The Role of the Size and Location of the Tumors and of the Vertebral Anatomy in Determining the Structural Stability of the Metastatically Involved Spine: a Finite Element Study

Abstract

Vertebral fractures associated with the loss of structural integrity of neoplastic vertebrae are common, and determined to the deterioration of the bone quality in the lesion area. The prediction of the fracture risk in metastatically involved spines can guide in deciding if preventive solutions, such as medical prophylaxis, bracing, or surgery are indicated for the patient. In this study, finite element models of 22 thoracolumbar vertebrae were built based on CT scans of three spines, covering a wide spectrum of possible clinical scenarios in terms of age, bone quality and degenerative features, taking into account the local material properties of bone tissue. Simulations were performed in order to investigate the effect of the size and location of the tumoral lesion, the bone quality and the vertebral level in determining the structural stability of the neoplastic vertebrae. Tumors with random size and positions were added to the models, for a total of 660 simulations in which a compressive load was simulated. Results highlighted the fundamental role of the tumor size, whereas the other parameters had a lower, but non-negligible impact on the axial collapse of the vertebra, the vertebral bulge in the transverse plane and the canal narrowing under the application of the load. All the considered parameters are radiologically measurable, and can therefore be translated in a straightforward way to the clinical practice to support decisions about preventive treatment of metastatic fractures.

Figure 1

Three-dimensional reconstructions (first row) and simulated planar X-rays images (second row) of the three spines used to build the finite element models.

Figure 2

Exemplary finite element models of the same vertebra (L4 of “spine 1”) in which a tumoral lesion has been added (highlighted in red). For each model, the tumoral size (in mm, cm³ and in percentage of the volume of the vertebral body) as well as the coordinates of the centroid (in mm) with respect to the center of the vertebral body are reported. In the model in the top left corner, loading and boundary conditions are shown.

Figure 3

Relative importance of the various features (bone quality, vertebral level, tumor size, the coordinates of the tumor centroid (“tumor pos. X”, “tumor pos. Y” and “tumor pos. Z”) in determining average axial displacement (left), vertebral bulge (center) and LICN (right). In all charts, the importance calculated for the three individual spines as well as for the pooled data of all spines is shown.

Figure 4

Effect of the tumor size in determining average axial displacement (left), vertebral bulge (center) and LICN (right). In all charts, the results of all simulations of the three individual spines as well as regression lines for each spine and for the pooled data of all spines are shown.

Figure 5

Effect of the bone quality (in average Hounsfield units for each vertebra) in determining average axial displacement (left), vertebral bulge (center) and LICN (right). In all charts, the results of all simulations of the three individual spines as well as regression lines for each spine and for the pooled data of all spines are shown.

Figure 6

Effect of the vertebral level in determining average axial displacement (left), vertebral bulge (center) and LICN (right). In all charts, the results of all simulations of the three individual spines as well as regression lines for each spine and for the pooled data of all spines are shown.

Figure 7

Effect of the Z coordinate of the position of the tumor centroid in determining average axial displacement (left), vertebral bulge (center) and LICN (right). In all charts, the results of all simulations of the three individual spines as well as regression lines for each spine and for the pooled data of all spines are shown.