Journal of Biomechanics

Editorial Board and Publication Information

2012-05-11

Microbubble-induced sonoporation involved in ultrasound-mediated DNA transfection in vitro at low acoustic pressures

Yuanyuan Qiu, Chunbing Zhang, Juan Tu, Dong Zhang — 2012-04-13

Abstract: In the present work, human breast cancer cells MCF-7 mixed with polyethylenimine: deoxyribonucleic acid complex and microbubbles were exposed to 1-MHz ultrasound at low acoustic driving pressures ranging from 0.05 to 0.3MPa. The sonoporation pores generated on the cell membrane were examined with scanning electron microscopy. The transfection efficiency and cell viability were evaluated with flow cytometry. The results showed that ultrasound sonication under the current exposure condition could generate cell pores with mean size ranging from about 100nm to 1.25μm, and that larger sonoporation pores would be generated with the increasing acoustic pressure or longer treatment time, leading to the enhancement of transfection efficiency and the reduction of cell viability. The simulations based on the Marmottant model were performed to test the hypothesis that the microstreaming-induced shear stress might be involved in the mechanisms of the low-intensity ultrasound induced sonoporation. The calculated shear stress resulting from the micro-streaming ranged from 15 to 680Pa corresponding to the applied acoustic pressures 0.05−0.3MPa, which is sufficient to induce reversible sonoporation. This study indicates that the shear stress related bio-effects may provide a base for strategies aimed at targeted drug delivery.

Biomechanical characterisation of ovine spinal facet joint cartilage

Mohd Juzaila Abd Latif, Zhongmin Jin, Ruth K. Wilcox — 2012-04-09

Abstract: The spinal facet joints are known to be an important component in the kinematics and the load transmission of the spine. The articular cartilage in the facet joint is prone to degenerative changes which lead to back pain and treatments for the condition have had limited long term success. There is currently a lack of information on the basic biomechanical properties of the facet joint cartilage which is needed to develop tissue substitution or regenerative interventions. In the present study, the thickness and biphasic properties of ovine facet cartilage were determined using a combination of indentation tests and computational modelling. The equilibrium biphasic Young's modulus and permeability were derived to be 0.76±0.35MPa and 1.61±1.10×10−15m4/(Ns) respectively, which were within the range of cartilage properties characterised from the human synovial joints. The average thickness of the ovine facet cartilage was 0.52±0.10mm, which was measured using a needle indentation test. These properties could potentially be used for the development of substitution or tissue engineering interventions and for computational modelling of the facet joint. Furthermore, the developed method to characterise the facet cartilage could be used for other animals or human donors.

Our capricious vessels: The influence of stent design and vessel geometry on the mechanics of intracranial aneurysm stent deployment

2012-04-09

Abstract: There is a growing interest in virtual tools to assist clinicians in evaluating different procedures and devices for endovascular treatment. In the present study we use finite element analysis to investigate the influence of stent design and vessel geometry for stent assisted coiling of intracranial aneurysms. Nine virtual stenting procedures were performed: three nitinol stent designs ((i) an open cell stent resembling the Neuroform, (ii) a generic stiff and (iii) a more flexible closed cell design), were deployed in three patient-specific cerebral aneurysmatic vessels. We investigated the percentage of strut area covering the aneurysm neck, the straightening induced on the cerebrovasculature by the stent placement (quantified by the reduction in tortuosity), and stent apposition to the wall (quantified as the percentage of struts within 0.2mm of the vessel).The results suggest that the open cell design better covers the aneurysm neck (11.0±1.1%) compared to both the stiff (7.8±1.6%) and flexible (8.7±1.6%) closed cell stents, and induces less straightening of the vessel (−5.1±1.6% vs. −42.9±9.8% and −26.9±11.9% ). The open cell design has, however, less struts apposing well to the vessel wall (56.0±6.4%) compared to the flexible (73.4±4.6%) and stiff (70.4±5.1%) closed cell design. With the presented study, we hope to contribute to and improve aneurysm treatment, using a novel patient specific environment as a possible pre-operative tool to evaluate mechanical stent behavior in different vascular geometries.

Sagittal plane hip motion reversals during walking are associated with disease severity and poorer function in subjects with hip osteoarthritis

Kharma C. Foucher, Bryan R. Schlink, Najia Shakoor, Markus A. Wimmer — 2012-04-12

Abstract: A midstance reversal of sagittal plane hip motion during walking, or motion discontinuity (MD), has previously been observed in subjects with endstage hip osteoarthritis (OA) and in patients with femoroacetabular impingement. The goal of the present study was to evaluate whether this gait pattern is a marker of OA presence or radiographic severity by analyzing a large IRB approved motion analysis data repository. We also hypothesized that subjects with the MD would show more substantial gait impairments than those with normal hip motion. We identified 150 subjects with symptomatic unilateral hip OA and Kellgren–Lawrence OA severity data on file, and a control group of 159 asymptomatic subjects whose ages fell within 2 standard deviations of the mean OA group age. From the gait data, the MD was defined as a reversal in the slope of the hip flexion angle curve during midstance. Logistic regressions and general linear models were used to test the association between the MD and OA presence, OA severity and, other gait variables. 53% of OA subjects compared to 7.5% of controls had the MD (p<0.001); occurrence of the MD was associated with OA severity (p=0.009). Within the OA subject group, subjects with the MD had reduced dynamic range of motion, peak, extension, and internal rotation moments compared to those who did not (MANCOVA p≤0.042) after controlling for walking speed. We concluded that sagittal plane motion reversals are indeed associated with OA presence and severity, and with more severe gait abnormalities in subjects with hip OA.

Detecting and quantifying global instability during a dynamic task using kinetic and kinematic gait parameters

Eytan M Debbi, Alon Wolf, Amir Haim — 2012-04-13

Abstract: Objectives: Instability during gait can be identified in many different ways. Recent studies have suggested utilizing spatiotemporal parameters to detect instability during gait. Detecting instability using kinetic and kinematic gait parameters has not yet been examined fully. In addition, these studies have not yet identified measures that are capable of assessing the magnitude of instability. The objective of the present study was to identify kinetic and kinematic gait parameters that can best identify instability and quantify its magnitude.Methods: Ten healthy men underwent successive gait analysis testing under three controlled settings: (1) Stage 0 instability (control setting), (2) Stage 1 instability and (3) Stage 2 instability. The levels of instability were precisely applied with the use of a controlled perturbation device (AposTherapy System). Differences between all stages and between stages were identified using Friedman and Wilcoxon tests.Results: Stride-to-stride variability (STSV) in kinetic and kinematic measures increased significantly between stages 0 and 1 or between stages 0 and 2 for almost all parameters (all P<0.05). A significant increase between stage 0 and both stages 1 and 2 was found for knee flexion moment, knee varus moment, knee flexion angle and hip adduction angle. The increase between stages 1 and 2 was variable. Only the knee varus moment parameter showed a significant increase in STSV between stages 1 and 2 (P=0.026).Conclusions: Almost all kinetic and kinematic gait parameters are sensitive to changes in global instability in a dynamic task. The most sensitive are parameters measured at the knee. Of these, STSV in knee varus moment can be used to quantify the magnitude of dynamic instability.

Mechanism of fractures of adjacent and augmented vertebrae following simulated vertebroplasty

Jaw-Lin Wang, Chun-Kai Chiang, Ya-Wen Kuo, Wen-Kai Chou, Been-Der Yang — 2012-03-26

Abstract: Percutaneous vertebroplasty (VP) is a minimally invasive procedure that is used to treat osteoporosis-induced vertebral compression fractures (OVCFs). Frequently observed complications are fractures of adjacent and augmented vertebrae. In the present work, mechanisms for these fractures are presented.Fresh 4-level osteoporotic thoracic motion segments were tested. Both ends of the specimen were mounted. The lower level of the free vertebra was compressively fractured and followed by an injection of a 3.5mL of a PMMA bone cement. Three steps of fatigue loading (5Hz for 5h) were incrementally and vertically applied on the specimens from 650N to 950N to 1150N. Specimens of intact, compressively fractured, cement augmented and post-fatigued loading were radiographed for the measurement of deformations of the vertebra, the canal, and the foramen. At the end of fatigue loading, the vertebrae were sliced for micro morphologic analysis.The largest height loss after fatigue loading was at the posterior region of the augmented vertebra. In the augmented vertebra, fissures were found along the bone-cement interface. These fissures split the cement and the trabeculae and propagated into the vertebrae and the endplates. The compactness ratio of the trabeculae region of the adjacent cranial vertebra was higher than that for intact and adjacent caudal ones. We attribute the fracture of the augmented vertebra, following simulated VP, to the initiation of fissures along the cement-bone interface, which, in turn, may be due to uneven deformation of the vertebra. Fracture of the adjacent cranial vertebra is attributed to collapse of its trabeculae.

Perturbation-dependent selection of postural feedback gain and its scaling

Seyoung Kim, Christopher G. Atkeson, Sukyung Park — 2012-03-26

Abstract: In this study we examined whether the selection of postural feedback gain and its scaling is dependent on perturbation type. We compare forward pushes applied to the back of a standing subject to previous work on responses to support translation. As was done in the previous work, we quantified the subject's response in terms of perturbation-dependent feedback gains. Seven healthy young subjects (25±3yr) experienced five different magnitudes of forward push applied by a 1.25m-long pendulum falling from the height of 1.4m toward the center of mass of the subject's torso. The loads on the pendulum ranged from 2 to 10kg. Impulsive force, ground reaction forces and joint kinematics were measured, and joint torques were calculated from inverse dynamics. A full-state feedback control model was used to quantify the empirical data, and the feedback gains that minimized the fitting error between the data and model simulation were identified. As in previously published feedback gains for support translation trials, gradual gain scaling with push perturbation magnitude was consistently observed, but a different feedback gain set was obtained. The results imply that the nervous system may be aware of body dynamics being subjected to various perturbation types and may select perturbation-dependent postural feedback gains that satisfy postural stability and feasible joint torque constraints.

Reduced knee joint moment in ACL deficient patients at a cost of dynamic stability during landing

2012-03-22

Abstract: The current study aimed to examine the effect of anterior cruciate ligament deficiency (ACLd) on joint kinetics and dynamic stability control after a single leg hop test (SLHT). Twelve unilateral ACLd patients and a control subject group (n=13) performed a SLHT over a given distance with both legs. The calculation of joint kinetics was done by means of a soft-tissue artifact optimized rigid full-body model. Margin of stability (MoS) was quantified by the difference between the base of support and the extrapolated center of mass. During landing, the ACLd leg showed lower external knee flexion moments but demonstrated higher moments at the ankle and hip compared to controls . The main reason for the joint moment redistribution in the ACLd leg was a more anterior position of the ground reaction force (GRF) vector, which affected the moment arms of the GRF acting about the joints . For the ACLd leg, trunk angle was more flexed over the entire landing phase compared to controls and we found a significant correlation between moment arms at the knee joint and trunk angle . The consequence of this altered landing strategy in ACLd legs was a more anterior position of the center of mass reducing the MoS . The results illustrate the interaction between trunk angle, joint kinetics and dynamic stability during landing maneuvers and provide evidence of a feedforward adaptive adjustment in ACLd patients (i.e. more flexed trunk angle) aimed at reducing knee joint moments at the cost of dynamic stability control.

Site specific inelasticity of arterial tissue

Eoghan Maher, Michael Early, Arthur Creane, Caitríona Lally, Daniel J. Kelly — 2012-03-26

Abstract: Understanding the mechanical behaviour of arterial tissue is vital to the development and analysis of medical devices targeting diseased vessels. During angioplasty and stenting, stress softening and permanent deformation of the vessel wall occur during implantation of the device, however little data exists on the inelastic behaviour of cardiovascular tissue and how this varies through the arterial tree. The aim of this study was to characterise the magnitude of stress softening and inelastic deformations due to loading throughout the arterial tree and to investigate the anisotropic inelastic behaviour of the tissue. Cyclic compression tests were used to investigate the differences in inelastic behaviour for carotid, aorta, femoral and coronary arteries harvested from 3–4 month old female pigs, while the anisotropic behaviour of aortic and carotid tissue was determined using cyclic tensile tests in the longitudinal and circumferential directions. The differences in inelastic behaviour were correlated to the ratio of collagen to elastin content of the arteries. It was found that larger inelastic deformations occurred in muscular arteries (coronary), which had a higher collagen to elastin ratio than elastic arteries (aorta), where the smallest inelastic deformations were observed. Lower magnitude inelastic deformations were observed in the circumferential tensile direction than in the longitudinal tensile direction or due to radial compression. This may be as a result of non-collagenous components in the artery becoming more easily damaged than the collagen fibres during loading. Stress softening was also found to be dependent on artery type. In the future, computational models should consider such site dependant, anisotropic inelastic behaviour in order to better predict the outcomes of interventional procedures such as angioplasty and stenting.

Flow resistance analysis of extracranial-to-intracranial (EC–IC) vein bypass

Y. Zhang, S.F. Sia, M.K. Morgan, Y. Qian — 2012-03-14

Abstract: Although brain bypass surgery has often been selected to treat internal carotid arteries (ICA) which are restricted by aneurysm or artery stenosis, its effectiveness has not been quantitatively evaluated. The purpose of this study is to propose an innovative approach for the evaluation of brain extracranial-to-intracranial (EC–IC) vein bypass surgery, based on the analysis of flow resistance in vein bypasses and within their contralateral carotid arteries through the use of computational fluid dynamics (CFD). Seven patients who underwent vein bypass surgery were examined with the use of high-resolution; computed tomography angiogram (CTA). The reconstructed three-dimensional (3D) geometries were segmented to create CFD calculation domains. Colour Doppler ultrasound (CDU) was used to measure blood flow velocities at the common carotid arteries (CCA), in order to determine inflow conditions. Based on the pipe flow theory, pressure drop was expressed as where A and B represent flow resistance coefficients and represents blood mass flow rate. The CFD results revealed that for a healthy ICA, the average values of A and B were 0.013088Pa/(ml/min)2 and 3.105Pa/(ml/min), respectively. For the vein bypass, an average value of A was 0.0143Pa/(ml/min)2 and B 3.402Pa/(ml/min), which was approximately that of a healthy ICA. However, in the case of a bypass utilising a venous conduit possessing a large-sized valve or existing size alteration, the flow resistance in that bypass would be higher than those found in the healthy ICA. An imbalance of flow resistances may impose conditions that could predispose hemodynamic failure or distal aneurysm development.

Sensitivity of maximum sprinting speed to characteristic parameters of the muscle force–velocity relationship

Ross H. Miller, Brian R. Umberger, Graham E. Caldwell — 2012-03-14

Abstract: An accumulation of evidence suggests that the force–velocity relationship (FVR) of skeletal muscle plays a major role in limiting maximum human sprinting speed. However, most of the theories on this limiting role have been non-specific as to how the FVR limits speed. The FVR is characterized by three parameters that each have a different effect on its shape, and could thus limit sprinting speed in different ways: the maximum shortening velocity Vmax, the shape parameter AR, and the eccentric plateau Cecc. In this study, we sought to determine how specifically the FVR limits sprinting speed using forward dynamics simulations of human locomotion to examine the sensitivity of maximum speed to these three FVR parameters. Simulations were generated by optimizing the model's muscle excitations to maximize the average horizontal speed. The simulation's speed, temporal stride parameters, joint angles, GRF, and muscle activity in general compared well to data from human subjects sprinting at maximum effort. Simulations were then repeated with incremental and isolated adjustments in Vmax, AR, and Cecc across a physiological range. The range of speeds (5.22–6.91ms−1) was most sensitive when Vmax was varied, but the fastest speed of 7.17ms−1 was attained when AR was set to its maximum value, which corresponded to all muscles having entirely fast-twitch fibers. This result was explained by the muscle shortening velocities, which tended to be moderate and within the range where AR had its greatest effect on the shape of the FVR. Speed was less sensitive to adjustments in Cecc, with a range of 6.23–6.70ms−1. Increases in speed with parameter changes were due to increases in stride length more so than stride frequency. The results suggest that the shape parameter AR, which primarily determines the amount of muscle force that can be produced at moderate shortening velocities, plays a major role in limiting the maximum sprinting speed. Analysis of muscle force sensitivity indicated support for previous theories on the time to generate support forces in stance (, Journal of Applied Physiology, 89, 1991–1999) and energy management of the leg in swing (, Journal of Biomechanics 16, 79–83) as important factors in limiting maximum speed. However, the ability of the knee flexors to slow the rotational velocity of the leg in preparation for footstrike did not appear to play a major role in limiting speed.

Geometry strongly influences the response of numerical models of the lumbar spine—A probabilistic finite element analysis

Frank Niemeyer, Hans-Joachim Wilke, Hendrik Schmidt — 2012-03-21

Abstract: Typical FE models of the human lumbar spine consider a single, fixed geometry. Such models cannot account for potential effects of the natural variability of the spine's geometry. In this study, we performed a probabilistic uncertainty and sensitivity analysis of a fully parameterized, geometrically simplified model of the L3–L4 segment.We examined the impact of the uncertainty in all 40 geometry parameters, estimated lower and upper bounds for the required sample size and determined the most important geometry parameters.The natural variability of the spine's geometry indeed strongly affects intradiscal pressure, range of motion and facet joint contact forces. Deriving generalized statements from fixed-geometry models as well as transferring those results to different cases thus can easily lead to wrong conclusions and should only be performed with extreme caution. We recommend a sample size of≈100 to obtain reasonable accurate point estimates and a sufficient overview of the remaining uncertainties. Yet, only few parameters, especially those determining the disc geometry (disc height, end-plate width and depth) and the facets' position (intra-articular space, pedicle length, facet angles), proved to be truly important. Accurate measurement and modeling of those structures should therefore be prioritized.

Evidence of changes in load sharing during isometric elbow flexion with ramped torque

Killian Bouillard, Antoine Nordez, Paul W Hodges, Christophe Cornu, François Hug — 2012-03-14

Abstract: This study aimed to: (1) test the repeatability of Supersonic Shear Imaging measures of muscle shear elastic modulus of four elbow flexor muscles during isometric elbow flexion with ramped torque; (2) determine the relationship between muscle shear elastic modulus and elbow torque for the four elbow flexor muscles, and (3) investigate changes in load sharing between synergist elbow flexor muscles with increases in elbow flexor torque. Ten subjects performed ten isometric elbow flexions consisting of linear torque ramps of 30-s from 0 to 40% of maximal voluntary contraction. The shear elastic modulus of each elbow flexor muscle (biceps brachii long head [BBLH], biceps brachii short head [BBSH], brachialis [BA], and brachoradialis [BR]) and of triceps brachii long head [TB] was measured twice with individual muscles recorded in separate trials in random order. A good repeatability of the shape of the changes in shear elastic modulus as a function of torque was found for each elbow flexor muscle (r-values: 0.85 to 0.94). Relationships between the shear elastic modulus and torque were best explained by a second order polynomial, except BA where a higher polynomial was required. Statistical analysis showed that BBSH and BBLH had an initial slow change at low torques followed by an increasing rate of increase in modulus with higher torques. In contrast, the BA shear elastic modulus increased rapidly at low forces, but plateaued at higher forces. These results suggest that changes in load sharing between synergist elbow flexors could partly explain the non-linear EMG-torque relationship classically reported for BB during isometric efforts.

Models incorporating pin joints are suitable for simulating performance but unsuitable for simulating internal loading

S.J. Allen, M.A. King, M.R. Yeadon — 2012-03-14

Abstract: Simulation models of human movement comprising pin-linked segments have a potential weakness for reproducing accurate ground reaction forces during high impact activities. While the human body contains many compliant structures such a model only has compliance in wobbling masses and in the foot–ground interface. In order to determine whether accurate GRFs can be produced by allowing additional compliance in the foot–ground interface, a subject-specific angle-driven computer simulation model of triple jumping with 13 pin-linked segments was developed, with wobbling masses included within the shank, thigh, and trunk segments. The foot–ground interface was represented by spring-dampers at three points on each foot: the toe, ball, and heel. The parameters of the spring-dampers were varied by a genetic algorithm in order to minimise the differences between simulated GRFs, and those measured from the three phases of a triple jump in three conditions: (a) foot spring compression limited to 20mm; (b) this compression limited to 40mm; (c) no restrictions. Differences of 47.9%, 15.7%, and 12.4% between simulation and recorded forces were obtained for the 20mm, 40mm, and unrestricted conditions, respectively. In the unrestricted condition maximum compressions of between 43mm and 56mm were obtained in the three phases and the mass centre position was within 4mm of the actual position at these times. It is concluded that the unrestricted model is appropriate for simulating performance whereas the accurate calculation of internal forces would require a model that incorporates compliance elsewhere in the link system.

Effective property of tooth enamel: Monoclinic behavior

Cunyou Lu, Toshio Nakamura, Chad S. Korach — 2012-03-09

Abstract: Human tooth enamel possesses a unique morphology characterized by a repeated cell arrangement, which is composed of varying orientations of hydroxyapatite crystals. In the past, various investigators have reported diverse mechanical properties based on isotropic or orthotropic mechanical models in their experimental and numerical studies. However, these models are insufficient to capture the accurate microstructural effects on the enamel mechanical response. In this paper, a monoclinic anisotropic model, which offers correct descriptions of enamel deformation behaviors, is introduced. The model takes into account the 3D orientation changes of the hydroxyapatite crystals and their spatial elastic property variations. The proposed approach is based on a unit-cell and periodic boundary conditions, and it utilizes the collective deformation characteristics of many rods to determine 13 independent material constants required for the monoclinic model. These constants are necessary to utilize the effective property model to study various mechanical conditions such as abrasion, erosion, wear and fracture of whole tooth enamel.

Effect of cytoskeletal elastic properties on the mechanoelectrical transduction in excitable cells

2012-03-09

Abstract: This paper addresses the possible mechanism of stretch on cell electrochemical potential change, based on the physicochemical properties of cytoskeletal network. Synthetic polyelectrolyte gel was used as an experimental model of the cytoskeleton. Gel samples with different density of network cross linking were studied. Triangular axial deformations of samples were applied. Simultaneously, the electrochemical (Donnan) potential of the gel was measured between a micropipette electrode pinned into the swollen gel, and a reference electrode in the outer solution. We found that axial deformation shifts the gel potential toward depolarization. The extent of gel depolarization showed a close negative correlation with the Young modulus of the gel. We suggest that the underlying mechanism is likely to be a universal process of counterion adsorption on charged polymer filaments due to the decrease of distance between polymer filaments owing to gel elongation.

The low permeability of healthy meniscus and labrum limit articular cartilage consolidation and maintain fluid load support in the knee and hip

Joseph M. Haemer, Dennis R. Carter, Nicholas J. Giori — 2012-03-05

Abstract: The knee meniscus and hip labrum appear to be important for joint health, but the mechanisms by which these structures perform their functions are not fully understood. The fluid phase of articular cartilage provides compressive stiffness and aids in maintaining a low friction articulation. Healthy fibrocartilage, the tissue of meniscus and labrum, has a lower fluid permeability than articular cartilage. In this study we hypothesized that an important function of the knee meniscus and the hip labrum is to augment fluid retention in the articular cartilage of a mechanically loaded joint. Axisymmetric hyperporoelastic finite element models were analyzed for an idealized knee and an idealized hip. The results indicate that the meniscus maintained fluid pressure and inhibited fluid exudation in knee articular cartilage. Similar, but smaller, effects were seen with the labrum in the hip. Increasing the fibrocartilage permeability relative to that of articular cartilage gave a consolidation rate and loss of fluid load support comparable to that predicted by meniscectomy or labrectomy. The reduced articular cartilage fluid pressure that was calculated for the joint periphery is consistent with patterns of endochondral ossification and osteophyte formation in knee and hip osteoarthritis. High articular central strains and loss of fluid load support after meniscectomy could lead to fibrillation. An intact low-permeability fibrocartilage is important for limiting fluid exudation from articular cartilage in the hip and knee. This may be an important aspect of the role of fibrocartilage in protecting these joints from osteoarthritis.

Evaluation of a robot-assisted testing system for multisegmental spine specimens

2012-03-05

Abstract: Mono- and multi-segmental testing methods are required to identify segmental motion patterns and evaluate the biomechanical behaviour of the spine. This study aimed to evaluate a new testing system for multisegmental specimens using a robot combined with an optical motion analysis system.After validation of the robotic system for accuracy, two groups of calf specimens (six monosegmental vs. six multisegmental) were mounted and the functional unit L3-4 was observed. Using rigid body markers, range of motion (ROM), elastic zone (EZ) and neutral zone (NZ), as well as stiffness properties of each functional spine unit (FSU) was acquired by an optical motion capture system. Finite helical axes (FHA) were calculated to analyse segmental movements. Both groups were tested in flexion and extension. A pure torque of 7.5Nm was applied. Statistical analyses were performed using the Mann–Whitney U-test.Repeatability of robot positioning was −0.001±0.018mm and −0.025±0.023° for translations and rotations, respectively. The accuracy of the optical system for the proposed set-up was 0.001±0.034mm for translations and 0.075±0.12° for rotations.No significant differences in mean values and standard deviations of ROM for L3-4 compared to literature data were found.A robot-based facility for testing multisegmental spine units combined with a motion analysis system was proposed and the reliability and reproducibility of all system components were evaluated and validated. The proposed set-up delivered ROM results for mono- and multi-segmental testing that agreed with those reported in the literature. Representing the FHA via piercing points determined from ROM was the first attempt showing a relationship between ROM and FHA, which could facilitate the interpretation of spine motion patterns in the future.

Sensitivity of model predictions of muscle function to changes in moment arms and muscle–tendon properties: A Monte-Carlo analysis

David C. Ackland, Yi-Chung Lin, Marcus G. Pandy — 2012-04-16

Abstract: Hill-type muscle models are commonly used in musculoskeletal models to estimate muscle forces during human movement. However, the sensitivity of model predictions of muscle function to changes in muscle moment arms and muscle–tendon properties is not well understood. In the present study, a three-dimensional muscle-actuated model of the body was used to evaluate the sensitivity of the function of the major lower limb muscles in accelerating the whole-body center of mass during gait. Monte-Carlo analyses were used to quantify the effects of entire distributions of perturbations in the moment arms and architectural properties of muscles. In most cases, varying the moment arm and architectural properties of a muscle affected the torque generated by that muscle about the joint(s) it spanned as well as the torques generated by adjacent muscles. Muscle function was most sensitive to changes in tendon slack length and least sensitive to changes in muscle moment arm. However, the sensitivity of muscle function to changes in moment arms and architectural properties was highly muscle-specific; muscle function was most sensitive in the cases of gastrocnemius and rectus femoris and insensitive in the cases of hamstrings and the medial sub-region of gluteus maximus. The sensitivity of a muscle's function was influenced by the magnitude of the muscle's force as well as the operating region of the muscle on its force-length curve. These findings have implications for the development of subject-specific models of the human musculoskeletal system.

A least-squares identification algorithm for estimating squat exercise mechanics using a single inertial measurement unit

Vincent Bonnet, Claudia Mazzà, Philippe Fraisse, Aurelio Cappozzo — 2012-03-09

Abstract: This study investigated the possibility of estimating lower-limb joint kinematics during a squat exercise performed in the sagittal plane based on data collected from a single inertial measurement unit located on the lower trunk. The human body was modeled as a three-degrees-of-freedom planar chain and the relevant joint angles (ankle, knee, and hip) are represented by Fourier series. A least-squares approach based on the minimization of the difference between the measured and estimated linear accelerations and the angular velocity of the lower trunk was used to solve the related analytical problem. The approach was validated on ten healthy young volunteers (ten trials each) using a force plate and a stereophotogrammetric system to collect reference data. The root mean square differences between the estimated joint angles and those reconstructed with the stereophotogrammetric system were lower than 4° with correlation coefficients higher than 0.99. The ankle joint resultant vertical force component was estimated with an accuracy of about 3% and a high correlation coefficient of r=0.95, whereas much lower percentage accuracies were found for the horizontal force and couple components. The latter accuracies were similar to those affecting these force and couple components as estimated through inverse dynamics and the stereophotogrammetric data in conjunction with the same mechanical model, which suggests that only minor errors were introduced by the proposed algorithm and measurement tools.

The mechanical behavior of PMMA/bone specimens extracted from augmented vertebrae: A numerical study of interface properties, PMMA shrinkage and trabecular bone damage

M. Kinzl, A. Boger, P.K. Zysset, D.H. Pahr — 2012-03-05

Abstract: Recently published compression tests on PMMA/bone specimens extracted after vertebral bone augmentation indicated that PMMA/bone composites were not reinforced by the trabecular bone at all. In this study, the reasons for this unexpected behavior should be investigated by using non-linear micro-FE models. Six human vertebral bodies were augmented with either standard or low-modulus PMMA cement and scanned with a HR-pQCT system before and after augmentation. Six cylindrical PMMA/bone specimens were extracted from the augmented region, scanned with a micro-CT system and tested in compression. Four different micro-FE models were generated from these images which showed different bone tissue material behavior (with/without damage), interface behavior (perfect bonding, frictionless contact) and PMMA shrinkage due to polymerization. The non-linear stress–strain curves were compared between the different micro-FE models as well as to the compression tests of the PMMA/bone specimens. Micro-FE models with contact between bone and cement were 20% more compliant compared to those with perfect bonding. PMMA shrinkage damaged the trabecular bone already before mechanical loading, which further reduced the initial stiffness by 24%. Progressing bone damage during compression dominated the non-linear part of the stress–strain curves. The micro-FE models including bone damage and PMMA shrinkage were in good agreement with the compression tests. The results were similar with both cements. In conclusion, the PMMA/bone interface properties as well as the initial bone damage due to PMMA polymerization shrinkage clearly affected the stress–strain behavior of the composite and explained why trabecular bone did not contribute to the stiffness and strength of augmented bone.

Simulation of lower limb axial arterial length change during locomotion

2012-03-05

Abstract: The effect of external forces on axial arterial wall mechanics has conventionally been regarded as secondary to hemodynamic influences. However, arteries are similar to muscles in terms of the manner in which they traverse joints, and their three-dimensional geometrical requirements for joint motion. This study considers axial arterial shortening and elongation due to motion of the lower extremity during gait, ascending stairs, and sitting-to-standing motion. Arterial length change was simulated by means of a graphics based anatomic and kinematic model of the lower extremity. This model estimated the axial shortening to be as much as 23% for the femoropopliteal arterial region and as much as 21% for the iliac artery. A strong correlation was observed between femoropopliteal artery shortening and maximum knee flexion angle (r2=0.8) as well as iliac artery shortening and maximum hip angle flexion (r2=0.9). This implies a significant mechanical influence of locomotion on arterial behavior in addition to hemodynamics factors. Vascular tissue has high demands for axial compliance that should be considered in the pathology of atherosclerosis and the design of vascular implants.

Optimizing whole-body kinematics to minimize valgus knee loading during sidestepping: Implications for ACL injury risk

C.J. Donnelly, D.G. Lloyd, B.C. Elliott, J.A. Reinbolt — 2012-03-05

Abstract: The kinematic mechanisms associated with elevated externally applied valgus knee moments during non-contact sidestepping and subsequent anterior cruciate ligament (ACL) injury risk are not well understood. To address this issue, the residual reduction algorithm (RRA) in OpenSim was used to create nine subject-specific, full-body (37 degrees of freedom) torque-driven simulations of athletic males performing unplanned sidestep (UnSS) sport tasks. The RRA was used again to produce an optimized kinematic solution with reduced peak valgus knee torques during the weight acceptance phase of stance. Pre-to-post kinematic optimization, mean peak valgus knee moments were significantly reduced by 44.2Nm (p=0.045). Nine of a possible 37 upper and lower body kinematic changes in all three planes of motion were consistently used during the RRA to decrease peak valgus knee moments. The generalized kinematic strategy used by all nine simulations to reduce peak valgus knee moments and subsequent ACL injury risk during UnSS was to redirect the whole-body center of mass medially, towards the desired direction of travel.

Improving the validation of finite element models with quantitative full-field strain comparisons

F. Gröning, J.A. Bright, M.J. Fagan, P. O'Higgins — 2012-03-02

Abstract: The techniques used to validate finite element (FE) models against experimental results have changed little during the last decades, even though the traditional approach of using single point measurements from strain gauges has major limitations: the strain distribution across the surface is not captured and the accurate determination of strain gauge positions on the model surface is difficult if the 3D surface topography of the bone surface is not measured. The full-field strain measurement technique of digital speckle pattern interferometry (DSPI) can overcome these problems, but the potential of this technique has not yet been fully exploited in validation studies. Here we explore new ways of quantifying and visualising the variation in strain magnitudes and orientations within and between repeated DSPI measurements as well as between the DSPI measurements and FEA results. We show that our approach provides a much more comprehensive and accurate validation than traditional methods. The measurement repeatability and the correspondence between measured and predicted strains vary to a great degree within and between measurement areas. The two models used in this study predict the measured strain directions and magnitudes surprisingly well considering that homogeneous and isotropic mechanical properties were assigned to the models. However, the full-field comparisons also reveal some discrepancies between measured and predicted strains that are most probably caused by inaccuracies in the models' geometries and the degree of simplification of the modelled material properties.

Robust estimation of physiological cross-sectional area and geometric reconstruction for human skeletal muscle

Dongwoon Lee, Kajeandra Ravichandiran, Ken Jackson, Eugene Fiume, Anne Agur — 2012-03-14

Abstract: Understanding muscle architecture is crucial to determining the mechanical function of muscle during body movements, because architectural parameters directly correspond to muscle performance. Accurate parameters are thus essential for reliable simulation. Human cadaveric muscle specimen data provides the anatomical detail needed for in-depth understanding of muscle and accurate parameter estimation. However, as muscle generally has non-uniform architecture, parameter estimation, specifically, physiological cross-sectional area (PCSA), is rarely straightforward. To deal effectively with this non-uniformity, we propose a geometric approach in which a polygon is sought to best approximate the cross-sectional area of each fascicle by accounting for its three-dimensional trajectory and arrangement in the muscle. Those polygons are then aggregated to determine PCSA and volume of muscle. Experiments are run using both synthetic data and muscle specimen data. From comparison of PCSA using synthetic data, we conclude that the proposed method enhances the robustness of PCSA estimation against variation in muscle architecture. Furthermore, we suggest reconstruction methods to extract 3D muscle geometry directly from fascicle data and estimated parameters using the level set method.

In-vivo transducer to measure dynamic mitral annular forces

2012-04-09

Abstract: Limited knowledge exists regarding the forces which act on devices implanted to the heart's mitral valve. Developing a transducer to measure the peak force magnitudes, time rates of change, and relationship with left ventricular pressure will aid in device development. A novel force transducer was developed and implanted in the mitral valve annulus of an ovine subject. In the post-cardioplegic heart, septal-lateral and transverse forces were continuously measured for cardiac cycles reaching a peak left ventricular pressure of 90mmHg. Each force was seen to increase from ventricular diastole and found to peak at mid-systole. The mean change in septal-lateral and transverse forces throughout the cardiac cycle was 4.4±0.2N and 1.9±0.1N respectively. During isovolumetric contraction, the septal-lateral and transverse forces were found to increase at peak rate of 143±8N/s and 34±9N/s, respectively. Combined, this study provides the first quantitative assessment of septal-lateral and transverse forces within the contractile mitral annulus. The developed transducer was successful in measuring these forces whose methods may be extended to future studies. Upon additional investigation, these data may contribute to the safer development and evaluation of devices aimed to repair or replace mitral valve function.

A platform for dynamic simulation and control of movement based on OpenSim and MATLAB

Misagh Mansouri, Jeffrey A. Reinbolt — 2012-04-05

Abstract: Numerical simulations play an important role in solving complex engineering problems and have the potential to revolutionize medical decision making and treatment strategies. In this paper, we combine the rapid model-based design, control systems and powerful numerical method strengths of MATLAB/Simulink with the simulation and human movement dynamics strengths of OpenSim by developing a new interface between the two software tools. OpenSim is integrated with Simulink using the MATLAB S-function mechanism, and the interface is demonstrated using both open-loop and closed-loop control systems. While the open-loop system uses MATLAB/Simulink to separately reproduce the OpenSim Forward Dynamics Tool, the closed-loop system adds the unique feature of feedback control to OpenSim, which is necessary for most human movement simulations. An arm model example was successfully used in both open-loop and closed-loop cases. For the open-loop case, the simulation reproduced results from the OpenSim Forward Dynamics Tool with root mean square (RMS) differences of 0.03° for the shoulder elevation angle and 0.06° for the elbow flexion angle. MATLAB's variable step-size integrator reduced the time required to generate the forward dynamic simulation from 7.1s (OpenSim) to 2.9s (MATLAB). For the closed-loop case, a proportional–integral–derivative controller was used to successfully balance a pole on model's hand despite random force disturbances on the pole. The new interface presented here not only integrates the OpenSim and MATLAB/Simulink software tools, but also will allow neuroscientists, physiologists, biomechanists, and physical therapists to adapt and generate new solutions as treatments for musculoskeletal conditions.

Accuracy and precision of hind limb foot contact timings of horses determined using a pelvis-mounted inertial measurement unit

Sandra D. Starke, Thomas H. Witte, Stephen A. May, Thilo Pfau — 2012-04-09

Abstract: Gait analysis using small sensor units is becoming increasingly popular in the clinical context. In order to segment continuous movement from a defined point of the stride cycle, knowledge about footfall timings is essential. We evaluated the accuracy and precision of foot contact timings of a defined limb determined using an inertial sensor mounted on the pelvis of ten horses during walk and trot at different speeds and in different directions. Foot contact was estimated from vertical velocity events occurring before maximum sensor roll towards the contralateral limb. Foot contact timings matched data from a synchronised hoof mounted accelerometer well when velocity minimum was used for walk (mean (SD) difference of 15 (18)ms across horses) and velocity zero-crossing for trot (mean (SD) difference from −4 (14) to 12 (7)ms depending on the condition). The stride segmentation method also remained robust when applied to movement data of hind limb lame horses. In future, this method may find application in segmenting overground sensor data of various species.

Stance and swing phase detection during level and slope walking in the cat: Effects of slope, injury, subject and kinematic detection method

Annette Pantall, Robert J. Gregor, Boris I. Prilutsky — 2012-04-09

Abstract: In quadrupeds, there have been limited comparisons of gait timing events detection (e.g., paw contact, PC and paw-off, PO) determined from kinematics and forceplates. The goal of this study was to investigate the effect of different slopes (0, −27, +27°), recovery times after ankle extensor nerve injury and repair (2, 6, 12 weeks), subjects and detection methods on accuracy of kinematically derived PC and PO timings during feline walking. Right hindlimb kinematics and ground reaction forces (GRF) of 4 cats walking along a sloped walkway with embedded forceplates were recorded. A total of 963 walking cycles were analyzed. Gait timings were determined from five kinematic methods based on displacements, velocities or accelerations of hindlimb markers. GRF based ‘gold standard’ timings for PC and PO were used to determine the systematic and random error of kinematic timing. Systematic errors between the kinematic methods differed significantly (p<0.05). Methods based on vertical paw peak acceleration and velocity gave the smallest systematic errors for PC and PO, respectively. The smallest random errors (standard deviations) for PC and PO were demonstrated by method based on paw horizontal displacement relative to greater trochanter: 13.4ms and 6.6ms, respectively. Effects of slope and subject on systematic errors of kinematic methods were significant, whereas effects of recovery time after nerve injury were not. It was concluded that timing of gait events can be determined consistently using kinematics, although adjustments must be made to account for the systematic error which varies according to subject and slope condition.

Noninvasive high resolution mechanical strain maps of the spine intervertebral disc using nonrigid registration of magnetic resonance images

David A. Reiter, Fadi A. Fathallah, Rida T. Farouki, Jeffery H. Walton — 2012-04-16

Abstract: High resolution strain measurements are of particular interest in load bearing tissues such as the intervertebral disc (IVD), permitting characterization of biomechanical conditions which could lead to injury and degenerative outcomes. Magnetic resonance (MR) imaging produces excellent image contrast in cartilaginous tissues, allowing for image-based strain determination. Nonrigid registration (NRR) of MR images has previously demonstrated sub-voxel registration accuracy although its accuracy and precision in determining strain has not been evaluated. Accuracy and precision of NRR-derived strain measurements were evaluated using computer generated deformations applied to both computer generated images and MR images. Two different measures of registration similarity—the cost function which drives the registration algorithm—were compared: Mutual Information (MI) and Least Squares (LS). Strain error was evaluated with respect to signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and strain heterogeneity. Additionally, the creep strain response from an in vitro loaded porcine IVD is shown and comparisons between similarity measures are presented. MI showed a decrease in strain precision with increasing CNR and decreasing SNR while LS was insensitive to both. Both similarity measures showed a decrease in strain precision with increasing strain heterogeneity. When computer generated heterogeneous strains were applied to MR images of the IVD, LS showed substantially lower strain error in comparison to MI. Results suggest that LS-driven NRR provides a more accurate image-based method for mapping large and heterogeneous strain fields and this method can be applied to studies of the IVD and, potentially, other soft tissues which present sufficient image texture.

Markerless Roentgen Stereophotogrammetric Analysis for in vivo implant migration measurement using three dimensional surface models to represent bone

2012-04-05

Abstract: Recent studies have shown that model-based RSA using implant surface models to detect in vivo migration is as accurate as the classical marker-based RSA method. Use of bone surface models would be a further advancement of the model-based method by decreasing complications arising from marker insertion. The aim of this pilot investigation was to assess the feasibility of a “completely markerless” model-based RSA in detecting migration of an implant using bone surface models instead of bone markers. A total knee arthroplasty (TKA) was performed on a human cadaver knee, which was subsequently investigated by repeated RSA measurements performed by one observer. The cadaver knee was CT scanned prior to implantation of the TKA. Tibia–fibular surface models were created using two different commercially available software packages to investigate the effect of segmentation software on the accuracy of repeated migration measures of zero displacement by one observer. Reverse engineered surface models of the TKA tibial component were created. The analysis of the RSA images was repeated 10 times by one individual observer. For the markerless method, the greatest apparent migration observed about the three anatomical axes investigated was between −2.08 and 1.35mm (SD≤0.88) for z-axis translation, and −4.57° to 7.86° (SD≤3.17) for Ry-axis rotation, which were well beyond out of the range of what is typically considered adequate for clinically relevant RSA measurements. Use of tibia–fibular surface models of the bone instead of markers could provide practical advantages in evaluating implant migration. However, we found the accuracy and precision of the markerless approach to be lower than that of marker-based RSA, to a degree which precludes the use of this method for measuring implant migration in its present form.

Modular and scalable load-wall sled buck for pure-lateral and oblique side impact tests

Narayan Yoganandan, John R. Humm, Frank A. Pintar — 2012-04-09

Abstract: A considerable majority of side impact sled tests using different types of human surrogates has used a load-wall design not specific to subject anthropometry. The use of one load-wall configuration cannot accurately isolate and evaluate regional responses for the same load-wall geometry. As the anatomy and biomechanical responses of the human torso depends on the region, and anthropomorphic test devices continue to advance and accommodate regional differences, it is important to obtain specific data from sled tests. To achieve this goal, the present study designed a scalable modular load-wall consisting of the shoulder, thorax, abdomen, and superior and inferior pelvis, and lower limb plates. The first five plates were connected to a vertical fixture and the limb plate was connected to another fixture. The width, height, and thickness, and the gap between plates were modular. Independent adjustments in the coronal and sagittal planes allowed region-specific positioning depending on surrogate anthropometry, example pelvis width and seated height. Two tri-axial load cells were fixed on the contralateral face of each plate of the load-wall to record impact force-time histories. The load-wall and vertical fixture design can be used to conduct side impact tests with varying vectors, pure-lateral to anterior and posterior oblique, by appropriately orienting the load-wall with respect to the surrogate. The feasibility of the design to extract region-specific biomechanical data was demonstrated by conducting pure-lateral and anterior oblique sled tests using two different surrogates at a velocity of 6.7m/s. Uses of this design are discussed for different applications.

Development and evaluation of multiple tendon injury models in the mouse

2012-03-09

Abstract: The mouse has proven to be an advantageous animal model system in basic science research focused on aiding in development and evaluation of potential treatments; however, the small size of mouse tendons makes consistent and reproducible injury models and subsequent biomechanical evaluation challenging for studying tendon healing. In this study, we investigated the feasibility and reproducibility of multiple mouse tendon injury models. Our hypothesis was that incisional (using a blade) and excisional (using a biopsy punch) injuries would result in consistent differences in tendon material properties. At 16 weeks of age, 17 C57BL/6 mice underwent surgery to create defects in the flexor digitorum longus, Achilles, or patellar tendon. Each animal received 1–2 full-thickness, central-width incisional or excisional injuries per limb; at least one tendon per limb remained uninjured. The injuries were distributed such that each tendon type had comparable numbers of uninjured, incisionally injured, and excisionally injured specimens. Three weeks after injury, all animals were euthanized and tendons were harvested for mechanical testing. As hypothesized, differences were detected for all three different tendon types at three weeks post-injury. While all models created injuries that produced predictable outcomes, the patellar tendon model was the most consistent in terms of number and size of significant differences in injured tendons compared to native properties, as well as in the overall variance in the data. This finding provides support for its use in fundamental tendon healing studies; however, future work may use any of these models, based on their appropriateness for the specific question under study.

Measuring dynamic stability requirements during sitting pivot transfers using stabilizing and destabilizing forces in individuals with complete motor paraplegia

Dany Gagnon, Cyril Duclos, Pierre Desjardins, Sylvie Nadeau, Michel Danakas — 2012-03-15

Abstract: Dynamic stability requirements have never been quantified when long-term manual wheelchair users transfer themselves in a seated position from an initial surface to a target surface, a functional task commonly referred to as sitting pivot transfers (SPTs). Ten individuals with spinal cord injury (SCI), who rely on a manual wheelchair for mobility, underwent a comprehensive biomechanical SPT assessment. SPTs performed toward a target seat of same height (even) and a seat 10cm higher than the initial seat (uneven), repeated three times for each task, were assessed. A dynamic equilibrium model, continuously measuring the theoretical forces required to move the center of pressure to the limit of the base of support (destabilizing force) and to neutralize the kinetic energy and stop the displacement of the center of mass at the limit of the base of support (stabilizing force) at each instance during the performance of SPTs, was used to identify the phases of greatest instability during the SPT tasks. The greatest levels of instability were reached around the time the buttocks lost contact with the initial seat and around the time the buttocks landed on the target seat (pre- and post-lift transition phases). These transition periods, characterized by the lowest destabilizing force (424.7–487.1N) and the greatest stabilizing force (24.2–33.2N), confirmed the greatest level of instability. The height of the target seat had no significant effect (p=0.278–0.739) on dynamic postural stability requirements during the SPTs. During SPTs towards even and uneven target seats, the greatest postural instability occurs during the transition phases in individuals with complete motor thoracic SCI.