Monday, October 31, 2011

Hot off the presses! Nov 10 J Biomech

The Nov 10 issue of the J Biomech is now up on Pubget (About J Biomech): if you're at a subscribing institution, just click the link in the latest link at the home page. (Note you'll only be able to get all the PDFs in the issue if your institution subscribes to Pubget.)

Latest Articles Include:

  • Editorial Board and Publication Information
    - J Biomech 44(16):IFC (2011)
  • The shocking truth about meniscus
    - J Biomech 44(16):2737-2740 (2011)
    The menisci of the knee are structures integral to the long term health of the knee joint. The primary function of these tissues is to distribute load across the tibiofemoral joint by increasing the congruency of the joint, thereby decreasing the resultant stress experienced by the articular cartilages. The menisci also play a secondary role in stabilizing the joint, particularly in the anterior cruciate ligament deficient knee, and also have roles in joint lubrication and proprioception. Also, an oft-cited role of this tissue is that of a shock absorber. We will review the literature supporting this shock absorption paradigm and describe the limitations and errors in the conclusions made by these studies. Consequently, we will show that the literature is inconclusive with no support for the shock absorption paradigm, which should therefore not be stated as a function of the menisci. We will describe how one of the three articles in support of this paradigm actually co! uld be interpreted to the contrary and support the idea that the menisci may play no significant role in shock absorption at the knee at all, with the two remaining papers being inconclusive.
  • Paretic muscle atrophy and non-contractile tissue content in individual muscles of the post-stroke lower extremity
    - J Biomech 44(16):2741-2746 (2011)
    Muscle atrophy is one of many factors contributing to post-stroke hemiparetic weakness. Since muscle force is a function of muscle size, the amount of muscle atrophy an individual muscle undergoes has implications for its overall force-generating capability post-stroke. In this study, post-stroke atrophy was determined bilaterally in fifteen leg muscles with volumes quantified using magnetic resonance imaging (MRI). All muscle volumes were adjusted to exclude non-contractile tissue content, and muscle atrophy was quantified by comparing the volumes between paretic and non-paretic sides. Non-contractile tissue or intramuscular fat was calculated by determining the amount of tissue excluded from the muscle volume measurement. With the exception of the gracilis, all individual paretic muscles examined had smaller volumes in the non-paretic side. The average decrease in volume for these paretic muscles was 23%. The gracilis volume, on the other hand, was approximately 11% ! larger on the paretic side. The amount of non-contractile tissue was higher in all paretic muscles except the gracilis, where no difference was observed between sides. To compensate for paretic plantar flexor weakness, one idea might be that use of the paretic gracilis actually causes the muscle to increase in size and not develop intramuscular fat. By eliminating non-contractile tissue from our volume calculations, we have presented volume data that more appropriately represents force-generating muscle tissue. Non-uniform muscle atrophy was observed across muscles and may provide important clues when assessing the effect of muscle atrophy on post-stroke gait.
  • Shear strength and toughness of trabecular bone are more sensitive to density than damage
    - J Biomech 44(16):2747-2754 (2011)
    Microdamage occurs in trabecular bone under normal loading, which impairs the mechanical properties. Architectural degradation associated with osteoporosis increases damage susceptibility, resulting in a cumulative negative effect on the mechanical properties. Treatments for osteoporosis could be targeted toward increased bone mineral density, improved architecture, or repair and prevention of microdamage. Delineating the relative roles of damage and architectural degradation on trabecular bone strength will provide insight into the most beneficial targets. In this study, damage was induced in bovine trabecular bone samples by axial compression, and the effects on the mechanical properties in shear were assessed. The damaged shear modulus, shear yield stress, ultimate shear stress, and energy to failure all depended on induced damage and decreased as the architecture became more rod-like. The changes in ultimate shear strength and toughness were proportional to the dec! rease in shear modulus, consistent with an effective decrease in the cross-section of trabeculae based on cellular solid analysis. For typical ranges of bone volume fraction in human bone, the strength and toughness were much more sensitive to decreased volume fraction than to induced mechanical damage. While ultimately repairing or avoiding damage to the bone structure and increasing bone density both improve mechanical properties, increasing bone density is the more important contributor to bone strength.
  • In vivo porcine left atrial wall stress: Effect of ventricular tachypacing on spatial and temporal stress distribution
    - J Biomech 44(16):2755-2760 (2011)
    Animal models of ventricular tachypacing (VTP) have been successfully used to reproduce the relevant features observed in patients with atrial fibrillation, such as increased atrial pressure and volume, ion-channel alterations and fibrosis. After performing VTP on a healthy Yorkshire pig, we measured an increase in volume of 60%, a two-fold rise in pressure, and a complex pattern of local mechanical, histological and biochemical changes, including a generalized stiffening of the wall. A protocol recently developed was employed to generate computational models of the porcine left atrium mechanics in healthy conditions and after VTP. Comparison of the stress distribution in the healthy vs. VTP case provided a map of how pressure overload affects and modifies left atrium mechanics. Overall, a positive increase in stress was computed after the VTP treatment. Regions of large increase in the stresses post-VTP were the appendage boundaries, the area around the lower pulmonar! y vein and the area in the front of the atrium towards the appendage. Due to the elevated stress, the back of the atrium mainly modified its mechanical response, while the appendage remodeled both its shape and its mechanical properties. Large changes in the shape of the mitral valve annulus could be observed as a consequence of the remodeling in the front of the atrium. The relation between local mechanical stress and remodeling that emerges from the results is in agreement with our hypothesis that the structural changes in the atrium are a consequence of a stress-mediated mechanism.
  • Fluid movement and joint capsule strains due to flexion in rabbit knees
    - J Biomech 44(16):2761-2767 (2011)
    Diarthrodial joints are freely moveable joints containing synovial fluid (SF) within a connective tissue joint capsule that allows for low-friction and low-wear articulation of the cartilaginous ends of long bones. Biomechanical cues from joint articulation regulate synoviocyte and cartilage biology via joint capsule strain, in turn altering the composition of SF. Joint flexion is clinically associated with pain in knees with arthritis and effusion, with the nociception possibly originating from joint capsule strain. The hypothesis of this study was that knee fluid volume distribution and joint capsule strain are altered with passive flexion in the rabbit model. The aims were to (a) determine the volume distribution of fluid in the joint at different total volumes and with flexion of rabbit knees ex vivo, (b) correlate the volume distribution for the ex vivo model to in vivo data, and (c) determine the strains at different locations in the joint capsule with flexion. D! uring knee flexion, ∼20% of anteriorly located joint fluid moved posteriorly, correlating well with the fluid motion observed in in vivo joints. Planar joint capsule principal strains were ∼100% (tension) in the proximal–distal direction and ∼−40% (shortening) in the circumferential direction, relative to the femur axis and 30° strain state. The joint capsule strains with flexion are consistent with the mechanics of the tendons and ligaments from which the capsule tissue is derived. The movement and mixing of SF volume with flexion determine the mechanical and biological fluid environment within the knee joint. Joint fluid movement and capsular strains affect synovial cell biology and likely modulate trans-synovial transport.
  • Large eddy simulation of the unsteady flow-field in an idealized human mouthâ€"throat configuration
    - J Biomech 44(16):2768-2774 (2011)
    The present study concerns the simulation and analysis of the flow field in the upper human respiratory system in order to gain an improved understanding of the complex flow field with respect to the process affecting drug delivery for medical treatment of the human air system. For this purpose, large eddy simulation (LES) is chosen because of its powerful performance in the transitional range of laminar and turbulent flow fields. The average gas velocity in a constricted tube is compared with experimental data (Ahmed and Giddens, 1983) and numerical data from Reynolds-averaged Navier–Stokes (RANS) equations coupled with low Reynolds number (LRN) κ–ω model (Zhang and Kleinstreuer, 2003) and LRN shear–stress transport κ–ω model (Jayaraju et al., 2007), for model validation. The present study emphasizes on the instantaneous flow field, where the simulations capture different scales of secondary vortices in different flow zones including recirculation zones, the laryngeal jet zone, the mixing zone, and the wall shear layer. It is observed that the laryngeal jet tail breaks up, and the unsteady motion of laryngeal jet is coupled with the unsteady distribution of secondary vortices in the jet boundary. The present results show that it is essential to study the unsteady flow field since it strongly affects the particle flow in the human upper respiratory system associated with drug delivery for medical treatment.
  • Variation in Young's modulus along the length of a rat vibrissa
    - J Biomech 44(16):2775-2781 (2011)
    Rats use specialized tactile hairs on their snout, called vibrissae (whiskers), to explore their surroundings. Vibrissae have no sensors along their length, but instead transmit mechanical information to receptors embedded in the follicle at the vibrissa base. The transmission of mechanical information along the vibrissa, and thus the tactile information ultimately received by the nervous system, depends critically on the mechanical properties of the vibrissa. In particular, transmission depends on the bending stiffness of the vibrissa, defined as the product of the area moment of inertia and Young's modulus. To date, Young's modulus of the rat vibrissa has not been measured in a uniaxial tensile test. We performed tensile tests on 22 vibrissae cut into two halves: a tip-segment and a base-segment. The average Young's modulus across all segments was 3.34±1.48 GPa. The average modulus of a tip-segment was 3.96±1.60 GPa, and the average modulus of a base-segment was 2.! 90±1.25 GPa. Thus, on average, tip-segments had a higher Young's modulus than base-segments. High-resolution images of vibrissae were taken to seek structural correlates of this trend. The fraction of the cross-sectional area occupied by the vibrissa cuticle was found to increase along the vibrissa length, and may be responsible for the increase in Young's modulus near the tip.
  • A massâ€"length scaling law for modeling muscle strength in the lower limb
    - J Biomech 44(16):2782-2789 (2011)
    Musculoskeletal computer models are often used to study muscle function in children with and without impaired mobility. Calculations of muscle forces depend in part on the assumed strength of each muscle, represented by the peak isometric force parameter, which is usually based on measurements obtained from cadavers of adult donors. The aim of the present study was twofold: first, to develop a method for scaling lower-limb peak isometric muscle forces in typically-developing children; and second, to determine the effect of this scaling method on model calculations of muscle forces obtained for normal gait. Muscle volumes were determined from magnetic resonance (MR) images obtained from ten children aged from 7 to 13 yr. A new mass–length scaling law was developed based on the assumption that muscle volume and body mass are linearly related, which was confirmed by the obtained volume and body mass data. Two musculoskeletal models were developed for each subject: one i! n which peak isometric muscle forces were estimated using the mass–length scaling law; and another in which these parameters were determined directly from the MR-derived muscle volumes. Musculoskeletal modeling and quantitative gait analysis were then used to calculate lower-limb muscle forces in normal walking. The patterns of muscle forces predicted by the model with scaled peak isometric force values were similar to those predicted by the MR-based model, implying that assessments of muscle function obtained from these two methods are practically equivalent. These results support the use of mass–length scaling in the development of subject-specific musculoskeletal models of children.
  • AFM membrane roughness as a probe to identify oxidative stress-induced cellular apoptosis
    - J Biomech 44(16):2790-2794 (2011)
    The morphological change of cellular apoptosis initiates from the change of membrane roughness. In order to identify cellular apoptosis in its early stage, atomic force microscope was adapted to reveal the change of membrane roughness in unprecedented details, providing an image in nanometer-scaled resolution. The mouse monocyte/macrophage cell line RAW 264.7 was the subject studied and subjected to apoptotic induction by hydrogen peroxide. A finding of the qualitative correlation between cell membrane roughness and oxidative stress level is disclosed stating that roughness is increasing with the increasing level of oxidative stress.
  • Strains at the myotendinous junction predicted by a micromechanical model
    - J Biomech 44(16):2795-2801 (2011)
    The goal of this work was to create a finite element micromechanical model of the myotendinous junction (MTJ) to examine how the structure and mechanics of the MTJ affect the local micro-scale strains experienced by muscle fibers. We validated the model through comparisons with histological longitudinal sections of muscles fixed in slack and stretched positions. The model predicted deformations of the A-bands within the fiber near the MTJ that were similar to those measured from the histological sections. We then used the model to predict the dependence of local fiber strains on activation and the mechanical properties of the endomysium. The model predicted that peak micro-scale strains increase with activation and as the compliance of the endomysium decreases. Analysis of the models revealed that, in passive stretch, local fiber strains are governed by the difference of the mechanical properties between the fibers and the endomysium. In active stretch, strain distribu! tions are governed by the difference in cross-sectional area along the length of the tapered region of the fiber near the MTJ. The endomysium provides passive resistance that balances the active forces and prevents the tapered region of the fiber from undergoing excessive strain. These model predictions lead to the following hypotheses: (i) the increased likelihood of injury during active lengthening of muscle fibers may be due to the increase in peak strain with activation and (ii) endomysium may play a role in protecting fibers from injury by reducing the strains within the fiber at the MTJ.
  • Assessment of the applicability of the Hertzian contact theory to edge-loaded prosthetic hip bearings
    - J Biomech 44(16):2802-2808 (2011)
    The components of prosthetic hip bearings may experience in-vivo subluxation and edge loading on the acetabular socket as a result of joint laxity, causing abnormally high, damaging contact stresses. In this research, edge-loaded contact of prosthetic hips is examined analytically and experimentally in the most commonly used categories of material pairs. In edge-loaded ceramic-on-ceramic hips, the Hertzian contact theory yields accurate (conservatively, <10% error) predictions of the contact dimensions. Moreover, the Hertzian theory successfully captures slope and curvature trends in the dependence of contact patch geometry on the applied load. In an edge-loaded ceramic-on-metal pair, a similar degree of accuracy is observed in the contact patch length; however, the contact width is less accurately predicted due to the onset of subsurface plasticity, which is predicted for loads >400 N. The Hertzian contact theory is shown to be ill-suited to edge-loaded ceramic-on-pol! yethylene pairs due to polyethylene's nonlinear material behavior. This work elucidates the methods and the accuracy of applying classical contact theory to edge-loaded hip bearings. The results help to define the applicability of the Hertzian theory to the design of new components and materials to better resist severe edge loading contact stresses.
  • Can aspect ratio be used to categorize intra-aneurysmal hemodynamics?â€"A study of elastase induced aneurysms in rabbit
    - J Biomech 44(16):2809-2816 (2011)
    Clinical studies suggest that aneurysm aspect ratio (AR) is an important indicator of rupture likelihood. The importance of AR is hypothesized to arise from its influence on intra-aneurysmal hemodynamics. It has been conjectured that slower flow in high AR sacs leads to a cascade of biological activities that weaken the aneurysm wall (Ujiie et al.,1999). However, the connection between AR, hemodynamics and wall weakening has never been proven. Animal models of saccular aneurysms provide a venue for evaluating this conjecture. The focus of this work was to evaluate whether a commonly used elastase induced aneurysm model in rabbits is suitable for a study of this kind from a hemodynamic perspective. In particular, to assess whether hemodynamic factors in low and high AR sacs are statistically different. To achieve this objective, saccular aneurysms were created in 51 rabbits and pulsatile computational fluid dynamics (CFD) studies were performed using rabbit specific inf! lows. Distinct hemodynamics were found in the low AR (AR<1.8, n=25), and high AR (AR>2.2, n=18) models. A single, stable recirculation zone was present in all low AR aneurysms, whereas a second, transient recirculation zone was also found in the superior aspect of the aneurysm dome for all high AR cases. Aneurysms with AR between 1.8 and 2.2 displayed transitional flow patterns. Differences in values and distributions of hemodynamic parameters were found between low and high AR cases including time averaged wall shear stress, oscillatory shear index, relative residence time and non-dimensional inflow rate. This work lays the foundation for future studies of the dependence of growth and remodeling on AR in the rabbit model and provides a motivation for further studies of the coupling between AR and hemodynamics in human aneurysms.
  • Modeling the impact of concomitant aortic stenosis and coarctation of the aorta on left ventricular workload
    - J Biomech 44(16):2817-2825 (2011)
    Coarctation of the aorta (COA) is an obstruction of the aorta and is usually associated with bicuspid and tricuspid aortic valve stenosis (AS). When COA coexists with AS, the left ventricle (LV) is facing a double hemodynamic load: a valvular load plus a vascular load. The objective of this study was to develop a lumped parameter model, solely based on non-invasive data, allowing the description of the interaction between LV, COA, AS and the arterial system. First, a formulation describing the instantaneous net pressure gradient through the COA was introduced and the predictions were compared to in vitro results. The model was then used to determine LV work induced by coexisting AS and COA with different severities. The results show that LV stroke work varies from 0.98 J (no-AS; no-COA) up to 2.15 J (AS: 0.61 cm2+COA: 90%). Our results also show that the proportion of the total flow rate that will cross the COA is significantly reduced with the increasing COA severity ! (from 85% to 40%, for a variation of COA severity from 0% to 90%, respectively). Finally, we introduced simple formulations capable of, non-invasively, estimating both LV peak systolic pressure and workload. As a conclusion, this study allowed the development of a lumped parameter model, based on non-invasive measurements, capable of accurately investigating the impact of coexisting AS and COA on LV workload. This model can be used to optimize the management of patients with COA and AS in terms of the sequence of lesion repair.
  • Direct numerical simulation of transitional flow in a patient-specific intracranial aneurysm
    - J Biomech 44(16):2826-2832 (2011)
    In experiments turbulence has previously been shown to occur in intracranial aneurysms. The effects of turbulence induced oscillatory wall stresses could be of great importance in understanding aneurysm rupture. To investigate the effects of turbulence on blood flow in an intracranial aneurysm, we performed a high resolution computational fluid dynamics (CFD) simulation in a patient specific middle cerebral artery (MCA) aneurysm using a realistic, pulsatile inflow velocity. The flow showed transition to turbulence just after peak systole, before relaminarization occurred during diastole. The turbulent structures greatly affected both the frequency of change of wall shear stress (WSS) direction and WSS magnitude, which reached a maximum value of 41.5 Pa. The recorded frequencies were predominantly in the range of 1–500 Hz. The current study confirms, through properly resolved CFD simulations that turbulence can occur in intracranial aneurysms.
  • Accurate determination of the structural elasticity of human hair by a small-scale bending test
    - J Biomech 44(16):2833-2837 (2011)
    This paper reports on a small-scale bending method for human hair. The test sample, which is elliptical in cross-section, is fixed to a hollow steel needle using resin to form a cantilever. A loading probe is used to subject this to a lateral load, where the load is applied parallel to either the long or short axis of the elliptical cross-section. From these tests, load–displacement relationships for the hair were obtained. From the experimental data and analysis, we found that the structural elasticity determined is independent of the direction of bending, and precise measurements of the structural elasticity of human hair with scattering of less than 5% were realized using this test scheme. Finally, changes in the structural elasticity of hair due to hair treatments were detected and the changes are discussed based on a theoretical model of the multi-layered structure.
  • Assessment of anatomical frame variation effect on joint angles: A linear perturbation approach
    - J Biomech 44(16):2838-2842 (2011)
    Although the interpretability and reliability of joint kinematics depends strongly on the accuracy and precision of determining the anatomical frame (AF) orientation, the exact dependency of joint angle error on AF misalignment is still not clear. To fully understand the behavior, this study uses linear perturbations to quantify joint angle error due to known modifications of the AFs, where the joint angles are calculated according to the Cardanic convention. The result is a functional representation of joint angle error with dependence on nominal joint angles and on the orientations of the alternative AFs relative to the nominal AFs. The results are validated using numerical analysis on knee joint angle data during walking. The derived relationship elucidates results from previous work studying this effect and allows AF differences to be inferred by joint angle curves when multiple sets of joint angle curves are collected simultaneously.
  • The use of magnetic resonance imaging to predict ACL graft structural properties
    - J Biomech 44(16):2843-2846 (2011)
    Magnetic resonance imaging (MRI) could potentially be used to non-invasively predict the strength of an ACL graft after ACL reconstruction. We hypothesized that the volume and T2 relaxation parameters of the ACL graft measured with MRI will predict the graft structural properties and anteroposterior (AP) laxity of the reconstructed knee. Nine goats underwent ACL reconstruction using a patellar tendon autograft augmented with a collagen or collagen–platelet composite. After 6 weeks of healing, the animals were euthanized, and the reconstructed knees were retrieved and imaged on a 3T scanner. AP laxity was measured prior to dissecting out the femur–graft–tibia constructs which were then tested to tensile failure to determine the structural properties. Regression analysis indicated a statistically significant relationship between the graft volume and the failure load (r2=0.502; p=0.049). When graft volume was normalized to the T2 relaxation time, the relationship wa! s even greater (r2=0.687; p=0.011). There was a significant correlation between the graft volume and the linear stiffness (r2=0.847; p<0.001), which remained significant with T2 normalization (r2=0.764; p=0.002). For AP laxity at 30° flexion, there was not a significant correlation with graft volume, but there was a significant correlation with volume normalized by the T2 relaxation time (r2=0.512; p=0.046). These results suggest that MRI volumetric measures combined with graft T2 properties may be useful in predicting the structural properties of ACL grafts.
  • The effect of decreasing computed tomography dosage on radiostereometric analysis (RSA) accuracy at the glenohumeral joint
    - J Biomech 44(16):2847-2850 (2011)
    Standard, beaded radiostereometric analysis (RSA) and markerless RSA often use computed tomography (CT) scans to create three-dimensional (3D) bone models. However, ethical concerns exist due to risks associated with CT radiation exposure. Therefore, the aim of this study was to investigate the effect of decreasing CT dosage on RSA accuracy. Four cadaveric shoulder specimens were scanned using a normal-dose CT protocol and two low-dose protocols, where the dosage was decreased by 89% and 98%. 3D computer models of the humerus and scapula were created using each CT protocol. Bi-planar fluoroscopy was used to image five different static glenohumeral positions and two dynamic glenohumeral movements, of which a total of five static and four dynamic poses were selected for analysis. For standard RSA, negligible differences were found in bead (0.21±0.31 mm) and bony landmark (2.31±1.90 mm) locations when the CT dosage was decreased by 98% (p-values>0.167). For markerless R! SA kinematic results, excellent agreement was found between the normal-dose and lowest-dose protocol, with all Spearman rank correlation coefficients greater than 0.95. Average root mean squared errors of 1.04±0.68 mm and 2.42±0.81° were also found at this reduced dosage for static positions. In summary, CT dosage can be markedly reduced when performing shoulder RSA to minimize the risks of radiation exposure. Standard RSA accuracy was negligibly affected by the 98% CT dose reduction and for markerless RSA, the benefits of decreasing CT dosage to the subject outweigh the introduced errors.
  • Comments on “Assessment of amputee socketâ€"stumpâ€"residual bone kinematics during strenuous activities using Dynamic Roentgen Stereogrammetric Analysis” (Volume 43, Issue 5, 2010)
    - J Biomech 44(16):2851-2852 (2011)
  • Volume 44, 2011. Author/Subject Index
    - J Biomech 44(16):2853-2870 (2011)

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