Dimensional Accuracy of Alginate Impressions Using Different Methods of Disinfection With Different Concentrations

A dental impression is a negative imprint of an oral structure used to produce a positive replica of the structure for use as a permanent record or in the production of a dental restoration or prosthesis.¹ Alginate is one of the most widely used dental impression materials due to its ease of mixing, the flexibility of the set impression, its accuracy when properly handled, and low cost. The accuracy of an impression material is vital to the production of a well-fitting restoration and, thus, to its longevity.² However, alginate is affected by the reactions of syneresis and imbibition after removal from the mouth, as has been observed in several studies; therefore, stone casts must be fabricated as soon as possible to avoid dimensional changes.^3,4

The dimensional stability of an impression material reflects its ability to maintain the accuracy of the impression over time.^4-6 Maintaining dimensional stability of dental impression materials is critical if the impression cannot be cast (in stone) soon after removal from the mouth. Therefore, the dimensional changes in the alginate impression material must be limited to an allowable range of up to only 0.15%.⁷ Although the hydrophilic nature of irreversible hydrocolloid is valuable for making impressions in a moist environment, this attribute limits its use, as microorganisms present in blood and saliva tend to become embedded in alginate impression material posing a risk to dental practitioners, patients, and laboratory personnel.^8,9 This necessitates the disinfection of impression materials to avoid cross infection while maintaining the accuracy of the impression material.

Disinfectants must be effective as antimicrobial agents without adversely affecting the dimensional accuracy of the impression material and the resulting gypsum cast on which the denture will be fabricated.⁹ The comparison of different methods of disinfection of irreversible hydrocolloid material and their dimensional accuracy is worth studying. Several types of disinfectant have been suggested for use in dental practice, such as sodium hypochlorite (NaOCl), iodophor, phenol, and glutaraldehyde. Sodium hypochlorite is the most widely used disinfectant despite the increasing availability of other disinfectants due to its effectiveness, cost efficiency, and abundance. It fulfills most of the requirements of an ideal disinfectant and has a powerful cleaning action, performing as a broad-range antimicrobial agent that is effective against gram-positive bacteria, gram-negative bacteria, fungi, spores, and virus (including HIV). Many studies have examined sodium hypochlorite as a disinfectant solution but no consensus has been found for its recommendation.^9-15 The present study, therefore, aimed to evaluate and compare the dimensional stability of alginate impressions disinfected with sodium hypochlorite using spray and immersion methods.

An edentulous acrylic maxillary master model was constructed. Reference points (A, B, C, and D) for cast measurement were made using posts with grooves scored onto the occlusal surfaces in the shape of an “x.” The posts were placed in the approximate position of the incisal papilla, left and right second molars, and center of the hard palate (Figure 1). Measurements were recorded using digital calipers. Three readings were taken for each linear measurement (A–B, A–C, A–D, B–C, B–D, and C–D). The measurements were done between intersects of the “x” on the posts of each model. Eighteen measurements were done for each model. The mean of the three linear measurements was taken from the gypsum plaster casts and compared to those recorded from the master model.¹⁰

Five custom-made trays were constructed using light-cure acrylic (Vertex™ Light Curing Trayplate, Vertex Dental, vertex-dental.com). The trays were made by uniformly covering the model with 3 mm of wax with “stops” cut out in the approximate locations of the palatine fovea and left and right premolar positions. This would enable accurate positioning of the tray and allow a uniform thickness of the impression material. Impressions of the master model were taken and allowed to set for 5 minutes after which they were removed. A total of 200 impressions of the master model were taken using an irreversible hydrocolloid impression material. The impressions were divided into five groups of 40.^10,11 The impressions were retained on the tray mechanically by the excess alginate engaging the tray holes.¹¹ After impressions were taken from the master model, they were rinsed with tap water and treated as follows:

• First group (n = 40) of impressions: sprayed with 1% NaOCl solution (10 puffs for 15 seconds), then sealed in plastic bags for 5 minutes 
• Second group (n = 40): immersed in solution of 1% NaOCl for 5 minutes
• Third group (n = 40): sprayed with 5.25% NaOCl solution (10 puffs for 15 seconds), then sealed in plastic bags for 5 minutes
• Fourth group (n = 40): immersed in solution of 5.25% NaOCl for 5 minutes
• Fifth and final group (n = 40): received no disinfection procedure acting as control group^10,12-14

Alginate impression material (Italgin – Chromatic Alginate, BMS Dental, bmsdental.it) was portioned, mixed, and manipulated according to the manufacturer's instructions. In this study, commercially available Clorox^® bleach was used. The chemical solution contains 5.25% NaOCl. The brand name “Clorox” is derived from combining the names of the ingredients that form sodium hypochlorite (NaOCl), chlorine (Cl), and sodium hydroxide (NaOH). To obtain a 1% solution, the 5.25% NaOCl solution was diluted with distilled water.¹⁰ The dilution of the solution was done in the following manner: 190.5 ml of the concentrated solution was measured using a cylindrical measure and poured into a volumetric flask (1000 ml), and the distilled water was added until the demarcating line was reached on the flask. The dilution process was done on the same day the study was conducted.

All the impressions were rinsed under tap water for 10 seconds and then sealed in plastic bags to prevent water evaporation, and all of them were casted immediately. A type III gypsum product was used to pour the impressions. The gypsum product was portioned, mixed, and manipulated according to the manufacturer's instructions, then hand-spatulated for 10 seconds and poured into the impressions with the aid of a vibrator. The casts were allowed to set for 45 minutes before being removed from the impressions. The casts were ready for measurement after the setting of the gypsum. One operator recorded the measurements using digital calipers. The mean of the three linear measurements taken from the gypsum plaster casts was obtained from impressions that were disinfected, and this was compared to the control, which was not disinfected.¹⁰

For descriptive statistics, the mean, standard deviation, percentages, and graphs were achieved. For analytical statistics, repeated analysis of variance (ANOVA) was used to test the significant differences between the control group and the four methods of disinfection. Post hoc test using least square difference (LSD) was carried out if there were any overall significant differences after ANOVA; this was done to compare means of the control group and the four methods of disinfection. The level of significance for all tests was α = 0.05.

Table 1 shows that the highest means of distances were obtained from casts retrieved from impressions immersed in 5.25% NaOCl solution, indicating that the most dimensional changes occurred at this concentration. The mean distances from casts obtained from impressions that were sprayed with NaOCl (1% and 5.25%) solution showed similar values to the control group. The means in A–B, A–C, and B–D distances in all disinfection methods were approximately equal to the mean of the control group. However, the means of distances A–D (5.25% and 1%), B–C (5.25%), and C–D (5.25% and 1%) by immersion method showed higher values than control.

Significant differences were seen in the means of distances A–B, A–C, and A–D by the four different methods of disinfection when compared to control, while no significant differences in means of distances B–C and B–D were observed; hence, no LSD was calculated. Significant difference, however, was found in the mean of distance C–D by the four different methods of disinfection when compared to control (Table 2).

As shown in Table 3, a significant difference was seen in the distances A–B and A–D between control and disinfection by immersion of both 1% and 5.25% NaOCl solution. However, significant differences were observed in distances A–C and C–D between control and immersion in 5.25% NaOCl solution.

In this study, the dimensional accuracy of alginate was not affected after disinfection using the spraying method with 1% and 5.25% NaOCl solution. These observations were in agreement with studies conducted by Guiraldo et al,⁹ Oderinu et al,¹¹ Rueggeberg et al,¹⁵ Hamedi et al,¹⁶ and Suprono et al.¹⁷ In the case of immersion of alginate impressions in both 1% and 5.25% NaOCl solution there were significant changes in dimensional accuracy of the alginate material. As expected, less dimensional changes occurred in the alginate impression after immersion in the less concentrated solution after disinfection with 1% NaOCl. Oderinu et al similarly reported that the immersion method caused dimensional variability using 1% of NaOCl for disinfection.¹¹ As in this study, Rueggeberg et al and Hamedi et al reported that there were significant changes in alginate after immersion with more concentrated 5.25% NaOCl solution.^15,16 Conversely, numerous other authors found that the mean dimensions measured between points did not differ significantly compared with the control group.^10,12,18-21

Some studies concluded that dimensional changes observed after disinfection of irreversible hydrocolloid impressions materials were not clinically significant.^22-25 However, as was found in this study, the most accurate casts were retrieved from impressions disinfected by spray method rather than immersion.^22,25 The dimensional changes of alginate impressions  disinfected by either spraying or immersion using different concentrations of NaOCl solution could be attributed to either a reaction between the hypochlorite absorbed into the impression and dental stone or a direct effect of the hypochlorite on the alginate in relation to surface quality.

A limitation of this study could be the accuracy of measurement of distance between reference points, even though the type of digital caliper used in this study has been shown to produce good reproducibility between repeat readings for each linear measurement.¹⁰ Additional tools such as measuring microscopes, micrometers, and dial gauges also could be used to verify results. Another limitation is that direct comparison of results with other studies was difficult due to a variety of brands of alginate materials used, the type of disinfection protocols applied, and the measuring techniques employed. Further research to evaluate the effect of NaOCl on the physical properties of the different brands of irreversible hydrocolloid impression material available is recommended.

Within the limitations and based on the findings of this study it can be concluded that disinfection of irreversible hydrocolloid impression material is more appropriate using NaOCl spray rather than immersion, and the spray method did not affect the dimensional stability of alginate as much as the immersion method did.

Ghada Hassan Babiker, BDS, MSc
Department of Oral Rehabilitation
Faculty of Dentistry
Khartoum University
Khartoum, Sudan
Department of Prosthodontics
Faculty of Dentistry
Al-Neelain University
Khartoum, Sudan

Nadia Khalifa, BDS, MSc, PhD
Department of Oral Rehabilitation
Faculty of Dentistry
Khartoum University
Khartoum, Sudan
Department of General and Specialist Dental Practice
College of Dental Medicine
University of Sharjah
Sharjah, United Arab Emirates

Mohammed Nasser Alhajj, BDS, MSc
Department of Oral Rehabilitation
Faculty of Dentistry
Khartoum University
Khartoum, Sudan
Department of Prosthodontics
Faculty of Dentistry
Thamar University
Dhamar, Yemen