Lumbar spinal stenosis is the narrowing of the spinal canal with consequent neural compression, commonly resulting in leg pain, low back pain, and functional disability, the severity of which varies widely among patients.1,2 Stenosis occurs most frequently in patients around the age of retirement, with a 3:2 ratio of men to women.2,3 In 2007, nearly 38,000 operations were performed for lumbar spinal stenosis in the US Medicare population alone, at a total hospital bill of $1.65 billion.4 With the increasing aging of the population, the prevalence of stenosis can be expected to continue rising.1,4
Patients with mild-to-moderate symptoms should first receive multimodal nonsurgical treatment except in the presence of motor deficits or cauda equina syndrome, which are indications for immediate surgery.1,5 Patients not improving after 3 to 6 months of nonsurgical treatment should be offered the option of surgical treatment, and surgery achieves the best results if performed within 1 year of the onset of symptoms.5 The clinical outcomes of surgical decompression have generally been good in properly selected patients. For example, the SPORT (Spine Patient Outcomes Research Trial) for stenosis without spondylolisthesis showed a significant advantage on all primary outcomes for posterior decompressive laminectomy over continuing nonsurgical care by the 3 month follow-up period which was maintained at 4 years.2,6 In the Maine Lumbar Spine Study's 8 to 10 year report, about half of each the decompressive laminectomy and nonsurgical patients had improved low back pain and were satisfied with their current state, but significantly more surgical than nonsurgical patients had improved leg pain and said they would still choose the same initial treatment.7 Thus, surgical decompression is an effective treatment for lumbar stenosis in patients who have not shown adequate improvement from conservative care.
Despite good clinical outcomes, traditional decompression has a number of shortcomings, both intraoperative and postoperative. First and most obviously, traditional open decompression has all the usual morbidity issues resulting from tissue damage from the standard posterior midline approach. The development of minimally invasive approaches has sought to adequately address this with muscle-sparing techniques. Second, it can be difficult to access the neural foramen with either open laminectomy/laminotomy or various minimally invasive surgery (MIS) decompression techniques, due to inadequate visualization and to the linear configuration of the decompression instruments. This is critical given 30% of patients enrolled in the SPORT trial were diagnosed with a component neuroforaminal stenosis.2 As a consequence of the difficulty of access and visualization, decompression of the neural foramen often remains incomplete.8 Foraminal stenosis has often been reported as the most common structural cause of failed back surgery, ranging from 25% to 58%.9
The difficulty of accessing a stenotic neural foramen with linear configured decompression instruments during traditional or MIS surgery results in resection of part or the entire facet joint. Although this allows for effective decompression and clinical relief of symptoms, it can lead to instability. The facet joint normally serves to guide vertebral motion and to resist compression, shear, and rotation forces. Biomechanical studies have shown a positive correlation between posterior element removal and increase in motion at the surgical site, with greatest changes occurring in axial rotation and extension.10 In addition, destruction of the facet joint transfers axial loads to the anulus and anterior longitudinal ligament, which may accelerate disc degeneration.11 This may cause instability and nonphysiological motion, possibly leading to neural trauma, facet fracture, disc disruption, or spondylolisthesis. In their retrospective study, Hopp and Tsou reported that 57 of 344 (16.6%) of patients who had decompression for stenosis had to undergo additional fusion surgery because of complications mostly due to instability.12 To avoid such complications and the need for reoperation, it has been recommended that patients undergoing decompression also be fused if there is preoperative instability or disc damage, or if the decompression is extensive or involves complete facetectomy.8,12,13
The patient registry of the Scoliosis Research Society is probably the only database on stenosis that is sufficiently large to reliably assess safety. The SRS recently reported on the morbidity from over 10,000 cases of first-time surgery for lumbar stenosis submitted in the past 4 years.14 The overall complication rate was 7%. Minimally invasive procedures were used for 12% of the patients. Patients who underwent minimally invasive procedures had a lower complication rate than those who underwent traditional procedures (5.8% vs. 7.6%, P = 0.01), though the authors cautioned that this may have been due to differences in the severity of the stenosis or the number of levels decompressed.
Current Minimally Invasive Techniques for Decompression
MIS for stenosis involves less damage of soft tissue and results in shorter hospitalization recovery times with no increase in complication rates compared to traditional laminectomy.15 Thus minimally invasive techniques have the potential to improve the outcomes and value of surgical treatment. Several techniques have been developed to minimize the damage to soft tissues for access and/or to reduce the amount of structures removed to perform the decompression including laminotomy, the use of tubular retractors, microendoscopic laminotomy, laminoplasty, and foraminotomy.
Laminotomy has been shown to be effective in several studies. Oertel et al reported on a medium-term study of 102 patients who received unilateral laminotomy for bilateral decompression. They reported 85% excellent to fair results using the Finneson and Cooper scale at a mean 5.6-year follow-up. The reported complication rate was 9.8% with a reoperation rate of 11.8%, including among others 7 patients for recurrent stenosis and 2 for spinal instability.16 Thomé et al randomized 120 patients with 207 levels of stenosis without disc herniation or instability to bilateral laminotomy, unilateral laminotomy, or laminectomy.17 The outcomes were good and comparable for unilateral laminotomy and laminectomy, but bilateral laminotomy provided significant improvements in Visual Analog Scale (VAS) pain, Short-Form 36 (SF-36), patient satisfaction, and a reduction in complication rates. In the unilateral laminotomy group, the perioperative morbidity was 17.5%, with a 7% rate of dural tears and 2 patients presenting with postoperative hematomas. In their study on 75 patients with focal lumbar stenosis with and without deformity who underwent laminoplasty (bilateral decompression through a unilateral approach), Kelleher et al reported mean Oswestry Disability Index (ODI) reduction from 50 before surgery to 24 at mean 32-month follow-up, with 8 (11%) patients requiring revision.18 Celik et al reported on 74 patients with 5-year follow-up after microdecompressive laminotomy compared to total laminectomy and reported a lower rate of perioperative complications and postoperative stability, but similar clinical outcomes between groups as measured by VAS pain scores, ODI, and walking distance without pain (P > 0.05 for all outcome measures).19
The use of a unilateral approach using a tubular retractor system is an effective MIS method. Rahman et al conducted a retrospective study of 126 patients treated with either tube-based minimally invasive or open decompression and showed less estimated blood loss (EBL), shorter operating times, shorter hospital length of stay, and fewer and less severe complications in patients treated with the MIS technique.20
Pao et al used microendoscopic laminotomy and reported that 40 of 50 (80%) patients had good or excellent results with ODI scores improving from 64.3 ± 20.0 to 16.7 ± 20.0 and Japanese Orthopedic Association score improved from 9.4 ± 6.1 to 24.2 ± 6.0. There were 11 surgical complications including 2 wrong-level operations and transient neuralgia in 4 patients.21 Costa et al conducted a large retrospective study using unilateral microdecompression and reported clinical benefit in 329 of 374 (88%) of patients who completed follow-up as measured by VAS pain scores and Prolo Economic and Functional scale.22 VAS pain scores decreased from 8.9 before surgery to 4.2 after a mean follow-up period of 30 months. Improvement as measured by the Prolo scale was demonstrated in 78.6% of patients with a mean improvement of 3 ± 2 points. No patients required surgical stabilization. Khoo and Fessler compared microendoscopic laminotomy to open decompression in 25 patients each.23 The microendoscopic laminotomy group had longer operating time, less blood loss, shorter hospital stay, and fewer narcotics, but the clinical outcomes were similar in the 2 groups. Specific outcomes were not reported due to the brief follow-up period as reported by the authors. Good clinical outcomes have also been reported for other microdecompression procedures, primarily for central and lateral recess stenosis.24–26 However, as Weiner et al and Palmer et al point out, the ipsilateral lateral recess is often difficult to decompress without extensive facet removal.25,26 This technique also requires extensive knowledge of the microanatomy and considerable experience with a microscope.25
Laminoplasty has been studied often in the cervical spine but rarely for lumbar stenosis. Kawaguchi et al reported on 54 patients receiving expansive laminoplasty.27 Expansive laminoplasty seeks to create more central canal space by inserting a bone graft to lengthen the lateral laminar length on the side of the central canal stenosis. The length of the lamina out laterally is increased by cutting 1 lamina superiorly to inferiorly and creating a groove on the opposite lamina to effectively create a hinge. Wires are used to lift up the posterior elements of the vertebra, impinging ligamentum flavum is removed, and exposed epidural spaces are shielded with fat graft before securing the wired bone graft. The wires are then used to secure bone graft from the spinous process of the level being decompressed into the lateral recess space created. Autograft and allograft are placed posteriorly to create arthrodesis. The authors reported an average recovery rate of 69% for patients with degenerative stenosis as measured by Japanese Orthopedic Association score. Five (9%) patients returned with spondylolisthesis. The primary factors associated with a poor recovery were advanced age and insufficient decompression of lateral stenosis.
Foraminotomy has also been poorly studied. Baba et al reported on 31 patients with foraminal stenosis receiving foraminotomy and lateral laminotomy and claimed improvement of radicular symptoms and intermittent claudication with only 1 patient requiring restabilization at the operated level.28 Patond and Kakodia conducted a study comparing foraminotomy, undercutting facetectomy, and interlaminar fenestration on 16 patients and reported good outcomes in 73% of patients (table of results not available) and an uneventful 2-year follow-up.29 Iwatsuki et al performed bilateral interlaminar fenestration and unroofing of the intervertebral foramen with a microscope via a unilateral approach in 47 single-level patients, and reported that 45 patients assessed their outcomes as “very successful” using a simple questionnaire.30 Patient symptoms were evaluated using the neurogenic claudication outcome score. Mean neurogenic claudication outcome score before surgery was 29.8 (range, 8–48) and at the 2-year follow-up visit was 83.2 (range, 32–100).
Interspinous process spacers have also been frequently used to successfully treat mild-to-moderate stenosis, but these are not reviewed here, since their mechanism of action does not involve removal of the compressing bone or tissue.
MIS Decompression Using a Flexible Microblade Shaver System
While many of the previous “minimally invasive” techniques allow for a muscle-sparing access, they still require significant resection of the facet joints and midline structures to decompress bone and tissue directly impinging on the nerves. The iO-Flex system (Baxano Inc., San Jose, CA) may be considered truly minimally invasive because it is designed to provide a complete decompression while leaving other structures, particularly the facet complex and midline bone and ligaments, intact. This system of flexible instruments is able to traverse the foramen and perform a thorough decompression of the lateral recess and neural foramen using a ventral to dorsal, rather than medial-to-lateral approach. This system allows for a bilateral decompression from a unilateral approach using either a tube-based or open exposure.
The central canal ligamentum flavum and bony decompression is achieved via standard unilateral hemilaminotomy. A cannulated probe is passed from medial to lateral through the dorsal and caudal aspect of the neural foramen (Figure 1). A nitinol wire with an exchange hitch on the proximal end is passed through the guide, where it exits the skin lateral to the access site and is captured in the system's distal handle. The approximate lateral exit point of the wire can be controlled with the cannulated probe.
Because the exiting nerve root cannot be visualized directly, it is imperative that its location be determined before proceeding with the decompression. Neural localization is achieved using the NeuroCheck device coupled with EMG stimulation. The device is pulled into the foramen and current is applied to the ventral and dorsal channels sequentially (Figure 2). A ratio algorithm denotes the position of the device relative to the nerve root. The neural localization device is replaced with a flexible microblade shaver (MicroBlade Shaver instrument) on the wire and pulled into the foramen. Using gentle upward tension on both handles, a bimanual reciprocating motion like that of Gigli saw is used to debulk the targeted bone and soft tissue (Figure 3). Morselized bone and ligament adhere to the instrument and are removed with it. The adequacy of tissue removal is assessed by fluoroscopy and direct palpation with a probe. The operative site is then irrigated, checked for hemostasis, and routine closure is performed. Figure 4 demonstrates preoperative magnetic resonance imaging of a typical case and pre- and postdecompression intraoperative lateral fluoroscopic images.
An early postmarket pilot study was performed to assess clinical outcomes with the first generation of this flexible microblade shaver system. A total of 9 patients were treated at 3 sites (Table 1). There were 2 males and 7 females with an average age of 68.7 years (range, 46–83). A total of 13 levels were decompressed with an average (range) operating time of 124 (84–204) minutes. Mean EBL was 110 mL (75–150) and hospital stay averaged 1.3 days (0–3). There were no reported dural tears; however, 1 patient experienced postoperative transient neuropathy which resolved 96 days post surgery. Patient outcomes were assessed using the VAS for leg and back pain, ODI, Zurich Claudication Questionnaire, and Physical Component Score of the SF-36 Questionnaire. Median outcomes are reported as they can be more representative of central tendency in such a small patient population (Table 2). At the 1 year follow-up time point, leg pain decreased by 5.6 cm (73%), ODI improved by 18 points (50%), Zurich Claudication Questionnaire physical function improved by 34 points (72%) and symptom severity by 18 points (31%), and Physical Component Score of SF-36 improved by 9 points (36%). These preliminary results are encouraging as all median outcome measures showed improvement over the minimally clinically important difference values for the respective outcome measures.31
An Institutional Review Board approved retrospective medical chart review was conducted at 2 centers to assess acute device and procedural outcomes using the flexible microblade shaver system (Table 3). A total of 132 levels were decompressed in 67 patients from September 2008 to April 2010. The number of males and females was roughly equal and the average (range) age was 66.2 (40–89). All patients had lateral recess and foraminal stenosis with or without central stenosis; 2 patients also had a facet cyst, 5 patients had concomitant degenerative scoliosis, and 13 patients had spondylolisthesis and stenosis. Twenty-eight levels were revisions from previous decompression surgery (26 levels) or X-STOP failures (2 levels). Operating time averaged 135 minutes, mean EBL was 127 mL, and the average length of stay was 2.3 days. Inadvertent durotomies occurred at 7 levels; 6 during access to the central canal with traditional surgical instruments, and 1 at the axilla of the L5 nerve root after introduction of the shaver.
Sixteen patients from this series were treated by the author. In this subset, an average of 2.3 levels per patient was decompressed. Average (range) operating time was 174 (105–314) minutes, and the mean (range) hospital stay was 3.8 (1–8) days. Short-term clinical outcomes have been excellent, with VAS pain decreasing from a mean (range) of 7.5 (5–8) before surgery to 1.7 (0–5) after surgery at an average of 3.5 month follow-up. Visual analog pain scores were not routinely collected at the other participating center and are therefore not available. Standing anteroposterior and lateral flexion and extension radiographs were obtained on all patients in this subset and showed no progression in subluxation or deformity as of the last follow-up visit.
All 67 patients from this retrospective study have remained clinically stable and no patient has undergone additional surgery as of the last follow-up time point. Additionally, the author has decompressed 127 foramens in 45 patients since January 2009. Access with the cannulated probe was achieved in 98% of stenotic foramen attempted. All patients have remained clinically and radiographically stable, no patient has returned for additional surgery and there have been no cases of neurologic impairment. Although the follow-up interval is short, these outcomes are promising.
The ultimate goal, regardless of the technique used, is to perform an effective decompression of the affected nerve root. The secondary aspect of morbidity comes from the destructive nature of the approach—removing too much of the facet joint or ligaments. This collateral damage of the exposure is the rationale for developing minimally invasive techniques. Current MIS techniques for decompression have successfully shown to shorten hospital recovery times, reduce intraoperative complications, and minimize soft-tissue trauma in patients. However, they may fail to provide an adequate decompression in patients with bony foraminal stenosis. In a retrospective review of 65 cases presenting with foraminal stenosis, Jenis et al reported that 63 (97%) underwent decompression with concomitant arthrodesis due to decompression at apical region of scoliosis, spondylolisthesis, or potentially destabilizing decompression.32 Furthermore, nearly 50% of these patients had already undergone decompression surgery.
There have been recent criticisms of the additional performance of fusion after decompression for stenosis.4,13 Although decompression is widely recognized as an effective treatment of stenosis, the indications for additionally performing fusion (with or without instrumentation) have been criticized as less clear and convincing.4,33 The increased rate of complications and the high costs associated with performing an additional fusion have been a source of concern,4,13 as the Medicare system faces the threat of looming financial unsustainability. One study reported that 39% of Medicare patients treated for stenosis received fusion, but 223 of these 523 patients (43%) did not have spondylolisthesis.34 Aside from patients with coexisting scoliosis or spondylolisthesis, one likely reason that fusion is often performed after decompression for stenosis is that traditional forms of decompression (including many “minimally invasive” decompressions) remove too much bone and tissue. Fusion is then performed to prevent spinal instability. It has been pointed out that it is technically easier to perform a destabilizing decompression and an additional corrective fusion than it is to just perform a thorough decompression that does not destabilize the spine.13
Current MIS techniques allow for a muscle-sparing decompression with shorter hospital stays and fewer intraoperative complications but may fail to provide an adequate facet-sparing foraminal decompression. MIS decompression using a flexible microblade shaver system potentially represents a way to perform an effective decompression in patients with lumbar spinal stenosis. Preservation of the posterior elements has been shown to preserve normal spinal motion.10 Therefore it stands to reason that if these elements are preserved while performing a clinically effective decompression, the motion segment will remain stable, reducing the need for additional fusion procedures. While many of the MIS techniques presented here have purported benefits, more clinical data are needed to characterize their longer term effectiveness.
- Traditional decompression involves a wide decompressive laminectomy with partial facet resection.
- MIS techniques allow for preservation of midline structures, reduced operating time and hospital stay, but still require partial facet resection to decompress the lateral recess and neural foramen.
- Inadequate decompression of the neural foramen in the most common structural cause of failed back surgery syndrome.
- The iO-Flex system allows for a facet-sparing decompression of the lateral recess and neural foramen by performing an intraforaminal decompression from ventral to dorsal, rather than medial to lateral.
The author would like to thank Robyn Capobianco and Michael Hanna, PhD (Mercury Medical Research and Writing), for providing medical writing services; and Joyce Lin for data analysis. Additionally, Lawrence Dickinson, MD; Ronnie Mimran, MD; and Jeff Randall, MD (Pacific Brain and Spine; Castro Valley, CA) and Joseph Stern, MD (Vanguard Brain and Spine, Greensboro, NC) contributed patients to the iO-Flex system studies.
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