Author + information
- John A. Ormiston, MBChB⁎ (, )
- Susann Beier, MSc,
- Brett Cowan, BE(Hons), MBChB and
- Mark W.I. Webster, MBChB
- ↵⁎Mercy Angiography Unit, 1st Floor, Mercy Hospital, 98 Mountain Road, Epsom, Auckland 1003, New Zealand
Dr. Hsiao's letter highlights major differences in the behavior between different metal alloys used in stent construction. Most coronary stents, including all those tested in our recent study on stent longitudinal integrity (1), are balloon expandable and made from plastically deformable metal. In response to an applied force, these stents initially deform in their narrow elastic range and could return to their undeformed state if the force were removed (Fig. 1). For stent deployment, when the force, or more correctly the stress, passes a critical level called the yield point, permanent plastic deformation remains after the force is removed, and the stent remains expanded (Fig. 1). Longitudinal distortion of a balloon-expandable stent is also plastic deformation, and the stent does not “spring back” when the distorting force is removed. The propensity for longitudinal distortion with plastically deformable stents is related mainly to stent design.
By contrast, self-expanding stents are usually made from Nitinol, a nickel–titanium alloy. Nitinol exhibits “superelasticity,” or elastic behavior, over a wide range of applied stresses that generally do not reach their yield point (Fig. 1). These stents are pre-stressed within their elastic range during manufacture and are designed to return to their original shape during deployment. If stress sufficient to permanently distort a balloon-expandable stent is applied to a Nitinol stent, the extended elastic range usually makes the distortion reversible. The self-expanding stent “springs back.”
The great majority of contemporary coronary stents are balloon expandable, and depending on design, are subject to longitudinal stent distortion, a troublesome but infrequent procedural complication. Nitinol stents are infrequently used during percutaneous coronary intervention, but would be unlikely to suffer from longitudinal distortion. Longitudinal strength is but one desirable mechanical characteristic of a stent, and others include ease of delivery, flexibility, radial strength, side-branch access, and radio-opacity.
- American College of Cardiology Foundation
- Ormiston J.,
- Webber B.,
- Webster M.