Patient Population

Institutional approval for this study was obtained from Weill Cornell Medical College. Data for this retrospective case series were extracted from the prospectively collected hospital database. We sought all patients who had undergone minimally invasive laminectomy for decompression of LSS between 2004 and 2011. Demographic and perioperative data on these patients were collected. For the demographic data, age, sex, body mass index (BMI), smoking status, history of diabetes, and steroid medication use were recorded. Operative and anesthesia reports as well as hospital charts were reviewed to add data on estimated blood loss, number and levels of surgery, complications, and length of hospital stay. The presence of preoperative spondylolisthesis was determined on radiographic studies. Postoperative radiographs were evaluated to assess changes in the amount and grade of previous spondylolisthesis and/or the development of new slippage. Standardized radiographs were used for assessments, both preoperatively and postoperatively. Patients who had presented with preoperative spondylolisthesis and primarily mechanical back pain and/or instability on flexion/extension films were not included in this study. Comparisons were made between the outcomes of minimally invasive laminectomies in our cohort and the outcomes of other decompressive surgical interventions for LSS, as published in the literature.

Operative Procedure

With the patient lying prone under general anesthesia, a small skin incision is made overlying the target level approximately 1.5 cm lateral to the midline. An 18- or 19-mm tubular retractor is placed over a series of tubular dilators for retraction, and under the microscope, the inferior edge of the lamina and the inferior edge and base of the spinous process are exposed (Fig. 1). The decompression is performed step by step starting with an ipsilateral partial laminectomy using a 3-mm curved matchstick drill bit and bayonet-shaped 2- and 3-mm Kerrison punches (Fig. 2). To achieve appropriate visualization of contralateral structures, the operating table is tilted away from the surgeon and the tubular retractor is angled medially. Next, the spinous process and the contralateral lamina are undercut using the drill and rongeurs (Fig. 3). The ligamentum flavum is completely exposed bilaterally and cranially until its insertion under the lamina. During the laminectomy, the ligamentum flavum protects the dura. Next, we insert a nerve hook and then a Kerrison rongeur in the midline where the two leaves of the ligamentum flavum meet, an area that is typically identified by the presence of epidural fat (Figs. 4 and 5). These techniques minimize risks of injury to the dura. The ligamentum flavum is then removed (Fig. 6). Great care is taken to completely decompress the lateral recess and entirely remove the yellow ligament. This also allows visualization and decompression, if needed, of the contralateral exiting and traversing nerve root.

An 18- or 19-mm tubular retractor is placed over a series of tubular dilators for retraction, and under the microscope, the inferior edge of the lamina and the inferior edge and base of the spinous process are exposed. Illustration by Thomas Graves. Copyright Roger Härtl. Published with permission.

The decompression is performed starting with an ipsilateral partial laminectomy using a 3-mm curved matchstick drill bit and bayonetshaped 2- and 3-mm Kerrison punches. Illustration by Thomas Graves. Copyright Roger Härtl. Published with permission.

The operating table is tilted away from the surgeon, and the tubular retractor is angled medially to achieve appropriate visualization of contralateral sublaminar structures. Next, the spinous process and the contralateral lamina are undercut using the drill and rongeurs. The ligamentum flavum is completely exposed cranially until its insertion under the lamina. Illustration by Thomas Graves. Copyright Roger Härtl. Published with permission.

A nerve hook is inserted in the midline where the two leaves of the ligamentum flavum meet, an area that is typically identified by the presence of epidural fat. Illustration by Thomas Graves. Copyright Roger Härtl. Published with permission.

The ligamentum flavum is then carefully removed using Kerrison rongeurs. Illustration by Thomas Graves. Copyright Roger Härtl. Published with permission.

Dural exposure and decompression after complete removal of the ligamentum flavum. The contralateral approach allows visualization and decompression, if needed, of the contralateral exiting and traversing nerve root. Illustration by Thomas Graves. Copyright Roger Härtl. Published with permission.

Adequate decompression is always confirmed by identifying certain landmark structures and achieving particular surgical end points, as follows: contralaterally and caudally by palpating the pedicle using a ball tip instrument, cranially by confirming complete removal of the yellow ligament and the absence of bony compression of the dura mater, and ipsilaterally by complete removal of the ligamentum flavum and by clear identification of the lateral aspect of the thecal sac and inspection of the traversing nerve root. As the next and final step, the table and retractor are brought back into the initial position, and the ipsilateral decompression is completed.

Clinical Outcome Evaluation

The preoperative and latest available follow-up Oswestry Disability Index (ODI)¹⁴ and visual analog scale (VAS) scores for back, buttock, and leg pain were collected and compared. Functional outcome was evaluated using Macnab's criteria.³⁵ Patients without late follow-up outcome scores recorded in the hospital's database were contacted by phone, and the ODI and VAS pain scores and Macnab's level of satisfaction were assessed. These patients were also asked if they had undergone any further lumbar spine surgery at outside medical centers.

A 12-point improvement on the ODI^3,20,22 and 3-point improvement on the VAS were regarded as the minimum clinically important difference (MCID).^15,16,22 As for functional outcome, 4 levels were defined according to Macnab's criteria: excellent (no pain; no restriction of activity), good (occasional back or leg pain of sufficient severity to interfere with the ability to work), fair (improved functional capacity but handicapped by intermittent pain of sufficient severity to curtail work), or poor (no improvement or insufficient improvement to enable increase in activities; further operative interventions required).³⁵

The patients who required another operation and underwent a second surgery at our and other institutions were identified. The indication for the second operation and a description of the procedure were recorded.

Radiographic Evaluation

In patients with postoperative radiographic studies, the latest study was used to assess changes in the amount and grade of previous spondylolisthesis and/or development of new slippage.

Statistical Analysis

Patients with preoperative radiographic studies, who had been evaluated for the presence or absence of preoperative spondylolisthesis, were divided into 2 groups: patients with concomitant preoperative spondylolisthesis and those without. We reported results for the entire group of patients as well as for each of the 2 groups separately. Comparisons were made between the 2 different groups and between each group and the entire study group.

Continuous variables are shown as the mean ± standard deviation. Percentages were calculated for categorical variables. Differences in categorical variables were assessed using the chi-square test. Differences in continuous variables were tested using the independent t-test, and differences for repeatedly measured variables were assessed using the repeated measures t-test. All analyses were performed using appropriate statistical software (SPSS, version 18.0.0.1, SPSS Inc.). A p value of < 0.05 was considered statistically significant.

Patient Demographics

One hundred ten consecutive patients, 58 males and 52 females, with isolated LSS who had undergone minimally invasive laminectomy surgery for decompression were included in our study. The mean age at surgery was 68.5 years (range 32–89 years; Table 1), and the mean BMI was 28.6 kg/m². A history positive for smoking, diabetes, and steroid medication use was present in 11.8%, 14.8%, and 4.6% of patients, respectively. In 97 patients with available preoperative imaging, spondylolisthesis Grade I was present in 51 patients (52.6%), whereas 46 patients (47.4%) did not have preoperative spondylolisthesis.

Surgical Details

One-, 2-, and 3-level surgeries were done in 57 patients (51.8%), 47 (42.7%), and 6 (5.4%), respectively. A total of 169 levels were operated on in 110 patients, and the most common level of surgery was L4–5 (50.3% of cases). The L3–4 level made up 29% of cases; L5–S1, 11.2%; and L2–3, the remaining 9.5%. The median estimated blood loss for 1-, 2-, and 3-level surgeries was 25, 50, and 150 ml, respectively (Table 2). More than half of the patients (56%) were discharged on either the day of surgery or the first postoperative day, while the mean duration of hospitalization was 2.2 ± 2.63 days (range 0–13 days).

Clinical Outcome

The mean preoperative ODI was 45.3 ± 19.62 (range 0–88; Tables 3 and 4), which decreased to 28.9 ± 21.76 at a mean follow-up of 28.8 months. An MCID (defined as improvement on the ODI by 12 or more points^3,20,22) was achieved in 54.8% of patients (Table 5). The median preoperative VAS back, buttock, and leg pain scores were 8, 4.5, and 4.6, respectively. At the latest follow-up, the median VAS back, buttock, and leg pain scores decreased to 3.5, 0, and 0, respectively. Three-point improvement on the VAS was regarded as the MCID.^15,16,22 The MCID for VAS back, buttock, and leg pain was achieved in 50.0%, 45.2%, and 52.4% of the patients, respectively. All 4 scores indicated statistically significant improvement at the latest follow-up. A comparison of ODI and VAS outcome scores in patients with and without preoperative spondylolisthesis showed no significant difference between the 2 groups (Tables 4 and 5).

Functional outcome, as assessed using Macnab's criteria,³⁵ was excellent and good in 71.6% of the patients. A comparison of Macnab's results in patients with and without preoperative spondylolisthesis showed no significant difference (Table 6).

Sixteen cases with dural injuries were not associated with CSF leakage and were intraoperatively repaired using DuraSeal. There were no intraoperative and/or inpatient complications. At a mean follow-up of 28 months, the reoperation rate at the same level (with or without surgery at a different level) for the entire study group was 12.9%. Among the 11 reoperated patients, 7 (8.2%) underwent repeat decompression without fusion at the same level (with or without surgery at a different level), whereas in 3 patients (3.5%) a fusion was performed at the same level. Comparison of total reoperation rates as well as reoperation with fusion in patients with and without preoperative spondylolisthesis showed no significant difference between the 2 groups (Table 7).

Radiographic Results

Because postoperative imaging was not routinely done in this retrospective case series, only a limited number of patients (41) had available follow-up imaging. In patients with preoperative spondylolisthesis the mean slippage prior to surgery was 5.5 ± 3.04 mm, which increased to 6.4 ± 3.5 at the latest follow-up. This increase did not reach statistical significance. None of the imaged patients without preoperative spondylolisthesis demonstrated any slippage at the latest follow-up. Although we report the radiographic follow-up for the study group, we emphasize that radiographic outcome was not used to draw definite conclusions in this study. Instead, clinical outcome has been mainly used as evidence to draw possible conclusions.

Learning Curve

The first 55 patients were grouped into an early experience group and the second 55 cases into a late experience group. Operative time, estimated blood loss, dural injury rates, clinical outcome, Macnab's criteria, and reoperation rate were compared between these 2 groups. Comparisons showed no statistically significant difference in operative time, estimated blood loss, clinical outcome, and Macnab results in the 2 groups. Dural injuries occurred in 15 cases (27.3%) in the early experience group and in only 1 case (1.8%) in the late experience group (p < 0.0001). Moreover, 9 patients (23.7%) in the early experience group needed reoperation, whereas only 2 patients (4.2%) in the late experience group underwent a second operation (p = 0.008). The follow-up duration for clinical outcome and Macnab's criteria estimation was 45.7 ± 17.03 months in the early group and 15.5 ± 7.03 months in the late experience group. Similarly, the follow-up duration for reoperation was 44.5 ± 18.34 months in the early group and 15.5 ± 7.03 months in the late experience group. Nonetheless, the period between the first and second operation was found to be similar and not statistically different between the early and late experience groups. The median interval between the first surgery and the reoperation was 16.9 months (minimum 0.2; maximum 39.7).

Background

Several less-invasive surgical techniques have been introduced for direct decompression of the lumbar spine. The introduction of a less-invasive retractor system dates back to 1991 when Caspar and colleagues proposed the use of less-invasive specular retractors for discectomies.^7,17 Independently, Foley and Smith, in 1997, described the tubular microendoscopic discectomy in the lumbar spine, in which endoscopes were used through tubular retractors to perform a discectomy.¹⁸ They had worked on this approach in the laboratory since 1994, and the first clinical case was performed in early 1996 (K. Foley, personal communication, 2004). The METRx tubular retractor system was introduced in 2003 and permitted the use of the microscope, which allows excellent illumination and visualization of pathology. The use of tubular retractors to access spinal pathology has been associated with several biomechanical and clinical advantages: First, the approach through a minimally invasive retractor can spare attachments of paraspinal ligaments and muscles, which stabilize the spine. Minimizing injury to paraspinal ligaments and muscles allows for postoperative strengthening of back muscles and maintains stabilization of the motion segment. Second, the minimally invasive approach allows contralateral decompression of the spinal canal, lateral recess, and foramen without another incision while also sparing part of the lamina, spinous process, and interspinous ligaments.^2,67

The anatomical description and clinical results of a unilateral laminotomy for contralateral decompression were first reported by Spetzger in 1997.^55,56 Similar observations were also described by McCulloch in 1999.^38,59 Adopting tubular retractors for this approach led to excellent decompression of the contralateral lateral recess and even foraminal disc herniations and contralateral foraminal stenosis.^2,67 For example, we reported that synovial cysts can be resected safely and without compromising the facet joint by approaching them from the contralateral side and from “normal” dura and anatomy via tubular retractors.²⁸ Biomechanical and laboratory results indicate that laminectomy via tubular retractors or bilateral laminotomies are associated with less destabilization than an open bilateral laminectomy.^{1,5,6,12,23,34,57} In their biomechanical study, Lee et al. demonstrated that bilateral laminotomy was associated with significantly less hypermobility and stiffness reduction than a full laminectomy, and they proposed that this could be attributed to preservation of the central posterior osteoligamentous structures in laminotomy.³⁴ In summary, minimally invasive laminectomy via tubular and similar retractors achieves effective neural decompression with simultaneous preservation of the stability of motion segments. Figure 7 features a case of LSS in which was achieved complete decompression and a favorable outcome after treatment via this technique.

Images obtained in a patient with severe LSS at L4–5 treated using minimally invasive laminectomy through tubular retractors. Left: Preoperative axial T2-weighted MR image of the lumbar spine at the L4–5 level showing severe stenosis. Right: Postoperative MR image at the same level showing marked and effective decompression of the neural elements.

Minimally Invasive Decompression of LSS

Minimally invasive laminectomy through tubular retractors has been shown in previous studies to be a feasible and effective technique for treating LSS.^{8,30,39,41–45} In the present study we evaluated the outcome of this procedure in a larger group of patients with a longer duration of follow-up. The results were compared with those of other decompressive procedures published in the literature (Table 8). Our study produced results corroborating the findings in previous studies on the same technique. Parikh et al. and Pao et al. reported higher ODI and pain score improvements than we did in the present study;^44,45 this may be explained to some extent by the shorter follow-up duration in those studies (9 and 15 months, respectively) compared with ours (28 months).

There is wide variability in the clinical outcome of different decompressive procedures in the literature, which makes it difficult to draw definite conclusions (Table 8). Nonetheless, comparisons between the minimally invasive laminectomy through tubular retractors and open laminectomy suggest that the former can have clinical outcomes comparable to those for the open procedures, while also having the benefits of minimally invasive surgery.^{36,46,54,60–64,66} Moreover, previous studies reporting reoperation rates for minimally invasive laminectomy suggest that there is an overall lower reoperation rate associated with this technique compared with that for open laminectomy.^{30,36,39,45,60–63}

Lumbar Spinal Stenosis and Spondylolisthesis

Little research has been done to compare the outcome of different decompressive procedures in patients with and without preoperative spondylolisthesis. Clinical results of the current investigation showed no significant difference in outcome for patients with and without preoperative spondylolisthesis. However, it must be remembered that our cohort included only patients with “stable” spondylolisthesis, without movement on flexion/extension films, and with neurogenic claudication in the absence of mechanical back pain. The radiographic results corroborated the clinical outcomes. They showed no significant progression of the slippage after surgery in patients with preoperative spondylolisthesis. Similarly, in patients without preoperative spondylolisthesis, no slippage developed after surgery. However, we reemphasize that although we have reported on the radiographic presence and extent of slippage in patients with available follow-up imaging, no direct conclusions have been made based on the radiographic results per se given the limited availability of these studies. The radiographic findings have been reported to provide a more comprehensive picture of the study group and have been discussed and interpreted with caution. On the contrary, in the current study we have mainly focused on clinical outcome and have used it to provide possible evidence and draw careful conclusions. The clinical outcome included pain and functional outcome scores, satisfaction level, as well as the reoperation rate. Palmer et al., who studied the feasibility of minimally invasive laminectomy via tubular retractors in 2002, also reported no radiographic progression of the slippage at 3 months' follow-up in 8 patients with preoperative slippage.^42,43 Given the small sample size and the short follow-up, conclusions for that study were limited. Findings in the current study are consistent with those of Pao et al. and Müslüman et al., who reported no progression of spondylolisthesis following a microsurgical unilateral approach for tubular bilateral decompression after 2 years' follow-up.^39,44 The reoperation rates in our study compare favorably with those of Kim et al., who reported a 15% total reoperation rate and 7.1% reoperation rate with fusion following bilateral tubular decompressive surgery via a unilateral approach using tubular retractors.³⁰ Those authors concluded that decompression without fusion is more cost-effective than instrumented fusion for a selected group of patients whom they defined as those with leg-dominant pain and stable Grade I spondylolisthesis.

A small number of studies have looked at the outcome in patients with preoperative spondylolisthesis treated using other minimally invasive decompressive techniques such as the full-endoscopic interlaminar approach. These studies all reported very low reoperation rates and/or minimal progressive spondylolisthesis postoperatively.

Standard open laminectomy, with or without fusion, is currently the most common procedure to treat LSS. However, conventional open laminectomy in the presence of preexisting spondylolisthesis can be associated with a high rate of subsequent instability requiring delayed fusion surgery. Radiographic results in Iguchi et al.'s study demonstrated the development of new spondylolisthesis in 6% of the patients without preoperative slippage at 10 years' follow-up, and the average progression of slippage was 2.3 mm.²⁶ In Epstein's study, progression from Grade I to Grade II spondylolisthesis at the 2-year follow-up was seen in 31.4% of cases.¹³ In a well-designed prospective study on open laminectomy in 58 patents with preoperative slippage, Blumenthal et al. found that the reoperation rate with fusion was 37.5% at 3 years' follow-up.⁴ Current surgical guidelines state that the “best medical evidence available in the literature confirms the utility of fusion for improving patient outcomes following decompression for stenosis associated with spondylolisthesis.”⁵¹

Despite all the variations in outcome among the different decompressive procedures, minimally invasive laminectomy through tubular retractors seems to result in clinical outcome and reoperation rates comparable to those for other decompressive procedures.^{8–11,25,32,36,40,46,53,54,58,60–66} However, the radiographic results suggest that this technique is associated with a lower instability rate than the full-open procedures, probably because of the preservation of posterior elements of the lumbar spine. Results from a cost-utility study showed that tubular retractor–guided decompression without fusion in this setting is more cost-effective than conventional decompression and fusion (Table 9).³⁰

We divided the patients into early and late groups, and no significant differences in operative time, estimated blood loss, and clinical outcome were observed between the two. In contrast, the CSF leakage rate and reoperation rates were higher in the early group. These differences are probably attributable to the learning curve associated with this technique.

Study Limitations

Finally, several limitations of the current study should be noted. It is a retrospective case series based on a prospectively collected database. The most important limitation lies in the fact that it was not designed to compare the outcome of different decompressive procedures. Thus, we attempted to compare the results of this study with those in the published literature. The wide variety of outcome measures used in these studies complicated the interpretation. In addition, because a number of between-group analyses may have been underpowered given the available sample size, the lack of observed statistical differences between the groups should be interpreted with caution. The current study provides preliminary estimates of differences between groups that can be used to power larger studies. The p values for between-group comparisons in the range of 0.06–0.20 should be considered as potential evidence for underpowered evaluations, and readers may wish to consider these differences as suggestive of nonsignificant trends. Another limitation of the study lies in the fact that comparison between preoperative and follow-up imaging was possible in only a small group of patients. For a small number of patients, neither pre- nor postoperative imaging was available in the hospital database; therefore, the presence or absence of spondylolisthesis was not determined in this group. Consequently, the small sample size for the radiographic comparison necessitates interpretation with caution.