, and inhibitor. Assuming a bimolecular reaction between amine and peroxide and on the basis of reaction kinetics, an equation was obtained expressing setting time in terms of content of amine, peroxide, and inhibitor. The experimental data were found to fit the equation at a high level of statistical significance (p less than 0.001).

Correlation between tensile and bond strength of composite resins

The purpose of the present work was to measure the bond strength of five composite resins (two microfilled and three conventional) to human etched enamel and to correlate the findings with previously published results on the tensile strength of the same resins. The two microfilled composites showed a significantly lower bond strength than the three conventional products. The coefficient of correlation between tensile and bond strength was 0.74.

Tensile strength of restorative resins

The purpose of the present work was to measure the tensile strength of restorative resins and to study the effect of the method of measurement on the recorded results. A direct pull method using dumb-bell shaped specimens was used. The tensile strength of the resins was also tested using the diametral compression method suggested by the A.D.A. It was found that the method of testing affects the results. Although the diametral compression method is a simple method, it cannot be considered reliable for all types of material. The tensile strength of the conventional composites was significantly higher than the tensile strength of the microfilled composites.

A qualitative and quantitative analysis of tertiary amines in restorative resins

Previous investigations have shown that the type and the quantity of tertiary amine influence the properties of restorative resins. It was the purpose of the present work to analyze qualitatively and quantitatively the amines in commercial restorative resins. The amines were extracted with hydrochloric acid from chloroform solutions of the monomers. For the qualitative analysis the acid amine solution was made alkaline, the amine extracted with methylenechloride and isolated. The method of analysis was NMR-spectroscopy. The quantitative determination was performed by means of UV-spectra of the acid amine solutions. Sixteen brands were investigated, and three species of amine were identified. The amount of amine varied between 0.3% and 4% by weight of the monomer. Iit is probable that the information obtained will lead to a better understanding of the nature of restorative resins.

Quantitative analysis of peroxides in restorative resins

It has been demonstrated previously by other authors that the quantity of benzoyl-peroxide influences the properties of restorative resins. The purpose of the present work was to analyse quantitatively peroxides in commercial restorative resins. The analysis was performed by iodometric titration. Sixteen brands were investigated, and it was found that the amount of peroxide varied between 0.3% and 2.6% by weight of the monomer. It is possible that the information obtained will lead to a better understanding of the nature of restorative resins.

A central nervous component in local muscular fatigue

It has been shown previously (this journal) that any activity, physical or mental, performed as "diverting activity" in the pauses between bouts of exhaustive musclar work has a positive effect on the recuperation after fatigue. It was concluded that this effect was primarily of central nervous origin (Setchenov, 1903) and not a circulatory effect (Weber, 1914). In the present experiments it is shown that the amount of work that can be performed before exhaustion is larger when the subjects work with their eyes open than when they work with closed eyes. Further, when complete exhaustion has been reached with closed eyes, opening of the eyes results in an immediate return of a working capacity amounting to 15-30% of that already performed. It is demonstrated, that patellar reflexes are brisker when the eyes are open than when they are closed. The same is the case during the diverse forms of "diverting activity" used in this and the previous article. The brisker myotatic reflexes are taken to mean an enhanced central nervous arousal, and the recuperative effect of "diverting activity" is consequently explained as being due to an increased facilitation of the neuro-motor system, which in fatigue is inhibited centrally through afferents from receptors in the fatigued muscles.

Recuperation after muscular fatigue by "diverting activities"

"Diverting activity" is defined as any physical or mental activity performed between or simultaneously with bouts of exhaustive, local muscular work. In the present experiments bouts of exhaustive work consisting of rhythmic lifting of weights were performed with the elbow flexors or with the flexors of the middle finger. Pauses of 2 min duration spent in complete rest or while performing diverting activities alternated between the bouts of work. As diverting activities were used: Physical activity, dynamic or static, performed with big or small muscle groups (other than the fatigued group), or mental activity (problem solving). It was found that the amount of work that could be performed after a pause with diverting activity was always larger than the amount of work performed after a passive pause. The beneficial effect was seen also when the blood flow to the exhausted muscles was interrupted by pneumatic cuffs. Determination of the blood flow in the exhausted muscles by means of Xe-133 clearance showed no systematic blood flow increases caused by the diverting activity. It is concluded that recuperation after local muscle fatigue is influenced by a central nervous factor (Setchenov phenomenon) that is largely independent of the local blood flow.

The stability of water in the pores of acid etched human enamel

A mechanism is proposed, explaining the fact that drying by compressed air for 5-10 sec. will remove acid and dissolved enamel from the relatively long and narrow pores of etched enamel surfaces. It is a consequence of the proposed mechanism that water in the liquid state is not stable in the pores, and accordingly, that condensation of water vapour on the etched and dried surfaces is not likely to occur. This was confirmed by studying tag formation in etched enamel surfaces under conditions where water vapour will condense on unetched tooth surfaces.