In some nonextraction cases, maxillary molar distalization is the method of choice to gain 2 to 3 mm of space in the dental arch to obtain a class I relationship² in both teens and adults.

The upper molars can be distalized by means of extraoral or intraoral forces.³ Extraoral traction with headgear has a long history of use in class II treatment since it has been designed to push distally the maxilla and the maxillary molars.^4,5 In recent years, several techniques have been developed to reduce the dependence on patient compliance, such as intraoral appliances with and without skeletal anchorage. However, even these devices can produce undesirable tipping of the maxillary molars and/or loss of anterior anchorage during distalization.^6,7 To achieve a tooth bodily movement implies that the applied force must pass through the center of resistance of the tooth or a sophisticated equivalent system of forces and moments needs to be applied to the tooth crown.⁸ A recent review of the existing literature⁹ assessed the efficacy of aligners in aligning and straightening the arches, with better results for mild to moderate crowding when compared to the results obtained with fixed appliances. More recently, it was stated that the overall available evidence regarding orthodontic tooth movement (OTM) control during CAT increased significantly, with three randomized controlled trials (RCTs) at grade A and an overall quality of evidence of moderate/high level, and that maxillary molar distalization of 2.5 mm and premolar extraction space closure (7 mm) are the most predictable and controlled movements with CAT.¹⁰

In 2014, Simon et al.¹¹ stated that maxillary molar distalization was the most predictable movement (88%) to perform with CAT. The authors started to focus on the key role of a correct staging of the planned movement and of the adoption of proper attachments during the whole distalization phase. Thus a highly significant element of bias in the 2012 study by Drake et al.¹² was the staging of 0.5 mm per aligner instead of the 0.25 mm recommended. In 2016, Ravera et al.¹³ confirmed the results of Simon et al.¹⁴ and demonstrated that distalization is efficiently achievable up to 2.5 mm on the first and second maxillary molars, with optimal vertical control of posterior teeth and any loss of anchorage on the anterior teeth. These results were obtained through the combination of staging, vertical rectangular attachments, and class II elastics (0.25–4.5 oz) for anchorage reinforcement.¹⁵ The use of attachments and elastics was previously described by expert clinicians.¹⁵ The application of composite attachments could be useful to improve the biomechanic efficiency of aligner therapy. Long vertical attachments located on the buccal aspect of the molars can create a sufficient moment to oppose the tipping movement.¹⁶ Thus long vertical attachments can provide good tipping control while molars are moving and then can increase posterior anchorage while retracting anterior teeth.

The need for a determined attachment combination was confirmed in a 2016 RCT by Garino et al.,¹⁷ who observed significant differences in the amount of distalization when comparing a five-attachment configuration (second and first molars, second and first premolars, and canine) with a three-attachment configuration (first molar, second and first premolars), with the first ones being most efficient. Controlling the tipping movement during molar distalization can be difficult because of the limited aligner-tooth surface in the direction of force application. The absence of long rectangular attachments on the second molar resulted in a probable loss of anchorage during the distalization of the first molar, with consequent reduced amount of distal movement of the second molar at the end of the treatment and significant tipping of the first molar. Furthermore, the absence of a proper anchorage preparation in the distal portion reduced the possibility of an adequate control of the retracting anterior teeth. As a result, the central incisors showed an uncontrolled tipping movement in the group with a three-attachment configuration.

Recently Gomez et al.¹⁸ demonstrated that when the aligner segment was displaced distally without attachments, a clockwise moment and distal inclination were produced on the upper canine. The presence of composite attachments helped counteract this inclination, producing a countermoment that in turn favored a bodily movement. In another finite element analysis study, Comba et al.¹⁹ demonstrated that the use of attachments on tooth surface counteracts the uncontrolled tipping during distalization through the generation of a countermoment that ends in the root uprighting. This moment is dependent from a complex force system and is generated by the active surfaces of attachments. When analyzing a couple of attachments located on the buccal surface of an upper canine, one located at the distocervical portion and the other located at the mesioincisal portion, compression areas were found on the mesial face of the cervical attachments and on the distal face of the incisal attachment. These outcomes validate Gomez findings.

The vertical pattern is an important point to consider while planning molar distalization. The distal movement measured in our study was associated with significant intrusion movements of the molars. The thickness of the aligners and the consequent occlusal force exerted on them might facilitate intrusion and explain the absence of any change of anterior vertical dimension while distalizing. Furthermore, Gomez et al.¹⁸ reported a marked tendency of “flaring” of the buccal and palatal flanks of the aligner segment during distal displacement. This finding is interesting because it could suggest an intrusive effect on the tooth.

The aligner therapy is a customized orthodontic treatment for both the patient and the orthodontist. The presence of composite attachments for the control of the maxillary molars during the distalization process is a choice of the prescribing clinician for most of the available systems in the market.

Mesiopalatal rotation of the upper first molar is present in about 95% of patients with angle class II, division 1 malocclusion and in 83% of them as a whole.^15,16 Mesiopalatal rotation of upper first molars often ends up in an intraarch loss of space.¹⁷ Frequently, this crowding occurs in the premolar and canine segments, thus potentially preventing the correct mesiodistal position of these teeth. On this basis, buccodistal rotation of maxillary molars can be considered a useful procedure to partially improve class II dental relationship. Molar rotation was indicated as one of the predictable movements controlled by aligners.²⁰

Despite the large use of class II elastics in everyday practice, little evidence is known about their effects. A recent systematic review stated that the current literature suggests using light forces (average, 2.6 oz) obtained with a 3/16-in diameter elastic and a rectangular 0.016- to 0.022-in stainless steel archwire.²¹ In aligner orthodontics, the use of 1/4-in diameter 4.5 oz was recommended^13,15 on the basis of expert clinician experience. However, as shown in Chapter 5, finite element analysis has shown the need for stronger class II elastics in CAT. Because class II elastics heavily rely on patient compliance, full-time usage is recommended. It has been described as an average period of 8.5 months for the correction of the class II discrepancy with elastics only, and the correction is usually obtained with predominant dentoalveolar effects. This is the average treatment time required to correct an end-to-end class II malocclusion according to existing literature.²¹

Please refer to Chapter 8 for specifics on extractions.

Please refer to Chapter 16 for specifics on mandibular advancement.

Please refer to Chapter 17 for specifics on orthognathic surgery.