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REVIEW ARTICLE |
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Year : 2018 | Volume
: 6
| Issue : 1 | Page : 19-23 |
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Clinical implications of elastomeric impression materials used for complete denture construction
Muhammad Rameez, Shoaib Razi, Fatima Farhan, Bharat Kumar, Haroon Rashid
Division of Prosthodontics, Ziauddin College of Dentistry, Karachi, Pakistan
Date of Web Publication | 21-Mar-2018 |
Correspondence Address: Dr. Haroon Rashid Division of Prosthodontics, Ziauddin College of Dentistry, Karachi Pakistan
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/dmr.dmr_10_17
The purpose of this article is to review and understand currently used elastomeric impression materials in the fabrication of complete dentures. The article focuses on polyether (PE), polysulphide, vinylpolysiloxanes (VPS) (addition silicones) and condensation silicone (CS) by comparing their clinical implications and properties. The choice amongst different elastomeric impression materials depend on the combination of their characteristics, clinician's comprehension of the notions and principles in impression making.
Keywords: Complete denture impressions, elastomeric impressions, impression manipulation, impression principles
How to cite this article: Rameez M, Razi S, Farhan F, Kumar B, Rashid H. Clinical implications of elastomeric impression materials used for complete denture construction. Dent Med Res 2018;6:19-23 |
How to cite this URL: Rameez M, Razi S, Farhan F, Kumar B, Rashid H. Clinical implications of elastomeric impression materials used for complete denture construction. Dent Med Res [serial online] 2018 [cited 2023 Apr 1];6:19-23. Available from: https://www.dmrjournal.org/text.asp?2018/6/1/19/228115 |
Introduction | |  |
Edentulism is defined as being without natural teeth and has become one of the important health issues because of its high prevalence.[1],[2] Over the past few decades, global burden of edentulism has been increased in low- and middle-income countries, possibly because of sociodemographic factors (low literacy rates), poor lifestyle habits (tobacco chewing and smoking), inferior quality of health-care services, and prevalence of certain medical conditions (arthritis, asthma, and diabetes).[3]
For edentulous individuals, complete dentures are one of the common treatment modalities for the replacement of missing teeth. In order for the treatment to be successful, it must be ensured that each clinical step of denture fabrication procedure is done by a trained clinician. Impression making for complete dentures is considered central to the function and clinical success of prosthesis.[4] Favorable impression technique along with suitable impression material influences the accuracy of oral tissues recorded. Various techniques have been considered for making complete denture impressions which include mucostatic,[5] mucocompressive, selective pressure,[6] functional,[7] and neutral zone impression.[8]
Selective impression technique involves exertion of moderate pressure on specific areas of an arch and minimal pressure on selected.[9] The static impression technique compresses the tissues minimally, while the functional impression technique is a selective pressure method capturing all muscular attachments in their most active position.[10] In the mucodisplacive approach, pressure is applied over mucosa during the impression-taking so that the shape of the tissues under loading is recorded.[11] The neutral-zone technique is based on the concept that for every individual, there exists within the denture space a specific area where the function of the musculature will not unseat the denture and where forces generated by the tongue are neutralized by the forces generated by the lips and cheeks.[8],[12] Until today, selective pressure technique is the predominantly used impression philosophy for complete denture impressions.[9],[13]
There is no single impression material that is ideal for all applications. Combination of materials may be used to improve the success of complete denture therapy.[14],[15],[16] Impression materials are usually classified as being flexible or rigid at the time of removal from the mouth.[17] [Table 1] shows the qualitative rating of mechanical and physical properties of elastomeric impression materials. | Table 1: Qualitative rating of mechanical and physical properties of elastomeric impression materials
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Traditionally, initial impressions for complete dentures have been recorded using impression compound or alginate with stock trays. The design of the custom-made tray for final impression of the edentulous arch will depend on the impression material used for taking secondary impression. Traditionally, zinc-oxide eugenol was used for final impressions;[15] however, elastomeric impression materials are now becoming the materials of choice for taking final impressions of edentulous arches.[18] The aim of the current paper is to review and understand the currently used elastomeric impression materials used for the construction of complete dentures.
Manipulation of Elastomeric Impression Materials | |  |
Elastomeric impression materials are available in two-paste system, as base paste and reactor liquid, two putty system and/or base putty and a reactor liquid.[19] [Table 1] shows the properties of different elastomeric impression materials. Manipulation of elastomeric materials can be done by three methods; hand mixing, static mixing, and dynamic mechanical mixing.[20]
Hand mixing
For two-paste system, equal quantities of base and catalyst paste should be taken on a glass slab or mixing pad. Stainless steel cement spatula is recommended for homogeneous mixing and it must be ensured that the final mix is free of any streaks of the catalyst or base paste and is uniform in color. In case of liquid components, base paste is dispensed on the manufacturers' supplied mixing pad and liquid catalyst is added with a dropper.
Condensation and addition silicone are also available in two-putty system which involves equal number of scoops of catalyst and base material. It is mixed by kneading the material with hand to get a uniform mix.
Static mixing
Static mixing of elastomers can be carried out with an auto-mixing gun device with two separate equally sized cylinders, one for base and another for catalyst, as well as a mixing tip. Manipulation using this method is termed as “flow division”[20] which involves two substreams of impression material, with no intermixing of two streams. Mixing tips commonly used have 11 or 12 elements, and are available in different diameters and lengths. The size of the mixing tips is also available in different sizes.
Dynamic mechanical mixing
The device used in dynamic mixing has a motor, two parallel plungers, and a mixing tip. Mixing is done by motor-driven impeller inside the tip. The impeller only mixes the material and does not propel it. Mechanical mixing using this device allows thorough mixing with little effort. Polyether and addition silicone materials are available with this dispensing system. Addition silicones and polyether materials are available in auto-mixed cartridges and static mixing tips or sausages and dynamic mixing tips with a dynamic mechanical mixer.[20],[21],[22]
The Use of Custom-Made Trays With Elastomers | |  |
Custom-made trays provide dimensional accuracy and stability as uniform thickness of material throughout the tray can be achieved.[23],[24],[25] The rigidity of custom-made tray reduces the potential for distortion and [26],[27] and thereby, accuracy of the master impression can be ensured. Heat-, cold-, and light-cured acrylic resins satisfy the requirements of custom-made trays and are considered to be the materials of choice for fabrication.[11],[28] Visible light-cured resins exhibit better dimensional stability immediately after curing, thus allowing instant clinical use after fabrication.[25],[29] If a cold-cured polymerization acrylic is to be used for making the custom-made tray, it should be fabricated 24 h prior to the impression procedure.[25],[30]
Trays are either spaced or close-fitting trays.[11] Spaced trays are fabricated using wax as a spacer, whereas close-fitting trays have no spacer or relief integrated within the trays.[31] Medium-body vinyl polysiloxane and irreversible hydrocolloids produce the highest pressures during impression taking and thus, spaced trays are recommended when using them.[11],[31] Light-bodied elastomers produce the lowest pressures and for them, close-fitting trays are usually suitable for impression making [11],[31]
Dimensional Accuracy | |  |
Dimensional stability and accuracy in terms of impression making have been a thesis of interest and have been researched since the invention of impressions.[32],[33],[34] Dimensional accuracy of impression depends on many factors, i.e., imbibition, syneresis, elastic recovery, storage, and compatibility with gypsum products. As per the guidelines of American Dental Association (ADA), impression made using elastomers to create precision castings should be able to produce fine details of 25 μm or less. It has been reported that clinicians, after making impressions with elastomers, defer the pouring of the impression for upto 72 h.[35] This delayed storage of the impression without pouring has substantial impact on the resulting accuracy of the cast and it is highly recommended that practitioner should know the tolerable time limit of impression storage.[36]
Previously, vinylpolysiloxanes (addition-cured silicones) are used to release hydrogen gas as a by-product which resulted in voids of the resulting casts.[37] The problem was resolved by incorporating palladium and platinum that worked as a scavenger to hydrogen gas.[38],[39] Polysulfides and condensation-cured silicone impression materials produce volatile by-products that result in shrinkage of impression. Vinylpolysiloxane cures through an addition reaction and shrinkage does occur, but less as compared to polysulfide and condensation-cured silicones. Polyether impression materials also show expansion following storage for longer periods.[40]
Impact of Impression Disinfection | |  |
Disinfection is a routine dental measure which should be taken to prevent contamination and control crossinfection. Dental procedures such as impression making of oral tissues expose the patient, operator, and the staff to a variety of infections if left unattended. The routine disinfectants used at dental practices are 0.5% sodium hypochlorite and 2% glutaraldehyde [41],[42],[43] and both have been shown to be effective in the prevention of crossinfection.[44] A study has shown that short-term exposure of elastomeric impressions with the disinfectants did not produce any distortions.[45] Some laboratories also disinfect the impressions which have been received from the dental practice; whether they have been previously disinfected or not, to rule out any risk of contamination and exposure to pathogens.
When elastomeric impressions are immersed in solutions containing quaternary ammonium compounds and glutaraldehyde plus an amino derivative more than once, it was noticed that the first immersion into the disinfectant did cause some expansion of impression materials, but the dimensional changes were minute and were still acceptable to the parameters set by the ADA specification.[23] However, the second immersion may cause changes which could lead to further dimensional changes.[46] Tensile strength, surface roughness, and wettability property of elastomers were assessed and results confirmed that the chemical sterilization had no effect on tensile strength, but microwave sterilization significantly altered the tensile strength negatively.[47] Vinylpolyether silicone impression materials are dimensionally stable for clinical use after disinfection for 30 min in glutaraldehyde and storage for up to 2 weeks.[23] A study conducted to assess radiofrequency glow discharge device for disinfection of the saliva-contaminated elastomeric impression material has also been found to be effective, handy, and rapid.[47]
Hydrophilicity | |  |
There are considerable differences in the hydrophilic behavior of the commonly used elastomeric impression materials that are used for final impressions. Vinylpolysiloxane impression material (VPS) is predominantly hydrophobic which can result in poor surface quality and voids in the impression.[48],[49] Its hydrophobicity can be explained by the material's chemical structure, which contains hydrophobic aliphatic hydrocarbon groups surrounding the siloxane bond.[50] However, their hydrophilic nature can be improved upon addition of intrinsic and extrinsic surfactants,[51] which significantly reduces the number of voids in stone.
Intrinsic surfactant is a component of preformulated hydrophilized VPS and is released from the surface upon contact with the wet surface. In contrast to intrinsic surfactant, the extrinsic surfactants such as polyether carbosilane does not increase the wettability of the impression materials but decrease the surface tension of the moist surface in contact.[52],[53],[54] Surfactants reduced the surface tension of the liquid in contact but do not change the surface properties of the VPS itself.[52] The oral mucosal tissues are in moist condition due to the action of major and minor salivary glands. It is recommended that moisture control is achieved prior to an impression with VPS so that desired surface details can be obtained.[55]
In contrast to hydrophilic VPS, polyethers are more likely to produce impressions with superior detail reproduction even in the presence of moisture.[56] The hydrophilic structures present in polyether impression material are represented by carbonyl (C = O) and ether (C-O-C) groups, while polysulfide impression material contains hydrophilic disulfide (-S-S-) and mercaptan groups (-S-H).[52] Research has shown that polyether impression material is considered to be the most hydrophilic among all the elastomeric impression materials, followed by polysulfides, and then silicones.[45],[57],[58]
Viscosity | |  |
Viscosity of an impression material is critical in recording the tissues in their static or functional form and thus, greatly influences the detailed reproduction of the alveolar ridges and adjacent structures in the dental cast. In 2006, Al-Ahmad et al.[59] concluded that an irreversible hydrocolloids and medium-bodied VPS produced the highest pressures over the tissues which were significantly higher than the pressures produced by both light-bodied polysulfides and light-bodied VPS.
Light-bodied VPS exerts lesser pressure over the tissues than zinc oxide eugenol impression paste and are recommended when impression of the denture-bearing tissues is to be recorded in resting state.[59] It has been shown in surveys that those dentures which were fabricated using a combination of heavy-, medium-, and light-bodied silicone materials were proffered by the patients.[60],[61] VPS is the most commonly used impression material for recording the final impressions of edentulous jaws in the United States.[13],[62]
Type IV Gypsum Compatibility and Wettability | |  |
Many studies can be found in literature which have focused on compatibility of impression materials with the gypsum products. Studies have concluded that some VPS impression materials were not compatible with type IV gypsum even though the details recorded using those materials were accurate.[63],[64] An in vitro study was carried out to check compatibility of gypsum with VPS (Aquasil, Dentsply, USA) and polyether (Impregum, 3M, USA) as per ADA standards, under dry and wet/moist conditions. There was no statistically significant difference in surface detail reproduction between the two materials in dry conditions, whereas under moist conditions, VPS performed better than polyethers, probably because of improved hydrophilicity of addition-cured silicones. However, dimensional stability of polyether was found to be better than that of VPS.[65],[66],[67]
Polyethers have the ability to transfer record accurately under the influence of saliva and blood; however, the best results can be achieved when conditions are dry. Reproduction of details on the cast can be obtained even if the impression is poured multiple times for a period of 1–2 weeks, but polyethers become rigid with passage of time which might lead to distortion of the cast while removing.[68],[69],[70]
Conclusion | |  |
Success of complete dentures relies on the accuracy of impression. A wide range of materials and techniques are available for the construction of complete dentures and the choice depends on the clinician's understanding and experience. Selective pressure impression technique is the most commonly taught and applied secondary impression technique. Silicone impression material should be considered as the material of choice for final impressions for complete dentures since unadjusted dentures made from silicone impressions were more comfortable, stable, and efficient for chewing. The combination of excellent physical properties, handling characteristics, and unlimited dimensional stability assures the popularity of these impression materials.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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