The influence of sagittal profile on the S2 alar-iliac (S2AI) screw trajectory in degenerative lumbar scoliosis patients: A computed tomography (CT) study

Abstract

Background The usage of S2 alar-iliac (S2AI) screw for spino-pelvic fixation had been gaining increasing popularity in degenerative lumbar scoliosis (DLS) patients owing to its considerable advantages. However, the recommended ideal angles for S2AI screw insertion varied in the literature. The purpose of this study was to investigate whether the optimal S2AI screw trajectory was influenced by sagittal profile in DLS patients. Methods Computed tomography (CT) scans of pelvis were performed in 47 DLS patients for 3-dimensional reconstruction of S2AI screw trajectory. Five S2AI screw trajectory parameters were measured in CT reconstruction images, including: 1) angle in the transverse plane (Tsv angle); 2) angle in the sagittal plane (Sag angle); 3) maximal screw length; 4) screw width; and 5) skin distance. The lumbar Cobb angle, lumbar apical vertebral translation (AVT); global kyphosis (GK); thoracic kyphosis (TK); lumbar lordosis (LL); sagittal vertical axis (SVA); sacral slope (SS); pelvic tilt (PT); and pelvic incidence (PI) were measured in standing X-ray films of the whole spine and pelvis. Results Both Tsv angle and Sag angle had significant positive associations with SS (P < 0.05) but negative associations with both PT (P < 0.05) and LL(P < 0.05) in all cases. Patients with SS less than 15°had both smaller Tsv angle and Sag angle than those with SS equal to or more than 15°(P < 0.05). Conclusions The decreased LL would lead to the backward rotation of the pelvis, resulting in a more cephalic and less divergent trajectory of S2AI screw in DLS patients.