Depictions of differential vertical asymmetry have been related to specific congenital or pathologic symptoms, such as hemifacial microsomia and condylar hyperplasia. Vertical maxillary asymmetry (VMA) is a condition localized to the nasal/maxillary/occlusal region, characterized by dentoalveolar asymmetry, canted nasal floor and occlusal plane, and asymmetric lip elevation. The asymmetry does not affect the orbits and the mandible. The potential etiology includes dentoalveolar compensation to localized skeletal or dental disturbances and possibly neuromuscular origins. Early treatment is recommended if the condition is properly diagnosed clinically and on the posteroanterior cephalography.

Asymmetry is the rhythmicexpression of functional design.

– Jan Tschischold

German calligrapher,typographer and book designer

The Three‐dimensional Nature of Asymmetry

The tenet that facial asymmetry is a common trait has been demonstrated in the cliché representation of differing appearance when the two rights and two lefts of a face are mirrored in the midline. Thus, a degree of asymmetry is acceptable in every face. Often, a small amount of asymmetry exists that may not influence the appraisal of facial attractiveness (Kaipainen et al. 2016). Treatment is often sought when a “measurable” asymmetry “disrupts” the general facial harmony.

Facial asymmetry has been reported extensively and mostly in the transverse dimension, ranging from mild midline deviations to extreme irregularities associated with recognized syndromes. However, its configuration in all planes of space is best captured by the spatial orientation of dentofacial traits in the three aeronautic rotational descriptors: pitch (posteroanterior up/down tip around a horizontal axis), roll (lateral up/down tip), and yaw (right/left rotation) (Ackerman et al. 2007; Ghafari 2012). Conceptually, pitch and roll comprise a vertical component and yaw predominantly includes a right/left discrepancy, such as midline deviations and subdivision malocclusions. The vertical component of pitch is revealed with the up and down position of posterior and anterior components of the occlusion relative to each other, often simplified in the relation of maxillary incisors and anterior gingival display to upper lip position at rest and in function. The normal inclination of the occlusal plane in the pitch direction favors a more downward tip of the anterior part, based on the normal inclination of the occlusal plane (9.5 degrees [range, 1.5–14.5 degrees]) (Downs 1952). Accordingly, deviations from this range do not begin at a flat (0 degrees) plane. Yaw and roll denote the more commonly defined dentofacial asymmetries.

The three rotational descriptors highlight the importance of their simultaneous assessment in various arrangements of malocclusion. More accurate evaluation is obtained with cone beam computed tomography (CBCT) images than traditional 2D lateral and posteroanterior cephalograms. Panoramic radiographs mainly illustrate roll, without inferences on pitch and only partial information on yaw.

Attempts have been made at quantifying the amount of deviation, including angular, linear, and proportional analyses that could “objectively” scrutinize such disruption. Non‐radiographic instruments have been developed that readily assess shifts from the norm, such as the deviation from the divine proportions [Ricketts’ “golden divider” (Ricketts 1982), Marquardt “golden decagon mask” (Marquardt 2002)].

Acceptable levels of midline deviation have been studied among laypeople, dentists, or orthodontics (Kokich et al. 1999). However, outside of the obvious asymmetry characteristic of pathologic syndromes, quantifying “acceptable” skeletal deviations may not be generalized. A hypothesis yet to be tested stipulates that less severe or mild asymmetries would be within one standard deviation, asymmetry becoming a significant dysmorphology beyond one standard deviation of a measurement. This premise must be balanced with the personal boundaries of comfort with and acceptance of asymmetric features.

A major shortcoming of a transverse cephalometric assessment in comparison with the sagittal evaluation is determining the potential concealment or exaggeration of the underlying skeletal discrepancy by the covering soft tissue thickness. Such assessment is possible on lateral cephalographs because soft tissue landmarks are properly defined and tegumental variations with hard tissue outlines are available.

Interaction between Maxillary and Mandibular Asymmetries

The association of maxillary and mandibular morphology has been reported during growth (Ghafari and Macari 2023) and treatment (Efstratiadis et al. 2005). Macari and Ghafari (2023) have demonstrated that not only mandibular position is affected by maxillary changes following sustained mouth breathing in young children, but also mandibular size. In craniofacial pathology, conditions have been classified under congenital (e.g. oral clefting, hemifacial microsomia, craniosynostoses) and acquired etiology (e.g. unilateral condylar hyperplasia, temporomandibular joint ankylosis or resorption, facial trauma or tumor). Regardless of the origin, considered in this review are commonly described maxillary asymmetries associated with mandibular asymmetry that involve the contribution of the orthodontist in their multidisciplinary treatment. They are classified in two basic types: maxillary asymmetry secondary to mandibular asymmetry, such as condylar hyperplasia and hemifacial microsomia, and mandibular asymmetry secondary to maxillary asymmetry, found with cleft lip and palate.

Maxillary Asymmetry Secondary to Mandibular Asymmetry

Condylar hyperplasia is a pathologic condition that usually sets in adolescence and may involve “normal” bilateral or unilateral condylar growth or abnormal enlargement of the condylar head (Machoň et al. 2023). Whereas bilateral hyperplasia has been hypothesized to occur in symmetrical Class III malocclusions (Genno et al. 2019), unilateral condylar growth causes mandibular asymmetry and compensatory maxillary asymmetry and progresses with rotational deviations in all planes, notably roll and yaw. The mandibular teeth, particularly the incisors, characteristically incline to the side of the hyperplastic condyle (Figure 12.10.1). Treatment of this condition involves high or total condylectomy (Wolford et al. 2002), more recently electrocautery of the condyle (Hashemi and Amirzargar 2022), and/or orthognathic surgery, depending on the time of diagnosis, severity of the facial dysmorphology, and the surgeon’s experience (Machoň et al. 2023; Wolford et al. 2002). Often, bone scintigraphy using the isotope technetium (Tc‐99m) is used to determine the active state or cessation of condylar cell proliferation (López et al. 2021). Frequently, the object of early orthodontic intervention is to maintain the symmetry in maxilla before further compensatory changes to the developing mandibular asymmetry.

Hemifacial microsomia is characterized by a deficient or missing mandibular ramus, lack of tissue on the affected side of the face, and microtia (total or partial absence of the external ear). The mandibular asymmetry is related to the severity of condylar and ramal defect, which ranges from mild with a small ramus but normal temporomandibular joint (level 1 of Pruzansky’s classification) (Figure 12.10.2) to the more severe pathology in which the joint and ramus are reduced or absent (Pruzansky’s classification 2 and 3) (Roberts 2000). Possible etiologies include a hemorrhage from the stapedial artery (about 6 weeks after conception) and the early loss of neural crest cells. Mandibular hypoplasia inhibits the symmetrical vertical growth of the maxilla, leading to a canted occlusal plane. Available studies indicate that the maxilla adapts to mandibular changes through dentoalveolar rather than skeletal compensation. In mild conditions, symmetry may be acceptable with sustained treatment (Figure 12.10.2). Early intervention with distraction osteogenesis in more severe conditions significantly improved the vertical maxillary asymmetry (VMA) 1 year post distraction through dentoalveolar modifications (Meazzini et al. 2012). On the other hand, in long‐term follow‐up of distraction osteogenesis until the completion of growth, treatment significantly improved vertical ramal asymmetry, but the results were gradually lost in time (Meazzini et al. 2005). Untreated patients maintained during growth the same ratio of affected/non affected ramal length as those who were treated. Thus, maxillary adjustments would also be influenced.

Mandibular Asymmetry Secondary to Maxillary Asymmetry

Representative of these conditions is the cleft lip and palate (CL/P) in which mandibular morphology, including asymmetry, adapts to the prevailing maxillary asymmetry. Studies of vertical mandibular asymmetry in patients with CL/P compared with control subjects who had malocclusions revealed greater mandibular asymmetry in the cleft patients, increasing with growth and peaking postpuberty (Figure 12.10.3) (Laspos et al. 1997; Kyrkanides et al. 2000). However, the reported average differences were within 2–3 mm (Figure 12.10.4), not necessarily clinically significant to warrant orthognathic surgery, which is often needed in the maxilla. Such decision is usually weighed by the severity of horizontal rather than vertical mandibular asymmetry. In addition, the reported statistically significant correlations in the vertical plane between the asymmetric growths of the jaws, and between vertical lower facial asymmetry and the vertical maxillary dentoalveolar measurements (Kyrkanides et al. 2000; Laspos et al. 1997) suggest that lower facial asymmetries develop in a pattern parallel with the dentoalveolar structures (Figure 12.10.3). The development of mandibular asymmetry may also be related to age and consequently to the timing and success of treatment for cleft‐associated maxillary problems.