Introduction

Symmetry may be defined as correspondence in size, shape and relative position of parts on opposite sides of a dividing line or median plane. Asymmetry is defined as a lack or absence of symmetry. The application of this definition to the human face illustrates an imbalance or disproportionality between the right and left sides of the face. No human face exhibits perfect bilateral symmetry. Although a mild degree of asymmetry is normal and acceptable in the average face, greater degrees of asymmetry have been correlated with clinical depression, low self‐esteem and other health problems associated with poor quality of life, such as neurosis and an inferiority complex.¹ A number of studies have been published in the psychology literature regarding facial asymmetry and perceptions of attractiveness.^2,3 These studies report that facial symmetry is an important factor in facial attractiveness.

Perfect bilateral body symmetry seldom exists, and mammals in particular have marked asymmetry in the size, shape and position of the internal organs. Brain function also exhibits asymmetry, a simple example being right and left‐handedness. Craniofacial asymmetry may result from discrepancies in the size, shape or relative position of individual bones in the craniofacial complex and/or the overlying soft tissues.

Peck et al.⁴ evaluated bilateral facial skeletal symmetry in 52 ‘exceptionally well‐balanced’ white Caucasian adults and observed that the degree of asymmetry was greatest in the lower facial third, and that asymmetry reduced as the cranium was approached. Severt and Proffit⁵ analysed data from patients with dentofacial deformities over an 18‐year period. Of the one‐third that exhibited facial asymmetry, the lower facial third was affected much more frequently than the middle or upper thirds. Only 5% had asymmetry of the upper face, 36% had asymmetry of the mid‐face, mainly just involving the nose, whereas 75% had a deviation of the chin.

Relationship between symmetry and proportion

Facial proportions and facial symmetry are separate yet interrelated entities. In fact, the classical Greek descriptions of symmetry (Greek ‘symmetria’ συμμετρία: a term blending the concept of symmetry with ‘ideal’ proportions) and proportion (Greek ‘analogia’ αναλογία: proportion) are entirely interwoven.

In the first century BC, the Roman architect Marcus Vitruvius Pollio wrote the 10‐volume De Architectura (‘On Architecture’). In Book III, Chapter 1, Vitruvius wrote:

‘The design of a temple depends on symmetry, the principles of which must be most carefully observed by the architect. They are due to proportion, in Greek ‘analogia’ (αναλογία). Proportion is a proper agreement between the measures of the parts of an entire work, and the relation between the different parts and the whole general scheme, in accordance with a certain part selected as standard. The principles of symmetry result from proportion. Without symmetry and proportion there can be no principles in the design of any temple. Thus, in the well‐proportioned human body there is a kind of symmetrical harmony … and so it is with perfect buildings.’⁶ [emphasis added]

The principles that Vitruvius described for architecture were based on those described by classical Greek sculptors. These same principles of correct proportions and symmetry are the foundation of diagnosis and treatment planning in the management of craniofacial deformities.

Clinical evaluation

Purpose of the clinical evaluation

When evaluating the craniofacial structures for asymmetry, the purpose of the clinical examination is threefold:

• Qualitative analysis: Identifying the specific location(s) of the facial asymmetry (skeletal and/or soft tissue).
  
• Quantitative analysis: Objectively quantifying the degree of facial asymmetry.

Having undertaken a thorough clinical examination, the third step is:

• Craniofacial growth assessment: To determine, as far as possible, the location(s), magnitude and direction of any further growth of the craniofacial complex.

As with all systematic clinical facial evaluation, an overall qualitative (visual, anthroposcopic) assessment is followed by a comprehensive quantitative (anthropometric and cephalometric) analysis. Having obtained the necessary data, a further qualitative assessment is carried out, to determine whether the results of the quantitative assessment corroborates the qualitative evaluation (i.e. look at the patient, then measure the face, and then look at the patient again).

The clinical evaluation should begin with the frontal facial examination.

Frontal facial examination

Bilateral symmetry

The face must be examined for bilateral symmetry with the patient in natural head position (NHP), bearing in mind that a small degree of asymmetry is present in most individuals and essentially normal. The clinician may need to make minor manual adjustments to the patient’s head position, as patients may have developed a compensatory head posture to minimize the aesthetic impact of their facial asymmetry, often tilting their heads to the right or the left, around the sagittal axis (Figure 10.1).

Facial midline (midsagittal plane)

With the patient in NHP, a true horizontal line may be drawn through the mid‐glabellar region (soft tissue glabella), midway between the eyebrows, perpendicular to a true vertical, e.g. a plumb line from the ceiling. From this true horizontal line, a perpendicular facial midline (midsagittal plane) may be constructed from glabella (Figure 10.2).

In less severe dentofacial deformities, the mid‐philtrum of the upper lip (midpoint of Cupid’s bow) will commonly be in the midline of the face, except in exceptional circumstances, e.g. cleft lip or severe nasal asymmetry distorting the upper lip. A line joining this point to soft tissue glabella forms the facial midline. In the symmetrical face, this line will extend to the midpoint of the chin (soft tissue pogonion). The midpoint of the nasal bridge and the nasal tip (pronasale) should be on this line if the nose is symmetrical (Figure 10.3).

With the lips slightly separated, the maxillary and mandibular dental midlines may be assessed in relation to the facial midline, in particular to the mid‐philtrum of the upper lip (midpoint of Cupid’s bow). Having obtained the patient’s permission, these points may be marked on the patient’s face with a skin pencil, in order to permit more accurate assessment (Figure 10.4).

Vertical reference lines/planes (Figure 10.5)

Having established the facial midline, further vertical lines may be drawn parallel to the facial midline through any pair of bilateral landmarks suspected of being asymmetrical. It is then possible to assess:

• The orientation of these lines in relation to the facial midline and the bilateral structures that they pass through.
  
• The horizontal distance of each of the bilateral landmarks to the facial midline.

Horizontal (transverse) reference lines/planes (Figure 10.6)

Horizontal reference lines may be constructed by connecting bilateral landmarks. It is then possible to assess:

• If the lines are perpendicular to the facial midline.
  
• The relative orientation of the various lines, which should ideally be parallel to one another, in the absence of transverse cants.
  
• The vertical level of bilateral structures to a baseline horizontal reference line, e.g. left and right cheilion (the angle of the mouth) to a horizontal line through subnasale (Figure 10.7). Such linear measurements may be used anthropometrically or on facial photographs and three‐dimensional (3D) facial soft tissue scans if landmark identification is reliable.

The presence of a cant in the transverse occlusal plane may be observed in relation to the interpupillary line (or a true horizontal plane) with the patient biting on a wooden spatula (or holding it against the maxillary teeth if there is a lateral open bite), both in the incisor/canine region and the premolar/molar region (Figure 10.8), or a Fox’s occlusal guide plane (Figure 10.9). In the absence of a transverse maxillary cant and/or vertical orbital dystopia, the transverse occlusal plane should be parallel to the interpupillary plane.

Following the frontal facial clinical examination, the asymmetrical face may be evaluated clinically and photographically from a number of different views.

Superior view (Figure 10.10)

The superior (bird’s eye) view is taken directly from above and behind, with the patient’s head hyperextended. This view is useful for assessment of the symmetry of the nasal bridge, and the horizontal projection of the forehead, orbits and particularly the zygomas and paranasal areas.

Submental view (Figure 10.11)

The submental (worm’s eye) view is taken directly from below, with the patient’s head hyperextended. This view is required to assess the symmetry of the mandibular body and the base of the nose, and is also useful for assessing any asymmetrical projection of the forehead, orbits, zygomas and paranasal areas.

Lateral view

The left and right profile views may be used to compare the contour of the left and right sides of the face. This view is particularly useful for assessment of asymmetry in the contour of the inferior border of the mandible.

Oblique lateral (three‐quarter) view

These views are taken with the patient in NHP, and are useful for assessment of asymmetry of the nose, zygomas, paranasal areas and the mandibular ramus and body outline, including the gonial angle.

Transverse occlusal plane view

This view is ideally taken with the patient in NHP. An orthodontic cheek retractor may be placed in order to retract the lips and cheeks (Figure 10.12); alternatively, an assistant may stand behind the patient and retract the cheeks using photographic retractors (Figure 10.13). A transverse cant of the maxillary occlusal plane may be assessed in relation to a true horizontal plane or to the interpupillary plane in the absence of vertical orbital dystopia.

Dynamic clinical evaluation

Mandibular lateral displacement

The basic position of the mandible in relation to the rest of the facial skeleton is referred to as the rest position. The muscles acting on the mandible are in a relaxed state and the condyles are in retruded, unstrained positions within the glenoid fossae. This overall position is also referred to as centric relation. In the rest position, the teeth are out of contact by 2–3 mm, referred to as the freeway space. When the teeth occlude into maximum intercuspation, the mandible should also be in a position of centric relation. Ideally, the path of closure of the mandible from the rest position to the position of maximum intercuspation of the teeth should be a simple hinge movement over the 2–3 mm of the freeway space, with the condyles rotating in their glenoid fossae.

Occasionally as the path of closure starts from the rest position, there is an initial, premature contact of the upper and lower teeth. This is uncomfortable for the patient, necessitating a displacement of the mandible to obtain maximum intercuspation of the teeth. If this displacement or functional shift of the mandible is in a lateral direction, then the patient may exhibit an apparent or pseudo‐ mandibular asymmetry (Figures 10.14 and 10.15).

It is important to evaluate the mandible on opening and closing, in order to check for displacements/functional shifts. Assess the patient from in front, and then stand above and behind the patient as they open and close the mouth, in order to assess any lateral mandibular movement on opening and/or closing. If a lateral mandibular displacement is present, the asymmetry should reduce as the patient opens the mouth, with the upper and lower dental midlines becoming better aligned (Figure 10.16).

With longstanding mandibular displacements, the pattern of muscular activity is sometimes such that the displacement cannot be identified by observation alone. Therefore, if no displacement is observed, it is important to double check by clinically manipulating the mandible into occlusion. After explaining to the patient what you are going to do, place the knuckle of your curled index finger under the patient’s chin and your thumb on the mental eminence. Ask the patient to relax, to open their mouth and roll the tip of their tongue towards the back of their throat, then apply gentle pressure to move the mandible up and down, aiming to keep the condyles in their retruded position within the glenoid fossae. Look for the initial, premature dental contact; the patient may be better able to identify this area and point it out to you. Having established the premature contact, ask the patient to bite together and the displacement will become evident.

It is absolutely vital to check for mandibular displacements in cases of mandibular asymmetry, as missing a lateral mandible displacement may lead to the incorrect diagnosis of a true (i.e. skeletal) mandibular asymmetry and inevitably to incorrect treatment. It is also important to bear in mind that a patient may present with a combined true mandibular asymmetry and a lateral mandibular displacement (Figure 10.17). The degree of true mandibular asymmetry must always be determined from the undisplaced position.

Asymmetrical facial animation

Some asymmetry in facial animation is relatively common. It is important not to mistake an asymmetry in facial animation with a dentoskeletal asymmetry, e.g. an asymmetrical smile where the levator muscles of one side cause greater elevation of the upper lip than the contralateral side may be mistaken for a transverse cant in the maxillary occlusal plane (see Chapter 23).