Search

Quantitative computed tomography (QCT)-based finite element (FE) models of the vertebra are widely used in studying spine biomechanics and mechanobiology, but their accuracy has not been fully established. Although the models typically assign material properties based only on local bone mineral density (BMD), the mechanical behavior of trabecular bone also depends on fabric. The goal of this study was to determine the effect of incorporating measurements of fabric on the accuracy of FE predictions of vertebral deformation. Accuracy was assessed by using displacement fields measured via digital volume correlation—applied to time-lapse microcomputed tomography (μCT)—as the gold standard. Two QCT-based FE models were generated from human L1 vertebrae (n = 11): the entire vertebral body and a cuboid-shaped portion of the trabecular centrum [dimensions: (20–30) × (15–20) × (15–20) mm3]. For axial compression boundary conditions, there was no difference (p = 0.40) in the accuracy of the FE-computed displacements for models using material properties based on local values of BMD versus those using material properties based on local values of fabric and volume fraction. However, when using BMD-based material properties, errors were higher for the vertebral-body models (8.4–50.1%) than cuboid models (1.5–19.6%), suggesting that these properties are inaccurate in the peripheral regions of the centrum. Errors also increased when assuming that the cuboid region experienced uniaxial loading during axial compression of the vertebra. These findings indicate that a BMD-based constitutive model is not sufficient for the peripheral region of the vertebral body when seeking accurate QCT-based FE modeling of the vertebra. [ABSTRACT FROM AUTHOR]

Changes in vegetation composition due to nitrogen deposition may influence the role of peatlands in the carbon cycle. The impact of nitrogen (N) on vegetation composition of the Mer Bleue Bog (Ontario) and the underlying mechanisms were studied using a coupled carbon (C) and nitrogen wetland model (PEATBOG). The model was applied to data from a long-term nutrient fertilization experiment and the results were compared with the observed C and N pools in plants, peat, and soil water after 8 years of fertilization with 1.6, 3.2, and 6.4 gN m y and additional phosphorus and potassium. The evaluated model was employed to simulate the vegetation dynamics in the peatland fertilized with different loads and concentrations of N. The model suggested a shift in plant functional types from moss-shrub dominated to graminoid dominated with increasing N load. Suppression of mosses by N fertilization was mitigated by daily deposition of N at a low concentration. Mosses became extinct in the simulations only when the concentration and load of N deposition were high, corroborating empirical results from the fertilization experiment. Tracking the deposited N in the system, the model indicated that it was primarily sequestered in the peat. Dissolved N concentrations in peat and export from the bog increased with N load and decreased with N concentration, leading to higher N uptake by vascular plants. The results further suggested that the observed detrimental effect of N on mosses was most likely due to toxicity caused by N uptake exceeding N assimilation. Exposure of mosses to high N concentrations in precipitation and evaporating water on moss tissues is thus a factor that needs attention when considering critical loads of N deposition on peatlands. [ABSTRACT FROM AUTHOR]