Hak Sun Kim, et al. comparable to that of the open anterior approach.4-6 How- ever, the advantages of the thoracoscopic approach include decreased disruption of the thoracic musculature, decreased pulmonary morbidity, and improved cosmesis. The results of endoscopic surgery generally depend on the availability of specific surgical instruments and imp- lants.7,8 Endoscopic spinal instrumentation, however, has been associated with a steep learning curve, and its current indications are limited to specific patient and deformity characteristics. Since VATS is considered one of the endo- scopic surgical specialties, it is reasonable to expect that its results will vary significantly according to the type of sur- gical implants and implants utilized. Even in the most ex- perienced hands, thoracoscopic spinal deformity correction has been associated with instrumentation failure (rod breakage, screw pullout) and a pseudarthrosis rate between 5% and 31%.9-11 To decrease the incidence of these complications, se- veral methods of increasing construct stiffness have been introduced. Dual rod constructs have been shown to increase screw pullout resistance, increase overall construct sta- bility, and improve control of the sagittal plane; however, they have been limited in the application to the thora- columbar and lumbar vertebra due to the larger vertebral size requirements and bulky profile. Thus alternatives to address this issue have been focused on using single rod, low-profile constructs with stiffness comparable to that of the dual rod counterparts. There are two basic types of surgical implants used in VATS for scoliosis correction that are classified according to the technique employed to reduce the rod to the screw. The first is the cantilever technique that utilizes a thin, more elastic 4.5 or 4.75 mm diameter rod that can be par- tially bended during insertion into a mono-axial screw implant. The second technique utilizes poly-axial reduction screws that can be easily applied for more rigid thick 5.5 mm rod implant systems. Due to the inherent differences in the two types of implants utilized, we hypothesized that there would be a difference in the surgical outcomes as well. Despite this important variable, no data exists regard- ing the clinical outcomes of VATS according to the differ- ent surgical implants that may be utilized. The purpose of the this study was to compare the results of VATS with the cantilever technique using a thin 4.5 mm rod system versus the poly-axial reduction screw tech- nique using a thick 5.5 mm rod system in terms of deformity correction. A single surgeon performed all surgical proce- dures. Detailed radiographs were taken at various stages of the treatment and at the follow-up, and they were compar- ed to evaluate the achieved correction. Operative parameters and patient-based clinical outcomes including complica- tions were assessed as well. MATERIALS AND METHODS A retrospective analysis of patients who were operated on for correction of thoracic adolescent idiopathic scoliosis by VATS between July 2001 and August 2005 were con- ducted. A total of 49 patients with adolescent idiopathic scoliosis (AIS) underwent VATS using the cantilever technique with 4.5 mm rod or the polyaxial reduction screw with a 5.5 mm rod. There were 24 patients in the 4.5 mm thin rod mono-axial screw group and 25 patients in the 5.5 mm thick rod poly-axial reduction screw group. All patients were operated on by the first author (Kim HS). All radiographic parameters measurements were made by the main corresponding author (Moon ES). The study was approved by the institutional review board and appropriate patient informed consent was obtained. All subjects had a Lenke type 1 curve.12 All patients in both groups had complete radiographic data, including pre- operative weight-bearing standing anteroposterior, lateral, side bending (right and left), and supine anteroposterior radiographs. These were compared with immediate post- operative (supine), post-operative (weight-bearing stand- ing) at 2 weeks, and final weight-bearing follow-up radio- graphs. The most recent follow-up weight-bearing antero- posterior and lateral radiographs were available for all of the patients and were included in the analysis. The parame- ters measured were the coronal curve correction, most cau- dal instrumented vertebra level, correction of the tilt angle of the most caudal instrumented vertebra (that is, the angle of tilt of the vertebra from the horizontal), coronal balance (measured as the lateral offset from the midpoint of the sacrum to the T1-sacrum plumb line), sagittal balance (measured in lateral projection as the offset from the mid- point of the sacrum to the T1-sacrum plumb line), and amount of thoracic kyphosis (T5-T12). Radiographic and clinical evidence of fusion was evaluated for all patients, which was defined as the objective radiographic appea- rance of mature, remodeled, bony trabeculae, and solid bony arthrodesis across the disc space with no subjective complaints of pain. Demographic data such as age, body mass index (BMI), Risser stage, and other parameters were evaluated and com- pared for each group. Operative parameters assessed includ- ed the operative time, estimated blood loss, transfusion rate, number of levels fused, and complication rate. The Scoliosis Research Society-22 (SRS-22) outcome questionnaire was administered to all patients pre-opera- tively, post-operatively, and at the time of final follow-up.10 Total scores and the individual domain scores for pain, self- image, function, mental health, and satisfaction were com- pared between the two groups. Absolute values and change 754 Yonsei Med J http://www.eymj.org Volume 51 Number 5 September 2010