Impression Techniques

By Muhammad Rafay Imran&

Yusra Mahar

Impression : A negative likeness or copy in reverse of the surface of an object ; an imprint of teeth and adjacent structure for use in dentistry.

Impression trays : A receptacle in to which suitable impression material is placed to make negative replica.

Impression trays can be classified broadly into stock trays and custom trays.

Stock Trays : Stock trays for partially edentulous patients may be perforated to retain the impression material or they maybe constructed with a rim lock for this purpose.

Another type of stock stray designed for the reversible type of hydrocolloids is water cooled trays. It contains tubes through which water can be circulated for the purpose of cooling the tray.

Disadvantages of Stock Trays

The peripheral borders may not be accurately recorded

Considerably more bulkier than a custom tray.

Custom Impression Trays

Peripheral borders can be precisely recorded in the impression Thickness of impression material can be controlled Custom trays are sometime needed for mouth that are abnormally or

unusual configuration

Impression Techniques

Physiologic impression technique McLean and Hindels method The Fluid wax method

Selected Pressure technique

Physiologic impression technique

This technique records the ridge position by placing an occlusal load on the impression tray as the impression is being made

These techniques produced a generalized displacement of the mucosa to a greater or lesser degree

This displacement was intended to record the tissue in the configuration it would assume when occlusal loading was applied to a partial denture in function.

McLean’s Impression Technique

A Custom impression tray is constructed over a preliminary cast Functional impression of distal extension ridge is made. Patient

applies some biting force with occlusal rims Then an alginate impression is made with the 1st impression held in

its functional position with finger pressure.

Hindels modification of McLean’s technique Main difference of this McLean’s technique is that impression of

edentulous ridge is not made under pressure but is an anatomic impression made at rest with ZOE paste.

As the hydrocolloid impression was being made finger pressure was through holes In the tray to the anatomic impression.

The Fluid Wax Method

Zinc Oxide eugenol and impression plaster are suitable impression materials for this technique as fluid wax adheres well to them

The anterior and posterior vibrating lines are marked as described in the conventional technique. These lines are marked in the patient’s mouth immediately after making the wash impression

Selected pressure impression technique The objective of this technique are :

The equalization of support between the abutment teeth and the soft tissue

To direct more force to those portions of the ridge which is able to absorb the stress without reverse response

Protect the areas of the ridge which is least able to absorb force.

Areas where relief is provided will be least displaced as the impression is recorded. In those areas of the tray where relief is not provided greater displacement of the underlying mucosa will occur.

Mandibular Arch posterior region Crest of Arch Buccal Shelf Lingual slope.

The end result of this selected pressure impression technique si that the denture base made impression will be closely adapted to and in firm contact with the tissue covering the buccal shelf area of the edentulous ridge.

The ridge crest on the other hand is lightly adapted to the tissue and effects of occlusal loading will be less in this area.