0

IN THIS ISSUE

### Research Papers

J Biomech Eng. 2016;138(4):041001-041001-8. doi:10.1115/1.4032551.

Prosthetic alignment, patient characteristics, and implant design are all factors in long-term survival of total knee arthroplasty (TKA), yet the level at which each of these factors contribute to implant loosening has not been fully described. Prior clinical and biomechanical studies have indicated tibial overload as a cause of early TKA revision. The purpose of this study was to determine the relationship between tibial component design and bone resection on tibial loading. Finite-element analysis (FEA) was performed after simulated implantation of metal backed (MB) and all-polyethylene (AP) TKA components in 5 and 15 mm of tibial resection into a validated intact tibia model. Proximal tibial strains significantly increased between 13% and 199% when implanted with AP components (p < 0.05). Strain significantly increased between 12% and 209% in the posterior tibial compartment with increased bone resection (p < 0.05). This study indicates elevated strains in AP implanted tibias across the entirety of the proximal tibial cortex, as well as a posterior shift in tibial loading in instances of increased resection depth. These results are consistent with trends observed in prior biomechanical studies and may associate the documented device history of tibial collapse in AP components with increased bone strain and overload beneath the prosthesis.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(4):041002-041002-9. doi:10.1115/1.4032627.

A study was made of the deformation of tendons when compressed transverse to the fiber-aligned axis. Bovine digital extensor tendons were compression tested between flat rigid plates. The methods included: in situ image-based measurement of tendon cross-sectional shapes, after preconditioning but immediately prior to testing; multiple constant-load creep/recovery tests applied to each tendon at increasing loads; and measurements of the resulting tendon displacements in both transverse directions. In these tests, friction resisted axial stretch of the tendon during compression, giving approximately plane-strain conditions. This, together with the assumption of a form of anisotropic hyperelastic constitutive model proposed previously for tendon, justified modeling the isochronal response of tendon as that of an isotropic, slightly compressible, neo-Hookean solid. Inverse analysis, using finite-element (FE) simulations of the experiments and 10 s isochronal creep displacement data, gave values for Young's modulus and Poisson's ratio of this solid of 0.31 MPa and 0.49, respectively, for an idealized tendon shape and averaged data for all the tendons and E = 0.14 and 0.10 MPa for two specific tendons using their actual measured geometry. The compression load versus displacement curves, as measured and as simulated, showed varying degrees of stiffening with increasing load. This can be attributed mostly to geometrical changes in tendon cross section under load, varying according to the initial 3D shape of the tendon.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(4):041003-041003-12. doi:10.1115/1.4032643.

Shear waves that propagate in soft solids, such as the brain, are strongly nonlinear and can develop into shock waves in less than one wavelength. We hypothesize that these shear shock waves could be responsible for certain types of traumatic brain injuries (TBI) and that the spherical geometry of the skull bone could focus shear waves deep in the brain, generating diffuse axonal injuries. Theoretical models and numerical methods that describe nonlinear polarized shear waves in soft solids such as the brain are presented. They include the cubic nonlinearities that are characteristic of soft solids and the specific types of nonclassical attenuation and dispersion observed in soft tissues and the brain. The numerical methods are validated with analytical solutions, where possible, and with self-similar scaling laws where no known solutions exist. Initial conditions based on a human head X-ray microtomography (CT) were used to simulate focused shear shock waves in the brain. Three regimes are investigated with shock wave formation distances of $2.54 m, 0.018 m$, and $0.0064 m$. We demonstrate that under realistic loading scenarios, with nonlinear properties consistent with measurements in the brain, and when the shock wave propagation distance and focal distance coincide, nonlinear propagation can easily overcome attenuation to generate shear shocks deep inside the brain. Due to these effects, the accelerations in the focal are larger by a factor of 15 compared to acceleration at the skull surface. These results suggest that shock wave focusing could be responsible for diffuse axonal injuries.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(4):041004-041004-10. doi:10.1115/1.4032628.

Studies on intervertebral disk (IVD) response to various loads and postures are essential to understand disk's mechanical functions and to suggest preventive and corrective actions in the workplace. The experimental and finite-element (FE) approaches are well-suited for these studies, but validating their findings is difficult, partly due to the lack of alternative methods. Analytical modeling could allow methodological triangulation and help validation of FE models. This paper presents an analytical method based on thin-shell, beam-on-elastic-foundation and composite materials theories to evaluate the stresses in the anulus fibrosus (AF) of an axisymmetric disk composed of multiple thin lamellae. Large deformations of the soft tissues are accounted for using an iterative method and the anisotropic material properties are derived from a published biaxial experiment. The results are compared to those obtained by FE modeling. The results demonstrate the capability of the analytical model to evaluate the stresses at any location of the simplified AF. It also demonstrates that anisotropy reduces stresses in the lamellae. This novel model is a preliminary step in developing valuable analytical models of IVDs, and represents a distinctive groundwork that is able to sustain future refinements. This paper suggests important features that may be included to improve model realism.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(4):041005-041005-9. doi:10.1115/1.4032804.

Bicycle helmet foam liners absorb energy during impacts. Our goal was to determine if the impact attenuation properties of expanded polystyrene (EPS) foam used in bicycle helmets change with age. Foam cores were extracted from 63 used and unused bicycle helmets from ten different models spanning an age range of 2–20 yrs. All cores were impact tested at a bulk strain rate of 195 s−1. Six dependent variables were determined from the stress–strain curve derived from each impact (yield strain, yield stress, elastic modulus, plateau slope, energy at 65% compression, and stress at 65% compression), and a general linear model was used to assess the effect of age on each dependent variable with density as a covariate. Age did not affect any of the dependent variables; however, greater foam density, which varied from 58 to 100 kg/m3, generated significant increases in all of the dependent variables except for yield strain. Higher density foam cores also exhibited lower strains at which densification began to occur, tended to stay within the plateau region of the stress–strain curve, and were not compressed as much compared with the lower density cores. Based on these data, the impact attenuation properties of EPS foam in field-used bicycle helmets do not degrade with the age.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(4):041006-041006-13. doi:10.1115/1.4032798.

Bone mechanical properties vary with age; meanwhile, a close relationship exists among bone mechanical properties at different levels. Therefore, conducting multilevel analyses for bone structures with different ages are necessary to elucidate the effects of aging on bone mechanical properties at different levels. In this study, an approach that combined microfinite element (micro-FE) analysis and macrocompressive test was established to simulate the failure of male rat femoral cortical bone. Micro-FE analyses were primarily performed for rat cortical bones with different ages to simulate their failure processes under compressive load. Tissue-level failure strains in tension and compression of these cortical bones were then back-calculated by fitting the experimental stress–strain curves. Thus, tissue-level failure strains of rat femoral cortical bones with different ages were quantified. The tissue-level failure strain exhibited a biphasic behavior with age: in the period of skeletal maturity (1–7 months of age), the failure strain gradually increased; when the rat exceeded 7 months of age, the failure strain sharply decreased. In the period of skeletal maturity, both the macro- and tissue-levels mechanical properties showed a large promotion. In the period of skeletal aging (9–15 months of age), the tissue-level mechanical properties sharply deteriorated; however, the macromechanical properties only slightly deteriorated. The age-related changes in tissue-level failure strain were revealed through the analysis of male rat femoral cortical bones with different ages, which provided a theoretical basis to understand the relationship between rat cortical bone mechanical properties at macro- and tissue-levels and decrease of bone strength with age.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2016;138(4):041007-041007-12. doi:10.1115/1.4032800.

This paper presents a mechanism for standing and sitting transformation of a wheelchair using a two-wheeled inverted pendulum concept with reduced torque requirement, in simulation studies. The motivation of this work is to design a compact standing mechanism to help an elderly/disabled person with functional limitation in lower extremities to maneuver in small and confined spaces and enable them to perform standard daily life routines independently. The wheelchair system at the upright standing position is tested with different travel distances, and the challenge is to control both sit-to-stand and stand-to-sit operations in a stable manner using flexible-joint humanoid. An additional spring/damping element is incorporated at each wheel to provide a comfortable ride for the user especially during stand-to-sit transformation task. A PD-fuzzy control with modular structure is implemented, and the performance of the system is observed through visual nastran 4d (vn4d) visualization software and simulation in matlab. The stand-to-sit performance tests have shown more than 38% reduction in tilt and back seat angles fluctuation in linear travel motion using a suspension system, while the initial tilt torque needed is 50% less than the amount required in previous designs.

Commentary by Dr. Valentin Fuster

### Technical Brief

J Biomech Eng. 2016;138(4):044501-044501-4. doi:10.1115/1.4032693.

Knee stiffness following joint injury or immobilization is a common clinical problem, and the rat has been used as a model for studies related to joint stiffness and limitation of motion. Knee stiffness measurements have been reported for the anesthetized rat, but it is difficult to separate the contributions of muscular and ligamentous restraints to the recorded values. in vitro testing of isolated rat knees devoid of musculature allows measurement of joint structural properties alone. In order to measure the effects of therapeutic or surgical interventions designed to alter joint stiffness, the opposite extremity is often used as a control. However, right–left stiffness differences for the normal rat knee have not been reported in the literature. If stiffness changes observed for a treatment group are within the normal right–left variation, validity of the results could be questioned. The objectives of this study were to utilize a new testing apparatus to measure right–left stiffness differences during knee extension in a population of normal rat knees and to document repeatability of the stiffness measurements on successive testing days. Moment versus rotation curves were recorded for 15 right–left pairs of normal rat knees on three consecutive days, with overnight specimen storage in a refrigerator. Each knee was subjected to ten loading–unloading cycles, with the last loading curve used for analysis. Angular rotation (AR), defined here as the change in flexion–extension angle from a specified applied joint moment, is commonly used as a measure of overall joint stiffness. For these tests, ARs were measured from the recorded test curves with a maximum applied extension moment of 100 g cm. Mean rotations for testing days 2 and 3 were 0.81–1.25 deg lower (p < 0.001) than for day 1, but were not significantly different from each other. For each testing day, mean rotations for right knees were 1.12–1.30 deg greater (p < 0.001) than left knees. These right–left stiffness differences should be considered when interpreting the results of knee treatment studies designed to alter knee stiffness when using the opposite extremity as a control.

Topics: Testing , Stiffness , Knee , Rotation
Commentary by Dr. Valentin Fuster