Abstract: BACKGROUND: Previous studies have shown the correlation between lumbosacral sagittal plane parameters and natural absorption of lumbar disc herniation. However, the lumbosacral sagittal plane parameters included lumbar lordosis Angle, lumbosacral joint Angle, sacral inclination Angle and many other parameters, and the effects of each parameter on the natural absorption of herniated disc were different. In addition, there are few studies on reabsorption of a specific segment of intervertebral disc herniation at present, and most of the measured data are obtained from DR Or CT, while the correlation between lumbosacral sagittal plane parameters measured from MRI and reabsorption after L5/S1 intervertebral disc herniation is rarely reported. 
OBJECTIVE: To study the corresponding changes of lumbar sagittal plane parameters after L5/S1 intervertebral disc herniation reabsorption and to screen out the lumbosacral sagittal plane parameters with the most significant changes during intervertebral disc reabsorption.
METHODS: In this study, 57 patients with lumbar disc herniation who had complete MRI image data from February 2011 to June 2019 were selected and met the diagnostic criteria for lumbar disc herniation and only received non-surgical treatment for reabsorption of L5/S1 protrusion segments. MRI measured the protrusion area of the maximum protrusion plane in the coronal plane, lumbosacral sagittal plane parameters [lumbar curvature index (LCI), lumbar Cobb Angle (α), L5/S1 disc Angle (β), intervertebral height measurement (D), lumbosacral joint Angle (LSA), sacral platform Angle (STA), sacral inclination Angle (SS), and lower lumbar lordosis Angle (Lowe LL))]. Besides, lumbosacral sagittal plane parameters were ranked in importance of variables by random forest model in R software, and then significant variables were fitted with multiple linear regression, and the changes between parameters before and after treatment were analyzed and compared by paired sample T test. 
RESULTS AND CONCLUSION: A total of 57 patients with L5/S1 lumbar disc herniation were included in this study, and the symptoms and imaging features of the patients were significantly relieved to a large extent. Before treatment, there were 4 cases of grade 1, 29 cases of grade 2 and 24 cases of grade 3. After treatment, there were 48 cases of grade 1 and 9 cases of grade 2. The random forest model suggested that intervertebral height, lumbar curve index, sacral inclination Angle, and lower lumbar lordosis Angle changed significantly in L5/S1 disc herniation reabsorption, and the order of their change significance was lumbar curve index > intervertebral space height > sacral inclination Angle > lower lumbar lordosis Angle. Lumbar curve index, lumbar Cobb Angle and sacral platform Angle increased, with statistical significance (P<0.05). There were no significant differences in disc Angle, intervertebral height, lower lumbar lordosis Angle, sacral inclination Angle and lumbosacral joint Angle (P>0.05). Lumbar curvature index was the most significant parameter of lumbosacral sagittal plane in herniated disc reabsorption. In addition, lumbar curve index, sacral inclination Angle, and lower lumbar lordosis Angle are commonly used clinically to describe the change of lumbar curvature, suggesting that L5/S1 disc herniation reabsorption is correlated with the change of lumbar curvature.

Key words: Random forest model, Lumbar disc herniation, Reabsorption, Lumbosacral sagittal plane parameters