Panoramic radiography is currently the most common imaging method used by dentists and facial surgeons. It plays a valuable role in the diagnosis of various facial, especially mandibular and dental, conditions, such as cysts, tumors, missing or supernumerary teeth, infectious dental lesions, or bony asymmetry, such as condyle hyperplasia [7]. Furthermore, it is easily available and exposes patients to a low level of radiation [8]. Despite these numerous advantages, multiple limitations of panoramic radiography have been described in the literature. Several studies have assessed the performance of panoramic radiography in the diagnosis of facial asymmetry by comparing different vertical and horizontal distances measured on dry skulls and panoramic radiography performed on those same skulls [7–13]. Turp et al. [9] compared three vertical measurements (condyle height, ramus height, and condyle plus ramus height) on 25 dry skulls and the panoramic radiographs of the same skulls. The correlation between radiologic and direct measurements for each parameter was low. The following factors were described as possibly involved in the low accuracy of diagnosis: the distortion of mandibular structures, especially posterior ones such as the condyle and the coronoid process, due to the positioning of these structures diagonally rather than perpendicularly to the X-ray beam [8]. This phenomenon was also explained by the imaging of an entire 3D horseshoe structure on a 2D film. Patient head positioning has also been described as an important factor influencing precision. The manufacturer’s listed magnification might be higher or lower on the radiograph owing to that position [10]. Nevertheless, it should be noted that the use of ratios, such as that obtained using the Levandoski method described above, moderates the effect of differential magnification, with the coronoid process and condyle being magnified to a similar degree [10]. The superposition of bone structures can also make measurement difficult.
Using a measurement method on a panoramic radiograph can be challenging. For consultations, panoramic radiographs are often printed on paper or X-ray film. Measurement using the Levandoski method described above is therefore performed manually. The results of this measurement can vary considerably from one examinator to another. In addition, the Levandoski measurement method, which to our knowledge is the only measurement method on panoramic radiography described in the diagnosis of CPH, has never been standardized in any software that could help to reduce the inter-examinator variation. Furthermore, this measurement method is based on imprecise landmarks. The positions of the Kr’ and Cd’ points, which are respectively located on the coronoid and condyle tip, can vary a lot, especially on a round-ended bony structure, the top of which is not an exact point but rather a curved line [14]. The same goes for the Go’ point that can be positioned anywhere on the lower border of the corpus of the mandible, posteriorly to the dental region. In addition, the superposition of bone structures can increase inaccuracy. In this work, we experienced this with the superposition of the zygomatic bone and the coronoid process hiding the coronoid top tip, leading to difficulties in determining the position of the Kr’ point.
Considering the limitations in the use of panoramic radiography as a reliable diagnostic method, authors have attempted to find a more precise way to diagnose CPH and, by extension, mandibular asymmetries. CT with 3D reconstruction has become increasingly available for dentists and facial surgeons. In recent years, it has been shown that a lower radiation dose could be as efficient as a higher radiation dose [15], which helps avoid the problem associated with radiation doses considered as limiting factors to the wider use of CT. Another advantage of 3D CT is that it permits the visualization of both hard and soft tissues simultaneously and that of craniofacial bones from different angles on rotating the 3D image [16]. 3D CT scans brings no magnification factor, which is one of the main limiting factors in panoramic radiography [17]. Inner craniofacial structures can also be observed. For the diagnosis of CPH, 3D viewing of the coronoid process and the zygomatic bone can help visualize the relationship between the two bone structures, and therefore, it can be an interesting tool for application in the preparations for surgical procedures. Despite these advantages, strong research-based evidence pertaining to the assessment of landmarks and measurement performance of 3D CT is lacking [18]. Smektala et al. conducted a systematic review of the literature to assess the accuracy of linear and angular measurements on 3D CT [19]. The researchers selected articles that compared landmark positioning and linear and angular measurements on dry skulls and 3D CT scans of the same skulls. The results of the review showed that although not always statistically significant, differences can be found between measurements on dry skulls and 3D CT scans. The limiting factors that could explain these differences are the nature and quality of the 3D image and the lack of a standardized selection of landmarks that could be precisely repeated and allow precise linear and angular measurements. The different measurement methods used for 3D CT scans also depend, apart from the coronoid length, on the rest of the mandibular anatomy that can change the coronoid or condyle length in a purely mathematical way.
In our study, we found a difference between the mean length ratios obtained using the Levandoski method and the scanographic measurement methods. The Levandoski measurement method tended to underestimate the length ratio, emphasizing the importance of using a scanographic measurement method at the slightest doubt in order to confirm the diagnosis of CPH. However, there is a statistical bias related to the use of multiple t-tests that would increase the risk of a Type 1 error [20]. Further studies with strong levels of proof are necessary.