INTRODUCTION

Transverse dental arch dimensions are important in the development of the dentition. Deviations from the normal occlusion include the lateral cross-bite and the scissors-bite. A relatively narrow maxilla can develop into a lateral cross-bite, but this process is gradual. Measurements of the transverse distance are a better method for scientific purposes if more subtle differences are being investigated.

Some authors have suggested that there is an increased prevalence of malocclusion not only for shorter periods of time,1–4 but also for longer periods of time.5–8 Altered intermaxillary transverse relations are one of the findings in studies that compare skeletal samples with contemporary samples. One aspect often discussed in this context is the change in dietary habits that have occurred during the last centuries.7910 The texture of food has become finer because of food processing, and is considered less demanding on the masticatory muscles and teeth. This lower chewing activity would lead to alterations in facial development11 and narrower maxillary arches, as has been shown in animal experiments.12–14 That the texture of food is less demanding has also been discussed in the context of short-term changes. Arch width and arch depth are dependent on each other. A change in a transverse dimension will affect arch depth if the arch perimeter remains unchanged.1516 One study on skeletal remains found shorter arch depths in a medieval sample compared with a modern group,17 and a shorter maxilla was found in a cephalometric investigation of medieval skulls.18

When comparisons are made with skeletal samples, the dimensional changes that occur post-mortem must be considered. Cranial dimensions have been found to shrink post-mortem. Shrinkage has been estimated to be in the range of 0.3% to 1.7%. Larger changes occur in the posterior parts of the mandible where there is no bony support in the transverse direction.19–22

The purpose of the present investigation was to examine differences in transverse arch dimensions and arch depth in the mixed dentition in four groups living under different conditions at different time periods. The study comprised a skeletal group, a group of Sami children born in the 1980s and brought up in a comparatively traditional way, a group of Norwegian children born in the 1960s, and a group of Norwegian children from the same area born in the 1980s.

MATERIALS AND METHODS

Measurements

Measurements in the skull group were made directly on the skulls. Measurements in the other groups were made on dental casts. All measurements were made by one of the authors (RL) using a sliding caliper.

The distance between the right and left maxillary first molars was recorded. The minimal intermolar distance at the gingival margin, the intermolar distance between the tips of the mesiobuccal cusps, and the intermolar distance between the central fossae were recorded. The intermolar distance in the mandible was measured as the distance between the tips of the first molar mesiobuccal cusps and the shortest intermolar distance as the distance between the lingual surfaces at the gingival margin. If the cusp tips were abraded, the assumed center of the abraded area was used.

Maxillary and mandibular intercanine distances were measured between the cusp tips of the deciduous canines and, in cases of abrasion, the assumed center of the abraded area was used (Figure 1). Some children lacked deciduous canines, therefore, the intercanine distance could not be registered. The permanent canine was never used.

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Arch depth was defined as the perpendicular distance from the first permanent molars to the incisors. A line connecting the mesial surfaces of the permanent first molars was the posterior measurement point, and the tangent to the buccal surfaces of the incisors was the anterior measurement point. The perpendicular distance between these lines was defined as arch depth (p in Figure 2).

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The angle between the left and right molar teeth was measured with a protractor parallel to the occlusal plane. The best fitting lines passing through the buccal cusps of the first permanent molar, the second deciduous molar, and the first deciduous molar were used. This intermolar angle was measured in both the maxilla and the mandible (Figure 2).

The difference between maxillary and mandibular arch widths was used to evaluate relative widths within the groups. The central fossae and the mesiobuccal cusps in the maxillary first permanent molars were used to determine whether mesiopalatal rotation of the molars might have a greater effect on the transverse distance measured at the mesiobuccal cusps. The number of registrations in the groups differs since measurements were made only when the measurement points could be accurately identified.

Comparisons between groups were made with the sexes pooled since sex in the skeletal sample was unknown. Separate values for the boys and the girls are shown in the modern cohorts. An analysis of variance (ANOVA) and the Games and Howell post hoc test were used to analyze the groups. This post hoc test was chosen since the sizes of the groups varied. The level of significance was set at P < .05.

Measurements were repeated on 15 models and 11 skulls one month or more after the first measurement. Systematic errors were tested for using paired t-tests. Measurement error was calculated using the formula Se = S_d²/2. The level of significance was set at P < .05.

The errors in the transverse measurements were 0.1 to 0.4 mm. The 0.4-mm errors were made in the skulls in the maxillary first permanent intermolar distance measured at the central fossae and in the mandibular first permanent intermolar distance measured at the mesiobuccal cusps. The measurement error in the perpendicular distance from the first permanent molars to the incisors (arch depth) was 0.2–0.3 mm, as was the error in the overjet measurement. The overjet measurement in the skulls also had an inherent error because of the placement of the jaws in relation to each other. Error tended generally to be larger for the skulls than for the models (0.0–0.1 mm). Errors in the angular measurements between the molar tooth rows were 0.7°–1.5°. The angular measurements between the molar tooth rows in the mandibles of the skulls had a systematic error (0.7°), the second registration being smaller than the first. No systematic errors were detected in the other measurements.

RESULTS

The intermolar and the intercanine distances are shown in Table 1. The intermolar and the maxillary intercanine distances did not differ among the groups. The mandibular intercanine distance was smaller in the skulls compared with the 1980s Sami group (P <. 01) or the 1960s Oslo group (P < .05).

TABLE 1. Intermolar and Intercanine Distances^a

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Arch depth and intermolar angle are shown in Table 2. Maxillary arch depth was smaller in the skulls compared with the 1980s Sami group (P < .05), the 1980s Oslo group (P < .001), or the 1960s Oslo group (P < .01). The same maxillary distance was also smaller in the 1960s Oslo group compared with the 1980s Oslo group (P < .05). The mandibular arch depth was smaller in the skulls compared with the 1980s Sami group (P < .01) or the 1980s Oslo group. The intermolar angles in the maxilla and in the mandible were smaller in the skulls compared with the other groups (P < .001).

TABLE 2. Arch Depth and Arch Form (Intermolar Angle)^a

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Table 3 presents the intermaxillary differences in the transverse dimensions and arch depth. The difference between the maxillary and the mandibular intermolar widths measured at the mesiobuccal cusps was larger in the skulls compared with the 1980s Oslo group (P <.05). The difference between the maxillary intermolar width measured at the central fossae and the mandibular intermolar width measured at the mesiobuccal cusps was larger in the skulls compared with the 1980s Sami group (P < .05) or the 1980s Oslo group (P < .001). The difference between the maxillary and the mandibular intermolar widths measured at the lingual surfaces was larger in the skulls compared with the 1980s Oslo group (P < .01). The difference between the maxillary and the mandibular intercanine distances was significantly different among the groups (P < .05), but there were too few skull measurements to detect any other differences with certainty. The intermaxillary difference in arch depth was smaller in the skulls compared with the 1980s Oslo group (P < .01). The overjet was smaller in the skulls compared with the other groups (P < .001).

TABLE 3. Differences Between Maxillary and Mandibular Transverse Distances, Arch Depths, and Overjet^a

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DISCUSSION

Transverse dental arch dimensions and arch depth were studied in the mixed dentition in different ethnic groups living at different periods of time. Four groups were examined: a skeletal sample; two groups from the Oslo area, one born in the 1960s and one in the 1980s; and a group of Sami descent born in the 1980s. Sex-pooled values were used since sex was unknown in the skeletal sample. The main findings were smaller arch depths and a different angle between the left and right molar teeth in the skeletal sample compared with the modern groups. The transverse intermaxillary relations differed between the 1980s Oslo group and the skeletal sample.

The form of the dental arch in the skulls differed compared with the arch forms in the modern groups. The differences in maxillary intercanine distance (deciduous canines) between the groups were nonsignificant, although the mean in the skull group was among the larger means when one percent shrinkage was assumed. In the mandible, however, the distance between the deciduous canines was shorter in the skulls compared with the modern samples, though not significantly so compared with the 1980s Oslo group. This becomes more evident when the intermaxillary differences in intercanine distances are compared. In the ANOVA, this difference varied among the groups (P < .05), but the number of measurements in the skull group (nine) was too small for a difference to be detected in further analysis. In some skulls, however, the intercanine distance could be measured in either the maxilla or the mandible. Taking these additional values into consideration did not change the overall impression that the transverse intermaxillary relation at the deciduous canines in the skulls differed from that in the modern groups. This is an important finding. The intermaxillary intercanine relation was one of the most influential factors in the development of a posterior crossbite in a study on three-year-old children.25 In another study of children from birth to three years of age, the teeth in the children that had an interfering contact in centric relation were the deciduous canines. This means that the intercanine relation is important in the development of a posterior cross-bite. These findings are related to sucking habits.26 Posterior cross-bite is also a part of a Class III malocclusion. The intercanine relation can have the same influence in a mild Class III malocclusion, but it has no influence on the centric relation in a severe Class III malocclusion.

A difference in the transverse intermaxillary relation at the molars was found mainly between the 1980s Oslo group and the skulls. In a previous study, a reduction in the transverse intermaxillary relation was found between children born in the 1980s and children born in the 1960s. This reduction was discovered when the sexes were analyzed separately in a larger sample that included Swedish children.27 Sex-pooled values were used in the present study, which reduced the power of the analysis. On the other hand, this means that the difference between the skulls and the children born in the 1980s is probably a true finding. The contemporary children were of the same age and the skeletal group was selected according to dental development. This inevitably led to a greater variation in age in the skeletal group. Dental arch size is dependent on the stage of eruption in the anterior part of the arch where tooth size is a determining factor.2829 In the posterior part, there is an annual increase in maxillary first permanent molar width of about 0.5 mm in the ages eight to 11 years.30 An increase in the width of the first permanent intermolar of 2.0 to 2.4 mm (female–male) in the maxilla and of 0.9 to 1.3 mm (female–male) in the mandible was found between eight and 13 years of age.31 Knott,30 on the other hand, found an increase of 1.5 to 1.7 mm (female–male) in the maxilla and 1.5 to 1.7 mm (female–male) in the mandible from nine to 13 years of age. That the range of ages of the children in the skeletal sample would be this large is unlikely. The standard deviation of the transverse arch measurements is, for instance, not larger in the skeletal sample than in the modern groups. The transverse intermaxillary registrations are less influenced by any differences in age.

In adults, the common finding is no change, or only small changes, in intermolar widths. In one study, no change was found between 26 and 45 years of age.31 In another, a decrease in mandibular intermolar width of 0.6 to 0.9 mm (male–female) was found between 17 and 48 years of age and the maxilla showed insignificant changes in the same direction.32 In a third study, a significant increase in maxillary and mandibular intermolar widths of 0.3 to 0.4 mm was found in a group between 23 and 34 years of age.33 An exception is the study by Harris,34 who also studied the largest time span from 20 to 55 years of age. He found an increase in both maxillary and mandibular arch widths of 2.3 to 2.4 mm. These differences have no obvious explanation. One interesting finding is that all these longitudinal data report change, or sometimes no change, in the same direction and of the same magnitude in both the maxilla and the mandible. The differences found in intermaxillary relation in the present study cannot be assumed to improve in the modern groups.

The things that were not found are also interesting. The Sami group was brought up in a more traditional way and children that were nomadic in the summertime were selected for this study. This does not mean that they were not influenced by modern habits. It can be assumed that modern facilities were available to them, but that they continued to practice some of their traditional habits. Greater chewing demands and a more traditional way of living have been suggested to contribute to a lower prevalence of transverse occlusal discrepancies.1335 The measurements made on the Sami children in the present study do not differ from those made in the other modern groups. One explanation could be that the diet of the Sami children was more similar than expected to the diet of the Oslo children. Another possible explanation for secular changes in the transverse dimensions is the improved nutritional status that has occurred during the last century, which is the likely cause of the increase in body height in modern populations.36–40 This change in body height is also apparently associated with a change in head shape.4142 The head was found to be narrower and longer in schoolchildren in Jena, Germany, in 1995 than in 1944, and their height also increased during this period.42 Although the correlation between width of the head and dental arch size is low,43–45 better nutrition, which can be assumed among the contemporary groups compared with the skeletal sample, might influence not only dental arch development but also body height and head shape.

The smaller intercanine width found in the mandible is a sign of more crowding in the skeletal group compared with in the modern groups. In a longitudinal study on mandibular incisors from the ages of seven to 10 years, a smaller intercanine distance was found to be associated with crowding.46 This must be related to tooth size but is nevertheless contrary to the findings of a lesser prevalence of crowding in skeletal samples.6

The arch depth was smaller in the skulls compared with the modern samples. The mean arch depth in the 1960s Oslo group was slightly smaller than in the two 1980s groups. This was because of a greater prevalence of caries in the deciduous molars in the 1960s Oslo group.47 Despite a low prevalence of caries, the arch depths in the skulls were smaller than in the other groups. This difference was larger in the maxilla than in the mandible. The intermaxillary difference in arch depth was greater in the modern samples than in the skulls and significantly so when the 1980s Oslo group was compared with the skull group. This contributed to the difference in overjet, which was smaller in the skulls than in the modern groups.

In a study comparing an adult modern sample and a 14th century skeletal sample considered to have deceased in their mid-twenties, maxillary and mandibular arch depths were found to be shorter in the skeletal sample. The difference in arch depth was larger in the maxilla than in the mandible, which agrees with the findings in the present study.17 In a study comparing Swedish men deceased in 1810, and representing an otherwise healthy population with Swedish inductees in the 1970s, arch depth was also found to be smaller in the skeletal sample.48 The difference in arch depth, however, was not larger in the maxilla than in the mandible.

The smaller arch depths found in skeletal samples are associated with more retroclined upper incisors, as has been found in cephalometric investigations. Luther18 found that the maxillary incisors were 11° more retroclined in a skeletal sample compared with modern controls. Varrela49 found a similar difference of 8°, as did Ingervall et al.48 In another study on skeletal remains, the sample was divided into a slightly abraded and a severely abraded group. The retroclination of the maxillary incisors in the severely abraded group was 13° greater than in the slightly abraded group.50 The inclination of the mandibular incisors is less clear-cut in comparisons of older and modern samples. Luther found more proclined mandibular incisors in the skeletal sample18 and Varrela49 found no difference between skeletal and modern samples, as did Ingervall et al.48 The previously-mentioned study in which skeletal remains were divided into slightly abraded and severely abraded groups did not report the inclination of the mandibular incisors, but the interincisal angle in that study indicated that the mandibular incisors were more retroclined in the severely abraded group, although to a lesser extent than the maxillary incisors.50 The arch depth measurements in this study indicate that the more retroclined maxillary incisors found in the skeletal samples were present in the mixed dentition. Between the times of mixed dentition and permanent dentition, arch depth decreases about 0.5 mm more in the mandible than in the maxilla51 and continues to decrease thereafter at a similar rate in the maxilla and in the mandible.52 That the difference in arch depth between the skeletal group and the three modern groups was larger in the maxilla than in the mandible or in the transverse dimensions is in accordance with a genetics study by Cassidy et al.53 They found that heritability was lower for maxillary arch depth than for mandibular arch depth or arch width. They concluded that arch size had a modest genetic component.

CONCLUSION

Transverse dental arch dimensions and arch depth were studied in the mixed dentition of a skeletal sample, a Sami group born in the 1980s, and two groups of Norwegian children from the Oslo area born in the 1960s and in the 1980s.

1. In the transverse dental arch dimension, a smaller mandibular intercanine distance was found in the skulls.

2. The intermaxillary difference in intermolar distance in the skull group was larger than in the 1980s Oslo group, indicating that the risk of developing a posterior cross-bite was greater for these Oslo children than for the other groups.

3. The intermaxillary difference in intercanine distance in the skull group was larger compared with the modern groups, indicating that the risk of developing a posterior cross-bite in the skull group was lower than in the modern groups. The number of skulls in this registration, however, was low.

4. The arch depths were smaller in the skulls compared with the modern groups. This is in accordance with the shorter arch depths that are found in adult skeletal materials when compared with adult modern samples. This indicates that the more upright maxillary incisor position found in skeletal samples is present at an early age and that dental attrition is not the only cause of this incisor position.

5. The overjet was smaller among the skulls, and although this is a less reliable registration, this finding was supported by the arch depth measurements in which the intermaxillary difference in arch depth was larger in the modern groups.

6. The dental arch form, as indicated by the angle between the left and right rows of molar teeth, was more acute in the skulls than in the modern groups.