6: Class I malocclusion
Mario Greco
Introduction
Class I malocclusions represent one of the most common conditions in the daily clinical practice and one of the most elective conditions to be treated with aligners, since the primary patients’ concern is often represented by crowded anterior teeth, especially in the mandibular arch.1
Working with clear aligners challenges the paradigm on which we, as orthodontists, based the traditional fixed mechanics approach. Working with aligners means that we need to plan everything in advance and not on a monthly basis, defining final teeth position from the beginning and spending more time in treatment plan design and staging than at the chairside.2–3
Diagnostic reference
When dealing with class I malocclusion, the first step to consider is the definition of the biologic limits of the arches. We should identify anterior, frontal, and vertical limits. All the limits represent both a morphologic limit (torque posterior and anterior) strictly connected to bone and teeth pattern of movement and an esthetic indication to define exactly the ideal teeth positioning in relationship to lips and face.
More specifically, during treatment plan in class I malocclusion a very schematic approach could be focused on the observation and respect of the following key points:
Substantially the esthetic indicators represent the limits in which teeth need to be moved on the horizontal plane (arch symmetry, face midline) and on the vertical plane (smile arc); the occlusal indicators are useful to define the proper Overjet needed to ensure anterior clearance and avoiding premature anterior contacts (causing posterior open bite) in relationship to dental size and anterior limit of dentition.
Treatment plan
The development of proper treatment plan in class I malocclusion starts from the definition of correct staging of movement to create a reliable digital setup and to reach a predictable result with high superimposition between the real and the digital settings.4 The ideal approach in terms of treatment staging should be based on curve of Spee leveling, incisor control, arch development, rotation control, attachment choice, and interproximal reduction (IPR).
Curve of spee leveling
To avoid anterior premature contacts, to create the proper cuspids and molars intercuspations, and to allow lower incisor leveling and correct anterior relationship related to guidance function, flattening of the curve of Spee is required. Moreover, the assessment of the amount of leveling will give information about the space needed for curve flattening.5
Incisor control
Having in mind the precise angular inclination of lower incisors according to the cephalometric references and using the superimposition tool (and/or the movement table tool), together with the grid tool of digital setup software, is possible to determine the amount of proclination or retroclination required to properly locate the lower incisors on the sagittal plane.6
Arch development
In terms of treatment approach, expansion represents a very common solution to treat crowding and transverse discrepancy. Buccal tipping movement is more predictable than bodily movement when planning arch expansion with aligners. This should be kept in mind when defining the buccolingual inclination of canine premolars and molars respecting the periodontal condition.7
Rotation control
Rotation of small teeth or round teeth such as premolars could be considered a difficult movement to achieve because of the reduced tooth surface on which the force can be applied. Complex rotations should be managed by first creating the mesial and distal spaces required to rotate teeth and then choosing the proper attachment.8
Attachment choice
Attachments represent a useful tool to increase the surface on which orthodontic forces could be applied (Fig. 6.1). See previous chapters for more details.9
Interproximal reduction
One common procedure in aligner technique is represented by the IPR, which ideally should be limited to 0.3 mm per interproximal point to avoid too wide enamel reduction. The management of IPR is fundamental not only for fixing crowding problems and finding more space but also to control the incisor inclination (i.e., creating space with IPR could represent a reliable system to upright upper or lower incisors), to compensate Bolton discrepancy by reducing teeth excess, and to create symmetric dimension between left and right sides.10
Class I conditions
Class I malocclusions can be divided into different categories following the principal condition that affects specifically one or more dimensions of the space (transverse or vertical) or which creates a determinate discrepancy. For this reason they will be discussed separately.
Dentoalveolar discrepancy
The most common condition is represented by crowding in the upper or lower arch or both. The clear aligner treatment (CAT) of crowding is highly predictable when approached with the proper staging such as expansion, small proclination, reduced IPR, and torque correction. Normally, being able to avoid extractions means that treatment options available are related to expansion (2 mm per quadrant) and IPR (0.3 mm maximum per interproximal space). The severity of crowding, particularly in the lower jaw, significantly affects the possibility of avoiding extraction treatment. Conditions in which it is reasonable to treat without extraction are as follows:
- ■ Light crowding, with normal amount of IPR (0.1/0.2 mm)
- ■ Mild to moderate crowding, with combination of expansion without changing intercanine width and maximum rate of 0.3 mm of IPR per interproximal space
- ■ Moderately severe crowding, combining the 0.3-mm IPR per interproximal space with torque correction of lower premolars to create a positive torque up to a maximum of -3 degrees of buccal torque inclination
- ■ Mild to moderate crowding, with combination of expansion without changing intercanine width and maximum rate of 0.3 mm of IPR per interproximal space
This means that when the crowding is lower than 4 mm per quadrant, the possibility to combine expansion and IPR could represent a reliable solution to recreate ideal alignment, but some options during the digital setup planning need to be controlled to avoid collateral effects, as follows:
- ■ Avoid excessive proclination of lower incisors by means of using the superimposition tool and the grid tool of the software and by favoring transverse expansion and consequently a more uprighted position of lower incisors (Fig. 6.2).
- ■ Place the lower premolars to a buccal crown torque next to zero to recreate space without changing the intercanine width (when on the occlusal picture of lower jaw, it is possible to observe the labial surface of premolars, torque correction could be achieved) (Fig. 6.3).
- ■ Combine class III elastics to create the proper OVJ (1.2 mm) and to favor the correction of crowding also in absence of real class III relationship (Fig. 6.4).
- ■ Create upper and lower jaw ideal shape to avoid black triangles and buccal facial corridors (Fig. 6.5).
- ■ Plan specific attachments (see Chapter XXX) (Fig. 6.6).
Tooth size discrepancy
The Bolton analysis is important because it allows the immediate visualization of the interarch and intraarch discrepancies. These discrepancies can affect the final overjet. Not considering the Bolton analysis in our treatment plans could lead to several unfavorable outcomes: anterior premature contacts with posterior open bite without reaching a proper class I intercuspation on both sides, excessive proclination of incisors, and uncorrected closure of upper diastemas. Therefore, the tooth size discrepancy analysis is crucial when designing orthodontic treatment. Othman and Harradine recommended a threshold of 2-mm discrepancy to be of clinical significance for restorative intervention (Figs. 6.7 and 6.8).11–12
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