Hot off the presses! Mar 15 J Biomech

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Latest Articles Include:

• Editorial Board and Publication Information
  - J Biomech 44(5):IFC (2011)
  
• Invariant hip moment pattern while walking with a robotic hip exoskeleton
  - J Biomech 44(5):789-793 (2011)
  Robotic lower limb exoskeletons hold significant potential for gait assistance and rehabilitation; however, we have a limited understanding of how people adapt to walking with robotic devices. The purpose of this study was to test the hypothesis that people reduce net muscle moments about their joints when robotic assistance is provided. This reduction in muscle moment results in a total joint moment (muscle plus exoskeleton) that is the same as the moment without the robotic assistance despite potential differences in joint angles. To test this hypothesis, eight healthy subjects trained with the robotic hip exoskeleton while walking on a force-measuring treadmill. The exoskeleton provided hip flexion assistance from approximately 33% to 53% of the gait cycle. We calculated the root mean squared difference (RMSD) between the average of data from the last 15 min of the powered condition and the unpowered condition. After completing three 30-min training sessions, the hi! p exoskeleton provided 27% of the total peak hip flexion moment during gait. Despite this substantial contribution from the exoskeleton, subjects walked with a total hip moment pattern (muscle plus exoskeleton) that was almost identical and more similar to the unpowered condition than the hip angle pattern (hip moment RMSD 0.027, angle RMSD 0.134, p<0.001). The angle and moment RMSD were not different for the knee and ankle joints. These findings support the concept that people adopt walking patterns with similar joint moment patterns despite differences in hip joint angles for a given walking speed.
• Finite element simulations of a focal knee resurfacing implant applied to localized cartilage defects in a sheep model
  - J Biomech 44(5):794-801 (2011)
  Articular resurfacing metal implants have recently been tested in animal models to treat full thickness localized articular cartilage defects, showing promising results. However, the mechanical behavior of cartilage surrounding the metal implant has not been studied yet as it is technically challenging to measure in vivo contact areas, pressures, stresses and deformations from the metal implant. Therefore, we implemented a detailed numerical finite element model by approximating one of the condyles of the sheep tibiofemoral joint and created a defect of specific size to accommodate the implant. Using this model, the mechanical behavior of the surrounding of metal implant was studied. The model showed that the metal implant plays a significant role in the force transmission. Two types of profiles were investigated for metal implant. An implant with a double-curved profile, i.e., a profile fully congruent with the articular surfaces in the knee, gives lower contact press! ures and stresses at the rim of the defect than the implant with unicurved spherical profile. The implant should be placed at a certain distance into the cartilage to avoid damage to opposing biological surface. Too deep positions, however, lead to high shear stresses in the cartilage edges around the implant. Mechanical sealing was achieved with a wedge shape of the implant, also useful for biochemical sealing of cartilage edges at the defect.
• Biomechanical outcomes and neural correlates of cutaneous reflexes evoked during rhythmic arm cycling
  - J Biomech 44(5):802-809 (2011)
  During walking cutaneous stimulation of the foot yields neural and mechanical reflexes that serve a functional purpose to correct or assist the ongoing movement. Concurrently, while cutaneous stimulation of the hand during rhythmic arm movement parallel the neural responses observed in the legs, studies of rhythmic arm movement have only limited mechanical measurements. Therefore it is difficult to determine whether reflex responses in the arms during rhythmic arm movement serve a functional purpose similar to those seen in the lower limbs. The purpose of this study was to explore the mechanical outcomes of stimulating a cutaneous nerve innervating the hand during arm cycling. We hypothesized that there would be measurable mechanical effects to cutaneous stimulation during arm cycling that function to correct or assist the task of arm cycling. Specifically, kinetic responses measured at the handle would be considered assistive if they were tangential to the arm cycling! path in the direction of forward progression. Also, limb kinematic responses would be considered corrective if they allowed limb movement that would result in removal of the limb from stimulus while not altering the kinetic profile at the handle necessary for arm cycling progression. Participants performed seated arm cycling while EMG was recorded from the arm and trunk muscles, kinematic data was recorded from the right arm, and kinetic data was recorded from the handle. Cutaneous reflexes were evoked by stimulating the superficial radial nerve. The results show that there are observable mechanical responses to cutaneous stimulation of the hand during arm cycling. Subjects responded to cutaneous stimulation of the hand during arm cycling with significant changes in backward and lateral forces at the handle as well as wrist abduction/adduction and wrist flexion/extension kinematics. These responses, related to the task and phase of movement, are consistent with the anatomi! cal location of the stimulus and are correlated to the neural ! responses. Therefore, these responses are comparable to functionally relevant responses in the legs during rhythmic movement. However, while there is a single observation of a kinematic corrective strategy, the kinetics measured at the handle are not tangential to the arm cycling path and therefore not considered an assistive response. Therefore, unlike the observations in the lower limbs, the mechanical responses during arm cycling are not clearly related to the functional context of the ongoing task.
• RandomPOD—A new method and device for advanced wear simulation of orthopaedic biomaterials
  - J Biomech 44(5):810-814 (2011)
  A 16-station wear simulator of the pin-on-disc type, called RandomPOD, was designed, built, and validated. The primary area of application of the RandomPOD is wear studies of orthopaedic biomaterials. The type of relative motion between the bearing surfaces, generally illustrated as shapes of slide tracks, has been found to have a strong effect on the type and amount of wear produced. The computer-controlled RandomPOD can be programmed to produce virtually any slide track shape and load profile. In the present study, the focus is on the biomechanically realistic random variation in the track shape and load. In the reference test, the established combination of circular translation and static load was used. In addition, the combinations of random motion/static load, and circular translation/random load were included. The pins were conventional ultra-high molecular weight polyethylene (UHMWPE), the discs were polished CoCr, and the lubricant was diluted calf serum. The U! HMWPE wear factor resulting from random motion was significantly higher than that resulting from circular translation. This was probably caused by the fact that in the random motion the direction of sliding changed more than in circular translation with the same sliding distance. The type of load, random vs. static, was unimportant with respect to the wear factor produced. The principal advantage of using the present random track is that possible unrealistic wear phenomena related to the use of fixed track shapes can be avoided.
• Indentation measurements of the subendothelial matrix in bovine carotid arteries
  - J Biomech 44(5):815-821 (2011)
  Artery biomechanics are an important factor in cardiovascular function and atherosclerosis development; as such, the macro-mechanics of whole arteries are well-characterized. However, much less is known about the mechanical properties of individual layers in the blood vessel wall. Since there is significant evidence to show that cells can sense the mechanical properties of their matrix, it is critical to characterize the mechanical properties of these individual layers at the scale sensed by cells. Here, we measured subendothelium mechanics in bovine carotid arteries using atomic force microscopy (AFM) indentation. To specifically indent the subendothelium, we evaluated three potential de-endothelialization methods: scraping, paper imprinting, and saponin incubation. Using scanning electron microscopy, histology stains, immunohistochemistry, and multiphoton microscopy, we found that scraping was the only effective de-endothelialization method capable of removing endoth! elial cells and leaving the subendothelial matrix largely intact. To determine the indentation modulus of the subendothelial matrix, both untreated and scraped (de-endothelialized) bovine carotid arteries were indented with a spherical AFM probe and the data were fit using the Hertz model. Both the endothelium on the untreated artery and the en face subendothelium had similar indentation moduli: E=2.5±1.9 and 2.7±1.1 kPa, respectively. These measurements are the first to quantify the micro-scale mechanics of the subendothelial layer, and constitute a critical step in understanding the relationship between altered subendothelial micromechanics and disease progression.
• Could intra-tendinous hyperthermia during running explain chronic injury of the human Achilles tendon?
  - J Biomech 44(5):822-826 (2011)
  Chronic tendinopathy of the human Achilles tendon (AT) is common but its injury mechanism is not fully understood. It has been hypothesised that heat energy losses from the AT during running could explain the degeneration of AT material seen with injury. A mathematical model of AT temperature distribution was used to predict what temperatures the core of the AT could reach during running. This model required input values for mechanical properties of the AT (stiffness, hysteresis, cross-sectional area (CSA), strain during running) which were determined using a combination of ultrasound imaging, kinematic and kinetic data. AT length data were obtained during hopping and treadmill running (12 kmph) using ultrasound images of the medial gastrocnemius (50 Hz) and kinematic data (200 Hz). AT force data were calculated from inverse dynamics during hopping and combined with AT length data to compute AT stiffness and hysteresis. AT strain was computed from AT length data during! treadmill running. AT CSA was measured on transverse ultrasound scans of the AT. Mean±sd tendon properties were: stiffness=176±41 N mm−1, hysteresis=17±12%, strain during running=3.5±1.8% and CSA=42±8 mm2. These values were input into the model of AT core temperature and this was predicted to reach at least 41 °C during running. Such temperatures were deemed to be conservative estimates but still sufficient for tendon hyperthermia to be a potential cause of tendon injury.
• Investigation of hemodynamics in the development of dissecting aneurysm within patient-specific dissecting aneurismal aortas using computational fluid dynamics (CFD) simulations
  - J Biomech 44(5):827-836 (2011)
  Aortic dissecting aneurysm is one of the most catastrophic cardiovascular emergencies that carries high mortality. It was pointed out from clinical observations that the aneurysm development is likely to be related to the hemodynamics condition of the dissected aorta. In order to gain more insight on the formation and progression of dissecting aneurysm, hemodynamic parameters including flow pattern, velocity distribution, aortic wall pressure and shear stress, which are difficult to measure in vivo, are evaluated using numerical simulations. Pulsatile blood flow in patient-specific dissecting aneurismal aortas before and after the formation of lumenal aneurysm (pre-aneurysm and post-aneurysm) is investigated by computational fluid dynamics (CFD) simulations. Realistic time-dependent boundary conditions are prescribed at various arteries of the complete aorta models. This study suggests the helical development of false lumen around true lumen may be related to the helic! al nature of hemodynamic flow in aorta. Narrowing of the aorta is responsible for the massive recirculation in the poststenosis region in the lumenal aneurysm development. High pressure difference of 0.21 kPa between true and false lumens in the pre-aneurismal aorta infers the possible lumenal aneurysm site in the descending aorta. It is also found that relatively high time-averaged wall shear stress (in the range of 4–8 kPa) may be associated with tear initiation and propagation. CFD modeling assists in medical planning by providing blood flow patterns, wall pressure and wall shear stress. This helps to understand various phenomena in the development of dissecting aneurysm.
• The influence of component design, bearing clearance and axial load on the squeaking characteristics of ceramic hip articulations
  - J Biomech 44(5):837-841 (2011)
  Squeaking of hip replacements with ceramic-on-ceramic bearings has put the use of this material into question despite its superior wear behavior. Squeaking has been related to implant design. The purpose of this study was to determine the influence of particular acetabular cup and femoral stem designs on the incidence of squeaking and its characteristics. The dynamic behavior of the stem, head and stem assembled with head was investigated by determining their eigenfrequencies using experimental and numerical modal analysis. Four different stem and three different cup designs were investigated. Operational system vibrations resulting in audible squeaking were reproduced in a hip simulator and related to the respective component eigenfrequencies. The applied joint load and bearing clearance were varied in the clinically relevant range. Stems with lower eigenfrequencies were related to lower squeaking frequencies and increased acoustic pressure (loudness), and therefore to a higher susceptibility to squeaking. Higher load increased the squeaking frequency, while the acoustic pressure remained unchanged. No influence of the clearance or the cup design was found. Stem design was found to have an important influence on squeaking characteristics and its incidence, confirming and explaining similar clinical observations. Cup design itself was found to have no major influence on the dynamic behavior of the system but plays an important indirect role in influencing the magnitude of friction: Squeaking only occurs if the friction in the joint articulation is sufficient to excite vibrations to audible magnitudes. If friction is low, no squeaking occurs with any of the designs investigated.
• Influence of arterial wall-stenosis compliance on the coronary diagnostic parameters
  - J Biomech 44(5):842-847 (2011)
  Functional diagnostic parameters such as Fractional Flow Reserve (FFR), which is calculated from pressure measurements across stenosed arteries, are often used to determine the functional severity of coronary artery stenosis. This study evaluated the effect of arterial wall-stenosis compliance, with limiting scenarios of stenosis severity, on the diagnostic parameters. The diagnostic parameters considered in this study include an established index, FFR and two recently developed parameters: Pressure Drop Coefficient (CDP) and Lesion Flow Coefficient (LFC). The parameters were assessed for rigid artery (RR; signifying high plaque elasticity), compliant artery with calcified plaque (CC; intermediate plaque elasticity) and compliant artery with smooth muscle cell proliferation (CS; low plaque elasticity), with varying degrees of epicardial stenosis. A hyperelastic Mooney–Rivlin model was used to model the arterial wall and plaque materials. Blood was modeled as a shear thinning, non-Newtonian fluid using the Carreau model. The arterial wall compliance was evaluated using the finite element method. The present study found that, with an increase in stenosis severity, FFR decreased whereas CDP and LFC increased. The cutoff value of 0.75 for FFR was observed at 78.7% area stenosis for RR, whereas for CC and CS the cutoff values were obtained at higher stenosis severities of 81.3% and 82.7%, respectively. For a fixed stenosis, CDP value decreased and LFC value increased with a decrease in plaque elasticity (RR to CS). We conclude that the differences in diagnostic parameters with compliance at intermediate stenosis (78.7–82.7% area blockage) could lead to misinterpretation of the stenosis severity.
• TGF-β1 diminishes collagen production during long-term cyclic stretching of engineered connective tissue: Implication of decreased ERK signaling
  - J Biomech 44(5):848-855 (2011)
  Cyclic stretching and growth factors like TGF-β have been used to enhance extracellular matrix (ECM) production by cells in engineered tissue to achieve requisite mechanical properties. In this study, the effects of TGF-β1 were evaluated during long-term cyclic stretching of fibrin-based tubular constructs seeded with neonatal human dermal fibroblasts. Samples were evaluated at 2, 5, and 7 weeks for tensile mechanical properties and ECM deposition. At 2 weeks, +TGF-β1 samples had 101% higher collagen concentration but no difference in ultimate tensile strength (UTS) or modulus compared to –TGF-β1 samples. However, at weeks 5 and 7, –TGF-β1 samples had higher UTS/modulus and collagen concentration, but lower elastin concentration compared to +TGF-β1 samples. The collagen was better organized in –TGF-β1 samples based on picrosirius red staining. Western blot analysis at weeks 5 and 7 showed increased phosphorylation of ERK in –TGF-β1 samples, which correl! ated with higher collagen deposition. The TGF-β1 effects were further evaluated by western blot for αSMA and SMAD2/3 expression, which were 16-fold and 10-fold higher in +TGF-β1 samples, respectively. The role of TGF-β1 activated p38 in inhibiting phosphorylation of ERK was evaluated by treating samples with SB203580, an inhibitor of p38 activation. SB203580-treated cells showed increased phosphorylation of ERK after 1 hour of stretching and increased collagen production after 1 week of stretching, demonstrating an inhibitory role of activated p38 via TGF-β1 signaling during cyclic stretching. One advantage of TGF-β1 treatment was the 4-fold higher elastin deposition in samples at 7 weeks. Further cyclic stretching experiments were thus conducted with constructs cultured for 5 weeks without TGF-β1 to obtain improved tensile properties followed by TGF-β1 supplementation for 2 weeks to obtain increased elastin content, which correlated with a reduction in loss of pre-! stress during preconditioning for tensile testing, indicating ! functional elastin. This study shows that a sequential stimulus approach – cyclic stretching with delayed TGF-β1 supplementation – can be used to engineer tissue with desirable tensile and elastic properties.
• Localized bimodal response of neurite extensions and structural proteins in dorsal-root ganglion neurons with controlled polydimethylsiloxane substrate stiffness
  - J Biomech 44(5):856-862 (2011)
  Neuron morphology, adhesion, axon or neurite outgrowth, and neuron–glial cell interactions are influenced by cues from growth factors as well as extracellular matrix molecules linked to the structural scaffolding. Both chemical and physical events affect neural response, as cells respond to soluble, and insoluble chemical cues and neuron–material interface cues including the mechanical properties of the substrate itself. Both types of cues directly influence neural behaviors and the array of structural elements including microtubules, actin filaments, neurofilaments, and filament-associated proteins. In this manuscript, we examine the relationship between the physical substrate environment and neuron morphology in primary dorsal-root ganglia (DRG) neuron co-cultures including glial cells and DRG neurons. By culturing DRGs on polydimethylsiloxane (PDMS) substrates of varying elasticity we found that structural protein responses, neurite extensions, and protein distr! ibutions varied between substrates, indicating a physical relationship between cultured surface rigidity and cellular morphology. In addition, we found higher cell densities for both DRG neurons and glial cells grown on semi-rigid polydimethylsiloxane substrates (PDMS ratio of base to curing agent of 35:1) than found on more rigid (15:1) or more flexible (50:1) substrates, indicating a localized bimodal response within a very small difference of elasticity on PDMS. These results imply that physiological relevancy may be best discovered by examining and replicating physical parameters such as tissue stiffness. This work is important in fields including biomaterials, neuron–material interactions, and neuroscience.
• Apparent transverse compressive material properties of the digital flexor tendons and the median nerve in the carpal tunnel
  - J Biomech 44(5):863-868 (2011)
  Carpal tunnel syndrome is a frequently encountered peripheral nerve disorder caused by mechanical insult to the median nerve, which may in part be a result of impingement by the adjacent digital flexor tendons. Realistic finite element (FE) analysis to determine contact stresses between the flexor tendons and median nerve depends upon the use of physiologically accurate material properties. To assess the transverse compressive properties of the digital flexor tendons and median nerve, these tissues from ten cadaveric forearm specimens were compressed transversely while under axial load. The experimental compression data were used in conjunction with an FE-based optimization routine to determine apparent hyperelastic coefficients (μ and α) for a first-order Ogden material property definition. The mean coefficient pairs were μ=35.3 kPa, α=8.5 for the superficial tendons, μ=39.4 kPa, α=9.2 for the deep tendons, μ=24.9 kPa, α=10.9 for the flexor pollicis longus (FP! L) tendon, and μ=12.9 kPa, α=6.5 for the median nerve. These mean Ogden coefficients indicate that the FPL tendon was more compliant at low strains than either the deep or superficial flexor tendons, and that there was no significant difference between superficial and deep flexor tendon compressive behavior. The median nerve was significantly more compliant than any of the flexor tendons. The material properties determined in this study can be used to better understand the functional mechanics of the carpal tunnel soft tissues and possible mechanisms of median nerve compressive insult, which may lead to the onset of carpal tunnel syndrome.
• Application of 1D blood flow models of the human arterial network to differential pressure predictions
  - J Biomech 44(5):869-876 (2011)
  A new application of 1D models of the human arterial network is proposed. We take advantage of the sensitivity of the models predictions for the pressure profiles within the main aorta to key model parameter values. We propose to use the patterns in the predicted differences from a base case as a way to infer to the most probable changes in the parameter values. We demonstrate this application using an impedance model that we have recently developed (Johnson, 2010). The input model parameters are all physiologically related, such as the geometric dimensions of large arteries, various blood properties, vessel elasticity, etc. and can therefore be patient specific. As a base case, nominal values from the literature are used. The necessary information to characterize the smaller arteries, arterioles, and capillaries is taken from a physical scaling model (West, 1999). Model predictions for the effective impedance of the human arterial system closely agree with experimenta! l data available in the literature. The predictions for the pressure wave development along the main arteries are also found in qualitative agreement with previous published results. The model has been further validated against our own measured pressure data in the carotid and radial arteries, obtained from healthy individuals. Upon changes in the value of key model parameters, we show that the differences seen in the pressure profiles correspond to qualitatively different patterns for different parameters. This suggests the possibility of using the model in interpreting multiple pressure data of healthy/diseased individuals.
• Does medio-lateral motion occur in the normal knee? An in-vitro study in passive motion
  - J Biomech 44(5):877-884 (2011)
  Medio-lateral translation during knee flexion continues to raise controversy. Small population sizes, small joint flexion ranges, less-reliable measurement techniques and disparate experimental conditions led to inconsistent reports in the past. To study this subject with more accurate and reliable measurements, we carried out femur and tibia tracking in 22 intact cadaver knees during passive joint motion using a state-of-the-art surgical navigation system. Trackers with active light-emitting diodes were fixed onto the femur and tibia, and an instrumented pointer was used to digitize a number of anatomical landmarks. International recommendations were adopted for anatomical-based reference frame definitions and joint kinematic analysis. For the first time, knee joint translations were reported in both the femoral and tibial reference frames, and over a flexion/extension arc as large as 140°. During flexion, in the femoral reference frame, the center of the tibial plat! eau moved 4.8±2.8 mm medially when averaged over the specimens. In the tibial frame, the knee center moved 13.3±5.7 mm laterally. The relative femoral-to-tibial medio-lateral translation was, on average over the specimens, nearly 20% of the width of the tibial plateau, and can be as large as 35%. Medio-lateral translation occurs in the natural normal knee joint.
• Numerical assessment of time-domain methods for the estimation of local arterial pulse wave speed
  - J Biomech 44(5):885-891 (2011)
  A local estimation of pulse wave speed c, an important predictor of cardiovascular events, can be obtained at arterial locations where simultaneous measurements of blood pressure (P) and velocity (U), arterial diameter (D) and U, flow rate (Q) and cross-sectional area (A), or P and D are available, using the PU-loop, sum-of-squares (∑2), lnDU-loop, QA-loop or new D2P-loop methods. Here, these methods were applied to estimate c from numerically generated P, U, D, Q and A waveforms using a visco-elastic one-dimensional model of the 55 larger human systemic arteries in normal conditions. Theoretical c were calculated from the parameters of the model. Estimates of c given by the loop methods were closer to theoretical values and more uniform within each arterial segment than those obtained using the ∑2. The smaller differences between estimates and theoretical values were obtained using the D2P-loop method, with root-mean-square errors (RMSE) smaller than , followed by! averaging the two c given by the PU- and lnDU-loops (RMSE ). In general, the errors of the PU-, lnDU- and QA-loops decreased at locations where visco-elastic effects were small and nearby junctions were well-matched for forward-travelling waves. The ∑2 performed better at proximal locations.
• An approximate simulation model for initial luge track design
  - J Biomech 44(5):892-896 (2011)
  Competitive and recreational sport on artificial ice tracks has grown in popularity. For track design one needs knowledge of the expected speed and acceleration of the luge on the ice track. The purpose of this study was to develop an approximate simulation model for luge in order to support the initial design of new ice tracks. Forces considered were weight, drag, friction, and surface reaction force. The trajectory of the luge on the ice track was estimated using a quasi-static force balance and a 1d equation of motion was solved along that trajectory. The drag area and the coefficient of friction for two runs were determined by parameter identification using split times of five sections of the Whistler Olympic ice track. The values obtained agreed with experimental data from ice friction and wind tunnel measurements. To validate the ability of the model to predict speed and accelerations normal to the track surface, a luge was equipped with an accelerometer to recor! d the normal acceleration during the entire run. Simulated and measured normal accelerations agreed well. In a parameter study the vertical drop and the individual turn radii turned out to be the main variables that determine speed and acceleration. Thus the safety of a new ice track is mainly ensured in the planning phase, in which the use of a simulation model similar to this is essential.
• Inlet boundary conditions for blood flow simulations in truncated arterial networks
  - J Biomech 44(5):897-903 (2011)
  In the context of patient-specific cardiovascular applications, hemodynamics models (going from 3D to 0D) are often limited to a part of the arterial tree. This restriction implies the set up of artificial interfaces with the remaining parts of the cardiovascular system. In particular, the inlet boundary condition is crucial: it supplies the impulsion to the system and receives the reflected backward waves created by the distal network. Some aspects of this boundary condition need to be properly defined such as the treatment of backward waves (reflected or absorbed) and the value of the imposed hemodynamic wave (total or forward component). Most authors prescribe as inlet boundary condition (BC) the total measured variable (pressure, velocity or flow rate) in a reflective way. We show that with this type of inlet boundary condition, the model does not produce physiological waveforms. We suggest instead to prescribe only the forward component of the prescribed variable in an absorbing way. In this way, the computed reflected waves superpose with the prescribed forward waves to produce the total wave at the inlet. In this work, different inlet boundary conditions are implemented and compared for a 1D blood flow model. We test our boundary conditions on a truncated arterial model presented in the literature as well as on a patient-specific lower-limb model of a femoral bypass. We show that with this new boundary condition, a much better fitting is observed on the shape and intensity of the simulated pressure and velocity waves.
• Evaluation of a flexible force sensor for measurement of helmet foam impact performance
  - J Biomech 44(5):904-909 (2011)
  The association between translational head acceleration and concussion remains unclear and provides a weak predictive measure for this type of injury; thus, alternative methods of helmet evaluation are warranted. Recent finite element analysis studies suggest that better estimates of concussion risk can be obtained when regional parameters of the cranium, brain and surrounding tissues are included. Lacking, however, are empirical data at the head–helmet interface with regards to contact area and force. Hence, the purpose of this study was to evaluate a system to capture the impact force distribution of helmet foams. Thirteen Flexiforce® sensors were arranged in a 5×5 cm array, secured to a load cell. Three densities of foam were repeatedly impacted with 5 J of energy during ambient (20 °C) and cold (−25 °C) conditions. RMS error, calculated relative to the global force registered by the load cell, was <1.5% of the measurement range during individual calibration! of the Flexiforce® sensors. RMS error was 5% of the measured range for the global force estimated by the sensor array. Load distribution measurement revealed significant differences between repeated impacts of cold temperature foams for which acceleration results were non-significant. The sensor array, covering only 36% of the total area, possessed sufficient spatial and temporal resolution to capture dynamic load distribution patterns. Implementation of this force mapping system is not limited to helmet testing. Indeed it may be adopted to assess other body regions vulnerable to contact injuries (e.g., chest, hip and shin protectors).
• Modeling costal cartilage using local material properties with consideration for gross heterogeneities
  - J Biomech 44(5):910-916 (2011)
  Contemporary computer models of the thorax designed to predict injury in automobile collisions model the costal cartilage as a homogeneous material using properties derived from local material characterization tests. No studies have validated the accuracy of these models in predicting the structural mechanics of costal cartilage. Two heterogeneities – the perichondrium and calcified regions – may affect the behavior of costal cartilage in a manner not accounted for by current models. This study sought to investigate the predictive ability of subject-specific models of whole costal cartilage segments, with the calcified regions modeled distinctly and with the perichondrium removed (from the physical specimens as well as from the simulations). Finite element models were developed in the case of five cadaveric costal cartilage segments. The properties of the cartilage were derived from indentation testing of each specimen, where the characteristic average instantaneou! s elastic moduli ranged from 8.7 to 12.6 MPa. Matched simulations and experiments were then performed, subjecting each specimen to cantilever-like loading with a dynamic posterior displacement of the sternal boundary (all other boundary degrees-of-freedom fixed). The models predicted the resulting peak anterior–posterior forces generated on the costal boundary with a minimum error of 1% and a maximum error of 36%. These results provide support to the previous implicit assumption that insight can be gained into the structural behavior of costal cartilage by observing the local material properties (when calcified regions are included and the perichondrium is removed). Future work includes the addition of the perichondrium, so as to model the whole costal cartilage composite structure.
• Effect of the fixator stiffness on the young regenerate bone after bone transport: Computational approach
  - J Biomech 44(5):917-923 (2011)
  Bone transport is a well accepted technique for the treatment of large bony defects. This process is mechanically driven, where mechanical forces play a central role in the development of tissues within the distracted gap. One of the most important mechanical factors that conditions the success of bone regeneration during distraction osteogenesis is the fixator stiffness not only during the distraction phase but also during the consolidation phase. Therefore, the aim of the present work is to evaluate the effect of the stiffness of the fixator device on the interfragmentary movements and the tissue outcome during the consolidation phase. A previous differentiation model (Claes and Heigele, 1999) is extended in order to take into account the different behaviors of the tissues in tension and compression. The numerical results that were computed concur with experimental findings; a stiff fixator promotes bone formation while the excessive motion induced by extremely flexi! ble fixators is adverse for bony bridging. Experimental interfragmentary movement is similar to that computed numerically.
• The effects of femoral graft placement on in vivo knee kinematics after anterior cruciate ligament reconstruction
  - J Biomech 44(5):924-929 (2011)
  Achieving anatomical graft placement remains a concern in Anterior Cruciate Ligament (ACL) reconstruction. The purpose of this study was to quantify the effect of femoral graft placement on the ability of ACL reconstruction to restore normal knee kinematics under in vivo loading conditions. Two different groups of patients were studied: one in which the femoral tunnel was placed near the anterior and proximal border of the ACL (anteroproximal group, n=12) and another where the femoral tunnel was placed near the center of the ACL (anatomic group, n=10) MR imaging and biplanar fluoroscopy were used to measure in vivo kinematics in these patients during a quasi-static lunge. Patients with anteroproximal graft placement had up to 3.4 mm more anterior tibial translation, 1.1 mm more medial tibial translation and 3.7° more internal tibial rotation compared to the contralateral side. Patients with anatomic graft placement had motion that more closely replicated that of the intact knee, with anterior tibial translation within 0.8 mm, medial tibial translation within 0.5 mm, and internal tibial rotation within 1°. Grafts placed anteroproximally on the femur likely provide insufficient restraint to these motions due to a more vertical orientation. Anatomical femoral placement of the graft is more likely to reproduce normal ACL orientation, resulting in a more stable knee. Therefore, achieving anatomical graft placement on the femur is crucial to restoring normal knee function and may decrease the rates of joint degeneration after ACL reconstruction.
• A novel method for determining articular cartilage chondrocyte mechanics in vivo
  - J Biomech 44(5):930-934 (2011)
  Work relating the mechanical states of articular cartilage chondrocytes to their biosynthetic responses is based on measurements in isolated cells or cells in explant samples removed from their natural in situ environment. Neither the mechanics nor the associated biological responses of chondrocytes have ever been studied in cartilage within a joint of a live animal, and no such measurements have ever been performed using physiologically relevant joint loading through muscular contractions. The purpose of this study was to design and apply a method to study the mechanics of chondrocytes in the exposed but fully intact knee of live animals, which was loaded near-physiologically through muscular contraction. In order to achieve this purpose, we developed an accurate and reliable method based on two-photon laser excitation microscopy. Near-physiological knee joint loading was achieved through controlled electrical activation of the knee extensor muscles that compress the articulating surfaces of the femur, tibia and patella. Accuracy of the system was assessed by inserting micro-beads of known dimensions into the articular cartilage of the mouse knee and comparing the measured volumes and diameters in the principal directions with known values of the beads. Accuracy was best in the plane perpendicular to the optical axis (average error=1%) while it was slightly worse, but still excellent, along the optical axis (average error=3%). Reliability of cell volume and sh! ape measurements was 0.5% on average, and 2.9% in the worst-case-scenario. Pilot measurements of chondrocyte deformations upon sub-maximal muscular loading causing a mean articular contact pressure of 1.9±0.2 MPa showed an "instantaneous" decrease in cell height (17±4.5%) and loss of cell volume (22.3±2.4%) that took minutes to recover upon deactivation of the knee extensor muscles.
• Three-dimensional micro-level computational study of Wolff's law via trabecular bone remodeling in the human proximal femur using design space topology optimization
  - J Biomech 44(5):935-942 (2011)
  The law of bone remodeling, commonly referred to as Wolff's Law, asserts that the internal trabecular bone adapts to external loadings, reorienting with the principal stress trajectories to maximize mechanical efficiency creating a naturally optimum structure. The goal of the current study was to utilize an advanced structural optimization algorithm, called design space optimization (DSO), to perform a micro-level three-dimensional finite element bone remodeling simulation on the human proximal femur and analyse the results to determine the validity of Wolff's hypothesis. DSO optimizes the layout of material by iteratively distributing it into the areas of highest loading, while simultaneously changing the design domain to increase computational efficiency. The result is a "fully stressed" structure with minimized compliance and increased stiffness. The large-scale computational simulation utilized a 175 μm mesh resolution and the routine daily loading activities ! of walking and stair climbing. The resulting anisotropic trabecular architecture was compared to both Wolff's trajectory hypothesis and natural femur samples from literature using a variety of visualization techniques, including radiography and computed tomography (CT). The results qualitatively revealed several anisotropic trabecular regions, that were comparable to the natural human femurs. Quantitatively, the various regional bone volume fractions from the computational results were consistent with quantitative CT analyses. The global strain energy proceeded to become more uniform during optimization; implying increased mechanical efficiency was achieved. The realistic simulated trabecular geometry suggests that the DSO method can accurately predict bone adaptation due to mechanical loading and that the proximal femur is an optimum structure as the Wolff hypothesized.
• Feasibility of a gait retraining strategy for reducing knee joint loading: Increased trunk lean guided by real-time biofeedback
  - J Biomech 44(5):943-947 (2011)
  The purpose of this feasibility study was to examine changes in frontal plane knee and hip walking biomechanics following a gait retraining strategy focused on increasing lateral trunk lean and to quantify reports of difficulty and joint discomfort when performing such a gait modification. After undergoing a baseline analysis of normal walking, 9 young, healthy participants were trained to modify their gait to exhibit small (4°), medium (8°), and large (12°) amounts of lateral trunk lean. Training was guided by the use of real-time biofeedback of the actual trunk lean angle. Peak frontal plane external knee and hip joint moments were compared across conditions. Participants were asked to report the degree of difficulty and the presence of any joint discomfort for each amount of trunk lean modification. Small (4°), medium (8°), and large (12°) amounts of lateral trunk lean reduced the peak external knee adduction moment (KAM) by 7%, 21%, and 25%, respectively, tho! ugh the peak KAM was only significantly less in the medium and large conditions (p<0.001). Increased trunk lean also significantly reduced the peak external hip adduction moments (p<0.001). All participants reported at least some difficulty performing the exaggerated trunk lean pattern and three participants reported ipsilateral knee, hip, and/or lower spine discomfort. Results from this study indicate that a gait pattern with increased lateral trunk lean can effectively reduce frontal plane joint moments. Though these findings have implications for pathological populations, learning this gait pattern was associated with some difficulty and joint discomfort.
• The influence of tibial component fixation techniques on resorption of supporting bone stock after total knee replacement
  - J Biomech 44(5):948-954 (2011)
  Periprosthetic bone resorption after tibial prosthesis implantation remains a concern for long-term fixation performance. The fixation techniques may inherently aggravate the "stress-shielding" effect of the implant, leading to weakened bone foundation. In this study, two cemented tibial fixation cases (fully cemented and hybrid cementing with cement applied under the tibial tray leaving the stem uncemented) and three cementless cases relying on bony ingrowth (no, partial and fully ingrown) were modelled using the finite element method with a strain-adaptive remodelling theory incorporated to predict the change in the bone apparent density after prosthesis implantation. When the models were loaded with physiological knee joint loads, the predicted patterns of bone resorption correlated well with reported densitometry results. The modelling results showed that the firm anchorage fixation formed between the prosthesis and the bone for the fully cemented and fully ing! rown cases greatly increased the amount of proximal bone resorption. Bone resorption in tibial fixations with a less secure anchorage (hybrid cementing, partial and no ingrowth) occurred at almost half the rate of the changes around the fixations with a firm anchorage. The results suggested that the hybrid cementing fixation or the cementless fixation with partial bony ingrowth (into the porous-coated prosthesis surface) is preferred for preserving proximal tibial bone stock, which should help to maintain post-operative fixation stability. Specifically, the hybrid cementing fixation induced the least amount of bone resorption.
• Comparing different approaches for determining joint torque parameters from isovelocity dynamometer measurements
  - J Biomech 44(5):955-961 (2011)
  Strength, or maximum joint torque, is a fundamental factor governing human movement, and is regularly assessed for clinical and rehabilitative purposes as well as for research into human performance. This study aimed to identify the most appropriate protocol for fitting a maximum voluntary torque function to experimental joint torque data. Three participants performed maximum isometric and concentric–eccentric knee extension trials on an isovelocity dynamometer and a separate experimental protocol was used to estimate maximum knee extension angular velocity. A nine parameter maximum voluntary torque function, which included angle, angular velocity and neural inhibition effects, was fitted to the experimental torque data and three aspects of this fitting protocol were investigated. Using an independent experimental estimate of maximum knee extension angular velocity gave lower variability in the high concentric velocity region of the maximum torque function compared t! o using dynamometer measurements alone. A weighted root mean square difference (RMSD) score function, that forced the majority (73–92%) of experimental data beneath the maximum torque function, was found to best account for the one-sided noise in experimental torques resulting from sub-maximal effort by the participants. The suggested protocol (an appropriately weighted RMSD score function and an independent estimate of maximum knee extension angular velocity) gave a weighted RMSD of between 11 and 13 N m (4−5% of maximum isometric torque). It is recommended that this protocol be used in generating maximum voluntary joint torque functions in all torque-based modelling of dynamic human movement.
• Mechanical properties of single bovine trabeculae are unaffected by strain rate
  - J Biomech 44(5):962-967 (2011)
  For a better understanding of traumatic bone fractures, knowledge of the mechanical behavior of bone at high strain rates is required. Importantly, it needs to be clarified how quasistatic mechanical testing experiments relate to real bone fracture. This merits investigating the mechanical behavior of bone with an increase in strain rate. Various studies examined how cortical and trabecular bone behave at varying strain rates, but no one has yet looked at this question using individual trabeculae. In this study, three-point bending tests were carried out on bovine single trabeculae excised from a proximal femur to test the trabecular material's strain rate sensitivity. An experimental setup was designed, capable of measuring local strains at the surface of such small specimens, using digital image correlation. Microdamage was detected using the bone whitening effect. Samples were tested through two orders of magnitude, at strain rates varying between 0.01 and 3.39 s−! 1. No linear relationship was observed between the strain rate and the Young's modulus (1.13–16.46 GPa), the amount of microdamage, the maximum tensile strain at failure (14.22–61.65%) and at microdamage initiation (1.95–12.29%). The results obtained in this study conflict with previous studies reporting various trends for macroscopic cortical and trabecular bone samples with an increase in strain rate. This discrepancy might be explained by the bone type, the small sample geometry, the limited range of strain rates tested here, the type of loading and the method of microdamage detection. Based on the results of this study, the strain rate can be ignored when modeling trabecular bone.
• Comparative diagnostic accuracy of knee adduction moments in knee osteoarthritis: A case for not normalizing to body size
  - J Biomech 44(5):968-971 (2011)
  Previous authors have questioned the practice of normalizing the external knee adduction moment during gait to body size when investigating dynamic joint loading in knee osteoarthritis (OA). The purpose of this study was to compare the abilities of non-normalized and normalized external knee adduction moments during gait in discriminating between patients with least and greatest severity of radiographic medial compartment knee OA. Subjects with mild (n=118) and severe (n=115) medial compartment knee OA underwent three-dimensional gait analysis. The peak external knee adduction moment was calculated and kept in its original units (Nm), normalized to body mass (Nm/kg) and normalized to body weight and height (%BW×Ht). Receiver Operating Characteristic (ROC) curve analysis indicated that non-normalized values better discriminated between patients with mild and severe knee OA. The area under the ROC curve for non-normalized peak knee adduction moments (0.63) was significa! ntly (p<0.05) greater than when normalized to body mass (0.58), or to body weight times height (0.57). Post-hoc analysis of covariance indicated the mean difference in peak knee adduction moment between OA severity groups (7.23 Nm, p=0.003) was reduced by approximately 50% (3.60 Nm, p=0.09) when adjusted for mass. These findings are consistent with the suggestion that non-normalized values are more sensitive to radiographic disease progression. We suggest including knee adduction moment values that are not normalized to body size when investigating knee OA.
• Identifying gait asymmetry using gyroscopes—A cross-correlation and Normalized Symmetry Index approach
  - J Biomech 44(5):972-978 (2011)
  Injury to a lower limb may disrupt natural walking and cause asymmetrical gait, therefore assessing the gait asymmetry has become one of the important procedures in gait analysis. This paper proposes the use of wireless gyroscopes as a new instrument to determine gait asymmetry. It also introduces two novel approaches: normalized cross-correlations (Ccnorm) and Normalized Symmetry Index (SInorm). Ccnorm evaluates the waveform patterns generated by the lower limb in each gait cycle. SInorm provides indications on the timing and magnitude of the bilateral differences between the limbs while addressing the drawbacks of the conventional methods. One-way ANOVA test reveals that Ccnorm can be considered as single value indicator that determines the gait asymmetry (p<0.01). The experiment results showed that SInorm in asymmetrical gait were different from normal gait. SInorm in asymmetrical gait were found to be approximately 20% greater than SInorm in normal gait during pre-! swing and initial swing.
• In vivo velocity vector imaging and time-resolved strain rate measurements in the wall of blood vessels using MRI
  - J Biomech 44(5):979-983 (2011)
  In this paper, we present a new approach for velocity vector imaging and time-resolved measurements of strain rates in the wall of human arteries using MRI and we prove its feasibility on two examples: in vitro on a phantom and in vivo on the carotid artery of a human subject. Results point out the promising potential of this approach for investigating the mechanics of arterial tissues in vivo.
• An improved cost function for modeling of muscle activity during running
  - J Biomech 44(5):984-987 (2011)
  This paper tries to improve a recently developed mass-spring-damper model of the human body during running. The previous model took the muscle activity into account using a nonlinear controller that tuned the mechanical properties of the soft-tissue package based on two physiological hypotheses, namely "constant-force" and "constant-vibration". Three cost functions were used, out of which one was based on the constant-force hypothesis and two others were based on the constant-vibration hypothesis. The results of the study showed that the proposed cost functions are only partially successful in capturing the experimentally observed trends of the ground reaction force and vibration. The current paper proposes an improved cost function that combines both above-mentioned hypotheses. It is shown that the improved cost function can capture all the trends that were observed in the measurements of the ground reaction force and vibration level. It is therefore advised t! o use the new cost function in place of the previous ones.
• Evidence of entropic elasticity of human bone trabeculae at low strains
  - J Biomech 44(5):988-991 (2011)
  We performed 60 microtensile tests on 6 single trabeculae excised from a human femur head at various maximum tensile loads. The obtained results show a clear dependence of the calculated stress–strain behaviour from the applied load and thus from the mean stress over the cross section of the trabecula. The pooled data were found in good agreement with a combination of both the model of the nonlinear stress–strain behaviour of collagen fibrils and that for the modulus of elasticity of staggered mineralized collagen. This circumstance could also suggest a realistic explanation of the extreme variability found in literature for the elastic modulus of the trabecular material. In particular, when the trabeculae are solicited with relatively low stresses, their mechanical properties are mainly affected by the entropic elasticity of collagen molecules. This work offers both experimental data and a reasonable interpretation of the behaviour of fully mineralized tissue at l! ow strains, that is up to about 0.1%.
• Erratum to: "The compressive mechanical properties of diabetic and non-diabetic plantar soft tissue" [Journal of Biomechanics 43(9) (2010) 1754–1760]
  - J Biomech 44(5):992 (2011)
  
• Erratum to: "D-15 evaluating the elbow joint laxity using Stewart platform mechanism: An experimental study" [Biomechanics 43 (Supplement 1) (2010) S73]
  - J Biomech 44(5):993 (2011)