Basic principlesin imp making

BASIC PRINCIPLES IN
IMPRESSION MAKING
Devan MM. Basic principles in impression making. Journal
of Prosthetic Dentistry. 2005 Jun 1;93(6):503-8.
ASWATI SOMAN
PG RESIDENT
PCDS

“Ideal impression must be in the mind of the dentist
before it is in his hand. He must literally make the
impression rather than take it”
- M.M. Devan
The impression problem would not be a problem if we were
taking impressions of casts. The problem is due to the fact that
the mouth is lined with displaceable tissue; displaceable tissue
that varies in degree of displaceability, according to:
(1) its thickness,
(2) its rigidity,
(3) the point, magnitude, and direction of the forces applied to it.

DEFINITION OF IMPRESSIONS
 An impression is a registration of mouth tissues made with
an impression material.
 It is a record, a facsimile of mouth tissues taken at
unstrained rest position or in various positions of
displacement.
 The impression form is the shape of the saddle which
will harness mouth tissues for the purpose of counter-
acting masticatory forces delivered at different levels of
jaw separation.

 The mucosal displacement recorded in an impression
will vary in degree and direction.
 For purposes of our present analysis let us somewhat
arbitrarily divide the impression area into two parts:
(1) the vault-ridge areas
(2) the flange-heel areas.

THE THEORY OF REST
IMPRESSIONS
 The facts cited in support of the theory of rest
impressions are irrefutable. However, the
interpretation of these facts and their unqualified
application in practice may be questioned.
 The facts are these:
The laws of hydrostatics tell us that water is
incompressible, i.e., you cannot take a given amount of
water and through pressure reduce its volume.
The mucoperiosteum is a semi-solid with over 80 per
cent of water in its composition, and if the known facts of
hydrostatics are utilized, the mucosa is incompressible
by any force that the muscles of mastication are able to
deliver.

While the mucoperiosteum
cannot be compressed, it may,
however, be displaced in the
absence of confining walls.
Any substance, no matter how
fluid, when enclosed in a rigid
container, takes on the same
rigidity as that container. Thus,
soft tissue, provided it is
completely contained, would
under these circumstances
become as rigid as bone.
Tissue is elastic and will not
remain passive in a displaced
position. It will try to recover its
unstrained rest form.
[A] Denture on a flat ridge that
has insufficient buccal extension
is like a loose piston which can
be easily withdrawn
[B] correct buccal extension give
firm retention like a closely fitting
piston (Tyson 1967)

CONTACTING DENTURE SURFACE
 This fact accounts for the lack of mucosal irritation in the presence
of the rough contacting surface found with mucostatic dentures.
 It is fallacious to argue, as some do, that minute tissue
registration in an impression will result in multiple miniature,
wedges that irritate the mucosa unbeknown to the patient
because it is below the threshold of pain response.

 A wedge cannot operate except in space, for a
wedge is an incline plane moving under an object.
 When the tissues are completely contacted, wedging action cannot
take place. There is no room available for movement.
 It is also wise to bear in mind that a relatively rough surface
increases the surface area in contact with the mucosa; and this
increases the forces of interfacial surface tension, which is directly
proportional to the area involved.

THEORY ANDPRACTICE
 In theory, rest impressions are plausible, but in practice, they may not
suffice for
retention and function.
 However, they are ideal during periods of rest which embrace the
greater part of the day and night.
 But under the hazards of a resistive bolus (where one-sided
horizontal forces prevail) such impressions may be dislodged or
rather dentures made from such impressions may be dislodged.
 Yet, this must be said in favor of rest, impressions: Given a satisfactory
end result, dentures made from rest impressions will remain
serviceable for a longer period of time than dentures built from
impressions taken during displacement.

 For displacement will subject osseous tissue to a continuous
stress. Displaced mucosa has a steady urge to return to rest
position. Then, again, displacement results in tensile and
shear forces to osseous tissue.
 Just as intermittent stress, rather than continuous stress, is
more physiologic to bone, likewise, bone is more counteractive
to compressive loads than to tensile and shear stresses.

RELATION OF DENTURE TO
MUCOSA
 A present-day denture does not merely rest upon its mu-cosal
base; it is attached to it, as veritably as the mucosa is attached
to the underlying bone.
 The difference is that in one case the attachment is fixed and in the
other it is not fixed.
 When an object is attached to a base, the base may be
employed to support or suspend the object. The dead weight of
the lower denture is supported by the lower base.

RELATION OF DENTURE TO
MUCOSA
 The dead weight of the upper is held in suspension by the upper
base. When the force is directed toward the base, the base is used
to support the object.
 When the force is directed away from the base, the base is
used to suspend the object. A base, to support an object,
must be hard and unyielding, possessing compressive
strength. A base, from which to suspend an object must be
tough, possessing tensile strength.

STRATEGY OF USING SOFT
TISSUE
 Our strategy should be to use mouth tissues to support the denture rather
than to hold the denture by suspension.
 If the plan is one of suspension, then the soft border tissues are useful. A
denture may be supported, suspended, or sustained by the mucosal base
in one of three ways:
 (1) A denture is supported when the force is basewise and perpendicular,
resulting in compressive loads.
 (2) A denture is suspended when the force is counterbase-wise, resulting in
tensile loads.
 (3) A denture is sustained when the force is basewise in one area and
simulta-neously counterbasewise in another area.
 This results in compressive, tensile, and shear stresses.

• The means are the forces of: (1) inter-facial surface tension,
(2) atmospheric pressure, (3) a combination of both.
• By means of the interposed salivary film, a denture is attached
to the mucosa as two pieces of moistened glass would be
attached, through interfacial surface tension
• By means of rarified air spaces between the denture and the
mucosa, a denture is attached to the mucosa, as a suction cup
is attached to a windshield of a car through the force of
atmospheric pressure.
MEANS OF ATTACHMENT

 Carrying a large deep air chamber in the middle of the hard
palate, a well-adapted denture is attached to the mucosa
through both the forces of surface tension and atmospheric
pressure.
 In function, atmospheric pressure is superior to inter-facial
surface tension as a retentive force, for forces horizontal as
well as parallel to the mean mucosal plane are resisted.
 Interfacial surface tension will resist only forces perpendicular
to the axes of surface tension forces.

 According to Stefan(1879)
 F interfacial surface tension,
 K-viscosity ,
 R-denture area
 H- width of space between denture base and mucosa,
 V - velocity of displacing force
F decreases with increase in H .
F increases with increase in R
To obtain maximum interfacial surface tension
 Saliva should be thin and even
 Perfect adaptation should be present between tissue and
denture base
 Denture base should cover large area

MUCOSTATIC SCHOOL
 The crux of Page’s announcement is this: Soft tissue should be
registered in an impression in unstrained rest position—any
other position will compel tissue to try to regain its rest
position—this leads to dislodgement of the denture.
 A denture may move with the mucosa without being detached
from it.
 Free movements of tissue are limited by two factors: (1) its
thickness, (2) its rigidity.
 Thickness and rigidity are in a state of constant flux
depending on fluid content and the stresses applied to
tissue.

MUCOSTATIC SCHOOL
 This would be true if mucosa had no thickness which, of
course, is inconceivable.
 As long as the mucosa has thickness, its rigidity of form is
conditioned by other factors than complete contact with the
denture base.
 It is condi-tioned by the point, magnitude, and direction of
masti-catory
forces applied to it.

FORCES OF RETENTION
 It depends primarily on the forces that produce
attachment of the denture to the mucosa.
 These forces include adhesion, cohesion, and
surface tension.

• Stability of a denture is its ability to remain
securely in place when it is subjected to horizontal
movements.
• For the denture to be stable it requires
FACTORS IN STABILITY

 Good retention
 Non interfering occlusion
Proper tooth arrangement
 Proper form and contour of the polished
surfaces
 Proper orientation of the occlusal plane
 Good control and co ordination of the patients
musculature.

COMMENTS ON NATURALAND
ARTIFICIAL DENTURES
 There is greater similarity than we commonly sup-pose
between natural and artificial dentures, Both are attached
to bone, one by means of the periodontal membrane, the
other by means of the mucoperiosteum.
 The presence of roots with the natural denture, the absence
of roots in the artificial denture is not their essential difference.

 The backbone of their difference is the difference between the
periodontal membrane and the muco periosteum
their difference in thickness and rigidity.
 The crux of the entire problem lies in the fact that, the
periodontal membrane can resist displacement much more
readily than the mucoperiosteum.

TISSUE REGISTRATION
 A registration other than at rest should be for purposes of
expediency only, for alveolar bone is best pre-served with rest
registrations.
 But, in the everyday practice of prosthetics, many cases will present
themselves where rest registrations will not suffice to effect
retention and comfort during chewing.
 It may be necessary to register in the impression a certain degree
and direction of tissue displacement, this, in anticipation of
dislodging forces during function.
 Rest registrations in the impression must be combined with flange
and tooth forms and placement that result in compressive loads.
 Otherwise the degree of mucosal displacement will be too great and
denture dislodgement imminent.

CONCLUSION
 In conclusion, let me say that the importance of the
impression phase has been overemphasized.
 Steps essential for retention have been played up while the
forces that lead to stability have not received the emphasis
that they merit.

REFERENCES
 Page H. From a manuscript of a summary of the paper
read at the 43rd annual session of the South California
Dental Society of Los Angeles. 1940 Jan.
 DeVan MM. An analysis of stress counteraction on the part of
alveolar bone, with a view to its preservation. D Cosmos
1935;77:109.
 Synge JL. Philos Trans R Soc Lond A. p.231.

Basic principlesin imp making

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