Introduction

Dental asymmetries are considered as one of the most difficult scenarios to manage in orthodontics. A correct differential diagnosis constitutes the cornerstone for any biomechanical application. These asymmetries could be classified by their location as either posterior or anterior, by their orientation in the occlusal, the sagittal, or the frontal plane, and by their etiology which could be dental, functional, or skeletal. Anterior asymmetries are generally more difficult to treat due to their interaction with smile esthetics. Posterior asymmetries, which are more discrete, have to be also considered for a better occlusal intermaxillary relationship and in the pursuit of excellence in orthodontics. The aim of this chapter is to identify different types of dental asymmetries, to develop a differential diagnosis, and to apply individualized treatment mechanics depending on each case.

The objectives of orthodontic treatment encompass dentofacial esthetics, functional occlusion, periodontal health, and stability. As Peck et al. (1990) mentioned, symmetry plays an important role in dentofacial esthetics. Asymmetry may be expressed as mandibular deviations, occlusal cants, dental midline shifts, or simple gingival height discrepancies (Sarver and Ackerman 2003). Perfect symmetry seldom occurs in nature, however, and the development of the human face and dentition is no different (Lundstrom 1961).

Some degree of facial and dental asymmetry occurs in virtually all individuals (Bishara et al. 1994; Ferrario et al. 1993; Ming 2006; Sheats et al. 1998; Smith and Bailit 1979; Yoon et al. 2013). Facial asymmetry can have dental, functional, or skeletal causes or a combination of the three (Bishara et al. 1994; Joondeph 2000). An asymmetry in a single dimension has the potential to cause other asymmetries in other dimensions within the craniofacial complex. The question then arises: What is a clinically acceptable asymmetry for the various dentofacial features? Beyer and Lindauer (1998) suggest midline asymmetry greater than 2.2 mm is discernible. Choi (2015) suggested that facial asymmetries within 3–4% and 3–4 mm are not clinically detectable.

According to Sheats et al. (1998), the most common asymmetries in patients presenting for orthodontic treatment were mandibular midline deviation from the facial midline, occurring in 62% of patients, followed by lack of dental midline coincidence (46%), maxillary midline deviation from the facial midline (39%), molar classification asymmetry (22%), maxillary occlusal asymmetry, (20%) and mandibular occlusal asymmetry (18%). Facial asymmetry (6%), chin deviation (4%), and nasal deviation (3%) made up the remainder.

Dental deviations should be treated as far as possible with intra‐arch mechanics, functional deviations with inter‐arch mechanics, and skeletal deviations with orthognathic surgery. While some degree of asymmetry is definitely part of natural appearance, symmetry is the ideal that we should treat toward (Pinho et al. 2007).

Classification of Dental Asymmetries

Facial asymmetry can be classified into skeletal, functional, or dental. Dental asymmetries, although somewhat associated with skeletal asymmetry, can be categorized into posterior and anterior depending on their location.

The posterior asymmetries can then be further classified into occlusal, sagittal, and frontal plane asymmetries, while anterior asymmetries can be divided into asymmetries of dental, functional, or skeletal origin.

Diagnosis of Dental Asymmetries

The diagnosis of asymmetry follows the traditional comprehensive orthodontic clinical examination including photographs, study models, occlusograms, and cone beam computed tomography (CBCT) (or panoramic radiograph, lateral cephalometric radiography, submental cephalometric radiography, and frontal cephalometric radiography, if a CBCT is unavailable).

The extra‐oral clinical examination of asymmetry is performed with the patient in the natural head position and involves utilizing some specific facial planes and landmarks, i.e. the interpupillary line, and a vertical line through the glabella perpendicular to the inter‐pupillary line, tip of the nose, and center of the philtrum and chin (Figure 11.2.1). The clinical examination for asymmetry can be performed by looking from a frontal view. In addition, looking from inferior view of the mandible can aid in determining the extent of deviation (Cheney 1961) (Figure 11.2.2). Alternatively, a straight instrument or piece of floss placed in line with the facial midline can aid in visualization of asymmetry (Figure 11.2.3).

A frontal facial photo is ideal to investigate asymmetry, where the anatomical landmarks and constructed lines can be drawn digitally and compared, which may be awkward when done in‐person. The presence of an occlusal cant should also be assessed at this stage. This can be done by asking the patient to bite on a tongue blade which is positioned in the canine area, and comparing it to the interpupillary line (Arnett and Bergman 1993) (Figure 11.2.4). Furthermore, a frontal examination of the patient smiling, with their mouth closed and then slightly opened, will also aid in observing any asymmetry of the smile or cant of the occlusal plane, respectively.

A clinical examination should involve screening for functional asymmetries such as mandibular dental midline deviations during opening, centric relationship, first contact of the occlusion, and in occlusion (Bishara et al. 1994). True dental or skeletal asymmetries will exhibit similar midline discrepancy in both centric relation and centric occlusion. Conversely, functional asymmetries due to occlusal interferences will show a midline discrepancy between centric relation and centric occlusion as the mandible shifts laterally.

Moving intra‐orally, the facial midline is coincident with the upper lip philtrum and the midpalatal suture, and these landmarks should be used when measuring the upper dental midline deviation. In both the upper and lower arches, the labial or lingual frena should not be used as a reference landmark, due to their alveolar insertion which adapts and alters based on tooth position (Figure 11.2.5).

Dental arch asymmetry should also be investigated at this point and can be further inspected using a dental cast. The dental cast can also be investigated for asymmetry via an occlusogram, which involves the use of a grid to aid in measurement of dental arch asymmetries (Faber 1992; Fiorelli and Melsen 1999; White 1982). This allows for discrete measurements of the left and right differences and asymmetries in different planes (Figure 11.2.6).

Dental arch asymmetries can be caused by skeletal factors, such as the rotation of the entire dental arch and the maxillary base, or by local factors such as the premature loss of primary teeth. Asymmetry of the buccolingual inclination of the posterior teeth, especially in cases with a unilateral crossbite, should be assessed to determine if an asymmetry is of dental, functional, or skeletal origin.

In certain cases, where the cause of asymmetry is unclear, a therapeutic diagnosis can be attempted. An outwardly apparent asymmetric occlusion may become symmetric after leveling and aligning of the arches or via the use of an occlusal splint. This usually manifests itself when there is an initial premature contact that causes a functional mandibular deviation.

The radiographic examination for asymmetry should start with conventional panoramic and cephalometric radiographs. The panoramic radiograph enables the clinician to view the condyles for size and shape discrepancies, as well as irregular cortical borders (Figure 11.2.7). While the condylar length and ramus length may be measured as well, positioning errors of the patient tends to make these measurements unreliable and is better evaluated using CBCT.

A lateral cephalometric radiograph can be used to identify mandibular asymmetry by observing the two borders of the mandible (Figure 11.2.8). This also has some reliability issues, however, as the patient’s head positioning may lead to small discrepancies between the borders of the mandible (Bishara et al. 1994). Hence only gross and obvious asymmetry can be reliably identified. Frontal and submental cephalometry has been the traditional go‐to method for evaluating skeletal asymmetry due to the right and left structures being equidistant from the film and X‐ray source. Hence, landmarks on either side of the midline can be measured and contrasted with greater accuracy (Figure 11.2.9).

With the development and subsequent popularization of CBCT, 2D radiographs, especially when facial asymmetry is involved, have largely become redundant. Faure et al. (2013