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Impression material selection depends on the procedure at hand and the clinician’s preference. Alginate, an aqueous hydrocolloid, was developed during the 1940s. Recently, ‘alginate substitutes’ have been introduced as an alternative to alginate materials.
Procedures and Impression Materials
Alginate impression materials are used to take impressions that will be used for pouring study models, orthodontic models, opposing arch models, models used to fabricate orthodontic and other appliances as well as for the fabrication of custom impression trays and bleaching trays, and are used during provisionalization. Alginate substitutes can be used for all of these purposes.
Materials and Properties
Currently available alginate substitutes are vinyl polysiloxane materials (VPS), also known as poly vinylsiloxane or addition silicone materials. These elastomeric materials are available from several manufacturers, relatively inexpensive and therefore can still be cost-effective as a substitute for alginates. Alginate substitutes were introduced to take advantage of the physical properties of VPS and to avoid some potential drawbacks of alginates. Advantages of alginate substitutes include their tear strength, dimensional stability, accuracy and ease of use.
Tear Strength, Elasticity and Strain-in-compression
The tear strength of alginate substitutes is higher than for alginates, which may be of importance in areas of undercuts as the set impression is removed, although lower than that of a traditional VPS impression material. In addition, the strain-in-compression of alginate substitutes allows for comfortable removal of the set impression, giving it a softer feel during removal similar to that of an alginate impression. The elasticity of alginate substitutes is also such that the set material recovers its shape following any distortion of the set impression during its removal.
Alginate substitutes, together with other VPS materials, are the most dimensionally stable impression materials. Alginates shrink as they dry out, and absorb/imbibe water and other liquids when exposed to these, while alginate substitutes do not and are less sensitive to temperature and humidity. Alginates also ‘dry out from within’ even if they are kept moist to prevent surface drying – water exudes from the internal areas of the impression to the surface, again resulting in shrinkage.
The dimensional stability of alginate substitutes allows for impressions to be stored for extended periods of time and poured later. Alginate substitutes can be poured up to 14 days after an impression is taken, allowing transport to the laboratory and pouring of the model at leisure. It has been recommended that alginate substitutes be poured at least 60 minutes after taking the impression, to allow for outgassing of the material first that might otherwise result in small surface porosities and imperfections. In contrast, with regular alginates, model pouring should occur within 30 minutes of the impression being taken (note however that extended-storage alginates that allow pouring 48 hours after impression-taking and storage for 100 hours).
Alginate substitutes allow for multiple pours, while the majority of alginate impression materials are suitable for only one pour or at most two pours. The ability to perform multiple pours is desirable for record models in addition to the models being used as well as in situations where the first pour resulted in errors.
A further aspect with model pouring is that, in contrast to alginate substitutes, if alginates are left attached to the set model for extended periods of time (instead of being removed shortly after the model has set), this can result in fracture of the model when the alginate is removed as it would have continued to shrink while attached to the model.
Accuracy and Detail Reproduction
Alginate substitutes are more accurate than alginates (although the level of accuracy demanded of an impression for indirect restorations is not required for the clinical indications for which alginates are used). They are also more suitable for impressions for removable prosthetics due to their level of accuracy.
Detail reproduction and smoother model surfaces are achieved using alginate substitutes. Their use has been shown to increase the accuracy of models of opposing dentitions used during the traditional laboratory fabrication of indirect restorations, resulting in superior results and potentially less chairside time for adjustments at the seating appointment. A smoother surface can also be expected to result in a smoother provisional restoration fabricated using an alginate substitute matrix, and the extended shelf life of the set impression means that the same impression can be used to fabricate a second provisional restoration should this become necessary.
Ease of Use
Substitute alginates are available with single-use mixing tips that mix the impression material in the required ratio as the two pastes are extruded from the double-barrel tube of impression material into the tip. This auto-mixing during extrusion increases accuracy by avoiding incorrect ratios of materials (paste/paste or, in the case of alginates, powder/water), avoids mess and clean-up, and makes substitute alginates easy to use. Some alginate substitutes are also available in packages designed for use with an automated mixing machine.
Disinfecting Alginate Substitutes
Alginate substitutes are VPS materials and must be disinfected accordingly. Prior to disinfecting, blood and bioburden should be cleaned off the impression as soon as the impression has been taken to avoid these from drying on the impression and then being difficult to remove. These materials should not be immersed in disinfectants (or cold sterilant) for extended periods of time as this will affect their dimensions and accuracy of the set material.
Only EPA-registered hospital-level disinfectants may be used and the disinfectant must have a tuberculocidal claim (a TB kill time is provided on the labelling). For VPS materials, a hospital level disinfectant such as 7.5% hydrogen peroxide is recommended and water-based disinfectants have been recommended as being preferable. Either immersion in the disinfectant – provided the recommended disinfection time for the material meets the immersion time required for disinfection. Alternatively, a spray technique may be used. After disinfection, the impression should be rinsed under running water to avoid residual disinfectant being incorporated in the mixed stone or plaster during model pouring and resulting in inaccuracies. The alginate substitute manufacturer’s recommendations for disinfection must be followed.
Alginate substitutes offer several advantages compared to alginates, and have been described as ‘superior replacements’ for these. They are dimensionally stable, offer the ability to perform multiple pours, are more accurate and result in improved detail reproduction. Although more expensive than alginate materials they are still relatively low cost and cost-effective, easy to use and reliable.
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3. Centers for Disease Control and Prevention. Guidelines for Infection Control in the Dental Healthcare Setting, 2003.
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5. Estafanous EW, Palenik CJ, Platt JA. Disinfection of bacterially contaminated hydrophilic PVS impression materials. J Prosthodont. 2012 Jan;21(1):16-21.
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Dr. Fiona M. Collins
Dr. Fiona M. Collins has authored and presented CE courses to dental professionals and students in the US and internationally, and is a consultant in the dental industry. She is a member of the American Dental Association and the Organization for Asepsis, Safety and Prevention. Fiona earned her dental degree from Glasgow University and holds an MBA and MA from Boston University. Dr. Collins can be reached at fionacollins [at] comcast [dot] net.