Study population

Data for this cohort study will be collected through the NORspine, which was established in 2006 and is a comprehensive clinical registry for quality control and research. Participation in the registration by either providers or patients is not mandated, nor is participation required as a necessary condition for a patient to gain access to healthcare or for a provider to be eligible for payment for the healthcare service. Patients operated between October 2006 and December 2011 will be screened for study eligibility. Follow-up time from the date of the operation (baseline) in this study is 12 months.

Inclusion criteria

1. Diagnosis of central lumbar spinal stenosis.

2. Operation in ≤2 lumbar levels with either open decompressive laminectomy, bilateral microdecompression or unilateral microdecompression for bilateral decompression in the time period between October 2006 and December 2011.

3. Included in the NORspine registry.

Exclusion criteria

1. History of lumbar fusion.

2. Previous surgery in the lumbar spine.

3. Discectomy as part of the decompression.

4. Associated pathological entities such as disc herniation, spondylolisthesis or scoliosis.

Primary outcome measure

The primary outcome measure is change in functional outcome between baseline and 12-month follow-up measured with V.2.0 of ODI,10 translated into Norwegian and tested for psychometric properties by Grotle et al.11 ODI is one of the principal condition-specific outcome measures used in the management of spinal disorders. It has been extensively tested, has shown good psychometric properties, and is considered applicable in a wide variety of settings. ODI contains 10 questions on limitations of activities of daily living. Each variable is rated on a 0–5-point scale, summarised and converted into a percentage score. Scores range from 0 to 100, with lower score indicating less severe pain and disability.

Secondary outcome measures

Secondary outcome measures are:

1. Changes in HRQL measured with the EQ-5D between baseline and 12-month follow-up;

2. Perioperative complications;

3. Duration of surgical procedures and hospital stays.

EQ-5D is a generic and preference-weighted measure of HRQL.12 The Norwegian version of EQ-5D has shown good psychometric properties.13 EQ-5D evaluates five dimensions: mobility, self-care, activities of daily living, pain, and anxiety and/or depression. For each dimension, the patient describes three possible levels of problems (none, mild-to-moderate and severe). This descriptive system therefore contains 3⁵=243 combinations or index values for health status. EQ-5D has been validated for patient populations similar to that in our study.13 Total score ranges from –0.6 to 1, where 1 corresponds to perfect health and 0 to death. Negative values are considered to be worse than death.

Surgeons provide the following complications and adverse events to the NORspine registry: intraoperative haemorrhage requiring blood replacement, unintentional durotomy, nerve injury, cardiovascular complications, respiratory complications, anaphylactic reactions and wrong level surgery. Patients report the following complications if they occur within 3 months of surgery: wound infection, urinary tract infection, pneumonia, pulmonary embolism and deep vein thrombosis.

Data collection and registration by the NORspine registry protocol

On admission for surgery, patients complete the baseline questionnaire, which includes questions about demographics and lifestyle issues in addition to the outcome measures. Information about marital status, educational level, employment status, body mass index and tobacco smoking is available in the NORspine registry. During the hospital stay, using a standard registration form, the surgeon records data concerning diagnosis, comorbidity, American Society of Anesthesiologists (ASA) grade, duration of symptoms, treatment and image findings. A questionnaire is distributed by regular mail 3 and 12 months after surgery, completed at home by patients, and returned in the same way. Patients who do not respond receive one reminder with a new copy of the questionnaire. Patients complete preoperative questionnaire data and postal follow-up questionnaires without any assistance from the surgeon.

Diagnostic imaging

In the NORspine registry surgeons provide data concerning preoperative diagnostic imaging and the results of these investigations. For patients with available preoperative MRI we review the images and perform a morphological grading of the severity of spinal stenosis as described by Schizas et al.14 This morphological grading from A to D is based on the cerebrospinal fluid/rootlet ratio as seen on axial T2-weighted MRI. The original publication defines four subgroups of grade A. We will not use these subgroups since they all are defined as no or minor stenosis. In the morphological grading A–D, we define grade A as no stenosis, grade B as relative stenosis and grades C and D as significant stenosis. The clinicians who review the preoperative MRI and perform the morphological grading of the severity of spinal stenosis will be blinded with regard to treatment allocation (laminectomy or microdecompression).

Surgical procedures

There is variation in the surgical management of lumbar spinal stenosis, and in the following only a general description is provided for each procedure. When a decompressive laminectomy (group 1) is performed the spinous process and the laminae of the involved level(s) as well as the medial aspects of the facet joints are resected.9 Microdecompression (group 2) can be performed using a bilateral or unilateral approach depending on the surgeon's preference and the individual patient's anatomy and symptoms. Unlike a decompressive laminectomy, the spinous process and the supraspinous and interspinous ligaments are left intact when performing a microdecompression.9 Bilateral microdecompression means resection of the bone from the inferior aspect of the cranial lamina, and, occasionally, from the superior aspect of the subjacent lamina. Resection of the medial aspect of the facet joint is performed to alleviate the lateral recess. Flavectomy is performed to expose the spinal canal. The same procedure is then repeated on the contralateral side. When performing a unilateral microdecompression for bilateral decompression, the spinous process is undercut in addition to the ipsilateral decompression. By angling the microscopic view and occasionally tilting the operating table following ipsilateral decompression, resection of the contralateral ligamentum flavum and the medial aspects of the contralateral facet joints are possible.9

Statistical analyses

This study will use mixed linear models to test the equivalence of the clinical effectiveness of microdecompression and laminectomy. If the population effect of treatment on changes is ≦8, the treatments are considered equal with respect to effectiveness. The minimal clinical important difference for change in the mean ODI score is considered to be in the range of 8–10 points.15–17 Assuming a correlation of 0.5 between baseline and follow-up measurements and an SD of 18 for the individual measurements, the study will have 90% power with 132 patients in each treatment group. The minimal clinical important difference for ODI in patients with lumbar spinal stenosis in the same study population will be analysed in a separate ongoing study. In the analyses of primary and secondary outcome measures adjustments for the number of levels operated (one or two), age, body mass index and preoperative ODI will be made. Supplementary analyses with adjustments for baseline covariates and for the propensity to receive microdecompression will be performed. We plan to conduct subgroup analyses to compare the clinical effectiveness of microdecompression and laminectomy in patients aged ≥70 years. In addition, we plan to conduct subgroup analyses to compare the clinical effectiveness of microdecompression and laminectomy in obese patients (body mass index ≥30). Statistical significance level is defined as p≤0.05 with no adjustments made for multiple comparisons. Baseline and follow-up measurements will be assumed to be normally distributed provided this assumption is confirmed by Q–Q plots. To evaluate the magnitude of change in EQ-5D score, effect sizes will be estimated according to the method of Kazis et al.18 An effect size of 0.8 or more is considered to be large.

Missing data

For the primary outcome (change in ODI between baseline and 12-month follow-up) we will perform a complete case analysis and a full information analysis using mixed linear models. In the complete case analysis for the primary outcome patients with missing ODI data at 12-month follow-up will be excluded. A study on an equivalent patient population showed no difference in outcomes between responders and non-responders.19

Study limitations

The main limitation of this study is that analyses are not based on randomised treatment assignments. However, the results are strengthened by the use of specific inclusion and exclusion criteria, the large sample size and the re-evaluation of the preoperative diagnostic imaging.20 Another potential weakness of the present study is the expected loss to follow-up of approximately 22%, which is relatively high.19 A third possible limitation is the growing tendency towards microdecompression, especially among neurosurgeons, during the study period.