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Rigid Impression Materials
Four main types of products form the group of impression materials
classified as non-elastic or rigid materials:
1. Impression plaster
2. Impression compound
3. Zinc oxide/eugenol impression paste
4. Impression waxes.
1- Impression plaster
Impression plaster is similar in composition to the dental plaster used to
construct models and dies.
Composition
It consists of β-calcium sulphate hemihydrate which when mixed with
water reacts to form calcium sulphate dehydrate.
Properties
The material is used at a higher water/powder ratio (approximately 0.60)
than is normally used for modeling plasters.
The fluid mix is required to enable fine detail to be recorded in the
impression.
The setting expansion of dental plaster is reduced to minimal proportions by
using anti-expansion agents. Potassium sulphate is the most common of
these and has the secondary effect of accelerating the setting reaction.
Freshly mixed plaster is too fluid to be used in a stock impression tray is
normally used in a special tray, constructed using a 1-1.5 mm spacer.
The tray may be constructed from acrylic resin or shellac.
The mixed impression material is initially very fluid and is capable of
recording soft tissues in the uncompressed state.
In addition, the hemihydrate particles are capable of absorbing moisture
from the surface of the oral soft tissues, allowing very intimate contact
between the impression material and the tissues. The fluidity of the
material, combined with the ability to remove moisture from tissues and a
minimal dimensional change on setting, results in a very accurate
impression which may be difficult to remove.
The water-absorbing nature of these materials often causes patients to
complain about a very dry sensation after having impressions recorded.
Disinfection of a plaster impression can be achieved with a 10 minute
soak in sodium hypochlorite solution.
The main differences between impression plaster and model plaster are:
- More rapid setting in order to avoid inconvenience/discomfort to
both the patient and dentist,
- Smaller setting expansion for greater accuracy,
- And much lower strength so that fracture can occur easily if the
material engages an undercut.
2- Impression compound
Impression compound is described as a rigid, reversible impression
material which sets by physical change.
Impression compound demonstrates thermoplastic behavior. Thermoplastic
solids can be transformed from a hard, solid material into a softened, moldable
material simply by raising their temperature to an appropriate level.
The process can be reversed by cooling to room temperature.
Classification
Two types of impression compound are available:
1. Type I (lower fusing) materials are impression materials.
2. Type II (higher fusing) materials are used for constructing
impression trays.
The difference in fusing temperature between type I and type II materials
naturally reflects a difference in composition of the thermoplastic
components of each.
The lower fusing, type I impression materials may be supplied in either
sheet or stick for many various colors.
Composition
In general impression compound is a mixture of waxes, thermoplastic
resins, organic acids, fillers and coloring agents.
Component Example Function
Thermoplastic
material (47%)
resins and waxes Characterizes the softening temperature
Filler (50%) Talc Gives "body" by
increasing viscosity of the
softened material; reduces
thermal contraction
Lubricant (3%) Stearic acid Improves flow properties
Applications
1. The sheet material (type I) is used for making a preliminary
impressions in edentulous ridges (mouth without teeth), normally
using stock trays.
2. The stick material (type I) is used for:
- border extensions on impression trays
- border molding,
- For recording individual tooth impressions of single crowns using
the copper ring technique.
- For check undercuts in inlay preparation.
3. Type II is used for making special tray.
Properties of impression compound
Thermal conductivity
Impression compound has very low thermal conductivity, i.e. they are
poor conductors of heat.
During softening of the material, the outside will soften first and
the inside last. So to ensure uniform softening the material should
be kept immersed for a sufficiently long time in a water bath.
Kneading of the material ensures further uniform softening.
Due to poor thermal conductivity, the layer adjacent to the mouth
tissues will remain soft. Removal of the impression at this stage
will cause serious distortion. Thus it is important to cool the
compound thoroughly before removing if from the mouth.
Coefficient of thermal expansion (CTE)
The CTE of a compound is comparatively high because of the presence of
resins and waxes. The contraction from mouth temperature to room
temperature is 0.3% and undergoes considerable shrinkage on removal
from the mouth.
Three factors combine to produce significant internal stresses within the
compound impression.
1. The high value of coefficient of thermal expansion.
2. The poor thermal conductivity.
3. The relatively large temperature drop from the softening
temperature to room temperature.
Dimensional stability
The gradual relief of internal stresses may cause distortion of the
impression. For the most accurate results, the model should be poured as
soon as possible after recording the impression.
Flow
The type I material should flow readily at just above mouth temperature,
whilst type II material should ideally not distort at mouth temperature.
The very high viscosity of impression compound is significant in
the degree of fine detail which can be recorded in an impression.
Compound is fairly rigid after setting and has poor elastic properties. A
large stress would be required to remove an impression from undercut
areas.
Manipulation
Sticks: Small amounts of compound are softened over a flame. When a
direct flame is used, the compound should not be allowed to boil or
ignite, otherwise, the plasticizers are volatilized.
Cakes: When a large amount of compound is to be softened, it is difficult
to heat the compound uniformly over a flame. The compound is softened
in warm water in a thermostatically controlled water bath (usually in
range of 60 to 70o C). After the compound is removed from the water
bath, it is usually kneaded with the fingers in order to obtain uniform
plasticity throughout the mass.
Precautions
Prolonged immersion in a water bath is not indicated; the
compound may become brittle and grainy because some of the low
molecular weight ingredients may be leached out.
Overheating in water makes the compound sticky and difficult to
handle.
Avoid incorporating water while kneading.
.
Advantages
1. The material can be reused a number of times (for the same
patient only) in case of errors.
2. Inaccurate portions can be remade without having to remake the
entire impression.
3. Accuracy can be improved by flaming the surface material
4. 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. Compresses soft tissues while making impression.
3. Distortion due to its poor dimensional stability.
4. Difficult to remove if there are severe undercuts.
3- Zinc Oxide/Eugenol impression pastes
These materials are normally supplied as two pastes which are mixed
together on a paper pad or glass slab.
Base paste.
Accelerator or catalyst paste.
There is normally a good color contrast between the two pastes, the zinc
oxide paste, typically, being white and the eugenol paste, a reddish brown
color. The enables thorough mixing to be achieved as indicated by a
homogeneous color, free of streaks, in the mixed material.
Composition of a Zinc Oxide–Eugenol Impression Paste
Components Percentage
Tube No. 1 (base)
Zinc oxide 87
vegetable or mineral oil 13
Tube No. 2 (accelerator)
Oil of cloves or eugenol 12
Gum or polymerized rosin 50
Filler (silica type) 20
Lanolin 3
Canada balsam 10
Accelerator solution (CaCl2) and color 5
Setting reaction
Some formulations do not contain water in the paste and the setting of the
material is retarded until the mixed paste encounters moisture in the
patient’s mouth. In other case, water provides additional acceleration.
When mixing the two pastes, a reaction between zinc oxide and eugenol
begins. The basis of the reaction is that the phenolic – OH of the eugenol
acts as a weak acid and undergoes an acid – base reaction with zinc oxide
to form a salt, zinc eugenolate.
Two molecules of eugenol react with zinc oxide to form the salt. It can
be seen that the ionic salt bonds are formed between zinc and the phenolic
oxygens of each molecule of eugenol.
- Setting time can be decreased by adding zinc acetate or a drop of
water or acetic acid (acetic acid is a more effective than water. It
increases speed of formation of the zinc hydroxide.
Properties
These materials are normally used to record the major impressions of
edentulous arches. The impression is recorded in special (custom) tray,
constructed on the model obtained from the primary impression, or inside
the patient’s existing denture. The periphery of the special tray or denture
needs to be modified with impression compound to ensure proper
contour of the impression and to give support to the paste in these
critical areas.
The thickness of paste used is normally around 1 mm.
For the majority of patients the zinc oxide eugenol impression pastes may
be considered nonirritant. Occasionally, eugenol may promote an
allergic response in some patients. For this type of patient, eugenol-free
zinc oxide impression pastes are available (to overcome the burning
sensation and irritation). The eugenol is replaced by an alternative
organic acid.
Dimensional stability
The dimensional stability is quite satisfactory. A negligible shrinkage
(less than 0.1%) may occur during hardening.
The thickness of paste used in normally around 1mm. this thin section of
material results in an insignificant dimensional change on setting and
subsequent storage of the impression.
The relatively low initial viscosity of the mixed paste, coupled with its
pseudoplastic nature, allows fine detail to be recorded in the impression.
Defects sometimes arise on the surface of the impression but these can be
corrected using an impression wax.
Biological considerations
Some patients experience a burning sensation in the mouth due to
eugenol. It can also cause tissue irritation. This may be overcome by
using a non-eugenol paste. Eugenol-free zinc oxide impression pastes are
available. The eugenol is replaced by an alternative organic acid.
Advantages
1. It has enough working time.
2. It can be checked in the mouth repeatedly without deforming.
4. It registers accurate surface details.
5. It is dimensionally stable.
6. Minor defects can be corrected locally without discarding a
good impression.
Disadvantages
1. It requires a special tray for impression making.
2. It is sticky in nature and adheres to tissues.
3. Eugenol can cause burning sensation and tissue irritation.
4. It cannot be used for making impression of teeth and undercut
areas as it is inelastic in nature.
4- Impression waxes
Impression waxes are rarely used to record complete impressions but are
normally used to correct small imperfections in other impressions,
particularly those of the zinc oxide/eugenol type.
They are thermoplastic materials which flow readily at mouth
temperature and are relatively soft even at room temperature.
Waxes can also be used to produce a mucocompressive impression of the
edentulous saddles.
These materials consist, typically, of a mixture of a low melting paraffin
wax and beeswax in a ratio of about 3: 1.
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