Noninvasive imaging predicts failure load of the spine with simulated osteolytic defects

Background: The clinical management of lytic tumors of the spine is current ly based on geometric measurements of the defect. However, the mechanical b ehavior of a structure depends on both ifs material and its geometric prope rties. Quantitative computed tomography and dual-energy x-ray absorptiometr y were investigated as noninvasive tools for measuring the material and geo metric properties of vertebrae with a simulated lytic defect. From these me asures, yield loads were predicted with use of composite beam theory. Methods: Thirty-four fresh-frozen cadaveric spines were segmented into func tional spinal units of three vertebral bodies with two intervertebral discs at the thoracic and lumbar levels. Lytic defects of equal size were create d in one of three locations: the anterior, lateral, or posterior region of the vertebra. Each spinal unit mas scanned with use of computed tomography and dual-energy x-ray absorptiometry, and axial and bending rigidities were calculated from the image data. Each specimen was brought to failure under combined compression and forward flexion, and the axial load and bending m oment at yield were recorded. Results: Although the relative defect size was nearly constant, measured yi eld loads had a large dispersion, suggesting that defect size alone was a p oor predictor of failure. However, image-derived measures of structural rig idity correlated moderately well with measured yield loads. Furthermore, wi th use of composite beam theory with quantitative computed tomography-deriv ed rigidities, vertebral yield loads were predicted on a one-to-one basis ( concordance, r(c) = 0.74). Conclusions: Although current clinical guidelines for predicting fracture r isk are based on geometric measurements of the defect, we have shown that t he relative size of the defect alone does not account for title variation i n vertebral yield loads. However, composite beam theory analysis with quant itative computed tomography-derived measures of rigidity can be used to pro spectively predict the yield loads of vertebrae with lytic defects. Clinica l Relevance: Image-predicted vertebral yield loads and analytical models th at approximate loads applied to the spine during activities of daily living can be used to calculate a factor of fracture risk that can be employed by physicians to plan appropriate treatment or intervention.