Impact of cervical range of motion on the global spinal alignment in ankylosing spondylitis patients with thoracolumbar kyphosis following pedicle subtraction osteotomy

Abstract Background Context The head's center of gravity (COG) plumb line (PL) and C7 PL could be simultaneously positioned over the pelvis in adult spinal deformity (ASD) with normal cervical mobility. However, the position of the head in relation to the global spinal alignment has yet to be investigated in ankylosing spondylitis (AS) patients with thoracolumbar kyphosis. Purpose The objective of this study was to analyze the position of head in relation to the global spinal alignment in AS-related thoracolumbar kyphosis. Study Design/Setting Retrospective single-center study. Patient Sample AS patients who underwent lumbar pedicle subtraction osteotomy (PSO) for thoracolumbar kyphosis from January 2010 to August 2016 were reviewed. Only patients with a visible ear canal on the preoperative, immediate postoperative and final follow-up radiographs were included. Outcome Measures The chin-brow angle (CBVA), cervical range of motion (ROM), lumbar lordosis (LL), thoracolumar angle (TLA), thoracic kyphosis (TK), L1 pelvic angle (L1PA), pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), TK+LL+PI, PI-LL, maximal kyphosis (MK), deformity angular rate (DAR), T1 pelvic angle (T1PA), T1 tilt, spinosacral angle (SSA), sagittal vertical axis (SVA) COG-C7, SVA COG-femoral head (FH) and SVA C7-S were evaluated. Data regarding the health-related quality of life, including the Oswestry disability index (ODI) and visual analogue scale (VAS)-back score, were also collected preoperatively, 2 years postoperatively and at the latest follow-up. Methods The cohort was divided into patients with occiput-trunk (OT) concordance or with OT discordance according to the SVA COG-C7 ≤ 30mm or > 30mm, respectively. There was no funding in this study and there are no conflict of interest-associated biases. Results A total of 43 patients (36 males and 7 females) with a mean age of 34.2 years (range, 18 to 59 years) were identified. There were 17 patients accompanied with OT concordance and 26 patients with OT discordance preoperatively. The cervical ROM was significantly lower (24.0° vs 56.1°, P Conclusions OT discordance in AS-related thoracolumbar kyphosis could be caused by the reduced cervical ROM. To maintain global spinal balance, the pelvis rotated further backward in response to the larger SVA COG-C7. Moreover, the larger SVA COG-C7 could be decreased after the lumbar PSO. Although there were radiographic differences between the patients with OT concordance and with OT discordance, there was no difference in clinical outcomes, and that a larger sample size and longer follow-up is needed.