Experimental investigation of the stent-artery interaction.

It is well acknowledged that stent implantation causes abnormal stretch and strains on the arterial wall, which contribute to the formation and progression of restenosis. However, the experimental characterization of the strain field on the stented vessel is scant. In this work, the balloon-expandable stent implantation inside an artery analogue was captured through two high-speed CCD cameras. The surface strain maps on the stented tube were quantified with a 3-D digital image correlation technique. The strain history at one specific reference point illustrated three stenting phases, including balloon inflation, pressurization and deflation. The surface strain distributions along one axial path were obtained at various time points to demonstrate the stent-vessel interactions. The radial wall thickness reduction history was used to evaluate the pressure-diameter relationship for the balloon. Results indicated that the expansion process of the balloon was significantly altered by the external loadings from both the stent and artery analogue. In addition, the repeatability of the stenting experiments was demonstrated through two tests with a change of 5% in the stent-induced maximum first principal strain. Moreover, a computational model of the stenting procedure was developed to recapture the stenting experiments. Comparison between experiments and simulation showed a difference of 7.17% in the first principal strain averaged over the high strain area. This indicated the validation of the computational framework, which can be used to investigate the strain or stress field throughout the computational domain, a feature that is not affected by experimental techniques.

On the importance of modeling stent procedure for predicting arterial mechanics.

The stent-artery interactions have been increasingly studied using the finite element method for better understanding of the biomechanical environment changes on the artery and its implications. However, the deployment of balloon-expandable stents was generally simplified without considering the balloon-stent interactions, the initial crimping process of the stent, its overexpansion routinely used in the clinical practice, or its recoil process. In this work, the stenting procedure was mimicked by incorporating all the above-mentioned simplifications. The impact of various simplifications on the stent-induced arterial stresses was systematically investigated. The plastic strain history of stent and its resulted geometrical variations, as well as arterial mechanics were quantified and compared. Results showed the model without considering the stent crimping process underestimating the minimum stent diameter by 17.2%, and overestimating the maximum radial recoil by 144%. It was also suggested that overexpansion resulted in a larger stent diameter, but a greater radial recoil ratio and larger intimal area with high stress were also obtained along with the increase in degree of overexpansion.

Performance of self-expanding nitinol stent in a curved artery: impact of stent length and deployment orientation.

The primary aim of this work was to investigate the performance of self-expanding Nitinol stents in a curved artery through finite element analysis. The interaction between a PROTÉGÉ™ GPS™ self-expanding Nitinol stent and a stenosed artery, as well as a sheath, was characterized in terms of acute lumen gain, stent underexpansion, incomplete stent apposition, and tissue prolapse. The clinical implications of these parameters were discussed. The impact of stent deployment orientation and the stent length on the arterial wall stress distribution were evaluated. It was found that the maximum principal stress increased by 17.46%, when the deployment orientation of stent was varied at a 5 deg angle. A longer stent led to an increased contact pressure between stent and underlying tissue, which might alleviate the stent migration. However, it also caused a severe hinge effect and arterial stress concentration correspondingly, which might aggravate neointimal hyperplasia. The fundamental understanding of the behavior of a self-expanding stent and its clinical implications will facilitate a better device design.

Robotically assisted perventricular closure of perimembranous ventricular septal defects: preliminary results in Yucatan pigs.

OBJECTIVE: Robotic systems allow surgeons to perform minimally invasive cardiac surgery in adults. Experience in the pediatric population, however, is limited. Perventricular closure of muscular ventricular septal defects has been reported in humans but requires a median sternotomy. The objective of this study was to assess the feasibility of robotically assisted closure of perimembranous ventricular septal defects by using the perventricular approach. METHODS: The procedure was attempted in 7 pigs with naturally occurring perimembranous ventricular septal defects. Echocardiography was performed to confirm the presence and assess the size of the defect. A 3-armed da Vinci system consisting of two 8-mm instrument ports and a 12-mm endoscopy port was used. A pericardiotomy was performed, and the right ventricular free wall was visualized. A spinal needle was advanced into the right ventricular cavity. By using echocardiographic guidance, a glide wire was advanced through the angiocatheter and manipulated through the defect into the left ventricle or the ascending aorta. A delivery sheath was advanced over the wire. An appropriately sized Amplatzer device was deployed through the sheath. RESULTS: The procedure was successful in 5 pigs. One device was removed because it was smaller than the defect and an appropriately sized device was not available. The placement failed in the second pig in the series. Four pigs were followed up for 1 to 4 months. Angiograms performed before the pigs were killed documented complete occlusion in 3 and mild-to-moderate shunt in 1. CONCLUSIONS: Robotically assisted perventricular closure with the Amplatzer Membranous VSD Occluder is feasible. This approach avoids the associated morbidities of cardiopulmonary bypass and median sternotomy. Further investigation and refinements are needed, however, before application of this approach in humans.

Intraoperative device closure of perimembranous ventricular septal defects without cardiopulmonary bypass: preliminary results with the perventricular technique.

OBJECTIVE: In infants undergoing closure of perimembranous ventricular septal defects, cardiopulmonary bypass remains one of the factors that prolongs hospital stay and morbidity. A new technique was used to close the defects under echocardiographic guidance without cardiopulmonary bypass to prevent the deleterious effects of bypass. METHODS: Recently, the Amplatzer membranous ventricular septal defect device (AGA Medical Corp, Golden Valley, Minn) was introduced. The device has a double-disc design with a short connecting waist. The left ventricular disc has an eccentric design to prevent encroachment on the aortic valve leaflets. Eight Yucatan miniature pigs with naturally occurring perimembranous ventricular septal defects underwent closure of the defect in the operating room by using the perventricular technique. After median sternotomy, a purse-string suture was placed on the free wall of the right ventricle. An angiocatheter was advanced in the right ventricle, and through the catheter, a wire was advanced from the right ventricle through the ventricular septal defect into the left ventricle. A delivery sheath and the dilator were advanced over the wire. The wire and catheter were removed, and an appropriately sized Amplatzer membranous device was advanced through the sheath. The device was deployed under echocardiographic guidance with the heart beating. RESULTS: The procedure was successful in all animals. There was no incidence of device embolization, heart block, or aortic insufficiency. Angiograms at 3 and 6 months revealed no residual defects and no aortic insufficiency. Pathologically, the devices were completely endothelialized when examined grossly. CONCLUSIONS: The perventricular technique appears to be excellent for closure of perimembranous ventricular septal defects in the operating room. The technique might be feasible in smaller babies, who are high-risk candidates for closure in the catheterization laboratory. Cardiopulmonary bypass and prolonged hospital stay are avoided.