Symptomatic lumbar spinal stenosis is the most common indication for spinal surgery. However, more than one-third of the patients undergoing surgery for lumbar stenosis report dissatisfaction with the results. On the other hand, conservative treatment has shown positive results in some cases. This trial will compare the outcomes of surgical versus non-surgical treatment for lumbar stenosis.

This is a three-centre randomised controlled trial in which 200 patients with symptomatic lumbar spinal stenosis will be randomised into one of two treatment arms. The patients in the surgical arm will undergo decompression of the neural structures by use of any open or minimally invasive (MIS) technique with or without fusion; the patients in the non-surgical arm will undergo imaging assisted percutaneous interspinous spacer insertion with fusion.

The primary outcome of the study will be the Oswestry Disability Index. Secondary outcomes will include motor amplitude and degree of denervation activity obtained by means of nerve conduction studies and electromyography.

Patient-reported outcome measures will be also used as secondary outcomes.

Inclusion Criteria:

1. Age 50-85 years.
2. Clinical symptoms of lumbar spinal stenosis (pseudoclaudication) indicating and motivating surgery. Numeric Rating Scale in lower limbs ≥3.
3. MRI with finding of lumbar spinal stenosis at 1-3 lumbar levels. Dural sac area ≤75 mm2 or degree of stenosis C or D according to Schizas's classification.34
4. The patient has given oral and written informed consent to participate.

Exclusion Criteria:

1. Degenerative deformity with Cobb angle >20°.
2. Symptomatic osteoarthritis in the lower limbs that affects and limits the patient's function.
3. Arterial insufficiency (claudication intermittent).
4. Former lumbar surgery other than disc hernia.
5. Conditions that affect the spine, such as ankylosing spondylitis, diffuse idiopathic skeletal hyperostosis, spondylodiscitis/infections, malignancy and neurological diseases.
6. Heart and lung diseases presenting a significant risk for surgery or making it impossible for the patient to take part in a percutaneous arm of treatment (American Society of Anesthesiologists (ASA) score >3).
7. Polyneuropathies.
8. Psychological factors rendering the patient incapable of inclusion in the trial (eg, drug addiction, dementia).