European Journal of Orthodontics 1 of 7 © The Author 2012. Published by Oxford University Press on behalf of the European Orthodontic Society.doi:10.1093/ejo/cjs031 All rights reserved. For permissions, please email: journals.permissions@oup.com

Introduction

Alginate is the most commonly used impression material in an orthodontic practice for making diagnostic and working casts. Alginates are irreversible hydrocolloid materials that were originally developed in the 1930s (Doubleday, 1998). The main advantages of alginates are the ease of use, the low cost, their hydrophilic characteristics, and the good patient acceptability (Frey et al., 2005; Murata et al., 2006). However, alginates have low tensile strength (Doubleday, 1998). They tend to have poor reproduction of surface detail, are not dimensionally stable on storage due to syneresis, and are usually best poured immediately (Doubleday, 1998). Some of the mechanical properties that can determine success or failure of an impression material are strain in compression, elastic recovery, and tensile strength (Frey et al., 2005). The requirements for these mechanical properties are described in American National Standard Institution/American Dental Association (ANSI/ADA) specifications no. 18-1992 for Alginate Impression Materials. Tensile strength becomes important when impressions are made of areas with undercuts. The higher the tear energy, the less likely it is for the material to be torn in an area with existing undercuts (Vrijhoef and Battistuzzi, 1986; Cohen et al., 1998). Materials with high tensile strength will display less of such side effects. Additionally, impression materials should also have optimal elastic recovery

The influence of mixing methods and disinfectant on the

physical properties of alginate impression materials

Karoline Dreesen*, Annelies Kellens*, Martine Wevers**, Pushpike J. Thilakarathne*** and Guy Willems**Department of Oral Health Sciences, Orthodontics, **Department of Metallurgy and Materials Engineering and ***L-BioStat, Katholieke Universiteit Leuven, Belgium

Correspondence to: Guy Willems, Department of Oral Health Sciences, Orthodontics, Faculty of Medicine, Katholieke Universiteit Leuven, Kapucijnenvoer 7, B-3000 Leuven, Belgium. E-mail: guy.willems@med.kuleuven.be

SUMMARY The aims of this in vitro study were to quantify the effect of manual versus automatic mixing and of using a disinfectant on mechanical properties of three different alginate impression materials. Two of the three alginates tested were especially developed for orthodontic use: Orthotrace® and Orthofine® while the third was a conventional alginate CA37FS®. Alginates were mixed by hand or automatically using a Cavex alginate mixer II®. Mixing was performed at room temperature using tap water. The material was allowed to set in a water bath at 35°C (±1°C), simulating intra-oral setting conditions, and half of the samples were disinfected before testing. For each tested material, 10 standardized samples were used. The disinfectant used was the CavexImpreSafe® that has a bactericide, virucide, and fungicide function. The specimens were exposed for 3 minutes in a 3% solution and were then tested according to the ISO 1563: 1990 (E) standard specifications. Descriptive statistics and three-way analysis of variance were performed, and a 5% significance level was used for statistical analysis. Evaluation of tensile strength and elastic recovery of different alginate samples, hand versus automatical mixing or disinfected versus not disinfected, resulted in significant differences for all materials except for Orthofine®. Considering detail reproduction, all three alginates evaluated reproduced the 50-mm line successfully without interruption. The mixing method can significantly affect the elastic recovery and tensile strength of the alginates tested while the effect of using a disinfectant is less explicit.

properties. Elastic recovery is the ability of the alginate material to recover its shape after it has been deformed during removal from the mouth. The greater the elastic recovery, the more accurate the impression material will be (Jorgensen, 1976). Finally, surface detail reproduction must be optimal. It allows appliances to be made on the casts and is essential in the production process of digital models, allowing for accurate and highly detailed casts. Although alginate is easy to manipulate, the correct handling (water/powder ratio, spatulation) affects the strength of the material. Therefore, it is imperative to follow the manufacturer’s prescriptions on mixing (Caswell et al., 1986; Johnson, 2002; Kohn, 2002; Frey et al., 2005). The most accurate way to dispense alginate is to weigh it because volumetric dispensing can differ from the recommended weight by 10–20% (Caswell et al., 1986). Using a special dispenser to measure the water also ensures that the correct amount of water is added to the mix.

Nowadays, high-speed rotary mixing instruments for alginate impression materials can be used in a dental practice. These instruments easily produce a fine paste low in air bubbles compared with paste mixed by hand. Therefore, it is estimated that paste obtained by this method possesses superior rheological properties by reducing the number and volume of porosities in the mixed alginate

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Table 1  Evaluated alingate impression materials and manufacturers’ information.

Product Type Total setting time Manufacturer Batch no. Mixing proportion

Powder (g) water (ml)

Orthotrace Extra fast set 2.10 min Cavex Holland BV 80214 7 15CA37FS Fast set 3.30 min Cavex Holland BV 81207 7 15Orthofine Extra fast set 2.00 min Zeist 86633 9 18

(Inoue et al., 2002; Hamilton et al., 2010). However, it has been noted that the working time of this automatically mixed paste is markedly reduced (Inoue et al., 2002).

In addition, dental impressions become contaminated with the microorganisms from saliva and blood of the patients that can cross-infect gypsum casts poured against them (Leung and Schonfield, 1983; Chau et al., 1995). The risk of transmitting pathogenic micro-organisms to dental laboratories via impressions has been considered a topic of importance for a number of years and this potential of cross-contamination between clinical area and laboratory must be reduced (Bergman, 1989; Owen and Goolam 1993; Sofou et al., 2002). Sterilization of impressions by dry or moist heat is unsuitable for alginates and therefore cold disinfection must be used for this purpose (Conner, 1991). As the necessity for disinfecting impressions has become apparent, it has also become clear that the process itself should have no adverse impact on the dimensional accuracy and surface texture features of the impression material and resultant gypsum cast (Ahmad et al., 2007). The ideal disinfection procedure must leave the physical and chemical properties of the impression material and gypsum unchanged to achieve optimal accuracy of the final casts and the appliances made on the casts. As alginate material impressions do not tolerate heat treatment, chemical disinfection has been the method of choice (Ahmad et al., 2007).

The aims of this in vitro study were to quantify the effect of hand-mixing (HM) and automatic mixing (AM) technique with and without the use of a disinfectant on mechanical properties (tensile strength, elastic recovery, and detail reproduction) of three different alginate impression materials.

Materials and methods

The materials used in this in vitro study were Orthotrace® (Cavex Holland BV, Haarlem, The Netherlands), an extra fast-setting alginate; CA37FS® (Cavex Holland BV) a fast setting alginate; and Orthofine®, an extra fast setting alginate (Postbus 92, Zeist, The Netherlands) (Table 1).

Each alginate was tested for elastic recovery, tensile strength, and detail reproduction. Ten different samples were taken according to the different mixing methods: HM

or AM and mixing method in combination with the use of the disinfectant. All materials were used according to the manufacturer’s recommendations, meaning that the powder/water ratio used for Orthotrace® and CA37FS® was 7 g/15 ml and for Orthofine® 9 g/18 ml. The mixing was performed on room temperature while further setting was allowed in a water bath at 35 ± 1°C to offer a heating rate of the sample comparable to the heating rate of impression materials in oral conditions. The water used for mixing was tap water at 23°C.

The disinfectant used was Cavex ImpreSafe® (Cavex Holland BV), especially designed for disinfecting dental alginate impressions. This disinfectant is bactericide, virucide (HIV, HBV, HCV, Influenza virus, Herpes virus, Rotavirus), and fungicide (complies with DGHM guidelines and EN1040, EN1275, EN13624, EN13727, EN14561, EN14562). Samples were exposed for 3 minutes in a 3% solution (30 ml disinfection solution for 1 l water) and afterwards abundantly rinsed with tap water before testing. According to the manufacturer’s information, using this disinfectant, which is mainly a mixture of quaternary ammonium salts and water in a ratio of 30–70 weight per cent, has no detrimental effect on the quality of the impression.

Mixing methods

The materials were mixed by using either the HM technique or the AM technique using a Cavex alginate mixer II® (Cavex Holland BV). The mixing time for the HM method was 20 s and for the AM method 10 s.

Measurements

The start of the mixing procedure was at time 0 s. After, respectively, 20 s (HM) and 10 s (AM), a specific mould was filled with a mixture of alginate impression material and the impression material was allowed to set in order to achieve a sample for testing. At time 60 s, the mould was placed in a water bath at 35 ± 1°C for 60 s for further setting offering a heating rate of the sample comparable to the heating rate of impression materials in oral conditions. At time 120 s, the mould was taken out of the water bath, the sample was taken out of the mould and at time 150 s, the

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Figure 1  Mould 68A to reproduce the ‘V’-shaped samples.

Figure 2  ISO stainless steel test block used in this study.

Figure 3  Effect of mixing technique and disinfectant on tensile strength of the three different materials tested. Vertical bars indicate 95% confidence interval for the mean. AM, automatic mixing; HM, hand mixing; D, disinfectant.

and before testing the elastic recovery, the samples were measured with a digital marking gauge. This was always performed by the same practitioner. The test to determine the elastic recovery consisted of two phases: firstly, the samples were gradually loaded up to 12 N, and secondly, the samples were gradually unloaded to allow for recovery from the deformation. At the end of the test, the samples were measured a second time with the digital marking gauge. The recovery from deformation is calculated as a percentage by using the following formula:

100 1 ,20

A B− × −

where A and B indicate sample height before and after deformation, respectively, and 20 being the height of the mould in millimetres.

Detail reproduction

Surface detail reproduction of the dental stones from the alginate impression materials was determined according to ISO (1990) specifications 1563. For determination, the ISO stainless steel test block was used (Ravensfield Design, Heywood, Lancashire, UK) (Figure 2). This test block is engraved with three horizontal and two vertical orientation lines; horizontal lines have a depth of 20 (b), 50 (a), and 75 (c) mm.

After mixing the alginate impression material (time 0 s), it was poured into the ring mould, the test block was centred above the mould, and manually pressed down on the mass of the impression material. At time 60 s, the test block was placed in a water bath of 35 ± 1°C for 60 s for further setting offering a heating rate of the sample comparable to the heating rate of impression materials under oral conditions. At time 120 s, the test block was taken out of the water bath and the ring mould was separated from the test block rendering samples for testing. Samples in the disinfection

various test were started. Samples in the disinfection group were, after the mould was taken out of the water bad, placed in the 3% disinfectant solution for 180 s and tested at time 330 s.

Tensile strength

For evaluating the tensile strength, 10 samples were made of each alginate/mixing method/disinfectant combination. For this test, a specific mould was used: mould 68A according to the ISO standards (Figure 1). The mould was filled with mixed alginate resulting in a ‘V’-shaped sample to standardize the tear location. A glass plate was placed on top of the mould to ensure a uniform thickness of the sample. When fully set, the sample was removed from the mould and placed in the Instron® 5567 (Instron European Headquarters, High Wycombe, England) for testing at a speed of 500 mm/minutes until rupture of the sample.

Elastic recovery

Elastic recovery was tested on cylindrical samples made with a specific split mould design that enabled easy recovery of the sample (ISO 1563:1990 (E) specifications). This mould has a height of 20 mm and an interior diameter of 12.5 mm. After mixing the impression material, the mould was filled with the mixture. After fully setting of the sample

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Figure 4  Effect of mixing technique and disinfectant on elastic recovery of the three different materials tested. Vertical bars indicate 95% confidence interval for the mean. AM, automatic mixing; HM, hand mixing; D, disinfectant.

Table 2  Mean elastic recovery (K) and tensile strength (N) of the tested impression materials (n = 10)

Product Elastic recovery % (SD) Tensile strength, N (SD)

HM + D HM AM AM + D HM + D HM AM AM + D

Orthotrace 94.53 (0.28) 94.56 (0.19) 95.10 (0.08) 95.61 (0.38) 27.76 (0.54) 34.58 (2.74) 37.12 (1.13) 30.15 (1.46)CA37FS 94.58 (0.32) 89.4 (0.57) 94.83 (0.17) 95.07 (0.47) 23.28 (1.75) 35.62 (0.72) 39.96 (1.04) 38.02 (2.00)Orthofine 96.40 (0.30) 94.92 (0.27) 94.89 (0.34) 96.43 (0.51) 16.99 (1.76) 21.17 (2.69) 23.35 (3.63) 22.65 (1.98)

HM, hand mixing; AM, automatic mixing; D, disinfection; SD, standard deviation; N, Newton.

considered significant. Tukey’s adjustments are used for multiple comparisons.

Results

The means and standard deviations of the tested mechanical properties are summarized in Table 2. The influences of mixing technique and disinfectant on different types of materials are illustrated in Figures 3 and 4. Table 3 shows the results of the analysis of variance and depicts that material, mixing technique and disinfectant had statistically significant influences on the elastic recovery and the tensile strength. There is a significant difference in the tensile strength and the elastic recovery of the three tested materials. Tensile strength is shown to be significantly higher for non-disinfected automatically mixed samples whereas the highest elastic recovery is reported for disinfected automatically mixed samples. However, significant interactions indicate that differences between types of materials depend on mixing techniques and the use of disinfectant.

Therefore, two-way and three-way interactions are tested. It is observed that hand-mixed and disinfected samples have lowest tensile strength for Orthofine®. For hand-mixed and disinfected samples, tensile strength of CA37FS® is significantly higher than for Orthofine® (P value = 0.0001) as well as for Orthotrace® (P value = 0.0001). For AM and

group were, after separating them from the test block, placed in the disinfectant solution of 3% for 180 s. Gypsum (Snow White Plaster No. 2, ISO Type I; KerrHawe S.A., Bioggio, Switzerland) was vacuum mixed according to the manufacturer’s instructions. After mixing, the gypsum was poured over the alginate samples, then the samples topped with the gypsum were carefully trembled to remove any remaining air bubbles out of the gypsum and the mix was left for setting. Subsequently, the surfaces of the gypsum impressions were inspected after 1 h by two independent examiners. To comply with the ISO specifications number 1563 for detail reproduction, the alginate impression materials had to reproduce the full 25-mm length of the 50-mm line (line a in Figure 2). A line that was fully reproduced was given a score 1, otherwise zero score was given

Statistical analysis

Descriptive statistics as well as a three-way analysis of variance were performed using SAS software, version 9.2 of the SAS System for WINDOWS (SAS Institute, Cary, North Carolina, USA). Main effects (e.g. the overall average effect of type of materials), as well as two-way and three-way interactions are verified. For instance, the two-way interaction between type of material and mixing technique implies that the difference between the materials depends on mixing technique. P values smaller than 5% are

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Table 3  Summary of three-way analysis of variance, elastic recovery and tensile strength

df Contrast Difference (95% CI) F value P value

Elastic recovery  Main effects    Type of material 2 395.81 P < 0.0001

CA37FS OF −2.19 (−2.38, -2.00) P < 0.0001CA37FS OT −1.48 (−1.67, −1.29) P < 0.0001OF OT 0.71 (0.52, 0.90) P < 0.0001

    Disinfectant 1 No Yes −1.49 (−1.62, -1.36) 527.26 P < 0.0001    Automatic or hand made 1 AM HM 1.25 (1.13, 1.38) 373.06 P < 0.0001  Two-way interactions    Type of material × disinfectant 2 120.54 P < 0.0001    Type of material × automatic or hand made 2 184.91 P < 0.0001    Disinfectant × automatic or hand made 1 124.26 P < 0.0001  Three-way interaction    Type of materials × disinfectant × automatic or hand made 2 182.33 P < 0.0001

Tensile strength  Main effects    Type of material 2 517.65 P < 0.0001

CA37FS OF 13.18 (12.12, 14.23) P < 0.0001CA37FS OT 1.81 (0.76, 2.87) P < 0.0001OF OT −11.4 (−12.40, −10.30) P < 0.0001

    Disinfectant 1 No Yes 5.49 (4.77, 6.21) 229.45 P < 0.0001    Automatic or hand made 1 AM HM 5.31 (4.59, 6.03) 214.47 P < 0.0001  Two-way interactions    Type of material × disinfectant 2 17.741 P < 0.0001    Type of material × automatic or hand made 2 35.351 P < 0.0001    Disinfectant × automatic or hand made 1 39.854 P < 0.0001  Three-way interaction    Type of materials × disinfectant × automatic or hand made 2 18.2 P < 0.0001

df, degrees of freedom; CI, confidence interval.P values less than 0.05 are statistically significant.

Table 4  Inter-observer agreement of the evaluation (no/yes) of detail reproduction

Assessor 1

Yes No

Assessor 2 Yes 26 2 28No 1 7 8

27 9 36

The two observers agreed on 92% (33/36) of the impressions. The chance-corrected agreement (Cohen’s kappa coefficient) equals 0.769 (SE = 0.17).

non-disinfected samples, tensile strength of CA37FS® is significantly higher than that for Orthofine® (P value = 0.0001). However, for AM and non-disinfected samples, there is no significant difference between CA37FS® and Orthotrace® (P value = 0.076).

Further, it is observed that automatically mixed and disinfected samples have highest elastic recovery for Orthofine®. Elastic recovery of CA37FS® for automatically mixed and disinfected sample is significantly smaller than that for Orthofine® (P value = 0.0001). Similarly, elastic recovery of CA37FS® for hand-mixed and disinfected

samples is significantly smaller than for Orthofine® (P value = 0.0001). However, elastic recovery of CA37FS® for automatically mixed and non-disinfected samples is not significantly different from Orthofine® (P value = 0.9999).

On average, elastic recovery is higher for Orthofine® while tensile strength is lower for that material.

Concerning the detail reproduction, 36 impressions were evaluated by two independent observers (Table 4). The two observers agreed for 92% (33/36) of the impressions. The chance-corrected agreement (Cohen’s kappa coefficient) equals 0.769 (SE = 0.17). Clinically, all three alginates could reproduce the 50-mm line. In fact, automatically mixed samples of Orthotrace® and CA37FS® could also reproduce the 20-mm line; this was equal to the ISO specifications number 4823 for detail reproduction of the addition silicones. They have to reproduce the full 25-mm length of the 20-mm line (line b in Figure 2).

Discussion

The statistical analysis shows that there are significant differences in the mechanical properties of the three alginate impression materials, the two mixing methods and the disinfectant. Alginate materials prepared with a method that produces less porosity may have improved properties such

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as recovery from deformation, strain in compression, compressive strength, surface detail reproduction, and dimensional stability (Hamilton et al., 2010).

When removing an impression from the mouth, the material must be sufficiently strong and elastic for the material not to be torn apart. By removing the impression by force can indeed produced which can cause ruptures and distortions in the impression.

Impression materials should have high elastic recovery in order to record undercuts of teeth or residual ridges. The elastic recovery of the tree alginates tested was in the range of the minimum (95%) called for in ANSI/ADA specification no. 18-1992. Statistically significant differences were found among the products, CA37FS® hand-mixed samples had the lowest elastic recovery, probably due to inclusion of air lumps in the samples. Orthofine® did not show a difference between HM and AM; however, Orthotrace® and CA37FS® both showed an improvement with AM over HM. As pointed out by the one of the reviewers, clinically a de/increasing of the elastic recovery by 2% seems irrelevant. In contrast to these significant differences in the recovery from deformation among the materials evaluated, Murata et al. (2006) found that there were no significant differences in the recovery from deformation among the six different materials tested using only one standard mixing technique. In 2005, Frey et al. observed a statistically significant difference in elastic recovery among three alginate materials. All three materials reached an elastic recovery above the minimum of 95%. Further, two materials showed a statistically significant improvement with the AM method.

To cast an impression twice, the impression material must remain sufficiently strong and elastic for the cast to be removed without damaging the impression. During removal, there is a risk of damaging and/or distorting the impression. A material with a high tensile strength therefore gives better results when double-pouring impressions (Vrijhoef and Battistuzzi, 1986; Cohen et al., 1998). In orthodontics, impressions often are double poured to make study and working casts from the same impression. Tensile strength is also important when areas with undercuts have to be duplicated. The higher the tear energy, the less likely it is for the material to tear in an area with existing undercuts (Vrijhoef and Battistuzzi, 1986). When making working casts by taking impressions over brackets, it is important for the alginate material not to tear when removing it. Therefore, it is important that the impression material is not only strong enough to be removed over the brackets but also strong enough to stay attached to the tray. Orthofine® showed a significant lower tensile strength in comparison with Orthotrace® and CA37FS® and a statistical significant improvement in tensile strength was noticed with AM for all tested materials. Vrijhoef and Battistuzzi (1986), Cohen et al. (1998), and Frey et al. (2005), measured the tear energy by using ‘V’-shaped specimens of the same alginate products and observed similar results for these products.

Tear energy in this study was in the range of 100–300 J/m2. Besides this, a statistically significant improvement of one product was found in tear energy with mechanical mixing.

The AM procedure of this study was performed using the Cavex alginate mixer II®. According to the manufacturer, the Cavex alginate mixer II® will standardize the alginate mixing procedure and will give a smooth and homogeneous mixture free from air bubbles. Inoue et al. (2002) showed that pastes mixed automatically had a markedly shorter working and setting time compared with HM or semi-automatic mixing. When the material is mixed at high speed, the temperature of the paste increases slightly as a result of the mechanical friction between the material particles or between the material and the mixing vessel. The working time is reduced when automatically mixing. In this study, a reduced working time was also observed just like an improved consistency after mixing and the bubble-free texture when the AM procedure was used. Inoue et al. (2002) investigated the setting characteristics and the rheological properties of alginates mixed by three methods: a hand-mixing technique, a semi-automatic mixing instrument, and an AM instrument. They found almost no porosities using the AM instrument and concluded that in clinical use, the homogeneous mix produced by AM was preferred over hand mixing (Inoue et al., 2002). Frey et al. (2005) used the Alginator II (Cadco, Oxnard, CA), a semi-automatic mixer and observed similar findings. The present study showed that AM improved the tensile strength of all three alginates.

Disinfection of dental impressions has been considered a topic of importance for a number of years. The main requirements for a disinfectant are efficiency of the disinfecting solution in eliminating the pathogens and the influence of the disinfection treatment on the mechanical properties of the impression material (Rentzia et al., 2011). Chemical disinfection, preferably by immersion, seems to be the most reliable and practical method, provided that it does not adversely affect the dimensional stability of the impressions. Irreversible hydrocolloids tend to be superficially dissolved or deteriorated by sodium hypochlorite and dehydrated by ethanol. Therefore, hydrocolloids should be disinfected for a limited time period. Immersion seems to be more secure than spraying (Kotsiomiti et al., 2008).

Cavex ImpreSafe was chosen as disinfectant in this study. The manufacturer’s instructions state that Cavex ImpreSafe is suitable for disinfection of alginate impressions. The active ingredient is a quaternary ammonium salt (30% w) and a 3% solution is necessary to disinfect alginate impressions by immersion for 3 minutes. This study has shown that disinfection with Cavex ImpreSafe did not affect the ability of an alginate impression material to reproduce surface detail. However, Ahmad et al. showed in their study that disinfection of alginate impressions with Perform-ID® (Schülke and Mayr GmbH, Germany) had a significant

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effect on the ability to reproduce surface detail (Ahmad et al., 2007). Chemical disinfection of impressions must be limited in time because of the absorption of water (imbition) can affect the precision of the impression and may result in inaccurate casts (Imbery et al., 2010). ISO 1563 specifications state that the alginate impression and resultant cast shall be able to reproduce the 50-mm line without interruption when testing for reproduction of detail and compatibility with gypsum products (ISO 1563, 1990). Koski showed that alginate mixed with a vacuum mixer produced fewer surface defects and had better detail reproduction with cast gypsum than either hand or centrifugal mixing when comparing mixing techniques and devices with different alginate brands (Koski, 1977). The present study showed that 2 out of 3 alginate materials tested were also able to produce the full length of the 20-mm line.

Conclusion

Within the limitations of this in vitro study following statistical and clinical conclusions can be made. Though the differences between the materials and between the mixing methods are found to be significant, they are not clinically relevant. However, there is a preference for AM to standardize the alginate mixing procedure, to facilitate the mixing, to reduce the amount of air bubbles, to obtain a homogenous mixture, and thus to improve tensile strength. Furthermore, it can be concluded that the use of the particular disinfectant tested had no negative clinical effects on the impressions.

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