Objectives:

Considering sagittal balance is particularly important in ASD patients because they frequently show hypolordotic prior fusion. Therefore, the purpose of this study was to identify risk factors for aggravation of sagittal imbalance after posterior lumbar fusion in ASD patients.

Methods:

Fifty-nine patients who underwent revision posterior surgery for ASD between 2014 and 2018 were included. Patients were divided into two groups according to postoperative sagittal balance status determined by the pelvic incidence minus lumbar lordosis (PI-LL) value, based on the age-adjusted Schwab classification (group A: ideal correction, N = 20; group B: undercorrection, N = 39). Several radiographic parameters were measured in plain radiographs. Clinical results were analysed using the visual analog scale, Oswestry Disability Index, and EuroQol 5-domain.

Results:

Better preoperative PI-LL (p = 0.001), slippage of the vertebral body (p = 0.022), higher disc height (p = 0.048), and absence of L4-5-S1 fusion (p = 0.041) in the index surgery were significantly correlated with better postoperative sagittal balance in multivariate analysis. The PI-LL improved postoperatively from 19.4 to 12.5 in group A (p = 0.019) and remained unchanged (from 38.6 to 38.6, p = 1.000) in group B. Although clinical outcomes improved postoperatively in both groups, no inter-group differences were observed.

Conclusions:

Preoperative sagittal imbalance, rigid affected segments, and previously fused lower lumbar segment (L4-5-S1) are independent risk factors for aggravation of sagittal imbalance in ASD patients. Surgeons should strive to restore sagittal balance after ASD surgery under the above-mentioned conditions.