Because the cervical spine is more complex than the thoracolumbar spine and has a greater range of motion, it is responsible for many key physiological functions. It can be judged that the possibility of degenerative changes in the cervical spine is high [7]. The sagittal balance and physiological curvature of the cervical spine are important for the maintenance of its function, and are closely related to the biomechanical properties of the cervical spine[3, 7]. The treatment of cervical spine disease is a major challenge. To handle complex cervical spine diseases, it is necessary to comprehend cervical spine biomechanics and normative data on cervical alignment[5]. At present, increasing attention has been given to research on cervical sagittal plane parameters by spinal surgeons[13, 15–18]. Normal values of cervical sagittal parameters have been identified by recent investigations, and these normal values vary greatly[5, 13, 19, 20]. However, few studies have compared the variation characteristics and correlation of cervical sagittal plane parameters between patients with nonspecific neck pain and CSR, CSM.
The results ot this paper reflected that the C2-C7 sagittal axial distance (SVA) and spinal cranial angle (SCA) of NS-NP group were obviously greater compared to those of CSR and CSM groups, while the C2-C7 anterior convex angle, C7s and T1s of NS-NP group were obviously smaller than those of CSR and CSM groups. This result is similar to that of previous researches[13, 17, 18]. In people with degenerative cervical spondylosis, the physiological curve of the cervical spine becomes progressively straighter or more lordotic, which leads to a forward shift of head's center of gravity, caused progressive increase in C2-C7 SVA and SCA and an increase in C7s and T1s to compensate for the sagittal balance of the spine. In contrast to CSR and CSM patients, NS-NP patients may have mainly localized muscle fatigue or muscle stiffness, and patients often present with complaints of a localized neck pain[19–21]. However, in slowly progressive diseases, such as myelopathy, the nature of patients' complaints is less likely to be localized, and their perception of disability suggests that the sagittal position of cervical spine in the population of patients with degenerative cervical spondylosis is affected by large variations[17]. As reported by Jouibari et al.[13], there exist no differences in the changes in cervical sagittal parameters in patients with neck pain compared with the asymptomatic population, except for the decrease in T1s. This also better explains our results showing that NS-NP patients are less affected by changes in those parameters relative to cervical spondylosis cases and tend to present similar results as the normal population.
We performed a correlation analysis of those parameters, and showed a significant negative correlation between the C2-C7 Cobb angle, C2-C7 SVA and T1 slope and SCA angle, which is in accordance with previous researches[4, 10, 12]. In recent studies, a new cervical sagittal parameter, SCA, has gradually been proposed to assess the relationship of SCA with other cervical sagittal parameters and postoperative recovery indices. It has been proposed that SCA can be considered another key parameter to predict imbalance and that higher SCA is positively correlated with NDI in cervical spine patients[9, 10]. Wang et al. [9] found that patients with higher SCA had a lower T1 slope (T1s) ,both preoperatively, postoperatively stage. The results of this study also reflected that ,there exist a positive association between the C2-C7 Cobb angle and C7 slope, C2-C7 Cobb angle and T1 slope, C2-C7 SVA and SCA, and C7 slope and T1 slope, while the C2-C7 Cobb angle was negatively associated to C2-C7 SVA. These correlations implied that the sagittal curvature is closely related to the sagittal displacement of the cervical spine. In degenerative cervical spine diseases, cervical curvature changes are one of the most common radiographs[22, 23]. When the cervical spine is in prolonged flexion and the muscle balance along the cervical spine is altered, the neck muscles are subjected to abnormal mechanical loading, caused ligament and joint capsule laxity and lower cervical curvature, i.e., decreased C2-C7 angle[13, 24]. When the C2-C7 Cobb angle decreases, the center of gravity of the head (CGH) and C2 vertebrae moves forward, which will lead to the same increase of C2-C7 SVA to maintain balance and offset the adverse effects caused by CGH moving forward [3]. The research results in this paper also show that C2-C7 Cobb angle has a positive correlation with T1 and C7 slopes, which indicates that when this angle is reduced, T1 and C7 slopes will also be reduced to compensate for this and reduce the related imbalance effects [22, 25–27]. When the physiological curvature of the cervical spine is reduced, many parameters of the cervical spine will change, and there is correlation between these changes. Relevant experimental research results show that the changes of cervical sagittal position are closely related to the complex compensation mechanism, which is also related to the spinal alignment, such as thoracic kyphosis and sacral inclination [5, 27, 28]. This compensation mechanism is mainly realized through the excessive contraction of the spine and small intervertebral disc joints, which will lead to an increase in the speed of spinal degeneration and many corresponding clinical symptoms, such as low back pain and shoulder pain, which will have a very negative impact on the health of patients[25]. Hence, spine surgeons should consider the patient's cervical sagittal balance during surgery process and repair cervical curvature, and studying cervical sagittal balance may help spine surgeons develop better treatment strategies[26–28].
We also performed a comparative analysis of sex differences in different parameters. The age of patients in the NS-NP group was younger, and these findings are similar to those reported by Cohen et al.[1], which are more common in middle-aged and young people, such as office workers, medical workers and professional drivers, who has high risk to encounter neck and shoulder pain than others. Among patients with nonspecific neck pain, we found that female patients were often more common than male patients, which was similar to some reports[19, 20]. This may be because the anteroposterior (AP) diameter of the thorax is significantly smaller in women than in men with chronic neck pain. The size of the thoracic AP may be a predictor of neck pain, and the AP diameter of the uppermost thorax, which is the basis of head and neck motion fixation, is an important factor. The smaller the bottom, the more likely and frequent the head is to go beyond it, especially when the head is moving forward[29]. This explains the greater SCA in women than in men in the NS-NP group as well as the fact that the cervical spine compensates for the forward shift of the head's center of gravity by decreasing C7s and T1s[30].
This study reports for the first time the characteristics and correlation analysis of changes in cervical sagittal parameters between nonspecific neck pain cases and those with neurogenic and spinal cervical spondylosis. It further clarifies that these parameters has closel association with the cervical spine disease and emphasizes the meaning of cervical sagittal balance. Lateral radiographs of the cervical spine, as a simple, convenient, and noninvasive examination, are necessary to assess these kind of sagittal balance. Of course, there are also some shortcomings. First, this study only evaluated the local sagittal position parameters, which may have certain limitations on the conclusion. The use of sagittal radiographs of the whole spine can be enhanced. Second, this study is a single-institution center study. More research in this area is needed to supplement and confirm the above results.