Novel methods for the mechanical characterization of patches used in carotid artery repair.

Carotid endarterectomy (CEA) is one of the approaches available for the treatment of carotid artery disease, with carotid patch angioplasty the pertinent technique mostly preferred by vascular surgeons. This technique entails an arteriotomy succeeded by closure with a textile, polymer or biological tissue patch. In this work, we propose microbuckling and microindentation as novel methodologies for acquiring the mechanical properties of patches used in carotid artery repair. Regarding microbuckling, the patch is loaded by a sensitive dynamometer at one end and its motion is recorded, at three different levels of axial deformation: δ/ℓ = 0.1, 0.3 and 0.5 (in the post-buckling regime). The corresponding experimental loads are recorded, as well. Following pertinent closed-from equations, various material metrics are obtained, such as the Young's modulus of elasticity and the so-called frictional couple of the material. Regarding microindentation, the material's hardness number is measured with the aid of a durometer. Similar to microbuckling, indentation analytical expressions allow for the determination of key material properties, such as the modulus of elasticity, indentation forces and depths. Where possible, we perform microtension to verify acquired results. Results demonstrate that measured properties may vary substantially for materials which are of the same type, due to variations of the material microstructure, as observed with optical and scanning electron microscopes (SEM). Several commercial patches were tested in this work. To shortly present the main results, the microbuckling technique furnished (for the Young's modulus) 40.17 MPa for the B/Braun Aesculap cardiovascular patch and 71.49 MPa for the Vasutek Terumo, while the microindentation technique, for bovine patches, provided 6.356 MPa for the Xeno Sure and 4.701 MPa for the Vascu-Guard. A test type recommendation is provided, relating the type of the patch material to the method more plausible in each case, in order to achieve better measurement accuracy. Results of this study can contribute in establishing guidelines and criteria determining material selection in CEA.

Viscoelastic dynamic arterial response.

BACKGROUND: Arteries undergo large deformations under applied intraluminal pressure and may exhibit small hysteresis due to creep or relaxation process. The mechanical response of arteries depends, among others, on their topology along the arterial tree. Viscoelasticity of arterial tissues, which is the topic investigated in this study, is mainly a characteristic mechanical response of arteries that are located away from the heart and have increased smooth muscle cells content. METHODS: The arterial wall viscosity is simulated by adopting a generalized Maxwell model and the method of internal variables, as proposed by Bonet and Holzapfel et al. The total stresses consist of elastic long-term stresses and viscoelastic stresses, requiring an iterative procedure for their calculation. The cross-section of the artery is modeled as a circular ring, consisting of a single homogenized layer, under a time-varying blood pressure. Two different loading approximations for the aortic pressure vs time are considered. A novel numerical method is developed in order to solve the controlling integro-differential equation. RESULTS: A large number of numerical investigations are performed and typical response time-profiles are presented in pictorial form. Results suggest that the viscoelastic arterial response is mainly affected by the ratio of the relaxation time to the characteristic time of the response and by the pressure-time approximation. Numerical examples, based on data available in the literature, are conducted. CONCLUSIONS: The investigation presented in this study reveals the effect of each material parameter on the viscoelastic arterial response. Thus, a better understanding of the behavior of viscoelastic arteries is achieved.

The Effect of Strain Hardening on the Dynamic Response of Human Artery Segments.

BACKGROUND: When subjected to time-dependent blood pressure, human arteries undergo large deformations, exhibiting mainly nonlinear hyperelastic type of response. The mechanical response of arteries depends on the health of tissues that comprise the artery walls. Typically, healthy arteries exhibit convex strain hardening under tensile loads, atherosclerotic parts exhibit stiffer response, and aneurysmatic parts exhibit softening response. In reality, arterial dynamics is the dynamics of a propagating pulse, originating in heart ventricle, propagating along aorta, bifurcating, etc. Artery as a whole cannot be simulated as a lump ring, however its cross section can be simulated as a vibrating ring having a phase lag with respect to the other sections, creating a running pressure wave. A full mathematical model would require fluid-solid interaction modeling continuity of blood flow in a compliant vessel and a momentum equation. On the other hand, laboratory testing often uses small-length arteries, the response of which is covered by the present work. In this way, material properties that change along the artery length can be investigated. OBJECTIVE: The effect of strain hardening on the local dynamic response of human arteries (excluding the full fluid-structure interaction) is examined through appropriate hyperelastic models related to the health condition of the blood vessel. Furthermore, this work aims at constituting a basis for further investigation of the dynamic response of arteries accounting for viscosity. METHOD: The governing equation of motion is formulated for three different hyperelastic material behaviors, based on the constitutive law proposed by Skalak et al., Hariton, and Mooney-Rivlin, associated with the hardening behavior of healthy, atherosclerotic, and aneurysmatic arteries, respectively. The differences between these modelling implementations are caused by physiology, since aneurysmatic arteries are softer and often sclerotic arteries are stiffer than healthy arteries. The response is investigated by proper normalization of the involved material parameters of the arterial walls, geometry of the arteries, load histories, time effects, and pre-stressing. The effect of each problem parameter on the arterial response has been studied. The peak response of the artery segment is calculated in terms of radial displacements, principal elongations, principal stresses, and strain-energy density. The validity of the proposed analytical models is demonstrated through comparison with previous studies that investigate the dynamic response of arterial models. RESULTS: Important metrics that can be useful to vascular surgery are the radial deformation and the maximum strain-energy density along with the radial resonance frequencies. These metrics are found to be influenced heavily by the nonlinear strain-hardening characteristics of the model and the longitudinal pre-stressing. CONCLUSION: The proposed formulation permits a systematic and generalizable investigation, which, together with the low computational cost of analysis, makes it a valuable tool for calculating the response of healthy, atherosclerotic, and aneurysmatic arteries. The radial resonance frequencies can explain certain murmures developed in stenotic arteries.

Dynamic behavior of suture-anastomosed arteries and implications to vascular surgery operations.

BACKGROUND: Routine vascular surgery operations involve stitching of disconnected human arteries with themselves or with artificial grafts (arterial anastomosis). This study aims to extend current knowledge and provide better-substantiated understanding of the mechanics of end-to-end anastomosis through the development of an analytical model governing the dynamic behavior of the anastomotic region of two initially separated arteries. METHODS: The formulation accounts for the arterial axial-circumferential deformation coupling and suture-artery interaction. The proposed model captures the effects of the most important parameters, including the geometric and mechanical properties of artery and sutures, number of sutures, loading characteristics, longitudinal residual stresses, and suture pre-tensioning. RESULTS: Closed-form expressions are derived for the system response in terms of arterial radial displacement, anastomotic gap, suture tensile force, and embedding stress due to suture-artery contact interaction. Explicit objective functionalities are established to prevent failure at the anastomotic interface. CONCLUSIONS: The mathematical formulation reveals useful interrelations among the problem parameters, thus making the proposed model a valuable tool for the optimal selection of materials and improved functionality of the sutures. By virtue of their generality and directness of application, the findings of this study can ultimately form the basis for the development of vascular anastomosis guidelines pertaining to the prevention of post-surgery implications.