Article

A patient-specific finite element methodology to predict damage accumulation in vertebral bodies under axial compression, sagittal flexion and combined loads.

Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Vienna, Austria.
Computer Methods in Biomechanics and Biomedical Engineering (Impact Factor: 1.79). 08/2008; 11(5):477-87. DOI: 10.1080/10255840802078022
Source: PubMed

ABSTRACT Due to the inherent limitations of DXA, assessment of the biomechanical properties of vertebral bodies relies increasingly on CT-based finite element (FE) models, but these often use simplistic material behaviour and/or single loading cases. In this study, we applied a novel constitutive law for bone elasticity, plasticity and damage to FE models created from coarsened pQCT images of human vertebrae, and compared vertebral stiffness, strength and damage accumulation for axial compression, anterior flexion and a combination of these two cases. FE axial stiffness and strength correlated with experiments and were linearly related to flexion properties. In all loading modes, damage localised preferentially in the trabecular compartment. Damage for the combined loading was higher than cumulated damage produced by individual compression and flexion. In conclusion, this FE method predicts stiffness and strength of vertebral bodies from CT images with clinical resolution and provides insight into damage accumulation in various loading modes.

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    • "Inability to include effects of structural anisotropy, as is routinely accounted for in micro-FE models of trabecular bone by considering the fabric tensor, thus compromises the predictive strength of continuum-based FE models developed from clinical resolution QCT data [9]. Conversely, it can be logically expected that, incorporating direction-specific mechanical properties will improve the predictive ability of continuum-based FE models. "
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    ABSTRACT: Most studies investigating human lumbar vertebral trabecular bone (HVTB) mechanical property- density relationships have presented results for the superior-inferior (SI), or "on-axis" direction. Equivalent, directly measured data from mechanical testing in the transverse (TR) direction are sparse and quantitative computed tomography (QCT) density-dependent variations in the anisotropy ratio of HVTB have not been adequately studied. The current study aimed to investigate the dependence of HVTB mechanical anisotropy ratio on QCT density by quantifying the empirical relationships between QCT-based apparent density of HVTB and its apparent compressive mechanical properties - elastic modulus (Eapp), yield strength (?y), and yield strain (?y) - in the SI and TR directions for future clinical QCT-based continuum finite element modeling of HVTB. A total of 51 cylindrical cores (33 axial and 18 transverse) were extracted from four L1 human lumbar cadaveric vertebrae. Intact vertebrae were scanned in a clinical resolution computed tomography (CT) scanner prior to specimen extraction to obtain QCT density, ?CT. Additionally, physically measured apparent density, computed as ash weight over wet, bulk volume, ?app, showed significant correlation with ?CT [?CT=1.0568x?app, r = 0.86]. Specimens were compression tested at room temperature using the Zetos Bone Loading and Bioreactor system. Apparent elastic modulus modulus (Eapp) and yield strength (?y) were linearly related to the ?CT in the axial direction [ESI = 1493.8x(?CT), r = 0.77, p < 0.01; ?YSI = 6.9x(?CT) - 0.13, r =0.76, p < 0.01], while a power law relation provided the best fit in the transverse direction [ETR = 3349.1x(?CT)1.94, r = 0.89, p < 0.01; ?YTR = 18.81x(?CT)1.83, r = 0.83, p < 0.01]. No significant correlation was found between ?y and ?CT in either direction. Eapp and ?y in the axial direction were larger compared to the transverse direction by a factor of 3.2 and 2.3 respectively, on average. Furthermore, the degree of anisotropy decreased with increasing density. Comparatively, ?y exhibited only a mild, but statistically significant anisotropy: transverse strains were larger than those in the axial direction by 30%, on average. Ability to map apparent mechanical properties in the transverse and axial directions from CT-based densitometric measures allows incorporation of transverse properties in FE models based on clinical CT data, partially offsetting the inability of continuum models to accurately represent trabecular architectural variations.
    Journal of Biomechanical Engineering 05/2014; 136(9). DOI:10.1115/1.4027663 · 1.75 Impact Factor
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    • "A second limitation pertains to the damage that does not take in account a pre-existing micro-damage. Finally, we only tested rigid uniaxial compression although sophisticated loading cases were tested (lateral bending, flexion-extension, combined load cases) by Chevalier et al. (2008). However, Chevalier and Zysset (2012) showed that all major stiffnesses correlated well with the axial stiffness. "
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    ABSTRACT: Computer tomography (CT)-based finite element (FE) models of vertebral bodies assess fracture load in vitro better than dual energy X-ray absorptiometry, but boundary conditions affect stress distribution under the endplates that may influence ultimate load and damage localisation under post-yield strains. Therefore, HRpQCT-based homogenised FE models of 12 vertebral bodies were subjected to axial compression with two distinct boundary conditions: embedding in polymethylmethalcrylate (PMMA) and bonding to a healthy intervertebral disc (IVD) with distinct hyperelastic properties for nucleus and annulus. Bone volume fraction and fabric assessed from HRpQCT data were used to determine the elastic, plastic and damage behaviour of bone. Ultimate forces obtained with PMMA were 22% higher than with IVD but correlated highly (R^2 = 0.99). At ultimate force, distinct fractions of damage were computed in the endplates (PMMA: 6%, IVD: 70%), cortex and trabecular sub-regions, which confirms previous observations that in contrast to PMMA embedding, failure initiated underneath the nuclei in healthy IVDs. In conclusion, axial loading of vertebral bodies via PMMA embedding versus healthy IVD overestimates ultimate load and leads to distinct damage localisation and failure pattern.
    Computer Methods in Biomechanics and Biomedical Engineering 12/2012; DOI:10.1080/10255842.2012.744400 · 1.79 Impact Factor
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    • "Two sets of data were created: embedding in PMMA by adding voxels layers on the endplates 85 (Chevalier et al. 2008; Crawford et al. 2003) or cropping superior and inferior voxel layers of the 86 vertebrae to remove cortical endplates (Dall&apos;Ara et al. 2010). "
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    ABSTRACT: Every year, 500,000 osteoporotic vertebral compression fractures occur in Europe. Quantitative computed tomography (QCT)-based finite element (FE) voxel models predict ultimate force whether they simulate vertebral bodies embedded in polymethylmethacrylate (PMMA) or vertebral sections with both endplates removed. To assess the effect of endplate removal in those predictions, non-linear FE analyses of QCT-based voxel models of human vertebral bodies were performed. High resolution pQCT images of 11 human lumbar vertebrae without posterior elements were coarsened to clinical resolution and bone volume fraction was used to determine the elastic, plastic and damage behavior of bone tissue. Three model boundary conditions (BCs) were chosen: the endplates were cropped (BC1, BC2) or voxel layers were added on the intact vertebrae to mimic embedding (BC3). For BC1 and BC3, the bottom nodes were fully constrained and the top nodes were constrained transversely while both node sets were freed transversely for BC2. Axial displacement was prescribed to the top nodes. In each model, we compared ultimate force and damage distribution during post-yield loading. The results showed that ultimate forces obtained with BC3 correlated perfectly with those computed with BC1 (R^2=0.9988) and BC2 (R^2=0.9987), but were in average 3.4% lower and 6% higher respectively. Moreover, good correlation of damage distribution calculated for BC3 was found with those of BC1 (R^2=0.92) and BC2 (R^2=0.73). This study demonstrated that voxel models of vertebral sections provide the same ultimate forces and damage distributions as embedded vertebral bodies, but with less preprocessing and computing time required.
    Journal of Biomechanics 04/2012; 45(9):1733-8. DOI:10.1016/j.jbiomech.2012.03.019 · 2.50 Impact Factor