T1 Slope Minus Cervical Lordosis (TS-CL), the Cervical Answer to PI-LL, Defines Cervical Sagittal Deformity in Patients Undergoing Thoracolumbar Osteotomy

Global radiographic parameters measured including the cervico-thoracic pelvic angle (CTPA), the C2 pelvic angle (CPA), and the T1 pelvic angle (TPA). CTPA is a global angular measure of cervical sagittal alignment and a correlate of the C2-C7 plumb line. T1 pelvic angle is a measure of global sagittal alignment and a correlate of the C7 sagittal vertical axis. CPA is the angle of a line from C2 centroid to the femoral heads (FH) and a line from the FH to the middle of the S1 endplate. T1 pelvic angle is the angle of a line from the center of T1 to the FH and a line from the FH to the center of the S1 endplate. CTPA is the angle of a line from C2 centroid to the FH and a line from the FH to the center of T1. CTPA is the result of subtracting T1 pelvic angle from CPA.

(a) Preoperative x-rays of a patient with thoracolumbar deformity from the uncompensated cervical sagittal deformity (CSD) group. Though there is lordotic alignment of the cervical spine, there is a mismatch in T1 slope and cervical lordosis (TS-CL > 17°). Cervical plumb line, TS-CL, and CTPA are elevated. (b) Postoperative x-rays of the same patient from the uncompensated group. There has been good global correction, but there has been progression of the cervical deformity with kyphotic alignment of the cervical spine, an increase in the cervical plumb line, a progression in the mismatch of T1 slope and cervical lordosis, and an increase in the cervico-thoracic pelvic angle. Abbreviations: CL, cervical lordosis; TS, T1 slope; TPA, T1 pelvic angle; CTPA, cervico-thoracic pelvic angle; C2-C7 PL, cervical plumb line; SVA, sagittal vertical axis.

(a) Preoperative x-rays of a thoracolumbar deformity patient from the compensated group (no cervical deformity). The magnitude of the cervical lordosis matches that of the T1 slope (TS-CL < 17°), demonstrating that there is good cervical compensation for the large global deformity allowing for horizontal gaze. (b) Postoperative x-rays of the same patient from the compensated group. Good global correction has been achieved and reciprocal change has occurred, with a decrease in the cervical sagittal alignment. The magnitude of the cervical lordosis remains in balance with the T1 slope. Abbreviations: CL, cervical lordosis; TS, T1 slope; TPA, T1 pelvic angle; CTPA, cervico-thoracic pelvic angle; C2-C7 PL, cervical plumb line; SVA, sagittal vertical axis.