Introduction

Facial asymmetry is a common find in orthodontics and comprises differences in morphology and/or position between right and left sides of the face using as a reference the midsagittal plane. Slight or subclinical asymmetries are present in most human faces and frequently are not noticed during a casual observation. Curiously, previous studies even demonstrated that perfect symmetric faces were perceived as less attractive than those with slight asymmetry (Swaddle and Cuthill 1995; Kowner 1996; Severt and Proffit 1997). However, when the asymmetry exceeds the acceptable degree and becomes noticeable, it may negatively affect occlusion, facial, and smile esthetics and should be carefully considered before initiating the orthodontic or surgical treatment.

In general, facial asymmetries can be clinically diagnosed in 12–44% of the patients (Severt and Proffit 1997; Sheats et al. 1998; Gribel et al. 2014). Considering a sample with patients seeking orthognathic surgery consultation, 34% demonstrated a clinically apparent facial asymmetry (Severt and Proffit 1997). In most of the cases (74%), asymmetry was identified in the lower third of the face. The upper face was asymmetric in only 5% of the patients, while the midface was not symmetrical in 36% of the cases (Severt and Proffit 1997). When affecting the middle and lower third of the face, the asymmetry was primarily explained by nose tip and chin deviation, respectively (Severt and Proffit 1997).

The facial asymmetry may involve one or more planes of space. Differences in the position, rotation, or a combination of both may occur. The adequate localization of the dental, skeletal, soft tissue, and functional asymmetry is highly important for the treatment plan. This chapter will present a review in three‐dimensional (3D) face reconstruction for localization and listing of craniofacial regions of facial asymmetry problems.

Facial Asymmetry Diagnosis and Complementary Exams for Facial Asymmetry Localization

The identification of facial asymmetry is accomplished with patient interviews, clinical examination, and complementary imaging exams (Bishara et al. 1994; Burstone 1998; Legan 1998; Masuoka et al. 2007; Chia et al. 2008; Lee et al. 2010; Cheong and Lo 2011; Thiesen et al. 2015).

A careful intra and extraoral clinical examination is an essential tool to identify the asymmetry condition in vertical, anteroposterior, and transverse planes (Bishara et al. 1994; Burstone 1998; Legan 1998; Chia et al. 2008; Cheong and Lo 2011; Thiesen et al. 2015). During extraoral assessment, the patient should be in an upright position, looking forward, with teeth in occlusion and relaxed lips (Thiesen et al. 2015). Special attention should be given to the symmetry of the orbits and cheeks, chin deviations, leveling of lip commissures, mandibular lower border contour, gonial angle region, dental and facial midlines, inclination of the occlusal plane, gingival exposure, functional deviation of the mandible, and existing malocclusions (Bishara et al. 1994; Burstone 1998; Legan 1998; Chia et al. 2008; Lee et al. 2010; Cheong and Lo 2011; Thiesen et al. 2015). If the asymmetry is not properly localized, it may result in longer treatment time, delays related to changes in treatment direction, or eventually in unexpected or compromised outcomes (Lindauer 1998).

The presence of evident facial disproportion, midline deviation, inclination of the occlusal plane, and progressive and unilateral posterior open bite in the clinical evaluation suggests the presence of facial asymmetry and complementary diagnostic tools should be indicated to precisely locate the structures involved in the condition. Photographs, dental casts, radiographs, and tomography are examples of complementary exams indicated for asymmetry diagnostic and localization. Some specific cases, nuclear medicine tests are also recommended.

The conventional radiographs used in orthodontics, such as lateral and posteroanterior cephalometric and panoramic radiographs, provide limited information due to overlapping of the craniofacial structures, image magnification, and difficulty in standardizing the position of patient’s head during image acquisition (Pirttiniemi et al. 1996; Legan 1998; Van Elslande et al. 2008; de Moraes et al. 2011; Damstra et al. 2013; Thiesen et al. 2015). A previous study compared posteroanterior cephalometric radiographs with cone‐beam computed tomography (CBCT) exams to detect facial asymmetry in cases with chin deviation (Damstra et al. 2013). CBCT images were reliable and accurate while PA radiographs were not precise to assess asymmetries in the mandibular length (Damstra et al. 2013). Considering the limitations of a bidimensional assessment, the computed tomography, especially the full‐face CBCT, is the complementary diagnostic tool of choice in cases with facial asymmetry.

The North American guidelines for CBCT use, proposed by the American Academy of Oral and Maxillofacial Radiology, recommends the use of CBCT scans to assess facial asymmetry (AAOMR 2013; Garib et al. 2014). According to the appendix of the proposed guidelines, facial asymmetry can be clinically presented as chin, mandibular, or midline deviations or occlusal cant discrepancies as well as other dental and craniofacial asymmetries. The European evidence‐based recommendations for the use of CBCT, the SedentexCT guidelines, also propose the use of this exam for severe cases of skeletal discrepancies and orthognathic surgery planning (SEDENTEXCT 2012). In asymmetric cases with surgical indication, CBCT scan not only works as a valuable diagnostic tool but also allows the creation of 3D prototyped models to guide the surgery.

The CBCT exam provides a detailed 3D assessment of craniofacial asymmetries involving skeletal and soft tissue structures. Additionally, CBCT can better evaluate craniofacial morphology when compared with digital 2D images (de Moraes et al. 2011). The clinician should not expose the patient to multiple radiograph exams before taking a large field of view (FOV) CBCT to diagnose the asymmetry. The radiation dose of a large FOV CBCT exam is lower than the dose of multiple radiograph exams necessary to localize the asymmetry with better diagnostic information (Lorenzoni et al. 2012; Thiesen et al. 2015). In addition, the bidimensional images that clinician is familiar with can be easily generated from a CBCT exam taken with a large FOV.

For example, the coincidence between facial and dental midline can be assessed during clinical examination at smiling. The perpendicular line passing through the glabella, center of the interpupillary distance, or subnasal region can be used to determine the facial midline. The center of the chin and the tip of the nose frequently present variations and should not be used as reference (Bishara et al. 1994; Thiesen et al. 2015). If the clinical exam determines that a patient with 4mm midline deviation does not have functional shift, and that the etiology of the facial asymmetry is not dental, this is an indication to take a large FOV CBCT exam and it is important to know how to use this diagnostic record to localize the asymmetry properly. The CBCT exam will allow the localization of the asymmetry and the 3D quantification of the morphological error.

In 3D imaging analysis, patient’s face can be assessed in all three planes of space. Clinicians are encouraged to use the 3D face reconstruction to evaluate vertical, anteroposterior, and transverse aspects as well as the rotation around the three axes (horizontal, axial, and vertical). The three aeronautical rotational descriptors known as pitch (vertical rotation), roll (lateral rotation), and yaw (horizontal rotation) are frequently used in imaging analysis (Ackerman et al. 2007; Yatabe et al. 2019), as shown in Figure 9.1. Face reconstruction for skeletal asymmetry analyses and the steps for a thorough 3D diagnosis using CBCT will be presented in the next topic. The identification of adequate references is important in evaluating asymmetry. The virtual preparation of the image data allows the correction of head tilts, through the head orientation step, and facilitates visual and quantitative evaluation of symmetry (Cevidanes et al. 2011). The appropriate operation of the CBCT exam will help the clinician to determine the correct location of the asymmetry and if the case diagnosis a nasomaxillary unilateral hypertrophy, sinus hypoplasia, hemimandibular hypertrophy, mandibular elongation/unilateral condylar hyperplasia, unilateral condylar resorption, or craniofacial microsomia. Each of this list of problems will also be discussed in this chapter later.

Image Analysis in CBCT Scans for Skeletal Asymmetry

An important question to answer about patients with facial asymmetry is “Why do we want a thorough 3D diagnosis?”. The craniofacial complex morphology is oriented by three planes of space: mediolateral, superoinferior, and anteroposterior. Asymmetric patients usually show more than one plane of space affected, such as cases of chin deviation in the horizontal plane and vertical asymmetry in mandibular ramus (Figure 9.2). However, the intrinsic interaction between all planes of space in the craniofacial complex may hide variations in planes of space affected with less severity. The interaction between all planes should always be considered in 3D diagnosis and consequently in the treatment planning. The 3D image analysis using linear and/or angular quantification will be able to identify the plane of space with more severe asymmetry features as well as if other planes are affected and how this combination is demonstrated.

Before 3D evaluation of CBCT scans, some steps are necessary to enhance the accuracy of asymmetry diagnosis. These steps will be discussed in the next topics.

Head Orientation

The patient’s head position during tomographic acquisition can affect the diagnosis if head tilts occur (De Momi et al. 2006). Figure 9.3 shows a coronal view of CBCT scan and the rendering skull of the same patient before and after head orientation. Note that in the original image, the head shows a roll inclination. After the head orientation, the asymmetry in the mandible was properly revealed. The recommended position of the head for asymmetry diagnosis is with the horizontal plane of the head or Frankfurt plane perpendicular to the midsagittal plane. The components of the Frankfurt 3D plane consist of bilateral porion (Po) and bilateral infraorbitale (Or) (Ruellas et al. 2016). The midsagittal plane is determined through the alignment of glabella (G), crista galli (CG), and basio (Ba) (Ruellas et al. 2016). The position of the Frankfurt plane perpendicularly to midsagittal plane positions will automatically determine the coronal plane for each patient by software in image analysis. Figure 9.4 shows a 3D model obtained from a full‐face CBCT scan with the Frankfurt plane perpendicular to the midsagittal plane after the head orientation step.

Scroll Through All Cross‐Sectional Slices

Once the head is oriented with the Frankfurt plane perpendicular to the midsagittal plane, a navigation through all cross‐sectional slices should be carefully performed. During the navigation, a search for anatomical structures in midline and bilateral regions of the skull brings information of symmetric and asymmetric regions of the skull in terms of morphology and/or position. The assessment of skull symmetry consisted in visualizing specific landmarks or regions of interest positions in all slices.

Landmarks placed in the midline of the skull are useful to detect horizontal asymmetry, by checking their positions in axial and coronal slices related to the midsagittal plane such as a vertical line fixed in the Nasion (Figure 9.5).

The assessment of bilateral regions is recommended to identify morphologic asymmetries between right and left sides, vertical rotation (roll), and horizontal rotation of the jaws (yaw). The image analysis of right and left side consists into three approaches: (i) to check bilateral regions in axial and coronal slices relative to the horizontal line of reference, parallel to the Frankfurt plane, (ii) to check the latero‐medial distance of bilateral regions relative to midsagittal plane, and (iii) to check the anteroposterior alignment of bilateral regions relative to a reference line parallel to coronal plane. Figure 9.6 shows the assessment of bilateral regions of maxilla and mandible.

Assessment of the 3D Rendering Viewing from Different Perspectives

In the 3D rendered view, it is possible to assess mandibular yaw rotation and lateral width of the zygomatic arches. From the frontal perspective, mandibular roll and yaw rotations and the cant of the occlusal plane can be evaluated (Figure 9.7). Finally, from the lateral view, the assessment of the mandibular corpus, mandibular ramus, and condyle is performed.

List of Problems in Skeletal Craniofacial Asymmetries Through 3D Assessment

After a preliminary analysis of the CBCT scans, added by clinical signs observed in a clinical examination, the identification of a list of problems will enhance the asymmetry localization and guide further steps in image analysis by quantification of the problems detected. In this topic, a list of problems in skeletal craniofacial asymmetry will be presented through a 3D assessment. Later, asymmetry localization and quantification will be further discussed.