Preliminary results were determined for a database on 3-dimensional (3D) cephalometrics using McNamara’s analysis in an adult southern Chinese population based on cone-beam computerized tomography (CBCT). 3D dentoskeletal morphology was assessed from CBCTs from 80 (39 males; 41 females; 21–30 years) consecutive adult southern Chinese without gross craniofacial deformity or asymmetry, adopting 16 variables from McNamara’s cephalometric method. For variables in relation to maxilla to cranial base, mandible to cranial base and dentition, there were no significant differences between males and females. For variables in relation to mandible to maxilla, 8 of 11 showed significant differences between males and females: Cd(L)-Gn (♂: 127.65 mm; ♀: 119.56 mm, P < 0.01), Cd(R)-Gn (♂: 127.85 mm; ♀: 119.94 mm, P < 0.01), Cd(L)-A (♂: 99.38 mm; ♀: 94.18 mm, P < 0.01), Cd(R)-A (♂: 93.93 mm; ♀: 94.99 mm, P < 0.01), MxMD-DF(L) (♂: 28.26 mm; ♀: 25.40 mm, P < 0.05), MxMD-DF(R) (♂: 27.74 mm; ♀: 24.02 mm, P < 0.05), ANS-Me (♂: 71.09 mm; ♀: 65.84 mm, P < 0.01), and MD-P(L) (♂: 22.85°; ♀: 25.25°, P < 0.05). The method errors did not exceed 0.5 mm for any variables. A preliminary CBCT cephalometric database of the population was created. The significant sexual differences in the 3D McNamara’s analysis indicate that gender specific data should be made available. The sample size should be increased to create a more representative database.
In oral and maxillofacial surgery, orthodontics and dentofacial orthopaedics, it is important to base the diagnosis and treatment on cephalometric representative reference values obtained from the patient’s population . Population norms for various two-dimensional (2D) cephalometric methods have been published for southern Chinese populations , but no comprehensive database for three-dimensional (3D) images has been published. The aim of this study is to create a preliminary 3D database in a southern Chinese population based on the cephalometric method designed by M cNamara .
Materials and methods
Cone-beam computerized tomography (CBCT) data from 80 (39 males and 41 females) southern Chinese in Hong Kong SAR, China, aged 21–30 years were studied and the cephalometric measurements of their jaw dimensions evaluated. ‘Southern Chinese’ are defined as those Chinese whose ancestors originated from provinces south of the Yangtze River and who speak in different dialects from the northern Chinese. The images were retrieved retrospectively from the database of a hospital oral radiology unit, because of the retrospective nature of the study and because individual records were not identified, clinical ethical approval was not required.
The CBCTs of patients with gross craniofacial deformity or asymmetry were excluded from the investigation. Only CBCT images covering the following three dimensions were included in the study: from whole orbit to whole mandible (supero-inferior), both external auditory meatus (left-right), and facial surface of incisors to cervical spine (antero-posterior).
CBCT images were acquired using the i-CAT Classic system (Imaging Sciences International, Hatfield, PA, USA). The patients were positioned according to the manufacturer’s instructions, with centric occluded and lips closed. Lateral scout radiographs were made and they were used to check the structures defining the three dimensions included. Patients were scanned using the following protocol. The protocol was a double 360° rotation, 40-s scan, comprising 549 projections (voxel size of 0.4 mm) with a 15.0 cm (diameter) by 22 cm (height) field of view. The protocols were acquired using XoranCat acquisition software (version 3.1.77, Xoran Technologies, Ann Arbour, USA). Exposure parameters were controlled by automatic exposure control.
The CBCT data were exported from the XoranCat software in DICOM multi-file format and imported into SimPlant OMS Pro Standalone 12.03 (Materialise N.V., Leuven, Belgium). The program was run on an IBM-compatible laptop computer using MS Windows Vista.
The whole mandible was segmented from the skull and the cervical spine was removed to facilitate the measurements in this study. The first step in segmentation was applying thresholds of grey scales relevant to Hounsfield Units from 500 to 3071, which represented bone intensity in CT images in general. Applying this threshold range to CBCT dataset could minimize the background noise at the vertex and the occlusion that degrades the quality of the 3D model. A region growing technique was then used to create two masks that contained the mandible and the maxilla with the skull. The cervical spine was automatically excluded from the masks in this step. The mask that contained the mandible was edited manually by adjusting the thresholds to ensure that the condyles (which usually have lower Hounsfield Units than the rest of the mandible) were included. A 3D image was generated from the masks.
The images were then reorientated, if necessary, to ensure the midsagittal plane was perpendicular to the axial view of the images, whilst the Frankfurt plane was perpendicular and parallel to the coronal and sagittal views, respectively. The 3D image was free to rotate in any dimension in order to verify the positioned landmarks of McNamara’s analysis.
The landmarks were identified using the 3D cephalometry module of the software. 22 landmarks were included in this study. 18 landmarks were identified on the 3D image by a single click using a cursor driven by the mouse. Figure 1 shows landmarks identified on the 3D image in the antero-posterior position. Some structures, such as condylion left, condylion right and the facial surface of the lower incisor, were identified only when the 3D model of the mandible was displayed ( Fig. 2 ). Two landmarks (left and right pterygomaxillary fissures) were identified on the CBCT images in the axial views, whilst the sella turcica and sella turcica posterior midpoint were identified on CBCT images in sagittal views. 3D measurements, such as distances (in millimetres) and angulations, in McNamara’s analysis were calculated automatically by the software. Each measurement was saved for future reference during landmark relocation and measurement error calculation.
The landmark identification followed the work of S wennen . The standard cephalometric landmarks, pogonion (Pg) and point A (A), lying on the mid-sagittal plane, were used to define the anterior limit of the mandible and maxilla, respectively. Modified left and right condylions [Cd(L); Cd(R)], which are the most posterio-superior points of the left and right condyles, were used instead of the conventional condylion (Co) because of ease of location using the 3D images and supplementary tomographic images simultaneously. These points define the posterior limits of the mandible. The landmark identification was calibrated between investigators and only one investigator was involved in landmark identification in the study. By locating these landmarks on a 3D model, the dimensions of the McNamara’s analysis listed in Table 1 were measured.
|A. Maxilla to cranial base|
|1. NA-P perpendicular (nasion perpendicular to point A)||A vertical plane, nasion perpendicular, is constructed perpendicular to the Frankfurt plane and sagittal plane, and extended inferiorly from the nasion. Frankfurt plane is defined by lower border of the left [Or(L)] and right [Or(R)] orbits and the most superior point of the left or right external acoustic meatus, known as porion left [Po(L)] or porion right [Po(R)]. Sagittal plane is defined by point nasion (N), point sella turica (S) and point anterior nasal spine (ANS). The distance is measured from point A to the nasion perpendicular.|
|2. SNA||The angle between the line joining S point and N point and the line joining N point and A point.|
|B. Mandible to maxilla|
|3. Cd(L)-Gn and Cd(R)-Gn (Effective mandibular length)||A distance is measured from the left condylion [Co(L)] and right condylion [Co(R)] to the gnathion, respectively.|
|4. Cd(L)-A and Cd(R)-A (Effective midface length)||A distance is measured from the Cd(L) and Cd(R) to point A, respectively.|
|5. MxMD-DF left (L) and MxMD-DF right (R) (Maxillomandibular differences)||Effective mandibular length minus effective midface length on both sides.|
|6. ANS-Me (Lower anterior face height)||A line is measured from the anterior nasal spine to the menton.|
|7. MD-P left (L) and MD-P right (R) (Mandibular plane angle)||The angles between the anatomic Frankfurt plane and the left and right mandibular planes. Mandible planes are form by point menton, left gonion right gonion.|
|8. FA-A left (L) and FA-A right (R) (Facial axis angle)||A line is constructed from the basion to the nasion (NBa). A second line (the facial axis) is constructed from the posterosuperior aspect of the left (L) and right (R) pterygomaxillary fissure [PTM(L), PTM(R)] to the gnathion (the lowest point on the lower order of the chin along the midline). The facial axis angle is the angle between the NBa and the facial axis.|
|C. Mandible to cranial base|
|9. Pg-N (Pogonion to nasion perpendicular)||The distance is measured from the pogonion to the nasion perpendicular.|
|10. Ui-A (Upper incisor to point A)||A plane, A perpendicular, is constructed parallel to the nasion perpendicular through point A. The distance is measured from the most anterior surface of the upper incisor to the point A perpendicular.|
|11. Li-APg (Lower incisor to A-Pg line)||The distance is measured form the facial surface of the lower incisor to the A-pogonion line.|
For method error assessment, the cephalometric dimensions of 10 CBCT were measured twice by the same observer after an interval of 2 weeks.
Method errors were calculated by D ahlberg ‘s formula <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='M.E.=∑d2/2n’>M.E.=∑d2/2n−−−−−−−√M.E.=∑d2/2n
M.E. = ∑ d 2 / 2 n
. Where ∑ d 2 is the sum of the squared differences between the two sets of two mean values, and n is the number of double measurements. For statistical analysis, the t test for independent samples was used, and the levels of statistical significance were P < 0.05.
The cephalometric measurements are summarized in Table 2 . There was large individual variation for all variables for both genders. For variables in relation to maxilla to cranial base, mandible to cranial base and dentition, there were no significant differences between males and females.
|Range||95% Confidence interval of the mean||Range||95% Confidence interval of the mean||Sex|
|Maxilla to cranial base|
|Mandible to maxilla|
|Mandible to cranial base|