The ideal cervical balance provides the minimal muscular energy expenditure and reduces the ANP. Regardless of whether lordosis or kyphosis, cervical sagittal alignment is important for slowing down the ANP and maintaining global sagittal balance. The sagittal imbalance is considered associated with poor quality of life. This study highlighted the relationship between ANP and cervical sagittal alignment, and demonstrated C2-7 SVA and T1 slope were significant independent factors associated with ANP in patients with CK. The results provided suggestions for the treatment strategies of CK.
It is generally acknowledged that CK could easier lead to cervical disc degeneration compared to normal sagittal alignment of the cervical spine. However, little attention has been paid to ANP [11–13]. Previous researchers proved thoracic inlet parameters correlates highly with the other cervical parameters and had a critical effect on cervical sagittal alignment [14–16]. Therefore, it is necessary to discuss the impact of thoracic inlet parameters on ANP in CK. Sun et al found patients with sagittal imbalance of TIA have higher risk of degenerative cervical spondylotic myelopathy. When T1 slope was less than 18.5°, it showed significant diagnostic value for the incidence of degenerative cervical disease. Yet, most of the cervical curvature in their study were lordosis, CK was not included[17]. Jouibari et al had compared cervical sagittal parameters between patients with neck pain and healthy controls. In their study, T1 slope was signifcantly lower in neck pain group compared to healthy control group, but no difference in cervical lordosis between the two groups[18]. Lin et al analyzed 90 patients who underwent cervical surgery and found the larger C2-7 SVA, lower TIA, and higher NT values were independent predictors of high preoperative neck disability[19]. Therefore, it indicated that if patients with CK could keep the cervical sagittal balance, they would not suffered the neck pain. In other words, CK was a normal physiological state. This study was imilar to research of Le Huec et al. In their study, it had analyzed radiographic parameters of 106 asymptomatic participants to evaluate the sagittal balance and found almost one third of participants had CK[27].
Similar to PI, TIA is considered the constant morphologic parameter. NT was a constant parameter, A higher T1 slope means the TIA become larger. In the present study, the mean TIA of the ANP group and Non-ANP group were 73.5°±5.6° and 71.2°±7.0°, respectively. Multivariate logistic regression pointed out that TIA was not the risk factor in the development of ANP. Yet, this study showed that T1 slope were associated with ANP in patients with CK, which would help elucidate the occurrence of ANP. Therefore, in the patients with ANP, the center of gravity of the head to moved forward and aggravated the kyphosis. Furthermore, C2-7SVA was also increased.
C2-7SVA is thought to be another important indicators of cervical sagittal balance [20, 21]. It is well known that the threshold for cervical imbalance is C2-7SVA ≥ 40 mm.[22, 23]. In the present study, C2-7SVA in both groups ༜40 mm meaned the cervical spine was in basic equilibrium. It was insufficient to assess the cervical alignment parameters with C2-7SVA and cervical curvature alone. Previous studies has also proved the T1 slope is a parameter that represents the angle, whereas C2-7SVA represents global sagittal alignment. Hyun et al and Tang et al considered C2-7 SVA was the best indicator of cervical malalignment, which significantly impacted by the T1 slope and cervical curvature[24, 25].
The T1 slope has been the only value linking both the cervical and thoracic spine. It has closely correlation with thoracic kyphosis, TIA and C2-7 SVA[26]. A higher thoracic kyphosis often results in a higher T1 slope[21].. It has to increase the cervical lordosis to compensated. However, in patients with CK, it is completely different. Staub et al reviewed 103 adult spinal deformity patients to find the relationship between T1 slope and cervical lordosis. Then, it implied that the T1 slope minus cervical lordosis ranged form 14.5°to 26.5°could maintain the horizontal balance.[26]. In our study, the multivariate logistic regression analysis showed T1 slope was the risk factor of ANP. On account of this reason, A higher T1 slope might lead to the ANP. Our findings are similar to their results. However, cervical alignment has a C2-7 SVA in the normal range if cervical lordosis is high or the T1 slope is low. The worst mismatch is a higher T1 slope and lower cervical lordosis. Regarding to the CK, it is different compared to the cervical lordosis. When a smaller C2-7 SVA accompanied a lower T1 slope, it is easily to keep cervical sagittal balance and achieve the posterior neck muscles compensatory mechanism, which is getting the center of head position back to the spinal axis. Otherwise, a larger C2-7 SVA with a higher T1 slope lead to cervical malalignment, which could not achieve the posterior neck muscles compensatory mechanism and eventually causes the ANP. The translational mobility of upper and middle cervical levels in regional CK type are higher than global CK type, a larger C2-7 SVA and higher T1 slope may accelerate disc degeneration at transition zone. Furthermore, the transition zone and apex of the level have the higher risk to result in ANP, especially accompanied with a larger C2-7 SVA and/or higher T1 slope. For the global CK type, the angular motion was higher at the apex of kyphosis, a larger C2-7 SVA or a higher T1 may be the reason of the ANP. From the evidence we have found so far, a smaller C2-7 SVA accompanied a lower T1 slope were tolerable with patients with CK. On the other hand, a larger C2-7 SVA with a higher T1 slope is intolerable which may lead to the ANP. Therefore, this statistical results show that larger C2-7 SVA and higher T1 slope were significant independent factors associated with ANP in patients with CK.
In addition, the posterior neck muscles also play an important role in maintaining cervical curvature. For the purpose of minimizing the energy expenditure, patients with CK require strengthening the posterior paraspinal neck muscles, especially for patients with a higher T1 slope.
In the ANP group, the mean T1 slope and C2-7 SVA in the ANP group was significantly greater than in the Non-ANP group. Theoretically, a higher T1 slope and larger C2-7 SVA might increase the risk of developing cervical sagittal imbalance in patients with CK, and eventually cause the neck pain.
Cervical alignment is a complex problem which also impacts the kinematics of the cervical spine. With regard to the CK, the sagittal alignment has different effects correlated with appearance of CK types[30]. The position of spinal cord compression is at the transition zone of the regional CK type or apex of kyphosis of the global CK type. Furthermore, the apex of regional CK type is also at high risk. Extension segmental motion of the global CK type is increased in the upper cervical spine compared to the regional CK type whose position is lordosis. It is opposite when cervical spine is flexion. Nevertheless, multivariate logistic regression showed different CK types was not the risk factor for the ANP, in spite of disc degeneration caused by abnormal kinematics. Previous studies demonstrated almost 20% asymptomatic individuals have cervical disc degeneration[28, 29]. Therefore, similar to their study, disc degenration was also found in Non-ANP group.
Limitation
However, there are a few limitations in this study. Firstly, there is lack of the global spinal sagittal radiographs to estimate the mutual effect of the lumbar and thoracic spine and CK; second, the kinematic MRI is a better method compared to supine MRI in the assessment of cervical instability and degeneration; thirdly, owing to this retrospective study, it is difficult to control for the many variables inherent in the patients; forth, there is really small sample size in patients with CK. The long-term and large scale studies should be performed to define the relationship between CK and ANP. In the future study, we can explore the correlation global spinal balance and the development of ANP.