Reliability and Reproducibility of CBCT Assessment of craniomaxillary Changes Before and After Treatment for Class III Growing Patients – An convenient and intuitively Way for Evaluation

doi:10.21203/rs.3.rs-4890919/v1

Objectives

To evaluate reliability and reproducibility of 3-dimensional (3D) assessment of maxillary protraction treatment using voxel-based superimposition of cone-beam computed tomography (CBCT) models of the anterior cranial base in growing patients with skeletal class III malocclusion.

Methods

CBCT scans were performed before and after maxillary protraction treatment for Class III malocclusion. Three observers independently constructed 162(27*2*3) 3D virtual models from CBCT scans, which had been reoriented 3D models before treatment to natural head posture, of 27 patients in software. The anterior cranial base was used to register the 3D models pre- and port- treatment. Three observers independently identified 9 landmarks(Including those in the contralateral side)and recorded in three-dimensional coordinates in the 3D models. Each observers performed this three times on the pre- and post-treatment model. The mean value of the 3 sets of coordinates at different times was taken as the coordinates for each landmark. The intraobserver reliability and inter-observer reproducibility of the method for craniomaxillary changes were analyzed.

Results

The ICCs was > 0.90 for 25 (92.6%) out of the total 27 intraobserver assessments. The precision of the measurement method was within 0.3 mm in 21 (77.8%) cases. The interobserver reproducibility errors were < 0.3 mm in 21 of the 27 cases (77.8%).

Conclusions

The reliability and reproducibility of the method for assessment of maxillary protraction treatment in growing patients with skeletal Class Ⅲ malocclusion were judged to be excellent.

Skeletal Class III malocclusions are the abnormal relationship between maxilla and mandible due to uneven growth of the two bony structures. About 42–63% of patients belong to the maxillary hypoplasia with the mandible being of normal size or slight prognathic type¹. Maxillary protraction is recommended for such skeletal Class III patients with maxillary deficiency.

Traditionally the cephalograms pre-and post- treatment were compared by superimposing stable structures such as the anterior cranial base, to evaluate the effect of maxillary protraction and maxillofacial changes^2,3. However, the simple sagittal assessments using 2-dimensional (2D) lateral cephalogram are prone to errors where accurate determination of some landmarks is concerned, because of failure to the anatomic structures⁴ there. Also,as patients with skeletal malocclusion often are associated with facial asymmetry⁵, 2D assessment cannot be accurate or reliable. With the advent of imaging technology, cone-beam computed tomography (CBCT) has been preferred as the method of choice for maxillofacial imaging,and as a valuable tool for orthodontic treatment planning and clinical research. CBCT would provide a comprehensive analysis of any three-dimensional (3D) changes in the size, shape and position of the maxillofacial structures⁶ .In particular, it affords a more accurate assessment of the labiopalatal direction and the contact relationship between adjacent teeth ,when compared to 2D radiography⁷. Superimposition of virtual models is necessary for 3D measurements, a process that is much more complex than with 2D images. Identification of landmarks the first step, which is also prone to errors. Despite the difficulty, some researchers have made progress and achieved good reliability^8–11. For instance, Lagravère and coworkers¹² put forward an ELSA standardized coordinate system based on 4 landmarks: (i) midpoint between the geometric centers of the foramina spinosum; (ii & iii) the right (rSLEAM) and left superior-lateral border of the external auditory meatus (lSLEAM); and (iv) the mid-dorsum foramen magnum (MDFM). However, because the skull base is situated far away from the face, any discrepancy are amplified when the craniomaxillary features were mapped to the coordinate system. Later, the error is the 3D superposition measurements will be coamplified,which can become clinically significant.

Studying the growth of the craniomaxillary structure relies on measurements made to stable locations in the skull, such as that anterior cranial base¹³. The cribriform ethmoid plate growth ceased by the age of 2.That is, the pubertal growth spur has much less pronounced effect on the anterior cranial base¹⁴. With different methodologies (longitudinal cephalometry¹⁵, histology¹⁴and dry-skull measurements¹⁶), three groups of researchers reported that the sphenoethmoid synchondrosis growth stand-by age 7. Therefore, the pre-sphenoid region (the plane surface on the sphenoid bone, in front of sella turcica) ,which is considered stable after 7 years of age¹⁷, were noted as stable in craniomaxillary region of growing individuals. They may then be used as stable reference, as well as treatment changes.

Voxel-based registration of changes with 3D CBCT images has been used in the past few years to evaluate overall facial changes relative to the cranial base¹⁸. But then, regional craniomaxillary registrations were still controversial. This present study, aimed to evaluate the 3-dimensional craniomaxillary changes in growing patients, who had received orthodontic treatment for Class III malocclusion, and to examine the intraobserver reliability and inter-observer reproducibility of a voxel-based superimposition method.

Corresponding pairs of pre- and post-treatment CBCT scans of 27 children, aged 8–11 years before treatment, who had received protraction therapy for their Class III malocclusion at the Department of Orthodontics of the University of Hong Kong-Shenzhen Hospital were included in this study. All patients or their parents gave consent to participate in this study, after hearing a thorough explanation of the research objectives and procedures. The two scans were taken on average 24 to 26 months apart. The scan was included in the analysis based on : cervical vertebral maturation (CS1-CS3); clear and legible 3D images; Class III malocclusion The Exclusion criteria were: discernible craniofacial asymmetry; discernible mandibular asymmetry; temporomandibular joint disorders; history of maxillofacial trauma or surgery in the region; some systemic disease; and previous orthodontic treatment. The study protocol was approved by the Medical Ethics Commission of the University of Hong Kong-Shenzhen Hospital.

The parameters of the CBCT machine (Dental Volumetric Tomograph, KaVo 3D eXam, Imaging Sciences International LLC, Hatfield, PA, USA) were set at 120 kVp, 18.54mA, 23 x 17 cm field of view, 0.3 mm voxel size, and scan time of 8.9 s. Data were exported in DICOM format into a 3D imaging software (InvivoDental software 5.1.3, Anatomage Inc, San Jose, USA), in which a virtual model of the patient was created in a 3D coordinate system. The position and orientation of the 3D model in the coordinate system depended on the head position of the patient when the CBCT scanned. In the "section" module, using axial, coronal, and sagittal views on the left side, the 3D virtual model on the right side was oriented: in the lateral view, bilateral structures, such as the orbits, external auditory canals and other structures as much as possible overlap, and the Frank-fort horizontal plane was oriented horizontally^{Figure 1}.

The software offered a “superimposition” module to achieve fully automated voxel-based superimposition of the pre- and post-treatment scan, some stable anatomical area had to be defined first. This was done by clicking on the "voxel registration" button, which called up 3 boxes in the sagittal, coronal and axial view for the observer to select the stable region to guide the superimposition. The boxes were used to select the anatomical structures¹⁹ of the anterior cranial base in the 3D models ^{Figure 2}. After clicking on the "start" button and the automatic voxel-based superimposition of the 3D images began. Stable reference structure that guided the superimposition.The whole process of registration and superimposition took approximately 3 minutes.

A total of 9 landmarks (or 6 in total, if the left and right were considered as contributing to one landmark) and defined criteria were established for each landmark. A total of 9 landmarks were located by each observer independently on every surface-rendered 3D model ^{Table1 and Figure3}. To identify the landmarks, the observer would navigate to any (sagittal, coronal or axial) view and select the most appropriate slice in that view for registration. The three-dimensional coordinates of each landmark were then automatically recorded by the software. For those landmarks that might be difficult to define in a special view, the observer could call up the surface-rendered model to assist with the localization ^{Figure 4}.

Three observers (an orthodontist, an orthodontics master, and a dental radiologist) were trained and calibrated to complete the entire superimposing measurement process using a set of 30 CBCT scans not included in this study. Then, the 3 observers worked independently throughout, to define the 9 anatomical landmarks^Table1 in the superimposed CBCT volume (from pre- and post treatment for the same patient), as described above.The X, Y and Z coordinate of each landmark was recorded, from the sagittal, coronal, and axial views respectively. Then the values were exported into a spreadsheet (Microsoft Excel, Microsoft Corporation, Redmond, WA, USA) for analysis later. Each observer repeated the superimposing measurement 3 times at intervals of 5 days, yielding 90 sets by each observer.

Statistical analyses in a statistical software package (SPSS version 21.0; SPSS, Chicago. IL, USA) were done. The first was to calculate the difference in the value of the coordinates (i.e. changes arising from treatment and growth) for all landmarks that was done by subtracting the pre- and post-treatment coordinate values of that landmark. The second was to estimatehe intraobserver agreement by computing the Intraclass correlation coefficients (ICCs). The distribution and variance of the data were evaluated using the Shapiro-Wilk normality test and Levene’s homogeneity test. T-test and Mann-Whitney U-test were used for inter-observer comparisons, and paired t-test was used for intraobserver reliability assessments.

Voxel-based superimposition visually demonstrated, the morphological skeletal changes of the craniomaxillary region for these Class III patients after protraction therapy:

The intraobserver agreement, as was estimated by the ICCs for the coordinate difference of each landmark, was listed in Table 2. Generally, the results indicated excellent reliability for intraobserver assessments, with ICC > 0.90 for 25 (92.6%) of intraobserver assessments.

Regarding the ICCs for the coordinate difference ,low ICC scores were obtained for 2 bilateral landmarks indicating a relatively low reliability.They were the X coordinate of the right and left zygomatic suture, and the Z coordinate of the right and left pyriform aperture.

Table 3 showed the frequency counts of the difference in mean values of the coordinate (i.e. difference arising from treatment and growth for each landmark). This was used to assess the reproducibility of this measurement method. The precision of the measurement was better than 0.3 mm in 21 cases (77.8%). For the difference in mean values for the X, Y, and Z coordinates used to estimate interobserver reproducibility, similar ICC results were obtained.The interobserver reproducibility errors were < 0.3 mm in 21 of the 27 cases; only 6 cases (22.2%) showed error ≥ 0.3mm.

Melsen¹⁵ confirmed that by the age of 7, the growth of sphenoethmoidal and sphenofrontal suture of the anterior cranial base usually stops. The authors also pointed out that after 5 years of age, changes in sella turcica were most likely, to some degree, due to resorptive activity in the lower half of the posterior wall and the floor of the sella turcica. On the other hand, the anterior part of the sella turcica was the most stable, and resting (inactive) bone was observed in almost all subjects. The brain almost stops growing at 7–8 years of age, after which the anterior cranial base continues to grow and contributes to facial development. That growth occurs almost entirely due to increased pneumatization of the frontal and ethmoid bones²⁰. This present study used the voxel based superimposition of CBCT data of the anterior cranial base in growing patients, to examine the coordinate difference in position between the pre- and post- treatment for skeletal class III malocclusion. Our patients’ age was 8–11, and hence choice of stable reference was approriate. The 3D analysis was conducted in 5 steps: model construction, model reorientation, voxel-based superimposition, landmark definition, and quantitative measurement. The coordinate system were automatically created by the software while the models were constructed. Although the coordinates of various structures on the reconstructed models might differ, that difference could usually be resolved by reorienting the models, that is, the coordinates of the pre- and post- treatment landmarks were very little affected. The superimposition methods were fully automated, with voxel-wise rigid registration of the anterior cranial base structures that have completed growth and did not change further with age^21–23. The bilateral structural landmarks were involved for this study.: the nasion (N), the anterior nasal spine (ANS), A point (A), the right upper incisal alveolar ridge (rUIAR), the left upper incisal alveolar ridge (lUIAR), the right pyriform aperture (rPA), the left pyriform aperture (lPA), the right zygomatic suture (rZS), and the left zygomatic suture (lZS). The N, rPA, lPA, rZS, and lZS together reflected the changes in the upper part of the maxilloface. The ANS, A, rUIAR, and lUIAR together reflected the changes in the lower part of the maxilloface. In additional, the rUIAR and lUIAR reflected the changes in the premaxilla alveolar.

Arising from the data handing process of this research, errors might occur during the following 4 steps: a) model reorientation, b) the voxel-based superimposition, c)localization(identification) of landmarks. First, even if the head for CBCT scans were consistently positioned according to the protocol, scan data with slight variations in head position would still occur due to such factors as varying body positions and neck curvatures. This might lead to inter-observer difference for cranial base registration.In this research, proper automated voxel registration need these two CBCT scanings approximated as far as possible, Two CBCTS with significant differences cannot be performed voxel registrationand. And different operators, using the same patient scans, the same references for registration, same software may or may not achieve proper registration. However, as the subjects’ craniofacial characteristics were basically symmetrical, the method error could be minimized by model reorientation. Second, the voxel-based superimposition was semi-automated—the observers selected the area and then the computer automatically superimposed.The error in the voxel-based superimposition depended on the area selected by the observers²⁴. If in the first superimposition the region selected on the pre- and post- treatment model was not exactly the same, error would creep into the superimposition results. To overcome this, we continued the selection until the superimposition results were consistent. Third, there were 2 sources of error during landmark identification in this study: a) While it was relatively easy to identify the landmarks in the 3D virtual surface models, it could be difficult to select the best slice or region to localize the selected landmarks²⁵; b) The 3 spatial planes are interrelated. Adjusting the slice in one plane will result in movement of the reference line in another plane. Therefore, some experience on the part of the observers was essential, for which training using, 30 extra sets of scans was done beforehand for the observers. In the present study, although the 3 observers had different working backgrounds, that seemed to have little impact on the amount of errors in measurement. Training and calibration of the observers or assessors cannot be overemphasized in any studies. Other factors related to the precision and reproducibility of the 3D measurements would need to be further investigated, such as the voxel size, scanning time, and scanning range²⁶. In this study, the voxel size used was 0.3 mm, whereas some other CBCT studies used a voxel size of 0.5 mm and up to 3 mm. Recent studies^27,28 indicated that the smaller the voxels, the better the measurement accuracy, and the smaller the measurement error.

Inspite of the general reliability of the coordinate difference shown at Table 2a for the right and left zygomatic suture, overall the results gained better intraobserver reliability and inter-observer reproducibility of this method. The less-than-perfect reproducibility maybe due to the ambiguous definition criteria, including choosiing inconsistently the best perspective and slice. In addition, Table 2b showed that the reliability of the Z coordinate definition was inferior to the reliability of the X, Y coordinates definition, which can be related to that some landmarks are poorly recognizable in the Z coordinate. Therefore, reproducibility of the landmark is related to its characteristics. The choice of landmarks has an impact on the reliability and reproducibility of measurements²⁹.

In this study, we proposed a new 3D quantitative measurement method for the assessment of maxillary protraction treatment in skeletal Class III malocclusion. An orthodontist could spend about 5 minutes to overlap the pre- and post- treatment CBCT images, to demonstrate visually the therapeutic effect to patients and their parents³⁰. Orthodontists could intuitively explain to patients the location and extent of treatment changes, for better patient's recognition and satisfaction. The landmarks selected for the study largely represented changes in the maxillofacial changes. Table 3 showed the ICCs was > 0.90 for 25 (92.6%) of the intraobserver assessments. The precision of the measurement method was < 0.3 mm in 21(77.8%) cases. Table 3 showed the interobserver reproducibility errors were < 0.3 mm in 21 of the 27 cases. Overall, the reliability and reproducibility of the method were excellent. Other new landmarks need to be proposed and tested to determine whether this method can be used for other growth or treatment assessment.

Overall, an excellent intraobserver reliability and interobserver reproducibility for the assessment of craniomaxillary changes is possible using voxel-based superimposition of CBCT models when measurement of the anterior cranial base of growing patients with skeletal class III malocclusion patients is concerened. And it was also convenient and intuitively to explain the therapeutic process and improvement in craniomaxillary positions to the patient after maxillary protraction .

Ethics approval and consent to participate

All experiments were performed in accordance with relevant guidelines and regulations (Declarations of Helsinki).

Study protocol was approved by the Medical Ethical Commission of the University of Hong Kong-Shenzhen Hospital.

The informed consents from all subjects and/or their legal guardian(s) have been obtained for study participation.

Consent for publication

The informed consents from all subjects and/or their legal guardian(s) have been obtained for publication of identifying information/images in an online open-access publication.

Availability of data and materials

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Competing interests:

The authors declare that they have no Competing interest.

Funding:

This work was supported by grants from the Program of Key Science and Technology Research, Health Commission of Hebei Province (Project No. 20191074).

Clinical trial number:

Not applicable.

Authors' contributions

XiaoYing Hu ^1*made substantial contributions to design of the work,and the acquisition, analysis and interpretation of data, and revise the manuscript.

GaryShunPanCheung² contributed to the conception and analysis of data, and revise the manuscript.

YiYang Zhang³ involved in the interpretingand drafting the manuscript.

RuoNan Sun ⁴ drafted and performed the work and drafted the manuscript.

FuSheng Dong ⁵ have made substantial contributions to the conception, design of the experimental work and analysis of data.

Acknowledgements

None

Authors' information (optional)

XiaoYing Hu¹^*,orthodontist. There are about 6000 outpatient visits per year. She have finished the correction of over 1000 cases with Class III Malocclusion.During the management of these patients,she noted the deleterious effect of bad tongue habit on the development of Class III Malocclusion.The bad tongue habit alsocaused narrowing of the upper airway even in deciduous period of about 1-2years of age. So she consulted two experts to investigate the topics. The data show the craniofacial and upper airway characteristics with Class III malocclusion in 4 to5year-old children, and reflect the outcome of the early intervention quatitatively She also wish obtained to find a way to decrease the recurrence of Class III malocclusion.

Two experts：

Gary Shun Pan Cheung.²:formerly of the Faculty of Dentistry, The University of Hong Kong, Hong Kong. E-mail:[email protected]
FuSheng Dong ⁵: Key Laboratory, College of Stomatology, Hebei Medical University, Hebei 50017, China. E-mail: [email protected]

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Table 1. Landmarks selected for the study.

*Landmark name*	*Anatomic region*	*Lateral view*	*Axial view*	*Anteroposterior view*
1.Nasion (N)	Frontonasal suture	Anterior-most point	Middle-anterior–most point on the anterior contour	Middle point
2.Anterior nasal spine (ANS)	Median, sharp bony process of the maxilla	Point on the tip	Anterior-most point	Middle point in the anteroposterior slice determined by the lateral and axial view
3. A point (A)	Premaxilla	Posterior-most point on the curve of the maxilla between the anterior nasal spine and supradentale	Middle-anterior–most point on the tip of the premaxilla	Middle point in the anteroposterior slice determined by the lateral and axial views
4. Right upper incisal alveolar ridge(rUIAR)	Premaxilla alveolar	Anterior-inferior–most point	Anterior-most point	Middle point in the anteroposterior slice determined by the lateral and axial view
5.Left upper incisal alveolar ridge(lUIAR)	Premaxilla alveolar	Anterior-inferior–most point	Anterior-most point	Middle point in the anteroposterior slice determined by the lateral and axial view
6.Right pyriform aperture(rPA)	pyriform aperture	Anterior-most point in the Lateral slice determined by the Anteroposterior view and axial view	Anterior-most point	Lateral–most poin
7.Left pyriform aperture(lPA)	pyriform aperture	Anterior-most point in the Lateral slice determined by the Anteroposterior view and axial view	Anterior-most point	Lateral–most poin
8.Right Zygomatic suture (rZS)	Zygomaticomaxillary suture	Anterior-inferior–most point	Anterior-most point	Lateral-inferior–most point
9.Left Zygomatic suture (lZS)	Zygomaticomaxillary suture	Anterior-inferior–most point	Anterior-most point	Lateral-inferior–most point

Table 2a. Assessment of intraobserver reliability: (a)Intraclass correlation Coefficient(ICC) and confidence interval for repeated measurements (from the triplicate evaluation).

*Landmark*	*Intraclass Correlation*			*95% Confidence Interval*
*Landmark*	X	Y	Z	X	Y	Z
Nasion (N)	0.96	0.99	0.97	0.89-0.99	0.94-1.00	0.91-1.00
Anterior nasal spine (ANS)	0.96	0.98	0.98	0.90-0.99	0.92-1.00	0.92-1.00
A point (A)	0.97	0.92	0.99	0.92-0.99	0.81-0.96	0.95-1.00
Right upper incisal alveolar ridge(rUIAR)	0.95	0.98	0.97	0.86-0.98	0.90-1.00	0.90-1.00
Left upper incisal alveolar ridge(lUIAR)	0.94	0.98	0.98	0.85-0.98	0.92-1.00	0.92-1.00
Right pyriform aperture(rPA)	0.95	0.97	0.78	0.85-0.99	0.90-0.99	0.62-0.84
Left pyriform aperture(lPA)	0.96	0.97	0.81	0.89-0.99	0.91-1.00	0.74-0.89
Right Zygomatic suture (rZS)	0.91	0.96	0.94	0.80-0.95	0.89-0.99	0.85-0.98
Left Zygomatic suture (lZS)	0.90	0.95	0.94	0.80-0.94	0.86-0.99	0.83-0.98

Table 2b. Assessment of intraobserver reliability: (b)ICC values of changes in madibular measurements difference in X, Y, and Z coordinates.

Range	Coordinate difference
	X		Y		Z		Total
	n	(%)	n	(%)	n	(%)	n	(%)
ICC≥0.95	6	（66.7）	8	（88.9）	5	（55.6）	19	（70.4）
0.90≤ICC＜0.95	3	（33.3）	1	（11.1）	2	（22.2）	6	（22.2）
0.85≤ICC＜0.90	0	0	0	0	0	0	0	0
0.80≤ICC＜0.85	0	0	0	0	1	（11.1）	1	（3.7）
ICC＜0.80	0	0	0	0	1	（11.1）	1	（3.7）
Total	9	(100.0)	9	(100.0)	9	(100.0)	27	(100.0)

Table 3 Frequency of the difference in mean values for difference among measurement (mm) and for reproducibility of the measurement method (mm)

	Among measurement								Reproducibility of the measurement method
Range (mm)	Coordinate difference								Coordinate difference
	X		Y		Z		Total		X		Y		Z		Total
	n	(%)	n	(%)	n	(%)	n	(%)	n	(%)	n	(%)	n	(%)	n	(%)
≥0.3	2	-22.2	1	-11.1	3	-33.3	6	-22.2	2	-22.2	1	-11.1	3	-33.3	6	-22.2
0.15≤x＜0.3	7	-77.8	7	-77.7	6	-66.7	20	-74.1	7	-77.8	7	-77.7	6	-66.7	20	-74.1
≤0.15	0	0	1	-11.1	0	0	1	-3.7	0	0	1	-11.1	0	0	1	-3.7
Total	9	-100	9	-100	9	-100	27	-100	9	-100	9	-100	9	-100	27	-100

No competing interests reported.

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Reliability and Reproducibility of CBCT Assessment of craniomaxillary Changes Before and After Treatment for Class III Growing Patients – An convenient and intuitively Way for Evaluation

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