Overview
The understanding of preserving normal tissues to achieve surgical goals drives surgeons to adapt to minimally invasive spine surgery. The current posterior microscopic lumbar diskectomy is still considered a gold standard procedure all over the world because of its low morbidity and high success rate, but it is associated with muscle dissection and bone removal, which may create muscle weakness and instability and also may cause epidural scarring.
The surgeon should recognize that lumbar disk herniations can be managed by percutaneous endoscopic techniques to reach the disk pathology but limit tissue damage and further consequences of this tissue damage. Kambin and Sampson and Hijikata performed the initial nonvisualized posterolateral nucleotomy, and Kambin also described and illustrated the working zone of percutaneous endoscopic lumbar diskectomy (PELD). All the earlier percutaneous procedures concentrated on nonvisualized diskectomy to indirectly decompress the nerve root, and this was largely due to unavailability of working-channel endoscopes.
In 1997, Tsou and Yeung introduced the rigid rod-lens integrated working-channel endoscope. This allowed diskectomy with uniportal access under constant visualization. Because of recent advances in technology—refinement of optics, better mechanical instruments, bipolar flexible radio frequency (RF) electrodes, endoscopic lasers, and endoscopic burrs—the technique is being continuously refined to increase access to almost all types of disk herniations. The working cannula is now placed closer to the epidural space and the base of the herniation, allowing the surgeon to selectively target the extruded disk herniation.
In recent times, surgeons have described different approaches to increase the success of PELD by targeted placement of the cannula to increase visualization and targeted decompression. The transforaminal access is limited in certain situations at L5–S1. In such cases, an interlaminar PELD is done to address the herniated disk at this level. The results of endoscopic diskectomy are comparable to microscopic diskectomy with the added benefit of reduced tissue damage, fewer complications, less epidural scarring, and an earlier return to work. Recently, access to the difficult L5–S1 disk has been described through a transiliac route and migrated herniations. In this chapter, we will describe the anatomy, surgical technique, and technical advances for percutaneous endoscopic diskectomy.
Surgical Anatomy
Percutaneous endoscopic diskectomy is usually performed through the intervertebral foramen (IVF). Rarely, for the L5–S1 disk, it might be difficult to achieve a trajectory through the IVF to the herniation because of anatomic constraints: high iliac crest, large facet joint, large ala, and narrow foramen. The interlaminar access can be used for diskectomy. The anatomy pertinent to the IVF and interlaminar access is described for effective percutaneous endoscopic diskectomy.
Transforaminal Approach
The IVF is the gateway for a transforaminal approach. It is formed by two mobile joints: the intervertebral disk and the zygapophyseal joint ( Fig. 42-1, A ). The dimensions of the foramen change dynamically from each level and from ongoing degenerative diseases of the disk.
The IVF from L1 to L4 is more the shape of an inverted pear, and at L5, it is more oval. The superior–inferior dimensions of the IVF are greatest at L2–L3. This length decreases as we move caudally, with the shortest lengths found at L5–S1. The anterior–posterior (AP) dimensions change from 7 mm at L1–L2 to 9 mm at L5–S1.
The boundaries of the foramen are (see Fig. 42-1, A ):
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Roof: Inferior vertebral notch of the pedicle of cephalic vertebra, outer free edge of ligamentum flavum
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Floor: Superior vertebral notch of the pedicle of caudal vertebra, posterior margin of the inferior vertebral body
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Anterior wall: Posterior aspect of adjacent vertebral bodies with their disk, lateral expansion of the posterior longitudinal ligament (PLL) and anterior longitudinal venous sinus
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Posterior wall: Superior and inferior articular process of corresponding facet joint and lateral prolongation of ligamentum flavum
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Medial wall: Dura
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Lateral wall: Fascia overlying the psoas muscle
The IVF contains (see Fig. 42-1, B ):
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The exiting nerve root with the dural sleeve
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Lymphatic channels
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Segmental artery
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Communicating veins between internal and external venous plexi
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Recurrent meningeal (sinuvertebral) nerves
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Adipose tissue
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Ligaments
The nerve root or the dorsal root ganglia occupy the upper part of the foramen. This neural tissue may occupy 30% to 50% of the IVF in the upper part. The lower part is available for safe placement of the working cannula. The surgeon should always review foramen anatomy on both computed tomography (CT) and magnetic resonance imaging (MRI) scans before the procedure. It is important to review the foraminal sagittal MRI for abnormal large vessels in the lower part of the foramen, because if these are present, this vessel configuration will not provide a safe corridor to approach the disk; however, the success of the PELD does not depend on identification of these foraminal ligaments.
Triangular Safe Zone
A safe area for access to the herniated disk lies between the exiting and traversing nerve root. Kambin described this triangular safe zone in 1991 as being the annular zone bordered anteriorly by the exiting root, inferiorly by the end plate of the lower lumbar segment, posteriorly by the superior articular process of the inferior vertebra, and medially by the traversing root ( Fig. 42-2 ). The maximal safe area for insertion of the endoscopic sleeve is the medial end of the triangle (see Fig. 42-2 , inset ). The point of insertion is to be referenced according to accepted and best-visualized radiographic landmarks for safe placement of a working cannula. Mirkovic and colleagues investigated the IVF anatomy from L2 to S1 vertebrae to define the safe working zone and the largest working cannula that can be introduced. The average dimensions of the triangular safe zone were 18.9 mm wide and 12.3 mm in height with a 23 mm hypotenuse. A cannula with a diameter of 6.3 mm placed in line with the center of the pedicle and slightly cephalad to the disk midline or a 7.5-mm cannula placed in line with the medial one third of the pedicle and slightly cephalad to the disk midline provides safe and maximal intradisk access ( Fig. 42-3 ).
In another cadaveric study, by Wimmer and Maurer, the maximum safe cannula diameter is 8 mm on average from L1–L2 to L3–L4 levels. From L4–L5 to L5–S1 the diameter decreases to 7 mm, which is attributed to disk degeneration at these levels. In actuality, the working cannula diameter can be greater if the cannula is placed eccentrically in the safe zone, and distraction by the cannula occurs in the disk space. In a recent cadaveric study, the triangular safe zone dimensions were determined by Choi, wherein the height of the triangular safe zone was formed by the lateral zone of the thecal sac, not the medial pedicular line; the base was formed by the superior end plate of the inferior vertebra; and the hypotenuse was formed by the spinal nerve. The average dimensions were 13.41 mm wide by 26.8 mm high with a 25.49-mm hypotenuse. The triangle formed was right angled at the upper lumbar vertebrae and obtuse angled at the lower segments; this allowed larger diameter cannulas at lower lumbar levels compared with upper lumbar levels.
Min and colleagues described the exit zone of the IVF in cadaveric dissection and stated that the actual working zone was not a triangle but rather a trapezoidal space bound by the superior articular process, the exiting nerve root on the sides, and imaginary lines parallel to the disk space ( Fig. 42-4 ). Descending from upper to lower lumbar levels, the angle of the oblique side goes on decreasing, and the dimensions of the base go on increasing. The authors also recommended directly viewing the annulus endoscopically before blind annulotomy. The exiting nerve root is closer to the entry portal than the dura. This should lead the surgeon to place the cannula more eccentrically at lower lumbar levels. It is also recommended to place the cannula close enough to the facet joint to scrape it to avoid nerve injury.
Compared with the lower levels, at the upper lumbar level, at L1–L2 and L2–L3, the thecal sac lies against the medial wall of the pedicle. The upper lumbar disk is also more concave compared with the lower disk. The surgeon should target the more medial disk (6 to 9 cm) that is steeper at upper lumbar levels. The surgeon should remain lateral to the midpedicular line to prevent dural sac damage ( Fig. 42-5 ).
The surgeon should preoperatively evaluate the foramen morphology, because the hypertrophied facet joint and the bulging disk may limit the actual dimensions of the safe working zone. These anatomic variations of the safe zone and also the potential for any congenital root anomalies, low-lying root, or abnormal large vessel may further lead to distortion of the safe zone. So it is important that the surgeon perform the procedure with the patient under local anesthesia so as to continuously assess the patient physically (toe and foot movements) and observe the patient’s pain response while placing the working cannula.
Endoscopic Anatomy
Compared with joint arthroscopy or other endoscopies, there is no well-defined cavity for spinal endoscopy. Therefore the surgeon must create a potential space to dissect toward the pathology. Because the access is through a bony window with vital neural structures, the surgeon must preoperatively define the target lesion and the trajectory to the pathology. The surgeon should be able to recognize these vital neural structures and differentiate them from other structures endoscopically. Doing chromodiskography with indigo carmine can identify the disk pathology and differentiate it from other structures.
If the surgeon is outside the disk in a transforaminal approach, the periannular tissue is encountered first. The surgeon can differentiate between periannular and epidural structures by recognizing certain features. The periannular structures consist of loosely woven fibrous tissue with some fatty tissue overlying it. The periannular fat is stationary compared with the epidural fat ( Fig. 42-6, A and B ). Once this periannular fat is cleared off with an RF probe (see Fig. 42-6, C ), the superficial layer of annular fibers and lateral expanse of the PLL are visible, but the PLL cannot be differentiated (see Fig. 42-6, D ). Surgeons routinely perform the inside-out technique, first entering the posterior part of the disk completely and making space within the disk space; next, the herniated disk is pulled into this space and is drawn out through the cannula, a technique that might be helpful in contained herniations. The endoscope shows nuclear tissue that resembles blue fluffy cotton. Compared with the annular tissue, the nucleus is tough; it lies in layers and does not melt with the RF probe.
Interlaminar Approach
The interlaminar PELD at L5–S1 is feasible because of the peculiar anatomy at this level. Ebraheim and colleagues found that the interlaminar width and distance is greatest at L5–S1, and the free space in the spinal canal is also greatest there, because it contains only the thecal sac and sacral nerve roots. The ligamentum flavum is a 2- to 6-mm thick yellow structure that spans the interlaminar space. It is thinnest at this level and is the only barrier for the epidural space. The ligamentum flavum is an active ligament that has an essential biomechanical role, and it is an active barrier for the thecal sac. Therefore the integrity of this ligament must be preserved. The wide interlaminar space and the thin ligamentum flavum allow easy passage and maneuverability of the working channel at this level. On withdrawal of the working channel, the opening in the ligamentum flavum spontaneously closes and restores its function as a protective barrier.
The other most important feature at this level is the peculiarity of the S1 nerve root anatomy. In cadaveric dissection, the relationship of origin of lumbar spinal roots to the intervertebral disk was studied. It was found that the S1 nerve root originated above the level of the L5–S1 disk in 75% of cases and at that level in 25%, but it was never below that level. The angle of takeoff of the S1 nerve root from the thecal sac is relatively less than at the other lumbar levels. All these features will contribute in the type of herniation at L5–S1. Thus the most common herniation here is axillary herniation, which displaces the nerve root far into the subarticular region and creates a potential space between the thecal sac and the nerve root ( Fig. 42-7, A ). Shoulder disk herniation at this level is relatively uncommon, but if present, it also provides access by pushing the nerve root medially (see Fig. 42-7, B and C ).
Indications and Contraindications
Indications
Soft Lumbar Disk Herniation
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The ideal indication for transforaminal endoscopic diskectomy is an extraforaminal far-lateral disk herniation.
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Depending on the skill, this technique is useful for:
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Recurrent herniation
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Synovial cyst
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Biopsy and débridement of diskitis
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Foraminal stenosis
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Relative Contraindications
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Cauda equina syndrome
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Coagulopathy
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Instability
Operative Technique
Equipment ( Fig. 42-8 )
Spinal Endoscope
Anthony Yeung developed the first working-channel endoscope in 1997, and it was approved for use by the U.S. Food and Drug Administration (FDA) in March 1998. With a working-channel scope, the instruments are small and pass through the scope. These are off-axis scopes used to look around corners or edges at the depth of the operative field. An elliptical cross-section of the endoscope in a circular working cannula allows potential space for outflow of irrigation fluid. Recently, these endoscopes were modified to accommodate a large working channel, which allows passage of large-size forceps, chisels, and endoscopic burrs.
Related posts:
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Anterior and Posterior Cervicothoracic Junction Stabilization Techniques
Lumbar Facet Screw Fixation Techniques
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Primary Malignant and Benign Tumors of the Spine
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