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Biomedical subjects

A J de Gee

Publications and source records attributed to A J de Gee.

At least 19 recordsLinked to original sources
PubMed1996

Influence of filler parameters on the mechanical coherence of dental restorative resin composites.

Next to the presence of (chemical) coupling between filler and resin, filler particle size is important for the mechanical coherence of dental resin composites which are used for posterior restorations. In the range of the current composites a smaller particle size is desirable. The better mechanical coherence for composites with smaller particles found in an in vitro erosive wear test is probably related to the size of food fibres, which are part of the erosive medium. It appears that there is a critical value of the filler particle size (1.3-1.5 microns), under which the food fibres are not able to penetrate in the interparticle space, so the erosive capability of the erosive medium will be reduced.

Adhesiveness↗
PubMed1996

Influence of enzymes and plaque acids on in vitro wear of dental composites.

The purpose of this investigation was to test the effect of an enzymatic and an acidic medium on in vitro wear of dental resin composites and an amalgam. Wear was evaluated in two-body and three-body wear conditions. The third body was a food slurry either in water, an esterase solution or an acid solution at a composition and concentration as found in caries active plaque fluid. In the two-body tests the slurry was left out. Under three-body wear conditions neither the enzyme nor the acids showed any significant (P < 0.05) effect on wear. Enzyme inactivation from a large excess of food constituents in the slurry or the absence of an effect from surface diffusion rates of the acids resulted in the lack of an effect on wear. The two-body/acids wear rates were statistically (P < 0.05) lower than the two-body/water wear rates caused by increased lubrication from the acids between sliding surfaces. A significant (P < 0.05) increase in wear was found in the two-body/enzyme experiments, showing that the enzymatic activity can start directly at the outer surface of the composites.

Acids↗
PubMed1996

Light initiation of dental resins: dynamics of the polymerization.

Literature has shown that slowing down the curing rate of light-initiated resin composites by lowering the light intensity of the light source improved the integrity of the adhesive composite-cavity interface as a result of a more gradual contraction stress development. This study investigated how the curing rate could be controlled by varying the concentrations of the photoinitiator components in BisGMA-TEGDMA resin. Conversions and curing rates were evaluated by linear contraction experiments and Fourier transform infrared spectroscopy. Curing rates could be moderated significantly be reducing the concentrations of both photoinitiator and reducing agent without affecting the conversion, provided that the light exposure with a standard dental light curing unit lasts for 60 s.

Light↗
PubMed1996

Early and long-term wear of conventional and resin-modified glass ionomers.

Various studies have shown that glass ionomers are susceptible to brittle fracture and acid conditions and that they undergo long-term changes in their mechanical properties. Little information is available on how brittleness, acid susceptibility, and long-term changes are reflected in the wear characteristics of glass ionomers. The purpose of this study was to evaluate long-term changes in conventional glass ionomers, metal-reinforced glass ionomers (including a cermet), and (light-curing) resinmodified glass ionomers by wear experiments simulating the wear process in occlusal contact-free areas. The wear tests were conducted periodically over a period of one year. In addition, wear was determined after one year at a pH of 5 or 6, for assessment of acid susceptibility, and at a condition as found in the occlusal contact areas. All materials showed high early-wear rates which decreased significantly during the one-year test period. This long-term process may be related to a slow progression of the acid-base reaction extending over several months. At each stage, the resinmodified glass ionomers wore significantly faster than the acid-base setting glass ionomers. Most of these materials were not affected at a pH of 6.0, while at a pH of 5.0 only the conventional and the metal-reinforced glass ionomers showed increased wear. Direct contacts with the antagonist led to a significant increase in wear in comparison with contact-free wear, probably as a result of sub-surface fatigue phenomena. In view of the unfavorable wear characteristics of the resin-modified glass ionomers and the high early wear of the conventional glass ionomers, including the metal-reinforced glass ionomers, it was concluded that none of these materials can yet be recommended for use in high-stress-bearing situations.

Aluminum Silicates↗
PubMed1995

The influence of water sorption on the development of setting shrinkage stress in traditional and resin-modified glass ionomer cements.

OBJECTIVES: The aim of this study was to determine the setting stress development for some traditional and resin-modified glass ionomer cements and to assess the effect of early water exposure to this stress. METHODS: The development of the setting stress of the glass ionomer cements was determined in a tensilometer set-up as described earlier by Feilzer et al. (1987). RESULTS: The results of this study show the influence of water sorption on the development of setting shrinkage stress in bonded glass ionomer cements. When curing took place under isolated conditions (no hydration or dehydration), all the traditional glass ionomer cements investigated fractured spontaneously, either adhesively and/or cohesively, due to the developing stress. Early exposure to water led to stress relief and prevented spontaneous fracturing. For the light-cured products, no spontaneous failures were observed under isolated conditions. Stress relief due to water sorption reversed the contraction stress into an expansion stress. SIGNIFICANCE: Exposure of traditional glass ionomer cements to water at an appropriate time by the use of permeable matrix systems is advised. Whether the conversion of contraction stresses into expansion stresses as observed for the resin-modified products, is beneficial for a restoration requires further study.

Absorption↗
PubMed1995

Influence of light intensity on polymerization shrinkage and integrity of restoration-cavity interface.

The results of this study showed that the use of high intensity curing light units negatively affected the integrity of the restoration-cavity interface in class V restorations This is explained by the high reaction rates of light curing resin composites. The interfacial integrity was better preserved with low light intensity as it extends the visco-elastic stage of the setting materials, thereby moderating the setting stress development. The ultimate polymerization shrinkages for both conditions were equal, which suggested equal degrees of conversion and thus equal material properties. The results may alleviate the trend in using higher intensity light curing units and in particular the development of units with laser beams in an attempt to further increase conversion rates.

Composite Resins↗
PubMed1994

Silane treatment of filler and composite blending in a one-step procedure for dental restoratives.

The aim of this study was to investigate whether a one-step procedure of silane treatment of filler and composite blending could also produce the strong resin-filler integrity, as can be obtained for dental composites in two steps by pretreating the filler with a silane coupling agent before blending. Resin mixtures containing either a silane-functional methacrylate or a non-silane-functional methacrylate were blended with silane pretreated or non-treated filler particles. The resin-filler integrity of the composites was determined from the wear performance in three-body wear tests. For composites with the non-silane-functional methacrylate the wear rates dramatically decreased when the silane pretreated filler was used instead of untreated filler, showing the efficacy of the silane treatment. For composites with the silane-functional methacrylate the decrease was small, indicating comparable resin-filler integrity for the one-step and the two-step procedures.

Analysis of Variance↗
PubMed1994

Occlusal wear simulation with the ACTA wear machine.

This paper describes how the clinical conditions in stress bearing areas are thought to be simulated in the ACTA wear machine. The wear types simulated are erosive wear and contact sliding wear in the presence of a third-body medium, consisting of natural food substances. Wear due to surface fatigue may also be studied with the wear machine. Wear rates for a wide range of resin composites, an amalgam and a glass polyalkenoate (ionomer) cement obtained with the ACTA wear machine correlated with an average correlation coefficient of 0.90 with data collected from clinical trials. The results justify the experimental set-up as being one that includes a realistic simulation of the complexity of clinical wear.

Bite Force↗
PubMed1993

Setting stresses in composites for two different curing modes.

As a continuation of a study into the development of the polymerization shrinkage stress of chemically initiated composites (CC), the development of the polymerization shrinkage stress of light-initiated composites (LC) in relation to the configuration-factor was determined. During setting, the LC composites generated a higher polymerization shrinkage stress, developed higher cohesive strength to resist this stress and showed less flow than their CC analogues. Trying to find a comprehensive explanation for the differences in behavior of stress development in LC and CC composites, the effect of mixing-in of porosity was also investigated on LC composites. Mixing-in of porosity slowed down and decreased the shrinkage stress development. This was attributed to either oxygen inhibition due to admixed air or to an increase of free surface from the presence of pores within the bulk of the composite.

Composite Resins↗
PubMed1993

Polymerization contraction and conversion of light-curing BisGMA-based methacrylate resins.

The aim of this study was to investigate the polymerization contraction and the conversion of light-curing methacrylate resins based on bisphenol-A bis(2-hydroxypropyl)methacrylate (BisGMA) diluted with triethylene glycol dimethyacrylate (TEGDMA), methyl methacrylate (MMA), hydroxypropyl methacrylate (HPMA) or (+/-)-2-ethylhexyl methacrylate (EHMA). The contraction measurements were carried out with a linometer, a simple device to determine true linear polymerization contraction of liquid monomers at ambient temperature. The contraction increased with the amount of diluting monomer. The estimated conversion of the BisGMA-TEGDMA, calculated using the contraction, is consistent with literature values. The BisGMA-HPMA mixtures showed high conversions at moderate contraction.

Composite Resins↗
PubMed1993

Influence of shearing action of food on contact stress and subsequent wear of stress-bearing composites.

The influence of sliding action of the antagonist on occlusal three-body wear of composites and an amalgam was investigated in vitro by gradual change in the distance between the opposing substrates. When the distance was decreased from 10 microns to approximately 3 microns, wear increased significantly by a factor of two to three and was exclusively of erosive nature. At a slurry-film thickness of approximately 1 micron, direct contacts between the antagonist and protruding composite filler particles started to occur. This consequently slowed the erosive wear. Ultimately, direct contact phenomena predominated, decreasing the wear rate of the various materials to different degrees. Loss of material due to subsurface fatigue could not be demonstrated with a contact pressure of 45 MPa at which the experiments were performed. From this study, it can be concluded that minor alterations of the food-film thickness at the contact areas result in considerable changes in wear rates and wear-rate ranking of composite materials, which may partly explain inconsistencies among clinical trials.

Composite Resins↗
PubMed1990

Annealing as a mechanism of increasing wear resistance of composites.

The objective of this study was to examine the influence of a short-term exposure to heat (125 degrees C) on the wear resistance of composites. Both light- and chemically initiated materials improved by 20-60%. However, the improvement was also attained in the course of time if the materials had not been exposed to heat. The heat-induced improvement could not be explained by a continuation of polymerization but rather by stress relief, which is common for annealing processes. Polymerization shrinkage stresses, initially concentrated mainly around the filler particles, became more homogeneously distributed by the heat treatment. The long-term improvement of the non-heat-treated materials was based on the same mechanism, but proceeded more gradually.

Composite Resins↗
PubMed1990

Relaxation of polymerization contraction shear stress by hygroscopic expansion.

We studied relaxation by hygroscopic expansion of the interfacial polymerization shear stress of bonded resin composites. In the experimental set-up, resin composite-cured-to-glass strips bent due to the polymerization shrinkage. The strips were stored wet or dry. The curvatures of the bent strips were recorded, by the scanning of the glass surfaces with a contact profilometer, periodically over a period of two months. From the measured deflections, we calculated the maximum tangential bending stress in the resins near the adhesive interface, which provided an estimation for the maximum shear stress values occurring at both ends of the strips. In the particular experimental set-up, the shear stresses in Bis-GMA/TEGDMA and urethane dimethacrylate-based resins were either fully relieved or converted into an "expansion stress" by hygroscopic expansion. The hydrophobic tricyclodecane dimethacrylate-based resins showed very little stress relaxation.

Absorption↗
PubMed1990

[Bonding to dentin].

The development of adhesive systems is mainly concentrated on the possibility of chemical bond formation. Although glass-ionomer bonds chemically both to enamel and dentin and proved to be a successful restorative in class V restorations, the ultimate goal is to accomplish a strong and durable bond with the esthetic composite materials. The mechanism of chemical bonding to dentin apatite and collagen is discussed for a number of recently developed dentin bonding agents. Bond strengths up to 15 MPa can be obtained, but these are still not able to resist the forces of polymerization shrinkage of the composite materials.

Collagen↗
PubMed1989

The influence of mixing ratio on the working time, strength and wear of composites.

The influence of varying the paste-to-paste ratio of chemically initiated composites on the working time, diametral tensile strength and in vitro 'occlusal' wear was investigated. If the mixing ratio was varied, the working time was not equally affected for the various brands. This is explained by different relative inhibitor concentrations in the pastes. Within reasonable deviations from the 1:1 ratio, no significant influence on the diametral tensile strength occurred. Wear properties as determined by a simulated occlusal wear test on three posterior composites, was more sensitive to the mixing ratio.

Acrylic Resins↗
PubMed1988

Wear rates of composites, an amalgam, and enamel under stress-bearing conditions.

This article describes an in vitro technique that, within in a few days, can predict the long-term occlusal wear of composites. The laboratory data of our study and the clinical observations of various authors correlate well. For 19 different products, the wear relative to an amalgam under stress-bearing conditions is given within statistically justified boundaries.

Animals↗
PubMed1988

Exposure of dentists and assistants to mercury: mercury levels in urine and hair related to conditions of practice.

A study of 162 dentists' and their assistants' mercury levels in hair and urine, and of questionnaire items regarding mercury consumption, revealed some striking relations. The mercury concentrations in both hair (Hg-H) and urine (Hg-U) were somewhat higher in the dentists than the assistants. There was no relation between concentrations in hair and urine. The method of condensation of amalgam was positively related to Hg-U; the vibration method was negatively related. Hg-U was also positively related to number of fillings and hours in own practice. The relation of Hg-U to ventilation in the surgery applied only to the dentists.

Dental Amalgam↗