Abstract: BACKGROUND: Unilateral biportal endoscopy can more directly and clearly identify tissue structure characteristics at close range, while indirect X-ray fluoroscopy is difficult to locate the dura mater, nerve roots and intervertebral disc.
OBJECTIVE: To investigate the relationship between L5 and S1 nerve roots and intervertebral space under unilateral biportal endoscopy, and to provide theoretical basis for the treatment of lumbar spinal stenosis with unilateral biportal endoscopy.
METHODS: Totally 29 patients with lumbar spinal stenosis underwent lumbar CT myelography. The images were imported into Mimics 17.0 software to establish a three-dimensional model. The relevant parameters of L4/5 and L5S1 levels were measured: The distance from the intersection of the lower edge of upper lumbar lamina and the medial edge of the base of the lower articular process (point A) to the lateral edge of the dural membrane (a1), the upper edge of the starting nerve root (a2), the lower endplate of upper lumbar spine (a3), and the upper endplate of lower lumbar (or sacral) vertebrae (a4); the distance from the intersection of the upper edge of the lower lumbar (or sacral) vertebral lamina and the medial edge of the base of the superior articular process (point B) to the lateral edge of the dural membrane (b1), the upper edge of the nerve root origin (b2), the lower endplate of the upper lumbar spine (b3), and the upper endplate of the lower lumbar (or sacral) vertebral body (b4); the distance from the intersection point of the medial edge of the inferior articular process of the upper lumbar spine to the medial edge of the inferior lumbar (or sacral) superior articular process (point C) to the lateral edge of the dural membrane (c1), the upper edge of the nerve root origin (c2), the lower endplate of the upper lumbar spine (c3), and the upper endplate of the lower lumbar (or sacral) vertebral body (c4); the distance from the intersection of the upper lumbar spinous process and the lower edge of the lamina (point D) to the lateral edge of the dural membrane (d1); the distance from the upper edge of the nerve root to the upper endplate of the lumbar spine (n1) and the upper endplate of the lumbar spine (or sacral) (n2). The position relationship between the triangle area (C area) surrounded by A, B and C and nerve roots and intervertebral space was observed. All 29 patients underwent unilateral biportal endoscopic lumbar decompression, and were evaluated by visual analogue pain score, Oswestry disability index, and Japanese Orthopaedic Association evaluation score. The parameters were analyzed and their reliability was verified clinically. 
RESULTS AND CONCLUSION: (1) There was no significant difference in the relationship between the affected side and the healthy side in the same segment (P > 0.05). There were significant differences between A, B and C of L4/5 and L5S1 and the distance between the lateral dural edge and intervertebral space (P < 0.05) and had no significant difference with the distance from the upper edge of nerve root origin (P > 0.05). The distance between the upper edge of L5 and S1 nerve root and the intervertebral space of the corresponding segment was significantly different (P < 0.05). (2) In L4/5 and L5S1 segments, the projection of point A was mostly near the upper and lower endplates of L5. AB line projection was located near and outside the lateral edge of the dural membrane. (3) The level of intervertebral space in L4/5 segment was mostly above C area, above point A; L5S1 segments were all in region C, below point A, and most of them corresponded to the intervertebral space at the level of point C. (4) Most of L5 at the upper edge of nerve root origin was projected at the level of L4/5 intervertebral space, while S1 was projected above L5S1 intervertebral space. (5) The observed results were consistent with the intraoperative findings. (6) The obtained data can guide the positioning of unilateral biportal endoscopy, and the constant bone markers are exposed under unilateral biportal endoscopy microscope. At the L4/5 segment, above point A is the intervertebral space. The herniated disc is explored and removed, and the starting point of L5 nerve root is found at the intervertebral space level, and the nerve root canal is decompressed along the nerve alignment. At L5S1, point C was located at the intervertebral space level, and the disc was explored from this level. A window was opened slightly above point A to find the origin of S1 nerve root and decompress the nerve outward and downward. The location of intervertebral space and nerve root was determined by each marker point, and the scope of decompression was determined to make the operation more safe, accurate, effective, and efficient.

Key words: lumbar spine, spinal stenosis, unilateral biportal endoscopic technique, digital orthopedics, three-dimensional reconstruction, morphological parameter