Minimally Invasive Posterior Lumbar Disc Surgery

Article written by Woeltjen, Bonnie Lane PA-C, Jones, Stanley C MD

The History, Current Treatment, and Future Outlook of Minimally Invasive Posterior Lumbar Disc Surgery

Woeltjen, Bonnie Lane PA-C, Jones, Stanley C MD

Abstract:
The purpose of this review article is to report the history of minimally invasive lumbar spine surgery, placing a special emphasis on intradiscal surgery, identify current procedures and report the future outlook of such surgeries.

Origins of Minimally Invasive Lumbar Intradiscal Surgery
Current Concepts

Microdiscectomy
Percutaneous Discectomy (manual and automated)
Chemonucleolysis
Intradiscal Eletrothermal Therapy
Thermal Discoplasty
Endoscopic Lumbar Discectomy
Percutaneous Laser Discectomy

The Future of Lumbar Disc Surgery


Origins of Minimally Invasive Lumbar Discal Surgery

Throughout recorded history numerous attempts, some even successful, have been made to surgically repair the spine and it numerous disorders. The first laminectomy was performed in the United States in 1829.1 Infection continued to be the bain of any surgeon at that time and it wasnt until the age of antisepsis brought about by Semmelweis and Lister in the mid 1800s, that spinal and all other types of surgery came into its element.
Virchow may have first described traumatic lumbar intervertebral disc disease in 1857 2 but the introduction of radiography by Conrad Rontgen in 1895 led to the increasing popularity of the theory of herniated discs as a cause of back pain due to the visualization of narrowed disc spaces.2,3 Dandy first described and performed a discectomy in 1929, erroneously calling the offending material cartilage from the intervertebral disc.4 In 1934, Mixter and Barr theorized that the cause of their patients back pain was rupture of the intervertebral disc into the spinal canal.5 In 1938, Barr surgically confirmed the relationship between disk herniation and sciatica by following 83 patients.2,3,6 It was the establishment of this cause and effect relationship that served as a touchstone for further research of spinal pathophysiology. Disc pathology dominated discussions on the etiology of low back pain throughout the 20th century.3 Disc herniations became a major, and almost the only, differential diagnosis for back pain for many years.
Surgeons and researchers developed theories, techniques and equipment to further visualize and aid in removal of the offending disc herniation. In 1938 and 1942, Pool published his account of myeloscopy using a modified illuminated otoscope.7 In 1955, Malis used a binocular microscope in the operating room to aid his approach, ushering in the idea of using a much smaller operating field in spine surgery.2 Lindblom first described diagnostic lumbar discography in 1948.8 In the early 1960s, the enzymatic shrinkage of herniated discs was reported using chymopapain.11,12,13 Microdiscetomy was introduced and discussed by several authors in 1977 furthering the minimally invasive concept.2 In 1975, Hijikata published his account of percutaneous discectomy and Onik et al improved the technique in 1985 with different and more efficient cutting tools, creating an automated tool for combination cutting and suction of herniated disc material. 9,10,11 Working endoscopy was the last domain of spine surgery. Although Pool first visualized the dorsal nerve root with an endoscope, termed myeloscopy, in 1938, and Kambin, in 1973, combined endoscopic removal of a protruding disc with an open laminectomy, it wasnt until the late 1980s that the percutaneous endoscope, alone, became a true working entity in lumbar disc surgery.7,13,14 Kambin, with his posterolateral arthroscopic approach, Perez-Cruet et al describing their endoscopic posterior approach and finally, Mathews, Ditsworth and Yeung separately with their transforminal endoscopic approaches, all proving the versatility of the endoscope.1,13,20
Spine surgery and more specifically, intradiscal surgery, has made great gains in the last century. Through improved visualization, imaging, and surgical techniques, the intervertebral disk and its relationship to back pain is better understood.

Figure 1: MRI image of herniated disk


Current Concepts

Microdiscectomy
Microdiscectomy is the natural progression of the original discectomy after the introduction of the operating microscope in the late 1970s. The microdiscectomy was pioneered by Caspar and Yasargil and decribed in the medical literature in 1977.13 There are those who may question whether the microdiscectomy should be included in the minimally invasive category. Of all of the procedures that will be discussed in this paper, the microdiscectomy is, perhaps, the most open or invasive of them all. This procedure does require general anesthesia and a skin incision of up to 50mm. Closure requires fascial and skin sutures.
The procedure itself is straight forward. The incision is centered above the offending disc space. The lumbodorsal fascia is incised, and the subperiosteal dissection exposes the superior and inferior lamina surrounding the affected disc, the ligamentum flavum and the medial facet. A weighted Williams or Taylor retractor is used to maintain exposure. An operating microscope or loupes are used to further visualize the affected areas. Variable amounts of the medial facet and ligamentum flavum are removed based on accessibility to the disc herniation. A burr drill is used to thin the laminae and facets. Curettes are used to remove ligamentum flavum, Kerrisons are used for bone removal and finally, pituitaries are used to remove disc material and any free fragments in the epidural space. The amount of disc material to remove remains somewhat controversial and is based on surgeon preference and past experience.4,13 Once the offending materials are removed, exploration takes place. Finally, foraminotomy is completed and the fascia and skin are closed in the aforementioned manner.
This procedure is usually done on an outpatient basis with most patients discharged within 24 hours.4,13 Reported success rates have been 90-95% with a 1.5-6% complication rate.4,12 Indications for the microdiscectomy include back pain, with or without radiculopathy, a confirmed disk herniation by MRI or CT and failure of conservative therapy. Unlike most of the following minimally invasive procedures, a microdiscectomy can treat large (>50% of the canal space), sequestered, or migrated disc herniations.

Percutaneous Discectomy (manual and automated)
Percutaneous lumbar discectomies (PLD) have been performed for over 30 years with overall results ranging from disappointing to good results. The techniques and equipment used for PLD vary widely and have fallen in and out of favor within the surgical community. Hajikata et al first reported performing a percutaneous nucleotomy in 1975. The procedure included the use of 3-5mm cannulas from the posterolateral approach, curettes and time consuming manual removal of the nucleus pulposus with pituitary forceps.9 The theory was that the reduction of intradiscal pressure would reduce irritation of the nerve root and the nociceptive nerve receptors in the annulus.12 The procedure remained limited in use until 1985, when Onik et al developed a new and smaller type of aspiration probe which reduced risk of injury to the peripheral nerves and the annulus, facilitated easier removal of the nucleus pulposus with an all-in-one suction cutting device and decreased surgical time.10
Both procedures use a posterolateral approach to the affected disc on an outpatient basis. The automated procedure is performed while the patient is in a lateral decubitus or prone position. An 18 gauge hubless sheath with a central trocar is guided toward the affected disc. The trocar is removed and a smaller 2.5mm cannula with an inner blunt end sleeve is placed over the hubless sheath. Once correct placement is confirmed, the hubless sheath is removed leaving the 2.5mm cannula. A 2mm saw is threaded through the cannula and a hole is cut into the annulus for the aspiration probe to be inserted. The aspiration probe is a sharpened cannula fitted through an outer needle. Using suction to pull in disc material, the inner, sharpened cannula uses a slide-like cutting motion to slice the tissue which is then aspirated, along with irrigation, through the inner cannula to a collection bottle.10 The probe, which is pedal activated, is gently moved back and forth within the disc until no more disc material is aspirated and then the probe is rotated. When aspirated disc material decreases significantly the probe is removed from the disc space, usually within 20-40 minutes.11,30
Little changes have been made in automated discectomy until recently with new innovations in automation. The newest entry is the Dekompressor (Stryker Corporation, Kalamazoo, MI) introduced in 2002. The Dekompressor is a disposable, self-contained, battery-operated hand piece connected to a helical probe. The outer cannula measures 1.5mm with an inner rotating probe. When activated, the probe rotates creating suction to pull milled nucleus pulposus from the disk up the cannula to a suction chamber at the base of the hand-held unit. Approximately 0.5 to 2 cc of nucleus pulposus is removed. This efficient removal of disc material decreases surgical procedure times to approximately 30 minutes, with the actual time of use for the probe not exceeding 10 minutes.31 This procedure is done under fluoroscopic guidance. The Dekompressor technique has yet to be studied in a controlled clinical trial and results with this new automated technique are limited.



Figure 2: Fluoroscopic image of the Dekompressor probe within the nucleus pulposus

Percutaneous discectomy has a success rate with multiple authors reporting 60-87% good outcomes.27 Indications for this procedure include radicular pain, usually being greater than back pain, a contained herniated disc of <6mm, and failure of conservative measures. Contraindications include extruded or sequestered disc fragments, a greater than 50% compromise of the spinal canal by the herniated disc, previous surgery at the site of the HNP, and spinal stenosis or spondylolisthesis.2,12 Its benefits are numerous and include reduced procedure time, lower costs, quick recovery and low complication rates that can include discitis and possible nerve injury.
In our ongoing retrospective study of 52 patients, on which we have performed the Dekompressor technique, our results are promising with an overall 62% decrease in back pain and 59% decrease in leg pain from pre-operative assessment to a six month follow-up. 94% of our patients would have the procedure done again and reported excellent or good results. Two patients required open procedures at later dates.

Chemonucleolysis

In the early 1940s, chymopapain was derived from the papaya fruit by Jansen and Balls. By 1956, Thomas saw the potential for this enzymatic substance and began work on determining a use for this substance. He injected the chymopapain intravenously into the ears of rabbits and noted that the ears became floppy.2,13 Thomas confirmed the softening of the cartilaginous material in the ear due to the chymopapain. Smith et al picked up the torch and hypothesized that chymopapain could be used to treat chondroblastic tumors, which it did not, but they did find that when injected into the intradiscal space of rabbits, the nucleus pulposus disappeared but left the annulus intact.2,13 In 1963, Smith injected the first human patient with chymopapain to treat sciatica.

Figure 3 : Chymopapain is introduced into the center of the nucleus pulposus






Chymopapain works by depolymerizing the proteoglycan and glycoprotein molecules in the nucleus pulposus.2 These large molecules are responsible for water retention and turgidity. When exposed to chymopapain, the water content within the disc plummets, shrinkage follows and causes a reduction in disc height and girth. The bulging disc therefore shrinks.
For the procedure, the patient is placed in either a lateral or prone position. Under conscious sedation and guided by fluoroscopy, a 6 inch, 18 gauge needle is inserted posterolaterally and placed centrally within the disc. Discography, along with the pain provocation test, ensues for evaluation of the affected disc. Chymopapain is then injected into the nucleus pulposus in amounts ranging from 1000 4000 U. The number of units injected decreases if more than one disc is to be treated.2,14
Chymopapain has been in use for over 30 years. After approval by the U.S. Food and Drug Administration in 1983 early complications were reported, even though studies had shown a very safe track record.13 Anaphylaxis, reported in 1% of cases, proved to be the most severe complication.13 It became clear that good patient selection, proper surgical training and technique, preoperative hypersensitivity testing and antihistamine administration could greatly reduce the complication rate when using chymopapain. In fact, complications became almost non-existent in the late 1980s and 1990s when utilizing the aforementioned criteria.
Indications for chymopapain include those patients who present with radicular pain as the chief complaint, confirmation of the disk herniation via MRI, CT, or myelogram, and having failed conservative treatment.2,13,21 Kim et al found that patients with moderate to severe positive straight leg raise had a significantly higher success rate as compared to those with no or mild SLR pain.21 The younger the patient the better the outcome. Younger patients had a success rate ranging from 82.3% for those in their 30s to 94.6% for those in their teens. Patients 50 years and older had only a 71% success rate.21 Those patients in whom the pain provocation test was positive had a 91.7% success rate compared to those who did not experience pain provocation at a 73.1% success rate.21
Chemonucleolysis has been fraught with controversy since FDA approval in 1982. There are those who want to dismiss the procedure as dead due to complications in earlier use, (and lack of manufacture and distribution within the United States since 1999) but there are others who state that with the proper inclusion criteria, pre-procedure testing, and good technique, chemonucleolysis is a significant player in the minimally invasive category.2,13,21


Intradiscal Electrothermal Therapy (IDET)
IDET was developed and published by Drs Saal in 2000 to serve as an alternative to fusion for patients with chronic discogenic low back pain.8,22 There are several proposed causes for discogenic back pain. Some have stated that discogenic back pain is due to an internal disc disruption, most likely due to annular tears or fissures.2 Others feel the discogenic pain may be a result of degenerative disc disease.8 Even the developers of the current IDET procedure state that the pathophysiology of discogenic pain is complex and hard to pin down into one complete and true definition.21 What is agreed upon is that the intervertebral disc, particularly the annulus, has nociceptive nerve receptors, which increase when the disk degenerates, is injured or exposed to a variety inflammatory substances. This increase in nerve receptors causes increased and unremitting low back pain. 4,8,24 The IDET was therefore developed to modify collagen, making collagen thicker and causing it to contract decreasing the bodys ability to revascularize and reinnervate the area. The collagen modification could also lead to a reduction in size of annular fissures and increase the stability of the disc itself.4,2 IDET also thermocoagulates the nociceptors within the annular walls, thus destroying the ability to transmit nociceptive input.4,24



Figure 4: The heating tip is threaded around the interior of the annulus at the site of the annular tear.

The procedure itself is performed with fluoroscopic guidance, while the patient is under conscious sedation lying prone. As with many intradiscal procedures, discography, along with the pain provocation test, ensues for evaluation of the affected disc. A 17 gauge needle is inserted posterolaterally into the disc, generally from the patients less painful side. A 30 cm catheter with a flexible 5-6 cm heating tip is threaded circumferentially into the disc through the nucleus pulposus to the pathologic area of the annulus. After fluoroscopic confirmation, the catheter tip is heated to 90oC over a 13 minute period. Once at 90oC, the temperature is maintained for an additional 4 minutes. The catheter and needle are then removed. The patient is then transferred and observed in recovery before being discharged home the same day.2,4,8,24
Indications for the IDET include long-term low back pain, failure of conservative therapy, normal neurologic exam, negative straight leg raise, MRI confirmation of no neural compressive lesion, and positive pain provocation test.24 Exclusion criteria includes inflammatory arthritis, non-spinal conditions which mimic lumbar pain and any medical or metabolic condition which would preclude proper follow-up.8,19,24 Much debate still centers on this technique, as there have been limited independent studies and long-term follow-up of patients receiving IDET.8,22,23


Thermal Discoplasty
Thermal discoplasty or Nucleoplasty (ArthroCare, Sunnyvale, California) is another procedure that has broken into the minimally invasive field in the 21st Century. The first nucleoplasty was performed in 2000.
The procedure combines disc removal and thermal coagulation to decompress a contained herniated disc. With the patient in a prone or lateral position under sedation, a posterolateral approach guided by fluoroscopy is made with a 17 gauge obturater stylet. A discogram may take place at this time to confirm location and for a positive provocation test. Taking care not to contact the anterior annulus, the nucleus pulposus is first ablated with radiofrequency waves as the wand is advanced causing a molecular dissociation process converting tissue into gas which is removed through the needle. As the wand is withdrawn, coagulation takes place thermally treating the channel, which leads to a denaturing of nerve fibers adjacent to the channel within the nucleus pulposus. This process is repeated up to 6 times within an individual disc. 25,26 The patient is then sent to recovery and later sent home the same day.


Figure 5 : Nucleoplasty coagulation channels within the nucleus pulposus after ablation.

Indications for this procedure include low back pain with or without radiculopathy, MRI confirmation of contained herniated disc, and failed conservative therapy.25 Patients who should be excluded from receiving this procedure include those with spinal stenosis, a loss of disc height of 50%, severe disc degeneration, spinal fracture or tumor.25
Although this is a popular topic at association and society meetings, very little has been published in the medical literature regarding this procedure. As of the date of submission for publication, there have been no published outcome studies in the major medical journals per a search of NIH PubMed.

Endoscopic and Arthroscopic Lumbar Microdiscectomy
There have been several uses for the endoscope in lumbar disc surgery. Kambin reported the use of endoscopy from the posterolateral approach, in combination with an open laminectomy in 1973, to remove a protruding disc.2,14 This was expanded to a biportal, posterolateral procedure that laid the groundwork for the most commonly used procedure, the unilateral endoscopic approach. In 1986, non-visualized endoscopy alone was used to improve upon the Hijikata technique of percutaneous nucleotomy.13,15 Yeung reported his history since 1993 with endoscopy. His experience includes the visualization of the disc during discography and visualization during ablation using chymopapain and laser technology.18,19 Endoscopy is a wide umbrella that can cover a variety of working environments. Endoscopy can be used independently and solely in the operative field or concurrently with arthroscopic tools, lasers, microscope or other minimally invasive techniques for visualization.
Endoscopic approach is basically the same as an open discectomy, except performed through channels created by dilators positioned on the lamina with visualization by the endoscope itself.20,13 Dilators can vary in size from 14-22mm, with the newer, larger dilators having an expandable bottom creating up to a 40mm working space at the base. Theoretically, using any form of endoscopy has a smaller incision and less trauma to the underlying tissue due to the muscle splitting versus dissection argument. There is some disagreement though, from proponents of each procedure regarding these advantages.2,13
It is generally agreed upon that endoscopy has a very steep learning curve. The field of vision and working space is quite limited with the surgeon unable to visualize common landmarks. Initial endoscopic systems had extremely expensive disposable scopes. The importance of image orientation for the surgeon cannot be underestimated. Newer systems address these problems with larger viewing areas and expandable dilators. Unfamiliarity with endoscopic technique can lead to longer operative times and an increase in complications, namely dural tears.20 Indications for endoscopic discectomy include disc herniations and failure to improve with conservative therapy. Perez-Cruet et al report varying results in 150 cases with 77% of the patients with excellent and 17% with good results. Complications occurred in 6.4% of the patients (8 dural tears and 4 recurrent disc herniations.)20
Kambin et al pioneered the arthroscopic discectomy in 1983.14 His posterolateral approach differs from the posterior intralaminar approaches of other endoscopic systems. In the arthroscopic procedure, the patient is prone and entry with a spinal needle is made approximately 8-12 cm from midline at a 45o angle. Using one or two endoscopes and a 1cm incision, a guide wire, blunt ended cannulated obturator and finally, a working sheath, measuring from 6.4mm by 6.4 - 9.4 mm, are inserted into a triangular working zone via fluoroscopic guidance.14 The triangular working zone is made up of the spinal nerve superiorly, the articular process posteriorly, and the proximal vertebral plate, inferiorly.14 With the working sheath resting on the annulus, coring, cutting and grabbing instruments are inserted into the disc space while a high-resolution glass arthroscope visualizes progress. Manual removal of nucleus pulposus with straight and upcutting forceps follows. If bleeding causes a loss of visualization radiofrequency coagulation or laser ablation may be used.14 Small fragments beyond the annulus may also be visualized and removed if they have not migrated too far.15 The ability to access these fragments within the spinal canal set the arthroscopic discectomy procedure apart from other minimally invasive procedures, including pure endoscopy, for disc herniations.15 According to Kambin, arthroscopic microdiscectomy is the minimally invasive procedure to look at for patients who are usually considered to be beyond the indications for other minimal procedures.14 Newer advancements to the endoscopy allow surgeons to operate on patients with sequestered and free-fragmented discs and lateral recess stenosis, but the procedure has a high level of complexity.2
Kambin has reported satisfactory patient outcomes in 85-92% range, with very low numbers of complications, including post-operative wound infections and sympathetic mediated pain.14


Percutaneous Laser Discectomy
Medical lasers have been used since the early 1960s. Ascher, Choy et al published their experiences with the use of a neodymium:yttrium-aluminum-garnet (Nd:YAG) laser on the lumbar spine for nucleolysis.2,18,27 There are several types of lasers in use for the lumbar spine with the most common including the holmium:yttrium-aluminum-garnet (Ho:YAG) laser, the potassium-titanyl-phosphate (KTP), and the neodymium (Nd):YAG laser. The Ho:YAG laser is most commonly paired with the endoscope for disc ablation and removal capabilities.16,32 This laser-assisted technique combines two effective but limited approaches.
As the affected tissues absorb the laser, light is converted to heat. At 100oC, tissue vaporizes and ablation takes places. As a small amount of nucleus pulposus is vaporized, intradiscal pressure decreases, allowing the disc to return to its normal state.12,28 If any disc material needs to be removed, endoscopic tools can be used for the task.
The patient is placed in a prone or lateral position under conscious sedation. An 18 gauge 7 inch needle is introduced just anterior to the superior articular process and superior to the transverse process via a triangular safe zone. Using fluoroscopy, the needle is placed 1 cm beyond the annulus into the nucleus pulposus just parallel to the disc axis, preferably halfway between the superior and inferior endplates.28 If the procedure is endoscopically assisted, dilators are placed over the guide needle for visualization and the introduction of the endscope. Irrigation with saline allows for better visualization of the spaces. Depending on the type, the laser is either fired pulse style or continuously.28 The Ho:YAG laser is pulse fired. Newer laser models offer side-firing capabilities. This advancement helps to provide more control of laser placement, better observation, and can help reduce the risk of injury to several areas especially those anterior to the spinal column.2
Larger fragments which are more difficult to remove through the endoscope can be laser ablated. After firing the laser and adequate nucleus pulposus has been removed/ablated, the laser and dilators are removed. The incision can be closed with sutures or surgical adhesives. The patient is moved to recovery and sent home later in the day.
Patients eligible to receive laser discectomy include those with back and leg pain with a confirmed disk herniation. A ruptured annulus and lateral recess stenosis are not contraindications.18,28 Newer transforaminal procedures can treat those patients with fragments in the epidural space.18 In 2002, Tsou and Yeung reported the 9 year retrospective results of their percutaneous transforaminal approach, with an 88.1% excellent to good result.18 Other studies report success rates from 78-85% in retrospective studies.2,18,28 There is a scarcity of clinical trials regarding percutaneous lasers.
Negative aspects of the laser includes a steep learning curve for the surgeon and great initial expense (from $30,000 to $150,000).18,28 The use of lasers coupled with an endoscopic approach significantly increases the difficulty level for the surgeon.


Figure 6: Endoscopic view of the laser probe at the nucleus pulposus

Conclusions
Each of the presented procedures has its merits and flaws. It is the ability to treat all sorts of intradiscal maladies that makes the microdiscectomy the standard by which the other procedures within this paper are compared. We, the authors, feel that patient selection and surgeon skill is critical to a highly successful procedure. It is important to note the inclusion and exclusion criteria of each procedure to ensure a good outcome. When these criteria are disregarded or are creatively interpreted, results are disappointing and may be outright dangerous, as noted with the chemonucleolysis. We feel that several of these minimally invasive procedures will stand the test of time and clinical trials and prove to be valuable additions to the spine surgeons skill set.


The Future of Minimally Invasive Lumbar Disc Surgery

Newer uses for minimally invasive techniques include percutaneous fusions and expanding endoscopic procedures. The field of minimally invasive spine surgery is growing rapidly. The overall trend within the United States is toward more minimally invasive techniques. New procedures, products, and techniques are being introduced into the market at a brisk pace.
The Luxor (Stryker Spine), a mini open retractor system, to be introduced late this year, can be used for multiple procedures including one and two level decompressions, fusions, interbody fusions, and TLIFs. The system requires a single a 35mm incision (less than the traditional microdiscectomy), with a series of dilators to open the working space, and has distraction, compression and reduction capabilities. The system can be used with a variety of visualization techniques including fluoroscopy, endoscopy, operating microscope, and direct feel. Unlike other systems currently in use, the learning curve with this system is minimal and uses the same technique as the Wiltse open procedure.33
Additionally, the disc nucleus prosthesis or gel replacement may soon be available. These devices could, theoretically, be coupled with the dissolution of the nucleus by chymopapain and immediately followed by nucleus replacement. These have yet to be proven by controlled clinical studies.

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