Over the decades, several effective surgical options have been developed such as anterior cervical discectomy or corpectomy and fusion, posterior cervical laminoplasty, and posterior cervical laminectomy and instrumented fusion for treating cervical spondylotic myelopathy (CSM), herniated cervical disc (HCD), and ossification of the posterior longitudinal ligament (OPLL). Many studies have been conducted to compare the outcomes of these techniques clinically and radiologically.

Recently, cervical sagittal balance has been identified as an important determinant of radiological outcomes following cervical surgery3,12). However, few studies have investigated pre- and postoperative radiologic changes in cervical surgery and the risk factors for these changes, considering the relatively recent interest in the concept of sagittal alignment. Tang et al.17) played a pivotal role in highlighting the concept of cervical regional sagittal imbalance. They analyzed the relationships between radiographic parameters and health-related quality of life (HRQOL) score. Of several radiographic parameters, sagittal vertical axis (SVA) C2-7 showed a negative association with HRQOL scores after posterior cervical fusion17). A previous study reported that posterior ligamentous structures attached to C2 and C7 played important roles in clinical outcome, but this study only evaluated clinical outcomes such as neck pain and did not examine the effects on the change in radiographic sagittal parameters between the preoperative and postoperative status11).

Thus, we hypothesized that the destruction of these uppermost and lowermost cervical stabilizing structures by various means, such as surgical techniques, and the level of pathologic lesion might have significant influences on cervical spinal sagittal alignment and balance. This study aimed to investigate the risk factors for postoperative cervical sagittal malalignment and sagittal imbalance. In particular, we focused on the preservation of C2 and C7 posterior ligamentous structures during surgery.

Currently, the importance of cervical sagittal alignment has been acknowledged and sagittal imbalance after posterior surgery on the cervical spine has been considered a major source of pain and disability3). Sakai et al.13) suggested that preoperative cervical sagittal imbalance is a powerful predictor of kyphotic deformity after cervical laminoplasty and is related to poor postoperative clinical outcomes.

Suk et al.16) reported a significant decrease in cervical lordosis after laminoplasty and that the incidence of kyphotic change was significantly higher in the decreased preoperative cervical lordosis group (Cobb angle<10°). This study also showed the loss of cervical lordosis by measuring the change in C2-7 Cobb angles and comparing preoperative and postoperative values (Group A: −2.69±4.99, B: −6.1±7.24, C: −8.64±6.28). Although there were no statistically significant differences in change in C2-7 Cobb angle between the three groups with the ANOVA, change in C2-7 Cobb angle revealed a strong negative correlation with change in SVA C2-7. Thus, we concluded that the postoperative loss of cervical lordosis is an important risk factor for postoperative cervical sagittal imbalance.

Some previous studies have shown that C2-7 paraspinal muscle and ligament preservation can reduce the postoperative loss of cervical lordosis. Sakaura et al.14) reported the 8-years’ follow-up data of 20 patients after cervical laminoplasty from the C3 to C6 level and the patients did not show any postoperative kyphotic change. Michael et al.8) reported a more significant postoperative loss of cervical lordosis when C3 laminas were included as compared to when C4 laminas were included and C3 laminas excluded. C6/7 lesional cervical alignment also plays important roles in the maintenance of cervical sagittal alignment1). Sakaura et al.15) reported that the funicular structures of the nuchal ligament attached to C6/7 played important roles in decreasing the segmental alignment of C6/7 postoperatively. Lin et al.7) introduced a laminoplasty technique with unilateral muscle-ligament complex preservation and compared this procedure with the traditional open-door laminoplasty. Their modality showed superiority in maintaining cervical sagittal alignment, including SVA C2-7 and C3-7 cobb angle measurement. These previous studies indicate that the paraspinal muscles and nuchal ligament play important roles in the maintenance of cervical lordosis. Furthermore, C2 and C7 are the core attachment points of the paraspinal muscles and nuchal ligament for maintaining cervical lordosis. Our study results indicate a relationship between C2/7 musculoligamentous structure preservation and cervical alignment. Consequently, we assumed that C2-C7 ligament disruption during surgery can influence the postoperative loss of cervical lordosis, sagittal imbalance, and poor clinical outcomes2,9,14).

We considered the change in SVA C2-7 as a representative parameter of cervical sagittal imbalance. Previous studies focused on the C2-7 loss of lordosis as a radiographic parameter for evaluating postoperative change. A few studies have evaluated the relationship of SVA C2-7 with cervical sagittal imbalance, although many studies have suggested a negative relationship between SVA C2-7 and clinical outcomes18,20).

Comparing the change in SVA C2-7, C2-7 lordosis, and T1 tilt with differences among groups and considering C2-7 ligament structure disruption that causes cervical spine destabilization and loss of cervical lordosis, it is difficult to elucidate which of these first results in the loss of cervical lordosis. However, it can be ascertained that changes in C2 tilt, T1 tilt, and SVA C2 are correlated with and ultimately affect the forward pitch of cervical sagittal balance.

T1 slope has been used as an important parameter for predicting surgical outcomes after posterior cervical surgery, and some studies have reported a correlation between T1 slope and C2-7 lordosis and SVA C2-74-6,10,19). Patients with a higher preoperative T1 slope were more likely to exhibit a postoperative loss of C2-7 lordosis, which was greater at the degree when it occurred. However, postoperative kyphosis did not occur more frequently in patients with higher T1 slopes3). Our study revealed that T1 tilt has a negative correlation with change in SVA C2-7. It could be suggested that a high T1 tilt results in a large C2-7 lordosis and that posterior cervical spine surgery and disruptions of C2, 7 musculoskeletal structures lead to the forward shifting of the center of gravity and change of SVA C2-7. Thus, these three parameters: SVA C2-7, T1 tilt, and SVA C2-7 have a complex organic correlation.

The limitations of this study include the small number of enrolled patients and their further division into three separate groups. Moreover, the follow-up periods of each group showed some differences. Therefore, a larger number of patients and long-term follow-up seem necessary for more definitive conclusions.

We suggest that the increase in C2 and T1 tilt could result from C2-7 supra and interspinous ligament destabilization, which might influence the postoperative loss of cervical lordosis and increased C2 SVA. Additional multivariate linear regression analysis revealed that the loss of cervical lordosis and increase of C2 and T1 tilt are significant factors for increased change of SVA C2-7 after cervical laminoplasty.

In analyzing C2 and/or C7 ligament structure involvements during the surgery, we found that C2 and C7 preservation during laminoplasty plays an important role in minimizing the preoperative-postoperative change of SVA C2-7, which means maintenance of cervical sagittal alignment.