Consideration of suture line stresses in the selection of synthetic grafts for implantation

Electrospun fibroin/polyurethane hybrid meshes: Manufacturing, characterization, and potentialities as substrates for haemodialysis arteriovenous grafts

Several attempts made so far to combine silk fibroin and polyurethane, in order to prepare scaffolds encompassing the bioactivity of the former with the elasticity of the latter, suffer from critical drawbacks concerning industrial and clinical applicability (e.g., separation of phases upon processing, use of solvents unaddressed by the European Pharmacopoeia, and use of degradable polyurethanes). Overcoming these limitations, in this study, we report the successful blending of regenerated silk fibroin with a medical-grade, non-degradable polyurethane using formic acid and dichloromethane, and the manufacturing of hybrid, semi-degradable electrospun tubular meshes with different ratios of the two materials. Physicochemical analyses demonstrated the maintenance of the characteristic features of fibroin and polyurethane upon solubilization, blending, electrospinning, and postprocessing with ethanol or methanol. Envisioning their possible application as semidegradable substrates for haemodialysis arteriovenous grafts, tubular meshes were further characterized, showing submicrometric fibrous morphologies, tunable mechanical properties, permeability before and after puncture in the same order of magnitude as commercial grafts currently used in the clinics. Results demonstrate the potential of this material for the development of hybrid, new-generation vascular grafts with disruptive potential in the field of in situ tissue engineering. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 807-817, 2019.

Changing Etiology of Anastomotic Aneurysms

Anastomotic aneurysms in patients at the Seattle Veterans Administration Medical Center between 1974-1979 were analyzed to determine their etiol ogy. Thirty-nine aneurysms occurred in 29 patients with 19 aneurysms presenting in 15 patients who had vascular reconstruction at our institution. The site most commonly involved was the femoral anastomosis (5%/anas tomosis) followed by the iliac (2%) and the aortic (0.5%) anastomoses. Mechanical forces appear to be the most common cause (76%) followed by graft infection (23%) with suture failure being very uncommon (2%) Histo logic examination of the aneurysms thought to be produced by mechanical forces disclosed normal host artery wall in 1 group and aneurysmal degen erative change in the remainder. The generation of disruptive forces at anastomoses between compliant and stiff materials and their role in produc ing anastomotic disease is discussed. A disturbing feature is the incidence of commensal infections seen previously in other prosthetic surgery.

False Anastomotic Aneurysms

This retrospective study covers the period from 1991 to 2002, during which 3,623 patients were operated on because of aneurysmal or occlusive disease of aortoiliac and femoropopliteal segments. Among them, 87 patients (2.4%) developed a false anastomotic aneurysm in the 12-year follow-up period and were treated operatively.

Most frequently, in 53 patients (6.9%), a false anastomotic aneurysm developed after aortobifemoral bypass performed owing to aortoiliac occlusive disease. The cause of false anastomotic aneurysm was infection in 21 cases (24.7%); resection and revascularization were performed with a Dacron graft in 46 cases (52.9%), with a polytetrafluoroethylene graft in 10 cases (11.5%), and with the great saphenous vein in 16 cases (18.4%). Homograft implantation in 4 patients (4.6%) or extra-anatomic bypasses in 11 cases (12.6%) were performed when graft infection was suspected. Of 87 patients who underwent surgery, 74 (85.5%) had good early results without infection, reintervention, limb loss, and mortality.

The presence of infection as a cause of false anastomotic aneurysm and comorbidity increased the mortality rate significantly after the reoperation, whereas the type of graft used in treatment had no influence on early results.

Design and fabrication of a mechanically matched vascular graft

The study provides a pathway to design a mechanics-matching vascular graft for an end-to-end anastomosis to a host artery. For functional equivalence, we submit that the graft and a host artery should have equal inner deformed diameters, equal pressure-radius module, and experience equal axial forces when subjected to mean arterial pressure. These criteria for mechanical equivalence are valid for a large class of materials that can be considered as elastic incompressible and orthotropic solids. As an example, specific known artery properties were used to design or select a graft made from a new synthetic biomaterial to demonstrate that reliable and robust technology for graft fabrication is possible.

OPTIMAL DESIGN OF ARTERIAL BYPASS PROSTHESES DIAMETER

The objectives of this paper are: (1) to analyze how the bypass diameter and the degree of the stenosis affect the flow-induced wall shear stresses in the stenotic and bypass vessels and (2) to propose a clinically usable method for a proper selection of the bypass diameter. The problem is solved in the following stages: First, the steady pressure and flow within the vessels are obtained using general laws of mass and energy conservation. Then, on the basis of modified Womersley's theory to take into account the nonlinear mechanical properties of the vessels, the oscillating pressure and flow within the vessels are calculated. They are used to derive expressions for the reflection coefficients at the distal (converging) bifurcation and to calculate wall shear stresses.

We hypothesize that the minimal differences in the wall shear stresses of the normal artery, stenotic artery and the bypass graft mitigate the development of anastomotic neointimal hyperplasia and graft pseudo intima. Using cost functions, based on steady and mean wall shear stresses, we calculated the optimal value of the undeformed bypass diameter for various degrees of stenosis. We found that the sum of cross-sectional areas of the optimal graft and the stenotic artery (normalized with respect to the cross-sectional area of the normal artery) varies between 1.14 and 1.28, an interval, which is consistent with the findings of McDonald (1974) for the area of the major branches in the body. Thus we conclude that the results obtained are consistent with the stated hypothesis.

STRESS CONCENTRATION DUE TO ADVANCING HEART PULSE THROUGH A BLOOD VESSEL JOINT

The effect of a vessel joint of different properties on the heart pulse propagation and anastomosis junction is investigated. The system under consideration is modelled by an elastic piecewise-constant vessel consisting of two perfectly jointed vessels of different thicknesses and radii. A mathematical model is developed, to perform numerical calculations and to obtain some qualitative and quantitative estimations. The effect of different thicknesses and radii of jointed blood vessels on the concentrations of pressure, radial vessel wall displacement, bending moments and shear forces are analyzed in detail.

[Anastomotic pseudoaneurysms]

Anastomotic pseudoaneurysm is a form of false aneurysm, whose wall does not consist of all normal layers of arterial wall. Given the rising number of reconstructive vascular procedures, the increase of anastomotic pseudoaneurysm cases is expected. Therefore, identification of causes, clinical manifestations as well as factors which affect the outcome of operative treatment of anastomotic pseudoaneurysms is of great practical value. This retrospectively-prospective study included 87 surgically treated cases of anastomotic pseudoaneurysms in the period from 1991 to 2002. The most often localization of anastomotic pseudoaneurysms was the inguinal region (68-86.2%). In the majority of cases, they were caused by arterial degeneration in the anastomotic region--56 cases (65.9%) and infection--21 cases (24.7%). The most frequent manifestations of anastomotic pseudoaneurysms were bleeding due to rupture in 26 cases (29.9%) and chronic limb ischaemia in 22 cases (25.3%). An acute limb ischaemia was present in 17 cases (19.5%), the symptoms caused by local compression to the surrounding structures--in 9 cases (10.3%), and in 12 cases (13.8%), the only manifestation of anastomotic pseudoaneurysm was asymptomatic pulsatile mass. In 32 cases (36.8%), surgical treatment involved the resection of anastomotic pseudoaneurysm and graft interposition, whereas in 39 cases (44.8%), bypass procedure had to be performed after the resection. Comorbidity significantly increased mortality in the first 30 days. The use of Dacron graft in primary operation significantly improved early results of operative treatment. Absence of infection as the cause of anastomotic pseudoaneurysm is a statistically important prognostic factor of operative treatment, considering the graft patency, limb salvage, infection, need for reintervention and mortality.

Scholarly review of geometry and compliance: biomechanical perspectives on vascular injury and healing

Mechanical stress and strain upon cardiovascular tissue are important factors that influence the ultimate configuration of clinically observed disease entities. Although mechanical forces can stimulate cellular changes and response, structural or geometric alterations introduced by disease processes can, in turn, influence local hemodynamic conditions. Dynamic interactions of structural parameters, such as arterial compliance and geometry, can further contribute to the final determination of the mechanical conditions and outcome of the vessel. Manipulation of vascular compliance and geometry may, therefore, have desirable effects. In this article, fundamental vascular biomechanical forces are defined and their association with cellular response and clinical disease processes are introduced. The interplay between vascular geometry and compliance is emphasized, and the potential for mechanical solutions to vascular diseases are explored.

A model of stress-induced geometrical remodeling of vessel segments adjacent to stents and artery/graft anastomoses

The mismatch between the elastic properties and initial geometry of a host artery and an implanted stent or graft cause significant stress concentration at the zones close to junctions. This may contribute to the often observed intimal hyperplasia, resulting in late lumen loss and eventual restenosis. This study proposes a mathematical model for stress-induced thickening of the arterial wall at the zones close to an implanted stent or graft. The host artery was considered initially as a cylindrical shell with constant thickness that was clamped to the stent or graft, which was assumed to be non-deformable in the circumferential direction. It was assumed that the abnormal circumferential and axial stresses due to the bending of the arterial wall cause wall thickening that tends to restore the stress state close to that existing far from the junction. The linear equations of a cylindrical shell with variable thickness were coupled to an evolution equation for the wall thickness. These equations were solved numerically and a parametric study was performed using finite difference method and explicit time step. The results show that the remodeling process is self-limiting and leads to local thickening that gradually decreases with distance from the edge of the stent/graft. Model predictions were tested against morphological findings existing in the literature. Recommendations on stent designs that reduce stress concentrations are discussed.

Porous polyurethane tubes as vascular graft

A vascular graft with the inner diameter of about 3 mm was prepared from segmented poly (ether urethane) with an extrusion technique. To make the wall of the vascular grafts porous, NaCl salts were added to the polyurethane solution to be extruded and removed with water extraction after evaporating the solvent in the extruded tube. The wall was reinforced with elastic fiber to prevent dilation. The compliance of the vascular graft measured with the method of Hayashi et al. ranged from 0.2 to 0.3% mmHg -1. The initial Young's modulus was close to that of canine carotic artery, to which the porous polyurethane graft 4-cm long was anastomosed. Vascular grafts were occluded within 2 weeks after implantation, when their pore size was 0, 1.7, or 4.4 mum, whereas those with the pore size of 5.5, 7.4, and 30 mum were patent for longer than 4 weeks. When the vascular graft with the pore size of 30 mum was implanted for 6 months, the luminal surface was covered with neointima, but the endothelium-like cells appearing in the middle of the intima of the vascular graft were immature and sometimes had a very big nucleus. In addition, spindle-shaped, modified smooth muscle cells were noticed in the deep layer of the neointima, especially in the tissue where anastomotic intimal hyperplasia occurred.

End-stage renal disease (ESRD) and vascular access grafting: a critical review

End-Stage Renal Disease (ESRD) is a major disease state, costing the U.S. $9.5 billion in 1992, and increasing 10% yearly. The growth in the number of ESRD patients can be attributed principally to demographic trends: the aging of the general population and the improved treatment and increased survival rate of patients with diabetes, hypertension, and other illnesses that lead to ESRD. Moreover, improved dialysis technology has enabled older patients and those who previously could not tolerate dialysis due to other illnesses to benefit from this treatment. Three modalities exist for the treatment of ESRD: hemodialysis, peritoneal dialysis, and kidney transplant. This article reviews the medical treatments and the synthetic polymers used in the manufacture of vascular access grafts. We report on the development of a new, polyurethane-based microporous vascular graft, which displays self-sealing and improved compliance characteristics for use in vascular access grafting.

Stress reduction by geometric compliance matching at vascular graft anastomoses

An analysis is presented of stresses developed with different junctional configurations of end-to-end vascular graft anastomoses with severe compliance mismatch. Junctions of circular transverse sections and junctions by bias cuts (beveled ends) are compared with an anastomosis of a graft constructed with an elliptical transverse section and a bevel end cut vessel. This latter substitutes midwall inextensional deformations for extensional deformations that occur with the former, conventional configurations. Applications to end-to-side anastomoses are also discussed. A range of parameters are considered: i.e., vascular wall thickness/radius ratios between 0.1 and 0.5, locations from the anastomotic plane between 1/2 and 3/2 times the vascular wall thickness, host vessel axial stretch by external forces between 0 and 15%, maximal vascular circumferential stretch distal from the anastomosis between 0 and 25%, and perimeter locations at the anastomotic junction between 0 degree and 90 degrees. The graft constructed with an elliptic cross-section developed peak stresses that are orders of magnitude lower than those developed with conventional configurations. The introduction of matching geometric compliance that dominates at the anastomotic junction minimizes consequences of material mismatch between graft and vessel and has the potential to reduce suture line stress greatly. This analysis may suggest designs for experimental studies to confirm relationships between neointimal hyperplasia and suture line stress levels, and provide a relatively simple solution for reduction of such stresses at the anastomotic junction. Choices may be permitted of graft materials with optimal surface properties despite less favorable elastic properties.

Biomechanics of human common carotid artery and design of novel hybrid textile compliant vascular grafts

The mechanical properties and structure of a human common carotid artery were studied in order to develop criteria for designing and manufacturing compliant textile vascular grafts. The arterial wall comprised a composite of elastin and collagen fibers with the collagen fibers crimped. This structure led to a unique pressure-circumferential stretch ratio curve, the slope of which increased with an increase in strain. The increase in slope was particularly rapid at a stretch ratio above 1.4 or pressure above 120 mmHg. Based on the knowledge gained, a criteria for the design of biomechanically compliant arterial grafts was developed. An elastomeric prestretched polyurethane monofilament yarn with a low modulus of elasticity and a bulked polyester multifilament yarn with a high modulus of elasticity were combined and used as threads in the manufacture of grafts. Tubular structures of diameters in the range 4-6 mm were made by weaving. Transverse compliance and morphological and permeability properties of these grafts were determined and compared with those of a currently available woven commercial grafts and human carotid arteries. Results indicated that the compliance values of the hybrid grafts were comparable with those of the human carotid artery. Preliminary in vivo studies in dogs showed promising results: a thin, stable neointima developed within 6 months of implantation on the flow surface.

Compliance and diameter mismatch affect the wall shear rate distribution near an end-to-end anastomosis

The development of intimal hyperplasia near the anastomosis of a vascular graft to an artery may be related to changes in the wall shear rate distribution. Mismatches in compliance and diameter at the end-to-end anastomosis of a compliant artery and a rigid graft cause shear rate disturbances that may induce intimal hyperplasia and ultimately graft failure. The goal of this study is to determine how compliance mismatch, diameter mismatch, and impedance phase angle affect the wall shear rate distribution in end-to-end anastomosis models under sinusoidal flow conditions. Wall shear rates are obtained through flow visualization using a photochromic dye. In a model with a well-matched graft diameter (6% undersized), the compliance mismatch causes low mean wall shear rates near the distal anastomosis. Considering diameter mismatch, the wall shear rate distributions in 6% undersized, 16% undersized, and 13% oversized graft models are markedly different at similar phase angles. In the two undersized graft models, the minimum mean shear rate occurs near the distal anastomosis, and this minimum is lower in the model with greater diameter mismatch. The oversized graft model has a minimum mean shear rate near the proximal anastomosis. Thus in all three models, the minimum mean wall shear rate is observed at the site of the divergent geometry. The impedance phase angle, which can be altered by disease states and vasoactive drugs, has a minor effect on the wall shear rate amplitude far from the anastomosis but a more pronounced effect closer to the anastomosis. Mean wall shear rates under sinusoidal flow conditions are significantly lower than under steady flow conditions at the same mean flow rate, but they are fairly insensitive to phase angle changes. In order to avoid the divergent geometry that may cause lower wall shear rates, we recommend that compliance mismatch be minimized whenever possible and that graft diameter be chosen to match the arterial diameter at the relevant physiologic pressure, not at the reduced pressure present when the graft is implanted.

Early development of suture holding capacity in vascular anastomoses in the rat

Early development of suture holding capacity (SHC) of end-to-end anastomoses in the infrarenal aorta and inferior vena cava (IVC) of the rat was studied by measuring the resistance to tensile forces at 0, 3, 5, 8, 11, or 14 days postoperatively. Comparable anastomoses were histologically evaluated. The SHC of the aortic wall decreased following anastomoses and did not regain the strength of nonanastomosed vessels during the first two weeks. The SHC of the IVC also decreased following anastomoses, but regained the strength of nonanastomosed vessels by the 5th day. The relative ultimate strain of the arterial and venous perianastomotic walls decreased slightly, while the relative compliance increased in the aorta and decreased in the IVC. Perianastomotic media necrosis was found exclusively in the arteries. The reduced perianastomotic SHC in the aorta indicates impaired healing, compared to the IVC. Changes in biomechanical properties of the aortic wall following anastomosis may increase its susceptibility to anastomotic complications such as pseudoaneurysms.

In vivo determination of human arterial compliance: preliminary investigation of a new technique

PURPOSE

To obtain more detailed information about the dynamic mechanical properties of human arteries in vivo, using a new technique based on intravascular ultrasound. (IVUS).

METHODS

Arterial compliance was measured in the common and/or external iliac arteries of 6 patients using an IVUS device, concurrently obtained intraarterial pressure measurements, and a video motion analysis system.

RESULTS

Compliance decreased with increasing vessel diameter and mean arterial pressure. Pre- and postangioplasty measurements were obtained in 2 patients but did not demonstrate a consistent change in compliance following angioplasty.

CONCLUSION

We conclude that IVUS provides an accurate means for in vivo determination of human arterial compliance, the practical value of which needs to be established.

Abnormal wall strain at distal end-to-side anastomoses

Cyclic stretch has been demonstrated to induce proliferative and secretory activities by cultured arterial endothelial and smooth muscle cells, cellular processes that contribute to the development of intimal hyperplasia. A model of an end-to-side anastomosis was developed to examine the hypothesis that regions of the artery at such anastomoses are subjected to focally increased cyclic stretch, which may stimulate the development of intimal hyperplasia. Polytetrafluoroethylene grafts were anastomosed end to side to latex rubber tubes that have elastic properties similar to those of the human femoral artery. Pulse waves with physiologic pressure, rate, and contour were applied, and systolic and diastolic diameters were measured in two planes at longitudinal intervals. Circumferential strain imposed on the latex "artery" was calculated at each interval. Strain imposed perpendicular to the suture line was also measured. Circumferential strain was consistently maximal at a distinct region of the "artery" along the proximal third of the anastomosis (6.0 +/- 1.1% vs. 3.3 +/- 0.5% at other regions of the "artery"). The maximal strain across the suture line was found at precisely the same region (3.9 +/- 0.3% vs. 2.0 +/- 0.4%). The anastomotic region of the recipient artery in a distal end-to-side anastomosis is subjected to cyclic circumferential strains two times greater than those experienced by the remainder of the artery. This corresponds to a common location of intimal hyperplasia. Such strains may be a stimulus for intimal hyperplasia.

Laser and suture anastomosis: passive compliance and active force production

One carotid from each dog underwent a laser anastomosis and the other carotid an interrupted suture repair. One or eight weeks later (n greater than or equal to 4 dogs/time period); four rings (1 mm in length) containing the laser or suture anastomosis or the normal artery (two/dog) were removed. Using a photoelectric force transducer and lever system, the ring was stretched in increments and passive force measured. At each length, the arterial muscle was stimulated and active force measured. The mean laser and control passive length/tension (L/T) curves were not different (P greater than 0.05), but the suture curve was shifted downward (P less than 0.05). The mean laser and suture active L/T curves were similar at 1 week (greater than 0.05) and lower than the control curve (P less than 0.04). At 8 weeks, the laser-repaired vessels produced an active force similar to control muscle (P greater than 0.05) but the suture repairs could not generate this active force (P less than 0.05). These data suggest that the laser repair and normal artery are more mechanically compatible than the suture repair as studied by this method.

New arterial prostheses by filament winding

A new manufacturing method for vascular grafts, based on a filament winding technique, is unveiled. The concept that pilots this method is presented and analysed in detail alongside the experimental results. A basic feature of filament winding is its ability to produce a two-phase structure built of a continuous fibre-reinforced polymeric matrix, shaped according to the shape of a mandrel. This structure offers a number of advantages over common vascular graft designs, e.g. better control of the mechanical properties and closer match with anisotropic properties of native arteries, and more degrees of design freedom with respect to pore size, biodegradability and biocompatibility. The experimental section offers a range of potential constituent materials, and presents an example of a Lycra fibre/Pellethane matrix prototypical prosthesis.

Recurrent femoral anastomotic aneurysms. A 30-year experience

Of the 1771 patients who underwent aortofemoral bypass grafting (AFB) during the 30-year period of 1957-1986, 43 noninfected recurrent femoral anastomotic aneurysms (RFAA) developed in 28 patients. Thirty-six RFAAs were treated surgically, with one death and no amputations occurring. Seven small RFAAs (less than 2.0 cm) were treated expectantly without complications. Using univariate and multivariate analyses, clinical characteristics and other factors influencing results in patients with RFAAs were compared to two control groups: patients who had undergone AFB without the development of femoral anastomotic aneurysms (FAAs) and patients who had undergone FAA repairs but without recurrence of FAA. Comparative analyses suggested: 1) local wound complications after initial AFB or FAA repair increased risk of a RFAA (p less than 0.03); 2) development of an FAA within 4.5 years after AFB increased risk of a RFAA (p less than 0.0002); 3) following an FAA repair, risk of a RFAA was almost three times greater for women than for men (p less than 0.05); and 4) patients with arteriosclerotic heart disease (ASHD) were less likely to develop RFAA than those without ASHD (p less than 0.05). Among the 20 additional variables analyzed--including hypertension, smoking, diabetes mellitus, and etiology of primary vascular disease--no statistically significant influence on the development of RFAAs could be detected.

Effect of healing on compliance of porous Dacron grafts

The effects of tissue ingrowth on the compliance of porous Dacron grafts were studied in 8 mm external-velour warp-knit Dacron grafts implanted in the canine descending thoracic aorta. The compliance of these grafts was measured before implantation and after implant periods of two weeks and one, two, four, and six months. Compliance, which initially was 0.043 +/- 0.008% mm Hg, decreased less than 10% after implantation (no statistical difference at p less than 0.05). No anastomotic neointimal hyperplasia was observed. Although the total thickness of the graft wall increased two- to three-fold after implantation, the benign healing process observed in this study had very little influence on graft compliance followed to six months.

Experimental determination of mechanical shear stress about an anastomotic junction

The present study is undertaken to determine whether the elastic tube model originally developed by Kuchar and Ostrach (Biomedical Fluid Mechanics Symposium, pp. 45-69, 1966) accurately provides a first approximation of the biomechanics of the anastomotic junction. The experimental protocol involves the use of canine carotid arteries as the host vessel and several graft materials including autogenous and prosthetic substitutes. The host artery-graft combinations are perfused in vitro in a pulsatile perfusion apparatus which simulates the natural hemodynamic environment. This apparatus provides accurate dynamic measurements of radial wall motion (measured at various longitudinal increments), associated pressures and rates of fluid flow. These data are then applied to the theoretical model for calculation of anastomotic induced bending stresses. The results indicate that the predictions derived from the elastic model consistently overestimate the measured radial change adjacent to the anastomotic junction. As a result shear stresses based on elastic theory deviate from values derived from a numerical curve fit to the experimental data.

Anisotropic tensile viscoelastic properties of vascular graft materials tested at low strain rates

Mechanical matching of vascular grafts and host vessels has been suggested to be important in determining graft patency rates. In this context, we have examined the anisotropic viscoelastic properties of natural vessels and some synthetic replacements using low strain rate tensile testing of circumferential and longitudinal strips. The canine iliac artery and iliac vein were compared with 6 mm diameter woven and knitted Dacron grafts, expanded polytetrafluoroethylene (PTFE) grafts, and helically constructed prototype polyurethane grafts at wrap angles of 45, 60 and 75 degrees. A thick-walled pressure vessel analysis was used to approximate physiological stress levels, and SEM was used to correlate anisotropic properties with graft wall structure.

[Vascular replacement of small caliber arteries--a new bovine collagen prosthesis]

A new bovine collagenic prosthesis (Solcograft P) was tested in animal experiments. In 45 dogs the infrarenal abdominal aorta was substituted. During a follow-up period of up to 18 months the functional rate amounted to 88%. Joint-crossing iliaco-femoral bypasses were implanted in 31 dogs, 5 times in both sides. For the new bovine collagenic graft the functional rate ran up to 83%, during a follow-up period of 9 months. A wound infection was observed twice in the inguinal region thus exacting the removal of the prosthesis. An aneurysm was not observed. In our opinion the satisfying results are due to the compliance of the new prosthesis which is similar to natural artery.