Sample

The Dunedin Multidisciplinary Health and Development Study7 (DMHDS) is a longitudinal study of a birth cohort of children who were born at the Queen Mary Hospital in Dunedin, New Zealand, between April 1, 1972, and March 31, 1973. The sample that formed the basis for the longitudinal study was 1037 children, and they were assessed within a month of their third birthdays. Periodic collections of health and developmental data (including dental examinations) have been undertaken since then, and the current study uses data collected at age 26. Ethical approval for the dental section of the study was obtained from the Ethics Committee of the Southern Regional Health Authority. All participants signed an informed consent statement approved by that committee.

Measures

At age 12, only those Study members who were residing in the greater Dunedin area were dentally examined. The data used in the current investigation come from data collection phases at ages 12, 15, 18, and 26 in the Dunedin study and comprise both self-report measures and clinical orthodontic data (Table 1). Study members were asked to rate their dental appearance at each of ages 15, 18, and 26. At ages 15 and 18, they were asked if they had had orthodontic treatment, and whether that had been with fixed or removable appliance therapy (braces or a plate). At age 26, the subjects were asked to rate the outcome of their orthodontic treatment (if they had had such treatment). Orthodontic clinical data were collected at ages 12, 15, and 18, but no malocclusion information was collected at age 26.

TABLE 1. Self-reported Sociodemographic and Clinical Measures Used in the Current Investigation

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The Dental Aesthetic Index8 (DAI) was used to capture the clinical information at ages 15 and 18, but not at age 12, because the age-12 data collection took place in 1984 and 1985, and the DAI was not published until 1986. Nevertheless, all but four of the clinical characteristics that comprise the DAI items had actually been recorded at age 12 (Table 2). Only two of those items had weights greater than 1.0 in computing the DAI scale score, and one of them (the diastema) is relatively common, whereas the other (the anterior mandibular overjet) is not. Calculation of a mini-DAI score for each participant at age 12 was possible, and the validity of this approach was examined by comparing the age-12 mini-DAI and age-15 DAI scale scores of those Study members who had not yet received orthodontic treatment by age 15. Once the validity of that approach was determined, the mini-DAI score was used to allocate each Study member to a treatment-need category using the same category descriptions as those used with the DAI (Table 3). The cut-off scores for each treatment-need category at age 12 were determined by observing the distribution of scale scores among those untreated by age 15.

TABLE 2. Items Used in the Dental Aesthetic Index and Age-12 Mini-DAI

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TABLE 3. Categorization of Treatment Need Using the Mini–Dental Aesthetic Index (DAI) and DAI Scales

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Dental examinations at ages 18 and 26 were conducted using calibrated dental examiners, and DMFT and DMFS scores were calculated for each age. Dental caries increment was computed by comparing caries status at 18 and 26 years for each surface. The net increment was computed by subtracting the number of reversals from the gross increment. The tooth-loss increment due to caries between ages 18 and 26 was obtained by observing—for each tooth that had been present at 18—its presence or absence at 26, and ascertaining the reason for its absence at that age. Periodontal measurements were made at age 26 in two quadrants (quadrants 1 and 3 for Study members whose DMHDS identification number is odd, and quadrants 2 and 4 for those with an even identification number; the mix of odd and even ID numbers was approximately 50:50). Those measurements were made in only two quadrants because of time constraints. Three sites (mesiobuccal, buccal, and distolingual) per tooth were examined, and gingival recession (the distance in millimeters from the cemento-enamel junction to the gingival margin) and probing depth (the distance from the tip of the probe to the gingival margin) were recorded using an NIDR probe. Midbuccal measurements for molars were made at the midpoint of the mesial root. All measurements were rounded down to the nearest whole millimeter at the time of recording. Where the gingival margin was situated more than 1 mm coronally to the cemento-enamel junction, a negative value for gingival recession was recorded. Gingival bleeding was assessed for each tooth by recording the presence or absence of blood at any of the three probing sites 10 seconds after probing and was recorded as present or absent. Periodontal measurements were not conducted on Study members who reported a history of cardiac valvular anomalies or rheumatic fever. At the analysis stage, summing the measurements for gingival recession and probing depth computed the loss of periodontal attachment for each site. Third molars were not included in the analysis of the periodontal data.

An estimate of social class was obtained for each participant by using information collected at age 5 regarding parental occupation. This data was subsequently categorized into SES using standard New Zealand occupationally based indices,910 which employ a six-interval classification (where, for example, a doctor scores 1 and a laborer scores 6). The estimate for the male parent was used where possible, and participants with a score of 1 or 2 were allocated to the high SES class group, those with a score of 3 or 4 were allocated to the medium SES group, and those remaining were allocated to the low SES group. Students and homemakers were unable to be categorized and were allocated to the latter group.

Although basic dental care for New Zealand children is funded by the State until they reach 18 years of age, almost all orthodontic treatment is provided through the private sector. The bulk of that treatment is carried out by specialist orthodontists, although it has been estimated that general dentists treat up to 25% of all orthodontic patients.11 However, this analysis did not distinguish between individuals treated by orthodontists and those treated by dentists.

Key indicators

The equity of orthodontic treatment was examined using two key indicators: (1) the receipt of treatment by treatment-need category, and (2) the receipt of treatment by SES group. For the efficacy of orthodontic treatment, the key indicator used was the proportion of Study members who moved to a less severe treatment-need category following treatment. The definition of effectiveness of treatment was altered somewhat from that described above and was used here to refer to the longer-term success (or otherwise) of treatment. It was measured using Study members' (1) rating of the success of their orthodontic treatment, and (2) self-rated dental appearance (at age 26) relative to their peers. Investigating the efficiency of orthodontic treatment was beyond the scope of the current study. The safety of orthodontic treatment was examined using the following clinical indicators: (1) cumulative dental caries experience by age 26; (2) dental caries increment from ages 18 to 26; (3) gingival recession and periodontal loss of attachment by age 26; and (4) tooth loss by age 26.

The statistical analysis of the data commenced with the generation of descriptive statistics, followed by the examination and testing of bivariate associations using chi-square tests for categorical dependent variables and independent sample t-tests for continuous dependent variables.

Equity of orthodontic treatment allocation

The distribution of Study members across the orthodontic treatment-need categories is presented in Table 4 along with the percentage in each SES group who had been treated by ages 18 and 26. The proportion treated by age 18 increased across the categories of increasing severity of orthodontic treatment need. When treatment by age 26 was included, the proportion treated increased in each category, with two-thirds of those in the handicapping category having been treated by age 26. No clear SES differences were apparent. Some 26.1%, 27.0%, and 20.8% of the high, medium, and low SES groups, respectively, had obtained treatment by age 18 (P > .05), and this small gradient had all but disappeared by age 26.

TABLE 4. Number of Study Members Treated by Age 18 According to Mode of Treatment and Socioeconomic Status (Percentages in Parentheses)

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One-third of those with a handicapping malocclusion and more than half of those with a severe malocclusion had not been treated by age 26. This meant that 42% of those with a severe or worse malocclusion had obtained orthodontic treatment by age 18, with that increasing to approximately half (49%) by age 26.

Efficacy of orthodontic treatment

This issue can only be examined with respect to orthodontic treatment by the age of 18, as that was the last age at which the DAI was used (permitting allocation of each Study member to a treatment-need category using the score ranges in Table 2). Of the 115 Study members treated by age 18, 40 (34.8%) improved; ie, they moved from a more severe treatment-need category to a less severe one following treatment. Similarly, 40 (34.8%) stayed in the same category, and 35 (30.4%) had moved to a more severe category by age 18. When the 50 Study members in the minor category were omitted from this analysis (because they could only stay in the same category or move to a worse one), 40 (61.5%) of the remainder improved, 13 (20.0%) stayed the same, and 12 (18.5%) moved to a worse category. The percentage improving in each of the treatment-need categories was 51.4% for the definite, 73.3% for the severe, and 76.9% for the handicapping category.

Effectiveness of orthodontic treatment

The percentage of Study members who considered their orthodontic treatment successful increased across the increasing age-12 treatment-need categories (Table 5). The difference between Study members who had been treated and those who had not been treated by age 26 in the proportion rating their dental appearance as above average increased with increasing severity of age-12 orthodontic treatment need. In the severe category, for example, 37.0% of those who had not been treated rated themselves above average, compared with 63.2% of those who had received orthodontic treatment.

TABLE 5. Rating of Orthodontic Treatment and Age-26 Dental Appearance (Relative to Peers) by Receipt of Treatment by Age 26 and Age-12 Treatment Need Category (Percentages in Parentheses)^a

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Safety of orthodontic treatment

Data on clinical characteristics by age 26 and history of orthodontic treatment are presented in Table 6. There were no significant differences between those who had been treated by age 26 and those who had not in their caries experience, periodontal disease occurrence, or tooth loss, and this null finding held when fixed appliance treatment only was examined.

TABLE 6. Clinical Characteristics at Age 26 by History of Orthodontic Treatment

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Equity of orthodontic treatment

Was the uptake of orthodontic treatment by Study members fair? From the treatment-need perspective, it certainly appeared to be so, with the treated percentage increasing across the categories of increasing malocclusion severity. Moreover, there were no apparent SES differences, which runs counter to expectations, particularly in view of a recent report12 of profound SES differences in caries-associated tooth loss in the same cohort. The most likely explanation for the absence of SES differences is that, unlike elsewhere in New Zealand, Dunedin residents have access to considerably cheaper orthodontic treatment through the University of Otago School of Dentistry, and that would have reduced the financial barrier to treatment uptake among those of lower SES.

The fact that only about half of those with a severe or handicapping malocclusion had been treated by age 26 is of unclear significance in the absence of information on the desire (or lack thereof) for treatment among those who had not been treated by that age. Whether the situation is satisfactory clearly depends on a value judgment by the observer: is the glass half-full or half-empty?

Efficacy of orthodontic treatment

The current study's data suggest that orthodontic treatment is efficacious for most of those who undergo it, with almost two-thirds of those treated moving to a less severe malocclusion category. However, in examining the efficacy of treatment for those who had received it, it would have been ideal to have full DAI scores at age 12. This would have allowed the use of the DAI in an evaluative manner. That is, the change in DAI score for each orthodontically treated Study member could have been monitored assuming that the DAI would indeed have been sensitive enough to have reflected relatively minor, but still clinically and socially significant, changes in malocclusion. Unfortunately, that was not possible, and the categorical approach, which had to be used, was a compromise that may have had some bearing on the findings. For example, it is possible that some of those who were on the borderline between two categories may have actually moved to the more severe category (or vice versa) without having had much of a change to their malocclusion, whereas others may have undergone considerable change but remained in the same category. Nevertheless, there is sufficient evidence in the current study's data to support the assertion that orthodontic treatment is efficacious.

Longer-term effectiveness of orthodontic treatment

Together with the Study members' ratings of their orthodontic treatment, the self-ratings of dental appearance by age 26 offered compelling evidence of the longer-term effectiveness of treatment, with the differential between those who had and had not been treated increasing steadily across the categories of malocclusion severity. This was perhaps the most convincing of the findings from this study and suggests that orthodontic treatment may have psychosocial benefits for those who undergo it, particularly among those with a severe or handicapping malocclusion. The ultimate judge of the success of treatment is not the clinician, but the individual who has been treated. Any clinical indicators that are used must be considered secondary to the personal and social outcomes of treatment.

Safety of orthodontic treatment

This study provides no population-level evidence that orthodontic treatment is harmful to oral health with, on average, no differences in a range of important clinical parameters between those who had or had not been treated. It should be emphasized that this finding applies at the group level only. The possibility of greater caries and periodontal disease experience should be acknowledged for each individual who is treated, and thus the appropriate preventive measures should continue to be emphasized. The loss of one or more third molars was included as an indicator because it was hypothesized that, by virtue of having undergone diagnostic radiography for orthodontic purposes, orthodontic patients would be more likely to have had the presence of impacted third molars noted and, therefore, be more likely to have been referred for their extraction. This was not supported possibly, because of the age at which most of the treated Study members would have received their care.

In summary, this study examined the equity, efficacy, effectiveness, and safety of orthodontic treatment among participants in a longstanding cohort study. The equity of care was found to be acceptable given that there were no clear SES differences in uptake (although this may be due to a local dental school reducing financial barriers to treatment). The treatment was found to be efficacious for most of those who underwent it, and the longer-term effectiveness of treatment was convincingly demonstrated. The safety of treatment was demonstrated by the absence of differences (in a number of clinical indicators) between those who were and who were not treated.