The spine and the body function within a cone of equilibrium with focus in maintaining sagittal and coronal alignment with minimum energy expenditure. This happens with a harmonious relationship involving cervical lordosis (CL), thoracic kyphosis (TK), lumbar lordosis (LL), and pelvic anatomy. The purpose is mostly to maintain a mechanical balance in the sagittal and coronal planes centered from the center of the cranial mass, femoral heads, and lower extremities [1]. Many authors have reported that sagittal balance rather than coronal balance is significantly correlated with health-related quality of life, especially in patients who received surgical treatment [2-4]. Therefore, more attention is often paid to sagittal balance than coronal balance during spinal deformity assessment, surgical plan-making, and surgical procedure. Generally, sagittal imbalance results in increased muscular effort and energy expenditure, causing pain, fatigue, and disability [5]. Sagittal imbalance of the spine as a crucial factor in the pathogenesis of myelopathy is supported by several reports [6,7]. Multiple studies have described normative values for parameters of spinopelvic alignment in different populations of varying ages and pathologic conditions. In these studies, the interrelation of pelvic parameters, LL, TK, sagittal imbalance, and its effect on the quality of life and patient outcome has been well-established [5,8,9]. Many studies have reported that pelvic incidence (PI), a constant morphological parameter in each individual, has a significant influence on sagittal alignment of the lumbar spine or the thoracic spine, such as LL and TK [10]. The correlation between the cervical and thoracic spinal parameters is weaker than that between the lumbar and pelvic spinal parameters [11,12]. We started our study with the hypothesis that the overall sagittal balance of the thoracolumbar and lumbosacral spine is expected to influence the cervical spine, with the justification that the cervical spine is not an independent unit because it is connected to the thoracic spine. The aim of this study is to explore the correlation between the lumbosacral parameters and cervical sagittal balance, hoping that the results could provide guidance for changing these primary contributors in correction surgery.

In the past decades, much emphasis has been placed on identifying the sagittal profile of the thoracolumbar spine. In contrast to numerous discussions about the relationship between spinopelvic parameters and global sagittal balance of the spine, the correlation between spinopelvic parameters and cervical sagittal balance is unclear. The cervical spine is a relatively complex segment, and many factors influence its alignment and balance. The identification of compensatory mechanisms can be a precious point for the appropriate reconstruction of cervical spine alignment, especially in patients who have undergone spinal surgeries.

In terms of normal values, the measured values vary widely in different studies. It seems that factors, such as age, sex, race, and method of measuring parameters, cause this disparity. Nunez-Pereira et al. [14] studied the interdependences between occipitocervical and spinopelvic parameters. They reported mean values for O_CC₂ and C₂C₇ Cobb angles as −12.7° and −15.8° for asymptomatic subjects, −10.4° and 18.9° for patients with cervicalrelated symptoms, and −11.9° and −21.2° for post-surgical patients, respectively. In our study, O_CC₂ angle was −14.7° for the asymptomatic patients, −14.3° for the patients with lumbar-related symptoms, −14.4° for those with cervicalrelated symptoms, and −14.5° for post-surgical patients. The C₂C₇ Cobb angle was −15.4°, −16.8°, −17.1°, and −16.34° in our four study group, respectively. These results are similar to those of Nunez-Pereira et al. [14] relatively. Also, the total amount of CL (O_CC₇ Cobb angle) in our study was similar to that of previous studies (−29.8°±5.6°) [14,15]. A study of Guo et al. [15] showed slight differences with regard to age in the occipitocervical alignment of healthy volunteers. In our study, age correlation was observed only with C₂C₇ Cobb angle and C₂C₇ Harrison angle for patients with cervical-related symptoms. The age-related structural changes in the spine justify this difference.

Several studies have implied that the lumbar and pelvic parameters are correlated with the cervical region values. In our study, the mean value of LL was −52.2°±9.2°, SS was 40.7°±8.8°, PI was 55.1°±10.4°, and PT was 13.6°±5.1°. The analysis showed no correlation between the spinopelvic and cervical parameters in asymptomatic and lumbar patients. However, C₁C₂ Cobb angle correlated with the three spinopelvic parameters (LL: r=0.2, p=0.002; PI: r=−0.2, p=0.02; PT: r=−0.2, p=0.02), and the C₂C₇ Cobb angle correlated with PT (r=0.2, p=0.02) in cervical symptomatic patients. These results are consistent with those of Nunez-Pereira et al. [14] and Lee et al. [16]. Shao et al. [17] performed measurements in asymptomatic healthy volunteers and found that CL correlated with LL (r=0.1, p<0.01) and O_CC₂ Cobb angle correlated with SS (r=−0.1, p<0.05). In our study, regardless of the patients’ symptoms (n=420), C₁C₂ Cobb angle correlated with LL (r=0.1, p=0.01) and C₂C₇ Harrison angle correlated with PI (r=0.1, p=0.04) and PT (r=0.12, p=0.003). It seems that pain promotes compensatory flexion to increase spinal canal volume that leads to segmental alignment changes, and this can be a factor in the change of the correlation between spinal parameters in patients with cervical pathologies compared to the asymptomatic individual.

In our study, there was no significant correlation between CL (C₀C₇ Cobb angle) and spinopelvic parameters in asymptomatic, cervical symptomatic, and lumbar symptomatic patients. However, regardless of the patients’ symptoms (n=420), there was a significant relationship between C₀C₇ angle and PI and LL. This will clarify the key role of sample numbers in evaluating the correlation between the parameters.

Recently, Nunez-Pereira et al. [14] proposed C₇ slope as a predictor of the overall sagittal balance of the spine. They believe that if C₇ slope is altered on cervical radiographs, a full-length sagittal radiograph should be taken to rule out the overall sagittal imbalance. However, we found a negative correlation between C₇ slope and C₇ SVA, and, in contrast to Nunez-Pereira et al. [14], this correlation was not statistically significant. Thus, we believe that further studies with higher sample sizes are needed to examine the role of C₇ slope in the overall sagittal balance of the spine.

C₇ SVA represents the global spinal sagittal alignment; thus, it should have a specific impact on CL to maintain sagittal balance, independent of other parameters. Subjects who had a positive SVA displayed an increase in CL, regardless of whether their SVA was within the normal range of values. This is a compensatory mechanism to maintain a horizontal gaze in response to changes in sagittal global alignment [11]. Interestingly, C₇ SVA was the only parameter in our study that showed a significant correlation with the C₂C₇ Cobb angle in the four groups (r=−0.6 in asymptomatic; −0.8 in lumbar; −0.8 in cervical; and −0.6 in post-surgical subjects). However, there was no correlation between C₇ SVA and total CL (O_CC₇ Cobb angle). Matsubayashi et al. [18] found a similar significant correlation (r=0.5) in normal adults. They divided the C₂C₇ angle into the C2–C4 (middle cervical) and C5–C7 (lower cervical) angles and found that C₇ SVA correlated with the C5–C7 angle but not with the C2–C4 angle.

The cervical spine is considered as a unique segment and can be divided into two sub-segments with different roles and functions. For example, the highest level of cervical flexion and rotation is seen in the upper cervical segment (C₀C₁ and C₁C₂) whereas the lower segment (C₂C₇) is associated with lateral bending [19,20]. Therefore, the difference in correlation between these two cervical subsegments and the spinopelvic parameters is not surprising. According to our study results, it seems that spinopelvic parameters (such as, PI, PT, and LL) show greater correlation with the upper cervical segment (C₁C₂ Cobb angle) and that the total CL and the sub-axial parameters (C3–C7) are less affected.

The prospective design and the survey of symptomatic subjects parallel to the asymptomatic individuals are the benefits of our study. This heterogeneity in patient selection facilitates the understanding of the general functions of the cervical spine, but it should be kept in mind that standard sampling and large sample size are the prerequisites of assessment of correlation in any population. Thus, the number of subjects may have underpowered our results. There are structural differences between population groups and races [19-21]. Therefore, this should be considered when comparing the results of various studies in different regions and races. We have not studied the correlation between some of the occipitocervical parameters, such as occipital incidence and occipital slope. It is therefore recommended to study their effect on cervical alignment in subsequent studies. Further study with a larger number of cases in various spinal disorders in different races should also be performed to obtain actual spinal sagittal balance.

Our results indicate significant interdependence between the spinopelvic and cervical alignment, especially in cervical symptomatic patients. In addition, strong correlations were found between the C₇ SVA and C₂C₇ Cobb angle. Overall, the results of this study could help to better understand the cervical sagittal alignment and serve as preliminary data for planning surgical reconstruction procedures.