This study found that CL, SVA, T1S, and TS-CL were significantly improved after ACDF, while NT and TIA were relatively stable and not affected by ACDF, which was basically consistent with previous studies [4,5,6]; SCA, as a new parameter, increases in the postoperative measurement, while follow-up SCA is basically the same as preoperative, with no significant change. In addition, we also found that at the last follow-up, some parameters which improved significantly after surgery gradually changed to preoperative state, or even worse than preoperative. SVA and T1S improved significantly after surgery, but the orthopedic effect was lost at the last follow-up, which was close to preoperative level. It may be related to the changes in bearing weight and angle of adjacent segments, especially the lower cervical, after ACDF ; after fusion, the lower cervical and the intervertebral disc bear more weight, and due to orthopedic changes in the distribution of bearing angle and force, there are compensatory changes in the lower cervical, resulting in an increase in T1S compared with the preoperative level. In addition, we found that T1S, SVA, and SCA improved obviously after surgery in multi-segment group, while single-segment group showed no obvious improvement. Multi-segment ACDF can better improve cervical parameters and restore cervical sagittal balance. However, after multi-segment ACDF, cervical stability is poorer and the adjacent stage disease (ASD) incidence is higher than single segment ACDF.
As for the evaluation of postoperative ACDF efficacy, commonly used methods include pain-related VAS score, cervical JOA score, and cervical NDI score. Studies have compared the efficacy of each scoring method in evaluating the efficacy of cervical orthopedic surgery . Accordingly, cervical NDI score was used to evaluate postoperative spinal cord improvement and patients’ quality of life. Because of the long recovery period of CSM, we used the last follow-up NDI scores to compare with that before surgery. After ACDF, NDI score was significantly improved compared with those of preoperative, but some patient follow-up NDI scores increase compared to preoperative NDI. In our study, whether the NDI score increased after surgery was used as dependent variables, and cervical parameters was used as independent variables, performing a binary logistic regression analysis.
The thoracic 1 vertebra is the bridge between cervical and thoracic vertebra. T1S is a very important parameter of cervical segment, which is closely related to other parameters as a bond . T1S is not only closely correlated with CL and SVA, but also with NT. In addition, it is negatively correlated with SCA. Changes in T1S may reflect changes in the sagittal balance of cervicothoracic spine. Studies found that  there is a strong correlation between T1S and other parameters, and T1S changes earlier when cervical balance is destroyed. The majority of cervical imbalance is caused by disc herniation, neck muscle fatigue, surrounding ligament relaxation, and the dislocation of vertebral body alignment caused by long-term bending. These reasons may also lead to the increase of SVA and the decrease of CL. Studies  have found a significant negative correlation between C2–7 Cobb angle and cervical SVA. In asymptomatic Chinese population, CL was significantly correlated with other cervical sagittal parameters including TIA, T1S, NT, and C2–7 SVA .
SCA, a new parameter introduced in recent years, is mainly used to reflect the balance of the head and neck spine. SCA has a high correlation with T1S and CL, and a weak correlation with SVA, which is the link between cervical segment and cranium. It was found in previous studies  that TIA was approximately equal to the sum of T1S plus NT, which was also confirmed in this study. TIA and NT were not sensitive to surgical changes, and the postoperative changes of T1S were relatively large. However, this equation was valid both before and after surgery. When T1S changes significantly, TIA and NT make corresponding adjustments to adapt. This allows the balance of the upper spine to be maintained with minimal changes.
Studies [2, 14] found a correlation between NDI score and cervical parameters. Preoperative NDI score increased with the increase of C2–7 SVA and TS-CL, while high C2–7 SVA and low TIA were independent predictors of high preoperative NDI score. Preoperative T1S was positively correlated with preoperative NDI score, and postoperative follow-up T1S was positively correlated with follow-up NDI score. In addition, C2–7 SVA, and TS-CL were positively correlated with NDI score respectively. It can be considered that cervical sagittal parameters are closely related to NDI score. Cervical parameters can be used to evaluate the prognosis of patients after ACDF. Oversize T1S, SVA, and mismatched TS-CL will have adverse effects on the prognosis. This study also found a negative correlation between NDI score and SCA. SCA is an important parameter for maintaining sagittal balance of cervical spine and maintaining a normal value of 83° ± 9° can promote good prognosis .
It is believed that cervical curvature has an impact on surgical prognosis, and the improvement of cervical lordosis will affect the long-term efficacy of ACDF in the treatment of degenerative cervical disc disease . Kyphosis of the cervical can affect the outcome of surgery. Cervical kyphosis is associated with increased neck pain before and after cervical surgery . In this study, decreased cervical lordosis was a risk factor for deterioration of the last follow-up NDI score. Existing researches suggest that  patients with kyphosis are 18 times more likely to have cervical symptoms, a risk factor for neck pain. In addition, kyphosis may become a risk factor for spinal cord compression after cervical orthopedic surgery. Maintaining or reconstructing cervical kyphosis after ACDF surgery is helpful to obtain a good prognosis, and correcting kyphosis deformity as far as possible is still an accepted surgical option.
This study found that excessive T1S and NT were risk factors for follow-up NDI score greater than preoperative, which may lead to poor prognosis. Studies  have found that there is a linear relationship between T1S and postoperative NDI score. When T1S > 40° will lead to poor prognosis, T1S can be used as a parameter to evaluate prognosis. In addition, increased SVA will also affect the surgical effect . The increase of T1S is often accompanied by the deterioration of SVA. If they change together, a poor prognosis will be the result. In addition to the consideration of contact with spinal cord compression and decompression, the correction of cervical imbalance and the prevention of T1S gradual deterioration after surgery are also factors to be considered. T1S is highly correlated with other parameters and tends to deteriorate after surgery, which will lead to poor prognosis. Therefore, more attention should be paid to this parameter in the future.
Our study found that T1S is positively correlated with CL. Excessive T1S leads to the compensatory increase of CL, which can maintain the horizontal gaze and relieve symptoms. However, due to cervical imbalance, intervertebral disc degeneration, hyperosteogeny, and muscle weakness, some cases did not have compensatory changes. Their postoperative T1S increased, but CL did not increase or even decreased. The result was an increase in follow-up NDI score.
Advanced age is associated with increased NDI score, which may relate to a number of reasons. It was found that C0–7 cervical lordosis, C2–7 cervical lordosis, and T1S were associated with age change . In the elderly patients, cervical degeneration is severe and re-imbalance is likely to occur after surgery, leading to poor prognosis. The recovery rate of spinal cord in elderly patients is slower than that in middle-aged patients, and the clinical symptoms caused by long-term compression are not easy to recover, which affects the postoperative quality of life of patients. In this study, the correlation analysis of cervical parameters and NDI score was conducted in group according to age. We found that, compared with other two groups, the correlation between T1S and NDI score was stronger in the eldest group, and the correlation coefficient between other parameters and NDI score was smaller. In elderly patients, increased T1S is more likely to lead to increased NDI. However, they lack the compensatory ability for increased T1S.