The effect of disinfecting solutions on the dimensional stability of dental alginate impression materials

Abstract

Objectives

Dimensional changes occur in set dental alginate impression materials when immersed in disinfecting solutions. In this contribution the dimensional changes of two alginates in two disinfecting solutions, and for two specimen thicknesses, have been studied. The results were analyzed theoretically.

Methods

The dimensional changes of two commercial alginates (Blueprint Cremix and Hydrogum), have been measured, in distilled water and two disinfecting solutions (Perform ID/sodium hypochlorite), using a traveling microscope, at 5 min intervals over a period of 1 h. Samples of simple geometry have been studied, namely rectangular strips with thicknesses of 1.5 and 3 mm, respectively.

Results

In all cases, both alginates continuously shrank with time, in the three immersion liquids, over the hour of measurement, indicating transfer of water from the alginate into the external water or disinfecting solution. The t 1/2 shrinkage plots were generally linear, but with an intercept on the t 1/2 axis, indicating the possibility of an initial expansion at very short times. In most cases, the ratios of slopes for both thicknesses were 1.33–1.54, in contrast to the theoretical value of 2. Perform ID however gave anomalous results for the 1.5 mm thick samples. At 10 min their shrinkage was 1.34–1.72%, compared with −0.42% to 0.67% in the other two media.

Significance

The effects of thickness observed were not in accord with simple Fickian theory because of the various ions diffusing into and out of the alginate. Moreover, the water content of the alginate decreased consequent on the cross-linking process.

Introduction

Disinfection of dental impression materials is necessary to avoid cross contamination consequent on the presence of micro-organisms embedded on/in the impression materials from blood and saliva . The actual micro-organisms found have been reported, for example, by Jennings and Samaranayake , and Look et al. .

There are generally two disinfecting methods available, namely the use of aerosols or immersion in disinfecting solutions . Clearly, either method should not compromise the dimensional stability of the impression material. With alginate impression materials, dimensional changes are known to occur on immersion in water and artificial saliva . Often the alginate initially swells, but then shrinks until ultimately the process equilibrates with a net shrinkage. Other sources suggest alginates swell continuously with time, on immersion in water .

The situation is currently confused because there are a number of alginates commercially available; although the general constituents will be similar, there are likely to be detailed differences. Furthermore there are a number of disinfecting solutions available. The various studies reported use different alginates and disinfecting solutions, and differing test methods. Some of the studies are reported here.

Martin et al. studied three disinfecting solutions, 5.25% sodium hypochlorite, Perform ID and Sterilox, the latter both as 10% and concentrated solutions, with a range of impression materials, including one alginate. Changes in dimensions of cylindrical samples (4 mm high) were measured at three time intervals, namely after setting, after the manufactures guideline for immersion disinfection, and 24 h after immersion. In the case of the alginate, some of the solutions exhibited an initial expansion up to 10 min, and then subsequently contraction to give an overall net contraction.

Abour et al. studied the dimensional stability of casts made from an alginate impression material that had been immersed in undiluted sodium hypochlorite, for 30 min; untreated impressions were used as controls. Casts of the impression materials were poured after 30 min, and stored for 24 h before being measured. The results of this study indicated no significant differences in the dimensional stability of the casts between controls and disinfected samples.

Bergman et al. investigated the effects of six disinfectants (2% gluteraldehyde, 2–4% propanolol, 0.5% chlorhexidine, 5.0% sodium salt of sulfanochloramide, 1.0% benzalkonium chloride and a mixture of phenyl phenol and 0.5% chlorcresol), on four proprietary alginate impression materials. Distilled water was used as the control. Six stainless steel blocks, indented with three lines, were used to measure dimensional stability and surface detail of the set alginate materials. The results of this study showed significant dimensional changes, but no significant changes in surface detail.

Alginate impressions have also been immersed in iodophor, phenol gluteraldehyde, and glyoxal gluteraldehyde , and the effects on surface detail and dimensional changes were examined. This study used a mandibular arch for the master cast. The set impressions were rinsed in water for 10 s, to simulate clinical practice, immersed in the disinfectant, as stipulated by the manufacturers, and then poured to form casts. Dimensional changes were followed by a measuring microscope. It was concluded from the results that hydrocolloid materials showed no significant changes in accuracy compared with controls. However, disinfected samples were smoother than the corresponding controls.

Peutzfeldt and Asmussen investigated three types of disinfectants with three commercial alginates. Deionized water was used as a control. The immersion time for all the samples was 1 h, after which they were stored in a humid environment for 24 h, and then casts were made. The results contra-indicated the use of immersion systems containing 70% ethanol, due to the resultant excessive shrinkage, attributed to dehydration of the alginates. Otherwise no significant dimensional changes were observed with other immersion systems, although it was noted that Blueprint exhibited expansion.

In another study by Jones et al. , alginate impression materials showed clinically unacceptable dimensional changes when impressions were immersed for more than 10 min in a 2% gluteraldehyde solution. It was also noted that the disinfected impressions exhibited discoloration.

The above list of studies is not exhaustive, but gives an indication of the differing results obtained with the various methods used. It is noteworthy that no investigators seemed to have monitored dimensional changes with time to get dimensional change profiles (Minagi et al. is an exception). Also no investigation has studied the effect of the dimensions (e.g. thickness) of alginate specimens on their dimensional changes during immersion in solutions.

Hence, the aims of this study were:

  • (1)

    To investigate the effects of two disinfecting solutions, and distilled water as a control, on the dimensional stability of two commercial alginates made from samples of two different thicknesses. The dimensional changes were measured as a function of time, up to 1 h, to determine the nature of dimensional change profiles.

  • (2)

    To investigate the effect of specimen size (e.g. thickness) by plotting the results (% linear shrinkages) against t 1/2 to establish the role of diffusion in the material/specimen.

  • (3)

    To analyze the data in terms of Fickian diffusion theory and the Gibbs modification of this theory.

Materials and methods

Materials

The two commercial alginates studied were Blueprint Cremix (Blueprint, Dentsply, USA) and Hydrogum (Ivoclar Vivadent, Liechtenstein). The two disinfecting solutions were (1) Perform ID (PID, Schulke and Mayr, Germany). This material consists of a powder containing ∼20 g potassium peroxomonosulfate, 15 g sodium benzoate, ∼10 g tartaric acid. One scoop of powder was added to 1000 ml of water to form the disinfecting solution. It is claimed by the manufacturer that impression materials, including alginates, will be stable during the disinfecting period of 10 min; (2) sodium hypochlorite solution (NaOCl, Milton Laboratories, Rivadis, France); 500 ml of this reagent was added to 1000 ml water. Distilled water (DW) was used as a control immersion medium.

Methods

Sample preparation

Modeling wax molds were used, with rectangular cavities 60 mm × 10 mm, to make samples of two thicknesses, 1.5 and 3 mm, respectively, for the purpose of determining the effect of thickness. Each alginate was mixed according to the manufacturer’s instructions (the water/alginate ratio for Hydrogum was 15 ml/7 g, and 34 ml/14.7 g for Blueprint Cremix) and introduced into the mold, standing on a glass plate. Then another glass plate was pressed on to the mold, squeezing out the excess material. After setting (2 min for Hydrogum and 1½ min for Blueprint Cremix, as per manufacturer’s instructions), the sample was removed from the mold and any flash carefully cut away. Five samples per material, per test were used for all procedures.

Measuring dimensional changes

As soon as a given sample was set, it was placed in a flat-bottomed polytetrafluoroethylene (PTFE) mold. The sample was anchored on a fixed pin at one end of the trough, and a second pin was carefully inserted into the other end of the sample, taking care not to penetrate the sample completely. Then the sample was covered with the appropriate liquid (30 ml).

Dimensional changes were measured with a traveling microscope (Chesterman) to ±0.001 cm, for each material in DW and the two disinfecting solutions. First a reading was obtained for the fixed pin. Then the microscope was focussed on the opposite edge of the sample, closest to the movable pin, and the second reading was taken, to obtain a baseline reading. The subsequent readings were taken every 5 min up to 1 h, on the sample edge closest to the movable pin.

Initially the data were plotted as percentage linear shrinkage against time, followed by percentage linear shrinkage against the square root of time ( t 1/2 min 1/2 ); the latter was to establish the role of diffusion in the samples.

Statistical analysis

The Mann Whitney test was used for a comparison between: Blueprint Cremix and Hydrogum at 1 h, and sodium hypochlorite/Perform ID at 10 min (and 1 h), for the two thicknesses 1.5 and 3 mm.

Materials and methods

Materials

The two commercial alginates studied were Blueprint Cremix (Blueprint, Dentsply, USA) and Hydrogum (Ivoclar Vivadent, Liechtenstein). The two disinfecting solutions were (1) Perform ID (PID, Schulke and Mayr, Germany). This material consists of a powder containing ∼20 g potassium peroxomonosulfate, 15 g sodium benzoate, ∼10 g tartaric acid. One scoop of powder was added to 1000 ml of water to form the disinfecting solution. It is claimed by the manufacturer that impression materials, including alginates, will be stable during the disinfecting period of 10 min; (2) sodium hypochlorite solution (NaOCl, Milton Laboratories, Rivadis, France); 500 ml of this reagent was added to 1000 ml water. Distilled water (DW) was used as a control immersion medium.

Methods

Sample preparation

Modeling wax molds were used, with rectangular cavities 60 mm × 10 mm, to make samples of two thicknesses, 1.5 and 3 mm, respectively, for the purpose of determining the effect of thickness. Each alginate was mixed according to the manufacturer’s instructions (the water/alginate ratio for Hydrogum was 15 ml/7 g, and 34 ml/14.7 g for Blueprint Cremix) and introduced into the mold, standing on a glass plate. Then another glass plate was pressed on to the mold, squeezing out the excess material. After setting (2 min for Hydrogum and 1½ min for Blueprint Cremix, as per manufacturer’s instructions), the sample was removed from the mold and any flash carefully cut away. Five samples per material, per test were used for all procedures.

Measuring dimensional changes

As soon as a given sample was set, it was placed in a flat-bottomed polytetrafluoroethylene (PTFE) mold. The sample was anchored on a fixed pin at one end of the trough, and a second pin was carefully inserted into the other end of the sample, taking care not to penetrate the sample completely. Then the sample was covered with the appropriate liquid (30 ml).

Dimensional changes were measured with a traveling microscope (Chesterman) to ±0.001 cm, for each material in DW and the two disinfecting solutions. First a reading was obtained for the fixed pin. Then the microscope was focussed on the opposite edge of the sample, closest to the movable pin, and the second reading was taken, to obtain a baseline reading. The subsequent readings were taken every 5 min up to 1 h, on the sample edge closest to the movable pin.

Initially the data were plotted as percentage linear shrinkage against time, followed by percentage linear shrinkage against the square root of time ( t 1/2 min 1/2 ); the latter was to establish the role of diffusion in the samples.

Statistical analysis

The Mann Whitney test was used for a comparison between: Blueprint Cremix and Hydrogum at 1 h, and sodium hypochlorite/Perform ID at 10 min (and 1 h), for the two thicknesses 1.5 and 3 mm.

Results

Fig. 1 is an example of a linear shrinkage (%) versus time plot, showing typical error bars, for the 1.5 mm samples immersed in 1% NaOCl for the two alginates studied.

Fig. 1
Dimensional changes of Blurprint Cremix and Hydrogum in 1% sodium hypochlorite – 1.5 mm samples.

Figs. 2–6 plot percentage linear shrinkage (average of five samples) versus t 1/2 for the two alginates immersed in the three immersion media, respectively, each graph having plots for the two thicknesses (1.5 and 3 mm). Plotting against t 1/2 is to establish the role of diffusion in the shrinkage process. Error bars have been omitted to emphasize the general character of the dimensional changes. Note with Fig. 3 , Hydrogum in DW, the 1.5 mm samples have an initial period where there is expansion, followed by a reversal into a linear shrinkage.

Fig. 2
Effect of thickness (1.5 and 3.0 mm) on dimensional changes as a function of t 1/2 – Blueprint Cremix in distilled water.

Fig. 3
Effect of thickness (1.5 and 3.0 mm) on dimensional changes as a function of t 1/2 – Hydrogum in distilled water (th = regression line for each thickness).

Fig. 4
Effect of thickness (1.5 and 3.0 mm) on dimensional changes as a function of t 1/2 – Hydrogum in sodium hypochlorite.

Fig. 5
Effect of thickness (1.5 and 3 mm) on dimensional changes as a function of t 1/2 – Blueprint in sodium hypochlorite.

Fig. 6
Effect of thickness (1.5 and 3 mm) on dimensional changes as a function of t 1/2 – Hydrogum in Perform ID.

Tables 1 and 2 show statistical analysis data, using the Mann Whitney test, for a comparison between: Blueprint Cremix and Hydrogum at 1 h, and sodium hypochlorite/Perform ID at 10 min (and 1 h), for the two thicknesses 1.5/3 mm. Table 3 summarizes the percentage linear shrinkages at 10 min (manufacturer recommended time for immersion disinfection), and 1 h (the period of measurement to obtain the nature of the shrinkage profiles), for the two alginates (of two thicknesses) in the three media. Table 4 summarizes the slopes and intercepts of the data in Figs. 2–5 , that is the figures where there is a major linear region ( Fig. 6 , for Hydrogum in Perform ID, has markedly different profiles). The following equation applies to these linear regions:

ΔLL(%)=S(t1/2t1/20)ΔLL(%)=S(t1/2t1/20)
Δ L L ( % ) = S ( t 1 / 2 − t 0 1 / 2 )

where Δ L / L (%) = % shrinkage, S = slope of the linear region, and t 0 1/2 = intercept of line with the t 1/2 axis.

Table 1
Comparison between Blueprint Cremix and Hydrogum (at 1 h).
Medium 1.5 mm 3.0 mm
Air NS NS
Distilled water S 1 NS
Sodium hypochlorite S 2 NS
Perform ID NS NS
S 1 – Blueprint > Hydrogum.
S 2 – Blueprint > Hydrogum.
NS – no significant difference; probability of the null hypothesis >0.05.
S – significant difference; probability of the null hypothesis <0.01.

Table 2
Comparison between sodium hypochlorite and Perform ID at 10 min (and at 1 h in parentheses).
Material 1.5 mm 3 mm
Blueprint NS (NS) S 1 (S 1 )
Hydrogum S 2 (NS) S 3 (S 3 )
NS – no significant difference; probability of the null hypothesis >0.05.
S – significant difference; probability of the null hypothesis <0.01.
S 1 – Hypochlorite > Perform ID.
S 2 – Hypochlorite < Perform ID.
S 3 – Hypochlorite > Perform ID.

Table 3
Percentage linear shrinkage values at 10 min (and 1 h in brackets).
Alginate Thickness (mm) Distilled water (%) Na hypochlorite (%) Perform ID (%)
Hydrogum 1.5 −0.42 * (0.2) 0.25 (1.6) 1.72 (3)
3.0 0.20 (0.7) 0.42 (1.3) 0.06 (0.5)
Blueprint 1.5 0.66 (2.0) 0.67 (2.4) 1.34 (2.3)
3.0 0.32 (1.3) 0.42 (1.7) 0.12 (0.6)

* Signifies expansion.

Table 4
Analysis of graphical data giving the slope, slope ratio and intercept on t 1/2 axis, for the 1.5 and 3 mm alginate samples (derived from % linear shrinkage versus t 1/2 plots).
Figure Material Slope Intercept on t 1/2 axis (min −1/2 )
1.5 mm 3.0 mm Ratio, S 1.5 mm 3 mm
Fig. 2 Blueprint (DW) 0.33 0.23 1.44 0.90 1.94
Fig. 3 Hydrogum (DW) 0.14 0.10 1.40 6.40 0.86
Fig. 4 Hydrogum (NaOCl) 0.20 0.15 1.33 1.70 1.90
Fig. 5 Blueprint (NaOCl) 0.40 0.26 1.54 1.21 1.21
Fig. 6 Hydrogum (PID) 0.50 0.10 5.00 0.00 1.92

Discussion

Fig. 1 , a typical example of the percentage shrinkage versus time plot, shows evidence that Blueprint shrank more than Hydrogum over the duration of the experiment (1 h). However, the statistical analysis data in Table 1 indicates that there were no significant differences between the percentage shrinkages of Hydrogum and Blueprint, except with the 1.5 mm samples immersed in distilled water and sodium hypochlorite, at 1 h. From the practical viewpoint, there are statistical differences between thicknesses in disinfecting solutions at 10 min ( Table 2 ), and the effect of thickness is much greater in Perform ID for both materials (for example see Fig. 6 ). More important however are the differences between release profiles (vide infra).

The percentage linear shrinkage graphs against square root of time, in Figs. 2–6 , representing the three immersion media and two sample thicknesses, generally gave linear plots, but not always going through the origin. In the case of Hydrogum in DW ( Fig. 3 ), the 1.5 mm samples appeared to shrink less than the 3 mm thick samples, the converse of what was seen in all the other Figs. 1, 2, 4, 5 and 6 . This was due to the 1.5 mm samples initially expanding, and going through a maximum, before reverting to a linear contraction. Consequently, the 1.5 mm samples have a higher shrinkage slope than the 3.0 mm samples, but actually lower shrinkage values ( Table 3 ), due to the samples initially expanding; shrinkage began after t 1/2 ∼ 3 min 1/2 , at a value of ∼−0.5%. Such behavior has been noted previously for some alginates, immersed in water, by Nallamuthu . It should be noted that the nature of the plots is similar in all cases, i.e. there is a usually a transitory initial portion, followed by a continuous steady shrinkage. Clearly there are individual differences between the various systems.

The ratios of the shrinkage values of the 1.5 mm and 3 mm samples, at t = 10 min ( t 1/2 = 3.20) (calculated from the data in Table 3 ; for example 0.25/0.42 in NaOCl = 0.6), were 0.6 and 2.1 for Hydrogum, and 1.6 and 2.1 for Blueprint, in NaOCl and DW, respectively. There were however marked differences with both alginates in PID, with respect to (i) the shrinkage profiles (see for example Fig. 6 ), and (ii) the ratios of the shrinkage values of the two sample thicknesses, at t = 10 min ( t 1/2 = 3.20). With the 1.5 mm samples the plot was curved concavely to the t 1/2 axis, suggesting there might have been an initial period of expansion. Clearly, shrinkage of the 3 mm samples was very much less. Most importantly, the shrinkages were very different at 10 min ( Table 3 ), there being a factor of ∼29-fold for Hydrogum, and 11.2 for Blueprint. This would seem to be a distinct disadvantage of Perform ID, and also indicates that a constituent of Perform ID was having a powerful effect, possibly on the structure of the set alginate. The constituent could possibly be potassium peroxomonosulfate, presumably added to confer a low pH 3.5. Low pH values are known to have an anti-viral effect .

From Figs. 2–6 , the ratios of the two slopes were calculated ( Table 4 ) in order to see if simple diffusion theory applied to these alginates immersed in the three media (it is assumed that linear shrinkage is proportional to mass loss). The theory below demonstrates that the ratio of the slopes of the two different thickness plots is theoretically 2. It is based on Fickian diffusion theory ; the linear part of an uptake/desorption (shrinkage) plot is given by:

Mt=2M(DtπL2)1/2
M t = 2 M ∞ D t π L 2 1 / 2
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on The effect of disinfecting solutions on the dimensional stability of dental alginate impression materials
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