Multimodal Magnetic Resonance Of Paraspinal Muscle Fiber Types Transformation In Rats With Discogenic Low Back Pain

Objectives:1.Using multimodal magnetic resonance research to dynamically observe the changes in the quantitative parameters of the structure and function of the paraspinal muscles of DLBP rats.2.To explore the law of muscle fiber transformation in paraspinal muscles of DLBP rats,and to provide experimental basis for the clinical diagnosis and treatment of low back pain.Methods:1.Experimental animal grouping and sample size estimation:Firstly,the rats were divided into 30 day,90 day and 180 day group,including three subgroups:Normal group,Sham group,and DLBP group,a total of 9 groups.Then,according to the principle of minimum sample size,use PASS software to estimate the sample size of SD rats,and the total sample size required for the 9 groups was about 90.Finally,the rats were randomly divided into the 9 groups（n=10 in each group）by using the random number table method.2.Establishment and function of DLBP rat model:Under the guidance of X-ray fluoroscopy,the L4/5 and L5/6 intervertebral discs of the rats in the DLBP group were punctured through the posterior approach,and sterile PBS was injected to destroy the nucleus pulposus.This animal model was used to explore the changes in the structure and function of the paraspinal muscles in DLBP rats,as well as the correlation between multimodal MRI parameters and pathological parameters.3.Behavior:30 days,90 days,and 180 days after modeling,conduct behavioral tests on rats in the Normal group,Sham group,and DLBP group,including tail suspension test,grasping test,wire hang test,and limb grip strength test and exhaustive swimming test,and behavioral tests were used to quantify the degree of low back pain and paraspinal muscles performance of DLBP rats.4.Imaging:30 days,90 days,and 180 days after modeling,the rats in the Normal group,Sham group,and DLBP group were scanned by functional magnetic resonance,including MENSA,MAGIC,IDEAL-IQ,BOLD,DTI sequence,and the changes of CSA relative values,T₁,T₂values,PDFF values,R₂^*values,FA values,MD values,eigenvalues（λ₁,λ₂,λ₃）of the paraspinal muscles at the L4/5 and L5/6 intervertebral disc levels in three groups of rats are observed.5.Histology:30 days,90 days,and 180 days after modeling,the three groups of rats that had completed behavioral and imaging evaluations were euthanized,and the paraspinal muscles at the modeling level were obtained for HE,immune Fluorescence and immunohistochemical staining,the pathological changes of paraspinal muscles morphology,muscle fiber type transformation（proportion of type I and type II muscle fibers）,capillary density（CD）,and capillary fiber ratio（C/F）were observed in the three groups of rats.Results:1.Behavioral results:At 90 and 180 days after modeling,compared with the rats in the Normal group and the Sham group,the resting time and hunching time of the rats in the DLBP group increased,and the struggling time decreased;the drop time of the wire hang test decreased;the drop time of the grasping test decreased;the grip strength of the limbs decreased;the exhaustive swimming time decreased（all P<0.05）.The resting time,hunching time and struggling time of the tail suspension test between the Normal group and the Sham group rats at each time point;the drop time of the wire suspension test,the grip strength of the limbs,and the exhaustive swimming time were not obvious statistical difference（all P>0.05）.2.Imaging results:30 days after modeling,except for the T₂values of the multifidus muscle at the L4/5 level of the rats in the DLBP group was significantly reduced（P<0.05）,the CSA relative values and T₁values of the paraspinal muscles at the L4/5and L5/6 intervertebral disc levels,T₂values,PDFF values,R₂^*values,FA values,MD values,eigenvalues（λ₁,λ₂,λ₃）were not significantly different from those in Normal group and Sham group（all P>0.05）.90 days after modeling,compared with the rats in the Normal group and the Sham group,the CSA relative values,T₁values,λ₁values,λ₂values andλ₃values of the paraspinal muscles of the rats in the DLBP group at L4/5 and L5/6 levels,and the FA values of the erector spinae muscle at the L4/5 level,the MD values of the erector spinae muscle at the L4/5 and L5/6 levels had no significant difference（all P>0.05）,but the T₂and FA values of the paraspinal muscles at the L4/5 and L5/6 intervertebral disc levels were significantly decreased,and the PDFF and R₂^*values were significantly increased（all P<0.05）.180 days after modeling,compared with the rats in the Normal group and the Sham group,there were no significant differences in the CSA relative values,T₁values,andλ₁values at the L4/5 and L5/6 levels of the paraspinal muscles of the rats in the DLBP group（all P>0.05）,but the T₂and FA values of the paraspinal muscles at the L4/5 and L5/6intervertebral disc levels were significantly reduced,and the PDFF,R₂^*,MD,λ₂andλ₃values were significantly increased（all P<0.05）.From 30 days to 180 days after the model,the T₂values and FA values of the rat paraspinal muscles in the DLBP group gradually decreased,and the PDFF values,R₂^*values,MD values,λ₂values andλ₃values gradually increased.Except the CSA relative values of paraspinal muscles in the DLBP group was significantly different between L4/5 and L5/6 layers,although the other parameters were different in sides and layers,there were no significant differences（all P>0.05）.There was no significant difference in each MRI quantitative parameter between the Normal group and the Sham group at each time point（all P>0.05）.3.Histological results:30 days after modeling,compared with the Normal group and the Sham group,There was no significant difference in the proportion of type I and type II muscle fibers,capillary density CD and capillary fiber ratio C/F（all P>0.05）.At 90 days and 180 days after modeling,the proportion of type I muscle fibers in the paraspinal muscles of rats in the DLBP group was significantly lower than that of the Normal group and the Sham group,while the proportion of type II muscle fibers was significantly higher than that of the Normal group and the Sham group,and the capillary density（CD）and capillary fiber ratio（C/F）values decreased significantly,and the differences were statistically significant（all P<0.05）.There was no significant difference in the ratio of type I and type II muscle fibers in the paraspinal muscles between the Normal group and the Sham group at each time point（all P>0.05）.4.Correlation analysis results:R₂^*values was moderately negatively correlated with I%values,CD values and C/F values（r=-0.477,P<0.001;r=-0.589,P<0.001;r=-0.516,P<0.001）;FA values was moderately positively correlated with I%values,CD values and C/F values（r=0.590,P<0.001;r=0.454,P<0.001;0.001;r=0.444,P<0.001）.Conclusions:1.With the prolongation of the course of IVDD in rats,the fat content in the paraspinal muscles increased,the type of muscle fibers in the paraspinal muscles transforms from type I to type II,the density of capillaries decreased,the structure and function of the paraspinal muscles were impaired,and muscle performance decreased.2.The increase of R₂^*values and the decrease of FA values in multimodal MRI quantitative parameters of paraspinal muscles in DLBP rats may be related to the decrease of type I muscle fiber ratio and capillary density.To a certain extent,R₂^*values can reflect the change of paraspinal muscles capillary density,and FA values can represent the transformation of paraspinal muscles fiber type.3.Multimodal MRI can be used as an effective means for non-invasive quantitative evaluation of the structure and function of DLBP paraspinal muscles in vivo.The changes of MRI quantitative parameters such as R₂^*values and FA values can indicate the pathological changes of DLBP paraspinal muscles.