Research Papers

J Biomech Eng. 2012;134(3):031001-031001-8. doi:10.1115/1.4005851.

In vitro cultures with insulin-like growth factor-1 (IGF-1) and transforming growth factor-β1 (TGF-β1) have previously been shown to differentially modulate the growth of immature bovine articular cartilage. IGF-1 stimulates expansive growth yet decreases compressive moduli and increases compressive Poisson’s ratios, whereas TGF-β1 maintains tissue size, increases compressive moduli, and decreases compressive Poisson’s ratios. The current study’s hypothesis was that sequential application of IGF-1 and TGF-β1 during in vitro culture produces geometric and compressive mechanical properties that lie between extreme values produced when using either growth factor alone. Immature bovine articular cartilage specimens were harvested and either untreated (D0, i.e., day zero) or cultured in vitro for either 6 days with IGF-1 (D6 IGF), 12 days with IGF-1 (D12 IGF), or 6 days with IGF-1 followed by 6 days with TGF-β1 (D12 SEQ, i.e., sequential). Following treatment, all specimens were tested for geometric, biochemical, and compressive mechanical properties. Relative to D0, D12 SEQ treatment enhanced volumetric growth, but to a lower value than that for D12 IGF. Furthermore, D12 SEQ treatment maintained compressive moduli and Poisson’s ratios at values higher and lower, respectively, than those for D12 IGF. Considering the previously described effects of 12 days of treatment with TGF-β1 alone, D12 SEQ induced both growth and mechanical property changes between those produced with either IGF-1 or TGF-β1 alone. The results suggest that it may be possible to vary the durations of select growth factors, including IGF-1 and TGF-β1, to more precisely modulate the geometric, biochemical, and mechanical properties of immature cartilage graft tissue in clinical repair strategies.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2012;134(3):031002-031002-9. doi:10.1115/1.4005692.

Quantification of plantar tissue behavior of the heel pad is essential in developing computational models for predictive analysis of preventive treatment options such as footwear for patients with diabetes. Simulation based studies in the past have generally adopted heel pad properties from the literature, in return using heel-specific geometry with material properties of a different heel. In exceptional cases, patient-specific material characterization was performed with simplified two-dimensional models, without further evaluation of a heel-specific response under different loading conditions. The aim of this study was to conduct an inverse finite element analysis of the heel in order to calculate heel-specific material properties in situ. Multidimensional experimental data available from a previous cadaver study by Erdemir (“An Elaborate Data Set Characterizing the Mechanical Response of the Foot,” ASME J. Biomech. Eng., 131 (9), pp. 094502) was used for model development, optimization, and evaluation of material properties. A specimen-specific three-dimensional finite element representation was developed. Heel pad material properties were determined using inverse finite element analysis by fitting the model behavior to the experimental data. Compression dominant loading, applied using a spherical indenter, was used for optimization of the material properties. The optimized material properties were evaluated through simulations representative of a combined loading scenario (compression and anterior-posterior shear) with a spherical indenter and also of a compression dominant loading applied using an elevated platform. Optimized heel pad material coefficients were 0.001084 MPa (μ), 9.780 (α) (with an effective Poisson’s ratio (ν) of 0.475), for a first-order nearly incompressible Ogden material model. The model predicted structural response of the heel pad was in good agreement for both the optimization (<1.05% maximum tool force, 0.9% maximum tool displacement) and validation cases (6.5% maximum tool force, 15% maximum tool displacement). The inverse analysis successfully predicted the material properties for the given specimen-specific heel pad using the experimental data for the specimen. The modeling framework and results can be used for accurate predictions of the three-dimensional interaction of the heel pad with its surroundings.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2012;134(3):031003-031003-9. doi:10.1115/1.4006089.

An experimental program has been carried out in order to investigate the mechanical behavior of porcine corneas. We report the results of inflation tests on the whole cornea and uniaxial tests on excised corneal strips, performed on 51 fresh porcine eyes. Uniaxial tests have been performed on specimens cut from previously inflated corneas. The cornea behavior is characterized by means of elastic stiffness, measured on both average pressure-apex displacement and average uniaxial stress-strain curves; and by means of transversal contraction coefficient, peak stress, and failure stress measured on uniaxial stress-strain curves. Uniaxial tests performed on excised strips allowed to measure the anisotropy in the corneal stiffness and to compare the stiffness of the cornea with the one of the sclera. Viscous properties of the cornea have been obtained through uniaxial relaxation curves on excised corneal strips. The relevant geometrical parameters have been measured and, with the aid of the elastic thin shell theory, a stress-strain curve has been derived from the average inflation test data and compared with similar data available in the literature. The experimental system has been developed in view of future applications to the mechanical testing of both porcine and human corneas.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2012;134(3):031004-031004-10. doi:10.1115/1.4006115.

Revision surgeries of total hip arthroplasty are often caused by a deficient structural compatibility of the implant. Two main culprits, among others, are bone-implant interface instability and bone resorption. To address these issues, in this paper we propose a novel type of implant, which, in contrast to current hip replacement implants made of either a fully solid or a foam material, consists of a lattice microstructure with nonhomogeneous distribution of material properties. A methodology based on multiscale mechanics and design optimization is introduced to synthesize a graded cellular implant that can minimize concurrently bone resorption and implant interface failure. The procedure is applied to the design of a 2D left implanted femur with optimized gradients of relative density. To assess the manufacturability of the graded cellular microstructure, a proof-of-concept is fabricated by using rapid prototyping. The results from the analysis are used to compare the optimized cellular implant with a fully dense titanium implant and a homogeneous foam implant with a relative density of 50%. The bone resorption and the maximum value of interface stress of the cellular implant are found to be over 70% and 50% less than the titanium implant while being 53% and 65% less than the foam implant.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2012;134(3):031005-031005-8. doi:10.1115/1.4006200.

The interactions of small leucine-rich proteoglycans (SLRPs) with collagen fibrils, their association with water, and their role in fibrillogenesis suggests that SLRPs may play an important role in tendon mechanics. Some studies have assessed the role of SLRPs in the mechanical response of the tendon, but the relationships between sophisticated mechanics, assembly of collagen, and SLRPs have not been well characterized. Decorin content was varied in a dose dependent manner using decorin null, decorin heterozygote, and wild type mice. Quantitative measures of mechanical (tension and compression), compositional, and structural changes of the mouse patellar tendon were evaluated. Viscoelastic, tensile dynamic modulus was increased in the decorin heterozygous tendons compared to wild type. These tendons also had a significant decrease in total collagen and no structural changes compared to wild type. Decorin null tendons did not have any mechanical changes; however, a significant decrease in the average fibril diameter was found. No differences were seen between genotypes in elastic or compressive properties, and all tendons demonstrated viscoelastic mechanical dependence on strain rate and frequency. These results suggest that decorin, a member of the SLRP family, plays a role in tendon viscoelasticity that cannot be completely explained by its role in collagen fibrillogenesis. In addition, reductions in decorin do not cause large changes in indentation compressive properties, suggesting that other factors contribute to these properties. Understanding these relationships may ultimately help guide development of tissue engineered constructs or treatment modalities.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2012;134(3):031006-031006-9. doi:10.1115/1.4005849.

Syrinxes are fluid-filled cavities of the spinal cord that characterize syringomyelia, a disease involving neurological damage. Their formation and expansion is poorly understood, which has hindered successful treatment. Syrinx cavities are hydraulically connected with the spinal subarachnoid space (SSS) enveloping the spinal cord via the cord interstitium and the network of perivascular spaces (PVSs), which surround blood vessels penetrating the pial membrane that is adherent to the cord surface. Since the spinal canal supports pressure wave propagation, it has been hypothesized that wave-induced fluid exchange across the pial membrane may play a role in syrinx filling. To investigate this conjecture a pair of one-dimensional (1-d) analytical models were developed from classical elastic tube theory coupled with Darcy’s law for either perivascular or interstitial flow. The results show that transpial flux serves as a mechanism for damping pressure waves by alleviating hoop stress in the pial membrane. The timescale ratio over which viscous and inertial forces compete was explicitly determined, which predicts that dilated PVS, SSS flow obstructions, and a stiffer and thicker pial membrane—all associated with syringomyelia—will increase transpial flux and retard wave travel. It was also revealed that the propagation of a pressure wave is aided by a less-permeable pial membrane and, in contrast, by a more-permeable spinal cord. This is the first modeling of the spinal canal to include both pressure-wave propagation along the spinal axis and a pathway for fluid to enter and leave the cord, which provides an analytical foundation from which to approach the full poroelastic problem.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2012;134(3):031007-031007-6. doi:10.1115/1.4006340.

Repeatedly and consistently measuring the mechanical properties of tendon is important but presents a challenge. Preconditioning can provide tendons with a consistent loading history to make comparisons between groups from mechanical testing experiments. However, the specific mechanisms occurring during preconditioning are unknown. Previous studies have suggested that microstructural changes, such as collagen fiber re-alignment, may be a result of preconditioning. Local collagen fiber re-alignment is quantified throughout tensile mechanical testing using a testing system integrated with a polarized light setup, consisting of a backlight, 90 deg-offset rotating polarizer sheets on each side of the test sample, and a digital camera, in a rat supraspinatus tendon model, and corresponding mechanical properties are measured. Local circular variance values are compared throughout the mechanical test to determine if and where collagen fiber re-alignment occurred. The inhomogeneity of the tendon is examined by comparing local circular variance values, optical moduli and optical transition strain values. Although the largest amount of collagen fiber re-alignment was found during preconditioning, significant re-alignment was also demonstrated in the toe and linear regions of the mechanical test. No significant changes in re-alignment were seen during stress relaxation. The insertion site of the supraspinatus tendon demonstrated a lower linear modulus and a more disorganized collagen fiber distribution throughout all mechanical testing points compared to the tendon midsubstance. This study identified a correlation between collagen fiber re-alignment and preconditioning and suggests that collagen fiber re-alignment may be a potential mechanism of preconditioning and merits further investigation. In particular, the conditions necessary for collagen fibers to re-orient away from the direction of loading and the dependency of collagen reorganization on its initial distribution must be examined.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2012;134(3):031008-031008-9. doi:10.1115/1.4006197.

Based on the fact that malignant cancerous lesions (neoplasms) develop high metabolism and use more blood supply than normal tissue, infrared thermography (IR) has become a reliable clinical technique used to indicate noninvasively the presence of cancerous diseases, e.g., skin and breast cancer. However, to diagnose cancerous diseases by IR, the technique requires procedures that explore the relationship between the neoplasm characteristics (size, blood perfusion rate and heat generated) and the resulting temperature distribution on the skin surface. In this research work the dual reciprocity boundary element method (DRBEM) has been coupled with the simulated annealing technique (SA) in a new inverse procedure, which coupled to the IR technique, is capable of estimating simultaneously geometrical and thermophysical parameters of the neoplasm. The method is of an evolutionary type, requiring random initial values for the unknown parameters and no calculations of sensitivities or search directions. In addition, the DRBEM does not require any re-meshing at each proposed solution to solve the bioheat model. The inverse procedure has been tested considering input data for simulated neoplasms of different sizes and positions in relation to the skin surface. The successful estimation of unknown neoplasm parameters validates the idea of using the SA technique and the DRBEM in the estimation of parameters. Other estimation techniques, based on genetic algorithms or sensitivity coefficients, have not been capable of obtaining a solution because the skin surface temperature difference is very small.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2012;134(3):031009-031009-8. doi:10.1115/1.4006198.

Fluoroscopic imaging is commonly used for assessing relative motions of orthopaedic implants. One limiting factor to in vivo model-based roentgen stereophotogrammetric analysis of total knee arthroplasty is the need for 3D models of the implants.The 3D models of the implant components must be reverse-engineered, if not provided by the company, which makes this method impractical for a clinical study involving many types or sizes of implants. This study introduces a novel feature-based methodology that registers the features at the implant-bone or implant-cement interface of the components that have elementary shapes. These features include pegs with hemispherical heads, and straight, circular or curved edges located on flat faces of the box of the femoral component or the stem geometry of the tibial component. Software was developed to allow easy registration of these features through a graphical user interface. The accuracy and precision of registration for multiple flexion angles from 0 to 120 deg was determined with reference to registered poses of the implants through experiments on bone replica models and also on a cadaver specimen implanted with total knee prostheses. When compared to an equivalent bi-planar model-based registration, the results were comparable: The mean accuracy of this feature-based method was 1.45 deg and 1.03 mm (in comparison to 0.95 deg and 1.32 mm for the model-based approach), and the mean precision was 0.57 deg and 0.26 mm (in comparison to 0.42 deg and 0.44 mm for the model-based approach).The methodology and the developed software can easily accommodate different design of implants with various fixation features. This method can facilitate in vivo kinematic analysis of total knee arthroplasty by eliminating the need for 3D models of the implant components.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2012;134(3):031010-031010-13. doi:10.1115/1.4005980.

Acoustic vaporization dynamics of a superheated dodecafluoropentane (DDFP) microdroplet inside a microtube and the resulting bubble evolution is investigated in the present work. This work is motivated by a developmental gas embolotherapy technique that is intended to treat cancers by infarcting tumors using gas bubbles. A combined theoretical and computational approach is utilized and compared with the experiments to understand the evolution process and to estimate the resulting stress distribution associated with vaporization event. The transient bubble growth is first studied by ultra-high speed imaging and then theoretical and computational modeling is used to predict the entire bubble evolution process. The evolution process consists of three regimes: an initial linear rapid spherical growth followed by a linear compressed oval shaped growth and finally a slow asymptotic nonlinear spherical bubble growth. Although the droplets are small compared to the tube diameter, the bubble evolution is influenced by the tube wall. The final bubble radius is found to scale linearly with the initial droplet radius and is approximately five times the initial droplet radius. A short pressure pulse with amplitude almost twice as that of ambient conditions is observed. The width of this pressure pulse increases with increasing droplet size whereas the amplitude is weakly dependent. Although the rise in shear stress along the tube wall is found to be under peak physiological limits, the shear stress amplitude is found to be more prominently influenced by the initial droplet size. The role of viscous dissipation along the tube wall and ambient bulk fluid pressure is found to be significant in bubble evolution dynamics.

Commentary by Dr. Valentin Fuster

Technical Briefs

J Biomech Eng. 2012;134(3):034501-034501-4. doi:10.1115/1.4006025.

Dynamic mechanical properties of placenta tissue are needed to develop computational models of pregnant occupants for use in designing restraint systems that protect the fetus and mother. Tests were performed on 21 samples obtained from five human placentas at a rate of 1200 %/s using a set of custom designed thermoelectrically cooled clamps. Approximately half of the samples from all five subjects were tested within 48 h of delivery. The remaining samples were frozen for 5–7 days and then thawed before testing. True failure stresses and strains were not significantly different between fresh and frozen samples (p-value = 0.858 and 0.551, respectively), suggesting that soft tissue may be stored frozen up to a week without adversely affecting dynamic material response.

Commentary by Dr. Valentin Fuster

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