Patient-specific biomechanical modeling using Finite Element Analysis (FEA) is pivotal for understanding the structural health of bones, optimizing surgical procedures, assessing outcomes, and validating medical devices, aligning with guidance issued by standards and regulatory bodies. Accurate mapping of image-to-mesh-material is crucial given bone's heterogeneous composition. This study aims to rigorously assess mesh convergence and evaluate the sensitivity of material grouping strategies in quantifying bone strength. Subject-specific geometry and nonlinear material properties were derived from computed tomography (CT) scan data of one cadaveric human vertebral body. Linear tetrahedral elements with varying edge lengths between 2mm and 0.9mm were then generated to study the mesh convergence. To compare the effectiveness of different grouping strategies, three approaches were used: Modulus Gaping (a user-defined absolute threshold of Young's modulus ranging from 500 MPa to 1 MPa), Percentual Thresholding (relative parameter thresholds ranging from 50% to 1%), and Adaptive clustering (unsupervised k-means-based clustering ranging from 10 to 200 clusters). Adaptive clustering enables a constant number of unique material properties in cross-specimen studies, improving the validity of results.Introduction
Method
Functional Spine Units (FSUs) play a vital role in understanding biomechanical characteristics of the spine, particularly bone fracture risk assessment. While established models focus on simulating axial compression of individual bones to assess fracture load, recent models underscore the importance of understanding fracture load within FSUs, offering a better representation of physiological conditions. Despite the limited number of FSU fracture studies, they predominantly rely on a linear material model with an annulus fibrosus Young's modulus set at 500 MPa, significantly higher than stiffness values (ca. 4 MPa) utilized in other FSU and spine section biomechanical models. Thus, this study aims to study the effect of varying annulus fibrosus stiffness on FSU fracture load, aiming to identify physiologically relevant biomechanical parameters. Subject-specific geometry and material properties of bones were derived from computed tomography (CT) image data of five human cadaveric FSU specimens. The annulus fibrosus and nucleus pulposus were manually recreated and assigned linear elastic material properties. By subjecting the model to axial compression, the fracture load of the FSU was deduced from the peak of the force-displacement graph. To explore the effect of stiffness of the annulus fibrosus on simulated fracture load, we conducted a parameter study, varying stiffness values from the high 500 MPa to a more physiologically relevant 25 MPa, aiming to approximate values applied in FSU kinematic models while achieving bone fracture.Introduction
Method
Multiple myeloma (MM) is a chronic, malignant B-cell disorder, with a less than 50% 5-year survival rate [1]. This disease is responsible for vertebral compression fractures (VCFs) in 34 to 64% of diagnosed patients [1], and at least 80% of MM patients experience pathological fractures [3]. Even though reduced DXA-derived bone mineral density (BMD) has been observed in MM patients with vertebral fractures [4], the current quantitative standard method is insufficient in MM due to the osteo-destructive bone changes. Finite-element (FE) analysis is a computational and non-destructive modeling and testing approach to determine bone strength using 3D bone models from CT images. Thus, this study aimed to assess the differences in FE-predicted critical fracture load in MM patients with and without VCFs in the thoracic and lumbar segments of the spine. Multi-detector CT (MDCT) images of two radiologically assessed MM patients (1 with VCFs and 1 without VCFs) were used to generate three-dimensional (3D) models of the whole spine. For each subject, the thoracic segments, 1 to 12 (T1-T12) and lumbar segments, 1 to 5 (L1-L5) were segmented and meshed. Heterogeneous, non-linear anisotropic material properties were applied by discretizing each vertebral segment into 10 distinct sets of materials. A compressive load was simulated by constraining the surface nodes on the inferior endplate in all directions, and a displacement load was applied on the surface nods on the superior endplate [2]. This analysis was performed using ABAQUS version 6.10 (Hibbitt, Karlsson, and Sorensen, Inc., Pawtucket, RI, USA). The MM subject with VCFs had originally experienced fractures in the T4, T5, T12, L1, and L5 segments whereas the MM subject without VCFs experienced none. The former displayed large and abrupt differences in fracture loads between adjacent vertebrae segments, unlike the latter, which exhibited progressive differences instead (no abrupt changes between adjacent vertebrae segments observed). Results from this preliminary study suggest that segments at high risk of fracture are collectively involved in an unstable network, which place the vertebral segments with high values of fracture loads (peaks) as well as the adjacent segments at risk of VCF. For instance, the high fracture load at T11 places T10, T11 and T12 at risk of fracture. Accordingly, T12 has already fractured, and T10 and T11 remain at risk. The relative changes between adjacent vertebrae segments that indicate instability (extremely high fracture load values) enables ease of identification of segments at high fracture risk. Clinicians would be able to work with pre-emptive treatment strategies in future as they can focus on more targeted therapy options at the high-risk vertebrae segments [3].
