The influence of impression trays on the accuracy of stone casts poured from irreversible hydrocolloid impressions

Dimensional changes of extended-pour alginate impression materials

INTRODUCTION

Recently, manufacturers have marketed alginate impression materials, claiming dimensional stability for up to 100 hours to allow shipping to a dental laboratory for digital model fabrication. The purpose of this study was to evaluate the dimensional changes of these newer materials after storage at different temperatures and times.

METHODS

Two extended-pour alginates (claiming 100 hours of accuracy) (Kromopan; Lascod, Florence, Italy; and Triphasix; Parkell, Edgewood, NY), 2 traditional alginates (Jeltrate; Dentsply-Caulk, York, Pa; and Kromatica; Matech, Sylmar, Calif), and 1 vinyl polysiloxane (VP Mix; Henry Schein, Melville, NY) were used to impress a scored aluminum die. The distance between the score lines was measured on the impressions at 10 minutes, 24 hours, and 100 hours after mix, and the percentages of dimensional change were calculated. Temperature effects were studied by storage at cold (-9°C), room (22°C), or hot (46°C) temperatures for 8 hours.

RESULTS

All alginates had statistically significant dimensional changes at 24 and 100 hours (0.69%-6.13%). VP Mix exhibited no statistically significant changes for any storage condition. The Kromopan (100 hour) and Kromatica (traditional) alginates were the most stable (0.85%-2.22% at 100 hours). Triphasix (100 hour) and Jeltrate (traditional) were the least stable (1.53%-4.73% at 100 hours). Cold-temperature storage resulted in the most dimensional changes (1.96%-4.73% at 100 hours). Room-temperature storage resulted in the least dimensional changes (0.97%-1.53% at 100 hours).

CONCLUSIONS

All alginate impression materials in the study had significant changes at 24 and 100 hours in all storage conditions tested.

Evaluation of dental arch reproduction using three-dimensional optical digitization

STATEMENT OF PROBLEM

Numerous investigations have been done to determine the most accurate method to reproduce a dental arch. Investigations have used different definitive cast configurations, materials, and measuring techniques to evaluate dimensional change, leaving clinicians undecided as to the most accurate method to reproduce a dental arch.

PURPOSE OF STUDY

The purpose of this investigation was to evaluate the effect of impression tray selection on accuracy of reproductions of a dental arch using a 3-dimensional optical digitizer.

MATERIALS AND METHODS

Impressions were made of a stone cast of a dental arch using vinyl polysiloxane impression material in custom and stock impression trays (n=12). Custom trays, designed with full palatal coverage, were fabricated on a separate standard stone cast using 2 layers of pink baseplate wax as the spacer and tissue stops on 1 anterior tooth and both first molar teeth. Impressions were poured with type IV gypsum and allowed to set with the tray inverted (n=5) or not inverted (n=5). The standard cast and resulting casts were digitized using an optical digitizer. Custom software was used to align and process the 3-dimensional images created by the digitizer. Each image was superimposed onto the standard cast image, curves were created of the percent of points less than a defined distance between the 2 superimposed images, and the area under the curve was calculated. A weighted average area was calculated for each cast, and these values were analyzed using a 2-way ANOVA (alpha=.05).

RESULTS

The main effect of tray type, laboratory setting technique, and the interaction between these 2 characteristics was analyzed. None of these effects was significant.

CONCLUSION

The differences in accuracy of reproductions of a dental arch, regardless of the impression and laboratory techniques, were not significant when evaluated using a 3-dimensional optical digitizer.

Laser digitization of casts to determine the effect of tray selection and cast formation technique on accuracy

STATEMENT OF PROBLEM; Several studies have attempted to determine the ideal combination of dental materials and laboratory techniques to produce the most accurate dental cast. Most have made use of 2-dimensional manual measuring devices, which neglect to account for the dimensional changes that exist along a 3-dimensional surface.

PURPOSE

The purpose of this study was to determine the effect of impression tray selection and cast formation techniques on the dimensional change of a dental cast with the use of new, 3-dimensional optical digitizing technology.

MATERIAL AND METHODS

Multiple impressions of a machined steel die that resembled a dental arch were made with custom and stock impression trays and vinyl polysiloxane impression material. The impressions were poured in type V artificial dental stone and allowed to set with the tray inverted or noninverted. The steel master die and stone casts were digitized with the Steinbichler Comet 100 Optical Digitizer, which was developed at the Minnesota Dental Research Center for Biomaterials and Biomechanics. Three-dimensional images of the stone casts were aligned to the 3-dimensional image of the master die and analyzed with AnSur-NT software. Multiple measurements of the master die and stone casts were analyzed to determine the accuracy of the 3-dimensional technology and of the impression and cast fabrication techniques. Planar distances between the center of each crown preparation were measured, as were crown heights. Data were analyzed with analysis of variance (P<.05), and root mean square error values were determined.

RESULTS

Casts were compared with a total of 45 significance tests, of which only 4 yielded P<.05. There was no pattern to these results, which suggests that they were false-positive findings.

CONCLUSION

Results obtained with the use of new optical digitizing technology indicated that neither impression tray type nor cast formation technique affected the accuracy of final casts.

A new method to simplify and increase the precision of maxillary cast mounting procedures in fully adjustable or semiadjustable articulators

This study introduces a new impression procedure that permits mounting the maxillary stone cast in an articulator for any patient treatment and is applicable to any dental arch with any impression material. A transfer tray was designed that makes it possible to transfer the patient's maxillary impression to the articulator in a one-stage procedure without the need for making record bases and occlusion rims or the need to use a face-bow bite fork.

Custom impression trays: Part I--Mechanical properties

Dimensional stability of custom impression trays is an important factor in determining the degree of accuracy achieved in forming a master cast. Such trays must remain stable over time and must not exhibit permanent deformation when a completed impression is removed from the oral cavity. Measurement of the mechanical properties allows comparison between various tray materials and is useful in interpreting data on stresses incurred during removal of the completed impression. In Part I of this three-part series, the various mechanical properties of five tray resins: one autopolymerizing polymethyl methacrylate, one light-polymerizing, and three brands of thermoplastic resins were recorded and compared. The thermoplastic resins studied in this investigation exhibited lower measured values for the strength and elastic modulus properties than the light-polymerizing resin and the autopolymerizing polymethyl methacrylate resin studied.

Use of a scanning laser three-dimensional digitizer to evaluate dimensional accuracy of dental impression materials

A steel die was machined to represent a dental arch with teeth prepared for complete crowns. Impressions and casts were made from the die, and critical dimensions were measured by two methods--one using micrometers and the other a scanning laser three-dimensional (3-D) digitizer. The digitizer recorded measurements were more precise than the micrometer's and virtually eliminated operator error because the acquisition of data was automatic. The digitizer could also be useful for high-precision analyses of the dimensional accuracy of dental impression materials, but the cost and complexity of the instrument makes it less desirable for low-precision work.

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).