Longitudinal evaluation of dental arch asymmetry in Class II subdivision malocclusion with 3-dimensional digital models

Introduction

Class II subdivision malocclusions with their asymmetric occlusal relationships often pose treatment difficulties. The aim of this study was to evaluate the longitudinal changes of dental arch asymmetry in untreated subjects with Class II subdivision malocclusion.

Methods

From 706 files from the University of Michigan Growth Study, longitudinal records of 17 untreated subjects with Class II subdivision malocclusion were included this study. Dental arch changes at 3 consecutive longitudinal intervals, defined by the cervical vertebral maturation method, were analyzed on digital dental models. The average ages of the subjects were 12.4, 15.1, and 19.1 years at the 3 time periods, respectively. Maxillary and mandibular reference lines were constructed and used for the intra-arch asymmetry measurements. The Friedman test and analysis of variance with repeated measures were used to determine dental arch asymmetries at the P <0.05 level.

Results

All subjects were found to have a type 1 Class II subdivision malocclusion characterized by distal positioning of the mandibular first molar on the Class II side. No statistically significant intra-arch asymmetry changes were found for the maxillary and mandibular dental arches in any time period. Between the baseline and the final follow-up, the data indicated decreases in maxillary and mandibular intercanine arch widths and arch lengths symmetrically.

Conclusions

The results of this study indicate that the dental arch asymmetry in patients with Class II subdivision malocclusions did not improve or worsen with growth.

Angle described Class II subdivision as a separate group of Class II malocclusions characterized by a unilateral Class II molar relationship. Since Class II subdivisions account for up to 50% of all Class II malocclusions and are among the most common dental asymmetries in the orthodontic population, the location and extent of asymmetry are concerns.

The etiology of the asymmetry can be quite complex. It could be dentally related, skeletally related, or a combination of both. Several studies have already reported that the primary factor contributing to a Class II subdivision malocclusion is the distal positioning of the mandibular first molar on the Class II side. A secondary contributor is the mesial positioning of the maxillary first molar on the Class II side. Depending on the origin of the malocclusion, 2 types of Class II subdivision malocclusion can be observed: type 1, characterized by distal positioning of the mandibular first molar on the Class II side, and type 2, characterized by mesial positioning of the maxillary first molar on the Class II side. In type 1 Class II subdivision malocclusions, coincidence of the maxillary dental midline with the facial midline and deviation of the mandibular midline were observed. Type 2 has the opposite characteristics.

Kurt et al evaluated the posterior vertical dimensions of the mandible and concluded that the mandible in Class II subdivision malocclusions does not exhibit unusual skeletal positioning or skeletal asymmetry. Alavi et al concluded that Class II subdivision malocclusions result mainly from unilateral distal positioning of the mandibular first molars irrespective of whether this was due to dentoalveolar or skeletal asymmetry. Rose et al confirmed that Class II subdivision malocclusions occur from distal positioning of the mandibular first molars on the Class II side, but they did not evaluate the anteroposterior positioning of the maxillary first molar as a possible contributing factor to the malocclusion. Azevedo et al evaluated Class II subdivision patients with apparent facial asymmetry and reported that the main component of Class II subdivision is dentoalveolar—primarily distal positioning of the mandibular first molar on the Class II side, and secondarily mesial positioning of the maxillary first molar on the same side. Similarly, Janson et al concluded that Class II subdivision malocclusion was mainly dentoalveolar, and the primary contributor was the distal positioning of the mandibular first molars on the Class II side without unusual skeletal or positional asymmetries. Recently, Minich et al evaluated whether Angle Class II subdivision malocclusions have skeletal or dental asymmetries between the Class II and Class I sides using cone-beam computed tomography. They concluded that there were significant skeletal and dental differences between the Class I and Class II sides, and the dental asymmetries accounted for about two thirds of the total asymmetry.

Based on the principle that occlusal changes can occur in growing and nongrowing subjects with normal occlusion, and because no studies have been published concerning dental arch asymmetry changes in patients with Class II subdivision malocclusion, we conducted this study to evaluate longitudinal growth changes in dental arch asymmetry and to contribute to the knowledge of dental arch changes over time.

Material and methods

The sample was selected retrospectively from the files of the University of Michigan Growth Study; 706 study casts were evaluated for longitudinal records of orthodontically untreated subjects with Class II subdivision malocclusion.

Sample selection was based exclusively on the initial anteroposterior dental relationship, regardless of any other dentoalveolar or skeletal characteristic. The selection criteria were the following.

  • 1.

    A Class II molar relationship of half a premolar in width or greater on 1 side and a Class I molar relationship on the other side from the buccal aspect, with the mesiopalatal cusp of the maxillary first molar occluding with the mesial triangular fossa of the mandibular first molar from the lingual aspect.

  • 2.

    All permanent teeth present up to the first molars.

  • 3.

    No supernumerary or congenitally missing teeth, excluding third molars, or teeth with extensive restorations or gross decay.

  • 4.

    No ectopic teeth or anomalies in tooth shape.

  • 5.

    No previous orthodontic treatment.

  • 6.

    No crowding or, at most, symmetrical crowding up to 3 mm.

  • 7.

    No anterior or posterior crossbite.

  • 8.

    Clear lateral cephalograms at 3 time points available for all selected subjects. Additionally, casts with fractured cusps or severe attrition, or with molars or premolars on which more than two-thirds of the occlusal surface had been reconstructed, were excluded.

A final sample size of 17 pairs of dental casts (8 boys, 9 girls) and a total of 68 sides (maxillary and mandibular, right and left) were included for analysis. All subjects were of European-American ancestry. The assessment of the stages of cervical vertebral maturation on the lateral cephalograms for each subject was performed by 1 investigator (B.Y.). The baseline assessment corresponded to stage 3, the second assessment corresponded to stage 4, and the final follow-up corresponded to stage 5. The mean ages at 3 times were 12.4 (±0.1), 15.1 (±0.6), and 19.1 (±0.6) years, respectively.

All plaster models were transformed into digital format by a 3-dimensional (3D) model laser scanner (D250 3D Dental Scanner; 3Shape A/S, Copenhagen, Denmark). The digital models were analyzed by 1 investigator (B.Y.) using the 3Shape Orthoanalyzer software (version 1.0; 3Shape A/S). Cast measurements were based on those of Alavi et al and Sinclair and Little.

On the maxillary digital model, 2 points—the first at the distal aspect of the incisive papilla and the second adjacent to the second rugae—defined the median raphe that was used as the dental midline. Unlike the study by Alavi et al, the posterior border of the raphe near the fovea centralis was not preferred because of poor determination in the digital models. An anterior reference line was constructed perpendicular to the median raphe through the midpoint between the central incisors using the horizontal default position of the software with grids.

The median raphe was transferred to the mandibular model as follows. The top view in the inspection section of the program was selected, the maxillary model was hidden, and the mandible was then visible. Plane offset was adjusted to +50 mm. During this adjustment, the horizontal default position with grids was kept stable. Hence, the median raphe was transferred to the mandible. The anterior reference line was constructed perpendicular to the transferred median raphe through the midpoint between the central incisors using the horizontal default position of the software with grids as well.

Horizontal measurements were made at 90° to the median raphe from the buccal cusp tips of the posterior teeth, the cusp tips of the canines, and the midincisal edges of the anterior teeth bilaterally; vertical measurements were made at 90° to the anterior reference line from the same tooth landmarks. The linear measurements made from bilateral the tooth landmarks ( Table I ) to the horizontal and vertical reference lines on the maxillary and mandibular dental models are shown in Figures 1 through 4 .

Table I
Definitions of abbreviations for cast landmarks
Abbreviation Definition
U1/L1 Midincisal edge of upper/lower central incisors
U2/L2 Midincisal edge of upper/lower lateral incisors
U3/L3 Upper/lower canine cusp tip
U4/L4 Upper/lower first premolar buccal cusp tip
U5/L5 Upper/lower second premolar buccal cusp tip
U6MB/L6MB Upper/lower first molar mesiobuccal cusp tip
U6DB/L6DB Upper/lower first molar distobuccal cusp tip

Fig 1
Linear measurements from bilateral tooth landmarks to the vertical reference line on the maxillary dental model. MR , Median raphe.

Fig 2
Linear measurements from bilateral tooth landmarks to the horizontal reference line on the maxillary dental model. ARL , Anterior reference line.

Fig 3
Linear measurements from bilateral tooth landmarks to the vertical reference line on the mandibular dental model. MR , Median raphe.

Fig 4
Linear measurements from bilateral tooth landmarks to the horizontal reference lines on the mandibular dental model. ARL , Anterior reference line.

Additionally, intercanine width (distance between cusp tips or estimated cusp tips in cases of wear facets), intermolar width (distance between mesiobuccal cusp tips or estimated cusp tips in cases of wear facets), and arch length (sum of the right and left distances from mesial anatomic contact points of the permanent first molars to the contact points of the central incisors) were measured ( Figs 5 and 6 ).

Fig 5
Intercanine width, intermolar width, and arch length on the maxillary dental model.

Fig 6
Intercanine width, intermolar width, and arch length on the mandibular dental model.

Asymmetries were calculated by determining the absolute difference between homologous measurements as related to the reference lines. The same procedure was followed to compute the anterior and buccal segment anteroposterior asymmetries. Dental midlines were evaluated by posteroanterior radiographs. On these, a reference midline was determined geometrically so that it passed through the root of crista galli perpendicular to a line connecting the most superior points on the orbital contours.

Ten randomly selected sets of digital models were redigitized and remeasured 2 weeks after the first set of recordings to calculate the method error by the same investigator (B.Y.). The casual error was calculated according to Dahlberg’s formula. The systematic error was calculated with paired-samples t tests at P <0.05. The mean differences of each variable between the successive tracings were compared with the paired t tests. No significant differences were found between the 2 sets of measurements.

Statistical analysis

All data analyses were carried out using SigmaStat (version 3.5; Systat Software, San Jose, Calif). Descriptive statistics, including means and standard deviations, were calculated for between-stage changes. Normal distribution of the data was verified with the Kolmogorov-Smirnov test. Data without normal distribution were compared with nonparametric statistics with the Friedman test, and analysis of variance with repeated measures was used for normally distributed data. All values were considered significant at P <0.05.

Results

Means and standard deviations for all data are listed in Tables II and III . All subjects were found to have a type 1 Class II subdivision malocclusion characterized by distal positioning of the mandibular first molar on the Class II side. No statistically significant intra-arch asymmetry changes were found for the maxillary and mandibular dental arches in any time period.

Table II
Statistical comparisons of absolute differences between horizontal measurements of upper and lower bilateral dental landmarks, arch length, and intercanine and intermolar width measurements with repeated-measures analysis of variance or Friedman tests
Measurement (mm) Baseline Assessment 2 Final follow-up P
Mean SD Mean SD Mean SD
Upper
U1 to MR 0.208 0.754 0.228 0.920 0.194 1.463 0.790
U2 to MR 0.005 0.769 0.005 0.858 0.310 2.370 1.000
U3 to MR 0.088 1.253 0.119 1.137 0.311 1.317 0.954
U4 to MR 0.062 0.940 0.255 0.985 0.509 1.475 0.589
U5 to MR 0.578 1.169 0.196 1.243 0.000 1.621 0.289
U6MB to MR 0.584 1.184 0.258 1.669 0.093 1.409 0.338
U6DB to MR 0.639 1.408 0.026 1.678 0.174 1.514 0.327
Intercanine width 34.265 1.564 34.274 2.164 34.086 2.021 0.707
Intermolar width 51.495 1.529 50.112 6.482 51.538 1.677 0.838
Arch length 70.356 3.439 69.373 2.826 68.915 2.782 0.080
Lower
L1 to MR 0.225 2.823 0.054 2.715 0.114 2.911 0.790
L2 to MR 0.060 2.408 0.709 3.030 0.343 3.117 0.205
L3 to MR 0.144 2.242 0.574 2.658 0.388 2.420 0.662
L4 to MR 0.194 1.744 0.047 1.991 0.126 1.968 0.790
L5 to MR 0.051 1.383 0.098 1.877 0.599 2.242 0.322
L6MB to MR 0.494 1.305 0.064 1.986 0.452 2.278 0.197
L6DB to MR 0.603 1.456 0.230 1.844 0.080 1.938 0.443
Intercanine width 26.259 1.286 25.976 1.719 25.664 1.299 0.080
Intermolar width 44.486 2.041 44.251 2.210 44.095 2.119 0.217
Arch length 60.846 2.504 59.546 2.347 58.931 2.372 <0.001
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Apr 6, 2017 | Posted by in Orthodontics | Comments Off on Longitudinal evaluation of dental arch asymmetry in Class II subdivision malocclusion with 3-dimensional digital models
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