The relationship between the mandibular incisor teeth and the lower lip

Complete dentures

The relationship teeth

and

N. Joseph lTandenberg

ktween

the man&b&r

incisor

the louver lip Nassif, lieutenant AFB, Calif.

Colonel,

USAF (DC)*

M

any prosthodontists insist that artificial anterior teeth should be placed in positions similar to those occupied by the natural teeth. Placing teeth in these esthetics, and physiologic support positions enhances phonetics, denture stability, for the lips, tongue, and cheeks. Most literature related to positioning of artificial anterior teeth emphasizes the positions of maxillary teeth .I-7 Little information is available regarding the positions of mandibular anterior teeth. Some, apparently, feel that lower natural teeth are not readily visible and that they play a minor role in denture esthetics. However, in speaking, people usually display more of the lower than the upper teeth.‘jl “t I conducted a study of 125 dentulous individuals making observations as they spoke. Twenty-nine per cent showed only lower teeth, 43 per cent showed more lower than upper teeth, 11 per cent showed more upper than lower teeth, and 7 per cent showed only upper teeth. Nine per cent showed no teeth when speaking. The purpose of this article is ( 1) to show the relationship of the lower incisor teeth to the lower lip and to other structures and (2) to suggest aids for positioning artificial lower incisor teeth. These suggestions are based upon a statistical analysis of measurements taken from lateral cephalometric radiographs. METHOD Fifty lateral cephalometric radiographs were made of 25 men and 25 women, 19 to 30 years of age. These radiographs were made using a Broadbent-Bolton radiographic cephalometer. The criteria for selection were that each subject have ( 1) a Class I molar relationship (Angle’s classification) and (2 j no previous orthodontic therapy. Condensed gree of Master

from a thesis prepared for partial fulfillment of the requirements of Science at The Ohio State University, Columbus, Ohio.

The views expressed herein are those of the author and do not necessarily views of the United States Air Force or the Department of Defense. *Chief, Prosthodontic Services, 392nd Aerospace tBrewer, A.: Personal communication, 1964.

Medical

for the dereflect

the

Group.

483

484

Nassif

Fig. 1

Fig. 1. The Frankfort plane is located on cephalometric radiographs by passing two points (P. and 0.). Point 0. (orbitale) is the lowest point in the inferior bony orbit. Point P. (machine porion) is the upper margin of the radiopaque the metallic ear posts that were placed in the external auditory meatus of the

J. Prosth. Dent. November, 1970

Fig. 2

a line through margin of the image made by subjectsrr

Fig. 2. The lower facial plane (L.F.PL.) is perpendicular to the Frankfort plane (F.PL.) and passes through the subnasion (S.). Subnasion (A’.) is the anatomic point where the nasal septum merges with the upper lip. The chin prominence (C.P.) is the most anterior aspect of the soft-tissue chin.

Before the radiographs were made, each subject was instructed to close in centric occlusion and to relax his lips. Tracings from the film were made on acetate paper of the integumental profiles and of selected skeletal structures visible on the radiographs. Landmarks were used to establish two reference planes (Figs. 1 and 2) . One reference plane, the Frankfort plane, was used to plot two angular measurements. Five linear measurements were made from the other reference plane, the “lower facial plane” (Fig. 2) . The correlation coefficient was used in this study to show what relationship exists between the lower lip and selected structures. The correlation coefficient is expressed as a single figure. g It is calculated so that its numerical value is between t-1 and -1. A positive value means that as one measurement increases in value the other one also increases in value. A minus value means that as one measurement increases in value the other one decreases in value. The closer the correlation coefficient is to 51, the more closely the position of one landmark will depend upon the position of the other landmark. Two measurements with a correlation coefficient of 0.80 show a closer degree of relationship than does a correlation coefficient of 0.40. RELATION BETWEEN CHIN PROMINENCE AND INCLINATION OF THE LOWER INCISORS The Frankfort plane (Fig. 1) was used with each of two other planes in plotting linear (Fig. 2) and angular (Fig. 3) measurements. “Chin prominence” is defined as the most anterior aspect of the soft-tissue chin (Fig. 2). The lower facial plane is defined in the legend for Fig. 2. In my research study, 1 found

V&mlc 24: Numhcr5

Relationship

hetweeu

mandibular

inckors

and lower

1ij.1 485

Fig. 3

Fig. 4

Fig. 3. Determination The (A.)

of the lower incisor inclination (Frankfort plane angle, F.M.Z.A.). angle formed by the Frankfort plane (F.PL.) and the axial plane of the lower incisor is shown as F.M.Z.A. For the 50 subjects, this angle varied 27 degrees (Table I).

Fig. 4. -4 comparison between variations in chin prominence (A. and B.) and lower incisor inclination. Subject A (solid outline) whose chin prominence is greater than that of subject B (broken outline) shows less incisor inclination (solid outline). For subject B, less chin prominence is reflected by a greater incisor inclination (broken outline). The straight broken line indicates the lower facial plane (L.F.PL.). Table I. Summation of 50 subjects

of compiled

data

made

from

cephalometric

Mean

(mm.)

radiographs

Standard

Linear and angular measurements

Range

S-I” Chint F.M.1.A.f. F.L.L.A.$

2.9 .-0.1 462 13.8

(mm.) .--

12.5 20.3 73.8 68.2

6.2 7.9 61.1 40.9

deviation

(mm-1 2.0 4.7 5.7 9.7

*The distance from the supramentale to the most anterior surface of the lower incisor (Fig. 9). +The linear distance from the most anterior aspect of the soft-tissue chin to the lower facial plane (Fig. 2). $The angle formed by the Frankfort plane and the mandibular incisor axial inclination plane (Fig. 3). $The angle formed by the Frankfort plane and the lower lip plane (Fig. 7). teeth

that chin prominence varied from being located on the lower facial plane to being located 20 mm. posterior to it (Table I). Subjects whose chin prominences were on or near the lower facial planes tended to have incisors that were upright. The lower incisors were found to have more labial inclination as the distance increased between chin prominences and the lower facial planes (Fig. 4). Statistically, the correlation coefficient was -0.50. often the cervical margin of artificial lower incisor In clinical application, teeth is placed too far labially. A method will be suggested for making an initial determination of the inclination of artificial lower incisor teeth for trial dentures. Wax occlusion rims made at the established vertical dimension of occlusion

486

J. Prosth. Dent. November, 1970

Nassif

Q -.-----------7 ( . o.-“/ F ‘-P

Fig. 5

L

/ 477

I

Fig. 6

“----A. 1’ ----C.P.

Fig. 5. The clinical Frankfort plane passes through the two landmarks (P. and 0.). Point P. is the highest point in the roof of the external auditory meatus. Point 0. is the lowest palpable (C.P.) is the most anterior point point in the inferior margin of the orbit. 10 Chin prominence of the soft-tissue chin. The inclination of the lower incisor (A.) is shown by a broken line drawn through the long axis of the tooth. Fig. 6. Determination of the horizontal distance between the chin (C.P.) and the subnasion (S.). The head of the subject is inclined until the Frankfort plane (F.PL.) is parallel with the floor. A straight edge (A.) is held perpendicular to the Frankfort plane (F.PL.) with its edge passing through the subnasion (S.).

are placed in the mouth of the patient. Observe the subject from the profile side, and locate the Frankfort plane (Fig. 5). Palpate the lower border of the orbit, and mark it on the patient’s skin. Mark also, on the skin, the level of the roof of the external auditory meatus (Fig. 5). With the patient standing, tilt the head until the Frankfort plane is parallel to the floor. With the patient’s jaws closed in centric jaw relation, hold a straightedge perpendicular to the floor so that it passes through the subnasion (Fig. 6). Determine the distance between the straightedge and the chin. The greater this distance, the more labially inclined should be the artificial lower incisors.

RELATION BETWEEN INCLINATION AND THE LOWER LIP PLANE

OF THE LOWER INCISORS

The plotting of the inclination of the lower incisor teeth has been discussed (Fig. 3) . The determination of the “lower lip plane” is shown in the legend for Fig. 7. The correlation coefficient was plotted for the angle formed by the lower lip plane and by the F rankfort plane and for the angle formed by the axial plane of the lower incisor and the Frankfort plane (Fig. 8). The correlation coefficient between these two angles was -0.01; therefore, no predictable relationship exists.

RELATION BETWEEN INCLINATION AND THE SUPRAMENTALE The bones

OF THE LOWER INCISORS

“supramentale” is the theoretical junction (Fig. 9) .I1 The distance from the supramentale

between alveolar and basal to the most anterior surface

Relatiomlli/~

betweart

mandibular

incisors

and

lower

lip

487

Fig. 8

Fig. 7

Fig. 7. Determination of lower lip plane (L.L.PL.) point in the mentolabial The angle formed by the ation of 53 degrees (Table

the angle of the Frankfort passes through two points sulcus. Point B. is the lowest Frankfort plane and the lower I).

and lower lip planes (F.L.L.A.). The (A. and B.). Point A. is the deepest point in the margin of the lower lip. lip plane (L.L.PL.) had a range vari-

Fig. 8. Relationship between the inclination of the lower incisor (A.) and the lower lip plane (L.L.PL.). This study showed a correlation coefficient of -0.01 between angles F.M.I.A. and F.L.L.A. Therefore, the lower lip plane cannot be used as a reliable guide to determine the inclination of the lower incisors.

Fig. 9

\ h

I-

l.

Fig. 10

A.

y--B. I Fig. 9. Determination lies the (S. from

of the distance that the most anterior surface of the lower incisor (I.) in front of the supramentale (S). The supramentale (S.) is the most posterior point in bony profile between the alveolar crest and the bony chin. The horizontal distance to 1.) from the supramentale (S.) to the labial surface of the lower incisor (I.) ranged 2 to 12 mm. (Table I).

Fig. 10. The relationship of the labial surface of artificial lower incisors (I.) to the labial border of a complete denture (A.). If the 50 subjects employed in this study should become endentulous, the labial surface of the artificial lower incisors would lie anterior to line B. Incisors positioned behind line B. would be posterior to the natural tooth positions.

488

J. Prosth. Dent. November, 1970

Nassif

Fig. I1

Fig. 12

Fig. 11. Profile view of a lower lip. The lower lip sulcus (L.L.S.) in the concavity between the lower lip and the chin. The lower anterior surface on the vermillion border of the lower lip.

Fig. 13 is the most posterior surface lip tip (L.L.T.) is the most

Fig. 12. The distances from the lower lip tip (L.L.T.) and the lower lip sulcus (L.L.S.) to the lower facial plane (L.F.PL.) are shown. These distances were directly related to each other in many of the 50 subjects. Fig. lip tion the

13. Profile view of a lower lip. The positions of the lower lip tip (L.L.T.) and the lower sulcus (L.L.S.) were interdependent. Each 1 mm. of change in the anteroposterior posiof the lower lip tip (L.L.T.) would find the lower lip sulcus (L.L.S.) position moved in same direction by approximately 1.5 mm.

of the lower incisor was measured on a line parallel to the Frankfort plane. This distance was plotted against the angle formed by the axial plane of the lower incisor and the Frankfort plane (Fig. 3). The correlation coefficient was -0.65. Generally speaking, the more the lower central incisor inclines forward, the greater will be the anterior distance of the labial surface to the supramentale (Fig. 9). Of the 50 subjects measured in this study, the most anterior surface of the lower incisor was always anterior to the supramentale (Table I). For purposes of clinical application, assume that the 50 subjects employed in this study became edentulous and that complete dentures were made for them. The mucosa covering the supramentale would contact the inner aspect of the labial border of complete dentures. The most labial surfaces of the artificial lower incisors would be positioned anterior to the inner aspect of the labial denture borders (Fig. IO), that is, provided you subscribe to the concept of placing the artificial teeth in that position occupied by the natural teeth.

SIMILARITIES IN PROFILES OF LOWER LIPS This part of the cephalometric study was made to determine whether similarities existed among profiles of lower lips. Two cephalometric points on the profile of the lower lip were selected as landmarks and were designated as the “lower lip sulcus” (mentolabial sulcus) and the “lower lip tip” (Fig. 11). Data were as-

Table II. Summation of 50 subjects Linear and angular measurements

of compiled

data rrradr from cephalomctric !

/ /

Range

(mm.)

6.51

L.T.T.* L.1,:r.t L.L.S.$ S#

tLower

lip tip-the

$Lower (Fig. I?).

lip

/ Standnrd

i

Mean

‘1.7

distance

sulcus----the

SSupramentale-the chin to the lower facial

from

distance

distance from plane (Fig. l,>)

(mm.)

the most anterior

the lower from

the

surface

?.L’ 2.5 ri.9 3 ‘2

lip

the most posterior

sulcus

surface

central

of the mandibular

lip tip to the lower lower

dcziation

imm.)

1.5.7 2.0 10.1 “1.9

-2.i 6.9 1 .l -- 20.3 13.8 -- 30.5

*Lower tooth tip-the distance from incisor to the lower facial plane (Fig. 1 i )

radiographs

facial

plane

to the lower

(Fig. facial

of bone above

12). plane

the bony

sembled on the distances between these two landmarks and the lower facial plane (Table II and Fig. 12). The correlation coefficient of the distances of the lower lip tip and the lower lip sulcus to the lower facial plane was 0.70. Each 1 mm. change in the position of the lower lip tip would find that of the lower lip sulcus changed in the same direction by approximately 1.5 mm. (Fig. 13). The findings based on the two landmarks agree with those of other investigatorsl” I3 who found that there are similarities in many profiles of lower lips. The prosthodontist should develop perceptive abilities enabling him to capture these similarities in edentulous profiles of patients requiring complete dentures. SIMILARITIES

IN PROFILES OF MANDIBULAR

DENTOSKELETONS

Two landmarks on the profile of the dentoskeleton were selected (Fig. 14), are defined in the These two landmarks, the “lower tooth tip” and supramentale, legend for Fig. 14. Their distances from the lower facial plane were plotted, and the resulting correlation coefficient was 0.80. This indicates similarities in the dentoskeletal profiles in most of the 50 subjects (Fig. 15) . The clinical significance is as follows: (1) The profile of mandibular complete dentures should be directly related to dentoskeletal profile provided the thickness of mucosa overlying the supramentale is fairly uniform; (2) profiles of mandibular complete dentures should be similar in appearance; and (3) diagnostic casts of dentulous mouths can be used to demonstrate these similarities. SIMILARITIES BETWEEN DENTOSKELETON

THE LOWER

LIP AND

THE UNDERLYING

The profile of the lower lip has two landmarks that maintain a close relationship to each other (Fig. 12). Likewise, two landmarks on the profile on the dentoskeleton show a close relationship to each other (Fig. 15) . Each of the two landmarks on the lower-lip profile were analyzed for its degree of relationship to each landmark on the profile of the dentoskeleton (Fig. 6) . That is, correlation co-

490

J. Prosth. Dent. November, 1970

Nassif

I --:--1.r.r. I -------I

-1.1.1.

S.

d, Fig.

14

I-I I

Fig. 15

--S.

L.f.PL.

of

I I -- -~A!J.‘-~.~~,~ I 1.1s ’ e--q --_ : k --L.F.PL. I I Fig. 16

Fig. 14. Profile view through the most anterior is more anterior). the bony chin.

the mandibular central incisor. The lower tooth tip (L.T.T.) is surface of the mandibular central incisor tooth (or other incisor tooth that The supramentale (S.) is the most posterior surface in the concavity above

Fig. 15. Profile view through the mandibular tooth tip (L.T.T.) and the supramentale (S.) A high correlation coefficient (0.80) indicates

central incisor. The distances from the lower to the lower facial plane (L.F.PL.) are shown. that these two distances are interdependent.

Fig. 16. Each landmark

on the lower lip (L.L.T. and L.L.S.) is directly related to each landmark on the dentoskeleton (L.T.T. and S.). This means that an anteroposterior change in the and the lower lip sulcus (L.L.S.) will find a similar position of the lower lip tip (L.L.T.) directional change in the position of the lower tooth tip (L.T.T.) and the supramentale (S.).

efficients were determined between the lower lip tip and (1) the lower tooth tip (0.68) and (2) the supramentale (0.78). Likewise, correlation coefficients were determined between the lower lip sulcus and (1) the “lower tooth tip” (0.52) and (2) the supramentale (0.74) . The preceding correlation coefficients (in parentheses) show a predictable degree of relationship between the distances of the profiles of the lower lips and of the profiles of the dentoskeletons. Concerning clinical significance, there is, generally speaking, a predictable degree of relationship between the lower lip and the dentoskeleton (Fig. 16). More forward positions of the lower tooth tip and of the supramentale will usually find the profile of the lower lip in a correspondingly more forward position. This would indicate that the profile of the labial surface of the mandibular complete denture directly influences the profile of the lower lip.

SUMMARY

AND CONCLUSIONS

1. Lateral cephalometric radiographs of 25 men and 25 women with Class I molar relationships were used to study the relationships of lower teeth to anatomic landmarks. 2. Correlation coefficients were used to show the relationships among lower incisor teeth, the lower lip, the chin, and the underlying dentoskeleton. 3. When the chin was found to be more prominent, the lower incisors tended to be more upright. 4. When the lower incisor teeth were found in a less upright position, their most anterior surfaces tended to lie further in front of the supramentale.

Relationship

betzuee~~ n~andibular

incisors

and lower

lip

491

5. Two landmarks were found to maintain similar anteroposterior relationshil>s to each other in many subjects. These were (a) the lower lip tip to the 10wc~1~511 sulcus, and (b) the lower lip tip and thtl lower lip sulcus to the dentoskelctori. These findings indicate that dentists should possess artistic‘ abilities for positionin? artificial anterior teeth. 6. The clinical significance of the cephalometric findings were related 10 edentulous patients. and judgment are necessary factors in order for 7. Esthetic skill, experience, the dentist to be able to position the artificial incisor teeth in positions similar to those occupied by the natural tecth.“~” *Boucher,

C. O., Postgraduate

course, Oct., 1966.

References 1. Martone, A. L.: Effects of Complete Dentures on Facial Esthetics, J. PROSTH. DEWT. 14: 231-255, 1964. 2. Boucher, C. 0.: Swenson’s Complete Dentures, St. Louis, 1964, The C. V. Mosby Company, pp. 215-286. 3. Swenson, W. A.: Complete Dentures, St. Louis, 1959, The C. V. Mosby Company, pp. 169-232. 4. Hickey, J. C., Boucher, C. O., and Woelfel, J. B.: Responsibility of the Dentist in Complete Dentures, J. PROSTH. DENT. 12: 637-653, 1962. 5. Hughes, G. A.: Facial Types and Tooth Arrangements, J. PROSTH. DENT. 1: 82-95, 1951 6. Payne, S. H.: The Try-in, D. Clin. N. America, 1960, pp. 333-342. 7. McGee, G. F.: Tooth Placement and Base Contour in Denture Construction, J. PROSTH. DEST. 10: 651-657, 1960. 8. Silverman, S. I.: Physiologic Factors in Complete Denture Esthetics, Dent. Clin. N. Amer., 1967, pp. 115-122. 9. Hill, A. B.: Principles of Medical Statistics, New York, 1961, Oxford University Press. 10. The Academy of Denture Prosthetics: Glossary of Prosthodontic Terms, J. PROSTK. DENT. 20: 447-480, 1968. 11. Graber, T. M.: Implementation of the Roentgenographic Cephalometric Technique, Amer. J. Orthodont. 44: 906-932, 1958. 12. Lindquist, J. T.: The Lower Incisor-Its Influence on Treatment and Esthetics, Amer. J, Orthodont. 44: 112-140, 1958. 13. Subtelny, J. D.: The Soft Tissue Profile, Growth and Treatment Changes, Angle Ortho. dont. 31: 105-122, 1961. 14. Rayson, J. H., Rahn, A. O., Wesley, R. C., Henderson, D., Ellinger, C. W., Lutes, M. and Frazier, Q. Z.: Placement of Teeth in a Complete Denture: A Cephalometric Study J, Amer. Dent. Ass. 81: 420-424, 1970. LIEUTENANT COLONEL NASSIF : 392m AEROSPACE MEDICAL GROUP (MSD) VANDENBERG AFB, CALIF. 93437