A Magnatic Resonance Imaging Morphometric Study of Lumbar Epidural Fat Distribution: Implications for Lumbar Epidural Lipomatosis

Background. To the best of our knowledge, no published English literatures has provided detailed parameters about the normal epidural fat and other contents in lumbar spinal canal. Our objective was to quantify reference data of epidural fat and the contents of lumbar spinal canal to guide the diagnosis of lumbar epidural lipomatosis. Methods. Results. the mean anteroposterior ( AP ) diameters ± standard deviation(SD) of lumbar vertebral body ( V ) (cid:0) dural sac ( DS ) (cid:0) epidural fat ( EF ) each measured lever on the mid-sagittal MRI and the mean cross-sectional area ± SD of lumbar spinal canal ( SC ) (cid:0) DS of each measured lever on the axial MRI were showed. The mean AP diameters of V and DS are showed obvious signicant difference between men and women ( P (cid:0) 0.05). The mean AP diameters of EF is showed no signicant difference between men and women (P (cid:0) 0.05). Also The mean area of lumbar SC (male 316.7 mm2, female 306.4 mm2 ) and DS (male 198.6 mm2, female 189.2 mm2) are showed obvious signicant difference between men and women ( P (cid:0) 0.05). The growth trend of the thickness of epidural fat in lumbar spinal canal is showed. Conclusion. Our investigation provides insight into the anatomy of epidural fat and gives the relevant parameters of lumbar spinal canal and its contents on MRI. MRI is the most sensitive imaging test to diagnose lumbar epidural

) dural sac ( DS ) epidural fat ( EF ) each measured lever on the mid-sagittal MRI and the mean crosssectional area ± SD of lumbar spinal canal ( SC ) DS of each measured lever on the axial MRI were showed. The mean AP diameters of V and DS are showed obvious signi cant difference between men and women ( P 0.05). The mean AP diameters of EF is showed no signi cant difference between men and women (P 0.05). Also The mean area of lumbar SC (male 316.7 mm2, female 306.4 mm2 ) and DS (male 198.6 mm2, female 189.2 mm2) are showed obvious signi cant difference between men and women ( P 0.05). The growth trend of the thickness of epidural fat in lumbar spinal canal is showed.
Conclusion. Our investigation provides insight into the anatomy of epidural fat and gives the relevant parameters of lumbar spinal canal and its contents on MRI. MRI is the most sensitive imaging test to diagnose lumbar epidural lipomatosis Background Spinal epidural lipomatosis (SEL) is a well-recognized and complex condition caused by the excessively accumulated fat in spinal epidural space. Since Lee et al. 1 rst mentioned this disease in 1975, an increasing number of SEL patients were diagnosed. SEL is usually composed by idiopathic SEL and secondary SEL, the secondary SEL affects the thoracic spine more frequently than it affects the lumbar region, whereas idiopathic SEL most often occurs in the lumbar region. 2 Secondary SEL is often associated with chronic steroid therapy and endocrinopathies. Idiopathic SEL has been associated with obesity 3,4 which representing about 24.5% of reported cases 5 . Also with the development of radiological technology and the increasing number of overweight and obese patients 6 , the number of SEL patients would have an obvious augment. The clinical manifestations of SEL include back pain, lower extremities weakness, paresthesias, ataxia, and less frequent bowel or urinary incontinence, 7 reduced the patients' life quality distinctly. As such, this disease has been taken a high attention in recent years.
The pathogenesis of SEL is still unclear. As we known, epidural fat is an important part lled in spinal epidural space and distributed along the spinal canal, it protects dura and its contents from the effects of detrimental forces in icted on the vertebral column 8 . However, if the epidural fat enlarges abnormally, it would encroach on the spinal canal and compresses the neural elements, and this process can be clearly reacted on MRI [9][10][11] . As mentioned in published studies, SEL is usually occurred in thoracic spinal canal (thoracic epidural lipomatosis TEL) and lumbar spinal canal (lumbar epidural lipomatosis LEL), extremely rare in cervical spinal canal (cervical epidural lipomatosis CEL) 12 . In consideration of the greater space in lumbar spinal canal than in thoracic spinal canal,the percentage of epidural fat in TEL and LEL must be obviously different. But how much the threshold value is can we make a diagnosis for this disease form a MRI image? Many experts [13][14][15] had done some correlation studies, but few of them gave detailed parameters about the thoracic or lumbar spinal canal and its contents.
In this study, we aim to make a morphometric study to specify a more visualized and accurate measurement for the distribution of lumbar epidural fat. We think those data will give a higher comprehension degree of LEL to spine surgeons.

Subjects
This study was approved by the institutional review board of Huzhou Central Hospital. Retrospective analysis of the Chinese patients who presented to the department of orthopedics at our institution between January 23, 2019 and January 23, 2020, requiring lumbar spine MRI scans. For this study, about 112343 patients were taken lumbar MRI examination as a result of medical examination trauma backache or any other complaint requiring lumbar investigation, our selection criteria were as follows: 1. normal lumbar MRI without disease like spondylitis, diskitis, disk herniation, epidural hematoma, extradural tumor, pathologic fracture, previous lumbosacral surgery or radiation therapy and any other structural lesions; 2. age is between 20 and 60-year-old; 3. other conditions which may in uence the measuring precision and accuracy. Finally, we found out 178 content lumbar MRI cases.

MRI procedure
The MRI examinations were performed using a 1.5-T superconducting magnet. The following sequences were investigated, focusing on the lumbar spine:

Measurements and indexes
In order to establish a reproducible measurement of the lumbar spinal canal ( SC ) and its contents for each image, we measure the anteroposterior ( AP ) diameters of lumbar vertebral body ( V ) dural sac ( DS ) epidural fat ( EF ) on the mid-sagittal MRI scanning and the cross-sectional area of lumbar SC and DS on the axial plane.

Statistics
Graphical representation of our data were created using SPSS statistical software program 22.0 (SPSS Inc). The means ± SD and Standard's t test (two tailed t-test, The signi cance level α=5%) were calculated for all parameters.

Results
In our 178 cases, the mean AP diameters± SD of V DS EF of each measured lever on the mid-sagittal MRI were showed in Fig 3. As the parameters showed the mean AP diameters ± SD of V DS EF are about 29.5 The growth trend of the mean AP diameters of EF and SC the in lumbar spinal canal is showed in Fig 5. Discussion LEL is characterized by increased deposition of normal adipose tissue in the extradural space and created a direct mechanical compression to the dural sac. 7 The clinical manifestations of this disease depend greatly on the level of canal compromise. 16,17 Different from TEL, most of the generating process of LEL (especially the early stage of LEL) is silently and easy to ignore because of the greater space of the lumbar spinal canal where epidural fat need a signi cant growth to give an enough stress to the dural sac to cause medical symptoms 18 . So the relevant parameters of normal lumbar spinal canal and its contents need to be clear. For the published studies, Quint et al. 15 measured the data in MR images of 28 normal control patients and found a mean sagittal epidural fat thickness of 4.6 mm (range 3-6 mm) at T7 lever. Borre et al. 14 measured MR images in 2528 patients at S1 lever and gave a MRI grading of LEL. However, as we known, few of English literatures gave detailed parameters about the normal epidural fat of lumbar spinal canal. We did this study and wanted to update the relevant parameters of lumbar epidural fat and contents in lumbar spinal canal to get a higher comprehension degree to LEL.
The diagnosis of LEL needs clinical symptoms, imaging studies and surgical evaluation, particularly the imaging studies. MRI is supposed to the most sensitive and speci c tool to assess fatty tissue 19 . LEL cannot be diagnosed on plain x-ray images, but it can help rule out other causes like degenerative disease or tumors. Myelogramsand CT scans were used to diagnose LEL before the MRI appeared, myelography is helpful to distinguish the intradural or extradural obstruction at the level of the canal compression. CT scans can differentiate the cause of the compression based on the density of the tissue in the epidural space as adipose tissue has a density ranging between − 80 and − 120 HU on CT scans. But some experts consider the two methods were not speci c for SEL and could be misleading 20 . MRI allows excellent resolution of cerebrospinal uid, spinal cord, and nerve roots, especially EF which has a pathognomonic appearance on MRI ( increased signal intensity on T1-weighted images, and intermediate signal intensity on T2-weighted images ) 21 . This advantage made MRI an excellent tool to measure the fat thickness in both axial and sagittal planes and obviate the need of myelography 19 . To improve the repeatability and reliability of this study in our measure method, we chose the MRI image.
Measurement of epidural fat thickness in the mid-sagittal plane is considered a proper means to evaluate epidural fat in the spine. 2 Recent years, a three-round Delphi survey was made to develop a list of radiologic criteria for describing lumbar spinal stenosis and found that only the AP diameter was considered an established and important criterion with a high level of agreement 22 . To get more exact data of the lumbar spinal canal and its contents, we measured the relevant data of AP diameter in midsagittal MRI image. Also in consideration of the "Y-sign" which is very characteristic and is only found on axial MR imaging of LEL, we measured the cross-sectional area of normal lumbar SC and DS (the area of EF is disrupt and the shape is irregular in axial MR imaging which made the accurate data di cult to measure, so in this study we didn't measure the parameters.) From the results of our study, the epidural fat presented along the lumbar spinal canal undulately. The LEL is a rare and complex condition that can lead to severe physical and functional limitation. With the testing technology updating and the deeper understanding to this disease, patients would be increased obviously in future. As such, the anatomy parameters of lumbar epidural fat need more concern, the MRI test is necessary. However, the results of this investigation also should be kept within the context of its limitations. Firstly, the factor of race was not considered. The variation of lumbar epidural fat thickness in this study might, at least in part, originate from this diversity. Secondly, the sample size of this study is limited. Thirdly, our study was limited to a MRI morphometric analysis rather than a cadaveric study.

Conclusion
In conclusion, our investigation provides insight into the anatomy of epidural fat in lumbar spinal canal and gives the relevant parameters of and its contents on MRI. MRI is the most sensitive imaging test to diagnose LEL.