The dental impression

The dental impression at the dentist’s

For many patients the dental impression is an uncomfortable affair but perhaps after reading this text you will understand better why the individual steps are so important. A possible remedy to this method could be digital dental impressions.  Whilst these are ready for the market, the machines are still extremely expensive and one mustn't forget: at the end of the day it's you who pays for all this investment. 

Impression in dentistry, is a medical term for a dental impression, which is a negative copy of the patient’s mouth and is made with an elastic impression material. Impression trays are used to carry impression materials for taking dental impressions. The impression tray is filled with plaster in order to obtain an accurate model of the patient’s mouth. The dental prosthesis is subsequently fabricated on the basis of this plaster model. The quality of any dental prosthesis, including factors such as marginal tightness, exact fit, and height, depends on the quality of the impression. So the dental impression is an important step in the creation of your replacement teeth - your dentist will certainly not bother you with this.

There are two types of impression trays: prefabricated and individualized.

Prefabricated trays are factory produced impression trays made of metal or plastic. They are available in a variety of different shapes (for toothed, partially toothed, and toothless jaws).

Individualized trays are custom made by the dental technician according to the patient’s jaw. In order to do this, a preimpression is initially taken with a prefabricated tray. This provides the technician with a rough overview of the jaw, from which the individual tray can be fabricated. These trays are useful in cases of unusual jaw anatomy, such as large or small jaws, or after tumor surgeries.

Partial trays are often used for provisional prosthesis. Why doesn't the dentist use a partial tray - wouldn't it be more comfortable for you? Impressions should always be taken with full impression trays. As a rule, partial trays are not precise because then the position of the plaster models cannot be precisely aligned with one another. With a partial tray, the dental technician obtains partial models; these can be swiveled and thus improperly positioned, leading to the fabrication of an ill-fitting dental prosthesis. A full impression yields complete jaw models, thus forming a so-called three-point contact. Moreover, the models cannot be swiveled and the technician can fabricate an exact-fitting dental prosthesis.

In order to obtain precise impression margins, the impression material must flow freely around the tooth margins. This flow should not be impeded by gums, blood, or saliva immediately surrounding the tooth. In order to achieve this, any bleeding must be stopped with various methods (eg., cautery) and the gums must be adequately deflected from the tooth surface. The latter may be achieved with a gingival retraction cord. Only then will precise impression margins be obtained.

Gingival retraction Cords are a dental aid used to retract and keep the gums (gingiva) away from the tooth during a dental procedure, particularly crown preparation. Gingival retraction Cords are available as braided or twisted cotton threads of various thicknesses. The thread is usually impregnated or soaked in a substance that can constrict blood vessels (adrenalin) or stop bleeding (aluminium chloride, aluminium sulfate, aluminium hexachloride, or aluminium hexahydrate).

The crown margins are ideally placed in the sulcus (space between the gums and the tooth), the risk of secondary caries is there negligible, and the junction between the crown and root is not visible. In order to obtain a precise tooth margin in an impression and avoid an ill-fitting crown with visible margins, Gingival retraction Cords are placed around a tooth into the sulcus. The gums are pulled aside by means of the cord. Without this, the gums would be injured during tooth grinding process.

Immediately after grinding, a second cord is inserted; the reason for inserting a second set of sutures is described as follows. By inserting a second set of sutures, the gums are pulled back once again. By removing this second set of sutures immediately before taking the impression, the impression material will mirror the margins accurately. The technician fills this impression with plaster and creates a negtaive replica of the teeth. For this, the margins of the ground tooth have to be clearly visible to the technician on the model, which requires the impression material to flow all around this region when the impression is being made.

As soon as the impression is taken, the Gingival retraction Cord is removed so that the gums again make contact with the tooth. Retraction pastes or ointments are often used as substitutes for sutures. These are usually injected into the gums, perform the same functions as the sutures, and can be rinsed away just before the impression is made.

Why does the dentist sometimes take 2 impressions of the same jaw? A preimpression prevents teeth from touching the impression tray. This event is termed push through. If the metallic tray is exposed through the impression material because of push through, the impression must be repeated - notice the two arrows marking the areas where the teeth have pushed through. In the event of push through, the respective tooth is slightly deflected, resulting in an inaccurate impression. The dental technician will then inevitably fabricate an ill-fitting dental prosthesis; a long cutting process and/or leaking crown margins would be the consequence.

After the preliminary work has been completed, the actual impression is taken. The impression material is placed at exactly the right spots with a fine syringe, following which it is appropriately distributed with an air blower. The individualized impression tray, which is loaded with impression material, is then inserted into the mouth. It is imperative that the impression remains in the mouth for as long as the manufacturer has specified. That is why a lot of dentists use a stopwatch. Finally, the impression is removed from the patient’s mouth, inspected with medical magnifying glasses, properly packaged, and sent to the dental technician. After an appropriate period of time, the dental technician will pour plaster into the impression. If this period of time is not observed, the impression will distort, again resulting in the fabrication of an ill-fitting dental prosthesis!

The requirements for impression materials are good fluidity and moisture tolerance. This allows for exact replication of the required structures. Well-defined borders are important for achieving a precise fit. Furthermore, the material needs to harden quickly at a defined time point, but must remain malleable long enough to be worked on. Most materials are made up of two components that are mixed together immediately before usage, following which they begin to harden. In the past self-mixing materials activated by water were used for cost-saving reasons. It is almost impossible to achieve the correct concentrations while using this material. Inaccurate concentrations can cause distortion of the impression, ultimately resulting in the fabrication of a leaking prosthesis. Good quality impressions are achieved with preproduced cartridges. Although material usage is higher, the materials are mixed at specifically defined rates. The technician obtains a precise impression and the patient receives a tight, well-fitting prosthesis.

However, correct mixing and usage of appropriate materials are not the only important factors: packaging and transport of the impression is just as important, as is the amount of time the impression spends in the patient’s mouth. All of these steps should be carried out in accordance with the manufacturer’s specifications in order to avoid any distortions. Long transportation times (e.g., to a technician in a distant country) will rarely result in high quality work.

After the Imprint is taken, usually the dentist fabricates a temporary Prostheses. A temporary prosthesis protects a tooth after it has been prepared for a crown. With the introduction of the temporary prosthesis, no patient needs to leave the dental clinic with missing teeth. A temporary prosthesis can be fabricated and fixed in the patient’s mouth directly by the dentist, or it could be fabricated in a dental laboratory by the technician and fixed by the dentist. There are many types of prostheses;

There is a broad range of further possible treatments. The so-called instant prosthesis, is used as a quick replacement after a series of extractions or the loss of several teeth. The instant prosthesis is fixed to the neighbouring teeth by means of wires. However, because these wires may damage the neighbouring teeth, instant prostheses are only worn for short periods of time. Temporary prostheses can also come in very complicated designs.

One of the most used imprint materials in dentistry is Alginate. Salts of alginic acid are called alginates. Alginate is mixed with water to a certain consistency, and is then used to take impressions of the teeth and jaws. It has no harmful side effects and allows for a high level of precision and attention to detail in impressions. You can see how the powder is mixed by hand, alternatively, special alginate mixers can be used. Accurate processing is the key when using alginates. After removal from the mouth, the impression should rest for a few minutes, following which it should be thoroughly cleaned and immediately processed by pouring plaster into it. While pouring, a vibrating device is used in order to eliminate air bubbles from the plaster model. Finally, the impression is placed on a level surface for the remainder of the hardening process. When the plaster has completely hardened, the impression tray, including the alginate, is separated from the plaster model. This is how the technician gets an impression of the patient’s jaw in order to fabricate an accurate prosthesis.

There are a lot special Imprint Procedure, for example at Fabrication of a Root Post. Fabrication of a root post is preceded by making a special impression of the tooth, particularly one that is drilled for a root canal procedure. If a tooth has been destroyed by severe caries (more about this in the video caries) and there is hardly anything left of the crown, fixing a conventional prosthesis is no longer possible because the crown will not hold because of insufficient retention.
The solution to this problem is the placement of a post in the filled root canal. After the root canals are filled, the fillings in one or more canals are excavated (removed) up to a certain length. Depending on the type used, the post is either cemented immediately into the canal (known as a prefabricated post) or an impression of the tooth is taken, using which the dental technician will create a plaster model (negative replica of teeth) and fabricate the post (custom-made post).
This post will be inserted a week later. With the help of the black pins, the position and shape of the excavation will be transferred, thus aiding in post fabrication. In this way, the technician can produce individual metal posts. A week later, the posts will be cemented in place and the appropriate corresponding elements will be inserted in the prosthesis; this largely helps in maintaning stability and retention of the prosthesis.

Another special imprint technique is the Rerelining of a Prostheses. Relining is dental jargon referring to the relining of an ill-fitting prosthesis. A prosthesis sitting on the mucous membrane accelerates jaw bone resorption. Therefore, the prosthesis has to be adjusted after a while. It is important that the jaw bone is adequately stressed, if not, the bone begins to resorb rapidly and/or pressure points can develop. By relining the prosthesis, the loss of jaw bone is compensated by a plastic compound and partial overloading of the mucosa is eliminated.

First, impression material is applied onto the base of the prosthesis. While the compound is hardening, the dentist keeps pulling on the patient’s lips; furthermore, the patient uses his mimic musculature, swallows, and moves his tongue. This is done so that the dental technician can clearly recognize the borders of important anatomical structures (e.g., the tongue and labial frenula) on the impression. Then, the prosthesis is taken out of the mouth and filled with plaster. This provides the dental technician with an exact replica of the patient’s jaw. Before the old dental prosthesis can be lined with the aid of the plaster model, the plaster model must be processed. Using special silicone, the old dental prosthesis (along with the plaster model) is placed on a special fixture. The actual relining will subsequently be executed on this fixture. As soon as all the required parts have been adjusted properly, the dental prosthesis is removed from the plaster model. In the next step, the technician uses a pencil to trace important anatomical features on the plaster model. Individual details are reworked with a special plaster knife. Now the impression material is removed from the base of the prosthesis, and the base is roughened up. After the plaster model has been isolated with a special lacquer to facilitate easy separation of the new prosthesis, the resin compound is mixed and applied to both the plaster model and the dental prosthesis. Finally, the technician places the prosthesis in the fixture and allows the compound to cure under heat and pressure. After curing, the prosthesis is retrieved from the plaster stump and polished and finished. The dentist then checks the fit of the prosthesis in the patient’s mouth, and checks whether the bite is accurate. This explains the procedure of relining a prosthesis.

Despite precise work it may be possible that impression flaws occur and the dental impression will be repeated.  What are impression flaws?

Impression flaws is an umbrella term referring to all unwanted complications that may arise while and after taking dental impressions. One of the most common errors in dental impressions is the presence of air bubbles. By using a fine syringe most air bubbles are avoided. Distributing the impression material with the aid of an air blower further decreases the presence of air bubbles. Another important issue is well-defined margins, which are created during tooth cutting for crown placement. In many cases, blood, saliva, or the gums themselves obscure the preparation margins. Thus, an impression taken without the appropriate preliminary work and/or inadequate coverage of the preparation margins by the impression material will result in a dental impression with missing borders. That’s why dentists prepare the cut tooth carefully before taking the impression.

Furthermore, distortion of the impression can also result in an ill-fitting prosthesis.  This can happen if the material has exceeded its shelf-life, if the processing times are not observed, or if the impression material is handled incorrectly. In this example, because of the occlusion of the teeth, the impression of the upper jaw should be pulled from the opposite side of the cut teeth first (shown in blue). This way, the impression is pulled off in a longitudinal direction from the cut tooth and is less likely to become deformed. The teeth in the lower jaw are leaning inwards; therefore, the impression in the lower jaw should be prepared and pulled from the side of the cut teeth first. If the impression tray is removed on the wrong side, the impression material is more likely to become distorted. Your dentist takes such details into consideration to prevent this from happening.

Maybe now you can understand better why your dentist puts in so much effort in order to obtain a perfect dental impression - it's for your replacement teeth! A possible remedy to this method could be digital dental impressions.  Whilst these are ready for the market, the machines are still extremely expensive and one mustn't forget: at the end of the day it's you who pays for all this investment.