RIGID IMPRESSION MATERIALS (DENTAL MATERIALS)

rigid impression materials

impression plaster

presentation
it presents as powder mixed with water in water/powder ratio (w/p= 0.60), 100 g powder/60 ml water.
composition
1- calcium sulfate ?-hemihydrate.
2- potassium sulfate: to reduce expansion, and to accelerate the setting reaction.
3- borax: to reduce the rate of setting.
4- starch: to help disintegration of impression on separation from the plaster or stone cast.
after cast hardening, the impression and cast are put in hot water. the starch swells and the impression disintegrates, making it easy to separate the cast from the impression.
properties
1- setting time (5 minutes).
2- the mixed material has a very low viscosity, so it is mucostatic.
3- it is hydropinghilic.
4- it adapts to the soft tissue and recording their surface detail with great accuracy.
5- the dimensional stability is very good (a dimensional change during setting is 0.06 %).
6- a separating medium must be used between the impression plaster and the pouring plaster or stone.
7- the material is rigid once set, and thus unable to record undercuts.
8- patient complains very dry sensation after having impression recorded because of water absorbing nature of this material.
9- the material is best used in a special tray, made from acrylic (1.5 mm spacer).
uses
1- final impression for completely edentulous arch.
2- occlusal bite registration.
impression compound
impression compound is described as a rigid, reversible impression material which sets by physical changes. on applying heat, it softens and on cooling it hardens.
presentation
they supplied as sheet, stick, and cake.


composition
1- thermoplastic resins.
2- wax.
3- plasticizer: stearic acid: addition of plasticizer to overcome brittleness.
4- filler: talc, calcium carbonate added to:
a- overcome tackiness.
b- control degree of flow.
c- minimize shrinkage due to thermal contraction.
d- improve rigidity of impression material.
types
1- type i impression compound (lower fusing)
? sheet form material: it is softened using water bath, a temperature in range (55-60 °c), knead the material after it has been heated in water to ensure its being at a uniform temperature. storage in hot water should not be long that important constituents such as stearic acid may be leached out. overheating make the compound sticky and difficult to handle.
? stick form material: it is softened over a flame. the compound should not be allowed to boil otherwise, the plasticizers are volatilized.
2- type ii tray compound (higher fusing)
it is used to prepare a tray for making an impression. it is generally stiffer and has less flow than regular impression compound.
properties
1- it is mucocompressive.
2- because of high viscosity and low flow therefore, the reproduction of surface detail is not very good.
3- it is not used to record the undercut, because it is rigid once cooled.
4- poor dimensional stability. it has high value of coefficient of thermal expansion and undergoes considerable shrinkage on removal from the mouth. also because pressure is applied during formation of an impression (mucocompressive), residual stress exists in cool impression, the gradual relief of internal stresses may cause distortion of impression (the cast should be poured as soon as possible or at least within the hour).
5- impression compound has low thermal conductivity, therefore, time must be allowed during heating or cooling to allow impression compound to come to uniform softening.
6- this material can be reused a number of times for the same patient only, in case of errors.
7- the material has sufficient body to support itself to an extent especially in the peripheral portions.

disadvantages
1- difficult to record details because of its high viscosity.
2- compress soft tissues while making impression.
3- distortion due to its poor dimensional stability.
4- difficult to remove it if there are severe undercuts.
5- there is always the possibility of overextension especially in the peripheral portions.

uses
1- type i sheet form: it is used for recording primary impression of edentulous ridges using stock tray.
2- type i stick form: it is used for border molding of an acrylic special tray during fitting of the tray.
3- type ii tray compound: it is used to make a special tray (now largely replaced by acrylic tray).





zinc oxide eugenol impression material

zinc oxide and eugenol combination is widely used in dentistry
1- cementing and insulating medium.
2- temporary filling.
3- root canal filling material.
4- surgical pack in periodontal surgical procedures.
5- bite registration paste.
6- temporary relining material for dentures.
7- impression material for edentulous area.
classification
1- type i (hard).
2- type ii (soft).

available as
1- base paste (white in color).
2- accelerator or reactor or catalyst paste (red in color).
setting reaction
the setting reaction is a typical acid-base reaction to form a chelate. this reaction called chelation and the product is called zinc eugenolate.
1- zno + h2o zn(oh)2
2- zn(oh)2 + 2he zne2 + 2h2o

microstructure
the set material consists of a matrix of amorphous zinc eugenolate surrounding and holds the unreacted zinc oxide particles.
properties

factors controlling the setting time
a- particles size of zinc oxide powder: if the particle size is small, the setting time is less.
b- by varying the lengths of the two pastes.
c- by adding a droping of water, the setting time can be decreased.
d- longer the mixing time, shorter is the setting time.
e- high atmospheric temperature and humidity decrease the setting time.
f- cooling the mixing slab, spatula increase the setting time.
g- by adding a droping of oil or wax, the setting time can be increased.

2- it registers surface details accurately due to its good flow.
3- the material has mucostatic properties.
4- the material is rigid once set and cannot be used for making impression of teeth and undercut areas.
5- it requires a special tray for impression making it has adequate adhesion to acrylic tray.
6- it is dimensionally stable, a negligible shrinkage (less than 0.1%) may occur during hardening.
7- no separating medium is required before the cast is poured because it does not stick to the cast material.
8- the paste tends to adhere to the skin, so the skin around the lips should be protected with vaseline to make the cleaning process much easier.
9- eugenol can cause burning sensation and tissue irritation. non eugenol paste were developed, here the zinc oxide is reacted with a carboxylic acid.
10- it can be checked in the mouth repeatedly, and minor defects can be corrected locally without discarding a good impression.

manipulation
the mixing is done on oil impervious or glass slab. equal length of base paste and catalyst paste squeezed on to mixing slab and mixed until a uniform color is observed. the mixing time is 1 minute.
uses
1- final impression of edentulous ridge.
2- occlusal bite registration.

impression wax
impression waxes are rarely used to record complete impression but are used to correct small imperfection in other impression. waxes are generally used in combination with other impression materials
these materials consist of a mixture of low melting paraffin wax and beeswax in ratio about 3:1. it may also contain metal particles. the flow at 37°c is 100 %. these waxes are subjected to distortion during removal from the mouth. they should be poured immediately.
waxes have larger coefficient of thermal expansion of any material used in restorative dentistry.
uses
1- to make functional impression of free end saddles (class i and class ii removable partial dentures).
2- to record posterior palatal seal in dentures.
3- functional impression for obturators.