T-Bar Angular Offset: A Novel Angle for Predicting Postoperative Coronal Balance Using Intraoperative Stitched Radiographs.

Background And Objectives: Accurate intraoperative assessment of coronal alignment is critical to achieving favorable clinical outcomes in adult spinal deformity surgery. However, surgical positioning creates challenges in predicting standing coronal alignment. Gravity-based plumblines require an upright posture and are not possible intraoperatively. Intraoperative stitched radiography (ISR) is increasingly being used to evaluate coronal balance as an alternative to 36-inch films, which can be cumbersome. However, ISR does not allow for the measurement of absolute distances, which limits the intraoperative assessment of the coronal plane. We propose a novel angle to assess coronal balance on ISR: the T-bar angular offset (TAO). We evaluate TAO as a measure of coronal balance and assess the degree to which TAO measured on ISR can predict postoperative coronal balance.

Methods: Patients undergoing spinal fusion surgery with ISR were identified retrospectively. A virtual T-bar was created by drawing a line bisecting the S1 endplate perpendicular to a line connecting the acetabula. A second line was drawn from the origin of the first to the midpoint of the superior endplate of the upper instrumented vertebra (UIV) or C7. The angles subtended by these lines were defined as the UIV TAO and C7 TAO, respectively.

Results: Sixty-eight patients were identified, with a median (range) of 8 (3-21) levels fused. Intraoperative UIV TAO correlated with both postoperative UIV TAO (R = 0.695, P < .001) and postoperative UIV central sacral vertical line (R = 0.435, P = .002). Intraoperative C7 TAO did not correlate with either postoperative C7 TAO (R = 0.213, P = .22) or C7 central sacral vertical line (R = 0.063, P = .72).

Conclusion: TAO is a useful angular measure that can be used on noncalibrated ISR to predict the positioning of the UIV relative to the sacrum during spinal fusion surgery. However, it does not appear to be predictive of the coronal alignment of C7.