Sagittal imbalance in patients with lumbar spinal stenosis and outcomes after simple decompression surgery

Accepted Manuscript Title: Sagittal imbalance in patients with lumbar spinal stenosis and outcomes after simple decompression surgery Author: E Kyung Shin, Chi Heon Kim, Chun Kee Chung, Yunhee Choi, Dahae Yim, Whei Jung, Sung Bae Park, Jung Hyeon Moon, Won Heo, Sung-Mi Kim PII: DOI: Reference:

S1529-9430(16)30885-3 http://dx.doi.org/doi: 10.1016/j.spinee.2016.08.023 SPINEE 57139

To appear in:

The Spine Journal

Received date: Revised date: Accepted date:

14-3-2016 21-6-2016 16-8-2016

Please cite this article as: E Kyung Shin, Chi Heon Kim, Chun Kee Chung, Yunhee Choi, Dahae Yim, Whei Jung, Sung Bae Park, Jung Hyeon Moon, Won Heo, Sung-Mi Kim, Sagittal imbalance in patients with lumbar spinal stenosis and outcomes after simple decompression surgery, The Spine Journal (2016), http://dx.doi.org/doi: 10.1016/j.spinee.2016.08.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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TITLE

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Sagittal imbalance in patients with lumbar spinal stenosis and outcomes after simple decompression surgery

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E Kyung Shin, BS1, Chi Heon Kim, MD, PhD2,3,4, Chun Kee Chung, MD, PhD2,3,4,5, Yunhee Choi, PhD5, Dahae

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Yim, MS5, Whei Jung, BS1, Sung Bae Park, MD2,3,4,6, Jung Hyeon Moon, MD2,3, Won Heo, MD2,3, Sung-Mi

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Kim, RN2

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1

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National University College of Medicine; 3Department of Neurosurgery, Seoul National University Hospital,

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Seoul, Korea; 4Clinical Research Institute, Seoul National University Hospital, Seoul, Korea; 5Medical Research

Department of Medicine, Seoul National University College of Medicine; 2Department of Neurosurgery, Seoul

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Collaborating Center, Seoul National University College of Medicine, Seoul, Korea; 5Department of Brain and

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Cognitive Sciences, Seoul National University College of Natural Sciences, Seoul, Republic of Korea;

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Department of Neurosurgery, Seoul National University Boramae Hospital, Seoul, Korea

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Running head: sagittal imbalance in lumbar spinal stenosis

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Corresponding Author

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Chi Heon Kim, MD, PhD.

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Department of Neurosurgery

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Seoul National University College of Medicine

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101 Daehak-Ro, Jongno-gu

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Seoul, 110-744, South Korea

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Tel: +82-2-2072-3398

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Fax: +82-2-744-8459

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E-mail: [email protected]

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Conflicts of Interest and Source of Funding

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Page 1 of 22

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This work was supported by Grant No. 0420153090 (2015-0987) from the Seoul National University Hospital

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Research Fund. The authors appreciate the statistical advice from the Medical Research Collaborating Center at

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the Seoul National University Hospital and the Seoul National University College of Medicine The senior

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author (CHK) is a consultant of Richard Wolf GmbH. There is no grant from Richard Wolf GmbH. The other

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authors declare no conflict of interest concerning the materials or methods used in this study or the findings

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described in this paper. This study was approved by the institutional review board at the Seoul National

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University Hospital (H-1505-086-673).

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Author contribution

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Dr. Chi Heon Kim had full access to all the data in the study and takes responsibility for the integrity of the data

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and the accuracy of the data analysis. CHK, EKS, and CKC made the first draft of the manuscript. YC and DY

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did a statistical analysis. SBP, CHK, JHM, WH and CKC contributed in designing the study protocol. WJ, EKS

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and SK summarized clinical data of the patients.

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Abstract: Structured Abstract

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Background Context

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Lumbar spinal stenosis (LSS) is the most common lumbar degenerative disease, and sagittal imbalance is

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uncommon. Forward-bending posture, which is primarily caused by buckling of the ligamentum flavum, may be

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improved via simple decompression surgery.

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Purpose

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The objectives of this study were to identify the risk factors for sagittal imbalance and to describe the outcomes

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of simple decompression surgery.

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Study Design

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Retrospective nested case-control study

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Patient Sample

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This was a retrospective study that included 83 consecutive patients (M:F = 46:37; mean age, 68.5 ± 7.7 years)

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who underwent decompression surgery and a minimum of 12 months of follow-up.

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Outcome Measurement

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The primary endpoint was normalization of sagittal imbalance after decompression surgery.

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Materials and Methods

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Sagittal imbalance was defined as a C7-sagittal vertical axis (SVA) ≥ 40 mm on a 36-inch-long lateral whole

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spine radiograph. Logistic regression analysis was used to identify the risk factors for sagittal imbalance.

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Bilateral decompression was performed via a unilateral approach with a tubular retractor. The SVA was

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measured on serial radiographs performed 1, 3, 6 and 12 months postoperatively. The prognostic factors for

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sagittal balance recovery were determined based on various clinical and radiological parameters.

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Results

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Sagittal imbalance was observed in 54% (45/83) of patients, and its risk factors were old age and a large

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mismatch between pelvic incidence and lumbar lordosis. The 1-year normalization rate was 73% after

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decompression surgery, and the median time to normalization was 1 to 3 months. Patients who did not

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experience SVA normalization exhibited low thoracic kyphosis (HR, 1.04; 95% CI, 1.02 – 1.10) (p < 0.01) and

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spondylolisthesis (HR, 0.33; 95% CI, 0.17 – 0.61) prior to surgery.

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Conclusions

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Sagittal imbalance was observed in more than 50% of LSS patients, but this imbalance was correctable via

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simple decompression surgery in 70% of patients.

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KEY WORDS

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Spine; spinal stenosis; sagittal; balance; laminectomy; surgery; ligamentum flavum

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Introduction

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Lumbar spinal stenosis (LSS) is the most common lumbar degenerative disease [1]. Surgery is indicated for

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medically intractable patients, and decompression surgery is sufficient in most of these patients [1-3]. However,

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sagittal imbalance is infrequently associated with LSS [4-6]. Sagittal imbalance is associated with functional

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disability and pain [7], and postoperative sagittal imbalance is correlated with less satisfactory outcomes and

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mechanical failure [8]. Therefore, restoration of sagittal balance, which may require instrumented correction,

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must be considered when selecting the appropriate surgical method [9]. However, affected patients may exhibit

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a forward-bending posture, which is represented by a positive sagittal vertical axis (SVA), to reduce buckling of

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the ligamentum flavum [4-6]. This forward-bending posture may be improved via simple decompression surgery

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in some patients [10-12]. The present study identified the risk factors for sagittal imbalance and described the

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outcomes of decompression surgery without instrumentation in LSS patients without coronal imbalance [4, 13].

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Materials and Methods

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Patients

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The appropriate institutional review board approved this study (1505-086-673). The medical records of 138

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patients who underwent lumbar decompression surgery due to lumbar spinal stenosis between Sept. 2011 and

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Jun. 2014 were retrospectively reviewed. The following patients were eligible for the present study: 1) patients

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with severe central lumbar spinal stenosis caused primarily by a thickened ligamentum flavum (LF), 2) patients

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who were followed up for more than 12 months, 3) patients without coronal deformity, 4) patients without

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instability on dynamic radiographs (slippage < 4 mm and rotation < 12°), and 5) patients without severe

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foraminal stenosis (obliteration of perineural fat on T1 sagittal magnetic resonance imaging) [13-16]. Patients

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with lumbar coronal deformity according to the Scoliosis Research Society-Schwab classification [13] were

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excluded from this study. Eighty-three patients (M:F = 46:37; mean age, 68.5 ± 7.7 years) were included in the

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present study. All patients completed the Korean version of the Oswestry Disability Index (K-ODI, /45) [17] and

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a questionnaire regarding visual analogue pain scores for the back (VAS-back, /10) and leg (VAS-leg, /10).

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Patients were encouraged to ambulate the day of surgery and were discharged without a lumbar brace 2-3 days 5

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after surgery. Patients were allowed to sit in a chair, walk and perform light daily work, but heavy lifting was

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allowed only 3 months after surgery. Patients were scheduled to visit the outpatient clinic 1, 3, 6 and 12 months

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after surgery, at which time whole spine radiographs and the above questionnaires were repeated. Patients were

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followed-up for a mean of 15.8 ± 6.6 months (median, 12 months; range, 12 – 39). Clinically, a successful

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outcome was defined as a decrease in the K-ODI greater than 6.4 (minimum clinically significant difference in

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the ODI), as reported in previous studies [18].

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Radiographic measurements

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Preoperative whole spine radiographs were performed on all patients using 36-inch-long digital lateral

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radiographic films with the arms of each patient flexed to 60° and the hips and knees fully extended [19].

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Patients were asked to stand and look straight ahead during whole spine radiography [20]. The sagittal vertical

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axis (SVA), the degree of lumbar curvature between T12 and S1 (T12 – S1, LL), the degree of thoracic kyphosis

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(T5-T12, TK), the thoracolumbar angle (T10-L2, TL), the segmental angle at the most stenotic level (SA) and

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various pelvic parameters (pelvic incidence, PI; sacral slope, SS; pelvic tilt, PT) were measured via whole spine

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lateral radiography (Fig. 1). The measurements were performed using magnified (150%) images to ensure

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higher measurement reliability [21] using the tools in the indicated picture archiving and communication system

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(Marosis, version 5483, Infinitt Healthcare, Seoul, Korea), which is compatible with Microsoft Windows

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(Microsoft Corp., Redmond, WA, USA) [19, 22]. Two researchers who were blinded to patient information

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performed each measurement, and inter-observer variability was assessed using concordance correlation

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coefficients (CCCs) [6, 23].

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Surgical methods

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Surgery was performed for all spinal levels affected by severe stenosis with each patient in the prone position,

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using a midline 2-3 cm skin incision. Single-level surgery was performed in 57 (67%) patients, and double-level

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surgery was performed in 26 (33%) patients. A tubular retractor was attached to the lamina on the symptomatic

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side, and a unilateral laminotomy without LF removal was performed using a rotary drill. The contralateral 6

Page 6 of 22

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lamina was not drilled out. Care was taken not to damage the facet joint. The interface between the lamina and

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LF was identified, and the LF was detached from the bilateral lamina using a curved curette (Fig. 2). The

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detached LF was removed using a pituitary rongeur. Bilateral decompression was confirmed via direct

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visualization of the lateral recess. The bilateral facet joint was preserved because we included patients without

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severe foraminal stenosis. The wound was closed in a layer-by-layer fashion.

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Statistical analysis

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We defined sagittal imbalance as an SVA ≥ 40 mm on preoperative whole spine radiographs, according to the

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Scoliosis Research Society-Schwab classification [13]. The analysis was performed in two steps. First, the

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incidence of and risk factors for sagittal imbalance were identified. Second, the outcomes of sagittal imbalance

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after lumbar decompression surgery and the prognostic factors that determine these outcomes were analyzed.

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The following factors were considered in the analysis: age, sex, duration of symptoms (months), the number of

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severe LSS level(s), walking limitations due to neurogenic intermittent claudication (m), K-ODI and VAS-

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back/-leg results, the presence of spondylolisthesis [24], and preoperative radiological parameters (TK, TL, LL,

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SA, PI-LL, and pelvic parameters [PI, PT and SS]) [25]. Agreement between the two reviewers was assessed

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using concordance correlation coefficients (CCCs) and limits of agreement [26, 27].

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First, the characteristics of patients with sagittal imbalance were compared with those of patients without

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imbalance via the independent t-test or Wilcoxon rank-sum test for continuous variables and a χ2-test or Fisher’s

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exact test for non-continuous values. Logistic regression analysis was used to identify the risk factors for sagittal

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imbalance. The assumption of linearity for a continuous variable was evaluated using restricted cubic splines.

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Factors with p-values less than 0.2 on univariable analysis were considered for multivariable analysis. Because

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conventional significance levels, such as 0.05, can fail to identify significant variables, the significance level in

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the univariable analysis was set to 0.2 [28], and forward variable selection was used to control multi-co-linearity.

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Second, the time to sagittal imbalance improvement after decompression surgery was analyzed [23]. Sagittal

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balance was defined as an SVA < 40 mm on follow-up whole spine AP radiographs (Fig. 1). The time to sagittal

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balance was measured at predetermined times (1, 3, 6 and 12 months after surgery). Therefore, the exact sagittal

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balance time was not observed; rather, the nearest time between two assessment times was considered the 7

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sagittal balance time. These types of survival data are known as “interval-censored survival data”. A common

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approach to interval censoring entails imputing survival times based on a specific time point within each interval,

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such as the end (or beginning or midpoint), and then applying standard survival analysis methods, such as

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Kaplan-Meier curves, log-rank tests or Cox regression analyses. However, this may result in overestimation (or

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underestimation) of survival times and underestimation of error variances, which can lead to false-positive

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results [29]. Hence, proper statistical methods should be used to analyze interval-censored survival data. The

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expectation-maximization and iterative convex minorant (EM-ICM) algorithm was used to analyze the interval-

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censored data and estimate survival proportions [23, 30, 31]. The generalized log-rank test and proportional

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hazard model for interval-censored data were applied to verify the predictors of improvement after

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decompression surgery [32]. Hazard ratios and 95% confidence intervals (95% CI) were estimated via the ICM

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algorithm and bootstrap estimation with 1000 bootstrap resamples, respectively [33]. The proportion hazard

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assumption and linearity were checked using restricted cubic splines, assuming that sagittal balance occurred at

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the end of each interval. Factors with p-values less than 0.2 in the generalized log-rank test were considered for

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the multivariable analysis, and forward variable selection was applied. In addition to the factors used in the first

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analysis, the relationship between preoperative sagittal imbalance and clinically successful outcomes was

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considered for analysis [34]. Statistical analyses were performed using SAS, version 9.3 (SAS Institute, Cary,

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NC, USA), and R software, version 3.0.3 (http://www.r-project.org), and the intcox library.

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Results

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Table 1 describes the characteristics of the patients. Sagittal imbalance was observed in 45/83 (54%) patients.

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Preoperatively, the K-ODI was higher in patients with sagittal imbalance (SVA ≥ 40 mm) than in patients

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without sagittal imbalance, but there were no differences in walking distance or pain in the back and leg

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between patients with an SVA ≥ 40 mm and patients with an SVA < 40 mm (Table 1). Concordant correlation

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coefficients (CCCs) showed satisfactory correlation (> 0.8) for all radiological measurements except the

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segmental angle measurement (0.754) (Supplemental Table 1) [21, 35]. Therefore, the mean values of each

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parameter were used in the present analysis [21]. Univariable analysis revealed that the number of stenotic

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levels, age, K-ODI results, PI, PT, LL and PI-LL were suitable for multivariable analysis. Age, K-ODI results 8

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and PI-LL mismatches were found to be significant risk factors for sagittal imbalance (Table 2). Successful

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clinical outcomes after decompression surgery were observed in 28/45 patients (62%) with preoperative SVA ≥

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40 mm and in 29/38 (76%) patients with a preoperative SVA < 40 mm. Outcomes were not significantly

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associated with preoperative sagittal imbalance (p = 0.24). There were no cases of surgery-related complications,

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such as dural tears or surgical site infections. At the last follow-up, the K-ODI was significantly higher in

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patients with preoperative sagittal imbalance (median, 13; range, 0 - 36) than in patients without sagittal

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imbalance (median, 8.5; range, 0 – 26) (p = 0.003), while there was no difference between patients with and

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without sagittal imbalance in VAS-back (p = 0.15; median, 4 (range 0 – 10) vs. median, 2 (range, 0 - 8)) and

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VAS-leg (p = 0.06; median, 5 (range, 0-10) vs. median 2.5 (range, 0 – 7)). The 1-year normalization rate of

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sagittal imbalance after decompression surgery was 73%, and the median survival time was between 1 and 3

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months (Fig. 3). The median preoperative SVA for patients with improved sagittal imbalance was 70.5 mm

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(range, 42.0 – 147.6), and the median SVA at the time of improvement was 22.8 mm (range, -12.11 – 39.7). The

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median preoperative SVA for patients who did not experience SVA normalization was 75.1 mm (range, 45.6 -

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171.2), and the median SVA at 12 months was 71.2 mm (range, 42.1 - 165.9). The preoperative SVA was not

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different between patients who did and did not experience normalization (p = 0.23). Multi-variable analysis was

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performed for the presence of spondylolisthesis, K-ODI results, SA, PI, PT, TK, and PI-LL, which demonstrated

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a p-value < 0.2 in a generalized log-rank test. TK and the presence of spondylolisthesis were found to be

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significant prognostic factors in the multi-variable analysis. The preoperative degree of TK was 28.7 ± 7.9° in

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patients who experienced normalization and 16.3 ± 14.2° in patients who did not experience normalization (HR,

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1.04; 95% CI, 1.02 – 1.10) (p < 0.01). Concomitant spondylolisthesis was noted in 28/33 (85%) patients who

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experienced improvement and 12/12 patients who did not experience improvement (HR, 0.33; 95% CI, 0.17 –

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0.61).

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Discussion

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The present study identified common problems faced by patients with spinal stenosis and identified the risk

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factors for these phenomena, as well as their impact on surgical outcomes. Sagittal imbalance was observed in

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54% of patients. The risk factors for sagittal imbalance were older age and a large PI-LL mismatch. Patients 9

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with sagittal imbalance exhibited greater disability (K-ODI) than patients without sagittal imbalance. Sagittal

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imbalance normalized in 73% of patients after 12 months, and the median survival time was 1 to 3 months after

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simple decompression surgery. A low thoracic kyphotic angle and spondylolisthesis were associated with

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residual sagittal imbalance.

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Sagittal imbalance in patients with spinal stenosis

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Sagittal imbalance was emphasized in this study because it is associated with poor health quality and pain [5, 36,

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37]. Sagittal imbalance is a known risk factor for poor outcomes and mechanical failure after deformity surgery

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[38]. These findings suggest that restoration of sagittal balance is an important issue in deformity surgery [13,

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36-38]. However, this issue is emphasized less in patients with spinal stenosis than in patients undergoing

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deformity surgery [4-6]. Previous several studies have demonstrated that sagittal imbalance is observed

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frequently in patients who have undergone simple decompression surgery. The SVA was more than 50 mm in 40%

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[4, 5] of patients and more than 40 mm in 74% of patients in one of these studies [6]. The SVA was ≥ 40 mm in

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54% (45/83) of patients in the present study and ≥ 50 mm in 41% (34/83) of patients. Old age, PI/LL

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mismatches and high disability index scores were associated with sagittal imbalance, findings similar to those of

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previous studies [13, 37, 39, 40]. Other risk factors for sagittal imbalance noted in the literature were high pelvic

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tilt and low thoracic kyphosis [4]. Sagittal imbalance must be emphasized in patients for whom decompression

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surgery is planned because of its high incidence.

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Surgical treatments

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A combination of various factors, such as facet joint hypertrophy, osteophyte growth, and LF thickening and

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stiffness, cause spinal stenosis [15]. Neurogenic intermittent claudication is a common symptom of the disease

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because dynamic stenosis is primarily caused by buckling and posterior compression of the LF [41]. This

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symptom is generally relieved by sitting or lying down [42, 43]. The LF is stretched by lumbar flexion, and

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more than half of affected patients must assume a forward-bending posture to reduce pain during standing [4-6,

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44]. This forward-bending posture is represented by a positive SVA, and sagittal imbalance is defined as an SVA 10

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≥ 40 mm [13]. Removal of the LF may improve forward-bending posture [4-6, 20, 23]. Therefore,

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decompression surgery without instrumentation may be considered [1, 4].

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There are various types of decompression surgeries for spinal stenosis. Laminectomy is a standard surgical

4

technique [45]. Currently, minimally invasive surgical techniques, such as unilateral laminotomy, bilateral

5

decompression, bilateral laminotomy and split-spinous process laminotomy, are used in addition to standard

6

techniques [46]. A Cochrane review showed that the above techniques had similar effects regarding functional

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disability and leg pain [46]. The risks of iatrogenic instability and postoperative back pain may be lower with

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minimally invasive surgical techniques than with standard laminectomy; however, the differences in these risk

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are too small to be clinically significant [46]. The outcomes of sagittal imbalance after decompression surgery

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are reported in several papers, but the prognostic factors that determine these outcomes are not clear [4-6, 12].

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We must select patients in whom LF buckling was the major contributor to their forward-bending posture [6].

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Changes in spinal sagittal imbalance after decompression surgery

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Improvements in sagittal alignment have been reported with standard laminectomy [12]. The authors of that

15

study hypothesized that improvements in pain and function after decompressive laminectomy may facilitate

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restoration of upright posture, which was shown by decreases in the SVA [12]. However, the above study did

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not report the outcomes of patients with sagittal imbalance (SVA ≥ 40 mm or ≥ 50 mm). Three papers

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demonstrated changes in sagittal imbalance after decompression surgery [4-6]. Hikata et al. demonstrated that

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sagittal imbalance (SVA ≥ 50 mm) normalized after decompression surgery in 52% (23/44) of patients within 12

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months [4]. Failure of sagittal balance restoration was observed in patients with a preoperative SVA ≥ 80 mm [4].

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These researchers demonstrated that preoperative SVA did not influence surgical outcomes [4], but

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postoperative sagittal imbalance was associated with more back pain and low quality of life [4, 5]. In the present

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study, seventeen patients exhibited an SVA ≥ 80 mm, and 12 (71%) patients exhibited improved sagittal

24

imbalance after decompression surgery. However successful clinical outcomes were noted in only 9 (53%)

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patients. An SVA of ≥ 80 mm was also not associated with normalization of sagittal imbalance or clinical

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outcomes (p = 0.74 and 0.12, respectively). Fujii et al. demonstrated that sagittal imbalance (SVA ≥ 40 mm)

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normalized after decompression surgery in 43% (28/65) of patients and determined that the prognostic factors

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for radiological improvement were age (cut-off, 65 years) and PI-LL mismatches (cut-off, 21.5°) [6]. However, 11

Page 11 of 22

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pre- and post-operative sagittal imbalance were not correlated with clinical outcomes [6]. Notably,

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approximately 50% of patients with postoperative sagittal imbalance reported good quality of life scores [6].

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The residual SVA was larger in patients older than 70 years after decompression surgery [6]. The current criteria

4

for sagittal imbalance may not apply to some elderly patients because the SVA normally increases with age [40].

5

The authors of the above two papers performed decompression via a spinous process splitting approach [4, 6,

6

47]. Their outcomes may be unique to this technique. However, conflicting results were presented in these

7

papers regarding the influence of postoperative sagittal imbalance on clinical outcomes after surgery, which

8

means that a larger study is required to confirm the prognostic factors that determine clinical outcomes after

9

surgery in patients with sagittal imbalance.

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Another study used a unilateral approach and bilateral decompression. Dohzono et al. demonstrated that

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preoperative sagittal imbalance (SVA ≥ 50 mm) was not associated with improvements in the Japanese

12

Orthopedic Association Score but was associated with residual back pain [5]. Unfortunately, information

13

regarding the radiological outcomes of sagittal imbalance was not provided [5]. The present study described the

14

outcomes of unilateral and bilateral decompression surgery and may be unique in this regard. Sagittal imbalance

15

(SVA ≥ 40 mm) was improved in 73% (33/45) of patients, and the recovery rate was not low, even for patients

16

with a higher SVA (≥ 80 mm) (71%, 12/17). Notably, preoperative SVA was not correlated with clinical

17

outcomes. Statistical methods identified thoracic kyphosis as a prognostic factor. The degree of preoperative TK

18

was 28.7 ± 7.9° for patients who experienced normalization and 16.3 ± 14.2° who did not experience

19

normalization (p < 0.01). Normal thoracic kyphosis is approximately 30° [48], and the value for patients who

20

did not experience normalization (16.3 ± 14.2°) in the present study was sub-normal. These results were

21

obtained in patients with degenerative lumbar kyphosis [49, 50]. The degree of TK decreased to compensate for

22

their sagittal imbalance [49] and increased after instrumented correction [50]. This result implies that a sub-

23

normal TK may not be an indication for simple decompression surgery. However, no cut-off values for TK could

24

be calculated in the present study because of the small number of included patients. Another predictor of a poor

25

prognosis was the presence of spondylolisthesis. In summary, sagittal imbalance may not be corrected via

26

simple decompression surgery in patients with either sub-normal TK or spondylolisthesis.

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Limitations

2

First, this was a retrospective study that included a small number of patients who underwent a 1-year follow-up.

3

We identified the prognostic factors for recovery of sagittal balance, but larger prospective studies are necessary

4

to identify patients for whom decompression surgery is sufficient to facilitate recovery. Second, a prerequisite

5

for recovery of sagittal balance is sufficient lumbar extensor muscle strength [4]. However, the present study did

6

not assess lumbar extensor muscle strength [4, 51]. Patients may assume a forward-bending posture because of

7

inadequate back extensor muscle strength [52]. We need to prospectively collect data regarding the strength

8

necessary for recovery of sagittal balance. Third, a systematic review showed that surgical outcomes were not

9

different among various surgical techniques [46]. The improvements in sagittal alignment have been reported

10

with both standard laminectomy and minimally invasive surgical techniques [4-6, 12]. Although we used only

11

minimally invasive surgical techniques, the improvements in spinal sagittal alignment associated with these

12

techniques may be similar to those associated with both standard laminectomy and other minimally invasive

13

surgical techniques. We must perform a comparative study in the future. Finally, radiological analysis via

14

computerized imaging tools may have enhanced the reliability of our results. The present study used simple

15

Cobb’s method to measure curvatures and validated image viewer tools to perform the complex pelvic and C7-

16

SVA measurements, which showed high intra- and inter-observer reliability, even among unskilled observers

17

[19]. To improve reliability, all measurements were performed using images magnified to 150% of their normal

18

size [21]. All measurements were performed by 2 researchers, and their correlation coefficients were greater

19

than 0.8 for all measurements except the segmental angle measurement (0.754) (Supplemental Table 1). The

20

measurement methods were the same as those used in previous studies [4-6, 12]. Therefore, the present results

21

seem to be reliable, but computerized measurements would be more reliable than manual measurements.

22 23

Conclusion

24

Sagittal imbalance was commonly observed in patients with spinal stenosis but was recoverable in 70% of

25

patients within 1 year after simple decompression surgery. A larger study is anticipated to identify the prognostic

26

factors associated with sagittal balance recovery. 13

Page 13 of 22

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Figure Legends

2

Fig. 1. Radiological measurements

3

The sagittal vertical axis (SVA) is the horizontal distance from the C7 plumb to the posterior-superior corner of

4

S1. Lumbar curvature (LL), thoracic kyphosis (TK) and thoraco-lumbar curvature (TL) were measured between

5

the superior endplate of T12 and S1, between T5 and T12 and between T10 and L2, respectively, using Cobb’s

6

method in whole spine lateral radiographs. Pelvic parameters (pelvic incidence, PI; sacral slope, SS; pelvic tilt,

7

PT) were measured using the measurement tools included in the picture archiving and communication systems.

8

Fig. 2 Surgical methods

9

After unilateral laminotomy, which was performed with a rotary drill (shaded area), a curved curette was

10

inserted into the interface between the contralateral lamina and ligamentum flavum (LF). The curette detached

11

the ligamentum flavum from the contralateral lamina laterally, superiorly and medially (Curette 1). The same

12

procedure was performed on the ipsilateral lamina (Curette 2), and the detached LF was removed using forceps

13

(b). The dark line indicates the midline. Surgery was completed after removal of the residual LF with a Kerrison

14

rongeur. Postoperative magnetic resonance imaging performed 6 months after surgery shows that the spinal

15

canal was decompressed and that the facet joint was preserved (c).

16

Fig. 3. Residual sagittal imbalance

17

Sagittal imbalance normalized in 33/45 (73%) patients over 12 months. The probability of residual sagittal

18

imbalance was 0.58 during the first postoperative month, 0.42 during months 1 – 3, 0.29 during months 3 – 6

19

and 0.26 during months 6 – 12.

20 21 22

19

Page 19 of 22

1

Table 1. Characteristics of patients SVA ≥ 40 mm

SVA < 40 mm

(n = 45)

(n = 38)

Sagittal vertical axis (mm) 43.8 [-54.4,171.2] (median [min,max])

70.6 [42.0,171.2]

17.6 [-54.4,38.9]

< 0.01‡

Age (year)

68.5 ± 7.7

70.8 ± 6.2

65.7 ± 8.4

<0.01†

Sex (female, n (%))

37

22 (50)

15 (39.47)

0.33

12 [1,156]

6 [1,120]

0.33 ‡

Total (N = 83)

Duration of symptom (mo) 12 [1,156]

p-value

Number of stenotic levels

0.19

single, n(%)

57

28 (63)

29 (76)

double, n(%)

26

17 (37)

9 (24)

NIC (m)

500 [10,4000]

500 [10,4000]

500 [10,3000]

0.91 ‡

K-ODI (/45)

23 [8,43]

23 [11,43]

18 [8,37]

0.04 ‡

VAS-back (/10)

7 [0,15]

7 [0,10]

7 [0,10]

0.68 ‡

VAS-leg (/10)

7 [0,10]

7 [3,10]

7 [0,10]

0.47 ‡

Grade of spondylolisthesis

0.82

0, n(%)

9

5 (11)

4 (11)

1, n(%)

74

40 (89)

34 (89)

Pelvic incidence

51.9 [22.7,72.3]

55.4 [35.9,72.3]

49 [22.7,66.3]

0.02 ‡

Pelvic tilt

22.4 [6.4,75.4]

26.8 [6.5,46.4]

20.2 [6.4,75.4]

0.00 ‡

Sacral slope

27.4 [2.0,47.6]

27.4 [2.0,46.9]

27.6 [11.6,47.6]

0.78 ‡

Lumbar lordosis

-34.6 [-68.0,35.7]

-30.7 [-59.3,7.2]

-36.2 [-68.0,35.7]

0.03 ‡

Segmental angle

-9.5 [-28.8,1.0]

-8.9 [-24.0,1.0]

-9.7 [-28.8,-1.4]

0.22 ‡

Thoracolumbar

7.2 [-17.0,37.0]

8.10 [-17.0,37.0]

7.1 [-9.9,36.7]

0.70 ‡

Thoracic kyphosis

26.6 [-42.8,48.6]

26.7 [-4.4,47.4]

26.5 [-42.8,48.6]

0.88 ‡

PI-LL

16.2 [-11.7,93.5]

21.8 [-3.1,53.8]

9.4 [-11.7,93.5]

<0.01 ‡

2 3

†

Independent t-test 20

Page 20 of 22

1

‡

Wilcoxon rank sum test

2 3

21

Page 21 of 22

1

Table 2. Risk factors for sagittal imbalance OR [95% CI]

p-value

Age

1.110 [1.029, 1.197]

0.007

K-ODI

1.076 [1.003, 1.155]

0.042

PI-LL

1.057 [1.015, 1.102]

0.007

2

Hosmer-Lemeshow test: p = 0.42

3

C-statistics = 0.83

4

22

Page 22 of 22