Radiographic and diagnostic approaches for mandibular asymmetries in orthodontic practice: a narrative review

2.1 Research question

To delineate the scope of the search strategy, we formulated a central research question that orthodontists could reference for guidance: “What are the appropriate radiological methods for diagnosing mandibular asymmetries according to evidence-based medicine?”.

2.2 Justification

The rationale for conducting the current NR (narrative review) stemmed from the lack of a thorough and comprehensive overview in the literature concerning the diagnostic method of mandibular asymmetries in orthodontic patients during growth. The search procedures and inclusion/exclusion criteria for this purpose are based on the consolidated methodology for Systematic Reviews (SRs) [23], and the selected articles underwent analysis using a narrative approach [22].

2.3 Eligibility criteria

With the exception of reviews, all research on mandibular asymmetries diagnostic techniques in orthodontics was taken into account. The language was unrestricted and the publication year was limited from 2013 to 2023, to consider only the most up-to-date diagnostic methods. Studies were included if they met the following criteria outlined according to the PICO format: studies conducted in growing human subjects (Participants); studies analyzing mandibular asymmetry using radiographic examination (Intervention), studies evaluating different type of radiographic tools used for the diagnosis of mandibular asymmetry (Comparison), mandibular asymmetry calculated via 2D or 3D dataset (Outcomes).

2.4 Literature sources and search parameters

To assess the corpus of current literature on the subject, multiple database searches were carried out through November 2023. The development of a search strategy that incorporated all discovered keywords and free-standing terms was aided by a health sciences librarian. The Web of Science, Embase, Scopus, Liliacs and PubMed databases were utilized. Additionally, further research was conducted to validate each source of evidence listed in the reference list. The results of customizing the search approach for each database are presented in Table 1.

2.5 Data cleaning

2.6 Information synthesis

The findings from the selected papers had to be presented narratively to comply with the suggested method for HRs [22]. A methodology derived from previous studies published by other researchers was used to report the results [24]. To better address clinical indications, the results were organized and discussed into distinct domains that included all the data retrieved from the included studies.

4.1 OPT

Since OPT is the standard preliminary radiographic record for orthodontic purpose, it may be utilized as a screening radiographic tool to identify the need for more in-depth radiographic studies and to detect early vertical mandibular asymmetries [30]. Indeed, OPT can assess condyle or ramus height vertically with acceptable consistency and reliability [75, 76] making it suitable for preliminary detection of condylar or ramal asymmetry. However, OPT has limitations, including limited resolution and susceptibility to picture distortion, magnification, and the superposition of anatomic structures [75, 77]. Considering that posterior structures (i.e., condyle and ramus) are subjected to less vertical distortion compared to anterior regions, high-quality OPT can be considered a non-invasive, acceptable diagnostic tool for screening vertical asymmetry of the mandible [20, 78]. However, a standardized protocol, which involves two aspects, is warmly encouraged to avoid reduction of diagnostic accuracy. Firstly, it is important that the distance between the film and the X-ray tube’s focus point is the same in order to avoid vertical amplification [79]. Secondly, head posture during x-ray acquisition must be standardized since there is evidence [78] that a 10 mm shift in head position could result in a 6% vertical size difference. Such posture discrepancy can influence the analysis of asymmetry, especially considering that, according to Habets [20], mandibular posterior vertical asymmetry is defined as asymmetry index values higher than 3%.

The utilization of Habets’ Asymmetry Index [20], which focuses on linear and angular measurements, can enhance the precision of assessing mandibular conditions, providing orthodontists with a quantitative tool to evaluate the severity and direction of asymmetries. This index can be evaluated by direct tracing with a pencil, or with a computer software [32]. The benefit of the study presented is that it enables simultaneous evaluation of mandibular measures in individuals, facilitating the differential diagnosis of morphological and functional mandibular asymmetries.

4.2 PA cephalogram

The PA cephalogram has long been utilized in orthognathic and orthodontic diagnosis, as well as in surgical planning, to examine the transverse dimension of the craniofacial skeleton and dentoalveolar structures, hence to address asymmetry [24, 80]. In one study [27] PA cephalometric analysis in orthodontics has been compared with orthopantomography as a diagnostic tool for mandibular asymmetries, using the asymmetry index. When the measurement values were entered into the AI formula, the results did not indicate a significant difference between the PR and PA cephalogram images, suggesting that both methods can be used for diagnosis. However, measurements such as the condylar height (CH), ramus height (RH), and CH + RH showed statistically significant differences on the right and left sides in panoramic radiographs (PR) and posteroanterior cephalometric radiographs (PACR) [27]. Another study [26] compared PA cephalometry with OPT using linear and angular measurements, concluding that there is a strong diagnostic correlation between the two methods. Therefore, the combination of both techniques allows for initial diagnostic screening. Nevertheless, as for panoramic radiograph, also posteroanterior teleradiography is a 2D image of 3D structures, and the head position has a significant impact on the vertical dimension, making it unsuitable for distance measurements [80].

4.3 Latero-lateral cephalogram

In this section of the discussions, studies referring to diagnostic methods of mandibular asymmetry not commonly employed are evaluated. A lateral cephalogram can contribute to the diagnosis, follow-up, and management of various dentofacial deformities and growth anomalies by evaluating interactions among the skull, face, and teeth [81]. However, due to the overlap of the two sides’ structures and superimposed pictures, the lateral cephalogram is unable to compare the orofacial features on the right and left [30].

4.4 Computed tomography methodologies

The 3D anatomical reconstruction ensures excellent visualization of both hard and soft tissues and, consequently, allows for a better definition of the discrepancy of the same structure between both sides. In assessing facial asymmetry, CT scans offer high-resolution images that allow for precise visualization of different areas of the mandible facilitating the identification of asymmetrical growth patterns, structural abnormalities, and deviations from normal anatomical configurations. By employing cross-sectional imaging, CT facilitates the examination of specific regions such as the condyle, ramus, mandibular body, and symphysis, elucidating alterations in size, volume, and position. However, CT has two main limitations for the primary identification of mandibular asymmetry in growing subjects. Firstly, CT does not fully adhere to the ALARA (As Low As Reasonably Achievable) principle due to the inherent high exposure to ionizing radiation, which poses potential biological risks in pediatric populations [74]. Secondly, CT is not an ideal imaging modality for detailing and characterizing soft tissues. CT primarily excels in imaging dense structures such as bones due to its reliance on X-rays, which are more effectively absorbed by dense materials. Soft tissues, being less dense, may appear less distinct on CT scans, making it challenging to discern detailed features or subtle variations in soft tissue structures. Also, CT is more susceptible to artifacts that compromise the clarity and accuracy of soft tissue representations, impacting the precision of facial soft tissue analysis [21].

Positron Emission Tomography-Computed Tomography (PET-CT) and Single Photon Emission Computed Tomography (SPECT) are imaging modalities that involve the injection of a radiopharmaceutical tracer which accumulates in areas with increased metabolic activity, such as regions of high cell turnover, inflammation and metabolism. Both imaging systems evaluate the dynamic aspects of mandibular asymmetry which could be crucial for differentiating between developmental asymmetries and acquired asymmetries due to trauma or pathological conditions. However, both methods also have limitations, including lower spatial resolution compared to anatomical imaging modalities like CT [21] or MRI [40]. In this regard, three studies [41, 42, 43] reported that SPECT is not sufficient to achieve the diagnosis of mandibular asymmetry and should be employed in conjunction with other imaging techniques to provide a more comprehensive assessment of mandibular asymmetry. These findings, again, arise concern related to adherence to the ALARA principle, as the use of multiple imaging modalities may increase radiation exposure.

4.5 CBCT

Although CBCT generates higher radiation exposure compared to single conventional 2D radiographic examinations (OPT, PA), the radiation dose is lower than all additional radiographic examinations necessary for complete orthodontic records, with the advantage of providing more detailed diagnostic data on asymmetry [82, 83]. The American Academy of Oral and Maxillofacial Radiology and the SedentexCT guidelines recommend using CT scans to evaluate facial asymmetry [84, 85]. Moreover, with technological progress, it is possible to reduce the Field of View (FOV) of the CBCT and consequently reduce the amount of radiation. For this reason, most of the studies available in the literature addressing mandibular asymmetry are related to the usage of CBCT technology.

Several studies [54, 55, 56, 86] have confirmed a high degree of repeatability of CBCT for assessing asymmetry, achieving good diagnostic outcomes. However, the conclusions drawn from the studies were only based on 2D linear and/or angular measurements of the mandibular, ramus, and condyle. The problem of using 2D measurements of 3D dataset has been recently emphasized since linear measurements can underestimate the true diameter of any curved surface [18, 87]. In this regard, the concept of integrating volumetric data with surface analysis obtained from sophisticated 3D imaging systems has been introduced for analyzing 3D dataset from both quantitative and qualitative perspective [18, 58]. In particular, recent studies [18, 61, 62, 88] demonstrated that reverse engineering software enables a detailed assessment or the monitoring of treatment progress of morphological symmetry of any anatomical structure. Specifically, 3D bone structures derived from CBCT scans can be mirrored and overlaid, allowing for the measurement of Euclidean distances or root mean squared (RMS) differences between the surfaces of the anatomical structures [50]. The mirroring process is performed after identifying the anatomical plane that serves as a reference for the models’ speculation. Subsequently, the models are registered using a “Best-fit alignment” algorithm, and a surface analysis is performed and visualized with a color map that highlights differences between both anatomical sides (right-to-left or left-to-right) [18]. Different levels of tolerance are applied through a technique known as “surface-to-surface” analysis, facilitating the identification and characterization of asymmetries with precision.

4.6 Acute and chronic TMJ disorders and asymmetry

Disorders of the temporomandibular joint (TMJ) can be transient or chronic. Acute illnesses are transient and usually go away on their own or with little medical intervention. Treatment for chronic diseases might be more complicated and have a longer duration. Symptoms of TMD (temporomandibular joint disorder) can include pain, jaw dysfunction, and joint noises. Another acute condition could be trauma of TMJ, which could be seen through OPT firstly, but for a better imaging evaluation CT or MRI is recommended [40].

With a female-to-male ratio of 3–6:1, juvenile idiopathic arthritis (JIA) refers to a set of disorders characterized by joint inflammation (arthritis) [53]. A tiny, asymmetrical, and hypoplastic mandible, a skeletal open bite, a short mandibular ramus, an elevated gonial angle, and anterior facial convexity are among the classic indicators of inflammation present in the affected joints. It has been suggested that the primary underlying causes of maxillomandibular growth abnormalities are chronic inflammation and gradual disruption of the condylar cartilage during mandible development. Affected condyles have a shorter and frequently asymmetric mandibular ramus [89]. Patients with JIA frequently exhibit indications of erosion and flattening of the condylar head, varying in severity. There are differences in the degree of functional limitations according to how much of the condylar head has been affected by the articular injury, ranging from minor erosions and osteophytes to conditions in which the condylar head is completely absent [53]. Using two-dimensional (2D) cephalometric studies, dentofacial development deviation in JIA with TMJ involvement has been well-described [90]. However, compared to 2D methods, 3D imaging offers a better visibility of dentofacial features [91]. Over the past 20 years, 3D imaging techniques have become more common in the assessment and follow-up of dentofacial development deviation due to the advent of cone-beam computed tomography (CBCT) in the field of dentistry [53].

Moreover, the diagnosis and follow up of TMJ chronic disorder such as internal derangement could be done with MRI, with no biological side effect into the patient.

4.7 Clinical implications

In light of the findings retrieved from the included studies, pediatric dentists and orthodontists should follow an appropriate clinical strategy when using radiological tools for the diagnosis of mandibular asymmetry. In this regard, after clinical facial and occlusal examination, a preliminary detailed analysis of 2D data retrieved from OPT is necessary to discriminate potential signs of asymmetry and to identify the area/regions involved. CBCT scans are encouraged as a second level of investigation, due to several advantages: (1) the ability to perform measurements in the three dimensions in each investigated area, (2) obtain volumetric reconstruction and volumetric data for side-to-side comparison, (3) integrate volumetric data with the analysis of side-to-side surface dataset, aiding in distinguishing the area mostly involved by the developing asymmetry. PET-CT and SPECT should only be considered if active pathological conditions are suspected.

The results discussed in this narrative are based on a limited number of studies, with heterogenous methodological design. In this regard, the topic of diagnosing mandibular asymmetry appears to be underestimated in literature. Future studies are warmly encouraged to evaluate and compare the diagnostic effectiveness of different radiographic imaging systems for detecting different mandibular asymmetric conditions.