Cohesion: It is the physical attraction of like molecules to each other.
This occurs between the denture base and the mucosa and works to
maintain the integrity of the interposed fluid. Watery serous saliva
can form thinner film and is more cohesive than the thick mucus
Interfacial surface tension: It is the resistance to separation of two
parallel surfaces that is imparted by a film of saliva between them.
The thin film of saliva tends to resist the displacing force which
tends to separate the denture from the tissues.
Atmospheric pressure: It acts to resist the dislodging forces applied to
the dentures, provided they have effective peripheral seal. This
peripheral seal prevents the entry of air between the denture
surface and the soft tissue. When displacing forces act on the
denture, a partial vacuum is produced between the denture and the
soft tissues, which aids in retention. Retention due to atmospheric
pressure is directly proportional to the area covered by the denture
base. Atmospheric pressure is also referred to as emergency retentive
force or temporary restraining force.
Capillarity action: When there is a close adaptation between the
denture and the mucosa, thin film of saliva tends to flow and
increase its surface contact thereby increasing the retention.
Gravity: When a person is in upright posture, gravity acts as a
retentive force for the mandibular denture and a displasive force for
FIGURE 4-2 Thin film of saliva between the denture base
and the tissue surface aids in retention by adhesion,
cohesion, interfacial surface tension and atmospheric
• Apprehension or fear of embarrassment
Implant dentures: Retention is definitely enhanced in implant-retained
Ridge extension: It increases retention by increasing the surface area.
E.W. Fish (1948) gave three principal factors that affect the retention
of complete dentures, which are as follows:
(ii) A balanced harmonious occlusion
(iii) Properly formed polished surface
S. Friedman (1957) advocated three basic goals for achieving
retention, which are as follows:
(i) Maximal coverage without undue displacement of tissues
(ii) Development of good border seal
(iii) Adequate provision for resistance areas
It is defined as ‘the quality of a denture to be firm, steady or constant, to
resist displacement by functional horizontal or rotational stresses’.
It is the resistance to horizontal or rotational forces. Stability ensures
physiological comfort to the patient, whereas retention contributes to
Factors affecting stability can be categorized as biological, mechanical
• Residual ridge relationships
• Nature of soft tissues covering the ridges
• Importance of modiolus and associated structures
• Influence of orofacial musculature.
• Neuromuscular control and education of the patient
• Tooth position and teeth arrangement
• Relationship of the polished surfaces of the denture
base to the surrounding orofacial musculature
• Relationship of opposing occlusal surfaces
• Contour of polished surface of denture
The development of stability is limited by the anatomical variations of
the patient that determines the residual ridge height and
Large, square and broad ridges offer a greater resistance to the lateral
forces than small, narrow and tapered ridges.
The residual ridges with sufficient vertical height provide better
stability than the resorbed ridges.
Square or tapered arches tend to resist rotation of the prosthesis better
The shape of the palatal vault also contributes to the stability of the
A broad, flat palatal vault may enhance the stability by providing a
greater surface area of contact and long inclines approaching a right
angle to the direction of the force. The V-shaped palate provides least
vertical support and retention.
Normal dental relationships of the artificial teeth set on ridges
enhance the stability of the denture.
Stability in prognathic and retrognathic patients is compromised
because of of set ridge relations.
Nature of soft tissues covering the ridges
The presence of keratinized, firmly bound mucosa to the residual ridge
permits the tissues to resist stress favourably and enhance stability.
Hyperplastic or flabby tissues with excessive submucosa provide poor
Importance of modiolus and associated structures
• Modiolus or tendinous node is an anatomical landmark near the corner
of the mouth. It is formed by the intersection of several muscles of
the cheeks and lips (Fig. 4-3).
• A total of eight muscles form the modiolus. These are zygomaticus,
quadratus labii superioris, quadratus labii inferioris, caninus,
triangularis, risorius, buccinator and mentalis.
• The denture base must be contoured to permit the modiolus to
function freely as one muscle can influence all the other muscles.
• The superior fibres of the buccinator act to seat the denture, the
middle fibres control the bolus of the food and the inferior fibres
contribute to the mandibular denture stability.
FIGURE 4-3 Muscles comprising modiolus.
The lingual slope of the mandible approaches 90° to the occlusal plane
which enables it to effectively resist horizontal forces and provide
The lingual flange of the lower denture should incline medially to
allow for contraction of the mylohyoid muscle which lies beneath the
mucosa covering the lingual slope of residual ridge.
Influence of orofacial musculature
• The orofacial musculature plays an important role in enhancing the
• The basic geometric design of denture bases should be triangular. In
the frontal section, the upper and lower dentures should appear as
two triangles whose apexes correspond to the occlusal surface (Fig.
• The maxillary buccal flange should incline laterally and superiorly.
The mandibular buccal flange should incline laterally and inferiorly,
and the lingual flange should incline medially and inferiorly. Such
inclinations provide favourable vertical component to any
• The tongue should rest against a lingual flange inclined medially
away from the mandible and somewhat concave to direct the
seating action on the mandibular denture.
FIGURE 4-4 Basic denture design of upper and lower
Neuromuscular control and education of the
• The tongue plays an important role in the neuromuscular control. In
a completely edentulous patient, all the periodontal receptors are
lost and the sensory stimuli from the oral mucosa are used to learn a
• Tongue works primarily by the touch and pressure system in
contrast to skeletal muscle which function by kinaesthesis.
• Normal tongue position enhances the stability of the dentures, as it
completely fills the floor of the mouth. Its lateral borders rest over
the ridge, whereas its tip or apex rests on or just lingual to the lower
• Patient education regarding denture use and maintenance is
important for the stability of dentures.
Tooth position and teeth arrangement
• Anterior and posterior teeth should be arranged as close as possible
to the position once occupied by the natural teeth.
• The teeth in the denture should be arranged in the neutral zone or in
• Neutral zone is defined as ‘the potential space between the lips and
cheeks on one side and the tongue on the other side’.
• ‘That area or position where the forces between the tongue and
• Natural or artificial teeth in this zone are subjected
to equal and opposite forces from the surrounding
FIGURE 4-5 Teeth arranged in neutral zone.
Relationship of the polished surfaces of the
denture base to the surrounding orofacial
• Action of the musculature on the denture base generally results in
lateral and vertical dislodging forces. Such muscles should be
identified and their actions should be permitted without any
• The denture border must be extended to contact the movable
tissues. Optimal extension enhances the denture stability.
• The external surface should be developed to harmonize with the
associated functioning musculature of the tongue, lips and cheeks.
• E.W. Fish (1933) believed that the contours of the polished surface
provide the principal factor governing the complete denture
Relationship of opposing occlusal surfaces
• Harmony between the opposing occlusal surfaces contributes to
• The dentures should be free of any interference within the
functional range of movement of the patient, regardless of the type
• Balanced occlusion enhances the denture’s stability.
• The occlusal plane should be oriented parallel to the residual ridge.
If the occlusal plane is inclined, then the sliding forces may act on
the denture and reduce its stability.
• If the occlusal plane is tipped, then there will be a shunting ef ect and
• If the mandibular occlusal plane is too high, then it can result in
• Raised occlusal plane prevents the tongue from reaching over the
food table into the vestibule. This compromises the stability of the
Contour of polished surface of denture
• The polished surface of the denture should be in harmony with the
• These should not interfere with the action of the oral musculature.
• The proper contour of the denture flanges permits the horizontally
directed forces that occur during contraction of muscles to be
transmitted as vertical forces tending to seat the prosthesis.
• Impression should be accurate and should duplicate all the details of
• Impression should not distort upon removal and should be
dimensionally stable. The cast should be poured as soon as possible.
• The occlusal rims should be parallel to the ridge. The occlusal plane
should equally divide the interarch space.
• If the occlusal plane is inclined, then the sliding forces may
Stable denture bases enhance the stability of the dentures.
Support is defined as ‘the resistance to vertical forces of mastication,
occlusal forces and other forces applied in a direction towards the denturebearing areas’.
It counteracts the forces directed towards the ridge at right angles to
the occlusal forces. It involves the relationship between the intaglio
surface of the denture base and the underlying tissue surface under
varying degrees and types of function so as to maintain an established
occlusal relationship and to promote optimal function with minimum
tissue-ward movement and base settling.
(i) Initial denture support: This support is achieved by impression
procedures that provide optimal extension and functional loading
(ii) Long-term support: This support is achieved by directing the
occlusal forces towards the tissues resistant to remodelling and
Factors responsible for effective support of the
• Denture is extended to cover the maximal surface area without
• Tissues capable of resisting resorption are selectively loaded.
• The tissues capable of resisting vertical displacement are allowed to
make firm contact with the denture base during function.
• Compensation is made for varying tissue resiliency to provide
uniform denture base movement under function.
• Soft tissues, firmly bound to the underlying cortical bone and
covered by the keratinized mucosa, minimize the base movement,
decrease the soft tissue trauma and reduce the long-term resorptive
This principle is based on maximal extension of the denture to make a
positive contact with the soft, yielding peripheral tissues as limited by
muscle function or bony anatomical structures.
It states that under given constant occlusal force, a broader
denture-bearing area decreases the stress per unit area under the
denture base, thereby decreasing the tissue displacement and
reducing the denture base movement.
Maximal border extension during impression procedure is,
therefore, essential in providing adequate denture support (Fig. 4-6).
FIGURE 4-6 Snowshoe principle.
Impression techniques can be classified on the following basis:
(i) On the basis of pressure used during impression making:
• Selective pressure technique
(ii) On the basis of tray selection:
(iii) On the basis of type of impression:
(iv) On the basis of material used:
• Reversible or irreversible hydrocolloids
(v) On the basis of mouth opening:
(vi) On the basis of hand-manipulated functional movements:
• Dynamic functional movements
• Passive functional movements
Mucostatic impression technique
• This technique was first proposed by J.A. Richardson and later
• Failures in pressure technique lead to the popularization of
• Supporters of this technique describe interfacial surface tension as
the only significant way of retaining complete dentures.
• Impression should, therefore, cover only the area of the oral cavity,
where the mucous membrane is firmly attached to the underlying
• The main point of the mucostatic principle was concerned with
Pascal’s law which states that pressure applied on a confined liquid
will be transmitted throughout the liquid in all directions.
• According to this concept, mucosa being more than 80% water will
react as liquid in a closed vessel. However, this is not true as the
tissue fluids can escape under the border of the denture. Also, the
mucosa is not a closed vessel.
• Impression is made with an oversized tray with oral mucosa and
jaws in a normal and relaxed manner.
• It requires minimal pressure to be applied to the oral tissues during
seating of the impression tray and set of the impression material
and requires a material of high fluidity.
• This technique seeks to eliminate all distortion of the oral tissues
and thus create a denture base that models the unloaded tissues.
• Retention is entirely dependent on surface forces of adhesion,
cohesion and interfacial surface tension.
• For this, thin film of saliva is necessary.
• Border moulding is not done in this technique.
• Impression material of choice is impression plaster.
• It results in denture which is closely adapted to the mucosa of the
denture-bearing area but has poor peripheral seal.
• Tissue health and denture retention is compromised.
Mucocompressive impression technique
• This technique was popularized by Carole Jones. It records the
tissues in a functional and displaced form. The materials used for
this technique are impression compound, waxes and soft liners. It
appeared logical to make impression that would press the tissues in
the same manner as chewing forces.
• G. Tryde, K. Olsson, S.A. Jenson, R. Cantor, J.J. Tarsetano and N.
Brill (1965) advocated closed mouth technique so that the patient
could exert his/her own masticatory force during impression
• Proponents of this technique presume that the occlusal loading
during impression making is comparable with the occlusal loading
• The oral soft tissues are resilient in nature.
• As the tissues are recorded with pressure method in this technique,
the soft tissues tend to rebound as soon as the forces are relieved.
• Dentures made by this technique tend to get displaced due to tissue
• Due to continuous pressure on the tissues during function, there is
compromised blood supply to the tissues leading to increased ridge
• Dentures made from such impressions did not fit well at rest.
• Due to continuous pressure, the tissue will undergo resorption.
• Closed mouth technique does not permit border moulding.
• This technique was advocated by C.O. Bouchers and combines the
principles of pressure and minimal pressure techniques.
• The philosophy of this technique is that certain areas of the maxilla
and the mandible are by nature better adapted for withstanding the
additional forces of mastication.
• Here, the impression is extended over as much denture-bearing
areas as possible without interfering with the limiting structures at
• This is achieved by the design of the custom tray in which the
nonstress-bearing areas are recorded in a state of rest, whereas the
stress-bearing areas are recorded under pressure.
• Relief is given using wax in the custom tray, which should be
removed before making the final impression.
• The relief wax is applied on the primary cast before custom tray
capable of supporting and distributing the loads.
• This technique seeks to create a denture base that selectively loads
the oral tissues during functioning of the prosthesis, thereby
optimizing the stability and retention of the prosthesis.
• Opponents of this technique are of the opinion that it is impossible
to record certain areas with different pressure from that applied to
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