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Many dental appliances are constructed outside the patients mouth on models of the hard

and/or soft tissues. The accuracy of fit and the functional efficiency of the appliance

depends upon how well the model replicates the natural oral tissues. The accuracy of the

model depends on the accuracy of the impression in which it was cast. The impression stage

is the fi rst of many stages involved in the production of dentures, crowns,

bridges, orthodontic appliances etc. It is of great importance, therefore, that inaccuracies areminimized at this stage, otherwise they will be carried through and possibly compounded

later on. Impression materials are generally transferred to the patients mouth in an

impression tray. Thetray is required because the materials are initially

quite fluid and require support. Once positioned in the patients mouth, the materials undergo

setting by either a chemical or physical process.After setting, the impression is removed

from thepatients mouth and the model cast using dental plaster or stone.

16.2 Classification of impression materials

Many criteria may be used to classify impression materials. The most widely used and

understood method is to classify them according to chemical type. Hence, we have silicone

materials, alginates, etc. Most dentists are able to associate a material from a particularchemical group with a particular set of characteristics or properties which render it suitable

for some applications but not for others. Other methods of classifi cation are sometimes used

and these may be based upon consideration of the properties of the materials either before or

after setting. Before setting, the property most normally used to characterise materials is

viscosity. This may effect the fi ne detail which can be recorded in impressions of hard

tissues and may infl uence the degree of tissue compression or displacement achieved with

soft-tissue impressions. Thus, materials which are initially very fl uid are often classifi ed as

mucostatic impression materials because they are less likely to compress soft tissues, whilst

materials which are initially more viscous are classifi ed as mucocompressive. It should be

remembered however, that viscosity often varies with the applied stress (p. 19). Thus, certain

materials which appear fairly viscous whilst under low stress conditions may become more

fluid during the recording of the impression, when the material is placed under higher stress.

When a substance behaves in this way, it is said to be pseudoplastic.Another complicating

factor is the spacing of the impression tray. This controls the thickness of the impression

material and hence the pressure transmitted to the underlying tissues. A relatively fluid

impression material confi ned in a close-fitting impression tray will compress the soft tissues

to a greater extent than the same material used in a loosely-fi tting tray. Classifi cation of

materials according to viscosity is not, therefore as simple as it may seem. Figure 16.1 gives a

simplifi ed classification according to viscosity in which materials with the highest viscosity

are shown at the left of the figure and those with the lowest viscosity are shown on the right.

There are often signifi cant variations between different brands of the same type of materialand these variations can spread across the divisions between different levels of viscosity.

A more widely used classifi cation of materials involves consideration of the properties of the

set material. This factor is primarily responsible for governing the principal applications of

the materials. The properties which are most important are rigidity and elasticity, since they

determine whether an impression material can be used to record undercuts. When standing

teeth are to be recorded, or when the patient has deep soft-tissue undercuts, the set impression

material must be flexible enough to be withdrawn past the undercuts and elastic enough to

give recovery and an accurate impression. Hence impression materials are classifi ed as being

elastic or non-elastic. The term elastic as applied to impression materials is fairly

unequivocal since the materials which form this group all possess the ability to be stretched

or compressed and give a reasonable degree of elastic recovery following strain. They couldbe described as possessing rubbery characteristics. The term non-elastic however, is not a

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particularly good term with which to describe a group of products which in some cases are

clearly plastic (e.g. impression waxes) and in other cases are very rigid but show little

evidence of plastic deformation (e.g. impression plasters). It may be less confusing

if the terms rubbery and non-rubbery were used instead of elastic and non-elastic. However,

the latter terms have been used for many years and are therefore likely to be familiar to

dentists. Figure 16.2 lists the major groups of impression materials using the classifi cationreferred to above.