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Impressions for fixed partial denture

INDIAN DENTAL ACADEMY Leader in continuing dental education

Introduction To obtain an impression is the first step necessary for the indirect fabrication of a prosthesis. An acceptable impression must be an exact negative replica of the prepared tooth . The impression should reproduce the critical area where tooth and restoration meet so that the restoration can be correctly contoured. All teeth or tissues immediately surrounding the prepartion must be reproduced in the impression.

The properties of an ideal impressions are 1) Complete plasticity before cure 2) Sufficient fluidity to record fine detail. 3) The ability to wet the oral tissues. 4) Dimensional accuracy. 5) Dimensional stability. 6) Complete elasticity after cure. 7) Optimal stiffness.

The impression materials used in fixed partial denture construction include; 1)Hydrocolloid impression material a) reversible hydrocolloid 2)Elastomeric impression materials a) Polysulfide b) Addition silicone c) Condensation silicone d) Polyether

Impression techniques 1.Stock tray technique double mix single mix 2.Custom tray technique single mix 3.Closed bite double arch technique 4.Copper band technique

5.Reversible hydrocolloid technique

Custom Tray Fabrication For Elastomeric Impression Custom tray is made from autopolymerizing acrylic resin or vacuum formed thermoplastic resin. Tray rigidity is important for even slight flexing of the tray will lead to distorted impressions. Resin thickness of 2 or 3mm is needed to give adequate rigidity. Clearance between the tray and the teeth should be 2 to 3mm however greater clearance is needed for the more rigid polyether material .

Procedure: 1)Mark the border of the tray on the diagnostic cast with a pencil approximately 5mm apically to crest of the free gingiva. 2)Adapt a wax space to the diagnostic cast two layers for 2.5mm thickness approximately. 3)Apply a layer of tinfoil over the wax. 4)Mix autopolymerizing acrylic resin according to manufacturers recommendation 5)Adapt the resin to the cast when in dough stage. 6)After the material has polymerized ,remove it from the cast and trim with acrylic trimming bur.

Putty wash Method Double mix and single mix. Stock trays can also be used with medium and heavy bodied elastomers normally used with custom trays.

Advantages: 1)Eliminates time and expense of fabricating custom trays. 2) Metal stock trays are rigid and are not susceptible to distortion.

Disadvantages: 1) More impression material is required 2) Metal tray must sterilized

Technique: A trays is selected based on shape and size of the patients arch. Prepare the Stock Tray. Coat the tray evenly with adhesive on the inside.  Mix the high viscosity putty impression material according to manufactures instruction. Roll putty into elongated cylinder. Insert into the stock impression tray.

Cover putty with the spacer (a sheet of polyethylene) .

Insert and seat the tray with a rocking type of motion. Hold and wait until initial set (approximately 2min). Remove from the mouth with minimal side ward movement Wait and test for final set. When the impression rebound completely. Peel of the spaces. Remove excess impression material with a sharp knife.

Putty placed on the tray

Placing the spacer

Removal of the spacer from the tray

Making the final impression Mix the low viscosity impression material according to the manufactures instructions . Load the low viscosity impression material on to a syringe . Syringe the inaccessible area first eg disto lingual line angle For the stock tray double mix insert the low viscosity impression material into the tray without overfilling it.

For stock tray single mix technique the unset high viscosity material should already be in the tray and the preparation syringed with low viscosity impression material. Seat the tray . Wait for the final set. Remove the tray parallel to the preparation path of withdrawal. The impression is evaluated for the finish lines and for any distortion and tear.

Final impression

Custom tray method Mixing method-single mix Advantages . Less impression material is required than stock tray. The tray is used only once so sterilization is not a problem. Less distortion of the impression .

Disadvantages Construction of the custom tray is time consuming . The tray should for 24 hours to minimize distortion

Making the final impression. Medium or high viscosity is used in the tray and low viscosity in the syringe. The low viscosity material is mixed first then the high viscosity material. The syringe is loaded with the low viscosity material and syringed in the prepared site. The high viscosity material is placed in the custom tray and seated in the mouth. The impression is removed and evaluated.

Removal of the tray using the wings







synonyms :dual quad tray, double arch , triple arch ,accu -bite ,closed mouth impression. Requirements ďƒ˜The articulator should provide for a positive stop or there should be sufficient teeth to maintain the vertical dimension. ďƒ˜There should be sufficient

space distal to the terminal

tooth in the arch to allow tray approximation.

Advantages The physical deformation of the mandible is minimal The







intercuspation is captured. 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 Evaluate the fit of the tray in the patients mouth. Observe the complete bilateral closure and the patients comfort. Practice till the patient is familiar with the task. Making the final impression Mix the low viscosity material and load the syringe The high viscosity material is used 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. Wait for the material to set . Instruct the patient to open the mouth remove the tray by applying equal pressure bilaterally. Evaluate the impression.

Closed mouth impression

Copper band or tube method The copper band is used to salvage an impression of multiple preparation where there is only vague margins on one or two preparation that are not adequately replicated in the impression.

Fitting of the copper band to preparation ďƒ˜A copper band is selected and annealed by heating on a flame and quenching in alcohol. ďƒ˜Mark the finish line with the explorer and round off the edges. ďƒ˜evaluate the fit and cut orientation hole in top one fifth of

the facial surface .

Copper band with orentation holes

Copper band contoured

Evaluating the fit of the copper band

Making the compound plug The red stick compound is heated over the Bunsen flame . Insert the warm compound mass to fill approximately the top third of the copper band. 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. Remove 0.2mm of the compound to create space for the heavy body poly vinyly silicone. Drill a hole through the centre of the compound plug.

Making the impression. Make 4to 5 holes evenly distributed above the bottom of the copper tube for the retention of the impression material. Coat the internal surface sparingly with adhesive . Clean and isolate the preparation. Mix heavy viscosity material and inject in to the band and position it on the tooth. Wait the material to set remove the band and inspect the impression

Copper band with the final impression

Reversible hydrocolloid technique This impression technique requires

a special conditioning

unit which consists of 3 thermostatically controlled water baths. 1. A liquefaction bath . 2.A storage bath . 3.A tempering bath.

Hydrocolloide conditioner

Rimlock trays with stops

Procedure 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. Carefully remove the retraction cord and flood the tooth with water.

Inject the syringe material on the tooth 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 ,evaluate for accuracy.

Removal of the retraction cord

Injecting hydrocolloide into the sulcus

Final impression in reversible hydrocolloide

Summary A good impression is an exact replica

of each prepared

tooth and should include adequate

amount of the

unprepared surfaces adjacent to the margin. Impression technique and material should be selected on the basis of

biologic factor dictated by the anatomy and the

physiology of the mouth dictated by the orofacial tissues. Even







procedure for impression making ,it is the responsibility of the dentist to select the best possible procedure based on sound knowledge , for achieving the best possible result for the patient.

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