Definition :

“That quality of maintaining a constant character or position in the

presence of forces that threaten to disturb it.”

Or “The quality of a prosthesis to be firm, steady or constant, to resist

displacement by functional horizontal or rotational stresses.”

Denture Stability :

“The resistance of a denture to movement on its tissue foundation,

especially to lateral (horizontal) forces as opposed to vertical displacement

(termed denture retention)” . . . . . . .JPD 99, GPT – 7

INTRODUCTION:

Stability ensures the physiological comfort of the patient. Denture

instability adversely affects support and retention. A denture that shifts

easily in response to laterally applied forces that are generated during most

of the functions of the mouth, causes a disruption in the border seal and

prevents the denture base from correctly relating to the supporting tissues.

This results in deleterious forces on the edentulous ridges during function.

Factors contributing to stability :

- Fish in 1948 described a denture as having 3 surfaces :

1. The impression surface.

2. The occlusal surface.

3. The polished surface.

Though all these three surfaces are developed independently, they are

integrated to create a stable, functional and esthetic result.

- Thus the factors contributing to stability can be categorised under the

following headings :

1. The relationship of the denture base to the underlying tissues.

2. The relationship of the external surface and border to the surrounding

oro-facial musculature.

3. The relationship of the opposing occlusal surfaces.

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4. Education of the patient.

I–RELATIONSHIP OF THE DENTURE BASE TO THE

UNDERLYING TISSUES:

This will be discussed under the following headings :

1. Residual ridge anatomy.

2. Denture base adaptation.

3. The mandibular lingual flange.

1. Residual ridge anatomy :

Residual ridge height and conformation are limiting factors for

stability.

a) Large, square, broad ridges offer a greater resistance to lateral forces

than do small, narrow and tapered ridges. (Fig. 1)

b) Small and rounded irregularities also contribute favourably to stability.

Alveoloplasty should therefore be limited to removal of bone that would

prevent the fabrication of a successful prosthesis. Ex –in case of sharp

spicules, severe undercuts and insufficient inter-arch space. But removal

of all irregularities to create a smooth, even ridge would diminish the

potential stability.

c) The arch form: Square or tapered arches tend to resist rotation of the

prosthesis better than the ovoid arches.

d) Shape of the palatal vault : Stability is limited by the length and

angulation of the palatal ridge slopes. A steep or high arched palate

enhances stability by providing greater surface area of contact and long

inclines approaching at right angles to the direction of force. But vertical

forces tend to unseat the denture easily. (Fig. 2)

According to Burns DR et al, (JPD 95, Vol.73 (4))

Treatment alternatives to increase stability including

- Ridge augmentation or vestibular augmentation

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- Dental implants to provide anchorage for an all implant supported

prosthesis.

- Mucosa and implant supported overdentures

were assessed. The study indicated superior statistics with respect to

implant overdentures and a slight but significant improvement in the

soft tissue response.

According to W. Kalk et al. (IJP 1992 No. 3 Vol. 5)

Stability was assessed in 3 groups :

1. Who needed preprosthetic surgery but which was contraindicated,

had received new dentures.

2. Same as “1” but were treated with vestibuloplasty and lowering off

the floor of the mouth before denture fabrication.

3. Control groups without residual ridge related problem who were

treated with new complete dentures.

The least displacement of the mandibular dentures occurred in groups 2 & 3,

greatest in 1. Loosening caused by tipping of the mandibular denture was

least in group 2 because of elimination of the muscle attachments and

increase in the extensions.

2. Denture base adaptation :

- The relationship of the intaglio of the denture base to the underlying

tissues is dependent on the impression procedures of the clinician.

- Health of the tissues at the time of impression making is important.

Stability is compromised in the following cases :

- Inflamed mucosa.

- Distorted or displaced tissues

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- Hyperplastic tissue.

- Denture base adaptation may be improved by the use of tissue liners,

adhesives,and fixatives.

- Mucostatic impression techniques increase stability because of less

displacement of tissues during impression making and hence less

rebounding of the displaced tissues.

1. The mandibular lingual flange :

A properly formed denture base outline develops a seal than can be

maintained during most of the normal oral functions.

The labial and buccal flanges have well defined landmarks that can

be visually evaluated. The distolingual extension of the lingual flange is

developed arbitrarily. The lingual slope of the mandible is at 90 degrees to

the occlusal plane which is a desirable feature.

The posterior lingual flange can be extended more inferiorly than

anterior lingual flange, although posterior fibres of mylohyoid muscle attach

more superiorly on the mandible; they descend vertically to attach to the

hyoid bone. When contracted the muscle fibres extend medio-inferiorly

allowing the posterior flange to extend to/or beyond the mylohyoid ridge.

Anteriorly the muscle fibres are directed more horizontally to communicate

with fibres of the opposite side. When contracted the fibres tense the floor

of the mouth and limit the extension of the anterior lingual flange.(Fig.3&4)

- Any flange extensions beyond the mylohyoid ridge must incline

medially away from the mandible to allow for the mandibular mylohyoid

muscle contraction.

The degree of positive contact of firm ridge to flange may be

compromised by the presence of a thin mucosa overlying the bony ridge

slopes that don’t tolerate the stresses effectively and may require relief.

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According to C. H. Jooste and C.J.Thomas, (IJP 1992 Vol. 5 No.1)

Analysis of cineradiographic tracings of movements of marker placed

in the mandibular dentures with and without denture extensions during

chewing exercises revealed that an extension had a stabilizing effect on the

mandibular complete denture.

Sublingual Crescent Area :

Definition : The crescent shaped area on the anterior floor of the mouth

formed by the lingual wall of the mandible and the adjacent sublingual fold.

It is the area of the anterior alveolingual sulcus. …. GPT –7

Extension of the denture over the resting tissues of the sublingual

crescent area completes the border seal and increase the covering surface of

the dentures resulting in :

1. Increased retention by allowing the tongue to aid in holding the

dentures in place. (Fig.5)

2. Denture is more stable during normal tongue movements such as

swallowing, speaking and eating.

II- RELATIONSHIP OF THE EXTERNAL SURFACE AND

PERIPHERY TO THE SURROUNDING ORO–FACIAL

MUSCULATURE :

Actions of the musculature on the denture base generally result in

lateral and vertical dislodging forces. Factors involving musculature and the

polished surface of the denture can facilitate stability if :

1. Action of certain groups are permitted to occur without interference

by the denture base so that they won’t dislodge the prosthesis during

function.

2. Dentist recognizes that normal functioning of some muscle groups

can be used to enhance stability ie. alterations in external

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contours can lead to dynamic seating and stabilizing action directed

towards the prosthesis. (Fig.6&7)

The action of Levator Anguli Oris, Incisivus, Depressor Anguli Oris,

Mentalis, Mylohyoid and Genioglosus can dislodge the denture base if they

are not allowed to function freely. Proper border or muscle molding ensures

optimal border extensions.

The following factors will be considered :

1. The external surface of the denture

2. Influence of oro-facial musculature

3. Modiolus and associated musculature

4. The neutral zone

1. The external surface of the denture :

The location and form of the polished surfaces given by wax to obtain

convexity or concavity facially and lingually contribute to functional

stability of dentures.

Fish believed that the contours of the polished surface provided the

principal factor governing complete denture stability. He wrote “The shape

of the buccal, labial and lingual surfaces can wreck stability as completely

as a bad impression or a wrong bite.” Thus the horizontal forces exerted by

the tongue and cheek can act either as a placing or displacing agent. The

lingual and buccal borders of mandibular denture and the buccal borders of

maxillary denture can be made concave so that the tongue and cheek will

grip and seat the denture.(Fig.8 to 13)

2. Influence of Oro-Facial Musculature :

The basic geometric design of denture bases should be triangular ie. in a

frontal cross-section the maxillary and mandibular dentures should appear

as 2 triangles whose apexes correspond to the occlusal surface.(Fig.14)

- Maxillary buccal flange should incline laterally and superiorly.

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- Mandibular – laterally and inferiorly and its lingual flange medially

and inferiorly.

Such inclination provides favourable vertical component to any

horizontally directed forces.

The tongue should rest against the lingual flange which is inclined

medially away from the mandible and concave. A normal tongue position

has the following features :

a. It should completely fill the floor of the mouth.

b. The lateral borders should rest over the ridge on the occlusal

surface of the teeth.

c. The tip or apex rests on or is just to the lingual side of the

lower anterior ridge.

According to PAJ. Culver and I. Watt (BDJ 1973 Vol. 135)

Although it is recognized that the tongue and the oral musculature in

general play a large part in stabilizing the upper denture, great emphasis is

still placed in the standard text books and in several papers on the physical

mechanisms by which denture retention and stability is achieved. The

peripheral seal achieved by these mechanisms is in fact broken in function

and the tongue apparently pushes the denture up and hence stabilizes it.

3. Modiolus and the associated musculature :

The modiolus or the tendinous node near the corner of the mouth is

formed by the intersection of several muscles of the cheeks and lips

including :

- Zygomaticus

- Quadratus Labii Superioris

- Levator Anguli Oris (Caninus)

- Mentalis

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- Depressor Anguli Oris (Triangularis )

- Depressor Labii Inferioris

- Buccinator

- Risorious

- Orbicularis oris

As none of these muscles contain fibres that have more than one bony

attachment, they depend on fixation of the modiolus to allow isometric

contraction.

Contraction of triangularis, caninus, and zygomaticus fixes the

modiolus allowing the middle fibres of the buccinator to contract

isometrically, thus allowing it to control the food bolus on the occlusal

table. The superior fibres of the buccinator seat the maxillary denture and

the inferior fibres contribute to mandibular denture stability.

3. The neutral zone:

Definition : “The potential space between the lips or cheeks on one side and

the tongue on the other; that area or position where the forces between the

tongue and cheeks or lips are equal or neutralized.

The idea is to establish harmony between the polished surface of the

denture and the associated musculature. The musculature should

functionally mold not only the borders but the entire polished surface. The

teeth placed within the neutral zone are balanced. Thus this functional rather

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than anatomic placement of the teeth further enhances the stability of the

denture by minimizing active forces.

III – RELATIONSHIP OF OPPOSING OCCLUSAL SURFACES :

The dentures must be free of interferences within the functional range

of movement of the patient, (this refers to the positions through which the

lower jaw moves horizontally during normal speech, swallowing and

mastication.)

During both functional and parafunctional movements the occlusal

surfaces shouldn’t strike prematurely in localized area. Such contacts cause

uneven stresses to be transmitted to the dentures during function resulting in

lateral and torquing forces that destabilize the denture.

Theories of Occlusion:

1. Occlusion in Centric Relation :

According to “Woelfel et. al. (JPD 1962 Vol. 12) :

They showed that most functional closures of the complete denture

patients occurred in closed proximity to centric relation. For this reason the

relationship of the mandible to the maxilla should be recorded in the most

retruded position for maximum stability and efficiency.

For many patients the normal range of horizontal movements of

mandible is limited to centric relation. This is true in case of skeletal class

III patients. Excursive balance may not be necessary in such patients.

Patients with a wider functional range of movements as in skeletal class II

require consideration of premature occlusal contacts which occur when the

mandible dosen’t close in centric relation. In such cases, the horizontal

forces can be minimized by training the patient to place food bilaterally to

ensure simultaneous posterior teeth contact.

2. Balanced Articulation :

Definition :The bilateral, simultaneous anterior and posterior occlusal

contact of teeth in centric and eccentric positions……………GPT 7

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Or

The occlusal contacts of maxillary and mandibular teeth initially in

maximum intercuspation and their continuous contacts during movements

from this position along specific working, balancing and protrusive

guidance pathways developed on the occlusal surfaces of the teeth.

..Boucher

Stability of the dentures is partially dependent upon the contact of a

tooth in one part of the arch to balance tooth contact in another part of the

arch in case of artificial teeth. Natural teeth are surrounded by bone and

except for movements within the limits of their periodontal attachments they

can be considered fixed whereas artificial teeth are attached to a movable

base resting on soft tissues that can be displaced. When natural teeth are

present bone receives stimulation, tensile in nature which contributes to

normal bone physiology. But dentures can’t replace this stimulation.

Arranging artificial teeth to provide excursive balance minimizes

localized stress concentration and lateral dislodging forces by ensuring

multiple points of contact. On closure through a bolus of food, bilateral

posterior teeth contacts within the range of balance ensures good seating of

the prosthesis. Though proof lacks to support the validity of a balanced

articulation during chewing, it appears to be more important when there is

no food in the mouth. “Brewer” reported that in 24 hrs. test period, tooth

contact during chewing was only 10 minutes whereas non-chewing

activities amounted to 2-4 hours of contact.

The horizontal movements of the mandible generated by an

articulator simulate para-functional rather than functional jaw movements

and teeth are balanced to provide stability during these anticipated

movements.

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3. Lingualised Occlusion :

First described by S. Howard Payne in 1941. This form of denture

occlusion articulates the maxillary palatal cusps with the mandibular

occlusal surfaces in centric working and non-working mandibular positions.

Lingualised occlusion provides :

a. Limited range of excursive balance and

b. Directing the forces to the lingual side of the lower ridge

during working side contacts. This minimizes horizontal stress and

enhances denture stability by controlling leverages induced by

eccentric tooth contacts.

According to Curtis .M Becker et al.( JPD 1977, 38 (6)):

Using lingualised occlusion satisfactory occlusion is easily obtained,

and balanced occlusion can be accomplished.

Selection of artificial teeth :

The selection of anatomic, semianatomic or non-anatomic artificial

teeth depends :

- Partially on the chosen occlusal scheme

- Quality of the residual ridge ie. Height and conformation.

If balanced articulation is desired throughout a limited functional range of

movement for patients with deficient residual ridges, the use of non-

anatomic zero-degree teeth set on a curve may provide desired occlusal

contacts while eliminating the interlocking of opposing anatomic teeth.

The tooth position and occlusal plane :

A.Tooth position :

When forces act on a body in such a way that no motion results, there

is a balance or equilibrium. This should be the primary consideration with

the forces that act on the teeth and the denture bases with their resultant

effect on the movement of the base. “A stable base is the ultimate goal.”

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Total stability isn’t possible because of the yielding nature of the

supporting structures. ‘Lever Balance’ is the basis of balanced occlusion.

Some rules in teeth arrangement are :

1. The wider and larger the ridge and closer the teeth are to the ridge,

greater is the lever balance.

2. Wider the ridge,narrower the teeth bucco-lingually greater the

balance and vice-versa.

3. More lingual the teeth placed in relation to the ridge crest, greater the

balance; more buccal the placement of teeth, poorer the balance.

4. More centered the forces of occlusion antero-posteriorly greater the

stability of the base.

Tooth position as well as tooth contact complement each other for

total balance.

Maxillary anterior tooth position :

The arch curvature should correspond to curvature of alveolar ridge,

facial contour and maxillary lip position.

Arranging teeth into a square arch form on a tapering or ovoid

residual alveolar ridge causes canines to be labial to crest of maxillary ridge

than central incisors, resulting in bicuspids being more buccal to the ridge

than they should be. Working side occlusal pressure produces a displacing

tendency, the ridge crest acting as a fulcrum. (Fig.15&16)

Normal Anterior Alveolar resorption:

The labial axial inclination of the natural anterior teeth places the

incisal edges labial to the fulcrum line about which the tooth would tend to

rotate when under incisal force or when the occlusal contact area is anterior

to alveolar support. Therefore if prosthetic teeth were placed in exactly the

same position as natural they would be labial to the alveolar support. More

labial bone may be lost due to alveolectomy because of undercuts and also

residual ridge atrophy. (Fig.17)

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The net result is a mechanically unfavourable tooth position in

relation to the denture base foundation. However, this position is required

for esthetics and function. But if this acknowledged unfavourable

relationship is combined with the error of a square arch on a tapering ridge,

unnecessary torque and instability are created.

Mandibular anterior tooth position:

This must confirm to the maxillary arch.Errors in maxillary tooth

position will be transferred to the mandibular arch.

Posterior teeth position:a. Maxillary :

Normal posterior maxillary teeth have a buccal axial inclination while

mandibular teeth have a lingual axial inclination. The normal alveolar bone

resorption that takes place after extraction can result in a slight crossbite

relationship of the ridge crests. This isn’t difficult to visualize since the

bony support of the mandibular teeth is slightly buccal to the maxillary bony

support before extraction. Due to buccal plate reduction in surgical

procedures this cross-bite tendency is augmented. Finally any advances in

resorption process results in a complete crossbite ridge relation.(Fig.18&19)

The maxillary posterior teeth may be arranged too far buccally for the

following reasons :

1. Anterior square arch form set on an ovoid ridge which causes canines

and bicuspids to be placed buccally.

2. A slight crossbite relationship of the ridges due to the axial

inclination of the natural teeth before extraction.

3. Advanced alveolar atrophy leads to increased crossbite relation of the

ridge.

4. Placement of mandibular teeth slightly buccal to the crest of the ridge.

5. Tendency to avoid crossbite arrangement results in placing maxillary

posterior teeth in a buccal position.

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Working side occlusal pressure causes a displacing tendency because

the line of force is buccal to the fulcrum. More lateral pressure is exerted on

the residual alveolar ridge and this results in a more rapid resorption.

Perhaps, because we can usually give the patient more stability and

retention on the maxillary denture we tend to abuse it. Another reason for

avoidance of crossbite is the lack of understanding of its equilibration. A

crossbite is less efficient than the normal bucco-lingual teeth position.

However, a crossbite on a very stable base is better than a normal bucco-

lingual relation of teeth on an unstable damaging base.

b. Mandibular:

Many operators avoid ridge lap grinding made necessary by the limits

of space and thickness of the baseplates, by placing lower posterior teeth

buccal to the ridge. The tooth is positioned so as to have the lingual cusp

and fossa over the crest of the ridge. If poor lingual cusp contact exists on

the working side excursions, displacing torques develop. The buccal cusp of

the lower teeth are buccal to the center of the alveolar crest which acts as a

fulcrum, thus creating a displacing force. The lingual cusp creates no torque.

If the guiding inclines of the cusps are not reduced and equilibrated on a

suitable articulator, more lateral forces will be added.

Hence the buccal cusp and fossa of the mandibular posterior teeth

should be directly over the crest of the ridge. The difference between this

position and that mentioned before is 2mm in the lingual direction, and thus

results in more stability and less lateral force. This is because occlusal

pressure on the tooth falls close to the fulcrum and creates little or no

torque. But this requires the buccal aspect of the ridge lap and the base to be

ground in that area which in turn depends on the availability of space

between the ridges. (Fig.20 to 23)

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According to C. H. Jooste & C.J. Thomas( Jol. of oral Rehabilitation, 1992,

Vol. 19):

A study was conducted on 6 patients with previous denture

experience. Metal indicators were placed on either side of the mandibular

dentures and a Co-Cr alloy marker was inserted in the left bucco-posterior

area of the mandible in each case. In new dentures posterior teeth were

positioned upto the retromolar pad, over the slope of the posterior

mandibular alveolar ridge. After habituation had taken place a

cineradiographic recording was made of chewing. A 2nd recording was made

with the teeth removed form the inclines. Denture movement was observed

by measuring the distances between the markers on an analyzer projector.

The results showed a significant difference between the two values. The

movement was less after the removal of the teeth over the incline. These

results support the clinical observation that teeth placed over a basal tissue

incline have a destabilizing effect during complete denture function.

B) Occlusal Plane :

A study of functions of mouth during chewing shows an intimate

relationship between the tongue, mandibular posterior teeth and the

buccinator muscle. The occlusal plane if incorrectly located results in the

malfunction of the soft structures.

A mandibular occlusal plane that is too high can result in reduced

stability. A high occlusal plane forces the tongue into a new position higher

than its normal position. This causes the tongue to loose much of its

accuracy. The higher position causes the floor of the mouth to rise and

create undue pressure on the border of the lingual flange. This leads to

disruption of the normal position of the floor of the mouth and hence :

a. Partial loss of border seal

b. Lateral forces directed against the teeth are magnified

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c. The tongue is unable to reach over the food table into the buccal

vestibule making control of food bolus difficult. (Fig.24&25)

- A raised occlusal plane is usually present when the vertical

dimension of occlusion is increased excessively. Various anatomical

landmarks such as Stenson’s duct, retromolar pad should be used to

determine an acceptable level of occlusal plane.

- When excessive mandibular ridge resorption has occurred in

comparison to maxillary, the occlusal plane is too low. An occlusal plane

that is slightly low causes no problem.

Ridge Relationships :

A problem of stability is the offset ridge relations seen in prognathic

and retrognathic patients.

- In case of class III patients, sufficient mandibular posterior

occlusion must be developed so that the contact against maxillary

denture extends posteriorly more than half the distance from the incisive

papilla to the hamular notch. Without this contact the maxillary denture

would tip antero-superiorly, traumatize the maxillary anterior ridge and

loosen the maxillary denture.

- In case of severe posterior crossbite the normal tooth to tooth

position may be altered to provide a stable relationship.

- While some compromises in the ideal tooth to ridge and tooth

to tooth position relationships may be made, the range of such skeletal

cosmetic deficiency correction without surgical intervention is limited.

Patient Education :

- Every patient should be informed regarding the care and proper

use of the dentures.

- Patients disregard reasonable limitations in the use of their

dentures and this is often inconvenient and needs to be adjusted.

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- Failure to follow the dentist’s advise will eventually lead to

damage of the supporting tissues.

- In case of retracted tongue position the dentist should guide the

patient by showing the normal position and demonstrating its

significance.

- For occlusion in centric relation, simultaneous bilateral

chewing habits should be encouraged.

- Incising with anterior teeth should be strictly avoided. No

treatment however sophisticated can be successful without the patient’s

co-operation. (Fig.26)

Checking stability of the denture :

- Pressure is applied with the ball of the finger in the premolar –

molar regions of each side alternately. This pressure must be at right

angles to the occlusal surface. If pressure on one side causes the denture

to tilt and rise on the other side, it indicates that the teeth on the side on

which pressure was applied are outside the ridge.

- Patient is asked to make excursive movements in case balanced

occlusion has been provided.

CONCLUSION:

Stability prevents antero-posterior shunting of the denture base. It has

been cited as the most significant property in providing physiologic comfort

to the patient.

Denture instability adversely affects retention and support and results

in deleterious forces on the edentulous ridges during function and

parafunction. It is important to know the factors affecting stability. Though

to fabricate a perfectly stable denture may not be truely possible, we should

still try to achieve the maximum possible.

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BIBILIOGRAPHY :

1. Boucher – Prosthodontic treatment for edentulous patients

2. Fenn – Clinical dental prosthetics

3. Heartwell – Syllabus of complete denture

4. Sharry – Complete denture prosthodontics

5. Winkler – Essentials of complete denture prosthodontics.

6. Glossary of prosthodontic terms – VII edition

7. “Prospective clinical evaluation of mandibular implant

overdentrues part I” : Burns D.R. ; JPD 1995, 73 (4).

8. A comparison of different treatment strategies in patients with

atrophic mandibles – A clinical evaluation after 65 years : : W. Kalk ;

JJP 1992, 5 (3).

9. “The influence of retromyhohyoid extension on the mandibular

complete dentures” : C.H. Jooste and C.J. Thomas; JJP 1992, 5 (1).

10. “Denture movements and control – A preliminary study” :

P.A.J. Culver and J. Watt : BDJ 1973, 135.

11. Lingualised occlusion in removable prosthodontics Curtis. M.

Becker, Charles. C. Swoope, Albert. D. Guckes ; JPD 1977; 38 (6).

12. “Complete mandibular denture stability when the posterior

teeth are placed over basal tissue” : C.H. Jooste and C.J. Thomas ; Jol of

Oral Rehabil ; 1992, 19.

13. “A contemporary review of the factors involved in complete

dentures. Part II : Stabiltiy” J.E. Jacobson and A.J. Krol; JPD 1983, 49,

165-172.

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1) Definition

2) Introduction

3) Factors contributing to stability:

I) Relationship of denture base to underlying tissues :

Residual ridge anatomy

Denture base adaptation

Mandibular lingual flange, sublingual crescent area

II) Relationship of external surface and periphery to the surrounding

oro-facial musculature :

The external surface of denture

Influences of oro-facial musculature

Modiolus and associated musculature

The neutral zone

III) Relationship of opposing occlusal surfaces :

Theories of occlusion – Occlusion in centric relation

– Balanced articulation

– Lingualised occlusion

Tooth position

Occlusal plane

Ridge relationships

IV) Patient education

4) Conclusion

5) List of references

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