Elastomeric Impression Materials: Types, Properties, and Techniques

Elastomeric Impression Materials

The first elastomeric materials to be introduced to dentistry were the natural rubbers introduced as denture base materials in the 1850s. These were called vulcanite as they were converted into rubber from their natural latex by a process called vulcanization.

The first elastomeric or rubber-based ‘impression material’ to be introduced was polysulfide which was introduced in 1950. Interestingly, they were originally developed as an industrial sealant for gaps between concrete structures. This was followed by condensation silicone in 1955, polyether in 1965 and the addition silicones in 1975.

Read And Learn More: Basic Dental Materials Notes

Introduction of the elastomers were a considerable technological advance in the quality of dental services. Elastomers are soft and rubber-like and far more stronger and stable than the hydrocolloids. They are known as synthetic rubbers. The ADA Sp. No. 19 referred to them as nonaqueous elastomeric dental impression materials.

The term nonaqueous was used to differentiate them from agar and alginate (considered aqueous or water containing materials). Currently ISO 4823 simply refers to them as ‘elastomeric impression materials’. These materials are the most accurate and dimensionally stable impression materials available in dentistry.

Impression Materials In Dentistry

Chemistry and structure of Elastomeric polymers

• The term ‘elastomer’ is derived from the words elastic polymers. Thus elastomers are essentially polymers with elastic or rubber-like properties. Other polymers used in dentistry are the denture and composite resins. Gutta-percha is also a polymer (cis-polyisoprene) which is closely related to natural latex (trans-polyisoprene). Natural latex is currently used in dentistry to manufacture examination gloves and rubber dams. Gutta-percha is used as an endodontic obturation material.
• Elastomeric materials contain large molecules with weak interaction between them. They are tied together at certain points to form a three-dimensional network. On stretching, the chains uncoil, and on removal of the stress they snap back to their relaxed entangled state .
• Elastomers are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. As a result of this extreme flexibility, elastomers can reversibly extend from 5% to 700%, depending on the specific material. Without the cross-linkages or with short, uneasily reconfigured chains, the applied stress would result in a permanent deformation.

 

Polyether Rubber Impression Material

Polyether was introduced in Germany in the late 1960s. It has good mechanical properties and dimensional stability. Its disadvantage was that the working time was short and the material was very stiff. It is also expensive.

Polyether Available As

Available as base and accelerator in collapsible tubes, cartridges for static mixing and dynamic mechanical mixing devices. The accelerator tube is usually smaller. Originally, it was supplied in a single viscosity. A third tube containing a thinner was provided.

Currently, it is available in three viscosities.

• Light bodied
• Medium bodied
• Heavy bodied

Commercial examples Impregum (3M ESPE), Ramitec, Polyjel (Dentsply), Permadyne (ESPE).

Impression Materials In Dentistry

Polyether Composition Base

Polyether Reactor/accelerator paste

Polyether Chemistry And Setting Reaction

It is cured by the reaction between aziridine rings which are at the end of branched polyether molecule. The main chain is a copolymer of ethylene oxide and tetrahydrofuran. Crosslinking is brought about by the aromatic sulfonate ester via the imine end groups. The reaction is exothermic (4 to 5 °C).

Polyether Properties

• Pleasant odor and taste.
• The sulfonic ester can cause skin reactions. Thorough mixing is recommended before making an impression and direct skin contact should be avoided.
• Setting time is around 6–8 minutes. Mixing should be done quickly that is 30 seconds. Heat decreases the setting time.
• Dimensional stability is very good. Curing shrinkage is low (0.24%). The permanent deformation is also low (0.8–1.6%). However, polyethers absorb water and can change dimension. Therefore, prolonged storage in water or in humid climates is not recommended.
• It is extremely stiff (flexibility 3%). It is harder than polysulfides and increases with time. Removing it from undercuts can be difficult, so additional spacing (4 mm) is recommended. Care should also be taken while removing the cast from the impression to avoid any breakage.
• Tear strength is good (3000 g/cm).
• It is hydrophilic, so moisture in the impression field is not so critical. It has the best compatibility with stone among the elastomers.
• It can be electroplated with silver or copper.
• The shelf life is excellent — more than 2 years.
• It has excellent detail reproduction (20 microns).
• Many medicaments, such as aluminum sulfate and ferric sulfate, used on gingival retraction cords have been accused of causing inhibition of set of polyvinyl siloxane materials. However, studies have not found not any inhibitory effect.
• Material interactions Composite based provisional crown materials like Protemp 4 have been observed to have an inhibitory effect on the setting of polyvinyl siloxane materials. When a provisional crown is made directly in the mouth using a putty impression as a template, an oily residue called the oxygen inhibited layer (OIL) remains on the tooth and in the impression after separation of the provisional crown. Failure to adequately remove the OIL can result in impaired setting.

Impression Materials In Dentistry

Polyether Manipulation Of Elastomeric Impression Materials

There are many methods of mixing and using elastomeric impression materials depending on whether it is supplied in tube, cartridge or putty form.

The 5 main mixing techniques are

• Hand or manual spatulation
• Manual kneading
• Rotary table assisted mixing
• Static or extrusion mixing
• Dynamic mechanical mixing

Polyether Hand Mixing – Pastes in tubes

Hand or manual spatulation and is primarily used elastomers supplied in tubes.

Polysulfides and addition silicones

• Equal lengths of base and accelerator pastes are extruded on to the mixing pad alongside each other without touching. The accelerator paste is then incorporated into the base paste. Mixing is done using a tapered stiff bladed metal or plastic spatula. Just before loading the tray the material should be spread in a thin layer to release the trapped air bubbles. A streak free mix is obtained in 45 seconds.

Polyether Condensation silicone

• Unlike addition silicone, the quantity of catalyst paste needed is very little. The manufacturer usually marks the length required on the mixing pad. The two pastes therefore are of unequal length and diameter.

Impression Materials In Dentistry

For polyether

• The required amount of thinner (when supplied) may be added to the base and accelerator depending on the viscosity needed. Again, like condensation silicone, the quantity of accelerator needed is very little. The ratio is usually displayed on the mixing pad. The mixing should be done quickly. The mixing time is 30 seconds.

Kneading – putty

• Kneading is primarily employed for very heavy or putty consistency elastomers. In case of addition silicones, equal scoops of base and accelerator are dispensed. With condensation silicones, the required number of scoops of base and recommended amount of liquid or paste accelerator is taken. In either case mixing is done by kneading between the fingers. Mixing is continued until a streak free mix is obtained.

Rotary table-assisted mixing

• The technique is similar to that described for zinc oxide impression pastes. The pastes are dispensed on to a rotating table. The spatula is used to scoop and flatten the pastes alternately and continuously as the table rotates until a uniform mix is obtained.

Static Or Extrusion Mixing

• Static mixing also known as ‘extrusion mixing’ (ISO 4823:2015), has grown in popularity over the years, primarily because of its high accuracy and convenience. Extrusion mixing is a method by which two or more material components are extruded simultaneously from their separate primary containers through a special mixing tip from which the material components emerge as a homogeneous mixture.

Advantages of static mixing

• Shorter mixing time.
• More uniform proportioning and mixing.
• Less voids.
• Mix can be delivered directly to the tray or impression site.

The system consists of a gun with a dual plunger. The cartridges are loaded onto this device. A static mixing tip is then attached to the cartridge. The tip contains helical mixing blades on the inside. Forcing of the base and accelerator through the tip results in its mixing.

Precautions

• The initial portion should be discarded as material from the right and left tubes may not have extruded evenly. This can affect its setting characteristics.
• The material should be ejected in a continuous stream, avoiding lifting or skipping across segments. This reduces the chances of air entrapment and voids.

Dynamic Mechanical Mixing

• Another device is an electrically operated dynamic mechanical mixer. The base and catalyst are supplied in large plastic bags which are loaded in to the machine. On pressing a button, the material is mixed and extruded through the tip, directly into the impression tray.

Impression Techniques

• Impressions may be made in custom or stock trays. Elastomers do not adhere well to the tray. An adhesive should be applied to the tray and allowed to dry before making impressions. The adhesive cements provided with the various elastomers are not interchangeable. A slightly roughened tray surface will increase the adhesion. For putty impressions, a perforated stock tray is used. The perforations help retain the putty in the tray.
• In case of elastomers, the bulk of the impression should be made with a heavier consistency (to reduce shrinkage). Light bodied should only be used in a thin layer as a wash impression.

Classification Of Elastomeric Impression Techniques

Based on the viscosity used

• Single viscosity technique
• Dual viscosity techniques
  • Dual viscosity technique using light body-heavy bodied
  • Putty-wash technique

 

Based on the number of stages used

• One stage technique—Both viscosities are dispensed and allowed to set simultaneously.
• Two stage technique—This is usually employed with putty. In this technique a preliminary impression is made first with the ultra-heavy or putty viscosity. This is relined later by the lighter viscosity called as wash impression.

One-Stage Single Viscosity (MONOPHASE) Technique

• Tray used Resin custom tray with 2–4 mm spacing.
• Viscosity Medium only.
• Method The paste is mixed and part of it is loaded on to the tray and part into a syringe. The syringe material is then injected on to the prepared area of impression. The tray with material is seated over it. The material is allowed to set.
• This technique utilizes the principle of shear thinning. The same material when ejected under pressure through the syringe tip it exhibits pseudoplasticity and behaves like a material with lower viscosity. Shear thinning allows as single material to be used as both syringe and tray material.

Elastomeric Impression Materials

One-stage Dual Viscosity (Dual-Phase) Technique

• Tray used Resin custom tray with 2–4 mm spacing.
• Viscosity used (a) Heavy bodied and (b) light bodied.
• Method The two viscosities are mixed simultaneously on separate pads. The heavy body is loaded into the tray while the light bodied is loaded into the syringe. The syringe material is injected over the preparation. The tray containing the heavy body if then seated over it. Both materials set together to produce a single impression.

Two-stage Putty Reline (Two-stage Putty-wash Technique)

• Tray used Perforated stock tray.
• Viscosity used (a) Putty and (b) Light body.
• Method First a preliminary impression is made with putty in the stock tray. Before seating the tray in the mouth, a thin plastic sheet is placed over the putty (it acts as a spacer). After setting it is removed and kept aside. Mixed light bodied is loaded into a syringe and injected over the preparation. Static mixing tips fitted with special direct delivery tips can also be used for this purpose. Light viscosity material is also loaded into the putty impression. The preliminary impression is then seated over the injected material and held till it sets.

Two-stage Putty Reline Advantages

• No special tray required.
• Simple and quick as custom tray is not required.
• Accurate.

Two-stage Putty Reline Precautions

Some clinicians use the preliminary impression as a template to construct a provisional restoration (temporary crown). The same preliminary impression is then relined with the final wash material to make the final impression. If a composite based provisional restorative material [e.g. Protemp 4 (ESPE), Structur (VOCO)] has been used to construct the provisionals, a shiny oily layer called the air or oxygen inhibited layer (OIL) from this material forms a coating over the putty in this region. The OIL can affect both setting as well as bonding of the wash material. This can result in delayed setting, distortion or separation of the wash impression from the preliminary impression. This can be eliminated by rigorously wiping off the OIL or physically removing of a layer of putty with a trimmer.

Elastomeric Impression Materials

One-stage Putty Reline (one-stage PUTTY-WASH technique)

• Tray used Perforated stock tray.
• Viscosity used (a) Putty and (b) Light body.
• Method Unlike the previous technique, the putty and light body are dispensed and mixed simultaneously. The putty is loaded into a perforated stock tray whereas the light body is injected on to the prepared tooth. The tray is then taken to the mouth and pressed into position. The heavier putty forces the lighter material into the details. Both material set simultaneously to produce an accurate impression.

One-stage Putty Reline Advantage

• No special tray required. The technique is simple and quick.

One-stage Putty Reline Removal Of The Impression

The material is checked for set by prodding with blunt instrument. When set, it should be firm and return completely to its original contour.

The impression is dislodged from the mouth as quickly as possible for the following reasons

• Elastic recovery is better
• Tear resistance is higher.

However, rapid removal may be difficult as well as uncomfortable to the patient. Removal is facilitated by breaking the air-seal. This can be done by teasing the borders of the tray parallel to the path of insertion until the air leaks into the tray. Compressed air through an air syringe may also be used. In addition to the holding the tray handle, a finger on the buccal portion of the tray may be used to apply additional pressure to dislodge the tray.

One-stage Putty Reline Infection Control

• Rubber impression materials are disinfected by immersing in disinfectant solutions. 10 minutes in 2% glutaraldehyde or 3 minutes in chlorine dioxide solutions have been found to be satisfactory. Because of its tendency to absorb water, a spray of chlorine dioxide is preferred in case of polyether. Other disinfectants used are phenol and iodophor.

One-stage Putty Reline Impression Errors

Errors in impressions do occur and can result in inaccurate casts and prostheses. Most of the errors occur due to poor technique and a failure to understand the properties of the material.

Some of the impression errors that can occur are

• Air entrapment
• Fluid entrapment
• Seating trails
• Contamination from provisional crowns
• Contamination from latex gloves
• Rough or uneven surface
• Distortion

Elastomeric Impression Materials

One-stage Putty Reline Air entrapment

Voids in the impression mostly result from a trapping of air.

Voids occur from a variety of causes.

• Air entrapment due to faulty loading of the tray. Care should be taken when loading the tray. Material should be loaded in a continuous motion from one end of the tray to the other pushing the material ahead as it is ejected from the static mixing device. The tip should be in close proximity to the surface of the tray. Lifting the tip away from the tray and moving it from place to place in a discontinuous motion can result in air entrapment.
• Air entrapment from faulty placement of material around the prepared tooth. When placing the material on the prepared tooth using a syringe or a static mixing tip, the material should be extruded in close proximity to the sulcus and prepared tooth in a continuous motion around the prepared tooth starting from the sulcus and finish line through to the occlusal surface. Discontinuous and erratic motion with lifting of the tip from place to place can result in air entrapment.