Veryan has developed three dimensional (3D) stent technology based upon a helical shape that may be applied to a self-expanding stent platform. The initial objective was to investigate the presence of, and effects arising from, secondary flows in a 3D stented vessel and then to determine how the design could be applied to improve the biomechanical and flow characteristics of straight tubular stents. The focus to date has been on the superficial femoral and proximal popliteal (femoropopliteal) arteries.
Veryan has applied this technology to its proprietary stent platform and has developed the BioMimics 3D™ self-expanding Nitinol stent. The design of the BioMimics 3D stent is built on the principles underlying the latest generation of Nitinol stent technology - adequate mechanical radial support and plaque coverage, good flexibility, durability against fracture, clear visualization and delivery accuracy - with the addition of benefits designed to arise from 3D geometry.
The key advantages of BioMimics 3D stent technology are the potential improvement of flow dynamics and the biomechanical performance of the stent and the unstented vessel segments proximal and distal to the stent.
Veryan conducted a standard preclinical porcine carotid artery model to investigate and compare the effects of secondary flows on neointimal formation with 3D and straight Nitinol stents.8 Reduced levels of neointimal thickness (45% (P<0.001)) were observed in Veryan 3D modified stents compared to straight stents. This was observed in proximal, mid and distal stent segments, with the greatest reduction at the distal segment suggesting that the effect is more pronounced when the secondary flow is at its most developed.
A sample of histology cross sections of straight and 3D stents is shown
This evaluation also confirmed that Veryan's 3D stent technology can be applied to a straight Nitinol stent with successful delivery and deployment using a standard delivery system. The 3D stents imparted a 3D geometry on the native porcine vessel and secondary blood flows were evident by colour-Duplex ultrasound and angiography. Neointimal thickness was 45% less (P<0.001)) in Veryan 3D modified stents compared to the control straight stents. 8
Forces in the femoropopliteal artery and stent durability
During lower limb movement, the femoropopliteal artery is subjected to various forces including compression, torsion and bending depending upon the level of the vessel.
Axial compression is the primary loading mode for stents implanted in the superficial femoral artery.9,10 When the leg is straight, the artery is under tension – when the knee bends, the curvature of the artery increases and the artery shortens.9,10,11 When straights stents are deployed in the femoropopliteal artery, they are subject to a compressive force during leg bending that may result in shortening with the risk of kinking of the vessel, both within the stented segment and in the unstented segments proximal or distal to the stent . The suitability of a stent for use in the femoropopliteal may therefore depend on its ability to shorten in a controlled manner, without inducing strains which may lead to fatigue fracture or kinking.
Existing stents indicated for placement in the femoropopliteal artery have a straight tubular design and therefore tend to straighten any curvature present in vessels. This straightening effect may interfere with normal vessel shortening that occurs during limb movement. The fracture of Nitinol stents has frequently been reported in the femoropopliteal application.12 - 15
The BioMimics 3D stent has a three-dimensional geometry which is permanently imposed on a straight Nitinol stent substrate during the manufacturing process. The device is designed to improve biomechanical performance of the stented and unstented vessel segments.
The BioMimics 3D stent is designed to accommodate arterial shortening as the leg bends. This feature may reduce the risk of stent fracture and vessel kinking and injury as illustrated.
Impact of straight stents on femoropopliteal artery
Impact of BioMimics 3D stent on femoropopliteal artery
8. Shinke T, Robinson K, Burke MG, Gilson P, Mullins LP, O'Brien N, Heraty KB, Taylor C, Cheshire NJ, Caro CG, Novel Helical Stent Design Elicits Swirling Blood Flow Pattern And Inhibits Neointima Formation In Porcine Carotid Arteries, 81st Annual Scientific Session of the American-Heart-Association, S1054-S1054, 2008, ISSN:0009-7322
9. Cheng, C. P., Wilson, N. M., Hallett, R. L., Herfkens, R. . & Taylor, C. A. In Vivo MR Angiographic Quantification of Axial and Twisting Deformations of the Superficial Femoral Artery Resulting from Maximum Hip and Knee Flexion. Journal of Vascular Interventional Radiology 17, 979–987 (2006).
10. Cheng, C., Choi, G., Herfkens, R. & Taylor, C. The Effect of Aging on Deformations of the Superficial Femoral Artery Resulting from Hip and Knee Flexion: Potential Clinical Implications. Journal of Vascular and Interventional Radiology (2010).
11. Wensing, P. J. W. et al. Arterial tortuosity in the femoropopliteal region during knee fleion: a magnetic resonance angiographic study. 186, 133–139 (1995).
12. Allie, D., Hebert, C. & Walker, C. Nitinol stent fractures in the SFA. Endovasc Today 3, 22–34 (2004).
13. Iida, O. et al. Influence of Stent Fracture on the Long-Term Patency in the Femoro-Popliteal Artery. JACC: Cardiovascular Interventions 2, 665–671 (2009).
14. Scheinert, D. et al. Prevalence and clinical impact of stent fractures after femoropopliteal stenting. Journal of the American College of Cardiology 45, 312–315 (2005). 154. Schlager, O. et al. Long-Segment SFA Stenting—The Dark Sides: In-Stent Restenosis, Clinical Deterioration, and Stent Fractures. Journal of Endovascular Therapy 12, 676–684 (2005).