Intraoperative versus postoperative radiographic coronal balance for adult spinal deformity surgery

Background: Coronal malalignment in adult spinal deformity (ASD) has a close relationship with patient clinical outcomes. The purpose of this study is to evaluate the relationship between intra- and postoperative coronal radiographic parameters. A novel parameter, the central sacral pelvic line (CSPL), and its relation to the central sacral vertical line (CSVL) is explored. CSPL is a measure of spinal alignment referenced to the patient’s pelvis as an intraoperative proxy for CSVL. CSVL is difficult to measure intraoperatively, because a C7-plumb line (referenced to gravity) cannot be drawn in the supine position. Methods: 47 subjects ≥ 18 years old undergoing a spinal fusion of ≥ 6 levels from 2015 to 2017 were enrolled. The CSPL is defined as the perpendicular line bisecting the midpoint of the line that connects the superior aspects of the acetabuli. Two metrics describing coronal alignment were derived from each radiograph: (1) horizontal distance between the C7-plumb line and the CSPL at C7 (C7-CSPL) and (2) horizontal distance between the C7-plumb line and CSVL (C7-CSVL). Pearson’s correlation and linear regression analysis was used to study the relationship between the intraoperative C7-CSPL and the postoperative C7-CSVL. Results: On average, the intraoperative C7-CSPL distance was 32.1 mm, postoperative C7-CSPL 20.8 mm, and postoperative C7-CSVL 18.9 mm. 15/47 (32%) had intraoperative C7-CSPL measurements > 4 cm, requiring intraoperative correction. Of those 15, 10 patients (67%) still had a postoperative C7-CSVL < 4 cm. Linear regression modeling indicates that when intraoperative CSPL is < 7.7 cm on average, the postoperative C7-CSVL will < 4 cm—our threshold for adequate coronal alignment. Patients with intraoperative C7-CSPL > 5 cm had a 50% chance of having a postoperative C7-CSVL > 4 cm; patients with intraoperative C7-CSPL < 5 cm had a 3% chance of having coronal malalignment. There is a strong positive relationship between postoperative C7-CSPL and C7-CSVL (r = 0.80 and 0.85, respectively). Conclusion: In adult spinal surgery, the intraoperative coronal alignment measured using the novel C7-CSPL distance correlates well with postoperative C7-CSVL distance. This gives the surgeon an objective measurement of the correction they need after assessing initial intraoperative imaging. Our findings suggest an intraoperative C7-CSPL distance < 5 cm as a threshold value to predict postoperative C7-CSVL < 4 cm in 97% of patients tested.