Elsevier

Seminars in Orthodontics

Volume 20, Issue 4, December 2014, Pages 287-298
Seminars in Orthodontics

A three-dimensional evaluation of Class II subdivision malocclusion correction using Cartesian coordinates

https://doi.org/10.1053/j.sodo.2014.09.005Get rights and content

Our objective was to evaluate the reliability of a comprehensive 3-dimensional (3D) evaluation method of dento-skeletal changes using Cartesian coordinates. The coordinates were used to evaluate changes that occurred during non-extraction orthodontic treatment of Class II subdivision malocclusions, and more specifically, describe how the Class II side was corrected to a Class I relation while maintaining the Class I side. The sample consisted of 25 adolescent orthodontic patients diagnosed with a Class II subdivision, and treated non-extraction to a bilateral Class I relation. The pre- (T1) and post-treatment (T2) Cone Beam Computed Tomography scans were oriented using the cranial nerve canals to set the 3 planes of reference, and 19 landmarks were registered on maxillary, mandibular, and dental structures. The data consisted of 3D coordinates representing the distances to the reference planes. A Class I side along with a Class II side was made for each subject. Comparisons were made between sides and between T1 and T2. The data was analyzed using t-test and Pearson׳s Correlation, and the intra-observer reliability was tested by intraclass correlation coefficient. The vertical dimension showed the most variability, both between patients and when comparing T1 versus T2. Treatment effects on the maxillary and mandibular molars on the Class II side were significantly different than the effects seen on the Class I side. There was a weak association between the transverse change, mandibular midline and the antero-posterior change of the mandibular molar on the Class II side. Gonion and PNS showed a tendency for posterior displacement. The coordinate method was found to be reliable for longitudinal studies in 3 dimensions and allows the evaluation of amount and direction of treatment changes. The orthodontic correction of the Class II was due to a combination of refrained forward movement of the maxillary molar and canine, slight outward transverse movement of the maxillary molar, and slight mesial movement of the mandibular molar, all on the Class II side.

Introduction

In the late 1800s, as the concept of prosthetic occlusion developed, Edward H. Angle was among the first to extend it to the natural dentition. Angle׳s classification of malocclusion was an important step in the development of the specialty of orthodontics because it not only subdivided major types of malocclusion but also included the first clear and simple definition of normal occlusion in the natural dentition.1 Angle׳s postulate was that the maxillary first molar was key to occlusion and that the maxillary and mandibular molars should be related so that the mesiobuccal cusp of the maxillary molar occludes in the buccal groove of the mandibular molar. Angle׳s classification of malocclusion in 1899 defined Class II subdivisions as a Class I molar relationship on 1 side and Class II molar on the opposite side.2 According to his theory, subdivisions are caused by a distally erupting mandibular first molar on 1 side, sometimes accompanied by a mesially erupting maxillary first molar, resulting in the “mislocking” of the molars during their eruption.3 There is no consensus in the literature whether the etiology is dental, skeletal, or a combination of both. However, different authors agreed on the origin of the asymmetry being mostly mandibular dento-alveolar, with the maxillary complex playing a secondary role, and without significant skeletal asymmetry.4, 5, 6, 7, 8, 9 A recent report further fine-tunes the dental/skeletal contribution to the etiology of a Class II subdivision with dental asymmetries accounting for about two-thirds of the total asymmetry.10

Most patients seeking orthodontic treatment are looking for an improvement in esthetics. It is said that beauty is in the eye of the beholder and that statement reflects the subjective aspect of esthetics (ruled by fashion, cultural background, and personal taste). Esthetics, however, also follow symmetry and proportions. Correcting asymmetric occlusions can be challenging, and the etiology of the asymmetry may be dental, skeletal, or a combination of both.

Perhaps the most famous asymmetric malocclusion treated in orthodontics is the Angle׳s Class II subdivision. The challenge for the practitioner in the treatment of such malocclusion is to alter the occlusion in such a way that the Class I would remain static while changing the Class II side to a Class I relation. Janson et al.11 studied the dental changes of a sample of Class II subdivision patients treated with asymmetric extractions and intermaxillary elastics, evaluated using submento-vertex (SMV) and postero-anterior (PA) radiographs. The results were compared to a group of Class I patients and a group of non-treated Class II subdivision patients. Correction of the upper and lower midline was achieved without cant of the occlusal plane, and no significant skeletal changes or transverse effects attributed to asymmetric mechanics were found. The same investigators compared the treatment effects on Class II subdivisions with 3 or 4 premolars extracted.12 Significantly, there was less mandibular incisor retraction observed in the asymmetric extraction group as well as a greater mandibular incisor extrusion.

Geramy13 analyzed the changes that led to space closure, midline correction, and overjet reduction after removal of 1 premolar on the side of the subdivision and canine retraction, using different designs of looped archwires. Researchers concluded that the antero-posterior (A-P) and medio-lateral movements of the upper incisors increase from the Class I side to the Class II side and the supero-inferior displacements of the incisors result in canting of the occlusal plane. Sanders et al.14 used a 3-dimensional (3D) coordinate system to compare the degree of dento-skeletal asymmetry in 30 subjects with Class II subdivision malocclusion compared to 30 subjects with normal Class I occlusion. This was the first case–control study to find significant skeletal asymmetries among Class II subdivision patients, since past studies reported distal positioning of the mandibular molar as the main etiologic factor. If 3D imaging proved to be more effective to detect Class II subdivision malocclusion skeletal asymmetries, it might also be more sensitive than 2-dimensional (2D) imaging to evaluate treatment changes, as it has been proven to give measurements closer to reality.15, 16, 17, 18 Landmark reliability and the reliability of the method used are essential for the veracity of the results. Baumrind and Frantz19 tested the reliability of landmark identification on 20 lateral cephalograms, and their scatter plot graphs revealed that each landmark has its own characteristic and usually a noncircular envelope of error.19

Renee Descartes (1596–1650) was a French philosopher, mathematician, and writer whose work revolutionized the scientific world, allowing algebraic equations to be expressed as geometric shapes in a 2D coordinate system: the Cartesian coordinate system. This system can be adapted to the needs of 3D images, making it possible to specify each point uniquely in a plane by 3 numerical coordinates. Each are the signed distances from the point to 3 fixed perpendicular directed lines, in our case planes of reference, measured in the same unit of length. In addition to a high-level accuracy, effective reproducibility is essential for 3D measurements of CT images, and a certain landmark is useless if the reproducibility of the anatomical coordinate system itself is not addressed.

There seems to be a lack of evidence on how a Class II subdivision, treated using non-extraction, is able to finish with bilateral Class I molar. The aim of this project is to evaluate the reliability of a comprehensive 3D evaluation method of dento-skeletal changes using Cartesian coordinates. The coordinates are used to evaluate changes that occurred during non-extraction orthodontic treatment of Class II subdivision malocclusion, and more specifically, describe how the Class II side was corrected to a Class I relation, while maintaining the Class I side.

Section snippets

Material and methods

The Case Western Reserve University Institutional Review Board (IRB) approved this retrospective study. Subjects were selected from a sample of adolescent orthodontic patients from the Graduate Clinic of CWRU School of Dental Medicine. Before the initiation of treatment, a legal guardian signed the American Association of Orthodontists Informed Consent, including a paragraph consenting to the use of records for research purposes.

Data were derived from pre-treatment and post-treatment Cone Beam

Results

The sample consisted of 25 subjects with Class II subdivision malocclusion treated with comprehensive orthodontics (Table 2) with a majority being Caucasian (n = 22). The 25 subjects (13 males and 12 females) were treated with intermaxillary elastics, and only 1 had a combination of intermaxillary elastics and Forsus™ (3M Unitek Corporation, Monrovia, CA) due to poor compliance reasons. Of them, 11 had the malocclusion (Class II) on the left side and 14 had on the right side, and the

Discussion

Cone Beam Computed Tomography has proven to be a powerful tool to perform clinical research. According to Ballrick et al.,24 CBCT can be used to make clinically accurate measurements with acceptable resolution. For longitudinal study purposes, Wu et al.23 in 2011 found a reliable method to orient the 3D images using cranial nerve canals. When this method was applied to our study, it raised questions about the extent of the asymmetry in the Class II subdivision malocclusion subjects and that the

Conclusions

The coordinate system (based on CBCT scans) proved to be a reliable method for the longitudinal study of Class II subdivision malocclusion treatment changes. Relative to Basion, used as the 0,0,0 origin point, the results of this study suggest the following:

  • (1)

    The orthodontic correction of the Class II was due to a combination of refrained forward movement of the maxillary molar and canine, outward transverse movement of the maxillary molar, and mesial movement of the mandibular molar, all on the

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