D.M. 005

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                                 05
Dr. Hanan Zreiqat


   Impression material part II

Zeina Zaza

                    19/07/2009

                    21/07/2009
Dental Materials
Lecture 5 Sunday July 19, 2009

                          Impression Material Part 2

The Midterm exam will include the first three slides and first five lectures.
     Introduction
     Oral Environment
     Impression Material

There will be no labs the week of the exam, but there will be lectures.


Elastomers

      Rubber impression materials or elastomers are called rubber material
because they share similar properties with rubber. Usually they are used
make impressions for:
         – Bridges
         – Implants
         – Partial dentures or complete dentures
         – Indirect esthetic restorations
               – Indirect meaning it involves an extra step in the lab.
               – Usually you drill and fill a cavity with material in the
                   clinic, this is considered a direct step.
               – With indirect there is an extra step, the restoration is sent
                   to a lab technician.

Polymerization reaction

   – These elastomers usually set by a polymerization reaction. There are
     different materials but each one sets with the formation of long-chain
     polymers and cross-linking of chains.
   – In general they are not as sensitive to water loss or water gain such as
     alginate and agar.
   – They also have the ability to reboud. Usually when you put an
     impression material in a patients mouth and remove it, there is stress
     or pressure on the material and it tends to go back to its original
     position. This is called rebound.
   – They are able to rebound in different degrees.
   – The best rebound is addition silicon, which is why it is the most
     accurate.
   – There are also polyethers which are pretty accurate too.
   – The accuracy of addition silicon is better than polysulfides.
   – Because these materials are hydrophobic, the field should be as dry as
     possible.
   – Moisture on the surface such as water, saliva, or blood will
     accumulate on the surface of the impression material and will cause
     irregularity and inaccuracy.
   – To overcome this problem they have added surfactants to the material
     to increase the materials affinity to water.


Dispensing & Composition

       Polysulfide is one of the oldest rubber impression materials used, but
is rarely used nowadays.
       Usually it is supplied as two pastes, a base and catalyst, and
depending on the viscosity it separated into light, regular, and heavy
viscosities. The lighter flows the easiest.

The Composition

      Catalyst chemical composition:
            - Lead dioxide or copper hydroxide.

      Base chemical composition:
            - 80% low-molecular-weight organic polymer containing
              mercaptan reactive groups (-SH), and 20% reinforcing
              agents: titanium dioxide, silica, zinc sulfide.
            - reinforcing agents: titanium dioxide, silica, zinc sulfide
              make material a little bit stronger.
            - usually silica is used as a filler or reinforcing agent.
            - the main component is low-molecular-weight organic
              polymer.

Setting Reaction
      Mercaptan + lead dioxide      polysulfide + H2O

When you mix them them you end up with a polysulfide and water as a
byproduct.
This reaction can be accelerated if you increase the temperature or moisture.
And the material will set (become hard) faster. The material must be mixed
with the correct ratio in order for it to work correctly.

Q: What increase the reaction rate of polysulfides?
A: Increase in temperature or moisture.

This is how the material is manipulated.




These two pastes are mixed together in a sweeping motion until they blend
completely and become one color. Then it is loaded into a tray so you can
make an impression.


Clinical Considerations When Used

      - Sometimes you need to apply an adhesive to the tray so that the
        material sticks to the tray. Such as we did with the alginate.
      - You also need to allow for a 2mm bulk thickness.
      - Dry field.
      - For polysulfides, you need to use custom acrylic trays. The plastic
        trays and flexible and may break. Using the custom acrylic trays
        will keep the impression from being distorted when removed from
        the patients’ mouth.
Properties of Clinical Interest

      -   Polysulfides is rarely used nowadays.
      -   They have a bad taste and odor and it is difficult to remove stains.
      -   Setting time: 8-14 minutes
      -   Higher tear strength than hydrocolloids.
      -   Accuracy improves if impression is poured within 30 minutes
      -   Messy (orange solvent to remove stains)
      -   Can be irritant to oral mucosa

For all these reasons, polysulfides have been largely replaced by other
rubber materials, such as addition and condensation silicon.

Condensation Silicon

Developed as a replacement to the polysulfide material. It is easier to mix
and work with and it has a shorter setting time compared to the polysulfide.

Dispensing & Composition

      - Two pastes, base and catalyst.
      - Comes as light, medium, or heavy viscosity.
        - For an accurate model a light viscosity is used.
      - Base: dimethylsiloxane + filler (silica) as a reinforcing agent.
      - Catalyst: suspension of stannous octoate + alkyl silicate.

Setting Reaction

   When you mix the base and catalyst the material will start to set within 5-
   7 minutes due to a chemical reaction between the base and catalyst.

   Because the setting reaction produces ethyl alcohol which evaporates, the
   material must be poured immediately. It cannot be stored.

This is a major difference between addition and condensation silicon. When
you practice the condensation silicon in used, but for a real impression it is
better to use the addition silicon.
Addition Silicon

       Desirable clinical qualities:
          – Dimensional stability.
          – Accuracy.
          – Clean.
          – Easy to mix.
          – No foul odor or taste.
          –
However, they are among the most expensive. This is important to consider
if you have a private practice.

Dispensing and Composition

      - Light, regular and heavy viscosities and also putty.
      - Dispensed as cartridge with 2 chambers (pastes), or two putty-
        system
        – Base: low-molecular-weight silicon with vinyl groups (paste
           system), or low-molecular-weight silicon with silane hydrogens
           (putty) + silica filler
        – Catalyst: chloroplatinic acid

Setting Reaction

      - Sets within 3-7 min.
      - No alcohol byproduct by sometimes hydrogen is produced. If not
        poured immediately you wait an hour or two and pour after the
        hydrogen has escaped. The accuracy will not be affected.
      - The advantage of addition silicon is that you can pour it more than
        once.
      - To avoid hydrogen production, palladium powder is added to
        absorb the H.

The setting time is usually shorter than the condensation silicon
Different Techniques

   • Putty/wash technique
        – One step
              • Spread putty on a plate and mix wash.
              • Place them in a tray together.
              • Apply on the surface.
              • Take the impression.
              • The putty provides support for the wash.
        – Two step
              • Take impression with the putty.
              • Remove from patient’s mouth.
              • Mix wash and apply over putty that is in the tray.
              • Retake impression
                    • This allows for finer detailing with the wash
                       impression material.

The 1 step technique is quicker, but because the 1 step technique putty has a
high viscosity it might not be as accurate as the 2 step technique. In the end
it is up to the dentist to decide what they want to do. Usually the wash
material is supplied in a gun type container. This is to make the material
easier to work with and more accurate when mixing.

Clinical Use

        - Crown and bridge work.
        - Bite registration (rapid setting 1-2 minutes) when you do any
          type of restoration you need to register the bite of the patient.
        - It is spread over the occlusal surface of the teeth and the patient is
          asked to bite.
        - Sometimes wax is used.
        - Indirect composite inlays (silicon die technique).

Polyethers

Polyethers are used for crown and bridge work since they are very accurate
and also more hydrophilic than other silicones. Moisture on the surface does
not affect the material as much as it did with the other silicones.
Dispensing and Composition

      - Dispensing same as other rubber materials (2 tubes).
      - In addition it’s supplied in pouches of base and catalyst placed in
        mechanical mixer.
          - This is better than manual mixing because it allows for less
             error.
   • Composition:
       – Base: low- molecular-weight polyether with cation reactive
          group
       – Catalyst: aromatic sulfonic acid

Properties
        - There is a 3-5 minute setting time.
        - It is a little bit more rigid when compared to the addition and
           condensation silicones.
        - It is not as flexible; areas with an undercut are difficult to make a
           mold of. It would be better to use addition or condensation
           silicon for areas with undercut.

Other Materials

Impression Compound

       Cakes or Sheets- Usually used to make a primary impression for an
       edentulous patient.
       Greenstick- Secondary impression for the patient is made by using the
greenstick compound. It is used to make a more accurate impression
including how the muscles interact with the teeth they surround. This is
called border molding.

Composition and Properties

        - Thermoplastic resin and waxes
           - Heated and made soft, but not with a flame. The material is
               softened using hot water.
        - Fillers to reduce flow
        - Plasticizers
        - Organic acids or oils
        - Pigments
        - Properties
         – Softened at 60°C, remains plastic at 45°C, firm at 37°C
         – Heated in water not by flame.
         – If you used a flame you should place the material in hot water
           quickly to keep it soft and not burn the patient.
         – Should be poured ASAP to avoid distortion

Impression Plaster

         – Supplied as powder, mixed with water
         – Seldom used because it dries out the patients mouth.
         – Also the plaster needs to be divided into more than one part to
           be removed from the patients’ mouth. It is later reassembled
           after being removed.
                – This can create distortion and inaccuracy.
         – Composed of plaster gypsum (Ca hemihydrae).
         – Used for primary impression (high P:L).
         – Wash impression.
         – Easy to use.
         – Inexpensive.

ZNO Eugenol

Zinc oxide is the base, eugenol is the catalyst. One is brown and one is
white, they are mixed together until the colors mix and then placed into a
tray.
        - Secondary impression for complete dentures, or wash impression
           (mucostatic impression)
        - 2 tube paste system with different colors.
        - Zinc oxide (80%) and fillers, eugenol (15%) with oils, resin,
           fillers. In addition to chemical accelerator (zinc acetate).
        - Dispensed in 2 equal lengths and mixed to homogenous color
           forming zinc eugenolate
        - Initial set:3-6 minutes, final set to make sure it is completely
           set:10 minutes
        - To accelerate the setting, a drop of water or zinc acetate is added.
Properties

     - Brittle, so not suitable for areas with undercut.
     - Flows readily.
     - Accurate when it is fully set.
     - Eugenol, which is the oil in the mix, it can be irritating and cause a
        burning sensation in the mouth.
     - Once set it is dimensionally stable.

Impression Wax

   - Clinical uses:
          – primary impression material for edentulous patients
          – Bite registration for upper and lower teeth.
          – Baseplate wax used to be used for provisional crown and bridge
             work
          – Melted to correct voids in gypsum casts

Disinfecting

     - Always disinfect the impression material.
     - Wash to remove saliva or blood or and other debris.
     - Spray it with a disinfectant or soak it in a disinfectant.
     - The disinfectant should be compatible with the impression materials.
     - After taking the impression, it should be rinsed with water, excess
        water shaken off, and disinfected by spraying or soaking.
     - Protective gloves should be worn.
     - Rinse with water after disinfection is complete.

         Disadvantages of Spraying
              - Can get in your eye
              - Does not reach the entire surface

         Disadvantages of Soaking
              - Effects the accuracy

Before disinfecting the cast should be set for at least 24 hrs.
   • Solution used: Sodium (Na) hypochlorite, iodophors.
                        Disinfection of Impressions

Impression            Compatible Disinfectants                 Immersion
Material                                                       Time
Alginate and Agar     1:10 Na Hypochlorite, 1:213 idophors     10-30 minutes
hydrocolloid
Polysulfides          1:10 Na Hypochlorite, 1:213 idophors,    10-30 minutes
                      glutaraldehydes, complex phenolics
Silicones (addition   1:10 Na Hypochlorite, 1:213 idophors,    10-30 minutes
and condensation)     glutaraldehydes, complex phenolics
Polyethers            1:10 Na Hypochlorite, 1:213 idophors,    <10 minutes or
                      complex phenolics                        spray
Compound              1:10 Hypochlorite, 1:213 idophors        10-30 minutes
Zinc Oxide            1:213 idophors, glutaraldehydes          10-30 minutes
Eugenol


Sterilizing Trays

      - Trays should be properly sterilized before use.
      - Disposable trays are recommended if appropriate.
      - Sterilization can be achieved by heated steam, dry heat, chemical
        vapors.



                                                      Done By: Zaina Zaza

				
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