Active chin-to-floor distance is a predictive factor for the onset of chronic low back pain

The purpose of this population-based prospective cohort study was to investigate whether the chin-to-floor distance (CFD), measured in the prone position, can predict the 1-year onset of chronic low back pain (CLBP) in community-dwelling adults. A total of 1,920 adults (mean age 66.2 years; 62.1% female) without CLBP at baseline were selected from 3,358 participants in the Locomotive Syndrome and Health Outcomes in Aizu Cohort who underwent health examinations in 2008. The primary outcome was incident CLBP, defined as low back pain persisting for at least three months during the 1-year follow-up. CFD was defined as the vertical distance from the mandibular tip to the floor during maximal spinal extension in the prone position. Multivariable logistic regression was used to evaluate the association between CFD and CLBP onset after adjusting for age, sex, body mass index, smoking status, and lumbar flexion assessed by finger-to-floor distance. Receiver operating characteristic (ROC) analysis was performed to assess the predictive performance of CFD, and optimal cutoff values were determined using the Youden Index. During the 1-year follow-up, 238 participants (12.4%) developed CLBP. Mean CFD was significantly lower in participants who developed CLBP compared with those who did not (12.6 cm vs. 16.5 cm, p < 0.001). In multivariable analysis, each 1 cm decrease in CFD was associated with an 8% increase in the risk of CLBP onset (odds ratio 1.08, 95% confidence interval 1.06-1.11). The area under the ROC curve for CFD was 0.64 (95% CI 0.60-0.68). The optimal cutoff value was 13.0 cm, yielding a sensitivity of 60.1% and specificity of 63.7%. CFD values < 5.0 cm and > 24.5 cm demonstrated high specificity and sensitivity, respectively. Lower CFD values were modestly associated with an increased risk of developing CLBP. Extremely low CFD values may identify individuals at particularly high risk, while a threshold around 24.5 cm may serve as a practical screening or rehabilitation target.