Surgical decision making surrounding the use of modern minimally invasive surgery (MIS) in the treatment of degenerative lumbar spinal pathology is challenged by the relative lack of comparative effectiveness studies directly evaluating different approaches with comprehensive clinical and radiographic outcomes over long-term time points.1 Instead, studies more often compare a single MIS approach to its open counterpart as in the comparison of MIS and open transforaminal lumbar interbody fusion (TLIF),2–7 open and MIS posterior lumbar interbody fusion (PLIF),8,9 or the comparison of minimally invasive lateral interbody fusion (XLIF) to alternative anterior lumbar interbody fusion (ALIF) approaches.10,11 This leaves a gap in the understanding of optimal surgical approach in a patient for whom multiple MIS approaches may be viable. Two such MIS approaches that are often considered in clinical settings between similar patients and pathology are MIS TLIF and XLIF approaches, with MIS TLIF employing direct neural decompression and XLIF relying on indirect decompression as a mechanism for stenosis resolution. A direct comparison of these approaches is heretofore unreported. The purpose of this study was to evaluate clinical and radiographic outcomes following the use of either MIS TLIF or XLIF in the treatment of degenerative lumbar spondylolisthesis with spinal stenosis through two years postoperative. Clinical outcomes are presented in the current report, while radiographic outcomes are presented in a separate paper.
MATERIALS AND METHODS
A prospective, multicenter, institutional review board (IRB) approved study was undertaken to compare clinical and radiographic outcomes between two direct-visualization minimally invasive approaches for interbody fusion: Minimally invasive lateral interbody fusion [extreme lateral interbody fusion (XLIF®); NuVasive®, Inc., San Diego, CA] and MIS TLIF [MAS® TLIF (maximum access surgery TLIF), NuVasive, Inc.]. Patients were enrolled according to a relatively narrow set of inclusion criteria, some of which included being at least 18 years of age, diagnosed with degenerative spondylolisthesis with resultant stenosis at one or two contiguous lumbar levels between L1 and L5, with symptoms consistent with the pathology (typically radiculopathy and/or neurogenic claudication with or without back pain). Primary exclusion criteria included patients with any prior lumbar fusion surgery, isthmic/lytic spondylolistheses, and/or Grade IV facet degeneration,12 which is not a contraindication to indirect decompression in XLIF, but was selected out as a possible confounding radiographic variable. Complete inclusion and exclusion criteria are included in Table 1.
Each patient was introduced to the study if the treating surgeon maintained clinical equipoise about the patient with respect to the two treatment arms and the patient was confirmed to meet all inclusion and none of the exclusion criteria. A randomized study arm was first introduced to the patient, where surgical approach (XLIF or MIS TLIF) was determined for each case by a blinded allocation card by site. If the patient rejected participation in the randomized study, an observational enrollment option (without randomization between procedures) was presented where surgical procedure was determined by patient, not surgeon, preference.
Baseline data collection included patient demographic, diagnosis, past medical history, work status, symptom, and comorbidity information. Treatment data included procedure(s) performed, operative time (ORT), estimated blood loss (EBL), and length of postoperative hospital stay (LOS). Narcotic and medication usage, work status, and recreation levels were collected by patient-completed questionnaires at preoperative and all postoperative time points (6 weeks, 12 weeks, 6 months, 12 months, and 24 months). Patient-reported outcomes were collected at baseline and at each postoperative visit and included measures of disability [Oswestry disability index (ODI)), back and leg pain (visual analog scale (VAS)], quality of life [SF-36 physical and mental component scores (PCS and MCS, respectively)], patient satisfaction with their outcome (very or somewhat satisfied grouped as “satisfied” and somewhat or very dissatisfied grouped as “dissatisfied”), and willingness to repeat the same surgery had their outcome been known in advance (very or somewhat willing grouped as “willing” and somewhat or very unwilling to repeat surgery grouped as “unwilling”). Minimum clinically important difference (MCID)13,14 and substantial clinical benefit (SCB)15 using established thresholds for low back and leg pain, ODI, and SF-36 PCS were also computed.
Complications and reoperations were recorded at all relevant peri- and postoperative time points. Neurological status assessment of motor and sensory function was collected preoperatively and at each postoperative time point using a modified American Spinal Injury Association (ASIA) scale.16 Radiographic measures of restoration, alignment, indirect decompression, and fusion were collected on radiographs and magnetic resonance imaging (MRI), but are reported separately in a radiographic outcomes paper.17
Statistical analysis was performed using JMP v12.0 (SAS Institute, Inc., Cary, NC) and included descriptive statistics and comparative analyses using Fisher's exact and analysis of variance (ANOVA) testing to compare outcomes between the two groups (XLIF and MIS TLIF). Paired testing was used to compare preoperative to postoperative time points within groups. Statistical significance was defined at P < 0.05.
Patients were enrolled from the practices of 15 surgeons in the United States between October 2009 and November 2012, with the last follow-up completed in January of 2015. A total of 55 patients were enrolled in the study and treated according to the protocol: 29 (53%) with XLIF and 26 (47%) with MIS TLIF. Twenty-nine patients were enrolled in the randomized arm (15 XLIF; 14 MIS TLIF); 26 patients (14 XLIF 12 MIS TLIF) were enrolled in the observational arm. A randomization and treatment allocation chart is included in Figure 1. At 24 months, 42 patients (76%) were available for clinical and 39 (71%) were available for radiographic follow-up.
Demographic comparisons between patients in the two groups are included in Table 2. Mean age was similar (63 vs 64 yrs) for the XLIF and MIS TLIF groups, respectively, with similar distributions of sex and with the same average body mass index (BMI) between the groups. There were significantly more patients reporting diabetes mellitus in the XLIF than in MIS TLIF group (28% vs 4%, P = 0.027) and baseline low back pain (LBP) was significantly higher in the XLIF than in the MIS TLIF group (7.3/10 vs 5.7/10, P = 0.027). Between the randomized and nonrandomized cohorts, the former was noted to have higher BMI (31.4 vs 28.5, P = 0.047), higher percentage of smokers (31% vs 4%, P = 0.014), and higher (worse) baseline ODI scores (49% vs 36%, P = 0.001). Because other baseline parameters were similar, and because of relatively low numbers precluding separate meaningful analysis, data from randomized and observational cohorts were combined for analysis.
Surgical Technique: XLIF
The XLIF procedure utilizes a mini-open, 90° off-midline, retroperitoneal, transpsoas approach for anterior interbody fusion. The technique has been previously described,18,19 and is performed with the patient in the lateral decubitus position and, following skin and fascial incision, blunt dissection is used to pass through the superficial abdominal musculature. The retroperitoneal space is realized and digital guidance is used to lead the first of three sequential dilators to the lateral border of the psoas muscle. Blunt dissection by the dilator through the psoas muscle is performed under the guidance of integrated triggered electromyography with the ability to stimulate in directional orientations and provide real-time discrete-threshold responses (NVM5®; NuVasive, Inc.). This allows for localization, with both direction and distance information, of motor or mixed nerves in the vicinity of the approach corridor in relation to the approach instrumentation. Once the dilator has passed to the lateral border of the disc, sequential dilators are passed using the same technique followed by delivery of a split-blade, table-mounted retractor. Once the disc space has been accessed and the retractor placed, standard surgical techniques for annulotomy, discectomy, endplate preparation, implant trialing, and grafting are performed. Supplemental internal fixation is applied at the surgeon's discretion though, for this study, bilateral percutaneous pedicle screws were placed in all patients without facet fusion. Direct decompressions were not performed in XLIF patients.
Surgical Technique: MIS TLIF
MAS TLIF is a pedicle-based procedure for minimally invasive TLIF. The patient is placed in a prone position and fluoroscopy is then used to identify and mark the starting location of each pedicle. A percutaneous exposure is used to place a modular screw shank (i.e., without a tulip) into the ipsilateral pedicles immediately superior and inferior to the index disc. A small (approximately 1.5 inches) incision is made on the ipsilateral side of the approach to expose the two pedicle screw shanks and to allow for delivery of two retractor blades, one each oriented cranially and caudally, which are affixed to the pedicle screw shanks for immobilization as well as for the ability to distract using the retractor (Figure 2). Once secured, the cranial and caudal retractor blades are parallel to the disc space and realize a medial exposure trajectory to the disc space through the foramen. The center (medial) blade is then attached to the retractor, which is intended to elevate tissue while following the topography of the facet, lamina, and base of the spinous process, as it is retracted medially (Figure 3). Once the desired exposure is achieved, a facetectomy and decompression can be performed followed by distraction, disc space and endplate preparation, and implant insertion using standard TLIF techniques. Once the implant is placed, the screw tulips are attached, a rod is delivered, and compression and/or lordosis correction be applied. Contralateral percutaneous pedicle screw placement is used to complete the bilateral construct. As per the protocol of the current study, a contralateral decompression could only be performed in MAS TLIF if done through the ipsilateral incision (Figure 2). Six patients were treated with a tubular MIS TLIF retractor deviant from the protocol described above.
Treatment characteristics and outcomes are included in Table 3. A total of 36 XLIF levels were treated in the 29 patients enrolled, including 22 single- and seven two-level cases. A total of 29 MIS TLIF levels were treated in 26 patients and included 23 single-level and three two-level procedures. The most common level treated in both groups was L4–5 (in 83% of XLIF and 100% of MIS TLIF patients). A left-sided approach was more common in the XLIF patients; a right-sided approach was more often used in the MIS TLIF cohort. Total procedural time was similar between the XLIF and MIS TLIF groups, at 171 and 186 minutes, respectively. Hospital stay was also similar between the groups at an average of two days for each. Blood loss was significantly lower in the XLIF than in the MIS TLIF group, with 79% versus 27% of cases, respectively, resulting in <100 mL of blood loss, P < 0.001 (Figure 4).
Adverse events included postoperative hip flexion weakness in 31% of XLIF patients, all resolving within six months postoperative. One femoral neuropathy with 4/5 distal weakness was recorded in the XLIF group, with full resolution by the six-month time point. No new postoperative hip flexion or distal weaknesses were observed in the MIS TLIF cohort. Three patients in the XLIF and two patients in the MIS TLIF group experienced new postoperative lower extremity sensory changes. All sensory changes resolved by 12 months postoperative in both groups. New postoperative lower extremity changes are included in Table 4. Other adverse events included two instances of ileus in the XLIF group and three instances of dural tear, one intraoperative pedicle fracture, and one pseudoarthrosis (revised at 15 months postoperative) in the MIS TLIF group. No other revisions or readmissions were observed.
Within procedure groups, all clinical measures (LBP, leg pain, ODI, PCS, MCS) from preoperative to 24-month postoperative time points were all statistically significantly improved (P < 0.05). Baseline LBP was, on average, significantly greater in XLIF than in MIS TLIF patients (Table 2); though, on average, both groups improved significantly from baseline to 24 months postoperative, with 73% improvement (7.3/10 to 1.9/10) in the XLIF compared with 64% (5.7/10 to 2.1/10) in the MIS TLIF group, respectively (P = 0.045). Average worst leg pain improved from baseline to 24 months postoperative between both groups: 79% (7.0/10 to 1.5/10) in the XLIF group and 74% (6.8/10 to 1.8/10) in the MIS TLIF group, P = 0.889.
Disability (ODI) improved, on average, from 43% at baseline to 20% at 24 months (a 53% change) in the XLIF group, while the MIS TLIF patients improved from 44% to 19% (57%), P = 0.455. From baseline to 24 months postoperative, quality of life measured by PCS and MCS of the SF-36 improved in the XLIF group from 37.7 to 61.4 (63%) and from 51 to 67.2 (32%), respectively (P = 0.449). For MIS TLIF patients at the same time points, PCS improved 64% from 39.5 to 64.9 and MCS improved 33% from 52.2 to 69.2. Clinical results at each time point between the groups are shown in Figure 5. At 24 months postoperative, patient satisfaction was similar between the groups, with 91% of XLIF and 80% of MIS TLIF patients satisfied with their outcome (P = 0.393) and 100% and 90% of patients, respectively, willing to undergo the same procedure had their outcome been known in advance (P = 0.210) (Figure 6A and B).
From baseline to 24 months postoperative, medication usage, work status, and recreational level tended to improve in both groups similarly. Notable improvements included decreases in the use of strong narcotics (e.g., Percodan, Percocet, Morphine, and Demerol) (26% to 5% of MIS TLIF and 28% to 13% of XLIF patients) and muscle relaxants (38% to 25% in the MIS TLIF and 20% to 0% in the XLIF groups) and a trend toward return to higher forms of recreation (noncontact sports) in both groups (from 19% to 30% of MIS TLIF and 11% to 38% of XLIF patients). Complete data on these parameters are included in Tables 5 to 7.
MCID and SCB were similar at 12- and 24-month time points between the groups, with between 79% and 96% of XLIF and 65% and 84% of MIS TLIF patients meeting MCID across different parameters at last follow-up (Table 8). Similarly, SCB was met in between 75% and 97% of XLIF and 79% and 88% of MIS TLIF patients (Table 9).
Comparative effectiveness research on minimally invasive techniques has historically focused on the comparison between MIS and open approaches, with general agreement on the benefits of modern (direct visualization, nonendoscopic) MIS approaches in many patients.5,8–10 Where the tools available to surgeons today include a variety of different MIS approaches that are able to be used to treat similar pathology, specific and direct findings are needed to guide best practices. To the authors’ knowledge, this work represents the first multicenter longitudinal study comparing outcomes of two different minimally invasive approaches for degenerative lumbar pathology. One paper exists describing differences in results between TLIF/PLIF and LLIF, though this work relied on individual cohorts from the published literature in a nonsystematic fashion and, as such, retained a low level of evidence and conclusions were not able to be made from the limited data included.1 The general findings of the current study show that XLIF with bilateral percutaneous fixation without direct decompression and MIS TLIF with direct decompression and bilateral percutaneous fixation are largely equivalent in the treatment of degenerative spondylolisthesis with stenosis. These results are, as a whole, equivalent or superior to previous findings of mid- to long-term clinical outcomes for each of these procedures.5,7,20
Both XLIF and MIS TLIF groups experienced high mean improvements in pain, disability, and quality of life compared with prior reported results,21,22 with 73% and 64% improvements in back pain, 79% and 74% improvements in leg pain, 53% and 57% improvements in ODI, and 63% and 64% improvements in quality of life (PCS), respectively. Similarly, high rates of patients met MCID and SCB thresholds across outcome parameters and were consistent with other reports of XLIF in the treatment of degenerative spondylolisthesis.23,24 By way of comparison with previously published results, consider the work from Glassman et al.25 who reported in 2009 on outcomes following posterolateral fusion stratified by preoperative diagnosis. In 80 patients treated for spondylolisthesis, the complication rate was 45% and mean improvement in LBP, leg pain, ODI, and PCS were 3.3, 3.2, 20.3, and 7.7 points, respectively. For the same outcome variables, improvements in the current XLIF and MIS TLIF groups were 5.4 and 3.6, 5.5 and 5, 23 and 25, and 23.7 and 25.4 points. Glassman et al.25 also reported 60%, 63.8%, 71%, and 56.3% of patients meeting MCID across back pain, leg pain, ODI, and PCS, respectively. In the current study, the same measures for XLIF and MIS TLIF patients resulted in 96% and 77%, 82% and 84%, 79% and 68%, and 86% and 65% of patients meeting MCID, respectively. These results are substantially equivalent to those reported by Khajavi et al.23,24 and further strengthen the argument that MIS procedures are equivalent or superior to conventional approaches in terms of clinical outcomes with less procedural morbidity.
Weaknesses of the study include some slight differences at baseline between the groups (e.g., higher rates of diabetes and elevated preoperative LBP in the XLIF group and higher levels of disability, smoking, and BMI in observational compared with randomized patients), which were not able to be controlled for and which could indicate selection bias (for the observational cohort), though the vast majority of characteristics between the groups were similar. Also, the inclusion of an observational arm lowers the quality of the evidence somewhat, but all patients were enrolled according to narrow inclusion criteria with equipoise by the surgeon, where the patient made the decision about procedure performed in order to decrease any effects of selection bias by the surgeon. Also, there is a precedent for observational arms in postmarket research to increase enrollment and better reflect “real-life” treatment.21,22 However, other studies have generally benefited from larger observational and randomized cohorts. The relatively small sample sizes were also a limitation of the study being able to make more generalized or nuanced conclusions on potential differences between the procedures. The initial protocol estimated a requirement for 55 patients in each group in order to be powered to detect a difference from 36.1 [standard deviation (stdev) 10.8) to 31.1 (stdev 6.6)] on ODI. These enrollment goals were not met and, thus, the study was not powered for each statistical interaction. In addition, at last follow-up time points, a below 80% follow-up compliance was observed. XLIF is also not able to be regularly performed at the L5-S1 level, where L5-S1 TLIF is a common procedure. This likely contributed to low enrollment rates and potentially highlights the MIS TLIF group in levels less commonly treated than over the course of regular practice.
Despite different mechanisms of action, overall two-year clinical outcomes between XLIF and MIS TLIF were similar. Perioperatively, MIS TLIF patients had higher blood loss than XLIF patients, and a higher surgical complication profile; XLIF patients had higher rates of postoperative lower limb discomforts, although all were transient. Over the two-year follow-up period, reoperations were low in both groups, with only one revision for pseudoarthrosis in the MIS TLIF group and no other readmissions or revisions, and patient satisfaction was high. These results suggest that both XLIF and MIS TLIF are reasonable MIS approaches for the treatment of lumbar degenerative spondylolisthesis with stenosis.
- There is a paucity of published literature examining outcome differences between varying minimally invasive approaches, as current reports have typically described differences between minimally invasive and open procedures.
- This report is the first to compare outcome differences between two minimally invasive approaches (MIS TLIF and XLIF) in the treatment of common degenerative lumbar pathology.
- Clinical outcomes between the 29 patients treated with XLIF and 26 patients treated with MIS TLIF were significantly improved from baseline and similar between groups at 24 months postoperative.
The authors thank NuVasive, Inc. and its Clinical Resources team and for their material and financial support of this study.
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