l1-pelvic-angle: A Convenient Intraoperative Measurement to Attain Optimal Deformity Correction

Introduction: L1-pelvic angle (L1PA) is a measurement that can easily be obtained intraoperatively to assess sagittal alignment in patients undergoing adult spinal deformity (ASD) surgery. How L1PA correlates to the commonly utilized measures of global sagittal alignment, sagittal vertical axis (SVA) and T1-pelvic angle (T1PA) and patient reported outcomes (PROs), is understudied. 1) Evaluate the associations between L1PA and both SVA and T1PA, and 2) assess the clinical impact of L1PA.

Methods: A single-institution retrospective cohort study was undertaken for patients undergoing ASDsurgery from 2013-2017. Ideal L1PA was defined as (0.5xPelvic Incidence)-21. Pearson correlation was performed to compare L1PA, SVA, and T1PA. Univariate/multivariate regression was performed to assess the effect of L1PA on mechanical complications, controlling for age, BMI, and postoperative pelvic incidence-lumbar lordosis mismatch (PI/LL). Due to the overlapping nature of patients with pseudarthrosis and rod fracture, these patients were analyzed as a single group.

Results: A total of 145 patients were included with at least 2-year follow-up. Mean preoperative L1PA, SVA, and T1PA were 15.5±8.9°, 90.7±66.8mm, and 27.1±13.0°, respectively. Mean postoperative L1PA, SVA, and T1PA were 15.0±8.9°, 66.7±52.8mm, and 22.3±11.1°, respectively. 36 (24.8%) patients achieved ideal L1PA. Preoperative L1PA was linearly correlated with preoperative SVA (r2=0.16,p < 0.001) and T1PA (r2=0.41,p < 0.001), respectively. Similarly, postoperative L1PA was linearly correlated with postoperative SVA (r2=0.12,p < 0.001) and T1PA (r2=0.40,p < 0.001), respectively. Achieving ideal L1PA±5° was associated with a decreased risk of rod fracture/pseudarthrosis on univariate and multivariate regression (OR=0.33, 95%CI=0.12-0.86,p=0.024). We did not observe a direct association between achieving ideal L1PA and PROs in our sample.

Conclusion : L1PA was linearly correlated with SVA and T1PA, and achieving ideal L1PA was associated with lower rates of rod fracture/pseudarthrosis. L1PA may be utilized as a reliable measure for sagittal alignment and future studies are warranted to better define the clinical implications of achieving a normal L1PA.