Correlation Among Residual Pain, Spinopelvic Parameters and Area of Fat Inltration in Lumbar Multidus Muscles in Patients after Lumbar Surgery: A Cross-Sectional Study.

Background: Numerous studies have reported that pelvic functional training is benecial for improving low back pain (LBP) in patients with lumbar disc herniation (LDH) after lumbar surgery and that fat inltration of the lumbar multidus muscles (LMMs) is one of the most important reasons for residual LBP after surgery. However, little is known about the exact relationship among residual LBP, spinopelvic parameters and the area of fat inltration in LMM after lumbar surgery. This study aimed to conrm the relationship among residual LBP, spinopelvic parameters and the area of fat inltration in LMM and to investigate why pelvic functional training can relieve pain symptoms in patients with LDH after lumbar surgery. Methods: One hundred forty-three patients with LDH were involved in this study. Clinical data were collected from a system of digital medical records, including age, gender, course, and weight. On the MRI images, the cross-sectional areas (CSAs) of bilateral fat inltration in the LMM were measured using a picture archiving and communication system (PACS). On the X-ray, sacral slope (SS), pelvic tilt (PT) and pelvic incidence (PI) were also measured by PACS. Pearson correlation analysis was applied to analyse the differences between CSA of fat inltration in LMM and spinopelvic parameters, and ROC curves were used to reect the degree of fat inltration in LMM with spinopelvic parameters. Results: One hundred and twenty-ve patients met the inclusion criteria. SS and PI were positively correlated with CSA of fat inltration in LMM at L3-4 and L4-5 (p < 0.01). At L4-5, SS and PI demonstrated signicant positive correlation with the CSA of fat inltration in the LMM (0.5 < | r | < 0.8). PI also exhibited a signicant positive correlation with VAS (0.5 < | r | < 0.8), but SS had a low correlation with VAS (0.3 < | r | < 0.5). At L4-5, only the PI had a signicant ROC

Conclusions: The wider pelvic anterior tilt, the more severe fat the in ltration in LMM. Residual LBP can be relieved by spinopelvic correction training potentially due to the improvement of fat in ltration in LMM.

Background
Despite advances in spinal surgery including endoscopic surgery for lumbar disc herniation (LDH), recent studies on postoperative low back pain (LBP) have reported high rates of persistent opioid use many years after lumbar surgery [1] , and postoperative pain heavily lowers the quality of life of patients [2] [3] .
One in vitro study attributed this improvement achieved by spinopelvic exercise to activation of the endogenous anti-in ammatory cytokine interleukin 10 in the spinal cord [10] . However, little is known about the musculoskeletal reason why postoperative LBP can be relieved by spinopelvic rehabilitation.
Paraspinal muscles are inevitably affected by channel establishment during lumbar surgery [11] . As the most important paraspinal deep muscle, the lumbar multi dus muscle (LMM) reinforces lumbar lordosis during rotation, antagonize lumbar exion and provides segmental stabilization and proprioception to the lumbar spine [12] . More importantly, LMM is thought to be responsible for remnants of LBP after surgery [13] . Additionally, unlike posttraumatic muscle wastage for phasic muscles, fat in ltration always occurs in the LMM after muscle bres are damaged [14] . Based on the understanding of the above literature analysis, it is worth thinking about whether the effectiveness of spinopelvic rehabilitation relates to the improvement of fat in ltration in LMM for patients with postoperative LBP. Therefore, the aim of this study was to determine the relationship among residual pain, spinopelvic parameters and the area of fat in ltration in LMM for patients after lumbar surgery.

Participants
This study retrospectively followed 143 patients with LDH who initially presented to our clinic in the Pain and Rehabilitation Medicine Centre of The Six Medical Center of Chinese PLA General Hospital for PTED between March 2019 and March 2020. In all, 18 cases were excluded for the following reasons: 13 could not show complete spinopelvic parameters on X-ray, 3 had no records of magnetic resonance imaging (MRI), and 2 had diagnoses that were not in accordance with the description of medical records. Inclusion criteria were as follows: (1) patients met the diagnostic criteria of LDH according to the medical records and nal diagnosis; (2) preoperative disc herniation at the following disc levels: L3-4, L4-5 and L5-S1; and Demographic details, course and clinical data including sacral slope (SS), pelvic tilt (PT), pelvic incidence (PI) and cross-sectional areas (CSA) of bilateral fat in ltration in LMM were recorded from patients' medical records. For details, see Table 1. MRI and X-ray methods Three spinal doctors who had been trained professionally and were quali ed were responsible for collecting X-ray and MRI imaging data and for data processing. Lumbar MRI was performed used The abovementioned methods were referred to in the study by Kjaer [15] and Q Li [16] , and there is also evidence that this measure of fat in ltration in LMM, which uses MRI images, is reliable [17] [18] .
Spinopelvic parameters on X-ray were measured with the angle measure module in PACS. Speci c methods were as follows: SS, the angle between the superior plate of S1 and a horizontal line; PT, the angle between the sagittal pelvic thickness line and a vertical line through the femoral head; PI, the angle between a line perpendicular to the sacral plate at its midpoint and a line connecting the same point to the centre of the bicoxofemoral axis. For details see Fig. 2.
To reduce the error of measurement, all the mean values were calculated from the three physicians mentioned above.

Statistical analysis
Statistical analyses of the ndings were performed with the SPSS for Windows v.10.0 software program.
Descriptive statistics were analysed for clinical data results. The relationships among spinopelvic parameters, VAS and CSA of fat in ltration in LMM were analysed based on the Pearson correlation coe cient. To examine the predictive effect of spinopelvic parameters on the degree of fat in ltration in LMM, continuous variables were converted into dichotomous variables. For the degree of fat in ltration in LMM, the CSA of fat in ltration in LMM was dichotomized into mild and severe (< 50% vs ≥50%) based on the fat percentage in LMM described by Hildebrandt [13] (Fig. 3). Finally, ROC curves were utilized to determine the optimal cut-off point for spinopelvic parameters in predicting the degree of fat in ltration in LMM. Statistical signi cance was accepted as p < 0.05.

Results
One hundred and twenty-ve patients with LDH met the inclusion criteria.   Table 2 for details.  (Fig. 4). However, at L3-4 and L5-S1, the AUCs of the ROC curves were all < 0.7 for SS, PT and PI ( Fig. 4 and Table 3).  [19] . It is pragmatically classi ed as either nonspeci c or speci c by the German National Disease Management Guideline [20] , and residual LBP after surgery is often considered nonspeci c pain because there are no clear causal relationships among the symptoms, orthopaedic physical examination and imaging ndings in most cases. By de nition, residual LBP does not have a clear pathoanatomical cause after responsible focuses are removed by operation. Therefore, there are no speci c treatments that can be provided, and the advice for residual LBP management is quite similar between various countries, for example, nonsteroidal anti-in ammatory drugs, manual therapy, acupuncture, and physiotherapy [21] . However, this routine therapeutic schedule does not conform to the principle of precision medicine due to unknown pathogenetic causes.
Studies have con rmed that residual LBP after surgery can be improved signi cantly by functional exercise, including pelvic exercise [21] . Tatsumi et al believed that the incidence of LBP was remarkably correlated with anterior tilt of the pelvis [22] , and Hasebe et al reported that LBP was improved signi cantly after spinopelvic alignment was corrected by dynamic stretching on some phasic muscles [23] . In addition, it has been reported that a larger anterior pelvic tilt during gait loading may affect the aggravation of LBP by gait loading [24] . According to the above, we know that although the correlation between spinopelvic parameters and LBP has been con rmed by many studies, the speci c reason remains unclear. It is worth noting that lumbar multi dus muscles (LMMs) are considered to be the most important muscle for lumbar segmental stability [25] , and LBP is related to fat in ltration in LMMs [26] . However, fat in ltration may be an important feature of degenerating LMMs that are affected in lumbar operations [27] . Therefore, we assumed that residual LBP after surgery may be relieved by correcting spinopelvic alignment through improvement of fat in ltration of LMM, and it is worth studying the association among residual pain, spinopelvic parameters and area of fat in ltration in LMM.
In this study, PI had a signi cant positive correlation with VAS for LBP with correlation coe cient value of 0.569. At L4-5, SS and PI all had signi cant positive correlations with CSA of fat in ltration in LMM with correlation coe cient values of 0.582 and 0.694, respectively, for patients with residual LBP after surgery.
Thus, the larger SS/PI, the larger CSA of fat in ltration. In other words, according to the conclusion of a positive association between PI and pelvic anterior tilt embraced by Diebo [28] , the wider pelvic anterior tilt, the more severe the fat in ltration in LMM. Moreover, to further observe the relationship between spinopelvic parameters and fat in ltration in LMM, we found that PI exhibited a signi cant ROC curve with an AUC value of 0.836 for CSA of fat in ltration in LMM at L4-5, which means that PI can also to some extent re ect the degree of fat in ltration in LMM at certain lumbar levels. This result might explain why some studies found that a smaller anterior lumbar tilt and a larger anterior pelvic tilt affect the aggravation of LBP [24][29] [30] , and we believe that fat in ltration in LMM has a bridge effect.
There were several limitations in this study. The rst is that time factors and precedence relationships were not included because this is a cross-sectional study. Second, spinopelvic parameters were measured only in the sagittal plane, and rotation, scoliosis and lateral tilt of the pelvis were not taken into account. Furthermore, the approach may need to be different for males and females in future studies because there may be gender differences in the CSA of LMM according to Hides et al [31] . The above factors are essential for further evaluation.

Conclusions
The study investigated whether spinopelvic parameters are related to the degree of residual LBP and CSA of fat in ltration in LMM for patients with postoperative LBP. Our ndings suggest that at L4-5, PI had a signi cant positive correlation with VAS for LBP and CSA of fat in ltration in LMM and may also re ect the degree of fat in ltration in LMM. Therefore, residual LBP is potentially relieved by spinopelvic correction training based on the improvement of fat in ltration in LMM. We hope this study will enhance the precision of spinal postoperative rehabilitation.