IMPRESSION TECHNIQUES OF FPD
A variety of techniques have evolved over time and selection of the specific technique depends on experience and an evaluation of an individual patient. Time, expense and accuracy must all be considered in making making the selection. PUTTY WASH / STOCK TRAY TECHNIQUE:
Mixing method: double mix and single mix
Stock
trays can also be used with medium and heavy bodied elastomers
normally used with custom trays. If
a single mix technique tec hnique is anticipated antic ipated with a stock st ock tray, the po lyvinyl
siloxanes are used. Place
the patient in supine position and the operator at 9 o¶clock position.
Select the tray shape and size based on patient¶s patient¶s arch shape and size. Coat
the tray evenly with adhesive on the inside and rim Isolate
the areas to be impressed with cotton rolls and dry preparations with
short bursts of compressed air. air. The sulcu s should be dry, but do not des sicate the dentin.
Choose a braided cord to push the tissue aside and physically enlarge the sulcular space by measuring the sulcus depth and width with periodontal probe. Subgingival ubgingival areas of the f inish line that need tissue displacement.
Cut approximately 1 ± 1 ¼ inch of cord.
Choose a dull, blunted acking instrument with a width that fits comfortably into the sulcus.
Wrap
the cord around the tooth and from the buccal, grasp the two ends with
your thumb and forefinger. Slide the cord down toward the sulcus.
Choose the mesiofacial or distofacial line angle to gently pack 1mm of cord into sulcus. This holds the cord in place to facilitate further sulcular displacement.
Continue packing the proximal sulcus and proceed toward the lingual, not the facial surface. Continue around the lingual toward the opposite proximal surface.
Control the buccal buccal ends of the cord w hile pushing the the cord with a hand instrument toward the tooth.
Then roll it into the sulcus while pushing toward the already packed cord. This prevents the cord from being displaced.
Observe
the gingiva being displaced displaced laterally from the tooth. tooth. Do not pus h the
cord down apically; this strips the attached gingiva.
Determine the final length of the cord and allow 1mm overlap plus 2mm excess for uncomplicated removal. Cut the cord with sharp scissors to prevent tugging on the cord.
Complete the packing packing at the facial surface. This varies according to the position of the finish line, supragingival or subgingival.
Evaluate
tissue displacement; if you cannot see the finish line in a certain
area(s), then additional additional cord(s) with increasing diameter is/are indicated indicated . Use
alum (aluminum sulfate) to control of fluids from the walls of the
gingival sulcus. If
the gingival sulcus is healthy and not lacerated, this medicament is
sufficient. Use
epinephrine (8 percent) to control minor hemorrhage, but do not use on
patients with lacerated tissue, cardiovascular disease, diabetes, or hyperthyroidism. Consult a current Physicians' Desk Reference (PDR) for drug contraindications. contraindications.
If
no medicaments are applied to the dry cord, then slightly moisten the cord
with water. This will help prevent its sticking to the sulcular tissue upon removal. If
inflammation is present and seepage cannot be controlled, remove the cord
and repeat the above steps or use electrosection. If
inflammation is present and seepage cannot be controlled, then verify the
temporary restoration's fit and surface texture. Cement the provisional and allow gingival healing before making another impression. Observe
the finish line. If you cannot see all areas of the finish line, repeat
the steps. If you can see at least 0.5 mm below the finish line, continue. Leave
cord(s) in place for 8 to 10 minutes. While the cord is displacing the
gingiva, it is compressing the blood vessels. Prolonged Block
retention can cause belated gingival sloughing and migration.
out undercuts around the teeth in the arch with red rope wax. The
tissue is dry but not dehydrated when applying the wax.
This facilitates impression removal, reduces distortion, and averts tearing.
Prepare syringe: Lubricate
syringe "0" -ring lightly. Trim tip. Open orifice to increase rate of
flow or close orifice to decrease rate of flow. Orifice must be smooth and without cracks. Measure
arch length of tray to guide in dispensing the amount of elastomer
RULE:
Dispense one times the length of the tray for the low -viscosity elastomer (syringable).
Making f inal impression:
Choose a large mixing pad, approximately 6 by 8 inches. Some brands of impression material have mixing devices that resemble a caulkin g gun and do not need a mixing pad. These dispense material by placing a replaceable cartridge and mixing tip into the gun and squeezing the trigger to express the material.
Choose a spatula that is large enough to pick up the mixed impression material, yet sufficiently pliable to mix the material against the pad.
Place
the pad near the edge of a table about waist high.
Mix
the low-viscosity impression material according to manufacturer's
instructions. First
use a circular motion combining the two strands, then a figure eight
motion to blend and flatten the mixture onto the mixing pad. Flattening the mixture and limiting the number of times you lift the spatula from the pad reduces the number of voids in the mixture. Mix
according to manufacturer's instructions to obtain a streak-free mixture
in less than 1 minute. Load
the syringe by holding it at a slight angle while scraping the pad. Wipe
off excess at the tip with a paper t owel, screw on the tip, and insert the plunger. Express a small amount before handing to the dentist. While
the plunger is inserted into the syringe, the dentist initiates cord
removal.
NOTE:
If using stock tray with single mix method, refer to section o n stock tray, and
then continue. Grasp
the 2 mm excess of cord with forceps and slowly tease the top cord
toward the occlusal with a gentle continuous pressure. Repeat for all cords. Evaluate
the retraction site for seepage, hemorrhage, or debris.
Quickly
blow away seepage with short bursts of compressed air or dry with
cotton pledgets.
If seepage cannot be controlled, Syringe inaccessible areas first, e. g., distal
lingual finish line. Position
the syringe so the elastomer is ahead of the tip's orifice
Insert tray: For
stock tray (putty wash) double mixing technique, insert the low-viscosity
impression material into the tray without overfilling the tray. Fill
the inside slightly less than the dept h of the external borders.
Note
that the excess flows to the back of the tongue or throat and causes
discomfort. For
stock tray (putty wash) single mixing technique, the unset high-viscosity
impression material should already be in the tray, and the preparations syringed with low-viscosity impression material. Spread
the checks one at a time, first with the tray and then with an index
finger. Position
the tray over the arch. Seat from posterior to anterior, allowing the
excess to extrude anteriorly.
Apply force in a vertical direction until further seating is impossible. Evaluate final position and adjust tray quickly if necessary.
Evaluate f inal impression: Stabilize
the tray and wait the minimum time as suggested by the
manufacturer to achieve the final set. Extended periods are usually advocated by researchers. Perform
"clinical final set test".
Remove tray: Insert
two fingers under each side of the tray to break the seal.
Remove the tray parallel to the preparation(s) path of withdrawal and transfer to assistant. This is crucial with cross arched impression.
Rinse impression with ambient water, and dry with short, small bursts of compressed air.
Some
elastomers are hydroscopic, so remove all the water.
Retraction cord(s) remaining in the impression material are removed carefully. Check sulcus for residual impression material and remove any debris and clean the oral cavity.
Evaluate set impression: Elastomeric Note
material should be present 0.5 mm beyond visible finish line.
presence of bur marks, the junction of smooth root surface, and
continuous finish line.
There should be no tray show-through in any areas of the impression, except at tissue stops.
There should be no shiny smooth areas; if present, they suggest moisture contamination.
There must be no voids present; if present, they suggest mixing problems or contamination.
Disregard small voids in unimportant areas.
Review for tears (subtle undercuts to be "blocked out"). Polysulfides can distort without tearing, but sometimes reveal excessive distortion by a lightened colour.
There should be no thin areas leaving the finish line unsupported. These areas distort under the weight of the stone.
Making working cast: Position
the dowels into the impression. Insert one extra dowel pin on either
side of the working dies. This ensures die removal despite errors during sectioning.
Check dowel(s) position(s) in impression for parallelism and retention and add paper clips or orthodontic wire for retaining the second pour to the first. The Pindex System allows immediate pouring of impression.
Pour
first layer of stone. Wait for initial set of stone. Lubricate die areas.
Bead
and box impression. Pour second layer of stone for the base.
CUSTOM TRAY
Synonyms: Acrylic tray Mixing technique: Single
Advantages:
1. Less impression material is needed than with stock tray. 2. Because the trays are used only once, sterilization is not a problem. 3. A uniform thickness of impression material minimizes distortion resulting from curing shrinkage. 4. Pre curing of the tray material is not required.
Disadvantages :
1. Construction of the custom tray is time consuming. 2. The tray must "age" for 24 hours to minimize further distortion. 3. The monomer may be a sensitizer for some personnel.
Technique: Soak
replicas of the diagnostic casts in slurry water for 10 minutes.
Paint
a layer of tinfoil substitute (or petroleum jelly) on the cast as a
separating medium to prevent the resin from adhering to the cast.
Draw a pencil line on the cast as a guide in defining the tray exte nsions
Warm
and adapt two sheets of base plate wax (total thickness, 2 mm) to the
cast.
NOTE: For
more rigid materials, polyethers and poly vinylsiloxanes, 3 mm is
required to facilitate removal from the mouth and removal of the replica cast. Trim the excess wax to the pencil line with a knife.
Cover the wax with a thin tinfoil (or polyethylene) sheet to protect resin from wax during the exothermic cure.
Remove wax to create four wide-spaced hard tissue stops at least 3 mm" and located on non functional cusps. Stops may be placed on firm tissue as a last resort.
Measure
monomer (liquid) and polymer (powder) according to the
manufacturer's recommended ratio. Insert
both (liquid first, then powder) into a suitable paper cup; most kits
contain cups. Mix
according to manufacturer's recommendations.
Wait
until doughy stage is reached, not sticky to the touch.
Form
dough patty into a flattened shape approximately 4 mm thick.
Adapt flattened patty to tin-foiled diagnostic cast. Trim excess to pencil line while resin is still doughy. Use excess resin to form handle on the tray, this will help in removal from the mouth.
Moisten
the unset tray surfaces with monomer (liquid) before joining.
Observe exothermic stage of curing acrylic resin. Wait for
the final set according to manufacturer's instructions (approximately
15 minutes). Gently lift the tray from the cast. Remove the wax spacer with the protector. Observe that all wax has been re moved.
Trim and polish the tray using slow speed, continually checking for excessive heating of the acrylic resin.
Store
at room temperature for 24 hours before making impression to
minimize distortions.
The adhesive can also be placed now.
Gingival
displacement and management
See section on stock tray
Prepare syringe: See section on stock tray
Making f inal impression: Select
medium to high-viscosity elastomer to line the tray. Choose low-
viscosity elastomer for syringe preparations.
"Provide two large mixing pads, approximately 6 by 8 inches. Some brands of impression material have mixing devices that resemble a caulking gun that do not need a mixing pad or spatula. These work by inserting a replaceable cartridge and mixing tip into the gun and squeezing the trigger to express the material into a tray or directly into the syrin ge.
Obtain
two spatulas that are shaped to manipulate the mixed impression
material and pliable enough to mix the material against the pad. Place
the pads near the edge of a table about waist high.
Mix
the low-viscosity impression material according to manufacturer's
instructions. First use a circular motion combining the two strands, then a figure eight motion to blend and flatten the mixture. Flattening the mixture and limiting the number of times you lift the spatula from the pad reduces the number of voids in the mixture. Mix
according to manufacturer's instructions, obtaining a streak -free mixture
in less than 1 minute. Load
the syringe by maintaining a slight angle while scraping the pad.
Wipe
off excess at the tip with a paper towel, screw on the tip, and insert the
plunger. Express a small amount before handing to the dentist. Mix the higher-viscosity impression material. While
the plunger is inserted into the syringe, the operator initiates cord
removal. Grasp
the 2 mm excess of cord with forceps and remove the top cord slowly
with a gentle continuous tug toward the occlusal. Do the same for all cords.
Evaluate Quickly
tissue for seepage, hemorrhage, or debris. blow away seepage with short bursts of compressed air or dry with
cotton pledgets. If
seepage is evident, see section on stock tray, Syringe inaccessible areas
first, i. e. distal lingual finish line.
Allow the elastomer to extrude ahead of the tip's orifice.
Insert
high-viscosity impression material into tray without overfilling the
tray. Fill the inside slightly less than the depth of the outside borders. Note
that the excess flows to the back of the tongue or throat and causes
discomfort, and propels saliva under the tray causing bubbles.
The patient is seated in an upright position to avoid this.
Spread
the cheeks one at a time, first with the tray and then with your index
finger. Position Seat
the tray over the arch.
from posterior to anterior, allowing excess to extrude in an anterior
direction.
Continue seating in a vertical direction until the tray stops prevent further progress.
Evaluate
final position and adjust tray quickly if necessary.
Evaluate f inal impression: See section on stock tray.
Remove tray: See section on stock tray.
Evaluate set impression: See section on stock tray.
CLOSED BITE DOUBLE ARCH METHOD
Synonyms: Dual quad tray, double arch, triple tray, Accu -bite, closed mouth
impression
Minimum conditions:
1. The articulator must have a vertical dimension holding stop such as an incisal pin or other metal-to-metal contact. If the articulator's design does not provide for a positive stop, there must be sufficient natural teeth remaining to maintain vertical dimension. This approach is limited to single castings in patients with suitable inter digitation. 2. There should be sufficient space distal to the terminal tooth in the arch to allow tray approximation.
Advantages:
The physical deformation of the mandible during opening is minimized.
The shifting of teeth occuring during maximum intercuspation is captured.
Less
elastomeric impression material is needed so the patient is more
comfortable. Less
gagging may occur.
Disadvantages:
Tray is not rigid; depends on impression mate rial for rigidity.
Is
not a functionally generated technique, so it is limited to one casting
quadrant.
The distribution of the impression material is not uniform.
per
Technique: While
waiting for anesthesia or after completion of tooth preparation.
Evaluate the fit of the tray in patient's mouth. Position
the tray's crossbar distal to last tooth in the arch. Instruct patient to
close the mouth. Observe the complete bilateral closure and the patient's comfort.
Adjust the tray, select new tray size, change brand, or modify technique.
Practice
Gingival
until patient is fam iliar with tasks.
displacement and management:
See section on stock tray
Prepare syringe: See section on stock tray.
Tray is inserted to determine the clearance laterally and distally.
Making f inal impression:
Choose a large mixing pad, approximately 6 by 8 inches. Some brands of impression material have mixing devices resembling a caulking gun that do not need a mixing pad.
These are activated by placing a replaceable cartridge and mixing tip into the gun and squeezing the trigger thus expressing the material.
Select
a spatula that is sufficiently large to elevate the mixed impressio n
material, yet pliable enough to mix the material against the pad. Place the pad near the edge of a table about waist high. Mix
the low-viscosity impression material according to manufacturer's
instructions. First use a circular motion combining the two strands, then a smooth figure eight motion to blend and flatten the mixture onto the mixing
pad. Flatten the mixture and limit the lifts of the spatula from the pad to reduce the number of voids in the mixture. Mix
according to manufacturer's instructions to obtain a streak-free mixture
within 1 minute. Load
the syringe by maintaining it at a slight angle while scraping the pad.
Wipe
off excess at the tip with a paper towel, screw on the tip, and insert the
plunger. Express a small amount before handing to the dentist. While the assistant is inserting the plunger into the syringe, initiate cord removal. Grasp
the 2 mm excess of cord with forceps and tease the top cord slowly
with a gentle continuous pressure toward the occlusal. Repeat for all cords. Evaluate
site for seepage, hemorrhage, or debris.
Quickly
remove seepage with short bursts of compressed air or dry with
cotton pledgets. If
seepage is uncontrolled, see section on stock tray.
Syringe
inaccessible areas first, i.e. distal lingual finish line; interproximal.
Allow the elastomer to extrude ahead of the tip's orifice.
Mix
the high-viscosity elastomer and "overfill" bilaterally. Manually seat
tray on maxillary arch. For
quadrant trays, position the crossbar distal to the last tooth in that arch.
Instruct patient to slowly close mouth. Evaluate
complete closure by observing the interdigitation on the opposite
arch. Secure
patient's comfort.
Evaluate f inal impression: Wait
the time suggested by the manufacturer to achieve the final set, plus 2
minutes. Perform
clinical final set test.
Tray removal: Instruct Place
the patient to open. The impression adheres to one arch.
a finger on either side of the tray. Remove with equal pressure
bilaterally to minimize the distortion of the tray. Do not use the handle to remove tray. Remove residual impression material in sulcus or interproximal areas.
Rinse impression with ambient water, and dry with short bursts of compressed air. Retraction cord(s) in the impression material is/are removed carefully. Check sulcus for residual impression material.
Remove debris and clean the oral cavity.
Pour die:
Trim excess impression material with a surgical knife.
"Vibrate the die stone and overfill the prepared tooth. Position dowel pin and allow stone to set. Place a bead of wax around the edges of the die to remove undercuts.
Lubricate
the dowel pin and die stone with separating media.
Making working cast(s) and attaching to articulator: Box the
impression.
Choose hinge articulator with incisal pin or vertical dimension stop (Recommended for neutron occlusion and one uncomplicated casting)
Place
sticky wax around circumference of tray. Cut a strip of boxing wax in
half length-wise. Soften the wax uniformly over a gas flame.
Adapt and box both upper and lower arches at one time. Seal the area at the junction of boxing wax and tray to prevent the stone from entering the opposite side of the impression.
Cut small openings to accommodate the articulator so that correct positioning of the impression is possible.
Position
the tray, bisecting the inter articulator distance from top to bottom,
parallel to the table top, and centered from side to side. Pour the opposing side (side without dies) first and close articulator arm into unset stone. Evaluate
impression tray position and secure the dowel pins so they are not
interfacing with the closure of the articulator and on the working side. Wait 45 minutes for initial set of gypsum. Lubricate
the die(s) with a separating media. Pour prepared side (with die) of
impression with gypsum product.
Close articulator arms until incisal pin contacts the lower arm. Remove unset stone around tip of dowel pin(s) to facilitate removal.
Place
a rubber band around upper and lower arms of the articulator arm.
Wait 45 minutes.
Removing impression tray f rom articulator:
Remove rubber band and pull articulator arms apart slowly, separating the casts from the impression tray.
Evaluate
casts and separate die(s).
COPPER BAND TECHNIQUE
The copper tube or band is used to salvage an impression of multiple preparations when there are only vague margins on one or two preparations that are not adequately replicated in the impression. The patient's condition, the extent of the aberration evaluated, and judgement determines whether the copper band technique saves time or whether a remake of the original impression is more appropriate. The steps for a copper band impression of a single prepared tooth are:
Fitting copper band to preparation: Select
copper band diameter by trial and error by deforming the tubes to
semi ellipsoidal cross section and trying in.
Anneal selected tube by heating in flame and quenching in alcohol.
Mark
approximate position of finish line with a sharp explorer tip.
Cut with scissors and smooth rough edges with carborundum stone.
Impression materials and techniques: Evaluate
fit; band should extend approximately1 mm beyond finish line and
produce minimal tissue blanching. Do not remove from preparation. If
fit is tight at finish line area, rock the tube gently from side to side. A
space between finish line and copper band approximating the width of an explorer tip is optimum.
Cut orientation hole in top one-fifth of facial surface of tube.
Make compound plug:
Cover fingers with a light coat of petroleum jelly.
Gently
heat red stick compound over a Bunsen burner flame; compress the
warm mass with the lubricated fingers.
Evaluate
viscosity and temperature; reheat or temper in a w arm water bath if
necessary. Insert
warm compound mass and fill approximately the top one -third of
copper tube. Seat
and orient band onto preparation. Compress excess into hand lightly
with lubricated finger.
NOTE:
The amount of compound is critical to the success of this step. The compound should just touch the occlusal surface. Cool tube with water.
Using Backup
towel clamp, grasp top one-fifth of copper band and remove
from mouth. Evaluate preparation side of band . Only part of the occlusal surface should be impressed.
Relieve any excess by cutting. Slow speed with No. 6 or 8 carbide bur is suggested. Stop frequently to air cool or blow away debris. Remove 0.2 mm of compound from the impressed occlusal surface. This creates a space for the heavy body polyvinylsiloxane.
Using
a long shank No. 6 round carbide bur, drill a hole through the center
of the compound plug. This will decrease hydraulic pressure to facilitate seating of the copper band.
Making impression:
Cut approximately four or five evenly distributed holes with a sharp No. 4 or No. 6 round carbide 2 to 3 mm above the bottom of the copper tube. Be
careful not to heat the tube.
These holes will retain the polyvinylsiloxane impression material and provide a suitable space at the finish line area. This additional space prevents marginal tearing.
Coat the internal surface sparingly with adhesive. Spot the internal surface; do not coat the entire area. The adhesive can occupy too much space in the finish line areas.
Clean and isolate preparation.
Prepare
the syringe tip by securing a hole size approximating the diameter of
the shank of a long shank bur.
NOTE: Some
poly vinylsiloxane impression systems have an auto mixing device to
facilitate the next two steps. Mix
1 inch of heavy-viscosity, but not putty, elastomeric impression
material. Inject
the elastomeric into the copper band, filling the space completely from
the compound to copper band edge. Position Evaluate
fingers on top edge of band, orient and seat customized copper tube. tube position; if incorrect, reposition.
Stabilize band and Note
compress excess into plug¶ s hole with lubricated finger.
excess expressed at gingival area. Remove fingers; protect copper band
from movement and await final set. Grasp
top one-fifth of impression with sharp backup towel clamp and gently
remove from tooth. Evaluating
impression and making die: Evaluate impression; if the area
needed is accurately impressed, proceed. With a sharp surgical knife remove excess elastomeric. Using
masking tape, box the impression and pour in die stone.
Trim die stone to form elongated tapered cylinder base for convenient manipulation during wax up.
The defective areas can be relieved on the original full arch impression and the waxing of the margin created from the copper band die and transferred to the relieved die for adjustment of proximal contacts and occlusion.
