Research Papers

J Biomech Eng. 2018;140(12):121001-121001-12. doi:10.1115/1.4040943.

The purpose of the current study was to investigate the robustness of dynamic simulation results in the presence of uncertainties resulting from application of a scaled-generic musculoskeletal model instead of a subject-specific model as well as the effect of the choice of simulation method on the obtained muscle forces. The performed sensitivity analysis consisted of the following multibody parameter modifications: maximum isometric muscle forces, number of muscles, the hip joint center location, segment masses, as well as different dynamic simulation methods, namely static optimization (SO) with three different criteria and a computed muscle control (CMC) algorithm (hybrid approach combining forward and inverse dynamics). Twenty-four different models and fifty-five resultant dynamic simulation data sets were analyzed. The effects of model perturbation on the magnitude and profile of muscle forces were compared. It has been shown that estimated muscle forces are very sensitive to model parameters. The greatest impact was observed in the case of the force magnitude of the muscles generating high forces during gait (regardless of the modification introduced). However, the force profiles of those muscles were preserved. Relatively large differences in muscle forces were observed for different simulation techniques, which included both magnitude and profile of muscle forces. Personalization of model parameters would affect the resultant muscle forces and seems to be necessary to improve general accuracy of the estimated parameters. However, personalization alone will not ensure high accuracy due to the still unresolved muscle force sharing problem.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):121002-121002-10. doi:10.1115/1.4040600.

Transcatheter aortic valve replacement (TAVR) has emerged as an effective alternative to conventional surgical aortic valve replacement (SAVR) in high-risk elderly patients with calcified aortic valve disease. All currently food and drug administration approved TAVR devices use tissue valves that were adapted to but not specifically designed for TAVR use. Emerging clinical evidence indicates that these valves may get damaged during crimping and deployment—leading to valvular calcification, thrombotic complications, and limited durability. This impedes the expected expansion of TAVR to lower-risk and younger patients. Viable polymeric valves have the potential to overcome such limitations. We have developed a polymeric SAVR valve, which was optimized to reduce leaflet stresses and offer a thromboresistance profile similar to that of a tissue valve. This study compares the polymeric SAVR valve's hemodynamic performance and mechanical stresses to a new version of the valve—specifically designed for TAVR. Fluid–structure interaction (FSI) models were utilized and the valves' hemodynamics, flexural stresses, strains, orifice area, and wall shear stresses (WSS) were compared. The TAVR valve had 42% larger opening area and 27% higher flow rate versus the SAVR valve, while WSS distribution and mechanical stress magnitudes were of the same order, demonstrating the enhanced performance of the TAVR valve prototype. The TAVR valve FSI simulation and Vivitro pulse duplicator experiments were compared in terms of the leaflets' kinematics and the effective orifice area. The numerical methodology presented can be further used as a predictive tool for valve design optimization for enhanced hemodynamics and durability.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):121003-121003-8. doi:10.1115/1.4040602.

Developing precise computational models of bone remodeling can lead to more successful types of orthopedic treatments and deeper understanding of the phenomenon. Empirical evidence has shown that bone adaptation to mechanical loading is frequency dependent, and the modal behavior of bone under vibration can play a significant role in remodeling process, particularly in the resonance region. The objective of this study is to develop a bone remodeling algorithm that takes into account the effects of bone vibrational behavior. An extended/modified model is presented based on conventional finite element (FE) remodeling models. Frequency domain analysis is used to introduce appropriate correction coefficients to incorporate the effect of bone's frequency response (FR) into the model. The method is implemented on a bovine bone with known modal/vibration characteristics. The rate and locations of new bone formation depend on the loading frequency and are consistently correlated with the bone modal behavior. Results show that the proposed method can successfully integrate the bone vibration conditions and characteristics with the remodeling process. The results obtained support experimental observations in the literature.

Topics: Bone , Vibration , Density
Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):121004-121004-10. doi:10.1115/1.4040772.

Ascending stairs is challenging following transtibial amputation due to the loss of the ankle muscles, which are critical to human movement. Efforts to improve stair ascent following amputation are hindered by the limited understanding of how the prosthesis and remaining muscles contribute to stair ascent. This study developed a three-dimensional (3D) muscle-actuated forward dynamics simulation of amputee stair ascent to identify the contributions of individual muscles and the passive prosthesis to the biomechanical subtasks of stair ascent. The prosthesis was found to provide vertical propulsion throughout stair ascent, similar to nonamputee plantarflexors. However, the timing differed considerably. The prosthesis also contributed to braking, similar to the nonamputee soleus, but to a greater extent. However, the prosthesis was unable to replicate the functions of nonamputee gastrocnemius, which contributes to forward propulsion during the second half of stance and leg swing initiation. To compensate, the hamstrings and vasti of the residual leg increased their contributions to forward propulsion during the first and second halves of stance, respectively. The prosthesis also contributed to medial control, consistent with the nonamputee soleus but not gastrocnemius. Therefore, prosthesis designs that provide additional vertical propulsion as well as forward propulsion, lateral control, and leg swing initiation at appropriate points in the gait cycle could improve amputee stair ascent. However, because nonamputee soleus and gastrocnemius contribute oppositely to many subtasks, it may be necessary to couple the prosthesis, which functions most similarly to soleus, with targeted rehabilitation programs focused on muscle groups that can compensate for gastrocnemius.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):121005-121005-8. doi:10.1115/1.4040774.

In the absence of standardized symmetry assessments, quantifying symmetry based on the kinematic evolution of lower extremity joints can elucidate gait irregularities. The objective was to develop a novel cyclogram-based symmetry (CBS) method to quantify lower extremity joints' symmetry and assess the effect of six-month utilization of foot drop stimulator (FDS) on CBS of the lower limbs during hemiplegic gait poststroke. Twenty-four participants (13 stroke and 11 healthy controls (HC)) performed ten walking trials at a free cadence on level ground. Symmetry values were computed using geometric properties of bilateral cyclograms obtained from normalized sagittal ankle, knee, and hip kinematics. CBS and traditional temporospatial symmetry values were compared between the two groups using independent sample t-test. The effect of FDS utilization on the symmetry was assessed by a paired sample t-test computed at baseline and six-month follow up. The CBS method successfully showed that the HC group was significantly more symmetrical at the ankle (p = 0.001), knee (p = 0.001), and hip (p < 0.005) compared with the stroke group. The stroke group showed significant increment in the hip symmetry with FDS at the baseline but did not show any significant CBS changes at follow up. Pearson correlations revealed that hip and knee CBS had a significant influence on the overall walking speed. The CBS method presents a unique approach to calculate the symmetry based on the kinematics of lower extremities during gait.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):121006-121006-8. doi:10.1115/1.4040776.

A fluid–structure interaction (FSI) model of a left anterior descending (LAD) coronary artery was developed, incorporating transient blood flow, cyclic bending motion of the artery, and myocardial contraction. The three-dimensional (3D) geometry was constructed based on a patient's computed tomography angiography (CTA) data. To simulate disease conditions, a plaque was placed within the LAD to create a 70% stenosis. The bending motion of the blood vessel was prescribed based on the LAD spatial information. The pressure induced by myocardial contraction was applied to the outside of the blood vessel wall. The fluid domain was solved using the Navier–Stokes equations. The arterial wall was defined as a nonlinear elastic, anisotropic, and incompressible material, and the mechanical behavior was described using the modified hyper-elastic Mooney–Rivlin model. The fluid (blood) and solid (vascular wall) domains were fully coupled. The simulation results demonstrated that besides vessel bending/stretching motion, myocardial contraction had a significant effect on local hemodynamics and vascular wall stress/strain distribution. It not only transiently increased blood flow velocity and fluid wall shear stress, but also changed shear stress patterns. The presence of the plaque significantly reduced vascular wall tensile strain. Compared to the coronary artery models developed previously, the current model had improved physiological relevance.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):121007-121007-10. doi:10.1115/1.4040605.

Concussions are among the most common injuries sustained by goaltenders. Concussive injuries are characterized by impairment to neurological function which can affect many different brain regions. Understanding how different impact loading conditions (event type and impact site) affect the brain tissue response may help identify what kind of impacts create a high risk of injury to specific brain regions. The purpose of this study was to examine the influence of different impact conditions on the distribution of brain strain for ice hockey goaltender impacts. An instrumented headform was fitted with an ice hockey goaltender mask and impacted under a protocol which was developed using video analysis of real world ice hockey goaltender concussions for three different impact events (collision, puck, and fall). The resulting kinematic response served as input into the University College Dublin Brain Trauma Model (UCDBTM), which calculated maximum principal strain (MPS) in the cerebrum. Strain subsets were then determined and analyzed. Resulting peak strains (0.124–0.328) were found to be within the range for concussion reported in the literature. The results demonstrated that falls and collisions produced larger strain subsets in the cerebrum than puck impacts which is likely a reflection of longer impact duration for falls and collisions than puck impacts. For each impact event, impact site was also found to produce strain subsets of varying size and configuration. The results of this study suggest that the location and number of brain regions which can be damaged depend on the loading conditions of the impact.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):121008-121008-9. doi:10.1115/1.4041045.

A three-dimensional fluid-structure interaction computational model was used to investigate the effect of the longitudinal variation of vocal fold inner layer thickness on voice production. The computational model coupled a finite element method based continuum vocal fold model and a Navier–Stokes equation based incompressible flow model. Four vocal fold models, one with constant layer thickness and the others with different degrees of layer thickness variation in the longitudinal direction, were studied. It was found that the varied thickness resulted in up to 24% stiffness reduction at the middle and up to 47% stiffness increase near the anterior and posterior ends of the vocal fold; however, the average stiffness was not affected. The fluid-structure interaction simulations on the four models showed that the thickness variation did not affect vibration amplitude, glottal flow rate, and the waveform related parameters. However, it increased glottal angles at the middle of the vocal fold, suggesting that vocal fold vibration amplitude was determined by the average stiffness of the vocal fold, while the glottal angle was determined by the local stiffness. The models with longitudinal variation of layer thickness consumed less energy during the vibrations compared with the constant layer thickness one.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):121009-121009-16. doi:10.1115/1.4041043.

With the recent implementation of multiphasic materials in the open-source finite element (FE) software FEBio, three-dimensional (3D) models of cells embedded within the tissue may now be analyzed, accounting for porous solid matrix deformation, transport of interstitial fluid and solutes, membrane potential, and reactions. The cell membrane is a critical component in cell models, which selectively regulates the transport of fluid and solutes in the presence of large concentration and electric potential gradients, while also facilitating the transport of various proteins. The cell membrane is much thinner than the cell; therefore, in an FE environment, shell elements formulated as two-dimensional (2D) surfaces in 3D space would be preferred for modeling the cell membrane, for the convenience of mesh generation from image-based data, especially for convoluted membranes. However, multiphasic shell elements are yet to be developed in the FE literature and commercial FE software. This study presents a novel formulation of multiphasic shell elements and its implementation in FEBio. The shell model includes front- and back-face nodal degrees-of-freedom for the solid displacement, effective fluid pressure and effective solute concentrations, and a linear interpolation of these variables across the shell thickness. This formulation was verified against classical models of cell physiology and validated against reported experimental measurements in chondrocytes. This implementation of passive transport of fluid and solutes across multiphasic membranes makes it possible to model the biomechanics of isolated cells or cells embedded in their extracellular matrix (ECM), accounting for solvent and solute transport.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):121010-121010-10. doi:10.1115/1.4041165.

Mechanical instability of soft tissues can either risk their normal function or alternatively trigger patterning mechanisms during growth and morphogenesis processes. Unlike standard stability analysis of linear elastic bodies, for soft tissues undergoing large deformations it is imperative to account for the nonlinearities induced by the coupling between load and surface changes at onset of instability. The related issue of boundary conditions, in context of soft tissues, has hardly been addressed in the literature, with most of available research employing dead-load conditions. This paper is concerned with the influence of imposed homogeneous rate (incremental) surface data on critical loads and associated modes in soft tissues, within the context of linear bifurcation analysis. Material behavior is modeled by compressible isotropic hyperelastic strain energy functions (SEFs), with experimentally validated material parameters for the Fung–Demiray SEF, over a range of constitutive response (including brain and liver tissues). For simplicity, we examine benchmark problems of basic spherical patterns: full sphere, spherical cavity, and thick spherical shell. Limiting the analysis to primary hydrostatic states we arrive at universal closed-form solutions, thus providing insight on the role of imposed boundary data. Influence of selected rate boundary conditions (RBCs) like dead-load and fluid-pressure (FP), coupled with constitutive parameters, on the existence and levels of bifurcation loads is compared and discussed. It is argued that the selection of the appropriate type of homogeneous RBC can have a critical effect on the level of bifurcation loads and even exclude the emergence of bifurcation instabilities.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):121011-121011-7. doi:10.1115/1.4041527.

The purpose of this work is twofold: first, to synthesize a motion pattern imitating sit-to-stand (STS) and second, to compare the kinematics and dynamics of the resulting motion to healthy STS. Predicting STS in simulation inspired the creation of three models: a biomechanical model, a motion model, and performance criteria as a model of preference. First, the human is represented as three rigid links in the sagittal plane. This model captures aspects of joint, foot, and buttocks physiology, which makes it the most comprehensive planar model for predicting STS to date. Second, candidate STS trajectories are described geometrically by a set of Bézier curves which seem well suited to predictive biomechanical simulations. Third, with the assumption that healthy people naturally prioritize mechanical efficiency, disinclination to a motion is described as a cost function of joint torques, and for the first time, physical infeasibility including slipping and falling. This new dynamic optimization routine allows for motions of gradually increasing complexity while the model's performance is improving. Using these models and optimal control strategy together has produced gross motion patterns characteristic of healthy STS when compared with normative data from the literature.

Commentary by Dr. Valentin Fuster

Technical Brief

J Biomech Eng. 2018;140(12):124501-124501-8. doi:10.1115/1.4041044.

The present study investigated the day-to-day reliability (quantified by the absolute and relative reliability) of nonlinear methods used to assess human locomotion dynamics. Twenty-four participants of whom twelve were diagnosed with knee osteoarthritis completed 5 min of treadmill walking at self-selected preferred speed on two separate days. Lower limb kinematics were recorded at 100 Hz and hip, knee, and ankle joint angles, three-dimensional (3D) sacrum marker displacement and stride time intervals were extracted for 170 consecutive strides. The largest Lyapunov exponent and correlation dimension were calculated for the joint angle and sacrum displacement data using three different state space reconstruction methods (group average, test-retest average, individual time delay and embedding dimension). Sample entropy and detrended fluctuation analysis (DFA) were applied to the stride time interval time series. Relative reliability was assessed using intraclass correlation coefficients and absolute reliability was determined using measurement error (ME). For both joint angles and sacrum displacement, there was a general pattern that the group average state space reconstruction method provided the highest relative reliability and lowest ME compared to the individual and test-retest average methods. The DFA exhibited good reliability, while the sample entropy showed poor reliability. The results comprise a reference material that can inspire and guide future studies of nonlinear gait dynamics.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):124502-124502-6. doi:10.1115/1.4040601.

The OrthoSensor VERASENSE knee system is a commercially available instrumented tibial insert that provides real-time intraoperative measurements of tibial contact force and contact location to guide surgeons toward improving outcomes in total knee arthroplasty (TKA). However, the device has been used contrary to the manufacturer's instructions in several studies and lacks published information on accuracy. Therefore, the primary objectives of this study were to evaluate the device's error in tibial contact force when used according to and contrary to the manufacturer's instructions, and also to evaluate the device's error in anterior-posterior (A-P) and medial-lateral (M-L) contact locations. The error in tibial contact force in one-compartment distributed loading was evaluated by applying known forces in ranges within and exceeding that instructed by the manufacturer, with rezeroing as instructed by the manufacturer, and without rezeroing. The error in tibial contact location in one-compartment concentrated loading was evaluated by applying known forces at known locations on the articular surface. Exceeding the maximum allowable load and not rezeroing did not adversely affect the bias (i.e., average error) (p > 0.05). The maximum absolute bias without rezeroing was 2.9 lbf. Rezeroing more than doubled the bias. The maximum root-mean-squared error in tibial contact location was 1.5 mm in the A-P direction. The device measures tibial contact force with comparable error well above the maximum allowable load and without rezeroing, contrary to the manufacturer's instructions.

Topics: Stress , Errors
Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(12):124503-124503-6. doi:10.1115/1.4040775.

Application of tibiofemoral compression force (TCF) has been shown to produce anterior cruciate ligament (ACL) injuries in a laboratory setting. A new robotic testing methodology was utilized to predict ACL forces generated by TCF without directly loading the ligament. We hypothesized that ACL force, directly recorded by a miniature load cell during an unconstrained test, could be predicted by measurements of anterior tibial restraining force (ARF) recorded during a constrained test. The knee was first flexed under load control with 25 N TCF (tibial displacements and rotations unconstrained) to record a baseline kinematic pathway. Tests were repeated with increasing levels of TCF, while recording ACL force and knee kinematics. Then tests with increasing TCF were performed under displacement control to reproduce the baseline kinematic pathway (tibia constrained), while recording ARF. This allowed testing to 1500 N TCF since the ACL was not loaded. TCF generated ACL force for all knees (n = 10) at 50 deg flexion, and for eight knees at 30 deg flexion (unconstrained test). ACL force (unconstrained test) and ARF (constrained test) had strong linear correlations with TCF at both flexion angles (R2 from 0.85 to 0.99), and ACL force was strongly correlated with ARF at both flexion angles (R2 from 0.76 to 0.99). Under 500 N TCF, the mean error between ACL force prediction from ARF regression and measured ACL force was 4.8±7.3 N at 30 deg and 8.8±27.5 N at 50 deg flexion. Our hypothesis was confirmed for TCF levels up to 500 N, and ARF had a strong linear correlation with TCF up to 1500 N TCF.

Commentary by Dr. Valentin Fuster


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