# Comparative In Vitro Analysis of Composite Resins Used in Clear Aligner Attachments
## 3 Results
### Tribological Results
Although the worn surface areas were similar, the volumes involved and the maximum measured depths were found to be greater in AC compared to CF. Despite its higher strength, AC showed higher friction coefficients and greater wear volumes under reciprocating dry sliding (Table 3). This counterintuitive result indicates that higher filler content does not always equate to better wear resistance, especially in tribological systems where surface adaptability and energy dissipation play a crucial role. In particular, the higher presence of filler increases the abrasive component of wear, as evident in Figure 6, which reports the stereoscopic images of the abraded areas after the test and it clearly revealed evidence of abrasive wear. In the case of AC this wear is particularly pronounced, leading to regions of material loss caused by matrix deformation and tearing. As a result, filler particles released from the matrix contributed more actively to the wear process. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \
\begin{tabular}{c} **Wear Surface** \\ **(mm2)** \\ \end{tabular} &
\begin{tabular}{c} **Wear Volume** \\ **(mm3)** \\ \end{tabular} & **Max Depth (mm)** \\ \hline AC & 1.82 \(\pm\) 0.075 & 0.012 \(\pm\) 0.0055 & 36.88 \(\pm\) 3.642 \\ CF & 1.70 \(\pm\) 0.159 & 0.0070 \(\pm\) 0.0083 & 17.91 \(\pm\) 3.387 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mean wear surface, volume, and max depth for the wear traces for both composites. CF demonstrated lower friction, reduced wear track depth, and overall better wear performance, despite its lower mechanical stiffness. One plausible explanation lies in its higher polymer content, which may act as a self-lubricating or energy-absorbing layer during sliding (Figure 6b). Furthermore, its more elastic matrix likely redistributes localized contact stresses more effectively, reducing crack initiation and propagation. This suggests that CF could be particularly effective for applications involving dynamic or shear-loaded interfaces, such as aligner-attachment contact points or interproximal guidance zones. The results on wear behavior are further supported also by the surface roughness data before and after the wear test, as reported in Table 4. Before the test, the average surface roughness of the AC samples was higher than the CF samples. However, after the wear test the roughness in the worn area became comparable for the two materials. The results on wear behavior are further substantiated by the surface roughness measurements conducted before and after the wear test, as reported in Table 4. Prior to wear, the average surface roughness (Ra) and the total peak-to-valley height (Rt) of the AC samples were significantly higher than those of the CF samples, indicating a more irregular initial surface for the AC material. Specifically, the AC sample exhibited an Ra of 0.577 \(\pm\) 0.035 \(\upmu\)m and an Rt of 4.369 \(\pm\) 0.521 \(\upmu\)m, compared to 0.337 \(\pm\) 0.070 \(\upmu\)m and 2.862 \(\pm\) 0.549 \(\upmu\)m for the CF sample. This initial difference is consistent with the observed abrasive wear behavior, which was more pronounced in the AC sample. After the wear test, both materials showed a notable decrease in surface roughness, with Ra values converging to 0.247 \(\pm\) 0.036 \(\upmu\)m (AC) and 0.236 \(\pm\) 0.019 \(\upmu\)m (CF), and Rt values reducing to 2.537 \(\pm\) 0.650 \(\upmu\)m (AC) and 2.224 \(\pm\) 0.782 \(\upmu\)m (CF). This convergence suggests that the wear process smoothed the surfaces, reducing the initial topographical differences. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Average Roughness (Ra, \(\upmu\)m)** & **Total Peak to Valley Height (Rt, \(\upmu\)m)** \\ \hline  & **Unworn Zone** & **Worn Zone** & **Unworn Zone** & **Worn Zone** \\ \hline AC & 0.577 \(\pm\) 0.0352 & 0.247 \(\pm\) 0.03649 & 4.369 \(\pm\) 0.5206 & 2.537 \(\pm\) 0.6496 \\ CF & 0.337 \(\pm\) 0.0703 & 0.236 \(\pm\) 0.0189 & 2.862 \(\pm\) 0.5488 & 2.224 \(\pm\) 0.7815 \\ \hline \hline \end{tabular}
\end{table}