Prosthodontics

1. Principles and objectives of
impression making
Principles of impression making
Concepts to be followed to get a successful impression are :
1. The oral tissue must be healthy
2. The tray and impression
material should be made of
dimensionally stable material
3. A guiding mechanism should be
provided for correct positioning
of the tray within the mouth
4. Sufficient space should be
provided within the impression
tray for the selected impression
material
5. The borders must be in harmony with the anatomical and
physiological limitations of the oral structure
6. Physiological type of border molding should be performed
7. Selective pressure should be applied on the basal seat during
impression making
8. Impression must be removed from the mouth without
damaging the mucosa
9. The external shape of the impression should be similar to the
external form of complete denture

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10. Impression should include all of the basal seat within the
limits of health and function of the supporting and limiting
tissue
Objectives of impression making in complete
denture
An impression should be made with purpose of obtaining the following
characteristics in the denture to be fabricated.
 Retention
 Stability
 Support
 Aesthetics
 Preservation of remaining structures
Retention
It is defined as, “That quality inherent in the prosthesis which resists
the force of gravity, adhesiveness of foods, and the forces associated
with the opening of the jaws.” _ GPT
Retention is the ability of the denture to withstand displacement
against its path of insertion.

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Factors that affect retention-
o Anatomical factors
o Physiological factors
o Physical factors
o Mechanical factors
o Muscular factors
Anatomical factors
Size of the denture bearing area:
Retention increases with increase in size of the denture bearing area.
The size of the maxillary denture bearing area is about 24 cm^2 and that
of mandibular is about 14 cm^2. Hence the maxillary denture have more
retention than mandibular denture.

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Quality of the denture bearing area:
The displaceability of the tissue influence the retention of the denture. Tissue
displaced during impression making will lead to tissue rebound during denture
use, leading to loss of retention.
Physiological factors
Saliva : The viscosity of saliva determines
retention. Thick and ropy saliva gets
accumulated between the tissue surface of the
denture and the palate leading to loss of
retention. Thin and watery saliva can also lead to
loss of retention. Thin and watery saliva can also
lead to compromised retention.
Cases with ptyalism can lead to gagging and in
patients with xerostomia, denture can produce
soreness and irrigation.
Physical factors
o Adhesion
o Cohesion
o Interfacial surface tension
o Capillary attraction
o Atmospheric pressure
24cm 14cm

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Adhesion
Adhesion is defined as, “The physical attraction of unlike molecules to one
another.”
The role of saliva is very important for adhesion. Saliva wets the tissue
surface of the denture and the mucosa. A thin film of saliva is formed
between the denture and the tissue surface. This thin film help to hold the
denture to the mucosa. The amount of adhesion present is proportional to
the denture base area.
Cohesion
Cohesion is defined as l, “The physical attraction of like molecules to
each other.”
The cohesive forces act within the thin film of saliva. The effectiveness of
these forces increases with increase in denture bearing area. Watery serous
saliva can form a thinner film and is more cohesive than thick mucus saliva.
Interfacial surface tension
Interfacial surface tension is defined as, “The tension or resistance to
separation possessed by the film of liquid between two well adapted
surfaces.”

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It play a major role in the retention of a maxillary denture. It is totally
dependent on the presence of air at the margin of liquid and solid contact.
In mandibular denture there is excess saliva along the borders of a
mandibular denture there is minimal interfacial surface tension.
Capillary attraction
Capillary attraction is defined as, “that quality of surface tension that
is elevation or depression of the surface of a liquid that is in contact with a
solid.”
Atmospheric pressure
Retention produced by atmospheric pressure is directly proportional to
the denture base area

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Mechanical factors
o Undercuts
o Retentive springs
o Magnetic forces
o Denture adhesives
o Suction chambers and suction disks
Muscular factors
There is balance between force acting from buccal
musculature and tongue. This balance is obtained in
neutral zone.

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Stability
Stability is defined as, “The quality of a denture to be firm, steady, or
constant, to resist displacement by functional stresses and not to be
subject to change of position when forces are applied.” _ GTP
THE VARIOUS FACTOR AFFECTING STABILITY ARE:
o VERTICAL HEIGHT OF RESIDUAL RIDGE
o QUALITY OF SOFT TISSUE COVERING THE RIDGE
o QUALITY OF THE IMPRESSION
o OCCLUSAL RIMS
o ARRANGEMENT OF TEETH
o COUNTER OF THE POLISHED SURFACE
VERTICAL HEIGHT OF RESIDUAL RIDGE
The residual is should have sufficient vertical height to obtain good stability.
Highly resorbed ridges often is stability.

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QUALITY OF SOFT TISSUE COVERING THE RIDGE
The ridge provide a firm soft tissue base with adequate submucosa to offer
goods ability. Flabby tissues with excessive submucosa offer poor stability.
QUALITY OF THE IMPRESSION
 An impression should be as accurate as possible.
 The impression surface should be smooth and duplicate all the details
accurately.
 It should be devoid of voids and any rough surfaces.
 The impression should not warp on removal.
 The impression should be dimensionally stable and the cast should be
poured as soon as possible.
OCCLUSAL RIMS
The occlusion plane should be oriented parallel to the ridge. If the occlusal
plane is inclined, then the sliding forces may act on the denture, reduce its
stability. The occlusal plane should divide the interarch space equally.

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ARRANGEMENT OF TEETH
The teeth in the denture should be arranged in neutral zone. The neutral
zone is defined as, “The potential space between lip and cheeks on one
side and the tongue on other. Natural or artificial teeth in this zone are
subject to equal and opposite forces from this surrounding
musculature.”_GTP
COUNTER OF THE POLISHED SURFACE
The polished surface of the denture should be harmonious with the oral
structure. They should not interfere with the action of the musculature.

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