An Anthropological Study on the Mandible in the

Japanese, especially Measurements in Relation

to the Construction of an Articulator

by

Masahiro KOYAMA*

Introduction

Exact knowledge of the mandible in man which occupies the lower portion of his face is without doubt a very important one from the standpoint of anthropology,

anatomy and dentistry as well. Various researches dealing with the morphology and

measurements of the mandible in the Japanese have been published by MIYAMOTO [ 1 ]

(the Japanese in the central Japan), OTSUKI [2] (the Japanese in the northwest of

Japan ), OMORI [3] (the Japanese in Kyushu Island), VEDA [4] (the Japanese in general), IHARA [ 5 ] (the Japanese in general), inter alia. The author has conducted the present study with a twofold purpose : to further elucidate its morphology in the light of anatomy and, at the same time, to make certain specific measurements so as to obtain standards to be used for the construction of an articulator adapted to the Japanese.

MethodandMaterials

A careful selection was made of the Japanese mandibular bones housed in the Department of Anatomy, Jikei-kai College of Medicine, Tokyo, to obtain 106 male and 68 female specimens for measurement purposes. The selection was made on the fol-lowing criteria.

1. Those mandibles which were over 20 years with distinct sex. 2. Those which preserved the right and left central incisors and third molars as

well. 3. Those in which the alignment of teeth was fairly regular. Measurement spots were chosen in basic compliance with those given in a text-

book of anthropology by MARTIN [ 6 ], with additional spots based on the findings of other scholars. The values to be measured were expressed in terms of mm, degree in

g, depending upon the type of measurement.

A. Measurement spots and indeces (Figs. 1-6).

1. Width of articulator processMartin No. 65 2. Width of coronoid process65 (1)

* 小 山 正 宏: Dept . of Prosthetics, Nihon Unfiversity School of Dentistry, (Director: Prof

Setsuzo Okino).

Part of the paper was read before the General Session of Nihon Unfiversity Dental Society,

April, 1961, the 3rd meeting of Japan Prosthetic SoCiety, October, 1961, and the 47th

meeting of Japan Prosthetic Society, October, 1963.

51

52

3. Width of mandibular angle 664. Front width of mandibular angle 675. Length of mandibular body 686. Length of mandible 68 (1)7. Height of mentum 698. Height of mandibular body (mentum portion) 69 (1)9. Height of mandibular body (M2 portion) 69 (2)

10. Thickness of mandibular body 69 (3)11. Ramus of mandible 7012. Vertical height of ramus of mandible 7013. Height of coronoid process 70 (1)14. Minimal height of ramus of mandible 70 (2)15. Height of mandibular notch 70 (3)16. Width of ramus of mandible 7117. Minimal width of ramus of mandible 71 a18. Width of mandibular notch 71 (1)19. Weight of mandible 39 (1),20. Height of condyle21. Thickness of condyle22. Angle of mandibular ramus 7923. Coronio-condylar line inclination 79 (3)24. Angle of mandibular basis 79 (4)25. Horizontal inclination of articular axis 79 (5)

26. Index of length of mandible 162

=

Length of mandibular body (5)•~100/

Width of articular process (1)

27. Thomson mandibular index

=

Length of mandible (6)•~100/

Width of articulator process (1)

28. Index of height of mandibular body 162 (1)

=

Height of mandibular body (M2 portion) (9)•~100/

Height of mandibular ramus (11)

29. Index of mandibular ramus 163

=

Width of mandibular body (16)•~100/

Height of mandibular ramus

30. Index of mandibular ramus (variant) 163 a

=

Minimum width of mandibular ramus (17)•~100

/ Minimum height of mandibular ramus

31. Index of mandibular width 164

=

Width of mandibular angle ( 3 ) X 100/ Width of articulator process ( 1 )

32. Index of mandibular notch 165

=

Height of mandibular notch (15) •~ 100

/ Width of mandibular notch (18)

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Fig. 1. Measurements of the mandible (1, Nos. are those of Martin)

Fig. 2. Measurements of the mandible

(2, Nos. are those of Martin)

Fig. 3. Measurements of the mandible (3, Nos. are those of Martin)

Fig. 4. Measurements of the mandible (4, Nos. are those of Martin)

Fig. 5. Measurements of the mandible (5, Nos. are those of Martin)

Fig. 6. Measurements of the mandible (6, Nos. are those of Martin)

54

33. Index of thickness of mandibular body height 166

=

Thickness of mandibular body (10)•~100

/ Height of mandibular body (foramen of mentum) (8)

34. Index of condyle

=

Thickness of condyle (21)•~100/

Length of condyle (20)

Measurement Relative to the Construction of Articulator

As supplementary to the measurements of mandibular properties generally done in anthropology, additional measurements were effected on other properties which have bearings on the construction of a dental articulator. Fig. 7 gives the measurement devices.

35. Width of articular process36. Median distance between the tip of left top of mandible and the center of

mandibular arch37. Median distance between the tip of right top of mandible and the center

of mandibular arch (35, 36 and 37 form a mandibular triangle or Bonwill triangle, Fig. 8)

38. Median distance between the tip of left top of mandible and the occlusal

plane (Fig. 9)39. Median distance between the tip of right top of mandible and the occlusal

plane40. Median distance between the projected line of mandibular top and the center

of mandibular arch (Fig. 10)41. Balkwill angle (Fig. 11)

42. Distance between the central fossae of 2nd molars43. Median distance between the central fossa of left 2nd molar and the center

of mandibular arch44. Median distance between the center fossa of right 2nd molar and the center

of mandibular arch (42, 43 and 44 form an arch triangle , Fig. 12)45. Curvature of dental arch .

Morphological Observations

1. Form of mental region

a. Classification based on KLAATSCH [7]b. Classification based on NISHI [8]

2. Form of mental trigon

3. Mental foramen

a. Positonb. Anomaly

4. Form of coronoid process*5. Anomaly of articular process

6. Form of mandibular notch

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Fig. 7. Appliances used for the measure-ment of mandibular properties Fig. 8. Mandibular triangle

Fig. 9. Measurement of distance between the mandibular top and occlusal

plane

Fig. 10. Measurement of projected distance between the man-dibular top and its median

point

Fig. 11. Balkwill's angle Fig. 12. Triangle of dental arch

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7. Form of the lower rim of mandibular body

8. Form of the elevated portion of interior mandible*9. Form of mandibular angle

10. Forms of masseter tuberosity* and pterygoid tuberosity*11. Anomaly of mylohyoid groove

12. Form of mylohyoid line13. Appearance frequency of the digastric fossa

14. Mandibular elevation*

a. Form

b. Position15. Form of mental spine

(Note: Those items marked with asterisks are not entered in P. N. A. [9])

TABLE 1

Measurement of Various Properties of the Jaws in the Japanese

57

General Considerations of Results

Various measurements effected on 174 skulls of the Japanese males and females, housed in the Department of Anatomy, Jikei-kai College of Medicine, with a view to obtaining anthropological data having to do with the construction of a dental articulator have given the following results.

I. Comprehensive measurement data of general anthropological character together with their arithmetic means and standard deviations are given in Table 1.

II. Comprehensive measurement data having specific bearings on the construction of a dental articulator are given in Table 2.

III. Considerations on morphological data.1. According to KLAATSCH, the cases of "Positivkinn" occupy a greater por-

tion of the mental classifications, in common to males and females.2. According to NISHI [8], the majority of mental classifications are round chins,

in common to males and females.3. According to SCHULZ [12], the greater cases of mental triangle are stellar.4. According to SHIMA [13], the position of mental fossa is mostly P2 both in

males and females.5. Appearance of such anomalies as excessive mental fossae, double articular

condyle and mylohyoid groove is more frequent in males than in females.6. Most mandibular notches are uniformly depressed both in males and females.7. Notches on the anglar portion of mandible have more or less the same depth

in males and females.8. 80% of pterygoid tuberosity in males and females form a linear elevation.9. Mylohyoid line appears to be mostly linear.

10. The apearance of digastric fossa is to the extent (60%) both in males and females.

TABLE 2

Measurement of Specific Values for the Construction of Articulators

58

11. According to YAMADA [15], the forms of mandibular elevation are chiefly round or oblong.

Conclusions

Various results of the present study described above have drawn the conclusions as follows.

1. With the exceptions of width of condyle, height of mandibular ramus, angle of mandibular basis, horizontal inclination of articular axis, index of thickness of man- dibular body height, Thomson mandibular index, index of mandibular ramus, index of mandibular ramus (variant), index of mandibular width and index of condyle, other measurement results are definitely greater in males.

Fig. 13 (a). Measurements of arch curvature (3rd molars)

Fig. 13 (b). Measurements of arch curvature (2nd molars)

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2. On the other hand, Thomson mandibular index and index of condyle are appreciably greater in females.

3. When the measurements of MIYAMOTO [16], MIYASHITA [17], IMAI [18] and MARUYAMA [19], the present measurement by the author most closely resembles that of MIYAMOTO.

4. As regards the mandibular triangle, that of a male is far larger on three sides than the female counterpart. In terms of a distance between the mandibular tip of the latter is smaller than the former by 20 mm.

5. Males generally have a longer distance between mandibular tip and the occlusal

plane than females. 6. There is no appreciable difference in terms of sex in Balkwill angle. 7. As regards the arch triangle, only sexual distinction is possible on the one side,

that is, distance between the second molar and the central fossa. 8. In common to both sexes, a distance between the cusp tip and the occlusal

plane on individual teeth is fairly close to one another in males and females (Fig. 13) .

References

[1] MIYAMOTO, H. : Anthropological investigations into the bones of the modern Japanese, Part 1 : Study of the skull, J. Anthro. , 39 : 307, 1924.

[2] OTSUKI, Y. : An anthropological study of the Japanese skull in the northern region, J. Juzen Soc. , 36 : 230, 1931.

[3] OMORI, A. : Study on the mandibular bone of the Japanese living in Kyushu Island, J. Anthro. , 59 : 43, 1944.

[4] UEDA, Y. : Measurements on the mandible of the Japanese, J. Dent. Sci. , 38 : 8, 1949. [5] IHARA, M. : An anthropological study on the mandibular bone in the Japanese, J. Dent.

Sci. , 39 : 11, 1950. [6] MARTIN, R. : Lehrbuch der Antrhopologie, 2 Aufl., Gustav-Fischer, Stuttgart (1928). [7] KLAATSCH, H. : Kraniomorphologie and Kraniotrigonometrie, Anch. f. Anthrop. , N. F. ,

8 : 101, 1909. [8] NISHI, S. and NISHI, K. : Contribution to our knowledge of the mentum formation, J. Anat.,

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(1961). [10] FURUYA, Y. and MIYASAKA, G. : Medical statistics, 5 ed, Kanahara Pub. Co., Tokyo (1959). [11] ADACHI, B. : On the treatment of numerals, Oka Co. , Tokyo (1928) . [12] SCHULZ, H. E. : Ein Beitrag zur Rassenmorphologie des Unterkiefers, Z. f. Morph. u.

Anthrop. , 32 : 275, 1933. [13] SHIMA, G. : On the formation of jaws in the interred Koreans, J. Anat., 4: 845, 1931. [14] KEITER, F. : Vorschlage zur Methodik der Unterkieferbeobachtung, Anthrop. Anz., 6:

154, 1929. [15] YAMADA, E. : On Torus mandibulae in the Japanese, Shi-Shimpo, 22 : 69, 1929. [16] SHIMA, G. : Measurements on the mandible in the present-day Koreans, J. Anthro. 47 :

1, 1932. [17] MIYASHITA, K. : A study on the mandible in the Chinese, J. Manchu Med., 19 : 415, 20 :

61, 22: 249, 617, 1933-35. [18] IMAI, Y. : Anthropological and anatomical studies on the mandibular bone in the abori-

ginals of Formosa, J. Jap. Dent. Soc. , 32 : 1, 33 : 9, 1935-36. [19] MARUYAMA, Y. : Anthropological and anatomical studies on the mandibular bone in the

Formosans in Fukien district, J. Formosan Med. Soc. , 36 : 1285, 1937.