Chapter 28 Posterior Lumbar Foraminal Decompression

Since the first publication by Hijikata and colleagues [1] in 1975 on purely percutaneous intradiscal lumbar disc decompression, several developments have been made with this approach for different forms of lumbar, and later also cervical, disc pathologies. The original Hijikata technique, developed for Japanese patients, was adopted in Zurich after its presentation at the 1978 SICOT (Société Internationale de Chirurgie Orthopédique et de Traumatologie) meeting in Kyoto, where it was introduced as a unilateral posterolateral decompression technique for contained lumbar disc herniations. It became evident that the sizes of the original instruments would require adaptation to the larger dimensions of anatomical structures in European patients. Specifically, a stepwise modification of the diameter of the dilating cannulas was needed to reach a working channel of 6 mm. With the larger cannula, the introduction of arthroscopic control became possible in 1982. This arthroscopic control, when used with a biportal cannula approach [2], for the first time allowed simultaneous endoscopic control, or discoscopy [3], of intradiscal selective tissue removal in the posterior field of the intervertebral disc space. Through this process, the indirect decompression of subligamentarily contained lumbar disc herniation was standardized in its specific range of indications.

Other surgeons aimed for technical improvement of uniportal intradiscal disc decompression by higher abrasive activity, with new devices such as the automated nucleotome introduced in 1985 by Onik and associates [4]. Its ease of application made disc surgery available to a larger group, including nonsurgical physicians, explaining the somewhat uncontrolled boom of percutaneous disc decompression in the later 1980s. This overuse with unconvincing results contributed to a rapid decline in the reputation of percutaneous methods for disc decompression in general. Because the slim aspiration cannula vacuumed disc tissue from any direction, limited control of the direction of disc tissue removal was possible. The tissue removed was not necessarily aspirated from the posterior subanular zone. The same inherent problem existed with later centrodiscal ablative laser disc decompression, which clearly limited this application to indirect decompression of symptomatic contained protrusions and did not allow for calculable tissue removal in the posterior subligamentary zone. Such calculable removal was better achieved through biportal endoscopically controlled mechanical tissue extraction with pituitary forceps under endoscopic control. In this approach, direct visualization of tissue fragment removal became available and challenged conventional microdiscectomy in this range of indications. Owing to this experience, it became evident as early as the 1980s that intradiscal techniques could not be adapted for adequate removal of any transligamentary sequestrated herniation because of geometric constrictions. In these cases, conventional open techniques, such as minimal interlaminotomy or microdiscectomy [5], remained the “gold standards.”

Endoscopic applications in the area inside the anulus fibrosus were in clinical use with biportal percutaneous nucleotomy beginning in the mid-1980s, but by its nature the technique remained without direct therapeutic use for extradiscal tissue removal. In the United States, Kambin used his anuloscopy for medicolegal reasons from 1988 on for endoscopic documentation of the anular fibers before their trepanation with his trephine-cutter, which was then his standard method for uniportal entrance into the lumbar disc with decompressive forceps [13]. With this method he could avoid damage to the exiting roots during anular trepanation. His technique was further developed by Savitz [6] in the 1990s, in combination with laser-assisted fluoroscopic microdiscectomy. In our experience, the laser application did not offer convincing advantages over mechanical tissue removal with a biportal approach, but others found it advantageous for tissue shrinking in coaxial unilateral applications [7,8].

In 1990, on the basis of our experience with minimally invasive techniques such as intradiscal endoscopic control for pointed mechanical tissue removal, we tried to explore the anular and foraminal site locally at the end of our biportal interventions. However, with the 7.5-mm working cannula that was available at the time, such exploration was not successful. During the same period, we made the first pathoanatomical investigations into the use of rod-lens coaxial endoscopic devices derived from integrated cystoscopes for use in non-prefigured retroperitoneal compartments. These devices were the precursors of a 6-mm, open coaxial endoscopic technique for direct visualization of foraminal sequestrated herniations, introduced clinically for the first time worldwide in 1991 [9,10].

Development of similar methods followed in the United States in 1993 for lumbar decompressive applications. Later, the uniportal coaxial endoscopic procedure was adopted by others, such as Yeung and Chow [8], who developed a similar system with a slightly smaller working channel but a second channel for laser fiber application as an additional working tool for disc decompression and bony enlargement of the foraminal access (Fig. 28-1).

Figure 28–1 Foraminoscope as developed in cooperation with Karl Storz GmbH Tuttlingen, Germany. Straightforward 6-degree, telescope autoclavable, with a 3-mm instrument channel.

Anatomical and technical considerations

In a European of average height, the lower lumbar foraminal ports measure about 1.2 to 1.6 cm in craniocaudal height and 6 to 8 mm in dorsoventral diameter. The lumbosacral junction L5-S1 is slightly less wide, and owing to its location under the level of the iliac crests, it is difficult or even impossible to approach for any percutaneous lateral or transforaminal technique. If anatomical accessibility of the foraminal area seems questionable, a local transforaminal steroid injection under fluoroscopic control can help screen for this method of decompression.

The foraminal port is composed primarily of fat tissue, which surrounds the exiting and descending root with its sensory ganglion. The root then further descends lateroventrally into the fossa lumbalis and joins the plexus lumbalis. Other connecting venous vessels draining the peridural venous plexus pass the lower foraminal site and can cause local venous bleeding during surgical procedures. The lumbar segmental arteries, which are normally located somewhat cranial to the foramen and lateral to the pedicle, are usually not encountered during foraminoscopic surgical procedures.

For the clinical presentation of typical dysesthetic radicular pain (e.g., pretibial burning sensation in a L4-L5 foraminal compression), the compression of the sensory ganglion beyond the pedicle by ascending foraminal disc herniation is the typical anatomical correlation (Fig. 28-2). Affected patients, without additional herniation in the canal and therefore without compression of the lateral recess, hardly ever show root tension signs, such as positive response to Lasègue maneuver.

Figure 28–2 Intraforaminal herniation with an ascending sequestrated disc tissue that is compressing the exiting root with its ganglion in the subpedicular zone (arrow).

Because the posterior longitudinal ligament is covering only the medial part of the foraminal zone, we often see a combination of partially subligamentary contained (medial) and free (more lateral) sequestrated herniation. It is our experience that larger lateroforaminal fragments are more often seen patients older than 55 years who also have some twisting-instability and already visible facet changes.

Our first clinical experiences with posterolateral foraminoscopic exploration showed that because of fatty pads and venous bleeding, an efficient endoscopically controlled tissue removal would experience limited success without some temporary spacing in this anatomically not directly preconfigured “virtual optical chamber.” In experimental settings, it became evident that for other endoscopic applications, available gas media such as nitrogen and carbon dioxide were inappropriate because of their rapid dissipation into the surrounding tissue compartments in addition to their potential embolic complications. To avoid these complications, isotonic liquids such as saline and lactated Ringer solution were standardized for this purpose. In addition to providing some local spacing, the continuous flow of irrigation also keeps visibility clear. In cases with venous bleeding, the irrigation pressure can be raised above 40 cm H₂O column pressure, which will clear venous bleeding from the area. This slight hyperpressure could be optimized by introduction of posterior rubber valve-taps that allow tight penetration with fine 3-mm forceps through the working channel down to the working zone. Nevertheless, some dissipation of irrigation fluid into the retroperitoneal space, up to 600 mL during a 40-minute procedure, remains common. We have not seen any measurable intraoperative hemodynamic or postoperative clinical effect of this retroperitoneal volume load, which is reabsorbed within hours.

First using saline solution at room temperature, we found a specific positive effect for cooled irrigation (around 8° C). The use of cooled irrigation causes vasomotor contraction, by which fine arteriolar bleeding can also be markedly reduced, thereby reducing the necessary amount of irrigation volume from more than 4 L to around 2 L every 30 minutes. Consequently, the local hypothermia in the foraminal area, besides providing a more routine technical application, may have also contributed to a decrease in postoperative analgesic demands.

Indications

The target indication for foraminoscopic microdiscectomy since its introduction in 1991 has been lumbar foraminal and extraforaminal disc herniation with clinical root/ganglion compression syndrome that is resistant to conservative treatment or is rapidly evolving under conservative treatment. The incidence of this condition in our population is around 10% of all operative lumbar disc cases [11].

The diagnosis of this type of herniation is often delayed because it occurs outside the lumbar canal [5] and therefore often does not correspond to classic clinical signs, such as a positive response to Lasègue root tension maneuver, nighttime pain, and pain aggravated by lying on the side opposite the herniation site.

Radiologic Diagnosis

Conventional radiographs may show some chondrosis of the respective level, and antalgic scoliosis is almost never seen. Because of mechanical irritation lateral to the thecal sheet of the exiting root, myelography results remain negative.

The best diagnostic tools are computed tomography (CT) and magnetic resonance imaging (MRI):

CT often shows some additional bony facet disease.

MRI best shows the extent of the foraminal compression, with obliterated fat tissue around the exiting root and ganglion and the typical disc tissue signal of the sequestrated material in the paramedical sagittal plane on T1- or T2-weighted images.

In patients with previous surgery for mediolateral herniation, post-discography CT or gadolinium-enhanced MRI is the most reliable diagnostic tool.

Discography to screen for linear accessibility of the foraminal port can be of specific value in cases with a high iliac crest for patients being considered for surgery at the L4-L5 or L5-S1 level.

Operating room setup and anesthesia

Foraminoscopic microdiscectomy is performed exclusively under aseptic laminar-flow operating room conditions, like any other form of discectomy (Fig. 28-3).

Figure 28–3 Draping after disinfection. Large sheets are mandatory for biplanar fluoroscopy.

Routinely we give perioperative antibiotic prophylaxis with triple-dose cephalosporin (second-generation) for 24 hours. The choice of anesthetic technique ranges from local anesthesia in combination with neuroleptic sedation to general intravenous anesthesia without muscular relaxation in order to maintain muscular radicular pain response. We recommend this second type to all patients who feel considerable irradiating pain in the prone position, which we test for the day before surgery.

For local anesthesia we use 1% lidocaine with adrenaline for the skin and the subcutaneous and muscular layers down to the intertransverse level. We systematically avoid deeper infiltration to the triangular working zone, in order to maintain the radicular pain response during the introduction of the guide needle and its guidewire while pinning up the anulus/sequestrum in the triangular working zone. If necessary, once the guidewire is in place and the dilating cannulas are slipped over, some additional local anesthesia can be injected without further risk of radicular pinning-up.

Once the foraminoscope is in place, an additional small infiltration can be added directly to the anular fibers under endoscopic control. In order to avoid anesthesia of the sensory ganglion, we avoid free infiltration into the foraminal space. Experience shows that under cool irrigation with 8° C lactated Ringer solution, the local structures, such as periosteum of the adjacent vertebral rims, are not highly sensitive to pain, and the additional infiltration of the anular fibers is not routinely necessary.

Because the operating room time for freshly sequestrated foraminal herniations is relatively brief, local anesthesia is the best option for patients in whom the time between onset of radicular compression and foraminoscopic sequestrectomy has been relatively short. In patients with onset of symptoms more than 6 to 8 weeks previously or sequential plurifragmentary sequestration with stepwise-increasing symptoms over several months, general intravenous anesthesia without muscular relaxation is more suitable. In such patients, perifocal adhesions with scar formation may require more time for tissue mobilization under foraminoscopic control. Using general anesthesia in these cases ensures that no accidental mass-motion of the patient will interfere with the decompressive manipulations. Procedures involving decompression time of more than 30 minutes can be somewhat cumbersome for patients awake in prone position with only local anesthesia.

Procedure

1. The patient’s written informed consent is obtained.

2. The position of the patient on the operating table is straight prone, without any bending of the knees. To reduce intra-abdominal pressure with its effect on lumbar venous plexus, we lay relatively resistant foam blocks the anterior iliac crests. This maneuver also reduces the lumbar lordosis and lowers the iliac crests slightly in relation to L5, so that the linear percutaneous access is further facilitated at least down to L4-L5.

In patients in whom access to L4-L5 or L5-S1 limited, some lateral bending of the thoracolumbar transition to the opposite side can help the surgeon gain foraminal access and extract free fragments during the procedure.

3. Once the optimal position is achieved, the anatomical landmarks are inked with a water-resistant pen. The site of skin entry is the projection of the intervertebral space in relation to the end plate of the caudal adjacent vertebra. Placement of the skin entry site lateral 10-12 cm to the midline of in slim patients, or 14-16 cm in more obese patients, may be necessary (Fig. 28-4). For the trajectory, check axial control of the marked entry point to the foraminal working zone should be checked under fluoroscopy.

Figure 28–4 With patient in the prone position, the entry point as measured under fluoroscopy is marked with a waterproof pen.

In more obese patients, a CT slice obtained with a wide window including the soft tissue and skin at the respective disc level in prone position before surgery can help in calculating the lateral distance, thus avoiding the use of instruments that are too short (Fig. 28-5).

4. Once the site is disinfected, a 18-cm, 1.7-mm puncture needle (like those used for discography) with 1.2-mm guidewire is inserted under fluoroscopic control down to the triangular working zone. The different layers can be felt, such as the less resistant subcutaneous fat tissue, then the clearly resistant posterior fascia, and then the less resistant muscular layer (Fig. 28-6).

5. The tip of the needle often transfixes the sequestrum in the lateroforaminal zone and should be pinned into the anulus/disc for another 10 mm to avoid loosening of the position. The guidewire is then pulled out of the needle, and the longer, 31-cm guidewire is slipped down through the needle into the disc. The three gradual-dilation sleeves, 3 mm, 4.8 mm, 6 mm, are slipped stepwise over each other, while the guidewire is maintained as the central anchorage pinning the fragment and disc. As before, during introduction of the increasing diameters, the changing resistance of the different tissue layers can be felt (Figs. 28-7 and 27-8).

6. Once the larger, 6-mm working cannula is in place, the smaller cannulas are pulled out, leaving the guidewire in place at the tip. Now the working scope is introduced, gliding with its working channel on the guidewire under permanent irrigation (40-60 mL/min) (see Fig. 28-8).

7. The border of the up-pinned sequestrum can often be seen hanging into the end of the working cannula. Through retraction of the guidewire and introduction of the two 2.7-mm spoon forceps, the surgeon can snap the fragment and pull it back across the working channel. In cases with larger fragments, it may be necessary to extract the fragment by pulling it back together with the working scope across the remaining working cannula (Fig. 28-9).

8. Once the fragments hanging into the working space are mobilized and pulled out, a small free working space can be made by pulling back the working cannula with the optic by 3 to 4 mm and rinsing out the cannula for further inspection of the foraminal port (Figs. 28-10 and 28-11).

9. In the first phase, after removal of the free fragments, some venous bleeding is often seen. With irrigation (with the 8° C cooled isotonic lactated Ringer solution), such minor venous bleeding often stops spontaneously. To clear the view more quickly, the flow rate can be raised to 70 to 80 mL/min.

Figure 28–5 Computed tomography scan at the L4-L5 level with a left-sided latero/extraforaminal extrusion. The angulation here would be 45 to 60 degrees, with the respective distance at the skin level (white arrow) to measure for the optimal approach.

Figure 28–6 Introduction of guidewire and subsequent overslipping sleeves under fluoroscopic guidance.

Figure 28–7 The needle (black line) should point to the lower foraminal zone in order to avoid conflict with the exiting root in the subpedicular zone, as shown in this L4-L5 level sagittal cut through the foramen.

Figure 28–8 Fluoroscopic check of needle and cannula position.

Figure 28–9 Endoscopic view at the beginning: Some coagulum still overlies the sequestrum.

Figure 28–10 The sequestrum is mobilized under endoscopic guidance and pulled out piece by piece.

Figure 28–11 The endoscopic view after the working cannula is retracted by 3-4 mm. Larger fragments are pulled back.

If necessary, the procedure can proceed under slight overpressure (30-40 m H₂O) for 30 to 60 seconds; this overpressure is achieved by using a rubber tap at the end of the working channel. The tap has a small hole that allows for the introduction of slim instruments such as pituitary forceps but prevents backflow of irrigation fluid. The surgeon must be aware that with continuous irrigation and the additional overpressure technique, some irrigation liquid dissipates into the surrounding retroperitoneal paravertebral compartment. In a routine case we can lose up to 750 mL. We have never seen a hemodynamic reaction to this loss. It seems that this amount of liquid is silently reabsorbed and discharged. From lumbar vertebral traumatology, we know that the retroperitoneal space is large enough to bear hematomas of up to 1 to 2 liters without local functional impairment.

Using today’s available flow-rated irrigation pumps, we very rarely see coagulum considerably impairing the light intensity and, thus, the endoscopic view. The situation was different in the early 1990s, when only low-pressure pumps with low flow rates were available. Also, the time-consuming repositioning of the scope in the working zone is rarely necessary now under optimal continuous visual endoscopic control. Once the system is in place with visual orientation in the triangular working zone, the use of fluoroscopy is required for only a few exposures during the exploration of the upper foraminal port to check the remaining safe distance to the lower pedicular rim. A distance of 3 to 4 mm or at least 50% of the diameter of the working cannula should be maintained in order to avoid unnecessary mechanical squeezing of the exiting root and intraforaminal ganglion.

10. Once the first fragment extraction and decompression have been achieved in the lower lateral foraminal zone at the level of the intervertebral disc, the foramen is further explored under endoscopic control to the midforaminal and medioforaminal zone, following the specific needs of the procedure as documented in the preoperative imaging. During exploration of the upper foraminal zone, special attention is given to the exiting root and the ganglion.

11. The often cranially migrated fragments are mobilized with a small L-hook, pulled downward to the disc level, and extracted. In subligamentary, contained sequestrations, it is helpful to open the ligament by means of a side-cutting probe at the upper disc rim under endoscopic view before attempting mobilization of the fragments with the hook and pulling them downward for extraction.

After removal of the herniation, the free way out along the exiting root can be carefully palpated with the L-hook under endoscopic view. To improve direct inspection in the upper third of the foramen, some caudal tilting of the scope is helpful. In order to avoid root compression, it may be necessary to pull back the scope somewhat to decrease the danger of root entrapment between the end of the scope and pedicular border. A unilaterally nose-type cannula can help selectively guide the exiting root away during decompression.

In patients with relatively narrow foramina, some writers have recommended enlargement by trimming the anterior rim of the facet forming the posterior “wall” of the foramen. Various methods have been advocated, such as laser-cutting and drill-trimming. We found it difficult to use mechanical trimmers under endoscopic control because of bleeding of the trimmed bony surface. With a laser working directly on bone, such problems can be reduced with the laser’s partially hemostyptic effect. Nevertheless this technique has a considerable thermal potential for weakening the structural stability of the facet for anteroposterior stress moments by direct mechanical impairment or secondary osteodystrophic changes. Therefore, in our practice, endoscopic foraminal bony facet trimming is not a primary option.

12. After control of the decompression under endoscopic control and abundant irrigation, we routinely inject 40-mg triamcinolone at the anular tear zone under endoscopic control. The aim is immediate onset of local tissue decongestion and reduction of scar formation. This drug has been routinely used in interlaminar procedures for many years.

13. The working scope is retracted, and an adhesive bandage is put in place. In most cases, the small (5-mm) skin incision does not need a suture.