Linear dimensional changes during setting of two polycarboxylate cements

The linear dimensional changes of two polycarboxylate cements have been studied. The measurements were performed on specimens placed on a mercury bath, and were started 3 min after commencing the mix. The specimens were allowed to set at 37 degrees C under various environmental humidity. Specimens from one of the polycarboxylate cements (Durelon), were made using both the universal liquid and the thin liquid meant for cementation. The powder/liquid ratios (P/L) were either 2-5 or 1-5.. Specimens from the second cement (Poly-C), were made using the liquid meant for cementation only with a P/L of 1-5. The dimensional changes of Durelon were dependent on the powder/liquid ratio and the environmental humidity during setting. Under dry conditions this cement showed a maximum contraction of 4-60% after 14 days when a high P/L was used and 6-10% when a low P/L was used. After 1 day under wet conditions no significant differences between cements with high and low P/L were registered, and the mean contraction was 1-10%. The dimensional changes of Poly-C were dependent on the environmental humidity, and after 14 days under dry conditions a maximum contraction of 5-50% was registered and after 1 day under wet conditions a contraction of 0-60%. The contraction of the polycarboxylate cements tested in this study started earlier and were in general more pronounced than that observed for a zinc phosphate cement (Oilo, 1975).     

The influence of surface roughness on the retentive ability of two dental luting cements

Two series of brass cones and two series of dentine posts with varying surface roughness were produced. Maximum roughness value and arithmetical mean roughness were recorded for each cone. Brass crowns were cemented either with zinc phosphate cement (De Trey's zinc Cenment Improved) or with polycarboxylate cement (Durelon). A tensile stress was applied until the crown and the cone separated. The retentive force in relation to retention area was measured. The results showed that the retentive ability of both cements increased with increasing surface roughness. The increase in retention was greater for brass than for dentine. Differences were also found between the two cements; on smooth surfaces the zinc phosphate cement had a lower retentive ability than the polycarboxylate cement, but on rough surfaces the opposite relationship was observed.     

Effect of bevelling on the occurrence of fractures in the enamel surrounding composite resin fillings

Three different types of cavities: (1) with a 90 degree cavo-surface angle, (2) with a bevel at the cavity margin 0-25-0-50 mm wide, and (3) with a bevel at the cavity margin 0-5-1-0 mm wide, were prepared in extracted human teeth. Fifteen cavities of all three types were filled with Adaptic. Fifteen cavities of Type 2 and of Type 3 were filled with cosmic and the same number and types of cavities with prestige. The fillings were polished 10 min after starting the mix of the composite resins. Fractures in the enamel at the cavity margins were visualized by discoloration with methylene blue. Fractures were observed around all types of cavities, but, for all types of materials, an increased bevel at the margin gave an increased number of fillings without fractures in the enamel. However, great variations in the frequency of fractures were also observed; i.e. Adaptic showing a small frequency, Cosmic a medium and Prestige a great frequency of fillings connected with fractures at the enamel margin.     

Replica techniques for in vivo studies of tooth surfaces and prosthetic materials

In order to obtain a replica technique suitable for a complete or segments of a dental arch two different impression materials were combined with two epoxy resin model materials. The models were examined and photographed in an optical microscope and a scanning electron microscope. No systematic differences were found between the used combinations of materials except a tendency towards greater contraction of one of the model materials. The A-silicone impression materials combined with one of the epoxy resin material seemed to be the easiest to handle and to give the best models.     

Linear dimensional change during setting of a zinc phosphate cement

A method for studying the linear dimensional changes of dental cements during setting has been described. The measurements started 3 min after commencing the mix with specimens placed on a mercury bath. Measurements were performed at 37°C a.nd under various environmental humidity. The specimens were made of a fine grain zinc phosphate cement with high powder/ liquid ratio (3/1: g/ml) and low powder/liquid ratio (2/1: g/ml), and the measurements showed that the cement contracted during setting under both wet and dry conditions. Under wet conditions cements with both high and low powder/liquid ratio showed the same contraction, with 0.43% after 30 min reaching a maximum of 0-66% after 1 day. Under dry conditions the contraction was more pronounced, and somewhat dependent on the powder/liquid ratio. The cement with the high powder/liquid ratio showed 1.33% contraction after 30 min with a maximum of 3.83% after 7 days, whereas the corresponding values for cement with low powder/liquid ratio were 0.79% and 4.76%.     

Early erosion of dental cements

The disintegration in water of various unset glass ionomer cements, a polycarboxylate and a zinc phosphate cement was measured gravimetrically after exposure of the cements to a constant water jet. The test gave reproducible results with significant variations between the various types and brands of cements. For zinc phosphate and polycarboxylate cements, no weight loss was observed in the period from 4 to 8 min after commencement of mixing. All the glass ionomer cements showed a significant loss of weight at 4 min and a somewhat reduced weight loss at 6 min after start of mixing. Two cements, a filling and a luting material, showed reduced weight when exposed to a water jet even 8 min after start of mixing. The early erosion as recorded in the present study conforms with the setting of the glass ionomer cements.     

Fracture toughness measurements of some dental core ceramics: a methodologic study

Abstract— Fracture toughness is regarded as an important property of dental ceramics. The most widely used methods for fracture toughness (K_Ic) determination are based on assessment of cracks created by hardness indentations. Different formulas have been developed for K_Ic calculations and all these methods and formulas include empirical factors based on pure ceramics, i.e. non-composite ceramics. These factors may, however, vary for a specified method for materials with different and complex structure. An important question is whether the various proposed methods and formulas lead to approximately the same numerical K_Ic values or at least to the same ranking of materials. The aim of this work was to compare two indentation methods and various formulas for calculation of K_Ic values when used on four commercial composite dental ceramics. The two applied methods and the different formulas showed substantial differences in the obtained values for one and the same material and a different ranking of various materials. It is unknown which method gives the most correct K_Ic values for these ceramic materials.     

Adaptation of amalgams to cavity walls

The adaptation of amalgam fillings to the cavity walls has been studied. Cavities were made in human teeth and filled with four different amalgams: (1) amalgam with an expansion of approx. 10 mum/cm after 48 h; (2) amalgam with no dimensional change after 48 h (balanced); (3) amalgam with a contraction of approx. 10 mum/cm after 48 h; and (4) amalgam with a contraction of approx. 20 mum/cm after 48 h. The filling surface and parts of the surrounding enamel were planed, polished and studied under the microscope. The degree of contact between the amalgam and the cavity wall and the number and dimensions of the marginal defects were observed. Fillings made from expanding or balanced amalgams mostly had good contact with the cavity wall, and the marginal defects were small and scattered. Around fillings made from contracting material, a slit formation was observed, and in these areas fractures of both the amalgam and the enamel margins had regularly changed the slit to a V-shaped gap 10-20 mum wide. Greater defects exceeding 50 mum were also frequently observed. The results showed a gradual decrease in the quality of adaptation from fillings made of expanding material to those made of contracting material. It was further shown that the polishing procedure caused fracturing of both the unsupported enamel and amalgam margins thus widening the defects considerably.     

Bond strength of new ionomer cements to dentin

Abstract – The bond strength of two glass ionomer and two polycarboxylate cements to dentin was compared in vitro. The results showed that a new luting type of the glass ionomer cements had an increasing bond strength, with increasing storage time in water. After 3 d the values were comparable to those obtained for the polycarboxylate cements. It was also shown that the breakage of bond was due to a cohesive failure of the cements. The glass ionomer cements seem to be sensitive to moisture during setting.     

Linear dimensional changes of three luting agents

The linear dimensional changes during setting of a reinforced zinc oxide eugenol cement (Alumina EBA Crown and Bridge Cement), a composite resin cement (Epoxylite 9080 Crown and Bridge Adhesive) and a zinc silico phosphate cement (Fluoro-Thin cement), have been studied. The measurements were were performed on specimens placed on a mercury bath. The cements were allowed to set under various environmental humidity. The measurements of specimen length started 3 or 5 min after commencing the mix. The EBA-cement showed a maximum contraction of 0-4 percent after 7 days when setting under dry conditions. Under wet conditions the cement had an initial contraction the first day of setting, and thereafter an expansion. After 21 days specimens setting under 100 percent relative humidity showed 0-3 percent expansion, whereas specimens immersed in water attained the initial length. The composite resin cement showed a contraction of 1-0 percent after 1 day when setting under dry conditions. Under wet conditions the specimens had an initial contraction of 0-8 percent the first day of setting being reduced to 0-4 percent during the next 20 days. The zinc silico phosphate cement showed a maximum contraction of 3-0 percent after 7 days under dry conditions and 0-5 percent after 1 day under wet conditions.