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.
†Read in part at the Annual Meeting of the Orthopaedic Research Society, Anaheim, California, February 3, 1999, and the Summer Bioengineering Conference of the American Society of Mechanical Engineers, Big Sky, Montana, 1999.
‡NuVasive, 10065 Old Grove Road, San Diego, California 92131.
§Orthopedic Biomechanics Laboratory, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, RN 115, Boston, Massachusetts 02215.
#Department of Occupational Therapy, College of Medical Technology, Hokkaido University, 060-0812 Sapporo, Japan.
**Department of Orthopaedic Surgery, Children's Hospital, Hunnewell 2, 300 Longwood Avenue, Boston, Massachusetts 02115. E-mail address: firstname.lastname@example.org.