Oblique Fixation from Posterior Corner in Lumbar Spine Through Kambin’s Triangle: A Neuroimaging Anatomic Assessment

Background: Traditional lumbar interbody fusion has many limitations, such as large trauma, severe damage to the normal posterior structure, and long postoperative recovery period. With the advance of minimally invasive surgery and spinal endoscopy, new fusion technologies such as percutaneous endoscopic transforaminal lumbar interbody fusion (PE-TLIF) and endoscopic transforaminal lumbar interbody fusion (Endo-LIF) through Kambin’s triangle with less trauma, less bleeding and faster recovery have been developed. However, nerve root injury and dural tears are important complications, and Kambin’s triangle is not "safe". Moreover, fusion after decompression often requires placement of a 14-mm channel, removal of more articular processes, fixation with posterior percutaneous pedicle screw, and changes of intraoperative position or anesthesia, which are inconvenient. One-stop percutaneous endoscopic transforaminal oblique fixation from posterior corner in lumbar spine overcomes the above limitations, and realizes one-stop decompression, fusion and fixation in a single regular minimally invasive channel. The purpose of this study is to measure the neuroimaging anatomic parameters of the nerves related to oblique fixation from posterior corner in lumbar spine through Kambin’s triangle, to define and evaluate the safe working area in Kambin’s triangle, and to identify the optimal target area for endoscopic fusion and fixation. Methods: Sixty volunteers (27 males and 33 females) underwent lumbar MR examination (VISTA,3D-STIR-TSE Sequence) and the data were uploaded to Philips (Achieva 1.5T MR) workstation. Three working targets (P1, P2, P3) were preset for oblique fixation from posterior corner in lumbar spine. The distances from the working targets to exiting nerve roots and dural sac/traversing nerve roots in the coronal and sagittal planes, and the distances from the exiting roots to the dural sac/traversing nerve roots in the upper and lower endplate planes were measured and statistically analyzed. Results: In L1/2–L5/S1, the P values of paired t -test for the distances (c1,

The distances from exiting nerve roots to dural sac/traversing nerve roots in the upper and lower endplate planes (d1, d2) gradually increased in L1/2-L5/S1 (P<0.0001) and the means of d2 were greater than d1 (P<0.05). There was no statistically significant difference between the left and right sides in the upper and lower endplates in each segment (P=0.26).
In L1/2-L5/S1, the P values of paired t-test for the distances (s1, s2, s3, s4, s5, s6) from the projection points of posterior-inferior (posterior-superior) corner of upper (lower) vertebral body to exiting nerve roots in the sagittal planes passing the targets were all greater than 0.05. There was no statistically significant difference between both sides of the same segment, and thus the mean value was calculated. With the outward shift of the targets in the sagittal planes, s1, s3 and s5 gradually decreased (s1>s3>s5), and the same trend was found for s2, s4 and s6 (s2>s4>s6). The distances gradually increased in each segment from the smallest value in L1/2 to the largest value in L5/S1.

Conclusion:
Kambin's triangle can be used as a working area for oblique fixation from posterior corner in lumbar spine, but the actual safe area is smaller than theoretical prediction. The intersection point between the vertical line from the medial 1/3 of pedicle and the horizontal midline of the posterior margin of intervertebral disc (P2) is an ideal "target" for oblique fixation from posterior corner in lumbar spine. It is neuroanatomically feasible to achieve one-stop complete decompression, fusion, and fixation in a single channel under spinal endoscopy. Further biomechanical studies and clinical trials are needed to determine whether it can be a new option for posterior spinal fusion.

Background
Lumbar interbody fusion surgery has become a standard operation in the treatment of various degenerative lumbar diseases [1], mostly through the posterior approach. Traditional lumbar interbody fusion techniques, such as posterior lumbar interbody fusion, transforaminal lumbar interbody fusion, and oblique lumbar interbody fusion, have been widely used due to complete decompression, firm fixation and confirmed efficacy. However, the destruction of the paraspinal muscle, blood vessels, and nerves during operation has resulted in many postoperative complications [2].
At present, PE-TLIF and Endo-LIF through Kambin's triangle have attracted great interest due to less trauma, less bleeding, faster recovery, higher safety and fewer complications. However, they also suffer from drawbacks of incomplete decompression, steep learning curve, low fusion rate and high radiation risk [3][4][5][6][7]. In particular, injury to the traversing nerve roots and the dural sacs leads to aggravated nerve injury and leakage of cerebrospinal fluid. Fusion after decompression often requires placement of a 14-mm channel, removal of more articular processes, fixation with posterior percutaneous pedicle screw, and changes of intraoperative position or anesthesia, which are inconvenient. One-stop percutaneous endoscopic transforaminal oblique fixation from posterior corner in lumbar spine overcomes the above drawbacks, and realizes one-stop decompression, fusion and fixation in a single regular minimally invasive channel (Fig. 1).
The purpose of this study is to measure the neuroimaging anatomic parameters of the nerves related to oblique fixation from posterior corner in lumbar spine through Kambin's triangle, to verify its anatomical feasibility, to define and evaluate the actual safe working area in Kambin's triangle, and to identify the optimal target area for endoscopic fusion and fixation.

Materials
Sixty normal adult volunteers (22 males and 38 females; 20-75 years of age, mean age 37.8 years; male height 168-182 cm, mean 174.6 cm; female height 157-171 cm, mean 165.3 cm; without lumbar tumors, trauma, deformity or a history of lumbar surgery) underwent MR scan (VISTA,3D-STIR-TSE Sequence) of the lumbar spine (Philips Achieva 1.5T MR) in the East Hospital of Shandong Provincial Hospital from December 1, 2019 to February 29, 2020. Scan data were transmitted to the workstation for measurement.

Data Measurement
In oblique fixation from posterior corner in lumbar spine, three points were preset as targets to measure the distances from each target to exiting nerve roots and dural sac/traversing nerve roots, to select the ideal "target", and to indirectly define the safe operation area of oblique fixation from posterior corner in lumbar spine. The three targets were: Data to be measured in the coronary plane: (Figs. 2, 3) c1: the distance from P1 to ipsilateral exiting nerve roots c2: the distance from P1 to ipsilateral dural sac/traversing nerve roots c3: the distance from P2 to ipsilateral exiting nerve roots c4: the distance from P2 to ipsilateral dural sac/traversing nerve roots c5: the distance from P3 to ipsilateral exiting nerve roots c6: the distance from P3 to ipsilateral dural sac/traversing nerve roots d1: the distance from exiting nerve roots to dural sac/traversing nerve roots in the upper endplate plane d2: the distance from exiting nerve roots to dural sac/traversing nerve roots in the lower endplate plane

Statistical Methods
The data were compared with independent sample t-test in SPSS 25.0.

Results
Distances from targets to ipsilateral exiting and dural sac/traversing nerve roots in the coronary plane (Table 1)

Distances from targets to ipsilateral exiting and dural sac/traversing nerve roots in the coronary plane.
In L1/2-L5/S1, as the targets shifted laterally along the horizontal midline of posterior intervertebral disc, the distances to dural sac/traversing nerve roots gradually increased, while the distances to exiting roots gradually decreased. The distances from P1 to exiting nerve roots were significantly greater than those to dural sac/traversing nerve roots, with differences of 1-3 mm. The distances from P3 to exiting nerve roots were significantly smaller than those to dural sac/traversing nerve roots, with differences of 1-3 mm. However, the distances from P2 to exiting nerve roots and to dural sac/traversing nerve roots did not differ significantly in each segment, and the differences in mean values were within 1 mm. Therefore, we suggest P2 as the optimal target for percutaneous endoscopic transforaminal oblique fixation from posterior corner in lumbar spine.

Distances from posterior-inferior corner of upper vertebral body to exiting nerve roots in the sagittal planes passing the targets
In L1/2-L5/S1, the P values of paired t-test for the distances (s1, s3, s5) from posterior-inferior corner of upper vertebral body to exiting nerve roots on the sagittal plane passing the targets were all greater than 0.05. There was no statistically significant difference between the left and right sides of the same segment, and thus the mean values of both sides were calculated. The s1 values in L1/2-L5/S1 were (2.51±0.24), (2.64±0.59), With the outward shift of the targets in the sagittal plane, s1, s3 and s5 gradually decreased (s1>s3>s5). The values gradually increased in segments, with the smallest value in L1/2 and the largest value in L5/S1. We found that the mean values of s1, s3 and s5 in each segment of L1/2-L5/S1 were greater than 2.50 mm, which allowed placement of 5-mm diameter screws to fix the new cage for lumbar interbody fusion.
Distances from posterior-superior corner of lower vertebral body to exiting nerve roots in the sagittal planes passing the targets In L1/2-L5/S1, the P values of paired t-test for the distances (s2, s4, s6) from posterior-superior corner of lower vertebral body to exiting nerve roots on the sagittal plane passing the targets were all greater than 0.05. There was no statistically significant difference between the left and right sides of the same segment, and thus the mean values of both sides were calculated. The s2 values in L1/2-L5/S1 were ( , s2, s4 and s6 gradually decreased (s2>s4>s6). The values gradually increased in segments, with the smallest value in L1/2 and the largest value in L5/S1. We found that the mean values of s2, s4 and s6 in each segment of L1/2-L5/S1 were greater than the diameter (7.5 mm) of conventional working cannula used in percutaneous transforaminal endoscopic surgery, which allowed placement of lumbar interbody fusion cages with cross-sectional diameters of no less than 7.5mm.

Advantages of Imaging Measurement
Dry bone or cadaver specimen were used for anatomic measurement because of better visibility and accuracy. However, they suffer from limitations such as difficulty in obtaining specimens, insufficient number of specimens, and destruction of normal anatomical structures in specimen processing that affect the accuracy of measurement. In this paper, we adopted the imaging measurement method to successfully overcome the limitations in the measurement with dry bone and cadaver specimen. We also summarized the advantages of imaging measurement: (1) improvement of measurement efficiency, omission of measuring tools and reading data, which reduces the systematic error and random error to some extent; (2) non-invasive measurement, which preserves object integrity and improves the accuracy of measurement; and (3) sufficient objects can be obtained by imaging measurement.

Development, challenge and thinking of lumbar interbody fusion
Since Hibbs et al first reported the stability of spinal fusion surgery in 1911, and the Mercer proposed that the ideal method of spinal fusion was lumbar interbody fusion in 1936, anterior lumbar interbody fusion and posterior lumbar interbody fusion have achieved promising curative effects [8][9][10][11][12][13]. However, they suffer from problems like large trauma, high cost, slow recovery, severe damage to normal structures, and postoperative secondary back pain. With the development of minimally invasive concept, the emergence of transforaminal lumbar interbody fusion, extreme lateral lumbar interbody fusion, oblique lumbar interbody fusion, and minimally invasive transforaminal lumbar interbody fusion achieve limited improvement in the disadvantages and deficiencies of traditional open surgery, despite some advances in soft tissue destruction and blood loss.
At present, PE-TLIF and Endo-LIF through Kambin's triangle have attracted much attention due to less trauma, less bleeding, faster recovery, higher safety, and fewer complications. They also have drawbacks such as incomplete decompression, steep learning curve, low fusion rate, high radiation risk [7], and injury to exiting roots and dural sacs, in particular. Lumbar interbody fusion after decompression often requires the use of posterior percutaneous pedicle screws, additional approach and posterior injury, and changes in intraoperative position or anesthesia. Interbody fixation and fusion are often separated and not completed in one process. We propose that, with the help of percutaneous spinal endoscopy, it is possible to achieve one-stop endoscopic decompression, fusion and fixation in a single minimally invasive channel.
The majority of lumbar interbody fusion surgeries with integrated endoscopy system are performed through the transforaminal approach [14][15][16][17][18]. Anatomical studies have shown that the size of Kambin's triangle in each segment of the lumbar spine is safe for interbody fusion cages [19][20]. The key is how to fully expose and utilize the Kambin's triangle. Lumbar foraminoplasty can remove most of the bone in the superior articular process, fully expose the Kambin's triangle, and provide a safe channel for the placement of fusion cage. However, the place of foraminoplasty should be as far from the exiting nerve roots and as close to the lower half of the intervertebral foramen as possible, so as to obtain sufficient safe space and avoid exiting nerve roots injury [21,22]. Foraminoplasty close to the apex of superior articular process is likely to cause irritation and even damage to exiting nerve roots [15,23]. Ozer et al [24] believed that the anatomic variability of Kambin's triangle may be the main reason for nerve root injury in endoscopic spinal surgery. The Kambin's triangle was divided into three types by measuring 34 patients and 8 cadavers. Type I is a closed triangle and there is no available space between sides; type II is a small triangle; and type III is a normal triangle, as described by Kambin. Only 20.8% Kambin's triangles are type III, in which there are wide spaces between the sides. Nearly half (48%) of Kambin's triangles are type II, in which a narrow space exists in the triangle. Nearly one third (31.2%) Kambin's triangles are type I, in which the triangle is closed and there is no space between the sides. Therefore, the Kambin's triangle is not "safe". For one-stop decompression, fusion and fixation in a single minimally invasive channel through the Kambin's triangle, it is necessary to have a new understanding of Kambin's triangle, and to define and evaluate the actual safe working area in Kambin's triangle.

Significance of the actual safe area of Kambin's triangle
Kambin [25] first proposed the concept of "safe triangle" in 1973, therefore, lumbar "safe triangle" is also known as "Kambin's triangle" (Fig. 6A). Kambin's triangle" is a three-dimensional triangle-like structure surrounded by four boundary lines. The lower boundary is the upper endplate of lower vertebral body, the inner boundary is dural sac/traversing nerve roots, the posterior boundary is facet joint, and the outer-upper boundary is exiting nerve roots. Therefore, Kambin's triangle is an irregular wedge-shaped three-dimensional structure, rather than a "triangle". Kambin's triangle was described as a triangular zone surrounded by the upper endplate of lower vertebral body, dural sac/traversing nerve roots, and exiting nerve roots in literature [26][27]. Luis M et al [28] pointed out that Kambin's triangle is not a two-dimensional but a three-dimensional structure, and should not be simply called triangle, and a more appropriate description is "expanded transforaminal corridor", especially in the removal of articular process, pars interarticularis, and lamina. (Fig. 6B) For decades, Kambin's triangle has been considered to be a relatively safe working area [29]. Spinal surgeons have successively reported transforaminal lumbar interbody fusion, minimally invasive transforaminal lumbar interbody fusion, and endoscopic translaminar lumbar interbody fusion, which are lumbar interbody fusion through Kambin's triangle with safe pass and sufficient operating space [30][31][32][33].
Hoshide R et al [34] studied the inner area of lumbar Kambin's triangle. The areas in L1/2-L5/S1 were (66.82 ± 13.89), (90.95 ± 21.29), (127.07 ± 33.43), (167.99 ± 27.64), and (153.75 ± 31.91) mm 2 , respectively, which are all larger than the cross-sectional area (44.16 mm 2 ) of conventional working cannula (7.5 mm diameter) used in transforaminal endoscopic surgery. Since the cross-section of working cannula is circular, the corner of the triangle cannot be regarded as "operable" area, that is, there is a "invalid area" in Kambin's triangle. Therefore, the distances from the preset targets to the exiting nerve roots and the dural sac/traversing nerve roots can more accurately guide the selection of working cannula size and the design of new lumbar interbody fusion cage (Fig. 7), and can indirectly define the working "target" and safe area in one-stop percutaneous endoscopic transforaminal oblique fixation from posterior corner in lumbar spine (Figs. 6C, 6D). Theoretically, the positions where the 7.5-mm diameter cannula is tangent to the exiting nerve roots and the dural sac/traversing nerve roots in conventional transforaminal endoscopic surgery define the boundary of the actual safe area of Kambin's triangle. This is demonstrated by the green area in Fig. 8: the inner boundary is dural sac/traversing nerve roots, the outer-upper boundary is exiting nerve roots, the inner-upper boundary is the bone at the posterior-inferior corner of vertebral body, the outer-lower boundary is the outer surface of pedicle, and the inner-lower boundary is the bone at the posterior-superior corner of the vertebral body.

The ideal "target" for percutaneous endoscopic transforaminal oblique fixation from posterior corner in lumbar spine
In this paper, we measured the distances from the three preset targets to exiting nerve roots and dural sac/traversing nerve roots, as well as the distances from the exiting roots to the dural sac/traversing nerve roots in the upper and lower endplate planes in the coronal and sagittal planes of MRI images. We found that, except for P3 in L1/2, the mean distances from each target to the exiting and dural sac/traversing nerve roots in each segment of L1/2-L5/S1 were greater than the radius (3.75 mm) of conventional working cannula used in percutaneous endoscopic transforaminal surgery, which allows the placement of interbody fusion cages with cross-sectional diameters of no less than 7.5 mm. The distances from P1 to exiting nerve roots were significantly greater than those to the dural sac/traversing nerve roots (differences of 1-3 mm). The distances from P3 to exiting nerve roots were significantly smaller than those to the dural sac/traversing nerve roots (differences of 1-3 mm). However, the distances from P2 to exiting nerve roots and to dural sac/traversing nerve roots were similar in each segment (mean differences within 1 mm). Hence, P2 is the ideal "target" for percutaneous endoscopic transforaminal oblique fixation from posterior corner in lumbar spine, and the intersections between the vertical line 1/3 lateral from pedicle medial wall and the upper/lower endplates are the optimal points for screw placement.
In clinical practice of transforaminal endoscopic spine system (TESSYS), cannula was not introduced through the small external opening and corridor of Kambin's triangle. Instead, lumbar foraminoplasty was performed to grind the bone of superior articular process. Cannula was inserted directly through the enlarged foramen to the inner opening of Kambin's triangle. Larger working cannula can be used in segments to facilitate surgical operation, and the puncture point was far away from exiting nerve roots to reduce damage to exiting nerve roots and ganglions. Moreover, herniated and free nucleus pulposus can be removed, which is impossible with Yeung endoscopic spine system (YESS). Theoretically, the "optimal target or safe area" may shift inward in clinical practice due to Foraminoplasty, and contralateral decompression was reported by some spinal surgeons [35,36]. We believe that one-stop complete decompression, fusion, and fixation in a single channel can be achieved in the near future with the development of spinal endoscopy technology and the emergence of minimally invasive integrated interbody fusion cages.

Further Research
Comparison between imaging measurement and dry bone measurement and correlation analysis between each index and body length will be performed in subsequent studies. In addition, objective evaluation of bearing capacity by biomechanical studies of compressive stress, stretch stress, torsion stress and fatigue load between lumbar vertebral bodies should be performed before the preparation of new endoscopic fusion cages. The established cage model matching with lumber vertebral body will be subjected to three-dimensional finite element analysis. Finally, the feasibility of one-stop percutaneous endoscopic transforaminal oblique fixation from posterior corner in lumbar spine will be preliminarily determined.

Conclusions
The intersection point between the vertical line from the medial 1/3 of pedicle and the horizontal midline of the posterior margin of intervertebral disc (P2) is an ideal "target" for oblique fixation from posterior corner in lumbar spine. Kambin's triangle is not "safe". The actual safe area is smaller than theoretical prediction. The inner boundary is dural sac/traversing nerve roots, the outer-upper boundary is exiting nerve roots, the innerupper boundary is the bone at the posterior-inferior corner of vertebral body, the outer-lower boundary is the outer surface of pedicle, and the inner-lower boundary is the bone at the posterior-superior corner of the vertebral body. The study shows that it is neuroanatomically feasible to perform one-stop complete decompression, fusion, and fixation in a single channel under spinal endoscopy. Further biomechanical studies and clinical trials are needed to determine whether oblique fixation from posterior corner in lumbar spine through Kambin's triangle can be a new option for posterior spinal fusion. *p>0.05, there was no statistically significant difference between the two sides of each target in the same segment. *p>0.05, there was no statistically significant difference between the two sides of each target in the same segment. Table 3. Distances from posterior-inferior (posterior-superior) corner of upper (lower) vertebral body to exiting nerve roots on the sagittal planes passing the targets (x± s, n=60, mm) *p>0.05, there was no statistically significant difference between the two sides of each target in the same segment. † p<0.05, s2 in L5/S1 was statistically significant with other segments. Figure 1 The working sketch of one-stop endoscopic fusion and fixation Measurement schematic diagram (c1, c2, c3, c4, c5, c6, d1, d2) Actual measurements (c1, c2, c3, c4, c5, c6, d1, d2). A: Coronal image of lumbar spine in MRI; B: Data to be measured in A Figure 4 Measurement schematic diagrams1, s2, s3, s4, s5, s6 Actual measurements (s1, s2, s3, s4, s5, s6). A: P1, P2, P3 in axial plane. B: s1, s2 to be measured .C: s3, s4 to be measured. D: s5, s6 to be measured Actual work safety area in one-stop percutaneous endoscopic transforaminal oblique fixation from posterior corner in lumbar spine