## 4 Results and Discussion
### Basic Principles of Clear Aligners  
In order to understand the complexity and importance of the staging in treatment with clear aligners, it is advisable to emphasize the fundamental principles that characterize these systems and differentiate them from conventional multibracket fixed orthodontic systems [3].  
From the authors' perspective, there are three fundamental principles:  
1. A closed system.
2. Push surfaces.
3. Differential anchorage.  
A closed system is one that does not allow a release of forces at its ends. Multibracket fixed orthodontic systems are often considered open systems as they manifest excess energy in the most distal elements of the system. An example of this occurs during the alignment and leveling phase with nickel-titanium round archwires in cases with significant dental crowding. As the archwire regains its original shape and \"_pulls_\" the teeth toward the new arch form, it generates a set of forces (or energy) whose remnant is expressed in the distal part of the last bonded molar tube as excess arch.  
In contrast, clear aligner orthodontics is organized through a system in which the aligner completely covers the dental crowns. Since it lacks an open or free area in the scheme, the forces introduced into it will persist and can cause undesired movements if there is insufficient space in the arch (resulting in dental intrusion or the \"_watermelon seed effect_\", as described in the literature [27; 28]).  
The practical implication of working in a closed system is that the clinician must focus on continuously creating space in their planning [3], whether through planned expansion, buccal inclination of the crowns, interproximal enamel reduction, tooth extractions, or distalizations. The visual creation of space forms an essential part of the sequencing of movements, as we will explore further.  
In fixed multibracket orthodontic systems, we have two elements: the archwires, which constitute the active component as they are responsible for exerting forces on the teeth, and the brackets, which form the passive component by acting as a connection between the archwire and the teeth (although they also provide information about tip and torque through their prescription). In clear aligner orthodontic systems, the aligners constitute the active element as they apply pressure to the teeth through push surfaces [4]. Each aligner slightly modifies the position of each tooth by exerting forces on millions of points on the dental crowns, creating an appliance with a shape slightly different from the teeth it encompasses. The flexibility of the plastic allows for a controlled adjustment period called the \"_interphase_\" [29; 30], which is crucial for the efficient functioning of invisible orthodontics. If there is space between the teeth, dental movement occurs after a few days, aligning the shape of the aligner with the shape of the teeth. However, precise mechanisms detailing when an aligner has fully expressed its information over time have not yet been developed, causing the pattern of changing aligners to be based on the professional's decision. Because the aligner has the same shape and size as the teeth it encloses, dental anatomy significantly influences the effectiveness of tooth movement [31]. The design of wide push surfaces forms a relevant part of the staging of movements, as will be explained later.  
Differential anchorage [32; 33] is a highly valuable feature of clear aligner orthodontics that involves establishing a staging of movements where there is always a greater anchorage value (represented by unmoving teeth) than teeth in displacement [34; 35]. This concept minimizes the movements of teeth used as anchorage [36] and maximizes the movements of target teeth. Moreover, differential anchorage provides the flexibility to modify anchorage segments throughout the treatment according to the clinician's needs [37]. Differential anchorage is the core of the staging of orthodontic tooth movement.