Extra Cranial Carotid Arteries: A Unique Field
for Self-Expanding Stents

In this issue of the Journal, Mukherjee et al.1 report a series of 178 patients treated with carotid stenting utilizing self-expandable stents at the Cleveland Clinic Foundation from February 1998 until January 2000. Stainless steel stents (Wallstent, Boston Scientific/Scimed, Minneapolis, Minnesota) were implanted in 89 patients; nitinol stents (Smart stent, Cordis, Miami Lakes, Florida) were implanted in the other 89 patients. Stents were implanted utilizing the standard technique.2 Glycoprotein (GP) IIb/IIIa antagonists were liberally administered in 73% of patients treated with a stainless steel stent and in 89% of patients treated with a nitinol stent (p = 0.001). Despite the fact that patients were not randomized to implantation of each specific stent, the two groups resulted well matched with a difference only in the presence of a contralateral carotid occlusion (3% in the stainless steel group and 14% in the nitinol group; p = 0.009). Procedures were technically successful in all patients. The final post-procedural stenosis was slightly higher in the nitinol group compared to the stainless steel group (4.2% vs. 1.9%; p = 0.001). All patients had a clinical follow-up of up to six months analyzing the incidence of death, Q-wave myocardial infarction (MI) and stroke. The only events reported during hospitalization and follow-up were stroke, which occurred in 3.3% of patients treated with a stainless steel stent versus the 2.2% of patients treated with a nitinol stent. This difference was not statistically significant. The authors do not clarify the exact nature of the strokes. In particular, we do not know how many of these events were permanent or transitory. The authors conclude that in their experience, the performance of these two stents appeared comparable. As a result, should we expect any difference between these two types of self-expanding stents? Nitinol, an alloy composed of nickel and titanium, has in many ways optimal mechanical characteristics to be considered the ideal self-expanding stent. These features were recognized as early as 1969, when Dotter first attempted their use in dogs.3 Human medical application of nitinol was first proposed in 1975, due to the unique elastic properties of this alloy. Nitinol is a very elastic alloy, and it has been used to construct memory metal stents. This alloy is available as a metal with elastic memory (the one used for the nitinol stents implanted in this study) or as a metal with thermal memory. Initially, nitinol was constructed as a thermal memory alloy, able to be transformed from a crystalline state with low elasticity (martensite, Adolf Martens studied this crystalline phase) to a stiffer and elastic state (austenite, Roberts Austen studied this elastic phase) by a change in temperature. More recently, the memory elastic alloy has been developed. Nitinol stents with elastic memory can change their shape upon a mechanical deformation and are able to re-expand to their original shape with a certain recoil force. This alloy is the one used to construct most of the current vascular stents. The unique elasticity of nitinol makes this metal ideal for construction of stents able to withstand deformations present in the extra cranial carotid artery. The radial force of nitinol stents is usually 3 or 4 times superior to one of the self-expandable stainless steel stents. For the Wallstent, the radial force is 0.34 Newton; for the Smart stent, it is 1.83 Newton. It is unknown how much this difference contributes to the clinical outcome. The initial experience reported by Mukherjee1 seems to fail to show any clinical relevant difference, still a larger number of patients with a dedicated randomized study appears necessary to better settle this question. Another important difference between these two stents is the amount of longitudinal shortening with a much lower shortening for the Smart stent compared to the Wallstent. The construction of the Wallstent is such that in order to minimize the shortening, the angle between the wires needs to be decreased with a negative impact on radial force. The operator overcomes the shortening of the Wallstent by oversizing the device. This choice positively affects the radial force. Ultimately, we may notice that the average length of the implanted self-expandable stainless steel stents will be higher compared to one of the nitinol stents. The Mukherjee study does not give us any number to support the above consideration, and we do not have any data, which tells us whether this difference, if it exists, has any angiographic or clinical impact. One last but not irrelevant issue is that the current self-expandable stainless steel stent (Carotid Wallstent, Boston Scientific/Scimed) available in Europe, is the only self-expanding stent with a monorail guidewire lumen, which can be advanced inside a 6 French introducer and has a unique flexibility due to the 5.9 French profile in its larger distal segment. Many operators may prefer readily demonstrable benefits rather than advantages not yet supported by clinical studies.

1. Mukherjee D, Kalahasti V, Bhatt D, et al. Self-expanding stents for carotid interventions: Comparison of nitinol versus stainless steel stents. J Invas Cardiol 2001;13:732–735. 2. Yadav JS, Roubin GS, Iyer S, et al. Elective stenting of the extracranial carotid arteries. Circulation 1997;95:376–381. 3. Dotter CT. Transluminally-placed coilspring endarterial tube grafts. Long-term patency in canine popliteal artery. Invest Radiol 1969;4:329–332. 4. Civjan S, Huget EF, DeSimon LB. Potential applications of certain nickel-titanium (nitinol) alloys. J Dent Res 1975;54:89–96.