A comparison between 2D and 3D cephalometry on CBCT scans of human skulls

Abstract

The purpose of this study was to evaluate whether measurements on conventional cephalometric radiographs are comparable with 3D measurements on 3D models of human skulls, derived from cone beam CT (CBCT) data. A CBCT scan and a conventional cephalometric radiograph were made of 40 dry skulls. Standard cephalometric software was used to identify landmarks on both the 2D images and the 3D models. The same operator identified 17 landmarks on the cephalometric radiographs and on the 3D models. All images and 3D models were traced five times with a time-interval of 1 week and the mean value of repeated measurements was used for further statistical analysis. Distances and angles were calculated. Intra-observer reliability was good for all measurements. The reproducibility of the measurements on the conventional cephalometric radiographs was higher compared with the reproducibility of measurements on the 3D models. For a few measurements a clinically relevant difference between measurements on conventional cephalometric radiographs and 3D models was found. Measurements on conventional cephalometric radiographs can differ significantly from measurements on 3D models of the same skull. The authors recommend that 3D tracings for longitudinal research are not used in cases were there are only 2D records from the past.

Conventional cephalometry has been one of the standard diagnostic tools for analysing maxillofacial deformities and orthodontic problems, and evaluating growth and/or treatment changes. Since cone beam CT (CBCT) technology became available its popularity has increased rapidly. This 3D technology gives a realistic representation of the head of the patient and has expanded the diagnostic possibilities, enabling 3D simulation of surgical and orthodontic procedures . For certain types of patients, such as those with craniofacial anomalies, orofacial clefts or orthognathic cases, conventional cephalograms are no longer the optimal diagnostic tool.

Although the radiation dose of a CBCT scan is lower than that of a multi slice CT (MSCT) scan , a CBCT is not suitable for the regular, daily orthodontic patient. To image the full height of a patient’s skull, a CBCT device with a large field of view is required. Radiation doses of such a scan are 3–44 times greater than comparable panoramic examination doses, depending on the CBCT device used . For the patients mentioned above though, CBCT has many benefits. It has been shown that conventional cephalometric radiographs, which may be considered the ‘gold standard’, can be compared with constructed cephalometric radiographs from CBCT scans and thus the latter can be used for longitudinal research. The 3D characteristics are lost, however, because both conventional and constructed cephalograms provide a 2D representation of 3D structures.

New 3D technology is becoming more popular and the number of software programs to analyse 3D data is increasing rapidly, the next step in cephalometry is 3D cephalometry on a 3D radiographic model of the patient’s skull. It is important to know whether classic cephalometry, performed since the early 1930s on 2D cephalometric radiographs, is comparable with measurements on 3D constructed models of the patient’s skull. In longitudinal studies on growth or treatment outcome, it is important to know if data from 2D cephalometric analyses made in the past can be compared with data from 3D-cephalometric analysis, which will be more common in the future. To the authors’ knowledge, there are no studies dealing with the interchangeability of measurements on cephalometric radiographs and 3D measurements on 3D models constructed from CBCT scans. O lszewski et al. reported on cephalometric measurements on 3D models derived from MSCT scans. MSCT has a very high image quality but a tenfold higher radiation dose compared with CBCT. The image quality of the CBCT, especially for soft tissues, is significantly less compared with an MSCT. The aim of this study was to evaluate whether measurements on conventional cephalometric radiographs are comparable with measurements on 3D constructed models of human skulls derived from CBCT scans.

Material and methods

The sample consisted of 40 dry skulls obtained from the collection of the Department of Orthodontics and Oral Biology of the Radboud University Nijmegen Medical Centre. The skulls were selected from a larger sample according to the following criteria: presence of permanent upper and lower incisors; presence of first permanent upper and lower molars; and presence of a reproducible, stable occlusion. The mandible was related to the skull, based on the position of the condyle in the fossa and maximum occlusal interdigitation. The mandibular position was fixed with broad tape from the ipsilateral temporal bone around the horizontal ramus of the mandible to the contralateral temporal bone.

Radiography

Each skull was positioned in the cephalostat (Cranex Tome Ceph, Soredex, Tuusula, Finland) by fixing it between the ear rods. The ear rods were placed in the pori acoustici externi and the Frankfurt Horizontal was placed parallel to the floor. Cephalometric radiographs were taken according to the following radiographic settings. For bigger skulls ( n = 30), the adult settings were chosen: 70 kV, 10 mA, 0.6 s. For smaller skulls ( n = 10), the child settings were chosen: 70 kV, 10 mA, 0.5 s.

The same skulls were placed in the I-cat ® cone beam CT (Imaging Sciences International, Inc. Hatfield, PA, USA), on a foam platform with the Frankfurt Horizontal parallel to the floor. The skulls were placed in the centre of the CBCT scanner using the midline light beam to coincide with the midsagittal plane. The scan was taken for all skulls in the extended height mode (22 cm): 129 kVp, 47.74 mA, 40 s with a resolution of 0.4 voxel.

Cephalometry

The conventional radiographs ( Fig. 1 A ) were digitized with Viewbox ® (dHAL Software, Kifissia, Greece) to identify landmarks and to calculate distances and angles. 3D skull models were constructed ( Fig. 1 B and C) from the CBCT data with Maxilim ® (Medicim, Sint-Niklaas, Belgium). The same software was used to cephalometrically analyse the constructed 3D models.

Fig. 1
Cephalometric radiograph and 3D model of the same skull. (A) Conventional cephalometric radiograph, (B) CBCT-constructed and (C) traced 3D model.

For the cephalometric analysis 17 hard tissue landmarks ( Table 1 ) were identified and 12 (10 angular and 2 linear) widely used cephalometric measurements were used. Table 1 presents the landmarks, lines and planes. Table 2 presents the measurements that were used in the present study.

Table 1
2D and 3D landmarks, lines and planes used in the study.
S Sella Centre of sella turcica
N Nasion Most anterior limit of the frontonasal suture on the frontal bone in the facial midline
A A-point Deepest bony point on the contour of the premaxilla below ANS
B B-point Deepest bony point of the contour of the mandible above pogonion
ANS Anterior nasal spine The tip of the anterior nasal spine
PNS Posterior nasal spine The most posterior point on the bony hard palate
Pog Pogonion Most anterior point of the symphysis of the mandible
Gn Gnathion Most anterior inferior point of the bony chin
Go l Left gonion Most posterior inferior point of the left angle of the mandible
Go r Right gonion Most posterior inferior point of the right angle of the mandible
Is Incision superius The incisal tip of the most anterior upper incisor
UIA Upper incisor apex The root apex of the most prominent upper incisor
Ii Incision inferius Incisal point of the most prominent medial mandibular incisor
LIA Lower incisal apex Root apex of the most prominent lower incisor
NSL Nasion sella line Line from point S to point N
NSP Nasion sella plane Plane constructed by projecting NSL on the medial plane
ML Mandibular line Line between gonion and gnathion
MP Mandibular plane (3D) Plane between left gonion, right gonion and gnathion
NL Palatal line Line from ANS to PNS
NP Palatal plane (3D) Plane constructed by projecting the line through point ANS and PNS on the medial plane
BOP Bisected occlusal plane Line connecting the vertical midpoint between Is and Ii and the mesial contact between the first molars
BOP Bisected occlusal plane (3D) Plane connecting the vertical midpoint between Is and Ii and the mesial contact between the first molars on left side and first molars on the right side

Table 2
Cephalometric variables in this study.
SNA Angle between point S, point N and point A
SNB Angle between point S, point N and point B
ANB Angle between point A, point N and point B
NSL/NL Angle between line SN and line NL
NSP/NP (3D) Angle between NSP and NP
NSL/ML Angle between SN and ML
NSP/MP (3D) Angle between NSP and MP
NL/ML Angle between NL and ML
NP/MP (3D) Angle between NP and MP
ILs/NL or NP Relative inclination of upper incisors to NL or in 3D to NP
ILi/ML or MP Relative inclination of lower incisors to ML or in 3D to MP
Inter incisal angle Angle between the lines through long axis of upper and lower incisors projected on the medial plane
NSL/BOP Angle between line NSL and the BOP
NSP/BOP (3D) Angle between plane SN and the BOP
Is to A-Pog Distance in mm between point Is and the line A-Pog or in 3D the plane constructed from projecting line A-Pog to the medial plane
Ii to A-Pog Distance in mm between point li and the line A-Pog or in 3D the plane constructed from projecting line A-Pog to the medial plane

Statistical analysis

For both the conventional cephalometric radiographs and the CBCT-constructed 3D models, the same operator (OV) marked the landmarks five times, each time with an interval of 1 week. The intra-observer reliability was calculated using the Pearson correlation coefficient for the first and second measurement. The mean value of the five repeated measurements and their variance was used for further statistical analysis. For each measurement, the standard deviation was calculated as the square root of the mean variance. This standard deviation was compared with the standard deviation of the same measurement in the other group. Paired t tests were performed to compare the means of corresponding measurements on the cephalometric radiograph and on the 3D model of the same skull.

Results

Intra-observer reliability for both the conventional cephalometric radiographs and the 3D model is shown in Table 3 . The correlation coefficient between the first and second measurements ranged between 0.69 and 0.98, with an average of 0.91. The standard error for the conventional cephalometric radiographs was significantly smaller for nine measurements out of 12, as compared with the standard error of the measurements on the 3D models. Reproducibility of the measurements on the conventional cephalometric radiographs was higher, compared with the reproducibility of the measurements on the 3D models.

Table 3
Average standard error (SE) for 40 cases, with 95% confidence interval (CI) and intra-observer reliability for first and second measurements. P -value relates to the test for statistically significant difference between the two methods.
Conventional 3D model
Measurement error Measurement error P -value
Reliability SE 95% CI SE Reliability SE 95% CI SE for error
ANB (°) 0.92 0.53 0.47–0.59 0.98 0.27 0.24–0.30 <0.001
SNA (°) 0.96 0.57 0.52–0.64 0.87 1.05 0.93–1.17 <0.001
SNB (°) 0.95 0.45 0.40–0.50 0.84 1.02 0.91–1.14 <0.001
NL/ML (°) 0.97 0.76 0.67–0.84 0.98 0.81 0.72–0.90 0.194
NSL/BOP (°) 0.96 0.83 0.74–0.93 0.91 1.55 1.37–1.72 <0.001
NSL/ML (°) 0.95 0.66 0.58–0.73 0.87 1.12 1.00–1.25 <0.001
NSL/NL (°) 0.97 0.51 0.45–0.57 0.82 1.09 0.97–1.22 <0.001
ILi to ML (°) 0.93 2.62 2.33–2.91 0.69 3.82 3.39–4.24 <0.001
ILs to NL (°) 0.94 1.62 1.44–1.80 0.93 1.95 1.73–2.17 0.005
Interincisal angle (°) 0.95 2.96 2.63–3.30 0.90 4.42 3.93–4.91 <0.001
Is to A-Pog (mm) 0.98 0.33 0.29–0.37 0.97 0.36 0.32–0.41 0.086
Ii to A-Pog (mm) 0.97 0.52 0.46–0.57 0.73 0.95 0.84–1.05 <0.001
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Feb 8, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on A comparison between 2D and 3D cephalometry on CBCT scans of human skulls
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