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

Abstract

Background: Cervical kyphosis and C2-C7 plumb line (CPL) are established descriptors of cervical sagittal deformity (CSD). Reciprocal changes in these parameters have been demonstrated in thoracolumbar deformity correction. The purpose of this study was to investigate the development of CSD, using T1 slope minus cervical lordosis (TS-CL) to define CSD and to correlate TS-CL and a novel global sagittal parameter, cervical-thoracic pelvic angle (CTPA), with CPL.

Methods: A multicenter, retrospective analysis of patients with thoracolumbar deformity undergoing three-column osteotomy was performed. Preoperative and postoperative cervical parameters were investigated. Linear regression for postoperative values resulted in a CPL of 4 cm corresponding to a TS-CL threshold of 17°. Patients were classified based on postoperative TS-CL into uncompensated (TS-CL > 17°) or compensated cohorts (TS-CL < 17°); the two were compared using an unpaired t test. Logistic regression modeling was used to determine predictors of postoperative CSD.

Results: A total of 223 patients with thoracolumbar deformity (mean age, 57.56 years) were identified. CTPA correlated with CPL (preoperative r = .85, postoperative r = .69). TS-CL correlated with CTPA (preoperative r = .52, postoperative r = .37) and CPL (preoperative r = .52; postoperative r = .37). CSD had greater preoperative CPL (P < .001) and CTPA (P < .001). The compensated cohort had a decrease in TS-CL (from 10.2 to 8.0) with sagittal vertical axis (SVA) correction, whereas the uncompensated had an increase in TS-CL (from 22.3 to 26.8) with all P < .001. Reciprocal change was demonstrated in the compensated group given that CL decreased with SVA correction (r = .39), but there was no such correlation in the uncompensated. Positive predictors of postoperative CSD included baseline TS-CL > 17° (P = .007), longer fusion (P = .033), and baseline CTPA (P = .029).

Conclusions: TS-CL and CTPA correlated significantly with established sagittal balance measures. Whereas reciprocal change in cervical and thoracolumbar alignment was demonstrated in the compensated cohort, the uncompensated population had progression of their cervical deformities after three-column osteotomy.

Clinical relevance: The balance between TS-CL mirrors the relationship between pelvic incidence minus lumbar lordosis in defining deformities of their respective spinal regions.

Keywords: CPL; TS-CL; alignment; cervical kyphosis; thoracolumbar deformity.

Figure 1

Cervical radiographic parameters measured including the cervical lordosis (CL), the C2 slope, the cervical plumb line (CPL), and the T1 slope.

Figure 2

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.

Figure 3

(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.

Figure 4

(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.

Figure 5

Patient with thoracolumbar deformity after pedicle subtraction osteotomy with a long fusion to the pelvis; patient developed cervical sagittal deformity. T1 slope minus cervical lordosis (TS-CL) is 72°.