Improved estimates of strength and stiffness in pathologic vertebrae with bone metastases using CT-derived bone density compared with radiographic bone lesion quality classification

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  • 1 Center for Advanced Orthopedic Studies, Department of Orthopedic Surgery,
  • | 2 Department of Orthopedics,
  • | 3 Department of Medicine, and
  • | 4 Department of Radiology, Beth Israel Deaconess Medical Center, Boston;
  • | 5 Department of Neurosurgery, Brigham and Women’s Hospital, Boston, Massachusetts;
  • | 6 ARTORG Center for Biomedical Engineering Research, University of Bern;
  • | 7 University of Zurich & MRI Schulthess Clinic, Zurich;
  • | 8 Department of Orthopedic Surgery, Inselspital, Bern University Hospital, Bern; and
  • | 9 Clinique Bois-Cerf, Radiology Department, Lausanne, Switzerland
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OBJECTIVE

The aim of this study was to compare the ability of 1) CT-derived bone lesion quality (classification of vertebral bone metastases [BM]) and 2) computed CT-measured volumetric bone mineral density (vBMD) for evaluating the strength and stiffness of cadaver vertebrae from donors with metastatic spinal disease.

METHODS

Forty-five thoracic and lumbar vertebrae were obtained from cadaver spines of 11 donors with breast, esophageal, kidney, lung, or prostate cancer. Each vertebra was imaged using microCT (21.4 μm), vBMD, and bone volume to total volume were computed, and compressive strength and stiffness experimentally measured. The microCT images were reconstructed at 1-mm voxel size to simulate axial and sagittal clinical CT images. Five expert clinicians blindly classified the images according to bone lesion quality (osteolytic, osteoblastic, mixed, or healthy). Fleiss’ kappa test was used to test agreement among 5 clinical raters for classifying bone lesion quality. Kruskal-Wallis ANOVA was used to test the difference in vertebral strength and stiffness based on bone lesion quality. Multivariable regression analysis was used to test the independent contribution of bone lesion quality, computed vBMD, age, gender, and race for predicting vertebral strength and stiffness.

RESULTS

A low interrater agreement was found for bone lesion quality (κ = 0.19). Although the osteoblastic vertebrae showed significantly higher strength than osteolytic vertebrae (p = 0.0148), the multivariable analysis showed that bone lesion quality explained 19% of the variability in vertebral strength and 13% in vertebral stiffness. The computed vBMD explained 75% of vertebral strength (p < 0.0001) and 48% of stiffness (p < 0.0001) variability. The type of BM affected vBMD-based estimates of vertebral strength, explaining 75% of strength variability in osteoblastic vertebrae (R2 = 0.75, p < 0.0001) but only 41% in vertebrae with mixed bone metastasis (R2 = 0.41, p = 0.0168), and 39% in osteolytic vertebrae (R2 = 0.39, p = 0.0381). For vertebral stiffness, vBMD was only associated with that of osteoblastic vertebrae (R2 = 0.44, p = 0.0024). Age and race inconsistently affected the model’s strength and stiffness predictions.

CONCLUSIONS

Pathologic vertebral fracture occurs when the metastatic lesion degrades vertebral strength, rendering it unable to carry daily loads. This study demonstrated the limitation of qualitative clinical classification of bone lesion quality for predicting pathologic vertebral strength and stiffness. Computed CT-derived vBMD more reliably estimated vertebral strength and stiffness. Replacing the qualitative clinical classification with computed vBMD estimates may improve the prediction of vertebral fracture risk.

ABBREVIATIONS

AICc = Akaike information criterion with small sample correction; BM = bone metastases; BV = bone volume; HA = hydroxyapatite; IQR = interquartile range; LMM = linear mixed-effects model; PVF = pathologic vertebral fracture; SINS = Spine Instability Neoplastic Score; TV = total volume; vBMD = volumetric bone mineral density.

Supplementary Materials

    • Appendix (PDF 1,484 KB)

Illustrations from Hubbe et al. (pp 160–163). Copyright Ioannis Vasilikos and Roberto Ferrarese. Published with permission.

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