This study evaluated the predictors of the restoration of the sagittal spinal balance after short-segment decompression and fusion surgery in the elderly patients who had LLS with sagittal malalignment. 38 out of 82 patients recovered the sagittal morphology three months or later after surgery. Three types of preoperative predictors for the restoration of the sagittal spinal malalignment were found in this study, including the mass of lumbar paraspinal muscles, the sagittal radiographic spinal parameters, and some demographic characteristics.
LSS is caused by various factors, such as facet joint hypertrophy, osteophyte, or ligamentum flavum (LF) thickening 4, 19. The LF is stretched during lumbar flexion, and more than half of LSS patients might assume a forward-bending posture to reduce pain during standing, which would lead to sagittal malalignment 20, 21. After decompression and fusion of the stenosis levels, the trunk would no longer need to maintain an anterior trunk flexion position to relieve nerve compression. Previous studies also demonstrated that the trunk muscle weakness led to adverse clinical outcomes in LSS patients 22, 23. Our study confirmed that patients with a greater CSA of paraspinal muscle were more likely to restore the normal sagittal spinal alignment after short-segment fusion surgery. This might be related to the fact that the stronger paravertebral muscles would help to stabilize the trunk, maintain the proper posture, and generate movement, especially the multifidus and erector muscles 24. Shen et al. 22 found that the sagittal malalignment was associated with the loss of the back muscle mass, especially the paravertebral muscles instead of the appendicular skeletal muscle weakness. Therefore, the paravertebral muscles were more important in the pathogenesis of sagittal malalignment in LSS patients.
In this study, the CSA of the lumbar paravertebral muscle at five levels from L1 to L5 were separately measured and then averaged as an indicator of muscle mass. Previous studies only measured the CSA of one single level 25, which lacked reliability in the assessment of the overall lumbar muscle. Functional CSA is a better indicator of the actual mass of muscles than the total CSA 18. Therefore, this study could provide a reliable preoperative evaluation index for spinal surgeons. In addition, fCSA could better predict the occurrence of postoperative malalignment, and the AUC is up to 0.81. Nevertheless, the cutoff value could not be obtained due to the small sample size.
Legaye et al. 26 found that PI was almost a fixed value after adulthood despite a slight change as the sacroiliac ligament complex relaxed in the process of degeneration, and everyone had a matched LL. Through a large database study, Roussouly et al. 27 demonstrated that a greater PI was presented with a greater LL. Correspondingly, it required stronger muscle strength to maintain the LL. As the infiltration of fat in muscles and the reduction of fCSA, the muscle strength weakens, which would lead to reduction in LL, and anterior inclination of the trunk. The pelvis would tilt posteriorly for compensation, causing an increase in PT. Therefore, the fCSA of lumbar paraspinal muscle was negatively correlated with the postoperative PT.
PT is a more sensitive indicator in detecting the sagittal malalignment than SVA because the SVA could be maintained as a normal distance by pelvic retroversion. Previous studies have stated that PT is significantly correlated with VAS and ODI scores of patients 28. When SVA is still in the normal range, sagittal malalignment has already occurred in the body, so PT is of great significance for the early patients in the evaluation of sagittal malalignment. Similarly, patients with a great PI might have a considerable PT. Therefore, to eliminate this effect, we calculated the ratio of PT to PI (PTr) and achieved similar result 16. This is the first time to confirm a negative correlation between PT or PTr and fCSA after short-segment fusion surgery, which provides insight for application in the clinical setting.
Sex is another factor in influencing the trunk muscle strength as females experience a dramatic decline in hormones due to menopause at the beginning of sixties 29. In this research, female patients were also tending to maintain the sagittal spinal malalignment after surgery, which attribute to the lower fCSA and the higher FI in females 30. Therefore, when we make surgery strategy in elderly LSS patients with sagittal malalignment, the sex difference should be taken into consideration.
Our study also found that age was another risk factor for the failure to restore the normal sagittal spinal morphology after short-segment decompression and fusion surgery. This might be related to the muscle degeneration during the aging process. With an increase age, the paravertebral muscle would worsen, the LL would reduce, and the PT would increase to compensate for the increase of SVA. When these compensation mechanisms are exhausted, it would ultimately lead to the onset of sagittal malalignment. Previous study also found that a decrease in CSA of the paravertebral muscles and an increase in fatty infiltration with aging 31, 32, which is consistent with the results of the present study. Hence, in the elderly LSS patients with sagittal malalignment, the short-segment decompression and fusion surgery might not be the optimal option unless the muscle condition would be ameliorated.
Hence, we must declare that during the degenerative process, the paravertebral muscles, the spinal radiographic parameters, and the demographic characteristics of patients interact with each other. The debut factor and sequence of occurrence needs to be further investigated in future studies.
The limitations of present study are as follows: firstly, we included a relatively small sample size and did not derive cutoff values for fCSA of the lumbar paravertebral muscles in patients with postoperative imbalance. Prospective studies are needed to determine which factors would facilitate patients to regain the normal sagittal morphology after short-segment fusion. Secondly, our follow-up periods were relatively short, ranging from 3 to 30 months, but previous studies have shown that 3 months were sufficient to return most patients to normal sagittal alignment after surgery 33.