From Volume 16, Issue 3, July 2023 | Pages 137-144
This is the third article in a three-part series considering skeletal discrepancies in the vertical and transverse dimensions. Significant discrepancies in the transverse dimension are relatively rare but present challenges for the orthodontist, in terms of treatment planning, management and relapse.
CPD/Clinical Relevance: This article aims to increase awareness of the assessment, aetiology and management of patients presenting with facial asymmetry.
Assessment of facial attractiveness is subjective and is influenced by social, personal and cultural factors. Facial symmetry is the equilibrium of contralateral facial landmarks in dimension and form around the mid-sagittal plane.1 Asymmetry occurs when there is a difference in the size and relationship of the two sides of the face and can include either the soft tissues alone, or a combination of the craniofacial skeletal complex and the soft tissues. No individual face is truly symmetrical, but imbalance beyond a threshold becomes clinically significant, although this threshold is subjective.2 There is an increasing gradient of asymmetry along the vertical axis, with mandibular disproportion observed more frequently than maxillary disproportion.3 This article provides an overview of the assessment of the transverse dimension and discusses the aetiology and management of patients presenting with transverse skeletal asymmetry.
The face can be divided into equal fifths, using intercanthal distance to define a fifth (Figure 1). This intercanthal distance represents the central fifth, while the width of an eye (inner to outer canthus) on each side are the medial fifths, and the lateral fifth on each side is the distance from the outer canthus of the eye to the ear. While the alar base width often approximates the central fifth, the distance between the mesial margins of the irises often correspond to the distance between the commissures of the lips.
Bilateral symmetry of the facial halves is assessed along the facial midline, an imaginary vertical line perpendicular to the pupillary plane. The facial midline joins the glabella (a point equidistant from the eyebrows), the philtrum of the upper lip and the chin point in symmetrical faces. However, visualization of these points is imprecise. The assessment of midline structures from the superior (bird's eye view) and inferior (submental) views supplements diagnosis (Figure 2).
Dental compensation to camouflage the underlying transverse skeletal asymmetry often results in rotation of the occlusal plane.4 The occlusal cant can be illustrated with a ruler, a wooden tongue spatula or a Fox guide plane held between the teeth. Parallelism of the occlusal plane is assessed relative to the pupillary plane (Figure 3).
As the expression of asymmetry is often multi-dimensional, it is also essential to review the face in the sagittal and vertical planes.
The dental midlines should ideally correspond to the facial midline and be coincident. It is important to determine whether any dental centreline discrepancies are skeletal, dental (due to crowding), or functional (due to a crossbite) in origin.
A change in the dental midline from the retruded contact position to the intercuspal contact position suggests lateral mandibular displacement. Dependent on the direction, the mandibular functional shift can emphasize or disguise an underlying skeletal asymmetry.
The presence of transverse dento-alveolar compensation (such as buccal tipping of upper molars and lingual tipping of lower molars) suggests that a posterior crossbite is more likely to be skeletal in origin and if severe, will not be amenable to orthodontics alone (Figure 4). The absence of such transverse dento-alveolar compensation indicates that the posterior crossbite is more likely to be dental in origin (caused by crowding or a digit sucking habit, for example).
The presence of a cant to the occlusal plane (Figure 5) will also be apparent on intra-oral examination. There is often an upward slant towards the deviated side.
The classification of skeletal facial asymmetry is usually based on the aetiology or presenting morphology, although other classifications are based on time of onset or structures involved. A classification based on aetiology is illustrated in Table 1.5,6
| Congenital | Orofacial cleft Craniofacial microsomia Craniosynostoses Congenital muscular torticollis | |
| Acquired | Unknown | Hemi-facial atrophy Hemi-mandibular elongation/hyperplasia |
| Pathological | Tumour, cysts, infection | |
| Traumatic | Maxillary, mandibular and condylar fractures | |
| Pseudo-asymmetry | Functional displacement | |
Orofacial cleft comprises a group of in-utero anomalies where there is a failure of fusion of the embryonic facial prominences or their elements. Surgical scarring from palatal repair at 9–12 months may also hinder maxillary growth, inducing sagittal and transverse asymmetries. Cleft-affected individuals typically present with deviated anterior nasal spine and nasal septum, a Class III incisor relationship, occlusal cants and posterior crossbites.7
This congenital deformity affects structures derived from the first and second pharyngeal arches and affects both hard and soft tissue growth and development. There is varying severity and usually only one side of the face is affected. Craniofacial microsomia classification, by Pruzansky, was limited to mandibular and temporomandibular joint (TMJ) anomalies.8 The more widely used OMENS (orbit, mandible, ear, facial nerve, soft tissue) classification (Table 2) comprises the extent of anatomical deformity and the associated degree of functional impairment of the orbit, mandible, ear, nerve and soft tissue.9
| O0–3 | Orbit |
| M0–3 | Mandible |
| E0–3 | Ear |
| N0–3 | Facial nerve |
| S0–3 | Soft tissue |
Craniosynostosis describes the premature fusion and restricted growth potential of the affected cranial sutures. Compensatory growth occurs at other sutural sites to accommodate the growing brain. The extent and location of involvement dictate the characteristic calvaria shape. Beyond the cranial deformation, some patients with syndromic craniosynostosis (as in Crouzon's disease or Apert's syndrome) exhibit mid-face hypoplasia. This dysmorphology can lead to mid-lower facial asymmetry.10
Congenital muscular torticollis is abnormal unilateral shortening of the sternocleidomastoid muscle (SCM) in infants, leading to an ipsilateral head tilt to the shoulder, with rotation of the face and chin to the opposite side. In a small percentage of individuals, mild craniofacial asymmetry remains despite the resolution of the torticollis.
Parry-Romberg syndrome is a rare but self-limiting acquired disorder of unclear origin. There is unilateral progressive atrophy of the facial skin, subcutaneous tissue and occasionally, the osseocartilaginous facial structures.
Unilateral hyperplasia of the condyle can extend to include the condylar neck and terminate at the mandibular symphysis of the affected side, resulting in 3D enlargement described as hemi-mandibular hyperplasia.11 The condition typically presents before puberty, with progressive vertical and transverse displacement of the mandible, accompanied by compensatory asymmetric growth of the maxilla.
Unlike hemi-mandibular hyperplasia, hemi-mandibular elongation manifests solely in the transverse dimension, markedly displacing the mandible to the unaffected side. The absence of vertical elongation also means the lack of compensatory occlusal features like occlusal canting. An individual can manifest hybrid forms of hemi-mandibular hyperplasia and hemi-mandibular elongation.
Several pathological conditions may contribute to facial asymmetry. Conditions leading to height discrepancy in the TMJ (such as condylar resorption, condylar hyperplasia or enlargement) can manifest as deviation of the mandible. Proliferative lesions such as fibrous bone dysplasia may also present with asymmetry.
Untreated or poorly managed facial fractures can result in conspicuous facial deformity. The condyles are particularly vulnerable to injuries, typically caused by blunt force to the chin. Post-injury unilateral ankylosis of the joint can lead to the development of facial asymmetry with growth, characterized by deviation of the lower face to the affected side.
Premature contact of teeth upon closing leads to displacement of the mandible from the initial contact into maximum intercuspation. These occlusal interferences may be localized (mispositioned tooth, or unadjusted dental restoration) or generalized. Mild transverse maxillary discrepancy can give rise to maxillary-mandibular cusp-to-cusp occlusion, causing a dysfunctional lateral mandibular shift into a unilateral posterior crossbite. Clinically, there may be significant dental or facial midline shifts.12 Anecdotally, individuals with functional displacements can adapt and develop skeletal asymmetry.
Patients exhibiting facial asymmetry (Figure 6) should have extra-oral photographs taken from both left and right sides. Photographic birds-eye and worms-eye views of the mandible, as well as frontal view photographs of the patient in occlusion biting on a tongue spatula, are helpful additional records (Figure 3). Patients presenting with a mandibular displacement should have intra-oral photos taken in both retruded contact and maximum intercuspation positions.
The dental panoramic tomogram allows a comparison of the size and shape of hard tissue structures across left and right sides of the mandible, including the mandibular rami and condyles (Figure 7). Posterior-anterior cephalometric radiographs also allow comparison of left and right bony structures, using the mid-sagittal plane as a reference to locate any facial asymmetry (Figure 8). These radiographs can be examined via different approaches, including anatomic, bisection and triangulation methods.5 A lateral cephalogram can also be used, with two lower mandibular borders and accentuated gonial notching commonly identified in cases of mandibular asymmetry (Figure 9). Specific pathology associated with the temporomandibular joints can be investigated further with transcranial or transpharyngeal radiographic views.13
Two-dimensional radiographs should be interpreted with caution when assessing skeletal facial asymmetries. For example, the apparent side of a facial asymmetry can switch with an altered patient head rotation of only five degrees.14
Cone-beam computed tomography (CBCT) uses a 360-degree rotation of an X-ray tube around the patient's head to acquire a 3D image of the hard and soft tissue structures of the face. Anatomical structures can be assessed through 2D slices, or through three-dimensional (3D) reconstruction (Figures 10 and 11). CBCT enables precise comparison of right and left hard and soft tissue asymmetries.16 Additionally, digital 3D ‘image fusion’ processes, from two or more different imaging techniques, are realistic and accurate tools for both planning and evaluating orthodontics and orthognathic surgery.17
Laser surface scanning and stereophotogrammetry are non-invasive and reproducible imaging techniques that allow 3D analysis of facial morphology and symmetry. Stereophotogrammetry uses multiple photographs of the same object at different angles to acquire 3D images. It is possible to superimpose 3D images digitally to assess changes over time. Laser scanning has been used for quantitatively assessing facial symmetry in adolescents.15
Articulated study models with a facebow record demonstrate the relationship of the jaws in all three planes, allowing accurate assessment of a patient's functional occlusion and any arch asymmetries. Digital intra-oral scanning allows the production of 3D digital models, and a combination of intra-oral scanning and CBCT facilitates virtual diagnosis, as well as surgical planning if required.
Technetium radioisotope imaging (Figure 12) can be used to detect tumour pathology or hot spots of active growth in the condyles, allowing an assessment of whether the skeletal asymmetry is progressive or stable. This technique uses a short-lived gamma-emitting isotope in target tissue, imaged with a gamma camera. Results should be interpreted with caution, owing to a high number of false positives, but a difference of 10% of more between the two condyles is suggestive of continued growth and potential worsening of the asymmetry.13
During clinical planning, it is essential that the individual's concerns have been identified and an accurate diagnosis of the cause of the asymmetry has been established. Skeletal asymmetries can be difficult to diagnose and manage, particularly when compensatory growth has occurred on the contralateral side and in the opposing arch, secondary to the true defect. Timing of treatment is important, particularly if cessation of abnormal growth is required, or where an interceptive approach in the case of mixed dentition or early adult dentition may be appropriate. In all cases, it is important to establish realistic objectives and ensure that patient expectations are carefully managed.
Functional displacements of the mandible that lead to facial asymmetry may be treated with orthodontics alone. The aim of orthodontic treatment in such cases is to eliminate a crossbite associated with mandibular displacement, often through maxillary expansion (Table 3).
| Upper removable appliances with a midline expansion screw |
| Quadhelix appliance |
| Rapid maxillary expansion |
| Fixed appliances with archwire expansion, cross-elastics or auxiliary archwires |
In the case of early mixed dentition (patients aged 7–11 years), a posterior crossbite can be corrected using a quad-helix or expansion plate, with evidence suggesting the quad-helix is more effective, although there is insufficient evidence to draw conclusions regarding the stability of crossbite correction.19
A mild-to-moderate mandibular asymmetry identified in the young growing patient may be managed with early intervention using a hybrid functional appliance. Here the appliance is designed to stimulate differential dento-alveolar eruption and growth of the mandible on the affected side. Such treatment may simplify or reduce the need for future surgical treatment, but is dependent on excellent patient compliance and favourable growth.
Mild transverse skeletal asymmetries may be accepted and treated with orthodontic camouflage alone. Non-coincident dental centrelines can be managed using asymmetric extraction patterns, asymmetric elastics, anterior cross elastics and push-pull mechanics.
Pathological causes of transverse skeletal discrepancies require referral to the appropriate specialist team for further investigation and appropriate management. Surgical treatment of such cases should only be considered after stabilization of the primary pathology.
Young patients with craniofacial microsomia may be treated with distraction osteogenesis or a costo-chondral bone graft of the condyle. However, evidence suggests that there are no significant differences in the surgical outcomes when comparing patients with craniofacial microsomia treated with and without early mandibular distraction osteogenesis.20 In severe cases of hemi-mandibular hyperplasia or hemi-mandibular elongation, condylar shaving of the abnormal cartilaginous growth site may be performed.
Some facial asymmetries require a combined orthodontic-orthognathic approach at the end of growth. Pre-surgical orthodontics involves decompensation of the dental arches to allow maximal correction of the underlying skeletal asymmetry at surgery, as well as CL correction relative to the dental bases and levelling of any transverse occlusal cants. Mandibular surgical correction may require bilateral sagittal split osteotomy with asymmetric movements. An extended sagittal split osteotomy may be used when mandibular shape change is required, for example in hemi-mandibular hyperplasia.21 Transverse cants in the maxillary occlusal plane may be corrected with repositioning the maxilla with a Le Fort I osteotomy. More extensive facial asymmetries may require staged orbital and/or craniofacial surgeries.
In severe cases of mandibular asymmetry, distraction osteogenesis (Figure 13) can be used to differentially increase the length of the mandibular ramus and body. This is the preferred method of choice for patients with syndromes who require larger advancements of 10 mm or more, owing to improved functional and aesthetic results.22
Secondary surgical procedures may be necessary to address skeletal deficiencies or excess, through augmentation or reduction surgery. For example, recontouring the lower mandibular border helps compensate for the excess vertical growth seen in hemi-mandibular hyperplasia. A sliding genioplasty may be performed to reposition the chin point.23 Alternatively, overlay alloplastic implants can be used. Soft tissue deficiencies can be disguised by Coleman fat transfer (if mild), or a free flap transfer (for larger defects).
Following treatment, the retention phase aims to stabilize the corrected jaw relationships. Good intercuspation post-surgery can help to improve post-operative stability. A hierarchy of surgical stability in orthognathic surgery has been established and mandibular surgical movements are generally considered to be stable.24 The surgical procedures deemed highly unstable include maxillary expansion, downwards movement of the maxilla,24 and clockwise rotation of the mandible with bicortical screw rigid internal fixation in the sagittal direction.25 Vertical extension of the ramus, which may be necessary for addressing TMJ pathology or trauma, is also thought to be unstable.26
Accurately diagnosing the aetiology of a skeletal asymmetry is imperative prior to treatment planning. Functional asymmetries caused by an occlusal interference may be treated with orthodontics alone, while skeletal asymmetries may require growth modification or finite surgical correction once growth has ceased. The treatment should address the patient's concerns and achieve a functional and aesthetic outcome.
