Complete Denture Impressions

Study Notes

Impressions capture a negative image of the oral tissues.
A cast is a positive replica of the oral tissues created from the impression.
Two impressions are made for complete dentures: primary (preliminary) and secondary (final).

The primary impression is made for the first time for a patient and does not capture fine details.
Stock trays are used for primary impressions and are made by manufacturers.

The secondary impression is made for the same patient but records all fine details of the oral structures.
Special trays are made specifically for one patient and are discarded after the impression is taken.

Preservation of remaining natural structures: Emphasis on protecting existing oral tissues.
Retention: Ensuring the denture stays secure in the mouth.
Esthetics: Achieving an appealing appearance.
Stability: Maintaining the denture's position during function.
Support: Distributing forces evenly to prevent tissue damage.

Pressure on the impression translates to pressure from the denture base on the tissues.
Excessive pressure can damage tissues and cause bone resorption.
Requirements for an acceptable impression:
- Capture details adequately to prevent tissue injury.
- Relief for non-stress bearing areas to prevent damage.
- Avoid overextensions of the impression.

Stability is crucial to protect the supporting structures of the denture (soft tissues and bone).

Wider denture coverage allows for a more even distribution of forces, reducing pressure per unit area.

Operator's Position: The dentist's position should be comfortable and provide clear visibility of the oral cavity.
Patient’s Position: The patient should be comfortable and in a position that allows for proper access to the mouth.

Impression Tray Design: The tray should be appropriately sized and shaped for the patient's anatomy.
Sizing of Impression Trays: The tray should fit comfortably and provide adequate space for the impression material.
Width of Impression Trays: The tray should be wide enough to capture the full width of the arch.
Anteroposterior Extension of Impression Trays: The tray should extend far enough to capture the entire arch.
Length of Impression Trays: The tray should be long enough to capture the area from the anterior to the posterior.
Modifications of Impression Trays: Modifications (bending, cutting/trimming, or adding material) can be necessary to ensure a proper fit.

Hand-manipulated: The dentist manually guides the impression material around the borders of the denture.
Functional Movements: The patient performs specific movements (sucking, grinning, licking, swallowing) to help shape the impression material.

Special trays are used for the secondary impression and are custom-made for each patient.
The tray outline is marked 2-3 mm short of the denture outline.
Adjustments are made to the tray border as needed.

Minimal pressure is applied, capturing the anatomical form of the soft tissues.
Advantages: Less tissue distortion. Preferred for flabby or thin ridges.
Disadvantages: Mucosa is not static over time. Neglects the principle of distributing forces.
Requirements: Fluid impression material. Tray with perforations and a spacer to prevent pressure on the mucosa.

Definitive pressure is applied using biting force, capturing the functional form of the soft tissues.
Advantages: Allows for proper force distribution.
Disadvantages: Denture may not fit well at rest due to tissue rebound. May interfere with blood supply and accelerate resorption.
Requirements: Impression material that is not easy to flow.