Many impression materials are available for use in
dentistry .Impression materials are used for recording the form and
dimensions of the oral tissues for the fabrication of definitive restorations.
The recording of tissues in oral cavity is difficult because of many factors as
salivation, blood flow and sulcular fluid .The impression material is
introduced into the mouth in a plastic condition to set against the oral
tissues , then removed from the mouth .The set impression is a negative
replica of the oral tissues .A positive reproduction(cast) is obtained by
pouring a suitable die material into it . Making a cast in gypsum
materials from an impression helps dentists design and construct removable
and fixed prostheses. The accuracy of these final restorations depends
mostly on the impression materials and techniques used . therefore
, accurate impressions are necessary for development of any dental
prosthesis.
Brif history
In the 1940s and
1950s, hydrocolloids had been the favored impression materials. Since
the introduction of hydrocolloids in the mid‐1930s, the impression of
undercuts grew to be possible. In the 1950s, polysulfides and condensation
silicones (C‐type silicones) had been used reliably in fixed
prosthodontics. The most disadvantage of all these dental materials was the
shrinkage over a period of hours(1) . With hydrocolloids and
polysulfides , shrinkage used to be due to the evaporation of water, while
in condensation‐cured elastomers, it was the evaporation of low‐ molecular
weight products(2,3). In the late 1960s, polyethers was submit to the market.
Its high mechanical properties, good elastic recovery, and small shrinkage
made it most advantageous to hydrocolloids and C‐type materials. Ten years
later, the hydrophobic addition silicones [poly(vinyl siloxane)]
were introduced. The level of hydrophobicity was decreased by addition of
surfactants. PVS has high dimensional stability over time and temperature, even
in a moist environment. It is recognized for its high elastic recovery.
The last 20 years have brought considerable improvement in polyether and
poly vinylsiloxane categories, and now they seem to be the most acceptable
categories for most prosthodontic uses(1).
Clinical situations for using
impression materials
Impression materials
are used in fixed prosthodontics to make impressions of teeth that
prepared to receive indirect extracoronal restorations such as crowns and
bridges(FPD), removable prosthodontics (as making dentures) , dental
implants, orthodontics and maxillofacial prosthetics (prosthetic
correction of intra-oral and extra-oral defects due to trauma, congenital
defects, and surgical resection of tumors) .(4)
Classification of impression materials
Impression materials
are classified according to their behavior after setting into elastic
and non elastic. non elastic impression materials not indicated to be used
in fixed prosthodontics because they dont record undercats accurately , so
we use elastic impression materials which include hydrocolloids and
elastomers. (5)
Hydrocolloids
Hydrocolloids comprise
agar and alginate. agar is reversible hydrocolloid since it go
between viscous gel and low viscous sol repeately by warming and cooling
but, alginate once transformed into gel can not be turned into sol,
supposed irreversible hydrocolloid impression material(5).Agar and
alginate might be utilized separately or in mixing to record
crown impressions. Agar was first brought into dentistry for recording
crown impressions in 1937 via Sears(6)and was the principal elastic
impression material accessible. It isn't generally utilized in dental
practice today , due to the requirement for costly molding baths and water
cooled trays. Alginate unlike agar, doesn't require any unique condition .
It is much easier to use and inexpensive, but it used for taking
primary impression only for fabricating primary or study casts .Alginate
and agar produce impressions with perfect surface detail. They are both moderately hydrophilic
and are not dislodged from wet surfaces as effectively as the
elastomers(7) .However, these materials have two significant defects
.one of them , extremely poor dimensional stability due to synersis or
imbibition of water on remaining in dry or wet situations. Furthermore,
low tear strength which can be a disadvantage when trying to record the
gingival sulcus. Some work posted the utilization of mixing reversible and
irreversible hydrocolloid impression materials (8,9
Elastomers
Elastomers comprise polysulfides,
silicones(addition and condensation) and plyethers .
Polysulfides
The polysulfide
impression materials have the longest history of utilization in dentistry of
all the elastomers. they were first evolved as a sealant for holes between
sectional concrete structures.(1 0)They are accessible in different
viscosities namely , light bodied (low viscosity), medium or regular
bodied and heavy bodied (high viscosity).These are presently
unpopular materials. Polysulfide impression materials are usually low to
moderately hydrophilic and make an accurate impression in the presence of
some saliva or even blood. The setting reaction of polysulfides is
relatively long with setting times often more than 10 minutes (acceleration can
be made by adding a small drop of water to the mixture ).
Polyethers
Polyether impression
material was introduced in the late 1960s for use in dentistry .
Initially ,available only in a single "regular"viscosity, slight
modification of the viscosity is possible with the use of a diluent. More
recently a heavy light bodied system has been introduced.
Polyether impression materials tend to have a fast setting time of less
than 5 minutes and for this reason have been popular for the recording of
single preparations in general practice . (5) Like
addition cured silicones , they undergo an addition polymerization
reaction on setting which has no by – product resulting in a material
with high dimensional stability.
Properties and
handling characteristics
The ideal impression
material must exhibit certain characteristics in clinical and
laboratory environments .Clinically, it should produce a dimensionally
stable, accurate impression with optimal mechanical properties (optimal
yield strength, Young's modulus and coefficient of thermal expansion ) for
adequate elastic recovery and to resist tearing. Also, it should set
within a reasonable amount of time and demonstrate biocompatibility:
minimal amount of toxicity and hypoallergenic nature . It ought to be
hydrophilic for making a good impression. In laboratory environment ,it
should be dimensionally stable for accurate pouring of multiple casts
and shouldn't be affected by disinfection. Finally, reasonable cost is
recommended. (1 ,16) This ideal type of impression material is
difficult to obtain in reality. A detailed description of each property
will help in understanding how they interact. Lately, dentists have moved in
the direction of utilizing polyvinyl siloxanes and polyethers due to their
improved physical and mechanical properties (17-19). These
properties include improved dimensional stability, accuracy, wettability,
excellent elastic recovery , ease of handling, flexibility , tear resistance,
ability to pour impression several times and producing casts from one
impression and better ability to reproduce detail.(15)
Dimensional
accuracy
With elastomeric
impression materials, for example, polyvinyl siloxane ,polyether,
and polysulfide, the dimensional accuracy is generally a time dependent
process , with greater dimensional accuracy occurring immediately after
polymerization is complete but declining as the impression is stored for
long time . ( 18-20) Polyvinyl siloxane and polyether
impression materials remain dimensionally accurate for 1 to about fourteen
days. (1 8,1 9,21 ). Polysulfide impression material is
dimensionally accutate when poured within 1 to 2 hours of taking
the impression. (1 8,19) Practitioners must take this property
into consideration when choosing impression materials given the time
available to the practitioner to pour casts during office hours.
Hydrophilic
versus hydrophobic nature of impression materials
Hydrophilicity is one
of the main features of a modern impression material , as the
impression material is in close contact with wet hard and soft tissues.
The hydrophilic nature of an impression material relates to its ability to
work, flow in a wet environment .(22 )Hydrophilic materials exhibit a low
contact angle with water while hydrophobic materials show a contact angle
of 90° or greater . Hydrophilic materials contain the functional groups
[carbonyl (c=o) and ether (c–o–c)] that attract and interact with water
molecules.(23)
Dimensional
stability
The dimensional stability of an impression material reflects its capacity to
keep the accurate details of the impression over time.(25) This gives
the option to pour it at the convenience of the operator. Therefore
materials ought to have low shrinkage upon polymerizing and stay
stable. High impression dimensional stability materials typically can be
poured within 1 to about fourteen days after the impression is taken and
still produce an exact cast .(25,21 )Materials which have high dimensional
stability are the polyethers and polyvinyl siloxanes, unlike alginate
which has a low dimensional stability.The polysulfides suffer from
distortion over time . (
Wettability
Wettability of an impression material is related to the ability of the material
to flow into small areas and reproduce fine details.(25 ) Impressions
which wet the teeth well displace moisture and results in fewer voids.
Materials that have high wetting angle don't flow easily into small
areas and they are poorly used in fixed prosthodontics,while materials
that have low wetting angles flow extensively providing more accurate
impressions . (25,30) Finally, impression materials need to
readily flow into fine details in the range of 20 to 70 μm, which is
necessary for perfectly adapted bridges and crowns.
Elastic recovery
Elastic recovery of an
impression is refered to the ability of the material to return to its
original dimensions without permanent deformation upon removal from the
mouth.(25) There is no impression material has 100% elastic recovery
and for all impression materials the greater the depth of undercut, the
greater its permanent deformation.(21 ) PVS showed the best
elastic behavior, with over 99% elastic recovery, followed by polyethers
and polysulfides.(25)
Flexibility
Impression materials should be flexible to be easily removed from
the mouth when set . It is important to have an impression material
flexible enough to overcome the undercuts in the adjacent teeth.
Possible errors in impression taking
A high quality, accurate dental impression is important , as without it the
dental lab can't fabricate precise, well-fitting crown .Potential
impression errors ought to be distinguished before the impression is sent
to the lab.The most common dental impression eorrors include Poor tray
selection , inadequate impression material mixing , surface contamination , poor margin
detail ,internal bubbles, marginal tears,choosing the wrong impression material
and failing to keep the patient still.(33)
Poor tray selection
The selection of tray is important to capture the needed area
without distortion and provide the needed details. Trays ought to be
sufficiently large to cover all the teeth without contacting the soft
tissues. (figure1) Trays are accessible in many sizes small, mdium and large.
Also the shape of tray varies some are square while others are round .
Inadequate impression mixing
Once the impression
material is mixed , it should be uniform in color without
streaking. Streaking is more common with hand mixed materials than
cartridge materials. (figure 2) In case of hand mixing , the material
ought to be kneaded quickly to keep within the working time and yield a
uniform color when completed. Streaking may also happen if the automix
cartridge isnot bled prior to attaching the mixing syringe, allowing one
component to extrude out of the cartridge first .Therefore the cartridge
must be bleed right before a new automix tip is placed in order to ensure
that both base and catalyst are equally flowing to avoid mixing
problems. (33 ,36)
Surface
contamination
Surface contamination of an impression cause an unset ,tacky layer
of impression material.(figure 3) It might be due to core build up
material, adhesives or composites leaving a greasy covering on the tooth
preparation. Any of these can prevent the material from
setting effectively. Retraction cords and solutions containing aluminum
chloride or ferric sulfate can transfer sulfur to critical areas
preventing the setting reaction .Also utlizing the rubber dam can have the
same effect.(37,38) Impression can become contaminated with latex when
mixing by hand.
Poor marginal details
The margine is one of
the most critical parts of a dental impression.Without a precise
marginal impression , for example, overhanging , open margins or an
insufficiently fitting crown are almost occur . Commonly voids at the
margine due to inadequate retraction or accumulation of fluids prevent
flow of impression around the margins.(figure4) The usage of retraction cords
is the most perfect way to solve this problem.one procedure is to utilize
a double retraction cord.(39-41 )
Internal bubbles
Internal bubbles occur as a result of either air entrapment or fluid accumulation.(figure5) Fluid in the sulcus may interfere with capture of the restoration margins in the impression. Bubbles present on the margins ,may negatively affect the fit of the prosthesis.bubbles occuring due to fluid accumulation may be large enough to affect the long term success of the luting agent, that will fill now a wider space. The thicker the luting agent , the weaker the interface between the restoration and underlying tooth. .Achieving proper hemostasis will solve this problem.
References
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2-Clancy JM, Scandrett
FR, Ettinger RL. Long‐term dimensional stability of three
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3- Lin CC, Ziebert GJ,
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G. A clinical guide to applied dental materials. Elsevier Health
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Hydrocolloid technic for inlays and fixed bridges. D. Digest..
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AH. Accuracy and bond strength of reversible with
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Restorative Dental Materials. 10th ed.pp.281 -332. London : Mosby, 1997.
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