Introduction
The purposes of this study were to analyze long-term changes in malocclusion traits and to compare the development in orthodontically treated and untreated subjects.
Methods
The sample comprised 308 adolescents in the intermediate, late mixed, or early permanent dentition who were examined clinically at the ages of 8 to 17 years and again 25 years later. The treated subgroup of 58 subjects had received orthodontic treatment with fixed or removable appliances or both. All subjects had a full complement of teeth, except a subgroup of 19 who had premolar extractions as a part of their orthodontic treatment plan.
Results
The prevalence of maxillary overjet was significantly reduced in the untreated group and the treated subgroups. The prevalence of distal molar occlusion was significantly reduced in the subgroup treated without extractions. Comparison of treated and untreated groups in terms of changes over time showed that development was significantly more favorable in all treatment categories regarding maxillary overjet, and in the nonextraction category regarding distal molar occlusion. Subjects treated without extractions had less favorable development than did untreated subjects regarding molar crossbite.
Conclusions
The long-term benefit of orthodontic treatment, with or without extractions, was confirmed regarding maxillary overjet, and the lasting effect of nonextraction treatment was confirmed regarding the distal molar relationship. The pattern of changes in treated and untreated subjects indicated that long-term development and individual variation can to some extent conceal the effects of a brief orthodontic intervention.
Development of occlusion continues to some extent throughout life; in that perspective, orthodontic treatment might seem like a brief disturbance. Horowitz and Hixon discussed how orthodontic therapy and retention might temporarily alter the course of physiologic changes until the maturation process resumes, and Thilander noted that postretention changes generally cannot be distinguished from normal aging processes. It is therefore reasonable to ask whether orthodontic corrections have a permanent effect, or whether they eventually fade away in the following decades of slow change.
Previous longitudinal studies dealing with the development of untreated occlusion from adolescence into adulthood vary in form and extent, but large samples of untreated subjects with full dentition are inevitably rare in countries with adequate orthodontic services. Some studies are limited to 1 class of occlusion, and some focus on specific traits such as arch width or incisor relationship measured on lateral radiograms.
Various aspects of occlusal development in untreated, fully grown subjects have been described in longitudinal studies and extensive cross-sectional surveys.
Long-term changes after orthodontic treatment have been studied in a number of studies and reviews, often describing reversion of treatment achievements or general deterioration of normal occlusal and dental relationships obtained at the end of treatment. This phase of development is usually labeled “relapse,” although others believe that “physiologic recovery” would be a more appropriate term.
Longitudinal studies of occlusal development through treatment, retention, and postretention cover various ages and address different questions. These studies might relate their findings to the original orthodontic problem, extractions, other aspects of treatment, treatment changes or end-of-treatment measurements and scores, or length of retention.
Some studies evaluated the interactions between variables, whereas others described the mean changes of occlusal traits through treatment and posttreatment. One study compared long-term development of overbite and overjet in treated deepbite patients and a control group of untreated subjects with normal occlusion.
Some aspects of posttreatment and postretention changes have been made clear in these studies, but the search for a cause-and-effect relationship has been mostly unrewarding. The factors that can obscure the etiology of relapse include individual variations, as shown by Little et al in their landmark study, the confounding effects of normal aging processes discussed by Thilander, and the general lack of experimental research with randomization and proper control groups.
In this study, we compared changes in treated and untreated subjects in an attempt to describe the long-term net effect of orthodontic intervention on 4 important occlusal traits. However, because the subjects could not logically be randomly assigned to the respective groups, the orthodontic problems were more prevalent in the treated group initially, necessitating caution in this comparison.
Material and methods
In 1972 and 1973, an epidemiologic study of malocclusion and dental, skeletal, and sexual maturation was conducted on a random sample of 1641 children attending primary schools in Reykjavík, Iceland. The same subjects were contacted again later and asked to participate in a second round of the study. The new survey included a questionnaire described elsewhere and a repetition of the previous clinical examinations. Panoramic radiographs (Orthopantomograph, Siemens, Bensheim, Germany) were also taken at the second visit, and the radiographic data were matched to the simultaneous clinical examination and all discrepancies resolved. The second survey was carried out with 832 subjects in 1998, approximately 25 years after the first examination.
All subjects with congenitally missing teeth and those who had prematurely lost permanent teeth were excluded, keeping the focus on the occlusal development in orthodontically treated vs untreated subjects undisturbed from untimely tooth loss.
To eliminate the least mature of the initial group, we also excluded those who were in the early mixed dentition at the initial examination (T1), classified as dental stage (DS) 1 by Björk et al. This reduced the group in the second survey (T2) to a longitudinal sample of 308 subjects. This sample was then split into 3 subgroups of 250 untreated subjects, 39 subjects treated without extractions, and 19 subjects treated orthodontically with extractions of premolars. The subgroup of 19 comprised 9 subjects with maxillary premolar extractions only and 10 with 4 premolar extractions. Because this combination impaired uniformity and obscured developmental changes, we omitted the extraction group of 19 in comparisons regarding molar relationships.
Sex, age, initial DS, and treatment categories of the treated and untreated subjects are described in Table I . DS 4 refers to those with all permanent teeth mesial to the first molars fully erupted, DS 3 to those in the late mixed dentition, and DS 2 to those in the intermediate mixed dentition with all permanent incisors fully erupted.
| Untreated | Nonextraction treatment | Extraction treatment | Total | |
|---|---|---|---|---|
| n | 250 | 39 | 19 | 308 |
| Men | 90 | 17 | 5 | 112 |
| Women | 160 | 22 | 14 | 196 |
| DS 2 at T1 | 26 | 6 | 3 | 35 |
| DS 3 at T1 | 99 | 26 | 10 | 135 |
| DS 4 at T1 | 125 | 7 | 6 | 138 |
| Mean age at T1 (y) | 11.8 | 10.7 | 10.7 | 11.6 |
| Age range at T1 (y) | 7-17 | 8-17 | 8-15 | 7-17 |
| Fixed appliances | 0 | 15 | 17 | 32 |
| Removable appliances | 0 | 24 | 2 | 26 |
| Mean age at T2 (y) | 37.9 | 37.4 | 37.2 | 37.8 |
| Age range at T2 (y) | 33-43 | 34-42 | 34-41 | 33-43 |
The following traits were recorded: overjet of 6 to 9 mm and 9 mm and over, overbite of 5 to 7 mm and 7 mm and over, crossbite on at least 1 molar, and mesial and distal first-molar occlusion deviating by a half-cusp width or more from normal.
Most of the treated subjects were treated after T1, but, in a few cases, when treatment was already started at T1, the initial records from the orthodontic offices were used.
The selection process therefore resulted in a sample of 308 subjects who had been examined in the intermediate, late mixed, or early permanent dentition and again approximately 25 years later at the age of 33 to 44 years.
The T1 registration was done by an author (T.E.M.). He was also in charge of the T2 examination, when the registrations were done by a qualified hygienist, after training and calibration between the 2 of them. The registrations were carried out with a measuring instrument designed specifically to record malocclusion with the comprehensive method of Björk et al. The marking lines on the instrument are easily readable and limited to those that categorize the malocclusion traits in question. Since the examiner at T2 did not know the treatment history of the subjects, any systematic or interexaminer bias would have affected to the same degree both treated and untreated subjects, the groups that were compared in the study. The registration method has been tested positively for intraexaminer and interexaminer reliability and has been used in a number of studies dealing with epidemiology and development of occlusion.
Statistical analysis
The data were analyzed with SPSS software (version 15.0, SPSS Sweden AB, Kista, Sweden). Incisor relationship was expressed as grade 1 and grade 2 anomalies, and molar relationship as unilateral and bilateral anomalies. The Wilcoxon signed rank test was used to analyze change over time in each group, and the Mann Whitney U test to analyze differences between groups. P values below 0.05 were considered statistically significant.
Results
Prevalence of malocclusion traits at T1 and T2 in the untreated group and the treated group and subgroups is described in Table II . The Figure illustrates strong general reduction of overjet, reduced prevalence of distal molar relationship, moderately reduced prevalence of overbite, and some increase in the prevalence of crossbite.
Table III compares the untreated group of 250 subjects with each treated subgroup. All malocclusion traits had a higher prevalence in the treated group of 58 subjects at T1: significantly for overjet, distal molar occlusion, and molar crossbite. At T2, the untreated subjects had a lower prevalence of crossbite than did the nonextraction treated group.
| T1 | T2 | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 39 vs 250 | 19 vs 250 | 58 vs 250 | 39 vs 250 | 19 vs 250 | 58 vs 250 | |||||||
| Difference % | P value | Difference % | P value | Difference % | P value | Difference % | P value | Difference % | P value | Difference % | P value | |
| Overjet ≥6 mm | 10.9 | 0.033∗ | 58.8 | 0.000 ‡ | 26.6 | 0.000 ‡ | –2.2 | 0.532 | 0.5 | 0.925 | –1.4 | 0.657 |
| Deepbite ≥5 mm | 3.0 | 0.574 | 8.7 | 0.251 | 4.8 | 0.295 | 4.0 | 0.421 | –3.5 | 0.587 | 1.5 | 0.715 |
| Distal occlusion unilateral/bilateral | 14.5 | 0.067 | 28.6 | 0.003 † | 19.1 | 0.003 † | –7.6 | 0.220 | NA | NA | ||
| Mesial occlusion unilateral/bilateral | 0.2 | 0.951 | 2.9 | 0.451 | 1.0 | 1.00 | 1.1 | 0.725 | NA | NA | ||
| Molar crossbite unilateral/bilateral | 4.9 | 0.120 | 7.7 | 0.075 | 5.8 | 0.040∗ | 13.5 | 0.001 † | NA | NA | ||
Table IV describes the development of the treated groups, by using development in the untreated group as the baseline. The statistical analysis shows that, in both the extraction and nonextraction treatment categories, the development of overjet was significantly more favorable than in the untreated subjects. Nonextraction treatment affected distal molar relationship favorably, and molar crossbite developed adversely in the nonextraction treated group.
| Baseline T1-T2 change | T1-T2 change in treated subjects and comparison with baseline | ||||||
|---|---|---|---|---|---|---|---|
| n = 250 | n = 39 | n = 58 | n = 19 | ||||
| Untreated | Nonextraction treated | All treated | Extraction treated | ||||
| % | % | P | % | P | % | P | |
| Overjet ≥6 mm | –4.8 | –17.9 | 0.017∗ | –32.8 | 0.000 ‡ | –63.2 | 0.000 ‡ |
| Deepbite ≥5 mm | –3.6 | –2.6 | 0.921 | –6.9 | 0.404 | –15.8 | 0.083 |
| Distal occlusion unilateral or bilateral | –3.6 | –25.6 | 0.003 † | NA | NA | ||
| Mesial occlusion unilateral or bilateral | 1.6 | 2.6 | 0.810 | NA | NA | ||
| Molar crossbite unilateral or bilateral | 1.6 | 10.3 | 0.022∗ | NA | NA | ||
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