Role of Spinal Load in the Pathophysiology of Lumbar Spinal Stenosis

Disability related to sLSS will be assessed with the ODI, which is considered the gold standard of low back functional outcome tools. The ODI is a questionnaire comprising 10 self-administered items that quantify a patient's perceived level of functional disability. Each of the items relates to a different area of functional impairment and consists of six statements, which are scored from zero to five points.

Dynamic compensation is defined as the difference between static and dynamic sagittal spinal alignment. Six possible gait events are available to choose from for the definition of dynamic sagittal spinal balance (left and right; heel-strike, toe-off, midstance). Dynamic sagittal spinal balance may be defined as sagittal spinal balance during left/right midstance, left/right heel strike and/or left/right toe off. The most appropriate gait event to calculate dynamic sagittal spinal balance will be used.

The curvature of the lumbar and thoracic spine during stance and during walking will be computed from the marker data using MATLAB™ software. A cubic polynomial function will be fitted to the marker positions in each time frame, approximating an S-shaped spine curvature with thoracic kyphosis (TK) and lumbar lordosis (LL) curves. The workflow used to calculate sagittal spinal balance parameters is based on the calculation methods for radiological sagittal spinal balance parameters. The set of marker-based sagittal spinal balance parameters comprises LL, TK, sagittal vertical axis (SVA), spino-sacral angle (SSA), spine inclination (SI). The data obtained using motion capture will be used to estimate spinal load using musculoskeletal modeling.

Sagittal spinal balance parameters from EOS radiological images are calculated semi-automatically using the sterEOS software provided by the EOS company. This workflow allows the calculation of four sagittal spinal balance parameters: SVA, SSA, LL and TK. LL is calculated as the angle between the tangents on the superior L1 and inferior L5 vertebral endplates. TK is calculated as the angle between the tangents on the superior T4 and inferior T12 vertebral endplates. SVA is measured as the horizontal distance between C7 plumb line and the posterior-superior corner of the S1 vertebra. SSA is defined as the angle between the line connecting the center of the C7 vertebra to the center of the S1 endplate and the line parallel to the superior S1 endplate.

Muscle fatigue will be assessed as the decrease in median EMG frequency.

The fatigue exercise duration will be measured using a stopwatch. Time is stopped from the point where the patients starts the exercise by no longer supporting the torso with their hands until the termination of the exercise by supporting the torso.

The measurement of muscle atrophy allows quantifying abdominal and paraspinal muscle degeneration. The analysis will be performed using ImageJ image analysis software (Version 1.52t, National Institutes of Health, Bethesda, Maryland).

The measurement of fatty infiltration allows quantifying abdominal and paraspinal muscle degeneration. The analysis will be performed using ImageJ image analysis software (Version 1.52t, National Institutes of Health, Bethesda, Maryland).

For L1 to L5, the cross-sectional area (CSA) of the abdominal and paraspinal muscles on each side will be measured, including the multifidus and the erector spinae (longissimus and iliocostalis) muscles, and the CSA of the vertebral body. The relative CSA (rCSA) will be defined as the ratio between muscle CSA and vertebral body CSA and calculated for each level and side. The CSA of lean muscle in the region of interest will be defined as LeanCSA and measured on each side. The ratio of LeanCSA to the paraspinal muscle CSA will be defined as functional CSA (LeanCSA/CSA), represented as % muscle CSA and calculated for each level and side. Overall CSA, rCSA and LeanCSA will be computed as average CSA (aCSA), average rCSA (arCSA) and average LeanCSA (aLeanCSA) across all levels considering the muscle as a single unit for each side.

Stenosis severity will be classified according to Schizas (Grade A stenosis is the mildest, with abundant cerebrospinal fluid inside the dural sac. In grade B stenosis, the rootlets occupy the whole of the dural sac, but they can still be individualized. In grade C, no rootlets can be recognized but epidural fat can be visualized posteriorly. In grade D, in addition to no rootlets being recognizable, there is no epidural fat posteriorly).

Segmental instability will be determined as a relative shift in anteroposterior position of two adjacent segments between the upright standing radiograph and the lying MRI of more than 3mm.

The EQ-5D-5L comprises 5 questions/dimensions (mobility, self-care, usual activities, pain, depression) with Likert-scale response and a visual analogue scale where patients are asked to rate their overall health from 0 (the worst health imaginable) to 100 (the best health imaginable). Each dimension allows to choose five levels labelled as (1) 'not /no problems', (2) 'slight problems', (3) 'moderate problems', (4) 'severe problems', and (5) 'unable to' (mobility, self-care, usual activities), 'extreme' (pain/depression), or 'extremely' (anxiety/depression). Using the EQ-5D-5L, a total of 3125 different health states can be described. The final EQ index is a number between 0 and 1, with 0 indicating the poorest possible health (a state as bad as being dead) and 1 indicating the best possible health.

The Swiss Spinal Stenosis Questionnaire is a self-report outcome questionnaire for patients with LSS and assesses the intensity of symptoms, physical function, and patient satisfaction following treatment.

The Tampa scale is a 17-item self-report questionnaire based on the assessment of fear of exercise, fear of physical activity, and fear avoidance. It consists of two subscales. While activity avoidance revolves around the reflection of activities that may increase pain or cause injury, somatic focus investigates the reflection of beliefs and underlying serious conditions.

The COMI is a validated 11-item self-report questionnaire for assessing the main outcomes of importance to patients back problems.

During all tasks, inertial sensor data will be collected. Inertial sensors (Blue Trident IMU, Vicon, Oxford, UK) will be placed on the subjects' femora, thoracic and lumbar spine and on the sternum. Accelerations and angular velocity will be captured for all movement assessments.

Selected muscles of the legs will be assessed according to Janda's M5/5 levels of strength. In a sitting position: Hip flexion / M. iliopsoas (L2), Knee extension/ M. quadriceps femoris (L3). In a supine lying position: Foot extension (lift up) / M. tibialis anterior (L4), Lifting the big toe/ M extensor hallucis longus (L5). Foot extension (push down)/ M. gastrocnemius (S1).

Physical activity (PA) level will be assessed using an activity monitor (GENEActiv, Activinsights, Kimbolton, UK) during three periods of time (9 days each).

The Back performance scale is an assessment consisting of a series of five movement activities that require mobility of the trunk. The activities (sock test, pick-up test, roll-up test, fingertip-to-floor test, and lift test) all require sagittal-plane mobility and are scored from 0 (can easily do) to 3 (hard/limited to do.