Dental impression materials and tips for taking good impressions.

This presentation will consist of two halves: at first we will get some information about different kinds of dental impression material. They components, application, advantage and disadvantage. At the second part we will analyse a few rule of taking impression.

First I will present a definition of dental impression. Dental impressions are molds taken of the teeth and oral tissue, in other words dental impression is a negative mold, showing the teeth in reverse. The mold can be filled with plaster or another material that is then allowed to set to produce a positive cast of the teeth . The positive cast are use in diagnosis and treatment of dental conditions among other for making dentures, bridges, veneers, dental implants, orthodontic braces, mouthguards.

So taking a dental impression we use a special material. What prosperities should have a useful impression material. Namely should have accurate reproduce the shape of the field of prosthetic. Important for dentist is easy way to use with minimum of equipment, good elastic properties with no permanent deformation after strain. Adequate strength so that it will not break or tear on removal from the mouth. Have adequate shelf life for storage and distribution. Economically commensurate with the results obtained. Meaningful for patient is pleasant taste and odor, esthetic color no containing of toxic or irritant constitutes.

As I said earlier we can meet variety of impression material, which we can classified in different ways. I focus on classification by mechanical properties. At the first group we distinguish elastic material which contains hydrocolloid. Reversible hydrocolloid (agar) and irreversible hydrocolloid (alginate). Second subgroup are elastomeric like polysulfide, silicone polyether. Second group are non-elastic which include zone oxide eugenic, impression wax, impression compound, impression plaster.

Let’s start from hydrocolloid materials. Hydrocolloids are considered type of 2 phases system. Dispread phases – solute and dispersion medium solvent. Two state of hydrocolloids are distinguishable. Sol what meaning viscous liquid. Through gelation process it’s change state to gel (jelly like material). Gelation process occurs in two way. First way going by change of temperature and it is reversible process. Observable in agar. Second way going by chemical reaction, what we can observe in alginate material. It’s irreversible process.

Agar material provide accuracy impression of excellent and fine detail but because of the very complicated process of preparation and impression taking has been completely superseded by alginate. Agar is currently used only in the dental laboratory.

The word alginate comes from the tem algin. It’s a peculiar mucous extract yielded by certain brown seaweed (algae). Alginate mass have a lot of components. I will describe the most important. Basic components are sodium or potassium alginate, which constitute soluble salt, and calcium sulfate dihydrate which release calcium iron to react with alginate. Trisodium phosphate is retarder. Gelation reaction occur in two steps . At the beginning trisodium phosphate react with calcium sulfate dihydrate. It’s retardation reaction, which prolong a material’s work time. Next when all content of retarder react second reaction, gelation reaction starts. It’ s run between potassium alginate and calcium sulfate dihydrate. Gelation occur by agglomeration of dispread phrases to form fibrils and network pattern. The final gel structure is structure sodium alginate cross –linked with calcium iron. It is supplied as a powder that is packed in bulk or in tins or in weighed packets. Because of bonding time distinguish two types: fast setting and normal setting. In both type we work according with the same steps, but in first type we must do it faster. First proportioning powder and water. Use the specific measuring devices provided by the manufacturer for mixing. Mix the alginate for the specific amount of time and using a stiff spatula “swipe” the alginate mass against the sides of the bowl to avoid entrapment of air in the mix. Then thread the mass on a perforated impression tray and take impression. How to do it I will show later. Alginate material is most widely used by dentists, but is not recommended for making impressions for crowns and bridges, because not enough good reproduction fine details .

In contrast to alginate mass are elastomeric materials characterize high ability to reproduction fine detail and higher mechanical strength than alginate. However the elastomeric materials are less flexible. They cannot by use in place with a prominent protuberances of tooth and gingivae.

Elastomeric are two component chemically setting materials. They are delivered as two pastes. Rubber (elastomeric) can be divided according to the chemical composition (polysulphide, silicones condensation & addition, polyether). According to consistency each meaning that this material can be supplied in different consistencies according to % of filler: light body or syringe consistency, medium body, heavy body. Deferent type of consistency allows use a few impression techniques. I’ll be returning to this techniques latter.

All elastomeric characterize the same polymerization reaction, which run between synthetic liquid prepolymer plus catalyst. In results obtain crosslinked polymer. At the beginning polymerization process observed at the beginning chain lengthening , next crosslinking, polymerization shrinkage, heat evolution and at the finish residual monomer. All polymerization process continues for 24 hours after clinical set of the material. So important is as soon as casting plaster positive. Manipulation of elastomers consist of extrusion of tubes, in which there is provided material an appropriate amount of a base and catalyst. It is important to strict to the proportions which are indicated by manufacturer. Next step is mixing it to a smooth paste, and take a impression.

Now I briefly characterize a subgroup of elastomeric materials.

Polysulphide dental impression material are delivering in two past system. Past are in contrast colours, what help to evaluate the degree of mixing. Past are available in different consistence (light, regular, heavy). Important advantage of it is low price compare to other elastomers. The base past consist polysulphide pre-polymer with terminal pendant, mercaptan group and filler in concentration according to consistency. Catalyst paste consist lead dioxide suspended in an inert oil. Lead oxide have characteristic staining brow color and is toxic. Sometimes lead dioxide is replaced by organic hydro – peroxide sins of eliminate brow color. Polymerization reaction goes between mercaptan groups and lead compound. In results we get a polysulfide rubber. Most important advantage of polysulphide is it strength, can be cast several times. Impression should be removed quickly after setting-do not rock the tray. Adhesive must be thin and dry before adding the impression material. Wait 20 to 30 minutes before pouring the impression for the stress relaxation to occur in the material.

Similar type impression material is polyether, although is more expensive. Polyether pre-polymer with terminal ethylene imines group is find in base past, catalyst past consist sulphonate ester. Ring opening offset some of the polymerization shrinkage but f polyether materials have the biggest dimensions stability during storage. Is hydrophilic watch meaning good wet ability for easy cast forming. Polyether is very stiff, which makes it difficult to remove without rocking.

Silicones material are the most flexible from elastomers material, although less precise. Are two type based on the type of polymerization reaction: condensation silicones, addition silicones. Silicones overcome the disadvantages of polysulphide and polyether : bad odor, messy , stains clothes permanently, effort during mixing, curing shrinkage, high permanent deformation. Supplied form two pastes of contrast color in unequal sized metallic collapsible tubes. Condensation silicones composited of base paste (large tube) siloxane pre-polymer with terminal hydroxyl group, and catalyst (small tube) tin octoate orthoalkyl silicate. Additional silicones composited base paste siloxane pre-polymer with silane group. Catalyst paste siloxane pre-polymer with vinyl terminal group, chloroplatinic acid. Are not dimensionally stable and have to be very fast cast.

Last group of impression material are non-elastic material. Now they are not used for making impressions, only for occlusal registration. So that is way I do not be focus on this group.

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