Impression ProceduresImpression Procedures for Partially Dentate Patients for Partially Dentate Patients

SDS 321SDS 321

McCracken’s Removable Partial Prosthodontics. McCracken’s Removable Partial Prosthodontics. Chapter 15.Chapter 15.

An impression,An impression,

of a partially dentate arch must record accurately theof a partially dentate arch must record accurately theanatomic form of the teeth and surrounding tissues so that ..anatomic form of the teeth and surrounding tissues so that ..

.. a definite path of insertion .. a definite path of insertion can be selected,can be selected,

and …and …

.. support, retention and stability can be planned... support, retention and stability can be planned.

MaterialsMaterialsMaterialsMaterials

that could permanently deform on that could permanently deform on removal from the undercuts removal from the undercuts should not be used,should not be used, e.g., e.g., rigid rigid and and thermoplastic.thermoplastic.

Impression MaterialsImpression Materials(A review)(A review)

Rigid:Rigid: Impression plasterImpression plaster

Metallic oxide (ZnO Euginol) pasteMetallic oxide (ZnO Euginol) paste

Thermoplastic:Thermoplastic:

Modeling compoundModeling compound

Impression waxesImpression waxes

Elastic:Elastic: Reversible HydrocolloidReversible Hydrocolloid

Irreversible HydrocolloidIrreversible Hydrocolloid

Rubber base (Marcaptan, Silicon, Rubber base (Marcaptan, Silicon, Polyether)Polyether)

Hydrocolloid MaterialsHydrocolloid Materials(A reminder)(A reminder)

These materials are elastic enough to be withdrawn from the These materials are elastic enough to be withdrawn from the undercuts without permanent deformation or distortion.undercuts without permanent deformation or distortion.

One piece impression is possible.One piece impression is possible. No separating medium is needed for pouring.No separating medium is needed for pouring. Extremely accurate in tissue details when handled properly.Extremely accurate in tissue details when handled properly.

Reversible HydrocolloidReversible Hydrocolloid(Agar – Agar)(Agar – Agar)

The gelation of Agar is primarily a physical change in which a The gelation of Agar is primarily a physical change in which a latticework of fibrils forms as the temperature is lowered.latticework of fibrils forms as the temperature is lowered.

The gel can be dispersed by heating. It is liquefied at a The gel can be dispersed by heating. It is liquefied at a temperature compatible to oral tissues and then solidified to a temperature compatible to oral tissues and then solidified to a firm – yet elastic gel at about 100firm – yet elastic gel at about 100˚̊ F (38 F (38˚C)˚C)..

The gelation is accomplished by means of special water The gelation is accomplished by means of special water cooled impression trays.cooled impression trays.

Irreversible HydrocolloidIrreversible Hydrocolloid(Alginate)(Alginate)

These are not thermally reversible. The gelation is induced by These are not thermally reversible. The gelation is induced by a chemical reaction.a chemical reaction.

Powder: Sodium alginate & Calcium sulphate.Powder: Sodium alginate & Calcium sulphate. Add water for Add water for reaction to form a latticework of fibrils of insoluble Calcium reaction to form a latticework of fibrils of insoluble Calcium alginate.alginate.

Alginate is dimensionally stable for a brief period after its Alginate is dimensionally stable for a brief period after its removal from the mouth, hence should be poured removal from the mouth, hence should be poured immediatelyimmediately or stored for up to or stored for up to 15 minutes15 minutes in a saturated atmosphere. in a saturated atmosphere.

Procedure of Impression MakingProcedure of Impression Making

1. Select a suitable size perforated tray for obtaining an 1. Select a suitable size perforated tray for obtaining an initial impression.initial impression.

2. Add wax or modeling compound in the tuberosity & 2. Add wax or modeling compound in the tuberosity & palatal area for maxillary, or in the lingual flanges for a palatal area for maxillary, or in the lingual flanges for a mandibular impression.mandibular impression. The edentulous areas may also need The edentulous areas may also need modifications of the tray with impression compound.modifications of the tray with impression compound.

For the Final ImpressionFor the Final Impression

A better extended custom tray is normally fabricated A better extended custom tray is normally fabricated on on the diagnostic cast.the diagnostic cast.

Why to have Tissue stopsWhy to have Tissue stops

The custom tray is fabricated to have tissue stops for accurate The custom tray is fabricated to have tissue stops for accurate relocation of the trayrelocation of the tray and and for achieving even thickness of the for achieving even thickness of the impression material.impression material. These stops are placed on the teeth other These stops are placed on the teeth other than abutments and on the edentulous ridge distally.than abutments and on the edentulous ridge distally.

Tissue stops can be added intra-orallyTissue stops can be added intra-orally

by applying green stick compoundby applying green stick compoundto the chosen location on the trayto the chosen location on the trayand and then seating it in the mouththen seating it in the mouthunder gentle pressure.under gentle pressure.

For obtaining an ImpressionFor obtaining an Impression

Seat the patient in an upright position and check the tray Seat the patient in an upright position and check the tray border extensions.border extensions. For Class I & II it should cover the For Class I & II it should cover the retromolar pad or maxillary tuberosity.retromolar pad or maxillary tuberosity. Any overextensions Any overextensions should be relieved by trimming the tray, especially in the should be relieved by trimming the tray, especially in the frenum areas.frenum areas.

What to do !What to do !

Over-extendedOver-extended Under-extendedUnder-extended

Border moldingBorder molding

is carried out in the areas of under extension of the tray andis carried out in the areas of under extension of the tray and

for the distal extension saddles.for the distal extension saddles. The commonly used borderThe commonly used border

molding material is the tracing molding material is the tracing (green stick)(green stick) compound. compound.

Choice of an Impression materialChoice of an Impression material

Any of the elastic material (alginate, agar, polysulphide, Any of the elastic material (alginate, agar, polysulphide, polyether, or silicone) can be used.polyether, or silicone) can be used. However, However,

in the presence of deep undercutsin the presence of deep undercuts (bilateral lingually tilted (bilateral lingually tilted premolars)premolars) alginate should not be usedalginate should not be used as it may be stressed as it may be stressed beyond its elastic limit on removal.beyond its elastic limit on removal.

Before making the impressionBefore making the impression

Remove the excessive saliva from the mouth with an ejectorRemove the excessive saliva from the mouth with an ejector and and dry the rest seat preparations and dimpling (if any) with an dry the rest seat preparations and dimpling (if any) with an air syringe.air syringe. An excessive drying should be avoided as the An excessive drying should be avoided as the material may adhere to the dried tooth surface.material may adhere to the dried tooth surface.

.. also.. also

open inter-dental spaces should be occluded with soft wax open inter-dental spaces should be occluded with soft wax prior to inserting the loaded tray,prior to inserting the loaded tray,

otherwise the impression material will flow beneath the otherwise the impression material will flow beneath the contact points and lock the impression in place and distortion contact points and lock the impression in place and distortion will occur upon removal.will occur upon removal.

Mix the impression materialMix the impression material

By adding alginate powder to the desired amount of water – By adding alginate powder to the desired amount of water – spatulate rapidly against the walls of the bowl for one minute,spatulate rapidly against the walls of the bowl for one minute,

oror

By mixing the catalyst and base components of the rubber By mixing the catalyst and base components of the rubber base material uniformly.base material uniformly.

Place the material in the tray, try to avoid entrapping air and Place the material in the tray, try to avoid entrapping air and lock the alginate in the perforationslock the alginate in the perforations – – for rubber base for rubber base materials,materials, an adhesive should be applied to the tray before an adhesive should be applied to the tray before mixing the material.mixing the material. Do not over – load the trayDo not over – load the tray as the excess as the excess material may initiate the gag reflex.material may initiate the gag reflex.

For a bubble – free impressionFor a bubble – free impression

Alginate should be wiped over the teeth and into the deep sulci Alginate should be wiped over the teeth and into the deep sulci with a finger immediately prior to inserting the loaded tray.with a finger immediately prior to inserting the loaded tray. The rubber base materials can be poured over the teeth using a The rubber base materials can be poured over the teeth using a syringe.syringe.

Inserting the tray in the mouthInserting the tray in the mouth

Stand behindStand behind (for maxillary)(for maxillary) and in front of the patientand in front of the patient (for (for mandibular)mandibular) impression.impression. Retract the cheek with the tray and Retract the cheek with the tray and with your free handwith your free hand and and rotate the tray in the mouth.rotate the tray in the mouth.

Then,Then,

Retract the lip (upper or lower)Retract the lip (upper or lower) and and seat the tray anteriorly,seat the tray anteriorly, centering it according to the frenum and the sulcus, centering it according to the frenum and the sulcus, and then and then posteriorly.posteriorly. Let the lip drape naturally and complete the border Let the lip drape naturally and complete the border molding.molding. For lower impressionFor lower impression – – ask the patient to elevate the ask the patient to elevate the floor of the mouth, e.g., by licking his lips.floor of the mouth, e.g., by licking his lips.

Muscle trimming of the bordersMuscle trimming of the borders

While waiting for the material to setWhile waiting for the material to set

Hold the tray immobile for at least 3 minutes with Hold the tray immobile for at least 3 minutes with a light finger pressure applied over the left and a light finger pressure applied over the left and right pre – molar areas.right pre – molar areas. Any movement of the tray Any movement of the tray can cause an inaccurate impression.can cause an inaccurate impression.

Once the material is setOnce the material is set

Release the surface tension by lifting the lip (up or down) away Release the surface tension by lifting the lip (up or down) away from the tray.from the tray.

Remove the impression quickly (snap removal) in line with the Remove the impression quickly (snap removal) in line with the long axis of the teeth.long axis of the teeth.

Rinse it thoroughly under running tap water – free it from saliva Rinse it thoroughly under running tap water – free it from saliva or blood.or blood.

Examine the impression carefully for acceptance and disinfect it Examine the impression carefully for acceptance and disinfect it before pouring.before pouring.

An acceptable impressionAn acceptable impression

Another acceptable one !Another acceptable one !

Edentulous Edentulous VsVs Distal extension Distal extension

Now the unacceptable onesNow the unacceptable ones

Incorrect seating of the impression tray laterallyIncorrect seating of the impression tray laterally..

Not AcceptableNot Acceptable

In-complete seating of the impression tray –In-complete seating of the impression tray –

unsupported impression at the borders.unsupported impression at the borders.

Repeat the ImpressionRepeat the Impression

Under extended tray –Under extended tray – thin edge of unsupported thin edge of unsupported impression of the posterior lingual sulcus.impression of the posterior lingual sulcus.

Re-do border molding alsoRe-do border molding also

Over-extended border moldingOver-extended border molding resulting in the distortedresulting in the distorted sulcus reflection in the impression.sulcus reflection in the impression.

Finally, an acceptable impressionFinally, an acceptable impression

A maxillary impression taken withA maxillary impression taken with combination combination of impression of impression compound in the distal extension areas and overall alginate compound in the distal extension areas and overall alginate impression.impression. Alginate has been cut off on one side to show its thickness.Alginate has been cut off on one side to show its thickness.

Management of Gag reflexManagement of Gag reflex

Exaggerated gag reflex can frustrate both the patient and the Exaggerated gag reflex can frustrate both the patient and the dentist. dentist. It can also compromise the treatment plan.It can also compromise the treatment plan.

Its effective managementIts effective management is based on experience with combinations of clinicalis based on experience with combinations of clinical techniques, techniques, prosthodontic management,prosthodontic management, medication,medication,

andand psychologist referral,psychologist referral, oror a specialist referral. a specialist referral.Some Some simple measuressimple measures that usually works: that usually works:

Reassurance and kind handling.Reassurance and kind handling. Upright position of the patient.Upright position of the patient. Avoid overloading the tray.Avoid overloading the tray. Distract patient’s mind – talking, raising one foot Distract patient’s mind – talking, raising one foot

etc.etc.

Care of Alginate ImpressionCare of Alginate Impression

Should not be exposed to air (Should not be exposed to air (dehydrationdehydration).). Should not be immersed in water (Should not be immersed in water (imbibitionimbibition).). Should be stored in a humid atmosphere by wrapping in a Should be stored in a humid atmosphere by wrapping in a

damp paper towel or in 100% relative humidity (humidor).damp paper towel or in 100% relative humidity (humidor). Should be poured Should be poured immediatelyimmediately, or , or immersedimmersed in a solution of in a solution of

accelerator (accelerator (Pot. SulphatePot. Sulphate) to avoid) to avoid syneresis -syneresis - giving off a giving off a mucinous exudate that has retarding effect on gypsummucinous exudate that has retarding effect on gypsum and and results in a soft or chalky cast surface.results in a soft or chalky cast surface.

Procedure of Forming a CastProcedure of Forming a Cast((Without BoxingWithout Boxing))

1. Remove the alginate impression from the damp paper towel 1. Remove the alginate impression from the damp paper towel and shake out any moisture.and shake out any moisture.

2. Add 100 gm of stone (powder) into 28 ml of water and 2. Add 100 gm of stone (powder) into 28 ml of water and spatulate for one minute.spatulate for one minute. Place the bowl on the vibrator to Place the bowl on the vibrator to escape the entrapped air.escape the entrapped air.

3. Hold the impression tray against the 3. Hold the impression tray against the vibrator and add a small amount of mixed vibrator and add a small amount of mixed stone to the distal area.stone to the distal area. Let it vibrate Let it vibrate around the arch - from tooth to tooth.around the arch - from tooth to tooth. Continue to add small increments until Continue to add small increments until impressions of the teeth have been filled.impressions of the teeth have been filled.

4. Continue to add stone in larger portions 4. Continue to add stone in larger portions until the impression is completely filled.until the impression is completely filled.

5. Let the stone reach its initial set.5. Let the stone reach its initial set.6. The base of the cast can then be 6. The base of the cast can then be

formed with a new mix of stone. formed with a new mix of stone. The base should be at least 15 The base should be at least 15 mm thick.mm thick.

Forming the CastForming the Cast((With BoxingWith Boxing))

Apply beading wax to the Apply beading wax to the periphery of the impression.periphery of the impression. It should be placed 2-3 mm It should be placed 2-3 mm from the borders of the from the borders of the impressionimpression and and should be should be 4 mm wide.4 mm wide.

Alternately, beading can be Alternately, beading can be done with a 50 – 50 mix of done with a 50 – 50 mix of

plaster and pumice.plaster and pumice.

Box the ImpressionBox the Impressionwith Boxing wax sheetwith Boxing wax sheet

Hold the boxed impression on aHold the boxed impression on avibratorvibrator and and add mixed dentaladd mixed dentalstone in small increments untilstone in small increments untilcompletely filled.completely filled.

After the stone has completely set & the exo-therm completed,After the stone has completely set & the exo-therm completed, peal off the boxing wax and remove the beading wax.peal off the boxing wax and remove the beading wax.

Remove the impression gently Remove the impression gently off the cast with a plaster off the cast with a plaster knife.knife. For the border molded For the border molded impression,impression, the cast should be the cast should be dipped in warm water for easy dipped in warm water for easy removal.removal.

Trimming the CastTrimming the Cast

The sides of the cast are trimmed to be parallelThe sides of the cast are trimmed to be parallel - - any stone any stone blebs or nodules are carefully removed.blebs or nodules are carefully removed.The base can be trimmed for either orthodontic specifications The base can be trimmed for either orthodontic specifications ((for a record castfor a record cast) or to remove excess stone only ( ) or to remove excess stone only ( for a for a master or a working castmaster or a working cast ). ).

Land area of the castLand area of the cast

should be trimmed with a sharp knife to have uniform should be trimmed with a sharp knife to have uniform height of the sulcus reflection ( 2-3 mm ).height of the sulcus reflection ( 2-3 mm ).

andand

it should be 3-4 mm wide to provide it should be 3-4 mm wide to provide necessary strength to the cast.necessary strength to the cast.