1 Introduction

Epiduroscopic surgery of the lumbar spine utelizes the trans-sacral approach through the sacral hiatus. The procedure is called trans-sacral epiduroscopc laser decompression surgery. a novel procedure that aims to decompress nerve root, perform fragmentectomy or ablate sensitized sinuvertebral nerves growing into annular tears to manage pain generators located in the lumbar ventral epidural space. It employs a 0.9 mm ultra high definition fiber endoscope (epiduroscope), a 1 mm straight firing 550um Ho:YAG laser and a 1.2 mm flexible micro-forceps. these are all applied to the lumbar ventral epidural space through a 30 cm long 3.0 to 3.3 mm diameter steerable catheter.

This procedure is performed primarily to treat symptomatic lumbar disc herniation, either extruded or sequestrated. This indication has since expanded to include management of chronic discogenic back or leg pain from annular tear syndrome and it can be used to perform adhesiolysis in postoperative syndrome or failed back surgery syndrome. If available diagnostic imaging cannot ascertain the cause of chronic back and leg pain, direct epiduroscopic visualization may give away the reason. It has the benefits of a minimally invasive procedure and is done under local anesthesia with conscious sedation.

2 History

The history of SELD can be traced in as early as 1931, when Michael Burman, an orthopedic surgeon in New York Hospital for Joint Diseases, published a study using available arthroscopic instruments to visualize the contents of the spinal canal in cadavers. With some success to see dura and blood vessels and cauda equina, he reported that the ability to visualize the contents of the spinal canal might be especially important in establishing a diagnosis of tumor or inflammation [1]. From late 1930s to early 1990s, a small number of pioneering individuals worked to develop a minimally invasive way to improve preoperative diagnosis of spinal pain and improve its outcome using spinaloscopy (epiduroscopy) through the interlaminar and transforaminal route [1,2,3,4]. Their instruments, lenses and lighting capabilities, limited their progress. The emergence of fiberoptic technology with eventual miniaturization of the fiberoptic-endoscopic cable, refinements in video imaging/ recording and the discovery of the trans-sacral hiatus access using steerable microcatheter ushered in the advances in thechnique to allow therapeutic use of the trans-sacral approach [4,5,6,7,8,9].

Epiduroscopy has been accepted (evidence rating 2B+) [10] as one of the interventional procedures to mitigate painful lumbar spinal conditions from failed back surgery syndrome by mechanical adhesiolysis and accurate delivery of epidural analgesic and steroids [5,6,7,8].

The first use of a Holmium: Yttrium Aluminum Garnet (Ho:YAG) laser in combination with epiduroscopic visualization was published in 2002 by Reutten et al. for use in adhesiolysis [11, 12]. In a succeeding study, he reported 45.9% good outcome that was at par with results of other studies in managing post-surgery pain syndrome. He insisted that therapeutic intervention was possible, however, its therapeutic use was limited due to technical difficulties. Navigation of the endoscope was specially limited in access via the sacral hiatus [13]. In 1996, Witte et al. reported on the development of a laser-discotomy procedure with introduction of instruments via the sacral canal. They carried out morphometric analysis of 100 sacral bones to determine the diameters and curvatures of the instruments needed for laser-discotomy [14]. In 2002, Snoke received a patent (United States) for a method of epidural surgery through trans-sacral approach [9]. The expanded applicability of epiduroscopy to include more than just being able to perform adhesiolysis or accurate drug delivery system into becoming part of mainstream minimally invasive spine surgical procedure (epiduroscopic surgery) came about with the first publication about SELD by Lee, Lee and Lim in 2016. The study was able to confirm the value of SELD for treating lumbar disc herniation from data that indicated a reasonable assurance of immediate pain relief and an increase in function in life with a very low rate of morbidity and definite radiological decrease of the herniated disc size. It was shown to be a safe and effective treatment for symptomatic lumbar disc herniation [15].

To perform SELD, mastery of trans-sacral access anatomy, the sacral hiatus, the sacral canal, and the lumbar ventral epidural space (VES) is necessary.

3 Sacral Hiatus

The sacral hiatus (Fig. 26.1) is the entryway to access the spinal canal in a number of neurosurgical and anesthesia procedures in pediatrics and adult populations. It can be manually located by palpating the posterior midline dimple between the two sacral cornua (and sometimes the coccygeal cornua). Several studies reported that the sacral cornua may only be palpable in 38–79% of individuals [16]. The skin, subcutaneous tissue, and fat layer above the sacral hiatus may be 3–10 mm thick, and has no major vascular or neural structures in the way. It is closed by the superficial dorsal sacrococcygeal ligament (also called sacrococcygeal membrane) which has to be pierced in order to gain entry to the sacral canal [17]. Kilicaslan et al. (Turkish study) reported that bony anatomic variations do exist such as absent hiatus (0.3%), complete agenesis (1%), and bony septum (2.6%) in their study of 300 sacral bones [18].

Fig. 26.1
figure 1

The sacral bone with relevant landmarks for SELD

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The first step in SELD surgery is to gain entry to the sacral canal. The patient is positioned prone on a radiolucent Wilson frame to decrease the lumbosacral angle. A lateral C-arm projection is used to locate the sacral hiatus. Around 8–10 mL of 2% lidocaine is infiltrated on the skin down to the sacral hiatus and through the sacrococcygeal ligament. A 3–5 mm stab skin incision made over the sacral hiatus and a 4.0 mm metal introducer (Fig. 26.2) is applied through the sacral hiatus using 2-hand technique at 45° initially while gently decreasing the angulation as you go into the sacral canal until the mid-S3 vertebral body (Figs. 26.3 and 26.4). We have used the metal introducer effectively instead of the usual needle, guidewire, and serial dilation. If difficult entry will be encountered, a mallet may be used to gently tap the introducer through.

Fig. 26.2
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The 4 mm diameter metal introducer used to penetrate the sacral hiatus

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Fig. 26.3
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Insertion of the introducer trough the sacral hiatus. Initially at 45° then gradually decreasing the angulation to adopt to the path of the sacral canal under C-arm guidance. The yellow line represents the trajectory of the SELD catheter along the ventral epidural space (VES)

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Fig. 26.4
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Insertion of the metal introducer. (a) Infiltration of local anesthesia on the skin, subcutaneous tissue, and through the sacral hiatus; (b) two-hand technique for insertion of the metal introducer; and (c) the endpoint

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4 Sacral Canal and Location of the Thecal End

Before the introduction of the metal introducer into the sacra canal, it is important to know the location of the thecal end or the end of the dural sac in order to prevent inadvertent dural puncture. It is easy to identify the thecal end in the T1 sequence of the MRI study of each patient.

Porzionato et al. reported the mean distance between the hiatal apex and the dural sac to be 45–60.5 mm in adults and 31.4 mm in children [17]. A recent study by Park et al. reported this distance to be 62.8 ± 9.4 mm [19]. It is however more practical to use the sacral vertebral bodies as landmark for localizing the thecal end since it is easier to see them in the lateral projection of the c-arm X-ray. In 1991, Larsen and Olsen, using myelography, reported that 83.4% of dural sac on their subjects terminates in the upper half of S1 to the lower half of S2 vertebral bodies [20]. Aggarwal et al. (2009) found the dural sac to end at S2 (83.6%) in his cadaver study [21]. In 2006, Binokay et al. using MRI T2WI divided the sacral vertebral bodies into upper, middle, and lower thirds, and reported that the end of the dural sac was usually located at the upper one-third of S2 (25.2%) [22]. We found the study of Binokay to be comparable to our finding of 27.4% of the thecal end were located at S2A level. In our institutional study, we likewise divided the sacral vertebrae of S1 and S2 into thirds and designated letters A to C for each (Fig. 26.5). We have demonstrated the thecal end location to be at S1C and S2A levels in 51.2%. Another important observation we noted was that no thecal end was found to be at the level of S3; thus, it is safe to dock the metal introducer up to the mid-S3 level.

Fig. 26.5
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The S1 and S2 vertebra were divided into thirds and were designated letters A–C. The thecal end were located at the S1C and S2A in 51.2%. No thecal end was seen at S3 level

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5 Configurations of the Thecal End

The second and most important step in epiduroscopic surgery is the approach to the lumbar ventral epidural space (VES). The ventral compartment of the lumbar epidural space is where most degenerative pathologies that are amenable for SELD surgery may be found (Fig. 26.6). These includes: (1) Central-Paracentral disc herniation and can either be sequestrated or migrated (in expert operators, foraminal herniation may be managed also); (2) sensitized sinuvertebral nerves growing into areas of annular tears; (3) discal cysts, and (4) post-inflammatory or postoperative adhesions.

Fig. 26.6
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Degenerative pathologies found in the different compartments of the epidural space

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A common obstacle that a beginner may experience is the inability or difficulty to direct the steerable SELD catheter to the VES. With this in mind, we made a local institutional study to identify the different morphometric variations of the lumbosacral canal relevant to successful and uncomplicated epiduroscopic entry to the VES.

We observed two general types of configuration in the variations (shape and orientation) of the Thecal end (T-end). Type A with tapering T-end and type B with blunt T-end. These two types can further be classified by the orientation or direction of caudal attachment of the T-end; subtype 1 with dorsal oriented T-end, subtype 2 with ventral oriented T-end, and subtype 3 with neutral oriented T-end (Types A1, A2, A3, B1, B2, B3) (Fig. 26.7) . To try to explain the nature of this occurrence, we theorize that the differences in shape and orientation of the T-end have been influenced by the presence and distribution of the meningo-vertebral ligaments in the lumbosacral canal and by the caudal attachment of filum terminale externa. Scapinelli, basing on cadaveric studies, was able to confirm the presence of these ligaments that consists segmentally of ventral and lateral fibrous bands, connecting the outer surface of the dura to the endostium of the spinal canal. The most characteristic component is the ventral one, running from the anterior wall of the dural sac to the posterior longitudinal ligament and vertebral endostium. Due to their anchoring function, the ligaments at the level of the dural conus were significantly developed (sacrodural ligaments of Trolard and Hofmann) [23].

Fig. 26.7
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Configurations and variations of the thecal end. Two main types: Taper (type A) and Blunt (type B). Subtypes were classified according to the orientation or direction of the caudal attachment of the thecal end (b, c); subtype 1, dorsal oriented T-end; subtype 2, ventral oriented T-end; and subtype 3, neutral oriented T-end

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We also found out that the dorsal oriented T-end (A1 and B1) consists 44.2%; the ventral oriented T-end (A2 and B2), 9.8%, and neutral oriented T-end (A3 and B3), 46% of the study population. Furthermore, our measurements showed that the A1 and B1 has the widest average VES (antero-posterior distance) at the level of the T-end (5.3 mm) followed by A3 and B3 (3.3 mm). The A2 and B2 have the narrowest VES measurement at the T-end.

Among the various observations we noted in our study, the T-end, particularly its configuration, may be able to predict whether the steerable catheter will go into the dorsal or ventral epidural space. Our results reveal that the T-end which are tapering in shape and dorsal and neutral in orientation are favorable configurations for easy approach into the VES. Fortunately, based on the study, a significant number of patients have favorable configuration of T-end, 63% of subjects has tapering T-end, 90.2% of them has dorsal (subtype 1) and neutral orientation (subtype 3).

Taking the results of the our study in concert, the most favorable configuration of T-end to tSELD surgery may be “taper” in shape and “dorsal” in orientation (Type A1) while the least favorable may be “blunt” and “ventral” (Type B2). These results could be used by the operator, especially as beginner, as a perioperative guideline to be able to gain access to the Lumbar VES safely and successfully.

6 Sacral Canal and the Lumbar Ventral Epidural Space

The distal sacral canal contains extradural fat, vertebral venous plexus, lower sacral nerve roots, and the filum terminale externa only [24]. It is safe to enter the hiatus as long as one keeps in the midline aided by antero-posterior C-arm projection.

Once the tip of the SELD steerable catheter has reached the cranial endplate of S1 (Fig. 26.8a) under C-arm guidance, its location in the VES can be confirmed by performing an epidurogram on a lateral projection. The flow of the contrast media in the VES will closely follow the contour of the posterior body line and flows anterior to the superior articular process (roof of the foramen) that leaves the neural foramen to be clearly appreciated in lateral C-arm view(Fig. 26.8b). As soon as correct placement of the catheter in the VES is verified, epiduroscopy may begin. Gradual saline irrigation is necessary to dilate the VES and allow around 3–5 mm viewing distance for the epiduroscope.

Fig. 26.8
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Tip of the steerable SELD catheter at the level of the cranial endplate of S1 (a). Flow (blue arrow) of contrast media in the ventral epidural space (VES) characterized by (1) it closely follows the contour of the posterior body line, (2) it flows anterior to the superior articular process and the roof of the foramen (white arrows) is clearly appreciated in the lateral projection (b). Endpoint epidurogram is performed to confirm decrease in the size of disc herniation and mark the end of the SELD procedure (c).

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Epiduroscopy offers limited visualization to the operator. This limitation can be overcome by preoperative mastery of the patients’ symptoms and physical examination findings correlated with their MRI study. Identification of epiduroscopic structures may also prove difficult for the beginner, knowledge of normal epiduroscopic anatomy (Fig. 26.9) will be necessary to differentiate and recognize abnormal and pathologic structures (Fig. 26.10).

Fig. 26.9
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Tip of SELD catheter at the L5 cranial endplate level (a and b). Common normal epiduroscopic structures (c and d). (MVL Meningo-vertebral ligaments, PLL Posterior longitudinal ligament)

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Fig. 26.10
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Common pathologic epiduroscopic structures: the dura and root are seen being compressed by the herniated nucleus pulposus (HNP) underlying the bulge in the posterior longitudinal ligament (PLL) except in image in b where the HNP has gone through the thinned out PLL. The yellow arrow in d points to inflamed adhesions beside an inflamed and bulging PLL. The blue arrow in e points to a minimal bleeding that can be encountered during the procedure

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The strong points of this procedure were that it is performed on awake patients and it renders it very safe. It is possible to touch the disc herniation or pathologic level with the laser tip or with the catheter itself to verify concordant pain which will help in localization. Patients with degenerative disc disease may present with multiple pathologies at different disc levels and SELD will be able to reach and manage them through a single entry point (Fig. 26.11). The highest level that we have reached with ease is at the level of L1/2 disc space.

Fig. 26.11
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Cadaver dissection study demonstrating the ability of the SELD steerable catheter to reach and manage indicated lesions up to Lumbar 2/3 level. (Photos courtesy of Professor Daniel H. Kim, Mischer Neuroscience Institute, Memorial Hermann- Texas Medical Center)

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The subsequent stages of the SELD procedure involve application of Ho:YAG laser to vaporize the disc herniation to decompress the affected nerve root, morcellation and removal of larger pieces from the VES using 1 mm micro-forceps and determination of the end point. These are all done under direct epiduroscopic visualization with C-arm guidance. The discussion of the complete technique however is not covered by this article and will be discussed in succeeding sections.

7 Summary

Anatomic and morphometric studies were a requirement for the development of new surgical techniques and procedures. The article has shown in a concise but detailed manner the anatomic and morphometric considerations that are relevant to SELD. During the approach to the sacral canal, docking the metal introducer that is used as a trocar and working channel up to the mid-body level of S3 is safe. The VES is the working space for SELD. Review of the patients’ particular thecal-end configuration in the MRI T1 sequence is integral for the successful entry to the VES. Choosing a patient with a dorsal oriented and taper shaped t-end will make the approach easier for the beginner and safer for the patient as will the mastery of normal and patho-anatomic epiduroscopic structures.