Because the cervical spine is more complex than the thoracolumbar spine and has a greater range of motion, it supports the weight of the head and is responsible for many important physiological functions. Therefore, the cervical spine is more susceptible to degenerative changes[17]. Sagittal balance of the cervical spine and normal cervical curvature and alignment play a critical role in maintaining the biomechanical properties and normal movement of the cervical spine[17, 18]. Numerous studies have determined the sagittal parameters of the cervical spine in people with neck pain versus healthy subjects and have found that these parameters vary greatly[3, 7, 19, 20]. However, we are the first to propose a comparison of cervical sagittal parameters between patients with non-specific neck pain and patients with cervical spondylotic radiculopathy and cervical spondylotic myelopathy.
Our results showed that the C2-C7 sagittal axial distance (SVA) and spinal cranial angle (SCA) of patients in the NS-NP group were significantly greater than those in the CSR and CSM groups, while the C2-C7 anterior convex angle, C7s and T1s of patients in the NS-NP group were significantly smaller than those in the CSR and CSM groups. This is similar to the results of previous studies[7, 21, 22]. 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 the head's center of gravity, resulting in a 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, 20, 23]. 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 the cervical spine in the population of patients with degenerative cervical spondylosis is affected by large variations[22]. As reported by Jouibari et al.[7], there were 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 cervical sagittal parameters relative to patients with cervical spondylosis and tend to present similar results as the normal population.
We performed a correlation analysis of cervical sagittal parameters, and the results of our study showed a significant negative correlation between the C2-C7 Cobb angle, C2-C7 SVA and T1 slope and SCA angle, which is consistent with previous findings[2, 13, 15]. 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[12, 13]. Wang et al.[12] reported that patients with a higher SCA had a lower T1 slope (T1s) and C2-C7 Cobb angle, both preoperatively, postoperatively and at follow-up. The results of this study also showed positive correlations between the C2-C7 Cobb angle and C7 slope, C2-C7 Cobb angle and T1 slope, C2-C7 SVA and C7 slope, C2-C7 SVA and T1 slope, C2-C7 SVA and SCA, and C7 slope and T1 slope, while the C2-C7 Cobb angle was negatively correlated with C2-C7 SVA. These correlations indicate that the sagittal curvature of the cervical spine 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[24, 25]. When the cervical spine is in prolonged flexion and the muscle balance along the cervical spine is altered, the muscles and ligaments of the neck are subjected to abnormal mechanical loading, resulting in ligament and joint capsule laxity and loss of cervical physiological curvature, i.e., decreased C2-C7 angle[7, 26]. 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[18]. In addition, we found that the C2-C7 Cobb angle was positively correlated with the T1 slope and C7 slope, suggesting that when the C2-C7 Cobb angle decreases, the T1 slope and C7 slope exhibit a compensatory decrease to restore the imbalance caused by the CGH forward shift[24, 27-29]. 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[3, 4, 30]. In general, the implementation of compensatory mechanisms relies on excessive muscle contraction and excessive tension in the spine and small disc joints, which can further accelerate the progression of spinal degeneration and cause a series of related clinical symptoms, such as low back pain, neck pain, and shoulder pain[31]. Therefore, spine surgeons should consider the patient's cervical sagittal balance during cervical spine surgery and try to restore normal cervical physiological curvature, and studying cervical sagittal balance may help spine surgeons develop better treatment strategies[32-34].
We also performed a comparative analysis of sex differences in cervical sagittal 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 and computer workers, manual laborers, medical workers and professional drivers, who are more likely to experience 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[35]. 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[27].
This study reports for the first time the comparison of cervical sagittal parameters between patients with non-specific neck pain and those with cervical spondylotic radiculopathy and cervical spondylotic myelopathy. It further clarifies that these parameters are closely related to cervical spine disease and emphasizes the importance of cervical sagittal balance. Lateral radiographs of the cervical spine, as a simple, convenient, and noninvasive examination, are necessary to evaluate the sagittal balance of the cervical spine. Of course, there are also some shortcomings. First, the lack of clinical data in this study and the failure to compare preoperative and postoperative cervical sagittal parameters and some clinical outcomes in cervical spondylotic radiculopathy and cervical spondylotic myelopathy will be a problem that we need to address in our subsequent studies. Second, this study only evaluated the parameters of the local sagittal position of the cervical spine, which may have certain limitations on the conclusion. The use of sagittal radiographs of the whole spine can be enhanced. Finally, this study is a single-institution center study. More research in this area is needed to supplement and confirm the above results.