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 » Introduction
 »  Materials and Me...
 »  Observations and...
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Table of Contents    
ORIGINAL ARTICLE
Year : 2016  |  Volume : 64  |  Issue : 4  |  Page : 677-683

Clinical, radiological, and functional evaluation of surgical treatment in degenerative lumbar canal stenosis


Department of Neurosurgery, Sri Venkateswara Institute of Medical Sciences, Tirupati, Andhra Pradesh, India

Date of Web Publication5-Jul-2016

Correspondence Address:
Dr. Siva Subrahmanyam Chivukula
Department of Neurosurgery, Sri Venkateswara Institute of Medical Sciences, Alipiri Road, Tirupati, Andhra Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0028-3886.185378

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 » Abstract 

Background: Lumbar spinal stenosis is a frequent indication for spinal surgery. The clinical symptoms may not be accurately reflected on radiological studies. Treatment is aimed at not only obtaining immediate pain relief but also in preventing its long-term disabling sequelae. The walking ability needs to be correlated with functional outcome measures in assessing patients with symptomatic lumbar canal stenosis (LCS).
Aims and Objectives: To study and evaluate the outcome of surgery for degenerative LCS on a clinical, radiological, and functional basis, and to establish an association between various factors that may be influencing the development of LCS.
Materials and Methods: A prospective study of 48 patients was carried out at the Department of Neurosurgery, Sri Venkateswara Institute of Medical Sciences, Tirupathi, in whom the LCS was confirmed and measured on magnetic resonance imaging images by thecal sac cross-sectional area (CSA). Their functional assessment was done by motorized treadmill test for assessment of the time of appearance of first symptom and the maximum walking distance. The visual analog scale (VAS), Oswestry disability index (ODI), Japanese Orthopedic Association (JOA) score, and Short Form-36 (SF-36) score were also evaluated.
Conclusions: The post-operative VAS, ODI, JOA, SF-36 scores showed a significant improvement from the preoperative ones. Postoperative thecal sac measurements showed a significant neural decompression substantiating the results of improvement in the functional assessment scores after surgery. The treadmill test is a quantifiable means of dynamic function. It may be considered as a useful tool for the assessment of functional impairment and for the correlation of thecal sac CSA with lumbar canal stenosis. A combination of functional scores, treadmill test, and thecal sac CSA may be included in the preoperative assessment of outcome in patients with LCS.


Keywords: Cross-sectional area; lumbar canal stenosis; treadmill test


How to cite this article:
Prasad B, Ramesh Chandra V V, Devi B V, Chivukula SS, Pundarikakshaiah K. Clinical, radiological, and functional evaluation of surgical treatment in degenerative lumbar canal stenosis. Neurol India 2016;64:677-83

How to cite this URL:
Prasad B, Ramesh Chandra V V, Devi B V, Chivukula SS, Pundarikakshaiah K. Clinical, radiological, and functional evaluation of surgical treatment in degenerative lumbar canal stenosis. Neurol India [serial online] 2016 [cited 2019 Nov 21];64:677-83. Available from: http://www.neurologyindia.com/text.asp?2016/64/4/677/185378



 » Introduction Top


Lumbar canal stenosis (LCS) is a continuum of pathology and one of the most useful definitions of LCS is that described by Verbiest in 1954 as “a disproportion in the spinal canal between the size of the neural elements and the space available.”[1] Degenerative LCS is due to narrowing of the spinal canal, the lateral nerve root canals or the intervertebral neural foramina due to progressive hypertrophy of any of the surrounding osteocartilaginous and ligamentous elements. It may result in neurogenic or vascular compression of the contents of the spinal canal at one or more levels. The classic symptom of LCS is pseudoclaudication or neurogenic claudication. The clinical symptoms of lumbar spinal canal narrowing may not be accurately correlated with the radiological studies. Despite significant spinal narrowing, some patients may have only minor symptoms; on the other hand, even with moderate canal narrowing, significant symptoms are possible.

With advances in understanding of the clinico-functional and patho-radiological correlation, the treatment has changed from various non-operative modalities to decompression, and subsequently, decompression and fusion with or without instrumentation. The measurement of walking ability utilizing the treadmill test is a quantifiable means of dynamic functional assessment. It needs to be correlated with functional outcome measures in assessing patients with symptomatic LCS. Clinical studies in literature that perform the clinicoradiological and functional outcome assessment are few; studies from India are even more infrequent. This study was undertaken to assess the efficacy of surgery in bringing about a functional, clinical, and radiological improvement in cases having a degenerative LCS. A study of the factors that were influencing the surgical outcome was also undertaken.


 » Materials and Methods Top


The data was collected prospectively from 48 patients with degenerative LCS in the Department of Neurosurgery, Sri Venkateswara Institute of Medical Sciences (SVIMS), Tirupathi, Andhra Pradesh, India, from April 2013 to December 2014. Institutional Ethical Committee clearance was obtained for conducting the study (IEC No.; Roc. No. A and E/08/IEC/SVIMS/04,345 dated 12-05-2014).

Inclusion criteria

Patients included in the study suffered from degenerative LCS and were managed with various surgical procedures after failure of adequate conservative treatment for 3 months, were within the age range of 18–65 years, had radiological evidence of LCS with a thecal sac cross-sectional area (CSA) <100 mm 2 on magnetic resonance imaging (MRI) and experienced functional impairment as evidenced by the treadmill test (not able to walk for 20 min with 0° slope at 2 km/h).

Exclusion criteria

Patients with primary canal stenosis, LCS due to trauma/tumor, aged above 65 years, unfit for general anesthesia and treadmill testing (due to neurological or cardiovascular disroders, osteoarthritis of the knee or hip joint, those who had prior lumbar or lower extremity surgery, and those who had suffered from a major lumbar trauma), as well as those with peripheral arterial or nerve disease were excluded from the study.

A thorough clinico-neurological examination of all patients with the major complaints of neurogenic claudication, back pain, and leg pain was performed. Functional assessment of the patients was performed using motorized treadmill test, visual analog scale (VAS), Oswestry disability index (ODI), Japanese Orthopedic Association (JOA) score, and the SF-36 score recorded before surgery and after 6 months of treatment. Evaluation with the treadmill (AEROFIT HK, 3.8 HP) at a speed of 2 km/h with a 0° slope was performed after 10 min of resting. The first symptom time (FST), that is, the time it took for the first cardiovascular or claudication symptom to appear during the treadmill test, and its measured distance ( first symptom distance, FSD) were recorded. Further, the treadmill test was continued to record the maximum walking time (MWT) and its measured distance (MWD), that is, the maximum amount of time that the patient could walk and its related distance, respectively, were also noted. The treadmill test was aborted if the patient had walked for 20 min or if cardiovascular symptoms appeared [Table 1].
Table 1: Assessment of walking ability (treadmill test)

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All patients were subjected to radiological investigations which included lumbosacral X-rays lateral views (in flexion and extension) and anteroposterior views (SIEMENS, 500 mA, India) to assess the spinal instability. MRI (SIEMENS Magnetom Symphony/Aera, 1.5 Tesla, Germany) of the lumbosacral spine was done for the diagnosis of LCS. The thecal sac CSA was calculated both pre- and post-operatively after taking the narrowest segment into consideration and the findings were tabulated. Proper informed written consent was obtained from all the patients at the time of inclusion in the study as well before they underwent appropriate surgery.

Statistical analysis

All data sets were collected and entered into a spreadsheet (Microsoft Excel 2007) that included the pre- and post-operative values carried out after 6 months of surgery. Statistical analysis was done using SPSS for windows version 16.0 (IBM, New York, USA). Student's paired t-test was applied for comparison of similar variables before and after treatment. Comparison of mean scores among more than two groups was done using analysis of variance. Pearson's rank coefficient was calculated between variables to assess the correlation. A P ≤ 0.05 was considered statistically significant.


 » Observations and Results Top


Fifty patients met the inclusion criteria and underwent surgery. Two patients were excluded from the study due to associated comorbidities that developed following their surgery. A total of 48 patients who underwent surgery were followed for upto 6 months and were further evaluated with radiological and functional outcome scores in their postoperative period. The present study included a total of 22 (45.83%) male and 26 (54.16%) female patients with their mean age at the development of LCS being 45.72 ± 7.7 years. The average duration of symptoms was 20.33 months. All the patients (n = 48) presented with sensory complaints and limb pain (either unilateral or bilateral), 41 patients had back pain, and neurological examination revealed motor deficits in 38 patients. Morphological analysis revealed that lumbar spinal stenosis (LSS) occurred at mainly two levels in 28 patients and at a single level in only 10 patients. The involvement of the L4-5 level was consistently observed (47%). This was followed in incidence by the L3/4 (25.77%) and the L5/S1 (23.71%) level. Approximately 75% of the patients underwent a decompressive procedure and 25% of the patients needed decompression and fusion with instrumentation due to the presence of an associated listhesis.

Radiological assessment

The mean thecal sac CSA values at each level showed a significant improvement after surgery. Overall, the mean preoperative CSA value was 50.10 mm 2, and the overall mean postoperative value was 118.9 mm 2. The improvement in the CSA values following surgery was statistically significant (P < 0.005) [Table 2] and [Table 3].
Table 2: Correlation between radiological assessment of the cross-sectional area and the clinico-functional variables

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Table 3: Correlation between maximum walking distance and clinico-radiological variables

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Functional assessment

The preoperative VAS score (back pain) ranged from 6 to 10 with the average preoperative score being 7.96 ± 0.619; and, the postoperative VAS score (back pain) ranged from 2 to 5 with the average score being 3.81 ± 0.0762. The preoperative VAS score (leg pain) ranged from 6 to 10 with the average preoperative score being 7.54 ± 0.898 and the postoperative VAS score (leg pain) ranged from 1 to 4 with the average score being 2.38 ± 0.89. There was a significant decrease in the VAS score after surgery with P < 0.001. The preoperative ODI ranged from 42 to 75.55 with the average score of 60.51 ± 8.49 and the postoperative ODI ranged from 24 to 57.77 with the average score of 36.12 ± 9.3. A significant improvement was noted in the quality of life (P < 0.001) after surgery in terms of the ODI score.

The JOA scores showed an improvement after surgery from a mean preoperative value of 11.02 ± 0.88 to a mean postoperative value of 19.9 ± 1.15 (P < 0.05). The recovery rate of the patients following treatment was calculated by using the description of Hirabayashi et al.[2] Recovery rate was classified using a four-grade scale: Excellent, >90%; good, 75–89%; fair, 50–74%; and poor, below 49%. A total of 56.25% patients showed a fair outcome, and 43.75% showed a poor outcome with surgery in terms of the recovery rate. The physical and mental component summary (MCS) of SF-36 showed significant improvement in the physical component summary (PCS) scores (P < 0.002) but the MCS did not show any significant change (P < 0.805).

Assessment of walking ability (Treadmill Test)

The mean walking ability in terms of FST and MWD calculated by the treadmill test showed a significant improvement in the postoperative period both in terms of distance (in meters) and time (in seconds). The result was statistically significant (P < 0.005) [Table 1] and [Figure 1].
Figure 1: Comparison of preoperative and postoperative functional assessment using treadmill test. FSD: First symptom distance; MWD: Measured walking distance; FST: First symptom time; MWT: Measured walking time

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The incidence of complications pertaining to the surgical procedure were negligible in the study (n = 3) with a zero level incidence of wound healing complications (viz., cerebrospinal fluid leak, seroma, infections, etc.). Accidental dural tear occurred in three patients. This tear was directly repaired during surgery. One patient showed a pseudomeningocele formation without any incidence of an additional postoperative deficit at regular follow-up. One patient with the preoperative presence of urinary incontinence showed partial improvement in the follow-up period after instituting the appropriate treatment.


 » Discussion Top


The studies by Weinstein et al.,[3] and Malmivaara et al.,[4] reported that a multiple level stenosis is a more frequent finding than a single-level stenosis in patients needing surgery. Similar finding was observed in the present study as the incidence of multiple-level pathology was higher than a single level one (1 level stenosis, n = 10 and ≥2 level stenosis, n = 38).

In the present study group, the most common level of LCS was observed at the L4/5 (n = 46) level with the mean CSA being 50.97 mm 2. This finding was similar to the other studies included in the literature review that also confirmed that the most common involved vertebral levels are L4/5, L5/S1, indicating that these levels are affected more frequently by the mechanical stress. The survey of the clinical examination of the present sample showed a host of clinical symptoms with paucity of clinical signs. Thus, low back ache was present in 41 patients, motor weakness in 38 patients, whereas all the patients (n = 48) had leg pain (either unilateral or bilateral) and sensory symptoms. This indicates that the patients may have sensory or motor symptoms but the degree or the duration of the LCS is not congruous with the severity of symptoms. Corresponding clinical signs also may not be elicited by examination.

All the patients included in the study showed sensory deficit in some form, which improved in 77.08% patients with surgery. Only 79.16% patients of the study sample had mild to moderate motor deficit. These patients showed improvement of various grades in the follow-up period. This is in accordance with the literature review that reveals a motor deficit present in between 10% and 57% of the patients. Severe motor deficit due to the presence of LCS is usually rare.

Functional outcome assessment

The mean (±standard deviation) preoperative VAS score for leg pain was 7.54 ± 0.89, and back pain was 7.96 ± 0.47. The postoperative VAS score measured value for leg pain was 2.38 ± 0.89 and back pain was 3.81 ± 0.76. The reduction in the VAS scores was statistically significant (P ≤ 0.05) indicating the significant reduction in the severity of pain after surgery. Similar results showing postoperative pain improvement after surgical treatment were noticed in the studies by Yukawa et al.,[5] Zoubouli et al.,[6] Haro et al.,[7] and Sigmundsson et al.[8]

ODI for the assessment of functional impairment of patients in both the pre- and post-operative period showed a significant reduction in the ODI scores from the preoperative mean score of 60.51 ± 8.49 to the postoperative mean score of 36.12 ± 9.3 with the P value being <0.001. Similar results were observed by Yasar et al.,[9] Yukawa et al.,[5] Ng et al.,[10] Barz et al.,[11] Kuittinen et al.[12]

Clinico-functional and radiological associations

The present study revealed that 56.25% of patients had a fair outcome and 43.75% patients had a poor outcome in terms of the JOA score recovery rate. Though a significant improvement was observed in the JOA score after surgery, the results were inferior in terms of the recovery rate, when compared to the results of the study by Nath et al.,[13] in whom the outcome after 1 year was excellent in 64% and good in 24% patients. This can be attributable to the widespread degeneration (multilevel stenosis, n = 38) in the present study sample; and, it may also be due to the inclusion of some variables in the scoring system, such as running and weight lifting, in which female patients and patients in an advanced age may show lower scores despite being relieved of pain. Similar results showing postoperative improvement in the JOA scores were reported by Haro et al.[7] Haro et al.,[7] and Zanoli et al.,[14] observed a significant improvement in the SF-36 score after surgery (P ≤ 0.05). Our results for the PCS and MCS subsets of the SF-36 score showed improvement after surgery, but the change in the scores was statistically significant in terms of PCS (P < 0.002) subset only. Though an improvement was noticed in the postoperative MCS scores, the measured values were not statistically significant (P < 0.805).

This indicates that surgical treatment for degenerative LSS reduces pain and restores physical function significantly and to a considerable extent, also the mental health of the patients. This can be justifiable by a simple explanation that the degenerative disease is more common in the elderly patients who may have multiple comorbidities with considerable functional impairment and, therefore, expect support from their relatives in carrying out activities of daily living. Hence, they are vulnerable to psychological stress.

A better improvement in the duration of walking by patients was observed at 6 months of postsurgical follow-up (mean FST 517.17 ± 153.34 and mean MWT 802.32 ± 152.42) when compared to the preoperative scores (mean FST 117.67 ± 79 and mean MWT 408.96 ± 121.68). In addition, the walking distance reached with the corresponding duration on the treadmill test showed a significant improvement after surgery from the mean preoperative MWD of 230.21 ± 68.14 to the mean postoperative MWD of 446.88 ± 85.53. All these measured scores were statistically significant (P < 0.001) and may be comparable to the results obtained in similar types of clinical and functional outcome assessment studies in patients with LCS by Yasar et al.,[9] and Deen et al.[15],[16]

Radiological assessment of the patients by measuring the thecal sac CSA in MRI films showed improvement in the overall mean postoperative measured values (118.9 mm 2) when compared with the overall mean preoperative values (50.1 mm 2) at various stenotic levels. The results were statistically significant with a P < 0.05, showing radiological evidence of the effectiveness of neural decompression and also providing the object measurement of improvement in the canal diameter following surgery [Figure 2],[Figure 3],[Figure 4],[Figure 5],[Figure 6],[Figure 7],[Figure 8],[Figure 9].
Figure 2: Preoperative magnetic resonance imaging axial image showing L4/5 canal stenosis

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Figure 3: Postoperative magnetic resonance imaging axial image showing adequate L4/5 thecal sac decompression

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Figure 4: Preoperative magnetic resonance imaging sagittal image showing L4/5 canal stenosis

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Figure 5: Postoperative magnetic resonance imaging sagittal image showing thecal sac expansion after performance of a laminectomy

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Figure 6: Preoperative magnetic resonance imaging axial image showing the L3/4 canal stenosis

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Figure 7: Postoperative magnetic resonance imaging axial image showing the L3/4 thecal sac expansion after the laminectomy

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Figure 8: Preoperative magnetic resonance imaging sagittal image showing L3/4 and L4/5 canal stenosis

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Figure 9: Postoperative magnetic resonance imaging sagittal image after L3 and L4 decompressive laminectomy

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Clinico-radiological and functional correlation data

An attempt was made to establish an association between the variables included in the present study. The present study does not show any correlation between age and outcome scores, except of the ODI score (r = 0.37) and the JOA score (r = −0.42). This shows that the functional ability decreases with increments in age of patients with LCS. The age of the patients showed a negative correlation with the thecal sac CSA (r = −0.13) and MWD (r = −0.22). Observations from the present study were similar to that of the findings presented by Sigmundsson et al.,[8] who reported a negative correlation of thecal sac CSA with age.

In the present study, as the duration of symptoms increased, the thecal sac CSA on MRI decreased and an increase in the duration of symptoms showed a decremental response in the preoperative walking ability. This shows a negative correlation between the duration of symptoms, dural sac stenosis (r = −0.13), and the walking ability (r = −0.26).

Patients with their duration of complaints <2 years showed a positive correlation with their outcome measure scores (r = 0.4). This implied that patients with a shorter preoperative duration of symptoms may have a better chance of improvement with surgical decompression. Similar results were reported by Sigmundsson et al.,[8] and Ng et al.,[10] in their analysis of patients with LCS. However, the results do not coincide with that of Amundsen et al.,[17] in which the authors could not find any correlation of the duration of complaints with the postoperative outcome.

In the present study, there was a positive correlation between the MWD and the mean thecal sac CSA values measured on MRI (r = 0.35 and P < 0.01). A similar correlation was observed between the preoperative FST and MWD of individuals (r = 0.62 and P ≤ 0.01). Hence, it may be considered that the preoperative FST, MWD and CSA values may have a utility in the grading or in severity assessment of the patient's condition before surgery for LCS.

The measured walking distance on the treadmill test showed a negative correlation with impaired function as expressed by the ODI scores, both in the preoperative (r = −0.21) and postoperative period (r = −0.32), and particularly, in the preoperative leg VAS (r = 0.3). In the present study, other functional scores such as VAS-back pain, JOA score and SF-36 did not correlate with the walking ability or with thecal CSA on MRI.

The results in our study indicate that nebulous clinical findings resembling spinal stenosis are also relatively common in patients who have only a mild or even no narrowing of the spinal canal on the imaging. This is in accordance with the previous studies which have also noted a lack of correlation between the radiologically detectable stenosis and clinical findings.


 » Conclusions Top


Neurogenic claudication with paresthesia was the most common symptom at presentation in the patients suffering from LCS. There was only a minor incidence of motor weakness in the included patients. The VAS, ODI, JOA, SF-36 scores were effective in assessing the severity of the manifestations in LCS as well as in measuring the postoperative relief of neural compression.

As the treadmill test simulate the normal physical strains that an individual experiences in day-to-day life, the preoperative baseline walking ability assessment with the treadmill test can be considered as a tool for assessing the functional impairment of patients suffering from LCS. It may be a useful test in providing better preoperative counseling and has a possible role in the decision-making for deciding on surgical treatment by the surgeon prior to the counceling of the patient. Preoperative radiological measurement of neural compression provides an objective evidence for the determination of the status of the stenotic spinal level both during surgical planning and after the surgical procedure. The results from the present study show that VAS, ODI, JOA, SF-36, walking ability measurement and severity of stenosis (CSA) may be included in the preoperative assessment of patients with LCS who are being planned for surgical decompression.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
 » References Top

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Verbiest H. A radicular syndrome from developmental narrowing of the lumbar vertebral canal. J Bone Joint Surg Br 1954;36-B: 230-7.  Back to cited text no. 1
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Weinstein JN, Tosteson TD, Lurie JD, Tosteson A, Blood E, Herkowitz H, et al. Surgical versus nonoperative treatment for lumbar spinal stenosis four-year results of the Spine Patient Outcomes Research Trial. Spine (Phila Pa 1976) 2010;35:1329-38.  Back to cited text no. 3
    
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Malmivaara A, Slätis P, Heliövaara M, Sainio P, Kinnunen H, Kankare J, et al. Surgical or nonoperative treatment for lumbar spinal stenosis? A randomized controlled trial. Spine (Phila Pa 1976) 2007;32:1-8.  Back to cited text no. 4
    
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Zouboulis P, Karageorgos A, Dimakopoulos P, Tyllianakis M, Matzaroglou C, Lambiris E. Functional outcome of surgical treatment for multilevel lumbar spinal stenosis. Acta Orthop 2006;77:670-6.  Back to cited text no. 6
    
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Haro H, Maekawa S, Hamada Y. Prospective analysis of clinical evaluation and self-assessment by patients after decompression surgery for degenerative lumbar canal stenosis. Spine J 2008;8:380-4.  Back to cited text no. 7
    
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Sigmundsson FG, Kang XP, Jönsson B, Strömqvist B. Correlation between disability and MRI findings in lumbar spinal stenosis: A prospective study of 109 patients operated on by decompression. Acta Orthop 2011;82:204-10.  Back to cited text no. 8
    
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Yasar B, Simsek S, Er U, Yigitkanli K, Eksioglu E, Altug T, et al. Functional and clinical evaluation for the surgical treatment of degenerative stenosis of the lumbar spinal canal. J Neurosurg Spine 2009;11:347-352  Back to cited text no. 9
    
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Ng LC, Tafazal S, Sell P. The effect of duration of symptoms on standard outcome measures in the surgical treatment of spinal stenosis. Eur Spine J 2007;16:199-206.  Back to cited text no. 10
    
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Barz T, Melloh M, Staub L, Roeder C, Lange J, Smiszek FG, et al. The diagnostic value of a treadmill test in predicting lumbar spinal stenosis. Eur Spine J 2008;17:686-90.  Back to cited text no. 11
    
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Kuittinen P, Sipola P, Saari T, Aalto TJ, Sinikallio S, Savolainen S, et al. Visually assessed severity of lumbar spinal canal stenosis is paradoxically associated with leg pain and objective walking ability. BMC Musculoskelet Disord 2014;15:348.  Back to cited text no. 12
    
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Nath R, Middha S, Gupta AK, Nath R. Functional outcome of surgical management of degenerative lumbar canal stenosis. Indian J Orthop 2012;46:285-90.  Back to cited text no. 13
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Zanoli G, Jönsson B, Strömqvist B. SF-36 scores in degenerative lumbar spine disorders: Analysis of prospective data from 451 patients. Acta Orthop 2006;77:298-306.  Back to cited text no. 14
    
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Deen HG Jr., Zimmerman RS, Lyons MK, McPhee MC, Verheijde JL, Lemens SM. Measurement of exercise tolerance on the treadmill in patients with symptomatic lumbar spinal stenosis: A useful indicator of functional status and surgical outcome. J Neurosurg 1995;83:27-30.  Back to cited text no. 15
    
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Deen HG, Zimmerman RS, Lyons MK, McPhee MC, Verheijde JL, Lemens SM. Use of the exercise treadmill to measure baseline functional status and surgical outcome in patients with severe lumbar spinal stenosis. Spine (Phila Pa 1976) 1998;23:244-8.  Back to cited text no. 16
    
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Amundsen T, Weber H, Nordal HJ, Magnaes B, Abdelnoor M, Lilleâs F. Lumbar spinal stenosis: Conservative or surgical management?: A prospective 10-year study. Spine (Phila Pa 1976) 2000;25:1424-35.  Back to cited text no. 17
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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