Influence of implant density and flexibility index on curve correction after scoliosis surgery

Numerous studies proved that all pedicle screw constructs produce the best outcome in the surgical treatment of patients with scoliosis. However, the optimal amount and distribution of screws has not yet been defined. In recent studies on idiopathic scoliosis, the correlation between curve correction and implant density has been discussed with diversifying results.

The aim of this study was to detect a possible correlation of sagittal and coronal curve correction in dependence of metal load and flexibility index.

Twenty-six patients were included in this study with surgical correction by one surgeon between January 2014 and December 2017. Clinical data and radiographic images (preoperative, postoperative and one-year follow-up) were retrospectively analyzed and metal load, flexibility index, correction rate and correction index were consequently calculated. The Pearson correlation analysis was used for metal load-correction index and metal load-correction rate, correction of kyphosis-metal load and correction of lordosis-metal load. According to the mean metal load of 88%, patients were divided into two subgroups-a low-density group of 12 patients and a high-density group of 14 patients. Clinical and radiographic features were examined by an independent two-sided t-test.

Eight patients were male, 18 patients female. Ten suffered from neuromuscular and 16 from idiopathic scoliosis. Mean age was 17.1 years. Correction rate directly postoperative was 70.43%, at the follow-up 67.90%. Mean correction index directly postoperative was 3.40 and at the follow-up 3.23. Pearson correlation of metal load-correction index directly postoperative was - 0.188 and one year postoperative - 0.189. The correlation between metal load-correction rate immediately after the surgery was 0.324 and at the follow-up 0.285. Correlation for correction of kyphosis-metal load postoperative was - 0.120 and one year later - 0.178. Pearson coefficient of lordosis-metal load directly after the surgical intervention was - 0.214 and at the follow-up - 0.220. Dependency of flexibility index and correction rate showed a positive trend (Pearson flexibility-correction rate direct 0.616; flexibility index-correction rate follow-up 0.516). A statistically significant difference between the high- and the low-density group was detected in the correction rate directly postoperative (p = 0.047).

With an implant density over 70%, satisfactory surgical treatment can be achieved in idiopathic and neuromuscular scoliosis. No statistical significance between the high-density (88-100%) and the low-density (73-87%) group could be verified in curve correction, ICU stay and complications.