Post-euthanasia micro-computed tomography-based strain analysis is able to represent quasi-static in vivo behavior of whole vertebrae.

Three-dimensional image-based strain measurement in whole bones allows representation of physiological, albeit quasi-static, loading conditions. However, such work to date has been limited to specimens postmortem. The main purpose of this study is to verify the efficacy of deformable image registration of post-euthanasia strain to characterize the in vivo mechanical behavior of rat vertebrae. A micro-computed tomography-compatible custom loading device was used to apply 75 N load to a three-level caudal motion segment of a healthy rat. Loaded and unloaded micro-computed tomography scans were acquired in vivo and post-sacrifice. A micro-computed tomography-based deformable image registration algorithm was used to calculate vertebral strains live and post-euthanasia. No significant difference was found in the in vivo strains (-0.011 ± 0.001) and ex vivo strains (-0.012 ± 0.001) obtained from the comparisons of loaded and unloaded images (p = 0.3). Comparisons between unloaded-unloaded and loaded-loaded scans yielded significantly lower axial strains, representing the error of the method. Qualitatively, high strains were observed adjacent to growth plate regions in evaluating the loaded-unloaded images. Strain patterns in the loaded-loaded and unloaded-unloaded scans were inconsistent as would be expected in representing noise. Overall, live and dead loaded to unloaded comparisons yielded similar strain patterns and magnitudes. Point-wise differences in axial strain fields also supported this observation. This study demonstrated a proof of concept, suggesting that post-euthanasia micro-computed tomography-based strain analysis is able to represent the in vivo quasi-static behavior of rat tail vertebrae.

Can a partial volume edge effect reduction algorithm improve the repeatability of subject-specific finite element models of femurs obtained from CT data?

The reliability of patient-specific finite element (FE) modelling is dependent on the ability to provide repeatable analyses. Differences of inter-operator generated grids can produce variability in strain and stress readings at a desired location, which are magnified at the surface of the model as a result of the partial volume edge effects (PVEEs). In this study, a new approach is introduced based on an in-house developed algorithm which adjusts the location of the model's surface nodes to a consistent predefined threshold Hounsfield unit value. Three cadaveric human femora specimens were CT scanned, and surface models were created after a semi-automatic segmentation by three different experienced operators. A FE analysis was conducted for each model, with and without applying the surface-adjustment algorithm (a total of 18 models), implementing identical boundary conditions. Maximum principal strain and stress and spatial coordinates were probed at six equivalent surface nodes from the six generated models for each of the three specimens at locations commonly utilised for experimental strain guage measurement validation. A Wilcoxon signed-ranks test was conducted to determine inter-operator variability and the impact of the PVEE-adjustment algorithm. The average inter-operator difference in stress values was significantly reduced after applying the adjustment algorithm (before: 3.32 ± 4.35 MPa, after: 1.47 ± 1.77 MPa, p = 0.025). Strain values were found to be less sensitive to inter-operative variability (p = 0.286). In summary, the new approach as presented in this study may provide a means to improve the repeatability of subject-specific FE models of bone obtained from CT data.

µFEA successfully exhibits higher stresses and strains in microdamaged regions of whole vertebrae.

Micro-finite element (µFE) modeling has shown promise in evaluating the structural integrity of trabecular bone. Histologic microcrack analyses have been compared to µFE models of trabecular bone cores to demonstrate the potential of this technique. To date this has not been achieved in whole bone structures, and comparisons of histologic microcrack and µFE results have been limited due to challenges in alignment of 2D sections with 3D data sets. The goal of this study was to ascertain if image registration can facilitate determination of a relationship between stresses and strains generated from µFE models of whole vertebrae and histologically identified microdamage. µFE models of three whole vertebrae, stained sequentially with calcein and fuchsin, were generated with accurate integration of element sets representing the histologic sections based on volumetric image registration. Displacement boundary conditions were applied to the µFE models based on registration of loaded and unloaded µCT images. Histologically labeled damaged regions were found to have significantly higher von Mises stresses and principle strains in the µFE models, as compared to undamaged regions. This work provides a new robust method for generating and histologically validating µFE models of whole bones that can represent trabecular damage resulting from complex physiologic loading.