Asymmetry in biological organisms is ubiquitous.

Slight facial asymmetry can be found in normal individuals, even those with aesthetically attractive faces. Right–left differences exist in nature where two bilateral congruent parts are present in an entity ( Fig. 85.1 ).

Computerised images of a relatively symmetric face reveal the extent of asymmetry in a pleasing face.

(A) This image is the original photo of the patient. (B) This composite is made up of the patient’s left side with a flipped mirror image representing her right side. (C) This composite is similarly created with two right sides. The effect demonstrates how each half of the patient’s face is different. This can be an interesting way to begin the conversation about asymmetries.

Asymmetry of the craniofacial skeleton is a complex morphological variation resulting from variable aetiological factors, the interplay of genetically controlled growth disturbances and the multitude of environmental factors such that perfect bilateral symmetry is rare. Asymmetry of the face is often camouflaged by the compensating growth of the facial bones, soft tissue compensation, adaptive response to respiration and other functional needs of the stomatognathic system, and alteration in head posture. Facial asymmetry could involve craniofacial skeleton affecting the occlusion of the muscular or soft tissues of the face. Orthodontists deal with facial asymmetry of the skeleton and occlusion. A patient presenting with facial deformities of asymmetry may have camouflaged the appearance by alterations in hair, cosmetic procedures and head posture.

Humans frequently experience functional as well as morphological asymmetries; for example, right-handedness is more common (70%–95%) than left.

In general, there are two major forms of bilateral facial asymmetry.

The first form is fluctuating asymmetry, which is the variability of left–right differences among individuals. It refers to minor random deviations from perfect symmetry in bilaterally paired structures. This deviation from perfect symmetry is thought to reflect the genetic and environmental pressure experienced throughout development, with greater pressures resulting in higher levels of asymmetry. Fluctuations are thought to represent an inability to tolerate stressors, either genetic or environmental.

The second form of asymmetry is directional asymmetry, which is the average difference between the two sides of the face. It occurs when there is a greater development in one-half of the body. Although subtle facial directional asymmetry is present in healthy individuals, a substantial directional asymmetry is often associated with conditions that disrupt normal craniofacial development, such as cleft lip and palate, plagiocephaly, or craniosynostosis.

Facial asymmetry is relatively common. Epidemiological studies assessing facial asymmetries in orthodontic patients reported a clinical prevalence ranging from 34% in a study at the University of North Carolina in the United States, 23% in Belgium and 21% in Hong Kong. These findings were based on retrospective studies on the prevalence of dentofacial characteristics to determine the orthodontic treatment needs in different populations.

In Brazil, Boeck et al. assessed the prevalence of skeletal deformities in a sample comprising 171 patients needing orthodontic-surgical treatment. Their findings revealed a prevalence of 32% of asymmetries among the individuals assessed. Gribel et al. assessed mandibular asymmetries using cone-beam computed tomography (CBCT) of 250 class I subjects and found a prevalence of 44% of mild-to-severe asymmetries. Severt and Proffit conducted research with 1460 patients at the University of North Carolina. They reported that 34% of individuals exhibited facial asymmetry, with a deviation of the chin being the most remarkable asymmetry feature. They found the asymmetry distribution to be 74%, 36% and 5% for the lower, mid and upper face, respectively. The deviation of the lower face is more frequent and pronounced than the upper and mid-face. ^, The increased frequency of lower facial asymmetry is because the mandible is a movable bone that grows over a more extended period than the maxilla. A summary of studies from literature with reference to sample size/population, method of assessment and findings is compiled in Table 85.1 . ^{, ,}

Studies have revealed that the dominant side of asymmetrical human faces can be either left or right, depending on the focused part of the face. Most examinations of face asymmetry proved that domination of the left side is more common in the human population. Vig and Hewitt found in their radiographic investigation that the cranial base and maxillary regions were significantly larger on the left side; on the contrary, Shah and Joshi pointed out that the total facial structure was larger on the right side based on 43 college students with a mean age of 22 years old. Melnik completed a longitudinal study from the Burlington serial control group and showed that the side of facial dominance was a function of age: while at 6 years of age, the left side was larger, and at 16 years, the right side was dominant. In the photographic study, Ferrario et al . found that the lower part of the face was dominant on the right side in both men and women. Haraguchi et al. found a consistent tendency for dominance of the right hemiface and left chin deviation in Japanese orthodontic patients. A significant linear and angular dimensional asymmetry was observed in the mandibles during the mixed dentition phase, independent of age and gender, suggesting a possible adaptive response to functional demands. The left side of the mandible exhibited dominance, with over 25% of the population showing moderate to severe asymmetry. However, no association was found between the presence of mandibular dimensional or angular asymmetries and tooth development. While there are conflicting opinions regarding the side of dominance from studies, the methods used in these studies vary and are primarily two-dimensional (2D) projections of three-dimensional (3D) structures; therefore, the findings are questionable.

The treatment of facial asymmetry varies from no treatment to complex craniofacial surgery, depending upon the severity of the problem, social and aesthetic concerns, compromised functions of the stomatognathic system and associated morbidity. Additional procedures to orthognathic surgery, such as fat grafts and Botox injections, are often needed to mask soft tissue asymmetry.

Chin: The hallmark of facial symmetry

Facial symmetry is first evaluated with the chin point or menton, the hallmark of facial symmetry/asymmetry ( Fig. 85.2 ). Facial asymmetry has been perceived to be clinically relevant when chin/menton deviation is by 4 mm or menton deviations of 4.28 and more.

Chin: the hallmark of facial symmetry.

Facial symmetry is first evaluated with chin point or menton. Facial asymmetry has been perceived to be clinically relevant when chin deviates by atleast 4 mm and menton deviates by 4.28 mm or more. A girl with chin deviation to the corresponding disturbance of the occlusion.

The deviations on the chin necessitate comprehensive evaluation of the facial asymmetry.

The right shift of the chin correspondents to buccal crossbite and mid-line shift.

Haraguchi et al. defined asymmetry with a more than 2 mm deviation from the facial midline associated with any of the four landmarks (ANS, U1, L1 and Me) asymmetric measured on a posteroanterior (PA) cephalogram. Soft tissue compensation, such as the masseter muscle, can mask accurate clinical evaluation of facial asymmetry. Such subjects with skeletal laterality could be categorised as requiring no treatment on clinical evaluation alone. Therefore, subjects who exhibit no asymmetry in skeletal measurements but are subjectively judged as needing treatment should be accounted for soft tissue laterality.

The clinical evaluation of the nasal tip position and its deviation from the mid-sagittal plane can be effectively conducted with the patient in a slightly elevated head position. This technique is a reliable method for assessing nasal tip deviation. Deviation of the nasal tip from the sagittal plane may be caused by various factors, including past traumatic injuries to the nose, deviated nasal septal (DNS) cartilage, congenital nasal stenosis, deformities arising from unilateral cleft lip and nose, or complications resulting from prior rhinoplasty procedures ( Fig. 85.3 ).

• 1.

  Cranial base facial asymmetry: Computed tomography studies of craniofacial morphology in subjects with facial asymmetry revealed that even though facial asymmetry is accompanied by various degrees of cranial base asymmetry, the severity of cranial base is not dominant in determining the severity of facial imbalance. The compensatory growth of the maxillofacial region may camouflage the cranial base asymmetry.

• 2.

  Unilateral temporomandibular joint (TMJ) sounds and facial asymmetry: Facial asymmetry can be reflected by differences in the development of the mandible and glenoid fossa and the presence of unilateral TMJ sounds. Even if the affected side is dominant, there may be compensating mechanisms. Unilateral TMJ sounds are often linked to asymmetries in the glenoid fossa and the mandibular and maxillary structures, leading to changes in the size and angle of the mandibular ramus. However, the absence of TMJ sounds does not necessarily mean the absence of facial asymmetry.

• 3.

  Airway and facial asymmetry : Asymmetry of the craniofacial skeleton may also affect the upper airway structures and their function. Unilateral nostril and nasal fossa underdevelopment may be accompanied by larger paranasal sinuses.

• 4.

  Muscular asymmetry : In major facial asymmetries, soft tissue drapes over the skeletal deformities are also affected. In certain clinical situations, soft tissue compensations may mask the severity of the skeletal discrepancy. Facial asymmetries limited to soft tissue are rare. However, hypertrophy of the masseter muscle, neurofibromatosis and traumatic injuries affecting soft tissue may be more prominent and limited to the soft tissues of the face.

• 5.

  Functional asymmetry of the face : Facial asymmetry with a functional component can manifest as a deviation of the chin in centric occlusion, while the facial midline and chin line align upon opening the mandible ( Fig. 85.4 ). This clinical presentation often occurs during the mixed or early permanent stage of dental development and may result from mandibular deflection during closure due to premature contacts, such as incisor crossbite or unilateral posterior crossbite. If not intercepted, deflection in the closure path may lead to abnormal development of facial structures, contributing to skeletal deformity.

  

  Functional asymmetry of the face.

  Facial asymmetry with an isolated functional component can be seen as chin deviation in centric occlusion from retruded contact position, while on opening the mandible, the facial mid line and chin line coincide.

In this patient’s facial scan, nasal asymmetry clearly contributes to the perception of asymmetry even though the chin is relatively centred.

Functional jaw deviations could result from excessive mandibular growth, as seen in skeletal class III subjects, where incisor interferences may contribute to jaw deviation to either side.

It is pertinent to mention that no two facial asymmetries are the same. Combinations of the aforementioned variations of dental, skeletal, functional and muscular discrepancies may contribute to a multitude of phenotypes.

Facial asymmetry can affect the craniofacial skeleton in any or all of the three planes of space. Facial asymmetry is best evaluated in the full-face or frontal view. Several classification systems are designed to help with the surgical treatment of facial asymmetry.

Structural classification

Reyneke et al. suggested that once the growth of the facial complex is completed, the clinical presentation of the facial asymmetry is the overriding consideration in management, irrespective of the aetiology. In other words, based on his observations and practice, correction of the asymmetrical maxillomandibular bony complex does not differ appreciably for aetiologically different asymmetries with similar clinical presentations when growth is not a contributing factor ( Fig. 85.5 and Table 85.2 ).

Facial asymmetry.

Structural classifications by Reyneke.

Source: Based on the concept of Reyneke JP, Tsakiris P, Kienle F. A simple classification for surgical treatment planning of maxillomandibular asymmetry. Br J Oral Maxillofac Surg. 1997 Oct;35(5):349–51. doi: 10.1016/s0266-4356(97)90408-3. PMID: 9427444.

TABLE 85.2

The classification of facial asymmetry by Reyneke (categorisation of asymmetry)

Type I	Asymmetry is caused by asymmetry of the symphysis of the mandible.The maxilla and the body of the mandible are symmetric, with the dental midlines in the centre of the face.
Type II	Facial asymmetry in which the discrepancy is primarily in the body, ramus or condyle of the mandible. The maxillary dental midline coincides with the facial midline and the mandibular dental midline coincides with the symphysial midline.
Type III	Asymmetry in which the maxillary midline is still coincident to the facial midline, but the mandibular midline is asymmetric to the maxillary midline, and the symphysis is still more asymmetric to the mandible.
Type IV	The facial asymmetry in which the discrepancy involves the maxilla, mandible and symphysis. The maxillary midline is asymmetric to the facial midline, while the body of the mandible to the maxillary midline is further asymmetric (mandibular midline is asymmetric), and the mandibular symphysis is asymmetric to the body of the mandible.
Type C	Depicts facial asymmetry caused by a cant in the occlusal plane, while the maxillary and mandibular dental midlines and symphysis coincide.

Subtypes Ic, IIc, IIIc and IVc indicate that an occlusal cant discrepancy has been superimposed on type I, type II, type III and type IV.

TML classification

Transverse asymmetry, maxillary cant and lip cant (TML) classification. TML classification has been proposed by Kim and his group in 2014. It involves the categorisation of deviation with Transverse asymmetry (T), Maxillary cant (M) and Lip cant (L). Although the efficacy of the classification is now being evaluated, analysing such patients’ facial asymmetry with the new classification system presented in this study and employing surgical methods appropriate for each case would help to achieve a more harmonious aesthetic outcome. In this system, transverse hard tissue asymmetry was defined as deviations greater than 2.0 mm ( Fig. 85.6 and Table 85.3 ).

TML classification.

It involves a categorisation of deviation with Transverse asymmetry (T), Maxillary cant (M) and Lip cant (L). (A) Reference landmarks and lines of the soft tissue ( R : right, L : left). EC, External canthus; Pm, midpoint of pupil; SG, soft tissue gonion; UStm, upper stomion; LC, lip commissure; SMm, soft tissue mandibular midline point. (B) Reference landmarks and lines of the hard tissue ( R : right, L : left). M, Molar point; Mm, menton point; G, gonion; Lo, latero-orbitale; CG, crista galli; ANS, anterior nasal spine.

Source: Reproduced with permission from Kim JY, Jung HD, Jung YS, Hwang CJ, Park HS. A simple classification of facial asymmetry by TML system. J Craniomaxillofac Surg. 2014 Jun;42(4):313–20.

(C) Classification of facial asymmetry according to the combination of menton deviation and transverse asymmetry (T-group).

(D) Subclassification of transverse asymmetry (T-group) according to direction of angle prominence in soft vs. hard tissue.

(E) Classification of asymmetry according to the combination of menton deviation and maxillary canting (M-group).

(F) Classification of asymmetry according to the combination of soft tissue menton deviation and lip canting (L-group).

TABLE 85.3

TML classification

Transverse asymmetry group Classification of facial asymmetry according to the combination of menton deviation and transverse asymmetry *		Maxillary and lip cant group (M L) Classification of asymmetry based on the combination of deviation of menton and cant in soft and hard tissue
		a. Classification in hard tissue	b. Classification in soft tissue
		M0 : Neither maxillary cant nor menton deviation	L0 : Neither lip canting nor soft tissue menton deviation
T1 : Equal direction of menton deviation and transverse soft tissue asymmetry		M1 : Presence of menton deviation and maxillary cant with mental deviation and downward maxillary cant in opposite directions	L1 : Presence of soft tissue menton deviation and lip cant with mental deviation and downward maxillary cant in opposite directions
T2 : Opposite direction of menton deviation and transverse soft tissue asymmetry		M2 : Presence of menton deviation and maxillary cant with equal direction of mental deviation and downward maxillary cant	L2 : Presence of soft tissue menton deviation and lip cant with equal direction of mental deviation and downward maxillary cant
T3 : Absence of transverse asymmetry despite the presence of menton deviation		M3 : Presence of menton deviation without maxillary cant	L3 : Presence of soft tissue menton deviation without lip canting
T4 : Presence of transverse asymmetry without menton deviation		M4 : Presence of maxillary cant without menton deviation	L4 : Presence of lip canting without soft tissue menton deviation
Subclassification of transverse asymmetry according to the direction of angle prominence in soft vs hard tissue
H1 : Equal direction of transverse asymmetry in soft vs hard tissue	H2 : Opposite direction of transverse asymmetry in soft vs hard tissue

Classification based on ramus length and chin deviation

Hwang et al. have proposed a simple classification based on menton deviation and ramus length. Facial asymmetry is classified into four groups: RM (Ramus Menton), M (Menton), RA (Ramus Angle), and B (Bulkiness).

These groups are based on menton deviation and ramal length differences on frontal cephalograms. This classification allows the clinician to determine the cause of a given asymmetry and to formulate a specific treatment strategy for a facial asymmetry patient ( Fig. 85.7 ).

Classification based on ramus length and chin deviation.

Facial asymmetry is classified into four groups: RM (Ramus Menton), M (Menton), RA (Ramus Angle) and B (Bulkiness).

Source: Based on the concept of Hwang HS. A new classification of facial asymmetry. In: McNamara JA, editor. Early orthodontic treatment: is the benefit worth the burden? Ann Arbor: Center for Human Growth and Development, The University of Michigan; 2007. pp. 269–294.

The morphological asymmetry of the face usually involves maxillary and mandibular components and may extend deep into the sphenoid bone and cranial base in other instances. The morphology of the asymmetrical face may have embryonic and developmental origins, affecting facial development during early embryogenesis. Facial asymmetry can also be acquired due to pathological conditions, such as neoplasia of the condyle, or as a result of facial trauma , poorly united fractures of the facial bones or the condyle ( Fig. 85.8 ). In the literature, a number of causal factors have been highlighted in the development of facial asymmetries ( Table 85.5 ). ^,

Facial asymmetry subsequent to malunited panfacial fractures.

Courtesy: Dr Pritam Mohanty, Bhubaneswar.

Causes of facial asymmetry can be grouped into three main categories:

• I.

  Congenital, of prenatal origin

• II.

  Acquired, resulting from injury or pathological conditions of the jaws and facial tissues

• III.

  Developmental, arising during the development and of unknown aetiology

When facial asymmetry is part of skeletal malocclusion, there is a decrease of non-structural protein (NSP) genes in the masseter muscle. It has been found that NSP gene downregulation promotes the development of asymmetry. PITX2 expression differences also contribute to both skeletal and muscle development in facial asymmetry.

Evaluation of facial asymmetry and occlusion in situations that are not apparent requires considerable experience and comprehensive clinical observation. The point at which ‘normal’ asymmetry becomes ‘abnormal’ cannot be easily defined, for it is often determined by the clinician’s sense of balance and the patient’s sense of imbalance. ^, According to the study by Beyer and Lindauer, generally speaking, a 2 mm (2.2 ± 1.5 mm) or greater deviation of the dental midline appears to be easily detectable. However, in a recent study, Pinho concluded that midline shifts became perceptible when equal to or greater than 1.0 mm for orthodontists and 3.0 mm for prosthodontists; lay persons saw no alteration on digital alteration of pleasant smile. A systematic review on the subject concluded that, on average, orthodontists were able to detect midline deviations greater than 2.2 mm. On the other hand, laypeople were able to detect only midline deviations greater than 3 mm. Considering that either laypeople or professionals would be part of our society, the authors advised using the most restricted limit of 2.2 mm as a boundary of acceptable midline deviation. With respect to facial morphology, similar quantification stands true, with 4 mm of deviation being easily appreciable. ^,

Clinical examination of an apparent asymmetrical face should focus on the face’s overall balance when the patient is relaxed, talking and walking.

The face is examined for hard and soft tissue asymmetry in the upper third, mid-face and lower third of the face. The clinical examination is perceived in transverse dimensions, differences in vertical heights from right to left, the angulations of the lower border of the mandible, the cant of the occlusal plane, the zygoma and mid-face are assessed for symmetry/deviations. For example, the difference in vertical height of commissures of the oral cavity and cant of the occlusal plane is suggestive of differential skeletal heights of the mid-face.

Soft tissue compensations can partially mask skeletal asymmetry. Soft tissue features can also be quantified by measuring frontal facial photographs. Patients often present with concerns, with a chief complaint of facial asymmetry, after repeatedly seeing their images in the mirror or photographs of themselves. Furthermore, photographs are easy to take, feasible, inexpensive and provide an instant review.

Chin deviation, body inclination difference, gonial angle difference and lip canting should be carefully examined since these are significant variables that differentiate asymmetry from normal subjects. Chin deviation is the most influential factor in assessing facial asymmetry, especially in the lower third of the face. Asymmetry in the lower third of the face, the mandibular region, tends to be perceived more critically than asymmetry in the upper or middle third of the face.

Examination of the face in function is performed by careful observation during the smile, speech and chewing movements. The cant of the occlusal plane and asymmetrical vertical positioning of maxillary teeth (R-L) can be evaluated when the patient is asked to smile. Lip canting can be associated with mandibular asymmetry, maxillary height differences and dental or skeletal occlusal plane canting. At the resting position, lip canting (deviated labial commissure or alar base) on one side often indicates vertical skeletal asymmetry. The lip commissural angulations and lip posture are better evaluated in function while smiling and blowing.

The functional examination also involves the range of mouth opening, deviation of chin during mouth opening, movements of the condyle at TMJ, any crepitus and sounds indicating TMJ pathology, condylar growth and associated pathologies. The functional jaw movements can also be recorded through videography for repeated observations.

Postural head tilting might camouflage a deviated chin. Asymmetric patients tend to tilt their heads to compensate for a deviated menton. The more asymmetric the patients, the more likely they are to tilt their heads.

It has been observed that when a discrepancy is found between skeletal measurements and a subjective evaluation, the influence of soft tissue structures should not be underestimated with regard to facial asymmetry. In brief, the face and facial skeleton are examined for the features in roll, pitch and yaw, independently and in totality ( Fig. 85.9 ).

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