The normal spine exhibits an S-shaped physiological curvature, and the spine and pelvis jointly maintain trunk balance. Owing to the development of DLS, compensatory changes in spine and pelvis occurred to maintain trunk balance. The sagittal spinopelvic parameters can adequately reflect this balance and compensate for biomechanical changes of the spine after DLS [16, 17]. To the best of our knowledge, this study is the first report of the differences and correlations between spinopelvic parameters of different types of DLS patients.
In DLS, the female sex has been considered a risk factor for spondylolisthesis. In this study, 77.6% of double-level DLS patients were women, which is similar to that obtained by previous studies [18, 19]. Previous studies have shown that compared with single-level DLS, women with double-level DLS account for a larger proportion (approximately 70%) [14]. Interestingly, this study found that women in the anterior type accounted for a larger proportion of women in double-level DLS, while men and women had a similar proportion in the combined type group. Compared with the combined type, the anterior type group patients were older (over 69 years old), suggesting that these patients may gradually develop double-level spondylolisthesis owing to previous delayed treatment single-level spondylolisthesis. However, further research is still needed to confirm this process. A study has shown that the age of DLS patients is an important consideration factor for surgery because they may be associated with more comorbidities and a higher risk of postoperative complications [20]. However, other studies suggest that compared with young DLS patients, elderly patients have significantly improved postoperative symptoms, and there is no significant increase in complications and mortality [21, 22]. It can be seen that for elder DLS patients, surgical treatment has obvious advantages. In addition, many studies have showed that the lumbar intervertebral instability and nerve compression are the primary factors leading to low back pain and functional disabilities in DLS patients [23–25]. This study found that patients in both groups had varying degrees of chronic low back pain. The incidence of radiating leg pain and neurogenic claudication in the anterior type was significantly higher. The preoperative ODI and VAS scores showed that the quality of life of the anterior type patients was significantly lower and VAS low back scores was higher than those in the combined type. The above results indicate that the lumbar intervertebral instability is more serious in the anterior type and leads to more severe clinical symptoms and a significant reduction in quality of life. Therefore, for such patients, when making surgical plans, enhancing the stability of the spine, relieving the postoperative clinical symptoms, and improving the quality of life after surgery may be important considerations for surgical treatment.
Previous studies on spinopelvic parameters in DLS patients have been reported in the literature [26–28]. The most common level of slippage in single-level DLS patients is L4L5, but the levels of L3L4-L4L5 and L4L5-L5S1 is common in double-level DLS patients [14]. This study found that L3L4-L4L5 and L4L5-L5S1 levels of spondylolisthesis are common in double-level DLS, which is similar to that obtained in previous studies. Hitherto, few studies have been conducted on the characteristics of spinopelvic parameters in double-level DLS patients. Iguchi et al. [15] divided double-level DLS into three types (anterior type, posterior type and combined type), and studied pedicle-facet angles,facet joint shape༌and vertebra rotation direction; they found that the sagittal facet joint angle and W-shaped facet joint are the key factors for the vertebral body forward slip. Ferrero et al. [7] showed that compared with single-level DLS, C7 tilt, PI and PT were significantly higher in patients with double-level DLS, but LLmax was significantly lower, indicating that sagittal imbalance was more serious, lumbar lordosis was severely lost, and more compensatory mechanisms (such as forward flexion and pelvic retroversion) were activated in double-level DLS. However, they did not analyse the difference between the spinopelvic parameters of different types of double-level DLS patients. The present study found that C7 tilt, LLmax, PI and PT increased significantly in the anterior type. These results all suggest that a higher PI in the anterior type may be a risk factor for anterolisthesis of two adjacent vertebral bodies. As the spondylolisthesis progresses, LLmax gradually increases. The change in lumbar lordosis is accompanied by the inclination of the sacrum, and the SS changes accordingly. Because PT and SS have the same effect in evaluating the pelvic shape, according to the geometric relationship PI = PT + SS, the PT increased, and to maintain the sagittal balance of the spine, C7 tilt increased [29–31]. As the LLmax of increases, changes in thoracolumbar segment also easily cause lumbar lordosis changes, which in turn accelerate lumbar degeneration [32]. The combined type group may compensate each other because of antrolisthesis and retrolisthesis. Thus, the parameters C7 tilt, LLmax, PI, and PT are smaller than those of the anterior type, which may be one of the reasons why combined type patients have milder clinical symptoms and better quality of life than that of anterior type patients. To maintain the sagittal balance of the spine༌LLmax increased in DLS with angular changes in TKmax. However, owing to the relatively fixed thoracic spine and low mobility, the compensatory changes is not significant in elderly DLS patients. Hence, there was no significant difference in TKmax between the two groups. The ratios LLmax/PI and PT/PI are used to evaluate the relationship between spinopelvic parameters and PI, which is a morphological parameter unique to everyone [33]. The ratio LL/PI, as a spinopelvic parameter, is related to the occurrence and development of proximal junctional degenerations [34].This study found that the LLmax/PI ratio imbalance in the anterior type group patients was more obvious, further indicating that the anterior type may be due to the evolution of a single-level slippage. Unfortunately, probably because of the small sample size in this study, the ratio LLmax/PI in the two groups were not statistically significant. For combined type patients, spondylolisthesis may not be caused by high PI, but the imbalance of LLmax/PI ratio after spondylolisthesis causes another vertebral body to slip backward to maintain the body's balance. Previous studies have shown that PI has a positive correlation with LL, and LL has a linear correlation with PT and SS, whereas PT has a linear correlation with LLmax/PI [14]. In this study, there was a significant correlation between the spinopelvic parameters in the two groups. In the anterior type group, PI was positively correlated with LLmax, and LLmax and SS have a positive correlation. In the combined type group, LLmax was positively correlated with PI and SS, and LLmax/PI was positively correlated with SS.
Our study had several limitations. First, the low incidence of double-level DLS results in a small sample size, which may have a statistical impact. Secondly, this is a retrospective analysis, which is subject to some restrictions in data collection. The radiological data in this study are the results of preoperative examination; there were no intraoperative data and postoperative follow-up data, thus there was no study of postoperative spinopelvic parameters and patients' quality of life. Finally, no complete and sufficient posterior type patients’ data were collected in this study. In the future, we plan to cooperate with other hospitals to conduct multi-centre clinical research, expand the sample size, and further study multi-centre data to verify our research results.