Impression Materials
Taking an accurate and detailed impression of the dental arch after tooth
preparation is one of the most important skills in restorative dentistry. The lab
technician will not have more information about the patient’s mouth than the
details of the impression he received from the clinic. Dentists must cultivate
knowledge of material properties and the indicated techniques of the chosen
materials.
Types of impressions
A primary impression is taken for diagnostic purposes or to fabricate a
special tray for a secondary impression. It must be accurate and without
deformities but is not detailed enough to fabricate a well filling restoration
upon. A secondary or working impression is of high detail and it possible to
fabricate a well fitting restoration upon.
Types of Impression Trays
- Stock trays can be sectional or full arch trays. They can be perforated ornon-perforated. They come in pre-determined sizes and are chosenaccording to the size of the arch that will be recorded. Available materials are aluminum, plastic or stainless steel. Special models include the Rim-Lock impression tray.
- Special or custom trays are fabricated on top of a diagnostic cast of the patient so as to be perfectly customized to the patient’s arch size. They are made of self or light cured acrylic resin and can be perforated or nonperforated.
- The ‘triple-tray’ or dual bite impression tray is a unique sectional stock tray with borders made of plastic or metal. The tray body itself is a meshwork where the patient bites after loading the tray. With this instrument the preparation, opposing tooth and their occlusal relation in maximum intercuspation are recorded. With the proper mounting and die preparation, this is arguably the best impression and occlusal registration for single restorations in dentistry
An impression taken in polyether material (Impregum® by 3M) using dual bite tray for an onlay on
37. Note the sectional matrix used to isolate the distal proximal box
Classification of Impression Materials
Alginate is an irreversible hydrocolloid consisting of powder to which water is added.
Advantages
- Rapid setting
- Straightforward technique
Disdvantages
- Poor accuracy and surface detail
Recommendations
- Diagnostic casts
- Types of alginate modified by silicone may be suitable for opposing impressions
Precautions
- Must be poured within 15 minutes of impression taking
- Borders of impression must be trimmed before placing on bracket table or bench.
At elevated temperatures, reversible hydrocolloid changes from a gel to a sol. This change is reversible. Agar changes from gel to sol at 99°C (210°F) but remains a sol as low as 50°C (122°F), forming a gel only slightly above body temperature. These unique characteristics are very favorable for its use as an impression material.
Advantages
- Low cost
- Hydrophilic
- Long working time
- Low material cost
- No custom tray required
Disdvantages
- Low tear resistance
- Low stability
- Needs a special heating unit
Recommendations
- Multiple preparations
- Problems with moisture control
Precautions
- Must be poured immediately
- Used only with stone
Polysulphide polymerises by a condensation reaction. It most commonly comes as two tubes where the contents are mixed on a glass slab or paper pad. A notable brand is Coe-Flex by GC.
Advantages
- Variety of viscosities
- Long working time
- Flexible
- Good tear resistance
- Long shelf life
Disdvantages
- Reaction byproduct (condensation polymerization produces water and alcohol)
- High dimensional shrinkage
- Dimensionally unstable
- Sticky and messy to handle
- Long setting time a disadvantage for single preparation
- Toxic if trapped subgingivally
- Unpleasant smell and taste
- Alcohol byproduct dimensionally unstable
Recommendations
- Impressions for crown and bridge preparations
Precautions
- Must be poured within one hour
- Make sure no material is retained. This may happen in molar furcations or deep undercuts
Polyether is an elastic impression material which polymerizes by an addition reaction. It produces no byproduct and is dimensionally stable. It is also relatively hydrophilic and has a fast setting time. It comes as either two tubes for manual mixing or a cartridge for automixing to be dispersed by a gun or large dipenser. Polyether is available in one medium viscosity phase but its viscosity reduces when syringed around crown margins. This property is called shear thinning.
Advantages
- One viscosity and undergoes shear thinning to give accurate details
- Accurate
- Short setting time helpful with single preparations
- Dimensionally stable
- Good tear resistance
Disdvantages
- Bitter taste
- Occasional Allergies
- Rapid setting time
- Difficult with multiple preparations
- Stiff when set, so difficult when managing large undercuts
- Imbibes water, so must be stored in dry conditions
- Can only be poured once
Recommendations
- All working impression techniques using a special tray
Precautions
- Care needed to avoid breaking cast when separating from impression.
This is a polyvinyl siloxane based impression material. It polymerizes by condensation polymerisation, so it produces water and/or alcohol byproducts affecting its dimensional stability. It comes in several consistencies (putty, heavy, medium and light viscosities). The material is manually mixed with a catalyst paste to solidify.
Advantages
- Easy handling
- Variety of viscosities
- Good elastic recovery
- Good tear resistance
- Good surface detail
Disdvantages
- Dimensionally unstable
- Hydrophobic
- Poor wetting
Recommendations
- Can be used in the laboratory for making matrices, indices and other lab procedures for which it is perfectly indicated.
- Alternative to alginate for diagnostic casts which will not be poured immediately
- May be used for working impressions for economic reasons, although the same level of accuracy as polyether and addition silicone cannot be expected.
This is polyvinyl siloxane which reacts by addition polymerisation, hence leaving no byproduct and hence being very dimensionally stable. They are inherently hydrophobic but modern brands are relatively hydrophilic due to adding medical grade surfactants or hydrophilic copolymers. Addition
silicone is available in a wide range of viscosities: putty, heavy, medium, light and ultra-light. The putty consistency is provided as two base and catalyst putties to be manually mixed. The other viscosities are available as auto mix cartridges to be dispensed from a gun or large dispenser
Advantages
- Less hydrophobic than previously
- Nearly negligible polymerization shrinkage on setting
- Excellent surface detail
- Minimal permanent deformation
- Excellent dimensional stability
- Resistant to water absorption
- Biocompatible
- No unpleasant taste due to flavouring
Disdvantages
- Setting may be inhibited by powdered latex gloves
- Findings reported for setting inhibition due to hemostatic gingival retraction paste
- Expensive due to chloroplatinic acid catalyst
Recommendations
- All working impressions to fabricate restorations
Precautions
- Some brands caution delayed pouring
Vinyl Polyether is a formulation that combines properties of polyether and addition silicone.They are supplied as putty, heavy, medium, and light silicones. An additional benefit is that they do not have the bitter taste of polyether materials. Their supposed advantages need to be supported by
clinical studies. Lab studies suggest that they are similarly accurate to PVS in the short-term but less dimensionally stable in the long-term when stored in disinfectant.
Overdenture secondary impression with special tray border molded with putty addition silicone
(Elite® by Zhermack) and washed with vinyl polyether (Identica® by Kettenbach)
