The spinal stability system consists of three subsystems: passive spinal column, active spinal muscles, and neural control unit [1, 8]. The passive musculoskeletal subsystem includes vertebrae, facet articulations, intervertebral discs, spinal ligaments, and joint capsules, as well as the passive mechanical properties of the muscles. The active musculoskeletal subsystem consists of the muscles and tendons surrounding the spinal column. The neural and feedback subsystem consists of the various force and motion transducers, located in ligaments, tendons, and muscles, and the neural control center [2, 8, 9, 11]. DK is a manifestation of sagittal malformation characterized by a decrease of LL or an increase of TLK. With the growth of the aging population, numerous scholars have concentrated on DK, in which the performance of spinal stability system mainly depends on the changes of body shape, including BMI and paraspinal muscles [6, 10].
A variety of methods are available to assess paraspinal muscles, while those methods are complex and time-consuming [17, 18]. Takayama et al. [14] proposed the concept of LCIV, which made the measurement of paraspinal muscles volume more reliable, simple, and efficient. They found a positive correlation between LCIV and CSA, while there was a negative correlation between LIV and age. They pointed out that the degeneration of paraspinal muscles may lead to the loss of LL, and the decreased tension of posterior extensor muscles plays a pivotal role in LLL [19, 20, 20].
There have been few studies on "body shape" and sagittal imbalance. Obesity may not only be a risk factor related to the natural degradation of spine, but also play a role in promoting occurrence of DLS [3]. The anterior vertebral bodies provide the primary load-bearing capacity of the thoracic spine [211]. Wang et al. [2] pointed out that BMI was closely associated with the formation and progression of DS. However, a number of scholars demonstrated that the loss of sagittal sequence was associated with absolute muscle loss, while higher BMI indicates higher degree of "obesity" with fat infiltration [1,222]. Compared with healthy adults, obese patients exhibited weaker paraspinal muscles and difficulty in upright posture, which would accelerate the degeneration of intervertebral discs and facet joints, resulting in spinal deformities. Kim et al. [5] confirmed that there was no significant correlation between BMI and sagittal imbalance. In the present study, the TLK and LL in both groups were not affected by BMI, which was in accordance with Kim et al.’ findings. A previous study demonstrated that multifidus and erector spinae have different roles in affecting the spinal-pelvic alignment and maintaining the sagittal balance. Multifidus at the lower lumbar spine level is critical for maintaining the curvature of the lumbar spine, whereas erector spinae at the lower lumbar level mainly affects the pelvic parameters [8]. Studies confirmed the correlation between sagittal alignment and cross-sectional area of paraspinal muscles in patients with lumbar spinal stenosis [9, 13]. Mannion et al. [233] believed that scoliosis is thought to progress during growth because spinal deformity produces asymmetrical spinal loading, generating asymmetrical growth, etc., in a vicious cycle.
In the present research, it was found that the LCIV gradually increased from top to bottom in DK group and LCIV in DK group was remarkably lower than that in control group except for L4-5. On the one hand, the apical vertebrae was mainly distributed on thoracolumbar and upper lumbar spine with DK, and there was much less impact on the bottom of the lumbar spine contrasted with the control group. On the other hand, the iliac crest and sacrum provided the attachment point of sacrospinous muscle and further thichened the paravertebral muscles in this area, regardless for DK and control group [14,244]. Additionally, it was unveiled that there was a significant correlation between LCIV and LL. It was shown that the loss of LL or even kyphosis may lead to a significant decrease in the content and strength of paraspinal muscles. Takemitsu et al. [14] found that the strength of posterior extensor muscle was significantly lower than that of flexor muscles in DK patients. Hongo et al. [255] indicated that decreased strength of paraspinal muscles was found to be associated with loss of LL and kyphosis in the elderly, and the LL could be recovered by exercise.
Although DK has been scarcely reported in Western countries, it is common in Asian countries because of different lifestyles and working posture [266]. Full of consensus, the SRS-Schwab classification for evaluating the sagittal deformity was then proposed [277] Hence, we, in the present study, divided patients with DK into balance group and imbalance group according to PI-LL and PT parameters. The results showed that the content of paraspinal muscles in the balance group was significantly higher than that in the imbalance group. Correlation analysis revealed that there was a significant correlation between paraspinal muscle content and PI-LL, PT, which further confirmed the close relationship between sagittal balance and paraspinal muscle content in patients with DK. Therefore, for the DK patients requiring to undergo deformity correction, it is highly essential to enhance rehabilitation training program, in order to simultaneously promote the recovery [28,299]. Besides, the relationship among TLK, BMI, and LCIV was assessed in the present study. For normal patients, muscle content can be roughly estimated by measuring height and weight. However, for patients with DK, the degree of kyphosis can be predicted by BMI and MRI findings. Therefore, we assessed the muscle mass by the degree of kyphosis and BMI, which was found reliable for surgeons to evaluate patients with DK.
There are some limitations in the current study. Firstly, LCIV was not as accurate as CSA. For muscle tissues infiltrated by massive fat, it was difficult to identify the muscle apex, which might cause bias. Secondly, since patients with DK were the main target in the current study, it remains unclear whether our findings can be helpful for other types of deformity, such as scoliosis. Thirdly, there was an obvious correlation between LCIV and sagittal alignment, while the causal relationship could not be determined. Finally, there was no simplified parameter, such as LCIV representing CSA, analogous to fat infiltration. Wecould propose the quantitative metric, combined with LCIV, to characterized the paraspinal muscle more accurately. .