Eur Spine J DOI 10.1007/s00586-015-3841-1

ORIGINAL ARTICLE

Substantial clinical benefit of minimally invasive lateral interbody fusion for degenerative spondylolisthesis Kaveh Khajavi • Alessandria Shen Anthony Hutchison

•

Received: 28 October 2014 / Revised: 13 February 2015 / Accepted: 26 February 2015 Ó Springer-Verlag Berlin Heidelberg 2015

Abstract Purpose Conventional lumbar arthrodesis for the treatment of degenerative spondylolisthesis (DS) is associated with high complication rates and variable clinical efficacy. Modern minimally invasive (MIS) approaches may reduce the morbidity and produce greater clinical improvement compared to traditional surgical techniques. The objective of this study is to report radiographic outcomes and evaluate clinical improvements in the context of substantial clinical benefit for DS patients treated with a MIS 90° lateral, transpsoas approach for lumbar interbody fusion. Methods From 2005 to 2011, 60 consecutive patients were treated with MIS lateral interbody fusion for Grade I or II DS at a single institution. Mean patient age was 68 years, 75 % were female, and 30 % had undergone previous lumbar surgery. A total of 71 levels were treated, supplemental posterior fixation was used in 57 (95 %) cases, and 26 (43 %) patients underwent additional direct posterior decompression. Results Average follow-up was 20.3 months. Average ORT, EBL, and LOS were 206 min, 83 cc, and 1.29 days, respectively. Complications occurred in 3 (5 %) patients. Transient approach-related thigh/groin pain was observed in 5 (8 %) cases. There were no cases of pseudoarthrosis. At 1 year, LBP improved 71 %, LP improved 68 %, ODI decreased 52 %, and SF-36 PCS and MCS improved 43 and 21 %, respectively. Substantial clinical benefit was met K. Khajavi (&)
A. Shen
A. Hutchison Georgia Spine and Neurosurgery Center, 2001 Peachtree Rd Suite 550, Atlanta, GA, USA e-mail: [email protected] K. Khajavi
A. Shen
A. Hutchison The Institute for Neurosurgical and Spinal Research (INSPIRE) Foundation, Atlanta, GA, USA

by 94.7 % of patients on NRS LBP, by 84.6 % on NRS LP, by 83.7 % on ODI, and by 66.7 % on SF-36 PCS. Disc height increased 71 % and segmental lordosis increased 27.8 % at treated levels. Foraminal height, width, and volume increased 19.7, 18.0, and 39.6 %, respectively. Slip improved 60.7 % with interbody fusion only and further improved to 69.2 % after the placement of supplemental instrumentation. Conclusions MIS lateral interbody fusion in the treatment of DS resulted in significant improvements in clinical and radiographic outcomes, with a low complication rate and a high proportion of patients achieving substantial clinical benefit. Keywords Minimally invasive surgery
XLIF
Spondylolisthesis
Substantial clinical benefit

Introduction Degenerative spondylolisthesis (DS) is a common cause of chronic low back pain, especially in the aging population. Although conservative treatment scan is beneficial, studies have shown that surgically treated patients experience substantially greater long-term improvements in pain and other clinical indicators [1, 2]. However, there is currently no general consensus on the ideal surgical treatment paradigm to guide decision-making pertaining to the procedure, surgical approach, use of direct decompression, and fixation type that leads to optimal radiographic and clinical improvements in DS patients. Furthermore, the relative lack of literature that describes substantial clinical benefit (SCB) [3], the number of patients in a series that meet a high clinical improvement threshold, limits the ability to interpret and view in context historical results with respect to current practices.

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A modern, non-endoscopic, minimally disruptive (MIS), 90° lateral, transpsoas approach for thoracolumbar interbody fusion, first introduced in the literature by Pimenta [4] in 2006, has gained in popularity over the past decade. In contrast to conventional, open approaches for interbody fusion, the MIS lateral approach leaves posterior structures, as well as the anterior and posterior longitudinal ligaments (A/PLL) intact, which are theoretical advantages in treating spinal instability. Additionally, interbody cage implantation in MIS lateral interbody fusion resulting in ligamentotaxis [4, 5] has the potential to realign the segment and indirectly decompress neural elements prior to the application of supplemental fixation and/or direct decompression [6–8]. The use of this MIS lateral interbody fusion technique in the treatment of DS has also been previously reported with similar rates of arthrodesis, immediate postoperative radiographic correction, and improvement in clinical outcomes compared to conventional posterior or open approaches, though with lower peri- and postoperative complication rates [9–14]. However, there is a relative paucity of literature examining both specific radiographic parameters to improvement (e.g., individual contribution of the intervertebral cage and supplemental fixation on realignment) as well as clinical outcomes following MIS lateral interbody fusion for DS. Thus, the purpose of this study was to evaluate radiographic and clinical outcomes for MIS lateral interbody fusion in the treatment of degenerative spondylolisthesis and to present substantial clinical benefit measures compared to historical controls.

Materials and methods Outcome data were collected prospectively at a single institution as part of an institutional review board (IRB) approved prospective registry. Demographic and treatment data were collected through retrospective chart review. Inclusion criteria for the study including being consecutively treated patients with either Grade I or II degenerative spondylolisthesis [15] who underwent either a one- or two-level MIS lateral approach for anterior lumbar interbody fusion (XLIFÒ, NuVasive, Inc. San Diego, CA) between 2005 and 2011. A total of 60 patients met this criteria and were included for analysis. All patients presented with back pain, and all but one patient presented with leg pain. Wide-aperture polyetheretherketone (PEEK) intervertebral cages (CoRoentÒ XL, NuVasive, Inc.) were used, supplemented with rhBMP-2 (InfuseÒ, Medtronic Sofamor Danek, Memphis, TN) osteoinductive graft material. Neurophysiologic monitoring (NV M5Ò, NuVasive, Inc.) was used in all cases. Additional direct posterior

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decompression was performed at the surgeon’s discretion, primarily for significant radicular pain secondary to severe central or lateral recess stenosis or when it was felt indirect decompression would not adequately address the pathology. When an additional decompression was performed, it was typically performed through a small midline incision for a limited central decompression often aided by the use of an operating microscope. Additionally, the use of supplemental internal fixation became standard practice starting with the fourth patient in the series due to concerns of construct stability in this mobile pathology. Clinical outcomes included demographic, treatment, complication, reoperation, as well as disability [Oswestry Disability Index (ODI)], low back and leg pain [numeric rating scale (NRS LBP and LP)], and quality of life (SF-36 physical and mental component scores [PCS and MCS]) measures. Outcomes were collected preoperatively and at standard follow-up intervals: 1, 3, 6, 12, and 24 months postoperative, as applicable. Substantial clinical benefit thresholds for ODI were defined as a net improvement of 18.8 points, a 36.8 % improvement, or a final raw score of 31.3 points. SCB thresholds for SF-36 PCS were set as a net improvement of 6.2 points, a 19.6 % improvement, or a final raw score of C35.1 points. SCB thresholds for NRS LBP and LP were defined as a net improvement of 2.5 points or a final raw score of \3.5 points. SCB thresholds for percent improvement were set as 41.4 % for LBP and 38.8 % for LP [3]. Radiographic measurements were taken at all treated levels and compared for differences at preoperative, intraoperative, and at last follow-up time points. Measurements included disc height (average of anterior and posterior disc heights), foraminal height, foraminal width, segmental lordosis (degree between inferior vertebral endplate to superior endplate at level of listhesis), slip distance, slip percent (reference to inferior endplate) and slip grade. Radiographs were evaluated using Surgimap Spine (version 1.1.2.263) radiographic mapping software, with each radiograph individually calibrated to provide absolute measurements. Statistical analyses included frequency analysis to examine demographic and treatment variables and repeated measures ANOVA to examine changes in clinical and radiographic outcomes from pre- to all postoperative time points. All analyses were performed using SAS v9.3 (SAS Institute, Cary, NC) and statistical significance was accepted at the 0.05 level.

Results Mean follow-up of the 60 patients was 20.3 months. Average patient age at time of surgery was 67.7 years

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(range 52–86 years) and 75 % were female. Forty percent (40 %) of patients were smokers, 23 % had diabetes mellitus, and 30 % had previous lumbar surgery (laminectomy, fusion, or both). Slip was categorized as Grade I in 47 (78 %) patients and Grade II in 13 (22 %) patients. Fiftynine (59) of the 60 patients had both preoperative LBP and LP NRS scores C3. A total of 71 levels were treated, with 49 (82 %) 1-level and 11 (18 %) 2-level cases. Fifty (83 %) patients were treated at L4–5, 18 (30 %) patients were treated at L3–4, and two (3.3 %) were treated at L2–3. Percutaneous transpedicular fixation was utilized in 57 (95 %) patients and 26 (43 %) patients underwent an additional direct posterior decompression. Overall mean operative time (ORT), estimated blood loss (EBL) (including posterior procedures), and postoperative length of stay (LOS) were 206 min (range 65–426 min), 83 cc (range 10–1000 cc), and 1.3 days (range 1–4 days), respectively. Complete demographic and treatment information is included in Table 1. There were no intraoperative complications. Postoperative complications occurred in three (5 %) patients, and included one myocardial infarction in a patient with a preexisting heart condition, one instance of urinary retention, and one delayed mild dorsiflexion weakness, which completely resolved by 3 months postoperative without further intervention. Additionally, mild transient hip flexion weakness, likely due to muscle trauma following retractor passage through the psoas muscle, was observed in two (3.3 %) cases, and mild anterior thigh discomfort occurred in three (5 %) cases, all of which resolved without long-

term sequelae. There were no cases of hardware failure or pseudoarthroses requiring revision. Average preoperative slip was 8.1 mm (20.3 % of endplate), and improved 60.7 % to 3.2 mm (7.9 % of endplate) following placement of the interbody spacer alone. In patients who received supplemental posterior fixation, average slip improved an additional 0.7 mm (8.6 %). From preoperative, total average slip at last follow-up was corrected 69.2 %, to 2.5 mm (6.5 % of endplate). At treated levels, disc height increased 71 % from 6.6 to 11.3 mm and segmental lordosis increased 27.8 % from 25.9° to 33.1°. Foraminal height increased from 19.4 to 23.2 mm (19.7 %) and foraminal width increased from 12.8 to 15.1 mm (18 %). All radiographic changes from preoperative to last follow-up were statistically significant (p \ 0.001). From preoperative to 12 months postoperative, ODI improved 51 % from 43 to 21 % and NRS LBP and LP improved 71 % from 8.0 to 2.3 and 65 % from 7.7 to 2.7, respectively (p \ 0.001). SF-36 PCS improved 40 % from 31.2 to 43.6 (p \ 0.001) and MCS improved 19 % from 43.8 to 52.0 (p = 0.003). Using ODI, 83.7 % of patients reached the threshold for SCB. NRS LBP SCB was reached in 94.7 % of patients. Using NRS LP, 84.6 % of patients reached SCB. Using SF-36 PCS, 66.7 % of patients reached SCB. There were no statistically significantly different clinical outcomes between the patients who received and those who did not receive a direct decompression (p [ 0.05). Complete clinical outcomes are included in Figs. 1, 2, 3, 4, and 5. At last follow-up, 95 % of patients reported being either ‘‘satisfied’’ or ‘‘very satisfied’’ with their outcome, and 90 % indicated they would choose the same procedure again, given their outcome.

Table 1 Demographic and treatment data Number of patients

n = 60

Age (years), mean ± SD

64.5 ± 8.3

Female, n (%) BMI (m/kg2), mean ± SD

45 (75.0) 29.1 ± 4.8

Diabetes mellitus, n (%)

14 (23.3)

Tobacco use, n (%)

24 (40.0)

Discussion The primary technical goals for the surgical treatment of DS are generally agreed upon to be realignment and

Levels treated, n (%) L2–3

2 (3.3)

L3–4

18 (30.0)

L4–5

50 (83.3)

Additional posterior instrumentation, n (%)

57 (95.0)

Additional posterior direct decompression, n (%)

26 (43.3)

Estimated blood loss (cc), mean ± SD

83 ± 124

Operative time(min), mean ± SD

206 ± 64

Length of stay (days), mean ± SD

1.3 ± 0.7

n number of patients, SD standard deviation, BMI body mass index, m meters, kg kilograms, cc cubic centimeter, min minutes

Fig. 1 Oswestry postoperative

scores

from

preoperative

to

12 months

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Fig. 2 Numeric rating scale for low back pain from preoperative to 12 months postoperative

Fig. 3 Numeric rating scale for leg pain from preoperative to 12 months postoperative

Fig. 4 SF-36 physical component score from preoperative to 12 months postoperative

Fig. 5 SF-36 mental component score from preoperative to 12 months postoperative

stabilization of the lumbar spine, neural decompression, and restoration of intervertebral disc height, neuroforaminal volume, and segmental lordosis [16–18]. As

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these goals are particularly well met in treating DS with instrumented fusion, outcomes are generally superior to those of other degenerative conditions in this model [4, 8, 19]. This was further shown by Glassman et al. [19], who evaluated patient outcomes stratified by preoperative diagnosis and found that nearly all clinical outcome measures (pain, disability, quality of life) were most improved in patients treated for spondylolisthesis with fusion compared to those who were similarly treated for other degenerative diagnoses. The benefits of fusion compared to non-fusion procedures or non-operative care, however, may be reduced when performed using conventional surgical exposures, which are accompanied by high rates of approach-related morbidity. In the previously mentioned study by Glassman et al. [19], for example, despite incrementally improved outcomes in spondylolisthesis patients following open posterolateral fusion (PLF), these patients exhibited both the highest overall complication rate (45 %) and the highest rate of major complication (15 %), higher in both cases even then those patients treated for deformity. Of note, major complications have been shown to adversely affect long-term outcome and may account for the relatively low number of patients who met minimum clinically important difference (MCID; net improvement of 12.8 points for ODI, 4.9 points for SF-36 PCS, 1.2 points for LBP, and 1.6 points for LP) in Glassman’s study (between 34.8 and 88.2 % of patients met MCID at 1 year, depending on cohort and postoperative time point) [20, 21]. These and similar results have historically served as the benchmarks for outcome following lumbar fusion surgery. Benefits of MIS lateral interbody fusion include the ability for direct access to the anterior column for a thorough discectomy and placement of a wide intervertebral implant to maximize spinal realignment, stability, and indirect neural decompression [7, 8, 22]. In using PLF to treat DS, the posterior column only is accessed for segmental realignment and stability, a model which has been shown to be less effective than those addressing the anterior column (interbody fusion) in both the ability to correct deformities and maintain the long-term stiffness needed for fusion [22, 23]. While posterior and transforaminal lumbar interbody fusion (P/TLIF) address the anterior column, they require dissection of posterior bony, ligamentous, and muscular structures which are essential for the endogenous stability of the spine, particularly in an unstable condition such as DS [24–28]. Additionally, MIS lateral interbody fusion maintains the anterior and posterior longitudinal ligaments, which results in slip correction through ligamentotaxis even prior to the application of supplemental internal fixation, as the current results show [29]. Due to the location (90° lateral) and exposure (blunt dissection) of the surgical corridor in MIS lateral interbody

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fusion, the risks of both conventional anterior surgery [visceral, vascular, and reproductive (retrograde ejaculation) complications] and posterior approaches (infections, nerve root injury) are minimized. In direct anterior lumbar interbody fusion (ALIF), vascular injury rates have been reported in between 2.2 and 6.7 % of cases [30–33], visceral complication rates in around 5 % [34, 35], and retrograde ejaculation has been shown in as many as 10 % of men, though is regularly lower [32]. Overall complication rates following 360° fusion (similar to the treatment of patients in the current study) are generally reported between 14 and 77 % [36, 37]. In PLIF and TLIF, overall complications are typically reported between 19 and 46 % [38, 39]. A study of 119 patients who underwent singlelevel TLIF at Thomas Jefferson University Hospital for a variety of degenerative lumbar conditions found an overall complication rate of 46 %, neural injury rate of 11 %, infection rate of 5 %, and reoperation rate of 10 % [39]. Similarly, in a study of 251 PLIF patients (82 % of which were treated for DS), an overall complication rate of 24.7 % was observed, with 10.3 % of patients experiencing intraoperative complications (primarily dural tears), as well as an 8.3 % neural injury rate (90 % of which were motor deficits, and 62 % were evaluated as either severe or permanent injuries) [22]. By comparison, several larger series studies of MIS lateral interbody fusion have found overall complication rates of between 6.7 and 17.2 %, with neural injury rates between 0.7 and 2.9 %, and reoperation rates between 0 and 2.6 % [5, 40, 41]. The primary risk during the MIS lateral approach is injury to the nerves of the lumbar plexus, which is attenuated through the use of appropriate, directional neuromonitoring, strict adherence to proper surgical technique, and increased surgeon experience [42]. The previously mentioned reported rates of complications in MIS lateral interbody fusion are primarily from studies with mixed degenerative diagnoses, whereas in DS, complication rates tend to be elevated further than for non-deformity degenerative indications. In the current study, an overall complication rate of 5 % was observed, with 8.3 % of patients experiencing procedural side effects. These results are both notably lower compared to conventional approaches and are consistent with other reports of MIS lateral interbody fusion complications. In terms of clinical outcomes, Weinstein et al. [1] found a mean pain (VAS) improvement of 2.2 in patients treated primarily with posterior fusions for DS through the spine patient outcomes research trial (SPORT). By comparison, the mean improvement in pain for patients who underwent MIS lateral interbody fusion in the current study was over double (5.7 mean net pain improvement) that of patients enrolled in the DS arm of SPORT. In another comparison, Lauber et al. [13] treated 19 patients with TLIF and found an ODI improvement of 29 % at 2 years postoperative,

while VAS improved 36 % from 8.1 to 5.2. Overall slip reduction at 2 years postoperative for the 39 isthmic and degenerative patients was 35 % from 23 to 15 % of the inferior endplate. By comparison, in the current study, slip was improved from 20.3 to 6.5 % (68 % improvement) of the inferior endplate at 1 year postoperative. A more recent study by Park et al. [14] reported clinical outcomes in a series of 43 patients treated with MIS TLIF for DS or segmental instability. Average patient age was 57.7 years, 67 % were female, 21 % were smokers, and mean follow-up was 35.8 months. At last follow-up, ODI improved 54 % from 58 to 27 %, VAS LBP improved 57 % from 5.9 to 2.5, and VAS LP improved 78 % from 8.2 to 1.8. Fourteen (28 %) complications were observed. Clinical outcomes of these patients were similar to the current results, though with a somewhat higher complication rate. Several reports of MIS lateral interbody fusion in the treatment of DS specifically largely corroborate the current findings. First, Rodgers et al. [9] evaluated 1-year outcomes of 63 patients treated with MIS lateral interbody fusion and pedicle screw fixation, without direct decompression, for Grade II spondylolisthesis. In the study, mean hospital stay was 1.2 days and four (7.8 %) adverse events were observed while pain improved 75 % (from 8.7 to 2.2), disc height improved from 4.6 mm to 9.0 mm, slip improved 68 %, while 89 % of patients reported being either ‘‘very’’ or ‘‘somewhat’’ satisfied with their outcome and 93 % would have chosen to undergo their surgery again had their outcome been known in advance. Another study by Marchi et al. [11] evaluated 52 patients who underwent MIS lateral interbody fusion without supplemental fixation in the treatment of either Grade I or II DS. In this study, back and leg pain improved 60 and 57 %, respectively, while disability (ODI) improved 60 %. Mean slip reduction was 53 % and disc height and segmental lordosis increased 55 and 38 %, respectively. However, five patients required a direct posterior decompression following inadequate resolution of symptoms following MIS lateral interbody fusion alone. These reports suggest the reproducibility of results when utilizing MIS lateral interbody fusion for the treatment of DS. In less invasive alternatives for conventional exposures, there may be anecdotal perception of minimally invasive as equivalent to minimally effective [43]. This may be attributed to the paucity of mid- and long-term clinical outcome reports, due to the relatively new adoption of these procedures and a greater emphasis on complication profiles in the literature. The definition of an MIS procedure, in fact, has been proposed as, ‘‘one that by virtue of the extent and means of surgical exposure results in less collateral tissue damage, resulting in measurable decreases in morbidity and more rapid recovery than traditional exposures,

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without differentiation in the intended surgical goals’’ [44]. In the current study, ODI improved 51 %, VAS LBP improved 71 %, VAS LP improved 65 %, PCS improved 40 %, and MCS improved 19 %. Substantial clinical benefit, a higher threshold than MCID for determining the number of patients who met validated improvement thresholds, was met in between 66.7 and 94.7 % of patients, depending upon the outcome evaluated. In comparison with the DS patients treated with PLF reported by Glassman et al. [19], only between 56 and 73.8 % of patients met MCID, a much lower threshold in net improvement. Again, the DS patients treated with PLF were one of the surgical indications that elicited the greatest clinical improvement following fusion, and this improvement was substantially lower than the higher thresholds met by patients treated with MIS lateral interbody fusion in the current study. Of note, on methodology and future research, substantial clinical benefit, more so than MCID, is a crucial additional measure to the commonly used clinical outcome evaluations [pain (VAS), disability (ODI), quality of life (SF36), and satisfaction] that should be considered. Mean improvements in continuous outcomes, even when provided with variability, do not provide patient-level information on how many met clinically validated improvement thresholds. Additionally, from a communication standpoint, patients tend to have difficulties in translating the significance of clinical improvements in the commonly used HRQOL instruments. For example, a 50 % improvement in ODI postoperatively does not equate to being 50 % better, which is ultimately what patients want to know. Using SCB, one can more directly predict how many patients will achieve an outcome that they assess as a substantial improvement, an easier concept to understand. Thus, this measure provides both great clinical value and essential research-related information about a given patient series. From a historical perspective, the relative difficulty in meeting this threshold in an adequate number of patients may account for its infrequent use in the literature. To our knowledge, this is the first paper to report data regarding substantial clinical benefit in a large cohort of degenerative spondylolisthesis patients undergoing lumbar arthrodesis. Not all patients at the treating facility presenting with degenerative spondylolisthesis were treated with fusion. Direct decompression without fusion for spondylolisthesis was reserved for those patients with low-grade, largely stable listhesis, and in more elderly and spondylitic cases with predominance of leg, rather than back pain. In these cases, a decompression with preservation of at least 50 % of the facet joints was performed. Reflective of a different disease state, the 60 patients in the current series treated with interbody fusion tended to be younger, with more mobile disease and larger disc height, with the presence of

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substantial leg and back pain (59 of 60 cases had both leg and back pain greater than or equal to 3/10) secondary to Grade I or II degenerative spondylolisthesis. In the current series, a total of 43 % of patients underwent a limited direct decompression in addition to MIS lateral interbody fusion, though the frequency of the use of direct decompressions has dramatically dropped off from early to later cases, with approximately only 20 % of patients requiring direct decompression in similar patients in current practice. This is likely due to the experience and realization of the magnitude of improvement of radicular symptoms possible with indirect decompression in the MIS lateral fusion procedure alone. An additional direct decompression in the current series was decided on a case-bycase basis resulting from the extent of correction and indirect decompression achieved by the interbody procedure alone, as well as the distribution and severity of preoperative radicular symptoms. In addition, the decision to include interbody fusion along with a direct decompression and posterior pedicle screw and rod fusion posteriorly for spondylolisthesis has precedent in the literature of being a more favorable construct than pedicle screw and rod fixation alone. In a study by Suk et al. [45], the authors compared outcomes of decompression and posterolateral fusion to those of decompression, posterolateral fusion, and posterior lumbar interbody fusion for degenerative spondylolisthesis. In this study, the authors found that patients treated with interbody fusion had a lower rate of nonunion (7.5 vs. 0 %), greater reduction of listhesis (28.3 vs. 41.6 %), lower rate of deformity recurrence and loss of reduction, with excellent results far more common in the interbody fusion group (75 vs. 45 %). Limitations of the current study include the lack of a systematically reported fusion rate and will be an important factor in assessing long term. At this time point, the absence of symptomatic pseudoarthroses suggests that healing is progressing or complete in these patients. Finally, the lack of a control group or comparison arm does limit our ability to appropriate postoperative improvement to either the interbody procedure or the additional decompression of symptom-generating factors; however, the purpose of this study is to present evidence supporting the safety and efficacy of a minimally invasive fusion alternative for the treatment of degenerative spondylolisthesis in a group of properly selected patients and further investigation is warranted.

Conclusion In this study, MIS lateral interbody fusion for the treatment of degenerative spondylolisthesis resulted in significant improvements in clinical and radiographic outcomes that

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maintained over long-term follow-up with a low complication rate and hastened postoperative recovery. All of these measures appear to compare favorably to conventional and at least equivalently to alternative MIS surgical approaches. This builds on previously published work in assessing outcomes following lateral interbody fusion [46– 50], though this work represents a less heterogeneous sample, with the addition of more robust parameters (clinical, radiographic, satisfaction, and patient threshold measures). Notably, in patients treated with MIS lateral interbody fusion, an overall complication rate of 5 % was observed and between 66.7 and 94.7 % of patients met substantial clinical benefit, while in a similar group of patients who underwent open PLF for DS, a 45 % complication and 5 % revision rate was observed with 56 and 73.8 % of patients meeting only minimum clinically important difference [19]. This illustrates the promise of modern less-invasive approaches to both reduce approach-related morbidity and improve clinical outcomes. In addition, evaluating outcomes in the context of substantial clinical benefit offers a more comprehensive understanding of efficacy at the individual patient level, whilst driving the benchmark for clinical improvement past a minimal magnitude and towards one that the patient recognizes as major. Thus, substantial clinical benefit should not only be considered in further research endeavors, but also as a long-standing goal of treatment. Conflict of interest Dr. Khajavi is a consultant for NuVasive, Inc. and has received research grants from NuVasive, Inc., though none were received related to this work. Alessandria Shen, subsequent to this work, became employed directly by NuVasive for a short time and no longer has any conflicts. No other authors report any conflicts.

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