Introduction
In this study, we aimed to compare 8 candidate midsagittal planes (MSPs) constructed from different median landmarks to determine the most appropriate one for evaluating craniofacial asymmetry.
Methods
We included 30 patients (18 men, 12 women; mean age, 25.7 ± 6.03 years) who visited the National Health Insurance Service Ilsan Hospital in Gyeonggi-do, Korea, with a complaint of facial asymmetry. Four MSPs passing through 2 median landmarks perpendicular to the Frankfort horizontal plane and 4 other MSPs passing through 3 median landmarks were constructed. Menton, anterior nasal spine, and anterior nasal spine-to-posterior nasal spine line deviations were evaluated using these 8 MSPs. Eight MSPs from 30 subjects were shown to 6 experts, who selected the planes that they considered the most appropriate.
Results
Experts most frequently selected the plane passing through nasion and basion perpendicular to the Frankfort horizontal plane (66 of 180 times; P <0.05). In evaluating craniofacial asymmetry, using MSPs passing through 3 median landmarks in the cranial base can lead to underestimation of the asymmetry of the menton, anterior nasal spine, and anterior nasal spine-to-posterior nasal spine line.
Conclusions
We suggest using MSPs perpendicular to the Frankfort horizontal plane or a plane passing through anterior nasal spine in clinical practice.
Highlights
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We sought the most suitable midsagittal plane for evaluating craniofacial asymmetry.
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Planes perpendicular to the Frankfort horizontal plane or passing through ANS could be useful.
Patients’ clinical photographs and cephalometric radiographic images have been used to diagnose craniofacial asymmetry. Until the advent of computerized tomography (CT), 2-dimensional cephalometric analysis had been important in diagnosing patients with craniofacial malformation and malocclusion. However, this approach suffers from the limitations of 2-dimensional imaging: distortion or enlargement of images and difficulty in superimposition of anatomic structures. Analysis through 3-dimensional (3D) CT plays a major role in overcoming the limitations of 2-dimensional analysis.
When establishing an orthognathic surgery plan for patients with craniofacial asymmetry using 3D cephalometric analysis, construction of a midsagittal plane (MSP) is crucial, and the degree of asymmetry could be increased or reduced, depending on the way of constructing the MSP. Many studies have investigated methods for constructing a MSP; median landmarks and bilateral symmetric landmarks of the craniofacial area have commonly been used. However, in patients with craniofacial asymmetry, the craniofacial structures are likely to lack symmetry. Thus, constructing the MSP in patients with craniofacial asymmetry is more difficult than in the patients with facial symmetry, because the reference for determining the landmark with ideal symmetry or that passes correctly through the middle of the craniofacial area among the landmarks used for determining the MSP is acquired after constructing it.
However, to date, only a few studies of MSP configurations in patients with craniofacial asymmetry have been conducted. Thus, in this study, 8 MSPs passing through median landmarks or planes perpendicular to the horizontal reference plane were selected as candidate MSPs for evaluating craniofacial asymmetry on 3D CT images of patients with facial asymmetry. We planned to investigate which plane would be the most appropriate from among the 8 MSPs by having them assessed by experts.
Material and methods
This study was approved by the institutional review board of National Health Insurance Service Ilsan Hospital (NHIMC 2016-06-006-001) in Gyeonggi-do, Korea. Six orthodontists and oral and maxillofacial surgeons who had diagnosed more than 100 orthognathic surgery patients selected the MSP that they judged to be the most appropriate among the 8 candidate MSPs, based on images of 30 patients with facial asymmetry.
Among the 82 patients who visited National Health Insurance Service Ilsan Hospital from 2008 to 2014 with a complaint of facial asymmetry and who had CT imaging, 30 subjects with facial asymmetry were selected using the following criteria: (1) adults over 19 years old; (2) no systemic diseases; (3) no congenital deformities, including cleft lip and palate; (4) no temporomandibular joint disease; (5) no facial injury or fracture; and (6) no history of orthognathic surgery. The mean age of these patients (18 men, 12 women) was 25.7 ± 6.03 years (range, 19-43 years).
To obtain 3D CT images, the patients were scanned by multislice CT (SOMATOM Sensation 64-slice; Siemens, Malvern, Pa) in the supine position under the following conditions: 120 kV, 200 mA, scanning time 1 second, 512 × 512 pixels with 1-mm slice thickness. The images were saved as DICOM files and reconstructed with a 3D stereoscopic medical image diagnosis program (Simplant version 14.0; Materialise Dental, Leuven, Belgium).
Eleven reference points were set in the 3D images ( Fig 1 ); these landmarks and their definitions are presented in Table I . The Frankfort horizontal (FH) plane passing through the left and right orbitales and the median point on the left and right porions was set as the horizontal reference plane, and the line passing through anterior nasal spine (ANS) and posterior nasal spine (PNS) (A-P line), was set as the reference line for evaluating the yaw of the maxilla ( Fig 1 ; Table I ).
| Landmark | Definition |
|---|---|
| Na (nasion) | Most posterior point of curvature between frontal bone and nasal bone in the MSP |
| Cg (crista galli) | Most superior point of crista galli of the ethmoid bone |
| S (sella) | Center of sella turcica |
| Ba (basion) | Middorsal point of the anterior margin of the foramen magnum |
| Or Rt (orbitale right) | Lowest point of lower margin of the right orbit |
| Or Lt (orbitale left) | Lowest point of lower margin of the left orbit |
| Po Rt (porion right) | Most superior point of right external auditory meatus |
| Po Lt (porion left) | Most superior point of left external auditory meatus |
| Anterior nasal spine (ANS) | Most anterior point of nasal floor |
| Posterior nasal spine (PNS) | Most posterior point of nasal floor |
| Me (menton) | Most inferior part of the bony chin in the median plane |
| Reference plane | |
| Frankfort horizontal (FH) plane | Plane passing through right orbitale, left orbitale and midpoint of left and right porions |
| Reference line | |
| A-P line | Line passing through ANS and PNS |
Eight MSPs for this study were constructed ( Fig 2 ; Table II ). The horizontal distances from the 8 MSPs to menton and ANS (menton and ANS deviations) were measured ( Fig 3 ). Angles between the 8 MSPs and the A-P line (A-P line deviation) were measured ( Fig 3 ). The directions in which deviation occurred from each MSP were sorted into positive values and the directions of the nondeviation side into negative values.
| MSP | Definition |
|---|---|
| MSP 1 (FH-Na-Ba) | Passing through Na and Ba while perpendicular to the FH plane |
| MSP 2 (FH-Na-S) | Passing through Na and S while perpendicular to the FH plane |
| MSP 3 (FH-Cg-Ba) | Passing through Cg and Ba while perpendicular to the FH plane |
| MSP 4 (FH-Cg-S) | Passing through Cg and S while perpendicular to the FH plane |
| MSP 5 (Ba-Na-S) | Passing through Ba, Na, and S |
| MSP 6 (Ba-Cg-S) | Passing through Ba, Cg, and S |
| MSP 7 (Ba-Na-ANS) | Passing through Ba, Na, and ANS |
| MSP 8 (Ba-Cg-ANS) | Passing through Ba, Cg, and ANS |
Three-dimensional images of the craniofacial structures and 8 MSPs of the 30 subjects were shown to 6 experts. They were not given the definition of each plane, and the 8 MSPs were shown to them in a random sequence. They selected 180 MSPs that they considered the most appropriate planes. The horizontal distances from all MSPs to menton and ANS, and the angles to the A-P line were measured. The values and absolute values of differences in deviation measurements using the candidate MSPs were determined, and their mean values were calculated. The values are shown in Table III . Table IV and Figure 4 show the process involved in finding the values and absolute values of differences in the menton deviation measurements.
| Definition | Abbreviation | |
|---|---|---|
| Absolute value of differences in menton deviation measurements |
∑ |Menton deviation measured by candidate MSP -menton deviation measured by selected MSP| / 180 | AVDMe |
| Absolute value of differences in ANS deviation measurements |
∑ |ANS deviation measured by candidate MSP -ANS deviation measured by selected MSP| / 180 | AVDANS |
| Absolute value of differences in A-P line deviation measurements |
∑ |A-P deviation measured by candidate MSP – A-P deviation measured by selected MSP| / 180 | AVDAP |
| Value of differences in menton deviation measurements |
∑ (Menton deviation measured by candidate MSP -menton deviation measured by selected MSP) / 180 | VDMe |
| Value of differences in ANS deviation measurements |
∑ (ANS deviation measured by candidate MSP -ANS deviation measured by selected MSP) / 180 | VDANS |
| Value of differences in A-P line deviation measurements |
∑ (A-P deviation measured by candidate MSP – A-P deviation measured by selected MSP) / 180 | VDAP |
| Expert 1 | Expert 2 | … | Expert 6 | n = 180 | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Patient 1 | Patient 2 | … | Patient 30 | Patient 1 | Patient 2 | … | Patient 30 | Patient 1 | Patient 2 | … | Patient 30 | Mean | SD | ||
| Menton deviation | |||||||||||||||
| MSP 1 | 14.07 | 7.95 | 9.06 | 14.07 | 7.95 | 9.06 | 14.07 | 7.95 | 9.06 | 5.76 | 3.23 | ||||
| MSP 2 | 14.06 | 7.94 | 9.05 | 14.06 | 7.94 | 9.05 | 14.06 | 7.94 | 9.05 | 5.82 | 3.22 | ||||
| MSP 3 | 15.41 | 8 | 9.44 | 15.41 | 8 | 9.44 | 15.41 | 8 | 9.44 | 5.72 | 3.75 | ||||
| … | |||||||||||||||
| MSP 8 | 11.65 | 3.55 | 5.24 | 11.65 | 3.55 | 5.24 | 11.65 | 3.55 | 5.24 | 5.07 | 3.6 | ||||
| Selected MSP | MSP 1 (FH-Na-Ba) | MSP 3 (FH-Cg-Ba) | MSP 1 (FH-Na-Ba) | MSP 3 (FH-Na-Ba) | MSP 2 (FH-Na-S) | MSP 1 (FH-Na-Ba) | MSP 2 (FH-Na-S) | MSP 6 (Ba-Cg-S) | MSP 1 (FH-Na-Ba) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| VDMe | |||||||||||||||
| MSP 1 | 0 | −0.05 | 0 | 1.34 | 0.01 | 0 | 0.01 | 1.25 | 0 | 0.08 | 1.56 | ||||
| MSP 2 | −0.01 | −0.06 | −0.01 | −1.35 | 0 | −0.01 | 0 | 1.24 | −0.01 | 0.14 | 1.54 | ||||
| MSP 3 | 1.34 | 0 | 0.38 | 0 | 0.06 | 0.38 | 1.35 | 1.3 | 0.38 | 0.04 | 1.94 | ||||
| … | |||||||||||||||
| MSP 8 | −2.42 | −4.45 | −3.82 | −3.76 | −4.39 | −3.82 | −2.41 | −3.15 | −3.82 | −0.61 | 2.65 | ||||
| AVDMe | |||||||||||||||
| MSP 1 | 0 | 0.05 | 0 | 1.34 | 0.01 | 0 | 0.01 | 1.25 | 0 | 0.81 | 1.33 | ||||
| MSP 2 | 0.01 | 0.06 | 0.01 | 1.35 | 0 | 0.01 | 0 | 1.24 | 0.01 | 0.81 | 1.31 | ||||
| MSP 3 | 1.34 | 0 | 0.38 | 0 | 0.06 | 0.38 | 1.35 | 1.3 | 0.38 | 1.37 | 1.37 | ||||
| … | |||||||||||||||
| MSP 8 | 2.42 | 4.45 | 3.82 | 3.76 | 4.39 | 3.82 | 2.41 | 3.15 | 3.82 | 2.17 | 1.63 | ||||
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