Dr Ankita Savaliya, Dr Janani Iyer, Dr Jyoti Nadgere
Professor and HOD,
Department of Prosthodontics and Crown and Bridge,
MGM Dental College and Hospital, Navi Mumbai
In complete denture prosthodontics, instability of lower denture is most common problem faced due to long term edentulism. Various factors affect the stability of the denture, one of them being teeth placed in proper position. Arrangement of artificial teeth in neutral zone and recording the correct contour of the polished surface will reduce the interferences of teeth while functional movements. Correct polished surface recording form favorable angle with cheek, tongue and lip. Teeth placed in neutral zone enhances stability by equalizing forces exerted by muscles of tongue on one side and muscles of cheek and lip on the other side. Different techniques and materials are used to record the Neutral zone. The aim of this article is to understand this technique by practical implication in the described case series.
Citations : Savaliya A, Iyer J, Nadgere J. Neutral zone: a novel approach for resorbed ridge management-a case series. J Prosthodont Dent Mater 2020;1(1& 2): 82-89.
One of the major difficulties in complete denture wearer patient is the poor stability of the mandibular denture. Mandibular ridge resorption is 4 times greater than that of maxilla. The denture bearing area of maxilla is 24 cm and 14 cm in mandible. Presence of mobile tissues at the floor of the mouth and tongue on the other side compromises the stability of the mandibular denture. The potential space between the lips and cheeks on one side and tongue on other ;that area or position where the force between the tongue and cheeks or lips are equal is called as “NEUTRAL ZONE”. It is also referred as Dead space, Stable Zone, or Zone of Minimal Conflict.
Sir Wilfred Fish in 1933 brought to the notice the importance of polished surface on stability of the denture. Recording the correct contour of the polished surface and arrangement of artificial teeth in neutral zone will reduce the interferences of teeth while functional movements and the muscular forces from both sides will be equalized; which in turn will help in seating the denture in place. It is indicated in patients with resorbed mandibular ridges and compromised neuromuscular control. The aim is to fabricate a denture which is in harmony with the facial musculature to increase the stability and retention.
CASE REPORT 1
A female patient aged 57 years reported with the chief complaint of loose mandibular denture. Patient was wearing the same denture for last 7 years. On examination, hyperactive mentalis muscle activity was seen which was responsible for repeated unseating of denture. (Fig 1) The patient was diagnosed as a completely edentulous patient with a hyperactive mentalis muscle. (Fig 2) Therefore, it was decided to fabricate a new set of dentures using the Neutral Zone technique.
Primary Impression was made in stock tray using medium fusing impression compound (Prime Dental Products). Border molding was done with low fusing impression material (DPI Pinnacle tracing stick) and wash impression was made with Zinc Oxide eugenol paste (DPI Impression Paste). Temporary denture base were made by auto polymerizing acrylic resin(DPI autopolymerising resin) and wax rims of modeling wax were fabricated on it. Denture bases were checked in the patient’s mouth for border extensions and stability. Maxillary rim orientation was analyzed by assessing the esthetics, phonetics and function. Mandibular rim was adjusted at the established vertical dimension and centric relation was recorded. Facebow transfer was done using mounted on semi-adjustable Hanau articulator. (Fig 3)
A second new pair of denture bases were fabricated. Maxillary rim was duplicated with medium fusing impression compound. A superstructure was constructed over the lower denture base in the posterior region using auto polymerizing acrylic resin to maintain the established vertical dimension. (Fig 4) 20-gauge orthodontic wire loops were incorporated over the crest of the lower ridge to support the low fusing impression material to be used to record the neutral zone.
Patient was made to sit in an upright position and the duplicated maxillary rim was inserted in the patient’s mouth. It was reassessed for occlusal plane and lip support. Low fusing impression compound was tempered in hot water bath and adapted over the lower denture base around the supporting wire loops and placed in the patient’s mouth.
Patient was then instructed to perform the following series of actions for 10 minutes:
- To have a sip of warm water and swallow.
- Take frequent sips of water.
- Pout and smile, grin, lick the upper lip and purse the lips (Fig 5).
Patient is then told to talk aloud, pronounce the vowels and count from 60 to 70.
Neutral zone impression was seated on the master cast. Orientation grooves were carved in the ledge area of the cast, 2 on the buccal side and one in the lingual area. Boxing of the master cast with boxing wax was done. A thin layer of Separating media (Sodium alginate) was applied over the cast and impression plaster was poured to form indexes. Buccally the indexes were separated at the midline for easy removal. Lingually the index was fabricated in single piece (Fig 7).
The indexes were labelled according to their position and checked for accurate placement. A new denture base was fabricated over the mandibular master cast. The indexes were seated in position and molted modelling wax was poured in the space between the indices forming a new occlusal rim. Semi anatomic teeth were selected according to SPA factor and teeth arrangement was done confining to the space and it was rechecked by placing the plaster indices in place (Fig 8). Non balanced(Organic) occlusal scheme was followed.
Try in was done and centric relation, esthetics and phonetics were verified. Neutral zone verification was done during try in stage by flowing light body addition silicone (Flexceed light body) on polished surface of upper and lower trial denture and asked patient to do all the movements. (Fig 9)
After all the movements exposed region of wax were removed and Patients approval was taken. Denture was processed with heat cure acrylic resin by compression molding technique. After finishing and polishing denture insertion was done and minor occlusal adjustment were made by selective grinding technique. Patients follow up was done after 24 hours and minor modification was done (fig 10).
CASE REPORT 2
A female patient aged 56 years reported with the chief complaint of ill-fitting lowered denture. Patient was wearing the same denture for last 4 years. On examination, mandibular ridge resorption was seen (Fig 1) Mandibular ridge was narrow in width with reduced height. Patient was diagnosed as completely edentulous maxillary and mandibular ridges with moderate resorption of mandibular arch Therefore, it was decided to fabricate a denture using a Neutral Zone technique.
The primary impression, Final impression and Jaw relation was taken conventionally as described in the previous case. Lower mandibular denture base was prepared to record neutral zone same as described previously. Patient was instructed to do all the movements as described in the previous case (Fig2)
In this case we used Zinc Oxide eugenol paste as a wash impression material to record the neutral zone. (DPI Impression Paste) (Fig 3)
Teeth arrangement, Try in and denture insertion was done as described in the previous case. (Fig 4) Non balanced (Organic) occlusion philosophy was chosen for this patient.
Neutral zone philosophy is well described in the literature to achieve stability and retention in a severely resorbed cases. Various methods that can be used to record the neutral zone are swallowing, phonetics, sipping of warm water, slicking and pursing upper lip smiling, whistling, protruding tongue, opening and closing of mouth, blowing air and exercise movement of lip, cheek, tongue and facial muscle. We used Phonetics and swallowing methods to record neutral zone as it activates all the muscle involved in. Lott and Levin suggested that reading interesting topic loudly and rapidly is the ideal method as this causes muscles to be increasingly strained, and saliva secretion increases which will result in more swallowing action. This will record more natural movements of muscle as it reduces the patient’s focus on the occlusal rim. Tench proposed the use of thermoplastic low fusing impression compound material to record neutral zone as it has the advantages of being easily available, ease of manipulation and low cost. Low fusing impression compound is most commonly used to record tissue while functional manipulation as in border molding to record vestibule; hence we used this material to record neutral zone. Other materials that can be used are tissue conditioners, modelling wax, light cure resins, silicone and auto-polymerizing acrylic resin. Modelling wax is initially stable but gets distorted after some time, hence it is not used routinely. Resin (light cure / acrylic) can be used but it causes irritation due to residual monomer content. Tissue conditioner, addition silicone (Light body), zinc oxide eugenol paste can be used as a wash impression material. We used tissue conditioner and zinc oxide eugenol paste as a wash impression material because of its adequate flow and slow setting time of the material. These materials can be used to record neutral zone or as materials for wash impressions of neutral zone. The material should have a body; so that it can stay over the denture base with wire as a supporting medium; And sufficient viscosity so that oral musculature can shape it in proper contour and dimension via the functional muscle movements. Whichever material is used two factors are mandatory 1) maintain the established vertical dimension 2) slow setting time; so that the patient gets adequate time to mold it. For index fabrication silicone, stone, plaster, or modeling plastic impression compound can be used. We used dental plaster because of its rigidity, ability to record minute detail and ease of manipulation and availability. Indexing helps in preservation of recorded neutral zone. Verification of neutral zone at try in stage is an important step. Impression of an external surface (labial, buccal and lingual) determine the correct contour, thickness and shape of the polished surface of the denture. Dentist or the technician has a tendency to wax up the external surface of denture during wax up of the trial denture. By making an impression of the external surface; the exposed areas need to be cut off or reduces so as to maintain our neutral zone contour. Zinc oxide eugenol or light body addition silicone can be used in 2 steps for this procedure. First buccal and labial surface is recorded and secondly lingual surface is recorded. This procedure has two advantage:
1) It forms a ledge lingually in the lower anterior region; it should be replicated in final denture as tongue sits on this ledge and it keeps the denture in position.
2) External impression fills up the denture spaces and cheeks can easily push the food towards the occlusal surface; hence reduces or eliminates food accumulation on the buccal aspect.
This technique is contraindicated in patients with long term edentulism as the tongue size increases results in macroglossia and causes unseating of the lower denture. In severe neuromuscular incoordination.
Atrophic mandibular ridges with poor denture stability and discomfort is the major problem in complete denture prosthodontics. Neutral zone impression technique is highly effective in such cases. This impression technique utilizes muscle forces to record the neutral zone. This technique defines the polished surface of the denture and accurately place the teeth in the stable zone. It increases the stability of the denture, esthetics and comfort of the patient. The neutral zone technique is also used in patient with partial glossectomy, motor nerve damage, mandibular resection which have led to atypical movement and less favorable denture bearing area. This technique requires extra clinical appointment of the patient and chair side working time. Accurate methodology and material used improves masticatory efficiency and prognosis of the case.
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