This prospective single center study was approved by the Ethics Committee of Kuopio University Hospital, and the patients provided written informed consent. This study consisted of 102 LSS patients diagnosed by a treating surgeon who reviewed clinical and imaging findings (MRI). Out of the 102 patients, only 14 with distinct only lateral stenosis were included. None of the patients had central canal stenosis. Accordingly, 140 roots of the 14 patients (mean age 58, range 48 - 76 years, male 43%) were assessed. Initial radiological evaluation was made by a neuroradiologist with 15 years of experience (TS). Selection for surgery was made by an orthopedic surgeon or neurosurgeon in Kuopio University Hospital, Kuopio, Finland. Potential subjects who were not offered surgery were not included.
The exclusion criteria were: emergency or urgent spinal surgery precluding recruitment and protocol investigations; cognitive impairment prohibiting completion of the questionnaires or other failures in co-operation, and the presence of metallic objects in the body that prevented magnetic resonance imaging. A coexisting disc herniation was not an exclusion criterion, but the primary diagnosis of the study patients had to be LLSCS.
The inclusion criteria were: 1) the presence of severe back, buttock, and/or lower extremity pain and/or neurogenic claudication with radiographic evidence (magnetic resonance imaging) exiting nerve roots by degenerative changes (ligamentum flavum, facet joints, osteophytes, and/or disc material), and 2) the surgeon’s judgment in clinical and radiological evaluation that the patient had degenerative LSSCS with symptoms that could be relieved by operative treatment. In addition, all patients had a history of ineffective response to conservative treatment over three months. Patients with only back pain were not included.
Magnetic resonance imaging
MR imaging of the lumbar spine was performed with a 1.5 T imager (Vision; Siemens Medical Solutions, Erlangen, Germany) and a dedicated receive-only spine coil. All patients were imaged prospectively with the same study protocol for study purposes. The imaging protocol conformed to the requirements of the American College of Radiology for the performance of MRI of the adult spine
. The following sequences were used: (a) sagittal T1-weighted spin-echo (repetition time/echo time (TR/TE) 600/12 ms; flip angle, 150°; 4 mm sections; intersection gap, 0.4 mm; field of view (FOV), 290 mm; rectangular FOV, 80%; three signals acquired per data line; matrix 288 × 512), (b) sagittal T2-weighted fast spin-echo (3500/120; flip angle, 180°; echo train length of five; 4 mm sections; intersection gap, 0.4 mm; FOV 290 mm; rectangular FOV, 63%; two signals acquired; matrix 180 × 512), (c) transverse T1-weighted spin-echo (700/15; flip angle, 90°; 4 mm sections; intersection gap, 0.4 mm; FOV, 250 mm; rectangular FOV, 80%; two signals acquired per data line; matrix 288 × 512), and (d) transverse T2-weighted fast spin-echo (5000/120; flip angle, 180°; echo train length of 15; 4 mm sections; intersection gap, 0.4 mm; FOV, 250 mm; rectangular FOV, 100%; three signals acquired per data line; matrix 330 × 512).
The entire lumbar spine was studied on the sagittal images (T12-S1) including parasagittal imaging of all the neural foraminae bilaterally. Transverse images were obtained from the inferior aspect of L1 to the inferior aspect of S1, and the orientation of the sections was planned parallel to the major axis of each disc. In all sequences, a saturation band was placed over the abdominal vessels.
Image analysis was performed as previously described in detail
. Briefly, the lateral canal of the lumbar spine was divided into subarticular (entrance) and foraminal (mid) zones. The subarticular zone (lateral recess) was the most cephalad part of the lateral lumbar canal and located medial to or underneath the superior articular process. The foraminal zone was located below the pedicle. Each subarticular zone and foraminal zone was evaluated separately, bilaterally. The observer was blinded to the clinical and radiological reports and to the findings of any prior clinical examinations. The observer was, however, aware that all study subjects were symptomatic. Visual assessment and quantitative measurements were performed on an IDS5 diagnostic workstation (version 10.2P4; Sectra Imtec, Linköping, Sweden) using highly magnified images on 1024 × 768 and 1600 × 1200 displays.
In visual analysis, the grading system classified the lumbar nerve root canals into three grades: 0 = normal, 1 = narrowing without root compression and 2 = nerve root compression. In quantitative analysis, the minimal width of the subarticular (entrance) zone (lateral recess) and the cross-sectional (mm2) area of the foraminal zone (mid zone area) were measured. At the foraminal zone, no space below the line parallel to the lower end plate was included in area measurements as previously described in detail
. Repeatability of assessments has been previously studied and shown to vary from moderate to substantial
Assessment of preoperative symptoms and functional disability
The overall current low back and leg pain intensity was assessed by a self-administered Visual Analogue Scale (VAS) (range 0-100 mm) in a sitting position during study visits. This has been proved to be a valid index of experimental, clinical and chronic pain
. Back pain at rest (during the previous week) and leg pain when walking (during the previous week) were measured separately with a numeric rating scale 0-10 (NRS-11)
. The questions about pain were anchored on the left (0) with the phrase "No pain" and on the right (10) with the phrase "intolerable pain".
Subjective disability was measured by the validated Finnish version of the Oswestry Disability Index, where 0% represents no disability and 100% extreme debilitating disability
Depression was assessed with the Finnish version of the 21-item Beck Depression Inventory (BDI) with scores ranging from 0 to 63
The treadmill test was supervised by a physiotherapist. The patient was asked to keep a straight, upright position during walking (with a zero degree ramp). The starting speed was 0.67 m/s for the first 10 min (400 m), then 1 m/s for the next 10 min (600 m); maximum results were thus 1000 m in 20 min. If the patient was not able to start with a speed of 0.67 m/s, another test with a starting speed of 0.5 m/s was applied. Thus the walking distance scale was 0-1000 m.
Lumbar paraspinal and lower limb needle EMG were recorded pre-operatively by a neurophysiologist (SM or AP) who was blinded to the radiological data and clinical assessment. The EMG investigation included bilateral paraspinal muscles innervated by the L2-L5 posterior primary rami and a symptomatic lower limb muscles (roots L3 - S1). Examination of the paraspinal roots was performed using a monopolar needle electrode (Medtronic, 50 × 0.40 mm) and examination of the lower limb muscles using a concentric (Neuroline, 38 × 0.45 mm) needle electrode. Amplification was set at 50 μV/div, and the high and low-pass filters at 10 kHz and 20 Hz, respectively.
The aim of the needle examination was to detect the abnormal spontaneous activity associated with axonal damage (fibrillation and positive sharp waves)
. Our EMG data was scaled in the following way: 0 = none or no reproducible spontaneous activity, 1 = rare or occasional (two or more) trains of fibrillation potentials, 2 = frequent spontaneous potentials recordable at more than one depth, 3 = abundant spontaneous activity nearly filling the screen. Categories 1-3 were considered abnormal.
During paraspinal EMG, the patient would lie in the prone position supported by pillows underneath the abdomen. The L3-4 interspinal space was determined by first locating the interspinal space at the level corresponding to the iliac crest, then identifying the L3, L4, L5 and S1 spinal processes by palpation. At least 20 insertions were analyzed from each multifidus muscle. The aim of the examination was to detect the abnormal spontaneous activity suggesting lower motor neuron disorder, and thus serving as a sign of denervation. Abnormal activity indicating denervation was considered to be fibrillation potentials, positive sharp waves, and complex repetitive discharges.
Lower limb needle EMG was performed with the patient lying in the supine position, considering assessment of m. vastus lateralis, tibialis anterior, extensor hallucis longus and gastrocnemius (roots L3 -S1).
The recording of tibial H-waves and peroneal F-waves was performed with Keypoint EMG equipment (Skovlunde, Denmark), using the H-wave and F-wave programs (20 successive samples, 1 stimulus per second, stimulus duration 0.2 ms, high pass filter 20 Hz, low pass filter 10 kHz, sensitivity 0.2 mV/div, sweep 10 ms/div). The stimulus intensity for F-responses was adjusted to obtain supramaximal M-response amplitude. The F-responses were recorded using a surface electrode placed over the middle of the short toe extensor muscle. The reference electrode was placed over the first metatarsal bone on the dorsal surface of the foot. The stimulus intensity for H-responses was adjusted to get repeatable responses with identical latency. The stimulus site was the popliteal fossa and the H-responses were recorded by surface electrodes over the gastrocnemius muscle. The latencies of the minimum F-response (Fmin) and H-response were determined. The recorded values were compared to the normal material of the laboratory (120 normal values) taking the height of the subject into account. The deviation from the normal value was expressed as standard deviation (SD). SD values higher than 2.5 SD were considered abnormal.
Nerve root level specific (L2 – S1) EMG was abnormal if abnormal spontaneous activity associated with axonal damage (fibrillation and positive sharp waves) was found in the limbs or paraspinals. Specific nerve root involvement was defined by that nerve root innervated paraspinal muscles (roots L2 – L5) or lower limb muscles (vastus lateralis, tibialis anterior, extensor hallucis longus and gastrocnemius) (roots L3 – S1). The EMG findings were classified root by root as 1) normal (92/140 roots) or 2) abnormal (active paraspinal and/or limb lesion (48/140).
In statistical analyses, MRI and EMG were analyzed root by root. Associations between MRI findings with VAS, BDI, walking capacity and EMG were analyzed using Chi-squared tests, Spearman and Pearson correlation coefficients, t-tests, and when no assumption of normal distribution could be made, non-parametric tests were used. Statistical analysis was performed using SPSS for Windows (version 19.0; SPSS, IBM, Chicago IL, USA). Statistical significance was set at the P < 0.05 level.