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Stent designs that favor flexibility and deliverability may come at the price of increased risk of longitudinal distortion, according to a small bench study published online November 30, 2011, ahead of print in JACC: Cardiovascular Interventions. Awareness of this predisposition can guide stent choice in select lesions and spur careful technique, the authors say. Moreover, understanding the interplay of design characteristics can help optimize future stent iterations.

The findings come in the wake of recent cases reported in the literature (eg, Hanratty C. Eurointervention 2011;Epub ahead of print) and at meetings such as the recent Transcatheter Cardiovascular Therapeutics scientific symposium in San Francisco, CA.

For the current study, investigators led by John A. Ormiston, MB, ChB, of Mercy Hospital (Auckland, New Zealand), used bench testing to evaluate the force required to compress or elongate 7 different contemporary stent platforms. The results were correlated with stent design. 

The stents (with drug-eluting counterparts where applicable) included:

• Cypher Select (Cordis/Johnson & Johnson, Miami Lakes, FL)
• Liberté (Boston Scientific, Natick, MA)
• Vision (Xience V; Abbott Vascular, Santa Clara, CA)
• MultiLink 8 (Xience Prime; Abbott Vascular)
• Driver (Endeavor; Medtronic, Santa Rosa, CA)
• Integrity (Resolute; Medtronic)
• Omega (Promus Element and Ion; Boston Scientific)

Of the 7 stent platforms, metals included stainless steel, cobalt chromium, and platinum chromium. Strut thickness ranged from 81 µ to 140 µ, and number of connectors ranged from 2 to 6.

The Weakest Links

Overall, the Omega and Driver stents required the least force to produce a 5 mm shortening (0.40 ± 0.06 Newtons [N] and 0.71 ± 0.13 N, respectively), while the Cypher Select required the greatest force (1.33 N; P = 0.001 for Cypher Select vs. Omega and P = 0.013 for Cypher Select vs. Driver). The force required for the other stents fell into an intermediate range, with the Integrity, Liberté, Vision, and MultiLink 8 devices all needing more force than the Omega (P = 0.013 to 0.004).

Similarly, a standard compressive force of 0.5 N produced varying degrees of shortening and distortion, from no effect for Cypher Select to as much as 4 mm for Driver and 5 mm for Omega, manifested by strut overlap.

On the other hand, the force required to elongate a stent at least 5 mm did not differ for most devices. However, the Cypher Select was most resistant, requiring more force than each of the other devices (P values ranging from 0.015 to 0.006). When subjected to a standard elongating force of 0.5 N, stents with the greatest longitudinal stability elongated the least (no elongation for Cypher Select with 6 connectors), while those with the least longitudinal strength (2 connectors) elongated the most (Driver and Omega both 5 mm), producing severe distortion.

Besides the number of connectors, their alignment with the stent’s long axis also may affect longitudinal integrity, the researchers say. For example, the angulation of the connectors in the Element stent may contribute to its reduced resistance to longitudinal distortion, they add.

To help clinicians recognize and understand the effects of longitudinal distortion, the investigators also imaged stents subjected to deforming forces in a silicone “phantom” artery using microcomputed tomography.

Sacrificing Structural Integrity

Optimal stent design requires a balance of desirable characteristics, and the recent trend toward enhanced deliverability has had the unanticipated consequence of increasing the potential for longitudinal distortion, the authors observe.

Distortion typically occurs after initial deployment, during positioning of a post-dilation balloon or IVUS catheter, Dr. Ormiston and colleagues write. The problem has likely been under-recognized, in part because cardiologists have not been attuned to it and because it is difficult to see in more radiolucent stents, they add.

Longitudinal deformation can have clinical consequences, Dr. Ormiston and colleagues note. When struts are pushed together or pulled apart, regions of the artery are no longer fully scaffolded, which may reduce drug delivery and increase the risk of restenosis.

The authors advocate careful post-dilation where distortion is recognized. Moreover, “[f]or scenarios where distortion is more likely, such as ostial locations, the interventional cardiologist may choose to select a stent with higher longitudinal strength.” To that end, they add, “standardized testing of longitudinal strength should be available to cardiologists to aid in stent selection.”

Not a Trivial Problem 

“This issue has been bubbling up for at least 6 months, and to me as a practitioner, it’s not a trivial concern,” Jeffrey W. Moses, MD, of NewYork-Presbyterian/University Hospital of Columbia and Cornell (New York, NY), told TCTMD in a telephone interview.

There are several scenarios in which the potential for stent distortion can become an issue, Dr. Moses indicated. It can occur “if the stent is a little underdeployed, or if it is used in a calcified, tortuous lesion where a stiff device could potentially catch a strut,” he said.

Moreover, the problem cannot always be counteracted by post-dilation, Dr. Moses explained. “You can expand the stent, but you can’t ‘undistort’ it,” he commented. In fact, post-dilation itself is a major issue. “There’s a lot of manipulation, a lot of traffic [in stents]. The most elementary [example] is post-dilation with a noncompliant balloon. It seems that a wing balloon can grab these [distorted struts] and move them,” he said.

Stent distortion can have a range of consequences, Dr. Moses added. “If you have internal flaring of struts, it may not allow the stent to be deployed optimally,” he said. “Also, you can have uncovered dissections that become occlusive. Or [a distortion] may impair your ability to manipulate the vessel downstream.” Most dangerously, it can become a nidus for thrombosis, he added.

Perhaps the best way to minimize the risk is to be selective in the use of stents with a predisposition to distortion, Dr. Moses advised. “Why would you want to use a stent [with a built-in potential for distortion]?” he asked. “I would have to understand what overall advantage a design has to make me willing to deal with the potential consequences.

“I have to admit that sometimes the Element design seems to have a smoother profile and in certain calcific vessels I’ve found it easier to deliver than at least Xience,” he added. “But that’s an unusual circumstance.”

Trading Away Too Much for Flexibility?

In the quest for improved stent flexibility, “we may have traded off too much,” Dr. Moses suggested. “We [operators] are used to putting in a stent and having a predictable, excellent result. When you don’t get that, it creates a lot of confusion. You wonder what’s going on. Did the stent slip? Is it compression?”

Asked if the study findings are likely to result in altered stent designs, Dr. Moses responded, “I expect they already have them on the drawing board.”

Meanwhile, although the prevalence of stent distortion and its clinical impact remain unknown, the take-away message for clinicians is to be alert to the phenomenon, Dr. Moses said. “I don’t see the issue going away,” he added.

Study Details

The Instron universal testing machine (Norwood, MA) was used to exert compressive and elongating forces on the stents. Three examples of the 3-mm diameter stent of each platform design were tested.

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