## 4 Discussion  
The aim of the present clinical study was to compare the perception of pain caused by the activation of two different types of archives during the initial orthodontic alignment phase. No differences between the two archives were found concerning the quality of pain. The mean VAS values of the 0.012 SS group were lower than those of the 0.014 Ni-Ti group at each time point, with a statistically significant difference in the first three days of treatment (T1, T3, T4, T5 and T6), which were characterised by the higher level of pain perceived. This outcome disagreed with the common opinion that light, continuous forces exerted by super-elastic Ni-Ti wires induce low pain perception. Concerning the occurrence of pain, the present results showed that it began immediately after the activation of arches in both groups (96% of the 0.012 SS group and 88% of the 0.014 Ni-Ti group), and that in 58% of all subjects, a higher value of pain intensity was reached on the first day of treatment, with no significant difference between the groups. These data were in accordance with other authors who reported that pain started within 12 h and raised in the following 24-48 h [8; 36]. \begin{table}
\begin{tabular}{c c c c} \hline  & **0.012 SS** & **0.014 Ni-Ti** & \(p\)**-Value** \\ \hline T22 & 0.0 (0.0–1.0) & 0.0 (0.0–5.5) & 0.840 \\ \hline T23 & 0.0 (0.0–0.0) & 0.0 (0.0–1.0) & 0.659 \\ \hline T24 & 0.0 (0.0–0.0) & 0.0 (0.0–1.0) & 0.639 \\ \hline T25 & 0.0 (0.0–0.0) & 0.0 (0.0–1.0) & 0.445 \\ \hline T26 & 0.0 (0.0–0.0) & 0.0 (0.0–1.0) & 0.532 \\ \hline T27 & 0.0 (0.0–0.0) & 0.0 (0.0–1.0) & 0.496 \\ \hline T28 & 0.0 (0.0–0.0) & 0.0 (0.0–1.0) & 0.505 \\ \hline T29 & 0.0 (0.0–0.0) & 0.0 (0.0–1.0) & 0.259 \\ \hline T30 & 0.0 (0.0–0.0) & 0.0 (0.0–1.0) & 0.290 \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._ \begin{table}
\begin{tabular}{c c c c c} \hline  & **Compressive** & **Pain during** & **Pain during** & **Spontaneous** \\  & **Pain** & **Occlusion** & **Chewing** & **Pain** \\ \hline
0.012 SS (N = 25) & 20 (80\%) & 14 (56\%) & 20 (80\%) & 9 (36\%) \\ \hline
0.014 Ni-Ti (N = 25) & 19 (76\%) & 10 (40\%) & 22 (88\%) & 11 (44\%) \\ \hline \(p\)-Value & 0.733 & 0.258 & 0.440 & 0.564 \\ \hline \end{tabular}
\end{table} 
Table 3: Pain characteristics. Frequency reported in number of subjects. The \(p\)-value refers to the comparison between groups (\(\chi^{2}\) test). In 52% of subjects, the pain disappeared within 7 days of the appliance activation. Although no significant difference was found between the groups, 48% of subjects reported pain beyond 7 days and 9 subjects beyond 15 days after fixed appliance activation. Most of the studies that have evaluated the characteristics of pain during an initial dental alignment with a fixed orthodontic appliance performed a standard follow-up of 7 days [1; 11; 18; 33; 37; 38; 39]; however, the results obtained in the present investigation justified a longer follow-up period and suggested its application in further clinical trials. The Australian archive is a high-tensile SS wire that is heat-treated to yield a higher resilience than normal SS and produces high forces that dissipate over short periods. The Ni-Ti archives exert continuous forces and have a low modulus of elasticity, a high springback and a low deactivation force. Therefore, we could suppose that the heavier orthodontic forces expressed by the 0.012 SS archive, characterised by a faster decay of intensity after their activation, could cause a less traumatic biological response at the tooth level and, consequently, lower pain perception and faster pain reduction compared with the lighter but continuous forces expressed by the 0.014 Ni-Ti archive. One the most frequent methods used by orthodontists to reduce pain perception is the administration of NSAIDs such as acetylsalicylic acid and ibuprofen.