Introduction
The definition of spinal stenosis originated from the Greek word stenosis. The modern description of spinal stenosis was put forth by Antoine Portal in 1803, and laminectomy was first performed by Lane for the treatment of spinal stenosis due to cauda equina syndrome at the end of the 19th century. Spinal stenosis is a common condition in the elderly (>60 years of age) [
1]. The conventional surgical options for spinal stenosis include open decompressive laminectomy, foraminotomy, and fusion. The current gold standard treatment for lumbar spinal stenosis refractory to conservative treatment is facet-preserving laminectomy [
2], in which a midline lumbar incision is made with the bilateral dissection of the paraspinal muscles for exposing the lumbar bony structures. However, extensive dissection results in paraspinal muscle atrophy due to ischemia and denervation [
3,
4]. In addition, resection of the posterior bone and ligaments increases the risk of complications, such as prolonged hospital stay, postoperative pain and infection, and increased blood loss [
5-
7]. However, the degree of bony resection required to achieve effective decompression with minimal tissue damage while reducing risks of the aforementioned complications remains debatable.
Removal of the entire lamina is not typically necessary because the main cause of spinal stenosis is related to the enlargement of the facet joint and ligamentum flavum as well as the extrusion of the intervertebral disc. At the beginning of 21st century, several authors introduced various lumbar decompression techniques to preserve the posterior midline structures [
5,
7,
8], including unilateral laminotomy for bilateral decompression (ULBD). In ULBD, complete bilateral decompression is achieved with minimal damage to the midline structure by performing ipsilateral hemilaminotomy instead of bilateral laminectomy. This technique then evolved into microendoscopic decompression with the use of microscopy and tubular retractors [
9]. For several decades, open laparotomy for appendectomy with cholecystectomy in general surgery department and open rotator cuff repair were the main procedures for the treatment of rotator cuff tears. However, laparoscopic surgery replaced open laparotomy about 10 years ago, becoming the mainstay for cancer surgeries. In addition, arthroscopy has replaced conventional open techniques for the repair of rotator cuff tears. The basic concept of biportal endoscopic spinal surgery (BESS) is similar to concepts of arthroscopic and laparoscopic surgeries, in which a scope is inserted from one side for viewing and the surgical instruments from the other side for working purposes through triangulation.
Recently, several authors have reported the use of BESS for the treatment of lumbar degenerative diseases [
10-
17]. BESS is advantageous because paraspinal muscle atrophy can be prevented due to reduced muscle dissection and retraction compared with open decompression and because damage to the surrounding tissues of the neural structures is low since fine discrimination under highly magnified views is possible. Decompression of the contralateral traversing root is relatively easy due to wide angle available for the insertion of an arthroscope. Continuous saline irrigation during surgery through the portals helps protect the soft tissue from bone debris and wash away potentially invading microorganisms to prevent infection.
Deep muscle dissection is not required when approaching the spinal canal through the interlaminar space and foramen via the intervertebral foramen. Several articles have recently described approaches for transforaminal lumbar interbody fusion, including percutaneous biportal endoscopic decompression, irrigation endoscopic decompressive laminotomy, and BESS [
15]. Indications for BESS are nearly identical to those for general open spinal surgery, such as central and foraminal stenosis, adjacent segment degeneration, low-grade spondylolisthesis, and lumbar disc herniation except for high-grade spondylolisthesis, deformity correction, fracture, or pathologic conditions.
Outcome and Complications
There has not yet been a prospective comparison of clinical outcomes among BESS and other decompression techniques, but most articles introducing this technique have reported favorable clinical outcomes and minimal complications [
10,
12-
16,
18]. In a series with a mean follow-up of 28 months following BESS and interlaminar decompression in 94 patients diagnosed with spinal stenosis, the Macnab criteria showed a good or better outcome in 87% of the patients and the Oswestry Disability Index (ODI) improved from 64.2±10.0 to 23.0±20.8. In this study, dural tearing occurred in six cases, which was repaired by conservative treatment without any additional procedure [
16]. In another study of BESS and interlaminar decompression in 58 patients with spinal stenosis, the outcome was good or better in 81% of the patients. Eight patients experienced complications related to surgery, with one case of postoperative epidural hematoma, two cases of dural tearing, two cases of transient leg numbness, and three cases of postoperative headache [
12]. In a study of BESS and interlaminar decompression in 105 patients with spinal stenosis using a 30° arthroscope, ODI improved from 67.4±11.5 to 22.9±12.4, with two cases of dura tearing and one case of postoperative epidural hematoma [
13]. In all three studies, perioperative complications occurred at the beginning of the learning curve and no wound infection occurred. Although this study did not focus on spinal stenosis, a comparison of patients who underwent discectomy with BESS and open decompression demonstrated that the early postoperative Visual Analog Scale (VAS) score was superior in discectomy using the BESS technique compared with that in open decompression and was advantageous in terms of estimated blood loss and hospital stay. However, there was no significant difference in ODI and VAS score between the two approaches [
18].
To date, two technical notes have been published regarding the use of BESS with transforaminal approach, but no randomized controlled study has been performed. Transforaminal decompression in 21 patients with foraminal stenosis with a follow-up period of 14.8 months showed that leg VAS score improved from 7.5±0.9 to 2.5±1.2 and the Macnab criteria showed good or better outcome in 81% of the patients. One case of dura tear was also observed in this study [
10]. In another study of transforaminal decompression using another BESS technique, although there were only 12 cases and a short follow-up period, no complications of dural tearing or neurological damage occurred [
14]. BESS with transforaminal approach follows the same principle as facet-preserving microscopic foraminotomy, in which less than 50% of the superior articular process is removed. The superior articular processes can be resected based on the upper border of TP at the lower level of the vertebra. Although there has been no report on spinal instability following the removal of the facet joint, translational and rotational stability is achieved in the movable segment when more than 50% of the joint is removed [
19]. BESS with transforaminal approach seems to prevent spinal instability by preserving more than 50% of the facet joint. It is performed with an arthroscopic magnified view using continuous saline irrigation such that surgeons can visualize the pathology more clearly.
There have been only two articles reporting technical notes and clinical outcomes of lumbar interbody fusion through BESS [
15,
20]. In a study of 69 patients who underwent biportal endoscopic lumbar fusion, the followup period was not longer than 1 year, but VAS score improved from 8.12±0.63 to 2.79±1.24 and ODI improved significantly from 45.65±12.97 to 15.41±9.07. Perioperative complications occurred in five cases, with two cases of dural tearing and three cases of postoperative epidural hematoma. Recent studies of BE-TLIF have reported that TLIF is possible during BESS, but dural tearing and root injuries occurred as perioperative complications; thus, caution is required to avoid root damage during cage insertion.
In case of lateral and anterior lumbar interbody fusion, restoration of sagittal balance and indirect decompression of spinal stenosis can be achieved through disc height restoration and spondylolisthesis reduction, but there is a disadvantage that canal encroachment cannot be directly restored in cases with severe spinal stenosis. Although TLIF through a direct posterior approach enables direct decompression of the spinal canal, it may damage the posterior muscles and ligamentous structures, leading paraspinal muscles atrophy. Direct decompression of the spinal canal is possible with BE-TLIF using BESS while preventing muscle atrophy due to less muscle dissection and retraction. In addition, endplate preparation can be performed thoroughly through a magnified arthroscopic view, which is advantageous for interbody fusion.
According to Choi et al. [
21], the learning curve of BESS is relatively short and the complication rate in the early learning period is approximately 10.3%. Dural and root injuries and postoperative epidural hematoma occurred as complications with this new technique [
21]. To date, there have been 648 patients who underwent interlaminar decompression, transforaminal decompression, and interbody fusion through BESS in 14 articles related to the biportal endoscopic technique and lumbar spine in the PubMed database (https://www.ncbi.nlm.nih.gov/pubmed). However, complications occurred in 39 cases (6.0%), with 20 cases of dural tearing, which was the most common complications during BESS surgery. Postoperative headache occurred in three cases, postoperative hematoma in five, root injury in four, transient paresthesia in two, and postoperative incomplete decompression in four. One case of iatrogenic hydroperitoneum occurred in the retroperitoneal space filled with water after the ventral approach to the psoas muscle during transforaminal decompression.
One of the noteworthy features of BESS is that postoperative infection, a relatively common complication of conventional lumbar spinal surgery, has not been reported. In a review article, postoperative infection of the lumbar spine occurred in 1% of the patients who underwent open lumbar discectomy and 1.9%–4.4% of those who underwent lumbar fusion surgery [
22,
23]. The incidence of infection following spinal surgery is lower than that with MIS [
24]. However, there has been no high-quality randomized controlled study or systemic review regarding infection rates between conventional open spinal surgery and BESS; thus, evidence to suggest that the risk of spinal infection is reduced with BESS remains insufficient. However, a review of 14 articles related to the biportal endoscopic technique reported no instance of spinal infection in 648 patients who underwent BESS. One of the advantages of BESS is that continuous irrigation with saline is used at a greater frequency than with conventional open spinal surgery. Because of the various reasons mentioned above, we speculate that BESS may reduce the risk of spinal infection.
The overall complication rate of BESS is close to 6%, which is relatively high and most complications were root injury, dural tearing, and incomplete decompression. Turbid water and an obscure field due to failure of bleeding control and continuous irrigation may lead to the abovementioned complications for surgeons not familiar with the technique and visual field. However, an attempt to increase the saline pressure to control bleeding should be avoided because of the risk of increases intracranial pressure and delayed recovery from postoperative headache or general anesthesia [
21]. Rather than attempting to obtain a clear view by increasing the infusion pressure, it seems more sensible to attempt facilitating outflow by applying an extension or cross cut of the fascia incision through the working portal. A clear visual field obtained through smooth continuous irrigation can prevent the complications mentioned here.
Several articles have reported the advantages of BESS. First, dissection of the paraspinal muscle is minimized, resulting in less damage to the posterior ligament and soft tissue and less need for inevitable fusion surgery for oneor two-level spinal stenosis with stability. Second, minimally invasive spinal surgery, such as microscopic ULBD, has the advantage of preserving the facet and contralateral bony structures compared with conventional laminectomy, but incomplete decompression may occur due to a restricted visual field and movement of the instruments. However, BESS has the advantage of microscopic ULBD to achieve complete decompression under a magnified arthroscopic field without the restriction of instrument movement because of the use of an independent portal. Third, continuous saline irrigation during surgery is a great advantage for preventing infection as well as ensuring a clean field of view.