An investigation into the compatibility of some irreversible hydrocolloid impression materials and dental gypsum products. Part II. A refined discriminatory procedure

Part I of this study considered the ability of alginate and gypsum combinations to reproduce a line on a test block. Part II describes a refined discriminatory procedure in the evaluation of the compatible combinations. Statistical analysis showed this procedure to be consistent, and that it could be expected to be repeatable. On the basis of this analysis, it was found that one alginate was clearly superior to all others, and that it was possible to distinguish a further group of combinations within the group that reproduced the 0.050 mm line. Surface treatment of the alginates by fixing solutions showed that none of the treated materials could be improved to the extent of equating them with the best of the naturally compatible combinations. It is recommended that manufacturers pay more strict adherence to the requirements of the ISO Standard with regard to the availability and recommendation of suitable compatible gypsum products, and also that perhaps the actual Standard should be modified to be more realistic in its demands, and more discriminatory in its evaluation procedure.

Accuracy of addition silicones as a function of technique

Both addition and condensation silicones produced stone dies that were larger in diameter and shorter in height than the tooth preparation. For both materials, there was little change in the distance between stone die preparations compared with the standard. The most significant difference between types of silicone was that condensation silicones produced significantly shorter dies (-0.24% to -0.37%) than addition silicones (-0.08%). Among addition silicones, Cinch produced more than twice as much vertical change (-0.16%) than the other three products (-0.06%). Since castings made from a short die will not seat completely on the prepared tooth, these results support the use of three of the four addition silicones tested. The same accuracy of impressions was achieved for all techniques (that is, putty/wash, single mix, and double mix) when addition silicones were used, whereas the putty/wash technique produced the most accurate dies for the condensation silicones. The custom tray produced dies that were much more accurate in vertical dimensions (-0.03%) than the stock trays (-0.15% to -0.21%). Thus the custom tray is the impression tray of choice, even for addition silicones, which produce relatively little polymerization shrinkage and are dimensionally stable.

Comparison of five interocclusal recording materials

The initial resistance of interocclusal recording materials to closure changed from 0.5 N to 13.8 N, and a rapid rise in the working time was seen in all elastomers. The resistance exhibited by wax at 60 degrees C was about 7 N. The volumetric contraction of elastomers in polymerization was clinically slight. The dimensional stability of rigid materials, acrylic resin, and zinc oxide pastes was good. Elastomers maintained their reliability for a relatively long time when stored in a tightly sealed plastic bag.

Visco-elastic properties of dental elastomeric impression materials

The visco-elastic properties of a number of set elastomeric impression materials were determined using a torsional pendulum apparatus, i.e. the real (G1) and imaginary (G2) parts of the complex shear modulus and the mechanical loss tangent (Tan delta). Comparison with published data on tension and compression set for these materials showed that whilst there was no correlation between Tan delta and tension set, there was significant (greater than 99%) correlation with compression set. Treating a permanent set test as the application of a rectangular stress pulse of duration tau, and assuming (i) linear visco-elasticity theory and (ii) that G1 and G2 are independent of frequency. Fourier integral analysis predicts that Tan delta should be proportional to permanent set. Tension set does not conform to this prediction because the deformation is nonlinear. The compression set test is shown to involve shear strains and elastomers generally are Hookean in shear. Whilst the theoretical treatment applies qualitatively, there are major quantitative disparities, the reasons for which are discussed.

Accuracy and bond strength of combination agar/alginate hydrocolloid impression materials

The accuracy and bond strength of several combinations of agar and alginate hydrocolloid impression materials were assessed. In general, there was little difference in accuracy among agar hydrocolloids. Of the alginate hydrocolloids, products J and CA produced the most accurate casts in combination with any of the agar hydrocolloids. For all product combinations, however, distortion of the diameter of the improved stone die was evident. The buccal-lingual diameter was 0.32% larger, whereas the mesial-distal diameter was only 0.06% larger. Regarding bond strength, alginate hydrocolloids P, CA, and VA in combination with agar hydrocolloids DB and DG produced the best results of all combinations tested. Thus, considering both bond strength and accuracy, product CA in combination with either DB or DG produced the best overall results.

Effect of material bulk and undercuts on the accuracy of impression materials

A truncated cone-shaped chromium steel die was used to determine the influence of the bulk of elastomeric impression material and size of undercut on the dimension of stone dies. It was found that both conditions affect the accuracy of stone dies. The possible clinical implications of the inaccuracies were discussed.

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.