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### Research Papers

J Biomech Eng. 2009;131(4):041001-041001-9. doi:10.1115/1.3049860.

A number of recent studies have demonstrated the effectiveness of atomic force microscopy (AFM) for characterization of cellular stress-relaxation behavior. However, this technique’s recent development creates considerable need for exploration of appropriate mechanical models for analysis of the resultant data and of the roles of various cytoskeletal components responsible for governing stress-relaxation behavior. The viscoelastic properties of vascular smooth muscle cells (VSMCs) are of particular interest due to their role in the development of vascular diseases, including atherosclerosis and restenosis. Various cytoskeletal agents, including cytochalasin D, jasplakinolide, paclitaxel, and nocodazole, were used to alter the cytoskeletal architecture of the VSMCs. Stress-relaxation experiments were performed on the VSMCs using AFM. The quasilinear viscoelastic (QLV) reduced-relaxation function, as well as a simple power-law model, and the standard linear solid (SLS) model, were fitted to the resultant stress-relaxation data. Actin depolymerization via cytochalasin D resulted in significant increases in both rate of relaxation and percentage of relaxation; actin stabilization via jasplakinolide did not affect stress-relaxation behavior. Microtubule depolymerization via nocodazole resulted in nonsignificant increases in rate and percentage of relaxation, while microtubule stabilization via paclitaxel caused significant decreases in both rate and percentage of relaxation. Both the QLV reduced-relaxation function and the power-law model provided excellent fits to the data $(R2=0.98)$, while the SLS model was less adequate $(R2=0.91)$. Data from the current study indicate the important role of not only actin, but also microtubules, in governing VSMC viscoelastic behavior. Excellent fits to the data show potential for future use of both the QLV reduced-relaxation function and power-law models in conjunction with AFM stress-relaxation experiments.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041002-041002-8. doi:10.1115/1.3072889.

Hip resurfacing is an alternative to total hip arthroplasty in which the femoral head surface is replaced with a metallic shell, thus preserving most of the proximal femoral bone stock. Accidental notching of the femoral neck during the procedure may predispose it to fracture. We examined the effect of neck notching on the strength of the proximal femur. Six composite femurs were prepared without a superior femoral neck notch, six were prepared in an inferiorly translated position to create a 2 mm notch, and six were prepared with a 5 mm notch. Six intact synthetic femurs were also tested. The samples were loaded to failure axially. A finite element model of a composite femur with increasing superior notch depths computed maximum equivalent stress and strain distributions. Experimental results showed that resurfaced synthetic femurs were significantly weaker than intact femurs (mean failure of 7034 N, $p<0.001$). The 2 mm notched group (mean failure of 4034 N) was significantly weaker than the un-notched group (mean failure of 5302 N, $p=0.018$). The 5 mm notched group (mean failure of 2808 N) was also significantly weaker than both the un-notched and the 2 mm notched groups ($p<0.001$, $p=0.023$, respectively). The finite element model showed the maximum equivalent strain in the superior reamed cancellous bone increasing with corresponding notch size. Fracture patterns inferred from equivalent stress distributions were consistent with those obtained from mechanical testing. A superior notch of 2 mm weakened the proximal femur by 24%, and a 5 mm notch weakened it by 47%. The finite element analysis substantiates this showing increasing stress and strain distributions within the prepared femoral neck with increasing notch depth.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041003-041003-12. doi:10.1115/1.3002765.

This work describes and presents results from a new three-dimensional whole-body model of human thermoregulation. The model has been implemented using a version of the “Brooks Man” anatomical data set, consisting of $1.3×108$ cubic volume elements (voxels) measuring 0.2 cm/side. The model simulates thermoregulation through passive mechanisms (metabolism, blood flow, respiration, and transpiration) and active mechanisms (vasodilatation, vasoconstriction, sweating, and shivering). Compared with lumped or compartment models, a voxel model is capable of high spatial resolution and can capture a level of anatomical detail not achievable otherwise. A high spatial resolution model can predict detailed heating patterns from localized or nonuniform heating patterns, such as from some radio frequency sources. Exposures to warm and hot environments (ambient temperatures of $33–48°C$) were simulated with the current voxel model and with a recent compartment model. Results from the two models (core temperature, skin temperature, metabolic rate, and evaporative cooling rate) were compared with published experimental results obtained under similar conditions. Under the most severe environmental conditions considered ($47.8°C$, 27% RH for 2 h), the voxel model predicted a rectal temperature increase of $0.56°C$, compared with a core temperature increase of $0.45°C$ from the compartment model and an experimental mean rectal temperature increase of $0.6°C$. Similar, good agreement was noted for other thermal variables and under other environmental conditions. Results suggest that the voxel model is capable of predicting temperature response (core temperature and skin temperature) to certain warm or hot environments, with accuracy comparable to that of a compartment model. In addition, the voxel model is able to predict internal tissue temperatures and surface temperatures, over time, with a level of specificity and spatial resolution not achievable with compartment models. The development of voxel models and related computational tools may be useful for thermal dosimetry applications involving mild temperature hyperthermia and for the assessment of safe exposure to certain nonionizing radiation sources.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041004-041004-10. doi:10.1115/1.3072892.

Classical marker-based roentgen stereophotogrammetric analysis (RSA) is an accurate method of measuring in vivo implant migration. A disadvantage of the method is the necessity of placing tantalum markers on the implant, which constitutes additional manufacturing and certification effort. Model-based RSA (MBRSA) is a method by which pose-estimation of geometric surface-models of the implant is used to detect implant migration. The placement of prosthesis markers is thus no longer necessary. The accuracy of the pose-estimation algorithms used depends on the geometry of the prosthesis as well as the accuracy of the surface models used. The goal of this study was thus to evaluate the experimental accuracy and precision of the MBRSA method for four different, but typical prosthesis geometries, that are commonly implanted. Is there a relationship existing between the accuracy of MBRSA and prosthesis geometries? Four different prosthesis geometries were investigated: one femoral and one tibial total knee arthroplasty (TKA) component and two different femoral stem total hip arthroplasty (THA) components. An experimental phantom model was used to simulate two different implant migration protocols, whereby the implant was moved relative to the surrounding bone (relative prosthesis-bone motion (RM)), or, similar to the double-repeated measures performed to assess accuracy clinically, both the prosthesis and the surrounding bone model (zero relative prosthesis-bone motion (ZRM)) were moved. Motions were performed about three translational and three rotational axes, respectively. The maximum 95% confidence interval (CI) for MBRSA of all four prosthesis investigated was better than −0.034 to 0.107 mm for in-plane and −0.217 to 0.069 mm for out-of-plane translation, and from −0.038 deg to 0.162 deg for in-plane and from −1.316 deg to 0.071 deg for out-of-plane rotation, with no clear differences between the ZRM and RM protocols observed. Accuracy in translation was similar between TKA and THA components, whereas rotational accuracy about the long axis of the hip stem THA components was worse than the TKA components. The data suggest that accuracy and precision of MBRSA seem to be equivalent to the classical marker-based RSA method, at least for the nonsymmetric implant geometries investigated in this study. The model-based method thus allows the accurate measurement of implant migration without requiring prosthesis markers, and thus presents new opportunities for measuring implant migration where financial or geometric considerations of marker placement have thus far been prohibitive factors.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041005-041005-5. doi:10.1115/1.3049821.

There has been growing interest in the mechanobiological function of the aortic valve interstitial cell (AVIC) due to its role in valve tissue homeostasis and remodeling. In a recent study we determined the relation between diastolic loading of the aortic valve (AV) leaflet and the resulting AVIC deformation, which was found to be substantial. However, due to the rapid loading time of the AV leaflets during closure $(∼0.05 s)$, time-dependent effects may play a role in AVIC deformation during physiological function. In the present study, we explored AVIC viscoelastic behavior using the micropipette aspiration technique. We then modeled the resulting time-length data over the 100 s test period using a standard linear solid model, which included Boltzmann superposition. To quantify the degree of creep and stress relaxation during physiological time scales, simulations of micropipette aspiration were preformed with a valve loading time of 0.05 s and a full valve closure time of 0.3 s. The 0.05 s loading simulations suggest that, during valve closure, AVICs act elastically. During diastole, simulations revealed creep (4.65%) and stress relaxation (4.39%) over the 0.3 s physiological time scale. Simulations also indicated that if Boltzmann superposition was not used in parameter estimation, as in much of the micropipette literature, creep and stress relaxation predicted values were nearly doubled (7.92% and 7.35%, respectively). We conclude that while AVIC viscoelastic effects are negligible during valve closure, they likely contribute to the deformation time-history of AVIC deformation during diastole.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041006-041006-11. doi:10.1115/1.3049857.

A detailed three-dimensional finite element model of the face is presented in this paper. Bones, muscles, skin, fat, and superficial muscoloaponeurotic system were reconstructed from magnetic resonance images and modeled according to anatomical, plastic, and reconstructive surgery literature. The finite element mesh, composed of hexahedron elements, was generated through a semi-automatic procedure with an effective compromise between the detailed representation of anatomical parts and the limitation of the computational time. Nonlinear constitutive equations are implemented in the finite element model. The corresponding model parameters were selected according to previous work with mechanical measurements on soft facial tissue, or based on reasonable assumptions. Model assumptions concerning tissue geometry, interactions, mechanical properties, and the boundary conditions were validated through comparison with experiments. The calculated response of facial tissues to gravity loads, to the application of a pressure inside the oral cavity and to the application of an imposed displacement was shown to be in good agreement with the data from corresponding magnetic resonance images and holographic measurements. As a first application, gravimetric soft tissue descent was calculated from the long time action of gravity on the face in the erect position, with tissue aging leading to a loss of stiffness. Aging predictions are compared with the observations from an “aging database” with frontal photos of volunteers at different age ranges (i.e., 20–40 years and 50–70 years).

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041007-041007-7. doi:10.1115/1.3049843.

Brace application has been reported to be an effective approach in treating mild to moderate idiopathic adolescent scoliosis. However, little attention is focused on the biomechanical study of patient-specific brace treatment. The purpose of this study was to propose a design method of personalized brace and to analyze its biomechanical behavior and to compare the brace forces with the I-Scan measurement system. Based on a three-dimensional patient-specific finite element model of the spine, rib cage, pelvis, and abdomen, a parametric patient-specific model of a thoracolumbosacral orthosis was built. The interaction between the torso and the brace was modeled by surface-to-surface contact interface. Three standard strap tensions (20 N, 40 N, and 60 N) were loaded on the back of the brace to simulate the strap tension. The I-Scan distribution pressure measurement system was used to measure the different region pressures, and the equivalent forces in these regions were calculated. The spinal curve changes and the forces acted on the brace generated by the strap tension were evaluated and compared with the measurement. The reduction in the coronal curvature was about 60% for a strap tension of 60 N. The sacral slope and the lordosis were partially reduced in this case, but the kyphosis had no obvious change. The brace slightly modified the axial rotation at the apex of the scoliotic curve. The forces generated in finite element analysis were approximately in good agreement with the measurement. The design and biomechanical analysis methods of patient-specific brace should be useful in the design of more effective braces.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041008-041008-11. doi:10.1115/1.3049856.

Recently a cartilage growth finite element model (CGFEM) was developed to solve nonhomogeneous and time-dependent growth boundary-value problems (Davol, 2008, “A Nonlinear Finite Element Model of Cartilage Growth,” Biomech. Model. Mechanobiol., 7, pp. 295–307). The CGFEM allows distinct stress constitutive equations and growth laws for the major components of the solid matrix, collagens and proteoglycans. The objective of the current work was to simulate in vitro growth of articular cartilage explants in a steady-state permeation bioreactor in order to obtain results that aid experimental design. The steady-state permeation protocol induces different types of mechanical stimuli. When the specimen is initially homogeneous, it directly induces homogeneous permeation velocities and indirectly induces nonhomogeneous solid matrix shear stresses; consequently, the steady-state permeation protocol is a good candidate for exploring two competing hypotheses for the growth laws. The analysis protocols were implemented through the alternating interaction of the two CGFEM components: poroelastic finite element analysis (FEA) using ABAQUS and a finite element growth routine using MATLAB . The CGFEM simulated 12 days of growth for immature bovine articular cartilage explants subjected to two competing hypotheses for the growth laws: one that is triggered by permeation velocity and the other by maximum shear stress. The results provide predictions for geometric, biomechanical, and biochemical parameters of grown tissue specimens that may be experimentally measured and, consequently, suggest key biomechanical measures to analyze as pilot experiments are performed. The combined approach of CGFEM analysis and pilot experiments may lead to the refinement of actual experimental protocols and a better understanding of in vitro growth of articular cartilage.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041009-041009-7. doi:10.1115/1.3005149.

The mechanical behavior of human scapholunate ligaments is not well understood. Presently, intact scapholunate specimens were mechanically tested in linear distraction and torsion. Fresh bovine tendon grafts were used to reconstruct the scapholunate interval and the tests repeated. Tests yielded the following average values for intact specimens: linear stiffness $(48.9N∕mm)$, linear load retained at $100s$ (44%), torsional stiffness $(19.5Nmm∕deg)$, torque remaining at $100seconds$ (66%), torque-to-failure (1253.9 N mm), and angle-to-failure $(50.4deg)$. Tests showed the following average values for reconstructed specimens: linear stiffness $(5.4N∕mm)$, linear load retained at $100s$ (49%), torsional stiffness $(12.6Nmm∕deg)$, torque remaining at $100s$ (71%), torque-to-failure $(936.8Nmm)$, and angle-to-failure $(54.5deg)$. There were no statistically significant differences between the intact and reconstructed specimens, with the exception of linear stiffness. Biomechanically, this is the first study in the literature to quantify torsional stress relaxation, failure torque, and failure angle for the intact and repaired human scapholunate ligament. Surgically, reconstruction with bovine tendon may warrant further investigation as a method to potentially retain function and strength after scapholunate injury.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041010-041010-8. doi:10.1115/1.3005152.

Computational speed is a major limiting factor for performing design sensitivity and optimization studies of total knee replacements. Much of this limitation arises from extensive geometry calculations required by contact analyses. This study presents a novel surrogate contact modeling approach to address this limitation. The approach involves fitting contact forces from a computationally expensive contact model (e.g., a finite element model) as a function of the relative pose between the contacting bodies. Because contact forces are much more sensitive to displacements in some directions than others, standard surrogate sampling and modeling techniques do not work well, necessitating the development of special techniques for contact problems. We present a computational evaluation and practical application of the approach using dynamic wear simulation of a total knee replacement constrained to planar motion in a Stanmore machine. The sample points needed for surrogate model fitting were generated by an elastic foundation (EF) contact model. For the computational evaluation, we performed nine different dynamic wear simulations with both the surrogate contact model and the EF contact model. In all cases, the surrogate contact model accurately reproduced the contact force, motion, and wear volume results from the EF model, with computation time being reduced from $13minto13s$. For the practical application, we performed a series of Monte Carlo analyses to determine the sensitivity of predicted wear volume to Stanmore machine setup issues. Wear volume was highly sensitive to small variations in motion and load inputs, especially femoral flexion angle, but not to small variations in component placements. Computational speed was reduced from an estimated $230hto4h$ per analysis. Surrogate contact modeling can significantly improve the computational speed of dynamic contact and wear simulations of total knee replacements and is appropriate for use in design sensitivity and optimization studies.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041011-041011-11. doi:10.1115/1.3049477.

In the present study a direct comparison was made between in vitro total hip wear testing and a computational analysis considering the effects of time and a nonlinear stress-strain relationship for ultrahigh molecular weight polyethylene (UHMWPE) at $37°C$. The computational simulation was made correct through calibration to experimental volumetric wear results, and the predicted damage layout on the acetabular liner surface was compared with results estimated from laser scanning of the actual worn specimens. The wear rates for the testing specimens were found to be $17.14±1.23 mg/106 cycles$ and $19.39±0.79 mg/106 cycles$, and the cumulative volumetric wear values after $3×106 cycles$ were $63.70 mm3$ and $64.02 mm3$ for specimens 1 and 2, respectively. The value of the calibrated wear coefficient was found to be $5.32(10−10) mm3/N mm$ for both specimens. The major difference between the computational and experimental wear results was the existence of two damage vectors in the experimental case. The actual location of damage was virtually the same in both cases, and the maximum damage depth of the computational model agreed well with the experiment. The existence of multiple wear vectors may indicate the need for computational approaches to account for multidirectional sliding or strain hardening of UHMWPE. Despite the limitation in terms of describing the overall damage layout, the present computational model shows that simulation can mimic some of the behavior of in vitro wear.

Topics: Wear , Cycles , Simulation
Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041012-041012-8. doi:10.1115/1.3072895.

Numerical algorithms for subspace system identification (N4SID) are a powerful tool for generating the state space (SS) representation of any system. The purpose of this work was to use N4SID to generate SS models of the flowrate and pressure generation within an ex vivo vascular perfusion system (EVPS). Accurate SS models were generated and converted to transfer functions (TFs) to be used for proportional integral and derivative (PID) controller design. By prescribing the pressure and flowrate inputs to the pumping components within the EVPS and measuring the resulting pressure and flowrate in the system,_four TFs were estimated;_two for a flowrate controller ($HRP,f$ and $HRPP,f$) and two for a pressure controller ($HRP,p$ and $HRPP,p$). In each controller,_one TF represents a roller pump ($HRP,f$ and $HRP,p$),_and the other represents a roller pump and piston in series ($HRPP,f$ and $HRPP,p$). Experiments to generate the four TFs were repeated five times $(N=5)$ from which average TFs were calculated. The average model fits, computed as the percentage of the output variation (to_the_prescribed_inputs) reproduced by the model, were $94.93±1.05%$ for $HRP,p$, $81.29±0.20%$ for $HRPP,p$, $94.45±0.73%$ for $HRP,f$, and $77.12±0.36%$ for $HRPP,f$. The simulated step, impulse, and frequency responses indicate that the EVPS is a stable system and can respond to signals containing power of up to 70_Hz.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):041013-041013-11. doi:10.1115/1.3072888.

The implantation of a cemented or cementless femoral stem changes the physiological load transfer on the femur producing an effect on the bone called adaptative remodeling. The patterns of this remodeling are attributed to mechanical and biological factors, and those changes in bone mineral density have been determined in long-term densitometry studies. This technique has proved to be a useful tool able to quantify small changes in bone density in different femoral areas, and it is considered to be ideal for long-term studies. On the other hand, the finite element (FE) simulation allows the study of the biomechanical changes produced in the femur after the implantation of a femoral stem. The aim of this study was to contrast the findings obtained from a 5 year follow-up densitometry study that used a newly designed femoral stem (73 patients were included in this study), with the results obtained using a finite element simulation that reproduces the pattern of load transfer that this stem causes on the femur. In this study we have obtained a good comparison between the results of stress of FE simulation and the bone mass values of the densitometry study establishing a ratio between the increases in stress (%) versus the increases in bone density (%). Hence, the changes in bone density in the long term, compared with the healthy femur, are due to different load transfers after stem implantation. It has been checked that in the Gruen zone 7 at 5 years, the most important reduction in stress (7.85%) is produced, which coincides with the highest loss of bone mass (23.89%). Furthermore, the simulation model can be used with different stems with several load conditions and at different time periods to carry out the study of biomechanical behavior in the interaction between the stem and the femur, explaining the evolution of bone density in accordance to Wolff’s law, which validates the simulation model.

Topics: Stress , Bone , Density , Prostheses
Commentary by Dr. Valentin Fuster

### Technical Briefs

J Biomech Eng. 2009;131(4):044501-044501-5. doi:10.1115/1.3049859.

The complex structure and properties of biological tissues as well as their in situ environment often make it difficult to self-heal. A suitable replacement tissue may be created in vitro through tissue engineering approaches and mechanical stimulation of tissue constructs. A new biaxial bioreactor was designed, constructed, and evaluated for the purposes of developing constructs with specific functional characteristics. Once constructed and assembled, the bioreactor was tested for position accuracy and application of strain. Additionally, a tissue construct was tested in the chamber and compared with a nonstimulated construct. Results showed high position accuracy, but some loss between applied strain via grip movement and strain experienced by the scaffold. The tested construct exhibited an increase in cells and matrix deposition in comparison to the nonstimulated construct. This biaxial bioreactor will be useful for mechanically stimulating tissue constructs in two perpendicular directions to create implants for tissues requiring preferred compressive and tensile resistances.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):044502-044502-4. doi:10.1115/1.3072891.

Debate regarding the mechanisms of how the eye changes focus (accommodation) and why this ability is lost with age (presbyopia) has recently been rejoined due to the advent of surgical procedures for the correction of presbyopia. Due to inherent confounding factors in both in vivo and in vitro measurement techniques, mechanical modeling of the behavior of the ocular lens in accommodation has been attempted to settle the debate. However, a paucity of reliable mechanical property measurements has proven problematic in the development of a successful mechanical model of accommodation. Instrumented microindentation was utilized to directly measure the local elastic modulus and dynamic response at various locations in the lens. The young porcine lens exhibits a large modulus gradient with the highest modulus appearing at the center of the nucleus and exponentially decreasing with distance. The loss tangent was significantly higher in the decapsulated lens and the force waveform amplitude decreased significantly upon removal of the lens capsule. The findings indicate that localized measurements of the lens’ mechanical properties are necessary to achieve accurate quantitative parameters suitable for mechanical modeling efforts. The results also indicate that the lens behaves as a crosslinked gel rather than as a collection of individual arched fiber cells.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):044503-044503-6. doi:10.1115/1.3072894.

Syringomyelia is a disease in which fluid-filled cavities, called syrinxes, form in the spinal cord causing progressive loss of sensory and motor functions. Invasive monitoring of pressure waves in the spinal subarachnoid space implicates a hydrodynamic origin. Poor treatment outcomes have led to myriad hypotheses for its pathogenesis, which unfortunately are often based on small numbers of patients due to the relative rarity of the disease. However, only recently have models begun to appear based on the principles of mechanics. One such model is the mathematically rigorous work of Carpenter and colleagues (2003, “Pressure Wave Propagation in Fluid-Filled Co-Axial Elastic Tubes Part 1: Basic Theory,” ASME J. Biomech. Eng., 125(6), pp. 852–856; 2003, “Pressure Wave Propagation in Fluid-Filled Co-Axial Elastic Tubes Part 2: Mechanisms for the Pathogenesis of Syringomyelia,” ASME J. Biomech. Eng., 125(6), pp. 857–863). They suggested that a pressure wave due to a cough or sneeze could form a shocklike elastic jump, which when incident at a stenosis, such as a hindbrain tonsil, would generate a transient region of high pressure within the spinal cord and lead to fluid accumulation. The salient physiological parameters of this model were reviewed from the literature and the assumptions and predictions re-evaluated from a mechanical standpoint. It was found that, while the spinal geometry does allow for elastic jumps to occur, their effects are likely to be weak and subsumed by the small amount of viscous damping present in the subarachnoid space. Furthermore, the polarity of the pressure differential set up by cough-type impulses opposes the tenets of the elastic-jump hypothesis. The analysis presented here does not support the elastic-jump hypothesis or any theory reliant on cough-based pressure impulses as a mechanism for the pathogenesis of syringomyelia.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):044504-044504-5. doi:10.1115/1.3049531.

The triphasic theory on soft charged hydrated tissues (Lai, W. M., Hou, J. S., and Mow, V. C., 1991, “A Triphasic Theory for the Swelling and Deformation Behaviors of Articular Cartilage  ,” ASME J. Biomech. Eng., 113, pp. 245–258) attributes the swelling propensity of articular cartilage to three different mechanisms: Donnan osmosis, excluded volume effect, and chemical expansion stress. The aim of this study is to evaluate the thermodynamic plausibility of the triphasic theory. The free energy of a sample of articular cartilage subjected to a closed cycle of mechanical and chemical loading is calculated using the triphasic theory. It is shown that the chemical expansion stress term induces an unphysiological generation of free energy during each closed cycle of loading and unloading. As the cycle of loading and unloading can be repeated an indefinite number of times, any amount of free energy can be drawn from a sample of articular cartilage, if the triphasic theory were true. The formulation for the chemical expansion stress as used in the triphasic theory conflicts with the second law of thermodynamics.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):044505-044505-4. doi:10.1115/1.2948398.

Endovascular stent grafts for the treatment of thoracic aortic aneurysms have become increasingly utilized and yet their locational stability in moderate chest trauma is unknown. A high speed impact system was developed to study the stability of aortic endovascular stent grafts in vitro. A straight segment of porcine descending aorta with stent graft was constrained in a custom-made transparent urethane casing. The specimen was tested in a novel impact system at an anterior inclination of $45deg$ and an average deceleration of $55G$, which represented a frontal automobile crash. Due to the shock of the impact, which was shown to be below the threshold of aortic injury, the stent graft moved $0.6mm$ longitudinally. This result was repeatable. The presented experimental model may be helpful in developing future grafts to withstand moderate shocks experienced in motor vehicle accidents or other dynamic loadings of the chest.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2009;131(4):044506-044506-5. doi:10.1115/1.3049863.

Although the importance of knowing the magnitude of residual stress (RS) and its functional significance are widely recognized, there is still disagreement and confusion regarding the nature of physical mechanisms giving rise to RS in tissues and organs. Here an attempt is made to examine the various mechanisms which may be involved in producing RS, and to estimate their roles and significance based on previously published experimental observations. Two concepts are introduced. The first establishes a hierarchy of different possible RS producing mechanisms from the micro (local) level of the tissue space, through the meso-level of the whole tissue, to the macro (organ) one. Whereas micro-level RS seem to be present in all soft tissues, the existence of macro- and meso-level mechanisms are tissue and organ specific. The second concept introduced highlights the significance of tissue swelling as an RS producing mechanism in the local micro-level. The implications of RS mechanism hierarchy are discussed regarding the interpretations of commonly used experimental methods aimed to study RS or to estimate its magnitude. Of the three categories of RS mechanisms, the local micro-RS is the least understood. It is analyzed here in terms of the tissue’s multiconstituent structure, in the framework of mixture theory. It is shown that the micro-RS can stem either from interactions between the solid tissue constituents or between its solids and its fluidlike matrix. The latter mode is associated with osmotic-driven tissue swelling. The feasibility of these two mechanisms is analyzed based on published observations and measured data. The analysis suggests that under conditions not too remote from the in vivo homeostatic one, osmotic-driven tissue swelling is a predominant RS producing mechanism. The analysis also suggests that a true stress-free configuration can be obtained only if all RS producing mechanisms are relieved, and outlines a manner by which this may be achieved.

Commentary by Dr. Valentin Fuster

J Biomech Eng. 2009;131(4):045001-045001-6. doi:10.1115/1.3072890.

Knee osteoarthritis is a chronic disease that necessitates long term therapeutic intervention. Biomechanical studies have demonstrated an improvement in the external adduction moment with application of a valgus knee brace. Despite being both efficacious and safe, due to their rigid frame and bulkiness, current designs of knee braces create discomfort and difficulties to patients during prolonged periods of application. Here we propose a novel design of a light osteoarthritis knee brace, which is made of soft conforming materials. Our design relies on a pneumatic leverage system, which, when pressurized, reduces the excessive loads predominantly affecting the medial compartment of the knee and eventually reverses the malalignment. Using a finite-element analysis, we show that with a moderate level of applied pressure, this pneumatic brace can, in theory, counterbalance a greater fraction of external adduction moment than the currently existing braces.

Commentary by Dr. Valentin Fuster

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