The objective of minimally invasive spine surgery is to reduce damage to surrounding tissues while accomplishing the same goals as conventional surgery. Patient demand and marketing for these techniques is driven by the perception of better outcomes, in particular, a quicker recovery from surgery. Patients are expected to mobilize more quickly due to reduced postoperative pain associated with the minimal access approaches.
The purported advantages of minimally invasive procedures over traditional approaches to the spine are unproven. One of the major difficulties has been defining universally accepted outcome measures. Some benefits of minimal access surgery might be seen very early in the postoperative course, before many spine outcomes measurements are typically recorded.1 Another problem with studying new procedures is the potential for a learning curve accounting for complications.2 Most studies that have been done lack a control group. Well-controlled studies take time, effort and are expensive to perform. Patients may refuse randomization because of the perception that the “new” and “less invasive” technique is always better. Alternatively, patients may shy away from newer procedures or the prospect of randomization.
The effectiveness of minimally invasive spine surgery is one issue. However, the relatively limited visualization achieved through the minimal access working channel raises more immediate questions regarding patient safety. For example, do patients that undergo minimally invasive discectomy have a greater risk of recurrent herniation due to limited surgical exposure with consequent reduced disc removal? In minimally invasive spinal fusion, what is the rate of nerve root injury, cerebrospinal fluid (CSF) leak, misplaced instrumentation, and pseudoarthosis compared to the conventional procedure?
As we set out to try to answer these questions in a systematic analysis, an even more basic question emerged. Exactly what procedure, or group of procedures, should we review? A broad range of techniques have been described as “minimally invasive,” “minimal access,” “minimal exposure,” “minimal incision,” “percutaneous,” and “less invasive” spine surgery. These include refinements to established techniques (e.g., percutaneous pedicle screws) and novel procedures (e.g., vertebral augmentation). Minimally invasive spine surgery has evolved across a spectrum of approaches that vary in anatomic tissue handling such as muscle dilating and muscle splitting techniques as well as by retractor characteristics and visualization technologies (microscope, endoscopy).
For the purpose of this article, all posterior thoracolumbar surgery conducted through a tube, cylindrical retractor blades or sleeves via a muscle dilating or muscle splitting approach were operationally defined and bundled as “minimal access tubular-assisted spine surgery” (MAS). Traditional muscle elevating or striping techniques, even if by limited incision, were considered to be “open” surgery.
The purpose of this review is to attempt to answer the following 2 clinical questions:
- Does MAS decrease the rate of complications in posterior thoracolumbar decompression and/or fusion surgery compared with traditional open techniques?
- What strategies to reduce the risk of complications in MAS have been shown to be effective?
Materials and Methods
Electronic Literature Database
The literature search is outlined in detail elsewhere.2a We conducted a systematic search in MEDLINE, EMBASE, and the Cochrane Collaboration Library for literature published from 1990 through July 2009 comparing MAS with open or conventional microsurgery in patients undergoing posterior thoracolumbar decompression or fusion. We limited our results to humans and to articles published in the English language. Reference lists of key articles were also systematically checked. We excluded studies evaluating internal disc decompression technologies, and those involving tumors, revision surgeries, trauma, or vertebroplasty/kyphoplasty. Noncomparative studies such as case series were also excluded. Outcomes of interest included operating time, blood loss, the length of stay in the hospital (HLOS), CSF leak, infection, reoperation, dural tears, nerve injury, and medical complications, Figure 1.
Each retrieved citation was reviewed by two independently working reviewers (J.R.D, D.C.N). Most articles were excluded on the basis of information provided by the title or abstract. Citations that appeared to be appropriate or those that could not be excluded unequivocally from the title and abstract were identified, and the corresponding full text reports were reviewed by the two reviewers. Any disagreement between them was resolved by consensus. From the included articles, the following data were extracted: study design, patient demographics, surgical procedures rendered, spine segment treated, and patient safety outcomes.
Level of evidence ratings were assigned to each article independently by two reviewers using criteria set by The Journal of Bone and Joint Surgery, American Volume (J Bone Joint Surg Am)3 for therapeutic studies and modified to delineate criteria associated with methodologic quality and described elsewhere (See Supplemental Digital Content 1, individual study ratings, tables, individual study ratings, available at: http://links.lww.com/BRS/A414).
Patient safety outcomes were reported as the proportion of patients experiencing a poor outcome or the mean number of days spent in the hospital, on ICU, and on a ventilator. Data were summarized in tables and pooled complication rates were calculated, weighted by sample size. Qualitative analysis was performed considering the following three domains: quality of studies (level of evidence), quantity of studies (the number of published studies similar in patient population, condition treated, and outcome assessed), and consistency of results across studies (whether the results of the different studies lead to a similar conclusion).4 We judged whether the body of literature represented a minimum standard for each of the three domains using the following criteria: for study quality, at least 80% of the studies reported needed to be rated as a level of evidence I or II; for study quantity, at least 3 published studies were needed, which were adequately powered to answer the study question; for study consistency, at least 70% of the studies had to have consistent results. The overall strength of the body of literature was expressed in terms of the impact that further research may have on the results. An overall strength of “HIGH” means that further research is very unlikely to change our confidence in the estimate of effect. The overall strength of “MODERATE” is interpreted as further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate. A grade of “LOW” means that further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate, whereas “VERY LOW” means that any estimate of effect is very uncertain.2a
We identified 361 articles from our literature search evaluating complications associated with MAS decompression and fusion procedures. From these potential articles, we judged 19 to undergo full text review. After full text review, we excluded 6 of the articles for the following reason: 3 articles did not evaluate a minimal access approach; one reported on clinical outcomes but not patient safety, one used an internal disc decompression technique, and one did not segment data by treatment group for patient safety outcomes. More information on excluded articles can be found in Table 1, available at: http://links.lww.com/BRS/A414. The remaining 13 articles provide a comparison of patient safety between MAS and traditional or standard techniques, Figure 2. Three studies,5–7 all randomized controlled trials (RCTs) were graded a level of evidence I or II, whereas the remaining 10 studies were retrospective cohort studies (7 with concurrent controls,8–14 3 with historical controls15–17) graded level of evidence III, (Supplement Digital Content, Table 2, available at: http://links.lww.com/BRS/A414). No comparative studies of the thoracic spine were identified; therefore, all studies included were studies of the lumbar spine, Table 1. Further details for each study can be found in the Supplemental Digital Content, available at: http://links.lww.com/BRS/A414.
Operation Time and Hospital Length of Stay
Operating time varied widely across studies and depended in part on number of levels of intervention and treatment (discectomy or fusion), Table 1. In 5 studies evaluating single level discectomy, the mean operating time ranged from 47 to 166 minutes for the MAS group and 36 to 170 minutes for the open discectomy group.5–7,9,15 In 2 studies with a mixture of single and multilevel laminectomy, mean operating time was 109 and 110 minutes for the MAS group, and 88 and 157 for the open group.11,16 Wu et al14 evaluated 873 patients undergoing MAS for mostly single level discectomy and noted that the first 220 cases took on average 75 minutes to complete whereas the last 653 cases took 49 minutes. The mean operating time for patients undergoing posterior lumbar interbody fusion (PLIF) in one study was 192 minutes versus 149 minutes comparing MAS with open surgery.18 In another study reporting on operating time for transforaminal lumbar interbody fusion (TLIF), MAS took an average of 348 minutes compared with 312 minutes for open surgery.13 In this study, the first 6 cases lasted 1.8 hours longer than the last 6 cases. Scheufler et al17 noted that fusion time for TLIF took longer with increasing levels of fusion; for MAS 104, 175, and 205 minutes for single level, bilevel, and multilevel fusion, respectively, and for open TLIF 132, 192, and 245 minutes, respectively.
Mean hospital length of stay across studies for discectomy or laminectomy ranged from 0 to 5 days for MAS and from 0 to 7 days for open surgery.5,6,9,11,12,14–16 For fusion, mean length of stay was longer, ranging from 5 to 6 days for MAS and 7 to 11 days for open fusion.8,13,18
Estimated mean intraoperative blood loss varied among nonfusion patients with some studies reporting slightly higher blood loss during MAS versus open surgery5,15 and others reporting lower blood loss.6,9,14,16 In the large series by Wu et al, the first 220 patients receiving MAS lost a mean 72 mL of blood whereas the last 653 patients lost a mean 35 mL. Only one patient undergoing MAS out of 929 in 3 studies (0.1%) received a blood transfusion compared with 1.4% of open surgery, Table 2. Patients receiving fusion lost more blood than those without fusion with comparable interventions, Table 1. In all 3 studies reporting blood loss for fusion, MAS resulted in statistically less blood loss compared with open surgery.13,17,18
Three RCTs5–7 and 4 cohort studies11,14–16 reported reoperation rates in patients undergoing discectomy or laminectomy without fusion, Table 2. In the RCTs, 9.2% of patients receiving minimal access surgery and 7.7% receiving open surgery underwent reoperation. Arts et al7 reported repeat surgery in 10% of those receiving tubular discectomy compared with 7% with conventional microdiscectomy, mostly as a result of recurrent herniation. Righesso et al5 in one RCT included only patients that had greater than 2-year follow-up. Reoperation occurred in 4.8% of patients receiving MAS compared with 5.2% in those receiving open lumbar discectomy. Ryang et al6 reported their rates in patients with follow-up from 6 to 26 months. Reoperation occurred in 2 patients (6.7%) in the MAS group (1 with residual disc herniation and one with recurrent disc herniation) compared with 4 patients (13.3%) in the open group (3 with recurrent and one with residual disc herniations). Reoperation was reported less frequently in the 4 cohort studies with rates ranging from 0% to 2% in the MAS group and 0% to 12% in the open group (mean pooled rates, 0.4% and 1.2%), respectively, Table 2.
Among patients receiving lumbar fusion, 3 recent cohort studies8,13,18 reported reoperation rates from 11.1% to 14% in the MAS group compared with 6.9% to 26% in the open group (mean pooled rates, 12.8% and 10.6%, respectively, Table 3). Each study was prone to selection bias making a direct comparison between groups difficult. Patients receiving MAS in the study by Park et al18 were all private pay patients while those receiving open surgery were patients who paid via health insurance. Furthermore, the MAS group compared with the open group in this series had more female participants (75% vs. 55%) and more systemic disease (ASA Class 2, 69% vs. 48%). MAS was the favored treatment in Schizas et al13 for patients with Grade I spondylolisthesis whereas open fusion was favored for foraminal stenosis or degenerative disc disease. In the Bagan et al series, the MAS group consisted of patients that had fewer two-level fusions (14% vs. 32%), fewer revision surgeries (14% vs. 26%) and lower prevalence of diabetes (7% vs. 26%) and hypertension (25% vs. 42%) compared with the open group.
Dural tear was reported relatively infrequently in all studies. In patients receiving single level lumbar discectomy or laminectomy, the pooled rates of dural tear for the 3 RCTs5–7 were 9.2% for the MAS group and 7.7% for the open group. Across 4 cohort studies,11,14–16 dural tear is recorded in 2.4% (n = 21) in the MAS group and 0% (n = 0) in the open group, Table 2. Among patients across 3 cohort studies receiving PLIF or TLIF, dural tear occurred in 1.9% and 0% in the MAS and open groups, respectively, Table 3.13,17,18
Cerebrospinal Fluid Leak
One RCT reported 0.6% and 1.3% of patients having a CSF leak following discectomy in the MAS and open groups, respectively.7 CSF leak was reported in 5 retrospective studies; 3 studies of patients undergoing discectomy or laminectomy9,11,16 and 2 studies of patients receiving PLIF or TLIF.8,17 Recorded mean rates of CSF leak in the discectomy/laminectomy studies ranged from 3% to 16% following MAS compared with 3% to 5% after open surgery. The two studies reporting this complication with fusion documented no cases of CSF leak from MAS and from 0% to 16% in open surgery.
Only one study of discectomy reported nerve injury; the rate of occurrence was 1.8% and 1.9% for the MAS and open groups, respectively.7 Three studies8,13,17 on fusion reported nerve injury, with pooled estimated rates of 2.5% for MAS and 1.2% for open surgery, Table 3.
Infection was rare, being reported in only one study after discectomy or laminectomy.11 In that study, 2.6% of the MAS patients and 3.4% of the open surgery patients developed an infection. The pooled rate of infection in 3 studies after fusion8,17,18 was 1.8% for MAS and 0.9% for open surgery, Table 3.
Other Complications Reported
Other complications reported from discectomy or laminectomy include wound hematoma7 (n = 2), exploration started at the wrong level7 (n = 1), seroma5 (n = 1), and iliac vein injury requiring repair9 (n = 1) for MAS, and wound hematoma7 (n = 1), exploration started at the wrong level7 (n = 5), death11 (n = 1), and pseudo-meningocele11 (n = 1) for open surgery. For fusion, nonunion18 (n = 1), screw malposition18 (n = 1), cage migration18 (n = 1), and pseudoarthroses13 (n = 1) occurred in MAS. One nonunion was recorded in the open surgery.18
Strategies to Reduce the Risk of Complications in MAS
No studies were identified that evaluated strategies to reduce the risk of complications in MAS for thoracolumbar posterior discectomy or fusion. Two studies looked at the learning curve required for MAS and compared complication rates between the first and last groups of MAS cases. Wu et al14 found in mostly single level discectomy an overall complication rate of 6.8% in the first 220 cases versus 3.6% in the last 653 cases (P > 0.05). Dural tears occurred in 3.6% of the early and 0.9% of the late group, and infection occurred in 0.5% of both groups. Schizas et al13 compared the first 6 cases with the last 6 cases of MAS for TLIF and found no pattern of increased incidence of complications between these 2 groups.
The overall strength of evidence to assess whether MAS increases or decreases complications in posterior thoracolumbar discectomy or fusion is “Low,” that is, further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate, Table 4. The overall strength of evidence to assess the effectiveness of strategies to reduce the risk of complications in MAS is “Very Low” such that any estimate of effect is very uncertain.
Morbidity associated with surgical spine exposure has become a recognized component of an entity circumscribed as “failed back syndrome.” Postexposure paraspinal muscle denervation and atrophy has been recognized since the 1970s when it was first documented by EMG.19,20 In contemporary practice, paraspinal muscle atrophy is presumed to be present by qualitative means only. The pathophysiology of postoperative muscle atrophy may be caused by muscle denervation and pressure-induced muscle ischemia as documented by myosomal enzyme studies.21,22 Less invasive muscle splitting and percutaneous muscle dilating approaches theoretically cause less direct paraspinal muscle damage and limited studies have demonstrated less postoperative magnetic resonance imaging atrophy and muscle enzyme changes after fusion procedures done through muscle splitting approaches compared with traditional muscle stripping approaches.21,22
Therefore, the potential exists to improve surgical outcomes by developing and enhancing techniques and tools that limit exposure-related damage to paraspinal muscle and its innervation. However, minimally invasive spine surgery should not be performed merely because of a theoretical advantage: randomized controlled studies with adequate follow-up are needed for more objective comparison to conventional approaches.
This systematic review is limited by the quality of the studies comparing complications between MAS and open spinal decompression or fusion. There were only 3 RCTs—all comparing open discectomy to MAS.5–7 Two of these studies were small, with 30 or less patients in each treatment arm.5,6 Most studies were cohort studies which are subject to selection bias. For example, it is possible that more complex patients, perhaps patients more prone to perioperative complications, were chosen for open procedures. This was the case in the study by Bagan et al8 where the MAS group consisted of fewer two-level fusions, fewer revision surgeries and lower prevalence of diabetes and hypertension compared with the open group. Similarly, in the study by Park et al,18 MAS patients were all private-pay patients whereas those receiving open surgery were patients who paid via health insurance. In the article by Schizas et al,12 MAS was the favored approach for foraminal stenosis or degenerative disease while open surgery was favored for Grade 1 spondylolisthesis.
To determine the safety of MAS versus open surgery, large prospective trials that account for surgeon experience and patient comorbidity either through random allocation or statistical controlling are necessary. The study by Arts et al7 is the only study that met these criteria, but went further in that patients and observers were blinded to treatment. This study did not show a faster rate of recovery from sciatica after MAS discectomy. On the contrary, patients who underwent MAS fared slightly worse with regard to leg and back pain. Functional outcomes, assessed by Roland-Morris Disability Questionnaire also favored conventional microdiscectomy. Although differences between groups were statistically significant, they were substantially below the published minimal clinically important differences (3 to 5 points for the Roland-Morris Disability Questionnaire and 20 to 35 mm on the visual analog scale for pain).23–25 Thus, the authors could not conclude that one treatment was clinically superior to the other.
To facilitate blinding of patients and observers, Arts et al7 describe using a midline incision (2.5–3 cm) for both treatment groups. For MAS discectomy, this represents a modification of the technique, which typically involves at most a 2 cm incision placed approximately 1.5 cm from the midline on the symptomatic side.2 It is possible that this modification of MAS discectomy may result in more retraction rather than dilation of the paravertebral muscles. More importantly, a midline approach may compromise access and operative visualization for MAS discectomy.
Arts et al7 reported that the complication rate for conventional microdiscectomy versus MAS was statistically similar, but the study was not powered to detect a difference. The most common intraoperative complication in both groups was dural tear, and the rate was double for MAS (14/166 = 8%) compared to open (7/159 = 4%), P = 0.18. With the muscle-splitting approach and decreased potential (dead) space created during MAS, there may be less risk of symptoms such as spinal headaches or CSF fistulas.26 The rate of operative exploration for the disc herniation starting at the wrong level was apparently higher in the conventional microdiscectomy group (5/159 = 3.1%) compared with MAS (1/166 = 0.6%), p value not reported. One might have expected to observe higher rates of recurrent disc herniation after MAS, due to limited surgical exposure and consequent reduced disc removal. However, the amount of disc material removed and the rate of recurrent disc herniation within 1 year was similar in the 2 groups.
The lack of benefit for MAS compared to conventional microdiscectomy in this blinded RCT is perhaps not unexpected. After all, many surgeons consider conventional microdiscectomy a “minimally invasive” procedure, and in many centers it is performed on an outpatient basis. Proponents of MAS may argue that the benefit of a “less invasive” procedure is more likely to be found with more complex surgery, such as multilevel spinal fusion. However, in our systematic review of fusion studies, overall results with regards to operative time, complications and reoperation were similar. The only exception was that in all three studies that reported blood loss, MAS resulted in statistically less blood loss compared with open fusion surgery.13,17,18 The quality of these studies was limited. There were no RCTs, and as noted above, there are potential issues of bias in cohort studies. One conclusion that can be drawn from this review is the need for well-controlled prospective RCTs comparing MAS to conventional spinal fusion.
In this systematic review, we chose to include only studies that compared MAS to a defined control group. There are some dangers in excluding carefully collected prospective data, simply because a comparison group is lacking. For example, the Spine Patient Outcomes Research Trial (SPORT) study27 did not evaluate MAS, but its analysis of surgically treated patients provides interesting data regarding complications. The rate of dural tear in SPORT was 4% (in the SPORT observational cohort study it was only 2%28) and this is similar to the rate for the open microdiscectomy group in the study by Arts et al.7 SPORT reported no postoperative complications in 95% of patients and the reoperation rate by 1 year for all causes was only 4%.27 The rate of these complications is higher in the MAS discectomy group for all 3 RCTs (Table 2).
Many authors mention the significant learning curve associated with developing mastery of MAS decompression and fusion procedures, yet there exists very scant data that delineates methods to accelerate learning and minimize the potential for complications.2,29 Wu et al14 reported differences in the first 220 versus later 653 MAS discectomy procedures, during which the surgical time, estimated blood loss and complication rate were all significantly reduced. Schizas et al13 reported that the first 6 cases of MAS TLIF lasted 1.8 hours longer than the last 6 cases. McLoughlin and Fourney2 documented the MAS discectomy learning curve for a single group and emphasized the need for a learning period before conducting randomized series to avoid misleading results.
Innovation is key to the evolution of spine surgery. However, MAS should not be performed merely because it represents an “advance” in technology. To validate MAS, the safety and effectiveness must equal or surpass that of conventional approaches. This will only be proven with carefully designed studies coupled with long-term follow-up. There is a dearth of such work in the existing literature, and we need to do much better in the future. In the meantime, it is the critical responsibility of the surgeon to self-educate, rehearse, deliver competent care and assess outcomes. Most importantly, surgeons need to be aware of the limitations of the current data, and reflect this in discussions with the patient to improved informed choice.
We recommend patients be informed that the purported advantages of MAS over traditional approaches to the spine are unproven. The only large randomized trial has shown a lack of benefit for MAS versus conventional microdiscectomy, and actually identified a trend toward higher complications. The rate of complications was not significantly different, but the study was not sufficiently powered to detect a difference in complications. In general, cohort studies (which are more prone to bias) have shown better results for MAS with a lower rate of complications than randomized trials—such data should be interpreted with caution by patients and surgeons. Although it is not well studied for all MAS, surgeons should appreciate the learning curve for new procedures.
- The single large randomized trial reported less favorable outcomes for MAS discectomy compared with open microdiscectomy, but there was no significant difference in the rate of complications.
- The quality of studies comparing the results of conventional fusion versus MAS fusion is poor.
- The effectiveness of strategies to reduce the risk of complications in MAS has not been reported.
Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Web site (www.spinejournal.com).
The authors are indebted to Ms. Nancy Holmes, RN, for her administrative assistance, and to Ms. Erika Ecker, BS, for her assistance in searching the literature, abstracting data, and proofing.
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