The advantages of imaging measurement
Dry bone or specimen were used for anatomic measurement in the past because of its visibility and accuracy. However, there are certain limitations, such as the difficulty of obtaining specimens and the insufficient number of specimens, and in the process of handling specimens, the normal anatomical structures are easily destroyed, which affects the authenticity of measurement data, etc. In this paper, the author adopted the method of imaging measurement, and successfully solved the limitation in the measurement of dry bone and cadaver specimen. The author summarizes several advantages of imaging measurement: (1) it improves the measurement efficiency. Meanwhile, it omits the process of measuring tool measurement and the reader reading the measurement value, which reduces the systematic error and accidental error to some extent.(2) imaging measurement does not destroy the measuring object, which preserves its integrity and improves the accuracy of the measurement result to some extent.(3) we could obtain enough objects through imaging measurement.
Development, Change and Limitation of Spinal Fixation Technology
The clinical application of pedicle screw fixation technique is a significant development in spinal surgery. The fusion and fixation of the lumbosacral vertebra using screws from the pedicle to the vertebral body, which was first applied by Boucher in 1959, have achieved good clinical results . Steffee and Roy-Camille et al reported that pedicle screw technique for posterior short-segment internal fixation of spine obtained good results in 1986 [6, 7]. The advantages of pedicle screw rod spinal fixation system, which can simultaneously fix the three columns and reconstruct the stability of spine in a timely and reliable manner, have been gradually popularized in clinical application. In the past ten years, pedicle screw technology has developed rapidly and gradually been used in the fixation of thoracic and even cervical vertebrae. The application of computer navigation system has greatly improved the accuracy of screw implantation . The research and clinical application of percutaneous pedicle screw surgery has also begun .
On account of complex, important anatomy structure and limited exposure in the back of spine, there are lots of difficulties in accurate positioning and screw setting. So it is easy to damage the important surrounding structures due to slightly careless. Incorrect screw placement may perforate the canal wall of the pedicle, damage the spinal cord, nerve roots, or break through the anterior cortex of the vertebral body, damage the anterior major vessels or viscera with disastrous consequences.
The stability of PS fixation declines in patients with osteoporosis, who was losing a large amount of bone, so as to affects the surgical effect . Therefore, how to ensure the strength of pedicle screw fixation in patients with osteoporosis has become a challenge in the field of spinal surgery. In order to solve the clinical problem, Cortical bone channel (cortical bone trajectory, CBT) scheme was suggested by Santoni et al in 2009 and applied to the lumbar pedicle nailing . Compared with PS, CBT screw has the characteristics of smaller diameter, shorter length and tighter thread [12, 13]. The screw is placed at an inward and outward tilt to maximize the contact with cortical bone. It has the advantages of strong control, less trauma, fewer complications, less intraoperative bleeding and lower postoperative infection rate [14–16], which has a broad clinical value and application prospect. However, there are still risks and postoperative complications in clinical practice, including failure of internal fixation, pedicle splitting, pseudarthrosis and dural tear. Some scholars [17–19] pointed out that the early complications of CBT screw technology mainly included postoperative screw loosening, intraoperative articular process fracture, dura mater breakage and pedicle splitting.
Translaminar facet joint screw fixation technology is a fixation method setting the screw, who was known as Magerl screw, through the base of the contralateral spinous process, contralateral lamina, articular facet joint. Jang JS et al reported that this screw has the advantages of firm fixation, fewer complications, less trauma and shorter operation time when combining with lumbar interbody fusion in the anterior approach . However, the fixation strength of facet joint screw is limited, So it is often used when the structure of the posterior column is unbroken.
Interspinous internal fixation technique is a minimally invasive non-fusion internal fixation technique for lumbar posterior approach. This technology has the advantages of simple operation, low trauma risk and short operation time . This technique is suitable for young patients with good physiological curvature in lumbar spine, which can effectively improve the symptoms of pain and maintain a certain degree of activity and then improving the quality of life. It can also improve the sagittal balance of spine and increase the diameter of the interbody foramen. Common complications include implant displacement and spinous process fractures, especially in patients with vertebral slippage and osteoporosis. Epstein  and Kim et al  found that this technology had poor clinical efficacy in patients over the age of 50, with a high incidence of complications and a high rate of reoperation, and its effectiveness and safety were questioned, suggesting in-depth evaluation of its clinical application.
Establishment of Percutaneous endoscopic transforaminal oblique fixation from posterior corner in lumbar spine
At present, PE-TLIF 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, 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. (Fig. 13).
Therefore, it is of profound clinical significance to study oblique fixation paths from posterior corner in lumbar spine, quantify its anatomical parameters, and then conduct biomechanical research on the basis of anatomy and try to develop a new type of endoscopic fusion-fixation device. In this study, the lengths of oblique fixation paths from posterior corner in lumbar spine and their angles in the sagittal and axial planes are measured comprehensively, and the feasibility of oblique fixation from the posterior corner of the vertebral body was illustrated. In this study, it was found that different target points and target areas of oblique fixation through posterior corner in lumbar spine resulted in different length of each path and different placement angles. At the same time, considering that the operating space in the safety triangle is small and there are important structures such as nerve roots and blood vessels, it is necessary to design a new type of endoscopic fusion-fixation device with a precise, compact and safe structure. In addition, several commonly used sizes and models should be designed, or even customized (Fig. 14).
the Placement of Screws in Percutaneous endoscopic transforaminal oblique fixation from posterior corner in lumbar spine (Fig. 1)
Posterior corner in lumbar spine was selected as the entry point for oblique fixation. The entry point in posterior-inferior corner is the bony area between the upper margin of the pedicle and the upper endplate. The entry point in posterior-superior corner is the bony area between the lower margin of the pedicle and the lower endplate.
Inward inclined angle, head tilt angle and tail tilt angle of screw placement, namely c1, c2, c3, a1, a2, a3, b1, b2 and b3, was measured and analyzed in this study. In the angle of the sagittal down-path, b1, b2 and b3 change significantly and consistently, showing a trend of decreasing first and then increasing, which is the smallest at L3[(46.08 ± 4.37)°,(36.21 ± 3.51)°,(22.47 ± 3.01)°] and the largest at S1[(69.49 ± 8.28)°,(61.23 ± 7.74)°,(36.60 ± 5.00)°]. In the angle of the sagittal up-path, a1 and a2 change in the same way, showing an increasing trend, while a3 goes down and then goes up, and it's the smallest at L4[(23.43 ± 4.29) °] and the largest at L5[(24.36 ± 2.94) °]. In the angle of axial plane, c1, c2 and c3 change in the same way, showing a trend of increasing first and then decreasing, in which c1 and c2 are the largest at L2[(40.07 ± 2.75)°,(55.1 ± 3.32)°], and c3 is the largest at L3[(73.11 ± 2.10)°], and all are the smallest at S1[(33.87 ± 2.94)°,(47.92 ± 3.54)°,(67.15 ± 3.60)°](Fig. 11,12).
Screw length were also measured and analyzed in this study, namely PA, PB, PC, PD, PE and PF. The medium group included PA, PB and PC, while the full-length group included PD, PE and PF, and the medium group < full-length group. In the medium group, PC path was the shortest [(38.56 ± 3.25) mm], PA path and PB path had little difference (P = 0.123), and there was no statistical significance. In the full-length group, PF path was the shortest [(43.86 ± 3.09) mm], and there was no significant difference between PD path and PE path (P = 0.177) (Fig. 6). Oblique fixation paths from posterior corner in lumbar spine increased successively from L1 to S1, among which PA and PD paths were the most obvious, followed by PB and PE paths. PC path and PF path first increased and then decreased (Fig. 7,8). PE was the optimal path for oblique fixation from posterior corner in lumbar spine, and its upward path first increased and then decreased, reaching the highest point at L3 [(46.76 ± 2.88)mm]; its downward path showed a trend of first decreasing and then increasing, with the lowest decline at L5[(45.66 ± 3.21)mm] (Fig. 9).
The PE Path should be the Optimal (long) Path
PE path was the longest of all the paths for oblique fixation from posterior corner in lumbar spine, with an average length of 45.95 mm, which was significantly longer than other paths in the same vertebral body (Fig. 5, 6, 7). Its upward path first increases and then decreases, reaching the highest point at L3. Firstly, its downward path shows a trend of decreasing and then increasing, with the lowest decline at L5 (Fig. 8). At the same time, the fixed angles (a2, b2, c2) of the PE path in the sagittal and axial planes are between a1-a3, b1-b3, and c1-c3, which may neither penetrate into the interbody space nor injure the anterior large vessels. The PE path is safe and easy to grasp in practice (Fig. 9, 11).
In addition, the fixation holding force strength of the PE path is higher because the bone canal was across the entire vertebral body from the posterior (upper and lower) corner of one side in lumbar spine to the anterior (lower and upper) corner of the contralateral side. The PE path is considered to be the optimal one for oblique fixation from posterior corner in lumbar spine.
Shortcoming and Further Research
In this study, the imaging measurement method is not compared with the dry bone measurement, and the correlation analysis between each index and body length is not carried out, which will be improved in the subsequent study. What needs to be emphasized is whether the entry point of oblique fixation posterior corner in lumbar spine is safe, and whether dural sac/traversing nerve roots and exiting nerve roots will be injured, which will be emphasized in the next article.
In addition, the biomechanical evaluation of compressive stress, stretch stress, torsion stress and fatigue load between the lumbar spine should be made in order to obtain objective evaluation for its bearing capacity before the new endoscopic fusion-fixation device are produced. Three-dimensional finite element analysis is also required after the device model is completed. Finally, the feasibility of one-stop fusion-fixation in lumbar spine under the microscope is preliminarily determined.