impressions

4- Impression wax

Waxes are thermoplastic materials, they do not set by chemical reaction . Normally used to correct small imperfection in other impression especially zinc oxide eugnol. They consist of a combination of a low melting paraffin wax and beeswax in a ratio of approximately 3:1 to ensure the wax flows at room temperature. A cast should be poured up immediately after taking the impression to avoid distortion that readily occurs in 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

Hydrocolloids

The colloid : state of matter in which individual particle of one substance are uniformly distributed in a dispersion medium of another substance. When dispersion medium is water it is termed a hydrocolloid.
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 .
A sol can be converted into gel in one of two ways:
Reduction in temperature (Reversible agar)
Chemical reaction ( Irreversible alginate )
Composition of Agar

Material

Percentage
purpose
Agar (seaweed extract)
12-15%
Colloidal particles as basis of the gel
Potassium sulfate
1%
Ensures set of gypsum materials
Borax
0.2%
Strengthens gel
Alkyl benzoate
0.1%
Antifungal agent
water
85%
Dispersing medium for the colloidal suspension
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.
Tear strength is not high.
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.
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).
Preparing the material for impression:
Gel heated to 71-100°C, 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 60-66°C.
3rd camber kept at 45-47°C to temper agar before use to prevent burns.
Tray is loaded and water hose attached and tray is seated on teeth.
Water (13 °C) is circuit through the tray
Advantage
Good surface detail.
Reusable .
Can be used with undercut.(not deep)
Disadvantage
Need special equipment.
Dimensional instability.
Evaporation or imbibitions.
Slow setting time.
Poor tear resistance.
Alginate
Common uses:
Diagnostic cast (study model)
Preliminary impression for complete denture
Partial denture framework
Custom trays for fluoride or bleaching
Composition
Material
Percentage
purpose
Na, K alginate
15-20%
Basis of the gel
Ca sulfate dihydrate
14-20%
Creates irreversible gel with alginate
Potassium sulfate
10%
ensures set of gypsum
Trisodium sulfate
2%
Retarder to control setting
Diatomaceous earth
55-60%
Filler to increase thickness and strength


Setting reaction/working time
Setting reaction 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
Control the Setting reaction
Controlling water temperature shortens or lengthens setting time
Changing P:L has adverse effect on materials property and strength
Allow extra 1-2 minutes, tear strength increases, and rebound from undercuts w/out deformation improves
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
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)
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*
Troubleshooting alginate impression
Premature set
Slow set
Voids
Distortion
Excess alginate at back of tray
Advantage
Simplicity of manipulation.
Little discomfort to the patient.
Short chair time.
Accurate reproduction of undercut area
Disadvantage
Not accurate in reproduction of hard object as rubber imp. Materials.
Affect hardness of surface of stone( potassium sulphate).
Poor dimensional stability.
Setting time dependent on operator handeling


Elastomers
Often called rubber materials since they have properties similar to rubber.
Clinical uses:
Bridges, Implants. Partial dentures complete dentures and Indirect esthetic restorations
Polymerization reaction
Involves formation of long-chain polymers and cross-linking of chains
General properties:
These are used a high degree of accuracy is needed, especially in crown and bridge work. They have two main advantages over the hydrocolloid;
Good tear resistance .
Dimensional stability.
They are mainly hydrophobic rubber based materials. All of these materials come in different viscosity ranging from low to high viscosity. The light boded material may be used as a wash impression over a medium or heavy-bodied material. There are two ways this can taking impression:
One stage impression: light body impression material is placed in a syringe , and placed over the areas high detail is required. Some is then squirted over the heavy-bodied impression material that has been loaded into an impression tray. The impression is then taken as normal.
two stage impression: an impression is taken with the heavy body impression material. This is then removed from patient mouth and inspected, the light body impression material is then prepared and again placed in a syringe. Then squirted over heavy body impression and then the impression replaced in its original placed..

Polysulfides

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 (causes polymerization and cross linking)
Uses
Crown and bridge impressions
Partial and complete denture impressions
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
need to Use tray adhesive
Used with custom trays
For all these reasons, Polysulfides have been largely replaced by other rubber materials


Silicon rubber
Condensation silicon Addition silicon

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)
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
Setting reaction
These material are supplied as 2 past. One past contain a polydimethyl siloxan polymer in which the methyl group are replaced by hydrogen and the other past contain a prepolymer in which the vinyl group replaced the methyl group. One of past contain a catalyst such as chloroplatinic acid .
Depending on the content of filler four viscosities are available
Impression making techniques
Putty/wash technique
One step
Two step
used with stock or special tray
Advantage
Adequate working and setting time.
Pleasant odour.
Adequate tear resistance.
Good elastic properties (used in case of sever undercut).
Disadvantage
Should poured immediately.
Poor dimensional stability.
Poor self life.
expensive
Polyethers
Also used for crown and bridge and over denture work since they are very accurate and also more hydrophilic than other silicons
Dispensing & composition
Dispensing same as other rubber materials (2 tubes)
In addition it’s 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
*mix well to avoid irritation from unmixed catalyst.
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


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 or impression immersed in disinfectant
Protective gloves should be worn
Rinse after disinfection is complete