dimensional change over time of extended-storage alginate

Original Article

Dimensional change over time of extended-storage alginate

impression materials

Mary P. Walkera; Jason Burckhardb; David A. Mittsc; Karen B. Williamsd

ABSTRACTObjectives: To evaluate the dimensional change over time of two extended-storage alginateimpression materials.Methods: Impressions were made of stainless steel dies in accordance with ADA SpecificationNo. 18 using three alginates: two extended-storage alginates and one conventional alginate. Theimpressions were stored for 30 minutes, 48 hours, or 100 hours (n 5 10 impressions/material/storage time). Following the respective storage times, dimensional change was measured bycomparing the length of the middle horizontal line in the impression with the same line on the dieand 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 alginatecontinuing to shrink over time and the extended-storage alginates expanding with increasedstorage time. The conventional alginate was most accurate after 30 minutes. In contrast, oneextended-storage alginate demonstrated minimal dimensional change at all storage times, andanother was most accurate after 100-hour storage.Conclusions: Evidence suggests that delayed pouring with dental gypsum should not adverselyaffect 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 andextended-storage applications. (Angle Orthod. 2010;80:1110–1115.)

KEY WORDS: Alginate; Extended storage; Dimensional stability

INTRODUCTION

Impression materials and in particular irreversiblehydrocolloid, also known as alginate, are some of themost commonly used dental materials. Alginateimpressions are used to generate gypsum casts used

for numerous applications, including treatment plan-ning for restorative and orthodontic care, and fabricat-ing removable prostheses. As with any hydrocolloid,alginates are approximately 85% water1,2 and areprone to distortion caused by expansion associatedwith imbibition (absorption of moisture) or shrinkagedue to moisture loss.2,3

In addition to water evaporation, impression shrink-age is related to syneresis and associated waterexudation onto the impression surface caused bycontinuing contraction of the colloidal skeletal networkeven in 100% humidity.1,4–7 Consequently, alginateimpressions are not dimensionally stable, leading todecreased dimensional accuracy over time. In previ-ous studies evaluating the effect of storage time,8–11

the recommended maximum storage time beforepouring of a conventional alginate impression withgypsum is typically as soon as possible but no longerthan 30 minutes.3,8–10 Recently, extended-storagealginate impression materials have been marketedwith claims that the materials exhibit dimensionalstability for up to 100 hours. These materials can beused for any alginate application but in particular are

a Professor, Director of Graduate Research Programs, Schoolof Dentistry, University of Missouri-Kansas City, Kansas City,Mo.

b Previous Resident, Department of Orthodontics and Dento-facial Orthopedics, School of Dentistry, University of Missouri-Kansas City, Kansas City, Mo; Private practice, Moorhead,Minn.

c Dental Student, School of Dentistry, University of Missouri-Kansas City, Kansas City, Mo.

d Professor, Department of Dental Public Health and Behav-ioral Science, School of Dentistry, University of Missouri-KansasCity, Kansas City, Mo.

Corresponding author: Dr Mary P. Walker, Director ofGraduate Research Programs, University of Missouri-KansasCity School of Dentistry, 650 E 25th St, Kansas City, MO 64108(e-mail: [email protected])

Accepted: May 2010. Submitted: March 2010.G 2010 by The EH Angle Education and Research Foundation,Inc.

DOI: 10.2319/031510-1501110Angle Orthodontist, Vol 80, No 6, 2010

recommended by the digital model companies.5,11

These companies will accept extended-storage algi-nate impressions, which then are poured and theresultant casts scanned or digitized. Although recentinvestigations included extended-storage alginate ma-terials,10–12 these investigations evaluated impressiondimensional change indirectly via measurements ongypsum casts, which incorporate the confoundingvariable of gypsum expansion.

Therefore, the purpose of this study was to directlyevaluate the dimensional stability of two extended-storage alginate materials by measuring dimensionalchange over time at intervals of 30 minutes, 48 hours,and 100 hours. The research hypothesis was thatextended-storage alginate impression materials willdemonstrate greater dimensional stability than aconventional alginate. The overall objective was todetermine whether extended-storage alginate materi-als produce clinically acceptable impressions aftermore than 30 minutes of storage time.

MATERIALS AND METHODS

The alginate impression materials used in this studyincluded one conventional alginate (JeltratePlus, Lot080218, Dentsply Caulk, Milford, Del) and two extend-ed-storage alginate materials (Alginmax, Lot 14785,Major Dental Products SpA, Moncalieri, Italy; andKromopan 100, Lot 0157382127.105, Kromopan USA,Des Plaines, Ill).

Specimen Preparation

Impression-making protocol followed American Den-tal Association Specification No. 18 (ADA 18) foralginate impression materials.13 Impressions weremade of standardized stainless steel test dies (similarto those described in the specification) scored withthree 25-mm horizontal lines and an orientation marker(Figure 1).

Two scoops of alginate were used for eachimpression, using the scoop provided by each manu-facturer. Before scooping, the powder was fluffed(container tipped back and forth twice) to reducepowder compaction. The necessary amount of waterwas measured with the cylinder provided by eachmanufacturer. The alginate powder was mixed withwater utilizing an electric alginate mixer (Alginator,Cadco Dental Products, Oxnard, Calif). Distilled ratherthan tap water was used because potential ionconcentrations that might be present in tap watercould possibly interfere with alginate chemical reac-tions.14 The mixed alginate was then placed into apolyvinyl carbonate (PC) ring mold (6 3 26 mm [height3 diameter]) coated with tray adhesive (Hold, WaterPik 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 adhesivewas removed with alcohol before a fresh coat ofadhesive was applied, to ensure optimal adhesion.15

The PC ring mold was then placed on a 60 3 60-mmglass plate sprayed with a separating agent to preventthe alginate from sticking to the glass. The ring moldwas then slightly overfilled, and the alginate vibratedwithin the ring mold for 10 seconds (Lab VibratorModel HV-1, Healthco Inc, Boston, Mass) to reduce airincorporation into the impressions. Twenty secondsbefore the end of the respective working time of eachmaterial, a clean test die sprayed with poly-tetrafluoro-ethylene separating agent (Dri-Film Lubricant, SyncoChemical Corp, Bohemia, NY) was centered andpressed into the alginate-filled PC ring mold. As perADA Specification No. 18, to more closely simulate theoral environment, this assembly was placed into a bath(Teledyne Hanau, Fort Collins, Colo) of distilled watermaintained at 35 6 1uC and loaded with a 1-kg massconditioned at the same temperature. Following thespecification protocol, 3 minutes after the manufactur-er’s stated setting time, the assembly was removedfrom the water bath and the set alginate impressionseparated from the metal die.

After impression removal, impressions were rinsedwith distilled water to simulate rinsing followingimpression removal from the mouth and the excessfluid shaken off. Impressions were stored at ambientlaboratory temperature according to the manufactur-ers’ recommendations in a 16.5 3 14.9-cm zip-lockplastic bag. If a humid storage environment wasrecommended by the manufacturer, as was the case

Figure 1. Diagrammatic representation of a stainless steel test die.

EXTENDED-STORAGE ALGINATE DIMENSIONAL STABILITY 1111

Angle Orthodontist, Vol 80, No 6, 2010

for JeltratePlus and Alginmax but not Kromopan 100,the impression was wrapped with a saturated, moistpaper towel. Although this protocol is not idealbecause of the potential for the impression to absorbmoisture from the paper towel, this protocol is used atmany dental schools, including our institution. More-over, this follows the alginate impression storage/shipping protocol recommended by one of the digitalmodel companies (OrthoCAD, Cadent Inc, Carlstadt,NJ). To control the amount of moisture, each towelwas saturated with 25 mL of distilled water. Threedifferent storage times were used: 30 minutes,48 hours, and 100 hours. Ten impressions were madewith each of the three alginate materials for eachstorage 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 fordimensional change. As a result, dimensional changeacross time (dimensional stability) was measuredusing the protocol for dental elastomeric impressionmaterials, as described in American National StandardInstitute/American Dental Association SpecificationNo. 19,16,17 in which the length of the middle horizontalline of the stainless steel die (Figure 1) is comparedwith the same line in the impression. The two cross-points (marked X and X9) served as the measurementbeginning and end points.

In developing the project protocol, it was noted thatthe alginate impressions were susceptible to moistureloss during measurements of the middle line using ameasuring microscope because of extended exposureto air and heat from the microscope lamp. To addressthis problem, impression measurements were madevia digital photographs integrated with image analyzingsoftware. Photographs of each impression were takenafter the respective storage time, with the die used tomake the impression included in the photograph(Figure 2). The plane of view of the camera (CanonEOS 20D, 100-mm macro lens, MR-14EX Ring Lite,Lake Success, NY) was 38.4 cm from the impressionand die surface. Using the resultant high-resolutiontagged image files (TIFs), the middle horizontal line ineach impression photograph was measured with animaging software program using a 3003 zoom(analySIS Imager 3.2, Soft Imaging System, Lake-wood, Colo), with each image pixel representing0.006 mm. Before any measurements were taken,each image was calibrated against the known length ofthe middle line of the die in the image. To account forevaluator measurement precision, the horizontal linemeasurement of the impression was made three timesto the nearest 0.01 mm. The average of the three

impression measurements was compared with themiddle line measurement of the die used to make theimpression. The percent dimensional change of thealginate impression from the metal die was computedusing the following equation: [(A–D)/D] 3 100, with A5 mean alginate impression measurement and D 5

die measurement.

Statistical Analyses

A two-factor analysis of variance (ANOVA) (a 5 .05)was used to compare percent dimensional change asa function of material and storage time. A plannedcomparison approach with individual one-factor ANO-VAs was then used to detect where differences existedbetween materials within each storage time. To controlfor family-wise error rate across the three one-factorANOVAs, the Bonferroni correction was used andeach test conducted at the .01 alpha level. Where theone-factor ANOVA indicated significant differences,Tukey’s post hoc analysis was used to explain pairwisecomparisons. Because investigators had no interest incomparing between storage times within materials,these comparisons were not made.

RESULTS

Mean and standard deviation (SD) values of percentdimensional change are presented in Table 1. Asindicated by the two-factor ANOVA, a significantdifference in percent dimensional change was notedas a function of material across storage time (P # .05)with no significant effect of storage time acrossmaterials (P . .05). Although all materials exhibitedshrinkage after 30-minute storage (as indicated bynegative percent differences), JeltratePlus continuedto shrink over time. In contrast, following initialshrinkage after 30 minutes, Alginmax and Krompan100 expanded with continued storage. According tothe one-factor ANOVAs, significant differences were

Figure 2. Representative photograph of alginate impression and die

used for image software–based measurements.

1112 WALKER, BURCKHARD, MITTS, WILLIAMS


observed between materials at 30-minute and 100-hour storage times (P # .01). After 30-minute storage,Alginmax and JeltratePlus exhibited significantly lessdimensional change than Kromopan 100. After48 hours, Alginmax exhibited less dimensional changethan JeltratePlus or Kromopan 100, but this differencewas not statistically significant. By 100 hours, Algin-max and Kromopan 100 demonstrated significantlyless dimensional change than JeltratePlus.

Images of each impression material at each storagetime are presented in Figure 3. Two observations werenoted when the photographs were analyzed. First, at30 minutes, all three materials demonstrated faintlines, with the lines becoming noticeably more visibleat 48 hours, and then fading again at 100 hours.Second, Kromopan 100 had more moisture present onits surface compared with JeltratePlus or Alginmax atall time intervals, but especially at 48 hours and100 hours.

DISCUSSION

Based on results of the current study, the researchhypothesis that extended-storage alginate impressionmaterials will demonstrate greater dimensional stabilitythan a conventional alginate was substantiated. Topotentially explain the behaviors of the materials in thisstudy, it appears that after 30-minute storage, all thealginate impressions exhibited shrinkage, most likelyas the result of material setting. With continuedstorage, the conventional alginate continued to shrink,supposedly as the result of syneresis across storagetime. In contrast, the extended-storage alginate mate-rials exhibited expansion after initial 30-minute shrink-age. This phenomenon might be related to recentlyreported differences in bound versus unbound waterassociated with higher filler:polymer and Ca:Na ratiosin extended-storage alginate materials.7

In terms of clinical application, results suggest thatthe conventional alginate impressions would be mostaccurate if poured within 30 minutes, which is inagreement with previous investigations.8–10,12 In con-trast, Kromopan 100 impressions were most accurateafter 100-hour storage, and Alginmax impressionsexhibited minimal mean dimensional change acrossstorage times (20.15% to +0.20%). Based on these

results, Kromopan 100 appears best suited forextended-storage applications, while Alginmax couldbe used for immediate and delayed applications. Toour knowledge, this information has not been previ-ously reported because no previous investigationsincluded the extended-storage alginates investigatedin the current study.

Despite the fact that significant differences werenoted between the dimensional accuracy of the testedmaterials, the question remains—Are these differenc-es clinically significant? Although no standard specif-ically addresses alginate dimensional accuracy orstability, according to ADA Specification No. 19,16 foran elastomeric impression to be classified as dimen-sionally accurate over time, the material should exhibitno more than 60.5% dimensional change upon settingand after subsequent storage. If this guideline is usedto evaluate the mean percent dimensional change ofeach material over time, the conventional alginate inthis study would be acceptable after 30 minutes and48 hours of storage. In contrast, Kromopan 100impressions would be acceptable after 48 hours and100 hours of storage, with Alginmax impressionsacceptable across storage times.

Even though extended-storage alginate impressionmaterials could be used for many aspects of dentistry,these materials are recommended when orthodontistsutilize digital model systems such as OrthoCAD,Cadent Inc, Carlstadt, NJ; emodelH, GeoDigm Corp,Chanhassen, MN; Ortho-graphics, Ortho Cast Inc,High Bridge, NJ.5,11 With these systems, a virtual set ofthree-dimensional models is generated by scans ofcasts poured from impressions sent to the company.18

Results of the current study suggest that the evaluatedextended-storage alginate impression materials exhib-ited minimal dimensional change at 100 hours and,based on the 0.5% dimensional change parameter,should produce clinically acceptable digital modelsfrom impressions shipped to the company.

As with any in vitro investigation, this study hadlimitations. For example, fairly large variability wasassociated with the current results. This is due, in part,to the inherent variability in an impression material thatis mostly composed of water. Variability might havebeen reduced if the same impression had beenmeasured across time, rather than separate impres-sions used for each time period. However, this wasruled out as a study approach because it wasimportant to make the results as clinically applicableas possible, and exposing the impression to airmultiple times is not what occurs in dental practices.Typically, the impression is made and stored and thenis poured with gypsum as soon as it is removed fromstorage. Another possible limitation of the study is thatimpressions were made using optimum conditions (eg,

Table 1. Mean (SD) Percent Dimensional Change of Alginate

Impressions as a Function of Material at Three Storage Times*

Material (n 5 10) 30 min 48 h 100 h

JeltratePlus a20.17 (0.10) a20.50 (0.44) a20.77 (0.68)

Alginmax a20.15 (0.13) a20.09 (0.35) b 0.20 (0.24)

Kromopan 100 b20.59 (0.32) a20.40 (0.22) b20.15 (0.39)

* Subset (a–b) indicates differences or similarities between

materials at each storage time (P # .01).



an electric alginate mixer and a dental laboratoryvibrator were used to minimize bubbles in the alginatematerial). Furthermore, to optimize the ability tomeasure dimensional change, any impression thatexhibited voids affecting the middle horizontal line wasnot included in the study. It is not known how voidsmight affect dimensional stability.

One other consideration is that impression shrink-age is typically counteracted by gypsum expansion.With minimal dimensional change of Kromopan 100(20.15%) and Alginmax (+0.20%) after 100-hour

storage, the potential for an oversized cast exists,depending on gypsum expansion. Thus, it might bemore appropriate to use low-expansion gypsum withthese particular alginates. A future study that includesAlginmax and Kromopan 100 impressions poured withgypsum to determine overall effects on the resultantdimensional accuracy of the cast would be valuable.

CONCLUSIONS

N Both extended-storage alginates—Alginmax andKromopan 100—demonstrated minimal dimensional

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.

1114 WALKER, BURCKHARD, MITTS, WILLIAMS


change after 100 hours of storage, whereas theconventional alginate, JeltratePlus, was most accu-rate after storage for only 30 minutes.

N Kromopan 100 seems best suited to serve as anextended-storage alginate because it was mostaccurate after 100 hours of storage.

N Alginmax appears to be a versatile extended-storagematerial that potentially can be used for bothimmediate and delayed pouring with gypsum in thatit demonstrated minimal dimensional change after30 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 alsoacknowledge technical assistance provided by Jim Thomas,Mark Dallas, and Nick Alderman.

REFERENCES

1. Cook W. Alginate dental impression materials: chemistry,structure, and properties. J Biomed Mater Res. 1986;20:1–24.

2. Giordano R 2nd. Impression materials: basic properties.Gen Dent. 2000;48:510–512, 514, 516.

3. Powers JM, Sakaguchi RL. Alginate hydrocolloids. In:Powers JM, Sakaguchi RL, eds. Craig’s Restorative DentalMaterials. St Louis, Mo: Mosby Elsevier; 2006.

4. Miller MW. Syneresis in alginate impression materials. BrDent J. 1975;139:425–430.

5. Santoro M, Galkin S, Teredesai M, Nicolay OF, CangialosiTJ. Comparison of measurements made on digital andplaster models. Am J Orthod Dentofacial Orthop. 2003;124:101–105.

6. Nallamuthu N, Braden M, Patel MP. Dimensional changes ofalginate dental impression materials. J Mater Sci MaterMed. 2006;17:1205–1210.

7. Fellows CM, Thomas GA. Determination of bound andunbound water in dental alginate irreversible hydrocolloid bynuclear magnetic resonance spectroscopy. Dent Mater.2009;25:486–493.

8. Cohen BI, Pagnillo M, Deutsch AS, Musikant BL. Dimen-sional accuracy of three different alginate impressionmaterials. J Prosthodont. 1995;4:195–199.

9. Chen SY, Liang WM, Chen FN. Factors affecting theaccuracy of elastometric impression materials. J Dent.2004;32:603–609.

10. Sedda M, Casarotto A, Raustia A, Borracchini A. Effect ofstorage time on the accuracy of casts made from differentirreversible hydrocolloids. J Contemp Dent Pract. 2008;9:59–66.

11. Alcan T, Ceylanoglu C, Baysal B. The relationship betweendigital model accuracy and time-dependent deformation ofalginate impressions. Angle Orthod. 2009;79:30–36.

12. Imbery TA, Nehring J, Janus C, Moon PC. Accuracy anddimensional stability of extended-pour and conventionalalginate impression materials. J Am Dent Assoc. 2010;141:32–39.

13. ADA. Council on Scientific Affairs. American Dental Asso-ciation Specification No. 18: Alginate Impression Materials.Chicago, Ill: ADA; 1992.

14. Bradna P, Cerna D. Impact of water quality on setting ofirreversible hydrocolloid impression materials. J ProsthetDent. 2006;96:443–448.

15. Smith SJ, McCord JF, Macfarlane TV. Factors that affect theadhesion of two irreversible hydrocolloid materials to twocustom tray materials. J Prosthet Dent. 2002;88:423–430.

16. ANSI/ADA. Council on Scientific Affairs. American NationalStandard Institute/American Dental Association Specifica-tion No. 19: Dental Elastomeric Impression Materials.Chicago, Ill: ADA; 1992.

17. ANSI/ADA. Council on Scientific Affairs. American NationalStandard Institute/American Dental Association Specifica-tion No. 19: Dental Elastomeric Impression Materials.Chicago, Ill: ADA; 2004.

18. Joffe L. OrthoCAD: digital models for a digital era. J Orthod.2004;31:344–347.



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