Orthodontic management of transverse cant of the occlusal plane

Introduction and definition

The occlusal plane (OP) can tilt in both the anteroposterior and vertical directions. When the OP is canted in the frontal plane and affects a person’s smile aesthetics, it usually results from facial asymmetry or differences in the vertical positioning of the right and/or left dental quadrants of the dental arches. More commonly, OP cant refers to a change in the inclination of the OP, indicating a tilt in the transverse direction when viewed from the front of a person ( Fig. 84.1 ).

Image, AltText currently not available — Figure 84.1

Although changes in the inclination of the OP in the sagittal plane are associated with growth and development, changes in the inclination of the OP in the vertical plane result from asymmetric growth of the craniofacial structures and lead to an asymmetric OP; this is defined as occlusal cant (OC).

Many terms have been described to denote this feature of occlusion: cant, canting and maxillary cant. Recently, Gateno et al. described it as roll malrotation. van Steenbergen and Nanda described these clinical features with the term ‘diverging occlusal planes’.

The ‘cant of an occlusal plane’ is evaluated in relation to a horizontal transverse plane formed by a line connecting R-L pupils when a subject is looking in front. A wooden spatula held between the buccal teeth (L-R) and its inclination can be visualised in relation to a horizontal reference plane, the most common being the inter-pupillary line. A cant of more than 4 degrees or above is considered clinically appreciable.

It has been reported that the detection rate for both untrained and trained observers surpassed 50% at 3 degrees of occlusal canting. Cants greater than 4 degrees were detected 90% of the time by untrained observers and 98% of the time by trained observers. Padwa et al. concluded that 4 degrees is the threshold at which OCs are detected with greater than 90% frequency, and 3 degrees is the threshold for greater than 50% frequency.

OCs of 0–3 degrees are seen in normal occlusion subjects. The cant up to 3 degrees will not be noticeable, and occlusal canting of this magnitude probably does not have detrimental effects on post-operative outcome. Any efforts to alter this inclination of cant with restorative or surgical procedures are not desired and may have an adverse impact on temporomandibular joint (TMJ).

A study by McAvinchey et al. based on various degrees of distortion concluded that the profession of the observer influenced the perception of facial asymmetry, with orthodontist as the most critical. A group of lay persons classified the upper limit of ‘normal’ as 5.6±2.7 mm. This group also felt that asymmetry greater than 11.8±4.0 mm required correction.

Aetiology of occlusal cant

Asymmetrical growth of the mandible

The asymmetrical growth of the mandible is the primary driver of the cant and/or cause of facial asymmetry; the maxilla follows and adapts the mandible to maintain occlusal contacts between upper and lower teeth. The consequence is a cant of the OP. Mild cants can be treated with orthodontics alone; however, if the cant is more severe, surgical options are the choice of therapy.

Facial asymmetry

OC is frequently observed in facial asymmetries, more so when the growth of the mandible on one side is different from another. For example, trauma to TMJ and subsequent unilateral ankylosis ( Fig. 84.2 ) lead to arrest of the growth, with average growth occurring on the unaffected side (contralateral side), and the chin deviates to the affected (ipsilateral) side. There is a biological compensatory adaptation of the maxillary complex to maintain occlusal contacts with mandibular dentition, resulting in the transverse cant of the OP. The vertical growth of the face on the side of trauma is much less, thus exhibiting a cant of the OP with a higher level on the affected side.

In contrast, subjects with unilateral condylar hyperplasia ( Fig. 84.3 ) show excessive growth of the condyle and ramus on the affected side, leading to the cant of the OP. The transverse cant direction is reversed compared to that seen in TMJ ankylosis, that is, the affected occlusal level is much lower than the normal side.

In growing children with cleft lip and palate, excessive scarring from surgery can inhibit maxillary growth in all three dimensions of space, more significantly on the complete cleft side. Fig. 84.4 shows an operated case of unilateral cleft lip and palate showing the reduced height level of the OP on the cleft side leading to transverse cant of the OP.

Faulty biomechanics

Canting of the OP can occur due to asymmetrical biomechanics during orthodontic therapy, such as using midline oblique elastics or class II elastics on one side and class III on the contralateral side. The lower incisors are the smallest teeth in the mouth and are susceptible to oblique elastic forces that travel between the upper and lower teeth. Using asymmetrical biomechanics during orthodontic therapy can lead to neglect of the OP. The upper and lower anterior teeth rotate counterclockwise, resulting in a canted OP ( Fig. 84.5 ).

Prevalence

Most facial asymmetries, more so those involving the vertical growth of the mandible, would exhibit cant of the OP. However, the cant of an OP may be observed independently of any significant clinically apparent facial asymmetry, which the clinician may first notice on diligent examination or on PA cephalometry evaluation. Severt et al. reported the clinically apparent facial asymmetry affecting canting of the OP in 41% (vertical asymmetry) subjects. Although many studies are available on traits of facial asymmetry, independent reporting of the cant of the OP has remained elusive.

Clinical presentation and diagnosis

Individuals with suspected or noticeable asymmetry are assessed through a comprehensive clinical examination, frontal photographs, posteroanterior (PA) cephalometry, orthopantomogram (OPG) and 3D imaging techniques. These techniques include cone beam-computed tomography (CBCT) and non-radiation 3D imaging of the face, such as 3dMD (3dMD LLC 3200 Cobb Galleria Parkway, #203 Atlanta, Georgia 30339, USA).

Clinical evaluation

Clinical evaluation on the cant of the OP is performed with a simple yet reliable clinical method. By placing a wooden tongue blade or ice cream stick across both sides of the first molars/premolars while a patient is seated upright, his face at eye levels of the operator, the left to right inclination of the spatula is evaluated in relation to an imaginary line connecting the pupils. The relationship between the transverse OP and the inter-pupillary line can be readily appreciated ( Fig. 84.4 ). The detailed extraoral examination is followed by an intraoral examination to record asymmetry in an overbite, arch form and levels of buccal occlusion.

Frontal photographs

The evaluation of frontal facial photographs is a diagnostic tool used to evaluate soft-tissue asymmetry and lip cant ( Fig. 84.4 ). A detailed clinical and radiological assessment of the TMJ is essential in such a patient.

Posteroanterior cephalogram

PA radiography is necessary for evaluating and objectively measuring occlusal plane cant ( Fig. 84.6 ). Analysis of PA radiograph allows easy visual comparison of asymmetry. The most commonly used asymmetry analyses are Grummons’ frontal analyses, which demonstrate the parallelism and asymmetry of facial points and planes according to a pre-determined mid-sagittal reference plane. The frontal analysis is useful in the quantitative evaluation of the right to the left asymmetry of the facial structures. However, the effectiveness of PA radiographs may be compromised by head rotation or improper landmark identification; therefore, close technical supervision and adherence to protocol are required when obtaining a PA cephalogram.

Reference planes

The reference lines for determining the cant are as follows:

1.
A true horizontal is represented by a tangent to the normal supraorbital rims.
2.
A vertical line is drawn through the crista galli and upper third of the nasal septum, representing the mid-sagittal reference plane.

The tilt of the occlusal plane to the true horizontal plane is measured as the angle of the cant of the occlusal plane.

Research studies have demonstrated that, in patients exhibiting facial asymmetry in the frontal plane, the degree of OC, when measured cephalometrically in relation to the true horizontal plane, is equivalent to the linear difference in millimetres between the vertical lengths (height) of the right and left sides of the maxilla. In a clinical context, the magnitude of OC may be evaluated by assessing the distance from the medial canthus to the canine.

In the case of patient ‘X’ ( Fig. 84.7 ), the measurement from the medial canthus to the right canine was recorded at 62 mm. In contrast, the distance to the left canine was recorded at 56 mm, leading to a total vertical discrepancy of 6 mm. The degree of cant is evaluated in relation to the true horizontal plane. In this PA cephalogram, the OP exhibited a cant of 6 degrees, corresponding to the clinically measured vertical discrepancy.

MDCT/CBCT

Three-dimensional computed tomography (CT) can provide information for use in diagnosis and treatment planning ( Fig. 84.8 ). Because of the complex 3D nature of facial asymmetry, a CT scan can be recommended in the evaluation of asymmetry cases that cannot be assessed using conventional methods. CBCT could be a preferred mode of diagnosis over MDCT owing to lower radiation doses.

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Orthodontic management of transverse cant of the occlusal plane

Introduction and definition