Hot off the presses! Jun 03 J Biom

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Latest Articles Include:

• Editorial Board and Publication Information
  - J Biom 44(9):IFC (2011)
  
• Posture shifting after spinal cord injury using functional neuromuscular stimulation—A computer simulation study
  - J Biom 44(9):1639-1645 (2011)
  The ability for individuals with spinal cord injury (SCI) to affect changes in standing posture with functional neuromuscular stimulation (FNS) was explored using an anatomically inspired musculoskeletal model of the trunk, pelvis and lower extremities (LE). The model tracked trajectories for anteriorly and laterally shifting movements away from erect stance. Forces were applied to both shoulders to represent upper extremity (UE) interaction with an assistive device (e.g., a walker). The muscle excitations required to execute shifting maneuvers with UE forces <10% body-weight (BW) were determined via dynamic optimization. Nine muscle sets were examined to maximize control of shifting posture. Inclusion of the Psoas and External Obliques bilaterally resulted in the least relative UE effort (0.119, mean UE effort=45.3 N≡5.4% BW) for anterior shifting. For lateral shifting, the set including the Psoas and Latissimus Dorsi bilaterally yielded the best performance (0.025,! mean UE effort=27.8 N≡3.3% BW). However, adding the Psoas alone bilaterally competed favorably in overall best performance across both maneuvers. This study suggests suitable activation to specific muscles of the trunk and LE can enable individuals with SCI to alter their standing postures with minimal upper-body effort and subsequently increase reach and standing work volume.
• Influence of interface condition and implant design on bone remodelling and failure risk for the resurfaced femoral head
  - J Biom 44(9):1646-1653 (2011)
  Resurfacing of the femur has experienced a revival, particularly in younger and more active patients. The implant is generally cemented onto the reamed trabecular bone and theoretical remodelling for this configuration, as well as uncemented variations, has been studied with relation to component positioning for the most common designs. The purpose of this study was to investigate the influence of different interface conditions, for alternative interior implant geometries, on bone strains in comparison to the native femur, and its consequent remodelling. A cylindrical interior geometry, two conical geometries and a spherical cortex-preserving design were compared with a standard implant (ASR, DePuy International, Ltd., UK), which has a 3° cone. Cemented as well as uncemented line to line and press-fit conditions were modelled for each geometry. A patient-specific finite element model of the proximal femur was used with simulated walking loads. Strain energy density wa! s compared between the reference and resurfaced femur, and input into a remodelling algorithm to predict density changes post-operatively. The common cemented designs (cylindrical, slightly conical) had strain shielding in the superior femoral head (>35% reduction) as well as strain concentrations (strain>5%) in the neck regions near the implant rim. The cortex-preserving (spherical) and strongly conical designs showed less strain shielding. In contrast to the cemented implants, line to line implants showed a density decrease at the centre of the femoral head, while all press-fit versions showed a density increase (>100%) relative to the native femur, which suggests that uncemented press-fit implants could limit bone resorption.
• Dynamic loading of immature epiphyseal cartilage pumps nutrients out of vascular canals
  - J Biom 44(9):1654-1659 (2011)
  The potential influence of mechanical loading on transvascular transport in vascularized soft tissues has not been explored extensively. This experimental investigation introduced and explored the hypothesis that dynamic mechanical loading can pump solutes out of blood vessels and into the surrounding tissue, leading to faster uptake and higher solute concentrations than could otherwise be achieved under unloaded conditions. Immature epiphyseal cartilage was used as a model tissue system, with fluorescein (332 Da), dextran (3, 10, and 70 kDa) and transferrin (80 kDa) as model solutes. Cartilage disks were either dynamically loaded (±10% compression over a 10% static offset strain, at 0.2 Hz) or maintained unloaded in solution for up to 20 h. Results demonstrated statistically significant solute uptake in dynamically loaded (DL) explants relative to passive diffusion (PD) controls for all solutes except unbound fluorescein, as evidenced by the DL:PD concentration ratio! s after 20 h (1.0±0.2, 2.4±1.1, 6.1±3.3, 9.0±4.0, and 5.5±1.6 for fluorescein, 3, 10, and 70 kDa dextran, and transferrin). Significant uptake enhancements were also observed within the first 30 s of loading. Termination of dynamic loading produced dissipation of enhanced solute uptake back to PD control values. Confocal images confirmed that solute uptake occurred from cartilage canals into their surrounding extracellular matrix. The incidence of this loading-induced transvascular solute pumping mechanism may significantly alter our understanding of the interaction of mechanical loading and tissue metabolism.
• Variability of tissue mineral density can determine physiological creep of human vertebral cancellous bone
  - J Biom 44(9):1660-1665 (2011)
  Creep is a time-dependent viscoelastic deformation observed under a constant prolonged load. It has been indicated that progressive vertebral deformation due to creep may increase the risk of vertebral fracture in the long-term. The objective of this study was to examine the relationships of creep with trabecular architecture and tissue mineral density (TMD) parameters in human vertebral cancellous bone at a physiological static strain level. Architecture and TMD parameters of cancellous bone were analyzed using microcomputerized tomography (micro-CT) in specimens cored out of human vertebrae. Then, creep and residual strains of the specimens were measured after a two-hour physiological compressive constant static loading and unloading cycle. Creep developed (3877±2158 με) resulting in substantial levels of non-recoverable post-creep residual strain (1797±1391 με). A strong positive linear correlation was found between creep and residual strain (r=0.94, p<0.001).! The current results showed that smaller thickness, larger surface area, greater connectivity of trabeculae, less mean tissue mineral density (TMD, represented by gray levels) and higher variability of TMD are associated with increasing logarithmic creep rate. The TMD variability (GLCOV) was the strongest correlate of creep rate (r=0.79, p<0.001). This result suggests that TMD variability may be a useful parameter for estimating the long-term deformation of a whole vertebral body. The results further suggest that the changes in TMD variability resulting from bone remodeling are of importance and may provide an insight into the understanding of the mechanisms underlying progressive failure of vertebral bodies and development of a clinical fracture.
• Patient-specific finite element analysis of the human femur—A double-blinded biomechanical validation
  - J Biom 44(9):1666-1672 (2011)
  Patient-specific finite element (PSFE) models based on quantitative computer tomography (qCT) are generally used to "predict" the biomechanical response of human bones with the future goal to be applied in clinical decision-making. However, clinical applications require a well validated tool that is free of numerical errors and furthermore match closely experimental findings. In previous studies, not all measurable data (strains and displacements) were considered for validation. Furthermore, the same research group performed both the experiments and PSFE analyses; thus, the validation may have been biased. The aim of the present study was therefore to validate PSFE models with biomechanical experiments, and to address the above-mentioned issues of measurable data and validation bias. A PSFE model (p-method) of each cadaver femur (n=12) was generated based on qCT scans of the specimens. The models were validated by biomechanical in-vitro experiments, which determine! d strains and local displacements on the bone surface and the axial stiffness of the specimens. The validation was performed in a double-blinded manner by two different research institutes to avoid any bias. Inspecting all measurements (155 values), the numerical results correlated well with the experimental results (R2=0.93, slope 1.0093, mean of absolute deviations 22%). In conclusion, a method to generate PSFE models from qCT scans was used in this study on a sample size not yet considered in the past, and compared to experiments in a douple-blinded manner. The results demonstrate that the presented method is in an advanced stage, and can be used in clinical computer-aided decision-making.
• Trajectory of the center of rotation in non-articulated energy storage and return prosthetic feet
  - J Biom 44(9):1673-1677 (2011)
  Non-articulated energy storage and return prosthetic feet lack any true articulation or obvious point of rotation. This makes it difficult to select a joint center about which to estimate their kinetics. Despite this absence of any clear point of rotation, methods for estimating the kinetic performance of this class of prosthetic feet typically assume that they possess a fixed center of rotation and that its location is well approximated by the position of the contralateral lateral malleolus. To evaluate the validity of this assumption we used a finite helical axis approach to determine the position of the center of rotation in the sagittal plane for a series of non-articulated energy storage and return prosthetic feet. We found that over the course of stance phase, the sagittal finite helical axis position diverged markedly from the typically assumed fixed axis location. These results suggest that researchers may need to review center of rotation assumptions when asse! ssing prosthetic foot kinetics, while clinicians may need to reconsider the criteria by which they prescribe these prosthetic feet.
• Experimental in vitro mechanical characterization of porcine Glisson's capsule and hepatic veins
  - J Biom 44(9):1678-1683 (2011)
  Understanding the mechanical properties of human liver is the most critical aspect of numerical modeling for medical applications and impact biomechanics. Many researchers work on identifying mechanical properties of the liver both in vivo and in vitro considering the high liver injury percentage in abdominal trauma and for easy detection of fatal liver diseases such as viral hepatitis, cirrhosis, etc. This study is performed to characterize mechanical properties of individual parts of the liver, namely Glisson's capsule and hepatic veins, as these parts are rarely characterized separately. The long term objective of this study is to develop a realistic liver model by characterizing individual parts and later integrating them. In vitro uniaxial quasi-static tensile tests are done on fresh unfrozen porcine hepatic parts for large deformations at the rate of 0.1 mm/s with a Bose Electroforce 3200 biomaterials test instrument. Results show that mean values of small strain! and large strain elastic moduli are 8.22±3.42 and 48.15±4.5 MPa for Glisson's capsule (30 samples) and 0.62±0.41 and 2.81±2.23 MPa for veins (20 samples), respectively, and are found to be in good agreement with data in the literature. Finally, a non-linear hyper-elastic constitutive law is proposed for the two separate liver constituents under study.
• Fluid–structure interaction of deformable aortic prostheses with a bileaflet mechanical valve
  - J Biom 44(9):1684-1690 (2011)
  Two different aortic prostheses can be used for performing the Bentall procedure: a standard straight graft and the Valsalva graft that better reproduces the aortic root anatomy. The aim of the present work is to study the effect of the graft geometry on the blood flow when a bileaflet mechanical heart valve is used, as well as to evaluate the stress concentration near the suture line where the coronary arteries are connected to graft. An accurate three-dimensional numerical method is proposed, based on the immersed boundary technique. The method accounts for the interactions between the flow and the motion of the rigid leaflets and of the deformable aortic root, under physiological pulsatile conditions. The results show that the graft geometry only slightly influences the leaflets dynamics, while using the Valsalva graft the stress level near the coronary-root anastomoses is about half that obtained using the standard straight graft.
• Long-term nano-mechanical properties of biomodified dentin–resin interface components
  - J Biom 44(9):1691-1694 (2011)
  Failures of dental composite restorative procedures are largely attributed to the degradation of dentin–resin interface components. Biomodification of dentin using bioactive agents may improve the quality and durability of the dentin–resin bonds. The aim of this study was to nanomechanically assess the reduced modulus of elasticity (Er) and nano-hardness (H) of major components of the dentin–resin interface (hybrid layer, adhesive layer and underlying dentin) biomodified by collagen cross-linkers at 24 h, 3 and 6 months following restorative procedure. Demineralized dentin surfaces were biomodified with 5% glutaraldehyde (GD) or 6.5% grape seed extract (GSE) prior to placement of adhesive systems and composite resin. Nano-measurements of the interface components in a fluid cell showed that both agents increased the Er and H of underlying dentin after 3 and 6 months when compared to a control. The mechanical properties of the adhesive and hybrid layers decreased o! ver time. Biomodification of the dentin–resin interface structures using GD and GSE can increase the mechanical properties of the interface over time and may contribute to the long-term quality of adhesive restorations.
• Computational fluid dynamics analysis of drag and convective heat transfer of individual body segments for different cyclist positions
  - J Biom 44(9):1695-1701 (2011)
  This study aims at investigating drag and convective heat transfer for cyclists at a high spatial resolution. Such an increased spatial resolution, when combined with flow-field data, can increase insight in drag reduction mechanisms and in the thermo-physiological response of cyclists related to heat stress and hygrothermal performance of clothing. Computational fluid dynamics (steady Reynolds-averaged Navier–Stokes) is used to evaluate the drag and convective heat transfer of 19 body segments of a cyclist for three different cyclist positions. The influence of wind speed on the drag is analysed, indicating a pronounced Reynolds number dependency on the drag, where more streamlined positions show a dependency up to higher Reynolds numbers. The drag and convective heat transfer coefficient (CHTC) of the body segments and the entire cyclist are compared for all positions at racing speeds, showing high drag values for the head, legs and arms and high CHTCs for the legs! , arms, hands and feet. The drag areas of individual body segments differ markedly for different cyclist positions whereas the convective heat losses of the body segments are found to be less sensitive to the position. CHTC–wind speed correlations are derived, in which the power-law exponent does not differ significantly for the individual body segments for all positions, where an average value of 0.84 is found. Similar CFD studies can be performed to assess drag and CHTCs at a higher spatial resolution for applications in other sport disciplines, bicycle equipment design or to assess convective moisture transfer.
• On the electrophysiological response of bone cells using a Stokesian fluid stimulus probe for delivery of quantifiable localized picoNewton level forces
  - J Biom 44(9):1702-1708 (2011)
  A Stokesian fluid stimulus probe (SFSP), capable of delivering quantifiable pN level hydrodynamic forces, is developed to distinguish the electrophysiological response of the cell process and cell body of osteocyte-like MLO-Y4 cells without touching the cell or its substrate. The hydrodynamic disturbance is a short lived (100 ms), constant strength pressure pulse that propagates nearly instantaneously through the medium creating a nearly spherical expanding fluid bolus surrounding a 0.8 μm micropipette tip. Laboratory model experiments show that the growth of the bolus and the pressure field can be closely modeled by quasi-steady Stokes flow through a circular orifice provided the tip Reynolds number, Ret<0.03. By measuring the deflection of the dendritic processes between discrete attachment sites, and applying a detailed ultrastructural model for the central actin filament bundle within the process, one is able to calculate the forces produced by the probe using ela! stic beam theory. One finds that forces between 1 and 2.3 pN are sufficient to initiate electrical signaling when applied to the cell process, but not the much softer cell body. Even more significantly, cellular excitation by the process only occurs when the probe is directed at discrete focal attachment sites along the cell process. This suggests that electrical signaling is initiated at discrete focal attachments along the cell process and that these sites are likely integrin-mediated complexes associated with stretch-activated ion channels though their molecular structure is unknown.
• Experimental determination of circumferential properties of fresh carotid artery plaques
  - J Biom 44(9):1709-1715 (2011)
  Carotid endarterectomy (CEA) is currently accepted as the gold standard for interventional revascularisation of diseased arteries belonging to the carotid bifurcation. Despite the proven efficacy of CEA, great interest has been generated in carotid angioplasty and stenting (CAS) as an alternative to open surgical therapy. CAS is less invasive compared with CEA, and has the potential to successfully treat lesions close to the aortic arch or distal internal carotid artery (ICA). Following promising results from two recent trials (CREST; Carotid revascularisation endarterectomy versus stenting trial, and ICSS; International carotid stenting study) it is envisaged that there will be a greater uptake in carotid stenting, especially amongst the group who do not qualify for open surgical repair, thus creating pressure to develop computational models that describe a multitude of plaque models in the carotid arteries and their reaction to the deployment of such interventional d! evices. Pertinent analyses will require fresh human atherosclerotic plaque material characteristics for different disease types. This study analysed atherosclerotic plaque characteristics from 18 patients tested on site, post-surgical revascularisation through endarterectomy, with 4 tissue samples being excluded from tensile testing based on large width–length ratios. According to their mechanical behaviour, atherosclerotic plaques were separated into 3 grades of stiffness. Individual and group material coefficients were then generated analytically using the Yeoh strain energy function. The ultimate tensile strength (UTS) of each sample was also recorded, showing large variation across the 14 atherosclerotic samples tested. Experimental Green strains at rupture varied from 0.299 to 0.588 and the Cauchy stress observed in the experiments was between 0.131 and 0.779 MPa. It is expected that this data may be used in future design optimisation of next generation interventiona! l medical devices for the treatment and revascularisation of d! iseased arteries of the carotid bifurcation.
• Effect of sub-optimal neuromotor control on the hip joint load during level walking
  - J Biom 44(9):1716-1721 (2011)
  Skeletal forces are fundamental information in predicting the risk of bone fracture. The neuromotor control system can drive muscle forces with various task- and health-dependent strategies but current modelling techniques provide a single optimal solution of the muscle load sharing problem. The aim of the present work was to study the variability of the hip load magnitude due to sub-optimal neuromotor control strategies using a subject-specific musculoskeletal model. The model was generated from computed tomography (CT) and dissection data from a single cadaver. Gait kinematics, ground forces and electromyographic (EMG) signals were recorded on a body-matched volunteer. Model results were validated by comparing the traditional optimisation solution with the published hip load measurements and the recorded EMG signals. The solution space of the instantaneous equilibrium problem during the first hip load peak resulted in 105 dynamically equivalent configurations of the ! neuromotor control. The hip load magnitude was computed and expressed in multiples of the body weight (BW). Sensitivity of the hip load boundaries to the uncertainty on the muscle tetanic stress (TMS) was also addressed. The optimal neuromotor control induced a hip load magnitude of 3.3 BW. Sub-optimal neuromotor controls induced a hip load magnitude up to 8.93 BW. Reducing TMS from the maximum to the minimum the lower boundary of the hip load magnitude varied moderately whereas the upper boundary varied considerably from 4.26 to 8.93 BW. Further studies are necessary to assess how far the neuromotor control can degrade from the optimal activation pattern and to understand which sub-optimal controls are clinically plausible. However we can consider the possibility that sub-optimal activations of the muscular system play a role in spontaneous fractures not associated with falls.
• Comparison of different hip prosthesis shapes considering micro-level bone remodeling and stress-shielding criteria using three-dimensional design space topology optimization
  - J Biom 44(9):1722-1728 (2011)
  Since the late 1980s, computational analysis of total hip arthroplasty (THA) prosthesis components has been completed using macro-level bone remodeling algorithms. The utilization of macro-sized elements requires apparent bone densities to predict cancellous bone strength, thereby, preventing visualization and analysis of realistic trabecular architecture. In this study, we utilized a recently developed structural optimization algorithm, design space optimization (DSO), to perform a micro-level three-dimensional finite element bone remodeling simulation on the human proximal femur pre- and post-THA. The computational simulation facilitated direct performance comparison between two commercially available prosthetic implant stems from Zimmer Inc.: the Alloclassic and the Mayo conservative. The novel micro-level approach allowed the unique ability to visualize the trabecular bone adaption post-operation and to quantify the changes in bone mineral content by region. Stress! -shielding and strain energy distribution were also quantified for the immediate post-operation and the stably fixated, post-remodeling conditions. Stress-shielding was highest in the proximal region and remained unchanged post-remodeling; conversely, the mid and distal portions show large increases in stress, suggesting a distal shift in the loadpath. The Mayo design conserves bone mass, while simultaneously reducing the incidence of stress-shielding compared to the Alloclassic, revealing a key benefit of the distinctive geometry. Several important factors for stable fixation, determined in clinical evaluations from the literature, were evident in both designs: high levels of proximal bone loss and distal bone densification. The results suggest this novel computational framework can be utilized for comparative hip prosthesis shape, uniquely considering the post-operation bone remodeling as a design criterion.
• Optical measurements of vocal fold tensile properties: Implications for phonatory mechanics
  - J Biom 44(9):1729-1734 (2011)
  In voice research, in vitro tensile stretch experiments of vocal fold tissues are commonly employed to determine the tissue biomechanical properties. In the standard stretch-release protocol, tissue deformation is computed from displacements applied to sutures inserted through the thyroid and arytenoid cartilages, with the cartilages assumed to be rigid. Here, a non-contact optical method was employed to determine the actual tissue deformation of vocal fold lamina propria specimens from three excised human larynges in uniaxial tensile tests. Specimen deformation was found to consist not only of deformation of the tissue itself, but also deformation of the cartilages, as well as suture alignment and tightening. Stress–stretch curves of a representative load cycle were characterized by an incompressible Ogden model. The initial longitudinal elastic modulus was found to be considerably higher if determined based on optical displacement measurements than typical values r! eported in the literature. The present findings could change the understanding of the mechanics underlying vocal fold vibration. Given the high longitudinal elastic modulus the lamina propria appeared to demonstrate a substantial level of anisotropy. Consequently, transverse shear could play a significant role in vocal fold vibration, and fundamental frequencies of phonation should be predicted by beam theories accounting for such effects.
• Correlation between vertical misfits and stresses transmitted to implants from metal frameworks
  - J Biom 44(9):1735-1739 (2011)
  An inappropriate prosthetic fit could cause stress over the interface implant/bone. The objective of this study was to compare stresses transmitted to implants from frameworks cast using different materials and to investigate a possible correlation between vertical misfits and these stresses. Fifteen one-piece cast frameworks simulating bars for fixed prosthesis in a model with five implants were fabricated and arranged into three different groups according to the material used for casting: CP Ti (commercially pure titanium), Co–Cr (cobalt–chromium) or Ni–Cr-Ti (nickel–chromium–titanium) alloys. Each framework was installed over the metal model with all screws tightened to a 10 N cm torque and then, vertical misfits were measured using an optical microscope. The stresses transmitted to implants were measured using quantitative photoelastic analysis in values of maximum shear stress (τ), when each framework was tightened to the photoelastic model to a 10 N cm! standardized torque. Stress data were statistically analyzed using one-way ANOVA and Tukey's test and correlation tests were performed using Pearson's rank correlation (α=0.05). Mean and standard deviation values of vertical misfit are presented for CP Ti (22.40±9.05 μm), Co–Cr (66.41±35.47 μm) and Ni–Cr–Ti (32.20±24.47 μm). Stresses generated by Co–Cr alloy (τ=7.70±2.16 kPa) were significantly higher than those generated by CP Ti (τ=5.86±1.55 kPa, p=0.018) and Ni–Cr–Ti alloy (τ=5.74±3.05 kPa, p=0.011), which were similar (p=0.982). Correlations between vertical misfits and stresses around the implants were not significant as for any evaluated materials.
• A method to perform spinal motion analysis from functional X-ray images
  - J Biom 44(9):1740-1746 (2011)
  Identifying spinal instability is an important aim for proper surgical treatment. Analysis of functional X-ray images delivers measurements of the range of motion (RoM) and the center of rotation (CoR). In today's practice, CoR determination is often omitted, due to the lack of accurate methods. The aim of this work was to investigate the accuracy of a new analysis software (FXA™) based on an in vitro experiment. Six bovine spinal specimens (L3-4) were mounted in a robot (KR125, Kuka). CoRs were predefined by locking the robot actuator tool center point to the estimated position of the physiologic CoR and taking a baseline X-ray. Specimens were deflected to various RoMpreset flexion/extension angles about the CoRpreset. Lateral functional radiographs were acquired and specimen movements were recorded using an optical motion tracking system (Optotrak Certus). RoM and CoR errors were calculated from presets for both methods. Prior to the experiment, the FXA™ software was verified with artificially generated images. For the artificial images, FXA™ yielded a mean RoM-error of 0.01±0.03° (bias±standard deviation). In the experiment, RoM-error of the FXA™-software (deviation from presets) was 0.04±0.13°, and 0.10±0.16° for the Optotrak, respectively. Both correlated with 0.998 (p<0.001). For RoM<1.0°, FXA™ determined CoR positions with a bias>20 mm. This bias progressively decreased from RoM=1° (bias=6.0 mm) to RoM=9° (bias<1.5 mm). Under the assumption that CoR location variances <5 mm are clinically irrelevant on the lumbar spine, the FXA™ method can accurately determine CoRs for RoMs>1°. Utilizing FXA™, polysegmental RoMs, CoRs and implant migration measurements could be performed in daily practice.
• Influence of material coupling and assembly condition on the magnitude of micromotion at the stem–neck interface of a modular hip endoprosthesis
  - J Biom 44(9):1747-1751 (2011)
  Hip prostheses with a modular neck exhibit, compared to monobloc prostheses, an additional interface which bears the risk of fretting as well as corrosion. Failures at the neck adapter of modular prostheses have been observed for a number of different designs. It has been speculated that micromotions at the stem–neck interface were responsible for these implant failures. The purpose of this study was to investigate the influence of material combinations and assembly conditions on the magnitude of micromotions at the stem–neck interface during cyclic loading. Modular (n=24) and monobloc (n=3) hip prostheses of a similar design (Metha, Aesculap AG, Tuttlingen, Germany) were subjected to mechanical testing according to ISO 7206-4 (Fmin=230 N, Fmax=2300 N, f=1 Hz, n=10,000 cycles). The neck adapters (Ti–6Al–4V or Co–Cr29–Mo alloy) were assembled with a clean or contaminated interface. The micromotion between stem and neck adapter was calculated at five reference points based on the measurements of the three eddy current sensors. The largest micromotions were observed at the lateral edge of the stem–neck taper connection, which is in accordance with the crack location of clinically failed prostheses. Titanium neck adapters showed significantly larger micromotions than cobalt–chromium neck adapters (p=0.005). Contaminated interfaces also exhibited significantly larger micromotions (p<0.001). Since excessive micromotions at the stem–neck interface might be involved in the process of implant failure, special care should be taken to clean the interface prior to assembly and titanium neck adapters with titanium stems should generally be used with caution.
• Shoulder and hip roll differences between breathing and non-breathing conditions in front crawl swimming
  - J Biom 44(9):1752-1756 (2011)
  The effects of breathing on body roll have been previously investigated for the roll of the whole trunk only. The purposes of this study were: to calculate separately the shoulder roll (SR) and hip roll (HR) of swimmers during front crawl for non-breathing and preferred-side breathing conditions; to assess the differences in the magnitude and temporal characteristics of these variables between non-breathing and preferred-side breathing conditions; and to examine their association with swimming performance (indicated by swimming speed). Twelve male swimmers who competed at national and international level performed two maximum 25 m front crawl trials: one non-breathing and one with breathing to their preferred side. Performance was recorded with four below and two above water synchronised cameras. SR and HR in both trials were calculated for the breathing and non-breathing sides. The timings of SR and HR peaks to each side and at the positions of neutral roll were also ! calculated. Swimming speed was significantly slower in the breathing trial (p<0.01). Swimmers rolled their shoulders and hips to the breathing side significantly more in the breathing than in the non-breathing trial (SR: p<0.01; HR: p=0.03). Nevertheless, there were no significant differences in the overall SR or HR between these trials. In the breathing trial, SR was higher in the breathing than in the non-breathing side (p<0.01) but HR was not significantly different (p=0.07). There was no evidence to suggest that temporal characteristics of SR or HR were associated with swimming performance.
• A novel multi-planar radiography method for three dimensional pose reconstruction of the patellofemoral and tibiofemoral joints after arthroplasty
  - J Biom 44(9):1757-1764 (2011)
  Determining the 3D pose of the patella after total knee arthroplasty is challenging. The commonly used single-plane fluoroscopy is prone to large errors in the clinically relevant mediolateral direction. A conventional fixed bi-planar setup is limited in the minimum angular distance between the imaging planes necessary for visualizing the patellar component, and requires a highly flexible setup to adjust for the subject-specific geometries. As an alternative solution, this study investigated the use of a novel multi-planar imaging setup that consists of a C-arm tracked by an external optoelectric tracking system, to acquire calibrated radiographs from multiple orientations. To determine the accuracies, a knee prosthesis was implanted on artificial bones and imaged in simulated 'Supine' and 'Weightbearing' configurations. The results were compared with measures from a coordinate measuring machine as the ground-truth reference. The weightbearing configuration was! the preferred imaging direction with RMS errors of 0.48 mm and 1.32° for mediolateral shift and tilt of the patella, respectively, the two most clinically relevant measures. The 'imaging accuracies' of the system, defined as the accuracies in 3D reconstruction of a cylindrical ball bearing phantom (so as to avoid the influence of the shape and orientation of the imaging object), showed an order of magnitude (11.5 times) reduction in the out-of-plane RMS errors in comparison to single-plane fluoroscopy. With this new method, complete 3D pose of the patellofemoral and tibiofemoral joints during quasi-static activities can be determined with a many-fold (up to 8 times) (3.4 mm) improvement in the out-of-plane accuracies compared to a conventional single-plane fluoroscopy setup.
• The turnover of mineralized growth plate cartilage into bone may be regulated by osteocytes
  - J Biom 44(9):1765-1770 (2011)
  During endochondral ossification, growth plate cartilage is replaced with bone. Mineralized cartilage matrix is resorbed by osteoclasts, and new bone tissue is formed by osteoblasts. As mineralized cartilage does not contain any cells, it is unclear how this process is regulated. We hypothesize that, in analogy with bone remodeling, osteoclast and osteoblast activity are regulated by osteocytes, in response to mechanical loading. Since the cartilage does not contain osteocytes, this means that cartilage turnover during endochondral ossification would be regulated by the adjacent bone tissue. We investigated this hypothesis with an established computational bone adaptation model. In this model, osteocytes stimulate osteoblastic bone formation in response to the mechanical bone tissue loading. Osteoclasts resorb bone near randomly occurring microcracks that are assumed to block osteocyte signals. We used finite element modeling to evaluate our hypothesis in a 2D-domain r! epresenting part of the growth plate and adjacent bone. Cartilage was added at a constant physiological rate to simulate growth. Simulations showed that osteocyte signals from neighboring bone were sufficient for successful cartilage turnover, since equilibrium between cartilage remodeling and growth was obtained. Furthermore, there was good agreement between simulated bone structures and rat tibia histology, and the development of the trabecular architecture resembled that of infant long bones. Additionally, prohibiting osteoclast invasion resulted in thickened mineralized cartilage, similar to observations in a knock-out mouse model. We therefore conclude that it is well possible that osteocytes regulate the turnover of mineralized growth plate cartilage.
• Estimation of centre of gravity movements in sitting posture: application to trunk backward tilt
  - J Biom 44(9):1771-1775 (2011)
  The aim of this study was to highlight, in sitting posture, the value of distinguishing between the movements of the vertical projection of the centre of gravity (CGv) and its difference from the centre of pressure (CP−CGv). A protocol for healthy, young, trained adults, consisting in tilting their trunk backward or keeping it vertical was used. A frequency analysis shows that statistically significant effects were only seen on CP−CGv movements: the RMS increased by 37% (p=0.004), while the MPF decreased by 5% (p=0.016), suggesting an increased muscular activity in these tilting postures. In contrast, no statistically significant effects on CP and CGv were reported. These data highlight the advantage, in sitting posture, of splitting overall CP displacements into basic components (i.e. CGv and CP−CGv), each of them having a biomechanical significance.
• Effect of tensile force on the mechanical behavior of actin filaments
  - J Biom 44(9):1776-1781 (2011)
  Actin filaments are the most abundant components of the cellular cytoskeleton, and play critical roles in various cellular functions such as migration, division and shape control. In these activities, mechanical tension causes structural changes in the double-helical structure of the actin filament, which is a key modulator of cytoskeletal reorganization. This study performed large-scale molecular dynamics (MD) and steered MD simulations to quantitatively analyze the effects of tensile force on the mechanical behavior of actin filaments. The results revealed that when a tensile force of 200 pN was applied to a filament consisting of 14 actin subunits, the twist angle of the filament decreased by approximately 20°, corresponding to a rotation of approximately −2° per subunit, representing a critical structural change in actin filaments. Based on these structural changes, the variance in filament length and twist angle was found to decrease, leading to increases in e! xtensional and torsional stiffness. Torsional stiffness increased significantly under the tensile condition, and the ratio of filament stiffness under tensile force to that under no external force increased significantly on longer temporal scales. The results obtained from this study contribute to the understanding of mechano-chemical interactions concerning actin dynamics, showing that increased tensile force in the filament prevents actin regulatory proteins from binding to the filament.
• Evaluation of residual stresses due to bone callus growth: A computational study
  - J Biom 44(9):1782-1787 (2011)
  Mechanical environment in callus is determinant for the evolution of bone healing. However, recent mechanobiological computational works have underestimated the effect that growth exerts on the mechanical environment of callus. In the present work, we computationally evaluate the significance of growth-induced stresses, commonly called residual stresses, in callus. We construct a mechanobiological model of a callus in the metatarsus of a sheep in two different stages: one week and four weeks after fracture. The magnitude of stresses generated during callus growth is compared with the magnitude of stresses when only external loads are applied to the callus. We predict that residual stresses are relevant in some areas, mainly located at the periosteal side far from the fracture gap. Therefore, the inclusion of these residual stresses could represent a significant impact on the callus growth and predict a different evolution of biological processes occurring during bone h! ealing.
• Analysis of pelvic movement in the elderly during walking using a posture monitoring system equipped with a triaxial accelerometer and a gyroscope
  - J Biom 44(9):1788-1792 (2011)
  The incidence of falls in the elderly is increasing with the aging of society and is becoming a major public health issue. From the viewpoint of prevention of falls, it is important to evaluate the stability of the gait in the elderly people. The pelvic movement, which is a critical factor for walking stability, was analyzed using a posture monitoring system equipped with a triaxial accelerometer and a gyroscope. The subjects were 95 elderly people over 60 years of age. The criteria for instability were open-eye standing on one leg for 15 s or less, and 11 s or more on 3 m timed up and go test. Forty subjects who did not meet both of these criteria comprised the stable group, and the remaining 55 subjects comprised the unstable group. Pelvic movement during walking was compared between the two groups. The angle, angular velocity, and acceleration were analyzed based on the wave shape derived from the device worn around the second sacral. The results indicated that pelv! ic movement was lower in all three directions in the unstable group compared to the stable group, and the changes in the pelvic movement during walking in unstable elderly people were also reduced. This report is the first to evaluate pelvic movement by both a triaxial accelerometer and a triaxial gyroscope simultaneously. The characteristics of pelvic movement during walking can be applied in screening to identify elderly people with instability, which is the main risk factor associated with falls.
• Compressive properties of cd-HA-gelatin modified intrasynovial tendon allograft in canine model in vivo
  - J Biom 44(9):1793-1796 (2011)
  Although we sometimes use the intrasynovial tendon allograft as a donor, the gliding ability of allograft prepared by lyophilization is significantly decreased. The gliding ability of the grafted tendon after tendon reconstruction is very important because the high gliding resistance causes more adhesion and leads to poor clinical results. We recently revealed that tendon surface treatment with a carbodiimide derivatized HA (cd-HA)-gelatin mixture for intrasynovial tendon allograft significantly improved its gliding ability. The purpose of this study was to investigate whether this cd-HA-gelatin treatment affects the tendon mechanical property or not. A total of 40 flexor digitorum profundus (FDP) tendons from canines were evaluated for compressive property by using indentation test. Indentation stiffness was measured for normal tendon, rehydrated tendon after lyophilization, rehydrated tendon after lyophilization that was implanted 6 weeks in vivo, and cd-HA treated r! ehydrated tendon after lyophilization that was implanted 6 weeks in vivo. The results for all groups showed no significant difference in the tendon compressive properties. The findings of these results demonstrate that cd-HA treatment for intrasynovial tendon allograft is an excellent method to improve the tendon gliding ability after lyophilization without changing the compressive property of donor tendon.
• Potential errors in fiber length measurements resulting from lever arm rotation during mechanical testing of muscle cells
  - J Biom 44(9):1797-1800 (2011)
  In single muscle cell preparations fibers are often suspended between connectors, extending perpendicularly from a force transducer, and the lever arm of a torque motor. The fiber does not move along a horizontal plane when shortened or lengthened by lever arm rotation. An error from the true length (TL) is introduced if the expected length (EL) is calibrated along this horizontal optical plane. Lever arm length (LAL), initial fiber length (FLi), connector length (CL), and the magnitude of EL all contribute to this error. A mathematical model was used to determine the TL during shortening (0.96–0.80FLi) and lengthening (1.10–1.50FLi) at a constant LAL of 13.6 mm. CL had the greatest impact on error. For FLi=2 mm at the longest CL modeled (15 mm), an expected shortening of 0.20FLi produced a true shortening of 0.17FLi, and an expected stretch to 1.50FLi resulted in a true stretch to almost 1.60FLi. Under these conditions, the true sarcomere length would be 4% and 6%! longer than expected during shortening and lengthening, respectively. Because of their non-linear nature, length errors at long CL's may result in an over-estimation of unloaded shortening velocity during slack tests and a left-ward shift in the passive tension-fiber length relationship at long fiber lengths. Measurement errors decreased dramatically with shorter CL's, becoming negligible (<1%) at CL=3 mm. We recommend that investigators keep CL as short as possible. Alternatively, we provide a method for adjusting the magnitude of the EL to yield a desired TL.
• An instrumented tissue tester for measuring soft tissue property under the metatarsal heads in relation to metatarsophalangeal joint angle
  - J Biom 44(9):1801-1804 (2011)
  Identification of the localized mechanical response of the plantar soft tissue pads underneath the metatarsal heads (i.e., sub-MTH pad) to external loading is key to understand and predict how it functions in a gait cycle. The mechanical response depends on various parameters, such as the external load (direction and rate), the sub-MTH tissue properties (anisotropy and viscoelasticity), and the configuration of the metatarsophalangeal (MTP) joint overlying the tissue. In this study, an instrument-driven tissue tester that incorporates a portable motorized indentor within a special foot positioning apparatus was developed for realistic in vivo mechanical characterization (i.e. tissue stiffness and force relaxation behavior) of the local sub-MTH pad with the MTP joint configured at various dorsiflexion angles associated with gait. The tester yields consistent results for tests on the 2nd sub-MTH pad. Measurement errors for the initial stiffness (for indentation depths �! �1 mm), end-point stiffness, and percentage force relaxation were less than 0.084 N/mm, 0.133 N/mm, and 0.127%, respectively, across all test configurations. The end-point tissue stiffness, which increased by 104.2% due to a 50° MTP joint dorsiflexion, also agreed with a previous investigation. In vivo tissue's force relaxation was shown to be pronounced (avg.=8.1%), even for a short holding-time interval. The proposed technique to facilitate study of the dependence of the local sub-MTH pad and tissue response on the MTP joint angle might be preferable to methods that focus solely on measurement of tissue property because under physiologic conditions the sub-MTH pad elasticity may vary in gait, to adapt to drastically changing mechanical demands in the sub-MTH region of the terminal stance-phase, where MTP joint dorsiflexion occurs.
• Kinematic performance of a six degree-of-freedom hand model (6DHand) for use in occupational biomechanics
  - J Biom 44(9):1805-1809 (2011)
  Upper extremity musculoskeletal disorders represent an important health issue across all industry sectors; as such, the need exists to develop models of the hand that provide comprehensive biomechanics during occupational tasks. Previous optical motion capture studies used a single marker on the dorsal aspect of finger joints, allowing calculation of one and two degree-of-freedom (DOF) joint angles; additional algorithms were needed to define joint centers and the palmar surface of fingers. We developed a 6DOF model (6DHand) to obtain unconstrained kinematics of finger segments, modeled as frusta of right circular cones that approximate the palmar surface. To evaluate kinematic performance, twenty subjects gripped a cylindrical handle as a surrogate for a powered hand tool. We hypothesized that accessory motions (metacarpophalangeal pronation/supination; proximal and distal interphalangeal radial/ulnar deviation and pronation/supination; all joint translations) would b! e small (less than 5° rotations, less than 2 mm translations) if segment anatomical reference frames were aligned correctly, and skin movement artifacts were negligible. For the gripping task, 93 of 112 accessory motions were small by our definition, suggesting this 6DOF approach appropriately models joints of the fingers. Metacarpophalangeal supination was larger than expected (approximately 10°), and may be adjusted through local reference frame optimization procedures previously developed for knee kinematics in gait analysis. Proximal translations at the metacarpophalangeal joints (approximately 10 mm) were explained by skin movement across the metacarpals, but would not corrupt inverse dynamics calculated for the phalanges. We assessed performance in this study; a more rigorous validation would likely require medical imaging.
• Effect of acetabular component anteversion on dislocation mechanisms in total hip arthroplasty
  - J Biom 44(9):1810-1813 (2011)
  Quantifying soft-tissue tension around the hip joint during total hip arthroplasty remains difficult. In this study, a three-dimensional computer-aided design model was developed to clarify how component position in total hip arthroplasty contributes to the primary cause of posterior dislocation in cases of flexion, adduction and internal rotation. To better understand the influences of anteversion angle of the acetabular component, its effects on the primary causes of dislocations and the range of motion were investigated. Three different primary dislocation mechanisms were noted: impingement of the prosthetic femoral neck on the cup liner; impingement of the osseous femur on the osseous pelvis; and spontaneous dislocation caused by soft-tissue traction without impingement. Spontaneous dislocation could be detected by calculating hip forces at any thigh position using the computer-aided design model developed. In computer analysis, a transition from prosthetic impinge! ment rate to osseous impingement rate occurred with increasing anteversion angle of the acetabular component. Spontaneous dislocation was detected at angles >10° of anteversion of the acetabular component when flexion occurred with extreme adduction and internal rotation. This study demonstrated the possibility of spontaneous dislocation that results not from prosthetic or bony impingement but from muscle traction with increased range of motion.
• Test–retest reliability of knee biomechanics during stop jump landings
  - J Biom 44(9):1814-1816 (2011)
  Studies that seek to determine the effects of an intervention on knee biomechanics during landing from a jump implicitly assume that the variables of interest are reliable both within and between data collection sessions. Currently, such reliability data are not available for a stop jump. Standard three-dimensional motion analysis was used to determine sagittal and frontal plane peak angles and moments and peak vertical ground reaction force within and between sessions for a stop jump. Twelve female recreational athletes participated in two data collection sessions spaced two weeks apart. Interclass correlation coefficients and coefficient of multiple correlation were used to determine within and between session reliability of peak knee flexion angle, peak internal knee extension moment, peak knee abduction angle, peak internal knee adduction moment and peak vertical ground reaction force. Overall reliability within a session (ICC (3,1) 0.631–0.881; CMC 0.672–0.958! ) and between sessions (ICC (3,k) 0.685–0.959; CMC 0.598–0.944) was good. Peak angles and moments were similar between sessions. The stop jump is less reliable within a session than a drop vertical jump reported previously in the literature. This is likely due to increased intrasubject variability between trials due to the less constrained nature of the task. Reliability of the stop jump is comparable to the drop vertical jump between sessions. Reliability of knee adduction moment is lower than reported for the drop vertical jump. The results of this study support the use of a stop jump task to evaluate knee biomechanics during landing in longitudinal studies with a repeated measures design.
• Anatomic variation in the elastic inhomogeneity and anisotropy of human femoral cortical bone tissue is consistent across multiple donors
  - J Biom 44(9):1817-1820 (2011)
  Numerical models commonly account for elastic inhomogeneity in cortical bone using power-law scaling relationships with various measures of tissue density, but limited experimental data exists for anatomic variation in elastic anisotropy. A recent study revealed anatomic variation in the magnitude and anisotropy of elastic constants along the entire femoral diaphysis of a single human femur (Espinoza Orías et al., 2009). The objective of this study was to confirm these trends across multiple donors while also considering possible confounding effects of the anatomic quadrant, apparent tissue density, donor age, and gender. Cortical bone specimens were sampled from the whole femora of 9 human donors at 20%, 50%, and 80% of the total femur length. Elastic constants from the main diagonal of the reduced fourth-order tensor were measured on hydrated specimens using ultrasonic wave propagation. The tissue exhibited orthotropy overall and at each location along the length of! the diaphysis (p<0.0001). Elastic anisotropy increased from the mid-diaphysis toward the epiphyses (p<0.05). The increased elastic anisotropy was primarily caused by a decreased radial elastic constant (C11) from the mid-diaphysis toward the epiphyses (p<0.05), since differences in the circumferential (C22) and longitudinal (C33) elastic constants were not statistically significant (p>0.29). Anatomic variation in intracortical porosity may account for these trends, but requires further investigation. The apparent tissue density was positively correlated with the magnitude of each elastic constant (p<0.0001, R2>0.46), as expected, but was only weakly correlated with C33/C11 (p<0.05, R2=0.04) and not significantly correlated with C33/C22 and C11/C22.
• A protocol for monitoring soft tissue motion under compression garments during drop landings
  - J Biom 44(9):1821-1823 (2011)
  This study used a single-subject design to establish a valid and reliable protocol for monitoring soft tissue motion under compression garments during drop landings. One male participant performed six 40 cm drop landings onto a force platform, in three compression conditions (none, medium high). Five reflective markers placed on the thigh under the compression garment and five over the garment were filmed using two cameras (1000 Hz). Following manual digitisation, marker coordinates were reconstructed and their resultant displacements and maximum change in separation distance between skin and garment markers were calculated. To determine reliability of marker application, 35 markers were attached to the thigh over the high compression garment and filmed. Markers were then removed and re-applied on three occasions; marker separation and distance to thigh centre of gravity were calculated. Results showed similar ground reaction forces during landing trials. Significant r! eductions in the maximum change in separation distance between markers from no compression to high compression landings were reported. Typical errors in marker movement under and over the garment were 0.1 mm in medium and high compression landings. Re-application of markers showed mean typical errors of 1 mm in marker separation and <3 mm relative to thigh centre of gravity. This paper presents a novel protocol that demonstrates sufficient sensitivity to detect reductions in soft tissue motion during landings in high compression garments compared to no compression. Additionally, markers placed under or over the garment demonstrate low variance in movement, and the protocol reports good reliability in marker re-application.
• Early osteoarthritis were only detected at the nanometer scale but not at the micrometer or millimeter scale
  - J Biom 44(9):1824-1825 (2011)
  
• Response to Letter to the Editor: "Structural and functional changes of the articular surface measured by atomic force microscopy"
  - J Biom 44(9):1825-1826 (2011)
  
• Letter to the Editor referring to the article "Mineral heterogeneity affects predictions of intratrabecular stress and strain" published in Journal of Biomechanics (volume 44, Issue 3, Pages 402–407)
  - J Biom 44(9):1826-1827 (2011)
  
• Response to 'Letter to the Editor referring to the article "Mineral heterogeneity affects predictions of intratrabecular stress and strain" published in Journal of Biomechanics (volume 44, Issue 3, Pages 402–407)'
  - J Biom 44(9):1827-1828 (2011)
  
• Corrigendum to: "Mechanics of microtubules by T. Hawkins, M. Mirigian, M. S. Yasar, J. L. Ross" [J. Biomech. 43(1) (2010) pp. 23–30]
  - J Biom 44(9):1829-1830 (2011)