A patient-specific finite element methodology to predict damage accumulation in vertebral bodies under axial compression, sagittal flexion and combined loads.
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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ABSTRACT: Computer tomography (CT)-based finite element (FE) models assess vertebral strength better than dual energy X-ray absorptiometry. Osteoporotic vertebrae are usually loaded via degenerated intervertebral discs (IVD) and potentially at higher risk under forward bending, but the influences of the IVD and loading conditions are generally overlooked. Accordingly, Magnetic Resonance Imaging was performed on 14 lumbar discs to generate FE models for the healthiest and most degenerated specimens. Compression, torsion, bending, flexion and extension conducted experimentally were used to calibrate both models. They were combined with CT-based FE models of 12 lumbar vertebral bodies to evaluate the effect of disc degeneration compared to a loading via endplates embedded in a stiff resin, the usual experimental paradigm. Compression and lifting were simulated, load and damage pattern were evaluated at failure. Adding flexion to the compression (lifting) and higher disc degeneration reduces the failure load (8-14%, 5-7%) and increases damage in the vertebrae. Under both loading scenarios, decreasing the disc height slightly increases the failure load; embedding and degenerated IVD provides respectively the highest and lowest failure load. Embedded vertebrae are more brittle, but failure loads induced via IVDs correlate highly with vertebral strength. In conclusion, osteoporotic vertebrae with degenerated IVDs are consistently weaker - especially under lifting, but clinical assessment of their strength is possible via FE analysis without extensive disc modelling, by extrapolating measures from the embedded situation.Journal of the Mechanical Behavior of Biomedical Materials 11/2014; 42. DOI:10.1016/j.jmbbm.2014.10.016 · 3.05 Impact Factor
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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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ABSTRACT: Constructing models based on computed tomography images for finite element analysis (FEA) is challenging under pathological conditions. In the present study, an innovative method was introduced that uses Siemens syngo(®) 3D software for processing models and Mimics software for further modeling. Compared with the slice-by-slice traditional manual margin discrimination, the new 3D modeling method utilizes automatic tissue margin determination and 3D cutting using syngo software. The modeling morphologies of the two methods were similar; however, the 3D modeling method was 8-10 times faster than the traditional method, particularly in cases with osteoporosis and osteophytes. A comparative FEA study of the lumbar spines of young and elderly patients, on the basis of the models constructed by the 3D modeling method, showed peak stress elevation in the vertebrae of elderly patients. Stress distribution was homogeneous in the entire vertebrae of young individuals. By contrast, stress redistribution in the vertebrae of the elderly was concentrated in the anterior cortex of the vertebrae, which explains the high fracture risk mechanism in elderly individuals. In summary, the new 3D modeling method is highly efficient, accurate and faster than traditional methods. The method also allows reliable FEA in pathological cases with osteoporosis and osteophytes.Experimental and therapeutic medicine 06/2014; 7(6):1583-1590. DOI:10.3892/etm.2014.1645 · 0.94 Impact FactorThis article is viewable in ResearchGate's enriched formatRG Format enables you to read in context with side-by-side figures, citations, and feedback from experts in your field.