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(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 fabrication
of baseplate for NZ impression
Clinic 4: NZ impression
Lab 4: NZ impression record mounted on lower master cast, orientation
grooves placed on master cast, putty index adapted around
NZ record and impression material removed and poured in wax
Finally, setting of teeth completed
Clinic 5: Try-in stage. Afterwards, NZ impression refined by tissue
conditioner applied to lower try-in denture
Lab 5: Processing, finishing and polishing
Clinic 6: Insertion of finished dentures
been described after insertion of the denture but using
hard relining material. 27,31
Discussion
Many approaches to set teeth have been advocated and
used in complete denture treatment. 20 However, there
is substantial debate on which of these provide optimal
position in the facio-lingual dimension and guarantee a
favourable outcome in terms of stability, facial support,
chewing efficiency, aesthetics and patient comfort. Some
of these approaches utilized biometric measurements and
location of relatively stable anatomical landmarks to set
teeth; 4 others relied on difference in resorption patterns
to set denture teeth where their natural predecessors
were thought to have been. 34 Some authors adopted a
mechanical concept and advocated setting teeth directly in
the centre of denture support area where the least amount
of leverage is present which in turn enhances the stability
of lower CD. 35 All of these approaches were and are still
being used and each of them proved to have advantages
and disadvantages when compared to others. Furthermore,
these approaches seem to work best when used with
patients who have; their oral and peri-oral musculature
unaltered for any reason, adequate neuromuscular control
and acceptable amount of residual ridge for support.
Unfortunately, the proportion of patients with these features
is dramatically decreasing and so the NZ concept has
become increasingly significant. These observations are
strongly supported by studies investigating the effect of
period of edentulism on position of neutral zone. It has
been found that NZ is closely related to the crest of residual
(Fig. 1) NZ baseplate with
acrylic pillars and wire loop
(Fig. 2) A: NZ impression taken with silicon. B: Putty index
adapted around master cast
ridge in patients who have been edentulous for less than
two years and significantly differs in those who were
edentulous for a period more than that. 16,17
Realizing the importance of the forces generated
by various oral structures on the teeth and polished
surfaces of CDs and their effect on the stability of CD
sheds light on the NZ technique. 1,10 It has been shown
that compromised retention, poor stability, phonetic
problems, inadequate facial support, inefficient
tongue posture/function and increased gagging are
all associated with functionally inappropriate setting of
denture teeth and physiologically inadequate contours
or volume of the denture base. 20
NZ technique has been criticized based on claims that
it is supported by empirical evidence. However, other
authors maintain that this is inaccurate as NZ technique
is based on significant clinical observations on the role
of destabilizing forces the muscles apply to CDs during
functional movements. Furthermore, the large number of
case reports accumulated in a short period of time and
clinical studies conducted by Stromberg & Hickey 36 and
Fahmy & Kharat 37 undermine this criticism and add to
the validity of NZ technique. Stromberg & Hickey 36 found
better patient adaptability to physiologically formed
denture bases when compared to conventional ones.
Fahmy & Kharat 37 found improved comfort and speech
clarity reported by patients upon wearing CD fabricated
using NZ technique when compared to conventional
CD. Moreover, Barrenas and Odman found less post
insertion problems and better patient acceptance in
NZ dentures when compared to conventional ones. 38
(Table 3) Summary of NZ impression clinical technique
Baseplate with acrylic pillars and/or wire loop is fitted in patient’s
mouth and checked for proper extensions and VDO
Baseplate is coated by adhesive and loaded with regular bodied
silicone impression material
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
which can be corrected by addition of impression material
Impression disinfected and sent to lab
| 10 | Smile Dental Journal | Volume 6, Issue 4 - 2011