H. N. Hashemi

Curved Beam Computed Tomography based Structural Rigidity Analysis of Bones with Simulated Lytic Defect: A Comparative Study with Finite Element Analysis

In this paper, a CT based structural rigidity analysis (CTRA) method that incorporates bone intrinsic local curvature is introduced to assess the compressive failure load of human femur with simulated lytic defects. The proposed CTRA is based on a three dimensional curved beam theory to obtain critical stresses within the human femur model. To test the proposed method, ten human cadaveric femurs with and without simulated defects were mechanically tested under axial compression to failure. Quantitative computed tomography images were acquired from the samples, and CTRA and finite element analysis were performed to obtain the failure load as well as rigidities in both straight and curved cross sections. Experimental results were compared to the results obtained from FEA and CTRA. The failure loads predicated by curved beam CTRA and FEA are in agreement with experimental results. The results also show that the proposed method is an efficient and reliable method to find both the location and magnitude of failure load. Moreover, the results show that the proposed curved CTRA outperforms the regular straight beam CTRA, which ignores the bone intrinsic curvature and can be used as a useful tool in clinical practices.

Hierarchical analysis and multi-scale modelling of rat cortical and trabecular bone.

The aim of this study was to explore the hierarchical arrangement of structural properties in cortical and trabecular bone and to determine a mathematical model that accurately predicts the tissues mechanical properties as a function of these indices. By using a variety of analytical techniques, we were able to characterize the structural and compositional properties of cortical and trabecular bones, as well as to determine the suitable mathematical model to predict the tissues mechanical properties using a continuum micromechanics approach. Our hierarchical analysis demonstrated that the differences between cortical and trabecular bone reside mainly at the micro- and ultrastructural levels. By gaining a better appreciation of the similarities and differences between the two bone types, we would be able to provide a better assessment and understanding of their individual roles, as well as their contribution to bone health overall.

Finite Element Analysis of the Knee: The Effect of Tibiofemoral Alignment and Weight on the Stresses in the Knee

Osteoarthritis (OA) is a degenerative disease of articular cartilage that affects millions of people [1]. Local biomechanical factors may severely affect the initiation and progression of OA due to changes in loading conditions at the knee cartilage. Body weight and the frontal plane tibiofemoral alignment are two biomechanical factors that could increase the overall loading at the knee. A normal knee will have a tibiofemoral angle approximately 7° valgus [2]. Deviation from this angle leads to a knee joint with a varus or valgus condition. The tibiofemoral angle is measured by the intersection made between the mechanical axis of the femur and the tibia in the frontal plane and affects the magnitude of the varus knee moment, Fig. 1A. Biomechanical studies have shown the varus moment is a key determinant in the load distribution at the knee [3, 4], Fig. 1A, and has been linked to OA progression [5, 6].© 2008 ASME

Effect of Blood Viscosity on Thrombosis Potential Near a Step Wall Transition

Thrombosis and hemolysis are two problems encountered when processing blood in artificial organs. Physical factors of blood flow alone can influence the interaction of proteins and cells with the vessel wall, induce platelet aggregation and influence coagulation factors responsible for the formation of thrombus, even in the absence of chemical factors in the blood. These physical factors are related to the magnitude of the shear rate/stress, the duration of the applied force and the local geometry. Specifically, high blood shear rates (or stress) lead to damage (hemolysis, platelet activation), while low shear rates lead to stagnation and thrombosis [1].Copyright

The Effects of Graft Size and Insertion Site Location During Anterior Cruciate Ligament Reconstruction on Intercondylar Notch Impingement

Intercondylar notch impingement is detrimental to the anterior cruciate ligament (ACL). Notchplasty is a preventative remodeling procedure performed on the intercondylar notch during ACL reconstruction (ACLR). This study investigates how ACL graft geometry and both tibial and femoral insertion site location affect ACL-intercondylar notch interactions post ACLR. A range of ACL graft sizes are reported during ACLR, from 6mm–11mm in diameter. Minor variability of up to 3mm in ACL insertion site locations is reported during ACLR. Several 3D finite element (FE) knee joint models were constructed using three ACL graft sizes and polar arrays of tibial and femoral insertion site locations. Each knee model was subjected to flexion, tibial external rotation, and valgus motion. Impingement force and contact area between the ACL and the intercondylar notch compared well with published cadaver study results. A 3mm shift in the antero-lateral direction of the tibial insertion site of the average and maximum size ACL increased impingement force by 155.4% and 242.9% respectively. A 3mm shift in the anterior-proximal direction of the femoral insertion site of the average and maximum size ACL increased impingement by 292.6%, and 346.2% respectively. Simulated notchplasties of 4mm and 5mm reduced graft impingement force by 89.4% and 100% respectively for the simulations with greatest impingement. For the kinematics applied, the results show that small differences in graft size and insertion site location may lead to large increases in impingement force and contact area. The study aims to improve ACLR success rates by understanding how minor variations in graft size and insertion site location affect intercondylar notch impingement. Because minor variations in insertion site location during ACLR are a known occurrence, the results of this study may support the argument for performing notchplasty during ACLR.Copyright

Uncovering the Mechanism of Soft Tissue Injuries Associated With ACL Tear

Anterior cruciate ligament (ACL) injury is a common and painful injury that occurs approximately 250,000 times annually in the U.S. [1]. Articular cartilage and meniscal injuries are also associated with ACL injuries [2]. ACL injuries can often lead to degenerative osteoarthritis of the articular cartilage [2]. An epidemiology study of athletic injuries by Majewski et al. [3] determined that out of 19,530 sports injuries, 20% were ACL injuries and 8% were medial collateral ligament (MCL) injuries.Copyright

The Effect of Rate Dependency on the ACL Failure Locus: A Subject Specific Study

A full or partial tear of the anterior cruciate ligament (ACL) is a common and painful injury that has been estimated to occur approximately 250,000 times annually in the U.S. [1]. Articular cartilage and meniscal injuries are also associated with ACL injuries [2]. ACL injuries can often lead to degenerative osteoarthritis of the articular cartilage [2]. An epidemiology study of athletic injuries by Majewski et al. [3] determined that out of 19,530 sports injuries, 20% were ACL injuries and 8% were medial collateral ligament (MCL) injuries.Copyright

Development of a failure locus for a 3-dimensional anterior crutiate ligament: A finite element analysis

This study investigates the combinations of loading which cause injury to knee joint ligaments. A 3D model of a knee, including bones, cartilage, menisci and ligament bundles was created from magnetic resonance images (MRI). Material properties for bone, cartilage, meniscus and ligament were all determined based on previous published work. The model incorporates a novel approach for accounting for prestrain in ligament bundles. The ligament bundle structures were resized based to their zero load lengths and strained to their reference lengths at full extension using FEBio (University of Utah). Previous studies investigating ligament failure used 1D nonlinear spring elements for ligament structures. The 3D ligament model will provide improved accuracy for locating bundle ruptures. By monitoring stresses and strains in ligament bundles during knee joint orientation simulations, ruptures can be virtually diagnosed. The results of these simulations can be used in clinical applications. In sports where anterior cruciate ligament (ACL) injuries are prevalent, training programs can be adapted to address the avoidance of harmful knee orientations. The ability to monitor where bundle ruptures occur will provide increased insight for practitioners in identifying more precise mechanisms of injury to ligaments and cartilage within the knee joint.

Development of a Failure Locus for a 3-Dimensional Anterior Crutiate Ligament: A Finite Element Analysis

This study investigated movement combinations which may cause injury to the anterior cruciate ligament (ACL). A 3-Dimensional finite element knee joint model, including bones and ligament bundles, was developed. Bone was modeled as rigid, and a transversely isotropic material was applied to the ligament structures. This study incorporates a novel approach for developing bundle specific prestrain within the ligament structures. The bundles were stretched from their zero load lengths to their reference lengths, producing a strain field mimicking in vivo conditions at full knee extension. A failure locus was created by performing multiple knee joint motion combination simulations until ligament failure. The locus shows which movement combinations of internal/external femoral rotation and varus/valgus angle cause failure within the ACL bundles at 25° of knee flexion. The 3D model provided improved accuracy for locating bundle ruptures. By monitoring stresses and strains within the ligament bundles during knee joint orientation simulations, ruptures were virtually diagnosed. The relationship between knee joint orientation and ligament rupture provides a spectrum for the propensity of ACL injury. The results highlight femoral external rotation relative to the tibia as an important factor related to ACL injury. The results also show the posterolateral bundle to be more susceptible to rupture than the anteromedial bundle. These results have various clinical applications. In sports where ACL injuries are prevalent, training programs can be adapted to address the avoidance of harmful knee orientations. Monitoring bundle rupture locations also increases insight for practitioners in identifying more precise injury mechanisms.Copyright

The Development of a Three Dimensional Anterior Cruciate Ligament Failure Locus: A Finite Element Analysis

A full or partial tear of the anterior cruciate ligament (ACL) is a common and painful injury that has been estimated to occur approximately 250,000 times annually in the U.S. [1]. Articular cartilage and meniscal injuries are also associated with ACL injuries [2]. ACL injuries can often lead to degenerative osteoarthritis of the articular cartilage [2]. An epidemiology study of athletic injuries by Majewski et al. [3] determined that out of 19,530 sports injuries, 20% were ACL injuries and 8% were medial collateral ligament (MCL) injuries.Copyright