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Anatomic study of the human mandible
Relatkmsbip
Between the Alveolar Border, the Mylohyoid Plate, and the Third Molar Tooth
Ridge, the Lingual
Introduction 1114 alveolar
border, tho mylohyoid ridge, and the third molar tooth usually consideration in textbooks of anatomy. It seems rcasonable and desirable, from a bioanatomic point of view, to treat these elements of the mandible as a unit, especially in situations where surgical interference is indicated. Cunningham, 1 .for example, refers to the mylohyoid line as one which begins at the level of that socket of the last molar tooth; Gray” and Grant” make no statement concerning this relationship ; I\IorrisS rcm;~rks that the mylohyoid line becomes more I)rornincnt as it approaches the alveolar border ; T’icrso1.4 merely mentions the -fact that the rnylohyoitl line is particularly prominent under the molars.
I-- receive separate
Materials
and Methods
One hundred sevontcen unselected, Asiatic, adult, tlentulous, dry mandibles were studied to det.crminc (1) the percentile range, in millimeters, I’rom the alveolar border to the mylohyoid ridge; (2) the percentile range, in millimeters, from the alveolar border to the apex of the third molar tooth; (3) the percentile rang,), in millimeters, of the apex of the third molar above and below the mylo.hyoid ridge; and (4) the thickness of the lingual plate of the mesial. and distal roots of the third molar. In addition to direct caliper measurements, the device shown in Fig. 1 was employed, with the dial as the upper element and the modified dental tool as the lower element. The scale on the dial of this instrument” is calibrated in X,oee inch units. In each measurement, to determine the thickness of the lingual plate, the operator scanned the area of the alveolus between the limbus and the apex until an of
From Dentistry,
CL.
the s.
Department of So&on, Massachusetts. Starr&t Co~~pany,
Jiioanatorny, Athol,
Tufts Massachusetts.
99s
lJnivcrsity, Model
School So.
106.
of
Medicine
and
School
ANATOMIC
STUDY
OF
HUMAN
MANDIBLE
999
absolute minimum reading for the area was obtained and this reading was recorded. The measurements were repeated on different days and the mandibles were selected, as in the first case, in a random order with. the data recorded in different books. An analysis was made to determine (I) the type of statistical operations which would be valid and (2) the extent of errors in measurement and technique. Samples of various measurements were taken
and compared with their corresponding repeated measurements. The san lples teste :d were checked to determine their adequacy by making sure that 8 the one pert enti1.e levels found by interpolation on a “t” scale agreed within pert entage unit of the percentage standard error of the arithmetic mea ,n of the entire population tested. Error values were increased in one ease w -here ther .e was a 2 per cent variation between the sample and the popul ation
12
4
3
DISTANCX
ON
AI’JO
QN
6
SUPCLZIOR OP
BASED
3
APclx
STANDARDIZfZD
BWZCT
a
7
P.lmxKm FROM
9
RlDGL
IN
EXPOSURES
MiZASUREMf’_NT3 b’iq.
II
Of-
OF 3.
12
I3
IA
-
MVLOHYOID
k~-RAV
IO
\\7
Mm.
00
MANDIBC~:S
MANDIRLES
ANATOMIC
STUDY
OF
HUMAN
MANDIBLE
1001
tested. Analysis on repeated measurements from samples of all areas measured showed a standard error of measurement of + 7/1,ooeinch in the 0 to 20’$“,o~~ inch range. The source of error is correlated to the accuracy of the individual measurements made.
Observations Fig. 2 shows that, for the most part, the distance from the alveolar border to the mylohyoid ridge varies from 3 to 9 mm., whereas the distance from the alveolar border to the apex of the third molar ranges, in general, from 7 to 12 mm. A study of Fig. 3 emphasizes the clinically significant point that the apex is within -3 t,o +5 mm. from the mylohyoid ridge. Figs. 4 and 5 show the distribution of minimum thicknesses of the lingual plate at the roots of the third molar tooth. It may be seen that, with the exception of the 0 to 15 THlGKNE!3S OF LINGUAL
PLATE
AT APEX
OF THIRD MOLAR O-15 CASES Meon Mode mdiin
Q
5 I2
%CASES
I I8
THiCKNESS d (10-3tNCHESb II 18 111’ 24 35 43 60 R 94 90 FALLtNG
BELOW
THICKNESS
STANDARD
WITH O-15 INCLUDED Mecm e1.97 Mode * 100-105 Medidn s95
ERROR
OF THE
MEAN
t 7
92-G-zO
d ON ARCVE
Fig.
ROOT
OMITTED
= 99.07 = 100-105 = 100
CORRECTED
I 0
MESIAL
SCALE
4.
group in Fig. 4, the population is quite normal, as indicated by the closeness of the median, mean, and mode. With the 0 to 1.5 group included, the population still may be normal, since the measurement errors were +7 units through the 200 unit range. Fig. 5 shows also a shift of the parameters (mean, mode, and median) to the lower minimum thicknesses.
iscussion From a practical point of view, it is obvious that the study presented here from a random sampling of mandibles indicates that the mean value for the distance from the, alveolar border to the mylohyoid ridge lies between 5
100%
SJ’l%Yk’OR,
CTTRBY,
ANI)
‘TOY
and 6 mm. and that t.he mean value of the distance from the apex of the third molar to the mylohyoid ridge lies just below 9 mm. (h’ig. 2). It is cspccially noteworthy tha-t the? apices of the third molars of 48 per cent of the population, examined. lie 1 to 10 mm. bdovrr the mylobyoid ridge (Fig. 3). An examination of Fig. 4 shows a “normaI” distribution of the thickness of the lingual plate of the me&l root of t,he third mola,r, as indicated by the mean, mode, and median. It. is interesting t,o observe that in about, 65 per cent of the cases meaxurcd the thickness of the lingual plate of the distal root of tho third molar lies below the loo/l,oo~ inch (Fig. 5). A study of P’igs. 4 and 5 THk3@JESS
QF LINGUAL
PLATE
Fig.
AT APEX
OF THIRD
MOLAR,
DISTAL
ROOT
5.
brings out the following points: first, a probability t,hat the lingual plate in the region of t,he dist.al root of the t,hird molar is thinner t,han t.he lingual plate in tho region of the mesial root of the same tooth; second, for 50 per cent of the cases this proba.bility factor would bo approximately 15 per cent and for 25 per cent of t.he cases the probability factor would be 6 per cent.; th.ird, the mean thickness of the, lingual. plate in the region of the mcsial root, of the t,hird molar would bc !jX,ooo inch; and, fourth, the mean t.hickness of the lingual plate in the region of the third molar distal root would bc ss,o,~e inch. The angle of the root from the central axis was considered in relation to mca,surement of the lingual p1at.e thickness. In general, the figures of t~hc limbus distributions suggest greater facilitation o-f measurements tha.n at the apex. These findings should be of practical service to the oral surgeon, since they indicate the possibility of encountering thin lingual pla,tes more frequently than might be ant.icipated. B’urthermore, these data provide the a.natomic basis for the differential diagnosis of infection in the oral cavity from that in th.e submaxillary triangle following extraction of a third molar tooth,
ANATOMIC
STUDY
OF
HUMAN
MANDIBLE
1003
Conclusions 1. On the basis of repeat measurements, it was found that the technique of measuring the mesial and distal roots of the third .molar tooth showed definite reproducibility from an unselected sample of mandibles. 2. This study indicates that a ‘(normal” pattern of correlation can be posited in the thickness of the lingual plate of the third molar tooth. In the total mandible population measured, approximately two-thirds possess distal roots with mesial plate thickness of less than lcO/l,oce inch 3. Clinical applications of the data here presented are strongly suggestive when one notes the range and distribution of the lingual plate thicknesses and the large number of apices which were found to be below the level of the mylohyoid ridge. References Cunningham’s Textbook of Anatomy, ed. 9, New York, 1951, Oxford J. C.: University Press, p. 201. A Method of Anatomy, ed. 5, Baltimore, 1952, Williams 65 Wilkins Grant, J. C. B.: Company, p. 693. Goss, C. M.: Gray’s Anatomy of the Human Body, ed. 25, Philadelphia, 1948, Lea & Febiger, p. 128. Huber, G. Carl: Piersol’s Human Anatomy, ed. 9, Philadelphia, 1930, J. B. Lippincott Company, p. 211. Applied Surgical Anatomy of the Orofacial Region, J. Oral Surg. 11: 185. Jacobs, M.: 199, 1953. Lindegard, 13. : Processus Alveolaris Mandibulae Hominis; An Osteometrie Study, Acta odont. scandinav. 10: 14-25, 1952. Shapiro, I-I. H.: Applied Anatomy of the Head and Neck, ed. 2, Philadelphia, 1917, J. B. Lippincott Company, p. 89. ed. 2, St. Louis, 1952, The C. V. Mosby Company, pp. 60, Sicher, H. : Oral Anatomy, 396, 453. 1953 Morris’ IIaman Anatomy, cd. 11 (edited by J. P. Terry, R. J., and, Trotter, M.: Schaeff’er), Philadelphia, 1953, The Blakiston Company, p. 1%.
1. Brash, 2. 3. 4. 5. 6. 7. 8. 9.
Between the Alveolar Border, the Mylohyoid Plate, and the Third Molar Tooth
Ridge, the Lingual
Introduction 1114 alveolar
border, tho mylohyoid ridge, and the third molar tooth usually consideration in textbooks of anatomy. It seems rcasonable and desirable, from a bioanatomic point of view, to treat these elements of the mandible as a unit, especially in situations where surgical interference is indicated. Cunningham, 1 .for example, refers to the mylohyoid line as one which begins at the level of that socket of the last molar tooth; Gray” and Grant” make no statement concerning this relationship ; I\IorrisS rcm;~rks that the mylohyoid line becomes more I)rornincnt as it approaches the alveolar border ; T’icrso1.4 merely mentions the -fact that the rnylohyoitl line is particularly prominent under the molars.
I-- receive separate
Materials
and Methods
One hundred sevontcen unselected, Asiatic, adult, tlentulous, dry mandibles were studied to det.crminc (1) the percentile range, in millimeters, I’rom the alveolar border to the mylohyoid ridge; (2) the percentile range, in millimeters, from the alveolar border to the apex of the third molar tooth; (3) the percentile rang,), in millimeters, of the apex of the third molar above and below the mylo.hyoid ridge; and (4) the thickness of the lingual plate of the mesial. and distal roots of the third molar. In addition to direct caliper measurements, the device shown in Fig. 1 was employed, with the dial as the upper element and the modified dental tool as the lower element. The scale on the dial of this instrument” is calibrated in X,oee inch units. In each measurement, to determine the thickness of the lingual plate, the operator scanned the area of the alveolus between the limbus and the apex until an of
From Dentistry,
CL.
the s.
Department of So&on, Massachusetts. Starr&t Co~~pany,
Jiioanatorny, Athol,
Tufts Massachusetts.
99s
lJnivcrsity, Model
School So.
106.
of
Medicine
and
School
ANATOMIC
STUDY
OF
HUMAN
MANDIBLE
999
absolute minimum reading for the area was obtained and this reading was recorded. The measurements were repeated on different days and the mandibles were selected, as in the first case, in a random order with. the data recorded in different books. An analysis was made to determine (I) the type of statistical operations which would be valid and (2) the extent of errors in measurement and technique. Samples of various measurements were taken
and compared with their corresponding repeated measurements. The san lples teste :d were checked to determine their adequacy by making sure that 8 the one pert enti1.e levels found by interpolation on a “t” scale agreed within pert entage unit of the percentage standard error of the arithmetic mea ,n of the entire population tested. Error values were increased in one ease w -here ther .e was a 2 per cent variation between the sample and the popul ation
12
4
3
DISTANCX
ON
AI’JO
QN
6
SUPCLZIOR OP
BASED
3
APclx
STANDARDIZfZD
BWZCT
a
7
P.lmxKm FROM
9
RlDGL
IN
EXPOSURES
MiZASUREMf’_NT3 b’iq.
II
Of-
OF 3.
12
I3
IA
-
MVLOHYOID
k~-RAV
IO
\\7
Mm.
00
MANDIBC~:S
MANDIRLES
ANATOMIC
STUDY
OF
HUMAN
MANDIBLE
1001
tested. Analysis on repeated measurements from samples of all areas measured showed a standard error of measurement of + 7/1,ooeinch in the 0 to 20’$“,o~~ inch range. The source of error is correlated to the accuracy of the individual measurements made.
Observations Fig. 2 shows that, for the most part, the distance from the alveolar border to the mylohyoid ridge varies from 3 to 9 mm., whereas the distance from the alveolar border to the apex of the third molar ranges, in general, from 7 to 12 mm. A study of Fig. 3 emphasizes the clinically significant point that the apex is within -3 t,o +5 mm. from the mylohyoid ridge. Figs. 4 and 5 show the distribution of minimum thicknesses of the lingual plate at the roots of the third molar tooth. It may be seen that, with the exception of the 0 to 15 THlGKNE!3S OF LINGUAL
PLATE
AT APEX
OF THIRD MOLAR O-15 CASES Meon Mode mdiin
Q
5 I2
%CASES
I I8
THiCKNESS d (10-3tNCHESb II 18 111’ 24 35 43 60 R 94 90 FALLtNG
BELOW
THICKNESS
STANDARD
WITH O-15 INCLUDED Mecm e1.97 Mode * 100-105 Medidn s95
ERROR
OF THE
MEAN
t 7
92-G-zO
d ON ARCVE
Fig.
ROOT
OMITTED
= 99.07 = 100-105 = 100
CORRECTED
I 0
MESIAL
SCALE
4.
group in Fig. 4, the population is quite normal, as indicated by the closeness of the median, mean, and mode. With the 0 to 1.5 group included, the population still may be normal, since the measurement errors were +7 units through the 200 unit range. Fig. 5 shows also a shift of the parameters (mean, mode, and median) to the lower minimum thicknesses.
iscussion From a practical point of view, it is obvious that the study presented here from a random sampling of mandibles indicates that the mean value for the distance from the, alveolar border to the mylohyoid ridge lies between 5
100%
SJ’l%Yk’OR,
CTTRBY,
ANI)
‘TOY
and 6 mm. and that t.he mean value of the distance from the apex of the third molar to the mylohyoid ridge lies just below 9 mm. (h’ig. 2). It is cspccially noteworthy tha-t the? apices of the third molars of 48 per cent of the population, examined. lie 1 to 10 mm. bdovrr the mylobyoid ridge (Fig. 3). An examination of Fig. 4 shows a “normaI” distribution of the thickness of the lingual plate of the me&l root of t,he third mola,r, as indicated by the mean, mode, and median. It. is interesting t,o observe that in about, 65 per cent of the cases meaxurcd the thickness of the lingual plate of the distal root of tho third molar lies below the loo/l,oo~ inch (Fig. 5). A study of P’igs. 4 and 5 THk3@JESS
QF LINGUAL
PLATE
Fig.
AT APEX
OF THIRD
MOLAR,
DISTAL
ROOT
5.
brings out the following points: first, a probability t,hat the lingual plate in the region of t,he dist.al root of the t,hird molar is thinner t,han t.he lingual plate in tho region of the mesial root of the same tooth; second, for 50 per cent of the cases this proba.bility factor would bo approximately 15 per cent and for 25 per cent of t.he cases the probability factor would be 6 per cent.; th.ird, the mean thickness of the, lingual. plate in the region of the mcsial root, of the t,hird molar would bc !jX,ooo inch; and, fourth, the mean t.hickness of the lingual plate in the region of the third molar distal root would bc ss,o,~e inch. The angle of the root from the central axis was considered in relation to mca,surement of the lingual p1at.e thickness. In general, the figures of t~hc limbus distributions suggest greater facilitation o-f measurements tha.n at the apex. These findings should be of practical service to the oral surgeon, since they indicate the possibility of encountering thin lingual pla,tes more frequently than might be ant.icipated. B’urthermore, these data provide the a.natomic basis for the differential diagnosis of infection in the oral cavity from that in th.e submaxillary triangle following extraction of a third molar tooth,
ANATOMIC
STUDY
OF
HUMAN
MANDIBLE
1003
Conclusions 1. On the basis of repeat measurements, it was found that the technique of measuring the mesial and distal roots of the third .molar tooth showed definite reproducibility from an unselected sample of mandibles. 2. This study indicates that a ‘(normal” pattern of correlation can be posited in the thickness of the lingual plate of the third molar tooth. In the total mandible population measured, approximately two-thirds possess distal roots with mesial plate thickness of less than lcO/l,oce inch 3. Clinical applications of the data here presented are strongly suggestive when one notes the range and distribution of the lingual plate thicknesses and the large number of apices which were found to be below the level of the mylohyoid ridge. References Cunningham’s Textbook of Anatomy, ed. 9, New York, 1951, Oxford J. C.: University Press, p. 201. A Method of Anatomy, ed. 5, Baltimore, 1952, Williams 65 Wilkins Grant, J. C. B.: Company, p. 693. Goss, C. M.: Gray’s Anatomy of the Human Body, ed. 25, Philadelphia, 1948, Lea & Febiger, p. 128. Huber, G. Carl: Piersol’s Human Anatomy, ed. 9, Philadelphia, 1930, J. B. Lippincott Company, p. 211. Applied Surgical Anatomy of the Orofacial Region, J. Oral Surg. 11: 185. Jacobs, M.: 199, 1953. Lindegard, 13. : Processus Alveolaris Mandibulae Hominis; An Osteometrie Study, Acta odont. scandinav. 10: 14-25, 1952. Shapiro, I-I. H.: Applied Anatomy of the Head and Neck, ed. 2, Philadelphia, 1917, J. B. Lippincott Company, p. 89. ed. 2, St. Louis, 1952, The C. V. Mosby Company, pp. 60, Sicher, H. : Oral Anatomy, 396, 453. 1953 Morris’ IIaman Anatomy, cd. 11 (edited by J. P. Terry, R. J., and, Trotter, M.: Schaeff’er), Philadelphia, 1953, The Blakiston Company, p. 1%.
1. Brash, 2. 3. 4. 5. 6. 7. 8. 9.