Imaging Sci Dent.  2019 Jun;49(2):159-169. 10.5624/isd.2019.49.2.159.

Use of an anatomical mid-sagittal plane for 3-dimensional cephalometry: A preliminary study

• Vernucci R
• Aghazada H
• Gardini K
• Fegatelli
• Barbato E
• Galluccio
• Silvestri

• KMID: 2450182
• DOI: http://doi.org/10.5624/isd.2019.49.2.159

Abstract

PURPOSE
Cone-beam computed tomography (CBCT) is widely used for 3-dimensional assessments of cranio-maxillofacial relationships, especially in patients undergoing orthognathic surgery. We have introduced, for reference in CBCT cephalometry, an anatomical mid-sagittal plane (MSP) identified by the nasion, the midpoint between the posterior clinoid processes of the sella turcica, and the basion. The MSP is an updated version of the median plane previously used at our institution for 2D posterior-anterior cephalometry. This study was conducted to test the accuracy of the CBCT measures compared to those obtained using standard posterior-anterior cephalometry.
MATERIALS AND METHODS
Two operators measured the inter-zygomatic distance on 15 CBCT scans using the MSP as a reference plane, and the CBCT measurements were compared with measurements made on patients' posterior-anterior cephalograms. The statistical analysis evaluated the absolute and percentage differences between the 3D and 2D measurements.
RESULTS
As demonstrated by the absolute mean difference (roughly 1 mm) and the percentage difference (less than 3%), the MSP showed good accuracy on CBCT compared to the 2D plane, especially for measurements of the left side. However, the CBCT measurements showed a high standard deviation, indicating major variability and low precision.
CONCLUSION
The anatomical MSP can be used as a reliable reference plane for transverse measurements in 3D cephalometry in cases of symmetrical or asymmetrical malocclusion. In patients who suffer from distortions of the skull base, the identification of landmarks might be difficult and the MSP could be unreliable. Becoming familiar with the relevant software could reduce errors and improve reliability.

Keyword

Cephalometry; Cone-Beam Computed Tomography; Dimensional Measurement Accuracy

MeSH Terms

Cephalometry*
Cone-Beam Computed Tomography
Dimensional Measurement Accuracy
Humans
Malocclusion
Orthognathic Surgery
Sella Turcica
Skull Base

Figure

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  Fig. 1 Landmarks selected for tracing the mid-sagittal plane on cone-beam computed tomographic (CBCT) images. A. Localisation of the nasion. B. Localisation of the midpoint between the posterior clinoid processes. C. Localisation of the basion.

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  Fig. 2 The anatomical mid-sagittal plane traced by the software on a patient's cone-beam computed tomography 3-dimensional reconstruction. A. Axial orientation. B. Sagittal orientation.

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  Fig. 3 Different views of the “Dist. to midline” function used to measure the distance from the right and left zygomatic suture to the mid-sagittal plane on cone-beam computed tomography. Note how the orientation of the skull in the 3-dimensional environment could lead to errors in the location of landmarks. Changes in the orientation help to locate the landmarks in the correct 3-dimensional position. A. Frontal orientation. B. Oblique orientation.

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  Fig. 4 Comparison of the inter-zygomatic distance measured on cone-beam computed tomography (CBCT) to values measured on PA cephalograms, stratified by operator. PA Zr-Zl: inter-zygomatic distance measured on posterior-anterior cephalograms, CBCT Zr-Zl: inter-zygomatic distance on CBCT, measured according the formula: Zr-MSP distance+Zl-MSP distance (see text). A. Operator 1. B. Operator 2.

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  Fig. 5 Raw data of right-side and left-side cone-beam computed tomography (CBCT) measurements compared with half of the inter-zygomatic distance measured on PA cephalograms. 2D.mid=distance between the 2D mid-plane and the Zr/Zl landmarks on posterior-anterior cephalograms, given by the formula 2D Zr-Zl/2; Zr-MSP=CBCT Zr-mid-sagittal plane (MSP) distance; Zl-MSP=CBCT Zl-MSP. A. Operator 1. B. Operator 2.

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  Fig. 6 Differences between measurements in millimetres, stratified by operator. A. Operator 1. B. Operator 2.

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  Fig. 7 Box-plots of the absolute differences show the distributions of the 3 sets of measurements assessed in this study stratified by the operator. Extreme values were measured in patient No. 13, who was considered to be an outlier and represented by dots. Most of the differences between measurements are above 0 for the total inter-zygomatic measurements and the right-side measurements, with the lower and the higher side of the box representing the first and the third quartiles of the distribution. The thick black line represents the median value, which is close to 0, indicating that the left-side measurements were more accurate than the right-side measurements.

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  Fig. 8 Three-dimensional axial orientation of patient No. 13 showing how, given the distortion of his skull base, the mid-sagittal plane was not suitable for dividing the skull into 2 perfectly symmetric anatomical halves: measurements of this patient were considered to be outliers in the statistical analysis.

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  Fig. 9 Percentage difference between cone-beam computed tomography measurements and the posterior-anterior cephalogram measurements, stratified by operator (see text for the formula). A. Inter-zygomatic distance. B. Left-side measurements. C. Right-side measurements.

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