This study was approved by the local ethics committee of Charité – Universitätsmedizin Berlin (EA4/011/10). Written informed consent was obtained from all study participants. The presented study is the result of a prospective cross-sectional study to characterize the changes in biomechanics, psychosocial risk factors as well as structural abnormalities and their interaction in cLBP. The study has been pre-registered at the German national trial registry (DRKS00027907XXX). Furthermore, the study was conducted in accordance to the STROBE guidelines for observational studies as well as in accordance with the declaration of Helsinki.
We included participants the minimum age of 18 and the maximum age of 65 years were included in this study. Knowledge of the German language was needed to be eligible. Participants were excluded if they suffered from neurological movement disorders, acute radiculopathy, systemic diseases requiring medication, or malignant diseases. Furthermore, a BMI > 28 kg/m was an exclusion criterion due to the usage of non-invasive measurement devices for spinal shape, which show an excellent agreement below this threshold but insufficient agreement in overweight persons. The recruitment of study participants employed a multifaceted approach to ensure comprehensive outreach and enrollment. Initial efforts involved collaboration with local healthcare providers and clinics specializing in spine disorders, where potential participants were identified based on diagnostic criteria and suitability for the study. Additionally, partnerships were established with local companies, leveraging their networks to reach individuals affected by LBP within the working-age population. A strategic media campaign was conducted to enhance visibility, utilizing press releases, television appearances, and targeted advertising to raise awareness about the study and encourage participation. Flyer distribution in community centers, health fairs, and medical facilities supplemented these efforts, providing accessible information to interested individuals. Furthermore, word-of-mouth referrals played a crucial role, with participants encouraged to share study details with peers and acquaintances. Throughout the recruitment process, stringent ethical guidelines were followed, including informed consent procedures and protection of participant confidentiality. In order to reduce potential bias, we formulated clear inclusion and exclusion criteria, ensured recruitment from diverse sources, especially socio-economically and geographically, to avoid location-based bias. Furthermore, cultural and language sensitivity in the recruitment process were strictly adhered to in order to ensure that participants thoroughly understand the questionnaires and clinical examination.
Participants that did not report any back pain during the measurements were characterized as asymptomatic. Participants who had suffered from back pain of the lumbar spine on nearly every day for at least 12 weeks were considered participants with cLBP and participants who had suffered from LBP for less than 12 weeks and/or in the form of intermitting pain periods were considered as participants with ncLBP.
Participants were asked to answer the following standardized and validated participant-reported outcome measurements (PROMs). Furthermore, participants underwent clinical examination by a resident of orthopaedic surgery with several years of clinical experience. The standardized examination included evaluation of acute symptoms and medical history as well as thorough analysis of back, hip and hamstring mobility. Lastly, participants back shape and function was analyzed by a health professional using a non-invasive device.
During the clinical examination, all study participants were asked to rate their back shape function on a scale of 1-10. 1 describes no subjective restriction of back function, while 10 represents severe restriction of spinal function.
The Ott test is used to assess the mobility of the thoracic spine. With the participant in upright standing position, the examiner marks the spinous process of the seventh cervical vertebra and a point 30 cm below. The patient is then asked to bend forward as far as possible while the examiner measures the increase in distance between the two marked points.
The Schober test is used to measure the flexion of the lumbar spine. During the test, the patient stands with their feet shoulder-width apart and the examiner marks the skin in between the posterior superior iliac spine, which corresponds fairly to S1 and a point 10 cm above. The patient is then asked to bend forward as far as possible while the examiner measures the distance between the two marked points.
The FFD test is used to assess the flexibility of the spino-pelvic mobility as well as the hamstrings. During the test, the patient stands with their feet together and reaches forward with their hands towards the floor. The examiner measures the distance between the fingertips and the floor.
The STS test is a clinical tool used to assess lower extremity strength. During the test, the patient sits on a chair and is asked to stand up and sit down as many times as possible in 30 s.
The Idiag M360® (MediMouse, Idiag AG, Fehraltorf, Switzerland) was used to evaluate back shape and function. The device is rolled from the spinous process of the seventh cervical vertebra along the spinous processes to the anal fold. For reproducible measurements, the spinous process of the seventh cervical vertebra and a reference point 2 cm below the junction of the left and right posterior superior iliac spines are marked. The validity and reliability have been established in previous studies. Measurements were performed in upright standing as well as in left and right lateral bending, flexion, and extension. Each posture was measured three times.
To evaluate the association between the SRI(independent variable), back shape and function parameters (dependent variables, i.e., FFD, Ott, Schober, STS, thoracic Cobb, thoracic bending to the left and right, lumbar cobb, lumbar bending to the left and right, thoracic kyphosis, thoracic extension and flexion, lumbar lordosis, lumbar extension and flexion), we used two separate linear regression models for each dependent variable. Both models included a nominal covariate for the respective groups (asymptomatic, cLBP, ncLBP) adjusted for age, sex (male, female) and body mass index (BMI). The analysis was divided into two models. Model A evaluated the main effects between the back pain groups and SA. Model B analyzed and evaluated the interaction between the group and SA. Furthermore, Model B tested different associations between SA and back functioning parameters. Model Parameters of Model B were only reported if at least small interaction effects of differential association between SA and functional or shape parameters were present.
The estimated mean differences between asymptomatic participants and the respective pain groups are reported together with 95% confidence intervals (CIs) and partial η ‘s as a measure of effect size. A partial η < 0.01 was considered a very small effect, a partial 0.01 ≤ η < 0.06 a small effect, a partial 0.06 ≤ η < 0.14 a moderate effect, and a partial η ≥ 0.14 a large effect. The interpretation of effect sizes was based on scientific evidence. Furthermore, p-values were examined for demographics and both linear regression models. A two-sided-significance level of 0.05 was used. Yet, the interpretation of p-values has to be performed with caution, as p-values are no effect sizes, are dependent on sampe size. Additionaly interpretation of p-values is difficult in situation of multiple testing without any adjustment for multiple test. Additionaly interpretation of p-values is difficult in the situation of multiple testing without any adjustment for multiple testing. R version 4.3.1 and SPSS version 27.0 were used for data analysis.
