PROSTHETIC DENTISTRY

FOR THE FIFTH YEAR

DENTAL COLLEGE THI-QAR
UNIVERSITY

Dr. Azhar Al Awadi

LECTURE TWO
 University of Thi Qar College of Dentistry 5th Year Prosthodontics Lec. 2 Dr. Azhar Imran Alawadi

Retention of Complete denture

Outcome of complete denture treatment depends largely, on the degree of retention and
stability of the dentures. Good retention helps the denture to remain in place when the jaws
are apart, as in laughing and speaking. Good stability prevents the dentures from skidding
when the jaws are brought together, as in chewing or swallowing. Successful integration
of the prosthesis with the patient’s oral functions depends largely on denture retention.
Integration means that the patient feels that the denture as part of his body.
A retentive denture contributes severely to patient acceptance of the finish denture.
Definitions:
Retention of the denture can be defined as:
- The resistance of a denture to dislodgment.
- The quality of a denture that holds it to the tissue
foundation.
- The resistance to the movement of a denture away from
its tissue foundation especially in a vertical direction.
- The resistance to removal in a direction opposite to that
of insertion.
- The attachment of the denture to the underling soft tissue
and it is the mean by which the denture holds in position in mouth at rest.
It is checked by firmly seating the denture in the mouth and trying to displace it with force at
right angle to its occlusal surface, if the denture resist displacement it is said that it has
retention.

There are several forces that try to displace the denture away from its foundation; retention
is the ability to resist all these forces. Dentures stay in place if the retentive forces acting on
the dentures go over the displacing forces.
Retentive forces offer resistance to vertical movement of a denture away from the underlying
mucosa and act through the three surfaces of a denture. These surfaces may be defined as
follows:

(1) Occlusal surface: that surface of a denture

which makes contact or near contact with the
corresponding surface of the opposing denture or
dentition.
(2) Polished surface: that portion of the surface of
a denture which extends in an occlusal direction
from the border of the denture and which includes
the palatal surface. It is that part of the denture base
which is usually polished, includes the buccal and
lingual surfaces of the teeth, and is in contact with
the lips, cheeks and tongue.
(3) Impression surface: that portion of the surface
of a denture that had its shape determined by the
impression. It includes the borders of the denture
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 University of Thi Qar College of Dentistry 5th Year Prosthodontics Lec. 2 Dr. Azhar Imran Alawadi

and extends to the polished surface.

The retentive forces that act upon each of these surfaces are of two main types, muscular
forces and physical forces.
.
Muscular forces. These forces are exerted by the muscles of the lips, cheeks and tongue
upon the polished surface of the denture and by the muscles of mastication indirectly through
the occlusal surface.
The successful muscular control of dentures depends on two factors:
• The design of the dentures.
• The ability of the patient to acquire the necessary skill.

The design of the dentures.

The oral and facial musculature supply
supplementary retentive forces, provided:
(1) The denture bases must be properly extended to
cover the maximum area possible, without
interfering in the health and function of the
structures that surround the denture.
(2) The polished surfaces of the dentures are
properly shaped.
The buccal flanges of the maxillary denture slope up and out from the occlusal surfaces of
the teeth and the buccal flanges of the mandibular denture slope down and out from the
occlusal plane, the contraction of the buccinators will tend to seat both dentures on their
basal seats.
The lingual surfaces of the lingual flanges should slope toward the center of the mouth so the
tongue can fit against them and perfect the border seal on the lingual side of the denture.
For Orbicularis oris muscle if we did not give the lower labial flange the correct thickness
and shape (concave) to receive this muscle, it will try to dislodge the denture.

(3) Occlusal plane is the average plane

established by the incisal and occlusal
surfaces of the teeth; it is not a plane but
represents the planar mean of the curvature
of the surfaces. The occlusal plane must be
at the correct level; externally should it be
with
The relaxed lower lip level or with
commissures of lips and internally with the
lateral border of the tongue and slightly
below the superior portion of the tongue.
The position of occlusal plane in denture
wearers should be as close as possible to the plane, which was previously occupied by the
natural teeth. Such position of the occlusal plane provides normal function of the tongue and
cheek muscles, thus enhancing the denture stability
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 University of Thi Qar College of Dentistry 5th Year Prosthodontics Lec. 2 Dr. Azhar Imran Alawadi

(4) The arch form of the teeth must be in the "neutral zone” between the tongue
internally and the cheeks and lips externally.
Conversely an incorrectly shaped denture results in the muscular force dislodging that
denture. In short, the muscles can either help or hinder denture stability and retention.
Patient’s skill

The patient’s ability to acquire the necessary skills to control new dentures is related to
biological age. In general, the older the patient, the longer the learning period. In the extreme
case, the elderly or senile patient may not be able to acquire this skill at all and so new
dentures may fail even though they are technically satisfactory.
It is for this reason that replacement dentures for an older patient should be constructed in
such a way that the patient’s skill in controlling the previous denture shapes can be
transferred directly to the replacements. This is achieved by copying the old dentures as
closely as possible. When dentures are first fitted, muscular control takes some time to
develop and is therefore likely to be inefficient. Thus, it is during this initial learning period
that the physical forces of retention are particularly important.
Physical forces
(1) Interfacial force is the resistance to separation of two parallel surfaces that is imparted
by a film of liquid between them. These forces act to keep the denture inside the patient’s
mouth because of thin film of saliva between the denture and mucosa. Interfacial
forces depend on:
Interfacial surface tension and Viscous tension.
Interfacial surface tension is the tension or resistance to separation possessed by the film of
liquid between two well adapted surfaces. This acts with the air-liquid interface acting
between two surfaces where a thin film of liquid holds the surfaces on the either sides. Thin
film of saliva resists the displacing forces, and this aids in retention.
It is dependent on the ability of the fluid to “wet” the rigid surrounding material. For
retention to happen effectively, there needs to be a thin film of saliva, and as there is excess
saliva in the borders of a mandibular denture, there is minimal interfacial surface tension
seen.

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 University of Thi Qar College of Dentistry 5th Year Prosthodontics Lec. 2 Dr. Azhar Imran Alawadi

Wettability
Wetting is the ability of a liquid to maintain
contact with a solid surface, resulting from
intermolecular interactions when the two
are brought together.
If the surrounding material has low surface
tension, as oral mucosa does, fluid will
maximize its contact with the material, so
wetting it readily and spreading out in a
thin film. If the material has high surface
tension, fluid will minimize its contact with
the material, with the result that it will form
beads on the material’s surface. Most denture base materials have higher surface tension
than oral mucosa, but once coated by salivary pellicle they display low surface tension that
promotes maximizing the surface area between liquid and base. The thin fluid film between
denture base and the mucosa of the basal seat therefore furnishes a retentive force by virtue
of the tendency of the fluid to maximize its contact with both surfaces.
Capillary action, or capillarity, is the ability of a liquid to flow against gravity where liquid
spontaneously rises in a narrow space such as a thin tube, or in porous materials such as
paper or in some non-porous materials such as liquefied carbon fiber.
Is the penetration of liquids into narrow crevices, is what causes a liquid to rise in a capillary
tube, because in this physical setting the liquid will maximize its contact with the walls of
the capillary tube. When the adaptation of the denture base to the mucosa on which it rests is
sufficiently close, the space filled with a thin film of saliva acts like a capillary tube in that
the liquid seeks to increase its contact with both the denture and the mucosal surface in this
way, capillarity will help to retain the denture.
Surface tension at the periphery of the saliva film will produce a slight negative pressure
beneath the denture
Interfacial viscous tension refers to the force holding two parallel plates together that is due
to the viscosity of the interposed liquid. The viscous force increases proportionally to
increases in the viscosity of the interposed fluid, and decreases as the distance between the
plates (i.e., the thickness of the interposed medium) increases. The interfacial viscous tension
depends on saliva viscosity.
Thick, high-mucin saliva is more viscous than thin, watery saliva—yet thick secretions
usually do not result in increased retention for the watery, serous saliva can be interposed in
a thinner film than the more cohesive mucin secretions.
(2) Adhesion
Adhesion is the physical attraction of unlike molecules for each other. Adhesion of saliva to
the mucous membrane and the denture base is achieved through ionic forces between
charged salivary glycoproteins and surface epithelium or acrylic resin. By promoting the
contact of saliva to both oral tissue and denture base, adhesion works to enhance further the
retentive force of interfacial surface tension. In patients with xerostomia (sparse or absent
saliva), the denture base materials seem to stick to the dry
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mucous membrane of the basal seat and other oral surfaces. Such adhesion is not very
effective for retaining dentures, and predisposes to mucosal abrasions and ulcerations due to
the lack of salivary lubrication. So saliva substitute can be helpful in this situation.
The most adhesive saliva is thin serous but contains some mucous components.
Thick and ropy saliva is very adhesive but tends to build up so that it is too thick in palatal
area and interferes with oral adaptation, in this situation patient should rinse out ropy saliva
every two or three hours.
(3) Cohesion
Cohesion is the force of attraction between like molecules, which maintains the integrity of
the saliva film. It occurs within the layer of fluid (usually saliva).
That is present between the denture base and the mucosa, and works to maintain the integrity
of the interposed fluid. Normal saliva is not very cohesive, so that most of the retentive force
of the denture-mucosa interface comes from adhesive and interfacial factors unless the
interposed saliva is modified (as it can be with the use of denture adhesive.
In general, the retention force directly proportional to the surface area of the surfaces and
inversely proportional to the distance or space between the surfaces.
(4) Atmospheric pressure Atmospheric pressure can act to resist dislodging forces applied
to dentures, if the dentures have an effective seal around their borders. This resistance force
has been called “suction”. When a perpendicular force is exerted on a properly extended
complete denture to dislodge it, pressure between the prosthesis and mucosa drops below the
outside pressure thus resisting displacement. Retention due to atmospheric pressure is
directly proportionate to the area covered by the denture base. For atmospheric pressure to be
effective, the denture must have a perfect seal around its entire border. Proper border
molding with physiological, selective pressure techniques is essential for taking advantage of
this retentive mechanism.

Obtaining optimum physical retention

The aspects of complete dentures that influence the amount of physical retention obtained
are:
• border seal
• area of impression surface.
• accuracy of fit.
Border seal
For optimum retention, the denture border should be shaped so that the channel between it
and the sulcus tissues is as small as possible.
It is not possible to maintain a close approximation between the border of a denture and the
mucosal reflection in the sulcus at all times because the depth of the sulcus varies during
function. The denture has to be constructed so that the border conforms to the shallowest
point that the sulcus reflection reaches during normal function. This means that for some of
the time when the patient is at rest the denture will be slightly underextended. If the denture
were extended further in an attempt to produce a more consistent seal in this area,
displacement might occur when the sulcus tissues moved during function. The problem of
achieving a constant border seal is overcome by extending the flanges of the denture laterally
so that they contact and slightly displace the buccal and labial mucosa to produce a facial
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 University of Thi Qar College of Dentistry 5th Year Prosthodontics Lec. 2 Dr. Azhar Imran Alawadi

seal. Along the posterior border of the upper denture as it crosses the palate, another
approach to creating the smallest possible space between denture and mucosa is adopted. A
groove known as a post-dam is cut into the working cast so that the posterior border of the
finished denture has a raised lip which becomes embedded a little way into the palatal
mucosa. However, although an enhanced posterior seal is achieved with a post-dam it differs
from the facial seal against the flanges in that even a small downwards movement of the
posterior border of the denture is likely to break the seal with a resultant loss of retention. If
the post-dam has width as well as depth the basic retention of the denture will be improved.
Area of impression surface
The degree of physical retention is proportional to the area of the impression surface. It is
important therefore to ensure maximum extension of the dentures so that the optimum
retention for a particular patient may be obtained.
Accuracy of fit
The thinner the saliva film between the denture and underlying mucosa, the greater the
forces of retention; therefore, it is important that the fit of the dentures is as accurate as
possible. A poor fit will increase the thickness of the saliva film and increase the likelihood
of air bubbles occurring within the film. These bubbles will further reduce the retention of
the denture. In addition, as the pressure of the saliva film drops due to displacing forces
acting on the denture, the air bubbles will expand and may extend to the border area,
resulting in a breaking of the border seal.

Other factors effect on retention of complete denture

Bony undercuts (mechanical factor)
If bony undercuts exist, retention may be enhanced by designing a
denture that utilises these undercut areas. In order to achieve this
without traumatizing the mucosa on insertion and removal of the
denture, special care is required in planning the path of insertion.
The resiliency of the mucosa and submucosa overlying basal bone
allows for the existence of modest undercuts that can enhance
retention. Exaggerated bony undercuts or ones covered by thin
epithelium may compromise denture retention by necessitating extensive internal adjustment
of the denture, less severe undercuts of the lateral tuberosities, maxillary premolar areas,
distolingual areas, can be extremely helpful to the retention of the prosthesis. Modest
undercuts can help in retention of complete denture because it will keep the denture in its
place (mechanically) and prevent vertical displacement of the denture.
Modest undercuts are very helpful in the retention of
complete denture especially when they present in the upper
anterior region, so in this case you should have rotational
path of insertion and the undercuts will prevent vertical
dislodgment of the denture. This is the most common
undercuts in the upper ridge, also we may have undercut in
the tuberosities.
In the lower jaw we should use the available undercuts in the
retromylohyoid space which give good retention and stability
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of the lower denture and in this case insert the denture first in retromylohyoid eminence and
then put the denture forward.
Parallel wall
Prominent alveolar ridges with parallel buccal and lingual walls provide significant retention
by increasing the surface area between the denture and the mucosa thus maximising
interfacial and atmospheric pressure forces.
Gravity
When a person is in an upright posture, gravity acts as a retentive force for the mandibular
denture and a displacive force for the maxillary denture. In most cases, the weight of the
prosthesis constitutes a gravitational force that is in significant in comparison with the other
forces acting on the denture. But if a maxillary denture is fabricated wholly or partially of a
material that increases its weight considerably (e.g., a metal base or precious metal posterior
occlusal surfaces), the weight of the prosthesis may work to unseat it if the other retentive
forces are themselves suboptimal. Increasing the weight of a mandibular denture (through
the addition of a metallic base, insert, or occlusal surfaces) may seem theoretically capable of
taking advantage of gravity.
Surgical factors: Usually we use these factors to increase the retention of the dentures
through various procedures, like vestibuloplasty, ridge augmentation, frenectomy & dental
implants.
Psychological factors:
The role of psychological factor on denture retention depends on several considerations
related to the patient himself which include:
1. Intelligence
2. Expectations
3. Apprehension
4. Gagging reflex
5. Previous denture experience.

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