Stent deformation

Computational fluid dynamics models were developed over the years that include the effects of leaflet motion and its interaction with the flowing blood (Bellhouse et al., 1973 Mazumdar, 1992). Several finite-element structural models for heart valves were also developed in which issues such as material and geometric nonlinearities, leaflet structural dynamics, stent deformation, and leaflet coaptation for closed valve configurations were effectively dealt with (Bluestein and Einav, 1993 1994). More recently, fluid-structure interaction models, based on the immersed boundary technique. 

In the series of Strecker stent placement in 21 patients, complications were reported in two. One stent compression and one stent dislocation occurred in two patients. Both stents were removed without problems (WiTT et al. 1997). In a smaller trial, one Strecker stent out of five dislocated 6 days after placement (ScHMiDT et al. 1999). In the series of Hauck et al. (1997), tumor ingrowth through the stent was seen in nine of 51 patients treated with Strecker or nitinol stents. Stent deformation occurred only with Strecker stents ( =12), but not in nitinol stents. Mucus retention was a frequent problem within the first 7 days after stent placement (39%). 

One of the most commonly used medical devices is the stent, (Fig. 21.1), small metallic structures that are expanded in blood vessels, functioning to maintain the patency (freedom from obstruction) of the vessel in which it is placed. Although the first use of stents was in vasculature (blood vessel systems), more recent applications include, for example, implantation between two vertebrae to increase the rigidity of the spine. A typical vascular stent is placed in its anatomical location and then either plastically deformed/expanded (stainless steel) or allowed to expand to a predetermined size, as a consequence of shape memory (nitinol). 

Shape-memory alloys (e.g. Cu-Zn-Al, Fe-Ni-Al, Ti-Ni alloys) are already in use in biomedical applications such as cardiovascular stents, guidewires and orthodontic wires. The shape-memory effect of these materials is based on a martensitic phase transformation. Shape memory alloys, such as nickel-titanium, are used to provide increased protection against sources of (extreme) heat. A shape-memory alloy possesses different properties below and above the temperature at which it is activated. Below this temperature, the shape of the alloy is easily deformed due to its flexible structure. At the activation temperature, the alloy can be changed by applying a force, but the structure resists this deformation and returns back to its initial shape. The activation temperature is a function of the ratio of nickel to titanium in the alloy. In contrast with Ni-Ti, copper-zinc alloys are capable of a two-way activation, and therefore a reversible variation of the shape is possible, which is a necessary condition for protection purposes in textiles used to resist changeable weather conditions. 

One interesting alloy of titanium and nickel, called Nitinol, exhibits shape-memory properties. Below a particular temperature (the transformation temperature), the crystal structure of the alloy is such that it can be plastically deformed (martensitic). As the alloy is heated, the crystal structure alters to one that is more ordered and rigid (austenitic), and the deformed metal reverts to its original shape. This effect has been exploited in a number of devices, including a stent (a device used to hold open passageways such as arteries). The stent is placed inside a small-diameter catheter for insertion into the body, where it expands on being warmed to bod y temperature. 

Another example of a biodegradable stent is the REVA (Reva Medical Inc., San Diego, CA), a tyrosine-derived polycarbonate, which after metabolism produces amino acids, ethanol, and carbon dioxide. The REVA is balloon expandable with a slide and lock design which allows the expansion of the stent without deformation (Figure 5C). Iodine is its source of radiopacity. It has thick struts of 200 microns. Preclinical data show complete re-endothelialization [81]. Currently, the paclitaxel-eluting REVA stent is under development. 

Fig. H (a) PVA-C heart valve stent in natural state and deformed state, (b) Four-part injection mold for the PVA-C heart valve stent. Reprinted fiom [7] with permission. Copyright 2002 Wiley Periodicals...

Tailored characterization methods for the SME were also developed for biomedical applications, such as for stents. A mechanical key characteristic for vascular stents is to withstand the compressive radial stresses over the lifetime of the application, i.e., maintain desirable thermomechanical characteristics with respect to recovery and deployment [63]. In a study on this topic, SME characterization methods were applied to a shape-memory stent from polymer networks, synthesized via photopolymerization of fert-butyl acrylate and PEG dimethacrylate [72]. The free recovery response of polymer stents at body temperature was studied as a function of Tg, crosslinking density, geometrical perforation, and deformation temperature. 

The free recovery process is illustrated in Fig. 18a, where the images were taken using a digital camera at a frequency of 1 Hz and measurements were made once the stents had unrolled and started to unfurl. Figure 18b shows that stents manufactured with a high amount of perforation initiated shape recovery sooner than their solid counterparts, despite similar times to complete recovery. It was concluded that the time for full shape recovery is highly dependent on Tg, crosslink density, and deformation (storage) temperature. 

Deformation and migration of Palmaz stents in three patients brought the suitability of the stent for use in the tracheobronchial system into question (Perini et al. 1999). The authors encountered stent collapse in three patients at 44 days, 78 days and 362 days after initial placement, respectively. Three stents also showed signs of complete or partial migration. The authors decided to no longer use this stent type in the tracheobronchial tree. 

Perini S, Gordon RL, Golden JA, LaBerge JM, Wilson MW, Kerlan RK (1999) Deformation and migration of Palmaz stents after placement in the tracheobronchial tree. J Vase Intervent Radiol 10 209-215... 

---