Is the chin-brow vertical angle always a reliable parameter for surgical planning in the correction of thoracolumbar kyphosis secondary to ankylosing spondylitis?

Abstract

The aim of this retrospective study was to investigate the correlation between chin-brow vertical angle (CBVA) and osteotomized vertebra angle (OVA) across diverse cervical ranges of motion (CROMs) and to examine the reliability of CBVA in surgical correction design. The authors analyzed all ankylosing spondylitis (AS) patients with thoracolumbar kyphosis who had undergone single-level lumbar or thoracic pedicle subtraction osteotomy between January 2015 and December 2019 and had at least 2 years of follow-up. The patients were categorized into 3 groups based on their CROM: group A, CROM ≤ 10°; group B, 10° < CROM ≤ 20°; and group C, CROM > 20°. The correlation between ΔCBVA (change in CBVA from preoperatively to immediately postoperatively) and ΔOVA (change in the Cobb angle of the osteotomized vertebra from preoperatively to immediately postoperatively) was evaluated across the 3 CROM groups, identifying the subgroups as the cervical ankylosis (CA) group and cervical nonankylosis (CNA) group. The Cobb angle from C2 to C7, CBVA, Cobb angle from C0 to C7, global kyphosis, sagittal vertical axis C7-S1, and pelvic tilt were measured to enable comprehensive intergroup and intragroup comparisons of preoperative and postoperative parameters. Among the 64 patients included in this study, a significant correlation between ΔCBVA and OVA was observed in patients from group A (p < 0.001) and group B (p < 0.001); however, no correlation was evident in group C (p = 0.31). Consequently, patients from groups A and B were amalgamated into the CA group, whereas those from group C were classified into the CNA group. Both subgroups attained satisfactory orthopedic outcomes following the surgical intervention. Notably, postoperative evaluations revealed significant kyphosis changes in the cervical spine in the CNA group (change 20.25°) compared to those in the CA group (change 4.97°). For AS patients with thoracolumbar kyphosis, CBVA is not consistently reliable for determining the OVA necessary for deformity correction, with its reliability closely linked to CROM. When the CROM is extensive (CROM > 20°) in patients with AS, the CBVA is not recommended as a reliable parameter for guiding the design of the OVA.