Evaluation of skeletal and dental asymmetries in Angle Class II subdivision malocclusions with cone-beam computed tomography

Introduction

The purpose of this study was to determine whether Angle Class II subdivision malocclusions have skeletal or dental asymmetries between the Class II and Class I sides.

Methods

A sample of 54 untreated Angle Class II subdivision patients with pretreatment photos and cone-beam computed tomography (CBCT) scans was used. The photos were used to identify the Class II subdivision malocclusion and to record the amount of crowding per quadrant. Landmarks were plotted on each CBCT volume so that direct 3-dimensional measurements could be made to compare the positions and dimensions of the skeletal and dental structures on the Class II side vs the Class I side.

Results

Significant differences were found for 2 skeletal measurements: the position of the maxilla relative to the cranial base, and the mandibular dimension from the mandibular foramen to the mental foramen. Statistically significant dental differences were found for the position of the mandibular first molars and canines in relation to the maxilla and the mandible. Statistically significant differences were found for the maxillary first molars and canines in relation to the mandible.

Conclusions

There were significant skeletal and dental differences between the Class I and Class II sides. The dental asymmetries accounted for about two thirds of the total asymmetry.

Class II subdivision malocclusions can be extremely challenging for diagnosis and treatment planning because many clinicians have difficulty in identifying the cause of the malocclusion. Since Angle Class II subdivision malocclusions possess characteristics of both Class I and Class II malocclusions, there is asymmetry between the right and left sides of the dentition. This asymmetry often requires asymmetric extractions or mechanics during treatment, which can be very complicated. The etiology of the asymmetry can be quite complex. It could be dental related, skeletal related, or a combination of both.

Any time a dental midline deviation or an asymmetric occlusion is observed, the clinician must check for skeletal asymmetries, dental asymmetries, and functional shifts. Manipulating the patient into centric relation or using an occlusal splint to verify the position of the mandible is an important first step in correctly diagnosing any type of asymmetry.

Previous studies used 2-dimensional radiographs (posteroanterior cephalograms, submentovertex view, and corrected oblique cephalograms) to evaluate Class II subdivisions for dental and skeletal asymmetries between the Class II and Class I sides of the skull and the dentition. These studies showed that the differences between the 2 sides were primarily dentoalveolar. The main factor is the distal positioning of the mandibular first molar on the Class II side with a mandibular dental midline deviation toward the Class II side (type 1). A secondary factor is the mesial positioning of the maxillary first molar on the Class II side with a maxillary dental midline deviation away from the Class II side (type 2). These authors found no statistically significant skeletal differences between the 2 sides, but they mentioned a tendency for mild skeletal asymmetries in Class II subdivisions and recommended further research to investigate these minor asymmetries.

The purpose of this study was to analyze Class II subdivision malocclusions for skeletal and dental asymmetries by using cone-beam computed tomography (CBCT).

Material and methods

In this retrospective study, we evaluated 54 pretreatment records (photos and CBCT volumes) of patients from the Department of Orthodontics of Case Western Reserve University in Cleveland, Ohio. Initial pretreatment photographs of 300 Angle Class II malocclusion patients were examined to search for Class II subdivision malocclusions that met the following criteria: (1) erupted permanent dentition from first molar to first molar in both arches; (2) no missing teeth, excluding third molars; (3) no craniofacial syndromes that would cause severe skeletal asymmetries; (4) one side of the arch with an Angle Class I molar relationship and the other side with at least a half-step Angle Class II molar relationship or greater; and (5) clear pretreatment photographs and CBCT volumes.

Once the patients were identified, the amounts of crowding per quadrant were estimated by the principal investigator (C.M.M.) and recorded so that the role of dental crowding could also be evaluated in the Class II subdivision malocclusions. Two groups were created: a noncrowded group (no crowding or ≤2 mm of crowding in any quadrant), and a crowded group (≥3 mm of crowding in any quadrant). Patients with excess spacing were not included in either group.

The scans were made by using the CBCT unit (Hitachi, Tokyo, Japan) at Case Western Reserve University’s Craniofacial Imaging Center. All images were taken at settings of 2 mA, 120 kV, and 12-in field of view. The CBCT volumes were analyzed and measured by using Dolphin 3D software (version 11.0 premium software; Dolphin Imaging & Management Solutions, Chatsworth, Calif).

Establishing a consistent method of orientation ensured that the coordinate systems would be the same for each image so that the measurements for each subject could be reliably compared. This was done in the Dolphin 3D software by setting the x-, y-, and z-axes on reproducible anatomic landmarks that could be easily identified.

From the right lateral view, the z-axis was set to the Frankfort horizontal, a line passing through the right porion and right orbitale. Then the y-axis was set perpendicular to the z-axis through the middle of the sella turcica by clipping the image from the sagittal view to obtain a clear view of the sella.

From the frontal view, the x-axis was defined by a line passing through the right and left orbitales. From the superior view, the z-axis was constructed by drawing a line from the crista galli through the middle of the sella and perpendicular to the x-axis through the middle of the sella.

By establishing the x-, y-, and z-axes, 3 planes in space were created that intersected at the origin. The origin (0, 0, 0) was located along the midsagittal plane, just below the sella, and at the level of Frankfort horizontal. After the reconstructed model was oriented and the axes and planes were defined, landmarks could be plotted in precise anatomic locations by using sagittal, coronal, and axial slices of the CBCT volume.

Twenty-one digital landmarks, skeletal and dental, were selected based on ease of identification and reproducibility ( Fig , Table I ). Three midline landmarks—origin, incisive foramen, and genial tubercles—were chosen to represent the middle of the cranial base, the maxilla, and the mandible, respectively. The other 18 landmarks were located on the right and left halves of the skull and dental arches.

Fig
Skeletal landmarks.

Table I
Landmark abbreviations
Landmark Specific name Generic name Abbreviation
1 Origin (0, 0, 0) Cranial base CB
2 Class II side foramen rotundum Maxilla II MxII
3 Class I side foramen rotundum Maxilla I MxI
4 Class II side mandibular foramen Mandible II MdII
5 Class I side mandibular foramen Mandible I MdI
6 Class II side superior, posterior, lateral condyle Condyle II CoII
7 Class I side superior, posterior, lateral condyle Condyle I CoI
8 Class II side foramen ovale Foramen ovale II FOII
9 Class I side foramen ovale Foramen ovale I FOI
10 Class II side mental foramen Mental foramen II MeFII
11 Class I side mental foramen Mental foramen I MeFI
12 Incisive foramen Incisive foramen IF
13 Midpoint between genial tubercles Genial tubercles GT
14 Class II side mesial buccal cusp tip of upper 6 Upper 6 II U6II
15 Class I side mesial buccal cusp tip of upper 6 Upper 6 I U6I
16 Class II side mesial buccal cusp tip of lower 6 Lower 6 II L6II
17 Class I side mesial buccal cusp tip of lower 6 Lower 6 I L6I
18 Class II side cusp tip of upper 3 Upper 3 II U3II
19 Class I side cusp tip of upper 3 Upper 3 I U3I
20 Class II side cusp tip of lower 3 Lower 3 II L3II
21 Class I side cusp tip of lower 3 Lower 3 I L3I

6 , First molar; 3 , canine.

The subdivision side is the Class II side, and the other side is the Class I side; the right side is always the Class II side in this study, and the left side is always the Class I side; therefore, “Class II side” and “right side” are used interchangeably in the study, as well as “Class I side” and “left side.”

Each landmark was assigned unique coordinates (x, y, z) when it was plotted in the Dolphin 3D software. These coordinates could then be copied and pasted into Excel (Microsoft, Redmond, Wash), with a spreadsheet designed so that the direct measurements in millimeters could be calculated between any 2 landmarks by using the distance formula. In 3-dimensional space, the distance between points (x 1 , y 1 , z 1 and x 2 , y 2 , z 2 ) is determined with this equation:

d=(x2x1)2+(y2y1)2+(z2z1)2
d = ( x 2 − x 1 ) 2 + ( y 2 − y 1 ) 2 + ( z 2 − z 1 ) 2

Measurements were also made in the Class II side of the maxilla and the mandible that were compared with the measurements in the Class I side of the maxilla and the mandible ( Table II ). These measurements were evaluated to determine whether there were any Class II side vs Class I side differences in the following.

  • 1.

    The positions of the maxilla or the mandible in relation to the cranial base (rotation of the maxilla or the mandible, when one side is ahead of or behind the other side).

  • 2.

    The positions of the maxilla and the mandible in relation to each other (intermaxillary measurement).

  • 3.

    Intramaxillary dimensions (one side larger or smaller than the other).

  • 4.

    Intramandibular dimensions.

  • 5.

    The intra-arch positions of the molars and canines relative to the maxilla (mesial or distal position of the teeth in the arch on 1 side).

  • 6.

    The intra-arch positions of the molars and canines relative to the mandible.

Table II
Paired-samples descriptive statistics and method errors
Measurement pair Mean SD Method error
1 CB to MxII 24.601 1.683 0.418
CB to MxI 24.219 1.965 0.723
2 CB to MdII 52.531 3.038 0.741
CB to MdI 52.185 3.206 0.439
3 MxII to MdII 46.004 3.857 0.694
MxI to MdI 45.855 3.668 0.427
4 MxII to IF 61.726 4.625 0.852
MxI to IF 61.867 4.680 0.927
5 MdII to MeFII 58.947 4.183 0.751
MdI to MeFI 59.569 3.925 1.097
6 MdII to GT 72.259 4.321 0.737
MdI to GT 72.701 3.996 0.886
7 FOII to MdII 33.238 3.726 0.687
FOI to MdI 33.024 3.413 0.613
8 CoII to MdII 35.252 3.469 0.747
CoI to MdI 35.380 3.329 0.845
9 MxII to U6II 55.265 5.066 0.366
MxII to L6II 55.585 5.252 0.352
10 MxII to U3II 69.199 5.487 0.409
MxII to L3II 67.099 5.859 0.379
11 MxI to U6I 54.767 5.052 0.542
MxI to L6I 56.616 5.330 0.434
12 MxI to U3I 68.858 5.367 0.533
MxI to L3I 68.428 5.864 0.548
13 MdII to U6II 45.083 3.640 0.776
MdII to L6II 44.435 3.689 0.794
14 MdII to U3II 66.632 4.521 0.783
MdII to L3II 65.347 4.348 0.829
15 MdI to U6I 43.141 3.233 0.713
MdI to L6I 45.714 3.409 0.647
16 MdI to U3I 64.861 3.746 0.587
MdI to L3I 66.729 3.969 0.688

Each measurement for the Class II side is paired with its counterpart from the Class I side to give 16 measurement pairs and 32 measurements and variables; all measurements are in millimeters.

Each measurement was a straight-line distance between the 2 points.

Fifteen of the 54 subjects were randomly selected, and the CBCT orientations, landmark identifications, and measurements were repeated by the same examiner (C.M.M.) to test for reliability.

The method error ( ME ), or casual error, was calculated by using Dahlberg’s formula : ME=(d2/2n),
ME = √ ( ∑ d 2 / 2 n ) ,
with d as the difference between the original and repeated measurements and n as 15 for the sample size that was repeated.

The method error tests for the reproducibility of each measurement and provides an estimate of the standard deviation around the mean of each measurement.

Statistical analysis

The null hypothesis was that no significant difference would exist between the Class II and Class I sides for the skeletal and dental measurements of Class II subdivision malocclusions. A second hypothesis was that there would be no significant difference in skeletal or dental measurements between the 2 sides when Class II subdivision malocclusions were separated into a noncrowded group with minimal or no dental crowding and a crowded group with moderate to severe dental crowding.

To test these hypotheses, descriptive statistics (means and standard deviations) were calculated for all 32 variables. Paired-samples correlations were used to determine whether there were relationships between the variables on the Class II side and the Class I side. Paired-samples t tests were run to evaluate any significant differences between the corresponding variables of the Class II and Class I sides. The alpha value was set at 0.05 for this study.

Statistical analyses were run by using SPSS software (version 15.0; SPSS, Chicago, Ill).

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Apr 7, 2017 | Posted by in Orthodontics | Comments Off on Evaluation of skeletal and dental asymmetries in Angle Class II subdivision malocclusions with cone-beam computed tomography
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