Setting reaction Mercaptan + lead dioxide polysulfide+H2O

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Impression material
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• The function of an impression material is to make
  a ve copy, which records the dimensions of the
  oral tissue and their relationships.
• The impression must be accurate so that the
  model, cast, or die will be accurate. That is the
  positive copy of the oral structure will be
  accurate.

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Initial requirement of an impression material

• Must be a semi-liquid material that will flow and
  adapt itself around the structure of interest.
• It must set and harden into a solid that is rigid
  enough to be removed from the mouth without
  becoming deformed.

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Other characteristics of an impression material

• Copy details accurately
• Dimensional stability after removal from the
  mouth
• Appropriate working time (from the start of mix)
• Appropriate time to harden in the mouth (setting
  time)
• Bio-compatibility/Aesthetic, odor/taste
• Chemically compatible with material used to pour
  cast or dies

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Impression trays

• Used to carry the impression material into the
  oral cavity
• Can be metal or plastic, for edentulous or for
  arches with teeth, pre-manufactured (stock trays)
  or custom-made (made in the lab. From resin).
• Trays need to be rigid enough to support
  impression material.
• For impression material to be retained in tray,
  tray maybe perforated, and adhesive spray can be
  used to enhance retention.

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Variety of trays
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• Study model
• a physical model of the oral tissue for study.
  Used in diagnosis and treatment planning.
• Cast
• a model requiring very exact replication of size
  and shape
• Die
• replica of a single tooth (cut from a cast)

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Classification of dental impression materials

• Inelastic / Elastic
• Mechanism of hardening
• Thermal behavior

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Impression material (rigid)
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Impression materials (aqueous)
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Impression materials (rubber)
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Inelastic impression materials (rigid)

• Plaster of Paris (Type I gypsum)
• Can not be removed over undercuts without
  impression being fractured
• Dental compound/impression compounds
• Softened by heat and pressed against tissue
  before it hardens. Thermoplastic softens at 60C,
  remains plastic at 45C and becomes firm at mouth
  temperature. Used for primary impression
• ZOE (Zinc oxide eugenol)
• Used as luting agent, temporary filling,
  secondary impression material.

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Hydrocolloids

• The colloid state represents a highly dispersed
  phase of fine particles within another phase,
  some where between solution and a suspension.
• The colloid can exist as a viscous liquid known
  as a sol or a solid known as gel.
• If the particles are suspended in water, the
  suspension is called a hydrocolloid (hydrosol,
  hydrogel
• Hydrocolloids come in two forms
• Reversible agar
• Irreversible alginate

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Composition of Agar
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Properties

• Flows well and adapts readily to hard and soft
  tissue contours
• Accurate reproduction (hydrophilic nature),
  however, the impression need to be poured
  immediately.
• Need to be stored in 100 humidity for short
  period of time if not poured immediately
• Distortion is more likely to occur if impression
  not poured within an hour.

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• Tear strength is not high.
• If compressed it might rebound if compression is
  not too sever otherwise deformation occurs.
• Material suffers from loss or gain of water
• Syneresis a process by which the gel contracts
  and some of the liquid is squeezed out, forming
  an exudate on the surface.
• Evaporation loss of water which causes the
  material to shrink, and impression is distorted.
• Imbibition uptake of water, this will swell and
  distort the impression.

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Clinical application

• The use of agar requires special equipment
  consisting of hydrocolloid conditioner (has 3
  water bath chambers) and water -cooled tray
  connected to a rubber hose that delivers water.
• Agar is supplied in two viscosities, thick and
  thin (depending on the amount of agar).

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• Preparing the material for impression
• Gel heated to 71-100C, becomes liquid (sol)
• Tray and agar syringe are liquefied in 8-12
  minutes in boiling water in 1st chamber
• 2nd chamber is used to store the agar at 60-66C.
• 3rd camber kept at 45-47C to temper agar before
  use to prevent burns.
• Tray is loaded and water hose attached and tray
  is seated on teeth.

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Hydrocolloid conditioner, water cooled tray with
hose, impression materials
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Alginate

• Inexpensive
• Easy to manipulate
• Requires no special equipment
• Reasonably accurate
• Common uses
• Diagnostic cast (study model)
• Preliminary impression for complete denture
• Partial denture framework
• Custom trays for fluoride or bleaching

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Composition/setting rxn/working time
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Setting rxn/working time

• Setting rxn occurs when the powder is mixed with
  water
• Ca sulfate dihydrate NaAlginate Ca
  alginate
• Working time total time from start of mixing to
  the final time at which an impression tray can be
  fully seated without distortion
• Regular set 2-3 minutes
• Fast-set 1.25-2 minutes
• Setting time elapsed time from the start of
  mixing until impression material becomes firm
  enough to resist permanent deformation.
• Regular set 2-5 minutes, Fast set 1-2 minutes

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Setting rxn/working time

• Controlling water temperature shortens or
  lengthens setting time
• Changing PL has adverse effect on materials
  property and strength
• Allow extra 1-2 minutes, tear strength increases,
  and rebound from undercuts w/out deformation
  improves

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Important considerations to ensure accurate
impression

• 2-4 mm bulk material in tray
• Snap action removal from mouth
• Allow extra 1-2 minutes after setting to improve
  tear strength
• Stored in a moist environment to avoid loss of
  water and deformation
• Disinfect in less than 10 minutes to avoid
  dimensional instability

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Impression making

• Fluff powder, wait 30 seconds for the dust to
  settle
• Measure powder and water according to
  manufacturer instruction
• Use spatula to stir water into powder to mix
• Stir vigorously to create homogenous mix (45
  seconds for regular, 30 seconds for fast setting
  alginate)

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• Tray should cover all anatomical features
• Utility wax can be added on tray borders to
  extend its coverage
• Adhesive spray is used to prevent separation of
  alginate from tray.
• Tray is loaded from posterior towards anterior
• Moisten and smooth alginate
• Posterior part of tray is seated first then
  anterior.
• tips to control gag reflex

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Alginate impression
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Troubleshooting alginate impression

• Premature set
• Slow set
• Voids
• Distortion
• Excess alginate at back of tray
• cause solution

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Criteria for acceptable alginate impression

• All teeth and relevant soft tissue recorded
• No large voids
• Free of debris
• No distortion
• Etc etc etc (refer to text book)

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Elastomers

• Often called rubber materials since they have
  properties similar to rubber.
• Clinical uses
• Bridges
• Implants
• Partial dentures complete dentures
• Indirect esthetic restorations

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Polymerization reaction

• Involves formation of long-chain polymers and
  cross-linking of chains
• General properties
• Not as sensitive to water as hydrocolloids
• Have a certain amount of rebound, the highest is
  for addition silicon and polyethers, medium for
  condensation silicon and poorest for Polysulfides
• Not wet well by water (hydrophobic) possible
  solution surfactants

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Polysulfides
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Dispensing Composition

• Supplied in two tubes as base and catalyst, equal
  lengths are mixed. Light, regular, heavy
  viscosities
• Chemical composition of base
• 80 low-molecular-weight organic polymer
  containing mercaptan reactive groups (-SH), and
  20 reinforcing agents titanium dioxide, silica,
  zinc sulfide
• Chemical composition of catalyst
• Lead dioxide or copper hydroxide

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Setting reaction

• Mercaptan lead dioxide polysulfideH2O
• The reaction is sensitive to temperature and
  moisture so increase in any will accelerate the
  setting.
• Also sensitive to correct mixing ratio

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Manipulation and dispensing
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Uses

• Crown and bridge impressions
• Partial and complete denture impressions
• Clinical considerations when used
• Used with custom trays
• Allow 2mm thickness
• Use tray adhesive
• Dry field

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Properties of clinical interest

• Setting time 8-14 minutes
• Higher tear strength than hydrocolloids
• Accuracy improves if impression is poured within
  30 minutes
• very unpleasant taste and odor
• 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

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Silicon rubber
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Classification according to polymerization rxn
Condensation silicon
Addition silicon
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Condensation silicon

• Developed as alternative to Polysulfides
• Has more desirable qualities in comparison
• Easy mix
• Better taste and odorless
• Shorter setting time (5-7 minutes)

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Dispensing Composition

• Two pastes, base and catalyst.
• Comes as light, medium, or heavy viscosity
• Base dimethylsiloxane filler (silica)
• Catalyst suspension of stannous octoate alkyl
  silicate

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Setting rxn

• Condensation reaction that produces ethyl alcohol
  as by product. The alcohol evaporates which
  causes dimensional instability.
• The material continues to contract with time, so
  needs to be poured within minutes.

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Addition silicon (PVS)

• Desirable clinical qualities
• Dimensional stability
• Accuracy
• Clean
• Easy to mix
• No foul odor or taste
• However, they are among the most expensive

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Dispensing 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

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Setting rxn

• Polymerization rxn of chain lengthening and
  cross-linking with reactive vinyl groups,
  producing a stable silicon rubber.
• No ethyl-alcohol by product
• Some addition silicons produce hydrogen as by
  product, manufacturers incorporated palladium
  powder that absorbs hydrogen
• setting time 3-7 minutes

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Impression making techniques

• Putty/wash technique
• One step
• Two step
• Pros and cons for each technique and possible
  solution

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Clinical use

• Crown and bridge work
• Bite registration (rapid setting 1-2 minutes)
• Indirect composite inlays (silicon die technique)

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Polyethers

• Also used for crown and bridge work since they
  are very accurate and also more hydrophilic than
  other silicons

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Dispensing composition

• Dispensing same as other rubber materials (2
  tubes)
• In addition its supplied in pouches of base and
  catalyst placed in mechanical mixer.
• Composition
• Base low- molecular-weight polyether with cation
  reactive group
• Catalyst aromatic sulfonic acid
• clinical tip mix well to avoid irritation from
  unmixed catalyst.

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Properties

• Stiff, difficult to remove from undercuts
• Short working and setting times
• Setting time 3-5 minutes
• Sensitive to moisture and temperature
• More hydrophilic (must not be stored in water or
  disinfectant)
• accurate

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Inelastic impression materials

• Impression compound softens with heat, hardens
  in the mouth.
• Cakes (sheets) and sticks
• Clinical uses
• Sheets
• Primary impression in metal trays
• To make custom trays
• Sticks are used for border molding

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Composition properties

• Thermoplastic resin and waxes
• Fillers to reduce flow
• Plasticizers
• Organic acids or oils
• Pigments
• Properties
• Softened at 60C, remains plastic at 45C, firm
  at 37C
• Heated in water not by flame
• Should be poured ASAP to avoid distortion

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Other impression materials

• Impression plaster seldom used
• Composed of plaster gypsum (Ca hemihydrae)
• Used for primary impression (high PL)
• Scored with a knife in the mouth then removed and
  reassembled in the lab (distortion?)
• Wash impression
• Easy to use
• inexpensive

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ZNO eugenol

• Secondary impression for complete dentures, or
  wash impression (mucostatic impression)
• 2 tube paste system, 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 set3-6 minutes, final set10 minutes
• To accelerate the setting, a drop of water or
  zinc acetate is added.

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Properties

• Brittle, so not suitable for areas with undercut
• Flows readily
• Accurate
• Eugenol can be irritant (burning sensation)
• Once set, dimensionally stable

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Impression wax

• Clinical uses
• Preliminary impression for edentulous patients
• Bite registration
• Baseplate wax used to be used for provisional
  crown and bridge work
• Melted to correct voids in gypsum casts

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Disinfection of impressions

• The disinfectant should be compatible with the
  impression materials.
• After taking the impression, it should be rinsed
  with water, excess water shaken off, and
  disinfectant sprayed (2 disadvantages?) or
  impression immersed in disinfectant
• Protective gloves should be worn
• Rinse after disinfection is complete

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Disinfecting casts

• Maybe necessary if impression was not properly
  disinfected, or if immersion of impression
  adversely affects the impression
• Casts should be set and stored for 24 hours
  before disinfection.
• Solution used Na hypochlorite, iodophors.

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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

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Reference dental materials, clinical
applications for dental assistants and dental
hygienists
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