Elastomeric impression materials

Question 1

Why do we do impressions on patients? (5)

Answer 1

• 1. To generate a copy and study the case
• 2. To diagnose the case
• 3. To plan the line of treatment.
• 4. To educate the patient about their dental needs.
• 5. To construct indirect restorations e.g. Inlay, Onlay, crown, bridge, complete or partial denture.

Question 2

Why is communication needed between the dentist and technician?

Answer 2

• For this type of work, communication between the dentist and the technicians and any people who are going to help you in this procedure is crucial.
• Key to successful use of these materials.
• Communication for better documentation, better understanding and success of the treatment.

Question 3

What are the steps to digital workflow?

Answer 3

• Scan
• Design
• Mill
• Fabricate
• Deliver
• Digital ready
• We don’t even need to send our scan to the technician as we have some printers that can be available chairside.

Question 4

Accuracy definition?

Answer 4

• Accuracy – refers to how close a measured value is to the true or accepted value. It is a measure of correctness. High accuracy means the measurements are close to the actual value. E.g. A dart hitting the bullseye on a dartboard is accurate.

Question 5

Precision definition?

Answer 5

• Precision – refers to the consistency or repeatability of measurements. It indicates how close multiple measurements are to each other, regardless of how close they are to the true value. E.g. Several darts landing close together on the dartboard, even if they are far from the bullseye, demonstrate high precision.

Question 6

What is more important, accuracy or precision?

Answer 6

• An ideal scenario is to have both high accuracy and high precision where measurements are both consistent and close to the true value.

Question 7

What are the different properties to look for in impression materials?

Answer 7

• Consistency
• Flow materials
• Setting time
• Dimensional stability
• Ease of handling – if we cannot manipulate
  the material properly, they are not going to
  be mixed easily with their components/catalysers
  so will not be able to have correct dimensional
  stability afterwards, will not reach the proper setting time, not going to reach the consistency you need to make the impression of the teeth of the patient.

Question 8

How can you classify impression materials?

Answer 8

• They can be classified depending on their:
  o Method of hardening
  o Type of impression
  o Rigidity.

Question 9

What are some examples of non-elastic (rigid) impression materials?

Answer 9

o Impression plaster
o Zinc oxide eugenol
o Impression compound
o Impression waxes

Question 10

What are some examples of elastomers (rigidity - compliant - elastic)?

Answer 10

o Polysulphides
o Silicones (Polysiloxanes)
o Polyethers

Question 11

What are rubber polymers?

Answer 11

• Rubber polymers: either chemically or physically cross-linked. The chain molecules in unstretched and stretched rubbers.
• They can be stretched and rapidly recover their original dimensions when the applied stress is released.
• Can easily go over the teeth when doing the impression and when you pull it out, the material can return to the shape. How much it returns will depend on what recoil byproduct is.
• Some release byproducts that can affect the dimensional accuracy.

Question 12

What are the advantages of compliant - elastic materials over hydrocolloid?

Answer 12

o Higher tear strength – when you pull the material out, the alginate can be between the teeth depending on how tight the neighbouring teeth are. But with these materials you have higher accuracy.
o Absence of syneresis or imbibition
o More dimensionally stable on storage.

Question 13

What is the presentation of compliant - elastic materials?

Answer 13

o Base – packaged as a paste in a tube, as a cartridge, or as a putty in a jar.
o Catalyst – also known as the accelerator, is packaged as a paste in a tube, as a cartridge, or as a liquid in a bottle with a dropper top.
o Machine mixes these so you don’t have external contamination while you manipulate the materials.
o One of the challenges here is that if you touch the catalyst with the gloves and its on the glove and not on the material, you will lose the proportion you need to achieve a better performance and best properties of the materials.

Question 14

What is consistency of complainant - elastic materials?

Answer 14

o Addition silicones consistencies can be:
 Extra-low
 Low (if come with syringe or wash).
 Medium (regular).
 Monophase
 High (tray)
 Putty (extra high)
o Polyether impression materials can be available in low, medium, monophase and high consistencies.

Question 15

What is the chemical composition of polysulphide?

Answer 15

rigidity - Complainant - elastic - elastomers
o Base – Mercaptan polysulphide
o Cross-linking agent – sulphur and/or lead dioxide.
o Catalysts – copper hydroxides, zinc peroxide, organic hydroperoxide
o Fillers – zinc sulphate, lithopone, or calcium sulphate dihydrate.

o It has a terminal and pendant mercaptan groups (-SH).
o The terminal and pendant groups of adjacent molecules are oxidised by the accelerator (lead dioxide) to produce chain extension and crosslinking, respectively.
o By-product is water so is non-toxic, not unpleasant to the patient.

Question 16

What are the advantages for polysulphide?

Answer 16

o Lower cost (compared to silicones and polyethers)
o Long working time – good if that is your work time. But can also be disadvantage.
o High tear strength.
o High flexibility
o Good detail reproduction.

Question 17

What are the disadvantages of polysulphide?

Answer 17

o Long time to set
o Unpleasant smell.
* The material is a really good material; however, the cross-linking agent (Sulphur) has a really unpleasant smell for both the dentist, and the patient so need to be careful for this choice of material.

Question 18

What is the chemical composition of polyether?

Answer 18

Rigidity - complainant - elastic - elastomers
o Base – polyether
o Cross-linking agent – sulphate
o Catalysts – glycol-based plasticizers.
o Fillers – silica
o By-product – NONE.
o When you lose components from your impression material, will result in dimensional inaccuracies.

Question 19

What are the mixing technique considerations for polyether?

Answer 19

o Material is very stiff, which make sit difficult to remove without rocking.
o When removing the impression, break the seal and rock slightly to prevent tearing.
o Water, saliva, and blood affect the polyether material
o Added moisture will increase the impression’s marginal discrepancy.

Question 20

What are the advantages of polyether?

Answer 20

o Highly accurate
o Good dimensional stability
o Stock or dual-arch trays
o Good surface detail.
o Pour within one week – kept dry.
o Multiple casts
o Good wettability – hydrophilic.

Question 21

What is the chemical composition of C-silicones?

Answer 21

Rigidity - compliant - elastic - elastomers
o Base – poly dimethyl siloxane with terminal OH group.
o Cross-linking agent – Alkyl ortho silicate or organo hydrogen siloxane.
o Catalysts – Organo tin compounds.
o Fillers – silica.
o By-product – ETHYL ALCOHOL.
o Not unpleasant to the patient as not strong smell or strong taste of alcohol.

Question 22

What are the mixing technique considerations/disadvantages for C-silicones?

Answer 22

o The material has a limited shelf life.
o The tray requires a special tray adhesive.
o No syneresis or imbibition but does respond with shrinkage over time.
o The material is more flexible, so there is more chance for distortion during removal.
o Wait 20 to 30 minutes before pouring of models for stress relaxation to occur.

Question 23

What are the advantages of C-silicones?

Answer 23

o Better elastic properties
o Clean, pleasant
o Stock tray – putty-wash (more individual to patient and with stock tray can still have precision and accuracy).
o Good working and setting time.

Question 24

What is the chemical composition of polyvinylsiloxanes?

Answer 24

Rigidity - compliant - elastic - elastomers
o Base: Paste 2 – Siloxane prepolymer (Silcone with hydrogen groups).
o Most expensive one
o Cross – linking agent – 2.
o Catalysts: Paste 1 – vinyl poly(dimethylsiloxane), prepolymer (Silicone with vinyl groups (C=C) and chloroplatinic acid catalyst.
o Fillers – silica.
o By-products – NO Bioproduct or H+. Depending on how long your using this material there is hydrogen by product.

Question 25

What are the mixing technique considerations/disadvantages of polyvinylsiloxanes?

Answer 25

o For dimensional stability, this is the best impression material. o Pouring of the model can be delayed up to 7 to 10 days. o Stiffness of the material makes removal of the tray difficult (difficult to remove material from the tray, not from the mouth). o Material dispensed using auto-mixing unit and mixing tips. o Used for monophase techniques.

Question 26

What are the advantages of polyvinylsiloxanes?

Answer 26

o Highly accurate. o High dimensional stability –pour up to1 week. o Stock or custom trays. o Multiple casts. o Easy to mix o Pleasant odour.

Question 27

What are the impression techniques for silicones?

Answer 27

o Simultaneous, dual-viscosity technique. o Single viscosity o Monophase o Putty-wash – have the 2 materials, then you manipulate them to 1 colour, then place onto tray, then go with low viscosity material and go around the part you have the teeth in the tray, then go with the syringe pistol (low viscosity impression material) around the cervical margin of the gums between the teeth, then you place the tray inside the patients mouth.

Question 28

What are the curing stages of silicones?

Answer 28

o Initial set – The first stage results in stiffening of the paste without the appearance of elastic properties (manipulation). Can have a spatula and touch it, until the mark of the spatula not going in, but returning to you. Then turns into it only being changed by cutting it. o Final set – the second stage begins with the appearance of elasticity and proceeds through a gradual change to a solid rubberlike mass. The material must be in place in the mouth before the elastic properties of the final set begin to develop. o Final cure – the last stage occurs from 1 to 24h

Question 29

What are the elastomers properties?

Answer 29

Many have great elastic recovery Flexibility Flow Shrinkage hours Tear strength

Question 30

Which elastomer has the best flexibility?

Answer 30

* Many have a good flexibility, but the polysulfide, it goes really well with the flexibility

Question 31

Which elastomer has the best flow?

Answer 31

* Flow you can compare and see how good the addition silicone and polyether are. So these materials are better at copying difficult areas between the teeth and cervical areas.

Question 32

Which elastomer has the least shrinkage?

Answer 32

* It’s important we don’t have shrinkage so addition silicone and polyether, gives you best result without shrinkage.

Question 33

Which elastomer has the best tear strength?

Answer 33

* Tear strength is when you remove these impressions, are they going to fall apart or not. Can see great results for addition silicone and polyether.

Question 34

What are silicones purposes as elastomers?

Answer 34

* Silicones are hydrophobic, manufacturers have made the addition silicone more hydrophilic by adding surfactants to the paste to reduce the surface tension of the silicone paste when it touches saliva and water. * This addition allows the impression material to wet soft tissue better and be poured in stone more effectively. * The hydrophilization of addition silicones is gained with the incorporation of non-ionic surfactants. These molecules consist of a hydrophilic part and a silicone compatible hydrophobic part.

Question 35

What is the order of dimensional changes in elastomers?

Answer 35

C-silicones> Polysulfides > Polyether > addition silicones.

Question 36

How should condensation silicones and polysulfide impressions be poured?

Answer 36

* For the greater accuracy, the models and dies should be poured immediately for condensation silicones and polysulfides.

Question 37

How should addition silicones be poured?

Answer 37

* Some addition silicones release hydrogen after setting, so one should not pour gypsum models and dies until after 1 or 2 hours. (look at word for table of properties of elastomers figures).