Impression materials
Dental impression materials are used for taking impressions in the mouth. They are intended for reproducing and transferring the outer contours of intraoral structures (teeth, jaws, soft tissues, fillings, restorations) three-dimensionally during impression-taking. An initially soft material is applied directly to the surface, avoiding air voids where possible, before setting rapidly (between one and a few minutes) in a supporting impression tray to create a female mould (impression). The impression is then cast (usually with plaster but possibly with acrylic resin) to produce a model.
In the past, plaster was used for taking impressions even though it was not elastic after setting. For this reason, a plaster impression of undercut structures (teeth) always had to be sectioned (fractured) in order to withdraw it from the mouth. Impression plaster was also used as a pick-up impression material for single-prep impressions taken using customised copper rings filled with thermoplastic (resin-based) impression material. However, after cooling and setting the rigid impression could not be removed from undercuts without being damaged.
For this reason, nowadays rigid setting, reversible thermoplastic materials are only used in non-undercut regions, e.g., resins for functional impressions on denture peripheries or waxes for bite registration.
Special zinc-oxide-eugenol impression materials may also be used when taking functional impressions.
For decades almost only impression materials which remain elastic after setting and, when sufficiently thick, virtually fully recover their original shape after removal from the mouth, have been in use. This property is referred to as “elastic recovery”.
These include alginate impression materials which, when mixed with water, form gel-like polysaccharides. As alginate impressions may dry out or absorb further water and swell, they are neither dimensionally stable nor can they be stored for longer periods. This also applies to reversible hydrocolloids.
In the meantime, so-called elastomeric alginates, which are generally irreversible materials and set to a rubbery-elastic consistency, are widely accepted. The chemistry behind the chain reactions can be divided into polymerization (e.g. polyether impression materials), polyaddition (e.g. A-silicones) and polycondensation (e.g. C-silicones, polysulfides). These impression materials are either mixed by hand or mechanically (refer to “Mixing systems for impression materials”) to combine two components (“base” and “catalyst”) homogeneously and void-free.
One-step putty-wash impression taken with regular and light body silicones over impression posts
Silicone impression materials (polyvinyl siloxanes, PVS) in particular are supplied in different consistencies for various impression procedures. When taking one-step or two-step putty-wash impressions, a kneadable (heavy body) putty exerts controlled pressure (usually in an impression tray) on a syringeable, light body, low viscosity precision impression material. This allows it to penetrate narrow gaps (e.g., intraorally in the gingival sulcus, interdental spaces, fissures) thus increasing the accuracy of detail (crispness) of the precision impression.
Materials which are also kneadable yet slightly less viscous, and with an extended setting time, are used for taking functional impressions for full dentures. They intentionally displace very compressible mucosal areas more than firm sections (varying mucosal resilience) thus balancing the pressure exerted which reduces the risk of denture sores.
Regular body silicone impression materials are either used with other materials or alone (monophase) e.g., for taking pick-up impressions for fixed/removable restorations either involving implants or not.
Pick-up impression taken over inner telescopes using monophase silicone
To ensure that the impression material flows smoothly onto surfaces, which are often moist, a degree of hydrophilicity (acute contact angle) is desirable.
The setting reaction of impression materials may be accelerated by specific additives or a warmer environment (e.g., intraoral) but can also be retarded by inhibitors or precooling. Processing encompasses the mixing time, working time (loading the tray with impression material and inserting it into the mouth) and setting time. Special chromatic impression materials change colour to indicate the changes.
Silicone impression materials which attain their final hardness very quickly can also be used for bite registration.
Nowadays, materials for analogue impressions are often no longer required as contactless optical procedures using intraoral scanners are increasingly in use (digital impression-taking).
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Wax build-up technique Wax build-up technique The various anatomical structures (such as cusp tips and slopes as well as marginal ridges) are usually built up one after another by adding small portions of wax (often using differently coloured waxes for didactic purposes). The firm, special waxes first have to be melted at room temperature. This can be carried out by warming small portions on differently shaped working tips of hand instruments in an open flame (such as a gas burner) or using electrically heated instruments which provide for more accurate temperature control and avoid contamination (e.g. electric wax-knife, induction heaters, wax dipping units). The wax is applied drop-by-drop to ensure that the warmer molten wax added last fuses seamlessly with the firm, cooler material. After hardening, the wax pattern can be reduced by sculpting, milling guidance surfaces or drilling to add retainers. Modern procedures include flexible, occlusal preforms for adding contours to soft wax. In addition, wax preforms, such as for occlusal surfaces or bridge pontics, are available in various shapes and sizes. Recently, irreversible, light-curing materials have been introduced for use instead of reversible thermoplastic waxes. Wax preforms To ensure that the wax pattern can be released without being damaged, model surfaces, opposing dentition and preparations must be hardened/sealed with special lacquer (applied by spraying, brushing or dipping). These waxes are mostly relatively rigid/elastic after cooling. Attaching wax sprues to a removable framework supported on double crowns using a hand instrument When employing the lost wax technique, prefabricated wax sprues, bars and reservoirs are attached to the patterns. Once the pattern has been released and its sprues waxed onto the crucible former, it is invested in a casting ring with refractory investment material. The wax can then be burnt out residue-free and casting completed. Unlike standard wax build-up techniques, a diagnostic wax-up is not intended for fabricating an indirect restoration, but rather for simulating the appearance and/or external contouring for producing orientation templates. |