Virag Srivastava et al

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REVIEW ARTICLE
The Neutral Zone: Concept and Technique
Virag Srivastava, NK Gupta, Amrit Tandan, Laxman Singh Kaira, Devendra Chopra
ABSTRACT
When all of the remaining natural teeth are removed; there
exists within the oral cavity a void that may be called the potential
denture space. The denture space is bounded by the tongue,
medially or internally, and by the muscles and tissues of the
lips and cheeks laterally or externally. Within the denture space,
there is an area that has been termed the neutral zone.
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.
Since these forces are developed through muscles contraction
during the various functions of chewing, swallowing and
speaking, they vary in magnitude and direction in different
individuals and in different periods of life. The way these forces
are directed against the dentures will either help to stabilize
them or will tend to dislodge them.
In summary, the neutral zone philosophy is based on the
concept that for each individual patient, there exists within the
denture space a specific area where the function of the
musculature will not unseat the denture and at the same time,
where the forces generated by the tongue are neutralized by
the forces generated by the lips and cheeks. Furthermore,
denture stability is as much or more influenced by tooth position
and flange contour as by any other factors.
Keywords: Denture space, Stability, Neutral zone.
How to cite this article: Srivastava V, Gupta NK, Tandan A,
Kaira LS, Chopra D. The Neutral Zone: Concept and Technique.
J Orofac Res 2012;2(1):42-47.
Source of support: Nil
Conflict of interest: None declared
INTRODUCTION
The goal of dentistry is for patients to keep all of their teeth
throughout their lives in health and comfort. If the teeth are
lost despite all efforts to save them, a reestablishment should
be made in such a manner as to function efficiently and
comfortably in harmony with the muscles of the
stomatognathic system and the temporomandibular joints.
The lower denture commonly presents the most
difficulties with pain and looseness being the most common
complaint.
1
This is because the mandible atrophies at a
greater rate than the maxilla and has less residual ridge for
retention and support.
2
The neutral zone technique is most
effective for patients who have had numerous unstable,
unretentive lower complete dentures. These patients usually
have a highly atrophic mandible and there has been difficulty
in positioning the teeth to produce a stable denture.
3
REVIEW OF LITERATURE
Various theories have been put forward to enhance stability
of mandibular denture. Majority of literature support that
posterior denture teeth should be arranged to occupy the
position of their natural tooth predecessors
4-8
or posterior
denture teeth should be arranged directly over the crest of
the edentulous ridge.
9-13
Weinberg
14
suggested that buccal cusps and central
fossae of mandibular posterior denture teeth should be
arranged directly over the crest of the edentulous residual
ridge.
Pound
15,16
recommended that the lingual surfaces of
mandibular posterior denture teeth should occupy an area
bounded by two lines originating from the mesial surface
of the mandibular canine and extending posteriorly to the
lingual and buccal aspects of the retromolar pad.
Lammie
17
suggested that mandibular posterior denture
teeth should be arranged over the buccal shelf to provide
increased tongue space and to facilitate the development of
vertical facial polished surfaces, against which an effective
facial seal can be achieved and maintained.
Wright et al
18
believed that posterior mandibular denture
teeth should be arranged directly over the center of the
denture stress-bearing area. This location may not correlate
with the crest of the edentulous ridge, particularly in the
presence of severe ridge atrophy.
Campbell
19
stated that mandibular posterior denture teeth
should be arranged slightly lingual to the crest of the
edentulous ridge, while the maxillary posterior denture teeth
should be arranged slightly buccal to the edentulous ridge.
Neutral zone is defined as 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.
20
It is also known as dead
zone
21
, stable zone
22
, zone of minimal conflict
22
, zone of
equilibrium
23
, zone of least interference
24
, biometric denture
space
25
, denture space
26
and potential denture space (Fig. 1).
27
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. Since these forces are developed through muscles
contraction during the various functions of chewing,
swallowing and speaking, they vary in magnitude and
direction in different individuals and in different periods of
life.
28
The way these forces are directed against the dentures
will either help to stabilize them or will tend to dislodge
them.
The more ridge loss, the less area of the denture base
and the less influence impression surface area will have on
the stability and retention of the denture. As the surface
10.5005/jp-journals-10026-1010
The Neutral Zone: Concept and Technique
Journal of Orofacial Research, January-March 2012;2(1):42-47
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JOFR
area of the impression surface decreases and the external
surface area increases, the development and contour of the
external surface become more critical.
The polished surfaces of the denture must exhibit a series
of inclined planes in relation to the muscles of the tongue
and cheeks. The palatal surface of the upper denture looks
inward and downward, while the lingual surface of the lower
denture looks inward and upward.
29
The forces on the external surface are constantly
changing in magnitude and direction during swallowing,
speaking and mastication. It is only when the mouth is
completely at rest that the forces are constant.
If a persons teeth were in contact all the time, the
external surface would be relatively unimportant in denture
stability. Conversely, if a person never brought his teeth
into contact, the occlusal surface would be relatively
unimportant and the stability would be dependent on the
forces on the external surface as transmitted to the
impression surface.
The only time teeth are in contact is during mastication
and swallowing. This means that the patient will only make
tooth contact during normal function. But the lips, cheeks
and tongue are constantly in function. This stresses the
significance of the horizontal forces exerted by the lips,
cheeks and tongue.
It seems reasonable that when the dentures are made,
the artificial teeth should be placed in the same relative
position to the musculature as the natural teeth. The term
relative position rather than exact position is used
because age, tonus, ridge resorption and other factors may
modify or alter the denture space and neutral zone so that
the artificial teeth should not necessarily be in the exact
same position as the natural teeth.
Fahmi
30
investigated neutral zone in relation to the crest
of the residual ridge in the anterior, premolar and molar
regions. He found that 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 period of
edentulousness, the more buccally or labially located was
the neutral zone.
CLINICAL AND LABORATORY PROCEDURES
After a thorough examination of the patient, stock trays are
selected for the patient. Primary impressions of the maxillary
and mandibular ridges are made with modelling compound.
While making the impressions, the patient is asked to do the
functional movements, including sucking and swallowing
movements. There should be minimal displacement of
tissues, while making impressions. After acceptable
impressions are made, they are poured in plaster.
Construction of the Acrylic Base
The acrylic base to be made in the neutral zone approach in
complete dentures has to serve two purposes. On this acrylic
base, occlusion rims will be constructed that will be formed
and molded by the patient to locate the individuals neutral
zone and to establish a tentative vertical dimension and
centric relation. Its second purpose is to serve as a tray for
the final impression which is obtained by a closed mouth
procedure. It is, therefore, extremely important that the
primary impression be carefully and meticulously developed
so that the resultant tray is accurate and stable.
Manipulation of Compound
To develop the body of the denture and register the neutral
zone by the use of modeling compound, there are three
important factors to be considered. First, the compound
must be very securely attached to the tray. Second, the
compound must be thoroughly and uniformly softened for
the muscles to mold the material. Third, it must be hard
enough so that it will not flow and will maintain its shape
as an occlusion rim until inserted into the mouth for forming
the neutral zone. The consistency of the compound should
be similar to that used when making a primary impression.
Instead of impression compound, low fusing green sticky
wax or the mixture of two
31,32
can be used to record the neutral
zone depending on operators choice to modify the properties.
A water bath, preheated to the adequate temperature, is
used to soften the material. It is then kneaded and rolled
into U shaped and adapted on the temporary denture base.
A Hanau torch can be used to heat and sear the compound
so that it will completely adhere to the tray. The compound
is tempered in the water bath. This keeps the compound
soft so that it can be molded. Repeated flaming, tempering
and shaping keep the compound soft, while it is shaped into
the form of an occlusion rim.
Fig. 1: Potential denture space
Virag Srivastava et al
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Locating the Neutral Zone
To locate the neutral zone for the lower arch, the patients
lips are lubricated with petroleum jelly. The tray with the
softened modeling compound is rotated and carefully seated.
Care must be used so that the lips do not press against the
compound until it is completely seated. The patient is
instructed to swallow and then purse the lips as in sucking.
Most patients will swallow without any problem. However,
some patients will have difficulty in swallowing correctly.
With these patients, it is necessary to practice before they
can deliver a proper swallow on command. The practice or
instructional procedure is as follows:
1. Remove the compound rim from the mouth and instruct
the patient to swallow. If the patient swallows correctly
several times, explain that this is exactly what should
be done when the material is put into the mouth. If the
patient does not swallow correctly, instruct the patient
to keep the lips together and swallow as if swallowing a
bolus of food.
2. If difficulty still persists, place a few drops of water on
the tongue by means of a syringe to help the patient
swallow.
3. Once the patient has swallowed correctly several times,
resoften the compound and proceed in locating the
neutral zone.
It is important to instruct the patient to keep the lips
together and swallow. We should not tell the patient to close
and swallow. By doing so, the patient may overclose and
press the compound into the maxillary ridge, distorting the
compound. If there are repeated impressions of the maxillary
ridge onto the compound, the patient is either overclosing
during swallowing or too much compound has been used.
Proper swallowing actions will mold the compound rim into
the neutral zone. Sufficient time is allowed for the compound
to harden and it is then removed from the mouth and
inspected.
If initially an excessive amount of compound is used, it
will be forced upward above the normal height of the
occlusal plane and because of excessive bulk of compound,
the tongue, lips and cheeks will be unable to mold the
compound into a neutral zone of proper width. Therefore,
any excess compound above the usual height of the occlusal
plane is removed with a sharp knife and the compound is
resoftened, placed back in the mouth and the patient is
instructed to suck and swallow. If additional compound has
been pushed up, it should be reduced and the procedure
repeated until the functions of swallowing and sucking no
longer force the compound to an excessive height.
In all cases, the compound will exhibit similar shapes
and contours, but there will be definite differences for each
patient. The lingual surface of the compound rim will be
shaped to the contour necessary to avoid interference with
functional tongue movements. The anterior segment of the
compound rim may have a labial, straight or lingual
inclination depending on the tonus of the muscles in the
lower lip and also the action of the tongue during
swallowing. The buccal surface will generally be inclined
to the lingual with a narrowing in the bicuspid area where
the modiolus functions. The lingual surface will be inclined
to the buccal.
Testing the Stability of Occlusion Rims
The occlusion rims are placed back into the patients mouth
and checked for stability by having the patient open wide,
wet the lips with the tongue, count from 1 to 100, and say
exaggerated oh ahs and ees. If these movements raise
the rim, the lack of stability must be caused by an improper
molding of the compound, as the tray or base was proved to
be stable. So, the lack of stability is because of the compound
on the base rather than the base itself. In such cases, the
compound is resoftened and the procedure is repeated until
a stable rim is achieved.
The next procedure is to test the outer edge of the rim
with the tip of the index finger in the bicuspid and incisor
regions. If pressure on the outer edges causes the opposite
side of the rim to lift up, then the rim must be narrowed
from the labial or buccal to where the vertical pressure will
not cause the rim to tilt. This will occur where there has
been extensive ridge resorption and where the residual ridge
is narrow buccolingually and labiolingually. If this is not
corrected and the teeth placed at this position, then the
vertical forces as in mastication will tilt the denture.
After the labial contour and curvature of the occlusion
rim have been established and if the width of the anterior
section is thicker than the incisal edges of the anterior teeth,
the occlusion rim should be narrowed by trimming from
the lingual.
The final test is to have the patient speak, swallow, wet
the lips and open wide without the rim moving or being
dislodged. We have therefore created a tray or base that is
not dislodged by muscle function and have placed on it a
body that is also not displaced by muscle function.
Final Impressions
To achieve optimum success in complete denture
prosthesis, the dentures should be both retentive and stable.
The retention of a denture is mainly dependent on the
accuracy of the impression and fit of the denture base to
the tissues.
The Neutral Zone: Concept and Technique
Journal of Orofacial Research, January-March 2012;2(1):42-47
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JOFR
Impression techniques can be either closed mouth or
open mouth. Both these can be incorporated into the neutral
zone approach, but a closed mouth technique is preferred.
The advantages with the use of a closed mouth technique
are as follows:
A more accurate functional molding of the borders can
be obtained, especially in the lower arch.
By having the patient to close gently and swallow, there
is more even distribution of pressure and impression
material with less likelihood of excessive pressure in
one area or another.
Fabrication of Tongue, Lip and Cheek Matrices
Prior to construction of the matrices, the casts must be
indexed so that the matrices will fit back into their proper
positions. Several circular holes are made on the labial and
buccal surfaces of the cast and a cross is made in the tongue
area of the lower model.
With the lower occlusion rim in place, the lower model
is lubricated and stone is placed on the lingual portion of
the model, forming an artificial tongue and, on the labial
and buccal of the lower model, completely encasing the
occlusion rim. These matrices are trimmed to the exact
height of the lower occlusal plane, which was established
in the mouth. This preserves the height of the lower occlusal
plane. The same is done for the upper model and occlusion
rim.
After the stone is set, the labial and buccal matrices are
split in the middle to facilitate removal. When two occlusion
rims are now removed, the matrices can be placed back into
position. Instead of stone, rubber base putty can also be
used to form the matrices
26
(Fig. 2).
The space between the matrices on the lower rim
represents the neutral zone and indicates where the teeth
should be positioned. The matrices on the upper indicate
the outer limits of the neutral zone and serve as a guide for
positioning the upper anterior teeth (Fig. 3).
After selection of the proper size, occlusal morphology
and material of the posterior teeth to be used, we go in for
the positioning or arrangement of teeth. The following are
a step by step sequence for arrangement of anterior and
posterior teeth:
1. The lower anterior teeth are set to the height of the labial
matrix and to the labial limit of the neutral zone.
2. The upper anterior teeth are set against the labial limits
of the upper matrix.
3. The lower posterior teeth are set against the tongue
matrix and against the template occlusally.
4. The upper posterior teeth are set to the buccal limits of
the neutral zone.
5. The upper posterior teeth will have to be rearranged
to assure maximum contact with the lower posterior
teeth.
6. The upper and lower posterior teeth are checked for the
buccal and lingual relationship to each other.
7. In order to avoid an edge to edge relationship which
might lead to check biting, the lower posterior teeth may
be moved buccally within the neutral zone, resulting in
a crossbite relationship.
The Trial Denture
After the verification and correction of the stability,
retention, vertical dimension, phonetics, centric relation and
esthetics are done. There is an important step to be completed
during the trial denture tryin is the making of external
impressions on the labial, buccal and lingual surfaces of
the dentures. These will determine the thickness, contours
and shape of the polished surfaces of the denture. With
conventional complete dentures, the technician or dentist
Fig. 2: Construction of matrices
Fig. 3: Potential denture space
Virag Srivastava et al
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JAYPEE
empirically waxes the external surface. By means of external
impressions, a physiologic molding is made so that the
external surfaces are functionally compatible with muscle
action.
The trial denture acts as a tray to be used for the accurate
secondary impression for forming the contours of the
external surface of the denture. The trial dentures are waxed
up so that there is just enough wax to hold the tooth in
position. The materials for the external impressions are either
zinc oxide eugenol, one of the conditioning materials or
light body addition silicone impression material.
Both the upper and lower trial denture external
impressions are done in two steps, either the labiobuccal
and than the lingual or palatal, or vice versa.
The impression material is placed on the lingual surfaces
of the lower denture, between the necks of the teeth and the
denture periphery. The upper trial denture is placed in
position, and the lower is then rotated into the mouth, taking
care not to wipe off any material on the lips. With the lower
trial denture in position, the patient is asked to close, purse
the lips as in sucking and swallow. This is repeated several
times. After the material has set, the trial dentures are
removed from the mouth, and the gross excess is cut away.
The impression material is then placed on the buccal and
labial surfaces of the lower trial denture, and sucking and
swallowing motions are repeated (Fig. 4). The same
procedure is then followed for the upper external
impressions.
The impression on the lingual of the lower trial denture
will frequently result in a very large and extensive ledge in
the anterior region. This should be duplicated exactly in the
processed denture. Experience has shown that practically
all patients do tolerate these contours which rarely have to
be reduced. As a matter of fact, these ledges seem to help to
retain the lower denture. The tongue sits on these ledges
and helps to keep the lower denture in position.
Another important reason for using this procedure is that
it tends to minimise the accumulation of food on the external
surface of the denture. With the use of external impressions
proper contours, which eliminate or minimize food
accumulation, are developed. The external impression tends
to fill out the denture space, thus making it easier for the
cheek to push the food back onto the occlusal surfaces of
the teeth. Finally, by duplicating these impressions in the
final denture, the operator has reproduced functionally
contoured external surfaces of the denture that will aid
immeasurably in the retention and stability of the dentures.
Processing of the Dentures
The laboratory procedures for investing, packing and
processing of the dentures using the neutral zone technique
are generally the same as for conventional dentures.
However, because of the materials used for the external
impressions, it is necessary to be especially careful in some
of the procedures.
When zinc oxide eugenol paste has been used for taking
the impression of the polished surface, the flasks should
not be allowed to remain in the boil-out tank for more than
5-minute. Reason being the zinc oxide eugenol paste gets
liquefied, if left for a longer time. This results in bleached
appearance of the processed denture.
After the dentures are being processed, they are mounted
on the articulator. Occlusal discrepancies are checked for
with the template and carbon paper. They are corrected, the
dentures are finished, polished and insertion is done.
CONCLUSION
The neutral zone philosophy is based on the concept that
for each individual patient there exists within, the denture
space, a specific area where the function of the musculature
will not unseat the denture and, at the same time, where the
forces generated by the tongue are neutralized by the forces
generated by the lips and cheeks.
In other words, we should not be dogmatic and insist
that the teeth should always be placed over the crest of the
ridge, or lingual to the ridge or buccal to the ridge. Placement
of the teeth should be dictated by the musculature and will
vary for different patients.
The neutral zone has not been given enough importance,
in the literature but as a determinant of occlusion, it cannot
be ignored. Complete and partial denture failures are often
related to noncompliance with neutral zone factors.
Regardless of the method of treatment, any part of the
dentition out of harmony with the neutral zone will result in
instability, interference with function or some degree of
discomfort to the patient. Thus, the neutral zone must be Fig. 4: External impression of polished surface
The Neutral Zone: Concept and Technique
Journal of Orofacial Research, January-March 2012;2(1):42-47
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JOFR
considered as an important factor while rehabilitating the
edentulous patients. The operator should try to neutralize
forces acting on complete dentures, which will make the
prostheses more functionally physiologically and
psychologically acceptable to the patient.
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ABOUT THE AUTHORS
Virag Srivastava
Senior Lecturer, Department of Prosthodontics, Institute of Dental
Sciences, Bareilly, Uttar Pradesh, India
NK Gupta
Professor, Department of Prosthodontics, BBD College of Dental
Sciences, Lucknow, Uttar Pradesh, India
Amrit Tandan
Professor, Department of Prosthodontics, BBD College of Dental
Sciences, Lucknow, Uttar Pradesh, India
Laxman Singh Kaira
Senior Lecturer, Department of Prosthodontics, Institute of Dental
Sciences, Bareilly, Uttar Pradesh, India
Devendra Chopra
Senior Lecturer, Department of Prosthodontics, Institute of Dental
Sciences, Bareilly, Uttar Pradesh, India
CORRESPONDING AUTHOR
Virag Srivastava, Senior Lecturer, Department of Prostho-
dontics Faculty Residence No. 94, Rohilkhand Medical College
Campus Bareilly, Uttar Pradesh, India, Phone: +91-8808022222
e-mail: drvirag@gmail.com