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Year : 2022  |  Volume : 33  |  Issue : 1  |  Page : 30-36
Current overview for chemical disinfection of dental impressions and models based on its criteria of usage: A microbiological study

1 Department of Orthodontics and Dentofacial Orthopaedics, Saveetha Dental College, Saveetha Institute of Medical and Dental Sciences, Chennai, Tamil Nadu, India
2 Department of Orthodontics, Saveetha Dental College, Saveetha Institute of Medical and Technical Sciences, SIMATS, Chennai, Tamil Nadu, India
3 Department of Microbiology, Saveetha Dental College, Saveetha Institute of Medical and Technical Sciences, Chennai, Tamil Nadu, India

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Date of Submission20-Jun-2020
Date of Decision28-Sep-2020
Date of Acceptance10-Oct-2020
Date of Web Publication09-Aug-2022


Objectives: The aims of this study were to compare the efficacy of two proven chemical disinfectants, glutaraldehyde and povidone iodine on dental impression and models by determining the reduction in the microbial load, and to compare changes in the physical properties of the models after adding the disinfectants. Materials and Methods: Irreversible hydrocolloid upper impressions of 90 patients were made and divided into 3 groups of 30 samples each; Group A––Control group; Impressions were run under clean tap water before pouring the model. Group B––2% Glutaraldehyde sprayed on the impression and left in situ for 10 min before pouring the model. Group C –10 ml of (5%) povidone iodine incorporated into the gypsum before pouring the model. Models from all three groups were subjected to microbiological assessment at three different time intervals, T0––24 h, T1––1 month and T2––3 months of storage by comparing the colony forming units (CFUs) of bacteria and fungi. The compressive strength of 5 models from each group was also analyzed in Newton's/mm2. Results: 2% Glutaraldehyde proved more effective than povidone iodine after 24 h of storage (T1), however at the end of 1 month (T1) and 3 months (T2) the Povidone group showed the maximum disinfection. Both the disinfectants caused a reduction in the compressive strength of the model with the povidone iodine group showing the maximum reduction. Conclusion: Although povidone iodine was the most effective disinfectant after 3 months, it showed a significant reduction in the compressive strength and caused discoloration of the model. 2% Glutaraldehyde proved to be the choice of disinfectant with minimal adverse effects.

Keywords: Antimicrobial activity, chemical disinfection, COVID-19, dental impressions, glutaraldehyde, povidone iodine

How to cite this article:
Chidambaram S R, George AM, Muralidharan N P, Prasanna Arvind T R, Subramanian A, Rahaman F. Current overview for chemical disinfection of dental impressions and models based on its criteria of usage: A microbiological study. Indian J Dent Res 2022;33:30-6

How to cite this URL:
Chidambaram S R, George AM, Muralidharan N P, Prasanna Arvind T R, Subramanian A, Rahaman F. Current overview for chemical disinfection of dental impressions and models based on its criteria of usage: A microbiological study. Indian J Dent Res [serial online] 2022 [cited 2023 Feb 4];33:30-6. Available from:

   Introduction Top

Disinfection of dental impression and dental models should be considered as an integral part of dental practice and is a subject of importance and concern especially with the severity of the ongoing Covid-19 pandemic.[1] The microbial flora of the oral cavity is rich and extremely diverse which reflects the abundance of local causative factors like blood, saliva, moisture, availability of surfaces on which microbial populations can develop.[2],[3] Literature states that microorganisms can be recovered from impression surfaces even after a 5-h incubation period although in reduced amounts from that recovered immediately after impression making.[4] In some countries, recommendations concerning disinfection of items sent to dental laboratories have existed for several years.[5] However, they have not been fully followed to their potential which should count as a potential source of cross-infection to the entire community.[6] Although sterilisation in an autoclave involves the complete removal of pathogenic organisms including spores, it is not the choice for dental impressions as it may compromise the dimensional accuracy and in no circumstances be a viable option.[4] The basic procedure followed by many practitioners as a mode of disinfection is the rinsing under running tap water however studies have proven that this procedure removes only 40% of the pathogenic microorganisms and poses a substantial risk for cross-infection[7]

Another important factor that must be considered in the process of disinfection is that dental impressions can cause cross contamination of dental models poured against them.[2] Dental models are made of gypsum material from the impression taken of the patients upper and lower arches. Gypsum models are hygroscopic and can absorb moisture from the atmosphere and this may favor the survival and growth of microorganisms.[8] It is therefore imperative that disinfection of all impressions as recommended by the Centre for Disease Control (CDC) is followed for all patients. (Centers for Disease Control. Recommended infection-control practices for dentistry. MMWR 1986; 35:237).

Various methods have been formulated for disinfection of impressions and resultant dental models. Use of sodium hypochlorite, glutaraldehyde, iodophor, chlorhexidine, ethylene oxide, UV rays, dry heat sterilisation and microwave has been made popular over the years in terms of disinfectants.[9] Among the above methods, the use of Glutaraldehyde and Povidone iodine have been proven to be beneficial and readily available[10] The other important criteria while selecting disinfectants are that they should not compromise the dimensional stability of the models and also the reproduction of surface details from the impression material.[11],[12]

Although many studies have proven the effectiveness of various disinfecting agents on dental impression and models, literature is scant with the effectiveness of these disinfecting agents over a longer duration of storage.[13]

Therefore this study was undertaken to compare two chemical disinfectants: -

  1. 2% Glutaraldehyde and 5% povidone iodine with a control group where the impressions were cleaned under clean running tap water and comparing the microbial load at 3 different time intervals-(T0-after 24 h, T1-after 1 month and T2-after 3 months of storage)
  2. To check if there is a reduction in the compressive strength of the models after applying chemical disinfectants.

   Materials and Methods Top

Well recorded upper alginate impressions of the patients who had reported to the department for treatment was used for the study. The study was conducted in the Department of Orthodontics in collaboration with the Department of Microbiology and approval from the Institutional Review Board (SRB/SDC/ORTHO/16/052) was sought and obtained. The sample size was calculated using the G-Power software (version 20) where a total of 15 samples per group were required for a 95% power.[14] In this study 30 samples were included in a group to have better statistical data and for the 3 groups a total of 90 models were assessed.

Alginate impressions were made with the Tropicalgin (Chromatic alginate impression material - Zhermack Italy) with ISO 1563 - ADA specification and it had a normal setting time of 2 min and 35 sec. The alginate was mixed according to manufacturer's instructions and impressions made were washed under clean tap water. All the procedures were handled by a single operator in a clean environment with sterile aseptic technique. The groups including the study models were treated by three different methods; -

Group A––Treated under running tap water before pouring the model.

Group B––2% Glutaraldehyde sprayed on the impression and left in situ in a plastic bag for 10 min before pouring the model. It is recommended that spraying of the Glutaraldehyde should be done in a ventilated area and after spraying, the impressions are placed in air-tight plastic bags to prevent toxicity.

Group C––10 ml of 5% povidone iodine mixed with the gypsum before pouring the model.

The models were poured with Type Class III Orthokal (Kalabhai Karson Pvt, Ltd).

The Orthokal was also mixed according to the manufacturer's instructions. After making the models, they were placed in disposable cardboard boxes stored in designated storage areas.

The study models were subjected for analysis on two parameters, the alteration of microbial load at TO, T1 and T2 and changes in the compressive strength.

Microbiological Assessment

The models were subjected to microbiological assessment at specific time intervals as designed in the study. To check for the microbial load, a sterile cotton swab was moistened with sterile water and used to wipe the cast at specific areas and with a standard dimension. The site chosen for this study was at three separate locations at the center of the palate at T0, T1, and T2 with a uniform dimension of 1 cm x 1 cm.

The sampling was carried out in a sterile lab environment with the Bunsen burner flame and inoculated onto the Blood agar plate [Figure 1] for evaluation of bacterial count; the blood agar plates are dried using a hot air oven after taking from the refrigerator. For evaluating the fungal count, Sabouraud dextrose agar (SDA) medium was used. [Figure 2] The procedure was followed for all the 90 sets of models, and kept in the incubator at 37 for 24 h to look for the bacterial growth [Figure 3]. For fungal growth the plates were incubated at room temperature for 48 h [Figure 4]. Assessment was done by counting the number of colony forming units (CFUs) per plate.
Figure 1: Swabs collected for checking bacterial growth in the three groups evaluated in the study

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Figure 2: Swabs collected for checking fungal growth in the three groups evaluated in the study

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Figure 3: Colony forming units in the blood agar medium for assessing bacterial load

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Figure 4: Colony forming units in the Sabouraud Dextrose Agar medium for assessing fungal load

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Assessment of compressive strength

The compressive strength of the material was assessed by preparing 2 x 2 x 1 cm of gypsum cubes. They were analysed on newly prepared samples after mixing 10 ml of glutaraldehyde and povidone iodine mixed with the gypsum during manipulation. Five samples of each of the groups were made and subjected to testing using a universal compressive testing machine (ACI). 150 X 150 mm of dimension concrete tube was tested. The total cross-section area was 22,500 sq mm. Compressive strength = Load x 1000/Sq of cross section area. Results were measured in Newton's/mm2.

Statistical analysis

The collected data were analysed with SPSS IBM statistics software (Version 20.0). For descriptive statistics, mean & S.D were used. The ANOVA test was done to determine the level of significance between bacterial and fungal CFUs within the three groups (Group A, B & C) at three different time intervals (T0, T1 &T2). The Tukey Honest Significant Difference (HSD) test was done to determine the level of significance between the groups for bacterial and fungal CFUs. In all the above statistical tests, the probability value 0.05 is considered as a significant level.

   Results Top

Results of the bacterial count [Table 1] and [Table 2] in the three different time intervals and the three study groups showed that there was a significant difference between the groups and at all 3 time intervals. Although the povidone iodine group showed the maximum disinfection after 1 month and 3 months of model storage but at the end of 24 h, the glutaraldehyde showed better disinfection with a CFU count of 0.17, whereas the povidone iodine had a higher CFU count of 0.20.
Table 1: Anova test of variance to determine bacterial CFUS at three different time intervals in the three study groups

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Table 2: Tukey HSD test to determine significant difference between the three groups for bacterial CFUS

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Results of the fungal count [Table 3] and [Table 4] at three different time intervals and the three study groups showed that there was a significant difference between the groups and at all 3 time intervals. The povidone iodine group showed the maximum disinfection after 1 month and 3 months of model storage. However, at the end of 24 h (TO), there was no difference in the disinfecting properties with both the glutaraldehyde and the povidone iodine group having the same CFU count of 0.17
Table 3: Anova test of variance to determine mean and sd of the fungal CFUS at three different time intervals in the three study groups

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Table 4: Tukey HSD test to determine significant difference when applied at the intra-group level for fungal

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Results of the compressive strength measured in Newton's/mm2 between the three groups showed that povidone iodine group showed the least compressive strength (28 N/mm2) followed by glutaraldehyde group (35.8 N/mm2) and then the control group (40.4 N/mm2) [Table 5]. There is a statistically significant difference seen in the values between all the three groups.
Table 5: Anova test of variance to determine mean and standard deviation of compressive strength between the three study groups

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   Discussion Top

Dental impressions can be a source of carrying microorganism responsible for causing bacterial, viral and fungal infections and the need to eliminate the microorganism should be considered of vital importance in dental practice.[15] Among the various methods of sterilisation and disinfection, chemical disinfection is the ideal choice for disinfecting dental impressions.[16] There are basically three methods of chemical disinfection, a) Spray disinfection, b) Immersion disinfection and c) Mixing the disinfectant in the gypsum before pouring the model.[17] Among the three methods, immersion disinfection should be considered the optimum method as all surfaces including the impression tray gets involved in the process of disinfection. However immersion disinfection is not the method of choice for hydrocolloids as they are extremely hydrophilic and can damage the integrity of the impression.[18],[19] A pre wash under clean running water to remove blood and saliva should be considered a mandatory procedure before applying the surface disinfectant.[12]

Studies have shown that although many disinfecting solutions are available in the market today, 2% glutaraldehyde and 5% povidone iodine have been proved to be most effective when tested for reduction in the microbial load at 1 and 24 h, respectively.[10] Results of this study clearly indicate that merely washing an impression under running tap water was inadequate to remove the pathogenic load and adding a disinfectant was mandatory. Results also showed that both glutaraldehyde and povidone iodine showed a statistically significant effect in reducing microbial load which was in concurrence to the study done by Ivanovski et al. who stated that both glutaraldehyde and povidone iodine were effective in eliminating microorganisms.[10] While comparing the effectiveness of two disinfectants tested after 24 h (T0), glutaraldehyde was shown to have a superior disinfecting property when compared to povidone iodine with a CFU count of 0.17 when compared to 0.20, respectively. When comparing the reduction in the fungal load after 24 h (T0), both glutaraldehyde and povidone iodine showed similar disinfecting properties with a CFU count of 0.17. However, with prolonged duration of storage of the dental model, 1 month (T1) and 3 months (T2) povidone iodine proved to have superior bactericidal and fungicidal properties compared to glutaraldehyde.

Glutaraldehyde is considered a high level disinfectant and is available in neutral, alkaline and acidic forms. Its main mechanism of action is against amines, amides and sulfhydryl groups.[20] Glutaraldehyde acts on proteins by denaturation and on nucleic acids by alkylation.[21] It can destroy all types of microorganisms such as bacteria, fungi (including spores) and viruses.[22] Spraying of 2% glutaraldehyde can be used in a wide range of impression materials such as irreversible hydrocolloids, zinc oxide eugenol paste, polysulfide, polyether and addition silicones. Another important criteria that should be taken into account with chemical disinfection is the time taken after application and the recommendation for glutaraldehyde is a 10 min wait duration to produce its effectiveness after spray application[15]

Povidone iodine comes under the classification of iodophors and also has bactericidal and virucidal properties. For fungicidal properties, it requires more contact time. Iodophors inactivate microbial proteins and enzymes and reduce the oxygen carrying capacity of aerobic microorganisms.[21] However exposure to organic material can neutralise its disinfecting property and hence more frequent applications are needed to provide effective disinfection especially as an antiseptic.[15] Hence the alternative method to mix the povidone iodine in the gypsum product before pouring the model was done in this study.


Other commonly accepted disinfectants are 1% sodium hypochlorite (NaOCl) and 2% chlorhexidine. Although sodium hypochlorite is an effective disinfectant causing bactericidal activity and low cost, the disadvantages are that 0.5% NaOCl takes 24 h to produce its complete effectiveness and shows mild dimensional changes. The limitations of 2% chlorhexidine include its poor effectiveness against viral and fungal microorganisms. Alcohols provide an intermediate level of disinfection and the commonly used alcohols are isopropyl alcohol and ethyl alcohol. They are contraindicated for impression disinfection as they can cause surface changes.[23],[24],[25]

A significant disadvantage from the results of this study was the reduction in the compressive strength of povidone iodine compared to Glutaraldehyde which was statistically significant (p < 0.05). The reduction of the compressive strength can be attributed to an increase in setting time due to the addition of povidone iodine to the gypsum product. Therefore disinfecting with povidone iodine mixed in the gypsum product would be the ideal choice in the making of study models where the reduction in compressive strength is not a major criterion. Glutaraldehyde should be the disinfectant of choice in working models which are subjected to fabrication of appliances and other adjuvant lab procedures.

Another criteria which is of relevant importance is the cross infection that can occur during transporting impressions and models back and forth from the dental laboratory. It is recommended that the disinfection status of the dental impression from the dental clinic should be conveyed to the lab as repetitive disinfections could cause changes in dimensional stability and surface details.[26] Spread of the microorganisms takes place mainly during trimming of the model.[2] Three minutes of microwave disinfection at 650W works as an effective and reliable method to reduce the microbial load of dental models.[27] In view of the rapid spread of SARS virus through fomites, it is recommended that all impressions should be sent to the laboratory in sealed dental pouches and communication to the laboratory sent through digital format.

The limitations of this study was that the disinfection properties were only analyzed for a period of 3 months and further studies need to be done for longer duration as in the case of orthodontic study models.

   Conclusion Top

Chemical disinfection should be considered a standard protocol and mandatory procedure for all dental impressions and casts and should be selected based on its usage criteria

  1. For working models where dimensional stability is the major criteria, 2% glutaraldehyde should be considered as the gold standard with an immersion duration for 10 min (additional silicone impressions). Alternatively immersing in an ultrasonic bath would be more effective in removing physical debris such as blood and saliva. For irreversible hydrocolloids, (Alginate) immersion methods should be avoided, as they are hydrophilic in nature and absorption would cause distortion of the dimensional stability, so spraying with 2% Glutaraldehyde and left in situ for 10 min is recommended choice.
  2. For study models where the duration of the antimicrobial effect is the major criteria, 5% povidone iodine mixed with gypsum before pouring the model should be the material of choice.
  3. Disinfection status of the dental impression from the dental clinic should be declared to the dental laboratory as repetitive disinfections could cause changes in dimensional stability and surface details.

It is of paramount importance for health care authorities to make policies of appropriate methods of cleaning and disinfecting dental impressions and models especially with the menacing effects of the ongoing COVID-19 pandemic.

Key Message

Disinfection of dental Impressions and models in the dental clinic should be a mandatory procedure to prevent cross infection, especially in the wake of the current COVID-19 pandemic. However, standardisation of the appropriate disinfectant is difficult because of the variations in the physical properties of different impression materials commonly used in clinical practice. Both povidone iodine and glutraldehyde proved to be effective disinfectant, however the choice between them depends upon the criteria of usage of the models prepared (Working model or study models)

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Conflicts of interest

There are no conflicts of interest.

   References Top

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Correspondence Address:
Dr. Ashwin Mathew George
Department of Orthodontics, Saveetha Dental College, Saveetha Institute of Medical and Technical Sciences, SIMATS, No. 162, Poonamallee High Road, Velappanchavadi, Chennai, Tamil Nadu
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijdr.IJDR_623_20

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]


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