Advanced training part II/05

Impressions as the basis for a long-term prosthetic restoration

1. November 2005

Univ.-Prof. Dr. med. dent. Bernd Wöstmann,

Geboren 1961, 1985 Staatsexamen, 1986 bis 1995 Wissenschaftlicher Mitarbeiter, dann Oberarzt der Poliklinik für Zahnärztliche Prothetik A der Westfälischen-Wilhelms Universität Münster,
1993 Habilitation, 1995 Hochschuldozentur für Zahnärztliche Prothetik an die Justus-Liebig-Universität in Gießen, 1998 apl. Professor, seit 2002 Univ.-Professor für Klinische Werkstoffkunde und Gerostomatologie in Gießen. Leiter der Forschungsstelle für Gerostomatologie in Gießen, 2. Vizepräsident des Dachverbandes der Gerontologischen und Geriatrischen Wissenschaftlichen Gesellschaften Deutschlands (DVGG), Boardmember des European College of Gerodontology, 1999 Friedrich-Hartmut-Dost Preis für besondere Verdienste in der akademischen Lehre.


Abstract

Despite rapid technical advances in CAD-CAM systems, impressions still have a very important role in restorative dentistry. Expressed in the terminology of modern information technology, impressions will probably still continue to
Fig. 1 Correct dimensions of the lumen of the crown when fabricating a cast crown
be the information carrier and information medium from the dental practice to the dental laboratory for the foreseeable future, as the majority of CAD-CAM systems currently available use a model in the initial stages and therefore require an impression.

Though there have been various improvements in impression materials in recent years, the prepared tooth still cannot be perfectly reproduced on the model using any of the materials and methods currently available, with the result that there is a fine line between the lumen of the crown in the impression being too large or too small. Consequently there is always a degree of unavoidable "marginal error" in the restoration.

The average marginal accuracy in dental restorations of approx. 50 µm attainable in laboratory tests is still only achieved clinically in exceptional cases. This applies mainly to restorations with infragingival or paragingival margins. If the crown margin is completely supragingival, a degree of accuracy can be attained that is comparable with that of laboratory tests, as impressively demonstrated in the study by Kern M. et al. [6].
Fig. 2 As a first approximation the distribution of the dimensions of the lumen of the crown reproduced in the impression can be regarded as normal (above). This also applies to the working model (below). (e) Acceptable range for the lumen of the crown. See text for explanation.
Biological tolerance limits for marginal discrepancies, however, are still virtually an unknown quantity [5], though the relationship between marginal inaccuracies (in particular with regard to overhanging and projecting margins) and periodontal damage has been adequately proven [3,11]. The weak point in any restoration is therefore still the marginal area. Unless proved otherwise by clinical research, it must be assumed that an increase in marginal inaccuracy will result in an increased risk of caries and periodontal pathogens.

Only factors resulting directly from patient treatment can be primarily responsible for this considerable discrepancy between what is technically possible and what is clinically attainable. Otherwise similar results would be recorded in laboratory tests. Factors relating to the working procedure have an important effect on the accuracy of fit of a restoration, though the effect of these factors has till now been more or less disregarded.


Factors relating to the working procedure
Fabrication of a fixed restoration involves numerous treatment and laboratory stages. As the lumen of the crown has to be slightly larger than the prepared tooth to allow space for cementing the restoration, it would be practical if the lumen of the crown could be continuously enlarged during the individual working stages (Fig. 1).
Fig. 3 Same situation as in Fig. 2, only with greatly reduced distribution range.
This is not possible, however, because of the materials involved. It is also not necessary, as any dimensional "error" in the impression can be compensated for in the dental laboratory by applying spacer, adjusting the setting expansion of the investment etc. A defined, and above all reproducible, impression of the lumen of the crown and consequently an accurate fit of cast restorations can only be attained by systematically standardizing the working procedure, from preparing the tooth to fitting the final cast restoration. The working procedure is described below.

Simplifying it to some extent, it is assumed in Fig. 2 that the dimensions of the lumen reproduced in the impression corresponds to a normal distribution curve around the actual lumen of the prepared tooth. The "width" of this distribution depends on the consistency, or rather inconsistency, of the ambient conditions when taking the impression. The lumen reproduced in the impression will differ depending on the type and temperature of impression material selected, the impression technique used, the choice of impression tray, the ambient temperature etc. The problem is that it is not easy to judge the dimensions of the actual negative reproduced in the impression.

The impression is then passed on to the dental laboratory where the model is fabricated. If the impression reproduces a lumen that is too small ( Fig. 2 a), this will tend to produce a model in the laboratory that is too small and vice versa ( Fig. 2 c).

Model fabrication in the laboratory also has a crucial effect on the shape of the subsequent model die depending on the ambient conditions. As a first approximation there is again a normal distribution curve ( Fig. 2 bottom).
Fig. 4 Same situation as in Fig. 3. The working procedure in the practice and laboratory has also been coordinated
The position of the curve maximum no longer relates, however, to the original tooth in the oral cavity but to the lumen of the impression supplied! The maximum of the distribution curve, which represents the size of tooth die, varies depending on the frequency given by the primary normal distribution curve ( Fig. 2 a-c). Using a purely mathematical statistical approach this produces a very broad distribution function overall ( Fig. 2 bottom red curve). As the subsequent working stages in the laboratory (waxing up, investing, casting) can generally be described as following the same pattern, it is not surprising that the overall distribution widens with each additional working stage. The entire problem now becomes clear: with each additional working stage there is a greater distribution of results. This distribution can never be reversed! It is a widespread misconception that one fault can be "compensated" for by another. Meiners [10] had already made a reference to this fact in 1985.
Fig. 5 Electrosurgical exposure of the preparation margin
 
Fig. 6 Application of Expasyl
 
Fig. 7 Expasyl dries the gingival sulcus very well, but seldom fully exposes the preparation margin
 
Fig. 8 Peridenta retraction sleeve in situ
To ensure that the crown can be cemented later, the lumen of the crown should be a little larger than the prepared original tooth so that there is sufficient space for the layer of cement ( Fig. 2 d). With the distribution assumed in Fig. 2, most of the model dies would also inevitably produce unacceptable crowns - assuming that the "acceptable range" for the size of the lumen of the crown is characterized by area (e) in Fig. 2. From a purely theoretically scientific and technical point of view there is only one way to attain improved results and avoid failures: the scatter range of the distribution has to be reduced. This allows results to be reproduced more easily and failures, which can be regarded as extreme values of the normal distribution, to be avoided (Fig. 3). From a purely practical point of view, the scatter range can be reduced by standardizing the working procedures, beginning with the selection of the impression technique for a specific situation to finishing the "impression taking" treatment stage with the patient.

A definite reduction in the scatter range of results can, however, only be obtained by standardizing procedures both in the dental practice and dental laboratory, as the working stage with the widest scatter range essentially determines the overall scatter range (the scatter range always increases with each working stage and never decreases again, see above!). Optimum results can only be attained if the working procedures in the dental practice and laboratory are coordinated (Fig. 4).


Clinical factors
Preparing the impression site
The success of an impression depends on perfect reproduction of the prepared teeth so that preparation margins can be clearly defined on the model in the laboratory. Naturally an impression can only be taken of areas that are accessible. With supragingival preparations it is generally very easy to see and dry the area to be reproduced in the impression [6,7]. In the case of infragingival preparation margins, the preparation margin has to be placed supragingivally using surgical measures (electrosurgery, laser surgery, Fig. 5) or the gingival sulcus retracted with a retraction cord. The retraction cord should preferably be preimpregnated with a vasoconstringent. Astringent retraction liquids - particularly metal salt based liquids - interact with conventional polyethers (Impregum, 3M ESPE, but not with P2, Heraeus Kulzer) and some A-silicones and could inhibit their setting. These should therefore be avoided. A retracted gingival sulcus can be dried effectively for a long period using Expasyl, Pierre Roland (Fig. 6 and 7). Contrary to the claim of the manufacturer, the material does not, however, produce adequate gingival sulcus retraction according to our tests. With single teeth the use of a retraction sleeve (Peridenta) for retracting the gingival sulcus is an excellent alternative to using a retraction cord (Fig. 8).

Waiting time between preparation and taking the impression
If the gingival sulcus cannot be completely and effectively dried after preparation, at least one week should be waited before taking the impression to allow full recovery of the periodonteum, which is virtually always damaged during infragingival preparation of the tooth by abrasive rotary instruments. Taking an impression in the days immediately following preparation should be avoided, as it would inevitably lead to traumatization of the existing granulation tissue accompanied by bleeding that is generally very difficult to staunch. The periodonteum is fully healed after about a week and an impression can then generally be taken without any problem.

Anesthetic
Impressions taken under local anesthetic are generally much more successful than those taken without the use of anesthetic [15]. If anesthetic is not used, it is usually impossible to prevent the patient experiencing pain during impression taking. Placing retraction cords and drying prepared abutments are often the main causes of pain. The reaction of the patient to the pain often leads to the retraction cord not being correctly placed or the teeth not being properly dried. In either case, the result is generally a relatively poor quality impression. The majority of anesthetics also contain vasoconstringent additives that cause a lack of blood in the anesthetized region and counteract gingival sulcus bleeding, producing favorable conditions for a successful impression.

Condition of the periodonteum and oral hygiene
The condition of the periodonteum and oral hygiene of the patient also affect impression taking, as a periodontally diseased tooth increases the likelihood of periodontal bleeding. As periodontal disease and associated gingival sulcus bleeding are directly related to the oral hygiene of the patient, ensuring good oral hygiene prior to prosthetic treatment is also extremely important with regard to impression taking. The poorer the oral hygiene, the greater the likelihood is of failure.
Fig. 9 Semi-customizable, autoclavable carbon fiber trays (Clan BV)



Material factors
Choice of impression tray
Metal stock trays are the type of impression tray recommended. Alternatively autoclavable carbon fiber trays (Clan BV) can be used. These are almost as stable as steel trays and are intended to replace semi-customizable Schreinemakers trays for fully dentulous jaws, which are unfortunately no longer available (Fig. 9). The use of flexible trays (even customized trays!) in combination with putty materials is obsolete, as these trays bend when the impression is inserted and the impression material sets in this condition. After removal of the impression from the mouth, this inevitably results in distortions of the whole negative that cannot be checked. Customized trays should therefore only be used with heavy body or monophase materials.

All types of impression tray should be coated with a thin layer of suitable adhesive before use. The adhesive should be compatible with the type of impression material used. Use of an appropriate adhesive solvent (e.g. Traypurol, Voco) makes cleaning the tray much easier.


Impression materials
Nowadays elastomer materials are mainly used for precision impression taking. Polysulfides are not used in Germany, which limits the range of materials to silicones and polyethers. As polyethers are very difficult to cut, they are unsuitable for use with the two-step putty wash or one-step putty wash techniques. Their intrinsic stickiness makes them ideal for pick-up impressions of primary units or transfer copings. There is a problem with disinfecting conventional polyethers (see above), as they tend to absorb water and swell when stored in water-based disinfectants. Though immersing the impression for a short time in a suitable disinfectant solution is acceptable, disinfecting it over a period of several hours should definitely be avoided [1,13].

Silicones are the most universally used impression materials. They are suitable both for taking impressions of prepared teeth and reproducing mucosal areas and can be disinfected without any problem.

Two different types of silicone are used in dentistry as impression materials. These are condensation-cured and addition-cured silicones that differ basically in their curing reaction. With C-silicones the inevitable evaporation of alcohol following polycondensation causes shrinkage of the C-silicone. Impressions taken with modern C-silicones can, however, be stored overnight without any problem [18]. The recommended method for preventing alcohol evaporation is to place the impression in a sealed bag (e.g. freezer bag) together with a cotton roll soaked in alcohol. Polyvinylsiloxanes (PVS), on the other hand, can be stored for an unlimited period. The biggest disadvantage of PVS is their intrinsic hydrophobic properties, though these have now been greatly reduced due to surface active additives.

PVS and polyethers are now usually supplied in cartridge systems. Impression materials are also available in foil bags for use with automatic mixing units (Pentamix, 3M Espe; Starmix, DMG; Plug&Press Dispenser, Kettenbach). These mixing systems guarantee homogenous, thorough mixing of the materials and produce extremely reproducible results. Use of these automatic mixing aids is strongly recommended.


Impression technique
The quality of an impression using conventional methods depends not only on the properties of the impression material used but also very much on the suitability of the impression technique.


Two-step impression techniques
When using two-step putty wash or one-step putty wash impressions it is very important to remove all undercuts and interdental septa to ensure a successful impression. If the undercuts are not removed, it inevitably leads to considerable displacement of the primary impression material and consequently errors in the impression! It is easier to cut out the undercuts if the putty impression is taken over a foil. Holes should be cut into the foil in the area of the prepared teeth to avoid flow folds. The main advantage of the two-step putty wash technique is that it provides excellent reproduction of the infragingival structures (Fig. 10 and 11), which is only attainable by a reduction in the accuracy of the geometrical reproduction of the prepared tooth due to the unavoidable effects of compression [4].
Fig. 10 Two-step putty wash impression of an infragingival preparation margin
 
Fig. 11 Similar situation as in Fig. 10 but a one-step putty wash impression. Note the much poorer reproduction of the preparation margin
 
Fig. 12 The customized tray should have "chimney-shaped" perforations for the open impression technique



One-step impression techniques
A one-step impression technique can be used to avoid the effects of compression caused by the two-step impression technique. In contrast to the two-step putty wash technique, the one-step technique produces more accurate impressions [2,8,9]. Clinically these techniques are not entirely suitable for reproducing infragingival preparation margins, as they do not produce the necessary pressure for compressing the material into the gingival sulcus. There is also always the possibility of bubbles forming in the impression (Fig. 11).

In principle the heavy body wash technique is also a one-step impression technique that uses a heavy body material instead of putty as the high viscosity component. Monophase materials can also be used. Use of a customized tray is recommended so that the low viscosity material can flow into small gaps and prevent the formation of flow folds. Highly accurate impressions can be attained using the one-step technique in conjunction with a customized tray and automatically mixed PVS or polyether [16,17].


Hydrocolloid impression technique
This is also a two-step technique that uses a reversible thermoplastic hydrocolloid as the impression material. The disadvantage of this technique compared with similar elastomer techniques is the relatively high expenditure on equipment. The high precision, which is a feature of hydrocolloid impressions, when the technique is correctly applied, and which is comparable to the accuracy attained by A-silicones and polyethers [14], is probably largely due to the high degree of standardization in the working procedure, which is an integral part of the technique. The main limitation of the hydrocolloid technique is the difficulty in reproducing infragingival areas accurately [12].


Types of impressions
The impression techniques described above illustrate the many different influencing factors on the quality of the finished impression during impression taking. As all the techniques and materials described have both advantages and disadvantages, it is advisable to select a technique that is suitable for the actual clinical situation (Tab. 1).


Partial crowns
Partial crown and inlay preparations are the most difficult shapes to reproduce. An exact fit of the casting is required as these preparations generally have complex geometrical shapes. A one-step technique with an addition-cured silicone or polyether is therefore recommended in this case. The two-step putty wash technique can only be recommended in certain situations for the fabrication of partial crowns because of its intrinsic compressive effect. Careful cutting of the impression is too time-consuming and often very difficult, as there are often undercuts in the prepared tooth.


Single crowns
The two-step putty wash impression technique is recommended for taking an impression of a tooth prepared for a single crown. The compressive pressure of the wash impression normally effectively forces the low viscosity impression material into the gingival sulcus. In comparison, with the sandwich or one-step putty wash impression technique there is an increased risk of flow folds. In theory the one-step technique can be used, but it is more time-consuming because it requires a customized tray. Fabrication of a customized tray is also extremely difficult, as all the undercuts have to be carefully blocked out.


Standard bridges
There is basically no difference between taking an impression of a single crown and taking an impression of the abutment teeth for fabricating a bridge. It is essential that C-silicones are cooled when used in cases of large restorations with several abutments so that the material does not cure too quickly, increasing the formation of endogenic stresses. The use of PVS is recommended in this case. If all, or virtually all, existing teeth are prepared, a one-step impression can be taken, as there are no undercuts on unprepared teeth that would make it difficult to fabricate a customized tray.


Adhesive bridges
There are no problems taking an impression for fitting an adhesive bridge, as the preparation margins of the prepared teeth are supragingival. A one-step or hydrocolloid impression is recommended to attain optimum accuracy
Großansicht
Tab. 1: Recommended indication of impression techniques for different clinical situations with prepared abutment teeth * = recommended, (*) = possible. 1 A heavy body wash impres-sion with silicone is more accurate!
and also ensure that the teeth are not pushed out of the rest position. It is also possible to take the impression using the one-step putty wash or sandwich technique, as long as the viscosity of the heavy body component is not too high.


Implants
There are other problems to be considered when taking an impression for fabricating an implant-borne restoration compared with taking an impression of prepared teeth. It is no longer necessary to reproduce an exact impression of the implant surface and margin in the same way as the preparation margin of a natural tooth, as the majority of modern implant systems use prefabricated precision components. As implants are osseointegrated and, unlike natural teeth, do not have any natural movement at all, it is especially important to fix the exact three-dimensional position of the implants and transfer this precisely to the working model when taking an impression of several implants. The pick-up technique using A-silicone or polyether and an impression tray with "chimney-shaped" perforations is recommended for taking implant impressions (Fig. 12).


Conclusion
Finally, the question is what further developments can be expected in impression taking. In future, high-tech procedures combined with "optical" impressions might be increasingly developed and improved, but this will not solve existing clinical problems. As all the impression techniques currently available only reproduce areas that are accessible, it does not matter whether an impression material or a video camera is used as the means of reproduction. This is basically the problem clinically with regard to impressions taking: accessible, visible areas can be reproduced without any real difficulty and the subsequent working and material chain is accurate enough to produce acceptable castings. Standard impression materials and techniques currently available generally produce excellent results. In order to exploit fully the options provided by these materials and techniques, it is now essential to focus more on improving clinical and procedural parameters that impair or limit the standard of quality attainable by the materials.

Prof. Dr. Bernd Wöstmann
Department of Prosthodontics - Dental Clinic
Justus-Liebig-University Giessen
35392 Gießen
Phone ++49 641 99-46143 or -46150
Fax ++49 641 99-46139
bernd.woestmann@dentist.med.uni-giessen.de

Bibliography
Interactive zm-Advanced-Training:
Thus it is done!

To the questionnaire


zm 95, No. 21, Nov. 1st. 2006, Page 32-39