Midline deviations
Dental asymmetry or centre line deviations may be the outcome of discrepancies in tooth dimensions or improper placement of dental units within normal/symmetrical underlying jaw bones or be the outcome of asymmetrical skeletal bases housing the dental units. ‘Centre line’ is the synonym for the midline, a term more often used by British clinicians.
The dental midline or centre line is evaluated at the junction of contact points of the mesial surface of the central incisors in each arch. It is the centre point of the dental arch, which marks the anterior limit and a reference point that divides the arch into two parts. Midline symmetry or deviations are recorded as coincident or non-coincident to mid sagittal plane and to each other ( Fig. 82.1 ).
The dental midlines must coincide with facial midlines.
Therefore, the dental midline is assessed in relation to the facial midline. Mid sagittal reference plane (MSR) is used as a reference plane to evaluate the deviations. MSR is an imaginary line that divides the face into two halves.
The maxillary dental arch midline is expected to coincide with the mid-sagittal line of the face and midline of the maxilla. Accordingly, the mandibular dental midline should be placed in the mid-dental arch of the mandibular symphysis and the face. Both maxillary and mandibular midlines are expected to coincide with each other and to the midline of the face which is represented by the mid sagittal reference (MSR) plane. The dental midlines are evaluated with (1) teeth in occlusion and (2) on the opening of the mandible.
Definition and evaluation
Dental midline shifts more significant than 2 mm is typically considered a concern because such deviations can negatively impact facial attractiveness. Research suggests a 56% likelihood that a layperson will give a less favourable attractiveness score when there is a 2-mm discrepancy between the dental and facial midlines ( Fig. 82.2 ).
A perfect coincidence of maxillary and mandibular midlines is desirable though rare.
Up to 1 mm of midline deviation is acceptable. (A) The frontal profile photo shows maxillary dental midline is coincident with MSR plane. (B) Frontal occlusion photo shows dental midlines are not perfect alignment. The lower midline is slightly shifted to right. Bold white line refers to MSR; red line: maxillary midline; yellow line: mandibular midline.
Effects of an altered tip of the incisors on the midline
Dental midline shifts may or may not be associated with alteration in the mesiodistal inclination of the incisors. In certain situations, altered angulations may compensate for underlying dentoalveolar or mild forms of skeletal asymmetry. In other instances, alteration in angulations may reflect a midline shift. A discrepancy of 10-degree change in incisors’ angulations could compromise facial aesthetics and is therefore considered unacceptable by orthodontists.
Thomas et al. conducted a study examining the attractiveness scale with the angulation of incisors. Their findings indicated that the mean acceptable midline angulation for male subjects was 6.6±4.58 degrees as assessed by orthodontists and 10.7±6.28 degrees as perceived by laypeople. For female subjects, the mean acceptable threshold was found to be 6.4±4.08 degrees for orthodontists and 10.0±6.18 degrees for laypersons (P <.001). It can be summarised that discrepancies of 10-degree tip of incisors were deemed unacceptable by 68% of orthodontists and 41% of laypeople.
Reference planes
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Mid-sagittal reference (MSR) plane.: The mid-sagittal reference plane of the face/head is also called the MSR plane. It corresponds to a median plane that divides the body into two halves. It is mainly constructed or follows the average mid-sagittal structures of the skull. MSR extends down from the head, in natural head position (NHP) or when a subject is sitting upright or standing vertically. The face is bisected by a mid-sagittal vertical line running through the head, the centre of the nose, lips and the chin. The pupils are equidistant to this line in the horizontal plane, vertical to mid-sagittal line.
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MSR plane can be visualised as an imaginary vertical or plumb line outside the face dropped at the glabella. The plumb line also bisects the dorsum of the nasal tip equidistantly unless there is an asymmetrical nose. MSR plane is easily visualised in dental clinical settings with a piece of long floss or a ruler ( Fig. 82.3 ).
Figure 82.3 Evaluation of midline shift in a clinical setting using a long piece of dental floss.
(A) Using a long piece of dental floss. (B) Using a ruler, we evaluate distance of midline from corners of the mouth.
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Burstone highlighted the significance of assessing the facial midline from the patient’s viewpoint. Many patients are particularly concerned about their facial midline, including the placement of the upper lip and the corners of the mouth. As a result, the centre of the philtrum is a reliable and practical reference for determining the position of the maxillary dental midline. The ‘V’ shape at the vermillion border is a well-recognised landmark appreciated by orthodontists and patients ( Fig. 82.4 ). Arnett and Bergman noted that the philtrum is a reliable midline structure and can be used as a reference point for midline evaluation. According to Miller, the maxillary midline is typically found at the centre of the mouth, and the philtrum is used as a reference in about 70% of individuals. However, the maxillary and mandibular midline alignment occurs in only approximately one-fourth of the population.
Figure 82.4 Patients perceive symmetry of dental midline in relation to lips.
The ‘V’ at the vermillion border forms a suitable landmark for the dental midline evaluation that orthodontists and patients much more readily appreciate.
Source: Reproduced with permission from Burstone CJ. Diagnosis and treatment planning of patients with asymmetries. Semin Orthod. 1998 Sep;4(3):153–64 .
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Another effective method to assess the dental midline is to examine the distance between the canine or first premolar tooth and the corners of the mouth. When the midline is correctly positioned, the patient will observe equal exposure of teeth on both the right and left sides. Patients tend to refer more to soft tissue markers, such as the philtrum and the corners of the mouth, when evaluating the dental midline rather than relying on references like a plumb line or dental floss placed across the face used by the orthodontist.
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Mid palatal raphe: The maxillary dental midline in the arch is evaluated as a part of symmetry. Mid palatine raphe and incisive papillae are used as reference points in cases of the normal shape of the arch. Marking points usually determine the maxillary model midline over the mid-palatal suture, from the incisive papilla to the most visible posterior landmark.
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Symphysis of the mandible.: The mandibular dental midline is evaluated in reference to MSR and in relation to symphysis of the mandible ( Fig. 82.5 A).
Figure 82.5 (A) The dental and skeletal midlines are evaluated in relation to MSR, which is constructed on a conventional PA cephalogram to assess incisor apical base discrepancy between upper and lower arches. An arbitrary skeletal mid-sagittal plane passes through the lower apical base midline. (B) Upper dental midline to the right of the lower midline. Skeletal problem with apical base discrepancy. (C) Upper midline to the right without an apical base discrepancy. Upper incisors are tipped towards the right. (D) Dental midlines correspond. The apical base discrepancy is masked by compensatory tipping of the upper incisors to the left side.
Source: Based on the concept of Burstone CJ. Diagnosis and treatment planning of patients with asymmetries. Semin Orthod. 1998 Sep;4(3):153–64 .
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The base of dentoalveolar structures: Midlines are evaluated on their respective skeletal bases, maxillary and mandibular bases at A point and B point, respectively ( Fig. 82.5 B).
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Angulation of the incisors: A diligent appraisal of the dental casts and clinical evaluation is performed to assess the axial inclinations of the incisors, which may contribute to midline discrepancy. Alternatively, incisors may have been inclined to compensate for the underlying dentoalveolar or apical base discrepancy. Under these situations, a mental graphic picture needs to be created for the positions of incisors by placing them in a correct mesiodistal angulation (tip) and visualising where the centres of the roots might be so located that a perpendicular can be dropped to the occlusal plane.
Fig. 82.5 C shows a midline discrepancy with the upper incisors to the right of the lower. By imagining the visualisation of uprighting the incisors to their axial inclinations, the midlines would correspond, and a dental midline discrepancy would be apparent. In contrast, in Fig. 82.5 D, a skeletal discrepancy is shown. Equalising axial inclinations would not help the midline coincide. The midlines become further apart as the teeth are upright. This is an example of a skeletal or apical base discrepancy.
Midline correction is easy to achieve once axial inclinations are corrected with a conventional orthodontic mechanism. In contrast to a situation where upper teeth have tipped to maintain a midline correction to compensate for the underlying apical base asymmetry, corrections of teeth angulations will aggravate the midline discrepancy. Such patients would require a surgical approach to correct apical base relationship, which will simultaneously correct the dental midlines.
Prevalence of midline deviations
Midline deviations in population surveys have been reported to be close to 20%. Midline deviations in the orthodontic population and patients with facial asymmetries are close to 60% in the range of 46% to as high as 78%. , ,
TABLE 82.1
Prevalence of midline shift in children
| S. no. | Author/year | Sample size/population | Age | Sample characteristics | Method of assessment | Finding (prevalence) |
|---|---|---|---|---|---|---|
| 1 | Sheats et al. (1998) |
Florida school study group
First screening: 5817 untreated children. Second screening: 861 untreated children |
First screening 9.3±0.8 years
Second screening 14.4±0.5 years |
School children | Florida school group from orthodontic screening examinations | Florida schools’ group non-coincident midline: 21% |
| 2 | Borzabadi et al. (2009) | 502 Iranian M: 249, F: 253 | Aged 11–14 years | Exclusion criteria: subjects with craniofacial anomalies (clefts and syndromes) and non-Iranian nationals | Clinical examination using a mouth mirror, ruler and a sliding digital calliper | Non-coincident dental midlines: 23.7% |
| 3 | Murshid et al. (2010) | 1024 adolescents M: 608; F: 416 | Aged 13–14 years | Jeddah, Saudi Arab | Clinical screening examination | Midline deviation: 24% |
TABLE 82.2
Prevalence of midline deviations in orthodontic subjects and with facial asymmetries
| S. no. | Author/year | Sample size/population | Age | Sample characteristics | Method of assessment | Finding (prevalence) |
|---|---|---|---|---|---|---|
| 1 | Sheats et al. (1998) | Virginia orthodontic study group: 229 children | Orthodontic study group children 6.4–63.1 years | Orthodontic treatment patients with a history of craniofacial deformity or other syndromes were excluded. | Orthodontic group findings from the clinical examination and visual assessments of slides and models that orthodontic residents had completed the treatment. |
Midline deviations from face midline: Maxillary midline deviation: 39%
Mandibular dental midline: 62% Non-coinciding dental midline: 46% |
| 2 | Bhateja et al. (2014) |
280 patients
M: 177; F: 103 Records of patients Aga Khan University Hospital, Karachi, Pakistan (January 2006–July 2012) |
Mixed dentition 78
Mean age 11.05±2.71 Permanent dentition 102 Mean age 18.62±7.92 years |
Subjects with dental and facial asymmetries.
No history of previous orthodontic treatment. Patients with craniofacial anomalies were excluded. |
Clinical examination
Visual assessment of the frontal facial photographs. Dental casts. |
Non-coincident midlines: 78%
Mandibular midline asymmetry: 67.5% |
| 3 | Jain et al. (2015) | 300 patients orthodontics patients | 13–30 years |
Inclusion criteria
No history of trauma No major local/systemic problems affecting facial structures No orthodontic or interceptive treatment carried out. |
Clinical examination |
Midline deviation: 77%
Upper dental midline shift: 21% Lower dental midline shift: 43% Upper apical base midline shift: 13% Lower apical base midline shift: 23% |
Maxillary midline deviations ranged from 21% to 39%, with an average of 30%. , , Midline deviations and asymmetries are more common in the mandible owing to its more extended period of growth, which makes it susceptible to being influenced by environmental factors affecting normal growth till adulthood. In the mandible, midline deviations are reported in the range of 43%–67.5%. The prevalence of midline shift in children and midline deviations in orthodontic subjects with facial asymmetries are tabulated in Tables 82.1 and 82.2 .
Clinical presentation of dental midline deviations
Jerrold and Lowenstein described the following major possible clinical situations on midline deviations.
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Group A: Local/dentoalveolar type of midline shift when the face is symmetric and there are no major skeletal deviations. Four clinical situations can exist.
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When the maxillary midline is off to the right or left side.
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A clinical situation in which both maxillary and mandibular midlines are coincident, but both are off from the centre line of the face in the same direction. Both have moved to the right or the left side.
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A clinical situation where the face, maxilla, mandible and maxillary midlines are coincident, but the mandibular midline is off to centre line or plumb line of the face. This clinical situation is seen during the late mixed dentition stage. Unilateral loss of deciduous canine in the lower arch can lead to a slight shift of the incisors towards the lost tooth side. Once detected, it is advised to perform the deciduous canine extraction on the contralateral side, which usually leads to spontaneous midline correction.
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While the facial midline is correctly centred, the maxillary midline and mandibular midlines are not in alignment due to the shift of each of them in their respective jaws in a direction opposite to each other. Say, when the maxillary midline is shifted to the right side, the midline of the mandibular dental arch has moved to the left side, thus compounding the severity of the problem.
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Group B: Dental asymmetry comprises a clinical situations of facial asymmetry, functional or true skeletal. The dental midline deviations may or may not be present. A major cause of such asymmetries in otherwise normal faces involves a lateral functional shift of the mandible.
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The functional lateral shift of the mandible: A simple test involves carefully observing a subject from the front. He is gently asked to open his mouth. If the chin deviation disappears upon opening the mandible and reappears upon closure of the mandible to centric occlusion. The cause is often a lateral shift due to premature contacts during closure in occlusion. Here, the mandible is rotated into a lateral eccentric posture, usually as a result of functional interferences. The maxillary midline is correct.
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On gentle manipulation and guiding the mandible during closure till the first point of dental contact is made, the face becomes relatively symmetric, and the patient should be able to tolerate this position for a short period without feeling the excessive strain in the temporomandibular joint areas or the muscles of mastication, a functional shift is suspected. However, if this condition is not treated well during early growth, the deformity may become skeletal in nature.
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Occasionally, in a situation as described above (1), the local tilting or shifting of teeth may lead to maxillary and mandibular midline coinciding while the chin remains deviated.
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Both maxillary and mandibular midlines are coincident but deviate from the midline of the face to the side of the chin deviation.
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The maxillary midline is off to the opposite side of the mandible, and the dental midline deviates towards the deviated chin.
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Both dental midlines are coincident, and yet both are off to one side of the facial midline while the mandible is rotated to the contralateral side. Aetiologically, the upper and lower teeth have shifted to one side as described, while the mandible has reacted to the functional interferences or cross bite, thus assuming an eccentric posture.
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The maxillary midline is off to one side, the mandibular midline coincides with the mid frontal plane, and yet the mandible has rotated or deviated to the opposite side.
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The maxillary midline is off to one side, the chin is off to the same side because of functional interferences and the mandibular midline is centred at the initiation of the treatment. In such situations, the lower midline moves to the opposite side following asymmetry correction.
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This situation is similar to the previous one except that instead of the mandibular midline being ‘centred’ at the initiation of treatment, it is off to the opposite side of the maxillary midline.
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