# Incisors Extrusion with Clear Aligners Technique: A Finite Element Analysis Study
## 4 Discussion  
The obtained results evidenced the high unpredictability of aligner orthodontics in performing anterior teeth extrusion. The discrepancy between digital treatment planning and the clinical results may reside principally in the way the digital setup is performed [8]--the general thought that moving crowns in the setup will result in coherent mechanics in the achieving of the desired tooth position is increasingly going to be rejected, while deepening our biomechanical knowledge of this treatment modality. The FE simulations of incisor extrusion are emblematic of this concept--planning a pure bodily extrusion of the four incisors, without any other movement that could generate undesired forces and displacements, will result in retroclination of anterior teeth, as well as buccal inclination and intrusion of the posterior ones, independently of the adopted attachments configuration. Furthermore, for the first time in aligner orthodontics, published clinical trials [3; 4; 5] have reported outcomes which are consistent with the biomechanical information obtained from the present FE analyses. This matching in knowledge did not occur for distalization, for which only one protocol was tested [9]. The results demonstrated some biomechanical flaws that need to be improved. Regarding open-bite treatment, with focus on upper incisor extrusion, it could be stated that the FE analyses were useful to obtain a biomechanical pattern, and that the obtained data are confirmed by the three retrospective studies by [PERSON] et al., [PERSON] et al., and [PERSON] et al. [3; 4; 5]. Furthermore, our simulations showed that the teeth presenting the worst inefficiencies were lateral incisors. Clinically, the lack of control on lateral incisors is an old issue for aligner orthodontics. No scientific paper has been published about this topic; however, it is common knowledge that the lateral incisors are the most difficult teeth to control with \begin{table}
\begin{tabular}{c c c} \hline \hline
**Tooth Deformation** & **MIN (mm)** & **Location** \\ \hline NOATT & 0.00 & Mesial surface of the root of element 1.3 \\ ATT1-2 & 0.00 & Distal surface of the root of element 1.3 \\ ATT3-7 & 0.00 & Mesial surface of the root of element 2.3 \\ OTT & 0.00 & Buccal surface of element 2.2 \\ RETT & 0.00 & Buccal surface of element 1.1 \\ PALAT & 0.00 & Mesial surface of element 2.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Minimum tooth deformation and locations. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Aligner Max** & **MAX (MPa)** & **Location** \\
**Pressure** & **MAX (MPa)** & **Location** \\ \hline NOATT & 0.14 & Buccal gingival margin of element 1.1 \\ ATT1-2 & 0.02 & Incisal surface of the attachment of element 1.2 \\ ATT3-7 & 0.17 & Buccal gingival margin of element 2.2 \\ OTT & 0.14 & Buccal gingival margin of element 1.2 \\ RETT & 0.25 & Gingival surface of the attachment of element 1.2 \\ PALAT & 0.19 & Gingival surface of palatal attachment of element 1.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Aligner maximum pressure locations. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Tooth Deformation** & **MAX (mm)** & **Location** & **Direction** \\ \hline NOATT & 0.04 & Buccal surface of element 2.2 & Buccal \\ ATT1-2 & 0.05 & Incisal margin of element 2.1 & Palatal \\ ATT3-7 & 0.04 & Incisal margin of element 1.2 & Buccal \\ OTT & 0.04 & Buccal gingival margin of element 1.2 & Buccal \\ RETT & 0.04 & Buccal gingival margin of element 2.1 & Palatal \\ PALAT & 0.04 & Incisal margin of element 1.1 & Palatal \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Maximum tooth deformation and locations. aligners, especially when extrusion movements are planned. For this reason, extra aligners are very often needed (Figure 3). The lack of control on the lateral incisors, as well as the discrepancy between planned movement and the clinical result, in our opinion, could be interpreted correctly by analyzing aligner deformation.