Retention and stability of complete dentures

Retention
and Stability of
Complete Dentures

Chapter 1
RETENTION AND STABILITY OF
COMPLETE DENTURES

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 Retention and stability of complete dentures

I- RETENTION:

Retention of a denture is its resistance to removal in a

direction opposite that of its insertion. It is the quality
inherent in a denture that resists the force of gravity, the
adhesiveness of food and the forces associated with the
opening of the jaws.

FACTORS AFFECTING RETENTION OF

DENTURES:

I. Physical Factors:

A number of physical factors combine to retain

complete dentures in position. These physical factors
include adhesion, cohesion, interfacial surface tension and
atmospheric pressure.

1. Adhesion:

Adhesion is the physical attraction of unlike

molecules to each other. It is also the molecular attraction
between surfaces of unlike bodies in contact. It acts when
saliva wets and sticks to the basal surface of the denture
and to the mucous membrane of the basal seat.

The effectiveness of adhesion depends on:

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 Retention and stability of complete dentures

- The close adaptation of the denture base to the supporting

tissues.

- Fluidity and consistency of saliva: The amount and

consistency of saliva affects denture stability and
retention. Moderate flow of serous type is quite effective,
provided that the denture base can be wetted. Adhesion of
saliva to the mucous membrane is not a problem because
saliva wets it very effectively.

- Thick ropy saliva adheres well to both denture base and,

mucosa. Since much of this type of saliva is produced by the
palatal glands under the maxillary denture. It builds up and
it is sufficient to force the denture out of their correct
position.

- The amount of retention supplied by adhesion is directly

proportional to the area covered by the denture. Maximum
extension of the denture base as determined by a
functionally trimmed impression is important for denture
retention. It is not expected that patients with small jaws
(basal seats) exhibit effective retention resulting from
adhesion as patient with large jaws.

2. Cohesion:

Cohesion is the physical attraction of like molecules

for each other. It occurs in the layer of saliva between the
denture base and the mucosa.

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 Retention and stability of complete dentures

- It is effective in direct proportion to the area covered by

the denture.

- Since saliva is a liquid, hence the layer of saliva must be

thin in order to be effective. Therefore close adaptation of
the denture base to the mucosa must be as close as possible.

3. Interfacial Surface Tension:

The interfacial surface tension is the tension or

resistance to separation possessed by the film of liquid
between two well adapted surfaces. One of its retentive
values is minimal distortion or displacement of the soft
tissues by the impression and the denture. Also, a perfect fit
of the denture base is essential.

Interfacial surface tension occurs in the thin film

of saliva between the denture base and the mucosa of the
basal seat. It is quite similar in its action to cohesion. It is
effective in direct proportion to the size of the basal
surface of the denture.

4. Atmospheric Pressure:

When an upper denture is inserted in the mouth of the

patient, air is expelled out from underneath the denture. If
the borders of the denture are properly adapted to the

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 Retention and stability of complete dentures

adjacent tissues, no air can get in. Thus the pressure

acting on the fitting surface of the denture is less than that
acting on the polished surface, the difference causes a
positive force holding the denture in place.

- The retentive force supplied by atmospheric pressure is

directly proportional to the area covered by the denture
base, for atmospheric pressure to be effective the denture
must have a perfect seal around its border.

- The retention achieved from adhesion, cohesion and

interfacial surface tension is able to resist only vertical
dislodging forces. Horizontal dislodging forces are
resisted by the retentive force supplied by atmospheric
pressure.

- Upper dentures are mainly retained by physical factors.

On the other hand, lower dentures are poorly retained by
physical factors; they are almost entirely retained by
mechanical forces.

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 Retention and stability of complete dentures

II. Intraoral Factors and Biomechanical

Considerations:
Number of biomechanical, intraoral factors,
influence the retentive quality of complete denture such
as:

1) Arch size:

The size of the maxilla and mandible determines the

amount of basal seat available for the denture. The greater
the arch size, the more the support and, the larger the
retention.

2) Disharmony in jaw sizes:

Some patients have a large maxillary arch and small

mandibular arch while others have the opposite
disharmony. In such cases, a problem with stability and
retention of the smaller arch is usually expected.

3) Arch form:

The arch may be square, ovoid or tapered and

opposing arches may not necessarily have the same form.
As square arch form indicates favorable prognosis, while
tapered arch is the least favorable.

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 Retention and stability of complete dentures

4) Ridge form:

The ideal form is a high ridge with flat crest and

parallel or nearly parallel sides (square ridge). This type
of ridge gives the maximum amount of support, retention
and stability. The V-shaped ridge provides less retention
to complete dentures, as the peripheral seal may be broken
in all areas simultaneously. The flat ridge also has poor
prognosis because the lack of vertical height affords little
resistance to the horizontal dislodging forces.

5) The vault form:

The vault form affects the retention of the maxillary

denture. A flat vault resists vertical displacement but it
provides little resistance to lateral displacement. The
narrow V-shaped vault is also unfavorable for the
retention of dentures. It is able to resist lateral
displacement however; vertical forces tend to unseat the
dentures. In most cases with a. V-shaped vault the
residual ridges are also V-shaped in cross section, thus the
problem is complicated. The solution involves the
development of perfect peripheral seal and proper
contouring of the denture polished surfaces so that the
cheeks and buccinator muscles can mechanically aid in
retention. The U-shaped palatal vault is the most
favorable for retention and stability of complete dentures.
It resists both vertical and lateral dislodging forces.

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 Retention and stability of complete dentures

6) The soft palate:

There are three classifications for the configuration of

the soft palate based on the degree of flexure the soft
palate makes with the hard palate, class I soft palate has a
gentle curvature and demonstrates little muscular
movement, hence it is the most favorable, it allows more
tissue coverage for producing the palatal seal .

Fig.2 Different curvatures of the soft palate

7) Undercuts on the basal seat:

The presence of undercut may cause problems of

denture retention and insertion.

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 Retention and stability of complete dentures

8) Interarch distance:
A small interarch distance enhances retention of
dentures, because the tongue fills the oral cavity more
completely thus, contacting the lingual and palatal
surfaces of the denture, providing an excellent seal.

9) Polished surface of the denture:

The Polished surface of the denture being shaped
concave promotes the neuromuscular control to be in favor
with denture retention.

10) Gravity:
This physical force is primarily concerned with the
mandibular prosthesis.

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 Retention and stability of complete dentures

II- STABILITY:

Stability is "The quality of prosthesis to be firm

steady or constant, in order to resist displacement by
functional horizontal or rotational stresses".

Denture stability "is the resistance of a denture to

movement on the denture foundation area".

FACTORS AFFECTING STABILITY OF

COMPLETE DENTURES:

I- Intra-oral factors:
A number of intra-oral factors influence the quality
of complete denture stability in relation to the basal seat
area these are:

1- Ridge form:
Residual ridges with flat crests and parallel or
nearly parallel sides resist lateral forces and enhance
denture stability, while flat ridges offer minimal
resistance to lateral stresses reducing denture stability.

2- Arch form:
Square arch form provides the most favorable
prognosis for denture stability; on the contrary ovoid
arch form is the least favorable condition.

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 Retention and stability of complete dentures

3- Vault form:
The shape of the vault of the palate affects the
stability of maxillary denture. The two extremes, very
flat, and very high V-shaped palatal vaults present
difficulties in denture stability. While a moderately high,
broad palate offers better stability.

4- The mucosa supporting the prosthesis:

Mucosa of even, medium thickness (2-3 mm)
composed of dense fibrous connective tissue and which
is firmly attached to the underlying bone offers the most
favorable prognosis. This nature of supporting mucosa
prevents the denture and mucosa from moving together in
relation to the underlying supporting bone and serves as a
cushion absorbing the forces created by the denture. If the
oral mucosa is thin, ulcerations and sore spots are more
likely to occur in the basal seat area. On the other hand,
thick flabby tissues, contribute to excessive vertical and
horizontal movements, reducing the stability of the
denture. In such condition, it is desirable to reduce these
tissues surgically to gain a more stable denture bearing
area.

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 Retention and stability of complete dentures

5- Interarch distance:
A small interarch distance in contrast to large
interarch distance, enhances stability as, the occlusal
surfaces of teeth are closer to the ridge, minimizing
undesirable leverage action and tongue forces.

6- The tongue:
The size, form and function of the tongue influence
denture stability. Small narrow tongue or extremely large
tongue adversely affects denture stability, while broad
thick tongue enhances denture stability.

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 Retention and stability of complete dentures

II- Mechanical factors:

Number of mechanical factors influence the quality

of complete denture stability, these are:

1- Position of posterior teeth:

Posterior teeth are generally placed to enhance

stability of the mandibular denture.

i- Buccolingually, the teeth should be placed over or

slightly lingual to the crest of the mandibular ridge,
however the lingual surface of the teeth should not exceed
the medial extension of the mylohyoid ridge, to provide
enough space for the tongue to move freely. Encroaching
upon the tongue space, causes cramping of the tongue,
and lateral movement of the denture whenever the tongue
moves.

Cramped tongue

ii- Anteroposteriorly, the teeth should never be positioned

on the upward incline of the mandible. Forces directed to
an inclined plane are more dislodging than forces directed
at right angle to the supporting basal seat.

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 Retention and stability of complete dentures

2- Lower occlusal plane:

i- Height of the lower occlusal plane should coincide with

a line extending from or slightly below the corners of
mouth anteriorly and the center of retromolar pad
posteriorly. This position usually places the occlusal plane
below the greatest convexity of the tongue. This position
allows the tongue to function in co-ordination with the
buccinator muscle to keep the food on the occlusal surface
of the teeth, and aid in lower denture stability.

Level of occlusal plane

ii- Direction of the occlusal plane: the plane of occlusion

should be parallel to the main direction of the denture
foundation area, thus the masticatory biting forces are
perpendicular to the supporting basal seat, minimizing
undesirable lateral forces.

3- Shape of the denture polished surface:

The denture polished surface should be properly

contoured so that, the tongue, cheek and lips seat rather
than unseat the dentures.

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 Retention and stability of complete dentures

4- Proper relief of the upper denture:

The tissue covering the median palatine suture is

usually thin and non displaceable, acting as shifting
fulcrum around which the upper denture can rotate.
Sufficient relief of the upper denture over the median
palatine raphe, to compensate for different degree of
tissue displaceability, improves upper denture stability.

5- Balanced occlusion:

Balanced occlusion in dentures means even

simultaneous contact between upper and lower teeth in
centric and eccentric position that is in harmony with
various mandibular movements. Occlusion is said to be
balanced when there is at least three widely separated
points of occlusal contact in any lateral or protrusive
position, such contact stabilizes dentures on their basal
seat.

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