## 4 Results and Discussion
### Macro-Staging and Micro-Staging
#### 4.3.2 Micro-Staging
The micro-staging was divided into a first phase of posterior expansion and anterior retraction to produce relative intrusion and molar derotation (**B**) and a second phase in which the isolated intrusion of first and second molars was planned (**C**). At the end of the first set of aligners, a reduction in gingival unevenness between the gingival margins of the molars and the anterior teeth was observed (**D**). Identify Interproximal Friction Points and Facilitate Plastic Push  
The second principle of orthodontics with clear aligners emphasized the importance of having large tooth-aligner contact surfaces so that the aligner could exert adequate force. The orthodontist must be able to identify the points or areas on the tooth crown where the aligner exerts pressure at each moment, modifying the way the tooth moves if these areas are small or create a lot of friction with adjacent teeth. An example illustrating this is the meso-distal root alignment of an upper central incisor (Figure 8). Since this is a tooth with a flat vestibular and palatal anatomy, and with minimal space on the mesial and distal sides (due to the presence of the contralateral central incisor and the lateral incisor), the only points of support for the plastic's push are the active surfaces of the optimized double root control attachment. As these are very small surfaces, the success of the root movement will be highly compromised. Adding an intrusive component to the lowermost end of the incisor while maintaining vertical height at the other end radically changes the region of the tooth where the aligner will push, increasing its push surface and, therefore, the success of the movement. Rotational movements of round teeth also play an important role in this regard. Numerous authors have reported low predictability of success in this movement due to frequent loss of tracking [60, 61]. We believe that this lack of effectiveness is associated with the way round teeth are derotated. On one hand, the anatomy of their surface makes it difficult to establish a perpendicular direction of forces in a pure rotation [61], depending on the active surface area of the attachment to support the aligner (again, a quite limited contact area) [15]. On the other hand, there is clear interproximal surface friction with adjacent teeth (which is detrimental in a closed system). Figure 8: Upper-right central incisor with distal root inclination (**A**). Planning micro-staging where only mesial root alignment is performed has few support points for the plastic and depends on the active surfaces of the optimized attachments (**B**). Adding an intrusive component in the region of the more extruded incisal edge and maintaining the vertical position of the more intruded region (in this case, the mesial part) adds much more support surface to the plastic (**C**), achieving a more predictable root uprighting (**D**). An interesting strategy is to change the type of planned rotation for a _\"hinge\"_ movement, which incorporates a bucco-lingual component and places the center of rotation of the tooth at its mesial or distal contact point. Combining this type of rotation with an expansive or proclination movement (mesial-out or distal-out rotation) has two major advantages: (1) it adds a large portion of the palatal surface as a pushing surface and (2) it creates interproximal space, reducing friction. A study by [PERSON] et al. [62], in which they combined expansion with mesial-out hinge rotation to derotate the upper first molars, achieved an 82% predictability. If the hinge rotation movement is performed inwards (mesial-in or distal-in rotation), it maintains the benefit of increasing the pushing surface compared to pure rotation (by extending the support area to a large part of the vestibular surface) but requires previously created space with interproximal teeth to avoid friction (presence of diastemas, interproximal reduction).