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Fusion Surgery for Lumbar Spinal Stenosis To the Editor: Försth et al. (April 14 issue)1 found that in patients with lumbar stenosis, even in patients with degenerative spondylolisthesis, decompression surgery plus fusion surgery does not result in better clinical outcomes than those of decompression alone. Although the authors are to be commended for their work, it is essential to examine the clinical limitations of their results. The primary conclusion of the study is that postoperative functional outcomes are independent of fusion. Although this conclusion may be true, in practice, the patient-specific anatomy should be studied to determine whether a biomechanical predisposition for instability exists. For example, biomechanical studies have shown that in patients with facet-joint edema, joints are likely to be unstable, even if they appear stable on dynamic imaging.2,3 Surgeons should have a low threshold for performing fusion in patients who have such biomechanical predispositions for instability. The authors also conclude that fusion leads to increased operating time, blood loss, and length of hospital stay. Although these results hold true for the open surgical techniques used in the trial, a major shift toward minimally invasive procedures has already substantially improved these variables,4,5 which should call into question the relevance of the findings by Försth et al. Najib E. El Tecle, M.D. Saint Louis University Hospital St. Louis, MO

Nader S. Dahdaleh, M.D. Northwestern University Chicago, IL nader​.­dahdaleh@​­northwestern​.­edu No potential conflict of interest relevant to this letter was reported.

1. Försth P, Ólafsson G, Carlsson T, et al. A randomized, con-

trolled trial of fusion surgery for lumbar spinal stenosis. N Engl J Med 2016;​374:​1413-23. 2. Fujiwara A, Lim TH, An HS, et al. The effect of disc degeneration and facet joint osteoarthritis on the segmental flexibility of the lumbar spine. Spine 2000;​25:​3036-44. 3. Caterini R, Mancini F, Bisicchia S, Maglione P, Farsetti P. The correlation between exaggerated fluid in lumbar facet joints and degenerative spondylolisthesis: prospective study of 52 patients. J Orthop Traumatol 2011;​12:​87-91. 4. Park Y, Ha JW. Comparison of one-level posterior lumbar interbody fusion performed with a minimally invasive approach or a traditional open approach. Spine 2007;​32:​537-43. 5. Wang J, Zhou Y, Zhang ZF, Li CQ, Zheng WJ, Liu J. Comparison of one-level minimally invasive and open transforaminal lumbar interbody fusion in degenerative and isthmic spondylolisthesis grades 1 and 2. Eur Spine J 2010;​19:​1780-4. DOI: 10.1056/NEJMc1606502

To the Editor: Before the publication of the studies by Försth et al. and Ghogawala et al. (April 14 issue),1 spine surgeons had relied on the results of a 25-year-old study involving 50 patients as a rationale for performing fusion in patients with lumbar spondylolisthesis.2 Spondylolisthesis (“slippage”) should not be confused with “instability” (>10 degrees of angulation or >4 mm of relative motion at the level of listhesis as measured on dynamic flexion–extension radiographs).3 In the study by Ghogawala et al., patients with greater than 3 mm of motion as seen on flexion– this week’s letters 597

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extension radiographs were excluded, whereas in the study by Försth et al., patients with “stable” slips and patients with “unstable” slips were included. The conclusions in both studies do not clearly differentiate between “stable” and “unstable” slips. There are obvious differences between patients who have a collapsed disk with fixed slips and patients who have a large disk, bulky fluid-filled facets, and more than 4 mm of dynamic instability. Ghogawala et al. and Försth et al. should be commended for showing that despite slip progression, a stable degenerative spondylolisthesis can be adequately treated with decompression alone, albeit with a 34% rate of revision surgery, as reported by Ghogawala et al. The ideal treatment of unstable degenerative spondylolisthesis remains unknown. Any implication that a stable slip should be treated the same as an unstable one could be misleading to the medical community. Brian W. Su, M.D. Mt. Tam Orthopedics and Spine Center Larkspur, CA brianwsu@​­gmail​.­com

Alexander R. Vaccaro, M.D., Ph.D. Rothman Institute Philadelphia, PA Dr. Su reports receiving consulting fees from Depuy, Med­ tronic, and Stryker and fees for product development from LINK; and Dr. Vaccaro, holding stock or stock options in Advanced Spinal Intellectual Properties, Bonovo Orthopaedics, Computational Biodynamics, Crosscurrent, Cytonics, Electrocore, Flagship Surgical, Flowpharma, Gamma Spine, Globus Medical, In Vivo, Innovative Surgical Design, Paradigm Spine, Progressive Spinal Technologies, Replication Medica, Rothman Institute and Related Properties, R.S.I., Small Bone Technologies, Spine Medica, Spinicity, Spinology, Stout Medical, Syndicom, and Vertiflex, receiving research support from AOSpine and Cerapedics, receiving consulting fees from DePuy, Ellipse, Gerson Lehrman Group, Globus, Guidepoint Global, Medacorp, Medtronic, Ortho­ bullets, and Stryker, receiving royalties from helping to design a product owned by Aesculap, Biomet Spine, DePuy, Globus, Medtronics, and Stryker Spine, and having served as a board or committee member for Advanced Spinal Intellectual Properties, Association of Collaborative Spine Research, Computational Biodynamics, Innovative Surgical Design, Progressive Spinal Technologies, R.S.I., Rothman Institute and Related Properties, and Spinicity. No other potential conflict of interest relevant to this letter was reported. 1. Ghogawala Z, Dziura J, Butler WE, et al. Laminectomy plus fusion versus laminectomy alone for lumbar spondylolisthesis. N Engl J Med 2016;​374:​1424-34. 2. Herkowitz HN, Kurz LT. Degenerative lumbar spondylolisthesis with spinal stenosis: a prospective study comparing decompression with decompression and intertransverse process arthrodesis. J Bone Joint Surg Am 1991;​73:​802-8. 3. Weinstein JN, Lurie JD, Tosteson TD, et al. Surgical versus nonsurgical treatment for lumbar degenerative spondylolisthesis. N Engl J Med 2007;​356:​2257-70. DOI: 10.1056/NEJMc1606502

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To the Editor: In the Swedish Spinal Stenosis Study (SSSS), Försth et al. state that there is no strong evidence to support the use of fusion in the treatment of degenerative lumbar spondylolisthesis. However, there are now two separate randomized, controlled trials (by Bridwell et al.1 and Ghogawala et al.) and one well-matched prospective cohort study by Herkowitz and Kurz2 that strongly suggest that patients who undergo fusion have better clinical outcomes and lower rates of reoperation than those who undergo decompression alone. The contradictory conclusions of the SSSS may be rooted in regional, cultural, and socioeconomic biases against fusion. Differences in surgical technique and experience may explain why the rate of reoperation reported by Försth et al. among patients who had undergone fusion (22%) is nearly 60% higher than the rate reported by Ghogawala et al. (14%). Other differences in the design of the study by Försth et al., including the lack of standardization of operative procedures, may have contributed to their aberrant findings as well. Occasionally, a strong clinical trial can disprove previously accepted medical knowledge. However, the SSSS has several shortcomings and contradicts other well-constructed studies. We therefore believe that readers should be skeptical about the conclusions reported by Försth et al. unless the findings are validated in further studies. Viren S. Vasudeva, M.D. John H. Chi, M.D., M.P.H. Brigham and Women’s Hospital Boston, MA vvasudeva@​­partners​.­org No potential conflict of interest relevant to this letter was reported. 1. Bridwell KH, Sedgewick TA, O’Brien MF, Lenke LG, Baldus C. The role of fusion and instrumentation in the treatment of degenerative spondylolisthesis with spinal stenosis. J Spinal Disord 1993;​6:​461-72. 2. Herkowitz HN, Kurz LT. Degenerative lumbar spondylolisthesis with spinal stenosis: a prospective study comparing decompression with decompression and intertransverse process arthrodesis. J Bone Joint Surg Am 1991;​73:​802-8. DOI: 10.1056/NEJMc1606502

To the Editor: Ghogawala et al. reported that 34% of the patients in the decompression-alone group had revision surgery because of sufficient instability, which is much higher than the 14% rate of reoperation in the fusion group. In an accompanying editorial, Peul and Moojen (April 14

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issue)1 also mention the remarkably different rates of reoperation between the two randomized, controlled trials in the Journal (those by Försth et al. and Ghogawala et al.) and considered the difference to be due to different thresholds for revision surgery, which was performed at the discretion of the surgeons. Indeed, although there are no universally agreed-on criteria for acceptable follow-up rates in randomized, controlled trials, the high dropout rate in the study by Ghogawala et al (25.7% in the 4-year follow-up), which may lead to problems of validity,2 and the fact that decompression was performed by means of complete laminectomy, a more invasive procedure than that performed in several cases in the study by Försth et al. (bilateral laminotomy with preservation of the midline structures was performed in 22 cases), may also be the cause of the different rates of reoperation. However, there is also disagreement among physicians on how postoperative instability and indications for revision surgery should be defined.3 We also found high variability in the reported rates of reoperation after two procedures.4 Minimizing repeat spinal surgery is an important goal for both surgeons and patients; the results of reoperation may be worse than the results of the original surgery, and the health care costs may be high,5 especially considering that most patients with lumbar stenosis are elderly. Patients should also be informed of the substantial risk of having an additional operation. Therefore, we suggest that a more detailed investigation regarding key concepts such as a standardized indication for revision surgery is needed to better interpret the findings of this trial. Kejia Hu, M.D. Huashan Hospital Shanghai, China kjhu14@​­fudan​.­edu​.­cn

Dehong Feng, M.D. Wuxi People’s Hospital Wuxi, China No potential conflict of interest relevant to this letter was reported. 1. Peul WC, Moojen WA. Fusion for lumbar spinal stenosis — safeguard or superfluous surgical implant? N Engl J Med 2016;​ 374:​1478-9. 2. Fewtrell MS, Kennedy K, Singhal, A, et al. How much loss to follow-up is acceptable in long-term randomised trials and prospective studies? Arch Dis Child 2008;​93:​458-61. 3. Burgstaller JM, Porchet F, Steurer J, Wertli MM. Arguments for the choice of surgical treatments in patients with lumbar spinal stenosis — a systematic appraisal of randomized controlled trials. BMC Musculoskelet Disord 2015;​16:​96.

4. Machado GC, Ferreira PH, Harris IA, et al. Effectiveness of surgery for lumbar spinal stenosis: a systematic review and metaanalysis. PLoS ONE 2015;​10(3):​e0122800. 5. Rajaee SS, Kanim LE, Bae HW. National trends in revision spinal fusion in the USA: patient characteristics and complications. Bone Joint J 2014;​96-B:​807-16. DOI: 10.1056/NEJMc1606502

Dr. Försth and colleagues reply: We are grateful for the interest in our randomized, controlled trial and that by Ghogawala et al. It is of great importance that the spine society reach a consensus regarding what type of surgery to perform for spinal stenosis, because this diagnosis will continue to grow in importance and put a strain on health care systems worldwide. There are certainly patients with spinal stenosis with instability who will have better outcomes if fusion is performed as an adjunct to decompression. The actual problem is that we currently do not have firm scientific support to identify these patients. Theoretically, specific appearances of facet joints or disks on preoperative radiologic images may indicate such instability. These issues are currently being evaluated in the radiologic follow-up part of the SSSS trial. The current concept of instability as it relates to the selection of the type of surgery is problematic in three ways. First, the term “instability” is vague and does not have a validated definition. Second, the commonly used method to detect instability — flexion–extension radiography — has low accuracy, with measurement errors of up to 4 mm.1 This low accuracy limits the usefulness of flexion–extension radiography when instability is defined by many surgeons as at least 3 mm of motion as seen on flexion–extension radiographs. Moreover, flexion–extension radiography has low reproducibility of findings and is highly dependent on the patient’s cooperation and pain status.2 Third, there is no clear evidence that a finding of radiologic instability is associated with pain and impaired function in the individual patient, and in several studies the presence of degenerative spondylolisthesis has not been shown to be associated with more pain and discomfort than are reported by patients without a slip.3 In the SSSS, we did not exclude any patients because of instability. Thus, our treatment groups included patients who would have been excluded in other studies, including the study by Ghogawala et al. Still, there was no benefit with fusion. However, we agree that there is a strong need to evaluate valid signs of

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instability, identify predictors of the development of an unstable segment after decompression alone, and develop methods for accurate biomechanical motion analysis. Repeated surgery is unavoidable when there are degenerative changes in the spine that continue to be present after surgery. New stenosis may occur at either adjacent segments (more common after fusion) or decompressed segments (more common after decompression alone). Treatment tradition (i.e., local or individual ways to manage recurrent symptoms), surgical technique, and clinical experience may influence the rate of reoperation. The selection of patients in the SSSS was performed pragmatically at several different major Swedish spine clinics, and all participating surgeons had long clinical experience. Accordingly, we regard both the internal and external validity of SSSS as high. Peter Försth, M.D., Ph.D. Karl Michaëlsson, M.D. Bengt Sandén, M.D., Ph.D. Uppsala University Uppsala, Sweden peter​.­forsth@​­surgsci​.­uu​.­se Since publication of their article, the authors report no further potential conflict of interest. 1. Shaffer WO, Spratt KF, Weinstein J, Lehmann TR, Goel V. 1990 Volvo Award in clinical sciences — the consistency and accuracy of roentgenograms for measuring sagittal translation in the lumbar vertebral motion segment: an experimental model. Spine 1990;​15:​741-50. 2. Cabraja M, Mohamed E, Koeppen D, Kroppenstedt S. The analysis of segmental mobility with different lumbar radiographs in symptomatic patients with a spondylolisthesis. Eur Spine J 2012;​21:​256-61. 3. Hasegawa K, Kitahara K, Shimoda H, et al. Lumbar degenerative spondylolisthesis is not always unstable: clinicobiomechanical evidence. Spine 2014;​39:​2127-35. DOI: 10.1056/NEJMc1606502

Dr. Ghogawala and colleagues reply: A fundamental question when treating a patient with single-level symptomatic lumbar stenosis with grade I spondylolisthesis is whether the spondylolisthesis is stable or unstable. A related and very relevant issue is whether simple decompression (laminectomy or partial laminotomy) renders a stable spondylolisthesis unstable. A patient with an unstable spine typically has mechanical low back pain with or without radiculopathy. In our study — the Spinal Laminectomy versus Instrumented Pedicle Screw (SLIP) study — we 600

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found that over 4 years of follow-up, instability developed in a third of the patients who were treated with laminectomy alone. These patients all underwent fusion at the level of spondylolisthesis and showed favorable changes in their outcome scores on the Medical Outcomes Study 36Item Short-Form Health Survey and Oswestry Disability Index after reoperation (see Table S7 in the Supplementary Appendix, available with the full text of our article at NEJM.org). Su and Vaccaro correctly state that many cases of stable lumbar spondylolisthesis can be adequately treated without fusion. However, the current literature does not provide enough guidance regarding which cases are stable and which are unstable. We have shown previously that motion on flexion–extension imaging, facet angle, and disk height are predictors of instability.1 As noted by El Tecle and Dahdaleh, identifying which specific radiographic features represent spinal instability calls for better anatomical studies to provide more predictive information for patients with grade I spondylolisthesis. We need more reliable and evidence-based information regarding which patients have unstable spondylolisthesis after decompression alone. Many patients can be treated without fusion, but clinical practice guidelines suggest that some patients would benefit from having the option of fusion of the spine when a decompression procedure is performed.2 Vasudeva and Chi correctly point out some shortcomings in the SSSS. The major difference between the SLIP study and the SSSS was the patient population. The SSSS included a heterogeneous population of patients with 1 to 2 levels of disease, whereas the SLIP study focused entirely on single-level grade I spondylolisthesis. It is worth pointing out that neither trial was appropriately powered to detect a between-group difference in the disease-specific Oswestry Disability Index outcome tool among patients with single-level grade I spondylolisthesis. Surgical techniques have become less invasive since the time both trials were conducted. In current practice, the operating time and length of stay are much shorter and the estimated blood loss is much less than what were reported in the SLIP study. Nevertheless, the SLIP study identified a specific population of patients who benefited significantly from lumbar spinal fusion. The exact technique for performing fusion will

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continue to evolve; the concept that must stand the test of time is that a significant number of patients with grade I degenerative spondylolisthesis benefit from stabilization of the spine. We strongly disagree with the editorial by Peul and Moojen, who suggest that lumbar fusion does not add value for patients with spondylolisthesis. The development of instability in patients after lumbar decompression would have probably increased health resource utilization (e.g., injections, physical therapy, and physician visits) in addition to lost productivity and the cost of revision surgery. Comprehensive economic studies are needed to examine the total health resource utilization among populations of patients treated with simple decompression, as compared with those treated with decompression plus lumbar spinal fusion. The SLIP investigators have formally requested collaboration with the Centers for Medicare and Medicaid Services and other payers to perform a long-term, 10-year analysis of comparative health resource utilization among the patients in the SLIP study who underwent simple decompression, as compared with those who underwent decompression plus fusion. Zoher Ghogawala, M.D. Lahey Hospital and Medical Center Burlington, MA zoher​.­ghogawala@​­lahey​.­org

Fred G. Barker II, M.D. Massachusetts General Hospital Boston, MA

Edward C. Benzel, M.D. Cleveland Clinic Foundation Cleveland, OH Since publication of their article, the authors report no further potential conflict of interest. 1. Blumenthal C, Curran J, Benzel EC, et al. Radiographic pre-

The editorialists reply: Hu and Feng propose that there should be standardized indications for revision fusion surgery. In the study by Ghogawala et al., it was unclear how instability was defined. In the study by Försth et al., which included a larger patient sample than that in the study by Ghogawala et al., the proportion of patients who had subsequent spine surgery after decompression was substantially lower than that in the study by Ghohawala et al., with no betweengroup difference, during 6.5 years of follow-up, which was 2.5 years longer than the follow-up by Ghogawala et al. Indeed, patients with spinal stenosis should be informed of the substantial risk of additional operations. If this revision rate exceeds 30%, which has not been observed in other highquality studies and spine registries, it is debatable whether the technique used was justifiable, and perhaps the indication for revision surgery should be questioned. Postsurgical instability is difficult to interpret without quantified metrics of pathologic biomechanical movement and correlated neurologic complaints. Accepted indications for reoperation include radiologically confirmed hypermobility, disk collapse that has created neuroforamen stenosis, worsening of slip between vertebrae, and progressive kyphosis. Recently, the debate over the topic of postsurgical spinal instability has intensified, and the establishment of standardized indications for fusion surgery would be in the best interests of both patients and society. Wilco C. Peul, M.D., Ph.D. Leiden University Medical Center Leiden, the Netherlands w​.­c​.­peul@​­lumc​.­nl

dictors of delayed instability following decompression without fusion for degenerative grade I lumbar spondylolisthesis. J Neurosurg Spine 2013;​18:​340-6. 2. Resnick DK, Watters WC III, Sharan A, et al. Guideline update for the performance of fusion procedures for degenerative disease of the lumbar spine. Part 9: lumbar fusion for stenosis with spondylolisthesis. J Neurosurg Spine 2014;​21:​54-61.

Wouter A. Moojen, M.D., Ph.D.

DOI: 10.1056/NEJMc1606502

DOI: 10.1056/NEJMc1606502

Medical Center Haaglanden The Hague, the Netherlands w​.­moojen@​­hagaziekenhuis​.­nl Since publication of their article, the authors report no further potential conflict of interest.

Stenting versus Surgery for Carotid Stenosis To the Editor: Rosenfield et al. (March 17 is- protection was noninferior to carotid endarterecsue)1 report that in the Asymptomatic Carotid tomy in asymptomatic patients. Similar results Trial (ACT) I, carotid-artery stenting with embolic were reported by Brott et al. (March 17 issue)2 for n engl j med 375;6 nejm.org  August 11, 2016

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