Amit Roychowdhury

Effects of trochanteric soft tissue thickness and hip impact velocity on hip fracture in sideways fall through 3D finite element simulations.

A major worldwide health problem is hip fracture due to sideways fall among the elderly population. The effects of sideways fall on the hip are required to be investigated thoroughly. The objectives of this study are to evaluate the responses to trochanteric soft tissue thickness (T) variations and hip impact velocity (V) variations during sideways fall based on a previously developed CT scan derived 3D non-linear and non-homogeneous finite element model of pelvis-femur-soft tissue complex with simplified biomechanical representation of the whole body. This study is also aimed at quantifying the effects [peak impact force (F(max)), time to F(max), acceleration and peak principal compressive strain (epsilon(max))] of these variations (T,V) on hip fracture. It was found that under constant impact energy, for 81% decrease in T (26-5mm), F(max) and epsilon(max) increased by 38% and 97%, respectively. Hence, decrease in T (as in slimmer persons) strongly correlated to risk for hip fracture (phi) and strain ratio (SR) by 0.972 and 0.988, respectively. Also under same T and body weight, for 75% decrease in V (4.79-1.2m/s), F(max) and epsilon(max) decreased by 70% and 86%, respectively. Hence, increase in V (as in taller persons) strongly correlated to phi and SR by 0.995 and 0.984, respectively. For both variations in T and V, inter-trochanteric fracture situations were well demonstrated by phi as well as by SR and strain contours, similar to clinically observed fractures. These quantifications would be helpful for effective design of person-specific hip protective devices.

Dynamic response of the pelvis under side impact load – a three-dimensional finite element approach

The pelvis is most susceptible to severe fractures in side impacts, arising from motor vehicle crashes. In recent years, car manufacturers are providing more importance to the protection of occupants in lateral impacts. This study was aimed to understand the dynamic response of the pelvis and establish its fracture threshold, using a three-dimensional finite element model, with 13,070 tetrahedral (trabecular bone) and 5,820 shell (cortical bone) elements through 3,704 nodes. These elements take care of bending stress due to out of plane loading. After a detailed modal analysis, thirty-eight load cases were simulated by varying the intensity of impact load, impact duration and density ( i.e . inertia effect) of bones, with 65% of the body weight acting on the sacrum. It was observed that the failure threshold load was near 3 kN. With better design of car-door and hippad, if side impact force was brought down to 3 kN (approximately), then the peak stress (162 MPa at 5 ms of impact time to peak load), within the pelvis, would not exceed the average compressive strength for the cortical bone.

Effects of body configuration on pelvic injury in backward fall simulation using 3D finite element models of pelvis–femur–soft tissue complex

Injuries due to backward fall apart from sideways fall are a major health problem, particularly among the aged populations. The objectives of this study was to evaluate the responses to changing body configurations (angle between the trunk and impacting floor as 0 degrees, 15 degrees, 45 degrees and 80 degrees) during backward fall, based on a previously developed CT-scan-derived 3D non-linear and non-homogeneous finite element (FE) model of pelvis-femur-soft tissue complex with simplified biomechanical representation of the whole body. Under constant impact energy, these FE models evaluated the pelvic injury situations on the basis of peak impact force (7.64-16.74 kN) and peak principal compressive strain (more than 1.5%), consistent with the clinically observed injuries (sacral insufficiency, coccydynia). Also the change in location of peak strain and increase in peak impact force for changing configurations from 0 degrees to 80 degrees indicated the effect of whole body inertia during backward fall. It was also concluded that the inclusion of sacro-iliac and acetabular cartilages in the above FE models will further reduce above findings marginally (9.2% for 15 degrees fall). These quantifications would also be helpful for a better design and development of safety structures such as safety floor for the nursing home or home for the aged persons.

Age Hardening Behavior of Wrought Al–Mg–Sc Alloy

Ageing of Al-6Mg alloy doped with varying concentration of scandium ranging from 0.2 wt% to 0.6 wt% has been carried out. Cold-rolled alloy samples are isochronally aged for 60 minutes at different temperatures. The cast and hot-rolled samples are also aged isochronally for 90 minutes at different temperatures up to 450°C. Isothermal ageing of cold-rolled samples is conducted at various temperatures for different periods of time ranging from 30 to 480 minutes. Hardness values of the differently processed alloys have been measured to understand the ageing behavior of Al-6Mg alloy with scandium addition. The hot-rolled alloys after ageing do not show any hardening response due to ageing. Ageing of cold-rolled alloys envisaged precipitation of Al3Sc which is not noted to be dislocation induced. The kinetics of precipitation of Al3Sc in Al–6Mg–Sc alloys are found to be controlled by the diffusion of scandium in aluminum.

Experimental validation of three-dimensional finite element model of pelvis-femur-soft tissue complex under side impact loading

An experimental validation with the pelvic bone or human cadaver has always been considered as a gold standard to assess the accuracy of the finite element predictions. The present study provides an approach in mechanical testing of pelvic bone and human cadaver under side impact load for the purpose of validating the previously developed finite element models of pelvis-femur complex and pelvis-femur-soft tissue complex, respectively. An experimental setup was designed and developed, instrumented with miniature piezoelectric load cell and miniature charge amplifier. PC based data acquisition system was used to record the impact force-time data. The support conditions of the experiments were applied on the finite element model and simulated under similar impact load. Considering all the load cases with pelvic bone and human cadaver, high correlation (r² = 0.966) and significance level (p < 0.0001) were obtained between the experimental and finite element results. It strongly suggests that the numerical results calculated by the finite element models could be used as a valid predictor of the actual results.

Three-dimensional finite element simulation of pelvic fracture during side impact with pelvis–femur–soft tissue complex

Pelvic injuries and fractures (abbreviated injury scale > 2) are caused by intrusion of car door structure during motor vehicle side impacts. The objective was to simulate automotive side impact situation for the occupant. This was performed with a computed tomography scan-based three-dimensional finite element (FE) model of human pelvis–femur–soft tissue complex with spring–dashpot–mass representation of whole body, using wide range of mechanical property variations. It was impacted with a 12-kg rigid impactor at 13.54 m s–1 to impart impact energy of 1100 J. The pelvic fracture condition was well-examined by the peak impact force (16.98 kN), peak strain (23.4%) and peak stress (189 MPa). It was concluded that this FE model could reconstruct clinically observed pelvic injury. Moreover, this FE model could reduce the need for modelling the complex whole body with a huge number of elements and nodes when pelvic fracture is the area of focus. Furthermore, this detailed FE model may be used for finding out different response corridors for automobile side impact, and the detailed responses may be used for developing occupant protective systems, better vehicle door structures and side impact dummies.

Effect of prior cold work on tensile properties of Al-6Mg alloy with minor scandium additions

Abstract Effect of aging on the mechanical properties of cold worked Al–6Mg alloy with minor additions of scandium is studied. Cast and mechanically worked samples are isochronally aged for 60 min at different temperatures up to 500°C. Evaluation of mechanical properties of the aged alloys is done at various strain rates of testing. The m-values (strain rate sensitivity) of the experimental alloys are desired from the tensile test results. The influence of scandium is much pronounced on yield strength than on the tensile strength. Alloys with higher scandium content have shown higher yield strength. The ‘m’ values are found to be comparatively high at peak aged condition of alloy with higher scandium content. The fracture of the experimental alloys occurs through microvoid coalescence. On étudie l’effet du vieillissement sur les propriétés mécaniques de l’alliage écroui d’Al-6Mg avec additions mineures de scandium. On a vieilli à durée égale pendant 60 minutes à différentes températures, jusqu’à 500°C, des échantillons moulés et formés mécaniquement. On fait l’évaluation des propriétés mécaniques des alliages vieillis par essai à différentes vitesses de déformation. On désire les valeurs de m (sensibilité à la vitesse de déformation) des alliages expérimentaux à partir des résultats de l’essai de traction. L’influence du scandium est beaucoup plus prononcée sur la limite d’élasticité que sur la résistance à la traction. Les alliages ayant une teneur plus élevée en scandium ont montré une limite d’élasticité plus élevée. On a trouvé que les valeurs de ″m″ étaient comparativement élevées sous la condition maximale de vieillissement de l’alliage ayant la teneur la plus élevée en scandium. La rupture des alliages expérimentaux se produisait par cupulation.

Prediction of subdural haematoma based on a 3D finite element human head model

Subdural haematoma (SDH) is a very common injury associated with in-house as well as traffic accidents. A high level of relative motion between the skull and the brain causes laceration in the cerebral bridging veins followed by acute SDH. This article describes finite element-based simulations and analysis of mechanical responses in the skull-brain complex in accident-like situations to study and assess SDH in the human head. A 3D finite element model of the human head was prepared by reconstructing CT images, and subsequently simulated at real life impact situations using explicit finite element codes. The Green-Lagrangian strain in bridging veins has been computed by post-processing finite element results for every possible orientation and position of bridging veins. It has been observed that occipital (back) impact is most likely to cause SDH, and that vein length plays the predominant role over vein angle in inducing strain. The possibility of vein rupture increases with head size.

AN ALGORITHM TO CALCULATE THE COMPONENTS OF ALL MUSCLE FORCES DURING EACH PHASES OF THE GAIT CYCLE, FOR THE PELVIC-FEMUR COMPLEX

With the help of finite element (FE) computational models of femur, pelvis or hip joint to perform quasi-static stress analysis during the entire gait cycle, muscle force components (X, Y, Z) acting on the hip joint and pelvis are to be known. Most of the investigators have presented only the net muscle force magnitude during gait. However, for the FE software, either muscle force components (X, Y, Z) or three angles for the muscle line of action are required as input. No published algorithm (with flowchart) is readily available to calculate the required muscle force components for FE analysis. As the femur rotates about the hip center during gait, the lines of action for 27 muscle forces are also variable. To find out the variable lines of action and muscle force components (X, Y, Z) with directions, an algorithm was developed and presented here with detailed flowchart. We considered the varying angles of adduction/abduction, flexion/extension during gait. This computer program, obtainable from the first author, is able to calculate the muscle force components (X, Y, Z) as output, if the net magnitude of muscle force, hip joint orientations during gait and muscle origin and insertion coordinates are provided as input.

Automobile frontal impact on knee-thigh-hip complex

The type of injury protection that are necessary to reduce the impact reaction force developed during automobile frontal impact on human kneethighhip complex depends on the impact energy and body resistance. To find out the nature of impact reaction force it is necessary to analyze the behavior of occupant lower extremity during frontal impact. Objective of this study is to develop a finite element (FE) model of knee-thigh-hip (KTH) complex and validate it experimentally under frontal impact load. A three-dimensional FE model of KTH complex with surrounding soft tissue was developed from the CT scan data. Bilinear, elasto-plastic and isotropic element-by-element material model for the pelvis and femur bone, and non-linear hyper-elastic material model for the soft tissue were considered. A cadaveric pelvic bone surrounded by soft tissue, fixed with the in house developed test-setup having piezoelectric based miniature load cell, was used for the experiment. Rigid impactor under 250J frontal impact energy, created maximum impact force of 6.85kN and 5.53kN at the contact regions for the FE analysis, whereas, it was 3.89kN for the experiment. The main causes of this difference may be due to energy loss during experiment, differences in bone properties, geometry and cortical bone thickness for the FE model and cadaveric pelvic bone.