Polyvinyl siloxanes in dentistry: an overview. Sensitive Teeth Relief

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Historically, various impression materials are used to record intraoral structures for the fabrication of definitive restorations. Impression materials too have gone through a tremendous phase of development. Knowing the physical and biological properties as well as the advantages and disadvantages of different impression materials, is a prerequisite for adequate practical application of dental materials and contributes to the success of prosthetic therapy. In the 1950s the rubber base materials, first in the form of the polysulfide and later the silicone, polyether began to be used as dental impression materials [1]. Elastomers are a group of elastic impression materials. General reviews of rubber impression materials by Craig (1977), Harcourt (1978), and McCabe and Storer (1980) have been published, as have a status report on polyethers (Council on Dental Materials and Devices, 1977) and an appraisal of addition silicone impression materials (McCabe and Wilson, 1978). There are four groups of Elastomers: polysulfides, polyethers, condensation and addition silicones. Addition silicones (polyvinylsiloxanes) have a moderately low-molecular weight silicone that contains silane groups that bind together in a network of chains that give material a rubber consistency. Polyvinyl siloxane (PVS) impression materials represent the state of the art in elastomeric impression materials in prosthodontics and restorative dentistry [2-5] used for recording the impressions of dentulous and edentulous arches, duplication of casts and bite registrations. Recently, new elastomeric impression materials with very high elastic recovery and high tear strength have been introduced.

Composition and chemistry of Polyvinyl siloxane

Polyvinylsiloxane (or PVS) is often called 'viny polysiloxane' (or VPS) as well. Since it is based on silicone chemistry, it has sometimes been referred to as an addition-silicone. Amorphous polymers of silicone and oxygen atoms (Fig 1) which are randomly coiled like spaghetti, (Fig 2) translucent or transparent because of the lack of crystallinity, tend to be flexible rather than brittle. Polyvinyl siloxane materials are a modification of the original condensation silicones. Both are based on the polydimethyl siloxane polymer; however the presence of differing terminal group's accounts for their different curing reactions [6]. Elastic properties can be obtained by cross linking and addition of the long chains. These oligomers are double-bond-functional silicones which become polymerized by free radicals from chloroplatinic acid.

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The base material contains a polymethyl hydrogen siloxane copolymer, which is a moderately low molecular mass polymer with silane terminal groups [6, 7]. The accelerator material contains the vinyl-terminated polydimethyl siloxane. This is also a moderately low molecular mass polymer but has vinyl terminal groups [6, 7]. The accelerator material also contains chloroplatinic acid as a homogeneous metal complex catalyst [8-10]. The mixture is filled with silica because that is the only thing with the right degree of hydrophilicity to be blended into the material. Polyvinylsiloxanes (PVS), used as dental impression materials, were formulated with the variation of loading combination of six types of fillers including nano-sized fumed silica. The fillers were blended with three types of silicone polymers together with cross-linker and inhibitor in base paste and with plasticizer and platinum catalyst in catalyst paste. By replacing parts of crystalline quartz with other fillers, the setting time became much faster. The test group in which quarter of quartz was replaced with fumed silica showed the most ideal working and setting time for clinical use. There was a negative correlation between pH and setting time (p < 0.05). Combining the fumed silica was effective in increasing the viscosity, tensile strength and maximum% strain. Combining the diatomaceous earth reduced the setting time and maximum% strain, and dramatically increased the viscosity and tensile strength. The best modulation of physical properties of PVS material was possible by combining fillers during the formulation. The base and accelerator pastes also contain fillers. Amorphous silica or fluorocarbons are used as fillers to add bulk and improve the properties of the paste. The filler is also normally silanated to increase the bond strength between filler and polymer, which better allows it to function as across--linker [11]. Colouring agents are added to distinguish the base and catalyst pastes and to aid evaluation of mixing. More recently, intrinsic surfactants have also been added in an attempt to negate the hydrophobicity of these materials[12].Compositions are proprietary however they are usually supplied in two equal-size tubes, often in cartridges for use in an automix dispenser gun. The base-to-catalyst ratio is 1:1. Newer addition silicones have been formulated to be more hydrophilic. Shelf life is usually about 2 years but is reduced by warm conditions. #1 Hyper-V VM Backup

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Setting Reaction

Poly vinyl siloxane + silane siloxane--Pt, Salt [right arrow] silicone rubber

On mixing, an addition reaction occurs between the silane and vinyl groups (Fig 3). There is crosslinking of a vinyl terminated polydimethyl siloxane catalyzed by a platinum salt (chloroplatinic acid). Hydrogen gas is a byproduct of the polymerization reaction. Several authors have reported hydrogen gas bubble formation on the surface of gypsum dies poured immediately from polyvinyl siloxane impressions [3, 13]. Hydroxide groups in many products produce hydrogen gas, resulting in small bubbles on the model surface (Fig-4) if pouring is not delayed by 30-60 minutes. Many of these addition silicones contain catalysts like palladium that absorb this hydrogen. Manufacturers have now eliminated the possibility of this side reaction by proper purification and accurate proportioning of the materials, and by the addition of palladium to the pastes as a hydrogen absorber [9, 10]. It is no longer necessary to wait for one hour before pouring these impressions.

Retardation of the setting reaction of addition silicones can occur if they come into contact with latex gloves or other rubber products. This was first encountered when putty consistency addition silicones were mixed with gloved hands. Slowing of the setting time may even result from contact of the impression material with the patient's teeth after the operator has touched the teeth with his gloves. It is theorized that residual sulfur in the gloves (which serves as an accelerator and/or surfactant in the manufacture of rubber latex) inhibits the action of the platinum salt catalyst [14]. Some researchers believe that it is the powder on gloves that is the source of the problem, but others question this [15]. In addition, sulfur-containing retraction cord additives such as ferric sulfate (found in Astringent) and aluminum sulfate may also produce this effect. Gloves and the inhibition of polymerization occasionally an inhibition or retarding effect is seen on polyvinlsiloxanes when they are used in a clinical setting. This phenomenon can occur after direct contact between the impression material and latex gloves, or a region of the mucosa previously touched by latex gloves [15, 16, 17, 18, 19].

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It was initially suspected that the corn starch powder used as a lubricant in the gloves was interacting [17] while other authors suggested atmospheric oxygen inhibition, or interactions with haemostatic agents were the cause. Several haemostatic agents were tested before DeCamargo et al [20] concluded that they were not the inhibitors. Jones et al [21] were able to show that provisional luting agents did not interfere with the polymerization. A sulphur compound has since been identified as being responsible for the retarding effect on polymerization. Zinc diethyl dithiocarbamate is an accelerator used in the manufacture of the latex gloves. It reacts with the platinum catalyst in the polyvinyl siloxane to cause a delay or total inhibition of polymerization [15, 19, 22] reported that even in concentrations as low as 0.005 per cent, total inhibition of polymerization of polyvinyl siloxane can be observed. It is also believed that the compound can remain on a previously gloved hand, and so washing gloves or washing hands after glove use is not recommended as a means of avoiding contamination. Interestingly, not all latex gloves will cause an inhibition of set. It has been observed that synthetic latex gloves do not produce this phenomenon, while some natural latex gloves do [18, 19, 23].

[FIGURE 6 OMITTED]

Advantages and Disadvantages

Advantages: Excellent dimensional stability, good tear strength, good working and setting times, excellent wettability, automixed system(Figure-5), short setting time, adequate tear strength, extremely high accuracy, minimal distortion on removal, dimensionally stable even after 1 week, If hydrophilic, good compatibility with gypsum. There are no reports of patient sensitivity to the addition silicones.

Disadvantages: Hydrogen gas release, inhibition of setting by sulfur-containing materials, expensive, Hydrophobic & hence requires a very dry field.

Each group of material has its advantages and disadvantages [24]. Their main advantages are low polymerization shrinkage, long-lasting dimensional stability and endurance, and an absence of toxic or allergenic behaviors [25-29].Impression detail (Figure 6&7)is influenced by factors such as viscosity, wettability [30-35], handling properties [36-38], and the presence of voids [39-43]. Two principal characteristics of the impression material are accuracy and dimensional stability [44-47].

Properties

Viscosity

Polyvinyl siloxanes are available in viscosities ranging from very low (for pouring, syringing or wash use), to medium, high and very high. The viscosity of the material increases with the proportion of filler present. Viscosity is also affected by the shear force placed on the material. The mixed base and catalyst pastes exhibit a decrease in their relative viscosities in response to high shear stresses. This is termed shear thinning. Thus a medium body impression material can possess sufficient viscosity to avoid excess flow if loaded into an impression tray, yet it can also exhibit an apparent lowered viscosity suitable for intrasulcular impressions, when it is expressed through an impression syringe tip. [7, 48]. The higher the viscosity of the material, the more pronounced is the effect of shear thinning. This phenomenon is suggested to be due to the extremely small filler particle size. [48]

Working and setting times

Modern polyvinyl siloxanes have a working time of two minutes and a setting time of six minutes (with slight variation) [3, 13]. They are more sensitive to temperature than polysulfide, can be extended by cooling or adding retarder. Ratio of base: Accelerator does not change working & setting time. These times are considered to be adequate if not ideal. Occasionally, situations will present which require extended working times and some methods of altering working and setting times have been reported in the literature. Alteration of the proportion of catalyst is to be avoided as this leads to variable results and has been suggested to facilitate the side reaction which produces hydrogen gas. Some manufacturers supply a retarder that can be incorporated into the mix to provide additional working time without compromising other properties [3].

[FIGURE 7 OMITTED]

The retarder is a small, reactive, tetracyclic vinyl molecule that polymerizes preferentially to the siloxane copolymers. This small molecule is cyclic and does not form a chain. It is thus a chain stopper, and temporarily prevents polymerization of the linear siloxane molecules. The retarder continues to polymerize until it is completely consumed and then the linear siloxane molecules polymerize causing the impression material to set. The most convenient and widely advocated method for extending working time is to refrigerate the materials before mixing. Gains of up to 90 seconds have been reported when the materials are chilled to 2[degrees]C [3, 49]. It is a good idea to store the addition silicones in a refrigerator and use them immediately after removal because the cool storage conditions act to lengthen the working time by about 1.5 minutes without adversely affecting the material's accuracy.

Reproduction of detail

Polyvinyl siloxanes are currently considered to reproduce the greatest detail of all the impression materials. The international standard for dental elastomeric impression materials1 states that a type III (light body) impression material must reproduce a line 0.020 mm in width. With the exception of the very high viscosity putty materials, all polyvinyl siloxanes (light, medium and heavy body) achieve this. Very low viscosity materials can reproduce lines 1-2 um wide [7, 50, 51]. Further, PVS materials have the best fine detail reproduction and elastic recovery of all available materials.[52]. It should be noted that the literature does not tend to support the use of putty and wash impression techniques for greatest accuracy in impressions. Wassell and Ibbetson [53] reported that heavy body and wash impressions were more accurate than putty and wash impressions. Frederick and Caputo [54] further showed that the putty and wash technique was significantly less accurate than polyether (heavy and light body) or reversible hydrocolloid impressions.

Dimensional stability

The accuracy of an impression material is dependent on the dimensional stability. There are a number of possible causes for dimensional changes in elastomeric impression materials. The major factors affecting the dimensional change of the impression are thermal contraction, polymerization shrinkage, and contraction due to the loss of volatile by products [55]. Polyvinyl siloxanes show the smallest dimensional changes on setting of all the elastomeric impression materials. Long term dimensional stability of polyvinylsiloxanes is reported in the literature. This is because they are not susceptible to changes in humidity, and they do not undergo any further chemical reactions or release any by-products. [3,6,7,13,52,56,57,58]. Tjan et al [59] evaluated the accuracy of monophase polyvinyl silicones and found that repeat pour at later time periods, did not affect the dimensional accuracy and stability of impression made with these materials. Polyvinyl siloxane impressions may be repoured to produce stone dies which are as accurate as the original, as many as seven days later [58]. The linear coefficient of thermal contraction is relatively high for all elastomers. When an impression is removed from the mouth, there is an element of shrinkage due to the decrease in temperature that occurs as the material moves from the mouth to the bench. Lower viscosity materials show the greatest change (0.02-0.05 per cent shrinkage) due to their lower filler content[56].Reheating an impression to 37[degrees]C before pouring has been demonstrated to improve the accuracy of the resultant die; however it is doubtful that this is clinically significant. [49,56] The addition silicones have excellent dimensional stability with shrinkage over 24 hours of only 0.05%.

Tear energy, elastic recovery and deformation

Polyvinyl siloxanes are frequently reported to be the most ideally elastic impression materials because they exhibit better elastic recovery and less permanent deformation than the other elastomers. They can absorb over three times more energy up to the point of permanent deformation than other elastomers, and if elongated to over 100 per cent (strain at tear), they rebound to only 0.6 percent permanent deformation [60]. Permanent deformation is related to the degree of cross-linking of the polymer strands, temperature, and the rate of applied stress. The ideal impression material should exhibit maximum energy absorption with minimal distortion. However, it is also desirable that the material tears rather than deforms past a critical point such as a margin. Polyvinyl siloxanes deform at much slower rates and tear at points of less permanent deformation than do the other elastomeric materials [60]. Blomberg et al [61] reported that polyvinyl siloxanes have sufficient elastic recovery to allow an impression to be poured only six minutes after removal from the mouth.

Marcinak and Draughn[62] evaluated the dimensional change in addition silicones by delaying the pouring of impressions from 2 h to one week. They concluded that these materials remained remarkably accurate even after one week, with the greatest change at any time being 0.3%. Lacy et al [27] investigated the time dependant accuracy of elastomeric impression materials and concluded that polyvinyl siloxanes were the most stable of elastomers.

Creep compliance

All elastomers are viscoelastic materials, implying that deformation and elastic recovery are time dependent as well. Therefore, the longer the material is deformed (as occurs when impressions are removed slowly from the mouth or separated slowly from a poured model), then the longer time it takes for elastic recovery and the possibility of permanent deformation becomes higher. Polyvinyl siloxanes have the least viscoelastic qualities thus requiring the least time for recovery from viscoelastic deformation.

Radiopacity

Radiopacity of impression materials is important for radiographic identification of excess material which may be accidentally swallowed, aspirated or left in gingival tissues. Presently, only the polysulphide materials exhibit significant radiopacity due to their lead dioxide content. Shillingburg et al [63] tested an experimental polyvinyl siloxane material containing 20 per cent barium sulphate to improve radiopacity. Although they were able to demonstrate qualities of radiopacity equal to that of the polysulphides, there were associated physical property drawbacks including hydrogen bubble formation in dies poured from the impressions, and evidence of long term breakdown limiting the shelf life.

Wettability

An impression material should have intimate contact with the tooth and underlying soft tissues and should not form bubbles or voids. Wettability is best with a hydrophilic material. Material should possess ability to displace moisture. According to O'Brien, wetting describes the relative affinity of a liquid for a solid. It is the degree to which a drop will spread on a solid surface, and can be quantified by observing the contact angle. High angles (greater than ninety degrees) indicate poor wetting, whilst a zero angle would indicate perfect wetting of the surface. Early forms of the addition silicones were hydrophobic, however most now available are hydrophilic because of inclusion of surfactants which reduce their contact angles with gypsum from approximately 90 to 50 degrees. This makes their contact angles with gypsum similar to those of the hydrophilic polyethers. [64]

When discussing the wetting characteristics of impression materials, it is important to distinguish between the ability of the material to flow around the soft and hard tissues of the mouth, and the ability of the material to be wet by gypsum slurry. Polyvinyl siloxanes are inherently hydrophobic, however in recent times; new 'hydrophilic' polyvinyl siloxanes have been introduced with manufacturer claims that they better wet moist dental surfaces. These new formulations have intrinsic surfactants added. Typically these are non-ionic surfactants of phenoxypolyethanol homologues [65-69]. There is no scientific evidence to indicate that polyvinyl siloxanes advertised as 'hydrophilic' can be syringed into a wet sulcus for an accurate impression [50, 65, 66]. It has also been shown that the newer hydrophilic materials perform no better than the original formulations of polyvinyl siloxane in wettability for pouring dies, if a compatible, extrinsic, spray-on surfactant is applied before pouring [65, 66].

This has been confirmed by Takahashi and Finger [70] who demonstrated that under a simulated clinically dry field, both the hydrophilic and original f ormulations of polyvinyl siloxane wet tooth structure with equal results. The application of an extrinsic surfactant to the surface against which an impression is to be made has also been suggested. Millar and co-workers [71] reported a significant reduction in the number of voids and an overall increased quality of polyvinyl siloxane impression when a modified polydimethyl siloxane wetting agent was applied to the prepared tooth surfaces before impressions were made.

Impression trays and adhesives

VPS adhesives (blue) for polyvinyl siloxane impression materials

The improved physical properties of modern elastomers and particularly polyvinylsiloxanes the use of stock trays for impressions has become common practice for reasons of cost and convenience [59]. Common stock trays made of polystyrene or chromium plated brass are reported to be suitably stiff to prevent flexure or distortion, although there remains some possibility of tray wall flexure with polystyrene trays[38,53]. The use of adhesives in trays has been shown to achieve higher material bond strengths for polyvinyl siloxanes than has mechanical retention [72, 73]. The adhesives used are usually polydimethyl siloxane and ethylsilicate. The adhesive reacts with the surface of the tray material and forms a chemical bond to the tray and to the impression material. It is generally recommended to wait for ten to fifteen minutes after application of the adhesive before making the impression [73].This allows time for the solvent to react with the tray material.

Chai et al. [38] reported that adhesive strength to acrylic resin (custom trays) was significantly lower than polystyrene or metal stock trays for the polyvinyl siloxanes. Investigations into their bond strengths with polyvinyl siloxane adhesives indicate that they bond better than polymethyl methacrylate materials provided that the air inhibited non-polymerized layer is removed with isopropyl alcohol or a carbide bur [72,74]. Sulong and Setchell [75] demonstrated that roughening the surface of the impression tray will significantly improve the effectiveness of polyvinyln siloxane adhesives.. In many clinical situations, the impression tray must be tried in the mouth prior to impression making and this leads to saliva contamination. If the tray has already been painted with adhesive, then subsequent application is recommended to maintain bond strengths. Contaminated adhesives have shown a drop in bond strength to one-fifth of their original amount.

Disinfection

Disinfection is the inhibition or destruction of pathogens and can be achieved by immersion of an impression into antimicrobial chemical solutions for 3 to 90 minutes depending on the agent. Sterilization is the total elimination of all micro-organisms and spores and requires immersion periods of 6 to 10 hours. Herrera and Merchant [76] tested the dimensional stability of different impression materials following immersion disinfection for thirty minutes. They observed that polyvinyl siloxane and polysulphide were unaffected after immersion in sodium hypochlorite, 2 per cent glutaraldehyde, 0.5 per cent povidone-iodine and 0.16 per cent halogenated phenol whilst polyethers were significantly unstable. Even after extending the immersion times to sixty minutes, Del Pilar Rios et al [77] agreed with the findings of Herrera and Merchant. Johansen and Stackhouse [78] demonstrated that polyvinyl siloxanes were able to be immersed in 2 per cent glutaraldehyde for 16 hours without any observed dimensional changes, whilst polyether materials showed dramatic distortions under the same conditions. Holtan et al. [79] measured the dimensional stability of polyvinylsiloxanes after sterilization procedures using a conventional steam autoclave, and an ethylene oxide gas autoclave. They determined that sterilization in ethylene oxide gas resulted in gas inclusions into the impression material which then formed bubbles in dies poured immediately from them. Waiting to pour dies for 24 hours after gas autoclaving prevented this problem. Impressions sterilized in the steam autoclave did undergo distortion that would have been significant enough to prevent a casting from seating. It was concluded that steam autoclaving was a suitable sterilization method if the impressions were not to be used for fixed prostheses.

Most recently, radiofrequency glow discharging has been advocated for use as a disinfecting procedure for polyvinyl siloxane impressions [80]. Whilst this procedure is claimed to clean and improve the wettability of the impression surface, it is not clear if glow discharging results in sterilization.

In Future

Continued efforts will be made to develop more effective single-viscosity addition silicone systems, probably by controlling the filler particle size and wettability of the filler by the silicone. Hydrophilic addition silicones should be developed further in order to improve the taking of impressions and pouring of casts. Because of the convenience and reduction in the number of voids, development of automatic mixing systems, static and/or mechanical, will continue. With the development of information on the visible curing composites of the BISGMA and urethane diacrylate, the commercial availability of visible-curing rubber impression materials is almost a certainty; the essentially unlimited working time at constant viscosity would be a major advantage for this type of system. The increased emphasis on disinfection of impressions will stimulate the development of systems that will disinfect polyethers without causing problems with dimensional change.

Conclusion

Polyvinyl siloxane impression materials are routinely used to record the impressions of dentulous and edentulous mouths. The composition has been changing from time to time to improve the clinical performance. It is evident that all materials change dimensionally over time. The present review suggests that addition silicones to a certain extent were helpful in improving their clinical performance and extending the use of the material in various dental applications.

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Hemchand Surapaneni (1) *, Pallavi Samatha Y (2)., Ravi Shankar Y., Sirisha Attili

Department of Prosthodontics (1), Department of Periodontics (2), Drs. Sudha & Nageswara Rao Siddhartha Institute of Dental Sciences, Chinaoutpalli, Gannavaram Mandal, Vijayawada, Krishna District 521286 Andhra Pradesh, India Department of Prosthodontics, GITAM Dental College and Hospital, Gandhi Nagar Campus, Rushikonda, Visakhapatnam 530045, Andhra Pradesh, India

Department of Prosthodontics, St. Joseph Dental College, Eluru, West Godavari District 521286 Andhra Pradesh, India

* Corresponding author, Dr. Hemchand Surapaneni, hemchand 18@yahoo.co.in

Received 27 December 2012; Accepted 16 June 2013; Available online 21 July 2013