Spine sagittal alignment has received focus over the past two decades[1]. The T1 vertebral body, fixed by the two sides of the rib, is the junction of a transitional region between the mobile, lordotic cervical spine and the rigid, kyphotic thoracic spine, all of which cause potential instabilities. T1S, as a parameter reflecting sagittal morphology of the T1 vertebral body, is associated with whole sagittal balance. The T1S larger than 25° or less than 13° indicated that the spine was unbalanced[2]. Studies found that the T1S had a linear relationship with CL[13–15]. These findings suggested that the T1S is an essential parameter in assessing cervical and global spinal sagittal balance[12, 16–18]. The value of the T1S is fundamental for surgical planning and outcome prediction[5, 8, 9, 19–23]. According to previous studies, age, spine global alignment, and thoracic alignment were associated with T1S[2, 11, 12, 24]. We initially analyzed 215 consecutive patients in the present study and found that older age, larger TLK, larger TK, larger CL, and larger SVA were associated with a high T1S. Pearson correlation analysis showed that the T1S had significant correlations with age, TLK, TK, CL, and SVA. Dichotomous variable logistic regression showed that TK, SVA, and CL were significantly related to the high T1S. These findings suggested that the T1S was influenced by lower adjacent segments and global spine alignment. CL compensated for the T1S to maintain horizontal gaze. These results were consistent with a previous study demonstrating that caudal spine segments had a sequential effect on cranial spine segments[25].
Some studies found that the T1S increased with age[10, 26]. However, there are also spine sagittal parameters other than age that have relationships with the T1S. Several studies reported the relationships between T1S and other global spine parameters. Lee et al.[1, 3] found a significant relationship between T1S and TK. A high T1S was related to a large CL to maintain the sagittal balance of the cervical spine[27]. We believe that age-related degenerative changes of the spine explained T1S change. Aging spines lose the characteristic shape of the vertebral body disks resulting in decreased lumbar lordosis or thoracolumbar kyphosis. Compensatory mechanisms for degenerative changes might influence the T1S.
We used a four-group classification to analyze the influence of ASD and thoracic compensation on the T1S. Compared with the Balance group, compensatory balance group and high SVA groups had lower LL and greater PT, suggesting that posterior pelvic rotation occurred in three ASD groups to compensate for lumbar deformity. Because of reasonable compensation in the compensatory balance group, the T1S was slightly reduced and similar to the balance group. In the high SVA group, the thoracic compensation group had decreased TK, restricting T1S enlargement. There was no significant difference in T1S between the balance and thoracic compensation groups. According to propensity score matching, compared with the balance group, the thoracic compensation group had posterior pelvic rotation and decreased TK. The increased T1S in the thoracic compensation group was not significantly different from the balance group. However, the thoracic decompensation group had no extra compensation ability to reduce the T1S. According to propensity score matching, compared with the balance group, the thoracic decompensation group showed limited pelvic posterior rotation, limited TK decrease, but increased T1S. These findings suggested a significant difference in T1S between the thoracic decompensation and balance groups. In our opinion, posterior pelvic rotation and thoracic compensation are two crucial factors protecting against T1S increases (Fig. 4).
Extension of the thoracic spine and pelvic retroversion are essential mechanisms to prevent the center of gravity from moving forward. Garbossa et al. [28]reported three types of spinal alignment, and active compensation mechanisms in the hidden imbalance state could maintain the balance of the global spine. However, an imbalanced spine state had anterior gravity lines despite active compensation mechanisms. In the present study, the compensatory balance group maintained the global spine and T1S balance well because of reasonable compensation. Extension of the thoracic spine in the thoracic compensation group prevented the T1S from over enlargement despite the balance of the global spine being destroyed. The thoracic decompensation group could not prevent the increase in T1S because of lack of compensation. In other words, the T1S reflected the compensation ability of the spine. Patients with high T1S might lack compensation ability for spine degeneration. Studies reported that high T1S induced postoperative kyphotic change after cervical laminoplasty[7–9, 19–21, 29]. These studies may help to explain that the poor outcome of high T1S patients after surgery may be related to lack of compensation.
This study has several limitations. First, a large sample cohort with long-term follow-up is more suitable to address this question. Second, lower limb parameters were not evaluated because of the range of whole-spine radiography. Finally, more deep studies are needed to validate our results further. Nevertheless, we believe that our results help understand the value of the T1 slope.