# 3D Printed Orthodontic Aligners--A Scoping Review
## 3 Results
### Summary of Findings
#### 3.2.2 Clinical Trials  
The identified clinical studies included four non-randomized trials and one randomized trial [8; 16; 31; 32]. The characteristics and main findings of the included clinical studies are summarized in Table 3.  
The clinical studies investigated different aspects of 3D-printed aligners in comparison with thermoformed aligners (industrial or in-house fabricated). Two clinical trials examined the impacts of in vivo ageing on mechanical properties. [PERSON] et al. investigated the impact on hardness, indentation modulus, elastic index, and chemical composition and concluded that 3D-printed aligners show higher stability compared to thermoformed aligners [8]. [PERSON] et al. concluded that some mechanical properties, such as Martens hardness and indentation modulus, were stable after one-week of in vivo ageing, whereas other variables such as indentation relaxation and elastic index were prone to ageing and showed significant differences compared to unused controls and thermoformed aligners [31].  
One study investigated the effects of intraoral ageing on dimensional stability and found no significant effect for either 3D-printed or thermoformed aligners after one week of clinical use [31]. However, they observed an increased thickness of about 0.3 mm compared to the virtual design. After one week of intraoral service, a non-significant increase of about 0.1 mm was observed due to ageing and water absorption. [PERSON] et al. studied the surface characteristics of 3D-printed aligners after exposition to an intraoral environment and found that they might be more prone to plaque accumulation and plaque retention due to increased material surface and thus fluid retention capacity compared to thermoformed aligners [32]. Only one study investigated the accuracy of planned tooth movements and reported an overall accuracy of 67.6%. The lowest accuracy was observed for the secondary premolar tip (17.6%), and the highest accuracy was observed for maxillary central incisor torque (100%) [33].  
All available clinical trials to date used Tera Harz TC-85; however, to date, no clinical trial has studied aspects of clinical efficiency compared to thermoformed aligners.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**Number of Participants**} & \multicolumn{2}{c}{**Number of Participants**} & \multicolumn{2}{c}{**} \\
**Author, Year** & **Study Design** & **Study Design** & **Aligency (3D-Printed Aligners)** & **(Vacuum-Formed Aligners)** & **Intervention/ Method** & **Primary Outcomes** & **Key Findings** \\ \hline [PERSON], [PERSON] [8] & Monocentric, non-randomized & 4/6 & 10 & 1 week of intracal & Martens hardness & No significant differences in mechanical properties after 1 \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline [PERSON], [PERSON] [32] & Monocentric, non-randomized & 12/12 & 12/12 & 1 week of intracal & & 3D-printed aligners showed significantly higher surface roughness compared & \\  & & & & & & \\  & & & & & & \\ \hline [PERSON], [PERSON] [31] & Prospective, monocentric, non-randomized & 6/12 & 24/48 & 1 week of intracal & & 3D-printed aligners showed increased thickness, while intracoral aging did not affect & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline [PERSON], [PERSON] [33] & Monocentric, no control group & 15/30 & 0 & Digital assessment of tooth movements & Accuracy of tooth movement & & \\  & & & & & (Tongue/tip/rotation of maxillary) incisors & & \\  & & & & & & \\ \hline [PERSON], [PERSON] [16] & Prospective, monocentric, non-randomized & 10/10 & 10/10 & 1 week of intracal & & Indentation modulus & 3D-printed aligners showed less force decay and higher wear resistance compared to control \\ \hline \hline \end{tabular}
\end{table}
Table 3: Characteristics and main findings of included clinical studies.