Background: The clinical management of lytic
tumors of the spine is currently based on geometric measurements
of the defect. However, the mechanical behavior of a structure depends
on both its material and its geometric properties. Quantitative
computed tomography and dual-energy x-ray absorptiometry were investigated
as noninvasive tools for measuring the material and geometric properties of
vertebrae with a simulated lytic defect. From these measures, yield
loads were predicted with use of composite beam theory.
Methods: Thirty-four fresh-frozen cadaveric
spines were segmented into functional spinal units of three vertebral
bodies with two intervertebral discs at the thoracic and lumbar
levels. Lytic defects of equal size were created in one of three
locations: the anterior, lateral, or posterior region of the vertebra. Each
spinal unit was 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
moment at yield were recorded.
Results: Although the relative defect size was
nearly constant, measured yield loads had a large dispersion, suggesting
that defect size alone was a poor predictor of failure. However,
image-derived measures of structural rigidity correlated moderately well
with measured yield loads. Furthermore, with use of composite beam
theory with quantitative computed tomography-derived rigidities,
vertebral yield loads were predicted on a one-to-one basis (concordance,
rc = 0.74).
Conclusions: Although current clinical guidelines
for predicting fracture risk are based on geometric measurements
of the defect, we have shown that the relative size of the defect
alone does not account for the variation in vertebral yield loads.
However, composite beam theory analysis with quantitative computed
tomography-derived measures of rigidity can be used to prospectively
predict the yield loads of vertebrae with lytic defects.
Clinical Relevance: Image-predicted vertebral
yield loads and analytical models that 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.