1. Neutral Zone in Complete Dentures:
Systematic Analysis of Evidence and Technique
• Ahmad A. Jum’ah, BDS(Hons), MSc/PhD (Clin) Student-Second year
Restorative Dentistry Department, Leeds Dental Institute, University of Leeds, UK
dnaahj@leeds.ac.uk
• Peter J. Nixon, Senior Consultant in Restorative Dentistry, Leeds Dental Hospital,
Leeds Teaching Hospitals Trust (LTHT), England, UK
Abstract
Neutral zone technique is a physiologic and functional approach that is widely and concisely described as a treatment
modality for unstable lower complete denture cases. It serves as a guide of where to set teeth and how to contour the
polished surface of the denture to ensure optimal stability, retention, facial support and aesthetics. In patients with
compromised support and poor denture adaptability, this technique is considered as a valuable tool in the prosthodontist’s
armoury especially where dental implants are contraindicated or unfeasible. The aim of this article is to describe the concept
and technique of neutral zone, discuss rationale, indications and to evaluate this technique from evidence-based perspective.
Abbreviations: NZ: Neutral zone, CD: complete denture, VDO: Vertical dimension at occlusion.
Introduction Analysis of functional forces
Stability of lower CDs is well recognized as a potentially Understanding the unique and synergistic interplay
difficult treatment aim to achieve. Looseness and discomfort and complex movements of muscles of cheeks, lips
are the most frequent complaints reported by patients and and tongue is the first step in construction of lower
they are quite often difficult to manage by dentists. CD that is stabilized rather than being dislodged by
movements of these structures.11,12 Description of forces
Neuromuscular control is said to be the key determinant applied to the lower CD purely on the basis of direction
of stability of lower CD as the area available for support is is an oversimplification, yet, it is quite useful for better
far less than maxillary support area. Size and position of understanding of the concept.12
prosthetic teeth and the contours of polished surface have
a crucial role in lower CD stability as they are subjected to The outward forces are principally generated by the
destabilizing forces from the tongue, lips and cheeks if they tongue and lingual frenum into which, genioglossus
are placed in hindrance with function of these structures.1 muscle is inserted. Teeth should be set and flanges should
be contoured in harmony with tongue size, position and
Throughout time, many concepts and theories emerged shape during rest and function. In rest position, the tongue
to describe where prosthetic teeth of CD should rests on lingual cusps of posterior teeth and lingual
be positioned. Some of them adopted mechanical flanges posteriorly and anteriorly. The tongue space
principles,2,3 others used biometric guides4 and a minority determined by position of teeth is far more important
advocated mathematical formulas based on natural teeth during function. Setting teeth too lingualy will encroach
position and dimensions.5 These dogmatic or arbitrary on this space and the tongue tends to dislodge denture
approaches have been challenged and found insufficient, in function. The height of posterior teeth is of a great
in fact not only by rigorous research, but also by failure importance in stability of lower CD as well. Having the
to restore function, aesthetic and comfort in patients with tongue resting on lingual cusps will reduce the horizontal
severely atrophic mandibular ridges (Class V Atwood’s6), (outward) force and apply force with vertical (downward)
patients with enlarged tongue and cases of marginal or component which enhances stability and retention.11
segmental mandibulectomy. To overcome such problem,
the neutral zone technique was advocated. Inward forces are generated by cheeks resulting from
contraction of the buccinator muscle that pushes food
The neutral zone, zone of minimal conflict,7 zone of bullous on top of occlusal surfaces of posterior teeth.
equilibrium,8 potential denture space9 and the dead Flanges contoured and teeth set too buccal are at
space10 are all terms used to describe the potential area increased risk of being dislodged by the action of this
where forces generated in an outward direction from the muscle. Anteriorly, lip muscles (mentalis and orbicularis
tongue are being neutralized or balanced by the inward oris) are the source of inward forces generated during
forces generated by lips and cheeks during functional speaking and swallowing. Contraction of these muscles
activities. Setting teeth and contouring polished surface to attain seal during these activities can destabilize lower
of lower CD within this zone, makes the prosthesis less CD with teeth and flanges placed too far labially. The
subjected to dislodging forces and adds more to stability.11 modiolus is a knot-like structure found in corners of the
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2. mouth where several muscles are inserted. Movement of articulator. In the lab, the lower occlusal rim is removed
this structure narrows the space available for flanges and from baseplate and substituted with a baseplate with
teeth. The modiolus produces quite strong inward forces acrylic pillars29 in the premolar regions and/or wire
in premolar region. Thus, contouring flanges in harmony loops13 on the remaining areas of the baseplate. The
with its’ functional movement is essential.11,12 pillars preserve the VDO recorded in bite registration
stage. It is essential the the pillars are relatively thin
Rationale bucco-lingually and are positioned directly over the
The rationale of using neutral zone technique is to ridge. The base plate is then fitted in the patient’s mouth
fabricate a lower CD that is optimally situated and in and VDO and extensions are checked. Then impression
harmony with the structures and forces discussed above. material such as compound11, plaster22, wax30, silicone31,
By doing so, these forces are more likely to be stabilizing polyether32 or tissue conditioner13,33 is applied to the
rather than unseating.11 The need for such a technique baseplate and retained by the wire loops and/or acrylic
that is based on physiologic concepts is significantly pillars. Before setting of material, patient is asked to
increasing as emergence of several factors (discussed perform functional movement such as, licking lips,
below) render a high proportion of conventionally made swallowing, pronouncing some words or combination
lower CDs unsatisfactory. of these. Care should be taken that the patient should
continue performing functional movements until the full
Increased access to dental care has led to patients losing setting of material; otherwise material might flow back
their teeth at a later stage of life.13 Compounded by and give inaccurate recording of the neutral zone. It is
increased life expectancy, this has led to the majority useful if the chosen material has relatively long working
of CD wearers to be elderly and has increased the time to allow the required movements to be carried out
proportion of those who have poor neuromuscular before the material becomes rigid. Also, it is worthwhile
control, poor adaptive capacity, severely atrophic to mention that it is better to perform the NZ record
ridges14 and atypical denture support area as a result while the upper occlusal rim or finished denture is fitted
of surgical interventions, poor planning for transition in the patient mouth as it may help to control recording
from partially dentate to edentulous state,15 untreated material and prevent it from being displaced in a labio-
edentulism for long period of time,16,17 trauma or occlusal direction.29
systemic diseases. Occasionally, patients with one or
a combination of these conditions can be successfully In the lab, the baseplate carrying recording material is
treated by CD constructed by conventional techniques.11 fitted on the master cast again and VDO is checked. A
putty or plaster index is made around the NZ record.
Indications Placement of three orientation grooves is recommended
• In general, neutral zone technique is indicated when as these help in repositioning the index on the master cast.
stability and patient’s acceptance of lower CD are in Impression material is then removed and replaced
question. Searching the literature, this technique is by wax; the use of the index will make sure that wax
found to be used in the following clinical situations: replicates the neutral zone record. Subsequently, teeth
• Severely atrophic mandibular ridge12,13,18-22 (Atwood’s V). should be set and flanges contoured according to the
• Patients with prominent and highly attached mentalis index that represents NZ.
muscle, lateral spreading of tongue as a result of poor
transition from dentate to edentulous state and severe NZ impression technique has various modifications, not
resorption.13 only in terms of materials used or retention provided by
• Patients with diminished neuromuscular control such as baseplate, but also in terms of the functional movements
those with a history of stroke,13 Parkinson’s disease13,23 performed and refinement of the procedure. A further
or patients with impaired motor innervation to oral and more defined NZ record can also be achieved in try-in
facial muscles as a result of brain surgery.18 stage. The wax below the teeth and covering the flanges
• Patients with atypical shape or consistency of oral can be cut back and tissue conditioning material or
and perioral structures. For example, patients who medium-bodied silicone applied. The patient is asked
have scleroderma,13 marginal21,24 or segmental25,26 again to perform functional movements. The dentures
mandibulectomy and partial glossectomy.27 are processed as usual. The same procedure has also
• NZ technique can be used to locate optimal position
for implants in cases of implant-supported or -retained (Table 1) Materials Used for NZ Impression
overdentures, which enhances the overall outcome of
Impression plaster
treatment.28
Impression waxes
Clinical technique Impression compound
Primary and secondary impressions are taken for Regular bodied silicone
maxillary and mandibular denture bearing areas as in Tissue conditioner
standard complete denture treatment. Bite registration
Polyether
is then performed as in conventional treatment. Master
casts with record blocks should be mounted on an Hard relining material
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3. (Table 2) Summary of clinical and laboratory stages of NZ
technique
Clinic 1: Upper & lower primary impressions using stock trays
Lab1: Casting primary models and construction of special trays
Clinic 2: Upper & lower secondary impressions
Lab 2: Casting master models and construction of record blocks
Clinic 3: Bite registration
Lab 3: Mounting master casts using CR record on semi-adjustable
or average value articulator. Removal of lower wax rim and fabrica-
tion of baseplate for NZ impression (Fig. 2) A: NZ impression taken with silicon. B: Putty index
adapted around master cast
Clinic 4: NZ impression
Lab 4: NZ impression record mounted on lower master cast, orien- ridge in patients who have been edentulous for less than
tation grooves placed on master cast, putty index adapted around two years and significantly differs in those who were
NZ record and impression material removed and poured in wax edentulous for a period more than that.16,17
Finally, setting of teeth completed
Clinic 5: Try-in stage. Afterwards, NZ impression refined by tissue Realizing the importance of the forces generated
conditioner applied to lower try-in denture by various oral structures on the teeth and polished
Lab 5: Processing, finishing and polishing surfaces of CDs and their effect on the stability of CD
Clinic 6: Insertion of finished dentures sheds light on the NZ technique.1,10 It has been shown
that compromised retention, poor stability, phonetic
been described after insertion of the denture but using
problems, inadequate facial support, inefficient
hard relining material.27,31
tongue posture/function and increased gagging are
Discussion all associated with functionally inappropriate setting of
Many approaches to set teeth have been advocated and denture teeth and physiologically inadequate contours
used in complete denture treatment.20 However, there or volume of the denture base.20
is substantial debate on which of these provide optimal
NZ technique has been criticized based on claims that
position in the facio-lingual dimension and guarantee a
it is supported by empirical evidence. However, other
favourable outcome in terms of stability, facial support,
authors maintain that this is inaccurate as NZ technique
chewing efficiency, aesthetics and patient comfort. Some
is based on significant clinical observations on the role
of these approaches utilized biometric measurements and
of destabilizing forces the muscles apply to CDs during
location of relatively stable anatomical landmarks to set
functional movements. Furthermore, the large number of
teeth;4 others relied on difference in resorption patterns
case reports accumulated in a short period of time and
to set denture teeth where their natural predecessors
clinical studies conducted by Stromberg & Hickey36 and
were thought to have been.34 Some authors adopted a
Fahmy & Kharat37 undermine this criticism and add to
mechanical concept and advocated setting teeth directly in
the validity of NZ technique. Stromberg & Hickey36 found
the centre of denture support area where the least amount
better patient adaptability to physiologically formed
of leverage is present which in turn enhances the stability
denture bases when compared to conventional ones.
of lower CD.35 All of these approaches were and are still
Fahmy & Kharat37 found improved comfort and speech
being used and each of them proved to have advantages
clarity reported by patients upon wearing CD fabricated
and disadvantages when compared to others. Furthermore,
using NZ technique when compared to conventional
these approaches seem to work best when used with
CD. Moreover, Barrenas and Odman found less post
patients who have; their oral and peri-oral musculature
insertion problems and better patient acceptance in
unaltered for any reason, adequate neuromuscular control
NZ dentures when compared to conventional ones.38
and acceptable amount of residual ridge for support.
Unfortunately, the proportion of patients with these features (Table 3) Summary of NZ impression clinical technique
is dramatically decreasing and so the NZ concept has
Baseplate with acrylic pillars and/or wire loop is fitted in patient’s
become increasingly significant. These observations are mouth and checked for proper extensions and VDO
strongly supported by studies investigating the effect of
Baseplate is coated by adhesive and loaded with regular bodied
period of edentulism on position of neutral zone. It has silicone impression material
been found that NZ is closely related to the crest of residual
While the patient is setting upright and comfortable the baseplate is
inserted in patient’s mouth
Patient is then asked to swallow few time, moisten lips, use tongue to
clear buccal sulci, smile, grin and purse lips
Before final setting of material, patient is asked to read loudly a
vocal passage
Once set, NZ impression removed and inspected for deficiencies
(Fig. 1) NZ baseplate with which can be corrected by addition of impression material
acrylic pillars and wire loop Impression disinfected and sent to lab
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4. The effect of various functional movements patients
perform during recording NZ on the location and
dimensions of NZ has been investigated by Makzoumi39.
This investigation concluded that NZ recorded whilst
patients perform a phonetic exercise is significantly
narrower when compared with a NZ record produced
during swallowing. This finding may be of a clinical
significance from two perspectives; first, the author used
modelling compound for the swallowing and used tissue
conditioner for phonetic technique which may indicate
that one of these materials is less reliable than the other
in recording NZ. Second, dentures fabricated utilizing
one functional exercise to shape the NZ may be unstable
during other functions. The authors’ preference is to as
patients to perform multiple tasks including swallowing,
using the tongue to moisten lips and finishing with
reading a speech articulation passage loudly.
(Fig. 3) Setting of teeth according to NZ record. Note the class II
arrangement of teeth From biomechanical perspective, NZ technique has
one disadvantage as teeth may be set far from the
Recently, Raja and Saleem19 published results of clinical denture support area. For example, in a case of
trial in which they compared patient acceptance of NZ excessive resorption of the anterior area of the mandible
dentures and conventional dentures in 128 patients. The accompanied by prominent and highly attached mentalis
authors concluded that there is no significant difference muscle, this will shift the NZ more lingually away from
in terms of patient’s acceptance between the two groups the crest of the ridge. This horizontal discrepancy can
as far as patients who have been edentulous for less increase the leverage forces on the denture and may
than two years are concerned. However, in patients who destabilize it.21 However, there is an agreement that
have been edentulous for more than two years, better these leverage forces are well counterbalanced by
results and patient acceptance were reported with NZ favourable and seating forces resulting from optimal
dentures. Unfortunately, the aforementioned studies can placement of teeth and polished surfaces of denture
be criticized in terms of design or information about being in harmony with the tongue, lips and cheeks.1,11,40
blinding and randomization which affects the quality of
evidence taken from these studies. Conclusion
NZ concept is considered as exceptionally important
The principle of the NZ concept has remained the
when considering treatment options for patients
same since it has been first described by Beresin and
complaining from unstable lower CD particularly
Schiesser. However, the technique has been subjected to
if implant treatment is not feasible. It aims to place
various modifications. Type of retention incorporated in
lower CD where forces generated by lips, cheeks and
the baseplate (acrylic pillars or wire loops13), recording
tongue have a stabilizing rather than dislodging effect.
materials used and further refinement to the initial
The principle technique used to record neutral zone
record are among the variations between clinicians.
is extensively recorded; yet it needs to be backed up
The authors’ preference is to use combination of thin
with high quality clinical trials to push it further up on
acrylic pillars in premolar region connected by a wire
the hierarchy of evidence. It is not a widely practiced
loop which maintains the VDO and provides maximum
procedure while the proportion of patients that may
retention at the same time. Medium or regular bodied
befit from is significant. This may be attributed to a lack
silicone impression material used along with adhesive
of experience and exposure to this technique during
for the initial record that is refined in the try-in stage by
undergraduate training and the associated increase in
tissue conditioning material is the personal preference of
chair time and laboratory costs.
the authors for purposes of NZ recording.
Acknowledgement
The authors would like to acknowledge with gratitude Dr.
Brian Nattress for his continuous support and cheif dental
technician, Carol Scholfield, for the skilled lab work.
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+97143624784 candidate@nicolasasp.ae www.dubaipostgraduate.com
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