relations between the mesiodistal crown diameters of the primary and permanent teeth of hong kong...

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~ Pergamon Archs oral Biol. Vol. 41, No. I, pp. 1 7, 1996 Copyright © 1996Elsevier ScienceLtd. All rights reserved Printed in Great Britain 0003-9969(95)00107-7 0003-9969/96$15.00 + 0.00 RELATIONS BETWEEN THE MESIODISTAL CROWN DIAMETERS OF THE PRIMARY AND PERMANENT TEETH OF HONG KONG CHINESE KEITH KWOK-WAH YUEN, ENDARRA LAI-KING TANG and LISA LAI-YING SO Department of Children's Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong (Accepted 4 September 1995) Summary--Relations between these diameters were studied on serial dental casts of 112 Hong Kong Chinese (61 males, 51 females)taken at mean ages of 5.68 and 12.31 yr. Size relations were studied in terms of correlation coefficient, difference, and ratio between corresponding pairs of individual teeth and corresponding groups of teeth in both dentitions. The coefficients of correlation were low to moderate (0.2-0.8), with those between groups of teeth consistently higher. As for size differences, incisors and canines were larger in the permanent dentition in both arches by 1.2-2.0 mm. Premolars were smaller than their primary predecessors except for the upper first premolar. When the absolute values of the differences were ranked, the second premolar-second primary molar differences were the greatest (2.0-2.8 mm) while those between the first premolar-first primary molar were the smallest (0.3-0.7 mm). When tooth groups were assessed, the permanent teeth were larger than their predecessors in the anterior segments but smaller in the posterior segments. The leeway space was larger in the mandibular arch (2.2 mm) than in the maxillary arch (0.3 mm). When diameters of five teeth on one side of the arch were combined, the permanent exceeded the primary by 3.5 mm in the upper arch but only 0.5 mm in the lower. The ratios between the permanent and primary tooth sizes showed the same trend as the differences. Key words: mesiodistal crown diameters, tooth size relations, correlation coefficients, differences, ratios, primary and permanent dentitions, Hong Kong Chinese. INTRODUCTION Animal, twin and family studies have indicated strong genetic control for tooth sizes (Osborne, Horowitz and De George, 1958; Bader, 1965; Garn, Lewis and Kerewsky, 1965). Some investigators have used dental size as a marker for genetic distance between populations (Garn, Lewis and Walenga, 1968). No conclusion has been reached as to the mode of inheritance for tooth size. Garnet al. (1965) suggested the possibility of an X-linked component to inheritance but this theory was not supported by other researchers (Bowden and Goose, 1969). The environmental factors that contribute to tooth size are also uncertain. The mother's health during preg- nancy, birth size and weight, and diet have been implicated as factors that would affect tooth size. It appears that tooth size is determined principally by genetics but the proportion and type of genetic control may vary between teeth, individuals and populations. It is likely to be polygenic in nature. Environment then acts on this genetic variability to produce continuous variation in tooth size (Hillson, 1986). The relation between the sizes of primary teeth and their permanent successors has been studied. Most of these studies were done on Caucasian populations (Moorrees et al,, 1957; Moorrees, 1959; Moorrees and Chadha, 1962; Clinch, 1963; Moorrees, 1964; Moyers et al., 1976; Garn, Cole and Wainright, 1977; Lysell and Myrberg, 1982), but there are some similar studies on other ethnic groups (Brown, Margetts and Townsend, 1980a, b; Steigman, Harari and Kuraita- Landman, 1982). In general, low to moderate corre- lation coefficients were found between pairs of corresponding teeth in both dentitions. There was a consensus that small primary teeth were likely to be followed by small permanent successors and large primary teeth by large permanent successors but the low correlation coefficients precluded their use for making reasonably accurate predictions (Moorrees and Chadha, 1962; Clinch, 1963; Garnet al., 1977: Brown et al., 1980b; Lysell and Myrberg, 1982). Size relations are important in the mixed dentition. They provide information on dental crowding and changing patterns of occlusion in the developing permanent dentition (Moorrees, 1964; Brown et al., 1980a). Size relations have been assessed in three ways, usually by examining the coefficients of corre- lation (Moorrees et al., 1957; Moorrees, 1959; Clinch, 1963; Moorrees, 1964; Garnet al., 1977; Brown et al., 1980b), but absolute differences and ratios have also been used (Moorrees, 1959; Moorrees and Chadha, 1962; Clinch, 1963; Moyers et al., 1976; Brown et al., 1980a; Steigman et al., 1982). Odontometric studies have been made on the mesiodistal crown diameters of primary and perma- nent teeth in Hong Kong Chinese (Sandar, Yen and

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~ Pergamon Archs oral Biol. Vol. 41, No. I, pp. 1 7, 1996 Copyright © 1996 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0003-9969(95)00107-7 0003-9969/96 $15.00 + 0.00

RELATIONS BETWEEN THE MESIODISTAL CROWN D I A M E T E R S OF THE P R I M A R Y A N D P E R M A N E N T

TEETH OF H O N G K O N G CHINESE

KEITH KWOK-WAH YUEN, ENDARRA LAI-KING TANG and LISA LAI-YING SO

Department of Children's Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong

(Accepted 4 September 1995)

Summary--Relations between these diameters were studied on serial dental casts of 112 Hong Kong Chinese (61 males, 51 females) taken at mean ages of 5.68 and 12.31 yr. Size relations were studied in terms of correlation coefficient, difference, and ratio between corresponding pairs of individual teeth and corresponding groups of teeth in both dentitions. The coefficients of correlation were low to moderate (0.2-0.8), with those between groups of teeth consistently higher. As for size differences, incisors and canines were larger in the permanent dentition in both arches by 1.2-2.0 mm. Premolars were smaller than their primary predecessors except for the upper first premolar. When the absolute values of the differences were ranked, the second premolar-second primary molar differences were the greatest (2.0-2.8 mm) while those between the first premolar-first primary molar were the smallest (0.3-0.7 mm). When tooth groups were assessed, the permanent teeth were larger than their predecessors in the anterior segments but smaller in the posterior segments. The leeway space was larger in the mandibular arch (2.2 mm) than in the maxillary arch (0.3 mm). When diameters of five teeth on one side of the arch were combined, the permanent exceeded the primary by 3.5 mm in the upper arch but only 0.5 mm in the lower. The ratios between the permanent and primary tooth sizes showed the same trend as the differences.

Key words: mesiodistal crown diameters, tooth size relations, correlation coefficients, differences, ratios, primary and permanent dentitions, Hong Kong Chinese.

INTRODUCTION

Animal, twin and family studies have indicated strong genetic control for tooth sizes (Osborne, Horowitz and De George, 1958; Bader, 1965; Garn, Lewis and Kerewsky, 1965). Some investigators have used dental size as a marker for genetic distance between populations (Garn, Lewis and Walenga, 1968). No conclusion has been reached as to the mode of inheritance for tooth size. Garne t al. (1965) suggested the possibility of an X-linked component to inheritance but this theory was not supported by other researchers (Bowden and Goose, 1969). The environmental factors that contribute to tooth size are also uncertain. The mother's health during preg- nancy, birth size and weight, and diet have been implicated as factors that would affect tooth size. It appears that tooth size is determined principally by genetics but the proportion and type of genetic control may vary between teeth, individuals and populations. It is likely to be polygenic in nature. Environment then acts on this genetic variability to produce continuous variation in tooth size (Hillson, 1986).

The relation between the sizes of primary teeth and their permanent successors has been studied. Most of these studies were done on Caucasian populations (Moorrees et al,, 1957; Moorrees, 1959; Moorrees and Chadha, 1962; Clinch, 1963; Moorrees, 1964;

Moyers et al., 1976; Garn, Cole and Wainright, 1977; Lysell and Myrberg, 1982), but there are some similar studies on other ethnic groups (Brown, Margetts and Townsend, 1980a, b; Steigman, Harari and Kuraita- Landman, 1982). In general, low to moderate corre- lation coefficients were found between pairs of corresponding teeth in both dentitions. There was a consensus that small primary teeth were likely to be followed by small permanent successors and large primary teeth by large permanent successors but the low correlation coefficients precluded their use for making reasonably accurate predictions (Moorrees and Chadha, 1962; Clinch, 1963; G a r n e t al., 1977: Brown et al., 1980b; Lysell and Myrberg, 1982).

Size relations are important in the mixed dentition. They provide information on dental crowding and changing patterns of occlusion in the developing permanent dentition (Moorrees, 1964; Brown et al., 1980a). Size relations have been assessed in three ways, usually by examining the coefficients of corre- lation (Moorrees et al., 1957; Moorrees, 1959; Clinch, 1963; Moorrees, 1964; Garnet al., 1977; Brown et al., 1980b), but absolute differences and ratios have also been used (Moorrees, 1959; Moorrees and Chadha, 1962; Clinch, 1963; Moyers et al., 1976; Brown et al., 1980a; Steigman et al., 1982).

Odontometric studies have been made on the mesiodistal crown diameters of primary and perma- nent teeth in Hong Kong Chinese (Sandar, Yen and

2 Keith Kwok-wah Yuen et al.

Brook, 1983; Otani et al., 1993) but these were cross-sectional studies and could not be used to reveal the size relations of the primary and permanent teeth. The present study was longitudinal in design and sought to examine the relations between the mesiodistal crown diameters of primary teeth and their permanent successors in the same individuals.

MATERIALS AND METHODS

Serial dental casts were obtained from 112 Hong Kong Chinese children (61 males, 51 females) at the average age of 5.68 yr (SD 0.44, range 4.64~.94) and 12.31 yr (SD 0.83, range 10.59 14.56), respectively. The children were drawn from 977 participants in the 5-year-old survey conducted by the Department of Children's Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong in 1986-1988. The sampling procedure for the survey has been described earlier (Wei, Holm and Tong, 1993). In summary, 1005 5-year-old Chinese children were selected from 14 kindergarten schools in the most densely populated areas of the territory. The schools were drawn at random with the chance of a particular school being chosen in accordance with the number of 5-year-old children attending. Twenty- eight children were unco-operative and only 977 were examined (520 boys and 457 girls) with mean age of 5.75 (SD 0.46 yr). Each child was examined for caries and a panoramic radiograph and impressions of their teeth were also taken.

Five hundred and five children were excluded from the present study due to the presence of approximal caries or restorations. During 1992-1993, a further survey of the remaining 472 children was scheduled. It was impossible to contact 248 children as they had moved. Fifteen had emigrated, nine were undergoing orthodontic treatment, and 58 refused to participate in the study. Of the 142 who had promised to co-operate, 30 did not turn up for appointment and they refused to come when follow-up calls were made. Hence, the study sample consisted of 112 children. Impressions were again taken and the crown diameters measured on the study casts.

The mesiodistal crown diameters of both primary and permanent teeth were measured by one of us (KKY) using calipers (TESA Digit-Cal SM) with digital output to the nearest 0.01 mm according to the method described by Moorrees et al. (1957). Teeth in which the measurement of the mesiodistal crown diameters could not be precise because of malfor- mation, caries, restorations, partial eruption, or plas- ter defects were excluded. Each tooth was measured twice and the average of the two readings was taken as the size for that individual tooth. The sizes of the pair of antimeric teeth were then averaged and used as the size of that tooth type for subsequent statistical analyses using the SAS/STAT ~-' statistical program (SAS Institute Inc., Cary, NC).

RESULTS

1. Representativeness o f the study sample

The study sample of 112 children comprised only 11.5% of the primary sample of 977. At the time of the 5-year-old survey, the present sample and the

primary sample were comparable in terms of age and sex distribution. The mean ages were 5.68 and 5.75 yr for the study and primary samples, respectively. The percentages of males in the two samples were 54.5% and 53.2%. In order to confirm the representativeness of the study sample, 50 children were randomly drawn from the primary sample and the primary tooth sizes in the study and random samples were compared using two-sample t-tests; p-values ranged from 0.39 (LD) to 0.94 (LA) with none of the values smaller than 0.05. Hence the present sample was taken to be representative of the primary sample and in turn the 5-year-old children in 198f~1988. This representativeness could be extended to the 12-year- old children in 1992 1993 because it was the same population that had grown from the age of 5 to 12 yr.

2. Method error

Measurement error was assessed by duplicate measurements of 25 sets of dental casts randomly drawn from each survey. The second measurement was made 3 months after the first. The method error was assessed by calculating the standard deviation of a single determination by the method of Dahlberg (1940), according to the following formula:

Method error = x/(YD 2)/2n

D, differences between the duplicate measurements; n number of double measurements.

Antimere sizes were not averaged in the assessment of method error. Thus the number of double measurements (n) for each tooth type was almost doubled. The method errors for individual primary teeth ranged from 0.05 to 0.12mm, with a mean of 0.09 mm, The UE and LE had the highest method error values (0.12 and 0.11 mm, respectively), fol- lowed by LC (0.10 mm). The method error for indi- vidual permanent teeth was slightly lower and ranged from 0.04 to 0.11 mm with a mean of 0.07 mm. The U6 had the highest method-error value (0.11 mm) followed by L6 (0.09mm). Method errors for five teeth grouped (central incisor to second primary molar/second permanent premolar) were 0.22 mm for the primary teeth and 0.17 mm for the permanent. These errors were smaller than, or similar to, those determined by other researchers and considered acceptable for studies of tooth size (Seipel, 1946; Moorrees et al., 1957; Margetts and Brown, 1978).

3. Mesiodistal crown diameters o f primary and permanent teeth

The descriptive statistics and sexual dimorphisms for the primary and permanent teeth are shown in Tables 1 and 2, respectively. The normality of the distribution of tooth size was assessed by Shapiro-Wilk tests with the sexes separated. None of the tooth sizes showed a tendency to non-normality at the 5% level. Hence, the distributions of the mesiodistal crown diameters of the primary and permanent teeth were sufficiently normal to allow the use of parametric statistical tests.

Preliminary analysis of the bilateral asymmetry of tooth size showed that four primary teeth were statistically asymmetrical at the 5% level. When expressed in terms of the percentage of the standard deviation of the corresponding tooth size, the

Tooth size relations of Hong Kong Chinese

Table 1. Mesiodistal crown diameters (mm) and sexual dimorphisms of primary teeth

Tooth Sex

Range Difference %

n Mean SD CV Max Min (M - F) Dimorphism

UA M 54 F 47

UB M 59 F 49

UC M 58 F 48

UD M 60 F 50

UE M 60 F 49

LA M 51 F 37

LB M 58 F 48

LC M 59 F 50

LD M 60 F 50

LE M 60 F 49

6.682 0.397 5.9 7.713 5.590 0.004 0.06 6.678 0.346 5.2 7.455 5.815 5.461 0.361 6.6 6.190 4.553 0.017 0.31 5.445 0.361 6.6 6.508 4.523 6.742 0.354 5.3 7.645 6.018 0.061 0.91 6.681 0.366 5.5 7.388 5.775 7.406 0.394 5.3 8.508 6.533 0.143 1.97 7.263 0.376 5.2 8.253 6.488 9.256 0.475 5.1 10.500 8.165 0.094 1.02 9.162 0.444 4.9 10.063 7.960 4.168 0.286 6.9 4.875 3.483 -0.046 - 1.10 4.214 0.278 6.6 5.020 3.575 4.667 0.342 7.3 5.468 3.928 -0.038 -0.81 4.705 0.313 6.6 5.325 3.953 5.900 0.320 5.4 6.543 5.135 0.019 0.32 5.881 0.346 5.9 6.570 5.001 8.184 0.464 5.7 9.073 7.235 0.089 1.10 8.095 0.468 5.8 8.985 6.898

10.295 0.473 4.6 11.333 8.903 0.149 1.47 10.146 0.404 4.0 11.070 9.323

asymmetry ranged from 5 to 12%. This finding agreed with previous findings and justified the use of averaged sizes from the antimeres (Seipel, 1946; Bar- rett, Brown and Mcdonald , 1963; Marget ts and Brown, 1978).

4. Coefficients o f correlation between tooth sizes o f primary teeth and their permanent successors

The correlation coefficients for the mesiodistal crown diameters of corresponding primary and per- manent teeth are listed in Table 3. Eighteen of the 20 coefficients in the two sexes were significant at the 5% level. Of these, 13 were highly significant (p < 0.001).

The coefficients for the L 3 : L C pairs in both sexes were not significant.

For males, the coefficients ranged from 0.3 to 0.6. The upper tooth-pairs had higher values than the corresponding lower tooth-pairs. The coefficients for females ranged from 0.2 to 0.7. The male coefficients were generally higher than the corresponding values for females except for the L 5 : L E pair. In both sexes, L 3 : L C pairs had the lowest coefficients.

The correlation coefficients between combined mesiodistal crown diameters of corresponding groups of teeth in primary and permanent dentit ions are listed in Table 4. The coefficients for the maxillary

Table 2. Mesiodistal crown diameters (mm) and sexual dimorphisms of the permanent teeth

Range . . . . Difference %

Tooth Sex n Mean SD CV Max Min (M - F) Dimorphism

U1 M 60 8.728 0.508 5.8 9.650 7.498 0.067 0.77 F 50 8.661 0,459 5.3 9.530 7.655

U2 M 58 7.175 0.600 8.4 8.483 5.753 0.052 0.73 F 48 7.123 0.499 7.0 8.115 5.520

U3 M 48 8.297 0.411 5.0 9.135 7.075 0.282** 3.52 F 45 8.015 0.400 5.0 8.950 7.058

U4 M 58 7.755 0.417 5.4 8.865 6.965 0.214" 2.84 F 48 7.541 0.430 5.7 8.398 6.618

U5 M 53 7.242 0.422 5.8 8.525 6.408 0.168 2.37 F 44 7.074 0.468 6.6 7,888 6.018

L1 M 60 5.476 0.328 6.0 6,398 4 .848 -0.056 1.01 F 49 5.532 0.317 5.7 6,135 4.830

L2 M 60 6.104 0.332 5.4 6,913 5.438 -0,022 -0.36 F 49 6.126 0.354 5.8 6,853 5.308

L3 M 55 7.287 0.366 5.0 7,980 6 . 1 9 5 0.365*** 5.27 F 51 6.922 0,427 6.2 7,748 5.760

L4 M 56 7.576 0.360 4.7 8.473 6,670 0.136 1.83 F 46 7,440 0.465 6.3 8.420 6,465

L5 M 53 7.443 0.378 5.1 8.295 6,633 0.167 2.30 F 45 7.276 0.396 5.4 7.975 6,190

*Significant dimorphism with p = 0.011. **Significant dimorphism with p = 0.001. ***Significant dimorphism with p < 0.001.

4 Keith Kwok-wah Yuen et al.

Table 3. Pearson's product-moment correlation coefficient (r) for the mesiodistal crown diameters of pairs of corresponding primary and permanent teeth

Male Female

Tooth pair r n p r n p

UI UA 0.6 53 <0.001 0.6 46 <0.001 U2 UB 0.5 56 < 0.001 0.3 46 0.049 U3 UC 0.5 46 < 0.001 0.5 42 < 0.001 U4 UD 0.6 57 <0.001 0.5 47 <0.001 U5 UE 0.6 52 <0.001 0.6 42 <0.001 LI LA 0.6 50 <0.001 0.5 36 0.001 L2 LB 0.5 57 < 0.001 0.5 47 0.001 L3 LC 0.3 53 0.066 0.2 50 0.105 LA LD 0.5 55 <0.001 0.3 45 0.023 L5 LE 0.4 52 0.002 0.7 43 < 0.001

groups ranged from 0.5 to 0.8 in both sexes. All were statistically significant at the 1% level, with nine o f them highly significant (p < 0.001). Highest values occurred in both sexes when the combined sizes of all five teeth were correlated. The figures for males were consistently larger than those for females.

For the mandibular arch, all coefficients were significant at the 5% level; eight were highly signifi- cant (p < 0.001). The coefficients ranged from 0.4 to 0.6, with no obvious sex differences. Tooth groups that included the lower canines had smaller co- efficients, owing to the statistically insignificant co- efficient for L3 :LC. In the maxilla, correlations were generally higher than for corresponding mandibular tooth groups except for combined incisor diameters.

5. Differences in the mesiodistal crown diameters of corresponding permanent and primary teeth

Differences between the mesiodistal crown diam- eters of corresponding permanent and primary teeth were obtained by subtracting the tooth size(s). Nor- mality assessment of the distribution of the difference between the tooth sizes of corresponding pairs and corresponding groups of permanent and primary teeth revealed that only one of them showed a significantly non-normal distribution (UI2345- U A B C D E , p = 0.046).

Fo r pairs of corresponding teeth, t-tests showed that sex differences were significant only for upper (p --- 0.014) and lower canines (p < 0.001). Hence, the descriptive statistics for the differences in the sizes of corresponding teeth in the primary and permanent

dentition listed in Table 5 are presented with the sexes pooled, except for the canines where data for each sex are also tabulated.

The differences in the mean values between perma- nent and primary teeth were significant in all in- stances (p < 0.001). For canines and incisors, the permanent teeth were larger than the corresponding primary predecessors. On the other hand the primary molars were larger than the permanent successors, except for U D where the succeeding U4 was slightly larger. In the upper arch, the permanent central incisor exceeded the predecessor by the greatest amount (2.0mm); this difference progressively de- creased posteriorly and was reversed at the second premolar ( - 2 . 0 mm). A similar trend was observed in the lower arch, except for a reversed position between the central and lateral incisors. The lateral incisor difference was 1.4 mm and the second premo- lar difference was - 2 . 8 mm. When ranked according to absolute values, the differences were smallest for the U 4 - U D and L 4 - L D pairs. The largest differences were found for the L5 -LE and U 5 - U E pairs with the primary teeth being larger.

As for the size differences between corresponding groups of teeth (Table 6), the incisor segments were larger in the permanent than the primary dentition in both arches, but the primary molar segments were larger than the succeeding permanent premolar seg- ments. The inclusion of the canines into the incisor and molar segments maintained the relation but the difference in the anterior segments was enhanced and the reverse difference in the posterior segments

Table 4. Pearson's product-moment correlation coefficient (r) for combined mesiodistal crown diameters of corresponding groups of primary and permanent teeth

Male Female

Tooth group r n p r n p

UI2 UAB 0.6 50 < 0.001 0.5 44 0.002 U123 UABC 0.6 39 < 0.001 0.5 37 < 0.001 U45 UDE 0.7 52 < 0.001 0.6 41 < 0.001 U345 UCDE 0.8 44 <0.001 0.7 38 <0.001 U12345 UABCDE 0.8 37 <0.001 0.7 33 <0.001 LI2 LAB 0.6 50 <0.001 0.5 35 <0.001 L123 LABC 0.5 45 < 0.001 0.4 35 0.040 L45 LDE 0.5 51 < 0.001 0.6 41 < 0.001 L345 LCDE 0.5 48 < 0.001 0.5 41 < 0.001 L12345 LABCDE 0.6 40 <0.001 0.5 29 0.002

Tooth size relations of Hong Kong Chinese 5

Table 5. Difference (mm) in mesiodistal crown diameters of primary teeth and their successors with sexes pooled

Difference (Permanent - Primary)

Tooth pair n Mean SD SEM CV

U1 UA U2 UB U3 UC*

U4 UD U5 UE L1 LA L2 LB L3 LC**

L4 LD L5 LE

99 1.99 0.39 0.04 19.6 102 1.69 0.51 0.05 30.4

M 46 1,51 0.38 0.06 25.1 F 42 1,31 0.37 0.06 27.9 M + F 88 1.41 0.38 0.04 27.2

104 0.31 0.38 0.04 122.2 94 -2.03 0.40 0.04 - 19.8 86 1.29 0.28 " 0.03 21.7

104 1.42 0.34 0.03 23.8 M 53 1.37 0.43 0.06 31.4 F 50 1.04 0.49 0.07 46.5 M + F 103 1.21 0.48 0.05 39.9

100 -0.65 0.49 0.05 --75.0 95 --2.84 0.42 0.04 - 14.7

*Statistically significant sex difference with p = 0.014. **Statistically significant sex difference with p < 0.001.

reduced. The combined diameters of the p r imary canines and molars were larger t han those of the succeeding pe rmanen t teeth by 0.3 m m on one side of the upper arch and 2.2 m m on one side of the lower arch. The tota l mesiodistal c rown diameters on one side of the upper arch exceeded those of their prede- cessors by 3.5 mm, while the difference in the lower arch was only 0.5 mm.

6. Ratios of the mesiodistal crown diameters of corre- sponding permanent and primary teeth

Rat ios were expressed with the size of pe rmanen t m e m b e r as the n u m e r a t o r and the size of the p r imary m e m b e r as the denomina to r . The dis t r ibut ions of the rat ios between 10 pairs of teeth and 10 pairs of groups of teeth were assessed by the Shapiro--Wilk test. N o n e of the 20 p-values was significant at the 1% level and only one of them was significant at the 5% level (U12345 :UABCD E , p =0 .034) . Hence, the distri- bu t ions of the rat ios were reasonably no rma l and parametr ic statistical tests were subsequent ly applied.

Sex difference in the rat ios were assessed with two-sample t-tests. Significant differences at the 5% level were found for the pairs of canines in bo th arches (p = 0.041 for pairs of upper canines and 0.002 for pairs of lower canines). The rat ios for pairs of cor responding pr imary and pe rmanen t teeth are presented in Table 7, with sexes pooled except for the

canines where the figures for males and females are also presented separately.

DISCUSSION

Prior study of the mesiodistal crown diameters in bo th dent i t ions of the same sample showed tha t bilateral asymmetry was insignificant and when the sizes for the ant imeres were pooled, the male teeth were larger t han those of the female except for the two lower incisors in bo th denti t ions. Statistically significant sex differences at the 1% level were found in only three pe rmanen t teeth, no t including the lower incisors, with the pe rmanen t canines topping the list. The percentage sexual d imorph i sm was 0.06-5.27% , with the pr imary teeth being less d imor- phic. This was generally similar to the reported range of 0 .4-3 .9% (Seipel, 1946; Moorrees , 1959; Moyers et al., 1976; Lysell and Myrberg, 1982). The pa t te rn of sexual d imorph i sm also agreed with previous studies (Seipel, 1946; Moorrees , 1959) in tha t the pe rmanen t canines were the mos t d imorphic and the incisors the least, bu t this t rend was not reflected by the p r imary teeth. W h e n the too th sizes were com- pared with those of o ther the popula t ions , the mesiodistal crown diameters of H o n g K o n g Chinese were found, on average, to be larger t han those of Swedes (Lysell and Myrberg, 1982), Icelanders

Table 6. Difference (mm) in the combined mesiodistal crown diameters of corresponding groups of primary and permanent teeth with sexes pooled

Difference (Permanent - Primary)

Tooth pair n Mean SD SEM CV

U l 2 UAB 94 3.67 0.74 0.08 20.2 U123 UABC 76 5.10 0.91 0.10 17.8 U45 UDE 93 - 1.68 0.66 0.07 - 39.0 U345 UCDE 82 - 0.30 0.80 0.09 - 267. I U 12345 UABCDE 70 3.46 1.14 0.14 32.9 LI2 LAB 85 2.68 0.53 0.06 19.6 L123 LABC 80 3.87 0.93 0.10 24.2 L45 LDE 92 - 3.45 0.74 0.08 - 21.5 L345 LCDE 89 - 2.21 0.05 0.11 - 47.4 L 12345 LABCDE 69 0.50 1.37 0.17 277.4

Keith Kwok-wah Yuen et al.

Table 7. Ratio in mesiodistal crown diameters of primary teeth and their successors with sexes pooled

Tooth pair

Difference (Permanent/Primary)

n Mean SD SEM CV

UI UA U2 UB U3 UC*

U4 UD U5 UE L1 LA L2 LB L3 LC**

L4 LD L5 LE

99 1.30 0.07 0.01 5. l 102 1.3I 0.10 0.01 7.5

M 46 1.22 0.06 0.01 4.9 F 42 1.20 # 0.06 0.01 5.0 M + F 88 1.21 0.06 0.01 5.1

104 [.04 0.05 0.01 5.1 94 0.78 0.04 0.00 5.0 86 1.31 0.08 0.01 5.9

104 1.31 0.09 0.01 6.5 M 53 1.23 0.08 0.01 6.5 F 50 I. 18 0,09 0.01 7.4 M + F I03 1.21 0.09 0.01 7.2

100 0.92 0.06 0.01 6.2 95 0.72 0.04 0.00 4.8

*Statistically significant sex difference with p = 0.041. **Statistically significant sex difference with p = 0.002. # Mean ratio significantly different from I with p = 0.038. Other mean ratios were significant with p < 0.001.

Axelsson and Kirveskari, 1983, 1984), and American whites (Moyers et al., 1976), but smaller than those of Australian aboriginals (Barrett et al., 1963; Margetts and Brown, 1978).

The coefficients of correlation between pairs of corresponding individual teeth were found to be low to moderate (0.2-0.7). A similar range of coefficients was found for Caucasians (Moorrees et al., 1957; G a r n e t al., 1977; Clinch, 1963; Lysell and Myrberg, 1982), and Australian aboriginals (Brown et al., 1980b). Highest correlation coefficients have usually been found between permanent and primary upper central incisors (Moorrees et al., 1957; Lysell and Myrberg, 1982; Brown et al., 1980b). This trend was not reflected in the present study. The highest co- efficients within these ranges had been said to be too low to enable a reasonably accurate prediction of the size of permanent teeth from their predecessors (Moorrees, 1959). The same conclusion may be applied to Hong Kong Chinese.

The coefficients between groups of teeth were also found to be low to medium (0.4-0.8) but higher than those between pairs of teeth. Higher coefficients were found between the sums of the sizes of all teeth. This agreed with similar trends found for Caucasians and Australian aboriginals (Moorrees et aL, 1957; Moor- rees, 1959; Moorrees and Chadha, 1962; Moorrees, 1964; Brown et al., 1980b). This might indeed reflect a greater genetic control over the size of tooth groups than over individual teeth mediated through compen- satory interactions between adjacent teeth during the early phase of odontogenesis as suggested by Harzer (1987).

The term 'leeway space' first coined by Nance (1947) deserves a separate discussion because of its implication in mesial migration of first permanent molars for the establishment of optimal occlusion. The amount of leeway space on one side of the upper arch has been reported to range from 0.7 to 1.5 mm and for the lower arch from 1.7 to 3.3 mm for various populations (Black, 1894; Seipel, 1946; Moorrees, 1959; Clinch, 1963; Brown et al., 1980a; Steigman et al., 1982). The mandibular leeway space was

consistently greater than the maxillary. We found the smallest maxillary leeway space ever reported (0.3 mm). The amount for the mandibular leeway space (2.2 mm) was at the medium of the reported range. The amount of mandibular in excess of maxil- lary leeway space was previously found to range from 0.8 to 2.0 ram. The corresponding figure here was 1.9 mm, which was among the highest, second only to that of female Australian aboriginals. This may con- tribute to the establishment of optimal occlusal re- lations in the transition from the mixed to the permanent dentition. Steigman et al. (1982) found negative leeway in the upper arch in 11% and in the lower arch in 2% of Israelis. We found a negative upper leeway in 34% and negative lower leeway in only 1%0 of our participants. This reflected the great individual variation in leeway space, especially in the upper arch, and the orthodontic assessment for space requirements of the permanent canine-premolar seg- ments should therefore be made on an individual basis.

Acknowledgements--We thank Dr J. Karlberg and Mr W. Wong of the Department of Pediatrics of the Faculty of Medicine, and Mr S. L. Lee of the Data Processing Unit of The University of Hong Kong for their advice and support in analysing the data. This study was supported with Research Grant no. 337/251/0006 of The University of Hong Kong.

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