Hak Sun Kim, et al. Moreover, a further significant loss of correction was observed during the subsequent follow-ups in the thin rod group with the average correction at the 2 year follow-up being only 51% (24.5O) as compared to 60% (18.7O) in the thick rod group. The fact that the only apparent variable between the two groups is the thin and thick rod instru- mentation system clearly demonstrates that it may be res- ponsible for the superior results seen in the latter group. It is reasonable to predict that as the surgeon’s experience grows, the curve correction with VATS will increase and the operative time will decrease.16,22 In one study of fifty patients who were treated with VATS, curve correction averaged 50% in the first forty patients and improved to 69% in the last ten patients.17 Wong, et al.6 reported a mean thoracic curve correction of 62% with VATS. Another recent paper reported that VATS allowed more discs to be excised and the anterior operative time significantly de- creased between the first 25 consecutive patients and sub- sequent 28 consecutive patients, while providing compa- rable correction of the thoracic deformity.15 The need for a thick rod system to attain maximum sta- bility is further stressed by the fact that several dual rod systems for an open approach have been developed by different authors to overcome the problem of rod fractures and loss of correction. Dual rod constructs have been shown to increase screw pullout resistance, increase overall con- struct stability, and improve control of the sagittal plane. These systems have rarely been used above T8 because they are bulky.23 Their application is limited to thoracol- umbar and lumbar deformities because of vertebral body size requirements, and an open anterior approach is usually required to ensure optimal implant positioning. Norton, et al.16 also reported the use of a 4.5 mm dual rod system for thoracoscopic correction of AIS ranging from T7 to L3 and achieved an overall 87.3% improvement in the Cobb angle. So as far as the Lenke type 1 AIS is concerned, the 5.5 mm thick rod with poly-axial screw construct seems to be a possible solution for attaining adequate stability while maintaining a low-profile and avoiding any post-operative loss of correction. The mean pre-operative kyphosis was similar in both groups. However, at the final follow-up, while the thick rod group showed an improvement in the angle by around 3O, the thin rod group had a loss of kyphosis by 1O. Though this difference was not found to be statistically significant and the values of kyphotic angles in both the groups re- mained within normal limits, it is our opinion that this finding needs to be further followed because as the number of patients in the series increases, this difference may be- come significant. In retrospectively reviewing the two groups of patients treated with either the thin or thick rod system, we found similarities in terms of blood loss, operative time, tilt angle of most caudal instrumented vertebra, thoracic kyphosis, and C7 sagittal plumb line offset. One weakness of the present study is that it was retros- pective in nature and therefore was not randomized. How- ever, aside from not being prospectively randomized, the two groups were similar in terms of age, pre-operative Cobb angle, fusion level, BMI, gender, and Risser staging. Hence, there was no significant statistical difference bet- ween both groups as far as the pre-operative parameters were concerned. Another argument that can be put forward is that the results will vary with varying fusion lengths. However in our study, the average fusion length for the thin rod group and the thick rod group is 6.2 and 5.9, res- pectively, which is similar to each other. Hence, this is unlikely to cause any variation. It may be argued that the variability in the results of the two groups may be dependent on the surgical skill of the surgeon and also may vary with the learning curve. These factors are unlikely to influence the results because the same author performed all the surgeries in both groups. Furthermore, the surgeries of both groups were carried out in an alternate fashion and so the learning curve unlikely led to any variation. Complications requiring a second sur- gical procedure occurred in 1 case of cap loosening in the thin rod group. This patient has a pre-operative Cobb angle of 40O, which was corrected to 18.4O post-op. At the time of loosening, the Cobb angle was at 24.6O. This complication was managed by a short procedure of replacing the cap by using the same thoracoscopic portals. At the latest follow- up, the Cobb angle was at 18.6O. In the thick rod group, 2 cases of saddle breakage of the reduction screw were seen in the early cases. The breaking of the long extension of the reduction screw head posed some difficulty in the initial cases and as a result, the saddle broke in two in these cases. However, this did not lead to any change in the amount of correction achieved or the final outcome. The author solved this problem by devising a custom-made instrument for breaking off the reduction screw saddle extension, which was used in the subsequent cases. Major complications in the form of vascular or neurological injury did not occur in either group. We are not aware of any reported cases of vascular or neurological injury associated with thoracos- copic instrumentation in literature, although this concern has been raised. SRS-22 questionnaire is an outcome questionnaire that is used to assess and discriminate outcomes among patients with idiopathic scoliosis. Assessments of pain, function, and range of motion form the basis of this tool. Many scor- ing systems are not sensitive enough to distinguish whether one procedure is superior to another because the long-term effect of a given treatment goes unrecognized. The SRS-22 758 Yonsei Med J http://www.eymj.org Volume 51 Number 5 September 2010