The patient is placed in the prone position on the radiolucent table with spine, hip and knee flexion under general anesthesia. The patient’s blood pressure was controlled to 90–110 mmHg/50–70 mmHg (mean arterial pressure≥70 mmHg) with anesthetic drugs to reduce intraoperative bleeding. It is important to reduce lumbar lordosis and increase foraminal space by flexing the hip and knee joints to reduce abdominal pressure and prevent epidural hemorrhage. For upper cervical spine tumors, the head should be fixed with head rest and adapters, the neck should be flexed and the upper back tilted downward to increase venous return and reduce intraoperative bleeding. For lower cervical or upper thoracic spinal tumors, the head should be immobilized with a head restraint and the shoulders should be pulled down with cloth tape.

The surgeon stood on the right side of the patient. A marked line was drawn along the middle line of the pedicle of the vertebrae. Another marked line was drawn along the spinous processes, interlaminar foramen could be marked with a circle zone and a parallel line was drawn along the intervertebral disc (Figure 2). Two points, 1.5 cm (1.0 cm for cervical) above and below the disc lower level at the inner edge of the pedicular line, were used as the body surface positioning points of the view portal and work portal, respectively. If the patient is obese or requires the use of pedicle screws, the 2 incisions should be wider and placed laterally from the midline (lateral line of the pedicle).

Under the guidance of C-arm fluoroscopy, 2 longitudinal skin incisions were made to form the view (caudal side) and work (cranial side) portals. The length of the skin incision was about 0.5–0.7 cm for view portal and 1.0 cm or larger for work portal so that the endoscope and instrument can be inserted properly, the saline flow can be maintained smoothly and the complete tumor mass can be removed easier. For cervical lesions, a deeper incision through layers of fascia is necessary and feel the feedback of resistance to avoid piercing the interlaminar space. The trocar with sleeve was inserted into the caudal incision (view portal) toward the interlaminar foramen and through the subcutaneous tissue, and fascia layer. The guide rod (or obturator) was inserted into the cranial incision (work portal) through the subcutaneous tissue, and fascia layer, too. Fluoroscopy determined that the 2 positioning of sleeve and rod, crossed at the above of LF of interlaminar foramen. Serial dilators were used to separate the back muscle and enlarge the work portal. A 30° endoscope, with 2.7 mm in diameter for cervical spine and with 4 mm for thoracic and lumbar spine, was inserted through the view portal. The irrigation system was connected to the sleeve and turned on for continuous irrigation. A natural gravity irrigation system (about 70–100 cm above the table) or pumping system (with pressure 30-40 mmHg) was used for continuous saline irrigation. The plasma radiofrequency (RF) wand was inserted through the work portal to create a small space enough to avoid obscuring the view of endoscope on the interlaminar surface and control bleeding so as to ensure the clarity of the surgical field. A blurred surgical field can cause neural structure injury, so it is important to maintain a clear view of the surgical field.

The LF was found if the direction is correct and partial laminectomy was performed at cranial side for better removal of the LF. Bilateral flavectomy was performed using curved curette, pituitary forceps and Kerrison punches. The surgical field was expanded to the periphery by adjusting the direction of the endoscope and fully expose the target point (tumor location) near the interlaminar space. After bilateral interlaminar decompression was performed, the thecal sac and extradural mass under the pedicle would be identified. Avoid to destruct the facet joint and partial resection of the base of the spinous process (over-the-top decompression) is allowed to increase the working space and better vision for the dura and intraspinal lesion. Epidural bleeding can be effectively managed by the low-voltage RF bipolar system under clear vision and by the continuous saline irrigation adding with epinephrine (0.1-mg epinephrine /1,000-mL normal saline). The extradural masses were meticulously dissected with a nerve dissector and removed with curved curette and pituitary forceps.

Intraoperative fluoroscopy determines the upper and lower boundaries of the tumor according to preoperative imaging data (CT or MRI). The dural sac and target tumor should be exposed after expending the surgical field. For IDEM lesions, a sharp scalpel was used to make a longitudinal incision of the tense dura, which was propped up by the tumor, with an extended incision according to the size of tumor. The incision also could be extended by a forceps (Figure 3). The water pumping system is closed or opened intermittently to prevent spinal hypertension or hydrocephalus. A duragen with appropriate size is used to block the cerebrospinal fluid (CSF) flow and placed on the cranial tumor pole. The adhesions were dissected with a nerve dissector and the tumor was removed with a forceps. The small vessels on epidural region and around the tumor could be coagulated by a smaller models of plasma RF wand for hemostasis. The dural origin is coagulated to reduce the recurrence rate. The incised dura was sutured with a 6–0-diameter prolene under endoscope (Figure 4) and covered with fibrin sealant and duragen to prevent the formation of liquor cysts. No drainage tube was needed if no obvious bleeding and outflow of CSF were found.

It is common for surgeons who are new to UBE to experience damage to the dura mater. A dura tear is the most common complication (1.9%–8.6%) in UBE and occurs most frequently in the thecal sac, axillar, tranversing root [18]. Wang et al. [19] reported that the incidence of intraoperative or postoperative CSF leakage is 2.49% due to tearing of the dura mater during UBE surgery for lumbar spinal stenosis. Causes of dural tears include adhesion of tumors or fibrotic scars, poor hemostasis or massive bleeding leading to blind procedure, instrument stab wounds, central fold tears, etc. For adhesion cases, meticulous use of a nerve dissector or angled curette is better than use of forceps or punches. For hemostasis, if the source of bleeding cannot be found, the endoscope can be advanced as close as possible to the suspected bleeding focus. This is accomplished by temporarily increasing hydrostatic pressure to flush out the blood, then using a small RF probe to coagulate the bleeding. A dura tear can occur when additional bone work is performed close to the exposed dura after the LF has been removed. The meningovertebral ligament is responsible for pulling the dura back, so this is not confirmed and there is a high possibility of making a dura tear during the removal of various epidural tissue [16]. In endoscopic spine surgery, hydrostatic pressure causes central folding of dura, and beginners have a high risk of tearing the dura because the working space is frequently restricted in inexperienced surgery [20]. When a dura tear occurs, water pressure should be decreased to prevent IICP and simple observation with absolute bed rest is recommended for injuries less than 4 mm [18,21]. For 5- to 10-mm tear, we trim the duragen into an elongated shape and inject half of it into the intradural space, like repairing a tire. Finally, cover another piece of duragen about 10 mm × 10 mm on it. Bone edges can be used to secure duragen patches (Figure 5).

For large dura tears of more than 10 mm, endoscopic repair will be performed. The pledgeted suture method can be used to reduce the suture numbers and avoid leakage of CSF (Figure 6). Postoperative management can be summarized as wound drainage was removed followed by wound sutures, prophylactic antibiotics, dexamethasone, elevated head position, and continuous lumbar subarachnoid drainage treatment [22].

The most common cause of postoperative hematoma is inadequate hemostasis, which leads to an unsatisfactory clinical outcome after surgery [16]. Postoperative hematoma may occur as epidural fibrosis, which can interfere with the expansion of the dural sac [23]. Hydrostatic pressure masking bleeding, tumor bleeding, bone bleeding, epidural vascular bleeding, muscle bleeding and patient factors should be considered when performing UBESST excision. Hydrostatic pressure of UBE and low blood pressure of patient may mask the bleeding and lead to postoperative bleeding and hematoma formation. Before wound closure, it is important to turn off the saline pumping system and check for bleeding regularly. Low-voltage RF is used to control bleeding in patients with highly vascular tumors, osteoporotic bone, or highly abundant epidural vessels. Bone wax can also be applied to the bleeding bone surface by using a burr tip. If the bleeding is from the endoscopic portal rather than within the surgical field, there may be bleeding from the muscle arteries, which should be confirmed and electrocoagulation performed. For patients with low platelet counts, a count of 50,000/μL is considered acceptable for nonneurological surgery, but higher platelet goals (closer to 100,000/μL) are recommended in patients with neurosurgical needs [24].

For patients receiving anticoagulants therapy, the last dose must be taken ≥48 hours before the procedure. When the patient’s estimated glomerular filtration rate is as low as 30–50 mL/min, it must be ≥72 hours. For patients receiving antiplatelet or antithrombotic therapy, it is recommended to stop taking the drug for 5 days. For patients with a high risk of thrombosis, low molecular weight heparin can be used instead. If antiplatelet or anticoagulant therapy is discontinued, therapy should be resumed within 48 hours of surgery based on the perceived risk of bleeding and thrombosis [25].

Spinal hypertension is a unique complication of endoscopic spinal surgery requiring water irrigation. It is often caused by intraoperative dural tear and high irrigation pressure [14]. When repairing the dura under endoscope, the water pumping should be closed or opened intermittently. If bleeding is not obvious, dry scope could be taken into consideration. Spinal hypertension should not be ignored as it is associated with serious neurologic complications such as headache, clumsiness, irritability, visual impairment, dizziness, tinnitus, seizures, near-death sensation, and dystonia. Treatment for the syndrome includes continued tracheal intubation, sedative medications, appropriate application of antihypertensive medications to control blood pressure, and necessary dehydrating medications and hormones. And symptoms and signs are generally completely relieved within 24 hours after surgery [14,26]. For intradural tumor excision surgeries, it should not take more than 60 minutes from endoscopic durotomy to endoscopic dural repair. When patient has a transient acute increase in blood pressure and bradycardia during surgery, stop water infusion and brain sponge compression for observation is recommended. When the durotomy to dural repair cannot be completed within 60 minutes or the patient's heart rate and blood pressure cannot be controlled, termination of the operation or conversion to open surgery should be considered, and a drainage tube should be placed before the wound is closed. Even if there is no dura tear, prolonged water pressure can cause postoperative back pain, neck pain, and sciatica.