Research Papers

J Biomech Eng. 2016;138(10):101001-101001-11. doi:10.1115/1.4034216.

Prior studies have assessed the effects of load carriage on the tibia. Here, we expand on these studies and investigate the effects of load carriage on joint reaction forces (JRFs) and the resulting spatiotemporal stress/strain distributions in the tibia. Using full-body motion and ground reaction forces from a female subject, we computed joint and muscle forces during walking for four load carriage conditions. We applied these forces as physiological loading conditions in a finite-element (FE) analysis to compute strain and stress. We derived material properties from computed tomography (CT) images of a sex-, age-, and body mass index-matched subject using a mesh morphing and mapping algorithm, and used them within the FE model. Compared to walking with no load, the knee JRFs were the most sensitive to load carriage, increasing by as much as 26.2% when carrying a 30% of body weight (BW) load (ankle: 16.4% and hip: 19.0%). Moreover, our model revealed disproportionate increases in internal JRFs with increases in load carriage, suggesting a coordinated adjustment in the musculature functions in the lower extremity. FE results reflected the complex effects of spatially varying material properties distribution and muscular engagement on tibial biomechanics during walking. We observed high stresses on the anterior crest and the medial surface of the tibia at pushoff, whereas high cumulative stress during one walking cycle was more prominent in the medioposterior aspect of the tibia. Our findings reinforce the need to include: (1) physiologically accurate loading conditions when modeling healthy subjects undergoing short-term exercise training and (2) the duration of stress exposure when evaluating stress-fracture injury risk. As a fundamental step toward understanding the instantaneous effect of external loading, our study presents a means to assess the relationship between load carriage and bone biomechanics.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):101002-101002-7. doi:10.1115/1.4034307.

Ureteral peristalsis can be considered as a series of waves on the ureteral wall, which transfers the urine along the ureter toward the bladder. The stones that form in the kidney and migrate to the ureter can create a substantial health problem due to the pain caused by interaction of the ureteral walls and stones during the peristaltic motion. Three-dimensional (3D) computational fluid dynamics (CFD) simulations were carried out using the commercial code ansys fluent to solve for the peristaltic movement of the ureter, with and without stones. The effect of stone size was considered through the investigation of varying obstructions of 5%, 15%, and 35% for fixed spherical stone shape. Also, an understanding of the effect of stone shape was obtained through separate CFD calculations of the peristaltic ureter with three different types of stones, a sphere, a cube, and a star, all at a fixed obstruction percentage of 15%. Velocity vectors, mass flow rates, pressure gradients, and wall shear stresses were analyzed along one bolus of urine during peristalsis of the ureteral wall to study the various effects. It was found that the increase in obstruction increased the backflow, pressure gradients, and wall shear stresses proximal to the stone. On the other hand, with regard to the stone shape study, while the cube-shaped stones resulted in the largest backflow, the star-shaped stone showed highest pressure gradient magnitudes. Interestingly, the change in stone shape did not have a significant effect on the wall shear stress at the obstruction level studied here.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):101003-101003-10. doi:10.1115/1.4034293.

Biofidelity response corridors developed from post-mortem human subjects are commonly used in the design and validation of anthropomorphic test devices and computational human body models (HBMs). Typically, corridors are derived from a diverse pool of biomechanical data and later normalized to a target body habitus. The objective of this study was to use morphed computational HBMs to compare the ability of various scaling techniques to scale response data from a reference to a target anthropometry. HBMs are ideally suited for this type of study since they uphold the assumptions of equal density and modulus that are implicit in scaling method development. In total, six scaling procedures were evaluated, four from the literature (equal-stress equal-velocity, ESEV, and three variations of impulse momentum) and two which are introduced in the paper (ESEV using a ratio of effective masses, ESEV-EffMass, and a kinetic energy approach). In total, 24 simulations were performed, representing both pendulum and full body impacts for three representative HBMs. These simulations were quantitatively compared using the International Organization for Standardization (ISO) ISO-TS18571 standard. Based on these results, ESEV-EffMass achieved the highest overall similarity score (indicating that it is most proficient at scaling a reference response to a target). Additionally, ESEV was found to perform poorly for two degree-of-freedom (DOF) systems. However, the results also indicated that no single technique was clearly the most appropriate for all scenarios.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):101004-101004-8. doi:10.1115/1.4034382.

Tendons are highly anisotropic and also viscoelastic. For understanding and modeling their 3D deformation, information is needed on their viscoelastic response under off-axis loading. A study was made, therefore, of creep and recovery of bovine digital extensor tendons when subjected to transverse compressive stress of up to ca. 100 kPa. Preconditioned tendons were compression tested between glass plates at increasing creep loads. The creep response was anomalous: the relative rate of creep reduced with the increasing stress. Over each ca. 100 s creep period, the transverse creep deformation of each tendon obeyed a power law dependence on time, with the power law exponent falling from ca. 0.18 to an asymptote of ca. 0.058 with the increasing stress. A possible explanation is stress-driven dehydration, as suggested previously for the similar anomalous behavior of ligaments. Recovery after removal of each creep load was also anomalous. Relative residual strain reduced with the increasing creep stress, but this is explicable in terms of the reducing relative rate of creep. When allowance was made for some adhesion occurring naturally between tendon and the glass plates, the results for a given load were consistent with creep and recovery being related through the Boltzmann superposition principle (BSP). The tendon tissue acted as a pressure-sensitive adhesive (PSA) in contact with the glass plates: explicable in terms of the low transverse shear modulus of the tendons.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):101005-101005-11. doi:10.1115/1.4034425.

In this work, we present a novel method for the derivation of the unloaded geometry of an abdominal aortic aneurysm (AAA) from a pressurized geometry in turn obtained by 3D reconstruction of computed tomography (CT) images. The approach was experimentally validated with an aneurysm phantom loaded with gauge pressures of 80, 120, and 140 mm Hg. The unloaded phantom geometries estimated from these pressurized states were compared to the actual unloaded phantom geometry, resulting in mean nodal surface distances of up to 3.9% of the maximum aneurysm diameter. An in-silico verification was also performed using a patient-specific AAA mesh, resulting in maximum nodal surface distances of 8 μm after running the algorithm for eight iterations. The methodology was then applied to 12 patient-specific AAA for which their corresponding unloaded geometries were generated in 5–8 iterations. The wall mechanics resulting from finite element analysis of the pressurized (CT image-based) and unloaded geometries were compared to quantify the relative importance of using an unloaded geometry for AAA biomechanics. The pressurized AAA models underestimate peak wall stress (quantified by the first principal stress component) on average by 15% compared to the unloaded AAA models. The validation and application of the method, readily compatible with any finite element solver, underscores the importance of generating the unloaded AAA volume mesh prior to using wall stress as a biomechanical marker for rupture risk assessment.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):101006-101006-11. doi:10.1115/1.4034490.

The propagation of mechanical signals through nonlinear fibrous tissues is much more extensive than through continuous synthetic hydrogels. Results from recent studies indicate that increased mechanical propagation arises from the fibrous nature of the material rather than the strain-stiffening property. The relative importance of different parameters of the fibrous network structure to this propagation, however, remains unclear. In this work, we directly compared the mechanical response of substrates of varying thickness subjected to a constant cell traction force using either a nonfibrous strain-stiffening continuum-based model or a volume-averaged fiber network model consisting of two different types of fiber network structures: one with low fiber connectivity (growth networks) and one with high fiber connectivity (Delaunay networks). The growth network fiber models predicted a greater propagation of substrate displacements through the model and a greater sensitivity to gel thickness compared to the more connected Delaunay networks and the nonlinear continuum model. Detailed analysis of the results indicates that rotational freedom of the fibers in a network with low fiber connectivity is critically important for enhanced, long-range mechanosensing. Our findings demonstrate the utility of multiscale models in predicting cells mechanosensing on fibrous gels, and they provide a more complete understanding of how cell traction forces propagate through fibrous tissues, which has implications for the design of engineered tissues and the stem cell niche.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):101007-101007-6. doi:10.1115/1.4034463.

The structure of the medial longitudinal arch (MLA) affects the foot's overall function and its ability to dissipate plantar pressure forces. Previous research on the MLA includes measuring the calcaneal–first metatarsal angle using a static sagittal plane radiograph, a dynamic height-to-length ratio using marker clusters with a multisegment foot model, and a contained angle using single point markers with a multisegment foot model. The objective of this study was to use biplane fluoroscopy to measure a contained MLA angle between foot types: pes planus (low arch), pes cavus (high arch), and normal arch. Fifteen participants completed the study, five from each foot type. Markerless fluoroscopic radiostereometric analysis (fRSA) was used with a three-dimensional model of the foot bones and manually matching those bones to a pair of two-dimensional radiographic images during midstance of gait. Statistically significant differences were found between barefoot arch angles of the normal and pes cavus foot types (p = 0.036), as well as between the pes cavus and pes planus foot types (p = 0.004). Dynamic walking also resulted in a statistically significant finding compared to the static standing trials (p = 0.014). These results support the classification of individuals following a physical assessment by a foot specialist for those with pes cavus and planus foot types. The differences between static and dynamic kinematic measurements were also supported using this novel method.

Topics: Arches , Bone
Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):101008-101008-10. doi:10.1115/1.4034306.

Most studies on football helmet performance focus on lowering head acceleration-related parameters to reduce concussions. This has resulted in an increase in helmet size and mass. The objective of this paper was to study the effect of helmet mass on head and upper neck responses. Two independent test series were conducted. In test series one, 90 pendulum impact tests were conducted with four different headform and helmet conditions: unhelmeted Hybrid III headform, Hybrid III headform with a football helmet shell, Hybrid III headform with helmet shell and facemask, and Hybrid III headform with the helmet and facemask with mass added to the shell (n = 90). The Hybrid III neck was used for all the conditions. For all the configurations combined, the shell only, shell and facemask, and weighted helmet conditions resulted in 36%, 43%, and 44% lower resultant head accelerations (p < 0.0001), respectively, when compared to the unhelmeted condition. Head delta-V reductions were 1.1%, 4.5%, and 4.4%, respectively. In contrast, the helmeted conditions resulted in 26%, 41%, and 49% higher resultant neck forces (p < 0.0001), respectively. The increased neck forces were dominated by neck tension. In test series two, testing was conducted with a pneumatic linear impactor (n = 178). Fourteen different helmet makes and models illustrate the same trend. The increased neck forces provide a possible explanation as to why there has not been a corresponding reduction in concussion rates despite improvements in helmets ability to reduce head accelerations.

Commentary by Dr. Valentin Fuster

Technical Brief

J Biomech Eng. 2016;138(10):104501-104501-7. doi:10.1115/1.4034169.

Current measures of knee joint laxity, such as those found clinically using the KT-2000 arthrometer, are not highly repeatable or reliable by Huber et al. (1997, “Intratester and Intertester Reliability of the KT-1000 Arthrometer in the Assessment of Posterior Laxity of the Knee,” Am. J. Sports Med., 25(4), pp. 479–485). In this study, a noninvasive in vivo magnetic resonance (MR) imaging-based measure of laxity, the knee loading apparatus (KLA) with anterior positioning frame, was evaluated with five normal subjects (repeatability study, n = 3). Effects of hormones and muscle guarding were considered. When compared to the KT-2000, the KLA was found to be more precise (±0.33 mm versus ±1.17 mm) but less reliable (Cronbach's alpha > 0.70 in 0/8 versus 5/8 load levels). Improved control of the initial subject position is recommended for future design iterations. The KLA shows promise as an accurate and reliable tool for measuring in vivo joint and ligament laxity.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):104502-104502-6. doi:10.1115/1.4034335.

Preventing dehydration during in vitro testing of isolated layers of annulus fibrosus tissue may require different test conditions than functional spine units. The purpose of the study was twofold: (A) to quantify changes in mass and thickness of multilayer annulus samples in four hydration environments over 120 min; and (B) to quantify cycle-varying biaxial tensile properties of annulus samples in the four environments. The environments included a saline bath, air, relative humidity control, and misting combined with controlled humidity. The loading protocol implemented 24 cycles of biaxial tensile loading to 20% strain at a rate of 2%/s with 3-, 8-, and 13-min of intermittent rest. Specimen mass increased an average (standard deviation) 72% (11) when immersed for 120 min (p < 0.0001). The air condition and the combined mist and relative humidity conditions reduced mass by 45% (15) and 25% (23), respectively, after 120 min (p < 0.0014). Stress at 16% stretch in the air condition was higher at cycle 18 (18 min of exposure) and cycle 24 (33 min of exposure) compared to all other environments in both the axial and circumferential directions (p < 0.0460). There was no significant change in mass or thickness over time in the relative humidity condition and the change in circumferential stress at 16% stretch between cycles 6 and 24 was a maximum of 0.099 MPa and not statistically significant. Implementation of a controlled relative humidity environment is recommended to maintain hydration of isolated annulus layers during cyclic tensile testing.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):104503-104503-9. doi:10.1115/1.4034383.

The vertebral strength and strain can be assessed in vitro by both using isolated vertebrae and sets of three adjacent vertebrae (the central one is loaded through the disks). Our goal was to elucidate if testing single-vertebra-specimens in the elastic regime provides different surface strains to three-vertebrae-segments. Twelve three-vertebrae sets were extracted from thoracolumbar human spines. To measure the principal strains, the central vertebra of each segment was prepared with eight strain-gauges. The sets were tested mechanically, allowing comparison of the surface strains between the two boundary conditions: first when the same vertebra was loaded through the disks (three-vertebrae-segment) and then with the endplates embedded in cement (single-vertebra). They were all subjected to four nondestructive tests (compression, traction, torsion clockwise, and counterclockwise). The magnitude of principal strains differed significantly between the two boundary conditions. For axial loading, the largest principal strains (along vertebral axis) were significantly higher when the same vertebra was tested isolated compared to the three-vertebrae-segment. Conversely, circumferential strains decreased significantly in the single vertebrae compared to the three-vertebrae-segment, with some variations exceeding 100% of the strain magnitude, including changes from tension to compression. For torsion, the differences between boundary conditions were smaller. This study shows that, in the elastic regime, when the vertebra is loaded through a cement pot, the surface strains differ from when it is loaded through the disks. Therefore, when single vertebrae are tested, surface strain should be taken with caution.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):104504-104504-6. doi:10.1115/1.4034426.

Located on the right side of the heart, the tricuspid valve (TV) prevents blood backflow from the right ventricle to the right atrium. Similar to other cardiac valves, quantification of TV biaxial mechanical properties is essential in developing accurate computational models. In the current study, for the first time, the biaxial stress–strain behavior of porcine TV was measured ex vivo under different loading protocols using biaxial tensile testing equipment. The results showed a highly nonlinear response including a compliant region followed by a rapid transition to a stiff region for all of the TV leaflets both in the circumferential and in the radial directions. Based on the data analysis, all three leaflets were found to be anisotropic, and they were stiffer in the circumferential direction in comparison to the radial direction. It was also concluded that the posterior leaflet was the most anisotropic leaflet.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):104505-104505-6. doi:10.1115/1.4034427.

Synthetic polyurethane foams are frequently used in biomechanical testing of spinal medical devices. However, it is unclear what types of foam are most representative of human vertebral trabecular bone behavior, particularly for testing the bone–implant interface. Therefore, a study was conducted to compare polyurethane foam microstructure and screw pullout properties to human vertebrae. Cadaveric thoracolumbar vertebrae underwent microcomputed tomography to assess trabecular bone microstructure. Spine plate screws were implanted into the vertebral body and pullout testing was performed. The same procedure was followed for eight different densities (grades 5–30) of commercially available closed cell (CCF) and open cell foams (OCF). The results indicated that foam microstructural parameters such as volume fraction, strut thickness, strut spacing, and material density rarely matched that of trabecular bone. However, certain foams provided mechanical properties that were comparable to the cadavers tested. Pullout force and work to pullout for screws implanted into CCF grade 5 were similar to osteoporotic female cadavers. In addition, screw pullout forces and work to pullout in CCF grade 8, grade 10, and OCF grade 30 were similar to osteopenic male cadavers. All other OCF and CCF foams possessed pullout properties that were either significantly lower or higher than the cadavers tested. This study elucidated the types and densities of polyurethane foams that can represent screw pullout strength in human vertebral bone. Synthetic bone surrogates used for biomechanical testing should be selected based on bone quantity and quality of patients who may undergo device implantation.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(10):104506-104506-4. doi:10.1115/1.4034489.

Platelet contractile forces play a major role in clot retraction and help to hold hemostatic clots against the vessel wall. Platelet forces are produced by its cytoskeleton, which is composed of actin and nonmuscle myosin filaments. In this work, we studied the role of Rho kinase, myosin light-chain kinase, and myosin in the generation of contractile forces by using pharmacological inhibitors and arrays of flexible microposts to measure platelet forces. When platelets were seeded onto microposts, they formed aggregates on the tips of the microposts. Forces produced by the platelets in the aggregates were measured by quantifying the deflection of the microposts, which bent in proportion to the force of the platelets. Platelets were treated with small molecule inhibitors of myosin activity: Y-27632 to inhibit the Rho kinase (ROCK), ML-7 to inhibit myosin light-chain kinase (MLCK), and blebbistatin to inhibit myosin ATPase activity. ROCK inhibition reduced platelet forces, demonstrating the importance of the assembly of actin and myosin phosphorylation in generating contractile forces. Similarly, MLCK inhibition caused weaker platelet forces, which verifies that myosin phosphorylation is needed for force generation in platelets. Platelets treated with blebbistatin also had weaker forces, which indicates that myosin's ATPase activity is necessary for platelet forces. Our studies demonstrate that myosin ATPase activity and the regulation of actin–myosin assembly by ROCK and MLCK are needed for the generation of platelet forces. Our findings illustrate and explain the importance of myosin for clot compaction in hemostasis and thrombosis.

Commentary by Dr. Valentin Fuster

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