12: The hybrid approach in class II malocclusions treatment

Francesco Garino, Tommaso Castroflorio, Simone Parrini

Introduction

Several protocols have been proposed for treatment of class II malocclusions. In nonextraction protocols, maxillary molar distalization can be used to correct molar relationships in patients with maxillary dentoalveolar protrusion and minor skeletal discrepancies.¹

The upper molars can be distalized by means of extraoral or intraoral forces.² 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.³

In the last decades, increasing numbers of adult patients have sought orthodontic treatment and expressed a desire for esthetic and comfortable alternatives to conventional fixed appliances. Clear aligner therapy (CAT) was introduced to answer this request.

In a review by Rossini et al. it has been stated that maxillary molar distalization up to 2.5 mm is one of the most predictable movements with CAT.⁴^,⁵ This high predictability was obtained through combination of staging, the use of proper attachment configuration,⁵ and full-day class II elastics (0.25 in, 4.5 oz) (see Chapters about Class II treatment and see Chapters 5 and 7). These results confirm what every orthodontist knows: Treatment success requires technical knowledge from the orthodontist as well as the cooperation of the patient.⁶ Class II treatments with CAT require mean treatment times of 18 to 20 months during which class II elastics need to be used all day from treatment beginning until class I canine relationship has been established.³^,⁷ Corrective devices should be comfortable, provide rapid and effective treatment, and favor patient compliance with orthodontic treatment. Clear aligners are comfortable and aesthetically acceptable as already discussed in the previous chapters,⁸^,⁹ and require strong patient compliance since they are removable. The existing literature showed that the mean duration of objectively measured wear was considerably lower than stipulated wear time among all removable appliances. Furthermore, compliance was found to be better in the early stages of treatment.¹⁰

Starting from these premises, the possible combined use of aligners and other orthodontic devices aimed to optimize patient adherence to therapy reducing the time required to wear class II elastics has been proposed. This kind of combined approach has been named hybridization of aligner therapy. Among others, temporary anchorage devices (bone-borne hybrid approach [see Chapter about miniscrews]) and tooth-borne distalization devices are the most popular hybridization approaches in CAT.

The application of forces in such distalizing appliances could be from buccal region, palatal region, or both, and they could be based on sliding mechanics or be friction free (e.g., in the Pendulum appliance).

Tooth-borne hybrid approach with distalizing device

Various types of molar distalization appliances are available and presented in the orthodontic literature, such as the Pendulum device, the Distal Jet, and the Carriere Motion 3D Appliance (CMA) (Henry Schein Orthodontics, Carlsbad, CA, USA).

These appliances are considered easy to install and can promote distal movement of the maxillary molars without the effect of maxillary orthopedic restriction.¹¹ However, most of these intraoral devices show undesirable reciprocal anchorage loss in the premolars and incisors during distal molar movement.¹² Furthermore, molar tipping is frequently observed in most of the cases.

The Distal Jet appliance is composed of two bilateral tubes connected to a Nance appliance. A bayonet wire is inserted into the lingual sheath of the first molar bands. On the tube there is a stainless steel coil spring and a clamp. The clamp can slide toward the molar and be tightened to compress the coil. The force exerted by the spring begins at 150 g and decreases as space is opened.¹³

The Pendulum appliance was introduced by Hilgers in 1992¹⁴ and is still one of the most used distalizing devices.¹⁴ It is a fixed appliance composed of a plastic pad–contacted palatal rugae. The distalizing force is produced by beta-titanium springs that extend from the palatal acrylic and fit into lingual sheaths on the molar tube, which gives greater control of these teeth.¹⁵

Both the Distal Jet and Pendulum appliances produce an increase in vertical dimension due to a backward rotation of the mandible.^16–18 These vertical changes comprise a slight opening of the mandibular plane angle (about 1 degree) and an increase in lower anterior facial height (2.2–2.8 mm).¹⁹ Ghosh and Nanda reported that the increase in lower anterior facial height was significantly greater in patients with higher pretreatment mandibular plane angles.²⁰ The increased lower facial height and mandibular plane angle could have resulted from driving the molars back into the “wedge.” These results suggest that the Pendulum may be contraindicated in patients with excessive lower facial height and/or minimal overbite.¹⁸ Similar results were reported for the Distal Jet appliance.¹⁶

The maxillary molar distalization obtained with those appliances is characterized by a great amount of molar distal tipping (in average >10 degrees).¹²

Whereas the Distal Jet produces a labial tipping of the upper incisors as a result of the uncontrolled counterforce acting on the premaxillae, the Pendulum appliance showed a more controlled inclination of the upper incisors with a mild crown buccal tipping.

CMA consists of two rigid bars bonded bilaterally to the maxillary canines and first molars. The canine pad with a built-in mesial hook used for placement of intermaxillary elastics is bonded to the anterior third of the clinical crown. Posteriorly, the molded pad with a ball-and-socket joint is bonded to the first molar at the center of its clinical crown to facilitate molar derotation and distalization.^21–23

The activation of the appliance is obtained by the use of two types of elastics: the first one being 0.25 in, 6 oz; the second one 0.19 in, 8 oz, to be used from the second month of treatment until the molar and canine class I relationships are established. Elastics should be worn 22 hours per day, changing elastics three times per day.²⁴

The principle of this appliance is similar to a cantilever-based fixed appliance previously shown by Nanda.²⁵ The author described that system as an effective way to correct molar class II in nongrowing patients. An active cantilever with information of molar tipback was applied at the upper arch, while in the lower arch the author used a multibracket fixed appliance and class II elastics. The undesired effects of class II elastics were controlled by the fixed appliance in lower arch and by the activation of the cantilever in upper arch.

Previous retrospective clinical studies demonstrated the possibility of obtaining a maxillary molar distalization between 1.6 and 5.1 mm²⁴ with the mean amount of molar tipping not exceeding 3.7 degrees when CMA was used in combination with fixed appliances as anchorage units on the lower arch.²⁴ Furthermore the treatment time had a mean duration of about 4 to 5 months.²⁴

There is a lack of high-quality evidence supporting or contrasting the use of CMA. In another retrospective study in which CMA effects were compared to other class II correction methods, CMA showed the same results obtained with class II elastics in terms of molar distalization but in less time.²⁶

One clinically and statistically relevant effect of treatment with CMA occurred in lower anterior facial height that was associated with a significant increase in the mandibular plane angle.²⁷

Proclination of the lower incisors resulting from the class II elastics mechanics was observed and resulted in a significant amount (4.2 degrees).²⁴

All the tooth-borne appliances mentioned earlier produce some side effects that need to be controlled during the hybrid aligner treatment. Excessive upper and lower incisor proclination could be difficult to control with aligners. According to Rossini et al., buccolingual tipping and torque control of upper incisors have a mean accuracy of about 50% of the planned movement.⁴ The proclination of lower incisors resulting from the use of CMA could be controlled using active aligners on the lower arch and applying a lingual radicular torque information on the lower incisors of at least 5 degrees.

Another side effect that can occur using tooth-borne distalization devices is the rotation of the occlusal plane due to the increase of the vertical dimension.

Khosravi et al.²⁸ in their study about overbite management with Invisalign aligners showed that overbite correction is mostly related to anterior teeth movement without any significant posterior intrusion and/or extrusion.²⁸ As described by Ravera et al.³ bite block effect of the aligner causes an intrusive effect on posterior teeth of 0.5 mm³. A similar value (0.6 mm) was described by Mantovani et al.²⁹ Therefore, only the 0.5- to 0.6-mm bite block effect should be considered to counteract the increase of the vertical dimension produced by tooth-borne distalization devices (average increase 2–3 mm).

On the basis of these considerations, tooth-borne distalization devices should be avoided in patients with excessive lower facial height and/or minimal overbite. Clinicians should be aware of the existing evidence related to the limited control of posterior intrusion, overbite correction, and buccolingual inclination provided by CAT.

Two clinical examples will be presented: one in a teen patient and the second in an adult patient.