PRESENTED BY:RUBINA

MDS 1ST YEAR

MODERATED BY:Dr. B.R.CHETAL

HOD

DEPARTMENT OF PROSTHODONTICS

Seminar on

TISSUE MANAGEMENT AND IMPRESSION PROCEDURES IN FPD

GENESIS INSTITUTE OF DENTAL SCIENCES AND RESEARCH, FEROZEPUR (PUNJAB).

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INTRODUCTION:

Because it is neither possible nor desirable to make patterns for fixed prostheses

directly in the mouth, an impression, or negative likeness of the teeth and surrounding

structures, is necessary to obtain a cast. This cast is then used to make a restoration in the

laboratory. To obtain the cast, an elastic impression material is placed in a tray that is

inserted into the patient’s mouth. When the material has set, it is removed from the

mouth. A suitable dental stone is then poured into the “negative” impression, and a

positive likeness or working cast is obtained.

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. If the impression does not reproduce this

critical area where tooth and future restoration meet, fabricating the restoration with

proper contours is not possible.

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 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.

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The patient’s mouth is a challenging environment in which to make an accurate

impression. Moisture control is probably one of the most important aspect 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.

When the preparation margins extend subgingivally, the adjacent gingival tissues

must be displaced laterally to allow access and to provide adequate thickness of the

impression material. This may require enlarging the gingival sulcus through mechanical,

chemical, or surgical means and must be done without jeopardizing periodontal health.

Improper manipulation of impression material and tissue displacement can lead to

permanent soft tissue damage.

PREREQUISITES OF IMPRESSION

Tissue Health:

After the teeth are prepared and a provisional restoration has been made the

health of the surrounding soft tissues must be reevaluated. Careful preparation will result

in minimal tissue damage; however, if a subgingival margin is needed, some tissue

trauma in the sulcular area may be unavoidable. The effects of this trauma can be

transient as long as the patient receives a properly made provisional restoration and

maintains adequate oral hygiene. However, if the provisional is poorly contoured, not

polished, or has defective margins, plaque retention will lead to a localized inflammatory

response. The combination of such tissue trauma in the presence of preexisting

periodontal disease can produce disastrous results.

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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. When correctly placed, maxillary

cotton rolls block salivary flow from the parotid gland

Periodontal disease must be treated and resolved before fixed prostheses are placed.

On occasion, a defective restoration will contribute to the inflammatory sulcular

response. If this is the case, a properly adapted and well-contoured polished provisional

must be fabricated and cemented on the prepared teeth; the focus must shift from the

teeth to the soft tissues, which must be re

In areas where only supragingival margins are present moisture control with arubber dam

is 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; an evacuator

must be placed where the saliva pools . in the maxill1ary arch, placing a sing cotton roll

in the vestibule immediately buccal to the preparation and a saliva evacuator in the op

posing lingual sulcus is 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 a maxillary roll does not stay in position but slips down, it can be retained

with a finger or the mouth mirror. When making a mandibular impression, placing

additional cotton rolls to block off the sublingual and submandibular salivary ducts is

usually necessary. Rolls on the buccal and lingual sides of the prepared teeth will help

with soft tissue retraction—the cotton on the buccal side displaces the cheek laterally, and

the cotton on the lingual side displaces the tongue medially. One or two cotton rolls

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placed vertically between the horizontally placed cotton rolls in the buccal vestibules will

help maintain the latter in position.

An alternative to multiple cotton rolls is placement of one long roll “horseshoe

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. In addition, two

cotton rolls should be placed in the maxillary and mandibular vestibules to control saliva

and displace the cheek laterally.

The tongue can be a problem when working in the mandibular arch. Saliva

evacuators may help eliminate excess flow, but most of these are displaced easily by a

“probing” tongue. If lingually placed cotton rolls continually become dislodged or, in

conjunction with a conventional saliva evacuator, fail to control moisture adequately, a

flanged-type evacuator (e.g., the Svedopter or the Speejectorf) should be considered. To

avoid the risk of soft tissue trauma, this device must be placed carefully. A cotton roll

between the blade and the mylohyoid ridge of the alveolar process will minimize

intraoral patient discomfort. Simultaneously, if properly positioned, this type of device

will providc a “stop” that prevents the flange from being displaced farther buccally,

allowing excellent lingual access to mandibular posterior teeth. Care must be taken not to

tighten the chin clamp excessively, because considerable discomfort can result from

pressure to the floor of the mouth. A disposable saliva ejector designed to displace the

tongue may also be effective.

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in addition to the pain control normally needed during tissue displacement, LA may help

considerably with saliva control during impression making.

Antisialogogs :

This group of drugs includes atropine, dicyclomine and methanthiline 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. The incidence of undiagnosed glaucoma in the general

population is high. 0.2 mg of clonidine is found to be effective in reducing the salivary

flow.

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.

Displacement Cord:

Some enlargement of the gingival sulcus can be obtained by placing a

nornmpregnated cord and leaving it in place for a sufficient length of time. The cord is

pushed into the sulcus and mechanically stretches circumferential fibers.

Placement is easier if a braided or knitted (e.g., Ultrapakt) cord is used However, larger

sizes of braided cord should be avoided because they doubleup” and can become too’

thick for atraumatic sulcular placement. In areas where very narrow sulci preclude

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placement & of the smaller sizes of twisted or braided cord, wool-like cords that can be

flattened are preferable for initial displacement of tissue.

Better sulcus enlargement can be achieved with a chemically impregnated cord or

by dipping the cord in astringent. These contain aluminum or iron salts and cause a

transient ischemia of the gingival tissue.

Many of the chemicals used for their astringent effect are stable only at narrow

ranges of low pH levels. The low pH levels have raised concern about the effect of acidic

solutions on tooth structure and, perhaps more importantly, on the smear layer.

Contact between the astringent and the prepared tooth surfaces must be minimized if the

smear layer is to be maintained. A nonacidic hemostatic agent can be used as an

alternative.

Step-by-step Procedure:

1. Isolate the prepared teeth with cotton rolls, place saliva evacuators as required, and dry

the field with air.

2. Cut a length of cord sufficient to encircle the tooth . Do not overdesiccate the tooth,

because this may lead to postoperative sensitivity.

3. Dip the cord in astringent solution and squeeze out the excess with a gauze square. An

impregnated cord can be placed dry but should be moistened in situ to prevent the thin

sulcular epithelium from sticking to it and tearing when it is removed.

4. If a nonbraided cord is used, twist it tightly for easier placement.

5. Loop the cord around the tooth and gently push it into the sulcus with a suitable

instrument.

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It is best to start in the interproximal area, because the cord can be more easily placed

here than facially or lingually. The instrument should be angled towards the tooth so the

cord is pushed directly into the area.

Tissue displacement must be done gently but with sufficient firmness to place the

cord just apical to the margin. Overpacking should be avoided because it could cause

tearing of the gingival attachment, leading to irreversible recession. Repeated use of

displacement cord in the sulcus also should be avoided, since this can cause gingival

recession.

Evaluation:

Difficulty with tissue displacement is often the result of gingival inflammation.

The inflamed and swollen tissue bleeds easily, preventing access by the impression

material. Initial assessment of cord placement can be a useful indicator of the amount of

displacement accomplished. When looking at the tooth preparation from the occlusal

aspect, one should be able to see the preparation margin circumferentially and the

uninterrupted cord, with no soft tissue folded over it, in contact with the tooth. If there is

any doubt, assessing displacement by removing the cord is a good idea. The entire

preparation margin should be clearly visible and will remain directly accessible for about

a minute.

Typically, if the result is acceptable, a second cord is quickly inserted to maintain the

displacement while the impression material is mixed. If the sulcus enlargement is not

favorable, the tissue health should be reassessed, particularly if adequate displacement

cannot be obtained by repeating the previous steps.

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Sometimes a double cord is helpful. First, a thin cord is placed and trimmed so

that its ends do not overlap. A second, larger cord is then placed in the normal manner

and removed. The thin, first cord remains during impression making. When using this

technique, the clinician should be careful not to damage the epithelial attachment.

On many occasions it is better to delay impression making and concentrate on how to

improve tissue health (e.g., by reassessing the quality of the provisional restoration and

reinforcing oral hygiene instructions) rather than attempting impression making under

adverse conditions. Minor hemorrhaging can sometimes be controlled with an astringent

or by infiltrating a local anesthetic directly into the adjacent gingival papillae.

Electrosurgery:

An electrosurgery unit may be used for minor tissue removal before impression

making. In one technique, inner epithelial lining of the gingival sulcus is removed, thus

improving access for a subgingival crown margin and effectively controlling post-

surgical hemorrhage provided the tissues are not inflamed. Unfortunately, there is the

potential for gingival tissue recession after treatment.

An electrosurgery unit 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

(cutting). For restorative procedures, an unmodulated alternating current is

recommended, because it will minimize damage to deeper tissues.

The following facts should be considered before attempting electro surgery

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1. It is contraindicated on or near 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.

2. It is not suitable on thin attached gingivae (e.g., the labial tissue of maxillary canines).

3. It should not be used with metal instruments, because contact could cause electric

shock. (Plastic mirrors and evacuation tubes are available.)

4. Profound soft tissue anesthesia is mandatory.

5. A thin wire electrode is best for sulcular enlargement. Gingival contouring is usually

performed with a loop electrode.

6. The instrument should be set to unmodulated alternating current mode.

7. The electrode should be passed rapidly through the tissue with a single light stroke and

kept moving at all times.

8. If the tip drags, the instrument is at too low a setting and the current should be

increased.

9. If sparking is visible in the tissue, the instrument is at too high a setting and the current

should be decreased.

10. A cutting stroke should not be repeated within 5 seconds.

11. The electrode must remain free of tissue fragments.

12. The electrode must not touch any metallic restoration: Contact lasting just 0.4 second

has been shown to lead to irreversible pulpal damage in dogs.

13. The sulcus should be swabbed with hydrogen peroxide before the displacement cord

is placed.

MATERIALS SCIENCE

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ELASTIC IMPRESSION MATERIALS

There is an extensive variety of materials for making a precision negative mold of

soft and hard tissues. In order of their historical development, they consist of the flowing:

1. Reversible hydrocolloid

2. Polysulfide polymer

3. Condensation silicone

4. Polyether

5. Addition silicone

Irreversible hydrocolloid is not sufficiently accurate for cast restorations. 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 details for the fabrication of clinically

acceptable fixed prostheses.

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 polyether 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.

Reversible Hydrocolloid :

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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.

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 soi 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 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. Reversible hydrocolloid’s lack of dimensional stability is due primarily

to the ease with which water can be released from or absorbed by the material (syneresis

and iinbibition). The accuracy of a reversible hydrocolloid impression is improved if the

material has as much bulk as possible (low surface area/volume ratio). This contrasts with

the elastomeric impression materials, whose accuracy is improved by minimizing bulk

(e.g., polysulfide and condensation silicone), because stresses produced during removal

are reduced.23 Therefore, an additional advantage of reversible hydrocolloid is that a

custom impression tray is not required.

Polysulfide polymer :

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known as rubber bases, were introduced in the early to middle 1950s. They were received

enthusiastically by dentists because they had better dimensional stability and tear strength

than hydrocolloid. Nevertheless, they should be poured as soon as possible after

impression making; delays of more than an hour result in clinically significant

dimensional change. There is a slight contraction of polysulfide during polymerization,

but the effects can be minimized with a custom impression tray to reduce the bulk of

the material. Generally a double-mix technique is used with a heavy-bodied tray material

and a less viscous syringe material. These polymerize simultaneously, forming a

chemical bond of adequate strength. The high tear resistance and enhanced elastic

properties of polysulfide facilitate impression making in sulcular areas and pinholes, and

it has improved dimensional stability over hydrocolloid (inferior to polyether and

addition silicone). Although it is the least expensive elastomer, it is not well- liked by

patients because of its unpleasant sulfide odor and long setting time in the mouth

Messy, Unpleasant odor, Long setting time Stability only fair Dimensional stability

Hydrophobic, Most impressions Delay pour of some Pleasant to use Poor welling

materials

Furthermore, high humidity and temperature dramatically reduce its working time,

which maybe so short that polymerization begins before it is inserted in the mouth,

resulting in severe distortion). Although air conditioning is common in dental operatories,

temperatures near 25° C (77° F) with humidity in excess of 60% can create problems.

Most polysulfide materials are polymerized with the aid of lead peroxides, which

explains this material’s typical brown color. The unpolymerized product is sticky and

should be handled carefully, because it stains clothing permanently. Alternatives to lead

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are available; copper hydroxide is the most common. Cu(OH)2-polymerized polysulfide

is light green and shares many of the characteristics of the Pb02-poly- merized material

(except for a reduced setting time).

Condensation Silicone :

Some of polysulfide’s disadvantages have been overcome by condensation

silicone, which is essentially odorless and can be pigmented to virtually any shade.

Unfortunately, its dimensional stability is less than that of polysulfide but greater than

that of reversible hydrocolloid. An additional advantage of this silicone is its relatively

short setting time in the mouth (about 6 to 8 minutes). As a result, patients tend to prefer

condensation silicone over polysulfide. In addition, condensation silicone is also less

affected by high operating room temperatures and humidity.

Silicone’s main disadvantage is its poor wetting characteristics, which stems fm

its being extremely hydrophobic (for this reason, it is used in commercial sprays that

protect automobile electrical systems from moisture). In this context, the prepared teeth

and gingival sulci must be completely free of moisture to make possible a defect-free

impression. Pouring without trapping air bubbles is also more difficult than with other

impression materials, and a surfactant may be needed. Silicone impression material is

available in a variety of viscosities. One technique involves a heavily filled

putty material that is used to customize a stock impression tray in the mouth, generally

with a polyethylene spacer. The spacer allows room for a thin wash of light-bodied

material, which makes the impression. The technique requires considerable care in

seating, however, to prevent strain in the set putty. If this happens, the impression will

rebound when removed from the mouth, resulting in dies that are too small.29 Care is

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also needed to avoid contaminating the putty surface with saliva, which will prevent the

wash impression from adhering properly.

Silicone and polysulfide have a dimensional instability that results from their

mode of polymerization. They are both condensation polymers, which as a by-product of

their polymerization reactions, give off alcohol and water, respectively. As a result,

evaporation from the set material causes dimensional contraction in both.

Polyether :

Polyether impression material, developed in Germany in the mid-1960s, has a

polymerization mechanism unlike those of the other elastomers. No volatile by-product is

formed, which results in excellent dimensional stability. In addition, its polymerization

shrinkage is unusually low compared with most room temperature— cured polymer

systems. However, its thermal expansion is greater than that of polysulfide.

With the high dimensional stability of polyether, accurate casts can be produced when the

material is poured more than a day after the impression has been made. This is especially

useful when pouring the impression immediately is impossible or inconvenient. Another

advantage of polyether is its short setting time in the mouth (about 5 minutes, which is

less than half the time required for polysulfide). For these reasons, polyether is used by

many practitioners. However, polyether has certain disadvantages. The stiffness of the set

material is one such disadvantage which causes problems when separating a zme cast

from the impression. Thin and single teeth, in particular, are liable to break unless the

practitioner uses great care. Polyether’s relatively short working time may limit the

number of prepared teeth that can be reliably captured in a single impression. Isolated

cases of allergic hypersensitivity to polyether elastomer have less reported (manifested

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as sudden onset of burning, itching, and general oral discomfort). Therefore, the allergic

patient’s record should carry a warning against polyether’s future use, and an alternative

elastomer should be chosen. Recent improvements in these materials have reportedly

reduced this problem.

Addition Silicone:

Addition silicone was introduced as a dental impression material in the 1970s.

Also known as poly(vinyl siloxane) (polysiloxane is the generic chemical expression for

silicone resins), it is similar in many respects to condensation silicone, except that it has

much greater dimensional stability (equivalent to polyether polymer), and its working

time is more affected by temperature. The set material is less rigid than polyether but

stiffer than polysulfide. As with the other materials previously described, adverse soft-

tissue responses have been reported. One disadvantage of this material is the setting

inhibition caused by some brands of latex gloves.36 The problem is most apparent if a

hand-mixed putty is used, but problems can occur if the tissues are touched with gloved

hands immediately before impression placement. If the putty system is used, gloves that

do not interfere with setting should be selected. Like condensation silicone, addition

silicones are hydrophobic. Some formulations contain surfactants, which gives them

hydrophilic properties imparting wettability similar to polyethers. However, these

products also expand like polyether when in contact with moisture. Addition silicone is

generally used as a two-viscosity system, although monophase formulations are also

available. It is easier to trap bubbles when using the monophase.

Manufacturer recommendations should be followed when a cast is being poured, and

pouring should be delayed with some of the earlier products. If this is not done, a

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generalized porosity of the cast surface caused by gas from the impression material will

develop. Newer products contain “scavengers” that prevent the escape of gas at the

polymer-cast interface. Addition silicone that contains scavenger material can be poured

immediately.

CUSTOM TRAY FABRICATION

A custom tray improves the accuracy of an elastomeric impression by limiting

the volume of the material thus reducing two sources of error. stresses during removal

Although reducing the bulk of an elastomeric impression material increases its accuracy,

the opposite is true for reversible hydrocolloid impressions. In hydrocolloid impressions,

dimensional change is due to water loss (or gain) from the surface of the impression. A

bulky hydrocolloid impression has a lower surface area/volume ratio and is therefore less

subject to dimensional change.

Step by step procedure:

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 to the diagnostic cast Two layers of baseplate

wax will result in a combined thickness of approximately 2.5 mm (the sheets should

be measured with a thickness gauge, because wax thicknesses vary).

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.

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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. 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, lingual cusps of the mandibular). If all teeth are

involved, a larger soft tissue stop (Fig. 14-14) 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 tripodal

arrangement in the arch. This wifi lend stability to the fray, and the 45-degree slope

will help center the fray during imsertion.

Visible light-polymerized custom tray material.

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 (see Fig. 14-10,]) or a wooden slab and roller (see Fig. 14-10, K)

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.

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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 Once it is fully seated

on its tops, a custom tray should be stable.

Cross section through a mandibular custom tray. Stops have been placed on the

noncentric cusps so that distortion will not interfere with the intercuspal relationship.

The 45-degree slope helps to center the tray. Space exists for the impression material.

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.

Evaluation:

The completed custom tray needs to be rigid, with a consistent thickness of 2 to 3

mm. It should extend 3-5 mm cervical to gingival margins and should be shaped to

allow muscle attachments. It should be stable on the cast with stops that can maintain

an impression thickness of 2 or 3 mm. The tray must be smooth, with no sharp edges.

Finally, the handle should be sturdy and shaped to fit between the patient’s lips .

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

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placed in boiling water for 5 minutes and allowed to cool to room temperature. A

light-polymerized tray can also be made

IMPRESSION MAKING

ELASTOMERIC MATERIALS

Step-by-step Procedure

Heavy Body-Light Body Combination

1. Try the custom tray in the mouth to verify its fit. Correct as needed.

2. Apply tray adhesive to extend a few millimeters onto the external surface of the

tray .

3. Isolate the abutment teeth and place gingival displacement cord in the sulcus.

4. On separate pads (one for the tray and one for the syringe material), disperse equal

amounts of base and accelerator.

5. Blend the two pastes thoroughly. Initially, the spatula is kept somewhat vertical

during mixing, which is changed gradually to a more horizontal position as the two

pastes become better incorporated. At this time, the spatula is wiped on a clean paper

towel. Mixing continues for another 10 seconds to ensure that the material is

homogeneous.

6. Load the syringe. This can be done by holding the barrel vertically and pushing it

through the mix and then angling and sliding it sideways over the mixing pad. The

syringec an also be loaded from the other end by picking up the mixing sheet,

forming a funnel, and expressing the material into the breech of the syringe.

7. Remove the displacement cord and gently dry the preparation with compressed air.

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8. Place the tip of the syringe nozzle so that it touches the margin and inject the

material slowly The tip should be inserted into the most distal embrasure first. This

will prevent the material from flowing down over the preparation and trapping air

bubbles. The tip is moved so that it follows the material rather than traveling ahead of

it. When all the margins and axial surfaces have been covered, the material is air-

blown into a thin layer. This improves the accuracy of the impression because the

light-bodied material has greater polymerization shrinkage than the tray material.

9. Syringe along any edentulous spaces, lingual concavities of the anterior teeth

(which are important for guidance), and occlusal surfaces of the posterior teeth

(which are important for obtaining an accurate articulation)

10. Seat the tray . It must remain immobile while the material undergoes

polymerization (6 to 12 minutes, depending on the material). Otherwise, strains will

form in the elastomer, which can cause distortion of the impression when it is

removed. The manufacturer’s recommendations for maximum working time and

minimum setting time should be followed. It is difficult to judge clinically when

elastomers start to develop elasticity. Any delay in seating the tray will result in a

distorted impression. It is tempting to remove the impression too soon, since the

patient may find it uncomfortable. However, premature impression removal is a

common cause of distorted impressions. Because setting times vary from batch to

batch, allowing the impression to set longer than what the manufacturer recommends

is a wise precaution.

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Single-mix Technique:

The same steps are performed for the single-mix technique as for the heavy body

—light body technique; however, as the name indicates, only one mix is used to load

the syringe and fill the tray. Most single-mix materials tend to produce a more

viscous combination with a slightly shorter working time.

Automix Technique

Most manufact:urers offer impression material in prepackaged cartridges with a

disposable mixing tip attached. The cartridge is inserted in a caulking gunlike device,

and the base and catalyst are extruded into the mixing tip, where mixing occurs as

they progress to the end of the tube. The homogeneously incorporated material can be

directly placed on the prepared tooth and impression tray. One of this system’s

advantages is the elimination of hand mixing on pads; the elimination of this variable

has been shown to produce fewer voids in the impression. Following the

manufacturer’s directions and bleeding the cartridge before inserting the tip are

crucial. Automixing is not available for the polysulfide polymers because these

materials are too sticky for proper combination.

Machine Mixing Technique :

An alternative method for improving impression mixing is to use a machine

mixer.* This system is convenient and produces void-free impressions.

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Evaluation :

The impression must be inspected for accuracy when it is removed.

(Magnification is helpful.) If bubbles or voids appear in the margin, the impression

must be discarded. An intact, uninterrupted cuff of impression material should be

present beyond every margin. Streaks of base or catalyst material indicate

improper mixing and may render an impression useless. If the impression passes all

these tests, it can then be disinfected and poured to obtain a die and working cast ).

SPECIAL CONSIDERATIONS

Certain modifications of the basic impression technique are sometimes needed,

particularly for making impressions with additional retention features such as

pinholes and post space.

Pin-retained Restorati-. Elastomeric impression materials are strong enough to

reproduce a pinhole without tearing. However, to avoid bubbles, they must be

introduced carefully into the pinhole with a lentulo or cement tube. With reversible

hydrocolloid, a special nylon bristle must be used for the impression.

Step-by-step Procedure

1. Apply a separating medium (e.g., die lubricant) to the pinholes and isolate and

displace the tissue in the conventional manner.

2. After mixing the light-bodied impression material, set aside a small amount for

placement into the pinholes.

Cement Tube

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3. Fill the tube and squeeze a small amount of material into each pinhole. Make sure

that no air is trapped in the base of the pinhole (insert an explorer into the material,

remove, and repeat the application).

Lentulo

4. Be sure that the slow-speed handpiece is rotating clockwise before picking up a

small quantity of impression material.

5. Spiral the material into the pinholes, rotating slowly while moving the lentulo

along the side of the pinhole.

6. Increase the speed of the lentulo while backing it out (to prevent the material from

being pulled out).

Prefabricated Plastic Pin

7. When making a reversible hydrocolloid impression of a pin-retained restoration,

use elastomer bristles to register the pin holes. The bristles can be modified as

necessary with a sharp scalpel to eliminate any inaccuracy relating to fit. Their

lengths should be adjusted so that they do not contact the impression tray. (A bristle

should extend 2 mm above the opening of the pinhole.)

8. Apply a separating medium to the pinhole before placing the bristle and completing

the impression.

Post-and-cores. - Elastomeric materials can be successfully used to make

impressions of the post space when endodontically treated teeth are being restored.

The procedure involves reinforcing the impression with a plastic pin or suitable wire

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DISINFECTION

When they are removed from the patient’s mouth, it must be assumed that all

impression materials have been in contact with body fluids. They should be

disinfected according to the recommended procedures for the material being used.

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 sprays. 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

glutaraldehyde 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.

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, cards, and saliva evacuators are needed for adequate moisture control.

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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 subgingival margins of prepared teeth.

However, a narrow cord impregnated with a mild astringent (e.g., Aid3) is

recommended. To protect the smear layer, excessive contact between hemostatic

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. All impression materials should be rinsed, dried, and

disinfected when removed from the mouth. Impressions made with polysulfide

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. When making pin-retained restorations, a cement tube, lentulo,

or nylon bristle is needed for an accurate impression of the pinholes or post spaces. In

this technique and others, a good impression is critical for an accurately fitting

restoration.

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REFERENCES:- Donovan T.E. et al: Review and survey of medicaments used with gingival

retraction cords. J.P.D.1985 vol.58 pg.525-531

Miller I.F:Fixed dental prostheses. J.P.D.1958 vol.8 pg.483-495

Ruel J. et al:Effects of retraction procedure on periodontium of humans.J.P.D.1980 vol.44 pg.508-514

Reiman B.Milford:Exposure of subgingival margins by non-surgical gingival displacement. J.P.D.1976 vol.436 pg.649-654

Benson D.W et al:Tissue displacement methods in fixed prosthodontics. J.P.D.1986,vol.55,pg.175-182

La Forgia A: Cordless tissue retraction for fixed prostheses. J.P.D.1967,vol.17,pg.379

A survey on the use of gingival retraction cords by dental professional. JIPS 2003,vol.3 pg.21,30

W.D.Mello, V.Chitre et al: Gingival retraction cords-their role in tissue displacement:A Review. JIPS2003,vol.3,pg.16

Shillingburg H.T etal. Fundamentals of fixed Prosthodontics.3r edi.quintessence pub.co

Benson etal. Tissue displacement methods in fixed prostho. J prosthet Dent;55:175-81.

Donovan etal. Review and survey of medicaments in retraction. J Prosthet 1985:53:525-30.

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