From Volume 12, Issue 1, January 2019 | Pages 29-31
Discrepancies between dental and facial midlines are not uncommon and these may have a dental or a skeletal cause. When planning orthognathic surgery, a decision must be made whether to accept or correct these. This paper presents a simple technique that uses graph paper to evaluate any midline discrepancies as an aid to pre-treatment discussion and explanation with the patient.
Facial symmetry is long established to be a principal component of an aesthetically pleasing face.1 A symmetric face is perceived as ‘more beautiful’ by layperson observation.2 Perfect bilateral symmetry, however, is rare and the face often has a mild degree of asymmetry, not perceptible on normal everyday observation.3
Prevalence of facial asymmetry in orthodontic patients has been reported as 12%–37% in the US.4,5 This increased to 50% when assessed radiographically.6 A CBCT study by Gribel et al in 2014 demonstrated a similar prevalence rate of 44% of mandibular asymmetries in 250 Class I patients.7
Asymmetry may lie at the hard or soft tissue level and involve skeletal, dental, muscular or functional components.8 Both developmental and environmental influences may be implicated when assessing the aetiology. Chia et al outline four main causes:
Often the true aetiology may not be clear and thus ‘asymmetry of development’ just becomes apparent throughout growth.
A thorough assessment of facial symmetry is important from the patient's first consultation. Transverse and vertical dimensions may be assessed from an extra-oral frontal view. The facial midline is constructed using trichion and glabella. Coincidence of the maxillary dental midline may then be assessed, both to the facial midline and the mandibular centreline. Nasal deviation may distort midline assessment and there may be an acquired postural compensation, for example, a head tilt that camouflages the extent of the asymmetry.10
Supplementary imaging techniques, such as photographs, tomography and bone scintigraphy, can help precisely and accurately determine the relative contribution of the structures contributing to the asymmetry. Plain film radiographic investigation may include lateral cephalograms or panoramic views. Overlapping of left and right sides affords limited value to lateral cephalograms. Panoramic views are useful to assess mandibular asymmetry but can also be affected by head positioning or a head rotation. Posterior-anterior radiographs are more suitable for assessment of asymmetries as bilateral structures can be compared.11 Cone beam computed tomography (CBCT) is undoubtedly the imaging technique of choice for the most accurate investigation.12 The higher associated radiation doses, however, may be of concern to the patient, who may have undergone numerous previous plain film radiographs up to the point of orthognathic surgical planning.
Orthodontic management of facial asymmetries is limited to the aim of achieving coincidence of the maxillary dental centreline with the mid-facial axis. However, when an orthognathic surgery approach is used, symmetry, or lack thereof, is important to determine from the outset. The treatment plan may aim to accept or correct a dental or skeletal asymmetry.
A means of graphically representing the radiographic tracing from a posterior-anterior cephalogram to detect and measure a facial asymmetry will now be described.
A 19-year-old male patient presented with a Class III incisor relationship on a moderate Class III skeletal base with increased vertical proportions. His malocclusion was associated with a 4 mm reverse overjet, 2 mm anterior open bite, and bilateral buccal crossbites without displacement.
He previously had undergone a course of upper fixed appliance treatment aged 13, involving extraction of ectopic maxillary canines.
The orthognathic surgery plan was for a 4 mm maxillary advancement with a 2 mm posterior differential impaction and mandibular set back to Class I.
On initial extra-oral examination, the upper centreline appeared to be 5 mm to the right of the mid facial axis (Figure 1). Further examination using trichion and glabella as a reference for the facial midline, indicated that the maxillary dental centreline was, in fact, correct to the mid facial axis. The nose and philtrum were deviated to the left, contributing to the appearance of an apparent discrepancy between maxillary dental and facial centrelines. The mandibular dental centreline was coincident with the maxillary centreline.
Due to the significant nasal deviation, it was difficult to establish the correct facial midline fully. There was a variance of opinions between clinicians involved in the patient's care with regard to the correct midline position. In addition, the patient himself was uncertain and needed further reassurance that the maxillary dental midline was correct to facial midline when there appeared to be such a visual discrepancy. Due to the size of the apparent midline discrepancy, further imaging was felt necessary to verify the coincidence of the centrelines. The patient was advised that this appearance of a centreline discrepancy may become more obvious to him following surgical maxillary advancement and thus may necessitate correction of the nasal deviation.
A posterior-anterior cephalogram was therefore taken (Figure 2). The radiograph was traced using a 0.7 mm tip pencil and this tracing was transferred onto 1 mm engineering graph paper (Figure 3). This allowed for ease of visualization and measurement of bilateral structures. The tracing indicated that the maxillary and mandibular dental centrelines were correct to each other and coincident with the facial midline. The analysis indicated that there was a panfacial asymmetry in the transverse plane, with the right half of the face being relatively wider than the left.
This case shows the importance of accurate diagnosis and treatment planning. It is essential to establish the correct position for the maxillary dental centreline using accurate reference points. A nasal and/or philtrum deviation can complicate the identification of the true facial midline and can result in a compromised final outcome.
CBCT is the imaging technique of choice for the most accurate investigation. The Sedentexct guidelines suggest their use for assessment of complex cases of skeletal abnormality.12 There is, however, a significantly higher radiation dose associated with CBCT, and the additional information obtained may not always justify the additional radiation exposure.
A posterior-anterior (PA) cephalogram offers an additional radiographic view that supplements the findings of other plain film views. It allows for the evaluation of vertical and/or transverse facial asymmetries, the width and transverse positions of the maxilla and mandible, and the relative vertical dimensions of bilateral osseous and dental structures.11 The midline structures are used to quantify the asymmetry as deviation to the left or right sides.13
Posterior-anterior cephalographs lack the standardized and reproducible technique of lateral cephalography.14 The head position is not easily reproducible and superimposition of structures can complicate accurate landmark identification. However, it has been shown that transverse measurements or widths are least affected by positional errors.15 The Ricketts PA cephalographic analysis is the most widely used quantifiable method of analysis. This provides age-dependent normative values.16 Measurement of a patient relative to normative values, however, often has limited use. In this case, it was more important to localize the site of asymmetry and plan its influence on the surgical outcome than to see how far the patient deviated from a normal range.
Thus by plotting the radiographic tracing on graph paper, it provided a clear visual aid to the identification of potential asymmetries. It assisted in discussion with the patient, enabling him to visualize the correct position of the centrelines and better appreciate the overall facial asymmetry and the likely effects of surgery. This approach maximizes the chances for optimal understanding and for obtaining truly informed consent from the patient.
Facial asymmetry is a common finding in orthodontic and orthognathic patients. An accurate assessment and diagnosis is imperative if an optimal treatment outcome is to be achieved. The use of a posterior-anterior cephalogram and graph paper tracing may prove to be a useful technique to examine the coincidence of dental and skeletal centrelines further and assess the overall asymmetry.
