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Hyung-Kyu Noh, Ho-Jin Kim, Hyo-Sang Park
Differences in positions of cone-beam computed tomography landmarks in patients with skeletal Class III facial asymmetry according to midsagittal planes.
- Korean J Orthod 2023;53:219-231
I appreciate the author’s hard work in clarifying the discrepancies in cone-beam computed tomography (CBCT) landmark positioning across various midsagittal planes (MSPs) in patients with skeletal Class III facial asymmetry. Since CBCT offers valuable insights that cannot be captured through cephalometrics alone, particularly in diagnosing asymmetry, this article holds significant value for its readers.
Q1. In the present study, the groups were divided into symmetric group and asymmetric group based on the amount of menton deviation using posteroanterior (PA) cephalograms. How was the MSP set in PA cephalograms?
Q2. Facial asymmetry is generally classified into 3 types: rolling-dominant type (R-type), translation-dominant type (T-type), and yawing-dominant type (Y-type), based on the distorted skeletal components that cause canting, translation, and yawing of the maxilla and/or mandible. Could you suggest which MSP would be more appropriate to examine each type of asymmetry?
Q3. The authors mentioned that the MSP based on the cranial part (MSP I or MSP II) generally reveals the most deviated maxilla and mandible, making it preferable for diagnostic screening. However, when there is a significant difference in the height of the orbit, it becomes challenging to establish the reference plane using bilateral landmarks such as frontozygomatic suture (FZS). How would you recommend adjusting the MSP in such a situation?
Q4. Although there is no consensus on the type of MSP for evaluating asymmetry, could you provide some insights into how you integrate these different types of MSP into a detailed workflow in your clinic?
A1. First of all, we would like to express our gratitude for your interest in our article. We used the menton deviation measured from the initial PA cephalogram as the group dividing criterion. We used the line connecting the bilateral FZS point (on the medial side) as the horizontal reference plane. The MSP of the PA cephalogram was defined as a plane that is perpendicular to this horizontal plane and passes through the crista galli. As the PA cephalogram is easily accessible and the first line of data, and analysis is done on planes on CT, we have used PA cephalogram for dividing groups. For the symmetry group, this line is also well matched to other CT planes, but not for asymmetry groups.
A2. It is a clinically interesting but difficult question to answer now. The literature has revealed these types of asymmetry patterns and suggested corresponding treatment strategies.1,2 However, to our knowledge, there is no study yet on how these skeletal/dental asymmetry characteristics are evaluated differently depending on the MSP. We believe that the mandibular asymmetry or the compensation of the mandibular teeth should be evaluated on the mandible’s reference plane,3 and thus would not be affected by the MSP types.2,4 On the other hand, the maxillary asymmetry or dental compensation of maxillary teeth is influenced by the choice of MSP. The yaw dominant type has prominent nasal alae deviation while no obvious deviation in roll and translation dominant types.5 This means that anterior nasal spine (ANS) may be deviated, and the ANS-associated planes may not be used. We are currently conducting a further study on this topic.
A3. As the questioner pointed out, the vertical position difference of the difference of the Frankfort horizontal (FH) landmark between the sides is often found in actual clinical practice and forms the basis for advocating the ANS-associated MSP. As soft tissue symmetry is the final goal of treatment, there is different vertical position of orbit and the middle structures are in the midline, ANS-associated MSP can be selected. Perhaps, in cases of severe craniofacial asymmetry accompanied by syndrome, the ANS-associated MSP that does not depend on the FH plane may be the clear alternative.6,7 However, if the difference is subtle, it may not be easy to choose between the 2 types of MSP. In such cases, soft tissue may be the most important criterion. It might be effective to choose the MSP that better reflects the actual head posture among the 2 MSPs.
A4. As the soft tissue is the most crucial criterion, it is helpful to record the natural head posture and compare it with the skull orientation according to each MSP. If facial scanning data is registered on the CBCT image and examined three-dimensionally, it is much more helpful. In case of nasal bridge deviation, the selection of MSPs can be influenced by the rhinoplasty or not. The next important thing is to reach an agreement with the oral surgeon. This is because the 2 main axes of treatment should have the same judgment criteria.
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