Comparison of Rod Thickness in VATS from the pre-operative values were compared between the groups. All scores were compared using the Mann-Whitney U test with the level of significance set at p < 0.05. One-way analysis of variance was used to examine the differences between the two groups for all demographic data, pre-operative, operative, and post-operative radiogra- phic parameters as stated previously. Selection criteria for each group This was a non-randomized study with all surgeries per- formed by a single surgeon (Kim HS). All cases included in this series were strictly Lenke type 1 curves. Any curve with a Cobb angle > 70O and kyphosis angle > 40O was excluded from the study. For the first 10 consecutive cases, poly-axial reduction screws with the 5.5 mm rod were used (thick rod group), followed by mono-axial screws with the 4.5 mm rod for the next 9 consecutive cases (thin rod group). The implants were then selected in an alternate manner for every 5 cases performed thereafter. Surgical technique The basic technique of thoracoscopic surgery involving the insertion of thoracoscopic portals, pleural dissection, dis- cectomies, and screw insertion were performed in the same way as described in other previous reports on the thoracos- copic management of AIS.13,14 Bony fusion was carried out by packing a morselized iliac crest bone graft into the disc space. In the thin rod group, the cantilever technique was used to achieve correction as described by Lonner, et al.,13 which involved prebending the contoured thin rod to the desired level of post-operative scoliosis and kyphosis, with sequential loading into the screws from proximal to distal and secured with locking nuts. In the thick rod group, once the screws were inserted, a rod was cut to the appropriate length and contoured so that it would easily pass through all of the poly-axial screw heads. This process was facili- Table 1. Demographic Data Number of patients (M / F) Age (yr)* Lenke classification 1A 1B 1C Body mass index Risser stage Fusion level Pre-operative major curve (degree)* Duration of follow-up (months) tated by adjusting the orientation of the screw heads using a customized tool made specifically for this purpose. Once the rod was positioned, it was sequentially captured to the screw heads using the inner nuts. The proximal screws were captured first followed by the distal and middle screws. The outer nuts were applied following the same se- quence. After tightening the nuts to the prescribed torque, the rod was rotated with customized derotators as needed for correction followed by final tightening. The remainder of the procedure was performed as described in other can- tilever technique reports. RESULTS Twenty-four patients (2 boys and 22 girls) were included in the thin rod group and 25 patients (1 boy and 24 girls) in the thick rod group. Tables 1-4 outline demographic data, curve characteristics, surgical procedures, and outcomes. Radiographic results In the thin rod group, the major curve was corrected from 49.8O pre-operatively to 15.8O (68.3% correction) at the im- mediate post-operative supine position, 23.2O (53.4% cor- rection) at post-operative 2 weeks in the standing position, and 24.5O (50.8% correction) at the time of the last follow- up. In the thick rod group, the major curve was corrected from 47.2O pre-operatively to 14.1O (70.1% correction) at the immediate post-operative supine position, 18.8O (60.2% correction ratio) at post-operative 2 weeks in the standing position, and 18.7O (60.4% correction) at the time of the last follow-up. It was observed that although there was no significant difference in coronal curve correction between the two groups in the immediate post-operative supine ra- diographs (p = 0.43), there was a significant difference in the correction at post-operative 2 weeks in the standing posi- Thin rod group 24 (2 / 22) 17.7 ± 5.2 (13 - 31) 16 6 2 18.8 ± 2.2 (17 - 21) 4.1 ± 1.5 (1 - 5) 6.2 ± 0.8 (5 - 8) 49.8 ± 9.3 (40 - 70) 32.38 ± 6.5 (24 - 46) Thick rod group 25 (1 / 24) 16.1 ± 4.1 (13 - 24) 17 5 3 19.5 ± 3.4 (16 - 25) 4.0 ± 1.3 (2 - 5) 5.9 ± 1.2 (5 - 8) 47.2 ± 9.9 (40 - 61) 37.8 ± 9.4 (24 - 60) *The data are given as the mean and the standard deviation, with the range in parentheses. Yonsei Med J http://www.eymj.org Volume 51 Number 5 September 2010 755