A Guide to Prosthetic Impressions

So last Friday my study day was lead by Dr Bill Sharpling at LonDEC and covered prosthetic impressions. There is a summary of what I learnt from the day.

Impression Techniques for Fixed Prosthodontics

  1. Putty-wash technique using medium bodied silicone and putty: either be 1 stage or 2 stage techniques. This is the turquiose and yellow impression in the picture above (the one that says hi!)
  2. Heavy and light bodied silicone. Again this can be 1 stage or 2 stage. This is the orange and teal and the royal blue and teal impressions in the picture.
  3. Monophase technique using a silicone or polyether such as Impregnum (see my previous post about Impregnum here). Here you use the same material for both syringing around the prep and in the tray, this is the light blue impression in the picture
  4. Injection technique. Using putty and light bodied silicone, scooping out the silicone when set and injecting the light bodied into the space when the tray is seated. This is the lower impression in the picture.

So, which is best technique?

According to studies done at King's College London, the best technique i.e. the one with the fewest air blows and other surface defects and which captured the preparations in good detail was the heavy and light bodied silicone technique but only when it's in combination with a custom tray

Having been taught this method at university I am pretty familar with it, however now I'm out in practice I have been using the putty-wash technique. 
I prefer this technique so far - it's much easier to capture the margins of the preps with no air blows and putty is a neat material to use whilst the heavy-bodied silicone (we used Doric) was a messy material which took 5 minutes to set, whereas the setting time of putty is much less. 

It can be argued different preparations would suit different methods. 
So whilst putty-wash is a good technique to capture margins of PFM or all ceramic crowns, the heavy-light bodied silicone technique is much better at capturing more subtle margins such as for all metal crowns or veneers. 
Putty-wash also tends to push the silicone off the occlusal surfaces of teeth so if your preparation ends on an occlusal surface for example a gold onlay, then it may be better to use the heavy and light bodied silicone where you can 'float' the heavy body onto the prep in order to stop pushing the light-bodied silicone off. 

The injection technique really suits lower posterior teeth as it prevents the tongue from brushing away the light-bodied silicone from lingual surface of the prep. 

Monophase techniques suit dentists who tend to work on their own without a nurse such as in those in Europe. 
I really found using the monophase technique difficult as trying to dispense the material from the syringe was a bit of an effort as you have to put alot of force into pressing the syringe.

Me attempting the monophase impression technique

A novel way to make a Custom tray without a Lab

1. Take a normal appropriately sized stock tray 
2. Take a spacer - you can buy these from labs or you can use something that you know is the correct measurement for the material you will use. When we did this we used an impression plastic bag (although remember it will need to be appropriately disinfected to go in the patient's mouth)
3. Using putty, take an impression but put the spacer on top of the putty so it is between the putty and the teeth
4. Seat fully and remove almost immediately 

And you will have a rudimentary custom tray! In the title picture, the bottom most impression was taken using a custom tray made in this way. 

Impression Techniques for Removable Prosthodontics

When taking impressions for removable prosthodontics, there will be 2 stages to consider: the preliminary impression and the secondary impressions.

The main aim of the preliminary impression is to achieve a massively overextending area so that the technique can make an effective custom tray to capture sulcus depths and other soft tissue landmarks e.g. the post dam. 

The material of choice for most dentists is alginate, but impression compound can be used for complete dentures. 

Here is a handy step by step guide to achieving a good preliminary impression:

  1. Use the correct sized tray either by trying it in the patients mouth or using calipers to measure the distance between the patient's tuberosities and matching them up with the tray (this saves you throwing the contaminated tray away if it doesn't fit)
  2. Mix the alginate to a super smooth consistency - the best way to do this is to use an alginate mixing machine e.g. Algamix. Mixing by hand is ok but you'll get better results with machine mixing. 
  3. Load a syringe full of alginate and syringe from the tuberosities forward to the labial frenum. For uppers place some in the roof of the mouth and for lowers do both buccal and lingual sides
  4. Load the tray and seat it posteriorly first
  5. DO NOT border mould at this stage - leave this for the secondary impression
  6. Fill in the lab form with all the requirements needed for the lab to make a suitable special tray

Requirements of a custom tray:

  • Close fitting 
  • Ending just shy of the sulcus depth (2-3mm)
  • Tissue stops if required - in the area of the 6's and the 3's
  • Perforated/unperforated 
  • Shellac/light cure acrylic
  • What material you intent to make the secondary impression with

Materials used for secondary impressions:

  1. Alginate - usually for partial dentures
  2. Silicones - combination of heavy and light bodied
  3. Zinc oxide eugenol/Kelly's paste - for complete dentures
For secondary impressions it is very important to BORDER MOULD in order to achieve good extensions. 
You can use wax, putty or greenstick around the periphery of your custom tray to achieve this. 

How to Border Mould

Ask the patient to do the following things when the tray is seated
  1. Smile
  2. Pucker their lips into a kiss
  3. Manipulate their cheeks yourself (always tell the patient what you're doing)
  4. Ask the patient to swing their lower jaw from left to right (only for the upper)
  5. Ask them to stick their tongue out and initiate a swallow (only for lower)

Tips to Prevent Gagging

  • Always sit the patient upright in the chair to prevent impression material running down their throat 
  • When taking upper impressions, have the patient's head in the crook of your arm so that you can tilt their head forwards whilst holding the tray in place
  • Encourage the patient to breathe through their nose
  • Seat the tray posteriorly first in order to move the bulk of the material anteriorly. This will also lead to better record of the sulcus depths
  • Distract the patient - talk to them, ask them to lick their lips or wiggle their toes, anything that you may think is effective
  • Never overload the tray, especially posteriorly

I hope you've found this guide useful! 
Which impression techniques do you use? Please leave your comments in the section below!
 Keep an eye out for future posts on some of my other study days!

Why not take a look at my other Clinical Guide posts?

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