defined in terms of the time required to complete the setting reaction. It is more normal, however, to use a definition which is based on the time to reach a certain degree of rigidity, hardness or elasticity. Setting times measured in this way often gives values which are significantly shorter than the times required to complete the setting reaction, indicating that setting continues slowly for some time after removal from the mouth. In order to obtain the optimum performance from any impression material, it is wise to leave the material in the patient’s mouth for an extra minute or so after setting has apparently been completed. This applies particularly to the elastic materials for which significant improvements in elasticity may occur after the apparent completion of setting.

For the convenience and comfort of both the dental surgeon and patient the most ideal combination of properties for an impression material is a long working time and short setting time. This can be achieved with materials which set by a chemical reaction providing the reaction rate is much quicker at mouth temperature than at room temperature.

16.4 Clinical considerations

Choice of impression material

The fundamental choice for impression materials is between rigid and elastomeric products. As a broad generalization, rigid materials are used

Fig. 16.8 Viscosity versus time curves for three impression materials. Material A is initially quite viscous and its viscosity slowly increases.

Material B is initially fluid (low viscosity) but its viscosity increases sharply. Material C is initially fluid and has an induction period during which viscosity is constant. This is followed by a rapid increase in viscosity.

when undercuts are not present amongst the surfaces to be recorded. The one exception to this is impression plaster which can be used in some circumstances where undercuts are present. The impression fractures during removal and then needs to be reassembled like a jigsaw before the master cast is prepared. Rigid impression materials can also be used in edentulous subjects where soft tissue (compressible) undercuts are present.

Both rigid and elastomeric materials are available which exhibit a wide range of accuracy and physical properties. The selection of a material for a specific clinical application is usually based on a combination of cost and required accuracy. The more accurate/dimensionally stable materials also tend to be more expensive and hence are used when required rather than routinely.

Impression trays

The purpose of an impression tray is to give rigid support to the impression material and to facilitate its introduction into the mouth. Trays can be either custom made or pre-formed (stock).

Custom trays are produced on study casts of a patient’s mouth that have been made using an impression material of lower accuracy/dimensional stability in a stock tray. They are designed to be rigid and to have optimal spacing between the tray and the tissues that are to be recorded to obtain the best results with the impression

material to be used in that tray. Custom trays should also have appropriate extensions in all directions to record the oral soft tissues.

Stock trays are produced in a variety of shapes and sizes so that the clinician selects the ‘best fit’ available for the patient. Such trays are often either too short or too long (underor overextended) in relation to the extent of the oral soft and hard tissues that need to be recorded for clinical purposes. Modification to the extension of a tray can be achieved by cutting it back (with disposable plastic trays) or by further extending the periphery using a rigid thermoplastic material like ‘green stick’ tracing compound. It may be possible to provide some support for tray extension into the vestibular sulcus using wax, but this material is too flexible to support adequately distal extension to impression trays. Inevitably, an impression material used in a stock tray will vary in thickness and hence the elastic properties of the material will not necessarily be optimal for that material. The accuracy of an impression in a stock tray depends upon the ability of an elastomeric material to perform adequately under less than optimal conditions.

Tray design must include some method for retaining the impression material attached to the tray. The methods commonly used include mechanical interlocks (holes perforating through the tray, wires attached to the base of the tray or increased bulk at the rim of the tray or ‘rim lock’) and the use of adhesives. The adhesives tend to be of the contact adhesive type, hence it is important that the adhesive layer has had adequate time to dry before the impression material is placed in the tray.

Trays designed for use with reversible hydrocolloids have an in-built water-cooling mechanism to accelerate gelation of this thermally softened material. Whilst it would be technically possible to use reversible hydrocolloids in uncooled trays, the gelation time would be far too long to make them clinically viable.

It has already been stated that trays need to be rigid to support the impression material. This can pose problems with very high viscosity impression putties and plastic stock trays.

Tissue management

When impressions are being recorded for fixed or removable prosthodontics it is important not only

to record the surfaces of the teeth with the impression but also the relationship between the teeth and the soft tissues surrounding them. For removable prosthodontics this can be achieved by careful impression technique and by smearing or syringing impression material into areas of the mouth which may be recorded badly during conventional placement of a loaded impression tray into the mouth.

With fixed prosthodontics (crown/bridgework) the preparations on the teeth commonly extend up to or beneath the gingival margin. It is necessary, therefore, to displace the gingival tissues away from the preparation prior to syringing in light bodied material around the teeth. This displacement can be achieved either by packing a retraction cord into the gingival crevice or by widening the crevice temporarily using either an electrosurgical unit or a soft tissue laser (this technique is known as troughing).

There are a wide variety of gingival retraction cords available, varying from silk suture material through custom made laid or braided cords to knitted cords. One manufacturer incorporates a thin copper filament in their cord to help it to remain in place within the gingival crevice. All are available in a range of diameters to cope with gingival crevices of varying depth and width.

The objective of placing a retraction cord is to displace the gingival tissue laterally away from the prepared tooth surface rather than apically. Consequently the packing technique needs to be directed to give lateral displacement of the cord, which is introduced in the crevice using a slim, flat-bladed instrument like a ‘flat plastic’ or using a custom designed cord packer.

There is some dispute about how many retraction cords should be placed within the crevice. Some authorities maintain that a very fine cord should be placed in the base of the crevice to help provide moisture control (the haemostatic cord) with a second layer at the coronal extent to displace the gingiva laterally (the expanding cord). The more superior cord is removed whilst the impression is recorded, whilst the fine cord is left within the crevice. One of the complications of the use of retraction cord is gingival recession exposing a restoration/tooth margin which can give aesthetic problems in the anterior portion of the mouth. The use of a dual cord technique is thought to increase the risk of recession and consequently a single large cord is advocated by other authori-

ties. Whichever approach is adopted, the expanding cord should not be left within the gingival crevice for more than 20 minutes to minimize the risk of recession. Care should be taken when removing cord from the crevice to ensure that the cord is wet. Dry cord will tend to stick to the mucosa lining the crevice and tear this away if it is removed, causing gingival bleeding.

Retraction cords are often used in association with a haemostatic agent to assist with moisture control. Some cords are impregnated with the haemostat whilst others are dipped in a haemostatic solution prior to placement. These agents include epinephrine and aluminium and ferric chlorides. Epinephrine containing cord must be used with great care as it contains large quantities of drug which can be absorbed through the gingival capillary bed and hence exert a systemic effect. This would be of particular concern in people with compromised cardiological function. They are probably best avoided. Some of the other haemostatic agents have a relatively low pH and can remove the smear layer/dissolve superficial dentine apical to the margins of the preparation. This may induce dentine sensitivity post-operatively in some teeth.

The alternative technique of troughing uses the cutting power of an electrosurgery unit or a soft tissue laser to widen the gingival crevice and produce haemostasis. The key to success with either instrument is to use a careful technique and, with the electrosurgery unit, a fine single wire cutting tip to ensure minimal soft tissue damage. There is no greater likelihood of gingival recession after troughing than in association with the use of conventional retraction cord unless the gingival tissues are particularly thin or there is an absence of attached gingiva. The electrosurgery unit is also particularly useful for removing areas of gingival overgrowth that may be present when replacing previous crowns that have had marginal deficiencies or where provisional restorations have been poorly contoured.

Impression technique

The specific technique used for each material will vary from product to product. However, there are some broad generalizations that are applicable to all. To obtain greatest accuracy the teeth need to be clean and dry. Salivary contamination is usually prevented using a combination of cotton rolls and

absorbent pads – ‘dry guards’. It has been suggested that a rubber dam can be used to provide isolation. This is only practical when an additioncured silicone rubber is not being used. The plasticizers in the rubber dam will poison the platinum catalyst in the impression material, inhibiting its set.

Material needs to be placed on both the occlusal surfaces of teeth (if present) and within the tray. If this is not done there is a tendency to entrap air within the occlusal surface of the tooth giving bubbles on the impression which will fill subsequently with the material from which the models are formed, producing inaccuracies on the occlusal surface of the prepared model. This is a particular problem when working with teeth with steep cusp angles and tortuous fissure patterns.

Obviously the impression needs to be placed in the mouth whilst the impression material is fluid and left in the mouth until it is set fully. During this time the impression tray needs to be kept as still as possible to avoid distortion of the setting material, producing distorted casts.

All manufacturers give guidance on the working and setting times of their products. These should be adhered to whenever possible to avoid removing an impression whilst it is still partially set. This is particularly important for elastomeric materials which do not fully develop their physical properties until set and hence will distort if removed too early.

Clinically it must be remembered that an impression material that is in direct contact with the lips/ tongue will set more rapidly than the bulk of the material in the tray. The material in contact with the lips will be raised fairly rapidly to mouth temperature whilst the bulk of material in the tray will remain closer to room temperature, giving differential setting. This can make a subjective assessment of the quality of set of a material difficult.

After removing the impression from the mouth it is necessary to check to see that the surfaces of the teeth are accurately represented in the impression. It is also wise to ensure that the material remains attached to the supporting impression tray. The forces required to remove the impression from the mouth can disrupt the adhesive bond between material and tray, particularly in the molar region of lower impressions. If this support is disrupted, the accuracy of the impression will be reduced, with the impression material recording the molar being lifted away from the tray,

distorting the occlusal plane and potentially giving grossly inaccurate occlusal contacts.

Impression material which extends beyond the tray distally will be unsupported and should be trimmed back to the extent of the impression tray. If this is not done, there may be problems with distortion of this distal extent of the model produced by the weight of the model-making material (commonly dental plaster or stone).

Cross-infection control

It is important that impressions are disinfected prior to their being sent to the laboratory for the manufacture of models and/or prostheses/appliances, to protect the laboratory staff from the transmission of infection from patients. Equally, work leaving the laboratory also ought to be disinfected to prevent transmission of organisms from the laboratory to the patient. The disinfection of impressions poses particular problems as some materials are water based (dental plaster and the hydrocolloids), some are subject to water uptake and/or loss if exposed to inappropriate conditions (hydrocolloids and poly-ethers) and the quality of the surface of gypsum-based die materials can be affected by the surface disinfectants used to prevent cross-infection.

The first stage in prevention of transmission from the patient to the technician is carefully to rinse the surface of the impression to remove gross contamination with blood or saliva. Current recommendations in the UK would then suggest that all impressions should be further disinfected by immersing them in sodium hypochlorite solution at a concentration of 10 000 parts per million available chlorine for 10 minutes. (Diluted household bleach at either 1 : 5 or 1 : 10 will achieve this, depending on whether the original solution was 5% or 10%.) The bleach should be washed off the surface of the casts and then models poured as soon as possible. This immersion time is sufficient to inactivate most oral micro-organisms (including human immunodeficiency virus (HIV) and hepatitis B virus (HBV)), whilst at the same time avoiding distortion of impressions. A similar regime can be used for the disinfection of the various trial stages during denture construction after they have been in the mouth. Aerosol-based disinfection processes are not satisfactory as a result of the potential to miss some areas of the impression with the spray and also the health and

safety risk of using powerful disinfectants in an aerosol form. Glutaraldehyde and sodium dichloroisocyanate solutions have also been advocated, but pose a greater health and safety risk to personnel in use than hypochlorite. The latter solution should be stored in air-tight containers and changed at least daily (more frequently with high rates of use) to maintain its disinfectant properties. Recently developed disinfectants such as sodium peroxymonosulphate (which is active in a 2% solution against bacteria, fungi and viruses) are likely to overcome many of the handling and storage problems of sodium hypochlorite. This product is effective with a 10 minute immersion period and can be used with the full range of dental impression materials, with the exception of the reversible hydrocolloids.

When managing patients who pose an established risk of spread of infection to laboratory personnel then greater efforts need to be made to sterilize rather than to disinfect impressions. Such patients would include those who have AIDS or those who are sero positive for HIV or HBV. Cold sterilization of impressions involves their immersion in 2% glutaraldehyde under acidic, neutral or alkaline conditions for 10 hours or with a phenolic buffer for 6.75 hours. This sterilization regime determines the impression materials that can be used for this group of subjects as the silicone rubbers are the only group of materials which shows adequate dimensional stability under these conditions. Despite careful rinsing there still tends to be some deleterious effects on the surface finish of gypsum-based model materials, giving softened powdery surfaces. When making dentures for such patients it is necessary to produce multiple copies of the working models as these will become contaminated with oral washings from the surface of the various stages of denture construction at registration and try in. It is not possible to sterilize a gypsum-based model and hence these need to be discarded after each clinical stage with the laboratory work being sterilized before being returned to the laboratory.

Commonly national dental organisations publish and regularly update guidelines on cross infection control. An example from the British Dental. Association is reproduced, with permission, as Appendix 1. The reader is advised to consult such guidance for country-specific and contemporaneous advice.