Metastatic Spine Disease Alters Vertebral Load-To-Strength Ratios in Cancer Patients Compared to Healthy Individuals.

Purpose: This study investigated the effect of bone metastasis on the biomechanical environment of human vertebrae in patients with metastatic spine disease through the metric of load-to-strength ratio (LSR). Specifically, we compared the patients' LSRs to age and sex-similar noncancer controls from the Framingham Heart Study. Methods: Derived from clinical CT data of 135 metastatic spine disease patients planned for radiotherapy and 246 normative controls from the Framingham Heart Study, individualized spinal musculoskeletal models and vertebral strength estimates were used to compute level-specific LSR under natural standing and three weight-holding conditions (standing + weight, flexion + weight, and lateral bending + weight). Results: Adjusted for age, BMI, and spinal region, osteosclerotic and mixed lesion vertebrae had higher strength than osteolytic and control vertebrae. The musculoskeletal models suggested breast, prostate, and male lung cancer patients had higher compressive vertebral loading, and female lung cancer patients had lower compressive vertebral loading than controls. Male patients had higher standardized LSRs in natural standing, while female patients had lower LSRs for all activities than controls. Independent of sex, vertebrae with osteosclerotic and mixed bone metastasis had lower LSRs than controls, while, for osteolytic bone lesions, males had higher and females lower LSRs than controls. Vertebrae with no observed lesion on CT had higher LSRs than controls in males and lower LSRs in females. Discussion: Our findings highlighted that primary cancer and lesion type differentially affected task-specific vertebral loading and strength, thus modifying the vertebral LSRs. Sex-mediated differences in LSRs between FHS controls and vertebrae with no observed metastatic lesions suggest that considering the latter as "normal" should be taken with care. Our initial assessment supports further examination of whether vertebral LSR measurements are associated with vertebral risk and, if so, what threshold values indicate risk. Level of Evidence: 3.