Age related changes in the lumbar spine can lead to narrowing of the spinal canal (spinal stenosis) causing leg and back pain. Spinal stenosis can be associated with a misalignment of the spine caused by forward slippage of a vertebrae over another. This instability is diagnosed using diagnostic imaging. With signs of instability the spine surgeon might choose a fusion of the vertebrae. This is a more complex procedure in relation to the simple decompression preformed without instability for spinal stenosis.

The purpose of this study is to identify characteristics of instability of the lumbar spine on diagnostic imaging, and investigate associations with surgical data and patient reported outcomes from the National Spine databases from Denmark and Sweden.

This will support spine surgeons in providing evidence-based surgical treatment for spinal stenosis with or without signs of instability

A multicenter retrospective cohort study of prospectively collected data from national spine surgery databases in Denmark and Sweden. This project is a collaboration between sector for spine surgery and research Middelfart Hospital and spine surgeons at Orebro Hospital and Lund Hospital in Sweden.

Data subtracted from Danish national spine surgery database, DaneSpine, from Middelfart Hospital, and Swedish Spinal Register, SweSpine, from hospitals in Malmö, Lund, Trelleborg and Karlskroga.

Both databases are clinical quality databases with a high response-rate. DaneSpine is administered by Danish Society of Spine Surgeons and is managed by secretary of DaneSpine. SweSpine is administered by Swedish Society of Spinal Surgeons and managed

Patients are invited to register when scheduled for surgery. Patients signs informed consent and receives follow-up questionaires 3 and 6 months, one, two, 5 and 10 years post-surgery. Questionaires are validated PROM (patient reported outcome measures) and PREM (patient reported experience measures).

The surgeon is responsible for registering surgical data, including complications and reoperations.

Data relevant for present study will be applied for at the respective societies and managed and stored in compliance with General Data Protection Regulation.

The EasyWiz software (Medical Insight, Chicago, IL, USA) is used for analysis of all MRIs and radiographs.

All imaging will be analyzed by 2 independent examiners. The interobserver reliability will be calculated.

Missing data such as variables reported as missing or non-reported will not be included in statistical analysis.

Statistics: significance of difference in mean values between groups for continuous normally distributed data unpaired t-test. Categorical variables normally distributed: Chi-squared/Fishers exact test. Multivariable regression analysis. Kappa statistics for interobserver reliability presented in the article.

Inclusion Criteria:

- MRI verified spinal stenosis patients with or with out degenerative spondylolisthesis (> 3mm anterior translation in lumbar MRI), who underwent surgery at L4/L5 level with either decompression alone or decompression with fusion between 2010 and 2017. Included ICD-10 diagnostic groups DM480 and DM431. Must have standing lateral radiographs preoperatively. Completed 2 year follow up.

Exclusion Criteria:

• previous surgery at the same level,
• malignancy or undergoing treatment for malignancy
• scoliosis Cobb angel >20 degrees

Consecutive patients with symptomatic, MRI-verifeis lumbar spinal stenosis referred to Spine Center of Southern Denmark between 2010-2017. And same study population referred to Spine Centers at hospitals in Malmö, Lund, Trelleborg and Karlskroga Sweden between 2010-2017.