Pericardial heterografts: a comparative study of suture pull-out and tissue strength

Impact of Clinically Relevant Elliptical Deformations on the Damage Patterns of Sagging and Stretched Leaflets in a Bioprosthetic Heart Valve

After implantation of a transcatheter bioprosthetic heart valve its original circular circumference may become distorted, which can lead to changes in leaflet coaptation and leaflets that are stretched or sagging. This may lead to early structural deterioration of the valve as seen in some explanted transcatheter heart valves. Our in vitro study evaluates the effect of leaflet deformations seen in elliptical configurations on the damage patterns of the leaflets, with circular valve deformation as the control. Bovine pericardial tissue heart valves were subjected to accelerated wear testing under both circular (N = 2) and elliptical (N = 4) configurations. The elliptical configurations were created by placing the valve inside custom-made elliptical holders, which caused the leaflets to sag or stretch. The hydrodynamic performance of the valves was monitored and high resolution images were acquired to evaluate leaflet damage patterns over time. In the elliptically deformed valves, sagging leaflets experienced more damage from wear compared to stretched leaflets; the undistorted leaflets of the circular valves experienced the least leaflet damage. Free-edge thinning and tearing were the primary modes of damage in the sagging leaflets. Belly region thinning was seen in the undistorted and stretched leaflets. Leaflet and fabric tears at the commissures were seen in all valve configurations. Free-edge tearing and commissure tears were the leading cause of valve hydrodynamic incompetence. Our study shows that mechanical wear affects heart valve pericardial leaflets differently based on whether they are undistorted, stretched, or sagging in a valve configuration. Sagging leaflets are more likely to be subjected to free-edge tear than stretched or undistorted leaflets. Reducing leaflet stress at the free edge of non-circular valve configurations should be an important factor to consider in the design and/or deployment of transcatheter bioprosthetic heart valves to improve their long-term performance.

Biomechanical Behavior of Bioprosthetic Heart Valve Heterograft Tissues: Characterization, Simulation, and Performance

The use of replacement heart valves continues to grow due to the increased prevalence of valvular heart disease resulting from an ageing population. Since bioprosthetic heart valves (BHVs) continue to be the preferred replacement valve, there continues to be a strong need to develop better and more reliable BHVs through and improved the general understanding of BHV failure mechanisms. The major technological hurdle for the lifespan of the BHV implant continues to be the durability of the constituent leaflet biomaterials, which if improved can lead to substantial clinical impact. In order to develop improved solutions for BHV biomaterials, it is critical to have a better understanding of the inherent biomechanical behaviors of the leaflet biomaterials, including chemical treatment technologies, the impact of repetitive mechanical loading, and the inherent failure modes. This review seeks to provide a comprehensive overview of these issues, with a focus on developing insight on the mechanisms of BHV function and failure. Additionally, this review provides a detailed summary of the computational biomechanical simulations that have been used to inform and develop a higher level of understanding of BHV tissues and their failure modes. Collectively, this information should serve as a tool not only to infer reliable and dependable prosthesis function, but also to instigate and facilitate the design of future bioprosthetic valves and clinically impact cardiology.

Biomimetic microstructural reorganization during suture retention strength evaluation of electrospun vascular scaffolds

Suture retention strength (SRS) is commonly used as a measure the ability of sutures to adhere implants to surrounding tissue. While SRS is widely employed, surprisingly its effects on graft microstructure have not been characterized. This is particularly germane to the broad utilization of electrospun implants in tissue engineering. These implants need to retain their initial nanoscale topography while simultaneously preserving clinically critical mechanical properties. We examined the suture-driven microstructural deformation of polycaprolactone electrospun to form both square and tubular SRS samples. The impact of fiber orientation (generally parallel or random orientation, orthogonally aligned) on the SRS of these vascular tissue equivalents was analyzed and compared to native and decellularized porcine vasculature. The initial state of the fiber clearly dictates the overall efficiency of scaffold utilization. SRS values for as-spun fibers at a thickness of 300 μm were found to be in the range of 1.59-4.78 N for the three orientations. Unexpectedly, random fibers provided the optimal SRS values based on both resistance to suture motion and the percentage of scaffold involvement. A "V-shaped" failure morphology is observed for both electrospun scaffolds and native tissue during SRS testing. Post-test fiber alignment in the tensile direction was visible in all initial fiber orientations similar to that of native tissue. These findings are significant as they allow us to employ new, counterintuitive biomimetic design criteria for nanofiber-based scaffolds in which reliable mechanical integration with the surrounding tissues via suture-based methods is important. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1525-1534, 2016.

Biomechanical evaluation of suture-holding properties of native and tissue-engineered articular cartilage

The purpose of this study was to determine suture-holding properties of tissue-engineered neocartilage relative to native articular cartilage. To this end, suture pull-out strength was quantified for native articular cartilage and for neocartilages possessing various mechanical properties. Suture-holding properties were examined in vitro and in vivo. Neocartilage from bovine chondrocytes was engineered using two sets of exogenous stimuli, resulting in neotissue of different biochemical compositions. Compressive and tensile properties and glycosaminoglycan, collagen, and pyridinoline cross-link contents were assayed (study 1). Suture pull-out strength was compared between neocartilage constructs, and bovine and leporine native cartilage. Uniaxial pull-out test until failure was performed after passing 6-0 Vicryl through each tissue (study 2). Subsequently, neocartilage was implanted into a rabbit model to examine short-term suture-holding ability in vivo (study 3). Neocartilage glycosaminoglycan and collagen content per wet weight reached 4.55 ± 1.62% and 4.21 ± 0.77%, respectively. Tensile properties for neocartilage constructs reached 2.6 ± 0.77% MPa for Young's modulus and 1.39 ± 0.63 MPa for ultimate tensile strength. Neocartilage reached ~ 33% of suture pull-out strength of native articular cartilage. Neocartilage cross-link content reached 50% of native values, and suture pull-out strength correlated positively with cross-link content (R² = 0.74). Neocartilage sutured into rabbit osteochondral defects was successfully maintained for 3 weeks. This study shows that pyridinoline cross-links in neocartilage may be vital in controlling suture pull-out strength. Neocartilage produced in vitro with one-third of native tissue pull-out strength appears sufficient for construct suturing and retention in vivo.

Mechanical testing of human cardiac tissue: some implications for MRI safety

PURPOSE

The effects of aging on tissue strength and its ability to withstand forces associated with MRI have not been investigated. This study aimed to determine the forces required to cause partial or total detachment of a heart valve prosthesis in patients with age-related degenerative diseases exposed to MRI.

METHODS

Eighteen tissue samples excised during routine heart valve replacement surgery were subjected to a suture pull-out test using a tensile materials testing machine. Five preconditioning cycles were applied before commencing the final destructive test. The test was complete when the sample ruptured and the suture was pulled completely free from the tissue. Results were compared with previously calculated magnetically induced forces at 4.7 T.

RESULTS

All tissue samples displayed a basic failure pattern. Mean forces required to cause initial yield and total rupture were 4.0 N (+/- 3.3 N) and 4.9 N (+/- 3.6 N), respectively. Significant factors determining initial yield were stenosed calcific tissue (p < .01), calcific degeneration (single pathology) (p < .04) and tissue stiffness (p < .01). Calcific degeneration (p < .03) and tissue stiffness (p < .03) were also significant in determining maximum force required to cause total rupture.

CONCLUSION

Specific age-related degenerative cardiac diseases stiffen and strengthen tissue resulting in significant forces being required to pull a suture through valve annulus tissue. These forces are significantly greater than magnetically induced < 4.7 T. Therefore, patients with degenerative valvular diseases are unlikely to be at risk of valve dehiscence during exposure to static magnetic field < or = 4.7 T.

Endothelial and Smooth Muscle Cell Seeding onto Processed Ex Vivo Arterial Scaffolds Using 3D Vascular Bioreactors

Biomaterials derived from ex vivo tissues offer a viable alternative to synthetic materials for organ replacement therapies. In this study, we describe the use of a tissue engineering scaffold derived from ex vivo arterial tissue to assess vascular cell adhesion within a three-dimensional perfusion bioreactor. With the aim of maximizing seeding efficiency, five methods for endothelial cell (EC) and three independent methods for vascular smooth muscle cell (VSMC) adhesion were explored. Seeded constructs were maintained in vascular bioreactors under pulsatile flow conditions, culminating at 165 ml/min at 1.33 Hz to validate cell attachment and retention over time. Progressive modification of the seeding and flow regime protocols resulted in an increased of EC retention from 5.1 to 634 cells/mm2. Seeding VSMCs as sheets rather than cell suspensions bound and stabilized surface EC matrix fibers, resulting in multiple cell layers adhered to the scaffold with cells migrating to the medial/adventitial boundary. In conjunction with the bioscaffold, the vascular perfusion system serves as a useful tool to analyze cell adhesion and retention by allowing controlled manipulation of seeding and perfusion conditions.

Resistance to tearing of calf and ostrich pericardium: Influence of the type of suture material and the direction of the suture line

The tearing of the valve leaflet of a cardiac bioprosthesis can cause early failure of this device, which is employed to replace a diseased native valve. This report involves the study of the behavior of 312 tissue samples (152 of calf pericardium and 160 of ostrich pericardium) treated with glutaraldehyde and subsequently subjected to tear testing. The samples were cut in the two principal directions: longitudinally, or root to apex, and transversely. They included a series of control samples that were left unsutured, and the remaining samples were repaired with the use of two different suture techniques: a running suture in the direction of the load and a telescoping suture perpendicular to the load. Four commercially available suture materials were employed: Pronova, nylon, Gore-Tex, or silk. The unsutured control samples of both types of pericardium exhibited a similar anisotropic behavior in the tear test. The mean resistance to tearing of the calf pericardium was 24.29 kN m in samples cut longitudinally and 34.78 kN m in those cut transversely (p =.03); the values were 28.08 kN m and 37.12 kN m (p =.002), respectively, in ostrich pericardium. The series repaired with the telescoping suture always exhibited greater resistance to tearing, with values that ranged between 44.34 and 64.27 kN for the samples of calf pericardium and from 41.65 to 47.65 kN for those obtained from ostrich. These assays confirm the anisotropic behavior of calf and ostrich pericardium treated with glutaraldehyde when subjected to tear testing, as well as the loss of this behavior in ostrich pericardium after suturing. Suturing techniques, such as the telescoping model, that provide a greater resistance to tearing should be studied for use in the design of the valve leaflets of cardiac bioprostheses made of biological materials.

Assessment of pericardium in cardiac bioprostheses. A review

Cardiac valve bioprostheses are assessed in terms of their present and future clinical utility. The problems concerning durability basically involve early failure due to tears in the valve leaflets and late failure mainly associated with calcification of the biological tissue. New strategies for selection and chemical treatment of the biomaterials employed are analyzed, and the available knowledge regarding their mechanical behavior is reviewed. It is concluded that the durability of these devices, and thus their successful use in the future, depends on the knowledge of the interactions among the different biomaterials of which they are composed, the development of new materials, and the engineering design applied in their construction.

Optimal bovine pericardial tissue selection sites. I. Fiber architecture and tissue thickness measurements

Use of bovine pericardium as an engineered biomaterial in the fabrication of bioprosthetic heart valves is limited, in part, by substantial intra- and intersac variations in its fibrous structure. To quantitatively assess this variability, we determined the fiber architecture of 20 whole BP sacs. Each sac was mounted on a prolate spheroidal mold, cleared and preserved in 100% glycerol, then sectioned into four equisized quadrants. This preparation method allowed for accurate intersac comparisons and minimized tissue distortions. The fiber architecture was evaluated by small-angle light scattering (SALS) using a 2.54-mm rectilinear grid resulting in approximately 1200 SALS measurements per quadrant, along with tissue thickness measured at 55 locations per quadrant. The fiber architecture was described in terms of fiber preferred directions, degree of orientation, and asymmetry of the fiber angular distribution. The BP sac fiber architecture demonstrated substantial intra- and intersac variability, with local fiber preferred directions changing by as much as 90 degrees within approximately 5 mm. Overall, most sacs revealed potential selection areas in the apex region characterized by a high degree of orientation, high uniformity in fiber preferred directions, and uniform tissue thickness. However, the size, location, and fiber orientation of these potential selection areas varied sufficiently from sac-to-sac to question whether anatomic location alone is sufficient for consistent localization of regions of high structural uniformity suitable for improved BHV design.

Effect of the suture on the durability of bovine pericardium used in cardiac bioprostheses

Our study of the different biomaterials used in the construction of biological cardiac prostheses has shown it to be of vital importance that the physical properties of the tissue and of the suture that anchors it to the rigid polymeric support are compatible. By means of dynamic tests, we have determined the fatigue curve in sutured bovine pericardial tissue, expressed by the equation log y = 1.27 +/- 0.18 (0.26 +/- 0.05) log t, where y is the initial fatigue stress (MPa) and t is the time (min) it takes to achieve permanent deformation of the tissue. By applying this correction, we determine a set of values for stress-time which, when compared with those obtained with a non-sutured sample, reveal a significant fall in this ratio and, thus, a decrease in the durability. The use of suture threads of lesser elasticity than the pericardium may play an important role in reducing the durability of the bioprosthesis constructed with these materials.

Mapping of glutaraldehyde-treated bovine pericardium and tissue selection for bioprosthetic heart valves

Glutaraldehyde-crosslinked bovine pericardium is widely used in bioprosthetic heart valve fabrication. In an attempt to set a scientific basis for more reproducible tissue selection, we produced and analyzed topographical maps of glutaraldehyde-treated bovine pericardium. Whole pericardia were divided into specific anatomical areas and their thickness was measured and mapped on templates. In each area, the suture holding power was determined in both parallel and perpendicular (to the base-apex line) directions; analyses of the tearing patterns in each fragment were used to evaluate predominant fiber orientation, and observations were confirmed by polarized light microscopy. Complete maps were superimposed graphically to aid in the selection of certain areas that would have known fiber orientation, high suture holding power, and suitable thickness. Our results describe regional heterogeneity of bovine pericardial structure and mechanical properties, specifically demonstrating variations in thickness, suture holding power, and collagen fiber orientation. Two areas of choice (representing about 35% of the total) were described as suitable for use in bioprosthetic heart valve fabrication.

Behaviour of the bovine pericardium used in cardiac bioprostheses when subjected to a real fatigue assay

The behaviour of bovine pericardium was studied using a fatigue assay. Twenty-three samples were assayed, maintaining the preset initial stress and measuring the time it took for the onset of load loss due to permanent deformation. The results indicated a mathematical relationship defined by the expression: log y = 1.3 - 0.211 log t, where y is the fatigue stress (MPa) and t the duration of the assay. The correlation coefficient was 0.948 (P < 0.001). The safety coefficient of the material diminished significantly as the period of time during which it was subjected to fatigue increased. The theoretical durability of the tissue was much greater than the real durability of the prostheses, which is determined by unsolved problems such as calcification and those derived from suture-related cutting.