<%@LANGUAGE="JAVASCRIPT" CODEPAGE="1252"%> Drug Eluting Stents
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Bare Metal Stents

In 1994, Johnson and Johnson produced the Palmaz-Schatz Balloon-Expandable Stent, the first stent approved by the FDA. In the past decade, over 25 companies have used various materials and designs in the construction of these bare metal stents. Over this time, stents achieved greater durability and flexibility. The development of stents was driven by the need to improve the efficacy of Plain Old Balloon Angioplasty, or POBA, procedures. In the 1970s and 1980s, interventional cardiologists searched for methods that would eliminate two of the main problems associated with angioplasty: elastic recoil and neointimal hyperplasia. Implantation of the stent appeared to reduce the occurrence of these potential symptoms.[3]

In the past, POBA surgery had the potential to cause an elastic recoil effect. This occurred in approximately 5-10% of patients within the first few hours, or even minutes, of surgery. After the procedure, the coronary artery experienced a rebound, and subsequent occlusion, at the site of the balloon insertion. Tearing of the vessel's inner lining and thrombosis of the region also contributed to this acute closure of the artery. Elastic recoil often led to severe complications, including myocardial infarction and the need for emergency bypass surgery. [1]

Neointimal hyperplasia is the immune system's reaction to the intrusion of angioplasty. Following the damage incurred on the endothelial barrier at the site of balloon inflation, the extracellular matrix can become exposed. This leads to the hyperproliferation of smooth muscle cells, a response that is prompted by growth factors and proteoglycans of the extracellular matrix. These cells move into the intima, where they cluster and form a lesion. The tissue in this region becomes thickened and scarred, and the artery undergoes a subsequent remodeling of its structure. The end result is a redevelopment of arterial blockage and obstructed flow, an event known as restenosis. In the past, this occurred in 40-50% of patients within three to six months after POBA. At the time, the most common treatment for restenosis was a repeat of the procedure. [7]

 

Materials and Design

Stents have a tubular, lattice structure and can be assembled from a range of metals, including nitinol, stainless steel, and cobalt chromium. Ideally, a stent should be spring-like and flexible, conforming to the shape of the arterial wall. The primary goal of the stent is to hold the inner wall in its newly compressed position, retaining the enlarged diameter. The stent diameter can range from around 2mm to 4mm, depending on the diameter of the individual vessel, as well as the specific condition and extent of disease. Stent length can range from around 8mm 38mm, depending on the extent of the coronary buildup.

The composite metals differ in degrees of strength and flexibility. Engineers choose specific materials and designs to create greater thromboresistance, and to make the stents both radiopaque and biocompatible. Radiopaque markers are used to help the physician implanting the stent visualize it using a fluoroscope. For example, Guidant, a primary manufacturer of stents, identifies cobalt chromium as a metal that is more radiopaque and durable than stainless steel. With this material, they are able to create thinner struts, which are generally associated with lower restenosis rates. Patterns of the struts differ from stent to stent. Among the numerous types of stents are the multicellular corrugated, coil, and serpentine designs. Some of these have a lining of carbon, platinum, or heparin, which are believed to decrease the potential for thrombosis. Long-term studies currently explore the relationship between stent design and rate of restenosis. Engineers continue to improve upon earlier structures. [4]

 

Procedure

The insertion of the stent involves a guiding catheter, guide wire, and balloon tipped catheter with attached stent. To access the blocked region of the coronary artery, surgeons thread the guiding catheter through the femoral artery and then thread the guide wire through the guiding catheter. The balloon tip ped catheter with stent is guided to the occluded region along the wire. The Mylar balloon is dilated and subsequently removed; the stent that originally surrounded it now remains pressed against the arterial wall. During this process, a flexible stent will insert with greater ease and deploy in a more uniform fashion. The scenario in which a stent delivery directly follows a POBA procedure occurs in about 70% of these surgeries. The other cases involve direct stenting, where stents serve as the initial treatment. Over time, the vessel lining grows into the metal lattice of the stent. [2]

To see an animation detailing the stent implantation procedure click here.

Demographics and Complications

Stents eliminated the concern for POBA's elastic recoil effect. Although the incidence of restenosis dropped to around 25% of patients, in-stent restenosis, or ISR, surfaced within 3 6 months after surgery. Again, this resulted from the body's tendency toward neointimal hyperplasia.

Working from bare metal stents, surgeons have employed various techniques in an attempt to diminish the effects of restenosis. They use brachytherapy, a dose of radiation emitted from a catheter that inhibits cell division at the occluded site. They have also used anti-coagulant drugs, given at the time of stent delivery, to reduce the restenosis rate. [3]

This interest in these anti-coagulant drugs, coupled with the desire to eliminate ISR, inspired the development of drug eluting stents.

Bare Metal Stents
 
 
   
   
   

 

Copyright © 2004 Nick Mark