Biomedical subjects

A Shirazi-Adl

Publications and source records attributed to A Shirazi-Adl.

At least 19 recordsLinked to original sources

PubMed2006

Effect of tibial tubercle elevation on biomechanics of the entire knee joint under muscle loads.

BACKGROUND: Anterior elevation of the tibial tubercle, known as Maquet procedure, is performed to reduce excessive patellofemoral contact stresses in knee joints with patellofemoral osteoarthritis and anterior pain. Previous investigations, however, have entirely focused on the likely effect of tibial tubercle elevation procedure on biomechanics of contact at the patellofemoral joint with no attention what-so-ever to associated alterations in biomechanics of the tibiofemoral joint. METHODS: Using a validated 3D nonlinear finite element model of the entire knee joint, the effect of 1.25 cm and 2.5 cm tubercle elevations on the entire knee joint biomechanics was investigated under constant quadriceps load of 411 N alone or combined with hamstrings co-activation of 205.5 N under joint angles of 0-90 degrees. FINDINGS: Results confirm the effectiveness of this procedure in reducing patellofemoral contact forces, especially at smaller flexion angles. Maximum contact stress substantially decreased at full extension but increased at 90 degrees. Substantial effects of tuberosity elevation on tibial kinematics, cruciate ligament forces, tibiofemoral contact forces and extensor lever arm were found. The posterior cruciate ligament and tibiofemoral contact forces at larger flexion angles considerably increased whereas the anterior cruciate ligament and tibiofemoral contact forces at near full extension angles decreased. Overall, the extent of changes depended on the magnitude of anterior elevation, joint flexion angle and loading considered. INTERPRETATION: Biomechanics of the tibiofemoral joint were significantly influenced by tibial tubercle elevation. Current results advocate the need for an integral view of the entire knee joint in management of various joint disorders rather than a view in which each component is considered and treated in isolation with no due attention to perturbations caused and associated consequences.

Biomechanical Phenomena↗

PubMed2006

Trunk biomechanical models based on equilibrium at a single-level violate equilibrium at other levels.

Accurate estimation of muscle forces in various occupational tasks is critical for a reliable evaluation of spinal loads and subsequent assessment of risk of injury and management of back disorders. The majority of biomechanical models of multi-segmental spine estimate muscle forces and spinal loads based on the balance of net moments at a single level with no consideration for the equilibrium at remaining levels. This work aimed to quantify the extent of equilibrium violation and alterations in estimations when such models are performed at different levels. Results are compared with those of kinematics-driven model that satisfies equilibrium at all levels and EMG data. Regardless of the method used (optimization or EMG-assisted), single-level free body diagram models yielded estimations that substantially altered depending on the level considered (i.e., level dependency). Equilibrium of net moment was also grossly violated at remaining levels with the error increasing in more demanding tasks. These models may, however, be used to estimate spinal compression forces.

Back↗

PubMed2006

Knee joint mechanics under quadriceps--hamstrings muscle forces are influenced by tibial restraint.

BACKGROUND: To investigate the role of quadriceps and hamstrings muscle recruitments on knee joint mechanics, measurement studies constrain the tibial anterior-posterior translation at a point away from the joint. This generates a restraining force perpendicular to the tibia thus introducing an artefact shear force that likely alters joint mechanics and forces in cruciate ligaments. METHODS: A 3D nonlinear finite element model of the entire knee joint, including tibiofemoral and patellofemoral joints, was used to investigate joint mechanics in flexion (0 degrees -90 degrees ) under isolated and combined hamstrings and quadriceps activation. The effect of tibial restraint at two locations on results was studied and compared with the reference boundary condition of tibia constrained by pure moments. FINDINGS: Tibial restraint by a force rather than a pure moment substantially influences the joint response. For identical forces, hamstrings have much greater moment generating capacity at larger flexion angles while quadriceps are more effective at smaller angles. INTERPRETATION: Tibial constraint by a restraining force rather than a pure moment causes an artefact force on the joint that vary with muscle forces and restraining location. These artefact shear forces, especially when placed closer to the joint, considerably reduce forces in cruciate ligaments; in anterior cruciate ligament at near full extension and in posterior cruciate ligament at larger flexion angles. The beneficial effects of muscle co-contraction in reducing anterior cruciate ligament forces at near full extension and in posterior cruciate ligament forces at near 90 degrees , however, disappear as the restraint on the tibia approaches the joint. The artefact forces could distort results and their interpretations.

Biomechanical Phenomena↗

PubMed2006

Wrapping of trunk thoracic extensor muscles influences muscle forces and spinal loads in lifting tasks.

BACKGROUND: An improved assessment of risk of spinal injury during lifting activities depends on an accurate estimation of trunk muscle forces, spinal loads and stability margin which in turn requires, amongst others, an accurate description of trunk muscle geometries. The lines of action of erector spinae muscles are often assumed to be linear despite the curved paths of these muscles in forward flexion postures. METHODS: A novel approach was introduced that allowed for the proper simulation of curved paths for global extensor muscles in our Kinematics-driven finite element model. The lever arms of global muscles at different levels were restrained either to remain the same or decrease only by 10% relative to their respective values in upright posture. Based on our earlier measurements, static lifting tasks at two trunk flexions (40 degrees and 65 degrees ) and three lumbar postures (free style, lordotic and kyphotic) with 180 N in hands were analyzed. FINDINGS: Muscle forces and spinal compression at all levels substantially decreased as the global extensor muscles took curved paths. In contrast, the shear force at lower levels increased. Allowing for a 10% reduction in these lever arms during flexion increased muscle forces and compression forces at all levels. Despite smaller muscle forces, wrapping of global muscles slightly improved the spinal stability. INTERPRETATION: Consideration of global extensor muscles with curved paths and realistic lever arms is important in biomechanical analysis of lifting tasks. Reduction in the erector spinae lever arms during flexion tasks could vary depending on the lumbar posture. Results advocate small flattening of the lumbar curvature in isometric lifts yielding smaller compression and shear forces at the critical L5-S1 level.

Abdominal Muscles↗

PubMed2006

Analysis of large compression loads on lumbar spine in flexion and in torsion using a novel wrapping element.

Axial compression on the spine could reach large values especially in lifting tasks which also involve large rotations. Experimental and numerical investigations on the spinal multi motion segments in presence of physiological compression loads cannot adequately be carried out due to the structural instability and artefact loads. To circumvent these problems, a novel wrapping cable element is used in a nonlinear finite element model of the lumbosacral spine (L1-S1) to investigate the role of moderate to large compression loads on the lumbar stiffness in flexion and axial moments/rotations. The compression loads up to 2,700 N was applied with no instability or artefact loads. The lumbar stiffness substantially increased under compression force, flexion moment, and axial torque when applied alone. The presence of compression preloads significantly stiffened the load-displacement response under flexion and axial moments/rotations. This stiffening effect was much more pronounced under larger preloads and smaller moments/rotations. Compression preloads also increased intradiscal pressure, facet contact forces, and maximum disc fibre strain at different levels. Forces in posterior ligaments were, however, diminished with compression preload. The significant increase in spinal stiffness, hence, should be considered in biomechanical studies for accurate investigation of the load partitioning, system stability, and fixation systems/disc prostheses.

Compressive Strength↗

PubMed2006

Model and in vivo studies on human trunk load partitioning and stability in isometric forward flexions.

To resolve the trunk redundancy to determine muscle forces, spinal loads, and stability margin in isometric forward flexion tasks, combined in vivo-numerical model studies was undertaken. It was hypothesized that the passive resistance of both the ligamentous spine and the trunk musculature plays a crucial role in equilibrium and stability of the system. Fifteen healthy males performed free isometric trunk flexions of approximately 40 degrees and approximately 65 degrees +/- loads in hands while kinematics by skin markers and EMG activity of trunk muscles by surface electrodes were measured. A novel kinematics-based approach along with a nonlinear finite element model were iteratively used to calculate muscle forces and internal loads under prescribed measured postures and loads considered in vivo. Stability margin was investigated using nonlinear, linear buckling, and perturbation analyses under various postures, loads and alterations in ligamentous stiffness. Flexion postures significantly increased activity in extensor muscles when compared with standing postures while no significant change was detected in between flexed postures. Compression at the L5-S1 substantially increased from 570 and 771 N in upright posture, respectively, for +/-180 N, to 1912 and 3308 N at approximately 40 degrees flexion, and furthermore to 2332 and 3850 N at approximately 65 degrees flexion. Passive ligamentous/muscle components resisted up to 77% of the net moment. In flexion postures, the spinal stability substantially improved due both to greater passive stiffness and extensor muscle activities so that, under 180 N, no muscle stiffness was required to maintain stability. The co-activity of abdominal muscles and the muscle stiffness were of lesser concern to maintain stability in forward flexion tasks as compared with upright tasks. An injury to the passive system, on one hand, required a substantial compensatory increase in active muscle forces which further increased passive loads and, hence, the risk of injury and fatigue. On the other hand, it deteriorated the system stability which in turn could require greater additional muscle activation. This chain of events would place the entire trunk active-passive system at higher risks of injury, fatigue and instability.

Abdomen↗

PubMed2006

Biomechanics of changes in ACL and PCL material properties or prestrains in flexion under muscle force-implications in ligament reconstruction.

The effects of changes in cruciate ligament material and prestrain on knee joint biomechanics following ligament reconstruction surgery by a tendon are not adequately known. A 3D nonlinear finite element model of the entire knee joint was used to investigate the joint response at different flexion angles under a quadriceps force while varying ACL and PCL initial strains or material properties. The ACL and PCL forces as well as tibiofemoral contact forces/areas substantially increased with greater ACL or PCL initial strains or stiffness. The patellofemoral contact force slightly increased whereas the tibial extensor moment slightly decreased with tenser or stiffer ACL. Reverse trends were predicted with slacker ACL. Results confirm the hypotheses that changes in the prestrain of one cruciate ligament substantially influence the force in the other cruciate ligament and the entire joint and that the use of the patellar tendon (PT) as a replacement for cruciate ligaments markedly alters the joint biomechanics with trends similar to those predicted when increasing prestrains. Forces in both ACL and PCL ligaments increased as one of them became tenser or stiffer and diminished as it became slacker. These results have important consequences in joint biomechanics following ligament injuries or replacement and tend to recommend the use of grafts with smaller prestrains (i.e. slacker than intact) when using the PT as the replacement material with stiffness greater than that of replaced ligament itself.

Biomechanical Phenomena↗

PubMed2005

Role of intra-abdominal pressure in the unloading and stabilization of the human spine during static lifting tasks.

The role of intra-abdominal pressure (IAP) in unloading the spine has remained controversial. In the current study, a novel kinematics-based approach along with a nonlinear finite-element model were iteratively used to calculate muscle forces, spinal loads, and stability margin under prescribed postures and loads measured in in vivo studies. Four coactivity levels (none, low, moderate, and high) of abdominal muscles (rectus abdominis, external oblique, and internal oblique) were considered concurrently with a raise in IAP from 0 to 4 kPa when lifting a load of 180 N in upright standing posture and to 9 kPa when lifting the same load in forward trunk flexions of 40 degrees and 65 degrees. For comparison, reference cases with neither abdominal coactivity nor IAP were investigated as well. A raise in IAP unloaded and stabilized the spine when no coactivity was considered in the foregoing abdominal muscles for all lifting tasks regardless of the posture considered. In the upright standing posture, the unloading action of IAP faded away even in the presence of low level of abdominal coactivity while its stabilizing action continued to improve as abdominal coactivity increased to moderate and high levels. For lifting in forward-flexed postures, the unloading action of IAP disappeared only with high level of abdominal coactivities while its stabilizing action deteriorated as abdominal coactivities increased. The unloading and stabilizing actions of IAP, hence, appear to be posture and task specific.

Abdominal Muscles↗

PubMed2005

Sensitivity of kinematics-based model predictions to optimization criteria in static lifting tasks.

The effect of eight different cost functions on trunk muscle forces, spinal loads and stability was investigated. Kinematics-based approach combined with nonlinear finite element modeling and optimization were used to model in vivo measurements on isometric forward flexions at approximately 40 degrees and approximately 65 degrees in sagittal plane with or without a load of 180N in hands. Four nonlinear (summation stress(3), summation stress(2), summation force(2) and muscle fatigue) and four linear (summation stress, summation force, axial compression and double-linear) criteria were considered. Predicted muscle activities were compared with measured EMG data. All predictions, irrespective of the cost function used, satisfied required kinetic, kinematics and stability conditions all along the spine. Four criteria (summation stress(3), summation stress(2), fatigue and double-linear) predicted muscle activities that qualitatively matched measured EMG data. The fatigue and double-linear criteria were inadequate in predicting greater forces in larger muscles with no consideration for their moment arms. Nearly the same stability margin was computed under these four cost functions. At the lower lumbar levels, the compression forces differed by <20% and the shear forces by <14% as various cost functions were considered. Smaller axial compression and anterior shear forces (by less than or approximately equal 6%) were computed when only the active components rather than the total muscle forces were taken as unknown in the summation stress(3) cost function. Overall, one single cost function of summation stress(2) or summation stress(3) rather than a multi-criteria one was found sufficient and adequate in yielding plausible results comparable with measured EMG activities and disc pressure.

Algorithms↗

PubMed2005

Analysis of articular cartilage as a composite using nonlinear membrane elements for collagen fibrils.

To develop a composite fibre-reinforced model of the cartilage, membrane shell elements were introduced to represent collagen fibrils reinforcing the isotropic porous solid matrix filled with fluid. Nonlinear stress-strain curve of pure collagen fibres and collagen volume fraction were explicitly presented in the formulation of these membrane elements. In this composite model, in accordance with tissue structure, the matrix and fibril membrane network experienced dissimilar stresses despite identical strains in the fibre directions. Different unconfined compression and indentation case studies were performed to determine the distinct role of membrane collagen fibrils in nonlinear poroelastic mechanics of articular cartilage. The importance of nonlinear fibril membrane elements in the tissue relaxation response as well as in temporal and spatial variations of pore pressure and solid matrix stresses was demonstrated. By individual adjustments of the collagen volume fraction and collagen mechanical properties, the model allows for the simulation of alterations in the fibril network structure of the tissue towards modelling damage processes or repair attempts. The current model, which is based on a physiological description of the tissue structure, is promising in improvement of our understanding of the cartilage pathomechanics.

Biomechanical Phenomena↗

PubMed2005

Biomechanics of the knee joint in flexion under various quadriceps forces.

Bioemchanics of the entire knee joint including tibiofemoral and patellofemoral joints were investigated at different flexion angles (0 degrees to 90 degrees ) and quadriceps forces (3, 137, and 411 N). In particular, the effect of changes in location and magnitude of restraining force that counterbalances the isometric extensor moment on predictions was investigated. The model consisted of three bony structures and their articular cartilage layers, menisci, principal ligaments, patellar tendon, and quadriceps muscle. Quadriceps forces significantly increased the anterior cruciate ligament, patellar tendon, and contact forces/areas as well as the joint resistant moment. Joint flexion, however, substantially diminished them all with the exception of the patellofemoral contact force/area that markedly increased in flexion. When resisting extensor moment by a force applied on the tibia, the force in cruciate ligaments and tibial translation significantly altered as a function of magnitude and location of the restraining force. Quadriceps activation generated large ACL forces at full extension suggesting that post ACL reconstruction exercises should avoid large quadriceps exertions at near full extension angles. In isometric extension exercises against a force on the tibia, larger restraining force and its more proximal location to the joint substantially decreased forces in the anterior cruciate ligament at small flexion angles whereas they significantly increased forces in the posterior cruciate ligament at larger flexion angles.

Biomechanical Phenomena↗

PubMed2005

Cruciate coupling and screw-home mechanism in passive knee joint during extension--flexion.

The screw-home mechanism and coupling between forces in cruciate ligaments during passive knee joint flexion were investigated for various boundary conditions, flexion axis alignments and posterior cruciate ligaments (PCL)/anterior cruciate ligament (ACL) conditions. A developed non-linear 3D finite element model was used to perform detailed elasto-static response analyses of the human tibiofemoral joint as a function of flexion angle varying from 10 degrees hyper-extension to 90 degrees flexion. The tibia rotated internally as the femur flexed and externally as the femur extended. The re-alignment of the flexion axis by +/-5 degrees rotation about the axial (distal-proximal) axis, transection of the ACL and changes in cruciate ligament initial strains substantially influenced the 'screw-home' motion. On the other hand, restraint on this coupled rotation diminished ACL forces in flexion. A remarkable coupling was predicted between ACL and PCL forces in flexion; forces in both cruciate ligaments increased as the initial strain or pretension in one of them increased whereas they both diminished as one of them was cut or became slack. This has important consequences in joint functional biomechanics following a ligament injury or replacement surgery and, hence, in the proper management of joint disorders.

Anterior Cruciate Ligament↗

PubMed2005

Effect of changes in cruciate ligaments pretensions on knee joint laxity and ligament forces.

The knee joint cruciate ligaments are reconstructed with the rationale to avoid joint instability, recurrent injury, damage to soft tissues and osteoarthritis. Wide range of procedures with different stiffness, pretension, orientation and insertion locations have been proposed with the primary goal to restore the joint laxity. Apart from the general lack of success in preservation of force in the reconstructed ligament itself, the concern, not yet addressed, arises as to the effect of such perturbation on the other intact cruciate ligament. The interaction between cruciate ligament forces is hypothesized in this work. Using a 3-D nonlinear finite element model of the tibiofemoral joint, we examined this hypothesis by quantifying the extent of coupling between cruciate ligaments while varying the prestrain in each ligament under flexion with and without anterior-posterior (A-P) loads. A remarkable coupling was predicted between cruciate ligament forces in flexion thus confirming the hypothesis; forces in both cruciate ligaments increased as initial strain or pretension in one of them increased whereas they both diminished as one of them became slack. Moreover, changes in laxity and in ligament forces as a cruciate ligament prestrained or pretensioned varied with flexion angle and external loads. These findings have important consequences in joint functional biomechanics following a ligament injury or replacement surgery and in selection of laxity matched or ligament force matched pretensioning protocols.

Anterior Cruciate Ligament↗

PubMed2005

Effect of load position on muscle forces, internal loads and stability of the human spine in upright postures.

A novel kinematics-based approach coupled with a non-linear finite element model was used to investigate the effect of changes in the load position and posture on muscle activity, internal loads and stability margin of the human spine in upright standing postures. In addition to 397 N gravity, external loads of 195 and 380 N were considered at different lever arms and heights. Muscle forces, internal loads and stability margin substantially increased as loads displaced anteriorly away from the body. Under same load magnitude and location, adopting a kyphotic posture as compared with a lordotic one increased muscle forces, internal loads and stability margin. An increase in the height of a load held at a fixed lever arm substantially diminished system stability thus requiring additional muscle activations to maintain the same margin of stability. Results suggest the importance of the load position and lumbar posture in spinal biomechanics during various manual material handling operations.

Computer Simulation↗

PubMed2005

Analysis of nonlinear coupled diffusion of oxygen and lactic acid in intervertebral discs.

The transport of oxygen and lactate (i.e., lactic acid) in the human intervertebral disc was investigated accounting for the measured coupling between species via the pH level in the tissue. Uncoupled cases were also analyzed to identify the extent of the effect of such coupling on the solute gradients across the disc. Moreover, nonlinear lactic production rate versus lactic concentration and oxygen consumption rate versus oxygen concentration were considered. The nonlinear coupled diffusion equations were solved using an in-house finite element program and an axisymmetric model of the disc with distinct nucleus and anulus regions. A pseudotransient approach with a backward integration scheme was employed to improve convergence. Coupled simulations influenced the oxygen concentration and lactic acid concentration throughout the disc, in particular the gradient of concentrations along the disc mid-height to the nucleus-anulus boundary where the solutes reached their most critical values; minimum for the oxygen tension and maximum for the lactate. Results suggest that for realistic estimates of nutrient and metabolite gradients across the disc, it could be important to take into account the coupling between the rates of synthesis and overall local metabolite/nutrient concentration.

Computer Simulation↗

PubMed2004

Spinal muscle forces, internal loads and stability in standing under various postures and loads--application of kinematics-based algorithm.

This work aimed to evaluate trunk muscle forces, internal loads and stability margin under some simulated standing postures, with and without external loads, using a nonlinear finite element model of the T1-S1 spine with realistic nonlinear load-displacement properties. A novel kinematics-based algorithm was applied that exploited a set of spinal sagittal rotations, initially calculated to minimize balancing moments, to solve the redundant active-passive system. The loads consisted of upper body gravity distributed along the spine with or without 200 N held in the hands, either in the front of the body or on the sides. Nonlinear and linear stability/perturbation analyses at deformed, stressed configurations with a linear stiffness-force relationship for muscles identified the system stability and critical muscle stiffness coefficient. Predictions were in good agreement with reported measurements of posture, muscle EMG and intradiscal pressure. Minimal changes in posture (posterior pelvic tilt and lumbar flattening) substantially influenced muscle forces, internal loads and stability margin. Addition of 200 N load in front of the body markedly increased the system stability, global muscle forces, and internal loads, which reached anterior shear and compression forces of approximately 500 N and approximately 1,200 N, respectively, at lower lumbar levels. Co-activation in abdominal muscles (up to 3% maximum force) substantially increased extensor muscle forces, internal loads and stability margin, allowing a smaller critical muscle coefficient. A tradeoff existed between lower internal loads in passive tissues and higher stability margins, as both increased with greater muscle activation. The strength of the proposed model is in accounting for the synergy by simultaneous consideration of passive structure and muscle forces under applied postures and loads.

Algorithms↗

PubMed2004

Effect of strain rate on tensile properties of sheep disc anulus fibrosus.

We investigated the effect of loading rate on tensile properties of sheep bone-anulus-bone specimens in axial direction. Disc anulus Samples with adjacent bone attachments were prepared from lateral, posterior and anterior regions of sheep lumbar spinal segments. The specimens were then tested at different strain rates under non-destructive cyclic tensile loading followed by destructive tensile loading. Each specimen was prepared by embedding the bony parts in the polymethylmetacrylate (PMMA) exposing the anulus portion to support tension. The results of non-destructive cyclic tests indicated a decrease in the hysteresis energy loss as strain rate increased. In the destructive tests, no significant differences in ultimate stress, ultimate strain and strain energy density were observed at different strain rates or annulus locations. However, there was a significant increase in the modulus at linear region as strain rate increased. Two major modes of failure were observed; rupture in the anulus mid-substance and at the anulus-endplate junction. The former failure was more frequent with no clear pattern across strain rates and locations, while the latter failure at anulus-endplate junction occurred primarily at slow strain rates.

Animals↗

PubMed2003

Finite element study of nutrient diffusion in the human intervertebral disc.

STUDY DESIGN: The diffusion of small nutrients in the intervertebral human disc was examined using a finite element model. OBJECTIVE: To investigate nutrient transport into the disc using a numerical approach. SUMMARY OF BACKGROUND DATA: The intervertebral disc is the largest avascular tissue in the body. Nutrients necessary for cellular survival diffuse from the blood supply around the disc margins to the cells. Limited analytical studies have been performed and compared with measurements. However, the studies have only considered supply through the center of the nucleus and have only examined single solutes. A more sophisticated model is required to investigate the solute supply. MATERIALS AND METHODS: An axisymmetric finite element model has been created to study the transport of three solutes, i.e., oxygen, glucose, and lactate, using nonlinear consumption-concentration and production-concentration rates. For each of them, data for the consumption/production rate, diffusivity, and concentration in the blood were taken from experimental measurements and used in the model. The effect of varying disc height, exchange area with the blood supply, solute consumption rates, and diffusivities was investigated. RESULTS: The model predicted that concentrations of oxygen and glucose, which are consumed by cells, fell towards the disc center. Concentration levels decreased with a decrease in fractional exchange area and diffusivity, or with an increase in disc height and consumption rate. In contrast, the concentration of lactate, produced by the cells, was highest in the center and fell towards the disc-blood vessel interface. The absolute values of concentrations were in agreement with available measurements in vivo and those computed by few available analytical models, indicating the reliability of the finite element simulations. CONCLUSIONS: Finite element methods can be used to predict concentration gradients of solutes throughout the disc in relation to changes in disc and endplate morphology, disc properties, and cellular activities. This study provides a foundation for investigating the effect of load-induced changes or effects of changes in cellular metabolism on disc nutritional supply.

Diffusion↗