INTRODUCTION

Automated percutaneous lumbar discectomy (APLD) was introduced in 1985 by Gary Onik et al.^1–3 Since the 1960s, many different techniques for percutaneous removal of the nucleus pulposus or its protruding components have been proposed; they may achieve the goal in different ways, with different types of instruments, with or without fiberoscopic vision, or different types of energy (radiofrequency [RF], laser, coblation, etc.).^4–12 The basic principle, shared by most percutaneous intradiscal decompressive procedures, including APLD, is that in an enclosed space a reduction in volume, even partial, confers a much greater reduction in pressure; this leads to decreased pressure upon the nerve root, and relief of sciatica, even without a radiographically evident reduction in total disc volume.¹³ After weeks or months, the partial vacuum causes the protruded portion of nucleus pulposus (or other disc material) to move away from the nerve root back towards the center of the disc, pushed by partially intact fibers and ligaments of the outer anulus; this process, along with regeneration of a more fibrous nucleus pulposus, favors restoration of the inner fibers of the anulus and decreases the tendency to further protrusion towards the spinal canal. The success of the procedure depends to a great extent on selecting lesions to treat: the protruding nucleus pulposus must be at least partially contained by the external fibers of the disc, without a large extrusion and migrated or sequestrated fragments.^14–16

For decades, minimally invasive treatments for disc protrusions have been opposed by the surgical community, despite the high preference of patients to undergo a less intrusive intervention. APLD seems to have suffered the drawback of having been the first nonchemical, nonmanual procedure to be used worldwide, as the technique was met with fierce opposition. It would be fair to state that 20 years ago, surgeons were not ready to embrace percutaneous procedures. Now that the neurosurgical and orthopedic communities have accepted the concepts of intradiscal decompression and minimally invasive procedures,¹⁷ other techniques less effective than APLD dominate the field. In all likelihood, this relationship stems from the fact that APLD is still burdened with old, biased, and superficial judgments that are in part substantiated by poorly conducted studies.^18,19 In most published series good results range from 60% to 85%,^20–26 depending on patient selection criteria, while poor results are reported in the only two randomized and controlled studies.^18,19 In 1993, Revel et al. reported a 37% success rate at 1 year in a study comparing APLD and chemonucleolysis.¹⁸ Chatterjee et al., 2 years later, found a 29% success rate with APLD when compared to open surgery.¹⁹ However these studies, like others reporting low percentages of good outcomes,^27–29 have limitations and features that make the patient populations and technical conditions not really suitable for a comparison, and their results unreliable. First, the numbers of patients are low: 32 treated by one operator¹⁹ and 69 treated by many operators in a multicenter study.¹⁸ The authors do not state how experienced the operators were, i.e. how many APLD procedures each had already performed. The technical learning curve for APLD is longer than one might expect. It is only after many procedures are performed that the surgeon can obtain sufficient quantities of nucleus pulposus, and from the correct location of the disc. For example, the L5–S1 level is approached safely and reliably only by operators having performed a minimum of 40–50 procedures at higher levels. It is highly likely that the operators in the two studies mentioned above were much more experienced in open surgery or chemonucleolysis. As well, these investigators did not have access to some of the technique modifications that are described later in this chapter.

Another critical issue relates to the population of patients that are being compared. It is undeniable that adhering to specific inclusion and exclusion criteria is crucial to obtain good results with APLD, chemonucleolysis, and open surgery. However, there is little overlap between the indications for open surgery or chemonucleolysis and those for APLD. Consequently, there is a huge inherent limitation in randomized trials attempting to compare APLD to either chemonucleolysis or open surgery. The patients recruited in the two trials mentioned above are likely to have extruded, noncontained protrusions, which are not good indications to perform APLD.

In almost 20 years, a minimum of 170 000, and very likely many more, procedures have been performed. It is reasonable to state that APLD ‘opened the way’ to the concept of minimally invasive spine surgery, and that the concept and the technique itself have stood the test of time. APLD is an effective and safe method to obtain an intradiscal decompression, for relief of discogenic radicular or low back pain. It remains the percutaneous procedure that removes the largest amounts of nuclear material from within the intervertebral disc. Another great advantage, when comparing APLD with physical techniques that blindly destroy the disc (such as laser, RF or Coblation®), is that the surgeon can verify directly and visually the quantity of disc material removed, and its ‘quality’ as well. The extracted nucleus pulposus can be observed as it passes through the transparent tubing that connects to a filter. How much nucleus is taken out and how degenerated it is are important procedural and prognostic pieces of information. For example, viewing the quantity of removed nuclear tissue and comparing that to the amount that was anticipated to be extracted from interpreting the preoperative imaging provides critical information to determine whether the probe worked in the correct intranuclear location. Observing blood coming from the disc could suggest the presence of unexpected degeneration, or of painful granulation tissue inside the disc, or prompt arrest of the procedure so as not to damage the endplate cartilage. Aspirated disc material can also be sent for histology or microbiology in selected cases.

APLD achieves a very good compromise between low invasiveness and the need to obtain discal decompression. Its clinical results remain among the most satisfying when dealing with minimally invasive percutaneous treatments.

SAFETY

One of the appeals of percutaneous procedures, other than obtaining a high proportion of good results in properly selected cases, is the limited associated tissue destruction. While open surgery is effective, it has well-known disadvantages, including epidural scarring, damage to bone, denervation of paraspinal muscles with consequent segmental instability, long postoperative inactivity, and the feared ‘failed back-surgery syndrome.’ Patients who experience this latter phenomenon are often untreatable and can be severely disabled. Indeed, they represent the best advertisement for the benefits of minimally invasive procedures, particularly given the high tendency of disc protrusions to self-heal.

When considering the issue of side effects and complications, it appears that APLD is an extremely safe technique. Of course, that assumes that the surgeon performing the technique has the experience and ability to convert it into a safe procedure. If the nucleotome is improperly placed, it can easily cut dura, nerve roots, vessels, and other soft tissues. However, once the nucleotome is safely within the disc, i.e. isolated from surrounding neural and vascular structures, it is unable, unlike other devices, to cut its way out of the disc space to cause injury to those structures. The operator must at every moment be absolutely sure of the anatomical position of the operating instruments. Obviously, a key prerequisite is the ability to view two-dimensional fluoroscopic images and reconstruct a three-dimensional anatomic picture in the mind’s eye of the operator. To achieve this objective the operator must have a perfect knowledge of radiological projections and a large amount of experience.

The mortality rate of the procedure is zero. Lesions of nerve roots, vessels, or the ureter are possible;^30–32 however, as previously emphasized, with thorough knowledge of and attention to radiographic landmarks for proper probe positioning, vascular, neural, or dural injuries are very unlikely. The only major reported injury following APLD occurred in Mexico and resulted in a cauda equina direct lesion. It is quite likely that there was little if any attention directed to the radiographic landmarks that allow the surgeon to stay out of a potentially harmful pathway;³³ moreover, the procedure was performed under general anesthesia, definitely contraindicated, for the reasons explained later.

A posteriorly placed colon can insinuate behind the psoas muscle.^34,35 For this reason the preoperative imaging studies, both computed tomography (CT) or magnetic resonance imaging (MRI), must be carefully examined to exclude the presence of such an anatomical condition, since bowel in the path of the instruments could be perforated, with the risk of peritoneal or disc infection or local abscess formation. If not available, a planning CT scan of the whole abdomen through the disc space of interest with large field of view (FOV) must be obtained. When the L5–S1 disc is being removed, two scan slices (at the L4–5 and L5–S1 levels) should be obtained because the entry point for the L5–S1 placement is at the L4–5 level to avoid the iliac crest. In addition, this preoperative, planning CT can provide other valuable information. At the L5–S1 level, special attention should be paid to the bifurcation of the iliac vessels; at upper levels, the scan ensures that the lower pole of the kidney or the sulcus of the pleural space will not be traversed.

Beginning in June 1987, in our institution more than 1250 patients (accounting for more than 1450 discs) were treated. We observed and reported an overall complication rate of less than 0.9%.³⁶ There were no injuries to nerve roots, dura mater, ureters, major vessels, or bowel. We suspect this extremely low complication rate stemmed from our singular use of only local anesthesia, with or without light sedation, and the avoidance of general anesthesia. There was one acute hematoma in the iliopsoas that occurred following injury to a small artery and which resolved without sequelae in approximately 1 month. Among the side effects observed were two cases of discitis, resulting in a rate of 0.16 %, similar to the rate published in large series of discography.^37–39 Since discitis is a major complication, special care must be taken during skin prep and draping. We also use prophylactic antibiotics and typically give 2 g of intravenous cephalosporin to cover Streptococcus epidermidis.

It should be emphasized that some of the potential complications and side effects one could observe following open surgery have never been reported following APLD. Advantages of APLD when compared to other percutaneous technologies are the internal cutting action of the device and its blunt external portion that obviates damage to structures other than the nucleus pulposus. Of course, these factors dictate that the inherent morbidity of APLD is lower than that of other percutaneous disc removal methods.

INSTRUMENTATION

Automated percutaneous lumbar discectomy utilizes a probe called Nucleotome® (Fig. 28.1), manufactured by Clarus Medical, LLC, for removal of the nucleus pulposus. The probe tip, excluding the handle, is 20.2 cm long and has an outer diameter of 2.2 mm. The blunt tip is an extremely important safety feature. Once the probe is inserted, the lack of a sharp end prevents it from piercing through the outer limits of the disc, even with an inadvertent hard push. This feature is unique and not a component of other instruments such as laser, RF probes, or manual biopsies, thus essentially removing the risk of lesion of vessels or other abdominal structures. The negative pressure for aspiration is generated by a console. A vacuum is created that draws nuclear material into the side port, which is located a few millimeters proximal to the distal tip of the probe. The cutting blade for fragmentation of nucleus pulposus aspirated through the port, works with a reciprocal, not rotatory motion. This type of movement is a safety feature because the ‘guillotine’ blade is contained within the probe. Consequently, only the nuclear material that is drawn into the port can be cut. The blade is pneumatically driven by a pressure pulse, generated by the same console that creates the vacuum that draws nuclear material into the side port. The console also controls the cut rate and the flow of irrigation fluid to the probe. Internal irrigation with sterile saline is a vehicle for easy aspiration. The reciprocal movement of the internal cutting blade also sequences the introduction of liquid inside the disc, to prevent accumulation of nuclear material and consequent clogging inside the probe, or an excess infusion of fluid within the disc. The cutting rate knob on the console allows for adjustment of between 60 and 180 cuts per minute. At the beginning of the procedure, the maximum cutting rate should be used to cut smaller pieces of disc and prevent the instrument from clogging. As the decompression proceeds, the amount of disc material aspirated diminishes, allowing the surgeon to ratchet down the cutting rate. This will allow more time for the negative pressure to draw disc material through the port before it is resected.

Fig. 28.1 The Nucleotome® probe has a blunt, rounded tip. Internal irrigation and cutting functions are incorporated. The aspirated nucleus pulposus enters the side port, and is resected by a pneumatically driven ‘guillotine blade’, which has a reciprocal, not rotary movement.

The fluids and solid material aspirated from the inner disc and exiting through the metallic probe are ultimately deposited into a filter in a disposable collection bottle. To reach that location the extracted nuclear tissue traverses a transparent plastic tube. Throughout this journey the nuclear material is clearly visible. As alluded to previously, there are tangible benefits that can be realized by real-time monitoring of the disc material as it flows through the transparent aspiration line, mainly the possibility to verify in which positions of the probe inside the disc the largest quantities of nuclear material are extracted. Moreover, the nucleus pulposus collected in the filter is available for quantitative and macroscopic qualitative evaluation, or even for histology examination. The operator must also use the transparent line to check for air: an excessive amount mixed with little or no nuclear material indicates a leak somewhere in the system, which decreases the effectiveness of the suction. The most common cause is an inadequate seal on the cannula, which allows air to be sucked into the disc space. This usually does not happen because anulus and extradiscal soft tissues make a seal around the probe and prevent air from being sucked into the disc. However, if anulus and tissues have less resilience because the patient is older, air will be sucked through the cannula preventing aspiration of the disc and thus rendering the procedure less effective (or longer); in these cases the straight cannula can be replaced with the larger, curved one.

If the cannula is not the cause, the air leak may be emanating from the fluid delivery line or even the probe. If a check of the fluid delivery line does not reveal the problem and all maneuvers fail to stop the leak, place the trocar back through the cannula and change both the cannula and the probe.

A sequence of devices is used for introduction of the probe inside the disc. After local anesthesia, the first device is positioned in the center of the disc. It is a flexible, 18-gauge stainless steel guide wire with a trocar point. Unlike bevel-pointed needles, a trocar point does not have a sharp cutting effect, thus limiting the risk of vessel or nerve injuries; these structures are more likely deviated by the trocar point rather than resected. In addition, unlike beveled needle tips, the symmetric trocar point follows a straight trajectory, without deflections that would render a precise positioning more difficult. This is particularly applicable when driving the instrument in relatively resistant tissues such as the anulus fibrosus. Moreover, the blunt trocar device is more difficult to push through soft tissues, which gives the surgeon a much better feeling of the tissue actually met and traversed, and allows for easy recognition of the muscular fasciae, anulus fibrosus, and nucleus pulposus.

Once the guide wire is positioned correctly, a cannula with a dilator inside is passed over the guide wire. The dilator is designed to protect soft tissues from surgical trauma; this prevents both bleeding and postoperative muscle spasm. Each single kit contains both a straight cannula, with an outer diameter of 2.8 mm, and a curved cannula, with an outer diameter of 3.8 mm. The reason for a larger diameter in the latter is that it is internally coated by a Teflon layer, which reduces friction and favors sliding of the flexible but straight probe. Once the cannula is positioned against the outer fibers of the anulus, the dilator is removed from the cannula and replaced by a trephine. An incision in the anulus fibrosus is made by means of the trephine, which is a few millimeters longer than the cannula; the same flexible trephine is designed to function with both the straight and curved cannula.

PATIENT POSITIONING AND SELECTION OF ENTRY ROUTE

The procedure can be performed with the patient in either the prone or lateral decubitus position. When the prone position is used, bolsters are placed underneath the patient’s abdomen to open the disc spaces posteriorly. This will provide for easy access and for better transmission of the decreased pressure in the center of the disc, consequent to the aspirating action of the probe, to the herniating nucleus pulposus. For the same reason, the patient is flexed when in the lateral decubitus position, which is the approach of preference for addressing pathology of the L5–S1 disc.

The entry route, as usual with every lumbar percutaneous approach to the disc space, is posterolateral (Fig. 28.2). Correctly positioning the guiding trocar is crucial to the result. The trocar must be placed with its tip in the midline in frontal view, at the junction of the middle and posterior thirds of the disc in lateral view, where the normal nucleus lies (Fig. 28.3