20: Overcoming the limitations of aligner orthodontics: A hybrid approach

Luca Lombardo, Giuseppe Siciliani

Introduction

Aligners were first introduced by Kesling¹ in 1945 to correct crowding. Later, Ponitz² reported the use of a removable plastic retainer (Essix, Dentsply, York, PA, USA). However, it was not until the 1990s, when Sheridan et al.³ combined these retainers with interproximal reduction (IPR), that they began to gain popularity. Then, in 1999, Zia Chishti and Kelsey Wirth, together with a computer specialist, founded Align Technology in Palo Alto, CA, USA.⁴ Since they launched their Invisalign brand into the market, the demand for orthodontic aligners has been growing among patients; especially adults, thanks to their esthetic properties and clinical efficacy.⁵

At first, aligners were marketed as an alternative to traditional fixed appliances in simple malocclusion cases involving slight crowding or minor space closure.⁶ Over time, however, the range of malocclusion cases that can be treated by means of invisible aligners has broadened. Clinical research has developed aligner-based solutions for even complex cases involving major rotation of the premolars, upper incisor torque, distalization, and/or extraction space closure.⁷ Despite the claimed efficiency of aligner treatment, however, its clinical potential still remains controversial among clinicians. Its advocates are convinced by the clinical evidence arising from successfully treated cases, while skeptics point to the significant limitations of the technique, especially in the treatment of complex malocclusions.^8–11 Orthodontics companies claim that aligners can resolve, without the use of additional techniques, rotations of 40 degrees at the upper and lower central incisors, 45 degrees in canines and premolars, 30 degrees in lateral incisors, and 20 degrees in molars. Extrusions and intrusions of 2.5 mm have been achieved in anterior teeth, and root movements of 4 mm and 2 mm have been reported in posterior teeth.¹²

Nevertheless, few studies have been published to support these claims, which are not always supported by the experience of other clinical practitioners. In fact, some orthodontists indicate that the number of patients who require some unplanned correction or even recourse to fixed orthodontics is closer to 70% to 80%.⁵^,¹³ Kravitz reported that Invisalign aligners had a mean accuracy of 41% in terms of achieving planned outcomes, with the most predictable movement being lingual contraction (47.1%) and the least predictable extrusion (29.6%).¹⁴

In the attempt to clarify the situation, Lagravère and Flores-Mir¹⁵ published the first systematic review on the subject in 2005. Since then, several authors have provided updated evidence on aligner efficacy.¹²^,^16–18 The most recent systematic reviews into the accuracy of orthodontic movements achievable with aligners have concluded that they are able to produce distal movement of the upper molars and resolve anterior crowding issues through incisor protrusion and by increasing the intercanine, interpremolar, and intermolar distances. On the other hand, removable aligners are far less effective at achieving transverse expansion via bodily movement of the posterior teeth. Furthermore, they are unable to perform canine and premolar rotations satisfactorily, and seem to fall short in terms of extrusion movements and control of overbite and occlusal contacts.

Bearing in mind this evidence, our clinical experience, and the ever-growing popularity of aligner treatment, we have developed a new hybrid approach using a combination of different devices to overcome some of the most common limitations of removable appliances.

Transverse expansion of the posterior teeth

Research has shown that aligners are unable to perform predictable bodily premolar and molar expansion. Digital setups tend to overestimate bodily expansion movements, and more tipping than planned occurs.19–21 However, in clinical cases (Fig. 20.1) in which the posterior sectors are greatly negatively inclined, it is possible to plan uncontrolled tipping of the upper and lower canines, premolars, and molars. Furthermore, the space needed to resolve crowding can be created by using aligners alone (Fig. 20.2) to exert pressure on the lingual surfaces of the teeth; this improves the archform by significantly increasing the intercanine, interpremolar, and intermolar distances (Fig. 20.3). In fact, Lombardo et al.²² have demonstrated that this vestibulolingual tipping can be achieved with a predictability of 72.9%.

Fig. 20.1Initial intraoral photographs of adult patient with class I malocclusion dentoalveolar contraction in both arches.

Fig. 20.2Intraoral photographs during aligner therapy with composite buttons.

Fig. 20.3Final intraoral photographs after 20-step aligner treatment.

That being said, in young patients with transverse deficits due to hypoplasia of the upper jaw (Fig. 20.4), it is not realistic to expect aligners to achieve skeletal alteration. Only an orthopedic approach, first on the deciduous teeth (Fig. 20.5) and then via skeletal anchorage²³ (Fig. 20.6), is able to normalize the maxillary dimensions and therefore permit correct eruption and improve the transverse and sagittal occlusal relationships. However, in such patients, aligners (Fig. 20.7) can be used as an efficacious tool for completing dental alignment and creating acceptable intercuspidation without decompensating the class III malocclusion (Fig. 20.8).

Fig. 20.4Initial intraoral photographs of a young patient with skeletal and dental class III and narrow upper jaw.

Fig. 20.5Rapid palatal expansion with arms for Delaire mask on deciduous second molars.

Fig. 20.6Hybrid expander with dental and skeletal anchorage in upper jaw and arms for Delaire mask.

Fig. 20.7Intraoral photograph during aligner therapy.

Fig. 20.8Final intraoral photographs after 11-step aligner treatment.

It is not only in children that such problems arise, however; in adult patients,²⁴ the predictability of transverse expansion via bodily movement of the premolars and molars is poor, and may be damaging in patients with thin periodontal tissues or gingival recession (Fig. 20.9). Hence in adults it is best to resolve issues of skeletal maxillary contraction via surgery or skeletal anchorage expanders (bone-bone rapid palatal expander) (Fig. 20.10). Only after the transverse deficit has been resolved should crowding be addressed, and in such cases the occlusion can be improved by means of aligners (Fig. 20.11), which can guide the extrusion of the teeth in a controlled fashion. This approach lessens the risk of premature contacts, unwanted vestibular movement, and worsening gingival recession (Fig. 20.12).