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IN THIS ISSUE

### Research Papers

J Biomech Eng. 2018;140(9):091001-091001-11. doi:10.1115/1.4039894.

The pelvic construct is an important part of the body as it facilitates the transfer of upper body weight to the lower limbs and protects a number of organs and vessels in the lower abdomen. In addition, the importance of the pelvis is highlighted by the high mortality rates associated with pelvic trauma. This study presents a mesoscale structural model of the pelvic construct and the joints and ligaments associated with it. Shell elements were used to model cortical bone, while truss elements were used to model trabecular bone and the ligaments and joints. The finite element (FE) model was subjected to an iterative optimization process based on a strain-driven bone adaptation algorithm. The bone model was adapted to a number of common daily living activities (walking, stair ascent, stair descent, sit-to-stand, and stand-to-sit) by applying onto it joint and muscle loads derived using a musculoskeletal modeling framework. The cortical thickness distribution and the trabecular architecture of the adapted model were compared qualitatively with computed tomography (CT) scans and models developed in previous studies, showing good agreement. The sensitivity of the model to changes in material properties of the ligaments and joint cartilage and changes in parameters related to the adaptation algorithm was assessed. Changes to the target strain had the largest effect on predicted total bone volumes. The model showed low sensitivity to changes in all other parameters. The minimum and maximum principal strains predicted by the structural model compared to a continuum CT-derived model in response to a common test loading scenario showed good agreement with correlation coefficients of 0.813 and 0.809, respectively. The developed structural model enables a number of applications such as fracture modeling, design, and additive manufacturing of frangible surrogates.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):091002-091002-7. doi:10.1115/1.4040121.

Topics: Stress , Cycles , Cartilage
Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):091003-091003-10. doi:10.1115/1.4040123.

Recent studies demonstrated an association between atypical femoral fracture (AFF) and long-term bisphosphonate (BP) use for osteoporosis treatment. Due to BP treatment, bone undergoes alterations including increased microcrack density and reduced tissue compositional heterogeneity. However, the effect of these changes on the fracture response of bone is not well understood. As a result, the goal of the current study is to evaluate the individual and combined effects of microcracks and tissue compositional heterogeneity on fracture resistance of cortical bone using finite element modeling (FEM) of compact tension (CT) specimen tests with varying microcrack density, location, and clustering, and material heterogeneity in three different bone samples. The simulation results showed that an increase in microcrack density improved the fracture resistance irrespective of the local material property heterogeneity and microcrack distribution. A reduction in material property heterogeneity adversely affected the fracture resistance in models both with and without microcracks. When the combined changes in microcrack density and tissue material property heterogeneity representing BP treatment were evaluated, the models corresponding to BP-treated bone demonstrated reduced fracture resistance. The simulation results also showed that although microcrack location and clustering, and microstructure significantly influenced fracture resistance, the trends observed on the effect of microcrack density and tissue material property heterogeneity did not change. In summary, these results provide new information on the interaction of microcracks, tissue material property heterogeneity, and fracture resistance and may improve the understanding of the influence of mechanical changes due to prolonged BP use on the fracture behavior of cortical bone.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):091004-091004-8. doi:10.1115/1.4040124.

Fenestrated endovascular aneurysm repair (FEVAR) is a minimally invasive method of abdominal aortic aneurysm (AAA) repair utilized in patients with complex vessel anatomies. Stent grafts (SG) used in this process contain fenestrations within the device that need to be aligned with the visceral arteries upon successful SG deployment. Proper alignment is crucial to maintain blood flow to these arteries and avoid surgical complications. During fenestrated SG deployment, rotation of the SG can occur during the unsheathing process. This leads to misalignment of the vessels, and the fenestrations and is associated with poor clinical outcomes. The aim of this study was to develop a computational model of the FEVAR process to predict SG rotation. Six patient-specific cases are presented and compared with surgical case data. Realistic material properties, frictional effects, deployment methods, and boundary conditions are included in the model. A mean simulation error of 2 deg (range 1–4 deg) was observed. This model was then used to conduct a parameter study of frictional properties to see if rotation could be minimized. This study showed that increasing or decreasing the coefficients of friction (COF) between the sheath and the vessel walls would decrease the amount of rotation observed. Our model accurately predicts the amount of SG rotation observed during FEVAR and can be used as a preoperative planning tool within the surgical workflow.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):091005-091005-10. doi:10.1115/1.4040019.

Cycling is a leading cause of sport-related head injuries in the U.S. Although bicycle helmets must comply with standards limiting head acceleration in severe impacts, helmets are not evaluated under more common, concussive-level impacts, and limited data are available indicating which helmets offer superior protection. Further, standards evaluate normal impacts, while real-world cyclist head impacts are oblique—involving normal and tangential velocities. The objective of this study was to investigate differences in protective capabilities of ten helmet models under common real-world accident conditions. Oblique impacts were evaluated through drop tests onto an angled anvil at common cyclist head impact velocities and locations. Linear and rotational accelerations were evaluated and related to concussion risk, which was then correlated with design parameters. Significant differences were observed in linear and rotational accelerations between models, producing concussion risks spanning >50% within single impact configurations. Risk differences were more attributable to linear acceleration, as rotational varied less between models. At the temporal location, shell thickness, vent configuration, and radius of curvature were found to influence helmet effective stiffness. This should be optimized to reduce impact kinematics. At the frontal, helmet rim location, liner thickness tapered off for some helmets, likely due to lack of standards testing at this location. This is a frequently impacted location for cyclists, suggesting that the standards testable area should be expanded to include the rim. These results can inform manufacturers, standards bodies, and consumers alike, aiding the development of improved bicycle helmet safety.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):091006-091006-11. doi:10.1115/1.4039987.

Wearable sensors embedded with inertial measurement units have become commonplace for the measurement of head impact biomechanics, but individual systems often suffer from a lack of measurement fidelity. While some researchers have focused on developing highly accurate, single sensor systems, we have taken a parallel approach in investigating optimal estimation techniques with multiple noisy sensors. In this work, we present a sensor network methodology that utilizes multiple skin patch sensors arranged on the head and combines their data to obtain a more accurate estimate than any individual sensor in the network. Our methodology visually localizes subject-specific sensor transformations, and based on rigid body assumptions, applies estimation algorithms to obtain a minimum mean squared error estimate. During mild soccer headers, individual skin patch sensors had over 100% error in peak angular velocity magnitude, angular acceleration magnitude, and linear acceleration magnitude. However, when properly networked using our visual localization and estimation methodology, we obtained kinematic estimates with median errors below 20%. While we demonstrate this methodology with skin patch sensors in mild soccer head impacts, the formulation can be generally applied to any dynamic scenario, such as measurement of cadaver head impact dynamics using arbitrarily placed sensors.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):091007-091007-12. doi:10.1115/1.4039824.

Successful designs of total hip replacement (THR) need to be robust to surgical variation in sizing and positioning of the femoral stem. This study presents an automated method for comprehensive evaluation of the potential impact of surgical variability in sizing and positioning on the primary stability of a contemporary cementless femoral stem (Corail®, DePuy Synthes). A patient-specific finite element (FE) model of a femur was generated from computed tomography (CT) images from a female donor. An automated algorithm was developed to span the plausible surgical envelope of implant positions constrained by the inner cortical boundary. The analysis was performed on four stem sizes: oversized, ideal (nominal) sized, and undersized by up to two stem sizes. For each size, Latin hypercube sampling was used to generate models for 100 unique alignment scenarios. For each scenario, peak hip contact and muscle forces published for stair climbing were scaled to the donor's body weight and applied to the model. The risk of implant loosening was assessed by comparing the bone–implant micromotion/strains to thresholds (150 μm and 7000 με) above which fibrous tissue is expected to prevail and the periprosthetic bone to yield, respectively. The risk of long-term loosening due to adverse bone resorption was assessed using bone adaptation theory. The range of implant positions generated effectively spanned the available intracortical space. The Corail stem was found stable and robust to changes in size and position, with the majority of the bone–implant interface undergoing micromotion and interfacial strains that are well below 150 μm and 7000 με, respectively. Nevertheless, the range of implant positions generated caused an increase of up to 50% in peak micromotion and up to 25% in interfacial strains, particularly for retroverted stems placed in a medial position.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):091008-091008-9. doi:10.1115/1.4039890.

Intervertebral disk (IVD) degeneration is commonly described by loss of height and hydration. However, in the first stage of IVD degeneration, this loss has not yet occurred. In the current study, we use an ex vivo degeneration model to analyze the changes in IVDs mechanical behavior in the first phase of degeneration. We characterize these changes by stretched-exponential fitting, and suggest the fitted parameters as markers for early degeneration. Enzymatic degeneration of healthy lumbar caprine IVDs was induced by injecting 100 μL of Chondroïtinase ABC (Cabc) into the nucleus. A no-intervention and phosphate buffered saline (PBS) injected group were used as controls. IVDs were cultured in a bioreactor for 20 days under diurnal, simulated-physiological loading (SPL) conditions. Disk deformation was continuously monitored. Changes in disk height recovery behavior were quantified using stretched-exponential fitting. Disk height, histological sections, and water- and glycosaminoglycan (GAG)-content measurements were used as gold standards for the degenerative state. Cabc injection caused significant GAG loss from the nucleus and had detrimental effects on poro-elastic mechanical properties of the IVDs. These were progressive over time, with a propensity toward more linear recovery behavior. On histological sections, both PBS and Cabc injected IVDs showed moderate degeneration. A small GAG loss yields changes in IVD recovery behavior, which can be quantified with stretched-exponential fitting. Parameters changed significantly compared to control. Studies on disk degeneration and biomaterial engineering for degenerative disk disease (DDD) could benefit from focusing on IVD biomechanical behavior rather than height and water-content, as a marker for early disk degeneration.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):091009-091009-10. doi:10.1115/1.4039676.

In-stent restenosis ails many patients who have undergone stenting. When the stented artery is a bifurcation, the intervention is particularly critical because of the complex stent geometry involved in these structures. Computational fluid dynamics (CFD) has been shown to be an effective approach when modeling blood flow behavior and understanding the mechanisms that underlie in-stent restenosis. However, these CFD models require validation through experimental data in order to be reliable. It is with this purpose in mind that we performed particle image velocimetry (PIV) measurements of velocity fields within flows through a simplified coronary bifurcation. Although the flow in this simplified bifurcation differs from the actual blood flow, it emulates the main fluid dynamic mechanisms found in hemodynamic flow. Experimental measurements were performed for several stenting techniques in both steady and unsteady flow conditions. The test conditions were strictly controlled, and uncertainty was accurately predicted. The results obtained in this research represent readily accessible, easy to emulate, detailed velocity fields and geometry, and they have been successfully used to validate our numerical model. These data can be used as a benchmark for further development of numerical CFD modeling in terms of comparison of the main flow pattern characteristics.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):091010-091010-11. doi:10.1115/1.4040127.

Computational fluid dynamics (CFD) modeling of myocardial bridging (MB) remains challenging due to its dynamic and phasic nature. This study aims to develop a patient-specific CFD model of MB. There were two parts to this study. The first part consisted of developing an in silico model of the left anterior descending (LAD) coronary artery of a patient with MB. In this regard, a moving-boundary CFD algorithm was developed to simulate the patient-specific muscle compression caused by MB. A second simulation was also performed with the bridge artificially removed to determine the hemodynamics in the same vessel in the absence of MB. The second part of the study consisted of hemodynamic analysis of three patients with mild and moderate and severe MB in their LAD by means of the developed in silico model in the first part. The average shear stress in the proximal and bridge segments for model with MB were significantly different from those for model without MB (proximal segment: 0.32 ± 0.14 Pa (with MB) versus 0.97 ± 0.39 Pa (without MB), P < 0.0001 — bridge segment: 2.60 ± 0.94 Pa (with MB) versus 1.50 ± 0.64 Pa (without MB), P < 0.0001). When all three patients were evaluated, increasing the degree of vessel compression shear stress in the proximal segment decreased, whereas the shear stress in the bridge segment increased. The presence of MB resulted in hemodynamic abnormalities in the proximal segment, whereas segments within the bridge exhibited hemodynamic patterns which tend to discourage atheroma development.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):091011-091011-7. doi:10.1115/1.4039989.

Examine the biomechanical effect of material properties, geometric variables, and anchoring arrangements in a segmental pedicle screw with connecting rods spanning the entire lumbar spine using finite element models (FEMs). The objectives of this study are (1) to understand how different variables associated with posterior instrumentation affect the lumbar spine kinematics and stresses in instrumentation, (2) to compare the multidirectional stability of the spinal instrumentation, and (3) to determine how these variables contribute to the rigidity of the long-segment fusion in a lumbar spine. A lumbar spine FEM was used to analyze the biomechanical effects of different materials used for spinal rods (TNTZ or Ti or CoCr), varying diameters of the screws and rods (5 mm and 6 mm), and different fixation techniques (multilevel or intermittent). The results based on the range of motion and stress distribution in the rods and screws revealed that differences in properties and variations in geometry of the screw-rod moderately affect the biomechanics of the spine. Further, the spinal screw-rod system was least stable under the lateral bending mode. Stress analyzes of the screws and rods revealed that the caudal section of the posterior spinal instrumentation was more susceptible to high stresses and hence possible failure. Although CoCr screws and rods provided the greatest spinal stabilization, these constructs were susceptible to fatigue failure. The findings of the present study suggest that a posterior instrumentation system with a 5-mm screw-rod diameter made of Ti or TNTZ is advantageous over CoCr instrumentation system.

Commentary by Dr. Valentin Fuster

### Technical Brief

J Biomech Eng. 2018;140(9):094501-094501-6. doi:10.1115/1.4040122.

Quantitative computed tomography (qCT) relies on calibrated bone mineral density data. If a calibration phantom is absent from the CT scan, post hoc calibration becomes necessary. Scanning a calibration phantom after-the-fact and applying that calibration to uncalibrated scans has been used previously. Alternatively, the estimated density is known to vary with CT settings, suggesting that it may be possible to predict the calibration terms using CT settings. This study compares a novel CT setting regression method for post hoc calibration to standard and post hoc phantom-only calibrations. Five cadaveric upper limbs were scanned at 11 combinations of peak tube voltage and current (80–140 kV and 100–300 mA) with two calibration phantoms. Density calibrations were performed for the cadaver scans, and scans of the phantoms alone. Stepwise linear regression determined if the calibration equation terms were predictable using peak tube voltage and current. Peak tube voltage, but not current, was significantly correlated with regression calibration terms. Calibration equation slope was significantly related to the type of phantom (p < 0.001), calibration method (p = 0.026), and peak tube voltage (p < 0.001), but not current (p = 1.000). The calibration equation vertical intercept was significantly related to the type of phantom (p < 0.001), and peak tube voltage (p = 0.006), but not calibration method (p = 0.682), or current (p = 0.822). Accordingly, regression can correlate peak tube voltage with density calibration terms. Suggesting that, while standard qCT calibration is preferable, regression calibration may be an acceptable post hoc method when necessary.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):094502-094502-5. doi:10.1115/1.4040125.

Many people with amputation utilize a prosthetic device to maintain function and ambulation. During the use of the prosthetic device, their residual limbs can develop wounds called pressure ulcers. The formation of these wounds has been linked to deformation and loading conditions of the skin and deeper tissues. Our research objective was to develop a complete profile of displacements on the gel liner at the interface with the socket during walking in transtibial amputees. Displacements for seven regions along the limb were quantified in addition to six calculations of displacement and three rotations relative to the prosthetic socket. The largest displacements were observed in the distal region of the gel liner, near the pin locking mechanism on the gel liner. Displacements were uneven throughout the liner with distal regions showing higher displacements. This mechanics-based information, combined with clinical information, will allow us to understand the local skin and muscle displacements, and will provide insights regarding localized tissue breakdown. Knowledge of how the liner displaces within the prosthetic socket can also help prosthetists modify designs to reduce these displacements, and reduce the potential for shear on the skin and in deeper tissues.

Commentary by Dr. Valentin Fuster
J Biomech Eng. 2018;140(9):094503-094503-11. doi:10.1115/1.4040126.

The tricuspid valve is a one-way valve on the pulmonary side of the heart, which prevents backflow of blood during ventricular contractions. Development of computational models of the tricuspid valve is important both in understanding the normal valvular function and in the development/improvement of surgical procedures and medical devices. A key step in the development of such models is quantification of the mechanical properties of the tricuspid valve leaflets. In this study, after examining previously measured five-loading-protocol biaxial stress–strain response of porcine tricuspid valves, a phenomenological constitutive framework was chosen to represent this response. The material constants were quantified for all three leaflets, which were shown to be highly anisotropic with average anisotropy indices of less than $0.5$ (an anisotropy index value of 1 indicates a perfectly isotropic response, whereas a smaller value of the anisotropy index indicates an anisotropic response). To obtain mean values of material constants, stress–strain responses of the leaflet samples were averaged and then fitted to the constitutive model (average $R2$ over $0.9$). Since the sample thicknesses were not hugely different, averaging the data using the same tension levels and stress levels produced similar average material constants for each leaflet.

Commentary by Dr. Valentin Fuster