Comparison of the Crown Dimensions between the Maxillary ... · Comparison of the Crown Dimensions between the Maxillary Second Deciduous Molar and the First Permanent Molar By Shintaro

Okajimas Folia Anat. Jpn., 73(4): 179-184, Ostober, 1996

Comparison of the Crown Dimensions between the Maxillary Second

Deciduous Molar and the First Permanent Molar

By

Shintaro KONDO, Eizo WAKATSUKI, Huang SHUN-TE, Chung SHENG-YEN,

Yoshinobu SHIBAZAKI and Masato ARAI

The First Department of Oral Anatomy, Showa University School of Dentistry, 1-5-8 Hatanodai, Shinagawa-ku,Tokyo, 142 Japan

Pediatric Dentistry and Orthodontics, School of Dentistry, Kaohsiung Medical College, 100 Shih-Chuen 1st Rd.,

Kaohsiung, Taiwan, R.O.C.

Department of Orthodontics, Showa University School of Dentistry, 2-1-1 Kita-Senzoku, Ohota-ku, Tokyo, 145

Japan

-Received for Publication, April 26, 1996-

Key Words: Odontometry, molariform teeth, cusp size, intercusp distance, Chinese

Summary: The morphologic relationship of the crown structures of the maxillary second deciduous molar (m2) and the first

permanent molar (M1) was investigated with odontometric methods. Materials used were 124 male dental casts taken fromChinese living in Kaohsiung (Taiwan). The mesiodistal and buccolingual diameters, cusp sizes, and intercusp distances

were measured. The mean crown diameters were larger in M1 than in m2. The tooth crown proportions were similar in m2

and M1. The intercusp distance was compressed buccolingually in m2. With respect to the relative cusp size, the paracone

was not significantly different between m2 and M1. The metacone was relatively larger in m2 than M1, while lingual cusps

(protocone and hypocone) were relatively smaller in m2 than in M1. Principal component analysis was performed toinvestigate factors influencing the variation in crown dimensions of the two teeth. Four components were extracted:

(1) buccolingual intercusp distances, (2) crown proportion (mesiodistal and buccolingual diameters), (3) mesiobuccal anddistolingual compression, and (4) protocone and mesiodistal intercusp distance. Analysis of principal component scores

indicated that the crown proportions of m2 and M1 were similar. The m2 had a smaller protocone and more buccolingually

compressed intercusp distances than did M11. The morphologic characteristics of the crown of m2 indicate that it is more

primitive and has developed less than M1.

The basic structures of deciduous teeth follow that of their peuinanent successors. The second de-ciduous molar (m2) resembles the first permanent molar (M1) rather than the second premolar (P2), its successor. The m2 and M1 have similar crown shapes and occlusal groove patterns and have the same main-cusp pattern. The m2 and MI are, however, not identical. Deciduous molars retain more primitive characteristics than do permanent molars. Therefore, comparison of these two teeth is valuable from a

phylogenetic point of view. Numerous morphologic studies have described

the quantitative relationships of crown dimensions and nonmetric crown characteristics of m2 and M1

(Butler, 1967a; Hanihara, 1956; Jorgensen, 1956; Nomura, 1974; Ozumi, 1960; Sugiyama, 1976; Yamada et al., 1986). Because these studies mainly analyzed the outline of the crown, the shape of each cusp was not clearly described. Therefore, the present odontometric study was performed to investigate the morphologic relationships of crown structures of m2

and Ml.

Materials and Methods

Male dental casts (56 deciduous and 68 mixed or permanent dentitions) were taken from Chinese living in Kaohsiung, Taiwan. Casts were used only if the teeth were fully erupted, had no anomalies of crown morphology, and the cusp tips, central pit, and occlusal grooves were not noticeably eroded.

Dimensions of the crown were measured with a sliding caliper to the nearest 0.01 mm. The dimensions of the crown included mesiodistal (MD) and bucco- lingual (BL) diameters, the cusp sizes (Pa, Me, Pr, Hy), and the intercusp distances (Pa-Me, Pa-Pr, Me-Pr). The MD and BL dimensions were measured with Fujita's method (1949). The distance from the central pit to one of the four corners of the crown was defined as a cusp size (Fig. 1). The sizes of the distal cusps (Me, Hy) were as defined by Yamada

179

180 S. Kondo et al.

lingual

Fig. 1. Measurement of crown dimensions and cusp sizes. MD: mesiodistal crown diameter, BL: buccolingual

crown diameter, Pa: paracone, Pr: protocone, Me: meta- cone, Hy: hypocone

et al. (1988, 1992). The sizes of the mesial cusps (Pa, Pr) were defined in a similar way. Teeth on the right side of the dental arch were usually measured; how-ever, when a tooth on the right side could not be measured because of absence, abnormality, heavy wear, or other reasons, the corresponding tooth on the left side of the arch was measured.

The following crown indices were calculated from the measurements:

Crown area (CrA) = MD x BL Crown index (CrI) = BL/MD x 100

Cusp size indices: Paracone index (Pa) = Pa/MD x 100

Protocone index (Pr!) = Pr/MD x 100 Metacone index (Me!) = Me/MD x 100

Hypocone index (HyI) = Hy/MD x 100 Intercuspal distance index:

Pa-Pr/Pa-Me = Pa-Pr/Pa-Me x 100

The indices between intercuspal distances and crown diameters:

Pa-Me/MD = Pa-Me/MD x 100 Pa-Pr/BL = Pa-Pr/BL x 100

Me-Pr/BL = Me-Pr/BL x 100

Percentage differences (%diff) between m2 and MI were calculated. Each %diff in the crown dimen-

sion was defined as (M1-m2)/m2 x 100. For indices, the %diff was defined as an absolute difference of

(M1-m2).

Statistical analysis Descriptive statistics, including distribution par-

ameters and multivariate statistical analysis, were

performed with SPSS statistical software (SPSS Japan Inc., Tokyo, Ver. 6.1J for Windows) on a personal

computer. Differences between measurements were analyzed with the two-tailed Student's t-test.

Results

All crown dimensions were greater in M1 than in m2 (P < 0.01, Table 1). The %diff of dimensions in order of decreasing magnitude was that of CA

(21.98%), Pa-Pr, Me-Pr, Pr, Hy (16.79% , 14.25% , and 14.39%), MD and BL diameters (10.14% and

10.71%), Me and Pa-Me (7.12% and 7.11%). The Cr! indices of m2 and M1 were not significantly

different. Of the cusp size indices, Pal had the smallest % diff (0.88%) and did not differ significantly between m2 and Ml. Other cusp size indices differed significantly between m2 and M1 (P < 0.01), of which

HyI had the largest %diff (2.02%). The indices of intercusp distance and crown diameter indicated that the intercusp distance was more compressed in m2 than in M1 and more compressed buccolingually than mesiodistally. Crown dimensions of M1 varied more than those of m2, except for Pr, Hy and the intercusp distances.

Principal component analysis (PCA) was per-formed to investigate factors influencing the variation in crown dimensions of the two teeth. To extract the variance components of the tooth shape, PCA per-formed after the variables were standardized. After Varimax rotation, each factor was interpreted. The total variance of the first four PCA factors whose eigen values were greater than 1 accounted for 76.2% of all variances (Table 2). Table 3 shows the basic statistics of PCA scores for the four factors. A factor loading whose absolute value is greater than 0.5 can serve as a guide for the interpretation of components. The first component (PC1) was concerned with the buccolingual intercusp distances (Pa-Pr, Me-Pr). The mean value of the PCA score for PC1 was greater in m2 than in M1 (P < 0.01). This result showed the buccolingual intercusp distances were more corn-

pressed in m2 than in Ml. The PC2 was bipolar, with negative weight for MD and positive weight for BL. The PCA score for PC2 tended to be larger in m2 than in Ml, but the difference was not significant. This result was related to the fact that Cr! index did not differ significantly between m2 and Ml. The PC3 was also bipolar, with negative weight for Hy, and

positive for Pa, and represented mesiobuccal and distolingual expansion or compression of the crown. Although the PCA score for PC3 tended to be larger

Crown Dimensions in Maxillary Deciduous and Permanent Molars 181

Table 1. Basic statistics of the crown dimensions and indices in M1 and m2

**: P<0 .01

t: The mean value of m2 was larger than that of Ml.

Table 2. Factor loadings after Varimax rotation

Table 3. Basic statistics of the PCA scores and results of a t-test

between M1 and m2

in m2 than in Ml, but the difference was not signifi-cant. The PC4 was concerned with Pr and Pa-Me, and the PCA score was larger in M1 than in m2

(P < 0.01). Therefore, m2 had a smaller Pr and larger Pa-Me than Ml. The results of PCA score analysis are summarized in Fig. 2.

Discussion

In many mammals, m2 resembles Ml (Butler, 1967a,b). The morphologic concordance between

m2 and M1 is termed isomorphy (Kraus et al. , 1969). Both these teeth usually have four main cusps and the same groove patterns. The proportional differ-ence between them has been described. The Cr! can be used to compare a tooth's crown proportions, for example, between persons of different ethnic groups

(Table 4). Kraus et al. (1969) reported that m2 of width (BL) was relatively greater than its length

(MD), while Nomura (1974) reached the opposite conclusion.

In the present study the difference in Cr! between m2 and M1 was not significant. This result agrees with those of Ozumi (1960) and Yamada et al. (1986). Most studies have found that the Cr! of m2 and M1 are approximately 110. Therefore, m2 is similar in tooth crown proportion to Ml.

With respect to the relationship between outside

182 S. Kondo et al.

Fig. 2. Plot of mean values of the PCA scores (PC1 and PC4) for m2 and Ml. The SD intervals for these are also shown.

Table 4. Crown indices in the m2 and M1 (male)

JPN, Japanese; JW, Japanese-American Whites, J13, Japanese-American Blacks; LB, Liberian Blacks AA, Australian Aborigines; TW, Taiwan Chinese

diff. NS: not significant, >>: P < 0.01

crown diameter and intercusp distance, the bucco-lingual intercusp distances were relatively smaller in m2 than in M1 (Pa-Pr/BL, Me-Pr/BL). The relative mesiodistal intercusp distance (Pa-Me/MD) did not differ significantly between m2 and M 1. Kamiakito

(1978) calculated occlusal surface indices in the de-ciduous molars and showed that occlusal table is relatively smaller in m2 than in Ml. These results are related to deciduous molars' having well-developed buccal and lingual cingula that surround the cervical region of the crown. These cingula are thought to be conservative. Various methods have been used to express varia-bility of cusp size in maxillary deciduous molars

(Jorgensen, 1956; Hanihara, 1961, Hanihara et al., 1970; Nomura, 1974). Yamada et al. (1988) proposed a simple method for expressing hypocone size. They measured the distolingual crown size with a sliding caliper. The distolingual crown size was defined as the distance from the central pit to the most disto-lingually protrusive point of the hypocone. This measurement is useful as an indicator of hypocone size because the measurement is closely related with hypocone variability. Yamada (1992) also measured metacone size in a similar way; however, he has not

yet measured the two mesial cusps. We measured the four main cusps to compare relative cusp size variability between m2 and Ml.

We found that the relative size of the paracone

(Pal) did not differ significantly between m2 and

Crown Dimensions in Maxillary Deciduous and Permanent Molars 183

Ml. The paracone forms and calcifies at the earliest developmental stage (Kraus & Jordan, 1965). This cusp is a successor of the single cone of the reptilian haplodont (Patterson, 1956). Thus, the paracone was first to differentiate ontogenetically and phylo-

genetically. An association between early formation and low cusp-size variability in anthropoid primates was noted by Corruccini (1979). Yamada (1987) measured distancesfrom the central pit to the pe- rimeter of the crown contour in maxillary molars. The radii of mesiobuccal regions were not significantly different. Therefore, the paracone is the cusp whose size is the most stable.

In the present study, the metacone was relatively larger in m2 than Ml. However, between these teeth absolute cusp size differed least. Nomura (1974) measured the cusp areas of the two teeth and found that the area of the metacone is larger than that of the paracone in m2. He also found that m2 has a relatively larger metacone than does Ml. Normura's results agree with those of the present study.

The first distal cusp (metacone in the maxillary; hypoconid in the mandible) appears earlier and be-comes larger in m2 than in M1 (Butler, 1967b). Larger cusps appear earlier in ontogeny than small basal cusps. Jorgensen (1956) reported that the meta-cone was somewhat larger than the paracone in Australopithecus. This configuration seems to be a

primitive trait in deciduous molars. In the present study, the protocone was relatively

smaller in m2 than in Ml. This cusp is more distal in m2 than in Ml, because the buccal region of the mesial marginal ridge is more prominent in m2

(JOrgensen, 1956). As a result of the distal shift of the mesiolingual protrusive point of the protocone, the distance from the central pit to this point is shorter in m2. Nomura (1974) also found that the area of the protocone is relatively smaller in m2 than in Ml.

We found that the hypocone was relatively smaller in m2 than in Ml. This cusp differentiates last onto-

genetically and phylogenetically (Kraus & Jordan, 1965; Patterson, 1965). Therefore, the hypocone varies most in the maxillary molars. Hanihara et al.

(1956, 1970) used two methods of comparison: the hypocone index and relative hypocone area. By any method of comparison, the hypocone was significantly smaller in m2 than in M1 Hanihara et al. speculated that in the differentiation phase the hypocone in m2 was not as developed as in Ml.

These results indicate that m2 has less developed lingual cusps and better developed buccal cusps than does Ml. The origin of lingual cusps differs for that of buccal cusps. In the molars of primitive primates there are two methods of new cusp differentiation

(Sakai, 1982). The two lingual cusps originate from

the lingual cingula. The metacone differentiates from the posterior slope of the paracone. Variability in cusp size corresponds with the phylogenetic origin of the cusp.

Butler (1967a) examined the prenatal growth pat-tern in the tooth germs of m2 and Ml. The m2 and

M1 had similar patterns despite M1 calcifying at a larger size. Butler concluded that the differences in external crown morphology between m2 and M1 reflected differences in growth gradients and enamel thickness. The enamel-dentin partition plane shows more primitive traits than does external crown mor- phology (Sakai & Hanamura, 1971; Korenhof, 1982). Therefore, m2 has thinner enamel and more con-servative external crown morphology than does Ml. As mentioned above, m2 and M1 have many mor-phologic similarities in crown proportion. These results are related to the permanent and deciduous molars series belonging to the primary dentition and lying in the molarization field.

Despite these similarities, there are differences between the two teeth. The m2 is a deciduous tooth, while M1 is a permanent tooth. The crown shape of m2 is more conservative than that of Ml. The cusps of m2 are not as developed as those of Ml. The mor-phologic characteristics of the crown of m2 indicate that it is more primitive and has developed less than Ml.

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