Variation patterns of cervical alignment in ankylosing spondylitis-related thoracolumbar kyphosis following pedicle subtraction osteotomy in the sitting position: an EOS-based investigation

OBJECTIVE Both unchanged upper cervical lordosis combined with decreased lower cervical lordosis and decreased upper cervical lordosis combined with decreased lower cervical lordosis have been reported to occur after correction surgery for adult spinal deformity. However, variations in cervical alignment after correction surgery in patients with ankylosing spondylitis (AS) have not been investigated. The current study aimed to investigate the variations in cervical alignment following the correction surgery in AS patients with thoracolumbar kyphosis. METHODS Patients with AS who underwent pedicle subtraction osteotomy (PSO) for thoracolumbar kyphosis from June 2016 to June 2019 with a minimum of 1-year follow-up were reviewed. Patients were grouped according to the presence (ossified group) and absence (non-ossified group) of total ossification of the anterior longitudinal ligament (ALL) in the lower cervical spine. Radiographic parameters, including thoracolumbar, craniocervical, and global radiographic parameters, were measured on lateral sitting EOS images. RESULTS Thirty-two patients (27 males and 5 females) with a mean follow-up of 1.5 years were identified. There were 21 patients in the non-ossified group and 11 patients in the ossified group. After PSO, both groups showed a decrease in the occiput-C7 angle (p < 0.001 for both). In the non-ossified group, the C2-7 angle decreased significantly (p < 0.001), while the occiput-C2 angle remained unchanged (p = 0.570). In the ossified group, the occiput-C2 angle decreased significantly (p < 0.001), while C2-7 angle remained unchanged (p = 0.311). In addition, the change in occiput-C2 was correlated with the osteotomy angle in the ossified group (R = 0.776, p = 0.005). CONCLUSIONS The variation patterns of cervical alignment following correction surgery for AS-related thoracolumbar kyphosis were different based on patients with or without total ossification of ALL in the lower cervical spine. When planning PSO for patients in the ossified group, restoration of the physiological upper cervical lordosis angle could be achieved by adjusting the osteotomy angle.