Ageing and BMI in Focus: Rethinking Risk Assessment for Vertebral Fragility and Pedicle Screw Loosening in Older Adults.

: Pathological vertebral fragility (path-VF) increases the risk of osteoporotic fractures and pedicle screw loosening (PSL) after posterior instrumented spinal fusion (PISF). While WHO body mass index (BMI) categories broadly identify risks related to underweight and obesity, fixed thresholds may inadequately reflect vertebral fragility risks among elderly patients, especially within the normal-weight range. This study investigates whether current BMI classifications sufficiently capture the risk of path-VF in older adults. : This retrospective study included 225 patients who underwent kyphoplasty or PISF (2022-2023). Path-VF was defined by non-tumorous fractures, screw reinforcement, or PSL within six months without prior reinforcement. Patients were grouped into the path-VF (n = 94) and control (n = 131) groups. HU and BMI values, BMI-related ORs, and age trends were analysed, and a logistic regression was performed. : Mean HU values were significantly lower in the path-VF group (71.37 ± 30.50) than in controls (130.35 ± 52.53, < 0.001). Path-VF females (26.26 ± 5.38) had a lower BMI than the control females (29.33 ± 5.98, = 0.002); no difference was found in males. Normal-weight females showed a borderline risk for path-VF (OR 2.03, = 0.0495). Obesity (OR 0.31/OR 0.37) and being male and overweight (OR 0.21) were protective (all < 0.05). BMI declined with age in path-VF males ( = 0.001) but increased in the controls ( = 0.023). A logistic regression identified a BMI < 22.5 kg/m and age > 67.5 years as significant risk thresholds. Notably, 20.2% of path-VF patients over 67.5 had a normal weight, suggesting a potentially overlooked subgroup. : The current WHO lower limit for normal BMI (18.5 kg/m) may underestimate the risk of path-VF in patients older than 67.5 years, potentially overlooking 24.7% of cases. The results offer a new approach for clinicians to interpret BMI values at the lower end of the normal range (<22.5 kg/m) with caution in elderly patients undergoing spinal surgery.