Stents by shayna nolan
Introduction
Stents provide support for hollow structures in the body, such as arteries and blood vessels, that experience restricted fluid flow. This can occur due to blockage or collapsing of the tissue. Coronary heart disease, renal failure and peripheral artery disease are a small number of conditions where stent use is often required. Stents provide unrestricted flow by keeping the affected vessel propped open, permitting maximum fluid flow [1].
Applications
Coronary Artery Disease
Coronary artery disease (CAD) is responsible for the highest number of deaths in the United States [6]. CAD occurs when blood flow through arteries is restricted due to accumulation of cholesterol and plaque on the inner area of an artery. This accumulation, also known as atherosclerosis, causes arteries to stiffen and shrink, and as a result blood and oxygen delivery to the heart is decreased. Angina and heart attack can occur if CAD is untreated [6].
CAD was originally treated by the performance of a balloon angioplasty. This method eliminates atherosclerosis by feeding a line to an area of blockage along the coronary artery, penetrating it with a deflated balloon. Next, by inflating the balloon to compress blockages and expand the artery, proper blood flow through the vessel is restored [7]. While this method does clear blockage temporarily, restenosis often occurs [4]. The first stent was introduced to maintain clear vessel passages after a balloon angioplasty [5].
Renal Failure
Renal failure occurs when kidney function is lost or decreased, and the body is unable to filter wastes from the blood on its own. Two options for treating kidney disease are kidney transplants or dialysis. Due to the shortage of available organ donors, dialysis can be a temporary or permanent method of filtering the blood [8].
Dialysis utilizes an external machine to filter impurities out of the blood. The machine is hooked up to a patient via fistulas in blood vessels. The fistulas permit higher blood flow to make the process efficient enough to clean the blood in a timely manner. However, sometimes these blood vessels will shrink and stent implantation is needed to allow necessary blood flow to secure the use of the external dialysis machine [9].
Types of Stents
Bare Metal Stents
Bare metal stents were first introduced to replace balloon angioplasty of coronary arteries [4]. Bare metal stents can either be self-expanding or balloon expandable, and can reach a diameter range of 2.25-25mm and length range of 8mm to 38mm depending on the specific characteristics of a patient [5].Stainless steel, nitinol, and cobalt chromium alloys are the primary constituents of bare metal stents [5,10]. In addition to material differences, different mechanical designs of the stent also impact its properties. One example of this is strut thickness. With an increased strut thickness, a greater level of vessel support is achieved, but at the same time more damage is caused to the vessel, and thus an optimization must be reached that is both supportive while not increasing injury level[11].
Coated Stents
Coated stents are utilized to optimize the surface characteristics of the stent to make it more compatible with the vessel. Common materials used to coat stents include: heparin, phosphorylcholine, and silicon carbide. These coatings are generally thought to decrease immune responses that can cause restenosis and thrombosis at the site of the stent [11].
Drug Eluting Stents
One of the main causes for restenosis after stent implantation is smooth muscle cells migrating and proliferating. Drug eluting stents are utilized to stop or impair cellular division and motility, thus minimizing the smooth muscle migration [2]. Paclitaxel is a commonly used for drug eluting stents, and is capable of interrupting cell replication [2]. Clinical trials have proven that Paclitaxel eluting stents significantly reduces restenosis in comparison to bare metal stents after a nine month period [2].
Obstacles
Biocompatibility
Despite the structural support of stents and the success rates achieved by both clinical trials and research, stents are overall incompatible with the body [4]. Restenosis in the stent is fueled by an automatic immune response to the initial placement of the stent. Smooth muscle cells hyperproliferate in an attempt to heal the wound caused by the placement of the stent, which can cause restenosis. This problem is most common in bare metal stents [4].
Drug eluting stents exhibit lower restenosis rates than bare metal stents, but are non-specific treatments. Paclitaxel and other anti-proliferating agents inhibit all cell proliferation, not just smooth muscle and extracellular matrix cells. As a result of this, healing times are drastically lengthened [4].
Future Work
Overlapping Bare Metal Stents
While stents are traditionally used to prevent the collapsing of vessels, current research is being applied to stabilizing aneurysms. Overlapping bare-metal stents are placed at the cite of an aneurysm in order to strengthen the weakened vessel by reducing the blood flow through the aneurysm. This controlled flow decreases stress on the vessel walls, and promotes platelet growth. As the vessel continues to strengthen, the aneurysm degrades [3].
Despite the high success rate of current findings, overlapping bare-metal stents still have significant development. Current obstacles faced in this area of research include varying blood flow and anatomical differences among patients [3].
History
-1969: First endovascular stent use by Charles Dotter [5]
-1986: The first coronary stent used [5]
-1990's: Stents become more effective than balloon angioplasty [5]
-1997: Stents used as rescue therapy in emergency angioplasty vessel ruptures [5]
-2003: Anti-proligerating drug eluting stents [4]
Sources
[1] Sudheendra, D.; Zieve, D. Stent. MedlinePlus, https://www.nlm.nih.gov/medlineplus/ency/article/002303.htm (accessed Feb 26, 2016).
[2] Stone, G. W.; Ellis, S. G.; Cox, D. A.; Hermiller, J. A Polymer-Based, Paclitaxel-Eluting Stent In Patients with Coronary Artery Disease. The New England Journal of Medicine. 2004, 350, 221–231.
[3] Zhang, P.; Liu, X.; Sun, A.; Fan, Y.; Deng, X. Hemodynamic Insight Into Overlapping Bare-Metal Stents Strategy in the Treatment of Aortic Aneurysm. Journal of Biomechanics. 2015, 48, 2041–2046.
[4] Jeewandara, T. M.; Wise, S. G.; Ng, M. K. C. Biocompatibility Of Coronary Stents. Materials. 2014, 7, 769–786.
[5] Yevzlin, A.; Asif, A. Stent Placement In Hemodialysis Access: Historical Lessons, the State of the Art and Future Directions. Clinical Journal of the American Society of Nephrology. 2009, 4, 996–1008.
[6] Coronary Artery Disease. MedlinePlus, https://www.nlm.nih.gov/medlineplus/coronaryarterydisease.html (accessed Feb 26, 2016).
[7] Angioplasty / Balloon Angioplasty. Cardiothoracic Surgery: University of Souther California, http://www.cts.usc.edu/hpg-angioplasty.html (accessed Feb 26, 2016).
[8] Kidney Failure: Symptoms, Signs, and Facts. MedicineNet, http://www.medicinenet.com/kidney_failure/article.htm (accessed Feb 26, 2016).
[9] Dialysis and Fistula/Graft Declotting and Interventions. Radiologyinfo.org, http://www.radiologyinfo.org/en/info.cfm?pg=dialysisfistulagraft (accessed Feb 27, 2016).
[10] Shah, S. N. Coronary Bare-Metal Stent . Medscape, http://emedicine.medscape.com/article/2009987-overview (accessed Feb 27, 2016).
[11] Hara, H.; Nakamura, M.; Palmaz, J. C.; Schwartz, R. S. Role Of Stent Design and Coatings on Restenosis and Thrombosis. Advanced Drug Delivery Reviews. 2006, 58, 377–386.
[12] http://clinicalgate.com/nonaortic-stents-and-stent-grafts/
