The document discusses the principles and objectives of dental impressions. It defines an impression as a negative replica of the surfaces of an object. The main objectives of taking an impression are retention, stability, support, aesthetics, and preservation of remaining structures. It describes different types of impressions like preliminary and functional impressions. Theories of definitive pressure, minimal pressure, selective pressure impressions are explained. Factors affecting retention like anatomical, physiological and physical factors are also summarized.

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Principles and objectives of impressions in
CD
Dr. M. Pavan Preetham
Reader
Dept. of Prosthodontics
SB Patil Institute for Dental Sciences & Research
Bidar

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IMPRESSION
 A negative replica or copy in reverse of the surface of an object .
– gpt 8
 • An impression can also be defined as an imprint of the teeth
and adjacent structures for use in dentistry. - gpt 4

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PRELIMINARY IMPRESSION
 A preliminary impression is an impression made for the purpose
of diagnosis or for the construction of a tray

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z History

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History • 1928 Pierre Fauchard made
dentures by measuring the mouth
with compasses and cutting bone
into an approximate shape.
• 1896 Green brothers introduced
mucocompressive theory.
• 1900-1929 Concepts like Rebase
impressions, border molding and
techniques for flabby tissues were
introduced.

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• 1930-1940 This era recognized the anatomy of the
denture bearing areas, and muscle physiology as
related to impression procedures. This is evident by
descriptions of border molding of dentures.
Several new impression materials were introduced :
reversible hydrocolloids, zinc oxide eugenol and zinc
oxide and oil of cloves
1950-1964 more emphasis on biologic factor of
impression making was given.
• 1951 Carl O Boucher introduced selective pressure
theory.

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z
Theories

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 Most of the impressions of the twentieth century are made with a
view to functional movement of the musculature.
 • Theoretically, the impressions are so formed that the complete
denture will require no change in contour.

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• Impression are
made with
– Definitive-pressure
impressions
– Minimal-pressure
impressions
– Selective-pressure
impressions
– Functional
impression

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Definitive-pressure impressions
 Because denture retention is tested most severely during
mastication, many dentist formerly considered it essential for the
tissue to remain in contact with the denture during chewing.
 However, dentures did not fit well at rest, because of » Tissue so
distorted, tend to rebound. » Tissues so abused will long
maintain the shape that they assumed on the day of impression.

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Minimal pressure impressions
 Richardson in 1896 had advocated plaster because it least
disturbed the tissues
 Addison (1944) descirbed the so called “muco- static
impression”
 Main point of the mucostatic principle concerned Pascal’s law,
which states that pressure on a confined liquid, will be
transmitted throughout the liquid in all directions.

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Selective pressure
 This principle is based on the belief that the mucosa over the
ridge is best able to withstand pressure, whereas that covering
the midline is thin and contains very little submucosal tissues
 This technique combines the principles of both pressure and non
pressure procedures

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 The technique utilizes a preliminary compound impression that
is generously relieved over the midline and incisive papilla
areas.
 • The final impression is taken, which acts as a wash and also
records the relieved areas with minimal pressure while the ridge
areas are undergoing considerable pressure

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 Thus, the midline and papilla sections of the denture will not
make contact with the mucosa when the denture is not in
function, they will not bear heavily when the patient is chewing

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Functional impression
 • This technique uses a complete denture that is delivered,
relieved on its internal aspects, and filled with a slow setting
impression material(tissue conditioning material).
 • The patient wears the prosthesis for several days, allowing the
tissues to be recorded “in function”

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 The completed impression then is converted to and reline
material via laboratory processing.
 • This technique seeks to create a denture base that models the
functionally loaded tissues

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BASIC
REQUIREMENTS
FOR IMPRESSION
MAKING
Basic
anatomy

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z
Objectives of
Impression
Making

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RETENTION STABILITY SUPPORT
ESTHETICS PRESERVATION
OF REMAINING
STRUCTURES

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Retention
 Retention is defined as the ability of denture to resist the
displacement against vertical forces
 Retention resists the adhesiveness of food, the force of gravity,
& the forces associated with the opening of jaws

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Factors affecting
Retention
Anatomical
factors
Physiological
factors
Physical
factors
Mechanical
factors
Muscular
factors

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Physical factors
Adhesion Cohesion
Interfacial surface
tension
Capillarity and
capillary attraction
Atmospheric
pressure and
peripheral seal

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 Adhesion is achieved by ionic forces between charged salivary
glycoproteins & surface epithelium or acrylic resin.
 Quality of adhesion depends on :-
 Close adaption
 Size of denture
 Type of denture bearing saliva area

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 Cohesion occurs within the layer of fluid (usually saliva ) that is
present between the denture base & the mucosa.

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 Interfacial surface tension is the resistance to separation of two
parallel surfaces that is imparted by a film of liquid between
them
 It is dependent on the ability of the fluid to wet the rigid
surrounding material .

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Oral & facial musculature supplement retentive forces
Atmospheric pressure Acts to resist dislodging forces
applied to the denture ,if the denture have an effective seal
around their borders.
Retention due to atmospheric pressure is directly
proportional to the area covered by the denture base.
In function, atmospheric pressure is superior to interfacial
surface tension as a retentive force, for forces horizontal as
well as parallel to the mean of mucosal plane are resisted.
Interfacial surface tension will resist only forces
perpendicular to the axis of surface tension forces

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STABILITY
It is the ability of the denture
to withstand horizontal forces.
• Vertical height of the residual ridge.
• Quality of soft tissue covering the
ridge.
• Occlusal plane
• Quality of the impression.
• Teeth arrangement.
• Contour of the polished surfaces.
Factors Affecting Stability

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SUPPORT
 It is the resistance to vertical forces of mastication & to occlusal
or other forces applied in a direction toward the basal seat .
 When the natural teeth are missing ,the alveolar ridge & their
covering of mucosal tissue become the supporting elements.

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ESTHETICS
 The thickness of the denture flanges is one of the important
factors that govern aesthetics.
 Thicker denture flanges are preferred in long-term edentulous
patients to give required labial fullness.
 Impression should perfectly reproduce the width and height of
the entire sulcus for the proper fabrication of the flanges.

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PRESERVATION OF REMAINING
STRUCTURES
De Van (1952) stated that, “the preservation of that which
remains is of utmost importance and not the
meticulous replacement of that which has been
lost”.
Impressions should record the
details of the basal seat and
peripheral structures in an
appropriate form to prevent injury
to the oral tissues

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