Discrepancy index relative to age, sex, and the probability of completing treatment by one resident in a 2-year graduate orthodontics program


The American Board of Orthodontics’ discrepancy index (DI) was designed to objectively quantify the complexity of a malocclusion before orthodontic treatment. In this study, we assessed the influence of age and sex on the DI distribution of a large mixed sample of patients. An additional objective was to ascertain the effectiveness of the DI for predicting the probability that 1 resident can complete the treatment of the malocclusion in a 24-month residency.


A group of 6 calibrated investigators independently determined the DI scores for 716 consecutive patients in the permanent dentition from the patient pool of Indiana University’s graduate orthodontics program over 7 years. The DI was scored and compared with the patient’s sex and age, and it was noted whether the patient was transferred to a second resident when the first one graduated.


The DI is not significantly related to sex or age, but it was a significant predictor for patients who required transfer to a second resident for completion of treatment.


The DI was a relatively stable index for measuring malocclusion complexity. It is independent of sex or age but is a consistent indicator of the greater time and effort required to complete treatment, because of the significant correlation to the necessity to transfer patient care to a second resident.

This study was a retrospective evaluation of the American Board of Orthodontics (ABO) discrepancy index (DI) relative to the patient’s sex and age at the start of treatment, and whether the patient was started and finished by the same resident. The complexity (severity) of a patient’s malocclusion is useful for predicting the clinical effort required to achieve optimal treatment. Several indexes have achieved some success in quantifying and categorizing the severity of malocclusions. Other indexes have distinct limitations. The peer assessment rating (PAR) index is used before and after treatment to assess the deviation from normal alignment and occlusion, and to calculate the degree of improvement. The index of orthodontic treatment need, dental health component, and standard component of aesthetic need are tools commonly used for determining orthodontic needs. The latter method contains a subjective component (esthetics) that is difficult to quantify reliably.

To assess the complexity of malocclusions for board case requirements, the ABO developed the DI. It is derived from evaluations of overjet, overbite, anterior open bite, lateral open bite, crowding, occlusion, lingual posterior crossbite, buccal posterior crossbite, and ANB, IMPA, and SN-GoGn cephalometric angles. The DI has evolved as the accepted method for quantifying the complexity of a malocclusion for the ABO Phase III clinical examination. A pilot study of the DI was undertaken to evaluate the complexity of cases treated by orthodontic residents to submit for board certification. Most of these patients were sufficiently complex to qualify for the ABO Phase III examination, supporting the value of the DI for selecting patients for board examinations.

Before development of the DI, a few studies examined some aspects of the relationships between initial severity of malocclusion, compared with sex, age, and the probability of finishing treatment in a 24-month residency. Using PAR scores, Firestone et al concluded that sex was a variable factor in the PAR score, but Grewe and Hermanson found no association between severity of the malocclusion and active treatment time. Using the malocclusion severity index, Hill reported that malocclusion prevalence was significantly higher in 9-year-olds than in 15-year-olds; they failed to take into account whether orthodontic treatment was performed. The index of complexity, outcome, and need (ICON) was age-dependent because malocclusion severity, as well as treatment need and complexity, increases with age. Willems et al reported that male patients tend to have higher PAR scores than do female patients. No studies specifically addressed whether orthodontic patients had started and finished with the same resident.

No index developed before the DI is suitable for routine assessment of a mixed population of patients, because the indexes vary with the age and sex of the patients. It is unknown whether the DI is affected by the patient’s sex or age. Furthermore, selecting patients who can be completely treated in a 2-year residency is an important aspect of the educational process, particularly for residents planning to use their patients for the ABO’s initial certification examination. In this study, we investigated the relationship of the DI to sex and age, and the value of the DI as a method for helping to select patients who can be completed by 1 resident in a 24-month orthodontic residency.

Material and methods

After institutional review board approval, 6 dental student investigators were trained in the DI method and calibrated with 20 sets of patient records. The cases selected represented the range of malocclusion complexity expected in the sample. The calibration patients were scored, and differences between the investigators were reconciled. The series of patients was repeatedly scored at 1 to 2 week intervals until all investigators were within ±5% of the collective mean.

The DI scores were calculated for 716 consecutive patients (total for the 6 calibrated investigators) completing full fixed appliance orthodontic treatment from 1998 through 2004. This was the same series of patients used for several clinical outcomes studies. The patients were started during the first 12 months of the 24-month graduate training program, without regard to the DI or to potential early board certification requirements. Information collected from patient records included the coded case number, sex, and age at start of treatment, and whether the patient was transferred to another resident. Data were entered into a spreadsheet and logged only by coded case number, not by patient name.

Statistical analysis

A Pearson correlation coefficient was calculated to evaluate the association between the patient’s age at the start of treatment with the DI scores. Two-sample t tests were used to evaluate the associations of the patient’s sex and 1 resident’s ability to complete treatment of the malocclusion with the DI scores.


One outlier was identified in the DI scores (score,152; next highest score, 78). No investigator recalled that any patient had a DI exceeding 100, so the outlier was excluded from further analysis or presentation in this report. Differential analysis of the components of the DI showed that the highest scores were for cephalometric measures, and the lowest scores were for buccal posterior crossbite. All variables are listed in Table I .

Table I
DI variables
n Mean SD Minimum Maximum
Overjet 716 1.9 2.2 0 25
Overbite 716 1.7 1.6 0 6
Anterior open bite 716 0.7 3.4 0 44
Lateral open bite 716 0.5 1.8 0 26
Crowding 716 2.2 2.0 0 7
Occlusion 716 2.5 3.0 0 16
Lingual posterior crossbite 716 0.5 1.2 0 9
Buccal posterior crossbite 716 0.2 0.8 0 8
Cephalometric angles 716 4.7 6.6 0 55
Other 716 0.8 1.5 0 10

Over the 7-year period when the patients were collected, there was no time trend in the DI scores (correlation, –0.06, Table II ). Of the 716 patients, 425 (59%) were female. Average DI scores (± SD) were 15.6 ± 11.1 for female patients and 15.8 ± 9.7 for male patients; these were not statistically significant ( P = 0.76). Patients were aged 19.0 ± 9.4 years (range, 9-66 years) at the start of treatment. Age at the start of treatment was not significantly associated with the DI (correlation, 0.04). Treatment duration averaged 2.6 ± 1.1 years (range, 0.3-7.5 years); 568 patients (80%) required transfer of care when the initial resident graduated. Patients with a higher DI had a longer treatment (correlation, 0.30), which was also evidenced by higher DI scores for the patients who required transfer of care when the initial resident graduated (16.6 ± 10.9) than those who did not (12.0 ± 8.0, P = 0.0001).

Apr 13, 2017 | Posted by in Orthodontics | Comments Off on Discrepancy index relative to age, sex, and the probability of completing treatment by one resident in a 2-year graduate orthodontics program
Premium Wordpress Themes by UFO Themes