Pedicle Morphology of Lower Thoracic and Lumbar Spine in Ankylosing Spondylitis Patients with Thoracolumbar Kyphosis: A Comparison with Fracture Patients

OBJECTIVE: The pedicle morphology of ankylosing spondylitis (AS)‐related thoracolumbar kyphosis patients may be different from that of individuals with normal spine due to the ectopic ossification and kyphotic deformity. However, there was no literature analyzing the pedicle morphology of AS patients with thoracolumbar kyphosis. Therefore, the present study aimed to investigate the pedicle morphology of lower thoracic and lumbar spine (T9‐L5) in ankylosing spondylitis (AS)‐related thoracolumbar kyphosis patients. METHODS: A retrospective review of AS patients with thoracolumbar kyphosis (AS group) and the patients with spinal or rib fracture (fracture group) who underwent CT scans of the lower thoracic and lumbar spine between February 2017 and September 2018 was performed. Patients with spinal tumor, spinal tuberculosis, severe degenerative spinal diseases including degenerative scoliosis, degenerative spondylolisthesis, degenerative spinal stenosis or history of previous spine surgery, or AS patients with pseudarthrosis which influenced the measurement of pedicle parameters were excluded. The measured parameters on CT images included transverse pedicle angle (TPA), transverse pedicle width (TPW), chord length (CL), pedicle length (PL), and sagittal pedicle angle (SPA). The intraclass correlation coefficient (ICC) was used to evaluate the agreement of radiographic parameters between observers. The independent sample t test was applied for the comparison of pedicle parameters between the two groups. The gender distribution between the two groups were compared using the Fisher's exact test. RESULTS: A total of 1444 pedicles of 53 AS‐related thoracolumbar kyphosis patients and 30 patients with fracture were analyzed. TPA was significantly smaller in AS group (p < 0.05). Significantly larger TPW was found in AS group in the lumbar spine (p < 0.05). TPW ≥ 7.5 mm was observed in 95.3%–98.1% of the pedicles at the levels of L3‐L5 in AS group. The CL and PL were significantly larger in AS group at the levels of T9‐L5 (p < 0.05). The CL ≥ 50 mm was found in 84.0%–96.2% of the pedicles in mid‐to‐lower lumbar spine in AS group. Significantly smaller SPA was found in AS group at the levels from L3 to L5 (p < 0.05). CONCLUSIONS: Pedicle screws with relatively large diameter of 7.5 mm and length of 50 mm could be used in mid‐to‐lower lumbar spine in the majority of AS‐related thoracolumbar kyphosis patients. Also, the insertion angle of pedicle screws in both the transverse and sagittal plane should be appropriately reduced in these patients. This study may help surgeons select the pedicle screws of appropriate size in AS patients.