The Benefits of Using spinal surgical osteotomes for ube

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Applicable to the most skilled spinal surgeons, there are still some difficulties and risks in the early implementation of UBE technology ( 10 ). Navigating the UBE learning curve is a concern for most surgeons who wish to use this technology. At present, there are still few studies on the learning curve of UBE technology. We adopt the cumulative summation (CUSUM) method to analyze the relationship between the number of repeated operations using UBE technology and the possibility of a successful single operation to provide a quantitative basis for determining the optimal number of repetitions in the learning process ( 11 ). In addition, the potential methods to shorten the learning curve of UBE were empirically summarized. Through all these, it may provide some references for doctors interested in performing UBE surgery.

Lumbar disc herniation is a common disease that presents as low back pain, lower limb pain, numbness, weakness, or claudication, with a lifetime prevalence of 12.2&#;43% ( 1 ). Conservative treatment can be tried for patients with mild symptoms and without progressive decline ( 2 ). However, for patients whose conservative treatment failed, surgery may be the best option ( 3 , 4 ). In recent years, unilateral biportal endoscopic (UBE) spinal surgery for the treatment of lumbar degenerative diseases and other diseases has gradually increased ( 5 &#; 8 ). It is generally believed that UBE surgery has the advantages of a wider field of vision, flexible operation, minimally invasive, and contributing to full nerve decompression and faster recovery ( 9 ).

The structures such as the inferior edge of the superior lamina, the root of the spinous process, the upper edge of the inferior lamina, the inner edge of the facet joint, and interlaminar ligamentum flavum were exposed using a plasma radio-frequency knife. Tools such as the power grinding drill, osteotome, and gun rongeur were used to remove bones of, for example, the lower edge of the upper lamina, the upper edge of the lower lamina, and the medial side of the facet joint. Then the ligamentum flavum was removed. The intervertebral disc that compressed the nerve was explored and removed. After confirming that there was no nerve compression or active bleeding, the instrument was removed and the incision was closed. A representative case is shown in .

The patient underwent general anesthesia and was placed in the prone position. With C-arm fluoroscopy, adjustments to the operating bed were made, so that the target intervertebral space is as perpendicular to the ground as possible. Taking the intersection of the upper and lower 1&#;1.5 cm of the target intervertebral space and the inner edge of the pedicle as the center, a 1&#;1.5 cm transverse incision was made. The left-hand incision serves as the observation channel (portal), and the right-hand incision serves as the working channel. The bilateral channels were dilated with a step-by-step dilator and the lower edge of the superior lamina and the interlaminar space can be touched by the dilator. The operator held the arthroscope in his left hand and the instrument in his right hand. Through the two channels, the camera lens and instrument will meet in the space around the interlaminar window in a continuous perfusion water environment.

This retrospective study was conducted in accordance with the guidelines of the Declaration of Helsinki and was approved by the ethics committee of the Second Hospital of Anhui Medical University (No. SL-YX-324(F1)). Patients with single segmental lumbar disc herniation treated in the Department of Orthopedics of the second Hospital of Anhui Medical University from November to May were studied, and UBE spinal surgery was performed entirely by the same doctor. All patients signed the informed consent form according to the standard of diagnosis and treatment before operation. The inclusion criteria that were used are as follows: (1) patients with single-segment lumbar disc herniation, who have clear surgical indications, (2) American Society of Anesthesiology (ASA) levels I&#;III, and (3) complete follow-up data can be obtained and the follow-up period is at least 18 months. The exclusion criteria are as follows: (1) patients with extreme lateral, very middle, or bilateral disc herniation, (2) patients with other serious diseases, (3) patients with previous lumbar surgery history, (4) patients with lumbar instability, lumbar infection, or lumbar tumor, (5) patients with the multisegmental lumbar disease need to be treated, (6) a patient whose operation is performed by another doctor. According to the above inclusion and exclusion criteria, a total of 97 patients were enrolled in this study.

The operation time showed a downward trend as a whole. The scatter chart of the operation time is shown in . The CUSUM analysis curve of the learning curve is shown in . CUSUM method showed that the curve reached the maximum in the no. 24 case, and then decreased gradually. So the cut-off point of the learning curve was selected as 24 cases. According to the cut-off point, the curve could be divided into two stages: the first stage was the learning stage in which the CUSUM value was increasing (the first 24 cases), and the latter stage was the proficiency stage in which the CUSUM value gradually decreased (after 24 cases).

All 97 patients underwent the UBE operation successfully. The follow-up time was 18&#;36 (22.6 ± 3.6) months. The operation time was 30&#;241 (97.9 ± 34.7) min. The estimated intraoperative blood loss was 10&#;50 (20.4 ± 5.0) ml. The postoperative hospital stay was 1&#;14 (4.4 ± 2.1) days. The VAS score of lower limb pain decreased from 5.75 ± 0.81 before the operation to 0.39 ± 0.28 at the last follow-up (P < 0.05). The ODI score decreased from 66.48 ± 4.43 before the operation to 14.57 ± 3.99 at the last follow-up (P < 0.05). The postoperative MacNab grade was grade 1 in 84 cases (86.6%), grade 2 in 7 cases (7.2%), grade 3 in 6 cases (6.2%), and grade 4 in 0 cases. The area of the dural sac at the narrowest part of the target segment increased from 89.34 ± 32.85 mm 2 to 140.86 ± 39.87 mm 2 (P < 0.05).

Discussion

The ideal management strategy for lumbar disc herniation remains controversial (13). Surgical treatment is a common method of treatment, which can be more effective than conservative treatment in patients with severe lumbar spinal nerve compression (14). With the main purpose of surgery to relieve nerve compression, there are many ways of performing the operation, namely, open surgery, microscopic surgery, and percutaneous transforaminal endoscopic surgery (15). Decompression combined with internal fixation is not superior to simple decompression in many cases (16). In recent years, there have been increasing reports of UBE surgery for lumbar disc herniation and lumbar spinal stenosis (5&#;8). In addition, the UBE technique can be used for nerve decompression of burst fracture (17), excision of the perispinal cyst (18, 19), clearance of epidural abscess (20), treatment of epidural lipomatosis (21), treatment of foraminal stenosis (8, 22), lumbar interbody fusion (6), and revision surgery (23).

The unilateral biportal endoscopic (UBE) technique uses an independent working channel, which can achieve complete decompression under a wide visual field of the arthroscopy (9). During the making of the channel, there is no need to strip away too much soft tissue. The lens and instrument are operated directly through soft tissue channels to the target. We found in our practice that even for obese patients, no significant difficulty was increased in the surgery. The channel provides less restriction for instrument movement, and the continuous perfusion of saline during the operation is a major advantage for infection prevention. Compared with microscope technology, UBE technology has a higher success rate, shorter operation time and hospital stay (24). In our study, 86.6% of patients got MacNab grade 1. Most surgeons choose a 30-degree arthroscopic lens, which can be used to observe the lateral structure of the lens because of its wide field of view (25). UBE technique can allow visualization of the contralateral spinal canal and intervertebral foramen (5). Compared with percutaneous transforaminal endoscopic surgery, the UBE technique has less radiation exposure (26). The injury of the multifidus muscle after UBE is minimal (27). Other advantages of the UBE technique are less destruction of the facet joint, lower incidence of complications, a lesser degree of postoperative back pain, and higher satisfaction (24, 28&#;30). UBE technique can essentially be used as an alternative to the microscope technique (31, 32). Compared with microscopic surgery, endoscopic surgery such as UBE has been found to contribute to less pain in the early stage after the operation (33). In addition, the implementation of UBE technology does not require the purchase of special lenses and instruments as seen in the percutaneous transforaminal endoscopic technique. UBE can use general arthroscopic lenses and open spinal surgical instruments, which is more conducive to wide acceptance in most hospitals.

The unilateral biportal endoscopic (UBE) technique requires both hands to operate the lens and surgical instruments and requires sufficient coordination of both hands and stable instrument operation with a single hand. In the early stage, it is difficult for spinal surgeons who have no experience in using arthroscopic equipment to coordinate the depth and direction of the lens, move the instruments quickly and smoothly in and out of the instrument channel and quickly acquire the field of vision. If the operation is performed incorrectly, complications such as dural injury often occur in UBE surgery (10). A total of 2 cases of dural injury were found in our patients, too. These have higher requirements for the surgeon's UBE technology, the cooperation of the surgical team, and the perioperative management, which have become a big obstacle to the further popularization and development of this technology. Navigating the learning curve quickly and safely is a core issue in the clinical application of UBE. At present, there are many studies on the learning curve of transforaminal endoscopy, but few studies exist on the learning curve of UBE technology.

As a new technology for the minimally invasive spine, UBE contributes a certain learning curve, which is mainly reflected in operation time and complications. CUSUM method is a quantitative analysis method for analyzing the learning curve of surgical techniques (11). Many other studies on the learning curve of surgical techniques are mostly based on the method of grouping all cases in order, which is subjective. And the cut-off point of the learning curve is often an integer multiple of the number of grouped cases, so the results are inaccurate. In our study, the CUSUM method is selected for the analysis. To obtain the operation time of each patient, the relationship between the operation time of each patient and the average value of the group is calculated, and the approximate parabola curve is obtained. At the highest point of the parabola curve, the learning curve is divided into two stages. According to the formula, the operation time of most cases before the highest point of the parabola is longer than the average operation time, and the operation time of most cases after the highest point is < the average operation time. There is no need for artificial subjective grouping in the study of the CUSUM method, which is more objective and accurate than the grouping method (34&#;36). According to the highest point of the CUSUM curve ( ), there were 1&#;24 cases in the early stage of this study and 25&#;97 cases in the later stage. This graph reveals that the initial curve is very steep, but it does not take too many cases to reach the highest point. With the increase in the number of cases (after 24 cases), the CUSUM curve of operation time showed a downward trend to be stable in the later stage. Evidence of the gradual decrease of operation time can also be seen in the scatter chart of operation time ( ). This shows that the difficulties encountered at the beginning of UBE technology, such as long operation time, are short-lived. After a period of learning and acclimation, the surgeons become more familiar with the surgical equipment and surgical procedures. Meanwhile, with the gradual optimization of the operating room procedures and the cooperation of other personnel, the learning curve gradually becomes more stable.

Through the comparison of the data of the two stages, there is no statistical difference in the general condition and preoperative index of the patients. But the operation time, postoperative hospital stay, and the proportion of Macnabcriteria1 grade in the second stage are all improved from those in the first stage, and the difference is statistically significant. This may be due to multiple reasons: the technique of the surgeon improves; the cooperation of fixed assistants gains understanding; anesthesia, nursing, and other surgical team cooperation are gradually optimized, and perioperative management is optimized. Although the operation time shortened with the learning stage, there was no significant difference in the incidence of postoperative complications, last follow-up VAS, ODI, and the area of the dural sac after operation between the two stages. This indicated that in our earliest cases, although it takes a longer time to operate, it still ensures a clinical effect and safety that is essentially the same as that in the mature stage. Looking at the CUSUM curve, it shows that in about 42&#;52 cases, the curve increased slightly again. This occurrence may be related to the increasing challenge of more difficult and complex cases after the surgical technique becomes proficient. Usually, as the technique is mastered, surgeons will unconsciously extend the application of the technique to more difficult cases that they may be reluctant to choose at an early stage (11). As we can see in our cases, the proportion of patients with the basic disease and the duration of preoperative symptoms in the second stage cases were higher than those in the first stage, although there was no significant statistical difference ( ).

What is the difference between the learning curve of the UBE technique and other invasive techniques such as percutaneous transforaminal endoscopic surgery? With regard to the learning curve of percutaneous transforaminal endoscopic surgery, the cut-off point reported in the early literature was about 40&#;70 cases (37, 38), while the cut-off point reported later in the new literature was about 20 cases (39, 40). However, like the early explorers of UBE technology in China, it only takes about 24 cases to master this technique skillfully, and its learning curve is shorter than that of transforaminal endoscopic surgery reported in the early literature. The shortening of the learning curve means that the operation time, hospital stay, operation costs, and complications can be reduced in a short time, which is more beneficial to patients and more likely to be recognized by surgeons. UBE technology provides the advantages of minimally invasive percutaneous transforaminal endoscopic surgery and flexible operation of open surgery, so it is currently being widely promoted in China.

What factors can optimize the learning curve? According to our experience, surgeons need rich experience in spinal surgery before carrying out this technique, and it is better to have experience in single-portal spinal endoscopy such as percutaneous transforaminal endoscopic surgery and double-portal endoscopic surgery such as arthroscopy surgery. At the same time, the surgeon must be trained in UBE technology. Our department has held UBE training using the plastic model and the fresh specimens of piglet spine many times, which is helpful for the surgeons to successfully overcome the steep learning curve of UBE technology. In the early stage, one should try to select the cases with typical, unilateral symptoms, clear surgical indications, less degeneration, less operative area complexity, and then gradually carry out the more difficult cases after gaining skill. Some special instruments needed in UBE, such as arthroscopy, plasma-mediated ablation probes, radio-frequency probes, and grinding drill, must be well prepared. Our general experience is for a right-handed surgeon to place arthroscope, water perfusion equipment, and other observation equipment on the left hand, while radio-frequency probe, grinding drill, and other energy power equipment on the right hand to avoid entanglement of the devices. Maintaining a clear field of vision requires the anesthesia team to provide an adequate degree of anesthesia, maintain normal blood pressure, and good muscle relaxation. This requires communication and coordination with the anesthesia team. During the operation, it is necessary to maintain the appropriate water pressure of the operating cavity. And one must pay attention to the appropriate perfusion pressure and the placement of the casing, and keep the effluent unobstructed at all times (41).

Early endoscopic spine surgery was used generally to treat disc herniation and was less invasive than traditional open techniques. Surgeons now have the surgical instruments and expertise to treat a wide range of spine pathologies beyond lumbar disc herniation. However, the technique requires specialized training and instruments, and there is a steep learning curve for beginners. Furthermore, there are potential risks and complications, including nerve injury, dura tear, postoperative hematoma, and infection. Surgeons need to adhere to established protocols and guidelines to ensure optimal patient outcomes. This article aims to describe in detail, with references to current literature, the essential surgical techniques used during UBE.

Unilateral biportal endoscopic spine surgery (UBE) was a pioneering technique, providing a less invasive alternative to conventional spine surgery in various spinal diseases. Forsts and Hausmann were the first to use an arthroscope intradiscally in the early s [ 3 ]. At the beginning of the 21st century, several authors introduced various spinal decompression techniques to preserve the posterior midline structures, including endoscopic spine surgery [ 4 6 ]. UBE has progressed due to the development of the endoscope and specialized surgical instruments [ 7 ]. The development of endoscopic instruments generated a subspecialty of minimally invasive spine surgery that shifts the point of visualization away from the surgeon&#;s eye or microscope and places it directly at the site of the spine pathology with an endoscope [ 8 ]. This technique has been used for various minimally invasive spinal decompression procedures, such as laminotomy for lumbar discectomy, unilateral laminotomy for bilateral decompression, and unilateral foraminotomy. Excellent clinical outcomes have been achieved through these techniques [ 7 11 ]. Additionally, they allow the visualization of the spinal structures via 2 small incisions on one side of the spine, thus minimizing tissue injury and enhancing postoperative recovery. Due to these advantages, UBE is increasingly widely performed, approximately one hundred UBE cases are performed annually at our institution.

In general, the indications for UBE are similar to those for conventional open and microscopic spinal surgery. When conservative treatment is ineffective or the neurologic symptoms of the patient worsen, a surgical procedure by UBE is recommended. The following describes the indications and contraindications for a surgical procedure by UBE: (1) spinal stenosis or foraminal stenosis; (2) hypertrophied ligamentum flavum (LF), ossification of ligamentum flavum (OLF) involving less than 50% of the spinal canal; (3) low-grade spondylolisthesis (I or II). The following are contraindications for a surgical procedure by UBE: (1) central lesion on the level of the spinal cord; (2) high-grade deformity; (3) tumor or vascular malformations; (4) severe dural ossification or severe stenosis; (5) high-grade spondylolisthesis (III or IV); (6) bilateral symptomatic foraminal-extraforaminal stenosis; (7) instability of the spinal column; (8) vertebral fractures or pathologic conditions because of the risk and technical challenge ( Table 1 ).

SURGICAL TECHNIQUE

1. Anesthesia and Patient Positioning

UBE is performed under general, epidural, or spinal anesthesia. However, in cervical surgery, it is performed under general anesthesia. General anesthesia is preferred in most cases, as it allows greater muscle relaxations, facilitates patient positioning, and reduces the risk of unintended patient movement during surgery. Careful consultation with an anesthesiologist is required before performing surgery at the spinal cord level, as intraoperative neurophysiological monitoring may be necessary.

Most UBE spine surgeries are performed in the prone position using a Wilson frame or Jackson table, although it is possible to change position depending on the surgical approach. It is important to reduce lumbar lordosis and increase the foraminal space by flexing the hip and knee joints. Also, an important key in patient positioning is to reduce abdominal pressure to prevent epidural bleeding.

Even cervical surgery is performed mostly using the prone position. To reduce the pressure on the abdomen, an H-shape pillow should be used to relax it. The neck should be flexed, and the upper back should slope downward. This improves venous return, reducing bleeding during surgery. To check the C6&#;7 level or lower, the head should be fixed by head fixation, and the shoulder should be pulled down using a plaster. During surgery, mean arterial pressure must be maintained below 80 mmHg to reduce intraoperative bleeding.

2. Localization and Portal Creation

When performing UBE lumbar spine surgery, the C-arm is used to check the target level and set it parallel to the endplate. At the junction of the medial pedicle line and the points 1 cm above and below the target disc space, 2 skin incisions are made. The appropriate distance between the 2 skin incisions is 2 to 3 cm apart, and the skin incisions are approximately located in the lower margin of the proximal pedicle and the midpoint of the distal pedicle ( Figure 1A ). The docking point for the discectomy and decompressive laminectomy is the inferior margin of the upper lamina. Obviously, in cases of obesity, a high-level disc, or hyperlordosis, it should be appropriately modified according to the patient. Since it is also different depending on the disc space angle, the angle must be determined using a preoperative radiologic image. The endoscopic portal size should be 7 mm or larger, and the working scope should be 9&#;10 mm or larger, so that the endoscope and instrument can be inserted properly, and the saline flow can be maintained smoothly. The direction of the skin incision can be either horizontal or transverse. To decompress the exiting nerve root or to remove up-migrated disc herniation and foraminal disc herniation on the contralateral side, 2 portals should be placed slightly below the routine portal. However, to decompress the traversing root or to remove the down-migrated disc on the contralateral side, 2 portals should be placed slightly above the routine portal. Modification of these portals can reduce unnecessary bone work.

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In a paraspinal approach, the upper and lower pedicles and the transverse process of the level are indicated using the C-arm. After adjusting the angle of the C-arm parallel to the endplate of the target level, the portal is made at the junction of the lateral margin of the transverse processes and the points 1 cm above and 1 cm below the target disc space ( Figure 1B ). The docking point is the isthmus. There is also a method to make skin incisions at an angle of about 30°&#;40° and check the preoperative computed tomography or magnetic resonance imaging (MRI) in advance to determine the distance of the skin incision from the midline before performing surgery ( Figure 2 ). After a skin incision is made, the docking point of the endoscope and the working instrument is set to the isthmus. However, at the L5/S1 level, a different location is required for the portal placement because of the iliac crest. In the L5/S1 Left side approach, the scope portal is the same as the routine portal and the working portal is 1 cm from the routine portal on the medial side. Each skin incision is made at the lateral margin of the L5 transverse process and the lateral margin of the sacral alar, and the distance between the incisions is approximately 2&#;3 cm. On the L5/S1 right side approach, the incision is made 1 cm from the routine portal placement on the proximal side ( Figure 1C ). Unlike other levels, the paraspinal approach at the L5/S1 level has a very restricted surgical field because of the prominent iliac crest, oblique pedicles, and more coronally oriented facet joint. The docking point of the endoscope and instrument is determined by the lateral border of the superior articular process (SAP), the lateral border of the sacral alar, and the osseous triangle at the base of the L5 transverse process.

In far-out syndrome decompression, the skin incision is made 1&#;2 cm lateral to the lateral margin of the vertebral body under C-arm fluoroscopy anteroposterior view confirmation. A skin incision is made 1 cm above and 1 cm below the intervertebral level, and the distance between the 2 skin incisions is about 2&#;2.5 cm. The landing point of the first dilator is very important. The aim in far-out syndrome decompression surgery is to remove the transverse process and pseudo-articulation of the sacral alar. It is important to place the first dilator through the working portal aiming at the junction of the SAP of S1 and the sacral alar and to place the endoscopic portal near the sacral alar or sacral notch for triangulation. Meticulous dissection and detachment should be performed around the bony structure, and saline flow should be maintained between the bony structure and soft tissue. The lateral aspect of the SAP, sacral alar, and even the lower border of the transverse process must be confirmed.

In revision surgery, it is important to make an incision that is slightly more lateral than when using a previous wound. Due to the characteristics of revision surgery, it is easy to lose orientation as a consequence of peridural scar tissue when entered through a previous incision; therefore, approaching from the lateral side and operating on the facet joint and lamina can be a safe procedure.

The skin incision for fusion using an endoscope is slightly different from the incision for decompression. After placing the C-arm parallel to the endplate, 2 skin incisions are made on the midline of the proximal and distal pedicle. Using the carinal lamina itself and the inferior margin as a docking point, a working and endoscopic portal is made approximately 3 cm away ( Figure 1D ). Pedicle screws are inserted using the previously made skin incisions. When performing modified far-lateral transforaminal lumbar interbody fusion, 2 skin incisions are made at the lateral border of the pedicle ( Figure 1E ).

In posterior cervical surgery, a skin incision is made vertically in the midline of the pedicle under C-arm fluoroscopic confirmation. It is made near the upper pedicle and lower pedicle, about 2 cm apart ( Figure 1F ). The operative angle is approximately 20°&#;25°. If the patient is obese, the 2 incisions should be wider and placed laterally from the midline. A #10 blade is used to make a deeper incision into the fascia until it touches the bone, with the guidance of the C-arm. Unlike lumbar, cervical surgery requires a deep enough incision because there are several layers of fascia and muscle. A wide blade is used because the dissection is safer with wider blades and can be performed without penetrating the interlaminar space.

3. Endoscopic Visualization

The initial docking point of the endoscope and the serial dilator is the location between the pathologic level of the spino-laminar junction and the inferior margin of the caudal lamina. Using the first serial dilator or muscle dissector, the paraspinal muscle should be sufficiently dissected on the lamina around the docking point. This is to guarantee sufficient saline patency. Muscle detachment should be performed from the lower border of the cranial lamina at the pathologic level to the upper border of the caudal lamina. It is performed by using the ablation mode of a radiofrequency (RF) probe and removing soft tissue with a muscle shaver; the endoscope and surgical instruments are triangulated. The outer layer of LF and bulky soft tissue should be removed using a Kerrison punch or pituitary forceps to confirm the landmark of laminectomy.

It is necessary to check whether the saline patency is smooth before laminectomy. A 3,000-mL saline bag is placed 80&#;100 cm above the patient&#;s back (100 cmHO injection pressure) or an automatic pressure pump is used. Water pressure should not exceed 30 mmHg if possible [ 12 ]. The use of working sheaths or cannulas to maintain smooth water flow during surgery in patients with excessive muscle mass or who are obese is also an essential surgical technique ( Figure 3 ). This allows the irrigation fluid to create a working space in the UBE. If a dura tear occurs, the intracranial pressure (ICP) can increase, and this increase is higher the closer it is to the cord level. Increased ICP can cause postoperative headaches, neck stiffness, seizures, and retroperitoneal fluid collection [ 13 ]. Even if there is no dura tear, high water pressure can cause postoperative back pain and neck pain, so low water pressure is recommended during UBE spine surgery ( Supplementary video clip 1 ).

The docking point in the paraspinal approach is the lateral edge of the isthmus. Under C-arm guidance, a guide pin and instruments are placed on the isthmus and the exit of the foramen. Using a Cobb elevator, the muscles attached to the lateral edge of the isthmus, the SAP of the facet joint, and the transverse process should be dissected to create a sufficient surgical field. Radicular arteries are distributed around the facet joint; it is therefore important to prevent bleeding by adequately coagulating the area with an RF probe before performing bone work. In far-out syndrome decompression, because the radicular artery runs over the sacral notch, greater caution is necessary when dissecting muscle around the sacral notch. When bleeding is unexpectedly severe, it is necessary to control the bleeding after confirming the bleeding site by placing the endoscope close to it. Occasionally, if the hypertrophy of the facet joint is extremely severe or if access to the lateral edge of the isthmus is challenging due to a decrease in intervertebral disc height, the isthmus can be reached by approaching the lateral edge of the SAP of the lower facet joint. The next step is to check the upper and lower transverse process.

In revision surgery, anatomical landmarks are often unclear due to overgrowth by scar tissue. The caudal border of the superior lamina, medial border of the facet joint, and upper border of the caudal lamina are undercut and dissected using a diamond drill, chisel, or small-head curve curette until the healthy dura of the traversing root is exposed. When the lateral margin of the traversing root is exposed, the outer annulus of the intervertebral disc is exposed by careful medial retraction. During the process of exposure, if there is adhesion between the dura and the disc space, a blunt dissector or a small nerve hook is used carefully to dissect the scar tissue and enable safe access.

In posterior cervical surgery, the surgical field is created by dissecting neck muscle using serial dilators. Endoscope and instrument insertion require intraoperative fluoroscopic confirmation because the interlaminar space can be penetrated and cause cord injury. First, surgeons should insert a 0° endoscope and a working instrument and check the saline flow patency. Using a natural drainage or pump system, it is safe to set water pressure below 30 mmHg. Next, the V-point where the superior lamina, inferior lamina, and medial aspects of the facet joint intersect should be checked with the endoscope and instrument after triangulation. Then the surgical field can be created by removing the remnant soft tissue around the V-point. Before bone drilling, it is recommended to expose the entire lamina using an RF probe.

4. Decompression

The anatomical landmark is checked by soft tissue dissection, and then a laminectomy is performed. The laminectomy is started from the lower border of the cranial lamina, using a drill or osteotome until a free margin of LF is obtained. Then, the V-shaped central fissure of the LF is distinguished from the lower border of the cranial lamina, bone work is performed until the cranial, lateral, and caudal sides are freely detached. To prevent fracture of the isthmus or inferior articular process, the proximal edge of the LF can be detached using curved curettes when the laminar isthmic space is narrow. The lateral margin of the nerve root and the dural sac are checked while removing the LF. Sufficient bone work and removal of the LF are done to reduce unnecessary traction. If additional bone work is required after LF, a drill can cause a dura tear; therefore, a small osteotome can be used as an alternative.

In the contralateral sublaminar approach, the LF on the contralateral side and the ventral side of the lamina should be detached using a freer or curette before contralateral decompression. Contralateral sublaminoplasty should be performed until the edge of the contralateral LF is free, and is performed generally until the medial side of the contralateral facet joint is exposed. Because a dura tear can occur as a result of a central portion defect of the LF, the base of the spinous process should be removed carefully. When the contralateral lamina is undercut using an osteotome or endoscopic drill, the LF is not removed to protect the neural structure. It is recommended to proceed between the LF and the ventral side of the lamina. When a lateral recess has a calcified lesion or bony structure, a straight or curved osteotome is used for decompression rather than a Kerrison punch. However, when removing the lesion or down-migrated disc around the exiting root of the contralateral side, the laminotomy area on the ipsilateral side of the lesion can be minimal ( Figure 4A ), but the upper portion of the lower lamina needs sufficient bone work for easy access. It is helpful to remove the upper portion of the contralateral lamina and the SAP. By removing the contralateral LF, the contralateral traversing nerve root can be identified, and by removing the foraminal ligament, the exiting nerve root can also be identified. The endpoint of decompression is the exposure of the medial border of the contralateral pedicle and restoration of dural pulsation. Adequate decompression may not be obtained if the medial side of the SAP is not exposed ( Figure 4B ). Furthermore, the authors advocate decompressing over 3 mm laterally from the lateral margin of the dural sac during continuous irrigation, because the dura shrinks under hydrostatic pressure. In contrast to the endoscopic view, the true lateral margin of the dura in its natural state may be located further laterally.

In a paraspinal approach, the lateral edge of the isthmus and the SAP tip are key structures. Using a Kerrison punch and drill, foraminoplasty is performed to decompress the neural structure and remove surrounding tissue. Then, the LF is detached and removed using an angled curette to expose the exiting nerve root and perform discectomy and additional bone work to decompress the neural structure. When soft tissue is dissected, the transverse process, isthmus, and facet joint are exposed. In the case of a hypertrophic facet joint, removing the SAP cranial tip with a diamond drill or osteotome to create sufficient space facilitates safe surgery. If the SAP tip is not removed sufficiently because of concerns regarding instability, the surgery becomes more difficult; therefore, it must be removed adequately. Even if the SAP tip is removed sufficiently, instability is not caused generally, but further study is required to confirm this. Discectomy is performed using a pituitary rongeur, curette, etc. Additional discectomy, often from the axilla region of the exiting root, may now be performed if the offending pathology is a herniated lumbar disc. In the approach at the L5/S1 level, drilling is first performed on the base of the L5 transverse process and the cranial and lateral sides of the SAP. If the SAP is too deep and too steep, it is difficult to access with a drill, so an angled instrument, such as a hockey stick chisel and an angled pituitary clamp can be useful. In obese patients, a 30° scope may be helpful rather than a 0° scope. Depending on the conditions, removing the sacral alar can be an important procedure for creating sufficient surgical space. To perform an L5 exiting root decompression and discectomy safely, sufficient space must be obtained. When the bone work is done, remove the LF using a Kerrison punch or curette. When anatomy is confused, discography can help to identify the anatomy. Some surgeons do not perform enough SAP resection because of the concern that excessive SAP removal could lead to instability in patients. However, this may lead to insufficient neural decompression and continued symptoms. According to biomechanical studies, resections of less than 75% do not result in segmental instability [ 14 15 ].

In far-out syndrome decompression, an endoscopic drill is used after exposing the lateral aspect of the SAP, the sacral alar, and the lower border of the transverse process. Bleeding occurs as the cancellous bone is exposed at times and bleeding should be controlled using an RF probe or bone wax. During the procedure, pseudo-articulation is identified and should be removed laterally as much as possible. After removal, the foraminal ligament covering the exiting nerve root must be checked and sufficiently removed. The LF attached under the transverse process should be detached and safely removed using a small Kerrison punch, angled curette, etc., and the exiting nerve root below this is checked. The annulus of the intervertebral disc can be identified and, if necessary, ventral decompression can be performed through discectomy.

Discectomy varies slightly depending on the location of the lesion and the characteristics of the disc. Generally, a retractor is used to sufficiently protect the root during discectomy, then an annulotomy is performed using an Indian knife, etc., and removal of the disc using pituitary forceps. Calcified discs are removed using a Kerrison punch or osteotome. During a discectomy, the nerve should be protected continuously, and it is also helpful to use scope retractors and assistant retractors. Expose the disc space by carefully performing dura retraction on the disc on the contralateral side as well as the ipsilateral side, and remove it using an angled hook, small pituitary forceps, and an angled upbite pituitary. Epidural bleeding control and annuloplasty should be performed using an RF probe, and the power of the RF probe must be lowered near the dura ( Table 2 ) [ 16 18 ]. Also, to reduce traction injury, it is important to perform root release intermittently. To reduce recurrence, internal disc decompression and nucleus pulposus must be adequately removed using an RF probe and annuloplasty is also performed.

Decompression in posterior thoracic surgery is comparable to decompression in lumbar surgery. The difference is that cord injuries must be avoided. The thoracic spinal canal is narrower than the lumbar spinal canal, the lamina is short and thick and overlaps the cranial and caudal lamina. Therefore, when bilateral decompression is performed through the unilateral approach, there is a high risk that the endoscope and working instruments excessively compress the cord, resulting in a thoracic cord injury. Therefore, it is necessary to sufficiently remove and undercut the base of the spinous process to expand the working space more than when performing lumbar surgery. To avoid neural injury during thoracic surgery by UBE, the LF is left in place as protection until all bone work is complete. Until the lateral edge of the thecal sac is checked out, which is naturally confirmed through epidural fat tissue, the remaining medial border of SAP (ipsilateral and contralateral) can be removed. The medial side of the facet joint, with as much remaining as possible for stability, overlaps the lateral end of the laminectomy. The 3 key steps in thoracic OLF removal are thinning, detaching, and removal. OLF is difficult and risky to remove with a Kerrison punch. When removing the OLF, it is important not to apply unintentional compression to the spinal cord. After exposing the OLF, the operator grinds until it is thin and translucent using a diamond drill. The thinned OLF should be detached from the thecal sac using a freer elevator and gently removed. If necessary, remove it piece by piece using a 1-mm Kerrison punch or small-sized pituitary forceps. If OLF removal fails due to dural ossification or severe adhesions, the floating method is a good alternative, leaving the OLF on the thecal sac. Above all, an important surgical tip is to experience sufficient lumbar spine surgery before thoracic spine surgery with UBE.

In posterior cervical foraminotomy surgery, partial laminectomy and facetectomy are performed at the V-point using a 3.5-mm diamond burr. Before using a drill, the V-point of the targeted lamina should be checked. The drill is used in the craniolateral direction from the inferolateral portion of the cranial lamina until the LF is detached. From the superolateral part of the caudal lamina, the bone is made thin in the caudolateral direction and is drilled until the dura is identified. According to the size and height of the pathologic lesion and level, the area of the foraminotomy can be extended to the lateral or craniocaudal side. It is possible to remove one-third to one-half of the medial side of the facet joint. However, if more than 50% of facet joint is removed, there is a substantial risk of instability. After flavectomy, the medial border of the pedicle and the dura and exiting nerve root should be checked. Once the exiting nerve root is identified, foraminal decompression is performed using a 1-mm Kerrison punch. If a protruded disc is visible around the nerve root, it is removed gently. If the workspace is narrow, a pediculectomy can be used to create enough space while reducing nerve root manipulation. Finally, the lateral edge of the pedicle should be checked to ensure appropriate foraminal decompression via the neural foramen using a ball-tip type hook. All surgical procedures should be performed safely to prevent spinal cord injury.

5. Unilateral Biportal Endoscopic Lumbar Interbody Fusion

In contrast to general decompression, it is not recommended to perform laminectomy using a drill during unilateral biportal endoscopic lumbar interbody fusion (ULIF). Local autobone can be collected for bone grafting by laminectomy using a Kerrison punch or osteotome. The inferior articular process is also removed and should be resected in several pieces because it may be difficult to remove through the working portal if it is resected in large pieces. Contralateral facetectomy performed across the base of the spinous process is helpful for spondylolisthesis reduction or correction of a lordotic curve. If the contralateral facet joint osteophyte is larger or to achieve greater reduction or greater lordotic curve in spondylolisthesis, a total facetectomy is performed by additionally making incisions on the contralateral side. These skin incisions are necessary even for percutaneous screw insertion on the contralateral side ( Figure 5 ).

The medial aspect of the SAP should be removed sufficiently to enable interbody cage insertion. If it is not sufficiently removed, excessive neural structure retraction may occur during cage insertion. A space of at least 8 mm from the lateral margin of the thecal sac must be maintained to insert the cage safely. The ipsilateral exiting nerve root should not be fully exposed before cage insertion to protect it during cage insertion. Angled endplate removers and pituitary forceps are used to remove the nucleus pulposus and cartilaginous endplate. Endplate preparation is completed on both the ipsilateral and contralateral sides using an angled endplate remover, which is essential for fusion. It is helpful to use a 30 degrees endoscope for endplate preparation to the contralateral side. During surgery on patients with high-grade spondylolisthesis or significant disc narrowing, the upper edge of the caudal vertebral body is removed with an osteotome to make a larger entry. By magnifying the endoscopic view, surgeons can determine when the endplate preparation is complete. To prevent bone graft loss during cage insertion, continuous irrigation should be paused. Before cage insertion, dilate the paraspinal muscles with a bar dilator to make it easier for cage insertion. Anchor to the caudal vertebral body edge with a specialized root retractor, and insert the cage into the annulotomy site with gelfoam to reduce bone graft loss and bleeding. After cage insertion, a foraminal decompression is performed by removing the foraminal ligament around the exiting nerve root on the ipsilateral and contralateral sides. If good pulsation of the nerve root and thecal sac is identified, it can be regarded as the endpoint of decompression. Perform percutaneous pedicle screw fixation using 2 ipsilateral and contralateral skin incisions. The distance between the exiting nerve root and the traversing nerve root on the ipsilateral side is measured by preoperative MRI. If it is more than 16 mm, a large-sized cage can be safely inserted without neural injury. However, if it is less than 16 mm, a smaller cage may be needed.

Modified ULIF is similar to routine ULIF. A skin incision is slightly more lateral than in routine ULIF and uses 2 short posterior lumbar interbody fusion (PLIF) cages rather than one long transforaminal lumbar interbody fusion cage. Two PLIF cages are inserted into the unilateral laminectomy and facetectomy area. After adequately retracting the dura toward the medial side, the first cage is inserted into the medial or contralateral side. Using the cage pusher, after pressing slightly further to the contralateral side the second cage is inserted into the empty space remaining. A fusion material such as a bone chip is pushed between the 2 cages.

6. Closure and Postoperative Care

During surgical drain insertion, the drain is inserted blindly or under endoscopic guidance. If adequate bleeding control is completed, surgical drain insertion may be skipped. Because maintaining adequate saline flow during the drain insertion is important for the instrumental portal patency, a drain line should be inserted via the instrument rather than the endoscopic portal for a clearer surgical view. Compression around the portal before suturing may help to minimize soft tissue water retention. After the appropriate surgery has been completed, the muscle is approximated and the skin incision is closed with absorbable sutures or a sterile strip. The wound is covered with a sterile dressing, and the patient is sent to the recovery room. Patients are observed in a recovery room for several hours before being moved to a general ward. Provide analgesics as required and encourage patients to walk as soon as possible. Pain usually subsides within 24 to 48 hours.

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