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)
