Influence of elastic nonlinearity on arterial anastomotic compliance

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.

Postprocedural peak systolic blood flow measurements correlate with the need for stent reintervention at 12 months

PURPOSE

Duplex surveillance of arterial stents has focused on detecting in-stent restenosis. Although velocity is commonly reported, that differs from blood flow, and patency of arterial prostheses is flow-dependent. Preliminary evaluation was performed to determine if postprocedure peak systolic blood flow (PSF) through stents correlates with rate of repeat intervention at 12 months.

MATERIALS AND METHODS

Retrospective review of consecutive patients undergoing arterial stent placement was performed. Demographics, comorbidities, stent size, postprocedure duplex information, and repeat intervention rates were recorded. PSF was calculated by using peak systolic velocity (PSV) and stent dimensions.

RESULTS

Consecutive stents (N = 35) were placed in 27 patients (mean age, 72.6 y ± 14). Twenty stents were free from repeat intervention (FR) and 15 required repeat intervention (RR) at 12 months. There was a significant difference between FR and RR groups with respect to initial in-stent PSV and PSF (92.5 cm/s for FR vs 43.7 cm/s for RR [P < .002]; 1,918 mL/min for FR vs 722 mL/min for RR [P < .0001]). PSF showed sensitivity, specificity, and accuracy rates of 92%, 82%, and 86.2%, respectively, for predicting repeat intervention, versus 83%, 71%, and 76% for PSV. Receiver operating characteristic curve analysis showed a greater area under the curve for PSF versus PSV (0.965 vs 0.859).

CONCLUSIONS

PSF from an initial postprocedure duplex study accurately correlates with need for repeat stent intervention at 12 months. PSV had a lower sensitivity, specificity, and accuracy. This preliminary finding must be confirmed by prospective studies in individual vascular beds and larger patient populations. A new approach to stent surveillance is suggested.

Mechanical analysis of end-to-end silk-sutured anastomosis for robot-assisted surgery

BACKGROUND

Robot-assisted anastomosis holds great promise for the future. To secure surgery quality, some key process factors, such as the force arrangement of sutures, should be provided because of the lack of haptic feedback in robotics systems

METHODS

A model of anastomosis is presented to establish the mechanical relationship between vessel and sutures. Stress distribution of the vessel loaded by the suture was then achieved through finite-element simulations, based on the material property test results. Further, experiments were performed to validate the reliability of the FEM simulation of the anastomosis process.

RESULTS

To avoid blood osmosis, the allowable lower limit of the suture tension was 0.05 N. To keep the tissue free from injury, the allowable upper limit of tension on the suture was 0.4 N.

CONCLUSIONS

The study provided meaningful results for directing the robot-assisted anastomosis procedure and design of the surgical tools.

Mechanical analysis of a prototype of small diameter vascular prosthesis: numerical simulations

This paper concerns a mechanical analysis of a prototype of a small diameter vascular prosthesis made of a fibre reinforcement silicone material. The theoretical approach is carried out for a neoHookean strain energy function augmented with unidirectional reinforcing that is characterized by a single additional constitutive parameter for strength of reinforcement. Numerical simulations based on a finite element model compare the compliance of different grafts and predict the degree of the compliance mismatch in an anastomosis between native artery and vascular prosthesis. Furthermore, specific applied strains on the prototype, viewed as arising surgical manipulation and implying telescopic shear have been simulated. Thus, for different fibre reinforcements, the stress gradient through the wall of the tubular structure is evaluated.

Investigation of 3-D mechanical properties of blood vessels using a new in vitro tests system: results on sheep common carotid arteries

In order to investigate the three-dimensional (3-D) mechanical properties of blood vessels, a new experimental device is described allowing in vitro static and dynamic measurements on segments of arteries with high technical performances. Static tests are applied to sheep common carotid arteries. Considering a thick-walled cylindrical model of orthotropic material under large deformations, a classical 3-D approach based on strain energy density is used to calculate the resulting mechanical behavior law in radial and circumferencial directions and stresses distribution throughout the wall thickness. Results are presented with reference to unloaded and zero-stress initial state thanks to simple measurements of inner and outer circumferences. A particular ratio relating the two main stresses (circumferential and longitudinal) is calculated that put into the forth the progressive modifications in the direction of the predominant stress in the wall and the specific radial location where these changes occur. We observe that this point location is a function of the test conditions of the specimen, i.e., stretching length and level of pressure.

Vascular response in the femoropopliteal segment after implantation of an ePTFE balloon-expandable endovascular graft: an intravascular ultrasound study

PURPOSE

To use intravascular ultrasound (IVUS) to document changes in vascular dimensions after placement of a balloon-expandable endograft.

METHODS

Thirteen patients (9 men; mean age 62 years, range 47-75) treated with an investigational polytetrafluoroethylene endograft for obstructive disease of the femoropopliteal segment were studied with IVUS immediately after endograft implantation and at follow-up. Corresponding IVUS cross sections were analyzed for changes in lumen, vessel, and plaque areas seen inside the endograft, in the anastomotic segment, and in the remote arterial segment.

RESULTS

A mean 6-month (range 1.5-9) follow-up was completed in 12 patients. Matched IVUS cross sections derived from within the endograft (n = 12) and at the endograft edges (n = 23) showed no change in lumen area (LA) in 17, reduction in 11, and dilatation in 7. Median changes within the endograft (+3%) were not significant (p = 0.28) and no neointima was found. Cross sections obtained at the anastomotic segment revealed a significant increase in LA (85%, p < 0.001), which was associated with a significant increase in both vessel area (VA) (42%, p < 0.001) and plaque area (PLA) (15%, p = 0.003) area. In the remote arterial segment, the change in LA was minimal (6%, p = 0.07), as were changes in the VA (9%, p = 0.04) and PLA (10%, p = 0.07).

CONCLUSIONS

Following endograft placement, luminal changes within the endograft, at the endograft edges, and at the remote arterial segments were minimal. Intimal hyperplasia was not observed in the endograft. The distinct LA increase at the anastomotic segments was determined by the extent of VA and PLA change.

Quantification of the fiber architecture and biaxial mechanical behavior of porcine intestinal submucosa

Porcine small intestinal submucosa (SIS) has been shown to serve as a remodelable tissue scaffold in a wide range of applications. Despite the large number of experimental studies, there is a lack of fundamental information on SIS anisotropic mechanical behavior and how this behavior changes postimplantation. As a first step in our study of remodeling biomaterials, we performed biaxial mechanical testing to quantify the anisotropic mechanical behavior and used small-angle light scattering (SALS) to quantify the gross fiber structure of fresh, unimplanted SIS. Structural results indicate that SIS displays primarily a single, continuous preferred fiber direction oriented parallel to the long axis of the intestine. Occasionally, two distinct fiber populations oriented at approximately +/-28 degrees with respect to the longitudinal axis could be distinguished. Consistent with this structure, SIS exhibited a nonlinear, anisotropic mechanical response with higher stresses along the longitudinal axis. Further, the circumferential stress-strain response was strongly affected by the maximum longitudinal strain level, but the maximum circumferential strain level only weakly affected the longitudinal stress-strain response. This asymmetric mechanical coupling suggests strong mechanical interactions on a fiber level. SIS stress-strain response also was similar to glutaraldehyde-treated bovine pericardium, attesting to the substantial strength of SIS in the fresh, untreated state. The results of this study will provide a basis for a future analysis of the structural and mechanical changes during the remodeling process.