negative likeness or copy in reverse of the surface of an object.
Ø Impression
positive replica of denture foundation tissues.
Ø Cast
OBJECTIVES OF IMPRESSION TAKING (5)
1) Preservation of the alveolar ridge
2) Retention
3) Stability
4) Support
5) Esthetics
Based on THEORIES OF IMPRESSION:
- Pressure theory
- Minimal pressure theory
- Selective pressure theory
Based on the POSITION OF THE MOUTH while making the impression:
- Open mouth
- Closed mouth
Based on the METHOD OF MANIPULATION FOR BORDER MOLDING:
- Hand manipulation
- Functional movements
- Was proposed on the assumption that tissues recorded under functional pressure (mastication) provided better support and retention for the denture.
PRESSURE THEORY (MUCOCOMPRESSIVE OR DEFINITE PRESSURE)
- Heavy bodied material (Impression compound)
PRESSURE THEORY (MUCOCOMPRESSIVE OR DEFINITE PRESSURE)
Better retention and support during occlusal functions like mastication
PRESSURE THEORY (MUCOCOMPRESSIVE OR DEFINITE PRESSURE)
o Excess pressure could lead to increased alveolar bone resorption – loose denture.
o Excess pressure result in transient ischemia
o Dentures which fit during mastication tend to rebound when the tissue resume to their normal resting state.
o Pressure on sharp spiny ridges or other bony areas often resulted in pain.
PRESSURE THEORY (MUCOCOMPRESSIVE OR DEFINITE PRESSURE)
- Interfacial surface tension was the only significant way of retaining complete denture. Retention is achieved through accurate tissue adaptation.
MINIMAL PRESSURE THEORY (MUCOSTATIC OR NONPRESSURE OR PASSIVE TECHNIQUE)
- Covers firmly attached mucosa – shorter flanges.
MINIMAL PRESSURE THEORY (MUCOSTATIC OR NONPRESSURE OR PASSIVE TECHNIQUE
- Equal transmission of pressure to all part.
MINIMAL PRESSURE THEORY (MUCOSTATIC OR NONPRESSURE OR PASSIVE TECHNIQUE)
Tissue health and preservation.
MINIMAL PRESSURE THEORY (MUCOSTATIC OR NONPRESSURE OR PASSIVE TECHNIQUE)
Give disadvantages of Minimal Pressure Theory
- Disadvantages:
o Shorter flanges prevent the wider distribution of masticatory stresses
o Reduced coverage, reduced retention
o Lack of border molding reduces effective peripheral seal therefore reduced retention
o Short denture borders provoke irritation to the tongue
o Shorter flanges reduce support for the face which affects the esthetics
o Shorter flanges, less lateral stability
o Patients with poor residual ridge were difficult to treat.
- Combines the principles of both pressure and minimal pressure techniques.
SELECTIVE PRESSURE THEORY
- Zones of basal seat: (3)
a) Primary stress bearing area
b) Secondary stress bearing area
c) Relief areas
- Advocates maximum extension within the comfort and functional limits of the surrounding muscle and tissue
SELECTIVE PRESSURE THEORY
o Considers the physiologic functions of the tissues of the basal seat and therefore appears more sound and appealing.
SELECTIVE PRESSURE THEORY
o Some feel that it is impossible to record areas with varying pressure.
o Since some areas are still recorded under functional load, the denture still faces the potential danger of rebounding and loosing retention.
SELECTIVE PRESSURE THEORY
CLASSIFICATION OF IMPRESSION MATERIAL
Based on ELASTICITY:
Ø Rigid – ZOE impression paste, impression compound, impression plaster
Ø Elastic – alginate, elastomeric impression material.
CLASSIFICATION OF IMPRESSION MATERIAL
Based on its PROSTHODONTIC USE:
Ø Preliminary impression material
Ø Final impression material
Modelling Compound (3)
- Impression compound
- Tray compound
- Stick compound
à Used both for preliminary impression material as well as final impression material.
Alginate
