Triangular-shaped incisor crowns and crowding

ORIGINAL ARTICLE Triangular-shaped incisor crowns and crowding Seung-Hoon Rhee, DDS, MSD,a and Dong-Seok Nahm, DDS, PhDb Suwon and Seoul, Korea The purpose of this study was to establish if there is a correlation between the shape of the labial crowns of the incisors and crowding. Plaster cast models of 69 untreated individuals (30 males and 39 females) were evaluated. The casts were selected randomly from the collection at Seoul National University and Ajou University. With the use of Little’s irregularity index, the sample was divided into 2 groups, a crowded group and a normal group. Repeated measurements of the maximum mesiodistal width of the incisal and cervical areas of the incisors were taken by means of a digital vernier caliper and a ratio of these measurements was calculated. The mean value for the crowded group was significantly larger in the incisal area (P < .01) and smaller in the cervical areas (P < .01) than corresponding values in the normal group. These ratios were correlated with the irregularity index (Pearson r 2 from 55% to 65%). The value obtained from the incisor width ratio in the normal group can be useful for the diagnosis and treatment of crowded malocclusion. (Am J Orthod Dentofacial Orthop 2000;118:624-8)

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rowding is a common characteristic of malocclusion. It appears in 37% of Class I malocclusions. Forty percent of children and 85% of adolescents have some degree of malalignment within the dental arches.1 Some of the possible etiologies of crowding reported in the literature are evolution, genetics, and environment. Many studies have been performed to determine the relationship of the teeth and dental arches to crowding. Some researchers have proposed that tooth size is correlated with crowding.2-5 Peck and Peck6-8 suggested that the determinant factor in lower incisor crowding was not only tooth size, but also tooth shape. Begg9 incorporated one of the theories trying to explain the increase of crowding in modern civilization into his philosophy of treatment. This theory stated that the lack of proximal and occlusal wear of the teeth was largely due to the modern diet. However, other authors have stated that the correlation of crowding and tooth size, or the MD/FL ratio, is weak, and that the correlation between crowding and dental arch size is more significant.10-15 Other factors, such as aging,16,17 tooth size differences between males and females,18,19 malocclusion,20 and race,21,22 have also been studied. Incisor crowns come in many shapes: square, triangular, ovoid, and mixed.23,24 It has been stated that the broader the contact, the more stable the tooth and the less likely it is to slip under pressure or tension.25 Tri-

Sixty-nine plaster cast models of untreated individuals with Class I malocclusion and Hellman’s dental age IVa were selected randomly from the collection of cast models at Seoul National University and Ajou University. By using the irregularity index as described by Little,27 36 plaster cast models from 15 males and 21 females were assigned to a crowded group (irregularity index > 3), and 33 plaster cast models from 15 males and 18 females were assigned to a normal group (irregularity index ≤ 3). One investigator measured the cervical maximum mesiodistal width (CMD) of each incisor at the level of the cementoenamel junction equal to one fourth of the labial crown length (Fig 1), as described by You23 and Ash.24 The incisal maximum mesiodistal width (IMD) was measured at the level of incisal edge. With the use of a digital vernier caliper, repeated measurements were taken of the following:

aFull-time Lecturer, Department of Dentistry, Ajou University, School of Medicine, Suwon, Korea. bProfessor, Department of Orthodontics, Seoul National University, School of Dentistry, Seoul, Korea. Reprint requests to: Seung-Hoon Rhee, Dept of Dentistry, Ajou University, School of Medicine, San 5, Wonchon-dong, Paldal-gu, Suwon 442-749, Korea (R.O.K.). Submitted, August 1999; revised and accepted, May 2000. Copyright © 2000 by the American Association of Orthodontists. 0889-5406/2000/$12.00 + 0 8/1/110812 doi:10.1067/mod.2000.110812

1. L1CMD: lower central incisor cervical maximum mesiodistal width 2. L1IMD: lower central incisor incisal maximum mesiodistal width 3. L1R: lower central incisor mesiodistal width ratio (L1IMD/L1CMD × 100) 4. L2CMD: lower lateral incisor cervical maximum mesiodistal width

624

angularly shaped incisors have a small contact area and thus a more unstable contact.26 The hypothesis tested in this study was that the labial crowns in the crowded group were more triangularly shaped than the crowns in the normal group. The purpose of this study was to determine whether there is a correlation between labial incisor crown shape and crowding. MATERIALS AND METHOD

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American Journal of Orthodontics and Dentofacial Orthopedics Volume 118, Number 6

Table I. Group

statistics and independent samples t test for equality of means between normal and crowding groups Normal (n = 33) Variable

Fig 1. Diagram representing measurements.

5. L2IMD: lower lateral incisor incisal maximum mesiodistal width 6. L2R: lower lateral incisor mesiodistal width ratio (L2IMD/L2CMD × 100) 7. LIRI: lower incisor irregularity index 8. U1CMD: upper central incisor cervical maximum mesiodistal width 9. U1IMD: upper central incisor incisal maximum mesiodistal width 10. U1R: upper central incisor mesiodistal width ratio (U1IMD/U1CMD × 100) 11. U2CMD: upper lateral incisor cervical maximum mesiodistal width 12. U2IMD: upper lateral incisor incisal maximum mesiodistal width 13. U2R: upper lateral incisor mesiodistal width ratio (U2IMD/U2CMD × 100) 14. UIRI: upper incisor irregularity index The mean measurements of the right and left antimere teeth were used in the statistical analysis. To test the systematic error of the measurements, 2 measurements were taken for each value. Systematic errors were estimated with a paired t test between 2 measurements. The t value of the mean difference was 1.61, and the mean difference was 0.03. Random errors were expressed as the coefficient of reliability (0.95). Therefore, the measurement error will have no effect on the interpretation of the study results.28 Two hypotheses were tested: (1) the null hypothesis (H0), which asserts that there would be no difference in incisor crown shape between normal and crowded groups, and (2) the alternative hypothesis (H1), which asserts that there would be more triangularly shaped incisor crowns in the crowded group than in the normal group.

L1CMD L1IMD L1R L2CMD L2IMD L2R LIRI U1CMD U1IMD U1R U2CMD U2IMD U2R UIRI

Mean 4.04 5.26 130.91 4.45 5.85 132.25 1.40 6.85 8.13 118.96 5.31 6.58 124.35 1.95

Crowding (n = 36)

SD

Mean

0.36 0.27 9.25 0.42 0.36 9.75 0.84 0.46 0.40 6.52 0.48 0.46 8.24 0.96

3.69 5.62 153.40 4.00 6.25 158.17 8.11 6.25 8.70 139.62 4.90 7.35 151.53 11.55

SD

Independent t test

0.34 0.35 15.43 0.47 0.30 16.68 2.38 0.40 0.45 8.96 0.54 0.58 17.26 2.55

4.12** –4.79** –7.41** 4.19** –5.09** –7.96** –15.90** 5.78** –5.48** –10.87** 3.39** –6.05** –8.46** –21.03**

**P < .01 significant (2-tailed).

The statistical tests used in this study were (1) mean and standard deviations and t tests for crowded and normal groups and (2) the Pearson product moment correlation, to establish a correlation between the irregularity index and the incisor width ratio (a 2-tailed analysis was used). RESULTS

The mean and standard deviations of the measurements for the crowded and normal groups, and t test results between the 2 groups, are presented in Table I. In the crowded group, the maximum mesiodistal incisal width was greater and the maximum mesiodistal cervical width was smaller; thus, the width ratios were larger than in the normal group (statistically significant at the P < .01 level). A comparison of male and female measurements showed that the teeth in males were larger than the teeth in females (Table II). Statistical significances were seen at both measurements of the lower lateral incisor, cervical and incisal maximum mesiodistal width (L2CMD, L2IMD), and in the upper central and lateral incisors at the incisal maximum mesiodistal width (U1IMD, U2IMD). To determine whether there was a correlation between incisor labial crown shape and crowding, we calculated the Pearson correlation between incisor width ratio, which represents shape, and the irregularity index, which represents crowding. Table III shows 2 measurements that were strongly correlated at the P < .01 statis-

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American Journal of Orthodontics and Dentofacial Orthopedics December 2000

Group statistics and independent samples t test for equality of means between males and females Table II.

Female (n = 39) Variable

Mean

SD

L1CMD L1IMD L1R L2CMD L2IMD L2R LIRI U1CMD U1IMD U1R U2CMD U2IMD U2R UIRI

3.80 5.42 144.15 4.08 5.98 148.84 5.23 6.47 8.30 129.19 5.01 5.83 138.11 6.74

0.40 0.34 17.52 0.51 0.36 20.68 4.14 0.52 13.18 0.56 0.65 20.51 0.95 15.03

Pearson correlation

Male (n = 30) Mean 3.93 5.48 140.69 4.39 6.17 141.77 4.47 6.63 8.59 130.44 5.21 7.18 139.09 7.26

Pearson correlation between irregularity index and incisor width ratio Table III.

SD

Independent t test

0.37 0.38 16.52 0.42 0.40 15.82 3.40 0.53 0.50 12.98 0.53 0.61 17.92 5.49

–1.33 –0.66 0.83 –2.65** –2.04* 1.56 0.81 –1.28 –2.44* –0.39 –1.53 –2.25* –0.21 –0.41

*P < .05 significant (2-tailed); **P < .01 significant (2-tailed).

tical significance level (from 0.74 to 0.81). Regression equation, r 2 (from 55% to 65%), scatter plot, and linear trend lines are presented in Figs 3 through 6. DISCUSSION

Crowding of the incisors, a common trait of malocclusion, is of special interest for stability and relapse in orthodontic treatment. Numerous studies have been done to discover the etiological factors related to crowding. Some of the factors considered in these studies were age, sex, tooth size, overbite, overjet, genetics, environment, third molars, muscle force, dental arch size, and growth and development.4,29-31 Generally, it is said that more triangularly shaped incisors have more crowding and more relapse due to the instability of the contact area.25,26 Thus, we attempted to determine the correlation between incisor crown shape and crowding. Many methods have been designed for the assessment of crowding, including the calculation of arch length discrepancy with the brass wire, irregularity index, and alignment score.32 The irregularity index can give a relatively accurate quantitative appraisal but it does not represent the discrepancy of winging, which appears frequently in crowding.27 Furthermore, it does not assess the vertical discrepancy of the contact areas, only horizontal displacement. Nevertheless, the irregularity index was considered suitable to estimate the amount of displacement of the contact point and was used to represent the amount of crowding. In his 500-model tooth measurement study, Ballard33 reported that the right and left mesiodistal tooth

LIRI UIRI

L1R

L2R

0.766**

0.770**

U1R

0.806**

U2R

0.743**

**Correlation is significant at the 0.01 level (2-tailed).

widths were different in 90% of the cases studied. This means that measuring only 1 side can cause a significant error; thus, we used the mean of both the left and the right measurements in this study. The mean values for incisal maximum mesiodistal widths were similar to those reported by Nahm,18 Suhr,2 and Jung and Yang.14 The crowded group showed a significantly larger mean value in the incisal maximum mesiodistal width than did the normal group. However, the cervical maximum mesiodistal width of the crowded incisors was significantly smaller than that of well-aligned incisors. The incisor width ratio in the normal group was smaller, and that of the crowded group was larger, than the values reported by You23 for extracted teeth. The width ratio of crowded incisors was significantly larger than that of the well-aligned incisors. The above result suggests that we cannot reject the alternative hypothesis (H1)—that is, that the labial crowns of incisors in the crowded group are more triangularly shaped than those in the normal group. Incisor width ratios of the normal group were 131% for lower central incisor, 132% for lower lateral incisor, 119% for upper central incisor, and 124% for upper lateral incisor. These values can be a useful aid in the diagnosis and treatment of crowded malocclusion as well as for retention protocol. During this study, we became interested in sex differences in the shape of the labial crowns of the incisors. The pooled sample was divided by sex and comparisons were made. When male and female measurements were compared, statistical significance was seen in both measurements of the lower lateral incisors, cervical and incisal maximum mesiodistal widths, and in both the upper central and the lateral incisors at the incisal maximum mesiodistal width. This is in accordance with findings in other studies.2,18,19,22,25,34 Females had more triangularly shaped lower incisors and lower crowding whereas males had more triangularly shaped upper incisors and upper crowding. However, these differences were not statistically significant. The Pearson correlation analysis showed significant correlation between measurements. The irregularity index showed a positive correlation with most

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Rhee and Nahm 627

Fig 2. Regression analysis of lower central incisor shape and lower irregularity index.

Fig 4. Regression analysis of upper central incisor shape and upper irregularity index.

Fig 3. Regression analysis of lower lateral incisor shape and lower irregularity index.

Fig 5. Regression analysis of upper lateral incisor shape and upper irregularity index.

of the measurements, although a negative correlation was shown for the cervical measurement. The lower irregularity index was highly correlated with lower lateral and central incisor width ratio, and the upper irregularity index was highly correlated with upper central and lateral incisor width ratio (from 0.74 to 0.81 in Table III). Scatter plot, linear trend line, regression equation, and r 2 were obtained with the irregularity index and highly correlated measurements. Correlations were made between the irregularity index and the lower central incisor width ratio (Fig 2), the lower lateral incisor width ratio (Fig 3), the upper central incisor width ratio (Fig 4), and the upper lateral incisor width ratio (Fig 5). Scatter plots showed a linear trend, and each r2 showed a relatively high value (from 55% to 65%); thus, we may infer that incisor width ratio is one of the features of crowding and can be useful for the diagnosis and treatment of malocclusion as well as for retention protocol.

In this study, the upper irregularity index was greater than the lower irregularity index when assessing the amount of crowding. This could be explained by the fact that the sample included models with large labial displacements of upper canines and others with loss of arch length and lingually positioned lateral incisors. Sanin and Savara35 proposed that incisor crowding could not result from the disharmony of teeth and jaws alone, but required multiple factors. The above results show a high correlation between incisor shape and crowding. However, the assessment of incisor crown shape as an etiologic factor in crowding, prevention of relapse, and stability after orthodontic treatment cannot be made with this result only. From previous studies, we can draw an inference that crowding is caused by multiple etiologic factors but we must not lose sight of the incisor crown shape, as shown by this study. Further studies must be done to determine the correlation of width ratio, stability, and relapse.

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CONCLUSIONS

The following conclusions may be drawn from this study: 1. The prevalence of crowding is higher in individuals with triangularly shaped incisors. 2. The width ratio of the incisors (IMD/CMD) is correlated with the irregularity index. 3. Therefore, the width ratio of the incisors can be a useful tool in the diagnosis, treatment, and retention protocol of crowded malocclusion. REFERENCES 1. Proffit WR. Contemporary orthodontics [Korean Edition]. Seoul: Jisung; 1993. 2. Suhr JE. A study of dental crowding and its relationship to mandibular incisor shape by model analysis in adolescents. Korean J Orthod 1994;24:247-73. 3. Adams CP. A comparison of 15-year old children with excellent occlusion and with crowding of the teeth, Angle Class I malocclusion, in respect of face size and shape and tooth size. Swed Dent J 1982;15:11-26. 4. Fastlicht J. Crowding of mandibular incisors. Am J Orthod 1970;58:156-63. 5. Norderval K, Wisth PJ. Mandibular anterior crowding in relation to tooth size and craniofacial morphology. Scand J Dent Res 1975;83:267-73. 6. Peck H, Peck S. An index for assessing tooth shape deviation as applied to the mandibular incisors. Am J Orthod 1972;61: 384-401. 7. Peck S, Peck H. Crown dimensions and mandibular incisor alignment. Angle Orthod 1972;42:148-53. 8. Peck S, Peck H. Orthodontic aspects of dental anthropology. Angle Orthod 1975;45:95-102. 9. Begg PR. Stone Age man’s dentition. Am J Orthod 1954;40: 298-312. 10. Gilmore CA, Little RM. Mandibular incisor dimensions and crowding. Am J Orthod Dentofacial Orthop 1984;86:493-502. 11. Puneky PJ, Sadowsky C, BeGole EA. Tooth morphology and lower alignment many years after orthodontic therapy. Am J Orthod Dentofacial Orthop 1984;86:299-305. 12. Smith RJ, Davidson WM, Gipe DP. Incisor shape and incisor crowding: a re-evaluation of the Peck and Peck ratio. Am J Orthod Dentofacial Orthop 1982;82:231-5. 13. Ahn BK. A statistical study of the effect of tooth size and dental arch size upon the crowding. Korean J Orthod 1990;20:217-26. 14. Jung MH, Yang WS. Causative factor and predictability of arch length discrepancy. Korean J Orthod 1997;27:457-71.

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