Neutral zone concept in complte dentures.ppt

Neutral zone concept
in complete dentures

Table of contents
 Introduction
 Definition
 Review of literature
 Anatomy : Muscles of mastication
Muscles of facial expression
Tongue
 Age changes

 Neutral zone concept
 Neutral zone and denture surface
 Steps in complete denture fabrication based on
neutral zone principle
 Summary
 Conclusion
 Reference

Introduction
 Complete dentures are primarily mechanical
devices, but since they function in the oral
cavity, they must be fashioned so that they are
in harmony with normal neuromuscular function.

 Failure to recognize the cardinal
importance of tooth position and flange
form and contour often results in dentures
which are unstable and unsatisfactory,
even though they were skillfully designed
and expertly constructed.

 When all of the natural teeth have been
lost, there exists within the oral cavity a
void which is the potential denture space .

 The neutral zone is
that area in the
potential denture
space where the
forces of the tongue
pressing outward are
neutralized by forces
of the cheeks and lips
pressing inward.

Definition
 NEUTRAL ZONE (GPT 8) : the potential space between
the lips and 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
 Acc to Beresin & Schiesser :The neutral zone is that
area in the mouth where, during function, the forces of
the tongue pressing outward are neutralized by the
forces of the cheeks and lips pressing inward.

Pound ,JPD,1954,4,6
 pointed out that in advanced resorption the
maxillary ridge moves lingually and the
mandibular ridge moves buccally.
 He stressed upon the importance of
polished surface and the need for healthy
state of ridges.
 He also stated that “tooth over the ridge
is the greatest fallacy.

Merkeley ,JPD,1959,9,567
 During contraction ,
the horizontal fibers of
the buccinator muscle
press against the
buccal surface of the
molars on the line
level with their
occlusal surfaces.
0
10
20
30
40
50
60
70
80
90
1st
Qtr
2nd
Qtr
3rd
Qtr
4th
Qtr
East
West
North

 Similar directed pressure is exerted by the
modiolus in the region of bicuspids .
 Upper 3rd of the buccal surface of the
lower bicuspids & molars are ground to
slope lingually .

Russell ,JPD , 1959, 9,180
 He showed to register the dynamics of the
musculature in a direct wax pattern .
 Stone index of this wax pattern was made.
 Lower denture is designed to fit within the
index .
 Opposing upper denture is made to
conform to the lower denture

Lundquist ,JPD,1959,9,44
 The buccinator muscle is effective
bilaterally as an aid in denture retention
and stabilization only if the patient is a
bilateral chewer .
 Alteration of the buccal contour of the
denture did not show any appreciable
variation in the electromyographic
recording

Freehette, JPD ,1961,11,1032
 There was an accompanying decrease in the
number of positive pressure stroke on the
balancing side during unilateral chewing on
either side.
 This emphasizes again the importance of
bilateral chewing as a means of augmenting the
stability of complete denture.

Shanahan ,JPD, 1962 ,12, 420,
 Patients with advanced resorption of mandibular ridge
need denture made from dynamic impression because
they provide more comfort and stability than denture
made from a static impression.
 Dynamic impression are made by physiologically
extending the denture base into prescribed areas of
extension for retention and then making an impression of
the denture supporting tissues while the various function
of the mouth are performed

Raybin ,JPD,1963,13,263.
 The development of the proper forms of the
polished surface of dentures must be based
upon accurate impressions of the tissue which
limit the extent of the dentures, and the teeth
must be arranged in relation to the ridges so the
slopes of the soft wax pattern for the polished
surface can have a favorable angle with the
cheeks , lips , and tongue .

Schiesser,JPD,1964,14,854
 the lower denture is instable if :-
1. The incisor teeth are set so far labially that the
lip causes the denture to rise
2. It is too wide in bicuspid region
3. The molars encroach on the tongue , and the
buccal and lingual flanges in the molar region
are parallel so that the tongue and buccinator
muscle will not hold them down

Stromberg & Hickey ,JPD,1965,15
,213
 Compared the retention of the denture
formed by the physiologic technique and
manual method .
 They concluded that there was no
significant difference between the
retention of the two forms .

Lott & Levin ,JPD ,1972,16,394
 They proposed the flange technique of
recording the neutral zone and denture
fabrication according to it.

Beresin and Schiesser,JPD
1976,36,357
The greater the ridge loss,
the smaller the denture
base area and the less
influence the impression
surface area will have on
the stability and retention
of the denture.

Razek & Abdalla,JPD,1981,46,484
 They concluded that
 The width of the neutral zone is minimum at the level of
the occusal plane and increases gradually as it goes up
and down
 The width of the neutral zone is also minimum at the
posterior (molar) region and increases gradually toward
the anterior .
 Width of the neutral zone increases as the vertical
dimension of occlusion increases and decreases as the
vertical dimension of occlusion decreases .

Levin ,JPD,1985,45,242
 When ridge is highly resorbed , thicker
lingual borders and incorporation of
sublingual extensions apparently are
advantageous .

Fahmy & Kharat, JPD,1990,64,
459-462
 Evaluated patients’ chewing efficiency and
satisfaction for complete dentures made with
either a conventional or neutral zone technique.
 Patients had better chewing efficiency when
wearing conventional dentures; however,
patients could not detect any difference in
masticatory performance between their
conventionally fabricated dentures and neutral
zone dentures.

 Patients did report greater comfort and
improved speech clarity with the dentures
fabricated using the neutral zone
technique compared with their
conventionally prepared dentures

Wright ,Int J Prosthodont
,1991,4,159-163
 Proper shaping of the polished surface of
a denture is important to facilitate the
intraoral manipulation and control of the
prosthesis by the patient .
 A method for assessment of polished
surface form and tooth position in
processed denture is described .

 Low- viscocity silicone impression material
is placed on the dentures , and the
patient’s chewing movement are used to
define those areas requiring modification

Fahmi , JPD, 1992, 67,805
 The neutral zone in relation to the crest of
the residual ridge was investigated in the
anterior , premolar & molar regions
 The position of the neutral zone in relation
to the alveolar ridge was found to be
highly affected by the period of
edentulousness .

 The longer the edentulousness ,the more
buccally / labially located was the neutral
zone .

Alfano & Leupold, JPD,
2001;85:621-3.
 A technique has been presented that
allows the practitioner to accurately record
the maxillomandibular relationship of a
patient.
 Because the patient functionally molds
the mandibular rim into the area of the
neutral zone, a more stable record base is
created.

Kokubo et al, 2002, JPD, 88,125-7.

G . Nishigawa et al, Journal of Oral
Rehabilitation 2003 ,30, 646
 Using the FEA program developed in this study,
the effect of the bucco-lingual position of the
artificial posterior teeth under occlusal force on
the denture supporting bone of the edentulous
patient can be examined
 it was revealed that the difference of the loading
point of the occlusal force affected the buccal
side of the alveolar bone of maxilla rather than
the palatal side of the alveolar bone

Makzoume,JPD,2004,92,563-8.
 Findings indicated that the location of the neutral
zone was not the same with the
swallowing/modeling plastic impression
compound technique and the phonation/tissue
conditioner technique.
 However, statistical significance does not
necessarily imply clinical significance, and the
results yielded by these 2 techniques may be
clinically acceptable.

 In general, the PNZ technique resulted in
impressions where the neutral zone
appeared to be narrower as the buccal
surface was located more lingual
compared to the SNZ technique.

Gahan & Walmsley
BDJ,2005,198,269
 Most effective for a highly atrophic ridge —
aids retention and stability.
 Uses muscle function to produce the
impression.
 Defines polished surfaces and tooth
position.

 Requires an extra clinical stage.
 Requires good communication with your
technician.
 Produces a lower denture that is in muscle
balance and in harmony with the
surrounding structures.

DENTURE SPACE
 Various structures of the oral cavity
change considerably when a patient
becomes edentulous.
 The lips and cheeks are no longer
supported by the teeth and bone, and
therefore show a tendency to "fall" into the
oral cavity.

 At the same time, the tongue will expand
into the space formerly occupied by the
teeth. In this way, characteristic spaces
develop in the oral cavity of the edentulous
patient forming the so called DENTURE
SPACE

DISLOCATING
MUSCLES
FIXING MUSCLES
VESTIBULAR
 Masseter
 Mentalis
 Depressor labii
inferioris
Buccinator
Orbicularis oris
LINGUAL
 Internal pterygoid
 Palatoglossus
 Mylohyoid
Muscles of tongue

Anatomy
 Muscles of mastication
 Masseter
Temporalis
Lateral pterygoid
 Medial pterygoid

 The posterior extension of the inferior buccal part of the
denture space is determined by the action of masseter muscle.*
 If masseter is relaxed while recording the impression, the
denture will tend to displace when muscle contracts as the
tissues covering the masseter muscle are displaced anteriorly.*
 Has no influence on neutral zone*
*Brill et al : JPD 1965,15,404
*Beresin VE, Schiesser FJ. The neutral zone in complete and
partial dentures.2nd ed. St Louis: Mosby; 1978. p. 15.

Significance
 Just as contraction of the
masseter muscle
determines the extension
of denture in lower
,posterior ,and buccal
part of the denture space
, the internal pterygoid
muscle determines the
extension of a denture in
the lower posterior lingual
part of the denture space.

Significance:
Cheeks are pressed against the
dental arches when buccinator
contracts. During chewing and
swallowing the muscle
rhythmically contracts with
muscles of mastication. It
assists in placing the food
between the teeth and returning
the food to occlusal table which
has escaped into the vestibule.*

 Any overextension will interfere with the
buccinator in function and result in either
displacement of the denture or pain from the
traumatized mucosa.

Significance
 Origin of the mentalis is located
closer to the crest of the residual
ridge than the mucosal reflection
in the alveololabial sulcus. The
bottom of the sulcus is lifted when
the muscle contracts thereby
reducing the depth and the space
of the oral vestibule can be
decreased considerable

 The denture must be
relieved over, and
contoured around them.
Extensions beyond their
crest will interfere with
the mentalis muscle
movement and lead to
denture instability

 Lammie* has shown that as
the alveolar ridge resorbs, the
ridge crest falls below the
origin of the mentalis muscle.
 As a result,the muscle
attachment folds over the
alveolar ridge and comes to
rest on the superior surface of
the ridge crest.
 The result is a posterior
positioning of the neutral zone
and, with it, the need to place
the lower anterior teeth further
lingually than had been the
position of the natural teeth
*Lammie GA. JPD,
1956;6:450-64.

Significance:
 It has the same characteristic course as the mentalis
i.e it originates near to the crest of the ridge and
extends down and below the alveololabial sulcus. It’s
contraction can reduce the denture space.*
 In action it pulls the modioli forward and tenses the
buccinator thereby applying pressure on the polished
surface.*
*Lott & Levin ,JPD ,1972,16,394

Significance
 It is active when the lips are pressed against
the teeth. Like buccinator, orbicularis oris
rhythmically contracts during chewing,
sucking and swallowing

Modiolus
 Orbicularis oris
 Zygomaticus major
 Levator labii superioris
 Levator aguli oris
 Buccinator
 Triangularis
 Risorius
 Mentalis

 Modiolus – “hub of wheel” in latin
 Forms a distinct conical
prominence in corner of mouth
 Situated at the corner of the
mouth it is in a strategic position
to unseat the lower denture and
sometimes the upper denture too.
This may occur if the arch form is
too wide and restricts the
movement of the modiolus.

 The modiolus is fixed
every time the
buccinator muscle
contracts ,which is a
natural
accomplishment of all
chewing efforts

 The contraction of the modiolus presses
the corner of the mouth against the
premolars so the occlusal table is closed
from in front .

 Modiolus is one of the
approaches to occlusal
plane determination when
it is related to the tongue
function and its relation to
the occlusal plane and
mandibular denture
stability

 Muscles form a V-
shaped strap that
press against the
bicuspid region .
 Denture will be
unstable when
bicuspid region is too
wide .
Scheisser ,JPD,1964,14, 854

 The lower denture
should be narrower in
the bicuspid region .

 Modiolus can mold a soft material on the
occlusal rim to correctly establish the shape
and the anteroposterior position of the arch
form of the anterior part thus establishing the
buccal limit to which the bicuspids must be
restricted.

Quadratus labii superioris
Levator anguli oris
Zygomaticus major
Risorius
Triangularis

Tongue
 Powerful and extremely adaptable
 It has two set of muscles: intrinsic and extrinsic
 They consist of the extrinsic and intrinsic groups of
muscles. The extrinsic muscles have their origin
external to the tongue, but their course terminates
within it. Their contraction causes the tongue to move
in relation to other oral structures. The intrinsic
muscles lie completely within the tongue, and their
activities sustain or alter tongue form.

Genioglossus
On the lingual side of the
mandible, also in the
midline, the insertion of the
genioglossus into the
superior genial tubercle can
appear surprisingly large
especially if resorption takes
place .

 A retracted tongue exposes the
floor and compromises denture
retention by losing the border
seal.
 A narrow dental arch
encroaches upon the tongue,
which can no longer occupy it’s
rest position and tends to push
the lower denture out.
 Occlusal plane placed at the
level of tongue helps stabilize
the denture and guide the food
on to it.

PALATOGLOSSUS MUSCLE
 The posterior lingual
part of the denture space
is further influenced by
the palatoglossus muscle.
 The mucosa covering the
lower part of the muscle
is lifted superiorly,
anteriorly, and medially.
 When the muscle
contracts , the terminal
part of the alveolingual
sulcus will be included in
this movement

It is essential that any
extension integrates with the
direction of insertion of the
mylohyoid muscle and is
inclined downwards and
medially at an angle of
approximately 45° to the
sagittal plane occupying the
cleft between mylohyoid
and hyoglossus muscles.

Freni
Their importance lies in the
fact that the denture
periphery must be relieved
around them otherwise pain
and ulceration follow.
In carrying this out the
operator must be aware that
the ‘notching’ of the base
that results can cause
structural weakness.

Maxilla and mandible
 Neither alveolar ridge
resorbs uniformly.
 Mandibular residual
alveolar ridges tend to
resorb more from the
lingual while maxillary
residual alveolar
ridges resorb more
from the buccal.

 Usually, the longer a patient is
edentulous, the greater is this interridge
facial/lingual and facial/palatal dimensional
disparity.

Lips and modioli
 Collapse of upper lip
 Reduced prominence of
philtrum and vermilion
border
 Drooping of corners of
mouth
 Modioli becomes
sagging, less active,
shape changes

Muscle attachment
 As the mandibular ridge resorbs the crest falls below
the level of the mentalis. As a result mentalis tends to
fold over and rests on the ridge.It pushes the neutral
zone posteriorly.
 The freni occupy a more superior position on the
ridge
 In some cases the buccinator attachment over the
external oblique ridge is lost after severe resorption .

Tongue
 In addition, it is estimated that tongue size increases
by approximately 10% in the edentulous patient.
 This lingual increase contributes further to the
confusion about optimum tooth placement, under the
dislodging forces.
Wright , JPD ,1961,5,629

 The position of the neutral zone in relation
to the alveolar ridge was found to be
highly affected by the period of
edentulousness .
 The longer the edentulousness ,the more
buccally / labially located was the neutral
zone
Fahmi , JPD, 1992, 67,805

 Since residual alveolar ridges are spatially
changing in a tight spaced functioning
stomatognathic apparatus, it appears prudent to
build prostheses that fit into current functional
spaces and use local forces to enhance
prosthetic function, stabilization and retention.

Denture surfaces
Sir wilfred fish described (1948) three
surfaces of a denture
 Impression surface
 Occlusal surface
 Polished surface

 With the neutral-zone concept, the
impression surface is called the ‘‘base’’
and the polished surface is called the
‘‘body’’ of the denture

Impression surface
That part of the denture in contact with the
tissues and on which the denture rests. The
retention of the denture depends on physical
forces developed by adhesion and atmospheric
pressure.

Occlusal surface
 It is that area in contact with the teeth, either natural
or artificial of the opposite side. The forces develop
by the muscles of mastication are received and
directed by the occlusal surface.
 The stability of the denture in occlusion is determined
by the fit of the impression surface against the tissues
and occlusal surface against each other.

Polished surface
This surface is constantly in contact with the cheek,
tongue and lips. With Residual Ridge Resorption the
impression surface decreases in size and the polished
surface becomes more critical for stability and
retention

 Since most of the time jaws are at rest stability is more
dependent on forces on external surface as transmitted to
impression surface . The only time teeth make contact is
during mastication and swallowing. In order to construct
dentures that function properly not only in chewing but also in
speaking and swallowing , one must develop the fit and
contour of the external surface as accurately as that of
impression surface and occlusal surface

ACTIVE MUSCULAR FIXATION
 When forces of the tongue are directed against a
lower denture, the denture will be easily dislocated if the
forces are not counteracted by equal forces exerted by
the musculature of the cheeks and lower lip. Their
antagonistic activity can be used to stabilize dentures.
 Similar action of antagonistic muscle groups
between the functioning genioglossus and orbicularis
oris muscles will fix a lower denture by opposing forces
on its anterior section

PASSIVE MUSCULAR FIXATION
 It is possible to fix a lower denture even if all the
muscles of the cheeks, lower lip, and tongue are
quite passive. The denture is fixed by the mass
and weight of these structures and through the
pressure exerted by muscle tonus.
 Two factors that help in this are
 1. The inclination of the polished surfaces.
 2. The position of the polished surfaces of the
denture between the cheeks and the lower lip on
the one side and the tongue on the other side.

INCLINATION OF POLISHED
SURFACES
 The basic geometric
design of denture base
should be triangular with
the apexes corresponding
to the occlusal surface.
The maxillary buccal
flange should incline
laterally and superiorly.

 The buccal flanges of the lower denture
must slope inferiorly and laterally, and the
borders must be extended out beneath a
fold of the buccinator muscle in the molar
region.

The lingual flanges must also definitely extend inferiorly
and medially below the anterior and lateral parts of the
tongue, and as far posteriorly as permitted by the range
of action of the tongue and the internal pterygoid muscle.
Such inclination will provide a favorable vertical
component to any horizontally directed forces.

Narrow artificial teeth permit the polished
surfaces to be automatically formed with
favorable inclined planes that can be wedged
below the tongue, lower lip, and cheeks. In this
way, these structures are brought to rest on the
polished surfaces, and their weight will force the
denture to remain on its foundation.

POSITION OF POLISHED SURFACES
 When a denture is
held in place by the
cheeks and lips, the
tongue should be
displaced with a
pressure equal to the
one with which the
cheeks and lips are
displaced.

 The denture must be placed in the denture space so
that there is an equilibrium between the inward pressure
exerted by the lips and the cheeks and the outward
pressure exerted by the tongue.
 The denture space has been given different names
because of this possible equilibrium. Some of these are:
the dead space,
the stable zone,
the neutral zone, and
the zone of minimal conflict.

 The of theory neutralization of forces that
stabilize dentures was made by Dr.
Russell Tench

 But the greatest controversy lies in
the arrangement of teeth. This
concept does not advocate
placement of teeth on the ridge.
Rather it is most of the time buccal
or labial to it. According to pound
“tooth over the ridge concept is a
fallacy”. *
*Pound ,JPD,1954,4,6

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