The position ridge
of the neutral
zone in relation
to the alveolar
F. M. Fahmi, BDS, MSca College of Dentistry, King Saud University, Kingdom of Saudi Arabia Included in this study are 21 edentulous patients. They were classified according to their period of edentulousness into two groups. One group was edentulous from r/i to 2 years. The other group was edentulous for more than 2 years. The neutral zone (NZ) was located for each patient by using record bases, and an impression compound as the occlusion rim. A 30-gauge stainless steel wire was adapted along the center of the alveolar ridge on the final cast. A 24-gauge stainless steel wire was adapted and stabilized over the center of each occlusion rim. An occlusal view radiograph was obtained of each recording base and its cast. Each film was placed in a viewing box and the relationship between the images of the two wires in the buccoiingual direction was studied in the anterior, premolar, and molar regions on the left and right sides. Where the two images coincided a zero score was given. In the case of buccal location a plus score was given, and where there was a lingual location of the (NZ), a minus score was given. All measures were taken in millimeters. Statistical analysis was performed between the two groups. The longer the period of edentulousness, the more bucally/lingually located was the NZ. (J PROSTHET DENT 1992;67:805-9.)
S
uccessful treatment of patients with complete dentures depends upon the proper positioning of the artificial teeth. Continuous resorption of the residual alveolar ridge and the forces from the perioral musculature imposes challenges in determining the tooth position. Weinbergi stated that buccal cusps and fossae of the posterior teeth should be directly over the crest of the ridge. This position is said to result in more stability and less lateral force since the occlusal pressure on the tooth falls close to the fulcrum and creates little or no ,torque. Hertwell and Rahn2 indicated that the posterior teeth should be positioned buccolingually on the residual alveolar ridge. Pound3 stated that invariably arranging the teeth over the crest of the residual ridge condemned patients by accentuating facial deformity, provoking phonetic problems, making food manipulation difficult during deglutition, and because of the instability of the mandibular denture. Robinson,4 Payne,5 Murray,G and Watt7 are of the opinion that artificial teeth should be positioned where the natural teeth grew. Brill et a1.sstated that forces are developed as a result of the contraction of muscles during function. These forces are directed against the denture and will either help stabilize it or tend to dislodge it. Beresin and Schiesserg have suggested that the denture teeth should be arranged in the neutral zone, where during function the forces of the tongue pressing outward are neutralized by the forces of cheeks and lips pressing inward.. The extraction of teeth causes losses from the buccal
aAssociateProfessor,Department of RemovableProsthetic Dental Sciences. 10/l/35884
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
sides of the maxillary alveolar ridges and from the lingual side of the mandibular alveolar ridges, which leads to smaller maxillae and larger mandibular residual ridges. Thus the residual alveolar ridge crest changes its location in a buccolingual direction after resorption. In addition, the tongue has a tendency to enlarge in the edentulous mouth. These factors influence the buccolingual position of the neutral zone (NZ). This study was undertaken to determine the center of the alveolar ridge crest with respect to the neutral zone in a buccolingual direction at the anterior, premolar, and molar regions on both right and left sides.
MATERIAL
AND
METHODS
Included in this study were 21 edentulous patients treated in the College of Dentistry, King Saud University, Riyadh, Saudi Arabia. These patients were edentulous for 6 months to 9 years. Final mandibular casts were prepared in dental stone (Columbus Dental, St. Louis, MO.). Undercuts were blocked, separating medium was applied to the casts, and recording bases were prepared in autopolymerizing acrylic resin (Dentsply Ltd., Webridge, England) by the sprinkle-on technique. The acrylic resin was allowed to polymerize in a water bath at 37O C and 20 psi for 15 minutes. Recording bases were separated from the casts, finished, and polished. The bases were tried in the patient’s mouth and were checked for comfort, retention, stability, and extension. Impression compound (Kerr-Sybron, Romulus, Mich.) was softened in a 65’ C water bath. The softened compound was kneaded and a roll was formed and attached to the crest of the mandibular recording base. A small amount of the compound was manipulated along the labial,
365
FAHMI
1. Stainless steel wire is adapted to the center of the alveolar ridge.
Fig.
Fig.
3. Record bases are placed on the final cast.
Table I. Distribution edentulousness
of patients based on period of
Period of edentulousness
No. of patients
O-2 years More than 2 years
12 9
Total
2. A 24-gauge stainless steel wire is adapted to the center of the neutral zone on the compound occlusion rim.
Fig.
buccal, and lingual slopes. The attached roll of compound was reheated in the water bath and was carried into the patient’s mouth. With the recording base firmly seated and the dentist’s fingers out of the patient’s mouth, the patient was instructed to swallow and suck several times. During function of the lips, cheeks, and the tongue, the forces exerted on the soft compound molded it into the shape of the neutral zone. After the compound cooled, the recording base with the compound rim was removed. A sharp knife was used to reduce the height of the compound rim level with the height of two thirds of the retromolar pads and the corners of the mouth. The crest of the alveolar ridge on the final casts was trimmed about 1 mm using a sharp knife. A 30-gauge stainless steel wire (Unitek Corp., Monrovia, Calif.) was adapted along the center of the alveolar ridge
806
z
(Fig. 1). The wire was stabilized in position using a commercial glue. The center of the buccolingual width of each compound rim was marked along its length. A 24-gauge stainless steel wire (Unitek) was also adapted and stabilized over the center of each occlusion rim, as shown in Fig. 2. The recording base was repositioned on the final cast (Fig. 3) and the occlusal surface of the compound rim was made horizontal using a parallelometer. An occlusal view radiograph was obtained of each recording base and its casts, using ultraspeed film (Film size 2% x 3 in, Eastman Kodak Co., Rochester, N.Y.). The source-to-object distance was 12 in and the central ray was directed at the center of the cast. Exposure parameters used were 90 kilovolts (peak), 15 mA, and 20 impulses. All the films were developed at the same time in an automatic processor. Fig. 4 shows an occlusal view radiograph, where the thinner wire’s image indicates the crest of the alveolar ridge and the thicker wire’s image denotes the center of the neutral zone (NZ). Each film was placed in a viewing box and the relationship between the images of the two wires in a buccolingual direction was studied. Where the two images coincided, a zero score was assigned. Buccal and labial locations of the NZ with respect to the ridge were assigned a positive value. Lingual locations of the NZ with respect
JUNE
1992
VOLUME
67
NUMBER
6
NEUTRAL
ZONE AND ALVEOLAR
RIDGE
Fig. 4. Occlusal film with thin wire representing the crest of the ridge and thick wire representing the center of the neutral zone.
Table
II.
Position of the neutral zone in relation to the alveolar ridge in different locations Parameters
No. of subjects
Period of edentulousness b-9*
1 2 3 4 5 6 7 8 9 10 11 12
0.5 0.5 0.5 0.5 1 1 1 1 1 1 1.5 1.5
Total
-3.5 -1.5 +3 0 0 -2.5 0 -1 0 0 -1
11
Mean
Left molar
0.9166
All measurements in millimeters. 0, NZ coincides with the alveolar. ridge: +, NZ is buccal *Period of edentulousness is lees than 2 years.
0 -6.5 0.5417
or labial with respect
to the ridge were assigned a negative value. Measurements were made with a millimeter ruler to an accuracy of 0.5 mm. RESULTS The patients were classified according to their period of edentulousness into two groups, as shown in Table I. The first group was edentulous for ‘/z year to 2 years. The sec-
THE JOURNAL
OF PROSTHETIC
DENTISTRY
Left premolar
Anterior
Right molar
Right premolar
0 -2 +3 -1.5 0 -2.5 +2 0 0 0 0 +2 +1 0.0833
-2.5 0 +1.5 0 -2 -2.5 0 -1.5 +2.5 +1 0 0 -3.5 -0.2917
0 -2 +1 0 -2 +2 0 0 0 0 0 0 -1 -0.0833
-1.5 0 0 0 +3 0 0 0 +1.5 0 0 0 3 0.25
to alveolar
ridge; -, NZ is lingually
located
with respect
to alveolar
ridge.
ond group had been edentulous for more than 2 years. The positions of the NZ with respect to the alveolar ridge centers for the first group in different locations-that is, the anterior, premolar right, premolar left, molar right, and molar left regions-are summarized in Table II. In 34 locations (56.7 9%) the NZ coincided with the crest of the alveolar ridge. In 15 locations (25%) the NZ was located lin-
807
FAHMI
Table
III.
Position of the neutral zone in relation to the alveolar ridge in different locations Parameters Period of edentulousness (Ye*
No. of subjects 1 2 3 4 5 6 I 8 9
2 2 3 4 5 5 6 6 9 42 4.6666
Total Mean
Left molar
Left premolar
+2 -2 +2 +2 +1 +1.5 +2 +3 +2 13.5 1.5
+2.5 0 +2 +2.5 +2.5 +3 +2 +3 +2 19.5 2.1667
All measurements in millimeters. 0, NZ coincides with the alveolar ridge; +, NZ is buccal or labial with respect *Period of edentulousness is m.ore than 2 years.
Table
IV. Period
O-2
of edentulousness
yr
t P Standard
Mean (mm) STD Mean (mm) STD
Left molar
0.09166 0.3588 4.6666 2.2360 5.76 0.005
Left premolar
-0.5417
to alveolar
Anterior
1.6020 1.5 1.4142 3.0347
0.0833 1.6353 2.1667 0.9014 3.4363
0.01
0.01
ridge; -, NZ is lingual
Right premolar
+3.5 -1 +2 +2 +5 +4 0 +4 +2 21.5 2.3889
with respect
+2.5 0 0 +2 +4 +2.5 0 +4 fl 16 1.7718
to alveolar
ridge.
Right molar -0.2917 1.5733 2 2.2361 2.7628 0.02
Right premolar
Total
-0.0833 1.0836 2.3889 1.9650 3.6923
0.25 1.0766 1.7778 1.6223 2.5974 0.02
0.01
0.9167 0.3589 4.6667 2.2361 5.76 0.005
deviation.
gually, and in 11 locations the NZ was located buccally or labially (18.3%). The relationship for the second group is shown in Table III. In this group the NZ coincided with the alveolar ridge (20%). On two casts the NZ was located lingually (5 % ), while it was located buccally or labially in relation to the crest of the ridge in a total of 30 casts (75 % ). A statistical analysis was undertaken between the two groups and for the different locations. The data were analyzed statistically by Student’s t test. The mean values, standard deviations, and degrees of significance are shown in Table IV. DISCUSSION The concept of the NZ is known as an important factor in complete denture fabrication.lO,ll Further study was undertaken to determine its dimensions at different occlusal heights.12 The present study investigated the relationship between the position of the NZ and the crest of the ridge in a buccolingual direction. It is aimed at supplying information to facilitate positioning the teeth in a region of minimum conflict, so that the stability of the denture is enhanced.
808
+2.5 -2.5 0 +2 +3.5 +1 +3 +5 +3.5 18 2
Right molar
Means, standard deviations, and statistical significance between all groups
More than 2 yr
STD,
Anterior
The mandible usually exhibits atrophy to a much greater extent than the maxillae. Many reasons for this are postulated. Most are based on the change of blood supply that occurs with increasing age following the process of bone resorption. The result will be a decrease in the denturebearing area, buccally and labially. The vestibules also become less pronounced as the adjacent muscles are positioned higher on the alveolar ridge. Anteriorly, the influence of the lip on the mandibular denture becomes more critical as resorption of the ridge increases or as the patient becomes older. Lammie13 has shown that as the ridge resorbs, the ridge crest falls below the origin of the mentalis muscle. When such resorption occurs, the muscle attachment folds over the alveolar ridge and comes to rest on the superior surface of the crest. The result is a posterior positioning of the NZ. Posterior positioning of the NZ may indicate the need to place the mandibular anterior teeth more lingually. The results of this study support the preceding theory for patients who were edentulous for a period of less than 2 years. However, the NZ was found to be labially located by a mean of 2 mm in patients who were edentulous for more
JUNE
1992
VOLUME
67
NUMBER
6
NEUTRAL
ZONE
AND
ALVEOLAR
RIDGE
than 2 years (Table IV). Accordingly, the anterior teeth should be positioned more labially to the crest of the ridge. However, consideration should be given to phonetics and esthetics. Wright14 stated that if the size of the mandibular teeth are too large or if the posterior teeth are set even 1 mm lingually, the tongue is deprived of approximately 1000 mm3 of its functional space. This can force the tongue into an abnormally retracted position. Transformation of the dentulous to the edentulous state causes a one tenth increase in tongue size. Wright et al. l5 have also indicated that the mandibular denture would receive more tongue pressure as a result of this increase in tongue size. This study also supports that hypothesis. The NZ in the most posterior locations was found to be located more buccally than lingually, ranging from 1.5 to 2.388 mm (Table IV). Slow resorption of alveolar bone over a period of years is less likely, however, to affect the ability of the long-term denture wearer to control a prosthesis. l6 The environment created by the musculature becomes more important as a retention factor and the ridge is not always the vital factor it used to be. SUMMARY The buccolingual relationship of the NZ to the crest of the residual alveolar ridge was studied in 21 edentulous patients. The correlation was studied in vitro from radiographic images obtained Ion occlusal films. The NZ in relation to the crest of the residual ridge was investigated in the anterior, premolar, and molar regions. The position of the NZ in relation to the alveolar ridge was found to be highly affected by the period of edentulousness (p = 005). The longer the period of edentulousness, the more buccallyflabially located was the NZ. The findings of
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
this study may be useful in arranging artificial teeth for complete dentures. REFERENCES 1. Weinberg LA. Tooth position in relation to the denture base foundation. J PROSTHET DENT 1958;8:398-405. 2. Heartwell CM, Rahn AO. Syllabus of complete dentures. 4th ed. Phil-
adelphia:
Lea & Febiger, 1986357. E. Lost fine arts in the fallacy of the ridges. J PROSTHET DENT 1951;1:98-111. Robinson SC. Physiological placement of artificial anterior teeth. J Can Dent Assoc 1969;35:260-6. Payne AGL. Factors influencing the position of artificial upper anterior teeth. J PROSTHET DENT 1971;26:26-32. Murray CJ. Re-estnblishing natural teeth position in the edentulous environment. Aust Dent J 1978;23:415-21. Watt DM. Tooth positions of complete dentures. J Dent 1978;6:14’7-60. Brill N, Tryde G, Cantor R. The dynamic nature of the lower denture space. J PROSTHET DENT 1965;15:401-16. Beresin VE, Schiesser FJ. The neutral zone in complete dentures. J
3. Pound 4 5. 6. 7. 8. 9.
PROSTHET DENT 1976;36:356-67. 10. Fish EW. An analysis of the stabilizing
tion. Br Dent J 1947;83:137-42. 11. Raybin NH. The polished surface
force in full denture
of complete
dentures.
construc-
J PROSTHET
DENT 1963;13:236-41. 12. Rasek M, Abdalla
F. Two-dimensional study of the neutral zone at different occlusal vertical heights. J PROSTHET DENT 1981;46:4OW34. 13. Lammie GA. Aging changes in the complete lower dentures. J PROSTHET DENT 1956;36:356-63. 14. Wright CR. Evaluation
of the forces necessary to develop stability in mandibular denture. J PROSTHET DENT 1966;16:414-30. 15. Wright CR, Swarm WH, Gradwin WC. Mandibular denture stability. Ann Arbor, Mich: The Overbek Company, 1961:32. 16. Anderson F, Storer R. Immediate and replacement dentures. 3rd ed. Oxford: Blackwell Scientillc Publications, 1981:216. Reprint requests to: DR. FAISAL FAHMI KING SAUII UNIVERSTTY COLLEGE CIF DENTISTRY P. 0. Box 60169, RIYADH 11545 KINGDOM OF SAUDI ARABIA
809
of the neutral
zone in relation
to the alveolar
F. M. Fahmi, BDS, MSca College of Dentistry, King Saud University, Kingdom of Saudi Arabia Included in this study are 21 edentulous patients. They were classified according to their period of edentulousness into two groups. One group was edentulous from r/i to 2 years. The other group was edentulous for more than 2 years. The neutral zone (NZ) was located for each patient by using record bases, and an impression compound as the occlusion rim. A 30-gauge stainless steel wire was adapted along the center of the alveolar ridge on the final cast. A 24-gauge stainless steel wire was adapted and stabilized over the center of each occlusion rim. An occlusal view radiograph was obtained of each recording base and its cast. Each film was placed in a viewing box and the relationship between the images of the two wires in the buccoiingual direction was studied in the anterior, premolar, and molar regions on the left and right sides. Where the two images coincided a zero score was given. In the case of buccal location a plus score was given, and where there was a lingual location of the (NZ), a minus score was given. All measures were taken in millimeters. Statistical analysis was performed between the two groups. The longer the period of edentulousness, the more bucally/lingually located was the NZ. (J PROSTHET DENT 1992;67:805-9.)
S
uccessful treatment of patients with complete dentures depends upon the proper positioning of the artificial teeth. Continuous resorption of the residual alveolar ridge and the forces from the perioral musculature imposes challenges in determining the tooth position. Weinbergi stated that buccal cusps and fossae of the posterior teeth should be directly over the crest of the ridge. This position is said to result in more stability and less lateral force since the occlusal pressure on the tooth falls close to the fulcrum and creates little or no ,torque. Hertwell and Rahn2 indicated that the posterior teeth should be positioned buccolingually on the residual alveolar ridge. Pound3 stated that invariably arranging the teeth over the crest of the residual ridge condemned patients by accentuating facial deformity, provoking phonetic problems, making food manipulation difficult during deglutition, and because of the instability of the mandibular denture. Robinson,4 Payne,5 Murray,G and Watt7 are of the opinion that artificial teeth should be positioned where the natural teeth grew. Brill et a1.sstated that forces are developed as a result of the contraction of muscles during function. These forces are directed against the denture and will either help stabilize it or tend to dislodge it. Beresin and Schiesserg have suggested that the denture teeth should be arranged in the neutral zone, where during function the forces of the tongue pressing outward are neutralized by the forces of cheeks and lips pressing inward.. The extraction of teeth causes losses from the buccal
aAssociateProfessor,Department of RemovableProsthetic Dental Sciences. 10/l/35884
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
sides of the maxillary alveolar ridges and from the lingual side of the mandibular alveolar ridges, which leads to smaller maxillae and larger mandibular residual ridges. Thus the residual alveolar ridge crest changes its location in a buccolingual direction after resorption. In addition, the tongue has a tendency to enlarge in the edentulous mouth. These factors influence the buccolingual position of the neutral zone (NZ). This study was undertaken to determine the center of the alveolar ridge crest with respect to the neutral zone in a buccolingual direction at the anterior, premolar, and molar regions on both right and left sides.
MATERIAL
AND
METHODS
Included in this study were 21 edentulous patients treated in the College of Dentistry, King Saud University, Riyadh, Saudi Arabia. These patients were edentulous for 6 months to 9 years. Final mandibular casts were prepared in dental stone (Columbus Dental, St. Louis, MO.). Undercuts were blocked, separating medium was applied to the casts, and recording bases were prepared in autopolymerizing acrylic resin (Dentsply Ltd., Webridge, England) by the sprinkle-on technique. The acrylic resin was allowed to polymerize in a water bath at 37O C and 20 psi for 15 minutes. Recording bases were separated from the casts, finished, and polished. The bases were tried in the patient’s mouth and were checked for comfort, retention, stability, and extension. Impression compound (Kerr-Sybron, Romulus, Mich.) was softened in a 65’ C water bath. The softened compound was kneaded and a roll was formed and attached to the crest of the mandibular recording base. A small amount of the compound was manipulated along the labial,
365
FAHMI
1. Stainless steel wire is adapted to the center of the alveolar ridge.
Fig.
Fig.
3. Record bases are placed on the final cast.
Table I. Distribution edentulousness
of patients based on period of
Period of edentulousness
No. of patients
O-2 years More than 2 years
12 9
Total
2. A 24-gauge stainless steel wire is adapted to the center of the neutral zone on the compound occlusion rim.
Fig.
buccal, and lingual slopes. The attached roll of compound was reheated in the water bath and was carried into the patient’s mouth. With the recording base firmly seated and the dentist’s fingers out of the patient’s mouth, the patient was instructed to swallow and suck several times. During function of the lips, cheeks, and the tongue, the forces exerted on the soft compound molded it into the shape of the neutral zone. After the compound cooled, the recording base with the compound rim was removed. A sharp knife was used to reduce the height of the compound rim level with the height of two thirds of the retromolar pads and the corners of the mouth. The crest of the alveolar ridge on the final casts was trimmed about 1 mm using a sharp knife. A 30-gauge stainless steel wire (Unitek Corp., Monrovia, Calif.) was adapted along the center of the alveolar ridge
806
z
(Fig. 1). The wire was stabilized in position using a commercial glue. The center of the buccolingual width of each compound rim was marked along its length. A 24-gauge stainless steel wire (Unitek) was also adapted and stabilized over the center of each occlusion rim, as shown in Fig. 2. The recording base was repositioned on the final cast (Fig. 3) and the occlusal surface of the compound rim was made horizontal using a parallelometer. An occlusal view radiograph was obtained of each recording base and its casts, using ultraspeed film (Film size 2% x 3 in, Eastman Kodak Co., Rochester, N.Y.). The source-to-object distance was 12 in and the central ray was directed at the center of the cast. Exposure parameters used were 90 kilovolts (peak), 15 mA, and 20 impulses. All the films were developed at the same time in an automatic processor. Fig. 4 shows an occlusal view radiograph, where the thinner wire’s image indicates the crest of the alveolar ridge and the thicker wire’s image denotes the center of the neutral zone (NZ). Each film was placed in a viewing box and the relationship between the images of the two wires in a buccolingual direction was studied. Where the two images coincided, a zero score was assigned. Buccal and labial locations of the NZ with respect to the ridge were assigned a positive value. Lingual locations of the NZ with respect
JUNE
1992
VOLUME
67
NUMBER
6
NEUTRAL
ZONE AND ALVEOLAR
RIDGE
Fig. 4. Occlusal film with thin wire representing the crest of the ridge and thick wire representing the center of the neutral zone.
Table
II.
Position of the neutral zone in relation to the alveolar ridge in different locations Parameters
No. of subjects
Period of edentulousness b-9*
1 2 3 4 5 6 7 8 9 10 11 12
0.5 0.5 0.5 0.5 1 1 1 1 1 1 1.5 1.5
Total
-3.5 -1.5 +3 0 0 -2.5 0 -1 0 0 -1
11
Mean
Left molar
0.9166
All measurements in millimeters. 0, NZ coincides with the alveolar. ridge: +, NZ is buccal *Period of edentulousness is lees than 2 years.
0 -6.5 0.5417
or labial with respect
to the ridge were assigned a negative value. Measurements were made with a millimeter ruler to an accuracy of 0.5 mm. RESULTS The patients were classified according to their period of edentulousness into two groups, as shown in Table I. The first group was edentulous for ‘/z year to 2 years. The sec-
THE JOURNAL
OF PROSTHETIC
DENTISTRY
Left premolar
Anterior
Right molar
Right premolar
0 -2 +3 -1.5 0 -2.5 +2 0 0 0 0 +2 +1 0.0833
-2.5 0 +1.5 0 -2 -2.5 0 -1.5 +2.5 +1 0 0 -3.5 -0.2917
0 -2 +1 0 -2 +2 0 0 0 0 0 0 -1 -0.0833
-1.5 0 0 0 +3 0 0 0 +1.5 0 0 0 3 0.25
to alveolar
ridge; -, NZ is lingually
located
with respect
to alveolar
ridge.
ond group had been edentulous for more than 2 years. The positions of the NZ with respect to the alveolar ridge centers for the first group in different locations-that is, the anterior, premolar right, premolar left, molar right, and molar left regions-are summarized in Table II. In 34 locations (56.7 9%) the NZ coincided with the crest of the alveolar ridge. In 15 locations (25%) the NZ was located lin-
807
FAHMI
Table
III.
Position of the neutral zone in relation to the alveolar ridge in different locations Parameters Period of edentulousness (Ye*
No. of subjects 1 2 3 4 5 6 I 8 9
2 2 3 4 5 5 6 6 9 42 4.6666
Total Mean
Left molar
Left premolar
+2 -2 +2 +2 +1 +1.5 +2 +3 +2 13.5 1.5
+2.5 0 +2 +2.5 +2.5 +3 +2 +3 +2 19.5 2.1667
All measurements in millimeters. 0, NZ coincides with the alveolar ridge; +, NZ is buccal or labial with respect *Period of edentulousness is m.ore than 2 years.
Table
IV. Period
O-2
of edentulousness
yr
t P Standard
Mean (mm) STD Mean (mm) STD
Left molar
0.09166 0.3588 4.6666 2.2360 5.76 0.005
Left premolar
-0.5417
to alveolar
Anterior
1.6020 1.5 1.4142 3.0347
0.0833 1.6353 2.1667 0.9014 3.4363
0.01
0.01
ridge; -, NZ is lingual
Right premolar
+3.5 -1 +2 +2 +5 +4 0 +4 +2 21.5 2.3889
with respect
+2.5 0 0 +2 +4 +2.5 0 +4 fl 16 1.7718
to alveolar
ridge.
Right molar -0.2917 1.5733 2 2.2361 2.7628 0.02
Right premolar
Total
-0.0833 1.0836 2.3889 1.9650 3.6923
0.25 1.0766 1.7778 1.6223 2.5974 0.02
0.01
0.9167 0.3589 4.6667 2.2361 5.76 0.005
deviation.
gually, and in 11 locations the NZ was located buccally or labially (18.3%). The relationship for the second group is shown in Table III. In this group the NZ coincided with the alveolar ridge (20%). On two casts the NZ was located lingually (5 % ), while it was located buccally or labially in relation to the crest of the ridge in a total of 30 casts (75 % ). A statistical analysis was undertaken between the two groups and for the different locations. The data were analyzed statistically by Student’s t test. The mean values, standard deviations, and degrees of significance are shown in Table IV. DISCUSSION The concept of the NZ is known as an important factor in complete denture fabrication.lO,ll Further study was undertaken to determine its dimensions at different occlusal heights.12 The present study investigated the relationship between the position of the NZ and the crest of the ridge in a buccolingual direction. It is aimed at supplying information to facilitate positioning the teeth in a region of minimum conflict, so that the stability of the denture is enhanced.
808
+2.5 -2.5 0 +2 +3.5 +1 +3 +5 +3.5 18 2
Right molar
Means, standard deviations, and statistical significance between all groups
More than 2 yr
STD,
Anterior
The mandible usually exhibits atrophy to a much greater extent than the maxillae. Many reasons for this are postulated. Most are based on the change of blood supply that occurs with increasing age following the process of bone resorption. The result will be a decrease in the denturebearing area, buccally and labially. The vestibules also become less pronounced as the adjacent muscles are positioned higher on the alveolar ridge. Anteriorly, the influence of the lip on the mandibular denture becomes more critical as resorption of the ridge increases or as the patient becomes older. Lammie13 has shown that as the ridge resorbs, the ridge crest falls below the origin of the mentalis muscle. When such resorption occurs, the muscle attachment folds over the alveolar ridge and comes to rest on the superior surface of the crest. The result is a posterior positioning of the NZ. Posterior positioning of the NZ may indicate the need to place the mandibular anterior teeth more lingually. The results of this study support the preceding theory for patients who were edentulous for a period of less than 2 years. However, the NZ was found to be labially located by a mean of 2 mm in patients who were edentulous for more
JUNE
1992
VOLUME
67
NUMBER
6
NEUTRAL
ZONE
AND
ALVEOLAR
RIDGE
than 2 years (Table IV). Accordingly, the anterior teeth should be positioned more labially to the crest of the ridge. However, consideration should be given to phonetics and esthetics. Wright14 stated that if the size of the mandibular teeth are too large or if the posterior teeth are set even 1 mm lingually, the tongue is deprived of approximately 1000 mm3 of its functional space. This can force the tongue into an abnormally retracted position. Transformation of the dentulous to the edentulous state causes a one tenth increase in tongue size. Wright et al. l5 have also indicated that the mandibular denture would receive more tongue pressure as a result of this increase in tongue size. This study also supports that hypothesis. The NZ in the most posterior locations was found to be located more buccally than lingually, ranging from 1.5 to 2.388 mm (Table IV). Slow resorption of alveolar bone over a period of years is less likely, however, to affect the ability of the long-term denture wearer to control a prosthesis. l6 The environment created by the musculature becomes more important as a retention factor and the ridge is not always the vital factor it used to be. SUMMARY The buccolingual relationship of the NZ to the crest of the residual alveolar ridge was studied in 21 edentulous patients. The correlation was studied in vitro from radiographic images obtained Ion occlusal films. The NZ in relation to the crest of the residual ridge was investigated in the anterior, premolar, and molar regions. The position of the NZ in relation to the alveolar ridge was found to be highly affected by the period of edentulousness (p = 005). The longer the period of edentulousness, the more buccallyflabially located was the NZ. The findings of
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
this study may be useful in arranging artificial teeth for complete dentures. REFERENCES 1. Weinberg LA. Tooth position in relation to the denture base foundation. J PROSTHET DENT 1958;8:398-405. 2. Heartwell CM, Rahn AO. Syllabus of complete dentures. 4th ed. Phil-
adelphia:
Lea & Febiger, 1986357. E. Lost fine arts in the fallacy of the ridges. J PROSTHET DENT 1951;1:98-111. Robinson SC. Physiological placement of artificial anterior teeth. J Can Dent Assoc 1969;35:260-6. Payne AGL. Factors influencing the position of artificial upper anterior teeth. J PROSTHET DENT 1971;26:26-32. Murray CJ. Re-estnblishing natural teeth position in the edentulous environment. Aust Dent J 1978;23:415-21. Watt DM. Tooth positions of complete dentures. J Dent 1978;6:14’7-60. Brill N, Tryde G, Cantor R. The dynamic nature of the lower denture space. J PROSTHET DENT 1965;15:401-16. Beresin VE, Schiesser FJ. The neutral zone in complete dentures. J
3. Pound 4 5. 6. 7. 8. 9.
PROSTHET DENT 1976;36:356-67. 10. Fish EW. An analysis of the stabilizing
tion. Br Dent J 1947;83:137-42. 11. Raybin NH. The polished surface
force in full denture
of complete
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construc-
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DENT 1963;13:236-41. 12. Rasek M, Abdalla
F. Two-dimensional study of the neutral zone at different occlusal vertical heights. J PROSTHET DENT 1981;46:4OW34. 13. Lammie GA. Aging changes in the complete lower dentures. J PROSTHET DENT 1956;36:356-63. 14. Wright CR. Evaluation
of the forces necessary to develop stability in mandibular denture. J PROSTHET DENT 1966;16:414-30. 15. Wright CR, Swarm WH, Gradwin WC. Mandibular denture stability. Ann Arbor, Mich: The Overbek Company, 1961:32. 16. Anderson F, Storer R. Immediate and replacement dentures. 3rd ed. Oxford: Blackwell Scientillc Publications, 1981:216. Reprint requests to: DR. FAISAL FAHMI KING SAUII UNIVERSTTY COLLEGE CIF DENTISTRY P. 0. Box 60169, RIYADH 11545 KINGDOM OF SAUDI ARABIA
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