Amhs oral Bid. Vol. 32. No. I. pp. 5-9. 1987 Printed in Great Britain. All rights reserved

1M03-9969 87 53.00 + 0.00 Copyrtpht T. 1987 Pergamon Journals Ltd

DIRECTIONALITY OF DENTAL TRAIT FREQUENCY BETWEEN HUMAN SECOND DECIDUOUS AND

FIRST PERMANENT MOLARS

PATRICIA SMITH, EDITH KOYOUMDJISKY-KAY& W. KALDERON and D. STERN

Hebrew University, Hadassah School of Dental Medicine, P.O. Box 1172, Jerusalem, Israel

Summary-Dental traits were scored for second deciduous molars (dm2) and first permanent molars (M 1) on of the dental arches of children The children

The overaIl frequency of the groups but the reIative frequency of of traits in two teeth showed as continuous oblique ridge,

and 7th cusp, in the early stages of development, in dm2 in Ml. Wrinkling, occlusal and marginal ridge cusps were in Ml; these appear

the relative frequency of in these teeth reflects

and twin studies of human populations is under multifactorial

control displays fluctuating asymmetry and Green, 1971; Bailit,

son and Kolakowski, 1974; the tooth in contrast, shows

marked In the molars, but not all traits in frequency the

first permanent molar (Ml) to second and third

and Mayhall, 1982) and this has been related to the greater developmental of M the key molar

as wrinkling and buccal are more common in the second and third molars in Ml. Similarly, comparison of the second deciduous

Ml shows that, despite marked overall morphological of these teeth, are consistently in dm2, whereas oth- ers are more frequent in (Dahlberg, 1963; Smith,

and Mayhall, 1982; Keiser, We postulate in trait frequency are related to ontogenetic history of traits

and coalescence which produce dentine-enamel junction. Similarly,

in external crown morphology dm2 and in growth gradients and enamel thickness between two teeth. Development of proceeds at

of dm2, mineralization the tooth is larger and the order of

of centres of is more vari- the external crown

of dm2 is more conservative of Ml. we may infer

in dm2 are phyio-

in M reflect

To test we compared dental trait in dm2 and Ml of individuals 7-l I

years. To is not population specific, we examined

of dental trait expression.

METHODS

and (1965) out there a similarity early of

Dental of were during study growth development Israel

dental and fossil out E. and in to representative successive Koyoumdjisky-Kaye, and (1976).

dental Their suggest onto- selected of children parents earlier features be most of African, and European

Many present the as as group Israeli children. junction obscured absent The were in plaster, alginate

outer surface 1963, impressions the The were by Sakai, Conversely, features the examiners and using

enamel cannot identified the after (1956) Hanihara dentine-enamel Kraus Oka and described Smith Only pointed that, to molar casts both and present the side

of monkeys identical human included; were as or molar germs. attributed later the teeth, metacone hypocoae differences crown of fully-formed as or in with teeth differences the of mineral- For tubercles marginal

5

6 PATRICIA SMITH et al.

cusps (also called metaconules) all stages of devel- opment, from small to large, were classified as present. The oblique ridge was classified as present if intact, and absent if absent or crossed by a devel- opmental groove. Any degree of expression of the Carabelli cusp, including pits or furrows, was scored as present. For the lower teeth, cusp pattern was divided into SY and all others; cusp number was categorized as five cusps and all others; wrinkling was defined as deflecting wrinkle or generalized wrinkling of the occlusal surface. The definition for protistylid included pits and deflecting grooves. For 6th and 7th cusps, double grooves located in the region of 6th and 7th cusps were also considered positive. MacNemar’s modified Chi-square test (Siegel, 1956) was carried out to determine the significance of directional trends between trait-frequency distribution in dm2 and Ml. Scoring techniques were standardized against a sub- sample and consistency of criteria checked by re- measuring subsamples and comparing scoring tech- niques between examiners. Overall concordance was better than 90 per cent.

RESULTS

Table 1 shows the frequency distribution, in the two categories of concordance and discordance of trait expression, for upper dm2 and M 1 in each group studied. All traits varied in frequency between the four ethnic groups, reflecting their different genetic make-up. However, despite variations in trait fre- quency, the pattern of concordance between dm2 and Ml for any one trait was similar in all four groups. Three patterns were identified: (a) presence or ab- sence randomly distributed, (b) trait present in dm2 when absent in Ml, (c) trait absent in dm2 but present in Ml.

The metacone falls into group (a). It was well developed in more than 94 per cent of all molars; the few cases with metacone reduction were randomly distributed between dm2 and Ml. Hypocone reduc- tion was more common; it was found in 39 per cent of either dm2 or Ml of North Africans, 23 per cent Druse and 27 per cent of Kurds. The highest fre- quency of discordance (22 per cent) was in the North

Table 1. Concordance and discordance between dm2 and Ml (upper teeth)

Group

Trait North

African Druze Kurds East

Europeans

Metacone Percentage normal in both teeth 94 100 100 100 Percentage reduced in both teeth 0 0 0 0 Percentage normal in dm2 only 3 0 0 0 Percentage normal MI only 3 0 0 0 Number examined 90 64 81 42

Hypocone Percentage normal in both teeth Percentage reduced in both teeth Percentage normal in dm2 only Percentage normal Ml only Number examined

:: a9 3 11 5 11 2 82 62

73 11 12 4

74

Occlusal tubercles Percentage normal in both teeth Percentage reduced in both teeth Percentage normal in dm2 only Percentage normal Ml only Number examined

67 48 70 I2 10 18 2:* 0 0

42’ 121 74 58 49

Continued oblique ridge Percentage normal in both teeth Percentage reduced in both teeth Percentage normal in dm2 only Percentage normal Ml only Number examined

2: 15 14

67’ 712 2 0

83 59

1 14 751 0

70

Marginal ridge cusps Percentage normal in both teeth Percentage reduced in both teeth Percentage normal in dm2 only Percentage normal Ml only Number examined

25 2 33 56 4 2

38. 40. 59 45

25 50

2!* 40

Carabelli’s cusp or pit Percentage normal in both teeth Percentage reduced in both teeth Percentage normal in dm2 only Percentage normal Ml only Number examined

64 58 5 10

28. 21 3 II

96 61

58

2: 16 73

95 0 3 2

40

51 5

:* 35

0 41 59* 0

41

14 47

0

:z*

59 2

25 14 41

*Denotes significant difference in distribution of trait between dm2 and Ml (p < 0.05).

Dental trait directionality

Table 2. Concordance and discordance between dm2 and Ml (lower teeth)

Group

North East Trait African Druze Kurds Europeans

Y parfern Percentage present in both teeth Percentage absent in both teeth Percentage present in dm2 only Percentage present in MI only Number examined

Fice cusps Percentage present in both teeth Percentage absent in both teeth Percentage present in dm2 only Percentage present in MI only Number examined

Wrinkling-rubercles Percentage present in both teeth Percentage absent in both teeth Percentage present in dm2 only Percentage present in Ml only Number examined

Sixth cusp Percentage present in both teeth Percentage absent in both teeth Percentage present in dm2 only Percentage present in Ml only Number examined

Seventh cusp Percentage present in both teeth Percentage absent in both teeth Percentage present in dm2 only Percentage present in MI only Number examined

Protosrylid/buccal pit Percentage present in both teeth Percentage absent in both teeth Percentage present in dm2 only Percentage present in Ml only Number examined

19 87 89 67 0 0 0 0

21, 130 II 33’ 0 0 0 0

47 46 44 21

93 91 92 88 0 0 0 0 I 9 8 I?’ 0 0 0 0

82 55 74 25

5 I2 I3 21 86 54 12 25 0 0 6 4 9 34. 9 50.

63 44 53 25

3 0 0 5 91 98 94 91

3 0 0 0 3 2 6 4

57 41 52 21

15 58 25; 2

84

0 99

0 I

94

15 66 17* 2

53

0 98

0 2

58

19 50 28’

74

0 96

2

I8 56 22’ 4

27

0 100

0 0

30

*Denotes significant difference between distribution of dm2 and MI (p < 0.05).

African group which showed trait frequencies closest to 50 per cent with discordance randomly distributed.

The oblique ridge and Carabelli cusp fall into group (b). The oblique ridge was continuous in both dm2 and Ml of only 5 per cent of North Africans, but 49 per cent of East Europeans. In all cases it was continuous in dm2 when interrupted or absent in M 1. Some degree of expression of Carabelli cusp was present on over 90 per cent of either dm2 or Ml of all groups. In discordant pairs, the trait tended to be present in dm2 when absent in Ml. Occlusal tubercles and marginal ridge cusps showed an oppo- site trend, defined as group (c). They also showed marked discordance, but the trait was generally absent in dm2 when present in Ml.

the Ml when present in dm2. Cusp number could be accurately determined in more individuals; discor- dance was due to the presence of four cusps in Ml when five cusps were present in dm2. Less than 20 per cent had 7th cusps or double grooves on both teeth. Discordance in this trait was due to its presence in dm2 when absent in Ml. Wrinking on the other hand was present in higher frequencies in Ml than in dm2 as were 6th cusps although these were present in few individuals. Protostylids and buccal pits were too infrequent for any assessment of distribution to be made.

DISCUSSION

The frequency distribution of traits in lower dm2 Of the upper molar traits described here, the and Ml is given in Table 2. As in the upper molars, hypocone, Carabelli cusp and oblique ridge were overall distribution of traits varied from group to present in dm2 with a higher frequency than in Ml; group, and discordance for any particular trait in- marginal ridge cusps and occlusal tubercles were creased as frequency approached 0.5. Cusp pattern present more frequently in Ml. In the lower molars, was 5Y for more than 90 per cent of all individuals Y pattern, 5th cusp and 7th cusp were more frequent where this could be accurately determined; discor- in dm2 and wrinkling or occlusal tubercles were more dance was always due to absence of the Y pattern in frequent in Ml. Kraus and Jordan (1965) reported

8 PATRICIA SMITH er al.

that the metacone can first be identified in dm2 at around 14 weeks in utero. and the hypocone and oblique ridge at 16 weeks. Mineralization begins at the apex of the mesiobuccal cusp at 14 weeks, fol- lowed by the mesiolingual cusp at 23 weeks. It is at this stage that the Carabelli cusp can first be recog- nized. Some three to five independent centres of mineralization can also be seen along the marginal ridge at this time although no separate cuspules are present before mineralization. Mineralization of the distobuccal cusp (the metacone) starts only at 28 weeks, and for the distolingual cusp (the hypocone) shortly afterwards. Coalition of centres of mineral- ization along the oblique ridge is completed at 32 weeks, and coalescence of all centres at 38 weeks.

In the lower dm2, all five main cusps appear early in morphogenesis with the 6th and 7th cusps appear- ing at 22 weeks in dm2, and 24 weeks in Ml. However, the later development of the distal portion of the tooth, the talonid (carrying the 5th and 6th cusps) is much slower than that of the mesial portion, the trigonid, and mineralization of M 1 is slow relative to that of dm2. Wrinkling, marginal ridge cusps and occlusal tubercles have not been identified with fea- tures that appear in the early stages of mor- phogenesis. Kraus (1963) and Kraus and Jordan (1965) emphasized that the three to five marginal ridge cusps that regularly appear on the upper molars during mineralization do not, unlike other features on the crown, appear during morphogenesis. They did not find wrinkling in human molar tooth germs even after all five main cusps had mineralized and united. Kraus and Oka (1967) postulated that wrinkling results from infolding of the inner enamel epithelium on the slopes of the cusps immediately prior to enamel formation. However it is not found on the mineralized dentine-enamel junction (Korenhof, 1978, 1982) and appears to reflect local- ized differences in the rate of formation and thickness of enamel.

The findings presented by Korenhof (1963, 1978, 1982) on comparison of the dentine-enamel surface with the outer crown surface also suggest that the development history of those traits showing greater frequency on the outer enamel surface of dm2 differs from that of traits less frequent in that tooth. Cara- belli cusp, metacone, hypocone, intact oblique ridges and hypoconulids are more common on the dentine-enamel junction than on the outer enamel surface and are more frequent on dm2. There is, however, little or no correlation between the presence of marginal ridge cusps, occlusal tubercles and wrink- ling on the outer and inner surfaces. Marginal ridge cusps and wrinkling are more common on the outer enamel surface than on the dentine-enamel junctions; both are more frequent in Ml than dm2.

Our study shows that in the four Israeli population groups, despite differences in trait frequency, the pattern of trait discordance between dm2 and Ml is similar and resembles that previously reported for American (Saunders and Mayhall, 1982) and South African caucasoids (Keiser, 1984). As the same pat- tern of directionality recurs in populations of different genetic background, this may be accepted as a generalization. Furthermore, the findings demon- strate that directionality along the tooth row is trait

specific, some traits increase in frequency, others consistently decrease in frequency.

Those traits present in dm2 in higher frequencies than in Ml develop early, whereas those more fre- quent in Ml appear later. The observations support the hypothesis of a relationship between ontogenetic timing and the appearance of a trait predominantly on dm2 or Ml.

Acknowledgements-Based on theses submitted by W. Kalderon and D. Stern to the Hebrew University in 1983 in partial fulfilment of the requirements for D.M.D. degrees and supported by grants in aid from the Israel Academy of Sciences and Hebrew University Alpha Omega fund.

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