Validating the definition of lumbar instability –a cross-sectional study with 420 healthy volunteers–

Lower back pain is thought to be caused by lumbar instability. To date, multiple definitions of radiological lumbar instability have been used without verifying the “normal range” of the lumbar segmental mobility. In addition, normative data for lumbar mobility in healthy individuals are required to establish an acceptable threshold for lumbar instability. This study aims to elucidate i) the prevalence of so-called radiological lumbar instability at each lumbar spine level in conventional criteria and ii) a practical radiological threshold for lumbar instability in healthy individuals. This study included participants who filled out a detailed questionnaire and underwent standard active dynamic radiography of the lumbar spine in the standing position. Participants with an Oswestry Disability Index of ≥ 20% and Roland–Morris Disability Questionnaire score of ≥ 14 points were excluded from the study. Intervertebral range of motion (IROM) and sagittal translation distance (ΔST) were measured at each intervertebral level. Nachemson’s criteria of radiological lumbar instability were applied. This study enrolled 420 participants (249 males and 171 females); 76% (320/420) met the criteria for radiological lumbar instability. The definition of lumbar instability based on IROM and ΔST was achieved by 0.2% and 1.7% of participants at the L5–S level, respectively.