An Investigation of the Mechanical Implications of Sacroplasty Using Finite Element Models Based on Tomographic Image Data

Sacral insufficiency fractures are an under-diagnosed source of acute lower back pain. A polymethylmethacrylate (PMMA) cement injection procedure called sacroplasty has recently been utilized as a treatment for sacral insufficiency fractures. It is believed that injection of cement reduces fracture micromotion, thus relieving pain. In this study, finite element models were used to examine the mechanical effects of sacroplasty.

Finite element models were constructed from CT images of cadavers on which sacroplasties were performed. The images were used to create the mesh geometry, and to apply non-homogeneous material properties to the models. Models were created with homogeneous and non-homogeneous material properties, normal and osteoporotic bone, and with and without cement.

The results indicate that the sacrum has a 3D multi-axial state of strain. While compressive strains were the largest, tensile and shear strains were significant as well. It was found that a homogeneous model can account for around 80% of the variation in strain seen in a non-homogeneous model. Thus, while homogeneous models provide a reasonable estimate of strains, non-homogeneous material properties have a significant effect in modeling bone. A reduction in bone density simulating osteoporosis increased strains nearly linearly, even with non-homogeneous material properties. Thus, the non-homogeneity was modeled similarly in both density cases. Cement in the sacrum reduced strains 40-60% locally around the cement. However, overall model stiffness only increased 1-4%. This indicates that the effects of sacroplasty are primarily local.