Rephrasing Stent Fracture - Coursework Example

1.5 Stent design Stent design can possible play an important role in SF (Figure 6). When a stent design centered on a closed cell design, it can be more prone to fracture when compared to an open-dell design. Suggestions indicate that an open-cell design allows for more “fexibility” which reduces the possibility of a stent fracture. While the suggestions seem feasible, there are no current studies that can support these claims. 2 When the thickness of the stent struts are decreased (from 100 to about 65µm), there is a noticeable increase in radial strength which results in a prolong stent life and improved deliverability. 3 Implantation can only be undertaken once stend have passed stringent simulation tests. Howeer, it is somewhat difficult to recreate the extreme scenario that the stent must pass through in order to pass the test. The test includes testing for vessel tortuosity along with rotational forces of about 70 times per minute for a number of years. Such parameters are difficult to integrate into a stent simulation test. 4 a b Figure 6: (a) Fully supported closed-cell design, (b) open-cell design 1. Current stents normally show problems in terms of standard lengths. When made longer, the stent tends to be more fracture prone. Customizing stents could possibly solve or minimize this problem. One of the more promising custom stent designs belongs to Xtent Custom. Known as the NX Co stent (Fugure 7), the unique feature of this design allows for multiple 6 mm inter-digitating segments. The use of such a segment allows the stent length to become “infinitely” variable. 5 In-lesion determination of stength length is then allowed instead of manufacturer reliance on fixed length. Using this process, short lesions of varied lengths and diameters may be treated using a single device when used in humans. Figure 7: The NX Stent design customized by Xtent Custom.5. The manipulation of the switch handle (on-right) is used to activate the inter-digitated separation at a chosen area, allowing the operator to then customize the stent length. Trimaxx stent on the other hand, allows the user to create a thinner stent strut. 3,6. Using three layers of tantalum (18µm) between the inner and outer layers of 316L stainless steel brings its thickness to 74µm, a thinness not achieved by other stents. 3,6. The high ductility, excellent radiopacity, and superior corrosion resistance of Trimaxx is based upon the characteristics of tantalum which has helped the manufacturers to create a thin strut. 3,6. Used by 100 patients, the evaluation and anlysis of ISR over a six month deployment showed that 25% ISR was the global low range 6. Figure 8 shows a cross sectional and top view of a Trimaxx Stent. Figure 8: The triplex metal of the TriMaxx stent metal plattform 6. Figure 9 shows a Tryton Side-Branch Stent, which was created by the Tryton Company.5 What makes their design unique is that the side branch fits perfectly with all the bifurcation lesins in different angulations. 5 Made of cobalt-chromium alloy, it was created with three different zones in mind. With a standard stent design, transition zone, and main vessel region, the trial study using 30 patients revealed a low major adverse cardiac events rate of 9.9% with a procedural success rate of 93.3% 5. a b Figure 9: (a) The design of the Tryton side branch stent (b) the Sideguard Ostium protection device 5. 1.5.2. Advanced coatings Even without strong scientific evidence, there is still a strong belief and support for the assumption that surface optimization is the key to reducing problems with in-stent restenosis (ISR). Solving that problem is believed to result in mettalic stent fracture rates in the range of 20-30 % 7. Using various surface modifications and inroganic coatings, the target of the experiment was to explore the stability of surface oxide-layers with the objective of reducing metal - ion release along with reduced surface thrombogenicity, which results in the promotion of endothelialization. The use of these biomaterials along with a corrosion resistant oxide surface along the lines of titanium oxide or chromium oxide should improve bioompatibility through the lessening of metal-ion release. Studies of in-vitro ion release from a coronary stent did not show clear clinical evidence regarding patient impact. Koster et al. 8 presented a direct connection between metal allergies and ISR in their studies. They proposed that patients sensitive to nickel and molybdenum resulted in higher rates of ISR than the non-sensitive patients. Their work then brought into question the kind of impact metal - ion release from stainless steel, the main material for stents, would have on the patient. The argument focused on nickel because of the high levels of molybdenum in 316L stainless steel. While not necessarily related to metal-ion release, the elemental proportions of the allow seems to be more influenced by the stability and regeneration abilities of oxide. While inorganic stents continue to be experimented upon and developed, the main concern, the prevention of ion-release, is not supported in the study since there is no proof of a relationship between nickel allergy and ISR development in patients. More large scale studies are required in order to achieve a more evidence driven conclusion to the study. 9. Surface modification also expored with Carbon coatings along with other methods that included a “diamond-like carbon (DLC)” 11,12. Made by BioDiamond in Germany, Gutensohn, et al. 13 studied the use of coronorary stainless steel stents, which were coated with simple DLC. The in-vitro study showed that the coating produced a significant decrease in both metal - ion release of nickel and chromium, while also lowering the platelet activation when compared to uncoated stents. While promising, the study results did not lead to a conclusion of improved biocompatibility, thus making the study fail to support the prevous conclusions achieved by the research. Using bare stainless steel stent and DLC coated stent in 347 patient, Airoldi et al. 14 followed the patients for six months withour seeing a significant difference in restenosis rates within the groups. While DLC might have reduced the rate of acute thrombosis, there was no long term value found in the value of the results. Perhaps because the DLC-coated stents are still in the development and marketing stage, it carries an advantage over the bare metal stents. On another note, when Saebe et al 15 experimented with flourine-doped DLC, the high ratio of albumin to fibrogen adsorption, combined with lowere platlet adhesion, resulted in a significant drop or stop of platelet adhesion and thrombus activation in controlled settings. All of the aforemention studies and experiments will hopefully help to enhance the future stent construction and mechanical integrity. The hope of an increased stent life, at this point, lies in the efforts to reduce restenosis rates in its existing configurations. 1.5.3 Grain refinements approach Cycilic stresses affect stent life due to systolic and diastolic pressure created in heart arteris (i.e. torsion, bending, tension, and compression). These are the factor which affect stents and often lead to the fracture and fatigue of the mechanis. 22,23. These are the reasons why stent integrity is of the utmost importance in the production of longer lasting stents. Each stent is affected by microstructures, surface condition, heat conditions, and the manufacturing process which can lenghten or shorten the lifespan of a stent. The suggestion or proposal of Al Mangour et all is to manufacture stents using cold spray 16,17. Described as Shot peening, the process involved metal spherical powders shot at a component or substrate. The resulting residual compressive stress seems to create a longer lasting product due to the increased parts longevity. The micron sized particles are accelerated to velocities of up to 1500 m/s using a supersonic gas jet towards a suitable substrate. 19-21. The stent strut sizes in the range of 70 µm reate three grains across thickness, which strengthens the material in terms of fatigue resistance. 22. Unfortunately magnetic susceptibility 23,24 results when martensitic transformation occurs. A process that will refine the grains and maintain austenitic structure of 316L has yet to be developed. It is believed that the cold spray process can help facilitate this development. Using a smaller grain size may provide an advantage in terms of wear properties and help with the stent fracture problem since the faitigue property will be improved. The use of the cold spray process is credited to Barua et al 26 who found that the process created a degradable mettalic stent based upon materials composed of iron and magnesium based alloys. Stents and other medical devices are created using casting and thermo-mechanical processes which help the manufacturer achive the necessary shape and mechanical properties.27. However, stent material is quite picky about the melt source due to the effects it can have on the homogeneity, porosity, and micro-cleanliness of the cast alloy. 22,27. Cold spray material seems to provide the hope that heat associated with conventional laser machining that causes grain coarsening can be avoided in the future along with the need for dross cleaning which is a result of laser machining. 27. References 1 Wholey. Designing the ideal stent. ENDOVASCULAR TODAY, 25-34. 2 Schillinger, M. et al. Does Carotid Stent Cell Design Matter? Stroke 39, 905-909, doi:10.1161/strokeaha.107.499145 (2008). 3 O’Brien, B. & Carroll, W. The evolution of cardiovascular stent materials and surfaces in response to clinical drivers: A review. Acta Biomaterialia 5, 945-958, doi:10.1016/j.actbio.2008.11.012 (2009). 4 Shaikh, F. et al. Stent fracture, an incidental finding or a significant marker of clinical in-stent restenosis? Catheterization and Cardiovascular Interventions 71, 614-618, doi:10.1002/ccd.21371 (2008). 5 Koo, B.-K. & Fitzgerald, P. J. Novel Coronary Stent Platforms. Korean Circ J 38, 393-397 (2008). 6 Abizaid, A. et al. Clinical and angiographic results of percutaneous coronary revascularization using a trilayer stainless steel-tantalum-stainless steel phosphorylcholine-coated stent: The TriMaxx trial. Catheterization and Cardiovascular Interventions 70, 914-919, doi:10.1002/ccd.21279 (2007). 7 Sydow-Plum, G. & Tabrizian, M. Review of stent coating strategies: clinical insights. Materials Science and Technology 24, 1127-1143, doi:10.1179/174328408x341816 (2008). 8 Köster, R. et al. Nickel and molybdenum contact allergies in patients with coronary in-stent restenosis. The Lancet 356, 1895-1897, doi:http://dx.doi.org/10.1016/S0140-6736(00)03262-1 (2000). 9 Norgaz, T. et al. Is There a Link between Nickel Allergy and Coronary Stent Restenosis? The Tohoku Journal of Experimental Medicine 206, 243-246 (2005). 10 Grill, A. Diamond-like carbon coatings as biocompatible materials—an overview. Diamond and Related Materials 12, 166-170, doi:http://dx.doi.org/10.1016/S0925-9635(03)00018-9 (2003). 11 Hauert, R. A review of modified DLC coatings for biological applications. Diamond and Related Materials 12, 583-589, doi:http://dx.doi.org/10.1016/S0925-9635(03)00081-5 (2003). 12 Roy, R. K. & Lee, K.-R. Biomedical applications of diamond-like carbon coatings: A review. Journal of Biomedical Materials Research Part B: Applied Biomaterials 83B, 72-84, doi:10.1002/jbm.b.30768 (2007). 13 Gutensohn, K. et al. In Vitro Analyses of Diamond-like Carbon Coated Stents: Reduction of Metal Ion Release, Platelet Activation, and Thrombogenicity. Thrombosis Research 99, 577-585, doi:http://dx.doi.org/10.1016/S0049-3848(00)00295-4 (2000). 14 Airoldi, F. et al. Comparison of diamond-like carbon-coated stents versus uncoated stainless steel stents in coronary artery disease. The American Journal of Cardiology 93, 474-477, doi:http://dx.doi.org/10.1016/j.amjcard.2003.10.048 (2004). 15 Hasebe, T. et al. Fluorine doping into diamond-like carbon coatings inhibits protein adsorption and platelet activation. Journal of Biomedical Materials Research Part A 83A, 1192-1199, doi:10.1002/jbm.a.31340 (2007). 16 Al-Mangour, B., Dallala, R., Zhim, F., Mongrain, R. & Yue, S. Fatigue behavior of annealed cold-sprayed 316L stainless steel coating for biomedical applications. Materials Letters 91, 352-355, doi:http://dx.doi.org/10.1016/j.matlet.2012.10.030 (2013). 17 Al-Mangour, B., Mongrain, R., Irissou, E. & Yue, S. Improving the strength and corrosion resistance of 316L stainless steel for biomedical applications using cold spray. Surface and Coatings Technology, doi:http://dx.doi.org/10.1016/j.surfcoat.2012.11.061. 18 Bagheri, S. & Guagliano, M. Review of shot peening processes to obtain nanocrystalline surfaces in metal alloys. Surface Engineering 25, 3-14, doi:10.1179/026708408x334087 (2009). 19 Gärtner, F., Stoltenhoff, T., Schmidt, T. & Kreye, H. The cold spray process and its potential for industrial applications. Journal of Thermal Spray Technology 15, 223-232 (2006). 20 Ghelichi, R. Coating by the Cold Spray Process: a state of the art. Frattura ed Integrita Strutturale 8, 30-44 (2009). 21 Irissou, E., Legoux, J.-G., Ryabinin, A., Jodoin, B. & Moreau, C. Review on cold cpray process and technology: Part I—Intellectual Property. Journal of Thermal Spray Technology 17, 495-516, doi:10.1007/s11666-008-9203-3 (2008). 22 Moravej, M. & Mantovani, D. Biodegradable Metals for Cardiovascular Stent Application: Interests and New Opportunities. International Journal of Molecular Sciences 12, 4250-4270 (2011). 23 Chen, X. H., Lu, J., Lu, L. & Lu, K. Tensile properties of a nanocrystalline 316L austenitic stainless steel. Scripta Materialia 52, 1039-1044, doi:10.1016/j.scriptamat.2005.01.023 (2005). 24 Susila, P., Sturm, D., Heilmaier, M., Murty, B. & Subramanya Sarma, V. Microstructural studies on nanocrystalline oxide dispersion strengthened austenitic (Fe–18Cr–8Ni–2W–0.25Y2O3) alloy synthesized by high energy ball milling and vacuum hot pressing Journal of Materials Science 45, 4858-4865, doi:10.1007/s10853-010-4264-3 (2010). 25 David H, K. Metals in medical applications. Current Opinion in Solid State and Materials Science 3, 309-316, doi:10.1016/s1359-0286(98)80107-1 (1998). 26 Barua, R. et al. Development of a Metallic Biodegradable Stent Based on Microgalvanic Corrosion. Journal of Medical Devices 8, 020913 (2014). 27 Poncin, P., and Proft, J. in Materials & Process for Medical Devices Conference (ASM, 2003). Read More