# Impact of Dental Model Height on Thermoformed PET-G Aligner Thickness--An In Vitro Micro-CT Study
## 4 Discussion  
Previous research revealed that predictability and accuracy of aligner treatments can be as low as 30-50% when compared with the initial setup [14; 15; 16; 17; 29]. Besides patient related factors, varying material thickness of aligners has been suspected to impact on predictability of aligner treatments [18]. Therefore, the present study aimed at evaluating homogeneity of thermoformed PET-G aligners, and whether base height of 3D-printed dental models also impacts on material thickness values at different measurement locations. The present study identified that aligners formed on narrow (N) models exhibited higher thickness values compared to those produced on higher (H) dental models. Material homogeneity was greater in the H compared to the N group. Additionally, thickness values varied with respect to the tooth types, and the measurement locations. Highest values were found at incisal edges and cusp tips, followed by palatal surfaces, and lowest values were seen at the facial aspects. Comparison between N and H groups did not reach statistical significance at most measurement locations. At palatal and facial surfaces, however, aligners from group N were significantly thicker compared to group H.  
According to literature, the area moment of inertia is calculated using the third power of material diameter in direction of the acting forces [30]. Therefore, a 10% reduction of aligner material thickness could in theory reduce exerted forces up to 30%. In the present study, facial surfaces showed the smallest thickness values (estimated mean thickness group H/N: 0.243 mm/0.268 mm), whereas palatal (0.332 mm/0.347 mm) and incisal surfaces (0.427 mm/0.434 mm) showed highest values. Therefore, thickness values were 38-43% higher at incisal and 20-25% higher at palatal compared to facial surfaces. Differences between the groups amounted to 9% at the facial and 4% at palatal faces. Thus, these differences appear to be in a clinically relevant range, potentially impacting on treatment predictability. Nonetheless, it must be noted that force transmission patterns from aligners are complex and not yet fully understood [31; 32]. This owes to the so called \"half shell shapes\" of aligners and differences in localization of contact areas to teeth and attachments. Additionally, material deformation when aligners are put on the teeth, and material swelling due \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Comparison** & **Est. Difference (SE)** & **T-Ratio** & \(p\)**-Value** \\ \hline cusp tip–facial & 0.116 (0.003) & 35.565 & \(<\)0.001 *** \\ cusp tip–facial cervical & 0.160 (0.003) & 48.827 & \(<\)0.001 *** \\ cusp tip–incisal & \(-\)0.051 (0.003) & \(-\)15.539 & \(<\)0.001 *** \\ cusp tip–oclusal & 0.105 (0.003) & 32.205 & \(<\)0.001 *** \\ cusp tip–palatal & 0.037 (0.003) & 11.162 & \(<\)0.001 *** \\ cusp tip–palatal & 0.045 (0.003) & 13.638 & \(<\)0.001 *** \\ cervical & 0.043 (0.003) & 13.262 & \(<\)0.001 *** \\ facial–incisal & \(-\)0.167 (0.003) & \(-\)51.103 & \(<\)0.001 *** \\ facial–oclusal & \(-\)0.011 (0.003) & \(-\)3.360 & 0.022 * \\ facial–palatal & \(-\)0.080 (0.003) & \(-\)24.402 & \(<\)0.001 *** \\ facial–palatal cervical & \(-\)0.072 (0.003) & \(-\)21.927 & \(<\)0.001 *** \\ facial cervical–incisal & \(-\)0.210 (0.003) & \(-\)64.366 & \(<\)0.001 *** \\ facial cervical–oclusal & \(-\)0.054 (0.003) & \(-\)16.622 & \(<\)0.001 *** \\ facial cervical–palatal & \(-\)0.123 (0.003) & \(-\)37.664 & \(<\)0.001 *** \\ facial cervical–palatal & \(-\)0.115 (0.003) & \(-\)35.189 & \(<\)0.001 *** \\ cervical & 0.156 (0.003) & 47.744 & \(<\)0.001 *** \\ incisal–palatal & 0.087 (0.003) & 26.701 & \(<\)0.001 *** \\ incisal–palatal cervical & 0.100 (0.003) & 29.177 & \(<\)0.001 *** \\ occlusal–palatal & \(-\)0.06875 & \(-\)21.043 & \(<\)0.001 *** \\ occlusal–palatal & \(-\)0.06067 & \(-\)18.567 & \(<\)0.001 *** \\ palatal–palatal cervical & 0.00809 & 2.476 & 0.187 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: \(Cont.\)to saliva exposure must be considered.