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TRANSCRIPT
Impressions in fixed partial denture 2009
INTRODUCTION:
Many Impression materials are suitable for use in dentistry. Impression materials are used to
record intraoral structures for the fabrication of definitive restorations. Accurate impressions are
necessary for construction of any dental prosthesis. The relationship between static and mobile
oral structures must be reproduced accurately for an optimum cast. Making a cast in gypsum
materials from an impression of dental anatomy aids dentists in designing and constructing fixed
prosthesis. The accuracy of these final restorations depends greatly on the impression materials
and techniques. The more common types of impressions are used for fabricating diagnostic and
master casts. Diagnostic casts are used to aid in treatment planning while master casts are used
for producing fixed partial dentures. Accurate impressions depend on identifying the applications
that do or do not fit each material’s characteristics. Materials used without adequate knowledge
of their characteristics can impair a successful outcome. Often, the choice of impression material
depends on the subjective choice of the operator based on personal preferences and past
experience with particular materials.
The dentist has a host of excellent impression materials available for making impressions in
fixed prosthodontics. With proper material selection and manipulation, accurate impressions can
be obtained for fabrication of tooth supported restorations. An acceptable impression must be an
exact record of all aspects of the prepared tooth. This means it must include sufficient
unprepared tooth structure immediately adjacent to the margins for the dentist and laboratory
technician to identify the contour of the tooth and all prepared surfaces. The contour of the
unprepared tooth structure cervical to the preparation margin is critical information that must be
available when the restoration is fabricated in the dental laboratory.1
All teeth in the arch and the soft tissues immediately surrounding the tooth preparation must be
reproduced in the impression. They will allow the cast to be accurately articulated and will
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Impressions in fixed partial denture 2009
contribute to proper contouring of the planned restoration. Particular attention is given to
reproducing the lingual surfaces of anterior teeth because they influence anterior guidance,
which determines the occlusal morphology of the posterior teeth. The impression must be free of
air bubbles, tears, thin spots, and other imperfections that might produce inaccuracies.
The patient's mouth is a challenging environment to make an accurate impression. Moisture
control is probably one of the most important aspects of successful impression making. Except
for the polyethers, all elastomeric impression materials are hydrophobic i.e. they do not tolerate
or displace moisture. Any moisture will result in voids. Consequently, saliva flow into the area
must be reduced and diverted to obtain the necessary dry field of operation.1
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REVIEW OF LITERATURE:
Fusayama T, Iwaku M, Daito K, Kurosaki N. and Takatsu T. in 1974 studied about the
accuracy of the laminated single impression technique with silicone materials. In this technique,
the heavy and wash types were mixed at the same time and the wash type was laminated in a thin
layer on the surface of the heavy type, and this is loaded in a tray and immediately impressed
upon the preparation. The accuracy of this technique was compared with single mix, double mix
without spacing and double mix with spacing. They found that the double impression technique
without spacing showed the greatest distortion with considerably decreased outer and increased
inner dimensions. The new laminated single impression technique produced the least distortion
though the difference in results between this technique and the double impression technique with
spacing was not statistically significant.2
Herfort T.W., Gerberich W.W., Macosko C.W., and Goodkind R.J. in 1978 compared the
tear strength of various elastomeric impression materials those included polysulfide, silicone and
polyethers. They found a direct relationship between shear modulus and difficulty in removing
impression from the mouth and the ease of removing an impression from the mouth was in the
order: light bodied silicones, light bodied polysulfides, heavy bodied silicones, heavy bodied
polysulfides and polyethers. On testing the maximum tear strength in 10-15 minutes, the
polysulfide materials showed three to six times the tear strength of silicones. The polyether
material displayed a tear resistance slightly higher than that of the silicones but one third to one
fifth as high as that of the polysulfides.3
Eames W B, Sieweke J C, Wallace S W and Rogers L B in 1979 studied the effect of bulk on
accuracy of various elastomeric impression materials. Trays were constructed using 2 mm plastic
on an Omnivac machine to provide 2, 4 and 6 mm spaces for impression materials. Their study
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showed that the interface space of 2 mm produced the most accurate impression than the 4 or 6
mm tray spaces.4
Lacy A.M, Fukui H, Bellman T and Jendresen M.D. in 1981 conducted a study to determine a
time –dependent accuracy of elastomer impression materials. The materials included were one
polyether, four polysulfide rubbers, and four polyvinyl siloxanes. The modes of impression
involved both putty/ wash systems and wash/ adhesive custom tray systems. The result of the
study showed that the polyether which was adhesively bonded to the custom trays showed
improved stability over condensation silicone and polysulfides used in the same fashion. They
concluded their study stating that: (1) polyvinyl siloxane (addition polymerization) silicones are
the most stable of elastomers currently available. (2) accuracy and consistency are best
maintained by use of custom tray and adhesives to retain polyvinylsiloxanes. (3) polysulfide
rubbers bonded to custom trays show a progressive increase in die diameter with time. (4)
polyether rubber is intermediate in stability to polysulfide or silicone systems and polyvinyl
siloxane when impression techniques involve adhesive bonding to custom-formed trays. (5)
polyvinyl siloxane putty/ wash systems may reveal some loss of accuracy of dies produced by
retrieval from multiple pours after 2-4 days, and (6) there appears to be no pronounced
differences between single mix and double mix techniques for polysulfides when both are both
are bonded to an acrylic resin custom tray.5
Wilson E G and Werrin S R in 1983 described the dual arch impression method with closed
mouth technique to obtain cast and die placement in centric occlusion in a single step. Complete
closure of the teeth was mandatory for registering the interocclusal relationship correctly. They
stated that the dual-arch or double-arch impression technique is extremely accurate and a viable
alternative to full-arch impressions. The accuracy of the interocclusal relationship was supposed
to be more likely because of the decreased number of steps in the technique.6
Valderhaug J. and Floystrand F. in 1984 studied the dimensional stability of
elastomeric impression materials in custom made and stock trays. The
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impression materials used for the study were polyether and silicone. Custom
trays were made of acrylic resin to obtain a thickness of 3 mm and non
perforated chromium plated brass trays of appropriate size were used as stock
trays. The distances between the abutment teeth were measured immediately on
removal of the impression and after 1 hour and 24 hours. Results indicated that
although ample amount of impression material was allowed (2-9 mm) the linear
dimensional stability of the impressions made in stock trays was not inferior to
custom trays.7
Vitre, Galburt and Maness in 1985 studied the biometric comparison of bur and electrosurgical
retraction methods. They recorded the free gingival tissue height after 4, 8, and 12 weeks. They
concluded that (1) the electrosurgical method showed more tissue loss at each time interval than
the bur method, (2) electrosurgical retraction showed more subject variability than the bur
method, (3) clinically, the tissue appearance of both group was indiscernable at the time intervals
tested, and (4) the electrosurgical method provided for a greater bulk of impression material at
the margin than the bur method.8
Tjan A H L, Whang S B and Sarkissian R in 1986 performed the clinically oriented evaluation
of the accuracy of commonly used impression materials. They used a simulated clinical model
and technique to compare the accuracy of stone casts produced from reversible hydrocolloid,
polysulfide rubber, polyether rubber, condensation curing silicone rubber and addition curing
silicone based rubber and to determine the effect of delay in pouring the impressions. They found
the impressions made in the elastomers were significantly better than reversible hydrocolloid
even though all impressions made were clinically acceptable. The data for addition curing
silicones indicated that equally accurate impressions were produced with putty wash and one
step impression technique.9
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Johnson G.H. and Craig R.G. in 1986 evaluated the accuracy of addition
silicones as a function of technique. They investigated four addition silicone
impression materials and one condensation silicone impression material with
three techniques i.e. putty-wash, single mix and double mix. They concluded
that the same accuracy was achieved with all techniques (i.e. putty-wash, single
mix and double mix) when addition silicones were used, whereas the putty-wash
technique produced the most accurate results for the condensation silicones.10
Cullen D R and Sandrik J L in 1989 conducted a study to determine the tensile strength of
elastomeric impression materials. They studied seven brands of elastomeric impression materials
including condensation and addition reaction silicones, polyethers and a light curing polymer
polyether urethane dimethacrylate. The tensile strength of each product was determined, and the
bond strength of various combinations within brands, reflecting various impression techniques
was tested. The study concluded that the reline technique, light-body cured to the previously
cured heavy-body putty, yielded results similar to the simultaneous-cure technique. No
advantage of one technique over the other was found in terms of bond strength. In general, the
tested elastomers exhibited greater tensile strength than bond strength.11
Gordon G.E. et al. in 1990 evaluated the effect of tray selection on the
accuracy of elastomeric impression materials. The tray systems selected for the
study were: a thermoplastic, custom-made (acrylic resin) and a stock tray
(plastic). Impressions were made of a stainless steel master die that simulated
crown preparation for a fixed partial denture and an acrylic resin model with
cross arch and anteroposterior landmarks in stainless steel that typify clinical
intra-arch distances. Results indicated that the thermoplastic custom tray
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produced dies as accurate as those of the custom acrylic resin tray. The stock
plastic trays consistently produced casts with greater dimensional change and
differed markedly from those of the standard for interpreparation, cross-arch,
and anteroposterior measurements. All of the tray systems performed similarly
in producing dies of nearly equal height for a given impression. In the
consideration of die height alone, tray selection may be unimportant. Regarding
the interpreparation dimensions, the stock tray routinely showed the greatest
change from the standard and produced dies ranging from 0.15%-0.33% change,
which represents a linear change of approximately 45µm to 100 µm greater than
the standard. Clinically, this change may result in fixed partial dentures that are
too long mesiodistally when constructed on casts made from stock trays.12
Gordon, Johnson and Drennon in 1990 studied the accuracy of stone casts made from
impressions using different tray and impression materials. The tray materials used were an
acrylic resin, a thermoplastic and a plastic as well as the impression materials used were an
addition silicone, a polyether and a polysulphide. The result of the study indicated that custom-
made trays of the acrylic resin and the thermoplastic material performed similarly regarding die
accuracy and produced clinically acceptable casts. The stock plastic tray consistently produced
casts with the greater dimensional change.12
Idris B, Houston F and Claffey N in 1995 compared the dimensional accuracy of one- and two-
step techniques with the use of putty/ wash addition silicone impression materials. They prepared
three metal tapered abutment preparations attached to a stainless steel base. These abutments
were made to simulate full crown preparations and were designed as follows: abutment A, full
crown preparation with no undercut; abutment B, V-shaped undercut 1 mm deep located below
the cervical finish line; and abutment C, wide c-shaped undercut of the same depth located below
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the cervical finish line. For measuring purposes all three abutments were prepared with cross-
grooves on the occlusal surfaces as reference points. For each technique 15 impressions of the
master model were made and poured in improved die stone. The interabutment and
intraabutment distances were measured with a toolmaker’s microscope. The result of their study
suggested that although statistically significant differences in accuracy can be found between the
techniques, these were not of sufficient magnitude to warrant the strong recommendation of one
technique rather than the other. Similarly, neither technique resulted in dies that deviated
sufficiently from the master model to cause clinically important difficulties in the fit of
castings.13
Livaditis G. S. in 1998 compared the methods and effectiveness of traditional fixed partial
denture impression systems that included the matrix impression system, in relation to the
registration of the finish lines and sulci of tooth preparation in the formation of full arch
impression. He believed that the matrix system cleanses blood and debris from the sulcus at the
critical moment similar to the copper-tube/ resin coping impression systems, virtually eliminate
tearing of the sulcular flange by developing the optimal configuration, deliver impression
material into the sulcus with speed and gentleness of a wash/reline impression but with the
accuracy of a copper-tube/ resin-coping system, eliminate collapsing forces to widen and hold
the sulcus open, use materials with preferred characteristics but short working times, allow
segmentation of a complex impression similar to the copper tube/ resin-coping system while
delivering a full arch elastomeric impression similar to the syringe/ tray system and allow fixed
prosthodontics impressions to be more predictable and simpler, although not necessarily time-
consuming.14
Millar B J, Dunne S M and Robinson P B in 1998 conducted an in vitro study of the number of
surface defects in monophase and two-phase addition silicone impressions. The number of voids
visible on the surface of impressions recorded in vitro were counted. Two monophase impression
materials in stock trays were compared with impressions recorded with two-phase addition
silicone materials in custom trays. A total of 200 automixed impressions, 50 with each material,
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were recorded of 50 dentoform molar teeth prepared for full veneer crowns. Impressions were
examined for surface voids anywhere on the prepared part of the tooth by a trained examiner.
They found no significant difference for voids between the monophase materials or between the
two-phase systems. However, both two-phase materials in custom trays had significantly fewer
surface voids than the two monophase materials used in stock trays. They concluded their study
saying that monophase addition-cured impression materials in stock trays carries an increased
risk of void formation on the surface of the impression when compared with two-phase addition
silicon materials in custom trays.15
Jokstad A. in 1999 compared the clinical performances among the different type of gingival
retraction cords. He used colored green (knitted, dl-epinephrine), white (twined, dl-epinephrine)
and blue (knitted, aluminium sulfate). Six criteria were formulated to evaluate the clinical
performance of retraction cords that included: (1) How easily was cord packed in a gingival
sulcus? (2) Did the cord fray during placement? (3) How rapidly did hemostasis occur? (4) How
much did the gingival sulcus dilate? (5) Was bleeding evident after removal of cords? (6) Did the
gingival sulcus remain dry after removal of cord?He concluded that knitted gingival retraction
cords were ranked better than twined cords, cords containing epinephrine performed better than
aluminium sulfate cords.16
Donovan T.E. and Chee W.L. in 2004 reviewed the contemporary impression materials and
techniques. They found most of the impressions sent to the dental laboratories showed the
deficiency recording the prepared subgingival margins of the preparation. This deficiency
resulted in the restorations with less than adequate marginal integrity. The second common
deficiency found was failure to follow basic principles inherent to the manipulation of
impression materials. Stock trays were used extensively and the importance of control of bulk
was ignored. Putty/ wash materials also were used extensively in an inappropriate manner,
resulting in impressions with less than optimal accuracy. Hence they urged the clinician to
review contemporary principles of impression materials and to familiarize themselves with
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various specialty impression techniques available and use them when indicated resulting in better
outcome.17
Kimoto K, Tanaka K, Toyoda M and Ochiai KT in 2005 studied the indirect latex glove
contamination and its inhibitory effect on vinyl polysiloxane polymerization. The purpose of
their study was to examine the surfaces of materials commonly used in restorative procedures
that were contaminated by indirect latex glove contact and then evaluated for the inhibition of
polymerization of VPS. The result of their study showed that particulate sulfur elements and
sulfur chloride compounds were present on the contaminated substrates whatever the way they
were cleaned. Those particulate sulfur and sulfur chloride compounds resulted in polymerization
inhibition of the tested VPS dental impression material.18
Samet N. Shohat M Linvy A and Weiss E I in 2005 evaluated the quality of 193 impressions
sent to commercial laboratories for the fabrication of FPD. The impression technique and
material used, tray type and number of prepared units were recorded for each impression. Data
relating to errors and faults, including defects in material polymerization, retention to tray, tissue
contact by tray, crucial areas beyond tray borders, heavy- bodied material exposure through the
wash material (for double step impressions), inadequate union of materials, retraction cords
embedded in impressions, and air bubbles, voids or tears along the margins were also
documented. Their study concluded that the impressions made with polyethers had the most
detectable errors, followed by condensation type silicones.19
Peregrina A, Land M.F., Wandling BS and Jhonston W.M. in 2005 studied the effect of
different adhesives on vinyl polysiloxane bond strength to two tray materials i.e.
autopolymerizing methylmethacrylate and light-polymerizing tray materials. They concluded
that for all impression materials tested, the universal spray-on adhesive consistently
demonstrated significantly lower bond strengths than all other adhesives. Equivalent or
significantly higher bond strength values were found for the universal paint-on adhesive than
with the adhesive supplied by the manufacturers of the impression materials tested.20
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DISCUSSION
IMPRESSION TRAYS:
According to Glossary of Prothodontic Terms, impression tray is defined as a device which is
used to carry, confine and control an impression material while making an impression.
Commonly used trays are Stock metal trays and Custom trays. The other types of trays available
are Rimlock trays, metal water coolant trays for reversible hydrocolloid impression and Dual
arch impression trays (Fig.2).
Classification of Impression Trays:
A. Based on whether they are prefabricated or individualized:
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1. Stock trays
2. Custom trays
B. Depending on presence or absence of perforations:
1. Perforated
2. Non perforated
C. Depending on whether they are meant for dentate or edentate individuals:
1. Dentulous trays
2. Edentulous trays:
3. Combination trays are also available for various partially edentulous situations based
on Kennedy’s classification.
D. Based on materials:
1. Metallic
2. Non metallic
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Impressions in fixed partial denture 2009
Fig.1
Stock trays are readily available and are rigid. They are of two types, Perforated and non
perforated trays. They are also available in Metal and Plastic materials. Stock trays are used for
taking putty-wash impression. For alginate, a perforated tray is preferred in order to retain the
impression. Non perforated trays may also be used but only in conjunction with an adhesive.
Non perforated trays are preferred for impression compound.
Advantages:
• Eliminates the time & cost of fabricating custom tray
• Metal stock trays are rigid & less susceptible to distortion
Disadvantages:
• Metal trays must be sterilized
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Impressions in fixed partial denture 2009
• More impression material required so thickness of the impression will be uneven
Custom trays (Individualized tray/special tray) are needed for atypical arch forms where an
appropriate stock tray is not available. Custom tray improves the accuracy of an elastomeric
impression by limiting the volume of the material, thus reducing stresses during removal and
thermal contraction. Although reducing the bulk of an elastomeric impression material increases
its accuracy, the opposite is true for reversible hydrocolloid impressions. Generally a custom tray
is made from autopolymerizing acrylic resin, although thermoplastic or photo polymerized resins
are sometimes used. Thermoplastic materials can be softened in a water bath and adapted either
manually or with a vacuum former with a heating element. The accuracy of impressions made
with a thermoplastic tray material or light-polymerized materials is comparable to that made with
an autopolymerized resin. Light-polymerized materials are convenient because a storage period
is not needed for the completion of polymerization.21
Requirements:
1. Should be rigid
2. Should be dimensionally stable
3. Should be easy to fabricate.
Advantages:
• Less impression material is required than for stock tray.
• Sterilization is not a big issue(single use)
• Less distortion of the impression.
Disadvantages:
• Time consuming
• The tray should age for 24 hours to minimize distortion
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Procedure of custom tray fabrication:
1. Using a pencil, mark the border of the tray on the diagnostic cast approximately 5 mm
apically to the crest of the free gingiva. Maxillary trays do not always necessitate
covering the entire palate, although this may be desirable if a removable appliance is
planned after completion of the fixed prostheses. Under no circumstances should the
posterior border extend farther than the demarcation between hard and soft palates.
2. Adapt a wax or other suitable spacer onto the diagnostic cast. Two layers of baseplate
wax will result in a combined thickness of approximately 2.5 mm.
3. Soften the wax by carefully heating it over a Bunsen burner or in hot water. Overheating
may melt it and produce an undesirable thin spot. Only light pressure should be applied.
4. After the second sheet of wax has been applied, trim it back until the pencil line is just
visible.
An alternative technique involves repeated dipping of the cast in molten wax. The cast is
thoroughly wetted and then dipped three or four times to obtain a sufficient and uniform
wax thickness (about 2 or 3 mm). This creates the space needed for the impression
material. Three stops are needed in the tray to maintain even space for the impression
material in the oral cavity. These are placed on noncentric cusps of teeth that are not to be
prepared (buccal cusps of the maxillary teeth and lingual cusps of the mandibular teeth).
If all teeth are involved, a larger soft tissue stop can be placed on the crest of the alveolar
ridge or in the center of the hard palate. Stops are made by removing wax at an angle of
45 degrees to the occlusal surfaces of three teeth that have a tripod arrangement in the
arch. This will lend stability to the tray, and the 45-degree slope will help center the tray
during insertion.
5. Apply a layer of tinfoil over the wax (which may melt from the polymerization heat of
the material) to prevent it from contaminating the inside of the tray.( Fig.2)
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Impressions in fixed partial denture 2009
6. Mix autopolymerizing acrylic resin according to the manufacturer's recommendations.
The use of vinyl gloves is recommended to prevent the development of sensitivity to the
monomer.
7. After the resin is mixed, set it aside until it is doughy (with the consistency of putty). A
template or a wooden slab and roller may help obtain a consistent thickness, although
with practice the resin can be thinned out accurately by hand. Care must be taken not to
stretch the material when manipulating it; thin areas in the resin may lead to a flexible
tray and produce distortions.
8. Gently adapt the resin to the cast. A handle made from the excess resin can be attached at
this time. If working time is unavailable, it can also be attached later with a separate
second mix of acrylic resin. Buccal ridges, which are helpful with impression removal,
can also be added.
9. After the material has polymerized, remove it from the cast and trim it with an acrylic-
trimming bur where the indentation made by the wax ledge is visible. All rough edges
should be rounded to prevent soft tissue trauma.
10. If necessary, fill defects in the stops with additional resin, wetting the set tray material
with monomer to ensure a good bond. To prevent the material from lifting up, some
pressure should be maintained during this phase.( Fig.3)
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Impressions in fixed partial denture 2009
Fig.2
Custom tray fabrication:
A. Armamentarium
B. Outlining the border of the tray on the diagnostic cast.
C. The tray should extend 3 to 5 mm from the gingival margin and about 3 mm beyond the most
distal tooth.
D. Softened baseplate wax is adapted to form a spacer. Typically two thicknesses will provide
the recommended 2 to 3 mm of space.
E. Spacer is trimmed to the pencil line.
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Impressions in fixed partial denture 2009
F. Wax is removed to form the tray stops. G, Covered with tinfoil. H, The foil is adapted to the
stops.
Fig.3I. Custom tray resin.
J. While it is still doughy, the resin is molded to a horseshoe shape (semicircle for maxillary
trays).
K. Wooden slab and roller used in an alternative method.
L. The resin is gently adapted to the cast, and the excess is trimmed. M and N, Resin is
moistened with monomer to attach the handle.
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Impressions in fixed partial denture 2009
O. When the resin has cured, the periphery is shaped with an acrylic-trimming bur.
P. Maxillary and mandibular custom trays.
Custom impression tray fabrication for fixed partial dentures has been greatly simplified with the
use of light-curing resins. These materials consist of a urethane dimethacrylate matrix, an acrylic
resin copolymer and microfine silica filler, and they are polymerized by exposure to 400 to 500
nm wavelength light in a photo-curing unit. Extended working time, lack of offensive odor,
limited equipment requirements, and decreased polymerization shrinkage are well-known
advantages of this material in comparison to autopolymerizing polymethyl methacrylate custom
tray resins
A modified light-curing method minimizes wax spacer heating and eliminates the need for tinfoil
adaptation or use of Air Barrier Coating material in the fabrication of light-cured custom trays.
PROCEDURE
This method requires the use of water in an electrical apparatus, the electrical power outlet that
supplies the light-curing unit must be equipped with an appropriately installed and tested ground
fault circuit interrupter. A suitable container for the water immersion procedure is the number
L16 transparent polypropylene container.
1. Mark the outline of the spacer and impression tray on the cast with a wax pencil.
2. Coat the cast with Model Release agent (Dentsply International, Inc) or petrolatum gel.
3. Soak cast in slurry water for 5 minutes to eliminate air. (Air released under the tray
during curing may produce distortion. Slurry water is recommended rather than tap
water to prevent erosion of the unprotected anatomical cast surface.)
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Impressions in fixed partial denture 2009
4. Place desired number of layers of softened baseplate wax spacer. Trim away excess
material to reveal outline for the spacer on the cast. Place occlusal or incisal stops as
needed.
5. Adapt Triad custom tray material to the cast and spacer. Trim to the outline of the tray on
the cast. Add a handle as desired.
6. Place cast with adapted tray material in the transparent propylene container. Add cool
water so that it just covers the highest point of tray material.
7. Place the container and its contents in the Triad curing unit.
8. Cure according to manufacturer’s recommended times. After the initial curing time,
remove the container and contents from the curing unit, remove the cast/tray from the
water bath, and gently separate the tray from the cast. Return the tray alone to the
container in an inverted position and adjust the water level until it just covers the highest
point of the tray material. Place the container in the unit and cure the tissue surface.
9. Trim, finish, and polish the custom tray. (Any additions of tray material can be cured in
the water bath as outlined above.) 21
With any system, tray rigidity is important, because even slight flexing of the tray will lead to a
distorted impression. For this reason, thin, disposable plastic trays are unacceptable. Resin
thicknesses of 2 to 3 mm are needed for adequate rigidity. It should extend about 3 to 5 mm
cervical to the gingival margins and should be shaped to allow muscle attachments. It should be
stable on the cast with stops that can maintain an impression. Clearance between the tray and the
teeth should also be 2 to 3 mm; however, greater clearance is necessary for the more rigid
polyether materials. To avoid distortion from continued polymerization of the resin the tray
should be made at least 9 hours before its use. When a tray is needed more urgently, it can be
placed in boiling water for 5 minutes and allowed to cool to room temperature. Direct contact of
custom or stock tray with gingival tissue in the critical portion of the impression can result in a
failed impression.12
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Impressions in fixed partial denture 2009
'Rimlock' Impression Trays offer dentists the chance to produce ideal impressions in a solid,
non-flexible tray, without the need to use impression adhesives. Rimlock trays incorporate a
unique 'beaded' edge that retains the impression material securely. They are of high quality
stainless steel that can be autoclaved. These are recommended for Putty wash and alginate
impressions. Its unique locking assures secure retention of material. Rim-Lock trays are available
in different sizes and arch configurations to suit practitioner’s needs and preferences.
Dual arch trays: The types of dual arch impression trays can be categorized as metal or plastic
with side walls or sideless and by the amount and location of the arch enclosed by the tray.
Metal trays are rigid and will not be deformed during the impression procedure. Some metal
trays are curved to approximate arch form while others are straight. Some trays can be bent to
more closely adapt to the particular arch form while others are too rigid. One type of metal tray
is disposable and has a fixed insert and very short side walls.
Plastic trays are flexible to varying degrees, depending on the shape and dimensions of the side
wall. So called sideless trays actually have very short side walls. They are the most flexible tray
design. One objective of this design feature was to produce a tray that would have the least
chance of impinging on anatomic structures thus avoid being deformed or displaced during the
impression procedure. Second objective of this tray design was to avoid problems with elastic
rebound from putty impression materials that are placed under loading by being forced between
the teeth, arch and walls of the tray during the impression procedure. Plastic trays come in many
sizes and shapes. Side walls can be long and made of thick, heavy plastic and are relatively
inflexible. Others have walls that are shorter or made of more flexible plastic. However none of
the plastic trays are as rigid as metal trays.
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Impressions in fixed partial denture 2009
Dual arch trays can also be categorized by the amount and location of the arch enclosed by the
tray. These categories include posterior sextant, three quarters of an arch, anterior sextant and
full arch.22
IMPRESSION MATERIALS:
There is an extensive variety of materials (Fig.4) for making a precision negative mold of soft
and hard tissues. In order of their historical development, the impression materials used in fixed
partial denture construction include:
1) Aqueous Hydrocolloids
a) Agar (reversible)
b) Alginate (irreversible)
2) Non-aqueous Elastomers
a) Polysulfide
b) Silicones
- Condensation
- Addition
c) Polyether
d) Hybrid Elastomers
Each material has advantages and disadvantages, and none is entirely free of shortcomings.
However, they all share one important characteristic: when handled correctly, they can produce
casts of sufficient accuracy and surface detail for the fabrication of clinically acceptable fixed
prostheses.
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Impressions in fixed partial denture 2009
Nevertheless, there are reasons for selecting one material over another. If it becomes necessary to
store the impression before a cast will be made, the polyethers and addition silicones are
preferable because they exhibit sufficient long-term dimensional stability, the other materials,
particularly the reversible hydrocolloids, must be poured immediately. If the impression will be
poured in epoxy or will be electroplated, reversible hydrocolloid should not be selected because
it is compatible only with die stone.
Depending on the manufacturer, many of the materials are available in cartridges for automixing
and tubes or containers for hand spatulation. The automixing products require no mixing pads or
spatulation, and training in their use is less time consuming. There may be less waste of material
associated with automixing and provide more bubble-free mix resulting in more accurate casts.
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Impressions in fixed partial denture 2009
Ideal characteristics of impression materials:
An ideal impression material should exhibit certain characteristics in the clinical and laboratory
environment. Clinically it should produce an accurate impression secondary to its adaptability to
oral structures, have a consistency that is dimensionally stable to resist tearing but results in an
Page 24
Fig.4 Various impression materials used in fixed partial prosthodontics
Impressions in fixed partial denture 2009
atraumatic removal, set within a reasonable amount of time, demonstrate biocompatibility to
include a hypoallergenic nature, and have a reasonable cost per use. In a laboratory setting, it
should be dimensionally stable for accurate pouring of multiple casts and should not affect
dimensional accuracy upon disinfection.
Reversible Hydrocolloid (Agar):
Reversible hydrocolloid (also called agar hydrocolloid or simply hydrocolloid) was originally
derived as a natural product of kelp. However, the material currently available is considerably
different.
History:
• In 1925, Alphous Poller of Vienna was granted a British patent for a totally different type
of impression material.
• Later Poller's 'Negacoll’ was modified and introduced to the dental profession as
'Dentacol’ in 1928.
• In 1937 introduced by Sears, Agar was first into dentistry for recording crown
impressions and was the first elastic impression material available.
Examples:
• Slate Hydrocolloid (Van R)
• Superbody
• Super syringe
Composition:
• Fillers; diatomaceous earth, clay, silica, wax, rubber, and similar inert powders.
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Impressions in fixed partial denture 2009
• Thymol and glycerine act as bactericide and plasticizer respectively.
If poured immediately, reversible hydrocolloid produces casts of excellent dimensional accuracy
and acceptable surface detail. At elevated temperatures, it changes from a gel to a sol. This
change is reversible-i.e., as the material cools; the viscous fluid sol is converted to an elastic gel.
Agar changes from gel to sol at 99° C (210° F) but remains a sol as low as 50° C (122° F),
forming a gel only slightly above body temperature. These unique characteristics (hysteresis) are
very favorable for its use as an impression material.
Reversible hydrocolloid is supplied in a range of viscosities. Generally a heavy-bodied tray
material is used with a less viscous syringe material. The required temperature changes are
effected with a special conditioning unit and water-cooled impression trays.
Advantages:
• Accurate reproduction of surface detail
• Hydrophilic
– displace moisture, blood, fluids
• Inexpensive
– after initial equipment
• No custom tray or adhesives required
• Pleasant in taste and flavor
• No mixing required
Disadvantages:
• Initial expense where special equipment is needed.
• Material needs to be prepared in advance.
• Thermal shock to the patients in few cases.
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Impressions in fixed partial denture 2009
• Tears easily upon removal from mouth.
• Dimensionally unstable and hence
– Should be poured immediately
– Only once can be poured and hence a single cast can be obtained.
• Difficult to disinfect
This impression technique requires a special conditioning unit (Fig.5) which consists of 3
thermostatically controlled water baths.
1. A liquefaction bath.
2. A storage bath.
3. A tempering bath.
Procedure:
Page 27
Fig.5 Special conditioning unit used for agar impression material
Impressions in fixed partial denture 2009
Select the correct size of water cooled impression trays. Place small modeling compound or
prefabricated stops in the tray to prevent over seating. Fill the impression tray with heavy bodied
material from the storage bath and place it in the tempering bath. Load the syringe material in the
syringe and replace it in the storage bath. Now carefully remove the retraction cord and flood the
tooth with water. Inject the syringe material on the tooth and cover the entire tooth. Remove the
impression tray from the bath wipe with the gauze and place it in the mouth. After seating cold
water is circulated through the tray until the impression material is set. Remove the impression
with a rapid motion, wash with cold water and evaluate for accuracy.
Irreversible Hydrocolloid (Alginate):
At the end of the 20th century, a chemist from Scotland noticed that certain brown seaweed
(algae) yielded a peculiar mucous extraction. He named it algin. This was later identified as a
linear polymer with numerous carboxyl acid groups and named anhydro-β-d-mannuronic acid
(also called alginic acid).
When the agar impression material became scarce because of World War II (Japan was a prime
source of agar), research was accelerated to find a suitable substitute. This result was present
alginate impression material. The general use of irreversible hydrocolloid far exceeds that of
other impression materials available because it is easy to manipulate, it is comfortable for the
patient and relatively inexpensive also.
Examples:
– Jeltrate (Dentsply/Caulk)
– Coe Alginate (GC America)
– Integra
– Superjel
– Tropicalgin
_ Xantalgin
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Impressions in fixed partial denture 2009
Classification of alginate:
I. According to setting time.
• Fast set 1.25 – 2min
• Regular set 3 – 4.5 min
II. According to concentration of filler
• Soft set
• Hard set
Composition:
Components: Function: Weight percentage:
Sodium / Potassium alginate Soluble alginate 15
Calcium sulfate dihydrate Reactor 16
Potassium titanium Fluoride Accelerator 3
Zinc oxide Filler particles 4
Diatomaceous earth Filler particles 60
Sodium phosphate Retarder 2
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Impressions in fixed partial denture 2009
Modified alginates:
1. Alginate in the form of sol containing the water but no source of calcium ions
Two component system (paste form)
- alginate sol
- calcium reactor
2. Alginates modified by the incorporation of silicone polymers (paste form) that results in fine
detail reproduction, good tear resistance and good dimension stability
3. Dustless alginate: Glycerin is incorporated to agglomerate the particles making the powder
denser so that no dust is formed when the lid is opened after tumbling.
4. Newer alginates: MILLENIUM ALGIN is considered an alginate of new generation with
maximum preciseness of 15 µ. The formulation was optimized to reduce natural contraction with
water loss. The impression can be stored for many hours in a Long Life Bag before casting the
model without undergoing any change for over 100 hours. Here no hand mixing is required so
that it saves time and material. Delivery options is by Cartridge or automix dispensing and
impressions remain stable where pouring can be done immediately or when it is convenient.
Impressions retain dimensional stability for months.
Manipulation:
Fluff or aerate the powder by inverting the can several times. This ensures uniform distribution
of the filler before mixing. Then measured amounts of powder and water are taken as specified
by the manufacturer (Fig.6). Mixing is done in a clean flexible plastic bowl with a clean wide
bladed, reasonably stiff metal spatula and it is better to use separate bowls for plaster and
alginate as plaster contamination can accelerate setting. The mixing is started with a stirring
motion to wet the powder with water. Once the powder has been moistened, rapid spatulation by
Page 30
Impressions in fixed partial denture 2009
swiping or stropping against the side of the bowl is done or vigorous figure-eight motion can also
be used.
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Fig.6 Hand manipulation of alginate impression material
Impressions in fixed partial denture 2009
• It can also be mixed with MIGMA™ Alginate Mixing Machine (Fig.7)
Properties:
Reproduction of Tissue Detail
• ADA specification requires the material to reproduce a line that is 0.075 mm in width.
• Detail reproduction is lower when compared to agar hydro-colloid.
Dimensional Stability:
• Set alginates have poor dimensional stability due to evaporation, syneresis and
imbibition. Therefore, cast should be poured immediately.
• If storage is unavoidable, keeping in a humid atmosphere of 100% relative humidity
(humidor) results in the least dimensional change.
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Fig.7 MIGMA™ Alginate Mixing Machine
Impressions in fixed partial denture 2009
Elasticity and Elastic Recovery
• Alginate hydrocolloids are highly elastic but less when compared to agar and about
97.3% elastic recovery occurs.
• Thus permanent deformation is more for Alginate (about 1.2%).
Tear strength:
• Varies from 350-700 gm/cm2
• Increased when the impression is removed with a snap
Compatibility:
• Pouring of stone mixture to fill the impression should start from one end of the arch.
After the impression has been filled with stone, it may be placed in either a humidor or a
2% potassium sulfate solution while the stone hardens in an atmosphere of approximately
100% relative humidity.
• Cast should be kept in contact with the impression, preferably for 60 min or minimum 30
min.
• If the cast is allowed to remain in contact with the hydrocolloid impression overnight, a
chalky stone surface may be produced.
Advantages:
• Inexpensive
• Easy to use
• Hydrophilic
– displace moisture, blood, fluids
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Impressions in fixed partial denture 2009
• Can be used in stock trays
Disadvantages:
• Poor tear resistance
• Dimensionally unstable
– Should be poured immediately within 10-12 minutes.
– Only single pouring possible.
• Lower detail reproduction
- Unacceptable for working cast of fixed prosthodontics but is used for partial frame
work impressions.
• High permanent deformation
• Difficulty in sterilization and disinfection
NON AQUEOUS ELASTOMERIC IMPRESSION MATERIALS:
These are synthetic rubbers that mimic natural rubber and initially called as rubber impression
materials. Currently it is referred as elastomers or elastomeric impression materials. ADA
Specification no. 19 identifies as “nonaqueous elastomeric dental impression materials”
Based on viscosity it has been classified as:
• low, medium, high, putty
• Single phase or monophase – single component material with sufficient shearing
potential that it can be used as the syringe material and the tray material.
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Impressions in fixed partial denture 2009
Polysulfide
• The is the first elastomer used for dental impressions
• Sometime it is referred as :
– By the type of material, such as rubber base impression material
– By the processing terminology, such as vulcanizing impression material
– By chemistry, such as mercaptan impression material or
– By the name of one of the first manufacturers, such as the Thiokol Corporation.
Polysulfide is supplied as Paste in collapsible tubes as base and accelerator where base is white
and accelerator is brown or grey in color.
Examples:
– Permlastic (Kerr)
– Omni-Flex (GC America) – copper hydroxide system.
– Coe-flex (GC-Amer)
– Neo-plex (Miles)
Composition:
1. Base
Polysulfide polymer
Lithopone and titanium dioxide (filler)
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Impressions in fixed partial denture 2009
Dibutyl phthalate (plasticizer)
Small quantity of sulfur
2. Catalyst (accelerator paste or reactor)
Lead dioxide,
Plasticizer,
Oleic and stearic acid as retarders
Available in 3 viscosities,
• Light body, medium body and heavy body.
Tray adhesive:
• Butyl rubber or styrene/ acrylonitrile dissolved in a volatile solvent such as chloroform or
a ketone
Manipulation:
With the proper lengths of the two pastes squeezed onto a mixing pad or glass slab, the
catalyst paste is first collected on a stainless steel spatula and then distributed over the
base, and the mixture is spread out over the mixing pad. The mass is then scraped up with
the spatula blade and again smoothed out. If sufficient force is applied and spatulation is
performed rapidly, the material will seem thinner and easier to handle and this
phenomenon is known as pseudoplasticity. Most accurate impressions are made using
custom acrylic trays because of uniform thickness of the material.
Mean working time
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Impressions in fixed partial denture 2009
• 6.0 min at 23 degree
• 4.3 min at 37 degree
Mean setting time
• 16 min at 23 degree
• 12.5 min at 37 degree
Elasticity improves with time, so longer the impression can remain in the mouth before removal,
greater will be the accuracy. Polysulfide ranks as one of the least stiff of the elastomeric
impression materials. Unset material has high level of viscosity. This thick consistency of the
uncured material helps displace an unwanted fluid present while seating the impression. Also,
the excess material extruded from the tray does not flow easily because of the high viscosity,
reducing the potential discomfort to the patient during seating of a tray.
Advantages Disadvantages
Long working time Requires a custom tray
Proven accuracy Must be poured in stone immediately
High tear resistance Potential for significant distortion
Less hydrophobic Odor offends patients
inexpensive to use Messy and stains clothes
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Impressions in fixed partial denture 2009
Long shelf life Second pour is less accurate
Condensation silicone:
It is also known as conventional silicone. In condensation reaction, polymerization reaction in
which the polymer chains all grow simultaneously and a reaction byproduct is formed.
Available in 3 viscosities
– Light body
– Medium body
– Putty body
Paste – two pastes in collapsible tubes
Putty – jars
Examples
– Speedex (Coltene/Whaledent)
– Primasil (TISS Dental)
– Accoe (GC-Amer)
– Xantopren (Unitek)
– Elasticon (Kerr)
– Cuttersil (Miles)
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Impressions in fixed partial denture 2009
Composition:
The base is a paste form, which is mainly Polydimethyl Siloxane.
The catalyst is of low-viscosity liquid to which colloidal silica filler is added to form a paste.
The polymerization occurs as a result of cross linkage between the orthoethyl silicate and the
terminal hydroxy group of the dimethyl siloxane, to form a three dimensional network. The
reaction is exothermic (1deg C. rise).
Properties:
• Setting time is 8-9 minutes
• Mixing time is 45 sec
• Tear strength 3000gm/cm lower than polysulfide
• Hydrophobic – area should be dried. Avoid air bubbles while pouring the cast
• It is stiffer and harder than polysulfide. The hardness increases with time.
• The spacing in the tray is increased to 3mm to compensate for the stiffness.
• Can be plated with silver/ copper. Silver plating is preferred.
• Shelf life is slightly less than polysulfides due to the unstable nature of the orthoethyl
silicates.
Advantages:
• Better elastic properties
• Clean, pleasant
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Impressions in fixed partial denture 2009
• Can be used in stock tray
- putty-reline technique
• Good working and setting time
Disadvantages:
• Permanent deformation is quite high up to 1-3%
• Poor dimensional stability
– high shrinkage due to polymerization and evaporation of ethanol
– pour immediately within 30 minutes
• Hydrophobic that cause poor wettability
Addition Silicones:
They are frequently known as polyvinylsiloxane or vinyl polysiloxane impression materials.
It is supplied in 4 viscosities:
– Light body
– Medium body
– Heavy body
– Putty
Examples:
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Impressions in fixed partial denture 2009
– Extrude (Kerr)
– Express (3M/ESPE)
– Aquasil (Dentsply Caulk)
– Genie (Sultan Chemists)
– Virtual (Ivoclar Vivadent
Composition:
Base paste contains polymethyl hydrogen Siloxane and filler.
Catalyst paste contains divinyl Polydimethyl Siloxane, platinum catalyst and filler.
Improper balance between the vinyl siloxane and silane siloxane contribute to the liberation of
hydrogen gas and to overcome this manufacturer adds palladium (scavenger) or wait for an hour
before pouring the impression.
Properties:
• Setting time – 5-9 min
• Mixing time – 45 sec
• Best dimensional stability among elastomers.
• Curing shrinkage 0.17%
• Permanent deformation 0.05-0.3%
• Tear strength -3000gm/cm
• Extremely hydrophobic
• Electroplated with silver or copper.
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Impressions in fixed partial denture 2009
• Low flexibility
• Harder than polysulfide – extra spacing 3 mm should be provided in the impression tray.
• Care should be taken while removing the cast from the impression to avoid any breakage.
• Shelf life ranges from 1-2 years
Advantages:
• Excellent reproduction of surface details
• Highly accurate
• High dimensional stability and hence can be poured up to one week
• Used for both Stock or custom trays
• Multiple casts
• Easy to mix
• Pleasant odor
Disadvantages:
• Expensive
• Polyvinyl siloxane contamination is usually a result of sulfur or sulfur compounds usually
seen in dental office in the form of latex gloves or rubber dams. Sulfur as a reactive
element may interfere with the silicon polymerization with the chlorplatinic acid catalyst
in VPS impression material. The sulfur containing chemical zinc diethyl dithiocarbamate
used during latex glove fabrication is a preservative and vulcanizing accelerator.18
• Short working time
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Impressions in fixed partial denture 2009
• Lower tear strength
• Possible hydrogen gas release that can lead to bubbles on die.
(Palladium is added to absorb the hydrogen gas)
Moisture control is required before impression making.
Modifications in PVS:
• Adding palladium scavenger to tie up any hydrogen gas
• Less hydrophobic, due to surfactant that has been added to overcome this drawback.
Most commonly, nonionic surfactants are used and various surfactant sprays are also
available. The mode of action is believed to be a diffusion controlled transfer of
surfactant molecules.
HYDROFLEX is having early wetting property and superior elasticity and tear strength.
Polyether:
First material introduced for dental impressions and introduced in Germany in late 1960s. It is
available as base and accelerator in collapsible tubes. The accelerator tube is usually smaller and
earlier it was supplied in single viscosity. A third tube containing a thinner was also provided.
• Available in 3 viscosities
– Light body
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Impressions in fixed partial denture 2009
– Medium body
– Heavy body
• Examples
– Impregum F (3M/ESPE)
– Permadyne (3M/ESPE)
– Pentamix (3M/ESPE)
– P2 (Heraeus Kulzer)
– Polygel (Dentsply Caulk)
Composition:
Base contains Polyether polymer, colloidal silica as filler and a plasticizer such as phthalate.
Catalyst paste contains the alkyl aromatic sulfonate, a colloidal silica filler and phthalate
plasticizer.
Polyether based polymer that is cured by the reaction between aziridine rings and the main chain
is a copolymer of ethylene oxide and tetrahydrofuran. Cross linking is brought about by an
aromatic sulfonate ester and it is an exothermic reaction of 4-5deg C.
Properties:
• Elasticity – stiffest of impression materials
– New formulations of medium or regular body are less stiff.
– When used in a thinner section decreases the stiffness.
• Impressions must be kept in cool, dry environment.
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Impressions in fixed partial denture 2009
• Sulfonic ester may cause skin reaction. To avoid this, mix thoroughly before making an
impression and direct skin contact should be avoided.
• Setting time is around 8.3 min.
• Mixing time - 30 sec.
• Heat increases the setting time.
• Is extremely stiff – flexibility 3%
• Its hardness is higher than polysulfides and increases with time. Removing it from
undercuts is difficult, so extra spacing 4mm should be given.
• Tear strength is good 3000gm/cm
• Hydrophilic – so moisture in the impression field is not so critical. It has the best
compatibility with stone.
• Can be electroplated with silver or copper.
• Shelf life > 2 years.
Advantages:
• Highly accurate
• Good dimensional stability
• Least hydrophobic
• Good surface detail
Page 45
Impressions in fixed partial denture 2009
• Pour within one week
– kept dry
• Multiple casts
• Good wettability
• Good shelf life
Disadvantages:
• Expensive
• Short working time
• Rigid so that it is difficult to remove from undercuts
• Bitter taste
• Low tear strength
• Absorbs water that changes dimension and leaches its components.
Modifications: Reducing the stiffness and producing polyether in low and heavy viscosities
have been the major changes.
Mixing systems:
Three types of mixing systems can be used for elastomeric impression materials:
1. Hand Mixing
2. Auto Mixing
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Impressions in fixed partial denture 2009
3. Mechanical Mixing.
Hand mixing(Fig.8):
Equal length of catalyst and paste are dispensed on paper pad/glass slab and the mixture
is spread up with the spatula blade until a uniform color is obtained.
All the 4 types of elastomers are available for mixing in this fashion.
In case of putties, the best mixing technique is to knead the material with base fingers.
Fig. 8
Automixing (Fig.9):
Here base and catalyst are in separate cylinders of the plastic cartridge that is placed in a
mixing gun.
The gun has 2 plungers advanced by ratchet mechanism to extrude equal quantities of
base and catalyst.
Page 47
Impressions in fixed partial denture 2009
The 2 components are forced through mixing tip that consists of internal spiral resulting
in a uniform mix.
Fig. 9
Mechanical mixing (Fig.10):
The catalyst and base supplied in plastic bags housed in a cartridge is inserted into the top
of mixing machine.
When the button is depressed, uniform mix is available from the plastic mixing tip that is
placed on front of the machine.
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Impressions in fixed partial denture 2009
Fig. 10
Comparison of Properties of impression materials:
Working time:
- longest to shortest
Agar > polysulfide > silicones > alginate = polyether
Setting time:
- shortest to longest
Alginate < polyether < agar < silicones < polysulfide
Stiffness:
- most to least
Polyether > addition silicone > condensation silicone > polysulfide = hydrocolloids
Tear strength:
- greatest to least
Polysulfide > addition silicone > polyether > condensation silicone >> hydrocolloids
Cost:
- lowest to highest
Alginate < agar = polysulfide <condensation silicone < addition silicone < polyether
Dimensional stability:
Page 49
Impressions in fixed partial denture 2009
- best to worst
Addition silicone > polyether > polysulfide > condensation silicone > hydrocolloid
Wettability:
- best to worst
Hydrocolloids > polyether > hydrophilic addition silicone > polysulfide > hydrophobic
addition silicone = condensation silicone
Compatibility with gypsum products:
- best to worst
Hydrocolloids > hydrophilic addition silicone > polyether > polysulfide > hydrophobic
addition silicone = condensation silicone
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Impressions in fixed partial denture 2009
IMPRESSION TECHNIQUES:
1. Copper tube and resin coping method.
2. Monophase / Single viscosity technique
3. Dual viscosity technique/Multiple mix single step technique
4. Putty wash technique
a. Single stage procedure/Simultaneous/Squash technique
b. Two stage procedure
c. Injection moulded technique
5. Dual arch impression technique
a. Monophase impression
b. Multiple mix technique
c. Laminar impression technique
d. Hydraulic pressure technique
6. Segmental impression technique
7. Matrix impression system
8. Wet Technique/ Reversible hydrocolloid technique
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Impressions in fixed partial denture 2009
9. Hydrocolloid Laminate technique
10. Impression using preformed crown shells
11. Functional check-bite impression
COPPER TUBE / RESIN COPING SYSTEM
It is a modified method of making impression with impression compound. Here rigid carrier of
impression material becomes the part of impression. It is a retraction less procedure where
retraction cords are not used.
A copper band (Fig.11) is selected and annealed by heating on a flame and quenching in alcohol
and mark the finish line with the explorer and round off the edges. Then evaluate the fit and cut
orientation hole in top one fifth of the facial surface. Now red stick compound is heated over the
Bunsen flame and the warm compound mass is inserted to fill approximately the top third of the
copper band then seat and orient on to preparation, and compress excess in to the band. The
compound should just touch the occlusal surface. Remove and evaluate the impression; only the
occlusal surface should be impressed and remove 0.2 mm of the compound to create space for
the heavy body poly vinyl silicone. Drill a hole through the centre of the compound plug.
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Impressions in fixed partial denture 2009
Fig.11
For making the impression, Make 4 to 5 holes evenly distributed above the bottom of the copper
tube for the retention of the impression material and coat the internal surface sparingly with
adhesive .Clean and isolate the preparation and mix heavy viscosity material and inject in to the
band and position it on the tooth and wait the material to set remove the band and inspect the
impression.23
MONOPHASE / SINGLE VISCOSITY TECNIQUE
In single viscosity (Fig.12), impressions are often taken with medium-viscosity impression
materials of polyether and addition silicone with resin custom tray of 2-4 mm spacing. Only one
mix with part of the material loaded in tray and another portion loaded in the syringe. When
medium-viscosity material is forced through the syringe the viscosity is reduced (shear thinning
effect) whereas viscosity of same material in the tray is unaffected. Success depends on the
pseudoplastic properties of these two materials.
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Impressions in fixed partial denture 2009
Advantages:
Easy to use
Simple technique
Excellent handling properties,
Accuracy
Medium viscosity material in a custom tray with 3mm spacer
Useful in cases where utmost accuracy is not needed
Disadvantages:
More polymerization shrinkage than heavy body
Surface reproduction is not very good
Selection of tray
In single viscosity technique resin custom trays are used.
Preparation of custom tray:
Adapt a wax or other suitable spacer to the diagnostic cast. Two layers of baseplate wax will
result in a combined thickness of approximately 2.0 to 4.0mm. Soften the wax by carefully
heating it over a Bunsen burner or in hot water. Three stops are needed in the tray to maintain
even space for the impression material in the oral cavity and these are placed on noncentric cusps
of teeth that are not to be prepared. Mix autopolymerizing acrylic resin according to the
manufacturer's recommendations and gently adapt the resin to the cast. A handle made from the
Page 54
Impressions in fixed partial denture 2009
excess resin can be attached at this time. After the material has polymerized, remove it from the
cast and trim it with an acrylic-trimming bur.
Making final impression:
Impressions are often taken with medium-viscosity impression materials of polyether and
addition silicone with resin custom tray. One mix with part of the material loaded in tray and
another portion loaded in the syringe.
Insertion of the tray:
Seat the tray with the impression material in patient’s mouth and hold and wait for the final set.
Removal of the tray:
Remove the tray parallel to the preparation path of withdrawal and the impression is evaluated
for the finish lines and for any distortion and tear.
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Impressions in fixed partial denture 2009
Fig.12
DUAL VISCOSITY TECHNIQUE:
Here low-viscosity material is injected with a syringe on the tooth surface and high-viscosity
material is mixed and placed in impression tray. After injecting the low-viscosity material, tray
containing high-viscosity material is placed in the mouth.
Selection of tray:
In dual viscosity technique too resin custom trays are used.
Preparing of custom tray:
Custom trays are fabricated the same way as for single viscosity technique.
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Impressions in fixed partial denture 2009
Making final impression:
Impressions are often taken with low-viscosity and high-viscosity impression materials of
addition silicone with resin custom tray. Here low-viscosity material is injected with a syringe
on the tooth surface and high-viscosity material is mixed and placed in impression tray. After
injecting the low-viscosity material, tray containing high-viscosity material is placed in the
mouth.
Insertion of the tray:
Seat the tray with the high-viscosity impression material in patient’s mouth and hold with gentle
pressure until the final set.
Removal of the tray:
Remove the tray parallel to the preparation path of withdrawal and the impression is evaluated
for the finish lines and for any distortion and tear.
Assistant’s role:
The role of assistant becomes important here as he or she mixes the light body material and loads
in a syringe. By the time the operator is injecting the light body material around the preparation
margin, the assistant will mix the heavy body material and load into the tray and will give to the
operator. Hence the work becomes easy and the quality of impression is improved.
PUTTY-WASH TECHNIQUE/RELINE TECHNIQUE
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Impressions in fixed partial denture 2009
The putty wash technique is commonly used in making impressions with silicone elastomers and
it is recommended to overcome the problems associated with polymerization shrinkage of the
condensation silicone impression materials. The instability of condensation type is attributed to
the production of an alcohol byproduct during polymerization. For this two-step putty/wash
technique was developed in which the use of a thin layer of wash material minimizes the amount
of alcohol byproduct and thereby retains the dimensional stability within acceptable limits. It is
also recommended for additition silicone impression materials even though these materials
appear to be dimensionally stable.
1. TWO STAGE PROCEDURE(Fig.13):
This is a 2 step procedure where a preliminary impression is made with stock tray where high or
putty consistency material is used before cavity preparation is made. These results in intraoral
custom made tray formed by the putty. Now space is provided for a low viscosity material by
cutting away some of the “tray” material or by using a thin polyethylene sheet, rubber or wax
sheet as a space. After cavity preparation, low-viscosity material is syringed into the area and
preliminary impression reinserted.
Advantages:
1) Eliminates time and expense of fabricating custom trays.
2) Metal stock trays are rigid and are not susceptible to distortion.
3) Impression of teeth can be captured with the wash material.
Disadvantages:
1) More impression material is required
2) Extra chair side time
3) Distortion
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Impressions in fixed partial denture 2009
Technique:
A Stock tray is selected based on shape and size of the patients arch. Now coat the tray evenly
with adhesive on the inside. Then mix the high viscosity putty impression material according to
manufactures instruction and roll putty into elongated cylinder and insert into the stock
impression tray. Cover putty with the spacer (a sheet of polyethylene) then insert and seat the
tray with a rocking type of motion and hold and wait until initial set (approximately 2 min). Now
remove from the mouth with minimal side ward movement and wait and test for final set till the
impression rebound completely. Then Peel of the spacer and remove excess impression material
with a sharp knife.
For making final impression, mix the low viscosity impression material according to the
manufactures instructions and load the low viscosity impression material on to a syringe .The
syringe material is loaded to the inaccessible area first like distolingual line angle, and insert the
low viscosity impression material into the tray without overfilling it. Seat the tray and wait for
the final set. Now remove the tray parallel to the preparation path of withdrawal and the
impression is evaluated for the finish lines and for any distortion and tear.24
Selection of tray:
Here Stock tray is selected based on shape and size of the patients arch.
Making final impression:
Mix the high viscosity putty impression material according to manufactures instruction and roll
putty into elongated cylinder and insert into the stock impression tray. Cover putty with the
spacer and seat the tray and hold and wait until it sets. Then Peel of the spacer and remove
excess impression material with a sharp knife.
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Impressions in fixed partial denture 2009
For making final impression, mix the low viscosity impression material according to the
manufactures instructions and load the low viscosity impression material on to a syringe. The
syringe material is loaded to the inaccessible area of prepared tooth first and the low viscosity
impression material is inserted into the tray without overfilling it. Seat the tray and wait for the
final set.
Insertion of the tray:
Seat the tray with the high-viscosity putty impression material in patient’s mouth with a rocking
type of motion and hold and wait until initial set. Then insert the low viscosity impression
material into the tray without overfilling it. Seat the tray and wait for the final set.
Removal of the tray:
Remove the tray parallel to the preparation path of withdrawal and the impression is evaluated
for the finish lines and for any distortion and tear.
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Impressions in fixed partial denture 2009
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Impressions in fixed partial denture 2009
Fig. 14
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Impressions in fixed partial denture 2009
2. SINGLE STAGE PROCEDURE/ Squash technique
For single stage procedure, stock tray with the unset putty material should already be loaded in
the tray and the preparation syringed with low viscosity impression material. Now seat the tray
with unset putty over the unset light body material and wait for the final set. Now remove the
tray parallel to the preparation path of withdrawal and the impression is evaluated for the finish
lines and for any distortion and tear. This method is unacceptable because it is impossible to
control the thickness of impression material and excess bulk is used. It is impossible to control
what material records the margin details of the preparations. Usually portions of the prepared
margin are captured in the putty, and putty materials are essentially deficient in their ability to
record marginal detail due to hydraulic pressure of putty pushes the syringe material in critical
areas.
Advantages:
a. Reduce chair side time
b. Saving of impression material.
Disadvantage:
a. Putty may displace wash material so that critical areas of the preparation will be
reproduced in putty.
b. Occasional ledges can be seen at the junction of the putty and wash material.24
Selection of tray:
Stock tray is selected based on shape and size of the patient’s arch..
Making final impression:
Here stock tray with the unset putty material should already be loaded in the tray and the
preparation syringed with low viscosity impression material
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Impressions in fixed partial denture 2009
Insertion of the tray:
Seat the tray with unset putty over the unset light body material and wait for the final set.
Removal of the tray:
Remove the tray parallel to the preparation path of withdrawal and the impression is evaluated
for the finish lines and for any distortion and tear.
Another Approach:
In this method putty material is used to fabricate a custom tray. It is fabricated in the same
manner as with PMM materials or light cure. One layer of base plate wax is placed over the
diagnostic cast as a spacer and wax is removed from nonfunctional cusps to provide occlusal
stops. A putty impression is made in a stock tray and a custom tray results. For making final
impression, mix the low viscosity material and load it into the syringe. Now syringe the material
onto the prepared teeth and Seat the tray and wait for the final set. Now remove the tray parallel
to the preparation path of withdrawal and the impression is evaluated for the finish lines and for
any distortion and tear.24
Preparation of tray:
Here putty material is used to fabricate a custom tray. It is fabricated with a putty impression is
made in a stock tray and a custom tray results.
Making final impression:
For making final impression, mix the low viscosity material and load it into the syringe . Now
syringe the material onto the prepared teeth and Seat the fabricated putty material custom tray
and wait for the final set.
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Impressions in fixed partial denture 2009
Insertion of the tray:
Seat the fabricated custom tray with low viscosity material in the mouth and wait for the final
set.
Removal of the tray:
Remove the tray parallel to the preparation path of withdrawal and the impression is evaluated
for the finish lines and for any distortion and tear.
INJECTION MOULDED PUTTY WASH TECHNIQUE: (Fig.15)
Preoperative full arch putty impression is recorded in a rigid plastic tray and the putty is prepare
to make the temporary restoration. Now the putty is modified by drilling a hole through the putty
from the outer surface of the tray to allow greater flow of light body material and buccal escape
channel is created. After tooth preparation, tooth is cleaned & retraction cord is removed. Now
putty is reinserted & checked for position on the tooth and mixed light body is injected thru the
hole into the preparation. Tray must be held while injecting light body to prevent it from lifting
off the tooth and wait for the final set.
Selection of tray:
Here rigid plastic trays are used to make preoperative putty impression to make the temporary
restoration.
Making final impression:
For making final impression mixed light body is injected thru the hole prepared in preoperative
putty impression into the preparation.
Insertion of the tray:
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Impressions in fixed partial denture 2009
Seat the fabricated custom tray with low viscosity material in the mouth and wait for the final
set.
Removal of the tray:
Remove the tray parallel to the preparation path of withdrawal and the impression is evaluated
for the finish lines and for any distortion and tear.
Most wash/reline procedures are used because of their simplicity. The theory supporting these
procedures is more subject to question, and success is more random than predictable. The
wash/reline methods generate unfavorable collapsing forces on the gingival tissue that affects the
degree of tissue displacement and registration of sub gingival tissues.
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Fig.15 Injection moulded putty wash impression technique
Impressions in fixed partial denture 2009
The wash/reline method provides even less control in the delivery of sulcular material. The low
viscosity reline material is expected to be driven into the sulcus by the heavier viscosity tray
material or the closely adapted lined tray. Contradictions occur in this concept. The same factors
that are expected to drive the impression material into the sulcus have an impact on the gingival
tissue collapsing the tissue against the prepared teeth and, in effect, inhibiting the entry of
impression material into the sulcus. The heavier the consistency of tray material, the greater will
be the impact on gingival tissue. The closer the adaptation of the lined tray the greater the
collapsing forces on the gingival tissue. All impression procedures should make detailed
reproductions of the prepared teeth and allow the flow of material into the sulcus, while
simultaneously facilitating displacement of gingival tissue.
In putty-wash technique, errors in manipulation may lead to inaccurate impressions which are the
following:
1. An excess bulk of wash material can result in dimensional changes proportional to the
thickness of the material during setting.
2. Undercuts or projections into the putty affect the accuracy of the impression.
3. Inaccurately seating the putty tray containing the wash material may lead to an excess
bulk of wash material around the abutment teeth, resulting in wash material being forced
away from the abutment tooth because of over seating and a non-uniform layer of wash
left around the abutment.
4. Some manufacturers provide a polyethylene spacer to be placed over the putty for
making the putty impression. This procedure has the major drawbacks of (1) no
landmarks, guiding planes, or posterior stops left in the putty impression, making
accurate replacement of the tray during the wash stage difficult, and (2) an inaccurate or
arbitrarily repositioned tray that cannot achieve a uniform wash space. For this a
technique is presented ensuring exact reseating of the putty impression tray and creation
of a uniform wash space, which are essential for accurate results.
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Impressions in fixed partial denture 2009
Technique to overcome the errors in manipulation –
The following technique is suggested to overcome the problems of errors in manipulation:
1. The putty impression is made with resin provisional restorations in place on the prepared
teeth and allowed to set. The provisional’s act as occlusal stops for seating of the
impression tray. If the restoration is a fixed partial denture, individual resin restorations
made on a stone cast are used so that a uniform impression is made of the edentulous
space.
2. When the impression is removed, the provisional restorations are detached from the set
putty. A wash space, equivalent to the dimensions of the provisional restoration, will be
created in the putty surrounding the prepared tooth. When design of the finish line is such
that an adequate bulk of wash material cannot be achieved because of the thin margins of
the provisional restoration, a strip of putty corresponding to the finish line should be cut
away.
3. To enable accurate reseating of the putty impression and venting away excess wash
material, all undercuts, projections into embrasures or tooth material should be cut away
from the putty before loading of the wash material.
4. The exact amount of wash needed to fill the wash space cannot be clinically calculated.
An excess of wash is usually present. To ensure venting out this excess, escape channels
should be cut into the putty from the gingival margin to the outer border of the tray. A
No. 8 round bur or No. 15 scalpel blade can be used for this purpose. The previous
removal of the embrasure projections will also assist in venting away excess wash
material.
5. Large embrasure spaces in the dental arch should be blocked out before impression
making. Utility wax can be used for this purpose. This block out facilitates removal of the
tray once the wash material has set, avoiding distortion on withdrawal, possible
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Impressions in fixed partial denture 2009
separation of the putty wash complex from the stock tray, and damage to teeth with
diminished periodontal support.25
DUAL ARCH IMPRESSION TECNIQUE
Synonyms:
Dual quad tray, triple arch, accu -bite, closed mouth impression, close bite double arch method.
The dual-arch technique can be used successfully as long as the operator understands the
indications and contraindications of the procedure. This technique captures the prepared teeth,
the opposing arch, and the occlusal articulation in maximum intercuspation simultaneously. This
technique should be used with a maximum of two prepared teeth. This should be used only with
patients that have existing anterior guidance. For this technique to be successful, the patient must
be able to close completely in MIP with the impression tray in place. In this technique rigid
metal trays are ideal. Cox et al supported the use of dual arch technique for the fabrication of 3
units FPD also.
Requirements
The articulator should provide for a positive stop or there should be sufficient teeth to maintain
the vertical dimension.
There should be sufficient space distal to the terminal tooth in the arch to allow tray
approximation.
Advantages
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Impressions in fixed partial denture 2009
• Clinical simplicity and accurate recording of MIP position.
• Here closed-mouth technique is used that eliminates any mandibular flexure that might be
associated with opening.
• Less impression material is needed.
• Less gagging may occur.
Disadvantages
• The tray is not rigid.
• Limited to one casting per quadrant.
• The distribution of impression material is not uniform
Technique –a. Dual arch-Multiple mix technique(Fig.16)
Evaluate the fit of the tray in the patient’s mouth and observe the complete bilateral closure and
the patient’s comfort. Now practice till the patient is familiar with the task. Make the final
impression by mixing the low viscosity material and load the syringe. Then the high viscosity
material is loaded in the tray. After the low viscosity material is syringed the tray is positioned
on the arch. Instruct the patient to close the mouth and observe for the interdigitation on the
opposite arch and wait for the material to set. Instruct the patient to open the mouth remove the
tray by applying equal pressure bilaterally and evaluate the impression.
Selection of tray:
Here custom dual arch trays are used
Making final impression:
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Impressions in fixed partial denture 2009
For making final impression, mix the low viscosity material and load the syringe. Then the high
viscosity material is loaded in the tray. After the low viscosity material is syringed the tray is
positioned on the arch.
Insertion of the tray:
The tray is positioned with high viscosity material on the arch and observed for the
interdigitation on the opposite arch and waited for the material to set.
Removal of the tray:
Remove the tray by applying equal pressure bilaterally and evaluate the impression.
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Impressions in fixed partial denture 2009
Fig. 16
b.Dual arch-Monophase Impression
In single viscosity/monophase, impressions are often taken with medium-viscosity impression
materials. Only one mix with part of the material loaded in tray and another portion loaded in the
syringe. When medium-viscosity material is forced through the syringe the viscosity is reduced
whereas viscosity of same material in the tray is unaffected. After the low viscosity material is
syringed the tray is positioned on the arch. Instruct the patient to close the mouth and observe for
the interdigitation on the opposite arch and wait for the material to set. Instruct the patient to
open the mouth remove the tray by applying equal pressure bilaterally and evaluate the
impression.
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Impressions in fixed partial denture 2009
Selection of tray:
Here custom dual arch trays are used.
Making final impression:
For making final impression, mix the low viscosity material and load the syringe. Then the high
viscosity material is loaded in the tray. After the low viscosity material is syringed the tray is
positioned on the arch.
Insertion of the tray:
The tray is positioned with high viscosity material on the arch and observed for the
interdigitation on the opposite arch and waited for the material to set.
Removal of the tray:
Remove the tray by applying equal pressure bilaterally and evaluate the impression.
c. Laminar impression technique
The laminar impression technique is a precise, rapid, and predictable alternative to traditional
methods of impression-making in fixed prosthodontics. A preliminary impression is made using
plastic “triple-arch” type trays and high-stiffness vinyl polysiloxane jaw relation registration
material. After tooth preparation, tissue management, and retraction this registration can be used
for provisional fabrication.
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Impressions in fixed partial denture 2009
Two holes are then drilled through the facial wall of the tray into the region of the preparation.
The tray is replaced in the mouth and light-bodied vinyl polysiloxane impression material is
injected into the holes by using an “automixing” gun system.
Advantages:
When the laminar impression technique is used, the occlusal registration, opposing arch
impression, and final impression are combined in one modular unit. By achieving multiple
objectives, a significant time-saving is achieved over more traditional methods. Precision
injection of the wash material avoids waste of excessive impression material. In addition,
injection into an open system does not generate compressive forces that can force impression
material intramucosally and cause a foreign body reaction until removed. The flushing action of
injecting a continuous flow of impression material aids in the removal of sulcular contaminants
and produces clear, detailed impressions of critical regions.11
D.Hydraulic pressure technique:
Low or medium viscosity impression material is injected around the prepared tooth and into the
preoperative impression of the unprepared tooth. The dentists reinsert the preoperative
impression and the patient closes into maximum intercuspation. The generated hydraulic
pressure forces the wash material into the sulcus and around the preparation. Vent holes can be
drilled on the buccal or lingual aspect of the impression material and tray to allow the escape of
excess wash material.22
SEGMENTAL IMPRESSION TECNIQUE
The segmental impression technique (Fig.17) offers a procedure that predictably permits making
a successful impression with multiple prepared teeth. With this technique the arch to be
impressed is broken down into easily managed segments. This usually is arbitrarily determined
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Impressions in fixed partial denture 2009
as two prepared teeth per segment. Individual custom trays for each segment with 1 mm of wax
spacer are used on the diagnostic cast. The tray should extend 3mm past gingival margin of the
prepared teeth because there are no occlusal stops and the gingival tissue must prevent over-
seating of the trays and the PMMA custom tray made 24 hrs in advance.
Procedure:
Here Light body is loaded into a syringe and one of the segmental trays. Now the impression
material is injected around the preparations and the tray with the material is seated into the place
and excess extruded material is removed from around the tray, and the impression material is
allowed to set. The tray is not removed. This procedure is repeated with each of the segments
until each segmental impression is in place. At this point an over-impression is made with a
compatible impression material in a stock tray. This impression is then handled in a conventional
manner. This technique has proven useful in extensive cases and moisture control is difficult in
specific patients.
Selection of tray:
Here both segmented custom trays with 1mm spacer are used to take multiple prepared teeth and
stock trays are used to take over impression.
Making final impression:
Here Light body is loaded into a syringe and one of the segmental trays.
Now the impression material is injected around the preparations and the tray with the material is
seated into the place and the impression material is allowed to set. The tray is not removed. This
procedure is repeated with each of the segments until each segmental impression is in place. At
this point an over-impression is made with a compatible impression material in a stock tray. This
impression is then handled in a conventional manner.
Insertion of the tray:
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Impressions in fixed partial denture 2009
The segmented tray is positioned with light body material and allowed to set and an over
impression is made with compatible impression material.
Removal of the tray:
Remove the tray parallel to the preparation path of withdrawal and the impression is evaluated.
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Impressions in fixed partial denture 2009
Fig. 17
MATRIX IMPRESSION SYSTEM
This is a new system that requires a series of three impression procedures, using three types
and/or viscosities of impression materials. It attempts to overcome the deficiencies of the older
systems and at the same time incorporate their best features
Series of impressions using three different viscosities of impression materials are made.
Step I: Matrix is made in occlusal registration elastomeric impression material over the
prepared teeth.
Step II: Definitive impression is made with high viscosity impression material in matrix.
Step III: Matrix impression(s) are seated in position and impression of entire arch is
made with medium viscosity impression material in stock tray (seated over matrix and
remaining teeth).
Procedure:
A matrix of occlusal registration elastomeric material is made over the tooth preparations. The
matrix may be made in one piece or in two or more sections, depending on the distribution and
complexity of the preparations. The matrix is trimmed to prescribed dimensions and, after the
retraction cord is removed, a definitive impression is made in the matrix of the preparations with
a high viscosity elastomeric impression material. After the matrix impression(s) is seated, a stock
tray filled with a medium viscosity elastomeric impression material is seated over the matrix and
remaining teeth to create an impression of the entire arch.
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Impressions in fixed partial denture 2009
This system effectively controls the four forces (relapsing, retraction, displacement, and
collapsing) that impact on the gingiva during the critical phase of making the impression when
attempting to register the sub gingival margins. The design of the matrix also gently forces the
high viscosity impression material along the preparations and into the sulcus where it cleanses
the sulcus of unwanted debris and fills the sulcus. The high viscosity material gently extends the
sulcus and does not permit it to collapse as the medium viscosity material in the stock tray is
seated for the pick-up impression. The matrix facilitates the formation of the optimum flange.
Tearing is virtually eliminated because of the improved configuration of the sulcular flange and
by the elimination of voids or contaminants in the sulcus.26
REVERSIBLE HYDROCOLLOID TECHNIQUE / WET TECHNIQUE: (FIG. 18)
This impression technique requires a special conditioning unit which consists of 3
thermostatically controlled water baths.
Inject the syringe material on the tooth cover the entire tooth and then remove the impression
tray from the bath wipe with the gauze and place it in the mouth. After seating cold water is
circulated through the tray until the impression material is set. Now, remove the impression with
a rapid motion, wash with cold water, evaluate for accuracy.
Procedure
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Impressions in fixed partial denture 2009
Select the correct size of water cooled impression trays and place small modeling compound or
prefabricated stops in the tray to prevent over seating .Fill the impression tray with heavy bodied
material from the storage bath and place it in the tempering bath. Now Load the syringe material
in the syringe and carefully remove the retraction cord and flood the tooth with water and inject
the syringe material on the tooth and cover the entire tooth. Remove the impression tray from the
bath and wipe it with the gauze and place it in the mouth. After seating, cold water is circulated
through the tray until the impression material is set. Remove the impression with a rapid motion,
wash with cold water, evaluate for accuracy.
Selection of tray:
Select the correct size of water cooled impression trays and place small modeling compound or
prefabricated stops in the tray to prevent over seating.
Making final impression:
Fill the impression tray with heavy bodied material from the storage bath and place it in the
tempering bath. Now Load the syringe material in the syringe and flood the tooth with water and
inject the syringe material on the tooth and cover the entire tooth. Remove the impression tray
from the bath and wipe it with the gauze and place it in the mouth. After seating, cold water is
circulated through the tray until the impression material is set.
Removal of the tray:
Remove the impression with a rapid motion, wash with cold water, evaluate for accuracy.
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Impressions in fixed partial denture 2009
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Impressions in fixed partial denture 2009
Fig. 18
HYDROCOLLOID LAMINATE TECHNIQUE:
Syringe agar : Injected around preparation.
Chilled Alginate mix: In tray, promptly seated on the top of agar.
Advantages:
- More accurate surface reproduction by agar.
- Agar more compatible with gypsum.
- Elimination of water-cooled impression trays.
- Economical than newer elastomeric impression materials
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Impressions in fixed partial denture 2009
Disadvantages:
- Stiff and therefore difficult to remove without rocking.
- Break seal and rock slightly to prevent tearing: Low tear resistance.
IMPRESSION USING PREFORMED CROWN SHELLS/ POLYCARBONATE
CROWN:
Prefabricated temporary crown shells (Fig. 19) are used for each tooth preparation & a final over
impression using a stock tray.
Procedure:
Adjust gingival margin and proximal contacts and coat internal & external surfaces of
provisional crown with an adhesive. Then mix regular body & fill crown shell. While filling
entrapment of air should be avoided and seat crown shell until it covers the finish line. Allow
material to set and finally make a pick up impression using regular body in a complete arch stock
tray.27
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Impressions in fixed partial denture 2009
FUNCTIONAL CHECK-BITE IMPRESSIONS:
The functional “checkbite-impression” is a combination record derived from adding functional
records to the static simple “checkbite-impression.” This static “checkbite-impression” is a
combined record used for making inlays, crowns, and fixed partial dentures. Because of its
simultaneous recording of prepared teeth and their precise relationship to the opposing teeth, the
cast restorations made from such a record need less adjusting in the mouth than castings made
from separate impressions and interocclusal records.
The basic “checkbite-impression” procedure is a prerequisite to the functionional “checkbite-
impression.” Therefore, the making of a simple checkbite-impression for a fixed partial denture
will be described starting at the point where the abutments have been prepared and the gingival
crevices packed. A unilateral checkbite-impression tray without the occlusal insert is tried in the
mouth. The patient is told about the procedure and what is meant by the request to “bite,” and is
cautioned to maintain contact between the upper and lower teeth until instructed to “open,” at
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Fig. 19 Prefabricated polycarbonate crowns
Impressions in fixed partial denture 2009
which time he is to separate the teeth forcefully but carefully. In the meantime, the dental
assistant mixes sufficient regular body rubber-base impression material to fill both the tray and
the syringe and the dentist injects impression material from the syringe over and around the
preparations while the assistant is filling the “checkbite-impression” tray on both sides of the
occlusal insert. The dentist places the tray in the mouth and instructs the patient to bite. The tray
is allowed to come to rest against the lips and is not held while the material sets. Then the patient
is told to open the mouth. The “checkbite-impression” obtained is an accurate static record of the
teeth and their occlusal relationship.
Making the functional check-bite impression:
After the gingival packing has been removed and it is certain that the abutments are dry, the
impression material from the syringe is injected over and around the preparations, the filled tray
positioned in the mouth, and the patient is instructed to bite. The impression material is allowed
to cure until prodding with an instrument shows that it is set, at which time the patient is told to
“open.”28
SALIVA CONTROL
Depending on the location of the preparations in the dental arch, several techniques can be used
to create the necessary dry field of operation (Fig.20). In areas where only supra gingival
margins are present, moisture control with a rubber dam is probably the most appropriate
method. However, in most instances a rubber dam cannot be used, and absorbent cotton rolls
must be placed at the source of the saliva. In the maxillary arch, placing a single cotton roll in the
vestibule immediately buccal to the preparation and a saliva evacuator in the opposing lingual
sulcus is usually sufficient. When working on a maxillary second or third molar, multiple cotton
rolls must sometimes be placed immediately buccal to the preparation and slightly anterior to
block off the parotid duct, which opens just anterior to the maxillary first molar. If the cotton roll
does not stay in position, it can be retained with a finger or the mouth mirror.
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Impressions in fixed partial denture 2009
When making a mandibular impression, placing additional cotton rolls to block off the
sublingual and submandibular salivary ducts is usually necessary. An alternative to multiple
cotton rolls is placement of one long roll "horse-shoe fashion" in the maxillary and mandibular
mucobuccal folds. However, when part of the cotton is saturated, the entire roll must be replaced.
The use of moisture-absorbing cards is another method for controlling saliva flow. These cards
are pressed paper wafers covered with a reflective foil on one side. The paper side is placed
against the dried buccal tissue and adheres to it. The tongue can be a problem when working in
the mandibular arch for that a disposable saliva ejector designed to displace the tongue may also
be effective. One more method is by using svedopter, which is a metal saliva ejector attached
with tongue deflector that is used for isolating mandibular teeth when the patients in a near
upright position.
When saliva control is especially difficult, a medication with antisialagogic action may be
considered. Dry mouth is a side effect of certain anticholinergics. This group of drugs includes
atropine, dicyclomine, and methantheline. Anticholinergics should be prescribed with caution in
older adults and should not be used in any patient with heart disease. They are also
contraindicated in individuals with glaucoma, because they can cause permanent blindness.29
Clonidine, an antihypertensive drug, has successfully reduced salivary output. It is considered
safer than anticholinergics and has no specified contraindications. However, it should be used
cautiously in patients who take hypertension medication.
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Impressions in fixed partial denture 2009
Fig. 20
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Impressions in fixed partial denture 2009
TRAY ADHESIVES:
Although stock trays often provide mechanical retention for elastomeric impression materials,
manufacturers typically recommend the use of an adhesive, whether a stock or custom tray is
used. The adhesive is applied to the impression tray and allowed to dry. Although minimal
information about adhesive composition for elastomeric impression materials is available, more
accurate and consistent impressions are obtained when adhesives are used. The minimally
required strength for adhesion between the elastomeric impression material and tray to prevent
debonding is not known. However, clinical experience has demonstrated that a strong bond is
desired to prevent undetected inaccuracies in impressions that contribute to non-fitting
restorations.20
However an impression made with a highly accurate material can be rendered absolutely useless
if it detaches from the tray, as the material is withdrawn from contact with the oral tissues.
During this withdrawal the bond between the impression material and the tray is highly stressed
in both tension and shear and often the adhesive is the main factor resisting these stresses. So it is
extremely important that the adhesive bond between the impression material and tray should be
of sufficient strength to withstand the forces generated during removal of the set material and
this bond is a critical factor in the production of a satisfactory final restoration.30
The adhesives furnished with the various types of rubber impression materials are not
interchangeable. Adhesives employed with the poly-sulfide rubber impression materials include
butyl rubber or styrene/acrylonitrile dissolved in a suitable volatile solvent such as chloroform or
a ketone. The base for the adhesive employed with the silicone rubber materials may contain
poly dimethyl siloxane or a similar reactive silicone, and ethyl silicate. The poly dimethyl
siloxane acts as an adhesive for the rubber, and hydrated silica forms from the ethyl silicate to
create a physical bond with the impression tray.
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Impressions in fixed partial denture 2009
Available in two types:
1. Paint on adhesives Eg Coltene, Kerr Universal VPS.
2. Spray adhesives Eg Sili spray.
Applied for 10-15 minutes before making impression
Block-out technique for impressions of teeth with increased open gingival embrasures:
The gingival embrassure that is normally filled with gingival papilla can become patent and
enlarged. The cause of these enlarged spaces is varied and can result from increased crown
length after periodontal surgical treatment, increased gingival recession, supra-eruption, or
improperly aligned and/or angulated teeth. Accurate impressions of teeth with these undercut
gingival embrasures can be a challenge. An impression with these clinical conditions can tear
when removed or result in a distorted impression and cast.
A common method to minimize the effect of undercuts has been to fill these areas with wax or an
alternative material, such as Oraseal caulking material or Cavit temporary stopping material.
This method prevents tears of the impression and the potential distortion. Waxes and other
commonly selected materials for blockout can be swiftly placed intraorally, but they are easily
displaced and could result in distortion.31
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Impressions in fixed partial denture 2009
DISPLACEMENT OF GINGIVAL TISSUES:
Tissue displacement is commonly needed to obtain adequate access to the prepared tooth to
expose all necessary surfaces, both prepared and not prepared. This is most effectively achieved
by placement of a displacement cord (generally impregnated with a chemical agent). Sometimes
gingival tissue is excised with a scalpel or with electrosurgery and must be done without
jeopardizing periodontal health but gingival retraction can cause recession and damage to free
gingiva.
Displacement Cord: Some enlargement of the gingival sulcus can be obtained by placing a non-
impregnated cord (mechanical) and leaving it in place for a sufficient length of time. The cord is
pushed into the sulcus and mechanically stretches the circumferential periodontal fibers where
single cord technique (Fig.21) is used for 1-3 prepared teeth with healthy gingival health and
double cord technique (Fig.22) is used for multiple prepared teeth with compromised gingival
health. Better sulcus enlargement can be achieved with a chemically impregnated cord (chemo
mechanical) or by dipping the cord in an astringent (e.g., Hemodent) where these materials
contain aluminum or iron salts and cause a transient ischemia, shrinking the gingival tissue. Even
so, the sulcus closes quickly (less than 30 seconds) after the cord is removed; therefore, the
impression must be taken immediately. A series of knitted and twined gingival retraction cords
was introduced that was impregnated with epinephrine or aluminium sulphate. Placement is
often easier if a braided (e.g., Gingibraid)* or a knitted (e.g., Ultrapak) cord is used. Epinephrine
should be used with caution, because it may cause a tachycardia," particularly if it is placed on
lacerated tissue. Unfortunately retraction with chemically impregnated cord is not always
sufficient for the reversible hydrocolloid impression technique, therefore electro surgery is
recommended.32
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Impressions in fixed partial denture 2009
Fig. 21
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Impressions in fixed partial denture 2009
Fig. 22
Electro surgery: An electro surgery unit is used for minor tissue removal before impression
making. In this technique, the inner epithelial lining of the gingival sulcus is removed, thus
improving access for a sub gingival crown margin and effectively controlling postsurgical
hemorrhage. Unfortunately, there is the potential for gingival tissue recession after treatment.
An electro surgery unit (Fig. 23) works by passage of a high-frequency current (1 to 4 million
Hz) through the tissue from a large electrode to a small one. At the small electrode, the current
induces rapid localized polarity changes that cause cell breakdown. It is contraindicated on
patients with any electronic medical device (e.g., a cardiac pacemaker, TENS unit, insulin
pump)" or patients with delayed healing as a result of debilitating disease or radiation therapy
and it is not suitable on thin attached gingiva (e.g., the labial tissue of maxillary canines).33
Another method of tissue removal which is comparable to electrosurgery, is use of a 12-fluted
flame shaped carbide bur. This method can provide adequate retraction for reversible
hydrocolloid impression techniques with less damage to the gingival tissues. Introducing copper-
tube or resin-coping procedure also keep the tissue retracted until complete set of impression
materials.
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Fig. 23 Electrosurgical unit
Impressions in fixed partial denture 2009
Retraction cord procedure seems to be less traumatic when compared to electrosurgery or
rotatory gingival curettage and lowers the risk of gingival recessions caused by the impression.
Various materials used are:
1. Heavy weight Rubber dams.
2. Aluminum shell.
3. Mechanical Pack of Zinc oxide eugenol.
4. Rolled cotton or synthetic cord.
Recent Advances: (Fig.24)
1. Gingifoam.
2. Expasyl TM.
3. Affinis/Magic foam cord.
4. Meroce1
5. Gel-cord.
6. Stay-put retraction cord.
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Impressions in fixed partial denture 2009
7. Comprecap.
8. Z-twist weave
9. Lasers.
Fig. 24
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Impressions in fixed partial denture 2009
DISINFECTION OF IMPRESSIONS:
After being removed from the patient's mouth, the impression is immediately rinsed with tap
water and dried with an air syringe. Suitable chemicals should be used, such as glutaraldehyde
solutions or iodophor sprays34 are most commonly recommended techniques for the impression
materials and chemical agents such as sodium hypochlorite can also be used. Some are perfectly
acceptable for one material but unsuitable for others. Because of its tendency to distort and
absorb moisture, polyether or "hydrophilic" addition silicone impression materials should be
sprayed and stored in a plastic bag rather than submerged and soaked in a gluteraldehyde
solution. Disinfection is an essential step for preventing cross-infection and exposure of
laboratory personnel. If it is performed properly, disinfection will not affect the accuracy or
surface reproduction of the elastomer. Special care must be taken with water-based materials and
polyether’s to insure that adequate immersion times are used to eliminate microorganisms but
that extended immersion times are avoided to prevent excess imbibition of the disinfecting
solution and distortion of impression.35
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POURING OF IMPRESSION:
One of the most important manipulative variables with impression materials is the time limit
after removal from the mouth to when the impression is poured. Water-based materials should be
poured within 10 minutes of removal from the mouth. The major component of these
impressions is water, which evaporates at room temperature. This water loss is accompanied by
distortion and is minimized by rapid pouring.
Condensation silicones produce ethyl alcohol as the by-product of the setting reaction, and
evaporation of the alcohol results in distortion. The identical phenomenon occurs with
polysulfide rubber, where the by-product is water. These materials should be poured within 30
minutes for maximum accuracy.
Polyether materials can absorb water from the atmosphere and thus should be poured within 1
hour for maximum accuracy. PVS impression material is stable because there is no volatile by-
product to the reaction and because they do not give off or absorb water. This dimensional
stability permits pouring of the impression at the convenience of the operator.24
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Impressions in fixed partial denture 2009
CONCLUSION AND SUMMARY:
An impression or negative likeness of the teeth and surrounding structures is used to obtain a
cast, on which the planned restoration is fabricated. A good impression is an exact negative
replica of each prepared tooth and must include all of the prepared surfaces and an adequate
amount of unprepared tooth structure adjacent to the margin.
Healthy soft tissues and the control of saliva flow are essential for a successful impression.
However, caution must be exercised to prevent injury to the gingiva. Cotton rolls, cords, and
saliva evacuators are needed for adequate moisture control. During the impression procedure,
using a local anesthetic to minimize discomfort and to reduce saliva flow is recommended. Both
mechanical-chemical and surgical methods for enlargement of the gingival sulcus can be used to
obtain access to sub gingival margins of prepared teeth. However, a narrow cord impregnated
with a mild astringent (e.g. Alcl3) is recommended. Deformation of gingival tissues occurs
during retraction and impression procedures which involves four forces:
1. Retraction: downward and outward movement of the free gingival margin.
2. Relapse: gingival cuff to go back to its original position
3. Displacement: downward movement of the gingival cuff
4. Collapse: tendency of the gingival cuff to flatten under forces
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Impressions in fixed partial denture 2009
To protect the smear layer, excessive contact between haemostatic agents and cut tooth structure
should be avoided. A custom acrylic resin tray should be used when making an impression with
any of the elastomeric materials.
This topic outlines the type of impression materials, ideal properties of impression materials and
several impression techniques for fixed partial denture. A number of ideal properties for
impression materials that includes accuracy, elastic recovery, dimensional stability, flow,
flexibility, workability, hydrophilicity, long shelf life, patient comfort and economics.
Impression materials vary considerably in relation to these ideal properties, and these differences
may provide a basis for the selection of specific materials in specific clinical situations. Special
attention is paid to polyvinyl siloxane impression materials because they have become the most
widely used impression materials (49.8%) in dentistry followed by condensation silicone
(24.4%) and Polyether (18.2%).
Hydrocolloids have a high hydrophilic nature (low contact angle of less than 90° but materials
with contact angle more 30° become adhesive in nature) that allows this material to capture
accurate impressions in the presence of saliva or blood. It has low wetting angle so it easily
captures full arch impressions. It has moderate ability to reproduce detail and costs relatively
little compared with other impression materials. It is not accurate enough for fixed partial
dentures. It has poor dimensional stability, must be poured within 10 to 12 minutes and is good
for only one pour per impression.
Polysulfide impression materials are generally low to moderately hydrophilic and make an
accurate impression in the presence of saliva or blood. Because the material has a low wetting
angle it makes a full arch impression than polyvinyl siloxane or polyethers. It reproduces detail
with excellent results but its dimensional stability is only fair. It may allow for more than one
pour and it is not a rigid material and impressions are easier to remove.
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Impressions in fixed partial denture 2009
Addition silicones involve the linking of a vinyl siloxane in the base material with a hydrogen
siloxane via a platinum catalyst. The reaction produces hydrogen, which is scavenged by the
platinum. Viscosity is altered by changing the amount of silica filler which produces either a
putty or less viscous wash material. Vinyl polysiloxane silicones are considered state-of-the-art
for fixed partial denture impressions because it shows accuracy of 96.86% over other
impressions. They are virtually inert after set, but cannot make impression in a wet environment
and they can be trimmed and poured in any die material. It has a moderately high wetting angle,
which makes it difficult to have an accurate full arch impression. It has an excellent ability to
reproduce detail of 25um or less and is dimensionally stable, which allows multiple pours of
accurate casts for several weeks after impression are made. The material is moderately rigid and
can be more easily removed.
Polyether impression material is moderately hydrophilic and captures accurate impressions in the
presence of some saliva or blood. Because their wetting angle is low, they capture a full arch
impression easier than polyvinyl siloxane. Their ability to reproduce detail is excellent and is
dimensionally stable and allows multiple pours of accurate casts for 1 to 2 weeks after
impression are made. They are rigid materials and may be more difficult to remove but they are
contraindicated in periodontally involved teeth. They do not tear easily which enables the dentist
to get good sub gingival detail without tearing the impression on removal.24
All impression materials should be rinsed, dried, and disinfected when removed from the mouth.
Impressions made with polyether polymer should be poured within 1 hour. Impressions made
with polyether or addition silicone have high dimensional stability and can be stored
considerably longer before pouring. In all techniques, a good impression is critical for an
accurately fitting restoration.
There are vast numbers of impression techniques available in the field of fixed partial
prosthodontics ranging from copper tube impression to sectional tray techniques. Each and every
technique has its own advantages and disadvantages, but still no impression technique is superior
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Impressions in fixed partial denture 2009
with all the qualities inherent in it and will be universally claimed superior. Some techniques can
be used with stock trays, some require the customized individual trays and some may require
special trays with added systems as water cooling systems and Rimlock trays. It is the role of
operator to select the appropriate technique for a particular clinical condition by evaluating the
various factors that include the trays available, the time required, the salivary flow and its
interference, the span of the restoration whether single unit crown and short span bridges or a
long span bridgework, the critical areas to be recorded and finally the economy.
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