Trabecular level analysis of bone cement augmentation: a comparative experimental and finite element study

Abstract

The representation of cement-augmented bone in finite element (FE) models of vertebrae following vertebroplasty remains a challenge, and the methods of the model validation are limited. The aim of this study was to create specimen-specific FE models of cement-augmented synthetic bone at the microscopic level, and to develop a new methodology to validate these models. An open cell polyurethane foam was used reduce drying effects and because of its similar structure to osteoporotic trabecular bone. Cylindrical specimens of the foam were augmented with PMMA cement. Each specimen was loaded to three levels of compression inside a micro-computed tomography (μCT) scanner and imaged both before compression and in each of the loaded states. Micro-FE models were generated from the unloaded μCT images and displacements applied to match measurements taken from the images. A morphological comparison between the FE-predicted trabecular deformations and the corresponding experimental measurements was developed to validate the accuracy of the FE model. The predicted deformation was found to be accurate (less than 12% error) in the elastic region. This method can now be used to evaluate real bone and different types of bone cements for different clinical situations.

Figure 1

A cement–augmented specimen inside the compression stage used during μCT imaging

Figure 2

The vertical slices taken from different sagittal positions

Figure 3

Comparison between the FE predicted deformation and the μCT image at: (a) the initial position, (b) Load Increment 1, (c) Load Increment 2, and (d) Load Increment 3. The green region indicates that FE images matches μCT image, while the blue regions represents “false negative” and red region represents “false positive” pixels

Figure 4

Error between FE predicted deformations and actual deformation measured using μCT for the manually and automatically segmented models

Figure 5

Local deformation for a single synthetic bone “trabecula” at the cement–trabeculae interface as predicted in the FE models and as observed in the μCT images at (a) The initial position, (b) Load Increment 1, (c) Load Increment 2, and (d) Load Increment 3

Figure 6

Comparison between the three contact algorithms in terms of (a) the stiffness and (b) the maximum von Mises stress at each load increment. *The case where the friction coefficient = 0.3 was stopped before Load Increment 3 due to computational restrictions

Figure 7

Comparison of the deformation after Load Increment 3 for (a) the frictionless model and (b) the tied contact model. This shows the relative displacement is larger near the interface between the trabecula and cement in the frictionless model than tied contact model