Impression Techniques in Fixed Prosthodontics

Impression Techniques in Fixed Prosthodontics

Describe in detail, with diagrams, the step-by-step preparation of a maxillary canine to receive a three-quarter crown. Explain and justify different impression techniques for fixed partial dentures.

Partial Veneer Crown Definition

A partial veneer crown is an extra coronal metal restoration that covers only part of the clinical crown. Partial veneer crowns include all tooth surfaces except the buccal or labial wall in the preparation.

Read And Learn More: Fixed Partial Denture Short Essay Question And Answers

Steps in the Preparation of Partial Veneer Crowns are

• Incisal reduction.
• Lingual reduction.
• Interproximal reduction.
• Proximal box or groove placement.
• Incisal offset placement.
• Facial bevel.
• Finishing the preparation.

Armamentarium

• High- and low-speed contra-angle handpiece.
• Burs as listed below.
• Utility wax and wax gauge to evaluate lingual reduction.

Incisal reduction

• Round-ended tapered diamond is used
• Reduce the incisal edge 1 mm at a 45° angle to the long axis of the tooth. Remove 1.0–1.5 mm following the facial contour of the tooth.

Lingual reduction

Done in two steps.

• Lingual surface reduction
  A football-shaped diamond is used to reduce the lingual surface in two planes, with a slight ridge along the center of the lingual surface incisogingivally. A clearance of at least 0.7–1 mm is required with the opposing tooth.
• Lingual–gingival reduction
  A round-ended tapered diamond is used to achieve a chamfer of 0.5 mm deep at the cervical finish line. The chamfer is extended to include the lingual line angles.

Interproximal reduction

Done in three steps.

• The proximal surface is reduced with a 169L carbide bur from the lingual to the facial surface with the contact point intact. The facial line angles must remain intact for good esthetic results.
• A light chamfer finish line is made on the proximal surface with a narrow chamfer diamond. This chamfer should merge with the lingual chamfer.
• The contact with the adjacent tooth is broken with a hatchet instrument from the facial surface, to form labial proximal extensions. The flare of proximal extensions is finished with a flame-shaped diamond.

Proximal grooves

A 167 carbide bur is used for groove placement at the proper alignment. The proximal grooves are placed parallel to the incisal two-thirds of the facial surface (169L carbide bur).

These grooves resist lingual displacement and should be a minimum of 3 mm long with 0.5 mm of the gingival finish line. The facial and lingual walls of the grooves should have a 2–5 degree incisal divergence.

The lingual wall of the proximal grooves should have a 2–5 degree incisal convergence with the lingual gingival wall.

The facial wall of the groove should be continuous with the proximal flare to add bulk to the facial margin.

Incisal groove

Inverted cone carbide bur is used.

A 0.5–1 mm groove is prepared within the dentin and is made parallel to the DEJ connecting the proximal grooves. The groove is not placed at the expense of the incisal edge.

Facial bevel

• Fine, flame-shaped diamond bur is used.
• A narrow bevel < 0.5 mm is prepared on the labial incisal finish line at right angles to the incisal two-thirds of the facial surface.

Finishing the preparation

• A carbide finishing bur is used.
• All the sharp and point angles are rounded to ensure the continuity of all finish lines.

Cingulum modification if needed for additional retention

• After paralleling a 170 bur to the long axis on the proximal grooves, a ledge is prepared in the cingulum.
• A pilot hole is cut in the ledge with a No. 1/2 round bur.

Different Impression Techniques

• Stock Tray/Putty wash (single mix and double mix).
• Custom tray (single mix).
• Closed bite/Double arch/Dual quad tray/Triple arch.
• Copper band.
• Reversible hydrocolloid (laminate and wet field technique).
• Matrix system.

Stock Tray/Putty wash

Polyvinyl siloxane is used.

Materials used

Reversible, irreversible hydrocolloids and elastomeric impression materials.

For accuracy, elastomeric impression materials are the most commonly used.

In this technique, a single impression with polyvinyl siloxanes or a double mix with medium and heavy-bodied elastomers can be done.

Single mix technique

Advantages of a single mix

• Less time required
• No need to fabricate a custom tray
• Metal stock trays are rigid and the chances of distortion are less.

Disadvantages

• Each time the tray is used it needs to be sterilized
• More impression material is required.

Technique

• After adjusting the chair position, a proper stock tray with the correct border extensions, and tray shape and size depending on the patient’s arch shape and size is selected.
• Apply tray adhesive on the inside and rim of the stock tray.
• Manipulation of impression material:
  Mix the high-viscosity putty impression material and roll the putty into the elongated cylinder.
  Place the putty in the stock tray and cover with polyethylene sheet.
• Seating of tray:
  Insert and seat the tray with a rocking-type motion.
  Seat the tray in the mouth without movement till the initial set occurs (approximately 2 minutes).
  For the 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.
• The setting of material and tray removal:
  Remove from the mouth with minimal sideward movement and ensure the material is set using a fingernail test (material rebounds completely).
• Trimming the impression:
  After removing the spacer, the excess impression material is removed with a sharp knife.

Double mix technique (PVS PS CS)

• After a stock tray is selected, tray adhesive is applied; the impression putty is mixed and placed in the tray.
• A polyethylene sheet is used to cover the putty material and the impression is seated in the patient’s mouth.
• After the complete set of the impression is ensured by fingernail testing, the tray is removed.
• Relieving the tray:
  A sharp hand instrument is used to remove the uniform amount of impression material from the tissue surface.
• After gingival retraction evaluate tissue displacement, check the finish line area(s), and leave cord(s) in place for 8–12 minutes.
• Manipulation of light-body material:
  After measuring the arch length of the tray, one times the length of the tray low-viscosity elastomer is dispensed, after trimming the tip of the syringe.
• Making a final impression:
  • A mixing pad, (6 by 8 inches) or an automatic gun dispensing system is used
  • The low-viscosity impression material is mixed with a circular motion combining the two strands, then a figure eight motion to blend and flatten the mixture onto the mixing pad (approximate mixing time less than 1 minute).
• Loading the impression material:
  • The syringe is loaded by holding it at a slight angle while scraping the pad
  • Screw on the tip, and insert the plunger
  • While the plunger is inserted into the syringe, the cord is removed
  • After evaluating the retraction site for seepage, hemorrhage, or debris, the first syringe is inaccessible areas. (For Example, distal lingual finish line)
  • The syringe is positioned so the elastomer is ahead of the tip’s orifice.
• Tray Insertion:
  • Insert the low-viscosity impression material into the tray slightly less than the depth of the external borders
  • Seat the tray from posterior to anterior, allowing the excess to extrude anteriorly
  • Seat the tray firmly in position
  • The tray should not be moved while the material is sitting.
• Final impression:
  • After the final set is over, the tray is removed
  • Rinse the impression with ambient water, and dry with short, small bursts of compressed air
  • Retraction cord (s) remaining in the impression material are removed carefully.
• Evaluate set impression:
• The area 0.5 mm beyond the visible finish line should be visible. There should be a no-show-through in any areas of the impression, except at tissue stops
• There should be no shiny smooth areas, no voids present
• Review for tears. There should be no thin areas leaving the finish line unsupported.

Custom Tray: PVS PE PS, CS

• Advantages
  • Less impression material is required compared to stock tray
  • Sterilization is not a problem
  • Less chances of impression material getting distorted due to curing shrinkage
  • Procuring of the tray material is not required.
• Disadvantages
  • Time-consuming due to tray fabrication
  • The tray must be used after complete curing to prevent further distortion
  • Monomer sensitivity during tray fabrication for some personnel.

Technique

Fabrication of tray

• The diagnostic cast is soaked in slurry water for 10 minutes and then painted with a layer of tinfoil substitute to prevent the resin from adhering to the cast
• The outline of the tray extensions is marked on the cast and two sheets of base plate wax are adapted to the cast
• Excess wax is trimmed and a thin tinfoil (or polyethylene) sheet is placed over the wax to protect the resin from wax during the exothermic cure.

Placement of tissue stops

• Four widely spaced hard tissue stops of 3 mm/2 mm are placed on non-functional cusps.

Manipulation of tray material

• The right proportion of monomer (liquid) and polymer (powder) are mixed and in the dough stage, it is flattened to approximately 4 mm thick
• The flattened putty is adapted to the tin-foiled cast; excess material is trimmed off and the handle is formed with excess resin.

Removal of the tray from the cast

• After the resin material sets (approximately 15 minutes), the tray is lifted from the cast. The wax spacer is removed (all wax needs to be removed).

Finishing the tray

• The tray is trimmed and polished. Gingival retraction is carried out and tray adhesive is applied.

Making final impression

• A mixing pad, (6 by 8 inches) or an automatic gun dispensing system is used
• The low-viscosity impression material is mixed with a circular motion combining the two strands, and then it is blended and flattened onto the mixing pad. (In less than l minute).

Loading the impression material

• The syringe is loaded by holding it at a slight angle while scraping the pad
• Screw on the tip, and insert the plunger
• While the plunger is inserted into the syringe, the cord is removed
• After evaluating the retraction site for seepage, hemorrhage, or debris, first syringe inaccessible areas (e.g., distal–lingual finish line)
• The syringe is positioned so the elastomer is ahead of the tip’s orifice.

Tray Insertion

The low-viscosity impression material is placed into the tray and the tray is seated firmly in position until the material sets.

Final impression

After the final set is over, the tray is removed; the impression is rinsed with water and dried with compressed air.

Closed–bite Double–arch Method

Synonyms: Dual quad tray, double arch, triple tray, Accu-bite, closed-mouth impression.

Materials used are polyvinyl siloxane and polyether.

Minimum conditions

• The articulator must have a vertical dimension holding stop, such as an incisal pin to maintain the vertical dimension.
• There should be sufficient space distal to the terminal tooth for tray approximation.

Advantages

• A functional impression is achieved
• Less elastomeric impression material is needed
• The physical deformation of the mandible during opening is minimized
• Less gagging may occur.

Disadvantages

• Tray is not rigid
• It is not a functionally generated technique, so it is limited to one casting per quadrant
• The distribution of the impression material is not uniform.

Procedure

• After evaluating the fit of the tray in the patient’s mouth, the tray is positioned accurately with the tray crossbar distal to the last tooth in the arch
• The patient is asked to close their mouth to observe the complete bilateral closure and the patient’s comfort.

Making final impression

• A mixing pad, (6 by 8 inches) or an automatic gun dispensing system is used
• The low-viscosity impression material is mixed with a circular motion combining the two strands then a figure-of-eight motion to blend and flatten the mixture onto the mixing pad (in less than 1 minute).

Loading the impression material

• The syringe is loaded by holding it at a slight angle while scraping the pad
• Screw on the tip and insert the plunger
• While the plunger is inserted into the syringe, the cord is removed
• After evaluating the retraction site for seepage, hemorrhage, or debris, first syringe inaccessible areas (e.g., distal-lingual finish line).

Tray insertion

• Insert the low-viscosity impression material into the tray slightly less than the depth of the external borders
• Seat the tray from posterior to anterior, allowing the excess to extrude anteriorly
• Seat the tray firmly in position
• For quadrant trays, position the crossbar distal to the last tooth in that arch.

Closed-mouth technique

Instruct patient to slowly close mouth and evaluate the interdigitation on the opposite arch.

Tray removal

• When the patient opens the mouth, the impression adheres to one arch
• The tray is removed by placing a finger on either side of the tray
• The handle is not used to remove the tray
• Residual impression material in sulcus or interproximal areas is removed
• Rinse the impression with water, and dry with small bursts of compressed air
• Retraction cord(s) remaining in the impression material are removed carefully.

Copper Band

Copper tube impressions are made when there are multiple preparations with vague margins.

Fitting copper band to preparation

• A copper band of adequate diameter is adapted to the prepared tooth
• This adapted band is annealed by heating in flame and quenching in alcohol
• The finish line area is marked with a sharp explorer tip
• The marked area is cut with scissors and smoothened with carborundum stone.

Checking the fit

• The copper band should extend at least 1 mm beyond the finish line with a slight gap between the finish line and the copper band
• Orientation holes are placed in the top one-fifth of the facial surface of the tube.

Making of compound plug

• The red stick compound is heated over a Bunsen burner flame
• Once the appropriate temperature is reached the warm compound mass is inserted on top of one-third of the copper tube
• The copper band is oriented and seated till the compound touches the occlusal surface of the band.

Removing the impression

• The impression is cooled in water
• A towel clamp is used to remove the copper band from the mouth.

Relieving the final impression

• A slow-speed handpiece with No. 6 or 8 carbides bur is used to remove 0.2 mm of the compound from the impressed occlusal surface creating a space for the heavy body polyvinyl siloxane
• A relief vent is placed with a long shank No. 6 round carbide bur, through the center of the compound plug.

Making impression (PVS)

• More relief vents are placed with a sharp No 4 or No 6 round carbide 2–3 mm above the bottom of the copper tube. These holes help to retain the polyvinyl siloxane impression material and provide a suitable space at the finish line area
• Some areas in the internal surface area are mildly coated with adhesive
• The prepared tooth is cleaned and isolated.

Final impression

• Either an automatic gun or a syringe loaded with heavy viscosity, the polyvinyl-siloxane impression is injected into the copper band, filling the space completely from the compound to the copper band edge
• After loading the impression material in the copper band the band is positioned and seated properly.

Removing the final impression
After the final set is completed a towel clamp is used to remove the impression by grasping on top one-fifth of the impression.

Laminate technique/Agar–alginate combination technique

• The agar material is syringed around the prepared tooth area and the alginate is used as tray material for the rest of the arch
• Alginate sets by chemical reaction and agar sets when it comes in contact with cool alginate.

Wet Technique

• Area to be recorded are flooded with warm water and immediately syringe material is injected while still the syringe material is liquid the tray material is loaded and seated. The pressure of the viscous tray material pushes fluid syringe material around areas to be recorded.

Matrix System

• Uses three types of impression consistencies as initially a quick setting matrix-forming material is used in a clear tray. This matrix impression is retrieved and trimmed to hold a high-viscosity polyether to record the prepared teeth surfaces and then a third less viscous tray material is used which encloses the matrix with light-body material and the rest of the arch.