Prosthodontics/Impressions

This is a learning resource about how to make impressions, for information regarding impression materials, please review the Wikipedia article on dental impression materials.

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Impressions are one of the most important stages in the construction of dentures. The dentures are made in the laboratory, and the technician/denturist has no physical access to the patient. The dentures are made to fit the study model. It is easy to see how a denture made on a study model which is not representative of the morphology of the patient's denture bearing area will have problems.

This page will divide impressions into preliminary/first impressions and second impressions, as is taught in dental schools since this method is more accurate. However, it should be noted that in general practice most dentists only bother with one impression because it saves time. Most of the time this will give acceptable results, as long as the technique is good.

First impressions

If 2 impressions are being carried out, first impressions are often not considered important, but the quality of the special tray is dependent upon the first impression.

Prepare patient

• Explain what is going to happen in terms they will understand ("take a mould/cast of your mouth to build the denture on"). Impression materials can be likened to plaster of Paris.
• Reassure patient that material will be in the mouth for only a few minutes, and they can breathe through their nose throughout.
• Make sure the patient is wearing a bib as some impression materials will leave permanent marks on patients' clothing.
• Get the chair into the correct position. Impression are taken with the clinician standing up and the patient sitting upright in the dental chair. Do not make impressions with the patient lying flat as it will cause more material to run to the back of the mouth and hence more stimulation of the gag reflex. The orofacial musculature is also slightly effected by lying flat or with the head held upright. Finally, it is easier for the clinician to position the loaded trays in the mouth if they are standing up.
• Lower impressions are done first as they are less likely to cause gagging. Get the easy one done first and it will be easier for the patient, instead of making them worry that the second one will be just as bad.
• For lower impressions, raise the chair so the mouth is at a level somewhere between elbow and shoulder height.
• For upper impressions, stand behind the patient and lower the chair to elbow height

Select stock tray

The square ("box") trays are for the partially dentate and the rounded trays are for the edentulous. Metal trays can be disinfected and re-used, plastic trays are single-use. First, have a quick look in the patient's mouth to get a general idea of their size and shape of their dental arches. Next, try in the stock tray without any impression material. It is better not to have any adhesive on the empty tray until you are sure that is the one you will use and it has been modified as necessary (see later). Trays are not stuffed in the mouth, as this can painfully stretch the lips. Strangely, some people are relatively large dental arches when compared to the size of their oral orifice. Instead, trays are carefully inserted one side in first to about halfway, then the corner of the mouth is gently retracted with a finger, and then the other side gently rotated into the mouth.

Modify stock tray

• Stock trays, especially lower stock trays, invariably require modification. Greenstick or wax can be used to modify the extension of tray. In the lower, it is vitally important to extend the tray distolingually, as frequently this area is left out of impressions. A lower denture which is extended distolingually in the lower ridge is the difference between a stable denture and a poor denture which moves constantly, especially in persons which a resorbed lower ridge. The objective is to get an impression which accurately records the full depth and width of the sulci. If the stock tray does not fully extend into a sulcus, it may be missed off the impression.
• In partially dental persons, the trays can be modified in the regions of the tray which will sit over the edentulous areas.
• In persons with odd-shaped dental arches, you may need to think on your feet. E.g. a short but wide arch, a large size tray can have the distal parts of the arches cut off with a bur, so it is wide but not too long that it sticks into the back of the mouth.

Choose impression material

• Most people use alginate for first impressions since it is cheap. Impression compound also tends to give overextended sulcus depth, and can be improved with a wash impression in alginate. If you will not be carrying out second impressions, you may want to use another material. After choosing the impression material, apply the corresponding adhesive to the tray (often done by the nurse while dentist talks to patient).

Mix impression material and apply to tray

• The mixing is usually done by a nurse, and is a skilled job that comes with experience. Make sure the older nurses impart their wisdom to any newer ones. With alginate, follow the ratio of powder to water recommended by the manufacturer. Note that water and room temperature have an impact on how quickly it will set. You will have to work faster in very warm weather as it is already setting before the tray reaches the mouth. Alginate is mixed vigorously and quickly, using the mixing spatula to compress the powder into the liquid against the side of the bowl. After a few moments, all the lose water will be gone and the mixing bowl can be tilted virtually on its side to facilitate more vigorous mixing.
• Some dentists have the nurse apply the impression material to the trays. This is fine as long as they know how you want it done. All trays should be filled level. Any heaping of impression material, especially in the upper, is useless and just makes the impression more uncomfortable for the patient.

Insert and position tray

• At this stage, you may want to "pre-load" some impression material into the sulci or palate. If you are repeating the impression because of a large air blow in the previous one, put some alginate into the area which before was missed.
• Keep some of the impression material out of the mouth to give a useful indicator of when the impression has set. Ideally, place a small bit of the material on the back of your gloved hand, as this is closer to the temperature in the mouth.
• While the nurse is mixing, ask the patient to take a sip of water and rinse out their mouth.
• Carefully but quickly insert the loaded tray as described previously. Position it over the dental arch and then firmly and slowly push it into position. Make sure it is horizontally level and the handle points out centrally. If the handle is rotated, the rest of the tray will be rotated and out of position. Do not push the tray as far as it will go, as this means you have made the teeth or alveolus contact with the tray. Aim to have impression material between the teeth/alveolus and the tray in all dimensions. Picture the tray sitting all around the denture bearing area, not resting on it. Standing up gives a much better view to see how to position the tray where you want.
• Once the tray is in position, gently move the patient's cheeks out and then down, which first releases air from the sulci and then pushes it out.
• The cheeks and the lips can be gently moved. This moves the muscles attachments and sculpts the sulci according to function.
• In lower impressions, the tongue is relaxed while the tray is being positioned. Do not try and fight the patient's muscles, they will win against the correct placement of the tray. Once the tray is in position, ask the patient to protrude the tongue for a moment.

Waiting for it to set

• While the impression material is setting, keep verbal communication with the patient to distract them and reduce gagging. Do not engage in continuous idle chatter, as this may appear like you are not paying any attention and are carelessly causing them extended discomfort. Every few moments, check the consistency of the impression material and tell them it is almost set.
• In upper impressions, the material may start to run to the back of the mouth and oropharynx and trigger the gag reflex. Impressions can even make some patients vomit. Once the upper tray is in place, the patient can carefully tilt their head forwards so no material runs backwards. However, do not let this maneuver change the position of the tray while it is setting, because you will just have to do it again. Breathing slowly and deeply though the nose, being distracted with the clinicians talk, and tilting the head forwards make upper impression much more comfortable in the vast majority of people.
• If they simply cannot tolerate an impression (rarely the case), it is worth re-assessing how well they will adapt to wearing a denture and perhaps altering the treatment plan accordingly.

Removing the impression

• Often a correctly placed impression will seem difficult to remove from the mouth. This is a good sign, but do not forcefully remove impressions as this can be painful for the patient. Lower impressions are removed by gently applying downward force to the tray handle. Rocking motions can encourage the suction to break. Upper trays are remove in the same way, but with upward force to the handle. If this approach is not working, ask the patient to make a seal with their lips around the tray handle, and have them forcefully blow out their cheeks. The increased air pressure in the mouth combined with outward movement of the cheeks tends to break the seal and then the denture can be removed as described above.
• Beware of very loose teeth during impressions. Extracting a tooth during an impression is very painful. Teeth which are more susceptible to this have virtually no bone support, but can still be very painful as there is no anesthesia and the gingiva is agonizingly torn away from the tooth with each movement the dentist makes while trying to free the tray. This is not pleasant. Identify any such teeth and consider the immediate denture approach.

Inspect the impression

First rinse off the impression with water to remove any blood or debris.

In edentulous patients, first impressions should record:

• Maxilla
  • Residual ridge, maxillary tuberosities and hamular notches
  • The full depth and width of the functional buccal and labial sulci
  • The hard palate and the anterior part of the soft palate
• Mandible
  • Residual ridge and retromolar pads
  • The full depth and width of the functional buccal and labial sulci, the frena and the external oblique ridges
  • The lingual sulcus, lingual frenum, mylohyoid ridge and retromolar area. Impressions should be recorded with the mylohyoid contracted.
  • Make sure the distolingual region is recorded

In partially dentate patients, first impressions should record:

• The teeth, palate, edentulous areas, and the buccal, labial and lingual sulci. Accurate detail of the sulci is only really needed in those areas in which there will be saddles.

Common errors:

• Tray not horizonally level, one side teeth are deeper into tray, and on the other the sulci depth may be missed
• Tray rotated, missing distal detail on one side
• Air blows, due to not releasing the air by moving the cheeks
• Poor detail of sulci, due to not pre-loading the sulci, or perhaps not pushing the tray far enough into position

Some errors in first impressions can be compensated for in the lab. A small airblow is easily filled with wax, but this is a guess as to the real surface of the mucosa. In general practice, the technician will not usually complain about the quality of the impression because they do not want to loose your future business, even though it might impact greatly on the quality of the special tray/denture they are able to make. Do not lower standards and send terrible impressions. Aim instead for perfection, and practice until it is achieved. Remember any time you might feel like you are saving by not repeating a poor quality impression is not really time saved if you end up giving the patient a poor denture which they are not happy with, and either demand a refund or you have to start over again. In dental school, come in to the prosthetics clinic or laboratory and practice taking impressions with your classmates (get permission from the tutors first, and make sure you tidy up). This is also a good exercise in empathy because a student soon learns just how unpleasant impressions can be for the patient. You may need to practice several times before you will do a good job on a real patient.

• When you are happy with an impression, place it in the disinfectant bath for the required time

After the impression

• Before letting the patient go, choose what type of special tray you want (see later)
• Ask the patient not to wear their existing denture for about 90 mins before the next appointment (second impressions). It gives the mucosa of the denture bearing area time to recover and assume its normal shape, rather than the artificial shape that the existing denture has created.
• Finally, make sure the patient is given a mirror to clean off any impression material from their face.

Send the impression to the lab

• Trim off any excess impression material with a scalpel, and you may want to mark with a pen the required extension of the special tray.
• Be aware of how alginate needs to be transported, in a damp, but not soaking, gauze which will prevent the impression drying out and changing its dimensions before it reaches the technician. Silicone is more stable in transit.
• Make sure you give the technician/denturist all the information he or she requires, and record that the the impression has been disinfected. Don't forget to specify closely adapted or spaced, and the date that the special trays are required for the second impressions.

Special trays

Alginate requires a spaced tray (3mm), and other materials ( e.g. Zinc oxide Eugenol) might require a closely adapted tray.

Partial denture design

Partial dentures are designed before the appointment for second impressions. Why? Because any rest seats, guide planes, undercut, etc may need to be carefully planned and actualized before taking the master impression. Partial denture design is a complex topic and will be dealt with in a dedicated learning resource.

Second impressions

An upper impression taken in a special tray. Note the poor detail of the upper left buccal sulcus

Second impressions are also called master impressions or working impressions.

Before the patient arrives

• Have a look at the special tray and make sure it is the type you requested.
• Often just by looking at the special tray on the study cast it can be seen that the frenal areas will need reducing

Communication with lab

• Ask for the registration rims to be made on the casts made from the second impression. Frequently people will try to save time by combining the registration stage with second impressions, but since in this case the rims have been made on the less accurate first impressions, this may introduces error.

References and further reading

• Barsby MJ, Johnson A, Welfare RD, Winstanley RB. Guides to standards in prosthetic dentistry - Complete and partial dentures. British Society for the Study of Prosthetic Dentistry, 1994.
• Heasman P, ed (2008). Master Dentistry Vol I: Restorative dentistry, paediatric dentistry and orthodontics (2nd ed.). Edinburgh: Churchill Livingstone. p. 112,122. ISBN 9780443068959.