# Lower Molar Distalization Using Clear Aligners: Bodily Movement or Uprighting? A Preliminary Study
## 1 Introduction  
Class III malocclusion could be considered one of the most challenging maxillofacial disorders an orthodontist has to deal with, regardless of the age of the patient [1, 2].  
The characteristics of Class III malocclusion have been widely documented and include: skeletal components with underdeveloped maxilla, overdeveloped mandible or a combination of them; dentoalveolar components with Class III molar relationship, negative anterior overjet or dental compensation (naturally proclined upper incisors and retroclined lower incisors to achieve normal overjet and function) [3, 4]. Correct diagnosis of the components is mandatory in order to provide an appropriate treatment plan.  
The prevalence of Class III malocclusion ranges between 0.7% and 19% among the world population, with a mean prevalence of 5.92% among Caucasians and higher values in some Asian regions [5, 6].  
The multifactorial etiology comprises genetical factors [7], that are not modifiable, and environmental factors such as bad habits (finger suction), swallow and breath disfunction, head or neck traumas; environmental factors alone cannot be responsible for the onset of the malocclusion, but they can play a role during the growth period if genetical tendency is present [8, 9, 10].  
Based on the patient's skeletal development stage, scientific literature suggests different approaches.  
The ideal timing of treatment is during the early mixed dentition age and the aim is to provide a more favorable environment for normal growth and to improve the occlusal relationship [11; 12]. The most validated approaches in growing patients are represented by orthopedic therapy using rapid palatal expander, followed by face mask, and functional therapy [13]. The closer the growth peak, the greater the dental effects rather than skeletal ones; consequently, the risk of having side effects, such as periodontal problems and dental compensation without correction of the skeletal malocclusion, increases [14].  
When an adult patient presents a Class III malocclusion that is treatable without surgery, the aim of the therapy is to correct the dental relationship, reaching molar and canine Class I, through lower molar and premolar distalization, and normalizing the anterior overjet.  
Lower molar distalization is a complex movement to obtain due to the high bone density and molar radicular morphology [15]. Widespread methods are represented by open coil springs on fixed appliances, inter arch elastics and lip bumper device [16; 17]. The main undesirable effect during lower molar distalization is anchorage loss in the anterior region, with lower incisor proclination that is disadvantageous to obtain the correction of the overjet [18; 19]. The introduction of temporary anchorage devices (TADs) improved the distalization mechanics reducing dental side effects, but mini screws present some disadvantages such as surgical invasiveness for the insertion procedure and high infection risk with consequent mini screw loss. Moreover, an increasing number of adult patients request a more comfortable and less visible treatment; clear aligner treatment (CAT) have been introduced to answer to this request and their efficacy is well demonstrated [20; 21].  
Efficacy of clear aligner in distalizing upper molars has been shown and they can distalize upper molars even more than 2.7 mm; the use of buccal attachments on involved and anchorage teeth is mandatory to obtain the desired movement [22; 23; 24].  
There are few studies in the literature that analyze lower molar distalization with CAT. It is reasonable that the same principles of upper distalization could be applied on lower distalization, with same feature and same anchorage strategies. The aim of this study is to assess the efficacy of Invisalign system in distalizing lower molars, without any auxiliary and evaluating the quantity of tipping and bodily movement.