Comparison of intermaxillary tooth size discrepancies ......

Abstract：The purpose of this study was to determine whether there is a prevalent tendency for intermaxillary tooth size discrepancies among different malocclusion groups. This study consisted of 60 subjects who served as
the normal occlusion group and 300 patients divided into 5 malocclusion groups (ie, Class I with bimaxillary
protrusion, Class II Division 1, Class II Division 2, Class III, and Class III surgery). Tooth size measurements
were performed on the models of normal occlusion and pretreatment models of patients by the Three
Dimension Measuring Machine. Moreover, tooth size ratios, analyzed as described by Bolton and the
Student t test showed no sexual dimorphism for these ratios in each of 6 groups, so the sexes were
combined for each group. Then these ratios were compared among different malocclusion groups. The
results showed no significant difference between subcategories of malocclusion, so these groups were
combined. There were now 120 cases in each of 3 categories: Class I, Class II, and Class III. A significant
difference was found for all the ratios between the groups, the ratios showing that Class III > Class I > Class
II. It demonstrated that intermaxillary tooth size discrepancy may be one of the important factors in the
cause of malocclusions, especially in Class II and Class III malocclusions. Thus this study proved the fact
that Bolton analysis should be taken into consideration during orthodontic diagnosis and therapy. (Am J
Orthod Dentofacial Orthop 1999;116:539-44)

Bolton,1 in 1958, developed a method of analyzing mesiodistal tooth size ratio between maxillary and mandibular teeth. In his study, he concluded that it would be difficult for proper occlusal interdigitation or coordination of arches in the finishing stage of orthodontic treatment without proper mesiodistal tooth size ratio between maxillary and mandibular teeth.

Stifer2 replicated Bolton study in Class I dentitions and reported similar results. Subsequently, other authors obtained the normal values of Bolton analysis of different races, eg, that of a Chinese population.3 These studies were performed on a mixture of treated and untreated subjects with good or excellent occlusion. However, up to now, for the study of tooth size of malocclusion, especially for the comparison of intermaxillary tooth size relationship among different malocclusions, few studies were available and the research results were controversial.
Arya et al4 showed that there were differences in tooth size between sexes, as reported by a number of
other authors. They attempted to show differences in tooth size between Class I and Class II malocclusions,
but failed to do so. In their study, the mean size of each tooth for the different groups (ie, Class I and Class II, boys and girls) were compared. Difference for individuals between different arches were not analyzed.

Lavelle5 showed that there was sexual dimorphism in tooth dimensions and in the ratio of upper to lower
arch tooth size. In addition, there was racial dimorphism between Negroids, Mongoloids, and Caucasians.
Lavelle also measured the ratio of upper to lower arch tooth size in different malocclusion types,as in the current study. The difference was in the method of analysis. Rather than tooth sizes being compared for individuals, the mean size of each tooth of male patients for each malocclusion type was stated. A pattern of contrast was found, which was different for the maxillary values, compared with mandibular values for different malocclusion categories. In this study, the mesiodistal crown dimensions for maxillary teeth were Class I > Class II Division1 > Class II Division 2 > Class III, cf, Class III > Class I > Class II Division 1 > Class II Division 2 for mandibular teeth. It can be inferred that as a general trend, the Bolton discrepancy would be greater in
Class III cases than other malocclusion groups but this was not analyzed for individuals.

The Sperry6 study analyzed the Bolton ratios for groups of Class I, Class II, and Class III cases. The skeletal patterns were not mentioned, although some of the Class III cases were treated surgically. Male and female subjects were not differentiated. The overall ratios showed that there was mandibular tooth-size excess for the Class III patients.
Crosby and Alexander7 analyzed the Bolton ratios for different occlusal categories. They did not differentiate
between sexes, and they did not include Class III patients. The relationship of malocclusion to skeletal pattern was not mentioned. They did not find a statistically significant difference in the prevalence of tooth size discrepancies among the different malocclusion groups.
The objectives of the current study were to determine (1) whether sexual dimorphism exists for tooth size ratios, and (2) whether there is a difference for intermaxillary tooth size discrepancies represented by anterior ratio, overall, and posterior ratio of Bolton, for Class I, Class II Division 1, Class II Division 2, Class III, and Class III surgery cases.

MATERIAL AND METHODS
The samples for the study consisted of 60 subjects with normal occlusion and 300 patients with varying malocclusions. Patients were selected randomly from clinical practice of the department of orthodontics, School of Stomatology, Beijing Medical University in the 1990s. Subjects with normal occlusion were selected from the study of Growth and Development in this department (project financially supported by National Nature and Science Foundation of China) whose models were recorded in the 1990s. All cases were Han nationality born and living in China. They were between 13 and 17 years old except Class III surgery patients who were from 17 to 23 years old.
Occlusion categories of all cases, which were classified by the Angle classification, coincided with skeletal categories. Skeletal types were assessed by ANB angle from cephalometric analysis, which meant in skeletal Class I, ANB angle was from 0o to 5o, cf, ANB angle > 5o for Skeletal Class II and ANB angle < –2o for skeletal Class III. All subjects were divided into 6 groups; each group consisted of 30 males and 30 females. These groups were normal occlusion, Class I with bimaxillary protrusion, Class II Division 1, Class II Division 2, Class III, and Class III surgery cases. The selection criteria of a Class III surgery case was severe skeletal Class III malocclusion mainly represented by severe mandibular prognathism and an ANB angle < –7o. In this study, the models were the major investigation focus.

The following selection criteria were used:
1. Good quality models of normal occlusion and pretreatment models of patients.
2. All permanent teeth had erupted and were present from right first molar through left first molar.
3. No severe mesiodistal and occlusal tooth abrasion.
4. No residual crown or crown-bridge restoration.
5. No tooth deformity (eg, conic-form lateral incisal teeth)

Each model was oriented in a YM-2115 Three Dimension Measuring Machine (similar to Yamada et al8 and Braun et al9) as shown in Fig 1. This sort of machine is used extensively in the precision machine tool industry. In this study, the device was a miniature system. It ran in the range of 150 mm ´ 200 mm ´ 100 mm and the accuracy of the 3 orthogonal axes was all 0.01 mm. The frictionless air bearing and touch trigger probe was used to identify the measuring point (ie, anatomic contact point of each tooth), and to record the corresponding X,Y, and Z oordinates automatically to a computer data file. Through the calculation of a computer program, the greatest mesiodistal diameters of all the teeth on each cast were obtained except the second and third molars. In order to determine the measurement error, 5 sets of models were measured again several days later by the same examiner. The result
showed no significant difference between the 2 mea- surements. Then tooth size ratios were analyzed as Bolton described:

In order to determine whether there is sexual dimorphism in the incidence of intermaxillary tooth size discrepancies, a Student t test was performed for each malocclusion group. Moreover, in order to compare intermaxillary tooth size discrepancies among different malocclusion groups, ANOVA and Student-Newman-Keuls Multicomparisons were performed. The software of above statistical analysis was SPSS (version 6.0), and the level of significance was P < .05.

RESULTS
Table I summarizes the means, standard deviations, and standard error of the tooth size ratios observed in
each group. It shows that there is no significant sexual dimorphism for all ratios of all groups. As to the
absolute value of tooth size ratios of different sexes, there is no prevalent trend. For instance, anterior ratio
of male patients (81.07 ± 3.52) was larger than that of female patients (80.97 ± 2.66) in Class II Division 2,
but that of male patients (80.31 ± 3.87) was smaller than that of female patients (80.80 ± 2.42) in Class II
Division 1.

ANOVA demonstrated that there were significant differences among the 6 groups (tables omitted, available from authors), thus Student-Newman-Keuls Multicomparison was performed to analyze the differences.

Because there was no significant sexual dimorphism for tooth size ratios, the sexes were combined for each group. Multicomparison was then performed between these groups. The results are shown in Table II. It was obvious that for the 3 tooth size ratios, there are no significant differences between Class III and Class III surgery, between normal occlusion and Class I bimaxillary protrusion, between Class II Division 1 and Class II Division 2.
Because there is no significant difference between subcategories of malocclusion, these groups are combined. There are now 120 cases for each category of Angle classification. Then the Multicomparison was performed between 3 new groups and the statistical results were summarized in Table III. It shows that the tendencies of anterior ratio, posterior ratio, and overall ratio were all Class III > Class I > Class II, the differences between these groups were significant at the level of significance P < .05. For example, overall ratios of the 3 categories were 95.59 ± 2.58, 93.39 ± 2.46, 92.06 ± 2.47, respectively.

DISCUSSION
When the whole malocclusion samples (ie, 150 female or 150 male patients) were combined into 1 group, our study shows that tooth size ratios of the malocclusion group are close to that of the normal occlusion group, as shown in Table I. For example, the overall ratio of the normal occlusion group for female patients is 93.11 ± 2.64; for the malocclusion group it was 93.69 ± 2.86. Zhu Xia and Xiying Wu10 also found no significant difference for tooth size ratios between the malocclusion group and the normal occlusion group after measuring mesiodistal tooth sizes of 1173 Han nationality cases on their models. Thus, only after comparing tooth size ratios among different classified malocclusion groups, can the law of nature be observed.
Sperry et al6 showed that the Class III group with mandibular prognathism had more patients with mandibular tooth-size excess for the overall ratio than the Class I and Class II groups (0.01 < P < .05). This conclusion was similar to a result of the present study. The similar result was that the overall ratio of Class III surgery was the highest among different malocclusion groups. However, the present study demonstrated that not only Class III surgery but also Class III nonsurgery had a greater frequency of mandibular tooth size excess than other malocclusion groups.
Lavelle5 showed that tooth sizes of Class III were the smallest among the 3 occlusion categories (ie, Class
I, Class II and Class III) for maxillary teeth; they were the greatest for mandibular teeth. This possibly indicated that tooth size ratios of mandibular teeth divided by maxillary teeth in Class III may be the greatest
among different malocclusion types. However, these ratios were not compared in his study. His result was only a kind of descriptive statistical result, which stated the mean size of each tooth of male patients for each malocclusion type and described a pattern of contrast. The present study compared these ratios and showed that anterior ratio, posterior ratio, and overall ratio of Class III malocclusions were all greater than other
occlusion categories.

Crosby and Alexander7 also compared the tooth size ratios among different malocclusion groups, as in the current study. They found that there were no significant differences among Class I, Class II Division 1, Class II Division 2, and Class II surgery groups. The current study also determined that no significant differences
exist between Class II Division 1 and Class II Division 2, but other results were different from their study. The present study showed that the 3 tooth size ratios were all Class III > Class I > Class II and the differences between these groups were significant. The differences of results between the current study and Crosby and Alexander7 study could be explained as follows.
First of all, the skeletal categories were not mentioned in Crosby and Alexander’s study although some of Class II cases were treated surgically. This may have an important effect on the selection of sample. For example, some skeletal Class II malocclusions can be converted to dental Class I malocclusions by forward movement of permanent first molar due to the premature loss of the deciduous second molar, so the Class I group may contain skeletal Class I and Class II patients. In the current study, skeletal categories were taken into account, and in order to simplify this study, the cases were selected by the criteria of occlusal categories coinciding with skeletal categories.

Second, Crosby and Alexander7 did not differentiate between sexes and did not mention the ratio of sexes in each group. In their study, it was not clear whether there was sexual dimorphism for tooth size ratios. The present study separated sexes and demonstrated that there was no sexual dimorphism for tooth size ratios, thus the sexes were combined in the ratio of 1:1 for each group.

Third, Crosby and Alexander7 did not include Class III patients in their study and only selected 20 to 30
cases for each group. However, in the current study, not only Class III but also Class III surgery patients were
included and 60 cases were contained in each of the 6 groups. As mentioned previously, because there was no
significant difference between subcategories of malocclusion, these groups could be combined. There were 120 cases in each category of Angle classification in the present study. Therefore, the samples of the current
study were greater and classification of malocclusion was more complete than their study.

Finally, the samples of the Class I group in the Crosby and Alexander7 study were composed of Class I malocclusion in which no prevalent clinical presentations were mentioned, but that of the present study
were made up of normal occlusion and Class I malocclusion with bimaxillary protrusion. This may be
another reason that led to the differences in results between their study and ours.

In clinical practice, clinicians often note the discrepancy of tooth size and skeletal size but seldom pay
attention to tooth size discrepancy between maxillary and mandibular teeth. The present study showed the
tendency of mandibular tooth size excess in Angle Class III malocclusion and the tendency of maxillary
tooth size excess in Angle Class II malocclusion. This indicated that it might be reasonable for orthodontists
to do interproximal stripping or tooth extraction in the mandibular dentition for Class III malocclusion and in
the maxillary dentition for Class II malocclusion.These results suggested that the Bolton analysis is important  and should be considered when diagnosing, planning, and predicting prognosis in clinical orthodontics. In the current study, the Class I malocclusion group only consisted of bimaxillary protrusion because of the  onsideration of our other studies. This was a defect of sample selection. But it demonstrated that Class I cases
with bimaxillary protrusion had no prevalent incidence of intermaxillary tooth size discrepancy. The samples of
other clinical presentations are needed to be added into the Class I group to further determine the relationship of Class I to Class II or Class I to Class III. For Class II and Class III malocclusions, this study showed a prevalent tendency of intermaxillary tooth size discrepancy.

 Although tooth size and tooth size ratios described by Bolton were different in different racial groups, and
the order was Negroids > Mongoloids > Caucasoids.8 However, there are little data in relation to the degree
and frequency of intermaxillary tooth size discrepancy in different racial groups for the same malocclusion
category. This study demonstrated a statistical tendency of intermaxillary tooth size discrepancies for different
malocclusion groups in a Chinese population. Certainly, further work is needed to explain the probable
existing racial differences for intermaxillary tooth size discrepancies in different malocclusion categories.

CONCLUSION

Mesiodistal tooth sizes were measured in a total of 360 sets of casts for normal occlusion, Class I bimaxillary
protrusion, Class II Division 1, Class II Division 2, Class III, and Class III surgery. Tooth size ratios were
analyzed as described by Bolton and compared among different malocclusion groups. The results showed:
1. Significant sexual dimorphism for the 3 tooth size ratios did not exist.
2. When tooth size ratios were compared, there were no significant differences between normal occlusion and Class I bimaxillary protrusion, between Class II Division 1 and Class II Division 2, between Class III and Class III
surgery patients.
3. The tendencies of the 3 tooth-size ratios for the 3 occlusion categories were all Class III > Class I > Class II at the level of significance P < .05. It suggests that the tooth size discrepancy between maxillary and mandibular teeth may be one of the important factors in the cause of malocclusions, especially in Class II and Class III malocclusions. In order to obtain optimal and stable treatment results, the Bolton analysis should be taken into consideration when diagnosing, planning, and predicting prognosis in clinical orthodontics.

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