Since the original introduction of the posterior lumbar interbody fusion (PLIF) procedure by Cloward in 1952,1,2 newer less invasive techniques have been developed to accomplish lumbar interbody fusion. As an alternative to PLIF, Harms and Rolinger introduced the transforaminal lumbar interbody fusion (TLIF) procedure in 1982 for the management of degenerative spinal disorders that necessitate interbody fusions.3 The impetus for the development of TLIF was to provide a more lateral approach to the disc space, thus reducing the amount of thecal sac and nerve root retraction.4 Additionally, in revision cases where scar tissue hinders identification of neural structures, avoidance of midline scar planes with the TLIF procedure is beneficial.5 Moreover, a circumferential fusion can be achieved from a unilateral approach with the TLIF procedure without having to expose the bilateral epidural space.5
Technological advances in access instrumentation, illumination, magnification, and need for minimizing approach-related morbidities through minimal-access exposures served as catalysts for the development of minimally invasive techniques in spinal surgery. Although clinical outcomes with both open PLIF and TLIF procedures have been favorable, the inherent muscle damage from subperiosteal dissections and retraction that have been demonstrated objectively through several studies is believed to adversely affect clinical outcomes.6–12 Foley et al introduced the minimally invasive surgery (MIS) TLIF procedure to reduce the approach-related muscle damage.13,14 Since its introduction, many investigators have reported significant advantages on open PLIF and TLIF. Such reported advantages include, but are not limited to, less intraoperative blood loss, less postoperative pain, decreased postoperative narcotic usage, early ambulation, and decreased length of stay in hospital.13–19
Materials and Methods
Although the data showing benefit of using lumbar arthrodesis techniques in the operative management for lumbar degenerative spondylolisthesis are clear,20 the specific benefits of various types of fusion techniques are less. Advocates for interbody fusion techniques point out the higher rate of fusion, graft under compression, facilitated deformity reduction, indirect decompression, and foraminal distraction, and a host of other benefits over posterior or posterolateral fusion alone.13–19 Indications for MIS TLIF commonly include Grade I or II spondylolisthesis, multirecurrent disc herniations, severe degenerative disc disease, postlaminectomy instability, pseudarthrosis revision, and trauma requiring interbody fusion.5,21 Higher grades of spondylolisthesis are feasibly treated, yet more challenging using either open or minimally invasive posterior interbody techniques. In short, the absolute indications for MIS PLIF do not differ substantively from its open counterpart.
In the same vein, similar contraindications exist for open and minimally invasive posterior interbody fusion techniques namely, bone quality unable to support an interbody construct or rare anatomic variations such as conjoined nerve roots within the neural foramen, for ipsilateral TLIF (open or minimally invasive) approaches.
Although subtle variations in technique do exist, essentially 2 versions of MIS TLIF approaches have been described—a fixed tube variant (e.g., METRx, Medtronic Sofamor Danek, Memphis, TN; Figure 1) and a mini-open expandable tube option (e.g., MaXcess, NuVasive, Inc., San Diego, CA; Figure 2). After the disc space has been accessed, however, removal of discogenic material and placement of interbody spacers for fusion are the same for both MIS techniques and open TLIF approaches.
The fixed tube option begins with placement of a guidewire down to the posterior elements, aligned to the disc space that is to be fused. The laterality of this incision is made either based on the pedicle screw angle desired (radiographically projected onto the skin surface) or a fixed distance from midline (roughly 3 finger breaths). As the skin restricts the ability to alter this approach angle, especially as one uses progressively smaller tubes, correct placement of this incision is critical to allow for disc preparation and graft placement. Sequential dilators follow the initial guidewire, which is removed after the first dilator is placed to prevent inadvertent advancement.
The mini-open variant is based on a Wiltse-type approach using an expandable tubular retractor to access the posterior elements, allowing decompression and placement of ipsilateral pedicle screws through a roughly 3-cm skin incision. Some authors perform a muscle-splitting approach down through the erector spinae muscles, whereas others use a similar dilator-based strategy as described previously.
Regardless of the method used, the retractor is then seated against the posterior elements. The remaining soft tissue visualized within the retractor is removed and the bone work is performed. If an expandable retractor is used, this can be opened to expose a greater surface area of the dorsal elements, but this should be minimized during the early stages of the procedure so as to limit the pressure exerted on the paraspinal musculature. Finally, though, the exposure can be expanded when the intradiscal graft is placed to allow for placement of pedicle screw instrumentation under direct vision using standard anatomic landmarks. The size of the fixed angled tubes limits direct screw placement and visualization, and so most authors perform a percutaneous technique after the interbody fusion has been completed.
If a posterolateral arthrodesis is to be performed on the contralateral side, a similar exposure on the opposite side may be performed; or a strictly percutaneous approach can be used on that side. Placement of contralateral screws can be achieved percutaneously or through placement of the expandable retractor. Figures 3 and 4 illustrate several key strategies for safe and accurate placement of percutaneous pedicle screws and prevention of interbody graft migration, respectively.
A literature search was performed using the National Center for Biotechnology Information databases using PubMed/MEDLINE search engines. Keywords included the following: minimally invasive, transforaminal, lumbar, interbody, fusion. Studies were included in the analysis on the basis of their inclusion of outcome results of MIS TLIF or if there were comparisons between open posterior approaches and MIS TLIF.
While reviewing the literature, we found 7 articles comparing MIS TLIF and open TLIF or PLIF and 8 articles that discussed outcomes of MIS TLIF without a comparative cohort. Mean duration of surgery for MIS TLIF versus open T/PLIF ranged from 156.2 to 348.2 minutes and 142.8 to 312.2 minutes, respectively. Blood loss ranged from 150 to 456 mL for MIS TLIF, and 366.8 to 1147 mL for open T/PLIF. Length of postoperative stay ranged in the MIS TLIF literature from 3 to 10.6 days and from 4.2 to 14.6 days for open T/PLIF. Incidence of complications ranged from 0% to 33.3% for MIS TLIF, and from 1.6% to 16.7% in the open T/PLIF literature. Outcome measures, as scored by visual analogue scale (VAS) and Oswestry Disability Index (ODI), improved significantly in all reports from pre- to post-surgery, with MIS TLIF improvements in VAS and ODI ranging from 62.0% to 87.7% and from 56.1% to 77.0%, respectively.
Despite the growing popularity of MIS TLIF procedures, there exist no randomized controlled studies comparing it with standard open interbody fusion techniques, such as PLIF or TLIF. Therefore, no class I data exist to suggest superiority of one technique over other. While MIS TLIF is still relatively young, several studies within the last 5 years have reported favorable outcomes comparable to open TLIF with added benefits associated with minimally invasive techniques such as decreased pain, shortened hospital stay, and less intraoperative blood loss. A summary of clinical studies comparing outcomes between MIS TLIF and open lumbar interbody fusions (PLIF and TLIF) and a review of retrospective studies on MIS TLIF are shown in Tables 1 and 2, respectively.
Schwender et al were among the first to report clinical outcomes with MIS TLIF in a retrospective series.16 The authors reported significant improvement in low back pain and radicular pain and 100% fusion rate in a series of 49 patients undergoing MIS TLIF with a mean follow-up of 22.6 months. A decrease in the mean preoperative VAS and ODI from 7.2 and 46 to 2.1 and 14 was noted in that study. This study offered early clues to the potential advantages of MIS TLIF over open TLIF.
Isaacs et al followed with a description of microendoscopic TLIF approach for single-level degenerative spondylolisthesis in 20 patients and reported less intraoperative blood loss, length of hospital stay, and postoperative narcotic usage compared with a similar cohort of 24 patients who underwent open PLIF procedures.15 Although patient functional outcomes with longer follow-up were not performed in this study, the proposed early postoperative benefits set precedence for many studies to evaluate longer term follow-up clinical functional outcomes.
Scheufler et al compared 53 patients who underwent percutaneous TLIF with a cohort of 67 patients who underwent a mini-open TLIF (oTLIF) for degenerative lumbar instability.30 At a 16-month follow up, the authors reported excellent and good clinical outcomes as assessed by VAS and standard functional outcome questionnaires in 46 (87%) patients but not different from their comparative cohort of patients who underwent oTLIF. Although they found no difference in intraoperative time, a statistically significant reduction in intraoperative blood loss and postoperative pain on day 2 were noted (P < 0.01).
Additional evidence of the added benefits of MIS TLIF when compared with open TLIF was demonstrated in a study by Schizas et al.26 In this prospective study of 36 patients (18 in each group) undergoing either MITLIF versus open TLIF for isthmic spondylolisthesis or degenerative disc disease, the authors reported a statistically significant reduction in blood loss and length of stay and equivalent outcomes in VAS and ODI in the MIS TLIF group compared with the open TLIF group. Interestingly, in contrast to other reported studies comparing open to MIS TLIF, the authors did not find any difference in operative time, postoperative pain, and narcotic use.
Similarly, Wang et al, in their prospective study of 85 patients (42 MIS, 43 open) undergoing a 1-level open or MIS TLIF for degenerative and isthmic spondylolisthesis, reported similar clinical outcomes as measured by VAS and ODI.24 Again the MIS group had a lesser intraoperative blood loss, lesser need for transfusion, reduced postoperative back pain, and shorter hospital course compared with the open group. As expected, the open group had a significantly lower radiation exposure.
Further comparative studies of MIS TLIF with open TLIF were performed by Peng et al.25 In their prospective study of 58 patients (29 in each group) with degenerative spondylolisthesis or mechanical back/radicular pain from degenerative discs, similar good long-term (2 years follow-up) clinical outcomes and fusion rates were observed in their patient groups. Intraoperative blood loss, total morphine use, and length of stay were all statistically significantly reduced with MIS TLIF but at the expense of increased fluoroscopic and operative time (P < 0.05).
Recently Shunwu et al reported superiority of MIS TLIF to open TLIF in a prospective cohort study of 62 patients (32 MIS, 30 open) who underwent 1-level TLIF for lumbar degenerative disc disease and spondylolisthesis by a single surgeon.23 At a minimum follow-up of 2 years, the authors reported statistically significant reduction of intraoperative blood loss (P = 0.001), time to ambulation (P < 0.001), and length of stay (P < 0.001). Moreover, they observed a statistically significant improvement in ODI (P = 0.021) and VAS (P = 0.008) in MIS group although there were significantly longer operative times with MIS.
In a retrospective study involving 40 patients treated for symptomatic spondylolisthesis, Park and Foley reported at a mean follow-up of 35 months improved ODI (55%–16%) and mean leg and back VAS scores (65 mm and 52 to 8 mm and 15 mm, respectively).28 The authors also described a unique percutaneous technique for reducing spondylolisthesis and achieved 100% reduction with a mean decrease in forward translation of 76%.
Villavicencio et al reported comparable outcomes in VAS, MacNab's criteria, and patient self reports in a retrospective series of 139 patients (76 MIS, 63 open) comparing MIS TLIF and open TLIF procedures.22 Statistically significant difference was noted in intraoperative blood loss (163 vs. 366.8 mL; P < 0.0001) and length of stay (3.0 vs. 4.2 days; P = 0.02). This study, on the other hand, reported a higher complication rate with MIS TLIF, which they attributed to the higher learning curve associated with MIS techniques.
The effect of MIS TLIF procedures in the elderly patient was studied by Lee et al in a retrospective review of patients undergoing a oTLIF procedure for degenerative/lytic spondylolisthesis and stenosis with instability.29 In this study with 27 patients with a mean age of 69.3 (all > 65 years) followed for a mean of 38.6 months, the authors observed statistically significant improvement in preoperative mean VAS (7.9–3.0) and ODI (51.4%–19.9%) and a mean subjective satisfaction rate of 72%, thus yielding a clinical success rate of 88.9%. However, the authors noted a 44.4% incidence of adjacent segment degeneration and a 77.8% fusion rate although no patient underwent a revision surgery.
The feasibility of MIS TLIF in revision lumbar fusion cases was assessed by Selznick et al in a retrospective study of 43 patients of which 17 underwent revision MIS TLIF or MIS PLIF.21 Compared with the 26 primary surgeries, the authors found a higher rate of incidental durotomy but did not find any significant difference in intraoperative blood loss or neurologic morbidity.
Clinical Outcomes With Modifications of MIS TLIF
Mummaneni et al described the oTLIF where an expandable retractor was used in a Wiltse-type approach.35 The advantage of the mini-open approach is simultaneous visualization of both pedicles that facilitates placement of pedicle screws.35 The available literature on outcome assessment in MIS TLIF versus oTLIF, though, is scarce. One such study by Scheufler et al mentioned earlier that compared MI-TLIF with oTLIF using a muscle-splitting (Wiltse) approach found comparable clinical results.30
Dhall et al retrospectively reviewed 42 patients who underwent a single-level mini-open or open TLIF (21 in each group) for degenerative disc disease or degenerative spondylolisthesis.27 At an average follow-up of 24 and 34 months for mini-open and open groups respectively, the authors reported a statistically significant reduction in operative blood loss and length of stay (P < 0.001) in favor of the mini-open groups. An equivalent improvement in the modified Prolo Scale was observed in both groups. More importantly, the authors reported a higher incidence of instrumentation-associated complications with the oTLIF. The complications in the group included 2 cases of transient L-5 sensory loss, 1 misplaced screw, 1 cage migration, and 1 pseudarthrosis. All 3 instrumentation-associated complications necessitated revisions.
The desire to further reduce the approach-related morbidity of lumbar arthrodesis has led to surgeons' interest in placing unique instrumentation constructs. Outcomes after unilateral pedicle screw fixation after a MIS TLIF have been studied by 2 investigators. Deutsch et al in a prospective study involving 20 patients who underwent 1-level fusion for degenerative disc disease.31 The authors reported statistically significant improvement of mean ODI (P < 0.005) and VAS (P < 0.005) scores. At 6-month follow-up, the authors found that 13 of 20 patients had some evidence of fusion on computed tomography, and no symptomatic pseudarthrosis was noted. Similarly, Beringer and Mobasser reported improved modified Prolo Scale scores in 8 patients who underwent a single-level MIS TLIF with unilateral percutaneous pedicle screws.32 Radiographic fusion was demonstrated in all 8 patients at 6 months. Similarly, Jang and Lee conducted a pilot study in 23 patients with degenerative spondylolisthesis and uni- or bilateral radiculopathy using a MIS TLIF using ipsilateral pedicle screw and contralateral facet screw fixation approach.34 At a mean follow-up of 19 months, the authors reported statistically significant improvements in mean numerical rating scale and ODI (P < 0.0001).
Another modification of MIS TLIF procedure, the cantilever TLIF (C-TLIF) was described by Anand et al in 2006 along with clinical, radiographic, and functional outcome assessments.33 The C-TLIF, performed with the operative microscope, avoids dural retraction by placing the structural allograft under the cortical apophyseal ring followed by placement of middle column autograft under compression. The authors performed a prospective cohort study on 100 patients with degenerative disc disease with or without spondylolisthesis who underwent C-TLIF. At a mean follow-up of 30 months, the authors observed a statistically significant reduction in pain scores (P < 0.05), ODI (P < 0.05), and treatment intensity scores (P < 0.05) and an increasing trend toward improvement on SF-36 scores. More importantly, there were no dural tears, neural injury, graft subsidence, malpositioned screws, or instrumentation failure and a fusion rate of 99%. A significant improvement in segmental sagittal lordosis from 2° to 9° and improvement in anterior and posterior disc height were achieved as well.
A goal of minimally invasive fusion techniques is to create a less tissue-destructive approach without compromising operative and clinical outcomes. Achievement of solid fusion is the ultimate goal of all spinal fusion techniques. The limited exposure inherent to MIS techniques that requires fusion has the potential to affect adequate bone grafting and graft site preparation to allow for arthrodesis to occur. Despite this concern, numerous studies on MIS TLIF have demonstrated excellent radiographic fusion.16,31,32,34,36 To delineate whether higher fusion rates are achieved with open TLIF procedures, Wu et al performed a quantitative meta-analysis on published studies that reported fusion rates.37 Comparing 8 MIS TLIF studies to 16 open TLIF studies, Wu et al found similar mean fusion rates of 90.9% and 94.8% with open TLIF and MIS TLIF, respectively. The use of recombinant bone morphogenic protein was higher in the MIS TLIF group (50% vs. 12%). Although superiority of one procedure over the other cannot be made, the study nevertheless suggests a noninferiority of one technique over the other in terms of achievement of fusion rates.
Many of the aforementioned studies are limited by small sample size and are frequently characterized by a single center experience. The ability to generalize these findings to larger groups of both patients and physicians remains the goal. Several large studies have been presented yet to the date of this publication remain unpublished although the findings appear noteworthy. One such study is a multicenter retrospective cohort study using prospectively collected data, performed at 7 academic centers across North America from members of the Degenerative Spine Study Group by Rampersaud et al (unpublished data). In this study, 80 patients with degenerative or isthmic Grade I-II spondylolisthesis underwent a 1-level MIS TLIF and 111 patients underwent either an open TLIF (42) or posterolateral fusion (69). Not surprisingly, the authors found a statistically significant difference in operative time, with the open cases requiring less time (43 minutes less than MIS) and less intraoperative blood loss in favor of MIS cases. Both groups showed a statistically significant improvement in clinical outcomes as measured by minimum clinically important difference and substantial clinical benefit at 1 and 2 years follow-up. It is noteworthy, however, that at 2 years after surgery there were significantly more patients that reached both minimum clinically important difference as well as substantial clinical benefit in the MIS group compared with the open group: 80.5% versus 62.1% and 68.3% versus 47.1%, respectively (P < 0.01). The results of this study support the noninferiority of MIS TLIF over traditional open techniques and in fact, suggest potential additional benefits and significantly better clinical long-term outcomes.
Limitations of Current Literature
The available outcomes literature on the results of patients undergoing MIS TLIF procedures are limited by heterogeneity of patient populations, small sample sizes, and variation in techniques.
The studies reported herein, albeit representative of small sample sizes and reflective of single-surgeon experiences, reveal a number of consistent trends as well as some interesting conflicting information. On the basis of available literature, there appears to be potential advantages with MIS TLIF over open TLIF or PLIF. Such benefits include an overall reduced intraoperative blood loss, less need for blood transfusions, and shorter length of stay in the hospital. There is a reported learning curve with MIS techniques in general that may prolong the operative time, although this appears to improve with experience. In terms of infections, instrumentation complications, fusion rates, and clinical outcomes, MIS TLIF appears to be comparable to open techniques. Randomized controlled studies would confirm this, but would be difficult to conduct in the current environment of increasing patient demand for less invasive procedures.
- MIS TLIF appears to provide significant advantages over open posterior interbody fusion techniques (TLIF or PLIF) in terms of intraoperative blood loss, need for blood transfusions, length of hospital stay, and postoperative pain.
- As with other minimally invasive techniques, MIS TLIF may result in slightly longer initial operative times, though this is variable with technique, training, and experience.
- There is a paucity of published data to support long-term superiority of MIS TLIF over open techniques, but the available data suggest decreased morbidity and at least comparable outcomes compared with its open counterparts.
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