Abstract: BACKGROUND: There are few articles on the treatment of lumbar spinal stenosis by unilateral two-channel endoscopic contralateral approach, and there is no in-depth analysis of the endoscopic location, decompression scope and adjacent relationship of various structures of this approach.  
OBJECTIVE: To observe the bony structure, nerve tissue, intervertebral space, attachment of ligamentum flavum and position of lumbar isthmus in lateral recess and foraminal area through three-dimensional CT imaging.
METHODS:  Totally 34 patients with lumbar spinal stenosis admitted to the Affiliated Hospital of Binzhou Medical College from June 2019 to July 2021 were enrolled. Lumbar CT myelography was performed before surgery, and the images were imported into Mimics 21.0 software to reconstruct lumbar three-dimensional CT model. Relevant parameters of L4/5 and L5S1 segments were measured: (1) The vertical distance from the intersection of the lumbar spinous process and the lower edge of the lamina (Q) to the lower edge of the contralateral same lumbar pedicle (a), the upper edge of the contralateral pedicle of the lower lumbar spine (b), the lower endplate of same lumbar (c), the upper endplate of lower lumbar vertebrae (d); (2) The vertical distance from superior articular process to the upper edge of same lumbar pedicle (e), the lower endplate of upper lumbar vertebrae (f); (3) The vertical distance from the lower endplate of lumbar vertebrae to the lower edge of the contralateral lumbar pedicle (g); (4) The vertical distance from the upper endplate of lumbar vertebrae to the upper edge of the same lumbar pedicle (h); (5) The vertical distance from the lower edge of the nerve root origin to the lower edge of the upper lumbar pedicle (i), the upper edge of the lower lumbar spine pedicle (j); (6) A 3 mm diameter guide rod was established through point Q and the lower edge of the contralateral same lumbar pedicle to measure the abduction angle of guide rod (k). Seven patients underwent unilateral biportal endoscopic lumbar decompression through the contralateral approach. The effect of surgery was verified by visual analog scale score, Oswestry disability index, and short form-36 health survey.  
RESULTS AND CONCLUSION: (1) The Q points and the upper margin of ligamentum flavum could be used as important localization markers under the endoscopy. The upper margin of the ligamentum flavum was used to locate the exiting nerve root, the upper edge of the foramina, the lumbar isthmus and the inferior articular process. The level of 1/3 of the contralateral foramen of L4/5 and the vicinity of L4 inferior endplate could be reached by Q point perpendicular to the posterior midline of spinous process, and then decompress the lateral recess at the level of the disc. (2) The lower edge of L5 nerve root origin was mostly projected at the level of L4/5 lower 1/3 of the foramen, and it was consistent with the L4/5 intervertebral projection, suggesting that the L5 walking nerve roots were mostly compressed near the level of the L4/5 disc. (3) The Q point, the lower edge of S1 nerve root origin and the L5 the exiting nerve root all corresponded to the level of the upper 1/3 of the L5S1 intervertebral foramen, and the corresponding lateral recess with dense nerve tissue in this area should be prudently decompressed. (4) The intervertebral spaces of L4/5 and L5S1 were projected near the level of the lower 1/3 of the same segment intervertebral foramen and under the Q point. The operation channel should be decompressed through Q point level with moderate tail tilt.  (5) When removing the hyperplastic superior facet osteophytes of L4/5 and L5S1, the superior facet should be retained at approximately normal height to avoid affecting the stability of lumbar spine. (6) All seven patients with contralateral approach surgery successfully completed the operation, and the follow-up time was 6-12 months. With the prolongation of postoperative rehabilitation time, the visual analog scale score and Oswestry disability index were significantly lower than those before surgery, and the score of short form-36 health survey was significantly improved than that before surgery. (7) It is suggested that the results of this study can guide unilateral biportal endoscopy for lumbar spinal stenosis through contralateral approach.

Key words: lumbar spinal stenosis, unilateral biportal endoscopy, three-dimensional CT, contralateral approach, mimics, lumber spinal canal decompression