## 4 Discussion  
Scoliosis correction surgeries are complex and challenging [6; 7; 10] and lack of satisfactory bone fusion can lead to numerous mechanical and clinical complications [8; 11; 14]. In the present study, it was hypothesized that greater preoperative soliotic curve flexibility is a protective factor for the occurrence of long-term mechanical complications. However, the findings do not statistically support this hypothesis, despite the apparent excess of failures among patients with nonflexible curves. A higher percentage of correction in the postoperative period was observed among curves classified as flexible (Tables 1-3). On average, 21.73 degrees of main soliotic curve correction was obtained, with an average correction of 70% of its value in degrees. The results of [PERSON]'s correlation between the difference in curves without and with traction or bending films and the percent correction in the postoperative period revealed a significant relationship only for the main curve, not the secondary curves. Additionally, the effect was weak, with an r of only 0.23 (Table 3d). When evaluating flexibility as a percentage, both the main and secondary curves showed a significant correlation with the correction achieved and with a more intense effect than when observing absolute values in degrees. Even so, for the comparative flexibility of the main curve, which showed the greatest effect, the value is considered only moderate (Table 3e). This fact is consistent with findings from previous studies [15; 16]. Among the hypotheses to explain this finding, the following stand out: technical variability between spinal X-ray images obtained with and without traction could perhaps underestimate the power of correction; the study sample had a large number of patients with severe and rigid curves, in whom the influence of traction during spinal X-ray examination could be smaller; and the reduction maneuvers, osteotomies and other reduction tactics used in different cases were not compared. On comparison of age, weight, height and number of instrumented levels with the percent scoliotic curve correction, only the last variable showed a statistically relevant result, though with a weak effect; a smaller number of instrumented levels was associated with greater correction (Table 5). This fact may indicate bias in the study given that the \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & & **Estimated Failures per Year** & \\ \hline
**Time** & **Patients at Risk** & **Number of Events** & **Probability, No** & **Probability, Yes** \\ \hline
1 st year & 321 & 9 & 0.973 & 0.0273 \\
2 nd year & 311 & 16 & 0.952 & 0.0485 \\
3 rd year & 93 & 19 & 0.939 & 0.0611 \\
4 th year & 38 & 21 & 0.899 & 0.101 \\
5 th year & 12 & 21 & 0.899 & 0.101 \\
6 th year & 3 & 21 & 0.899 & 0.101 \\ Variable & & Reference & RR 1 & 95\% CI 1 & \(p\)-value \\ Ambulation & & No & **0.27** & 0.11–0.65 & **0.003** \\ Sex & & Female & 1.34 & 0.54–3.31 & 0.5 \\ Main curve flexibility & & Flexible & 1.67 & 0.69–4.03 & 0.3 \\ Etiology & & & & \\ Neuromuscular & Idiopathic & **3.33** & 1.17–9.52 & **0.025** \\ Neuromuscular–Cerebral Palsy & Idiopathic & 2.24 & 0.78–6.40 & 0.13 \\ Congenital & Idiopathic & 0.00 & 0.00 & - \\ \hline \hline \end{tabular}  
* RR = relative risk, CI = confidence interval.  
\end{table} 
Table 6: Risk of failure per year of follow-up and Cox regression according to follow-up time, material failure, ambulation, sex, main curve, flexibility and etiology. sample is from a heterogeneous population with varying degrees of scoliotic curve severity and rigidity, as several studies comparing implant density and proximal and distal fixation with outcomes have shown results conflicting with those presented here [17; 18; 19; 20]. The probability of material failure was 2.73% in the first year after surgery and 10.1% over four years (example, Figure 1). The average time from surgery to diagnosis of the complication was 309 days.