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Current evidence of minimally invasive spine surgery in the treatment of lumbar disc herniations.

Throughout the years, variations in surgical approaches, new instrumentation, expanded use of imaging, advancements in biologic materials, gene therapy, and application of tissue engineering have been integral to the preventative, diagnostic, and therapeutic aspects of diseases and conditions affecting the spine.

Evolutions in treatment were motivated by many factors, paramount of which was to enhance patient outcome by facilitating quicker return to daily activities, diminishing intervention-associated pain and complications, and decreasing overall health care costs.

Based upon the development of instrumentation and imaging that allowed less tissue trauma compared with traditional open procedures, while providing adequate or enhanced visualization of the pathologic site and based upon the successful experience of outpatient spine surgery to assist early ambulation, the trend and evolution toward "minimal access" or minimally invasive spine surgery began to develop with greater intensity

Throughout this review, we are going to discuss several aspects involving the historical overview, anatomy, pathological process, classification, and initial treatment of herniated nucleus pulposus (HNP). We will the further our discussion in the different alternative minimally invasive treatments, including chemonucleolysis, manual percutaneous discectomy (MPD), automated percutaneous lumbar discectomy (APLD), percutaneous lumbar laser discectomy (PLLD), and microendoscopic discectomy (MED).

Historical Background

Ancient civilizations made tremendous contributions to the framework of government, science, art, and medicine. The foundations of many current medical disciplines are attributed to the teachings and practices of early Egyptian and Greek scholars; modern spine surgery is no different. (1)

Hippocrates (circa 460 to 370 BC), who is largely considered to be the father of spine surgery, was extremely active in medicinal teachings and writings that highlighted the art of sound reasoning and accurate observation. He was the first to discuss back pain and sciatica and correlate the site of pathology to associated symptoms. (1, 2)

His practices in the management of spine fractures, deformities, and back pain established a new dogma that stressed the importance and function of the spine. At the time, he utilized his designed board and ladder technique. (3)

In the ensuing years, many Greco-Romans contributed to the understanding of spine conditions. For example, Aulus Aurelius Cornelius Celsus (circa 25 to 50 AD) elaborated upon the association of spinal cord injury of the cervical spine and death. (1, 3)

Galen (circa 129-210 AD) drew conclusions between specific cervical levels of injury resulting in paralysis and loss of sensation below the level of injury. Moreover, Caelius Aurelianus (circa fourth century AD) provided the first clinical description of sciatica with regard to lower extremity radiculopathy. (4)

Later, it was Andreas Vesalius (1514 to 1564 AD), from Italy, who raised awareness of the intervertebral disc as an entity, as noted in his publication entitled De Humani Corporis Fabrica. (5)

Perhaps the earliest account of the removal of a herniated disc can be attributed to Krause, who, in 1909, resected a lumbar disc by way of a laminectomy followed by transdural disc resection. (2, 5)

In 1929, Schmorl reported his observation of "nodules" associated with the disc; he believed them to be a result of a prolapsed nucleus pulposus and associated with a degenerative or traumatized annulus fibrosus. Schmorl did not believe that such "nodules" had any clinical significance. (6)

Mixter and Barr in 1934, reported in the New England Journal of Medicine that sciatica was correlated directly with the occurrence of a herniated lumbar disc. The report also elaborated upon the operative management of a herniated lumbar disc by performing a discectomy by way of a laminectomy. The investigators' approach entailed excessive removal of the lamina and removal of the disc material by an intradural approach. (7)

In an effort to reduce surgery-associated morbidities, the operative microscope and microsurgical techniques were developed in the mid-1960s by Yasargil. This enabled smaller incisions with less blood loss, increased visualization of the site of pathology, decreased hospitalization, shorter postoperative recovery, and earlier returns to activities compared with previous operative interventional techniques. In 1968, Sachdev began using a binocular microscope to perform his lumbar microdiscectomies. (8)

Regardless of this, microdiscectomy technique did not gain momentum and worldwide acknowledgment until later. In 1978, Williams published his report describing the operative management of 532 patients, most being Las Vegas showgirls, undergoing lumbar microdiscectomy with removal of only the herniated disc fragment through an intralaminar window. (7-10)

Diagnosis and Classification

Ninety-five percent of all disc herniations involve the L4-L5 and L5-S1 levels, with the L5-S1 being the most commonly affected. (10-13) The peak incidence is between the fourth and fifth decades of life.

Just because a patient has a herniated disc on MRI doesn't mean that they are symptomatic, approximately only 5% of HNP's become symptomatic. There is a 3:1 male to female ratio with increased risk in male construction workers that utilize jackhammers and truck drivers. (10-14)

One of the key elements is to understand that this pathology is largely treated non-operatively, and that 90% of all patients will have significant improvement of symptoms within 3 months of non-operative care. There are multiple theories that attempt to explain this. One of them is that the initial local inflammatory response decreases with nonoperative treatment, therefore, creating more space for the nerve. A second theory describes a macrophage phagocytosis response; this appears to be larger in sequestered fragments. The third and last theory is that the herniated disc goes through hydration and dehydration phases. (8, 15)

The main function of the intervertebral disc is to allow spinal motion and provide intervertebral stability. It links adjacent vertebrae and is responsible for 25% of the spinal column disc height. (11, 12)

The disc has two parts. First, the annulus fibrosus, the outer structure that encases the nucleus pulposus, is composed of type 1 collagen organized in lammellae, which are obliquely oriented. It is characterized by high tensile strength, therefore, containing hoop stresses between vertebrae. It has a high collagen to low proteoglycan ratio, which gives it a lower percentage dry weight of the proteoglycans when compared to the nucleus. It is composed of mostly fibroblast-like cells, which are responsible for producing and maintaining its structure. (11-13)

Second, the nucleus pulposus is the central portion of the disc surrounded by the annulus fibrosis. It is primarily type 2 collagen creating a hydrated gel composed of 88% water and proteoglycans, with a low collagen to high proteoglycan ratio, therefore, having a high percentage of proteoglycan dry weight. This enables it to have a hydrophilic matrix, which is the primary responsible for resisting compression and maintaining disc height. Its viscoelastic matrix is also responsible for distributing forces smoothly to the annulus and endplates preventing potential damage. The nucleus pulposus is primarily composed of chondrocyte-like cells responsible for the matrix production, which have the ability to survive in hypoxic conditions. (12-14)

The disc is primarily avascular, and it obtains is blood supply from capillaries terminating at the end plates. This nutrition later reaches the nucleus pulposus through diffusion in pores around the endplates. (11-14)

The dorsal root ganglion gives rise to the sinuvertebral nerve. This particular nerve, then, innervates the superficial fibers of the annulus. It is important to understand that no nerve fibers extend beyond these superficial fibers. (16)

This normal innervation pattern is modified in discogenic pain. There are many chronic changes that occur at this level leading to excessive pain: 1. an increase in nerve terminals, especially the annulus fibrosus, 2. nerve extensions were found into the nucleus pulposus of the degenerated disc, and 3. postoperative discectomy patients have an increase in connections between the intervertebral disc and paravertebral muscles leading to local muscular denervation. (17)

There are different ways to classify HNP. (17) The first is to locate the herniation anatomically. There are four types anatomic types: central, posterolateral, foraminal (or far-lateral), and axillary. A central herniation is usually associated with back pain only and if it becomes significant has the potential of causing cauda equina. A posterolateral herniation is by far the most common, and the reason why it happens at this location is due to the inherited weakness of the PLL. It usually affects the traversing nerve root. For example, in the case of the L4-L5 level, it would affect the L5 root. A foraminal or far-lateral herniation is less common and usually affects the exiting nerve root below the pedicle. An axillary herniation is rare, and when it occurs, it usually can affect both the exiting and traversing nerve roots. (10-12, 17)

Another way to classify HNP is by its axial and sagittal characteristics. A bulge is a wider based out-pouching with broad area. A protrusion would be an eccentric bulging with an intact annulus. An extrusion occurs when disc material herniates through the annulus but remains in continuity with the disc space. A sequestration is when the disc material herniates through the annulus and is no longer in continuity with the disc space. (17)

The usual presenting complaint is acute or chronic intermittent lower back pain (LBP) associated with sciatica, which is radiating pain in a dermatomal distribution and classically described as a burning, stabbing, or electric sensation, sometimes accompanied with paresthesias. (18) This type of pain should be differentiated from the less well-defined deep aching pain that is confined to the sclerotomes of the spine, commonly known as referred pain. The presence of sciatica is a sensitive and specific symptom of HNP. Most symptomatic HNPs do not present with the so-called classic presentation of acute, unilateral, well-defined radicular pain after a strenuous episode of activity. (17-19)

Patients with a herniated disc may have a loss of lumbar lordosis, a functional scoliosis secondary to leaning away from the painful side, and the affected hip and knee may be slightly flexed and externally rotated to relieve tension on the nerve root. (17-20)

The classic straight leg raising (SLR) test or Lasegue test is thought to be a useful clinical test to demonstrate an inflammatory compressive process across a spinal nerve root. A positive crossed straight-leg-raise test may offer a higher specificity than a positive ipsilateral test, but the sensitivity is variable and some feel not reliable. These maneuvers are based on the premise that, normally, nerve roots have an excursion of 1.5 mm to 3 mm for L4, L5, and S1, and that under compression their excursion decreases. (18)

A clinical correlation is paramount when interpreting imaging studies. This is especially true for HNPs where there is a high rate of HNP found on MRIs in asymptomatic individuals. There are many occasions where this is not necessarily true. Kortelainen and coworkers reported that the most common symptom was an S1 radiculopathy; however, imaging or intraoperative findings showed that the most common herniation is the L4-L5 disk. (18)

In a classic literature review by Weber, nearly 90% of patient symptoms resolved within 4 to 6 weeks. Other studies have reported similar results. Therefore, no imaging is needed for patients who are clinically diagnosed with a HNP unless one of the red flags is discovered. If this is the case, then further work up is warranted before the 4 to 6 week observational period. (20)

Magnetic resonance imaging has become the examination of choice for diagnosing HNPs. It has the advantage of having no known side effects or morbidity, no radiation exposure, and is noninvasive. With MRI, it is possible to identify the separate constituents of the disc based upon the differing concentrations of water, proteoglycan, and collagen. (18, 20, 21)

Non-operative Treatment

To date, non-operative care of HNP includes a wide range of qualitatively different methods: lumbar supports, bed rest, oral analgesics and muscle relaxants, spinal manipulation, physical therapy, epidural steroid injections, and behavioral therapy. Not unsurprisingly, there are also either a wide range in the levels of success or little information avalable. (18, 21)

When reviewing the literature, it is difficult to assess the success of different types of non-operative treatment. The majority of studies also focus on LBP and not HNP, specifically. Several factors affect the outcome, such as selecting a homogeneous cohort, objective assessment of pain, psychosocial environment, cooperation with the study, and motivation of the patient to improve and resume their occupation. (18, 21, 22)

Surgical Indications

The primary rationale of any form of surgery is to relieve nerve root irritation or compression due to herniated disc material, but the results should be balanced against the likely natural history. Surgical planning should also take account of the anatomical characteristics of the spine and the anatomical characteristics of the herniation. (21, 22)

Ideally, it would be important to define the optimal type of treatment for specific types of herniations. For example, different surgical procedures may be appropriate if disc material is sequestrated rather than contained by the outer layers of the annulus fibrosus, and the choice of treatment should reflect these. (21)

When looking at non-operative versus operative treatments, the current update of the SPORT trial helps understand what to expect from surgery. In this randomized controlled trial (RCT), surgical candidates with imaging-confirmed lumbar intervertebral disc herniation meeting Spine Patient Outcomes Research Trial eligibility criteria were enrolled into prospective randomized (501 participants) and observational (743 participants) cohorts. (22)

Interventions were standard open discectomy versus usual non-operative care. Main outcome measures were changes from baseline in the SF-36 Bodily Pain and Physical Function scales and the modified Oswestry Disability Index. Carefully selected patients who underwent surgery for a lumbar disc herniation achieved greater improvement than non-operatively treated patients; there was little to no degradation of outcomes in either group (operative and nonoperative) from 4 to 8 years. (22)

Although the surgical techniques to address HNPs enjoyed a certain degree of popularity, they entailed a level of soft tissue and bony violation. In an effort to minimize such undesirable procedure-related events, individuals sought alternative methods.

Chemonucleolysis

In 1941, Jansen and Balls first isolated chymopapain from crude papain derived from the papaya fruit. Later in 1956, Thomas injected papain into the vein of a rabbit's ear and reported on the floppiness of that ear. It was only in 1964 that chemonucleolysis was reported as an alternative surgical treatment, when Smith injected chymopapain directly into the intervertebral disc of human subjects. (23, 24)

Chemonucleolysis hydrolyzes the noncollagenous proteins of the disc to depolymerize the nucleus pulposus, liberating chondroitin sulfate and keratin sulfate elements, thereby altering the characteristic integrity of the nucleus pulposus. This leads to a temporary increase in urinary mucopolysacarides. (24, 25)

Because collagen type 1 is not affected by chymopapain (unless at higher doses) only the nucleus pulposus is hydrolyzed. An intradiscal injection is performed utilizing fluoroscopy through a direct lateral or posterolateral approach, and the patient goes home on the same day. (24, 25)

Initially, chemonucleolysis showed promising results with an average of 75% success rate, ranging from 92% in Smith's original article to 76% in the largest trial. (23) There was a reported 2.4% complication rate and 0.8 anaphylaxis percentile. This technique very quickly developed traction, especially in Europe. Unfortunately none of the initial trials were controlled randomized trials and none utilized clinical outcome scores.

Despite the apparent efficacy of chemonucleolysis with chymopapain in open uncontrolled trials, it was not until 1978 that results from double-blind, placebo-controlled trials were published. In a study performed by August and associates, patients who were followed 6 weeks, 3 months, 6 months, and 1 year showed no statistical significant difference between groups, and success rates were noted to be 50%. (26)

An increase in radicular pain immediately after treatment was encountered in 16 patients (22%) in the chemonucleolysis group, as compared to none in the discectomy group. The efficacy of discectomy appeared to be definitely superior to that of chemonucleolysis. (26)

A later multicenter RT also performed by August and associates revealed that within a follow-up period of 1 year, 25% of patients required open discectomy following failed chemonucleolysis compared to 3% of patients in the discectomy group. Unfortunately, chemonucleolysis increased the amount of scar tissue formation, rendering open discectomy after a previous chemonucleolysis successful in only 44% of cases.

According to the most recent Cochrane review in 2007, when analyzing the efficacy of chemonucleolysis, only five RCT studies qualified. In each instance, a set dose of chymopapain was injected by standard technique and compared with standard discectomy. In all the trials, there was a poor description of the method of randomization, and the nature of these studies precluded blinding of the patients. The combined results from the five trials compared data from 680 patients with an average follow-up of 97%. All of the analyses showed consistently poorer results with chemonucleolysis and significantly better results with discectomy. (27)

Automatic Percutaneous Lumbar Discectomy

In 1984, after surveying the earlier techniques used for percutaneous discectomy, Gary Onik, a radiologist working with engineers from Surgical Dynamics, Inc. (Norwalk, CT), designed and introduced an automated percutaneous technique for disc removal. (28, 29)

The technique was simple. The patient is placed in the right lateral decubitus position. Using fluoroscopy, a lateral view of the patient's spine is obtained and the L4-L5 disk interspace identified. An entry point for the probe is selected on the skin about 10 cm from the midline. A 25 cm, one 8-gauge hubless stainless steel sheath with a central trocar is aimed obliquely toward the posterior margin of the intervertebral disk. A small 2.5 mm annulotomy is performed, and the nucleotome probe is advanced. The cutting instrument operates at up to 300 cycles/mm, enabling rapid aspiration of large amounts of material. (29)

Initially, the procedure was available only for patients with herniated discs at the L4-L5 level. Subsequently, special introduction instrumentation became available, which enabled safe puncture of the L5-S1 disc in approximately 85% of cases. (28-30)

In 1990, a prospective, multi-institutional study involving 327 patients undergoing treatment of lumbar disc disease was performed by 20 senior investigators. They reported a 75% success rate among patients monitored for more than 1 year. One case of discitis was reported, and no other serious complications were noted. (30)

Similar multicenter studies flourished in the early 1990s; the main one from China reporting their experience with 1,474 patients. Their reported success rate at 1 year was 83% (higher in protrusion versus sequestration, 86% versus 72%). The only complication reported was discitis (0.06%, nine patients). (31)

Following the hype of APLD, Chatterjee performed a significant clinical trial where he randomized 160 patients between microdiscectomy and APLD for the treatment of contained HNP. Patients who failed APLD after 6 months were offered microsmgical lumbar discectomy. Patients were followed for 3 weeks, 2 months, and 6 months. Four groups were established. The conclusions of this study revealed that APLD seems to be ineffective with only 33% excellent to good results at L4-L5 and 21% excellent to good results in L5-S1. A large percentage of patients were converted to microdiscectomy, and even when this occurred, patients outcomes were still inferior to primary microdiscectomies. (32)

In reviewing his personal series of 97 patients who had been monitored for more than 5 years, Maroon noted a success rate of 59%, which was substantially less than his own microsurgical success rate of 90% and was, incidentally, the same success rate he reported for his experience with chemonucleolysis in 1976. (29-33)

The swift rise in the application of APLD, similar to that of chemonucleolysis, has thus rapidly abated, because the overall success rates do not compare favorably with those of standard discectomy techniques

Manual Percutaneous Discectomy

In 1975, Hijikata, an orthopaedic surgeon from Japan, and coworkers introduced a percutaneous technique for disc removal, which they called "percutaneous nucleotomy." (34) Hult originally conceived the original concept of puncturing the annulus anteriorly to allow the disc to extrude into the retroperitoneum and not into the spinal canal. (35) Hijikata and coworkers, however, developed and applied specialized grasping forceps and curettes that they inserted through a cannula placed percutaneously, using fluoroscopic imaging, on the posterolateral aspect of the annulus. They subsequently reported a 72% success rate for 136 patients and postulated that the operation was successful because of the reduction of intradiscal pressure created by the fenestration in the outer annulus, so that nuclear material not removed with the instrumentation could extrude into the retroperitoneum. (34)

In a recent review of 600 cases that were evaluated retrospectively, Kambin and Schaffer (36) reported rates of satisfactory outcomes, as determined in patient self-evaluations, of 85% to 92%. Less than 2% of the cases required a second operation, and the investigators concluded that the technique offers the advantages of approximately 1-hour surgical time, negligible blood loss, avoidance of scarring in the epidural space, and anterolateral fenestration of the annulus for continuing relief of intradiscal pressure and nerve root decompression. (36)

Schaffer and colleagues (37) also used Hijikata's method but added discoscopy with an endoscope to allow direct observation of the instruments inserted via the contralateral side and to allow removal, under direct observation, of more of the nuclear material. Their success rate was 72% for 109 patients but with a complication rate of approximately 19%. (37)

There are many reasons why this technique never gained traction: it requires a large cannula size (5 mm to 8 mm in diameter); the potential for nerve root or vascular injury; the repeated entrance into the disc space, which might increase the risk of infection; the inability to relieve foraminal or lateral recess stenosis; the difficulty among obese patients; and the inaccessibility of the L5-S1 interspace in the majority of patients. (36-38)

Percutaneous Lumbar Laser Discectomy

The treatment principle of PLLD is based on the concept of the intervertebral disk being a closed hydraulic system. This system consists of the nucleus pulposus, containing a large amount of water, surrounded by the inelastic annulus fibrosus. An increase in water content of the nucleus pulposus leads to a disproportional increase of intradiscal pressure. A reduction of intradiscal pressure causes the herniated disk material to recede toward the center of the disk, thus leading to reduction of nerve root compression and relief of radicular pain. (39)

In PLLD, this mechanism is exploited by application of laser energy to evaporate water in the nucleus pulposus. Laser energy is delivered by a laser fiber through a hollow needle placed into the nucleus pulposus. The needle is placed into the intervertebral disk under local anesthesia. Apart from evaporation of water, the increase in temperature also causes protein denaturation and subsequent renaturation. This causes a structural change of the nucleus pulposus, limiting its capability to attract water, and therefore leading to a permanent reduction of intradiscal pressure by 57%. (39, 40)

In 1992, Choy and associates reported that an increase of intradiscal volume of only 1.0 mL causes the intradiscal pressure to rise by as much as 312 kPa (2,340 mmHg). (41)

Ascher and Heppner were the first to use the technique of percutaneous laser discectomy to treat lumbar disc disease. Their technique involved measuring the intradiscal pressure before and after laser discectomy using a saline manometer. (42) They postulated that the removal of even a small volume of tissue from the disc caused a corresponding decrease in intradiscal pressure. In his study, he introduced a neodymium:yttrium-aluminum-garnet (Nd:YAG), 1.06-m laser via a 400-nm fiber, and an 18-gauge needle. In their initial report, 9 of 12 patients initially exhibited improvement, but 5 of those patients subsequently required open operations, leaving 4 pain free.

In a follow-up publication in 1992, those investigators reviewed their results for 333 patients, with follow-up periods of up to 62 months. They observed good to fair responses for 261 patients (78.4%) and poor responses for 72 (21.6%). One hundred sixty patients experienced immediate pain relief during the procedure. (43)

The US Food and Drug Administration (FDA) approved percutaneous laser disc decompression for use in the USA in 1991. (44) The procedure is conducted under local anesthesia of the skin and underlying muscles. After assessment of the correct disk level by using fluoroscopy, a hollow needle is inserted 10 cm from the midline, pointing toward the center of the disk.

When the needle is in place, its correct position is verified by using biplanar fluoroscopy, sometimes in combination with CT imaging. A laser fiber (0.4 mm) is inserted through the needle into the center of the nucleus pulposus. Laser energy is then delivered into the nucleus pulposus to vaporize its content and reduce intradiscal pressure. (43, 44)

Differences can be found in the choice of laser type, power, pulse duration, and even total energy transmitted. A special note must be made on the trial that used a C[O.sup.2] laser or PLLD. C[O.sup.2] laser beams cannot be administered through a glass fiber. Therefore, in the study involved, a C[O.sup.2] laser beam was delivered into the disk by means of a fixed metal cannula. Four cases of thermal nerve root damage occurred due to heating of this cannula, presenting a total complication frequency of 8%.

In 1999, Hellinger, in Munich, began using the Ascher technique for Nd:YAG laser ablation; he reported his results for more than 2,500 patients treated for 13 years. He reported an overall success rate of approximately 80%. The actual resolution of sciatica may be longer than after conventional surgery, though immediate resolution of symptoms does occur according to the investigators. (45)

A systematic review published by Schenk in 2006 concluded that despite the fact that PLLD has been around for almost 20 years, scientific proof of its efficacy still remains relatively poor, though the potential medical and economic benefits of PLLD are too high to justify discarding it as experimental or ineffective on the sole basis of insufficient scientific proof. (44)

In a systematic review of percutaneous lumbar laser disc decompression that evaluated 33 publications, none of which were controlled, Manchikanti and coworkers concluded that based on USPSTF criteria, the indicated level of evidence for percutaneous lumbar laser disc decompression was II-2 for short-term and long-term relief. (46)

In the 2007 Cochrane Review update, Gibson and colleagues reveled that trials of percutaneous discectomy and laser discectomy suggest that clinical outcomes following treatment are at best fair and certainly worse than after microdiscectomy, although the importance of patient selection is acknowledged. (47)

Because the treatment principle of PLLD is based on the concept of the intervertebral disk being a closed hydraulic system, only contained herniations can be expected to respond to reduction of intradiscal pressure. Therefore, only contained herniations qualify for PLLD. The presence of disk extrusion or sequestered herniation is considered to be exclusion criteria. PLLD requires needle access to the intervertebral disk. For these reasons, patients with a narrowed intervertebral disk space or obstructive vertebral abnormalities are excluded. Any patient with severe progressive neurological deficit that require acute surgical intervention should not undergo PLLD. (47)

The most frequently described complication of PLLD is spondylodiskitis, both aseptic and septic. In order to avoid aseptic diskitis careful monitoring of patient complaints during the procedure is necessary, with adjustment of laser power, pulse rate, or pulse interval when heat sensations occur. (47)

Open Discectomy Versus Microscopic Discectomy

The standard of care remains an open partial discectomy in which the herniation is removed through a small annulotomy. The remainder of the stable disc is preserved. Throughout the years, many attempts have been made to refine the procedure to minimize the soft tissue dissection and ultimately scar tissue formation. Minimally invasive techniques have been developed with a focus on minimizing the soft tissue dissection, creating a laminotomy without creating instability, safely retracting the cauda equina and the individual nerve root, and finally excising the disc herniation. The use of magnification (microscope or magnification glasses) has assisted in achieving these goals. Microdiscectomy gradually has risen to become the standard operative technique for discectomy and has been shown to be superior to traditional open discectomy with regards to in-patient cost effectiveness, postoperative pain, and work days missed. (48-52)

In 1990, Barrios and associates performed a randomized study comparing standard open versus microsurgical lumbar discectomy. Patients were followed for an average of 3 years. He reported no difference in clinical outcomes or operative time with significantly less blood loss and comorbidities. (48)

In a randomized trial of 112 patients, Striffeler and coworkers compared their results between both techniques. They concluded that microsurgical lumbar discectomy had significantly higher excellent results compared to open discectomies. (49)

In 2012, Dasenbrock and associates performed a meta-analysis to compare of minimally invasive surgery versus open discectomy. (53) Data from six prospective RCTs comparing minimally invasive to open discectomy (with a total of 837 patients) were pooled in a meta-analysis. All minimally invasive discectomies were performed using tubular retractors. Non-significant higher complication rate was reported in minimally invasive surgery, which investigators attributed to limited visualization or the learning curve associated with this procedure. The current evidence suggests that both open and minimally invasive discectomy lead to a substantial and equivalent degree of short-term and long-term improvement in leg pain; the primary symptom of many patients with lumbar radiculopathy. The investigators concluded that minimally invasive discectomy may have many of its potential benefits, including a smaller incision and less paraspinal muscle injury, which may lead to reduced postoperative pain, a shorter hospital stay, and a faster recovery. There may also be a lower rate of surgical site infections, which may make this technique useful in obese patients. They also include that mimimally invasive discectomy requires a steep learning curve due to limited visualization and a restricted ability to extend the approach if necessary.

Wang and colleagues retrospectively reviewed the medical records of 120 patients (surgeon A with his first 60 patients and surgeon B with his first 60 patients) with sciatica and single-level L4-L5 disk herniation who underwent microscopic discectomy. (54)

Significant differences were observed in their operation time (p = 0.000), postoperative hospital stay (p = 0.026), and reoperation rate (p = 0.050) between the two groups. The investigators concluded that there a steep experience-related learning curve in the implementation of the approach. The surgeon's training level of minimally invasive spine surgery was an important factor for the success.

Microendoscopic Discectomy

Throughout the years, the endoscope has been used in various ways to examine or facilitate removal of herniated lumbar discs. In 1938, Pool at Columbia University first assessed disc pathological features and evaluated the dorsal nerve root with an endoscope. (48-51)

Schreiber and Suizawa, in 1986, used an endoscope to improve on the Hijikata technique of percutaneous nucleotomy. (50) They described a biportal approach, with working instruments on one side and an endoscope on the other. Two surgeons were required; one removed the disc while the other monitored progress with an endoscope. Disc material was initially removed blindly, creating a working space within the disc for insertion of the endoscope.

In 1993, Mayer and Brock reported a similar endoscopic technique using an angled scope, thus directing attention more dorsally in the region of annular tears. With the design of flexible up-biting instruments, better access to and observation of the dorsal annulus fibrosis was obtained. (51)

In 1995, Smith and coworkers presented their endoscopic working channel approach to far-lateral disc herniations. (55) Ditsworth, in 1998, (52) attempted to expand the patient pool for endoscopic techniques and described a transforaminal approach (the foraminoscopic approach). This was the first percutaneous technique to permit observation of the compressive disc and nerve root in the spinal canal. Ditsworth used a uniportal approach, with a small fiberoptic endoscope and a 6 mm working channel. The technique seemed well suited for the treatment of far-lateral disc herniations, but overall the limitations of the small size of the scope and foramen and the very steep learning curve have reduced the popularity of this technique.

The most popular and successful endoscopic system currently in use combines the technique of standard open microsurgical disc removal with endoscopic observation. In 1997, Smith and Foley, (55) who were the first to describe MED, presented results for their first 100 patients. They reported excellent results for 85 patients and good results for 11 patients, with a mean hospital stay of 9.5 hours and a return to work in 2 to 42 days

There were limitations to the initial MED system. The endoscope was not reusable, image quality was inconsistent, and the working space within the tubular retractor was limited.

The next generation MED system, called the METRx[R], was developed to address these limitations. Compared with the initial MED system, the METRx[R] system has additional advantages, including improved image quality, decreased endoscopic diameter, variable tubular retractor size, increased available working room within the tubular retractor, and address not only contained lumbar disc herniations but also sequestered disc fragments (Fig. 1). (56, 57)

In a multicenter, prospective study performed in Italy, 68 patients were randomized to either microendoscopic discectomy or microsurgical lumbar discectomy. They were followed for a period of 6 months, and 94% had excellent to good outcomes in both groups with no significant differences. (58)

Microendoscopic discectomy can be successfully performed on an ambulatory basis under local, spinal, or general anesthesia The patient is positioned prone with the spine flexed to aid in intraoperative exposure of the interlaminar space; abdominal compression is avoided by properly positioning the patient on a frame or rolls to reduce intraoperative venous bleeding. A set of fine-bayoneted instruments and a thin, tapered drill were designed to help simplify the procedure by optimizing working space. (57)

The lumbar level to be approached is confirmed by using lateral fluoroscopy, and then a 20-gauge spinal needle was inserted into the paraspinal musculature approximately one finger-breadth (1.5 cm) lateral to the midline on the symptomatic side of the patient at the appropriate disc level. A vertical incision is made at the puncture site. The incision length has to allow for the diameter of the respective tubular retractor. For example, the 16 mm tubular retractor required a 14-mm incision. A guide for tubular dilation is then placed through the incision and directed toward the inferior aspect of the superior lamina and medial facet junction under lateral fluoroscopic guidance.

The initial cannulated soft tissue dilator was inserted over the guide using a twisting motion while avoiding excessive downward force. Tip of the dilator was used to sweep the paraspinal musculature off the laminar edge while being careful not to enter the interlaminar space. Second, third, and fourth dilators were sequentially placed over the initial dilator down to the lamina. The flexible arm, which is secured to the table, is then attached to the tubular retractor to hold it firmly in place once positioned. The attachment of the flexible arm to the working channel is positioned 180[degrees] away from the surgeon. (57)

Because the surgeon is working through a tube, surgical orientation is of utmost importance. To help in this regard, the endoscopic orientation is adjusted such that the medial anatomy is on the top of the video monitor (12 o'clock), and the lateral anatomy is on the bottom (6 o'clock) (Fig. 2).

There is a learning curve to using the system efficiently and safely. Because most surgeons use an operative microscope or loop magnification, looking up at a video monitor to perform microdiscectomy seems cumbersome at first.

High-energy radiation light emitted from the illuminating fiber at the distal end of the scope may give rise to temperatures exceeding 41[degrees] C within 8 mm in front of the scope. Therefore, do not leave the endoscope tip in direct contact with the patient's tissue or combustible materials, or burns may result. The most frequent complications stated risks are dural tear, bleeding, neurological damage, damage to the surrounding soft tissue, and infection. (58-60)

In 2006, Xiaotao Wu and colleagues reported their follow up of 873 consecutive patients using MED. They followed patients from 2000 to 2003 and collected clinical outcomes. (61) They compared these results with a prior cohort of patients from 1998 to 2000 of 358 patients. They reported a significant difference in perioperative parameters.

Conclusion

With time, the acceptance of minimally invasive spine surgery has grown among surgeons and patients. The tenets that minimally invasive spine surgery embrace (e.g., diminished intraoperative soft tissue trauma, reduction of operative time and complications, smaller incision sites, and decreased hospitalization, and use of postoperative narcotics) contribute to an improved postoperative outcome as well as to a decrease in health care costs. Furthermore, with other minimally invasive technologies, the precision of the application of various instrumentation is increased. Recent comparative, clinical cohort studies have substantiated such advantages of minimally invasive spine surgery techniques over the traditional, open procedures

Many procedures have come and gone but only modifications of the original open discectomy have proven to be successful

Minimally invasive spine surgery procedures continue to evolve, expanding the repertoire of conditions that can be addressed safely and effectively; however, to assure optimal postoperative outcome and patient satisfaction, proper patient selection continues to be paramount.

Martin Quirno, M.D., Shaleen Vira, M.D., and Thomas J. Errico, M.D.

Martin Quirno, M.D., Shaleen Vira, M.D., and Thomas J. Errico, M.D., Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, New York, New York.

Correspondence: Martin Quirno, M.D., Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, 301 East 17th Street, New York, New York 10003; mquirno@gmail.com.

Disclosure Statement

None of the authors have a financial or proprietary interest in the subject matter or materials discussed, including, but not limited to, employment, consultancies, stock ownership, honoraria, and paid expert testimony.

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Caption: Figure 1 Cannulated METRx[R] endoscopic system. To view this figure in color, see www.hjdbulletin.org.

Caption: Figure 2 A, Hemilaminotomy view through endoscopic portal. B, Splitting of the ligamentum flavum. C, Anatomic view of the dura traversing nerve root and facet. D, Disc material being removed. To view this figure in color, see www.hjdbulletin.org.

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Author:Quirno, Martin; Vira, Shaleen; Errico, Thomas J.
Publication:Bulletin of the NYU Hospital for Joint Diseases
Article Type:Report
Date:Jan 1, 2016
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