With the increasing development and popularity of fusion surgery, the incidence of clinically diagnosed ASD is common. Some studies have shown that the incidence of ASD is between 4% and 31% [17, 18]. Therefore, scholars have performed in depth research on the risk factors for ASD with intentions to prevent its development, improve the patient’s quality of life after surgery and avoid the pain of secondary operations.
In recent years, an increasing number of studies have focused on the relationship between paraspinal muscles and spinal diseases. This is reasonable because the paraspinal muscle itself is connected with the bony structure, can support the body to complete many activities, and is a very important anatomical structure of surgical approach, therefore details cannot be ignored. At present, there is a lack of a unified method for evaluating the quality and quantity of the paraspinal muscles. It is common to use CSA to indirectly reflect the number of the paraspinal muscles, while FI or functional CSA is used to reflect the quality of paraspinal muscles [19.20]. Previous studies have proven the relationship between the paraspinal muscle and low back pain, neurological function, proximal junctional kyphosis, physical activity, etc. However, due to the disunity of different measurement methods, research results obtained from different centers sometimes greatly differ and even draw contradictory conclusions. Recently, some scholars began to study the relationship between paraspinal muscles and ASD [10–12]. However, the study has obvious limitations: it may be inappropriate to select only the cross section of the paraspinal muscles at the single segment level to represent the overall paraspinal muscle level because there is no known basic research [21–24] or theoretical support, and there may be poor comparability between different individuals at the same level due to factors such as development, living habits, pathological changes and so on. The accuracy of the FI measurement may be insufficient due to mainly relying on hand-drawn ROI or visual measurement. Based on this, this study measured the cross-sectional parameters of the paraspinal muscles at each level of the lumbar intervertebral disc, and the degree of FI was expressed by the "gray value" parameter with the help of professional image processing software.
Compared with the demographic data between the selected ASD patients and the control group, there was a statistically significant difference in BMD between the two groups. This is consistent with the results of previous studies . In this study, the CT value of the cancellous area of the vertebral body was obtained to represent the patient’s BMD. Previous studies have confirmed that the CT value of vertebral cancellous bone is positively correlated with the actual patient’s BMD , therefore, it is feasible to compare and analyze the CT value. The reason for this difference may be that patients with a lower BMD are more likely to have bone degeneration, destruction and spinal imbalance, and these patients have a higher risk of developing ASD than their peers after the first operation. This is consistent with previous perceptions that BMD is an important indicator that must be considered in patients undergoing spinal surgery at any stage.
We compared and analyzed the paraspinal muscle parameters of each intervertebral disc level (L1-S1) between the two groups. The results showed that there were significant differences in the rCSA and rFI of the MF muscle at the lower lumbar level between the two groups, but there was no significant difference in the quality and quantity of the ES muscle at each segmental level. This result confirms the important role of MF muscle in the development of ASD in patients, and the MF muscle at the lower lumbar level is more representative of the characteristics of the whole muscle. Then, we used univariate regression analysis to confirm that the smaller CSA of the MF muscle at L3-L4, the greater degree of FI at L4-L5 and L5-S1, and the lower BMD were all risk factors for ASD. It is worth noting that previous studies have shown a correlation between BMD and the degree of paraspinal muscle FI [27–29]. However, this correlation is not reflected in this study, considering that all subjects included in this study are patients with lumbar degenerative diseases, which is different from the normal people in previous studies.
Previous scholars have confirmed the effect of the number of segments in the first operation on the development of ASD [30–32]. We tried to divide all the patients into two groups according to the segment of the first operation and found that there were significant differences in the CSA and the degree of FI of the lower lumbar MF muscle between the two groups in the patients receiving single segment surgery for the first time. In the patients who received two-segment fusion for the first time, there was only a difference in the FI of the lower lumbar MF muscle between the two groups, but there was no difference in the CSA between the two groups. This may be because the longer fusion segment itself is a risk factor for the occurrence of ASD; on this basis, early changes in the paraspinal muscles can lead to the occurrence of ASD. Therefore, it can be considered that the FI of paraspinal muscles can reflect the changes in muscle characteristics more sensitively than the CSA. In actual clinical work in the future, it may be possible to directly determine the degree of FI of the lower lumbar MF muscle through intelligent image analysis software, which can be used as an evaluation index.
This study revealed the role of paraspinal muscles in the development of ASD. First, to prevent the occurrence of ASD, spinal surgeons should have a long-term vision, and the state of paraspinal muscles should also be taken into consideration when making surgical plans. Second, we should continue to emphasize the importance of low back muscle exercise and even set up a systematic exercise program for patients after the first lumbar fusion. Previous prospective studies on other spinal diseases have also shown that low back muscle exercise can effectively improve muscle quality [33–35].
This study also has some limitations. The sample size of this study was small, and there were only 33 cases in the ASD group. This is because there is a requirement for high-quality image data before the first operation. These images must be collected in this research center, hence limiting the number of sample size. In this study, all the segmental levels of each patient’s muscles were measured, compared and analyzed, which could have compensated, to a certain degree, for the small sample size. In addition, previous studies have shown that spinal-pelvic sagittal imbalance is also one of the risk factors for ASD [36, 37]. However, due to the overall lack of full-length films of the spine in our center, the measurement of pelvic parameters is limited.