Impression materials in fixed prothodontics

 

Many impression materials are available for use in dentistry .Impression materials are used for recording the form and dimensions of the oral tissues for the fabrication of definitive restorations. The recording of tissues in oral cavity is difficult because of many factors as salivation, blood flow and sulcular fluid .The impression material is introduced into the mouth in a plastic condition to set against the oral tissues , then removed from the mouth .The set impression is a negative replica of the oral tissues .A positive reproduction(cast) is obtained by pouring a suitable die material into it . Making a cast in gypsum materials from an impression helps dentists design and construct removable and fixed prostheses. The accuracy of these final restorations depends mostly on the impression materials and techniques used . therefore , accurate impressions are necessary for development of any dental prosthesis.

Brif history 

In the 1940s and 1950s, hydrocolloids had been the favored impression materials. Since the introduction of hydrocolloids in the mid‐1930s, the impression of undercuts grew to be possible. In the 1950s, polysulfides and condensation silicones (C‐type silicones) had been used reliably in fixed prosthodontics. The most disadvantage of all these dental materials was the shrinkage over a period of hours(1) . With hydrocolloids and polysulfides , shrinkage used to be due to the evaporation of water, while in condensation‐cured elastomers, it was the evaporation of low‐ molecular weight products(2,3). In the late 1960s, polyethers was submit to the market. Its high mechanical properties, good elastic recovery, and small shrinkage made it most advantageous to hydrocolloids and C‐type materials. Ten years later, the hydrophobic addition silicones  [poly(vinyl siloxane)] were introduced. The level of hydrophobicity was decreased by addition of surfactants. PVS has high dimensional stability over time and temperature, even in a moist environment. It is recognized for its high elastic recovery. The last 20 years have brought considerable improvement in polyether and poly vinylsiloxane categories, and now they seem to be the most acceptable categories for most prosthodontic uses(1).

Clinical situations for using impression materials 

Impression materials are used in fixed prosthodontics to make impressions of teeth that prepared to receive indirect extracoronal restorations such as crowns and bridges(FPD), removable prosthodontics (as making dentures) , dental implants, orthodontics and maxillofacial prosthetics (prosthetic correction of intra-oral and extra-oral defects due to trauma, congenital defects, and surgical resection of tumors) .(4)

Classification of impression materials

Impression materials are classified according to their behavior after setting into elastic and non elastic. non elastic impression materials not indicated to be used in fixed prosthodontics because they dont record undercats accurately , so we use elastic impression materials which include hydrocolloids and elastomers. (5)
Hydrocolloids

Hydrocolloids comprise agar and alginate. agar is reversible hydrocolloid since it go between viscous gel and low viscous sol repeately by warming and cooling but, alginate once  transformed into gel can not be turned into sol, supposed irreversible hydrocolloid impression material(5).Agar and alginate might be utilized separately or in mixing to record crown impressions. Agar was first brought into dentistry for recording crown impressions in 1937 via Sears(6)and was the principal elastic impression material accessible. It isn't generally utilized in dental practice today , due to the requirement for costly molding baths and water cooled trays. Alginate unlike agar, doesn't require any unique condition . It is much easier to use and inexpensive, but it used for taking primary impression only for fabricating primary or study casts .Alginate and agar produce impressions with perfect surface detail. They are both moderately hydrophilic and are not dislodged from wet surfaces as effectively as the elastomers(7) .However, these materials have two significant defects .one of them , extremely poor dimensional stability due to synersis or imbibition of water on remaining in dry or wet situations. Furthermore, low tear strength which can be a disadvantage when trying to record the gingival sulcus. Some work posted the utilization of mixing reversible and irreversible hydrocolloid impression materials (8,9

Elastomers 

Elastomers comprise polysulfides, silicones(addition and condensation) and plyethers . 

Polysulfides 

The polysulfide impression materials have the longest history of utilization in dentistry of all the elastomers. they were first evolved as a sealant for holes between sectional concrete structures.(1 0)They are accessible in different viscosities namely , light bodied (low viscosity), medium or regular bodied and heavy bodied (high viscosity).These are presently unpopular materials. Polysulfide impression materials are usually low to moderately hydrophilic and make an accurate impression in the presence of some saliva or even blood. The setting reaction of polysulfides is relatively long with setting times often more than 10 minutes (acceleration can be made by adding a small drop of water to the mixture ).

Polyethers

Polyether impression material was introduced in the late 1960s for use in dentistry . Initially ,available only in a single "regular"viscosity, slight modification of the viscosity is possible with the use of a diluent. More recently a heavy light bodied system has been introduced. Polyether impression materials tend to have a fast setting time of less than 5 minutes and for this reason have been popular for the recording of single preparations in general practice . (5) Like addition cured silicones , they undergo an addition polymerization reaction on setting which has no by – product resulting in a material with high dimensional stability.

Properties and handling characteristics 

The ideal impression material must exhibit certain characteristics in clinical and laboratory environments .Clinically, it should produce a dimensionally stable, accurate impression with optimal mechanical properties (optimal yield strength, Young's modulus and coefficient of thermal expansion ) for adequate elastic recovery and to resist tearing. Also, it should set within a reasonable amount of time and demonstrate biocompatibility: minimal amount of toxicity and hypoallergenic nature . It ought to be hydrophilic for making a good impression. In laboratory environment ,it should be dimensionally stable for accurate pouring of multiple casts and shouldn't be affected by disinfection. Finally, reasonable cost is recommended. (1 ,16) This ideal type of impression material is difficult to obtain in reality. A detailed description of each property will help in understanding how they interact. Lately, dentists have moved in the direction of utilizing polyvinyl siloxanes and polyethers due to their improved physical and  mechanical properties (17-19). These properties include improved dimensional stability, accuracy, wettability, excellent elastic recovery , ease of handling, flexibility , tear resistance, ability to pour impression several times and producing casts from one impression and better ability to reproduce detail.(15)


Dimensional accuracy

With elastomeric impression materials, for example, polyvinyl siloxane ,polyether, and polysulfide, the dimensional accuracy is generally a time dependent process , with greater dimensional accuracy occurring immediately after polymerization is complete but declining as the impression is stored for long time . ( 18-20) Polyvinyl siloxane and polyether impression materials remain dimensionally accurate for 1 to about fourteen days. (1 8,1 9,21 ). Polysulfide impression material is dimensionally accutate when poured within 1 to 2 hours of taking the impression. (1 8,19) Practitioners must take this property into consideration when choosing impression materials given the time available to the practitioner to pour casts during office hours.

Hydrophilic versus hydrophobic nature of impression materials 

Hydrophilicity is one of the main features of a modern impression material , as the impression material is in close contact with wet hard and soft tissues. The hydrophilic nature of an impression material relates to its ability to work, flow in a wet environment .(22 )Hydrophilic materials exhibit a low contact angle with water while hydrophobic materials show a contact angle of 90° or greater . Hydrophilic materials contain the functional groups [carbonyl (c=o) and ether (c–o–c)] that attract and interact with water molecules.(23)

Dimensional stability

The dimensional stability of an impression material reflects its capacity to keep the accurate details of the impression over time.(25) This gives the option to pour it at the convenience of the operator. Therefore materials ought to have low shrinkage upon polymerizing and stay stable. High impression dimensional stability materials typically can be poured within 1 to about fourteen days after the impression is taken and still produce an exact cast .(25,21 )Materials which have high dimensional stability are the polyethers and polyvinyl siloxanes, unlike alginate which has a low dimensional stability.The polysulfides suffer from distortion over time . (

 

Wettability


Wettability of an impression material is related to the ability of the material to flow into small areas and reproduce fine details.(25 ) Impressions which wet the teeth well displace moisture and results in fewer voids. Materials that have high wetting angle don't flow easily into small areas and they are poorly used in fixed prosthodontics,while materials that have low wetting angles flow extensively providing more accurate impressions . (25,30) Finally, impression materials need to readily flow into fine details in the range of 20 to 70 μm, which is necessary for perfectly adapted bridges and crowns.

 

Elastic recovery

Elastic recovery of an impression is refered to the ability of the material to return to its original dimensions without permanent deformation upon removal from the mouth.(25) There is no impression material has 100% elastic recovery and for all impression materials the greater the depth of undercut, the greater its permanent deformation.(21 ) PVS showed the best elastic behavior, with over 99% elastic recovery, followed by polyethers and polysulfides.(25)

Flexibility

Impression materials should be flexible to be easily removed from the mouth when set . It is important to have an impression material flexible enough to overcome the undercuts in the adjacent teeth.

 

Possible errors in impression taking

A high quality, accurate dental impression is important , as without it the dental lab can't fabricate precise, well-fitting crown .Potential impression errors ought to be distinguished before the impression is sent to the lab.The most common dental impression eorrors include Poor tray selection , inadequate impression material mixing , surface contamination , poor margin detail ,internal bubbles, marginal tears,choosing the wrong impression material and failing to keep the patient still.(33)

Poor tray selection

The selection of tray is important to capture the needed area without distortion and provide the needed details. Trays ought to be sufficiently large to cover all the teeth without contacting the soft tissues. (figure1) Trays are accessible in many sizes small, mdium and large. Also the shape of tray varies some are square while others are round .

Inadequate impression mixing 

Once the impression material is mixed , it should be uniform in color without streaking. Streaking is more common with hand mixed materials than cartridge materials. (figure 2) In case of hand mixing , the material ought to be kneaded quickly to keep within the working time and yield a uniform color when completed. Streaking may also happen if the automix cartridge isnot bled prior to attaching the mixing syringe, allowing one component to extrude out of the cartridge first .Therefore the cartridge must be bleed right before a new automix tip is placed in order to ensure that both base and catalyst are equally flowing to avoid mixing problems. (33 ,36)

Surface contamination 

Surface contamination of an impression cause an unset ,tacky layer of impression material.(figure 3) It might be due to core build up material, adhesives or composites leaving a greasy covering on the tooth preparation. Any of these can prevent the material from setting effectively. Retraction cords and solutions containing aluminum chloride or ferric sulfate can transfer sulfur to critical areas preventing the setting reaction .Also utlizing the rubber dam can have the same effect.(37,38) Impression can become contaminated with latex when mixing by hand.

Poor marginal details

The margine is one of the most critical parts of a dental impression.Without a precise marginal impression , for example, overhanging , open margins or an insufficiently fitting crown are almost occur . Commonly voids at the margine due to inadequate retraction or accumulation of fluids prevent flow of impression around the margins.(figure4) The usage of retraction cords is the most perfect way to solve this problem.one procedure is to utilize a double retraction cord.(39-41 )

Internal bubbles 

Internal bubbles occur as a result of either air entrapment or fluid accumulation.(figure5) Fluid in the sulcus may interfere with capture of the restoration margins in the impression. Bubbles present on the margins ,may negatively affect the fit of the prosthesis.bubbles occuring due to fluid accumulation may be large enough to affect the long term success of the luting agent, that will fill now a wider space. The thicker the luting agent , the weaker the interface between the restoration and underlying tooth. .Achieving proper hemostasis will solve this problem.

References 

 1- Hamalian TA, Nasr E, Chidiac JJ. Impression materials in fixed prosthodontics: influence of choice on clinical procedure. Journal of Prosthodontics: Implant, Esthetic and Reconstructive Dentistry. 2011 Feb;20(2):153-60. 

2-Clancy JM, Scandrett FR, Ettinger RL. Long‐term dimensional stability of three current elastomers. Journal of oral rehabilitation. 1983 Jul;10(4):325-33. 

3- Lin CC, Ziebert GJ, Donegan SJ, Dhuru VB. Accuracy of impression materials for completearch fixed partial dentures. Journal of Prosthetic Dentistry. 1988 Mar 1;59(3):288-91. 

4- Bonsor SJ, Pearson G. A clinical guide to applied dental materials. Elsevier Health Sciences; 2012 Dec 5. 

5- Wassell RW, Barker D, Walls AW. Crowns and other extra-coronal restorations: impression materials and technique. British dental journal. 2002 Jun;192(12):679-90. 

6- Sears AW. Hydrocolloid technic for inlays and fixed bridges. D. Digest.. 1937;43:230. 

7- Brown D. An update on elastomeric impression materials. British dental journal. 1981 Jan; 150(2):35-40. 

8- Heisler WH, Tjan AH. Accuracy and bond strength of reversible with irreversible hydrocolloid impression systems: a comparative study. The Journal of prosthetic dentistry. 1992 Oct 1;68(4):578-84. 

9- Appleby DC, Pameijer CH, Boffa J. The combined reversible hydrocolloid/irreversible hydrocolloid impression system. Journal of Prosthetic Dentistry. 1980 Jul 1;44(1):27-35. 

10- Craig R G. Restorative Dental Materials. 10th ed.pp.281 -332. London : Mosby, 1997. 

11-Pratten DH, Craig RG. Wettability of a hydrophilic addition silicone impression material.