Morphologic comparison of two neutral zone impression techniques: A pilot

study
Joseph E. Makzoumé, DDSa
School of Dentistry, St-Joseph University, Beirut, Lebanon
Statement of problem. Several studies have compared dentures fabricated using neutral zone and
conventional techniques. However, studies comparing swallowing and phonetic techniques for assessing the
location and shape of the neutral zone could not be identified in the literature.
Purpose. The purpose of this pilot study was to compare the outline form of the phonetic and swallowing
neutral zone impression techniques for the same subjects.
Material and methods. Nine denture wearers with advanced mandibular ridge resorption were included in
this study. For each subject 2 trays were prepared in autopolymerizing acrylic resin. One method used phonetics
and tissue conditioner to shape the neutral zone; the second method used swallowing and modeling plastic
impression compound. The resulting neutral zone impressions were leveled to the same occlusal height by
gently grinding the occlusal surface on sandpaper until it corresponded with landmarks (corners of the mouth,
two thirds of the height of the retromolar pads, bilaterally) noted on the cast. The impression was inverted onto
graph paper, and the contour was outlined with a lead pencil. One impression was made for each subject, for
each technique. The buccal contours of both neutral zones coincided at the median line. The maximum distance
between the zones was measured in a buccolingual direction in the anterior, premolar, and molar regions
bilaterally. When the location of the phonetic neutral zone in relation compared to the swallowing neutral zone
was buccally oriented, a plus score was given. When the phonetic neutral zone was lingually located, a minus
score was given. When the 2 lines coincided, a score of 0 was given. Measurements were made from direct
readings on the graph paper. Statistical analysis was performed using the Sign test (a=.05).
Results. Significant differences were noted buccally in the left molar (P=.031) and right molar (P=.003)
regions and also in the left and right premolar regions (P=.007), where the swallowing neutral zone was found
to be located buccal to the phonetic neutral zone. Significant differences were also noted lingually, in the right
premolar region (P=.015), where the swallowing neutral zone was found to be located lingual to the phonetic
neutral zone. There was no significant difference between the techniques for the anterior region.
Conclusion. Within the limits of this study, the phonetic neutral zone appears to be narrower posteriorly
compared to the swallowing neutral zone, thus limiting premolar and molar positioning. (J Prosthet Dent
2004;92:563-8.)

CLINICAL IMPLICATIONS
A denture fabricated with a mandibular impression made using a phonetic technique to
determine that the neutral zone will be narrower in the posterior region than a denture using
a swallowing technique to establish the neutral zone.

T he neutral zone is defined as ‘‘the potential space

between the lips and cheeks on 1 side, and the tongue
may have an adverse effect on the success of the prosthe-
sis. This is particularly true for patients with reduced
on the other; that area or position where the forces be- mandibular residual ridges, yielding flat or concave
tween the tongue and cheeks or lips are equal.’’1 This foundations due to severe bone resorption. A number
zone is referred to by various names, including the of techniques relying on function to develop the shape
dead space2 and zone of minimal conflict.3 Knowledge of the neutral zone and polished surface of mandibular
of the neutral zone concept may be advantageous dentures have been described.4-6 The concept considers
when fabricating complete dentures. Incorrect tooth the actions of the tongue, lips, cheeks, and floor of the
placement and arbitrary shaping of the polished surfaces mouth during a specific oral function, to push the soft
material into a position where buccolingual forces are
neutralized. Many materials have been suggested for
a
Private practice and Master Assistant, Post-Graduate Prosthodontics, shaping the neutral zone: modeling plastic impression
Department of Removable Prosthodontics. compound,5 soft wax,7 a polymer of dimethyl siloxane

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filled with calcium silicate,8 silicone,9 and tissue condi- sions of the borders. Borders were trimmed, and the
tioners and resilient lining materials.10,11 trays were reevaluated intraorally for stability by the cli-
Many techniques have been suggested using the pre- nician and confirmed by the subject, after opening the
viously described materials in conjunction with move- mouth wide, wetting the lips with the tongue, swallow-
ments including sucking,12 grinning and whistling,7 ing, and speaking. One of the trays was used to shape re-
and pursing the lips.13 The swallowing/modeling plas- silient lining material (Functional Impression Tissue
tic impression compound technique14 located the neu- Toner; Kerr Corp) using phonation. Swallowing was
tral zone, using swallowing as the principle modeling used to shape modeling plastic impression compound
function. Considering that a person swallows up to (Green Impression Compound Type 1; Kerr Corp) on
2400 times per day,15 and considering also that during the second tray, constructed with wire loops to retain
the entire swallowing sequence teeth come into contact the modeling material. None of the subjects wore a max-
for less than 1 second,16 it may be concluded that less illary denture during impression procedures. The result-
than 40 minutes of tooth-to-tooth contact occurs per ing impressions were leveled to the same occlusal height
day during function. Speech is also another important by gently grinding the occlusal surface on sandpaper un-
part of daily oral activities. During speaking, the mouth til it corresponded with landmarks noted on the cast.
is moderately opened, pressures of different magnitude The contours of both impressions were outlined on
and direction are generated, and forces are produced graph paper and compared. The recording of the pho-
with a greater horizontal than vertical component acting netic neutral zone was always performed before that of
on the dentures. Furthermore, although speaking causes the swallowing neutral zone, and an impression was
upward movements of the floor of the mouth similar to made for each subject using each technique.
swallowing, these movements are not as constant as
The phonetic technique
those found in swallowing.17 Thus, the phonation/tis-
sue conditioner technique uses phonation to develop The molding of the phonetic neutral zone (PNZ) was
a mandibular impression.8,13,14 Many studies have ana- developed progressively. One lateral segment was
lyzed the neutral zone13,18,19 and neutral zone dentures molded first (right or left), then the other lateral seg-
as compared with dentures made using conventional ment; and, finally, the anterior segment. The custom
techniques in the edentulous patient.8,9,12,20 It has tray was seated on the edentulous ridge, and 5 mL of tis-
been shown that neutral zone dentures are functionally sue-conditioning material mixed in a 1:1 ratio were in-
more stable than conventional dentures.6,8,9,12 jected with a syringe on the right lateral segment of
However, the author could not identify studies compar- the tray after the tongue was moved aside with a mirror.
ing the swallowing and the phonetic techniques for as- The subject was asked to pronounce the phoneme
sessing the location and shape of the neutral zone ‘‘SIS’’ 5 times followed by the phoneme ‘‘SO’’ once.
reported in the literature. The purpose of this study Both sounds had to be pronounced clearly, loudly,
was to compare the outline form of the phonetic and and vigorously to induce sufficient muscle contraction.
swallowing neutral zone impression techniques on the This phonetic sequence was repeated until the material
same subjects. had polymerized. The tray was removed from the
mouth, and excess tissue-conditioning material extend-
ing anterior to the premolar area was removed with scis-
MATERIAL AND METHODS
sors. The tray was reinserted intraorally, and the same
After institutional review board approval, 9 healthy procedure was repeated to mold the left lateral segment
edentulous subjects who wore complete dentures for at of the PNZ. Then the right lateral segment that was
least 2 years were included in this study. Ages ranged be- molded initially was removed from the tray and re-
tween 73 and 83 years, with a mean of 79 years. molded because the first impression was not considered
Requirements for selection were advanced mandibular reliable due to the fact that the first contact of the
ridge resorption (Class V, Atwood)21 and absence of clin- tongue with the soft material might be constrained, as
ical temporomandibular joint symptoms. During prelim- the tongue would try to avoid this initial contact.
inary evaluation, none of the subjects showed signs of Finally, the tray was reinserted and molding of the
phonetic problems with their existing dentures. Clinical PNZ was completed by injecting material in the anterior
assessment showed no abnormal swallowing habits. region and having the subject pronounce successively
All procedures were performed by a single clinician. the phonemes ‘‘DE, TE, ME, PE, SE’’ vigorously, until
A preliminary mandibular cast was made for each sub- the polymerization of the material was complete.
ject using an irreversible hydrocolloid (Aroma Fine Dust During the molding of each segment, whenever the sub-
Free; GC Europe, Leuven, Belgium) impression. Two ject swallowed or spoke sounds other than the pho-
custom impression trays were then prepared in autopo- nemes, the material was removed and the segment was
lymerizing acrylic resin (Formatray; Kerr Corp, Orange, remolded. The occlusal plane was then located accord-
Calif), placed intraorally, and evaluated for overexten- ing to the height of the lower lip at rest anteriorly, the

564 VOLUME 92 NUMBER 6

MAKZOUMÉ THE JOURNAL OF PROSTHETIC DENTISTRY

commissures laterally, and to a point located approxi-

mately two thirds of the height of the retromolar pad
posteriorly. Excess tissue-conditioning material was re-
moved with scissors. For research purposes, the impres-
sion was processed to replace the soft material with an
autopolymerizing acrylic resin (Formatray, Kerr Corp).
Using sandpaper (size 150#2/0; Greatwall Mould Co
Ltd, Shenzhen, China), the occlusal surface of the re-
sulting impression was leveled. The buccal and lingual
median lines were determined intraorally according to
the sagittal midface line and recorded first on the PNZ
impression, and then on the subsequent cast (Fig. 1).
The PNZ impression was inverted onto graph paper,
and the contour was outlined as a dashed line with
a lead pencil.
The swallowing technique
Modeling plastic impression compound (Green
Impression Compound Type 1; Kerr Corp) was softened
in a preheated water bath (Petra Electric, Burgau,
Germany) at 57°C.5 Water temperature was controlled
with a thermometer. The soft material was adapted to
the tray and formed into the shape of an occlusion rim.
Two and a half cakes of compound were used for each
subject. The modeling compound was reheated for 2 min-
utes in the water bath, and the tray was carefully placed
in the subject’s mouth without distorting the rim. The
subjects were instructed to swallow and then purse the
lips as in sucking, several times. To make swallowing eas-
ier, 1 mL of warm water was injected intraorally before
each swallow. After the material cooled, the tray was re- Fig. 1. Same cast and same median line for both neutral zone
moved from the mouth and excess compound forced to impressions for 1 subject. Top: phonation/tissue conditioner
an excessive height was trimmed away with a knife. The technique. Bottom: swallowing/modeling plastic impression
procedure was repeated as many times as necessary to compound technique.
perfect the impression according to the swallowing neu-
tral zone (SNZ) technique. Impression was deemed sat- The relationship between the buccal and the lingual
isfactory when 2 successive impressions produced similar contours was examined and measured in a buccolingual
shapes. The occlusal plane was then located as previously direction. The outlines were compared by measuring the
mentioned so that SNZ and PNZ impressions had the maximum distance between them in the anterior, pre-
same occlusal height. Using sandpaper (size 150#2/0; molar, and molar regions on the left and right sides.
Greatwall Mould Co Ltd), the occlusal surface of the When the location of the PNZ outline compared with
molded compound rim was leveled. The tray was reposi- the SNZ outline was buccally oriented, a plus score
tioned on the cast, and the median line was recorded on was assigned. For lingual location of the PNZ outline
the compound, ensuring that the PNZ and SNZ impres- in relation to the SNZ outline, a minus score was as-
sions used the same median line. The SNZ impression signed. When the 2 lines coincided, a score of zero was
was then inverted onto graph paper and placed on top assigned. Measurements were made 3 times directly
of the PNZ impression outline in a way such that the from the graph paper. The values refer to the maximum
buccal contours of both impressions coincided with distance between the contours. As differences between
the median line. The SNZ impression contours were the outlines were not all entirely lingual or entirely buc-
outlined with the same lead pencil as a solid line. The po- cal, the 1/2 sign was assigned considering the direc-
sition of the canine, determined clinically, was noted. tion at the point of maximum difference. The data
From the position of the canine to the rearmost limit were analyzed using the Sign test (a=.05).
of the drawing, the distance was divided into 3 segments:
RESULTS
the rearmost two thirds was the molar region, and the
anterior-most one third was considered to be the premo- The positions of the PNZ impressions with respect to
lar region. the SNZ impressions for the buccal contours and the

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Fig. 2. Tracings of phonetic neutral zone and swallowing neutral zone for 9 subjects. Solid line represents swallowing neutral
zone. Dashed line represents phonetic neutral zone.

Table I. Buccal position of phonetic neutral zone in relation Table II. Lingual position of phonetic neutral zone in
to swallowing neutral zone in different locations relation to swallowing neutral zone in different locations
No. of Left Left Left Right Right Right No. of Left Left Left Right Right Right
subjects molar premolar anterior anterior premolar molar subjects molar premolar anterior anterior premolar molar

1 21 22 0 20.5 22 22 1 21 20.5 21 21 1 0
2 21 21 0 0 21 21.5 2 22 1 0 0.5 1 20.5
3 21 20.5 20.5 21.5 21.5 22 3 0.5 22 20.5 20.5 0 1
4 0 21 0 0 21.5 21.5 4 20.5 0 21 20.5 1 0
5 21.5 0 0 0 0 23 5 0.5 0 0 0.5 1 0
6 0 21 0 0 21 21 6 0.5 0.5 1.5 1 1 1
7 20.5 22.5 21 0 21 21.5 7 0.5 0.5 0 0.5 0.5 1
8 0 21 0 0 21 21.5 8 21.5 0 0.5 1 0.5 21
9 20.5 21.5 20.5 0 21 20.5 9 21 20.5 21 20.5 0 0.5
Total 25.5 210.5 22 22 210 214.5 Total 24 21 21.5 1 6 2
Mean 20.6 21.1 20.2 20.2 21.1 21.6 Mean 20.4 20.1 20.1 0.1 0.6 0.2
SD 0.55 0.75 0.36 0.51 0.55 0.70 SD 0.98 0.86 0.83 0.74 0.43 0.71
SD, Standard deviation. SD, Standard deviation.
Measurements in mm. Score of 0 indicates PNZ coincides with SNZ; 1 score Measurements in mm. Score of 0 indicates PNZ coincides with SNZ; 1 score
indicates PNZ is buccal with respect to SNZ; 2 score indicates PNZ is lingual indicates PNZ is buccal with respect to SNZ; 2 score indicates PNZ is lingual
to SNZ. to SNZ.

lingual contours in different locations are summarized in located lingually in 18 locations (33.33%), and buccally
Tables I and II. When data from the anterior, premolar, in 25 locations (46.29%).
and molar regions were pooled, buccally, PNZ impres- When compared by segment, buccally, statistical
sion contours coincided with SNZ impression contours analysis showed significant differences in the left molar
in 18 locations (33.33%). PNZ impression contours (P=.031) and right molar regions (P=.003) and also in
were located lingually in 36 locations (66.66%). No buc- the left and right premolar regions (P=.007) where the
cal location of the PNZ contours with respect to the SNZ was found to be located buccal to the PNZ.
SNZ contours was noted. Lingually, PNZ impression Lingually, statistical analysis showed significant differ-
contours coincided with SNZ impression contours in ences in the right premolar region (P=.015) where
11 locations (20.37%). PNZ impression contours were the SNZ was located lingual to the PNZ. No significant

566 VOLUME 92 NUMBER 6

MAKZOUMÉ THE JOURNAL OF PROSTHETIC DENTISTRY

differences were noted when analyzing the anterior re- tion. The authors noted that the neutral zone could be
gion, on both sides of the median line, since the number reproduced with only limited variation and was within
of subjects with different scores was small. However, in the range of clinical acceptability and concluded that
13 locations (72.22%) the SNZ coincided with the there was no operator effect when making neutral
PNZ, and in only 5 locations (27.78%) was the PNZ lin- zone impressions. These authors also compared the re-
gually located. sults obtained by 1 operator with 2 impression materials
and 2 methods of application. The authors reported sig-
nificant differences among impressions when using dif-
DISCUSSION
ferent materials (Coe-Comfort versus Bio-Soft; B. L.
This investigation was not designed to determine Dental Co Inc, New York, NY) and different application
which of the 2 impression materials is better for repro- methods of the material (spatula versus injection). In the
duction, nor whether swallowing or speaking is a better present study, 2 modeling techniques and 2 materials
modeling function. The sole objective was to assess were used. The results confirm the variability of the
whether there was a significant difference in outline of neutral zone techniques.13,18 These findings should,
the impressions made with the 2 methods. Statistical however, be considered carefully as sample size was lim-
analysis of the buccal contours of both PNZ and SNZ in- ited, and interoperator and intraoperator variabilities of
dicated consistent differences in the left and right molar experimental procedures were not assessed, since 1 clini-
regions. The SNZ in the most posterior locations was cian made only 1 impression of each technique.
found to be located buccally (Table I). Two factors Since the PNZ impression was always made prior to
may be responsible for this observation. Either the com- the SNZ impression, bias may have been introduced as
pound was too viscous a material to be sufficiently the subject and/or the operator could be more comfort-
molded by the buccinator, or buccinator activity was in- able or precise in making the second impression. Ideally,
creased in speaking. Whichever is true, the question is the choice of the first impression technique should have
raised as to whether the SNZ denture on an advanced- been randomized. Furthermore, more elaborate tech-
resorbed residual ridge would be in equilibrium during niques could be developed to obtain similar occlusal
speech. The opposite question may also be raised as to plane for both techniques and to improve tracing tech-
whether a PNZ denture on an advanced-resorbed ridge nique. Further research is required to compare the func-
would be stable during swallowing. tional potential of mandibular dentures made using the
Significant differences were also noted in the left and swallowing/modeling plastic impression compound
right premolar region. The PNZ was found to be located technique and the phonation/tissue conditioner tech-
lingually (Table I). Accordingly, teeth may be posi- nique.
tioned more lingually, thus offering less lip and cheek
support. If a neutral zone technique is indicated for a pa- CONCLUSIONS
tient with a reduced alveolar ridge, it may be difficult to
support the patient’s lips if this is required to improve fa- Within the limitations of this study, the findings indi-
cial esthetics. It is interesting to note that in the premo- cated that the location of the neutral zone was not the
lar region, the PNZ buccal curves appeared more clearly same with the swallowing/modeling plastic impression
defined compared to SNZ curves (Fig. 2). The narrow- compound technique and the phonation/tissue condi-
ness of contour in this section is caused by the contrac- tioner technique. However, statistical significance does
tion of the zygomatic, caninus, and triangularis muscles, not necessarily imply clinical significance, and the results
which meet at the modiolus.2 Therefore, the corners of yielded by these 2 techniques may be clinically accept-
the mouth in speaking, when present with a resorbed re- able. In general, the PNZ technique resulted in impres-
sidual ridge, may lift the SNZ denture up or move it lat- sions where the neutral zone appeared to be narrower as
erally. the buccal surface was located more lingual compared to
Lingually, the only significant difference was noted in the SNZ technique.
the right premolar region where the SNZ was found to The author wishes to thank Dr Sheldon Winkler for his invaluable
be located lingually (Table II). Whenever the width of advice in this study, Drs Frank Schiesser and Pierre Klein for their
the neutral zone in this lateral section is not thick encouragement, Mr Fouad Nakhlé for his statistical assistance, and
enough to allow reduction from the lingual, tongue Dr Hani Ounsi for his help in editing the manuscript.
thrusting during speech may displace the SNZ denture,
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forces. Gerodontology 1998;15:113-9. 21. Atwood DA. The problem of reduction of residual ridges. In: Winkler S.
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Cytotoxicity of denture base resins: Effect of water bath and microwave

Noteworthy Abstracts postpolymerization heat treatments
of the Jorge JH, Giampaolo ET, Vergani CE, Machado AL, Pavarina AC, Carlos IZ.
Current Literature Int J Prosthodont 2004;17:340-4.

Purpose: This study compared the effect of two postpolymerization heat treatments on the cytotoxicity of
three denture base resins on L929 cells using 3H-thymidine incorporation and MTT assays.
Materials and Methods: Sample disks of Lucitone 550, QC 20, and Acron MC resins were fabricated under
aseptic conditions and stored in distilled water at 37°C for 48 hours. Specimens were then divided into three
groups: (1) heat treated in microwave oven for 3 minutes at 500 W; (2) heat treated in water bath at 55°C for
60 minutes; and (3) no heat treatment. Eluates were prepared by placing three disks into a sterile glass vial
with 9 mL of Eagle’s medium and incubating at 37°C for 24 hours. The cytotoxic effect from the eluates was
evaluated using the 3H-thymidine incorporation and MTT assays, which reflect DNA synthesis levels and cell
metabolism, respectively.
Results: The components leached from the resins were cytotoxic to L929 cells when 3H-thymidine
incorporation assay was employed. In contrast, eluates from all resins revealed noncytotoxic effects as
measured by MTT assay. For both MTT assay and 3H-thymidine incorporation, the heat treatments did not
decrease the cytotoxicity of the materials tested.
Conclusion: Resins were graded by 3H-thymidine incorporation assay as slightly cytotoxic and by MTT assay
as noncytotoxic. Cytotoxicity of the denture base materials was not influenced by microwave or water bath
heat treatment.—Reprinted with permission of Quintessence Publishing.

568 VOLUME 92 NUMBER 6