Impression Procedures Involved In Implant Supported Prosthesis

Impression Procedures Involved In Implant-Supported Prosthesis

Classify and discuss various impression procedures involved in implant-supported prosthesis.

Definition

An impression is defined as the negative replica of teeth and oral tissues.

Criteria for good impression

• Must accurately record the soft tissue and supporting areas
• Able to record the positioning of the implant components
• The resiliency difference between the implant and the mucosa should also be considered in the impression technique used
• Inaccuracies in impression can cause misfit of the prosthesis, uneven force distribution, abutment screw loosening and occlusal inaccuracies.

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Classification

General classification

• Direct transfer (open-tray impression technique)
• Indirect transfer (closed-tray impression technique).

Other systems

Virtual impression system

Other classifications

Are modifications of the above two techniques by utilizing different—

• Impression materials
• Tray
• Type of transfer – Abutment or implant level
• In the type of derived support.

Techniques

• Direct impression-transfer (open-tray impression technique)
• It is the most accurate type of implant impression transfer in which impression-transfer copings are picked with the impression. The transfer coping is disengaged through the impression tray opening after polymerization of the impression and removed along with the impression.

Armamentarium

• Custom tray/stock tray.
• Tray adhesive.
• Modelling wax.
• Medium-bodied and light-bodied polyvinyl siloxane impression material.
• Essential implant components alike transfer coping, lab analogs, abutments, and hex drivers.

Procedure

• A custom acrylic resin tray is fabricated from the primary cast with openings in the area where the implants are located or a stock tray is perforated in the region of the implant

• The healing abutments are removed and implant impression copings are placed. The approximation of components is verified with radiographs
• The impression tray is evaluated for good adaptation, uniform impression material space and adequate clearances for the impression coping to protrude through the openings of the tray
• The excessive openings are sealed with the modeling wax
• An adequate amount of tray adhesive is coated on the tissue surface of the impression tray
• The impression is made with a medium body or monophase polysiloxane impression material. The tray is seated intraorally with all the impression coping pins protruding through the opening of the tray
• After complete polymerization of the impression material the excess wax or impression material on the guide pins is removed from the impression pins for the drivers to gain access
• The impression copings are disengaged from the implants and an impression is removed from the patient’s mouth
• The impression is evaluated, disinfected and components are replaced with suitable laboratory analogs
• The master cast is made with type 4 gypsum products.

Advantages of Open Tray Technique

• Accurate
• The best technique for angulated implants
• Ideal for multiple implant restorations.

Disadvantages Of Open Tray Technique

• Accurate only with the use of custom trays hence, additional steps in the fabrication procedure of custom trays
• Not suitable in patients with restricted mouth opening and in posterior edentulous regions with reduced access.

Closed Tray Technique

In this technique, the impression copings remain in the mouth on the removal of the set impressions. After removing the set impression from the mouth the copings are transferred or oriented to the set impressions.

Procedure

• The transfer coping is fixed to the implant after removing the healing abutments
• The screw of the transfer coping is blocked with cotton balls or modeling wax
• Radiographs are made for verification of approximation of coping and implant
• Suitable stock or custom tray is selected
• Regular precaution of tray selection and adhesive application is followed
• Light-body viscosity material is injected around the implant and the tray is loaded with putty viscosity impression material and seated intraorally
• The set impression material is removed from the mouth and the components are approximated extra orally
• The evaluated impression is disinfected and a type IV gypsum cast is made.

Advantages

• Simple and easy to follow
• May not require a custom tray
• Is ideal in case of restricted mouth-opening
• Employed for simple implant prosthesis as single crowns and small span FPD with parallel implant abutments.

Disadvantages
Reorientation of copings can incorporate errors that can lead to inaccuracy and poor fit of the prosthesis.

Virtual Impression System

• This is a CAD-CAM system
• Eliminates the inaccuracies possible in direct and indirect transfer impressions.

Procedure

• An optical scanner reads and translates the codes embedded in the healing abutments
• This data is transferred to the CAD software, the implant abutments are virtually designed and the CAM milling apparatus produces the final titanium abutments
• The completed abutments are returned to the laboratory for the completion of the definitive prosthesis using conventional technique or by CAD–CAM procedure.

Advantages

• Offer options of both stock and custom abutments without their associated disadvantages
• Are custom-made for each patient and hence possess the optimal peri-implant soft tissue support
• Require no manipulation after they are machined resulting in a more precise fit.

Modification in techniques by type of impression materials used

• The most ideal materials for the implant prosthetic use (in the order of preference) are additional silicone, polyether, and condensation silicone (Tables 2 and 3)
• The putty and light-body materials are used in combination
• Ideally, auto-mixing devices are preferred for better material properties.

Impression trays

• Both stock and custom trays can be used
• The conventional method of evaluating the extension of the tray is followed
• In order to accommodate the transfer copings the tray materials must be easier to modify
• Tray material needs to be rigid.

Tray Adhesive

Composition – Silicone adhesive (styrene-acrylonitrile) dissolved in solvents, such as ketone, ethyl acetate, or chloroform and colorant.

Significance

• The adhesive ensures that the impression remains firmly attached to the tray upon removal from the mouth
• Inadvertent tray separation while removing the impression from the mouth can lead to distortion errors.

Procedure

• A thin layer of tray adhesive is applied onto the inner surface of the tray and allowed to air dry for at least five minutes, but no more than 30 minutes
• Mix and place the desired tray viscosity impression material directly into the coated impression tray in accordance with the manufacturer’s instructions.

Gingival Retraction

Before impression-making gingival retraction needs to be done.

Procedure

• Fix the desired abutment to the implant
• Fit the appropriate retractor impression cap over the abutments and on the exposed portion of the dental implant
• Move and verify the retractor cap to verify the snap-fit without impinging adjacent teeth
• Inject the light-body material into the impression cap and place heavy-body material, in the tray, over the impression caps
• Remove the impression from the patient and inspect for accuracy. Secure the impression cap so that it has not been dislodged or so that its position has not been distorted
• Snap the shoulder analog on the white nylon gingival retractor impression cap in the final impression. Ensure that no change of portion or distortion has occurred when snapping shoulder analog in the cap
• Pour the impression into a type 4 die stone. Place the reinforcement pin in the assembly as the impression cap/shoulder analog is filled with stone.

Disinfection of Impression

On intra-oral removal of impression immediately rinse with tap water, dry with an air syringe, and disinfect with suitable disinfectants.

Repeat impression

Impression must be repeated if:

• Visible streaks of base or catalyst material are evident
• The tray exposure in vital areas. This can be due to inaccurate seating of the tray
• Occurrence of voids, folds, or creases in critical areas
• Separation of the impression material from the impression tray.

Cast making

• There should be no impression material present between the critical implant components, impression coping, and abutment
• The corresponding abutment or analogues are carefully threaded onto the impression
• The soft tissue mask is injected around the implant region to simulate soft tissue. The impression is trimmed, boxed, and poured with type IV gypsum
• Once the stone has been set completely, the transfer copings or corresponding analogs are removed.