Comparison of aneurysmal and non-pathologic human ascending aortic tissue in shear

Applied use of biomechanical measurements from human tissues for the development of medical skills trainers: a scoping review

OBJECTIVE

The objective of this review was to identify quantitative biomechanical measurements of human tissues, the methods for obtaining these measurements, and the primary motivations for conducting biomechanical research.

INTRODUCTION

Medical skills trainers are a safe and useful tool for clinicians to use when learning or practicing medical procedures. The haptic fidelity of these devices is often poor, which may be because the synthetic materials chosen for these devices do not have the same mechanical properties as human tissues. This review investigates a heterogeneous body of literature to identify which biomechanical properties are available for human tissues, the methods for obtaining these values, and the primary motivations behind conducting biomechanical tests.

INCLUSION CRITERIA

Studies containing quantitative measurements of the biomechanical properties of human tissues were included. Studies that primarily focused on dynamic and fluid mechanical properties were excluded. Additionally, studies only containing animal, in silico , or synthetic materials were excluded from this review.

METHODS

This scoping review followed the JBI methodology for scoping reviews and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR). Sources of evidence were extracted from CINAHL (EBSCO), IEEE Xplore, MEDLINE (PubMed), Scopus, and engineering conference proceedings. The search was limited to the English language. Two independent reviewers screened titles and abstracts as well as full-text reviews. Any conflicts that arose during screening and full-text review were mediated by a third reviewer. Data extraction was conducted by 2 independent reviewers and discrepancies were mediated through discussion. The results are presented in tabular, figure, and narrative formats.

RESULTS

Data were extracted from a total of 186 full-text publications. All of the studies, except for 1, were experimental. Included studies came from 33 countries, with the majority coming from the United States. Ex vivo methods were the predominant approach for extracting human tissue samples, and the most commonly studied tissue type was musculoskeletal. In this study, nearly 200 unique biomechanical values were reported, and the most commonly reported value was Young's (elastic) modulus. The most common type of mechanical test performed was tensile testing, and the most common reason for testing human tissues was to characterize biomechanical properties. Although the number of published studies on biomechanical properties of human tissues has increased over the past 20 years, there are many gaps in the literature. Of the 186 included studies, only 7 used human tissues for the design or validation of medical skills training devices. Furthermore, in studies where biomechanical values for human tissues have been obtained, a lack of standardization in engineering assumptions, methodologies, and tissue preparation may implicate the usefulness of these values.

CONCLUSIONS

This review is the first of its kind to give a broad overview of the biomechanics of human tissues in the published literature. With respect to high-fidelity haptics, there is a large gap in the published literature. Even in instances where biomechanical values are available, comparing or using these values is difficult. This is likely due to the lack of standardization in engineering assumptions, testing methodology, and reporting of the results. It is recommended that journals and experts in engineering fields conduct further research to investigate the feasibility of implementing reporting standards.

REVIEW REGISTRATION

Open Science Framework https://osf.io/fgb34.

Minor aortic injury may be at risk of progression from uncontrolled shear stress: An in-vitro model demonstrates aortic lesion expansion

Background

Non-operative management is considered appropriate treatment for minor aortic injury, while blood pressure and anti-impulse therapy are routinely utilized to prevent higher grade aortic injury progression. However, a universal medical regimen for low grade intimal injuries has not been adopted and risks of low-grade injury progression not well described. The purpose of this study is to determine the fracture response of minimally damaged aortic tissue to the various applied forces. Our hypothesis is that internal circumferential shear within the aortic wall is a primary fracture mode. This knowledge may help guide clinical management to minimize risk of injury progression, including instituting standard medical regimens with anti-impulse therapy and β-blockade for such minor injuries.

Methods

Human ascending aortic tissue was obtained after aneurysm repair or heart transplant, stored at 4°C and tested within 48 hours. Minor injury was modeled with a small radial notch on the luminal aspect of aortic rings, circumferentially expanded under video acquisition and analyzed to determine lesion propagation.

Results

15 rings were obtained from 8 aneurysmal and 4 healthy aortas. All specimens demonstrated circumferential crack propagation. Propagation was longer (8.02 ± 5.92 mm vs 2.70 ± 1.23 mm) and initiation of crack propagation earlier in aneurysmal tissue (1.54 ± 0.17 versus 1.90 ± 0.17 times initial diameter).

Conclusions

Dilation of minimally injured aortic rings is associated with lesion expansion and injury progression in all specimens including healthy and aneurysmal tissue. This propagation illustrates the mechanical response to increased levels of internal shear, compromising structural integrity and increasing risk of aortic rupture in all injured aortas. Shear forces are routinely generated through normal circumferential aortic expansion with each pulsation, the magnitude of these forces determined by pulse and blood pressure. This suggests minor aortic injuries are not trivial and strategies to reduce shear stress be implemented in all such patients without contraindications to β- blockers.

Interstitial fluid-solid interaction within aneurysmal and non-pathological human ascending aortic tissue under translational sinusoidal shear deformation

The interaction shear force between internal interstitial fluid motion and the solid circumferential-longitudinal medial lamellae helps generate the shear stress involved in dissection of human ascending aorta aneurysmal or non-pathologic tissue. Frequency analysis parameters from the total shear stress versus time response to translational 1 Hz sinusoidal shear deformation over 50 cycles measure the interaction with respect to the three factors: tissue type, sinusoidal deformation amplitude and direction of the shear deformation. Significant 1, 3, and 5 Hz components exist in this order of descending magnitude for shear deformation amplitudes of either 25% or 50% of the specimen length. Evaporation tests indicate that the amount of free water in both aneurysmal and non-pathological tissue is nearly the same. The interstitial fluid-solid interaction under shear deformation is visible in the shoulders of the total shear stress versus time response curve that are caused by the 3 Hz component. During a single deformation cycle, the ratio of the amplitudes of the 3 Hz and the 1 Hz components measures the normalized amount of interaction. Under translational sinusoidal shear deformation at 25% amplitude, this interaction ratio is statistically smaller in non-pathologic than in aneurysmal human ascending aortic tissue in the circumferential direction. The frequency analysis parameters provide evidence that the structural changes in aneurysmal tissue induce an increase in the interstitial fluid-medial solid interaction shear force which contributes to the propensity for aneurysmal rupture. STATEMENT OF SIGNIFICANCE: Circumferential shear force between the interstitial fluid and medial lamellae within the human ascending aortic wall is demonstrably greater in aneurysmal than non-pathologic tissue. This force likely increases with medial elastin degeneration and may facilitate the dissection propensity in aneurysmal tissue. The 3 Hz component in frequency analyses of the total shear stress versus time curve produced by 1 Hz sinusoidal translational shear deformation measures the fluid-solid interaction shear force that is otherwise difficult to isolate. This non-standard examination of the interstitial fluid interaction helps clarify clinical mechanical implications of structural differences between aneurysmal and non-pathologic human ascending aortic tissue. The aneurysmal dissection susceptibility does not appear to depend on the amount of interstitial fluid or the wall thickness compared to non-pathologic tissue.

Ascending aorta mechanics in bicuspid aortopathy: controversy or fact?

Bicuspid aortic valve is the most common congenital cardiovascular defect, often associated with proximal aortic dilatation, and the ideal management strategy is debated. The inconsistency in previous and present guideline recommendations emphasizes the insufficiency of the maximal diameter as the sole criterion for prophylactic repair. Our ability to guide clinical decisions may improve through an understanding of the mechanical properties of ascending thoracic aortic aneurysms in bicuspid compared to tricuspid aortic valve patients and non-aneurysmal aortas, because dissection and rupture are aortic wall mechanical failures. Such an understanding of the mechanical properties has been attempted by several authors, and this article addresses whether there is a controversy in the accumulated knowledge. The available mechanical studies are briefly reviewed, discussing factors such as age, sex, and the region of mechanical examination that may be responsible for the lack of unanimity in the reported findings. The rationale for acquiring layer-specific properties is presented along with the main results from our recent study. No mechanical vulnerability of ascending thoracic aortic aneurysms was evidenced in bicuspid aortic valve patients, corroborating present conservative guidelines concerning the management of bicuspid aortopathy. Weakening and additional vulnerability was evidenced in aged patients and those with coexisting valve pathology, aortic root dilatation, hypertension, and hyperlipidemia. Discussion of these results from age- and sex-matched subjects, accounting for the region- and layer-specific aortic heterogeneity, in relation to intact wall results and histologic confirmation, helps to reconcile previous findings and affords a universal interpretation of ascending aorta mechanics in bicuspid aortopathy.