Abstract
Objectives:
To evaluate the dimensional change over time of two extended-storage alginate impression materials.
Methods:
Impressions were made of stainless steel dies in accordance with ADA Specification No. 18 using three alginates: two extended-storage alginates and one conventional alginate. The impressions were stored for 30 minutes, 48 hours, or 100 hours (n = 10 impressions/material/storage time). Following the respective storage times, dimensional change was measured by comparing the length of the middle horizontal line in the impression with the same line on the die and computing percent difference.
Results:
Significant differences in dimensional change were noted between materials across time (P < .05). All materials exhibited shrinkage after 30 minutes, with the conventional alginate continuing to shrink over time and the extended-storage alginates expanding with increased storage time. The conventional alginate was most accurate after 30 minutes. In contrast, one extended-storage alginate demonstrated minimal dimensional change at all storage times, and another was most accurate after 100-hour storage.
Conclusions:
Evidence suggests that delayed pouring with dental gypsum should not adversely affect dimensional accuracy of the generated casts with both extended-storage alginates. However, only one of the extended-storage materials appears suitable for both short-term and extended-storage applications.
Keywords: Alginate, Extended storage, Dimensional stability
INTRODUCTION
Impression materials and in particular irreversible hydrocolloid, also known as alginate, are some of the most commonly used dental materials. Alginate impressions are used to generate gypsum casts used for numerous applications, including treatment planning for restorative and orthodontic care, and fabricating removable prostheses. As with any hydrocolloid, alginates are approximately 85% water1,2 and are prone to distortion caused by expansion associated with imbibition (absorption of moisture) or shrinkage due to moisture loss.2,3
In addition to water evaporation, impression shrinkage is related to syneresis and associated water exudation onto the impression surface caused by continuing contraction of the colloidal skeletal network even in 100% humidity.1,4–7 Consequently, alginate impressions are not dimensionally stable, leading to decreased dimensional accuracy over time. In previous studies evaluating the effect of storage time,8–11 the recommended maximum storage time before pouring of a conventional alginate impression with gypsum is typically as soon as possible but no longer than 30 minutes.3,8–10 Recently, extended-storage alginate impression materials have been marketed with claims that the materials exhibit dimensional stability for up to 100 hours. These materials can be used for any alginate application but in particular are recommended by the digital model companies.5,11 These companies will accept extended-storage alginate impressions, which then are poured and the resultant casts scanned or digitized. Although recent investigations included extended-storage alginate materials,10–12 these investigations evaluated impression dimensional change indirectly via measurements on gypsum casts, which incorporate the confounding variable of gypsum expansion.
Therefore, the purpose of this study was to directly evaluate the dimensional stability of two extended-storage alginate materials by measuring dimensional change over time at intervals of 30 minutes, 48 hours, and 100 hours. The research hypothesis was that extended-storage alginate impression materials will demonstrate greater dimensional stability than a conventional alginate. The overall objective was to determine whether extended-storage alginate materials produce clinically acceptable impressions after more than 30 minutes of storage time.
MATERIALS AND METHODS
The alginate impression materials used in this study included one conventional alginate (JeltratePlus, Lot 080218, Dentsply Caulk, Milford, Del) and two extended-storage alginate materials (Alginmax, Lot 14785, Major Dental Products SpA, Moncalieri, Italy; and Kromopan 100, Lot 0157382127.105, Kromopan USA, Des Plaines, Ill).
Specimen Preparation
Impression-making protocol followed American Dental Association Specification No. 18 (ADA 18) for alginate impression materials.13 Impressions were made of standardized stainless steel test dies (similar to those described in the specification) scored with three 25-mm horizontal lines and an orientation marker (Figure 1).
Figure 1.
Diagrammatic representation of a stainless steel test die.
Two scoops of alginate were used for each impression, using the scoop provided by each manufacturer. Before scooping, the powder was fluffed (container tipped back and forth twice) to reduce powder compaction. The necessary amount of water was measured with the cylinder provided by each manufacturer. The alginate powder was mixed with water utilizing an electric alginate mixer (Alginator, Cadco Dental Products, Oxnard, Calif). Distilled rather than tap water was used because potential ion concentrations that might be present in tap water could possibly interfere with alginate chemical reactions.14 The mixed alginate was then placed into a polyvinyl carbonate (PC) ring mold (6 × 26 mm [height × diameter]) coated with tray adhesive (Hold, Water Pik Inc, Fort Collins, Colo) and allowed to dry for at least 4 minutes, as indicated by the manufacturer. When a ring mold was reused, any remaining adhesive was removed with alcohol before a fresh coat of adhesive was applied, to ensure optimal adhesion.15 The PC ring mold was then placed on a 60 × 60-mm glass plate sprayed with a separating agent to prevent the alginate from sticking to the glass. The ring mold was then slightly overfilled, and the alginate vibrated within the ring mold for 10 seconds (Lab Vibrator Model HV-1, Healthco Inc, Boston, Mass) to reduce air incorporation into the impressions. Twenty seconds before the end of the respective working time of each material, a clean test die sprayed with poly-tetrafluoro-ethylene separating agent (Dri-Film Lubricant, Synco Chemical Corp, Bohemia, NY) was centered and pressed into the alginate-filled PC ring mold. As per ADA Specification No. 18, to more closely simulate the oral environment, this assembly was placed into a bath (Teledyne Hanau, Fort Collins, Colo) of distilled water maintained at 35 ± 1°C and loaded with a 1-kg mass conditioned at the same temperature. Following the specification protocol, 3 minutes after the manufacturer's stated setting time, the assembly was removed from the water bath and the set alginate impression separated from the metal die.
After impression removal, impressions were rinsed with distilled water to simulate rinsing following impression removal from the mouth and the excess fluid shaken off. Impressions were stored at ambient laboratory temperature according to the manufacturers' recommendations in a 16.5 × 14.9-cm zip-lock plastic bag. If a humid storage environment was recommended by the manufacturer, as was the case for JeltratePlus and Alginmax but not Kromopan 100, the impression was wrapped with a saturated, moist paper towel. Although this protocol is not ideal because of the potential for the impression to absorb moisture from the paper towel, this protocol is used at many dental schools, including our institution. Moreover, this follows the alginate impression storage/shipping protocol recommended by one of the digital model companies (OrthoCAD, Cadent Inc, Carlstadt, NJ). To control the amount of moisture, each towel was saturated with 25 mL of distilled water. Three different storage times were used: 30 minutes, 48 hours, and 100 hours. Ten impressions were made with each of the three alginate materials for each storage time, for a total of 90 impressions.
Evaluation of Dimensional Change/Stability
The specification for alginate impression materials (ADA 18) does not include a measurement protocol for dimensional change. As a result, dimensional change across time (dimensional stability) was measured using the protocol for dental elastomeric impression materials, as described in American National Standard Institute/American Dental Association Specification No. 19,16,17 in which the length of the middle horizontal line of the stainless steel die (Figure 1) is compared with the same line in the impression. The two cross-points (marked X and X′) served as the measurement beginning and end points.
In developing the project protocol, it was noted that the alginate impressions were susceptible to moisture loss during measurements of the middle line using a measuring microscope because of extended exposure to air and heat from the microscope lamp. To address this problem, impression measurements were made via digital photographs integrated with image analyzing software. Photographs of each impression were taken after the respective storage time, with the die used to make the impression included in the photograph (Figure 2). The plane of view of the camera (Canon EOS 20D, 100-mm macro lens, MR-14EX Ring Lite, Lake Success, NY) was 38.4 cm from the impression and die surface. Using the resultant high-resolution tagged image files (TIFs), the middle horizontal line in each impression photograph was measured with an imaging software program using a 300× zoom (analySIS Imager 3.2, Soft Imaging System, Lakewood, Colo), with each image pixel representing 0.006 mm. Before any measurements were taken, each image was calibrated against the known length of the middle line of the die in the image. To account for evaluator measurement precision, the horizontal line measurement of the impression was made three times to the nearest 0.01 mm. The average of the three impression measurements was compared with the middle line measurement of the die used to make the impression. The percent dimensional change of the alginate impression from the metal die was computed using the following equation: [(A–D)/D] × 100, with A = mean alginate impression measurement and D = die measurement.
Figure 2.
Representative photograph of alginate impression and die used for image software–based measurements.
Statistical Analyses
A two-factor analysis of variance (ANOVA) (α = .05) was used to compare percent dimensional change as a function of material and storage time. A planned comparison approach with individual one-factor ANOVAs was then used to detect where differences existed between materials within each storage time. To control for family-wise error rate across the three one-factor ANOVAs, the Bonferroni correction was used and each test conducted at the .01 alpha level. Where the one-factor ANOVA indicated significant differences, Tukey's post hoc analysis was used to explain pairwise comparisons. Because investigators had no interest in comparing between storage times within materials, these comparisons were not made.
RESULTS
Mean and standard deviation (SD) values of percent dimensional change are presented in Table 1. As indicated by the two-factor ANOVA, a significant difference in percent dimensional change was noted as a function of material across storage time (P ≤ .05) with no significant effect of storage time across materials (P > .05). Although all materials exhibited shrinkage after 30-minute storage (as indicated by negative percent differences), JeltratePlus continued to shrink over time. In contrast, following initial shrinkage after 30 minutes, Alginmax and Krompan 100 expanded with continued storage. According to the one-factor ANOVAs, significant differences were observed between materials at 30-minute and 100-hour storage times (P ≤ .01). After 30-minute storage, Alginmax and JeltratePlus exhibited significantly less dimensional change than Kromopan 100. After 48 hours, Alginmax exhibited less dimensional change than JeltratePlus or Kromopan 100, but this difference was not statistically significant. By 100 hours, Alginmax and Kromopan 100 demonstrated significantly less dimensional change than JeltratePlus.
Table 1.
Mean (SD) Percent Dimensional Change of Alginate Impressions as a Function of Material at Three Storage Times*
Images of each impression material at each storage time are presented in Figure 3. Two observations were noted when the photographs were analyzed. First, at 30 minutes, all three materials demonstrated faint lines, with the lines becoming noticeably more visible at 48 hours, and then fading again at 100 hours. Second, Kromopan 100 had more moisture present on its surface compared with JeltratePlus or Alginmax at all time intervals, but especially at 48 hours and 100 hours.
Figure 3.
Representative photographs of alginate impressions after each storage time. JeltratePlus after (a) 30 minutes, (b) 48 hours, and (c) 100 hours; Alginmax after (d) 30 minutes, (e) 48 hours, and (f) 100 hours; and Kromopan 100 after (g) 30 minutes, (h) 48 hours, and (i) 100 hours.
DISCUSSION
Based on results of the current study, the research hypothesis that extended-storage alginate impression materials will demonstrate greater dimensional stability than a conventional alginate was substantiated. To potentially explain the behaviors of the materials in this study, it appears that after 30-minute storage, all the alginate impressions exhibited shrinkage, most likely as the result of material setting. With continued storage, the conventional alginate continued to shrink, supposedly as the result of syneresis across storage time. In contrast, the extended-storage alginate materials exhibited expansion after initial 30-minute shrinkage. This phenomenon might be related to recently reported differences in bound versus unbound water associated with higher filler∶polymer and Ca∶Na ratios in extended-storage alginate materials.7
In terms of clinical application, results suggest that the conventional alginate impressions would be most accurate if poured within 30 minutes, which is in agreement with previous investigations.8–10,12 In contrast, Kromopan 100 impressions were most accurate after 100-hour storage, and Alginmax impressions exhibited minimal mean dimensional change across storage times (−0.15% to +0.20%). Based on these results, Kromopan 100 appears best suited for extended-storage applications, while Alginmax could be used for immediate and delayed applications. To our knowledge, this information has not been previously reported because no previous investigations included the extended-storage alginates investigated in the current study.
Despite the fact that significant differences were noted between the dimensional accuracy of the tested materials, the question remains—Are these differences clinically significant? Although no standard specifically addresses alginate dimensional accuracy or stability, according to ADA Specification No. 19,16 for an elastomeric impression to be classified as dimensionally accurate over time, the material should exhibit no more than ±0.5% dimensional change upon setting and after subsequent storage. If this guideline is used to evaluate the mean percent dimensional change of each material over time, the conventional alginate in this study would be acceptable after 30 minutes and 48 hours of storage. In contrast, Kromopan 100 impressions would be acceptable after 48 hours and 100 hours of storage, with Alginmax impressions acceptable across storage times.
Even though extended-storage alginate impression materials could be used for many aspects of dentistry, these materials are recommended when orthodontists utilize digital model systems such as OrthoCAD, Cadent Inc, Carlstadt, NJ; emodel®, GeoDigm Corp, Chanhassen, MN; Ortho-graphics, Ortho Cast Inc, High Bridge, NJ.5,11 With these systems, a virtual set of three-dimensional models is generated by scans of casts poured from impressions sent to the company.18 Results of the current study suggest that the evaluated extended-storage alginate impression materials exhibited minimal dimensional change at 100 hours and, based on the 0.5% dimensional change parameter, should produce clinically acceptable digital models from impressions shipped to the company.
As with any in vitro investigation, this study had limitations. For example, fairly large variability was associated with the current results. This is due, in part, to the inherent variability in an impression material that is mostly composed of water. Variability might have been reduced if the same impression had been measured across time, rather than separate impressions used for each time period. However, this was ruled out as a study approach because it was important to make the results as clinically applicable as possible, and exposing the impression to air multiple times is not what occurs in dental practices. Typically, the impression is made and stored and then is poured with gypsum as soon as it is removed from storage. Another possible limitation of the study is that impressions were made using optimum conditions (eg, an electric alginate mixer and a dental laboratory vibrator were used to minimize bubbles in the alginate material). Furthermore, to optimize the ability to measure dimensional change, any impression that exhibited voids affecting the middle horizontal line was not included in the study. It is not known how voids might affect dimensional stability.
One other consideration is that impression shrinkage is typically counteracted by gypsum expansion. With minimal dimensional change of Kromopan 100 (−0.15%) and Alginmax (+0.20%) after 100-hour storage, the potential for an oversized cast exists, depending on gypsum expansion. Thus, it might be more appropriate to use low-expansion gypsum with these particular alginates. A future study that includes Alginmax and Kromopan 100 impressions poured with gypsum to determine overall effects on the resultant dimensional accuracy of the cast would be valuable.
CONCLUSIONS
Both extended-storage alginates—Alginmax and Kromopan 100—demonstrated minimal dimensional change after 100 hours of storage, whereas the conventional alginate, JeltratePlus, was most accurate after storage for only 30 minutes.
Kromopan 100 seems best suited to serve as an extended-storage alginate because it was most accurate after 100 hours of storage.
Alginmax appears to be a versatile extended-storage material that potentially can be used for both immediate and delayed pouring with gypsum in that it demonstrated minimal dimensional change after 30 minutes, 48 hours, and 100 hours of storage.
Acknowledgments
The authors thank Major Dental Products, Kromopan USA, and Dentsply Caulk for providing materials. The authors also acknowledge technical assistance provided by Jim Thomas, Mark Dallas, and Nick Alderman.
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