A. Shirazi-adl - Academia.edu (original) (raw)

Papers by A. Shirazi-adl

Biorheology, 2003

The fibril reinforced poroelastic models have been found successful in describing some mechanical... more The fibril reinforced poroelastic models have been found successful in describing some mechanical behaviors of articular cartilage in unconfined compression that were not understood previously, including the strong and nonlinear transient response, the strain-magnitude and strain-rate dependent cartilage stiffness and the depth-varying stresses and strains. It has been demonstrated that a better description for the mechanical behavior of cartilage is obtained by introducing a fibrillar matrix into a poroelastic model, in addition to the nonfibrillar matrix and water. This paper reports the development of the nonlinear fibril reinforced homogeneous and nonhomogeneous models and further explores the potentials of the models for investigation of cartilage mechanical response. Some comments are made in regard to further applications of the models and improved accuracy of the material representation.

Computers & Structures, 1989

ABSTRACT

Journal of Biomedical Materials Research, 1990

Advances in Bioengineering, 1999

The effects of dynamic loading rate on the load sharing of passive and active subsytems of the sp... more The effects of dynamic loading rate on the load sharing of passive and active subsytems of the spine were evaluated using a validated viscoelastic finite element (FE) model of a L2-L3 motion segment. Both the force controlled and hybrid force-rotation controlled simulations were performed to best simulate the in-vivo loading conditions. The stiffening response of motion segement at higher loading rate was predicted by both simulation strategies, while the interpretation of the hybrid simulations was much easier due to decoupling of the confounding interactions of load, posture and loading rate. The results collectively indicate that the loading rate markedly effects the distribution of stress/strain in the tissues and reduces the margin of safety of the spine. The results should be used in developing a better biomechanical basis for considering the role of trunk speed as a risk factor for low back disorders.

Mechanics in Biology, 2000

Computational biomechanics of the human spine under a novel compression loading that follows the ... more Computational biomechanics of the human spine under a novel compression loading that follows the curvature of the spine is performed by evaluation and comparison of the detailed response of the spine under various types of compression loading at different postures. The nonlinear finite element formulation of wrapping elements sliding without friction over solid body edges is developed and used to study the load-bearing capacity of thoracolumbar (T1-S1) and lumbosacral (L1-S1) spines under one or several wrapping compression forces. Follower load at the L1, axially-fixed compression at the L1, and combined axially-fixed compression plus constrained rotations are also considered for comparison. Moreover, for the detailed lumbosacral model, the effect of changes in the position of wrapping elements and in the lumbar curvature on results are considered. The idealized wrapping loading substantially stiffens the spine allowing it to carry very large compression loads without hypermobility...

Biomechanical Systems, 2000

1.1 Background: Occupational Lower Back Disorders 1.2 Finite Element Models of the Lumbar Spine 1... more 1.1 Background: Occupational Lower Back Disorders 1.2 Finite Element Models of the Lumbar Spine 1.3 Role of Combined Loading 1.4 Role of Facets and Facet Geometry 1.5 Role of Bone Compliance 1.6 Role of Nucleus Fluid Content 1.7 Role of Annulus Modeling 1.8 Time-Dependent Response Analysis Vibration Analysis • Poroelastic Analysis • Viscoelastic Analysis 1.9 Stability and Response Analyses in Neutral Postures 1.10 Kinetic Redundancy and Models of Spinal Loading 1.11 Future Directions

Theoretical and Applied Fracture Mechanics, 1999

Industrial epidemiological studies have shown that jobs requiring a higher speed of trunk motion ... more Industrial epidemiological studies have shown that jobs requiring a higher speed of trunk motion contribute to a higher risk of industrial low back disorders. Consideration of the loading dynamic characteristics, such as lifting at dierent speeds, requires modeling of the viscoelastic behavior of passive tissues. Detailed systematic analysis of the eects of loading rate has been lacking in the literature. A validated viscoelastic ®nite element model of a L2±L3 motion segment was used to identify the load sharing among the passive elements at dierent loading rates. Force controlled complex¯exion movement was simulated by applying load at the top of the upper vertebra without constraining any coupled sagittal rotation, whereas the lower vertebra was ®xed at the bottom. The load reached its maximum values of 2000 N compression, 400 N anterior shear, and 20 Nm¯exion in three dierent durations of 0.3, 3 and 30 s to represent fast, medium and slow movement. The global force±displacement response of the motion segment, forces in facet joints and ligaments, stresses and strains in anulus ®brosus, and intradiscal pressure were compared across dierent rates. The higher rate of loading while reaching a prescribed complex forward¯exion loading increased the intradiscal pressure and the stress in the anulus ®bers at the posterolateral innermost layers, but reduced the global displacements, ligament forces and facet joint forces. The distribution of stress and strain was markedly aected by the loading rate. Consideration of the time-dependent material properties of passive elements is essential to improve our understanding of the responses of the motion segment to dynamic loading conditions. Speed of the manual materials handling (MMH) tasks should be included as a risk factor in the biomechanical and epidemiological studies and guidelines for safe lifting.

Medical engineering & physics, 2014

An anatomically detailed eighteen-rotational-degrees-of-freedom model of the human spine using op... more An anatomically detailed eighteen-rotational-degrees-of-freedom model of the human spine using optimization constrained to equilibrium and stability requirements is developed and used to simulate several symmetric tasks in upright and flexed standing postures. Predictions of this stability and kinematics-driven (S+KD) model for trunk muscle forces and spine compressive/shear loads are compared to those of our existing kinematics-driven (KD) model where both translational and rotational degrees-of-freedom are included but redundancy is resolved using equilibrium conditions alone. Unlike the KD model, the S+KD model predicted abdominal co-contractions that, in agreement with electromyography data, increased as lifting height increased at a constant horizontal moment arm. The S+KD model, however, could not fully explain the CNS strategy in activating antagonistic muscles for most of the remaining tasks. Despite quite distinct activities in individual muscles, both models predicted L4-L...

Bio-medical materials and engineering, 2003

Proper isotropic and anisotropic friction constitutive equations are developed based on previous ... more Proper isotropic and anisotropic friction constitutive equations are developed based on previous friction measurements at cancellous bone-porous coated implant interfaces exhibiting nonlinear load-displacement curves. The simulated friction response is dependent on relative tangential displacements in both orthogonal directions. The interface constitutive matrix contains cross-stiffness terms identical in isotropic friction but different in anisotropic friction. These terms are due mainly to nonlinearity in response and vanish in unidirectional friction along a principal direction and in cases with Coulomb or linear friction. The interface ultimate resistance is evaluated by an elliptic criterion which becomes circular in isotropic cases. These constitutive relations are implemented in a finite element program which is employed to analyze a bone cube sliding on top of a porous-surfaced metallic plate, an experimental model used in our earlier measurements. The results for both isotr...

Journal of Orthopaedic Research, 2008

The primary orientation of collagen fibrils alters along the cartilage depth; being horizontal in... more The primary orientation of collagen fibrils alters along the cartilage depth; being horizontal in the superficial zone, random in the transitional zone, and vertical in the deep zone. Commonly used confined and unconfined (when with no underlying bone) testing configurations cannot capture the mechanical role of deep vertical fibril network. To determine this role in cartilage mechanics, an axisymmetric nonlinear fibril-reinforced poroelastic model of tibial cartilage plateaus was developed accounting for depth-dependent properties and distinct fibril networks with physical material properties. Both creep and relaxation indentation models were analyzed which results were found equivalent in the transient period but diverged in post-transient periods. Vertical fibrils played a significant role at the transient period in dramatically increasing the stiffness of the tissue and in protecting the solid matrix against large distortions and strains at the subchondral junction. This role, however, disappeared both with time and at loading rates slower than those expected in physiological activities such as walking. The vertical fibrils demonstrated a chevron-type deformation pattern that was further accentuated with time in creep loading. Damages to deep vertical collagen fibril network or their firm anchorage to the bone, associated with bone bruises, for example, would weaken the transient stiffness and place the tissue at higher risk of failure particularly at the deep zone.

Journal of Biomedical Materials Research, 1982

A comparative study of stress distributions in the component materials of a number of models of a... more A comparative study of stress distributions in the component materials of a number of models of a prosthetically resurfaced tibia is presented. Although the geometry is idealized to be axisymmetric, the loadings for which the finite element analyses are performed are considered to be nonaxisymmetric, simulating more realistically the loading conditions in vivo. The different models are chosen with the view of determining the influence of changes in the prosthesis design on the induced stress distribution in the component materials. The changes considered are in the thickness of the cement and the cement-bone composite layers, and in the shape of the prosthesis. Experimentally measured values of strains are compared with the analytically predicted values to check the validity of the assumptions used in the finite element modeling. The comparison of induced stresses in the different materials reveals the desirability, from a mechanical behavior point of view, of introducing a cement-bone composite layer and using a prosthesis with domed subsurface in the fixation system. It is shown that for a model incorporating these features, considerable reduction of stresses in the cement, in its bulk and at its interface with the prosthesis plate, is achieved. The reduced stresses can be expected to have beneficial effects on the long-term behavior of the cement and its interfaces in the fixation system.

Journal of Biomedical Materials Research, 1993

An apparatus was developed to measure load-displacement friction properties at the cancellous bon... more An apparatus was developed to measure load-displacement friction properties at the cancellous bone/porous-coated metal plate interface. Bone cubes were obtained from different proximal regions of four resurfaced cadaveric tibiae. Three different porous-surfaced metal plates (one fiber mesh and two bead) and a smooth-surface metal plate were used. In the presence of a constant normal contact pressure (0.10, 0.15, or 0.25 MPa), a variable tangential load up to the maximum resistance of the interface was applied and both relative normal and tangential displacements were recorded. Repetitive and fatigue loadings were also considered. Measured results show that the interface friction curve is highly nonlinear, exhibiting large relative tangential displacements in the range of 50-400 microns before the maximum load is reached. Relative displacements in the normal direction remain below 10 microns. The maximum resistance in friction is independent of the bone excision site, type of porous-surfaced metal plate, magnitude of normal load, placement of bone cubes on metal plates or vice versa, repetition of applied load, and conservation period of bone cubes in saline solution. The smooth-surfaced metal plate has significantly smaller friction resistance than porous-coated ones. The fatigue loading of up to 4000 cycles at 1 Hz, in the presence of 0.25 MPa contact pressure, slightly decreases the interface friction coefficient. Finally, the initial secant stiffness of the interface at 75% of the maximum resistance load is found to be larger for the bone cubes from the lateral and medial regions and for the metal plate with smooth surface.

Journal of Biomechanics, 2006