Time-dependent accuracy of elastomer impression materials. Part I: Condensation silicones

Evaluation of the properties of a new super quick-setting (2 min) polyether impression material

OBJECTIVES

Although a new super-quick setting polyether impression material has been commercially recently introduced, its properties have not been yet reported. Thus, it was the aim of this study to assess the dimensional stability, tear strength, and elastic recovery of the new material and to compare it with another commonly used polyether and polyvinyl siloxane.

MATERIALS AND METHODS

A new super-quick set polyether, a regular set polyether and a polyvinylsiloxane (PVS) impression material have been used in the study. Dimensional changes were measured using a modified mold as per ISO 4823:2000 after 1 h and 7 days. Tear strength was evaluated by subjecting specimens to tension until failure with a crosshead speed of 250 mm/min. Elastic recovery was measured by deforming specimens using a materials testing machine to a height of 16 mm (20% strain). The change in length (ΔL) was measured afterwards and elastic recovery was calculated in percentages.

RESULTS

Dimensional changes of the super quick and regular set polyether were comparable in both the vertical and horizontal dimensions after 24 h and 7 days. All the tested materials showed dimensional change values far below the maximum accepted ISO requirement (1.5%). The super quick setting polyether showed significantly improved tear strength (4.9 N/mm) in comparison to the regular set polyether (3.5 N/mm) and similar to PVS (5.2 N/mm). The elastic recovery of PVS (99.6%) was the highest among all the groups.

CONCLUSIONS AND CLINICAL RELEVANCE

The newly available super-fast set polyether offers a great potential for a reduced chair side time and comfort for both, the patient and the dentist. Super quick polyether showed as well improved tear strength, which is considered one of the shortcomings of the regular set polyether. In addition, the new polyether was as accurate as the regular set polyether and with good elastic recovery.

Accuracy of bite mark analysis from food substances: A comparative study

Aims and Objectives:

The aims and objectives of the study were to compare the accuracy of bite mark analysis from three different food substances-apple, cheese and chocolate using two techniques-the manual docking procedure and computer assisted overlay generation technique and to compare the accuracy of the two techniques for bite mark analysis on food substances.

Materials and Methods:

The individuals who participated in the study were made to bite on three food substances-apple, cheese, and chocolate. Dentate individuals were included in the study. Edentulous individuals and individuals having a missing anterior tooth were excluded from the study. The dental casts of the individual were applied to the positive cast of the bitten food substance to determine docking or matching. Then, computer generated overlays were compared with bite mark pattern on the foodstuff.

Results:

The results were tabulated and the comparison of bite mark analysis on the three different food substances was analyzed by Kruskall-Wallis ANOVA test and the comparison of the two techniques was analyzed by Spearman's Rho correlation coefficient.

Conclusion:

On comparing the bite marks analysis from the three food substances-apple, cheese and chocolate, the accuracy was found to be greater for chocolate and cheese than apple.

The elastomers for complete denture impression: A review of the literature

This article reviews the current trends in materials used for complete denture impression. Peer-reviewed articles, published in English and in French between 1954 and 2007, were identified through a MEDLINE search (Pubmed and Elsevier) and a hand search of relevant textbooks and annual publications. Emphasis was made on the characteristics of the elastomers, their manipulation, the different techniques used, and the quality of the impression obtained. The combination of excellent physical properties, handling characteristics, and unlimited dimensional stability assures the popularity of these impression materials.

Direct 3-D morphological measurements of silicone rubber impression using micro-focus X-ray CT

Three-dimensional computer models of dental arches play a significant role in prosthetic dentistry. The microfocus X-ray CT scanner has the advantage of capturing precise 3D shapes of deep fossa, and we propose a new method of measuring the three-dimensional morphology of a dental impression directly, which will eliminate the conversion process to dental casts. Measurement precision and accuracy were evaluated using a standard gage comprised of steel balls which simulate the dental arch. Measurement accuracy, standard deviation of distance distribution of superimposed models, was determined as +/-0.050 mm in comparison with a CAD model. Impressions and casts of an actual dental arch were scanned by microfocus X-ray CT and three-dimensional models were compared. The impression model had finer morphology, especially around the cervical margins of teeth. Within the limitations of the current study, direct three-dimensional impression modeling was successfully demonstrated using microfocus X-ray CT.

The effect of temperature changes on the dimensional stability of polyvinyl siloxane and polyether impression materials

STATEMENT OF PROBLEM

Dental impressions are commonly mailed to remote dental laboratories and can be subjected to extreme temperature changes; however, the effects of these temperature changes on the accuracy of elastomeric impression materials are unknown.

PURPOSE

This study evaluated the dimensional changes of polyether and polyvinyl siloxane impression materials as a function of storage temperature.

MATERIAL AND METHODS

A total of 144 impressions were made of a stainless steel die similar to ADA Specification No. 19. Half of the impressions were made in perforated custom trays and the other half in solid trays. Measurements of the vertical and horizontal lines on the impressions were made at 10 minutes, 1 hour, 24 hours, and 26 hours, and storage temperatures ranged from 4 degrees C to 40 degrees C.

RESULTS

Changes in storage temperature had a statistically significant effect on the dimensional stability of the horizontal and vertical lines. However, overall dimensional changes were extremely small.

CONCLUSIONS

Storing both impression materials at 4 degrees C for 24 hours and then allowing the impressions to reach room temperature resulted in a slightly expanded impression that partially compensated for the contraction that occurred from polymerization shrinkage. Similar results occurred for polyether material only, when stored at 40 degrees C.

Vertical distortion in distal extension ridges and palatal area of casts made by different techniques

A coordinate measurement machine with laser probe was used to measure the vertical distortion of the casts produced by use of three types of impression materials (irreversible hydrocolloid, condensation silicone, and addition silicone) and two types of trays (stock and custom trays). Results indicated that all impression groups showed positive vertical distortion (ranging from 0.00566 to 0.30299 mm) at the edentulous ridges and palatal area. The amount of the vertical distortion was greatest at the palatal area and was followed by the high edentulous ridge and the low edentulous ridge. Addition silicone, with either custom tray or stock tray, was the most accurate impression material. Condensation silicone was more accurate than irreversible hydrocolloid in custom tray impression. However, in stock tray impression the irreversible hydrocolloid was more accurate than the condensation silicone. The results suggest that, with careful manipulation, irreversible hydrocolloid with stock tray impression may provide a satisfactory cast for fabricating the framework of a distal extension removable partial denture.

Effect of tray space on the accuracy of monophasic polyvinylsiloxane impressions

The effect of tray space on the dimensional accuracy and stability of impressions made from four brands of monophasic polyvinyl siloxane material was assessed on the recovered stone casts by quantitative and qualitative methods. In general, both evaluation methods agreed that tray space and repeat pour did not affect the accuracy of the dies for individual castings. But disagreement was observed in the findings for fixed partial dentures (FPDs). Quantitative measurement of the interpreparation (IP) dimensions did suggest the possibility of a potential problem when a FPD is cast in one piece because of a significantly reduced distance between the two abutments. However, from the findings of this study it can be concluded that a rigid stock tray can be used with a monophasic polyvinyl siloxane impression material.

Dimensional stability of silicone-based impression materials

This study attempts to demonstrate that the polymerization reaction is not the only factor that affects the shrinkage of silicone-based impression materials because evaporation of the constituents also contributes to the shrinkage. These factors can be evaluated by the study of time-dependent dimensional changes. This is shown both by chemical kinetics and by experimental testing of condensation and addition polymerizing impression materials with different viscosities. Comparison of the different materials shows that the two contributions, polymerization shrinkage, and evaporation shrinkage, can be assessed separately by analysis of the time-dependent shrinkage diagrams. The instability due to the polymerization reaction is complete after a few hours, but the contribution of the constituent evaporation, if present, can have a significant long-term role.

Effects of reheating on the accuracy of addition silicone putty-wash impressions

The effect of the relatively high thermal contraction of addition silicones (polyvinylsiloxane) on the accuracy of stone casts was evaluated using the putty/wash relining impression technique and acrylic resin trays. Impressions were made at 37 degrees C and were divided into two groups. One group was poured at room temperature (22 degrees +/- 2 degrees C) and another group was reheated to 37 degrees C before pouring. Measurement of the dimensional changes failed to reveal improved accuracy of the stone casts poured at the same temperature as that reached during impression making. On the basis of this limited sample, it can be concluded that reheating the impression using the putty/wash relining technique did not improve the accuracy of the stone cast.

Live primates and dental replication: new problems and new techniques

Dental microwear analyses are beginning to provide new insights into the intricacies of jaw movement and tooth use in modern and extinct mammals. However, these analyses are also raising new questions that are best answered through studies of live animals with known diets. The process of taking dental impressions from live animals is a difficult one that presents problems different from those encountered in working with museum material. This report presents a review of some of those problems together with solutions that have been developed in the course of laboratory work with nonhuman primates. It also summarizes recent developments in dental casting.

Review of dental impression materials

Major advances in impression materials and their application have occurred during the last decade, with greater emphasis being placed on rubber impression materials than on dental compound, zinc oxide-eugenol, and agar and alginate. Of particular interest has been the effect of disinfection solutions on the qualities of impressions and the biocompatibility of impression materials. The principal advance in hydrocolloids has been the introduction of the agar/alginate impression technique, which has simplified the procedure and improved the quality of gypsum dies compared with those prepared in alginate impressions. The tear strength of some alginates has been improved, and some have been formulated so that the powder is dustless, thus reducing the health hazard as a result of patient inhalation of dust during the dispensing process. Polyether and silicone impression materials have been modified so that the working time, viscosity, and flexibility of the polyethers have been improved and, with the introduction of addition silicones, their accuracy has become exceptional. Although the early addition silicones liberated hydrogen after setting, thus delaying the pouring of models and dies, most addition silicones have been improved so that no hydrogen is released and dies can be poured immediately. The introduction of automatic mixing systems for addition silicones has simplified their manipulation, has reduced the number of voids in impressions, and has reduced the amount of material wasted. The incorporation of surfactants into addition silicones has made them hydrophilic, with wetting properties similar to those of polyethers, and has made pouring bubble-free gypsum dies easier.

This review is confined to published and unpublished information of the past decade. It will also suggest trends that should be anticipated in the near future based on this information. The review will not present information developed before 1975, which is available in several textbooks on dental materials by Craig (1985a), Phillips (1982), and Williams and Cunningham (1979).

Comparison study of elastic impression material

The study of four elastic impression materials indicated that master castings could be more accurately seated on casts made from MOD impressions than from full crown impressions. A discrepancy between clinical acceptance and statistical identity was also demonstrated. The study indicated that by altering just one variable, the pour time, a wide range of results is obtained, some clinically acceptable and others clinically unacceptable.

Improved accuracy by reheating addition-reaction silicone impressions

Addition-reaction silicone impressions were made at 37 degrees C in two cylindrical trays of a truncated-cone-shaped chromium-steel die. One tray size was used to obtain impressions with 1 mm thickness from tray to the buccal and lingual surface of the steel die while the second tray permitted a thickness of 4 mm. Stone dies were made from impressions after (1) cooling to 22 degrees C for 10 minutes or (2) cooling to 22 degrees C for 10 minutes and reheating to 37 degrees C for 30 minutes. Results revealed that reheating the impressions to mouth temperature before pouring the dies improved their accuracy.

Accuracy of four types of rubber impression materials compared with time of pour and a repeat pour of models

The accuracy of four types of elastomeric impression materials was studied as a function of model location, time of pouring, and repetition of pouring. There was little change in dimension among abutment preparations for all materials, for all times of pour, and with a repeat pouring. It appears that measurements between posts are not sensitive to detecting differences between materials and that other dimensions should be examined to discern where differences might lie. The diameter of the stone post was larger than the standard for the AS and PS materials and was unchanged or smaller than the standard for the CS and the PE. The vertical dimensions of stone posts were, in general, shorter than the standard for all materials, with the AS demonstrating the smallest change. The AS and CS products demonstrated the best recovery from undercuts and the least change in dimensions between an initial and second pour of an impression. The AS and PE were the least affected with delays of 1, 4, and 24 hours in pouring the impression.

Molar microwear and diet in the genus Cebus

Recent analyses have documented differences in dental microwear between primate species with different diets, especially between primate hard-object feeders and primate leaf-eaters. Thus far, these microwear differences have only been documented for primates with vastly different foraging strategies and geographic distributions. To see if similar differences could be documented for closely related species, dental replicas from Cebus apella, Cebus nigrivittatus, and Cebus capucinus were examined using scanning electron microscopy. Quantitative analyses reveal that (1) even for closely related species, microwear differences between facets of one species are still far less than those between homologous facets of different species; and (2) the dental microwear of Cebus apella, Cebus nigrivittatus, and Cebus capucinus are still significantly different from one another. Furthermore, the data suggest that the dietary differences between these species may center around the presence or absence of hard objects in the diet.

An evaluation of the time-dependent dimensional stability of eleven elastomeric impression materials

The dimensional stability of three polysulfides, one condensation-cured silicone, six addition-cured silicones, and one polyether, was evaluated when poured immediately and after storage for 1, 4, and 24 hours. Dimensional stability was determined by measuring the amount of gap width developed by a master coping when seated on the die that had been poured in the impression. The greatest accuracy occurred when the impressions were poured immediately. All the addition-cured silicone materials exhibited excellent dimensional stability for all storage times. Delayed pouring of artificial stone in the impressions made in these materials should result in very little change in die accuracy. The condensation-cured silicone material Polytrans has good accuracy if poured immediately. Delayed pouring results in a rapid loss of accuracy. The polysulfides have good accuracy only if poured immediately, with the exception of Omniflex, which can be stored up to 4 hours with only a minimum loss of accuracy. The polyether material Impregum expanded during storage. Since the dimensional change of the impression during storage is a characteristic of the material, a reduction in its bulk should lead to a desired amount of dimensional change. To achieve this desirable state, custom trays that will provide for the minimum recommended bulk of material should be fabricated. The complex geometry of the cavity preparation makes any prediction of the desirability of impression material expansion or contraction difficult, because a change in impression material dimensions may be beneficial to the fit in one portion of the casting but have an extremely adverse effect in another.(ABSTRACT TRUNCATED AT 250 WORDS)

Silver-plated dies. Part II: Marginal accuracy of cast restorations

This laboratory study simulated actual clinical crown fabrication. Crowns were finished to the point of clinical delivery with stone and silver-plated dies made from representatives of the four major groups of elastomers. A clinically acceptable technique for judging crown margin accuracy was used, and numerical scores were assessed. Scores were analyzed statistically to compare die types, materials used, and types of discrepancies. Results of this study indicate the following information about the materials tested. The marginal accuracy of crowns fabricated on the silver-plated dies is statistically as accurate as that of crowns fabricated on stone dies in all cases and significantly more accurate in some instances. Silver-plated dies fabricated from Reprosil produced more accurate crowns than all other dies tested. Acceptable crown margins were obtainable from either stone or silver-plated dies when judged by clinical criteria. Silver-plated dies can be used without compromising crown margin accuracy.

Laser-welded vs soldered nonprecious alloy dental bridges: a comparative study

The high cost of gold alloy has caused the dental profession to begin substituting nonprecious alloy for the framework in porcelain fused to metal bridges. Especially in long-span bridges it may be advantageous to make multiple castings and then join them for a better fit. As opposed to the highly successful soldering of gold, soldered nonprecious alloy bridges have a great failure rate in the mouth. Removal of and remaking of the bridges is thus the result. This study compares nonprecious units that have been laser-welded with those conventionally soldered. Seven identical bridges of three units were cast in a popular alloy composed of 74-78% nickel, 12-15% chromium, 4-6% molybdenum, and 1.8% maximum beryllium. One served as a control, while the remaining six were all cut in the same place. Of these, three were soldered with a gas oxygen torch. The other three were welded with a Nd-YAG laser. Better and stronger joints unlikely to fracture in the mouth were found with the laser-welded specimen.