Orthognathic surgery in asymmetric cases is challenging because of diversity and individuality. Clinical observations are of paramount importance and need to be systemically thorough. Three-dimensional diagnosis and virtual planning have been proven extremely helpful in facilitating treatment toward symmetry in difficult cases with increasing precision. Compared with orthognathic surgery in symmetric situations, asymmetries produce numerous pitfalls and provide opportunities for out-of-the-box procedures.
Key points
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Outline the clinical assessment of facial asymmetry for orthognathic surgery, how to assess, and the role of posture.
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Differences and advantages of 3D virtual planning for orthognathic surgery in asymmetric cases compared with symmetric situations.
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Surgical techniques and additional procedures in treating facial asymmetry with orthognathic surgery.
Introduction
Management of facial asymmetry through orthognathic surgery can be a challenge. Not only the direction of asymmetry needs to be corrected, but also the shape and form of individual skeletal parts are frequently affected by remodeling and may require additional adjustments.
Restoration of asymmetry may be appointed for both functional and aesthetic reasons. A proper definition of asymmetry or symmetry from both the clinician’s and the patient’s point of view is key when starting any surgical journey. The influence of symmetry on facial attractiveness is subject to discussion and seems to be related to facial normality. Slight asymmetries in proportional faces can contribute to attractiveness, and symmetric disproportional faces can be considered unattractive. Perception of these facial characteristics can differ between professionals and patients. Effective communication is, therefore, of utmost importance, and it might be useful to counsel patients with the use of a framework with categories of facial asymmetry. ,
Etiology
Apart from the definition and proper communication about perception, knowing the origin of facial asymmetry is helpful when aiming for correction. It can be categorized in various ways, such as based on anatomic region, congenital versus acquired, origin of tissue, or neurologic background. The nature of asymmetrical tissue seems to be most important for clinical impact and treatment decision in orthognathic surgery and can be subdivided into dental, skeletal, and soft tissue features.
Soft tissue asymmetries are differentiated in separate components (eg, muscle, fat, skin, or all components) and expressed in overgrowth/undergrowth, and/or neurologic background. Masseteric hypertrophy, lipomatosis, or hemifacial hyperplasia reflect these different layers and do not adjust to orthognathic corrections as a matter of course. Impairment of any motor nerve, such as the marginal branch of facial nerve, might influence lip dynamics and cause canting of the lip, which is not possible to correct with orthognathic surgery. The clinician should be aware of the limitations of orthognathic surgery and address this during consultation, as part of expectation management.
Dental asymmetries, for example, in primary failure of eruption, can mimic skeletal asymmetries in extra-oral assessment and can be a pitfall when starting orthognathic treatment without simultaneous three-dimensional (3D) evaluation of the underlying skeletal asymmetry or compensational changes.
Skeletal asymmetries are also the result of overgrowth or underdevelopment of the bony tissue, either congenital or acquired. The facial skeleton forms the basis of the resulting expression of the face, and should, therefore, be carefully studied at the beginning of the work up for a surgical treatment plan. The primary location of bony asymmetry should be detected, as well as any existing compensational changes.
Within the spectrum of skeletal facial asymmetrical disorders, the temporomandibular joint plays an important role. Forced bite or disc-displacement can mimic a (skeletal) asymmetry, which disappears when the mandibular condyles are guided properly into their fossa. This can be a challenge, especially in patients with persistent Sunday-bite or high muscle tension. It is advised to perform dynamic as well as static asymmetry assessments of the whole face, as differences can occur between them, which need to be addressed when planning corrective surgery.
A crucial factor in planning corrections for facial asymmetries is the presence or absence of progressiveness of disease. When ongoing changes are observed, no corrections should be performed until progression has ceased. Recognition of progression is key and clinicians should be aware that this can occur in both overgrowth conditions such as unilateral condylar hyperplasia (UCH) as in hypoplastic or degenerative disorders such as Parry–Romberg syndrome. Congenital asymmetrical conditions such as hemifacial hyperplasia, which grows proportional, or craniofacial microsomia, which is not progressive but can show increasing asymmetry due to continuous growth on the “normal” side, should be distinguished from this.
Detailed diagnostic criteria for all the asymmetrical overgrowth and undergrowth conditions are beyond the scope of this article. However, when assessing asymmetry, the abovementioned enumeration can be kept in mind. The following paragraphs will give a more detailed guidance towards clinical assessment, planning and pitfalls of orthognathic surgery in asymmetric patients.
Clinical assessment
Challenges in the Determination of Facial Asymmetry
With regard to its clinical assessment, facial asymmetry can be considered a variation in size, shape or arrangement of facial landmarks between 2 sides of the face, split by an imaginary facial midline. A reliable determination of this facial midline is essential in every attempt to assess facial symmetry, but it also represents an important issue, as frequently used reference points might not represent the true median sagittal plane.
Instead of being directional, asymmetric features can occur in the lower, mid-and upper parts of the face “fluctuating” between the left and right sides. As explained in the etiology section, they can originate from skeletal, soft tissue, dental and functional structures. As the degree of asymmetry can differ between soft tissues and underlying hard tissues, they can potentially mask or compensate for any existing asymmetry in another structure.
The most frequently used techniques for the clinical assessment of facial asymmetry make use of the natural head position (NHP). However, postural compensations for facial asymmetry, for example, by tilting the head slightly, are frequent and can seriously hinder clinicians in correctly characterizing facial imbalances.
Qualitative Evaluation of Asymmetries
The foremost step in assessing facial asymmetry is direct clinical examination. While a complete instruction on the clinical examination of the face falls outside the scope of this article, some clinical tips in the light of asymmetry are supplied. At first, a visual and palpable inspection of facial structures and contours should be performed. During this examination, it is important to keep the patients’ face completely unveiled. It can be beneficial to compare important midline landmarks (ie, soft-tissue glabella, nasal tip, and pogonion) to the true facial midline, either from standing behind the patient or from a worm’s-eye perspective. In challenging cases, it can be of additional value to “block” parts of the face with a piece of paper and mark the landmarks of the lower, mid, and upper facial parts individually, with the use of a marker, before doing a comparison to the facial midline. Another technique to evaluate an asymmetric lower part of the face is to guide the mandible laterally to align the chin with the facial midline and to evaluate subsequent occlusal and facial changes. The orientation of the occlusal plane can be evaluated through an extra-oral examination by holding a spatula between the canines and compare its orientation with the interpupillary line. It is essential to ensure a NHP in this step, which is further explained in the next paragraph on 3D planning. Spatulas with a stepwise increase in thickness on one side can be potentially useful to quantify any existing differences.
Intraorally, traditional occlusal traits should be evaluated. Special attention should be paid to any occlusal interferences that might cause functional mandibular shifts and therefore induce or mimic mandibular asymmetry. Next to the dental midlines, it can be of additional value to measure the amount of gingival show in the maxilla on both sides separately.
Quantitative evaluation of asymmetries
Although clinical examination is essential to evaluate qualitative facial asymmetry, it is less suitable for its quantification. Digital photography can have additional value to the clinical examination, but quantification is not possible as there is geometric distortion due to magnification errors. To quantify and deepen the evaluation of facial asymmetry, several techniques have been developed using laser surface scanning technology, 3D-stereophotogrammetry , and 3D radiographic imaging. One of the major advantages of quantification is the possibility to monitor asymmetries during follow-up examinations. Despite their advantages, these techniques should be considered supportive of the diagnostic process and not as a replacement for a comprehensive clinical examination.
Three-dimensional stereophotogrammetry can be used to objectify and quantify facial soft-tissue asymmetries in all 3 planes with several different techniques. The face can be mirrored using an arbitrary plane outside the face, followed by cranial base registration, which eliminates the problem of identifying a midsagittal plane. In another technique, the face is compared with an aligned perfect symmetric dummy. These methods do not need exposure to radiation, are low-cost, fast and landmark-independent. The latter is important as the traditional landmark-based evaluation of asymmetry, also in 3 dimensions, does not take entire facial structures into account and performs less than surface-based methods.
For skeletal asymmetry assessment, the use of traditional landmark-based radiography (two-dimensional) has long been deemed insufficiently reliable. To evaluate the complex nature of skeletal facial asymmetry, 3D evaluation is essential. Due to improvement in (cone-beam) radiological technologies, soft tissues can also be reliably evaluated with these techniques. With different methods such as surface area, distance, or volumetric measurements, morphologic asymmetries can be evaluated in detail.
Next-Generation Techniques
Machine learning techniques can be particularly useful to understand and analyze complex data with many variables. Modern learning techniques in orthognathic surgery are explored in many ways, for example, to assess the impact of orthognathic treatment on facial attractiveness and automated cephalometric landmark detection in 3D imaging. These techniques can be potentially useful for objectively determining facial asymmetry. Modern computer vision algorithms have been used to automatically process 3D images and construct high-dimensional statistical models of the facial shape. In other scientific efforts, an algorithm was designed that aids in the determination of the facial midline and allowing for real-time dynamic analysis of facial asymmetry. Despite the fact that these techniques are still in a developmental phase, they are specifically developed to overcome some of the mentioned difficulties in the assessment of facial asymmetry. It can be expected that it will significantly change the way we assess facial asymmetry in the next few years.
3D virtual planning
Head Position for Planning
Determining the head position for planning (HPP) is fundamental for 3D virtual planning. The HPP might be the patient’s NHP, but these are not identical by definition. Usually, the HPP is established using a combination of clinical assessment and clinical photographs. During the clinical assessment, important parameters to evaluate are the interpupillary line, the maxillary midline deviation compared with the facial midline, and the dental show at rest and smiling. Clinical photographs should be obtained at rest and smiling, in both frontal and lateral views.
The approach for determining the frontal HPP depends on the degree of asymmetry in the midface and upper face areas. In case of predominantly lower face asymmetry (eg, UCH), the HPP can often be established by assuming a horizontal interpupillary line and by using the symmetry of the upper midface and forehead ( Fig. 1 ). Then, the midsagittal plane is defined through the center of the nose bridge. The resulting midline deviation of the maxilla should be compared with the clinical measurements. If there is no consensus among these 2, either one of the head positions is chosen, or a compromise between the 2 is sought.
In case of extensive facial asymmetry (eg, craniofacial microsomia with orbital involvement), the interpupillary line and the symmetry of the midface and upper face areas are not reliable. In such cases, the NHP of the patient can be used for planning ( Fig. 2 ). To find a reproducible NHP, clinical photographs should be obtained on multiple occasions. The average head position on these photographs should be used as HPP. Using a mirror can assist the patient in finding the most natural and comfortable head position. The rotation of the camera should be standardized by using a tripod or by having a true vertical line visible on the photographs, such as a vertical laser line.
