Fatigue damage of collagenous tissues: experiment, modeling and simulation studies

Effect of bioprosthetic leaflet anisotropy on stent dynamics of Transcatheter Aortic Valve Replacement devices

The assessment of stent fatigue in Transcatheter Aortic Valve Replacement (TAVR) systems is critical for the design of next-generation devices, both in vitro and in vivo. The mechanical properties of the bioprosthetic heart valves (BHVs) have a significant impact on the fatigue life of the metallic stent and thus must be taken into consideration when evaluating new TAVR device designs. This study aims to investigate the relationship between BHV anisotropic behaviour and the asymmetric deflections of the stent frame observed during in vitro testing. An explicit dynamics finite element model of the nitinol stent with attached bioprosthetic valve leaflets was developed to evaluate the deflections of the TAVR device under haemodynamic loading. Our results demonstrate that pericardium behaviour plays a dominant role in determining stent frame deflection. The anisotropic behaviour of the leaflets, resulting from collagen fibre orientation, affects the extent of deflection encountered by each commissure of the frame. This leads to asymmetric variation in frame deflection that can influence the overall fatigue life of the nitinol stent. This study highlights the importance of considering both the flexible nature of the metallic stent as well as the leaflet anisotropic behaviour in the design and fatigue assessment of TAVR systems.

Mediators and moderators of a walking intervention to prevent neck pain among high-risk office workers: a secondary analysis of a randomized controlled trial

OBJECTIVES

To test hypothesized mediators and moderators of the benefits of an app-provided walking intervention for preventing neck pain in high-risk office workers.

METHODS

Ninety-one office workers at risk for developing neck pain participated in the primary clinical trial. Two hypothesized mediators (the total number of steps taken and the number of days that participants exceed the number of recommended steps) were tested using a smartphone application every month during a 6-month period. Four hypothesized moderators (number of working hours, level of work psychological demands, chair height adjustability, and body mass index) were assessed at baseline. Parallel mediation and moderation analyses were conducted using the Hayes PROCESS macro, model 4 and 1, respectively, with post-hoc Johnson-Neyman techniques.

RESULTS

The number of days that participants exceeded the recommended steps ‒ the specific number was tailored to each participant, but averaged 7735 steps/day ‒ mediated the benefits of the walking intervention for reducing the risk for neck pain at each of six assessment points (B's range -0.63 to -0.89, all p's < 0.05) over 6-month period. None of the hypothesized moderators evidenced statistically significant moderator effects of the walking intervention.

CONCLUSION

Workers should walk at rates greater than recommended levels on as many days as possible, rather than attempt to maximize walking within a limited number of days. Given that the walking program studied appeared to be similarly effective across multiple groups of workers, the findings suggest that regular walking can reduce the risk for developing neck pain among high-risk office workers.

Modeling fatigue failure in soft tissue using a visco-hyperelastic model with discontinuous damage

Soft tissue is susceptible to injury from single high-magnitude static loads and from repetitive low-magnitude fatigue loads. While many constitutive formulations have been developed and validated to model static failure in soft tissue, a modeling framework is not well-established for fatigue failure. Here we determined the feasibility of using a visco-hyperelastic damage model with discontinuous damage (strain energy-based damage criterion) to simulate low- and high-cycle fatigue failure in soft fibrous tissue. Cyclic creep data from six uniaxial tensile fatigue experiments of human medial meniscus were used to calibrate the specimen-specific material parameters. The model was able to successfully simulate all three characteristic stages of cyclic creep, and predict the number of cycles until tissue rupture. Mathematically, damage propagated under constant cyclic stress due to time-dependent viscoelastic increases in tensile stretch that in turn increased strain energy. Our results implicate solid viscoelasticity as a fundamental regulator of fatigue failure in soft tissue, where tissue with slow stress relaxation times will be more resistant to fatigue injury. In a validation study, the visco-hyperelastic damage model was able to simulate characteristic stress-strain curves of pull to failure experiments (static failure) when using material parameters curve fit to the fatigue experiments. For the first time, we've shown that a visco-hyperelastic discontinuous damage framework can model cyclic creep and predict material rupture in soft tissue, and may enable the reliable simulation of both fatigue and static failure behavior from a single constitutive formulation.

Low cycle fatigue properties of porcine aorta - Pilot study

INTRODUCTION

Biomechanical rupture risk assessment of aortic tissues is commonly based on computed stress to measured uniaxial static strength comparison. Loading of the arterial wall, however, is cyclic; thus, the static strength may not be a proper limit value. This study investigates the low cycle fatigue of porcine aortic samples tested in a circumferential direction.

METHODS

7 porcine descending aorta (both thoracic and abdominal) were harvested and 56 dogbone-shaped samples were prepared. Static strength was measured, the limit of engineering stress was chosen and then force controlled cyclic loading was performed up to 100,000 cycles. Efforts were made to obtain a sufficient number of points across the entire range of 0-100,000 cycles. Data were fitted by both linear and logarithmic law and extrapolated towards 1 cycle for validation against static strength/ultimate tension. Data dispersion was evaluated via normalised root mean square error.

RESULTS

Out of 56 samples from 7pigs, 28 samples from 4 pigs were successfully tested. There was a strong negative correlation between applied stress/tension and number of cycles to failure. The fitting of both linear and logarithmic values resulted in a similar accuracy (R²=0.72 and 0.71 for stress and R²=0.62 and 0.7 for tension, respectively), while predicting static failure properties was more accurate by logarithmic law. NRMSE was lower for absolute values (20-21%) than for relative values (27-30%).

CONCLUSIONS

Absolute values of cyclic strength and tension are less dispersed than relative ones. Logarithmic fits are more robust in predicting static strength from cyclic data, while linear fits serve as a lower limit estimation.

Fatigue Testing of Human Flexor Tendons Using a Customized 3D-Printed Clamping System

Improved surgical procedures and implant developments for ligament or tendon repair require an in-depth understanding of tissue load-deformation and fatigue properties. Cyclic testing will provide crucial information on the behavior of these materials under reoccurring loads and on fatigue strength. Sparse data are available describing soft tissue behavior under cyclic loading. To examine fatigue strength, a new technology was trialed deploying 3D-printing to facilitate and standardize cyclic tests aiming to determine tendon fatigue behavior. Cadaveric flexor digitorum tendons were harvested and mounted for tensile testing with no tapering being made, using 3D-printed clamps and holder arms, while ensuring a consistent testing length. Loads ranging between 200 to 510 N were applied at a frequency of 4 Hz, and cycles to failure ranged between 8 and >260,000. S–N curves (Woehler curves) were generated based on the peak stresses and cycles to failure. Power regression yielded a combined coefficient of determination of stress and cycles to failure of R2 = 0.65, while the individual coefficients for tissues of single donors ranged between R2 = 0.54 and R2 = 0.88. The here-presented results demonstrate that S–N curves of human tendons can be obtained using a standardized setting deploying 3D-printing technology.

Evaluation of Pericardial Tissues from Assorted Species as a Tissue-Engineered Heart Valve Material

Decellularized pericardial tissue is a strong candidate for a TEHV material as ECM is present to guide cellular infiltration and fixed porcine and bovine pericardial tissue have existing use in bioprosthetic heart valves. In this work, we compare the mechanical and microstructural properties of decellularized-sterilized (DS) porcine, bovine, and bison pericardial tissues with respect to use as a TEHV. H&E staining was used to verify removal of cellular content post-decellularization and to evaluate collagen fiber structure. Additionally, uniaxial and biaxial tension testing were used to compare mechanical performance and, for the latter, acquire constitutive model parameters for subsequent finite element (FE) modeling. H&E staining revealed complete removal of cellular content and good collagen fiber structure. Tensile testing showed comparable mechanical strength between the three DS pericardial tissues and considerably stronger mechanical properties compared to native tissues. Bovine and bison DS pericardial tissues showed the strongest mechanical performance in the FE models with bison demonstrating the overall best mechanical characteristics. The increased thickness of bovine and bison tissues coupled with the strong mechanical behavior and ECM structure indicates that these materials will be resistant to damage until sufficient cellular infiltration has occurred such that damaged tissue can be repaired.

The effects of walking intervention on preventing neck pain in office workers: A randomized controlled trial

OBJECTIVE

This study aimed to evaluate the efficacy of increased daily walking steps on the 6-month incidence of neck pain among office workers.

METHODS

Healthy office workers with high risk of neck pain were recruited into a 6-month prospective cluster-randomized controlled trial. Participants were randomly assigned at the cluster level, into either intervention (n = 50) or control (n = 41) groups. Participants in the intervention group were instructed to increase their daily walking steps to a designated level for a duration of 6 months. Participants in the control group received no intervention. The outcome measures included the 6-month incidence of neck pain as well as its pain intensity and disability level. Analyses were performed using multivariable logistic regression model.

RESULTS

Of the participants in the intervention and control groups, 22% and 34% reported a 6-month incidence of neck pain, respectively. After adjusting for confounders, a significant preventive effect of walking intervention was found (adjusted odd ratio 0.22, 95% confidence interval 0.06-0.75). No significant difference in pain intensity and disability level was found between those in the intervention and control groups.

CONCLUSION

An intervention to increase daily walking steps reduced onset neck pain in high-risk office workers. However, the walking interventions did not decrease pain intensity and disability in those increasing the number of daily walking steps compared to the control group.

A Proof of Concept Study of Using Machine-Learning in Artificial Aortic Valve Design: From Leaflet Design to Stress Analysis

Artificial heart valves, used to replace diseased human heart valves, are life-saving medical devices. Currently, at the device development stage, new artificial valves are primarily assessed through time-consuming and expensive benchtop tests or animal implantation studies. Computational stress analysis using the finite element (FE) method presents an attractive alternative to physical testing. However, FE computational analysis requires a complex process of numeric modeling and simulation, as well as in-depth engineering expertise. In this proof of concept study, our objective was to develop machine learning (ML) techniques that can estimate the stress and deformation of a transcatheter aortic valve (TAV) from a given set of TAV leaflet design parameters. Two deep neural networks were developed and compared: the autoencoder-based ML-models and the direct ML-models. The ML-models were evaluated through Monte Carlo cross validation. From the results, both proposed deep neural networks could accurately estimate the deformed geometry of the TAV leaflets and the associated stress distributions within a second, with the direct ML-models (ML-model-d) having slightly larger errors. In conclusion, although this is a proof-of-concept study, the proposed ML approaches have demonstrated great potential to serve as a fast and reliable tool for future TAV design.

Hemodynamics and Wall Mechanics after Surgical Repair of Aortic Arch: Implication for Better Clinical Decisions

Graft repair of aortic coarctation is commonly used to mimic the physiological aortic arch shape and function. Various graft materials and shapes have been adopted for the surgery. The goal of this work is to quantitatively assess the impact of graft materials and shapes in the hemodynamics and wall mechanics of the restored aortic arch and its correlation with clinical outcomes. A three-dimensional aortic arch model was reconstructed from magnetic resonance images. The fluid–structure interaction (FSI) analysis was performed to characterize the hemodynamics and solid wall mechanics of the repaired aortic arch. Two graft shapes (i.e., a half-moon shape and a crescent one) were considered. Material choices of the aortic arch repair included three commonly used graft materials (i.e., polytetrafluoroethylene (PTFE) synthetic graft, CorMatrix extracellular matrix, and pulmonary homograft) as well as one native tissue serving as a control. The pathological hemodynamic parameters, in terms of the percentage area of low wall shear stress (WSS), high oscillatory shear index (OSI), and high relative residence time (RRT), were quantified to be associated with potential clinical outcomes. Results have shown that the peak von Mises stress for the aortic arch repaired by the crescent graft was 76% less than that of the half-moon graft. Flow disturbance and recirculation were also minimized with the crescent graft. Moreover, pathological hemodynamic parameters were significantly reduced with the crescent graft. The graft material mismatch with the surrounding tissue aggregated the stress concentration on the aortic wall, but had minimal impact on flow dynamics. The present work demonstrated the role and importance of the graft geometry and materials on hemodynamics and wall mechanics, which could guide optimal graft decisions towards better clinical outcomes.

Diastasis Recti Abdominis-diagnosis, Risk Factors, Effect on Musculoskeletal Function, Framework for Treatment and Implications for the Pelvic Floor

Background:

Diastasis Recti Abdominis (DRA) can occur during pregnancy and postpartum. It is defined as an increase of the inter-recti distance (IRD) beyond normal values. The diagnosis of DRA is inconsistent within the literature and varies depending on measurement instrument and activity during measurement (rest versus active curl-up). DRA is characterized by the stretching of linea alba (LA) and contributes to a protrusion of the anterior abdominal wall due to increased laxity in the myofascial system that supports abdominal viscera. DRA has been postulated to affect lumbopelvic support and function due to laxity of the LA and altered angle of muscle insertion, but recent studies have not confirmed this. Risk factors for the development of DRA have been investigated in pregnancy to 12-months postpartum.

Objective:

Rehabilitation for DRA has been traditionally focused on reducing the IRD, but recent research has proposed that a sole focus on closing the DRA is suboptimal.

Results:

It is important alongside the rehabilitation of the abdominal wall that there is the consideration of the pelvic floor (PF). In healthy individuals, with the activation of the transversus abdominis, there is a sub-maximal co-contraction of the PF muscles. This co-contraction can be lost or altered in women with urinary incontinence. An increase in intra-abdominal pressure without simultaneous co-contraction of the PF may cause caudal displacement of the PF.

Conclusion:

The aim of this review is to bring the reader up to date on the evidence on DRA and to propose a rehabilitation framework for the whole abdominal wall in DRA with consideration of the impact on the PF.

Concise Review: Tissue Engineering of Urinary Bladder; We Still Have a Long Way to Go?

Regenerative medicine is a new branch of medicine based on tissue engineering technology. This rapidly developing field of science offers revolutionary treatment strategy aimed at urinary bladder regeneration. Despite many promising announcements of experimental urinary bladder reconstruction, there has been a lack in commercialization of therapies based on current investigations. This is due to numerous obstacles that are slowly being identified and precisely overcome. The goal of this review is to present the current status of research on urinary bladder regeneration and highlight further challenges that need to be gradually addressed. We put an emphasis on expectations of urologists that are awaiting tissue engineering based solutions in clinical practice. This review also presents a detailed characteristic of obstacles on the road to successful urinary bladder regeneration from urological clinician perspective. A defined interdisciplinary approach might help to accelerate planning transitional research tissue engineering focused on urinary tracts. Stem Cells Translational Medicine 2017;6:2033-2043.