Small sagittal vertical axis accompanied with lumbar hyperlordosis as a risk factor for developing postoperative cervical kyphosis after expansive open-door laminoplasty.

OBJECTIVE Preoperative positive cervical sagittal imbalance and global sagittal imbalance are risk factors for postoperative cervical kyphosis after expansive open-door cervical laminoplasty (ELAP). The purpose of this study was to investigate the relationship between the incidence of postoperative cervical kyphosis after ELAP and the preoperative global sagittal spinal alignment in patients with cervical spondylotic myelopathy (CSM) without spinal sagittal imbalance. METHODS Among 84 consecutive patients who underwent ELAP for CSM at the authors' hospital, 43 patients without preoperative cervical kyphosis (C2-7 angle ≥ 0°) and spinal sagittal imbalance (C2-7 sagittal vertical axis [SVA] ≤ 80 mm and C-7 SVA ≤ 95 mm) were included in the study. The global spinal sagittal parameters were measured on lateral whole-spine standing radiographs preoperatively and at 1 year postoperatively. The difference in preoperative global sagittal spinal alignment between the postoperative cervical lordosis group and the cervical kyphosis group was analyzed. RESULTS The incidence of postoperative cervical kyphosis after ELAP was 25.6% (11 of 43 cases). Thirty-two patients (16 men and 16 women; mean age 67.7 ± 12.0 years) had lordosis, and 11 (7 men and 4 women; mean age 67.2 ± 9.6 years) had kyphosis. The preoperative C-7 SVA and pelvic incidence minus lumbar lordosis (PI-LL) in the kyphosis group were significantly smaller than those in the lordosis group (p < 0.05). The smaller C-7 SVA accompanied by a small PI-LL, the "truncal negative offset," led to postoperative cervical kyphosis due to posterior structural weakening by ELAP. CONCLUSIONS In patients with CSM without preoperative cervical and global spinal sagittal imbalance, a small SVA accompanied by lumbar hyperlordosis is the characteristic alignment leading to postoperative cervical kyphosis after ELAP.