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| ORIGINAL ARTICLE |
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| Year : 2015 | Volume
: 2
| Issue : 2 | Page : 79-85 |
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Effect of duration of edentulism on neutral zone position in relation to alveolar crest ridge
Ajay Jain1, Ugrappa Sridevi2, Ugrappa Vijay Kumar3
1 Department of Prosthodontics, Faculty of Dentistry, AIMST Dental Institute, AIMST University, Kedah, Malaysia
2 Department of Oral Medicine and Radiology, Faculty of Dentistry, AIMST Dental Institute, AIMST University, Kedah, Malaysia
3 Private Practitioner, Hyderabad, Andhra Pradesh, India
| Date of Web Publication | 14-Jul-2015 |
Correspondence Address:
Ajay Jain
Senior Lecturer, Faculty of Dentistry, AIMST Dental Institute, AIMST University, Kedah
Malaysia
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/1658-6816.160770
Aim: The purpose of the study was to establish a relation between the crest of alveolar ridge and functionally obtained neutral zone and to determine the effect of duration of edentulism on the location of neutral zone in relation to the crest of residual alveolar ridge.
Materials and Methods: The study included three groups, Group I: 15 human subjects edentulous for 0 months to 2 years, Group 2: 15 human subjects edentulous for 2 years to 5 years and Group 3: 15 human subjects edentulous for more than 5 years. Neutral zone recording was performed for each human subjects and the bucco-lingual relationship of the crest of the mandibular alveolar ridge and position of neutral zone was examined. The results were analyzed by Kruskal-Wallis h-test and Chi-square independent test.
Results: The results suggested that the location of neutral zone varies from individual to individual depending on their musculature and there is significant relation to the duration of edentulism. As the duration of edentulism increases, there is more lingual positioning of neutral zone at the molar region of both side of the arch. At premolar region, there is no change in position of neutral zone, it remains constant since resorption of the alveolar ridge is directly under the buttress. In anterior region, there is more labial positioning of neutral zone as duration of edentulism increases.
Conclusion: This technique proved itself to be an easy and inexpensive way to determine the relationship between the crest of alveolar ridge and neutral zone. Incorporating this technique into the practice would be a great aid, which could be exploited by the clinicians for functional and psychological comfort of the patients. Keywords: Alveolar crest ridge, completely edentulous, edentulism, neutral zone, perioral musculature
How to cite this article:
Jain A, Sridevi U, Kumar UV. Effect of duration of edentulism on neutral zone position in relation to alveolar crest ridge. Saudi J Oral Sci 2015;2:79-85 |
How to cite this URL:
Jain A, Sridevi U, Kumar UV. Effect of duration of edentulism on neutral zone position in relation to alveolar crest ridge. Saudi J Oral Sci [serial online] 2015 [cited 2019 Dec 15];2:79-85. Available from: http://www.saudijos.org/text.asp?2015/2/2/79/160770 |
| Introduction | |  |
Complete denture prosthesis is primarily biomechanical artificial device, which is fit in the edentulous mouth to rehabilitate the handicapped edentulous jaws and to function with good retention and stability. The prosthesis must be made so that they are in harmony with normal neuromuscular function. [1] Successful treatment of patients with complete denture depends on the proper positioning of artificial teeth in relation to the basal seat and surrounding tissue. [2]
The concept of neutral zone in complete denture was introduced by Sir E. Wilfred Fish in 1931. He stated that the natural teeth occupy a "zone of equilibrium" with each tooth assuming a position, that is the resultant of various forces acting on it. [3],[4] Since then many others have helped to advance and develop both the theoretical basis and practical problems. Russel termed it as "reciprocal space". [5] Robert called it the "potential space". [6] While others named it as "dead space", "zone of minimal conflict", "denture space", "reciprocal zone," [7] and "zone of neutral muscular function". [8]
The soft tissues that form the internal and external boundaries of the denture space exert forces that greatly influence the stability of the dentures. The central thesis of neutral zone approach to complete dentures is to locate that area in the edentulous mouth where the teeth should be positioned so that the forces exerted by muscles will tend to stabilize the denture rather than unseat it. [1]
As the area of the impression surface decreases and the polished surface area increases, tooth position and contour of the polished surface become more critical. In other words, where more of the alveolar ridge has been lost, denture stability and retention are more dependent on correct positions of the teeth and contour of the external surfaces of the dentures. [9],[10]
Hence, this study was undertaken to establish a relation between the crest of alveolar ridge and functionally obtained neutral zone and to determine the effect of duration of edentulism on the location of neutral zone in relation to the crest of mandibular residual alveolar ridge.
| Materials and Methods | | |
This cohort study was conducted on 45 completely edentulous human subjects treated in A.B. Shetty Memorial Institute of Dental Sciences, Deralakatte, Mangalore, Karnataka, India, after getting ethical clearance from the committee. The subjects were informed about the study and informed consent was obtained.
The exclusion criteria were:
- Subjects with major osseous surgery or any congenital or acquired osseous abnormality found in mouth.
- Subjects with flabby ridges.
- Subjects with any pathology of tongue leading to enlargement of tongue.
- Subjects with history of wearing old dental prosthesis or faulty denture.
- Subjects with any facial asymmetry, hemihypotrophy, hemihypertrophy with evidences of muscular imbalance.
- Subjects with submucous fibrosis.
- Subjects with clinical temporomandibular joint disorders.
These subjects were divided into three groups:
Group 1:
15 Subjects edentulous for 0 months to 2 years.
Group 2: Subjects edentulous for 2 years to 5 years.
Group 3: Subjects edentulous for more than 5 years.
The position of neutral zone and its relation to the crest of the mandibular alveolar ridge in completely edentulous human subjects at different period of edentulism for three age groups, Group I, Group II, and Group III were determined at five locations at left molar, left premolar, anterior, right premolar, and right molar.
The mandibular master casts were obtained and duplicated with reversible hydrocolloid impression material (Agar-Agar) (Castogel, Bego, Germany) and poured with type III dental stone (Kalstone, Kalabhai Karson Pvt. Ltd., Mumbai, India) to obtain a pair of duplicated master casts so that these duplicated master casts could be modified for the study and the original master casts could be used for the denture processing. The duplicated master casts were modified at the crest of the ridge for the adaptation of wire and blocked out with modeling wax (Hindustan Dental Products, Hyderabad, India), followed by construction of record base utilizing autopolymerizing acrylic resin (DPI-RR Cold Cure, Mumbai, India) using sprinkle-on method. The record base was tried in the patient's mouth and checked for extensions, retention, stability, and comfort.
The neutral zone was recorded using low fusing impression compound [Figure 1] (DPI PINNACLE, Mumbai, India). The patient was instructed to do swallowing, sucking, and various other functional movements of the cheeks and tongue. During the function of the cheeks, lips and tongue the forces exerted on the material molded it into the neutral zone. After the setting of material, record base was taken out of the mouth. It is important that only an adequate amount of material be used for this procedure, as the usage of an excessive amount would force the material forward and upward above the normal height of the occlusal plane. The height of compound rim was leveled with 2/3 rd of the height of retromolar pads posteriorly and the corners of the mouth anteriorly with the help of sharp knife and sand paper (size: 400 fine). The crest of the alveolar ridge of the final casts was then trimmed about 1 mm using a sharp knife.
A 36-gauge stainless steel wire (Sendent S.S. wire soft) was adapted and stabilized along the center of the alveolar ridge [Figure 2]. The center of the bucco-lingual width of each compound rim was marked along its length and a 26 gauge stainless steel (Sendent S.S. wire soft) was adapted and stabilized over the center of each rim [Figure 3]. The wires were stabilized using cyanoacrylate resin. Two different gauge wires were utilized for radiographic interpretation.
Record base was repositioned on the master cast and occlusal view radiographs (X-Mind, Satelec Z.I., France) were obtained [Figure 4] of each record base and its cast using ultraspeed films (57 × 76 mm). This method minimized distortion of the radiograph that can cause an error in the measurement. [11] The object to source distance was 8 inch and the central was directed at the center of the cast. Exposure parameters used were 70 kV voltage, 8 mA current, and 2.5 s impulse. All the films were developed in automatic processor. | Figure 2: Adaptation of 36 gauge wire over the cast at crest of mandibular alveolar ridge
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 | Figure 3: Adaptation of 26 gauge wire over the center of the neutral zone recording
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Films were viewed, in a viewing box, for the relationship between the images of two wires in a bucco-lingual direction with thinner wire representing crest of alveolar ridge and thicker wire denotes center of neutral zone [Figure 5]. Where the two images coincided, a zero score was assigned. Buccal/labial location of neutral zone with respect to the ridge was assigned positive value and lingual location was assigned a negative value. Measurements were made with a millimeter ruler to an accuracy of 0.5 mm. | Figure 5: Film viewed in a viewing box for the relationship between the images of two wires in a bucco-lingual direction
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The basic data collected from 45 subjects was subjected to statistical analysis by using Kruskal-Wallis h-test and Chi-square Independent test. All the statistical analysis was done using SPSS version 16 and Microsoft Excel software (IBM SPSS Inc.). A value of P < 0.05 was considered statistically significant.
| Results | | |
Study involved in estimation of position of neutral zone and its relation to the crest of the mandibular alveolar ridge in completely edentulous subjects at different period of edentulism for three age groups, Group I (0-2 years), Group II (2-5 years), and Group III (>5 years) at five locations left molar, left premolar, anterior, right premolar, and right molar.
[Table 1] shows the position of neutral zone in relation to the alveolar crest ridge for group I. At left molar region, left premolar region, anterior region, right premolar region, and right molar region, the mean value of neutral zone in relation to crest of mandibular alveolar ridge are −0.83 mm, 0.00 mm, 1.46 mm, −0.06 mm, and −0.63 mm, respectively, with mean duration of edentulism is 0.93 years. | Table 1: Position of neutral zone in relation to the alveolar ridge crest in different locations for 0 month to 2 years
of edentulism
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[Table 2] shows the position of neutral zone in relation to the alveolar crest ridge for group II. At left molar region, left premolar region, anterior region, right premolar region, and right molar region, the mean value of neutral zone in relation to crest of mandibular alveolar ridge are −2.26 mm, −0.06 mm, 1.8 mm, −0.06 mm, and −2.06 mm, respectively, with mean duration of edentulism is 3.23 years. | Table 2: Position of neutral zone in relation to the alveolar ridge crest in different locations for 2 years to 5 years of edentulism
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[Table 3] shows the position of neutral zone in relation to the alveolar crest ridge for group III. At left molar region, left premolar region, anterior region, right premolar region, and right molar region, the mean value of neutral zone in relation to crest of mandibular alveolar ridge are −3.6 mm, 0.00 mm, 3.3 mm, 0.00 mm, and −3.8 mm, respectively, with mean duration of edentulism is 8.5 years. | Table 3: Position of neutral zone in relation to the alveolar ridge crest in diff erent locations for more than 5 years of edentulism
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At left molar region, 60% of subjects showed lingual positioning of neutral zone in group I and percentage increased to 80% in group II and 93.3% in group III, which is highly significant(P = 0.002) [Table 4]. At right molar region, 53.3% of subjects showed lingual positioning of neural zone in group I and percentage increased to 80% in group II and 93.3% in group III, which is highly significant (P = 0.001) [Table 5]. The correlation of percentage count of neutral zone locations with different durations of edentulism at the right and left molar regions is shown in Graph 1 [Additional file 1]. | Table 4: Percentage count, mean, SD, maximum and minimum value of neutral zone with different duration of edentulism at left molar region
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 | Table 5: Percentage count, mean, SD, maximum and minimum value of neutral zone with diff erent duration of edentulism at right molar region
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At left premolar region, 73.3% of subjects showed the position of neutral zone coinciding with the crest of the ridge in group I, 66.7% in group II, and 73.3% in group III. 15.6% showed lingual positioning and 13.3% showed buccal positioning that reflects there is no trend of the buccal and lingual positioning of the neutral zone [Table 6]. At right premolar region, 66.7% of subjects showed the position of neutral zone coinciding with the crest of the ridge in group I, 66.7% in group II, and 73.3% in group III. 17.8% showed lingual positioning and 13.3% showed buccal positioning that reflects there is no trend of the buccal and lingual positioning of the neutral zone [Table 7]. The correlation of percentage count of neutral zone locations with different durations of edentulism at the right and left premolar regions is shown in Graph 2 [Additional file 2]. | Table 6: Percentage count of neutral zone locations with different duration of edentulism at left premolar region
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 | Table 7: Percentage count of neutral zone locations with different duration of edentulism at right premolar region
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The position of neutral zone with different period of edentulism in anterior region [Table 8], showed a trend of normal physiological process where the mandibular anterior alveolar ridges resorbed centrifugally and showed standard deviation of 1.79749 at 0-2 years of edentulism, 1.42428 at 2-5 years of edentulism and 1.34519 for more than 5 years of edentulism, which is highly significant (P = 0.005). | Table 8: Mean, SD, maximum and minimum value of neutral zone with diff erent duration of edentulism in anterior region
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| Discussion | | |
Complete denture prosthesis is the conventional treatment option for all edentulous patients. Establishing support, harmonious occlusion with good retention, stability, esthetics, and phonetics are the prime objectives and goal for all prosthodontist. In order to construct dentures that function properly not only in chewing but also in speaking and swallowing, we prosthodontist must develop the fit and contour of the external surface just as accurately and meticulously as the fit and contour of the impression surface and occlusal surface.
Natural teeth are most movable part of the masticatory system. If outward horizontal forces from the tongue are greater than inward forces exerted by the buccinator muscle bands and the lips, the teeth will move horizontally until the opposing forces are equal. This is the neutral zone. As teeth erupt into the mouth, they are guided into a specific zone of neutrality that determines the horizontal position of each tooth in the arch.
The position of NZ with different duration of edentulism at right and left molar region showed a trend of normal physiological process where mandibular posterior ridges resorb centripetally and neutral zone was positioned lingually, which is in agreement with the study conducted by Martone [12],[13] in 1963. However, a small percentage of subjects have shown buccal positioning of neutral zone.
The position of NZ at right and left premolar region, showed a trend of normal physiological process where mandibular posterior ridges resorbed directly under the buttress and position of NZ was directly over the crest of the ridge, which is in agreement with the study conducted by Weinberg [14] in 1958. However, small percentage of subjects have shown buccal and lingual positioning of the neutral zone.
In anterior region, position of NZ with different period of edentulism showed a trend of normal physiological process, where anterior mandibular ridges resorbed centrifugally and the position of NZ located labially, which is in accordance with the study conducted by Lammie [15] in 1956 and Fahmi [2] in 1991.
Six subjects (40%) in the study group I, where the period of edentulism is within 6 months and five subjects (33.3%), where period of edentulism is within 1-year [Table 1]. In total, 11 subjects (73.3%) were within a 1-year period of edentulism, so it could be one of the reasons for lingual positioning of the neutral zone in 56.6% subjects. The results have shown a trend of lingual positioning of neutral zone increasing to 80% in group II and 93.3% in group III. This small percentage of buccal positioning of neutral zone may be due to increase in muscular forces from the tongue muscle and decreased muscular forces from cheek muscles, which is a resultant neutral zone in molar region.
The observations suggested that the location of neutral zone varies from individual to individual depending on their musculature and there is significant relation to the duration of edentulism. As the duration of edentulism increases, there is more lingual positioning of neutral zone at molar region of both side of the arch. At premolar region, there is no change in position of neutral zone, it remains constant since resorption of the alveolar ridge is directly under the buttress. In anterior region, there is more labial positioning of neutral zone as edentulism increases. This supports the study done by Bhargonde et al. [11]
| Conclusion | | |
This study brings to light the importance of the period of edentulism and its usefulness in the arrangement of teeth.
Correlation of percentage count of neutral zone locations with different duration of edentulism at right and left molar region.
Correlation of percentage count of neutral zone locations with different duration of edentulism at right and left premolar region.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]
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