Pre- and postoperative plain radiographs and low-dose spine CTs of 116 consecutive patients with AIS who underwent posterior corrective surgery with titanium-alloy (Ti-6AI-4V) during the period 2005-2009 were evaluated retrospectively. The patients were categorized into four period groups according to the date of operation (group 1: patients operated on in autumn 2005-2006, group 2: 2007, group 3: 2008, and group 4: 2009). As the number of patients operated on in autumn 2005 was only 7, these were included in the first period with patients operated on in 2006. Ninety four patients (81%) were female and 22 patients (19%) were male. The patients were identified from the database of the Orthopaedic Department and the database of the Radiology Department of our hospital. All patients had been examined with standing posteroanterior (PA) and lateral radiographs as well as with low-dose spine CT before surgery and six weeks postoperatively. The measurements performed on plain radiographs before and after surgery were: (1) Cobb angle of the major curve on standing PA-radiographs, (2) The degree of kyphosis at T5-T12 on standing lateral radiographs, and (3) Lower end vertebra tilt (LEVT), which is defined as the angle between the lower endplate of the lower end vertebra of the major curve and the horizontal plane, measured on standing PA-radiographs.
All CT-examinations were performed on a 16-slice scanner (SOMATOM Sensation 16, Siemens AG, Forchheim, Germany) according to our low-dose spine CT protocol: Slice collimation 16 × 0.75 mm, rotation time 0.75 s, pitch 1.5, tube voltage 80 kV and quality reference for the effective tube current-time product 25 mAs. The measurement of vertebral rotation was performed at the most rotated vertebra at the apex of the major curve as well as at one adjacent vertebra on either side of the apex. These measurements were performed at the same vertebral levels before and after surgery. The degree of vertebral rotation was measured according to the method of Aaro and Dahlborn . All measurements were performed by an experienced radiologist with specialized spine profile. The pre- and postoperative data which were collected from the medical and the radiological records of the patients included: age, gender, diagnosis, Lenke classification, date of operation, and the type of the operation.
The approval of the Regional Radiation Protection Committee to use the low-dose spine CT in the work-up of patients with AIS was obtained.
All operations were performed under general anesthesia with spinal cord monitoring using motor evoked potentials (MEP). The operations were performed through a standard posterior midline incision. After exposure of the posterior bony elements, estimated entry points for screws were defined by means of anatomical landmarks and small holes were created with an awl. Titanium markers were introduced into the holes with an estimated trajectory. Prior to making the screw canal, a fluoroscopic check of the accuracy of pin position and direction was made for each pedicle. The first part of the screw canal was made by means of a hand drill with a diameter of 3.2 mm, followed by a thin probe or feeler. The patency of the screw canal was checked with a feeler probe. Self-tapping screws with uniplanar screw head construct was regularly used. Pedicle screws used in this study were exclusively uniplanar. Curve correction was performed with a simple rod derotation manoeuvre of 90° of the concave rod. A firm pressure was applied to the hump on the convexity as well as applying pressure on direct vertebral rotation (DVR) handles attached to the heads of the 3-4 apical screws at the convexity, to counteract an increased vertebral rotation which otherwise often occur during the simple rod derotation. Before introduction of the stabilising rod on the convexity, DVR  was performed, in the apical area as well as in the caudal part of the construct if a significant rotation existed. As an attempt to improve the degree of deformity correction in stiffer curves, in situ concave rod bending and a more aggressive soft tissue release were performed when considered necessary. Prior to attachment of the convex rod, destruction of intervertebral joints as well as decortication of bony surfaces were performed.
All statistical analyses were performed by means of SPSS version 17. Data is presented as proportions (%) or as mean ± with standard deviations (SD) or with 95% confidence interval (95% CI). A linear regression analysis was performed to test for the occurrence of a trend of successively improved skill to correct the spinal deformity in all three planes during the study period 2005-2009. Pearson correlation was tested between patient number (order of the operation) and the degree of deformity correction. Independent sample t-tests were performed to test for the significance of differences in deformity correction between different study periods. Patients operated on with a rod diameter of 5.5 mm during 2005-2006 (n = 34), all were performed by or under supervision of the senior surgeon (AO), were divided into two groups (the first 17 and the last 17 patients) thereafter independent sample t-tests were performed to compare the magnitude of deformity correction among patients of the two groups. Spearman correlation and Mann Whitney-U tests were used to test the association between the correction of deformity and different variables such as age, gender and curve type. The statistical significance was set to P < 0.05.