WHAT IS NON-AQUEOUS ELASTOMETRIC IMPRESSION?
They are synthetic polymers with rubber properties after setting. Used to make the final impressions for indirect restorations and implants.
All teeth in the arch and the soft tissues immediately surrounding the tooth preparation must be reproduced in the impression. They will allow the cast to be accurately articulated (to check occlusion) and will contribute to proper contouring of the planned restoration.
If it becomes necessary to store the impression before a cast will be made, the polyethers and addition silicones are preferable because they exhibit sufficient long-term dimensional stability; the other materials, particularly the reversible hydrocolloids, must be poured immediately.
Requirements of an impression for a cast restoration:
1. It should be an exact duplication of the prepared tooth, including all of the preparation and enough un-cut tooth surface beyond the preparation to allow the dentist and the technician to be certain of the location and configuration of the finish line.
2. Other teeth and tissue adjacent to the prepared tooth must be accurately reproduced to permit proper articulation of the cast and contouring of the restoration.
3. It must be free of bubbles, especially in the area of the finish line and occlusal surfaces of the other teeth in the arch.
- High accuracy (very small contraction <0.5%)
- High dimensional stability
- Compatibility to stone
- Multiple pouring within 24 Hours after making impression
- High tear strength
- High elastic recovery
- Long shelf life
- Ease of use
- Commonly called rubber base
- Comes as two tubes of base and catalyst
- Will begin to shrink after one hour from removal
- Should be poured immediately
Although it is the least expensive elastomer, it is not well-liked by patients because of its unpleasant sulfide odor and long setting time in the mouth (about 10 minutes).
- Can have pronounced shrinkage (poor dimensional stability)
- Dies produced from this material can be undersized
- The above occurs due to the evaporation of the by- product of the condensation reaction. (H2O for polysulfides and ethanol for Condensation Silicones)
- Should be poured immediately
- Silicone impression material is available in a variety of viscosities.
- No volatile by-product is formed which results in excellent dimensional stability.
- Hydrophilic in nature.
- Pouring can be delayed to produce accurate casts even more than a day after the impression has been made.
Polyvinyl Siloxanes (Addition Silicone)
- Dimensional stability is quite high
- Least affected by pouring delay of any material (due to no volatile by-product formation)
- Surfactants have been added to the material to decrease it hydrophobicity
- Comes in different viscosities (consistencies)
Pouring should be delayed with some of the earlier products. If this is not done, a generalized porosity of the cast surface caused by gas from the impression material will develop.
Newer products contain “scavengers” that prevent the escape of gas at the polymer-cast interface.
Addition silicone that contains scavenger material can be poured immediately.
A) Stock trays
Stock trays can be used with these impression materials. Retention is provided by perforations, rim-locks, and/or tray adhesives.
B) Custom trays
A custom tray improves the accuracy of an elastomeric impression by limiting the volume of the material. Stops are needed in the tray to maintain even space for the impression material. These are placed on non-centric cusps of teeth that are not to be prepared.
A) Single Mix (mono-phase) Technique
The impression Material for the mono-phase technique has one viscosity for tray and syringe.
- Usually used with polyether
- Some of the material is applied to the syringe while the rest is applied to the tray
- Inject the material by the syringe around the prepared tooth
- Air drying to spread the material
- Place the tray
- Shorter working time is needed
B) Double Mix Technique
Common methods for making crown and bridge impressions are:
1. A simultaneous, dual viscosity technique
2. Putty-wash technique
1. A simultaneous, dual viscosity technique (Heavy Body-Light Body Combination)
- It’s a 4-handed technique
- Light body is injected by a syringe
- Heavy body placed by the tray
- On separate pads (one for the tray and one for the syringe material), disperse equal amounts of base and accelerator. Blend the two pastes thoroughly.
- THE LIGHT-BODIED MATERIAL IS THOROUGHLY SPATULATED.
- Mixture should be free of streaks and bubbles
- MEANWHILE, AN ASSISTANT MIXES THE HEAVY-BODIED MATERIAL.
- Low-consistency material is injected with a syringe into critical areas.
- Seat the tray loaded with the tray material (heavy-bodied), Tray must remain immobile while the material undergoes polymerization (6 to 12 minutes, depending on the material). Otherwise distortion of the impression can occur when it is removed.
– Automix Technique
Some manufacturers offer impression material in prepackaged cartridges with a disposable mixing tip attached.
- Elimination of hand mixing
- Produces void-free impression
The base and catalyst are extruded into the mixing tip, where mixing occurs as they progress to the end of the tube.
– Machine Mixing Technique
An alternative method for improving impression mixing is to use a machine mixer.
This system is convenient and produces void-free impressions.
2. Putty-wash technique
Silicone impression material is available in a variety of viscosities. It is a two-step impression procedure whereby a preliminary impression is taken in high or putty-consistency material.
- Washing Hands Is A Must! If the putty is used, it should be not be dispensed or mixed while wearing latex gloves. Because the sulphur granules in the powder will retard the setting reaction.
- Accelerator is added to the putty.
- Putty is placed in the tray.
- Spacer is placed over the putty. This allows room for a thin wash of light-bodied material, which makes the impression.
- Take the impression by putty material.
- Remove or trim some of the putty material by scalpel; or place a spacer to provide even thickness e.g. celluloid paper.
- Mix the light body material and place it in the tray space and re-seat the tray in mouth.
– Simultaneous Putty Wash Technique
- Placement of a dimple in the putty, filled with wash, before tray seating.
- Wash material capturing margins which is free of voids and tears.
- A good bond between putty and wash (i.e. no separation).
Manufacturers add colouring agents to the accelerator and/or base as an aid in determining the thoroughness of the mix.
Normally a different colour is used for each consistency of a particular product line so one can distinguish the wash (low) consistency from the tray consistency in the set impression.
Lingual View Of Impression
The impression material cervical to the margin is termed “flash”.
The more flash cervical to the impression, the easier it is to trim the dies during the laboratory phase of any project.
Less Ideal Impression
Evaluation of the Impression
- No air bubbles or voids especially at the margins.
- Intact uninterrupted cuff of the impression material should be present beyond every margin.
- Homogenous colour of the material. Presences of streaks indicate poor mixing.
- Good blend between heavy body and light body materials.
- No part of custom tray shown in the impression.
- The impression should not be separated from the tray.
The impression must be inspected for accuracy when it is removed.
– If bubbles or voids appear in the margin, the impression must be discarded.
- Voids on the margin of a preparation compromise the ﬁt and function of the ﬁnal restoration.
- Voids on occlusal surfaces make articulation of stone models difficult.
– Streaks of base or catalyst material indicate improper mixing and may render an impression useless.
A complete information about the impression material used is to be provided to the dental laboratory.
When they are removed from the patient’s mouth, it must be assumed that all impression materials have been in contact with body fluids and therefore should be disinfected.
Disinfection is an essential step for preventing cross-infection and exposure of laboratory personnel. If it is performed properly, disinfection will not affect the accuracy or surface reproduction of the impression material.
- After being removed from the patient’s mouth, the impression is immediately rinsed with tap water and dried with an air syringe.
- Suitable chemicals should be used, such as glutaraldehyde solutions 2% (10 minutes soak time) or iodophor sprays.
Because polyether is a hydrophilic material (has an affinity for water), it is better to spray it rather than to soak it.
Guides For Selection of Appropriate Disinfection Methods for Impressions Transported to Dental Laboratory
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