Neutral Zone
Impression Technique

The loose and unstable lower complete denture is one of the most common problems faced by denture patients. One of the methods used to solve this problem is the neutral zone technique. The neutral zone is the area where the displacing forces of the lips cheeks and tongue are in balance. It is in this zone that the natural dentition lie, and this is where the artificial teeth should be positioned. This area of minimal conflict may be located by using the neutral zone technique. The artificial teeth can then be set up in the correct positions. This technique is described below.

Primary impressions of the upper and lower jaws are taken in impression compound or impression plaster and the model is poured. On this model upper wax rims and a lower special tray are constructed. The special tray is a plate of acrylic adapted to the lower ridge, without a handle, with spurs or fins projecting upwards towards the upper arch. These help with retention of the impression material.

neutral1 neutral2

1) A lower edentulous arch

2) A lower acrylic special tray with metal spurs to aid retention of the impression material

The upper wax rim is adjusted as in normal registration for a complete denture. The lower special tray is placed in the mouth. Two occlusal pillars are then built up in self-cured acrylic (e.g. Total) on opposite sides of the lower arch. These pillars are moulded and adjusted to the correct height so as to give the usual 3mm freeway space.

neutral3 nuetral4

3) Occlusal pillars have been built up in green stick to the correct occlusal height

4) Establishing the correct occlusal height

A thick mix of viscogel is then placed around the rest of the lower special tray, distally and mesially to the occlusal pillars. The patient is then asked to talk, swallow, drink some water etc. After 5-10 minutes the set impression is removed from the mouth and examined. The viscogel material will have been moulded by the patient's musculature into a position of balance.

neutral5 netral6

5) The viscogel rim being moulded within the mouth

6) A completed viscogel impression

Indices are then constructed in the lab, by surrounding the impression with plaster. When the viscogel and the tray is removed, a gutter corresponding to the neutral zone is left behind. The teeth may then be placed into the neutral zone.

The resulting denture should feel more comfortable and be more stable and retentive because the denture should not interfere with or be displaced by the functions of the lips, cheeks and tongues.

neutral7 neutral8

7) A plaster index used to locate the teeth to the neutral zone.

8) The teeth positioned in the neutral zone leaving plenty of tongue space