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Polymer vs. Metal: The Battle of Bioresorbable Stents

Bioresorable (also known as bioabsorbable) stents are being held up as a promising alternative to standard drug eluting stents. But there's an internal battle (no pun intended) going on with bioresorable stents as well. Are polymer-based bioresorbable stents ideal for patients? Or should doctor's opt for their metal counterparts?

Bioresorable (also known as bioabsorbable) stents are being held up as a promising alternative to standard drug eluting stents. But there's an internal battle (no pun intended) going on with bioresorable stents as well. Are polymer-based bioresorbable stents ideal for patients? Or should doctors opt for their metal counterparts?

Cases were made for both at the 25th annual TCT Conference in San Francisco, but researchers agree that polymer platforms have more data behind them. However, metal platforms should not be discounted as they may offer significant advantages over polymers.
The Case for Polymers
Right now polymer-based scaffolds (which are made of PLLA) have simply outpaced metallic bioresorbable stents when it comes to the quantity of data available. Dr. Syed Hossainy, director of the innovation incubator at Abbott Laboratories, a division of Abbott Vascular, said, “The reason I'm on the side of favoring polymer-based scaffolds is derived from significantly more clinical data that's available. Some of it is in comparison with the leading drug eluting stent [DES] and second generation DES designs. Lastly, there is a significantly more understanding of the mechanistic function for PLLA-based platform as opposed to a metallic absorbable scaffold.”
 The Absorb Cohort B demonstrated a favorable comparison between the Absorb and standard drug eluting stents. (image courtesy Syed Hossainy / TCT Conference) 
Indeed Abbott has collected nearly five years of clinical trial data between two different designs for its Absorb bioresorbable scaffold. The Absorb Cohort B study (conducted by Abbott), demonstrated that the Absorb scaffold has a comparable clinical performance to XIENCE, the current standard for non-absorbable drug eluting stents. 
Another study conducted to compare the Absorb to the Absorbable Metallic Stent (AMS) which made of magnesium and manufactured by Biotronik, found that the quick absorption of magnesium likely caused less structural integrity as compared to the Absorb, and that PLLA, unlike magnesium, does not immediately lose load bearing ability as it breaks down.
As the magnesium scaffold breaks down it was also found to leave chemical byproducts that could lead to potential complications such as ectopic calcification (calcium deposits in the soft tissue), whereas PLLA naturally breaks down into water and carbon dioxide, which are both naturally absorbed by the body.
Metals Answer Challenges
Dr. Ron Waksman, a professor of medicine at the Georgetown University MedStar Heart Institute in Washington D.C., Listed a number of issues with polymers:
  • Polymers are thick
  • They prone to inflammation
  • They are slow to expand during implantations
  • They are less deliverable
  • They are have a storage shelf-life
  • They depend on accurate imaging
  • They are prone to fractures
  • They are limited in calcified lesions
  • They are limited in bifurcation
  • They are not free from thrombosis (formation of blood clots)
There are also several other challenges that PLLA bioresorbables must overcome including temperature sensitivity, poor deliverability, and stent security retention. Waksman also outlined some perceptual challenges on the parts of doctors and patients. “They may think, 'This is a big device that delivers poorly,' the struts are very thick,' 'this plastic stent won't have enough strength.' ”
“When we're talking about metallic we're talking primarily about iron and magnesium,” Waksman said, outlining how bioabsorbable metallic scaffolds can overcome the challenges presented by polymers:
  • Iron and magnesium alloy has a similar radial force to stainless steel and cobalt chromium stents.
  • The profile of metallic scaffolds is superior to PLLA and they are more deliverable.
  • Metallic bioresorbable scaffolds feel like metallic stents and bioabsorb within 6-12 months for magnesium and 12-36 months for iron.
A key point Waksman made was on the issue of scaffold malapposition – when a stent's struts separate from the vessel wall. He cited a cohort study, presented in 2011 at the American College of Cardiology of the Absorb that showed an 81.1% rate of malapposition. “In contrast in the magnesium the OCT demonstrated a very minimal malapposition post procedure at 6 months and 12 months...respectively the metal is performing better,” Waksman said, citing a year-long study of the Biosolve magnesium scaffold (also known as DREAMS) by Biotronik, which showed 97.3% of the stents still apposed after 12 months.
A 12-month study of Biosolve showed very little malapposition among implants. (image courtesy Ron Waksman / TCT Conference)
What about Patient Outcomes?
Differences in structure or perception aside, what doctors really want to know is what are the patient outcomes when it comes to polymer versus metal.
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A Biotronik-funded study on the Biosolve-I showed no instances of cardiac death or scaffold thrombosis, and low instances of target lesion failure, myocardial infarction, and target lesion revascularization up to 12 months for the Biosolve. Waksman also talked of the next generation Biosolve (the Biosolve-II) which will be more flexible and feature slower dismantling and absorption rates (magnesium currently absorbs faster than PLLA), and increased deployment diameter, and higher acute radial force among other features.
Polymer advocates may wish to note that the Biosolve-II will employ a PLLA coating for drug delivery - fueling speculation that a combination product may be the ideal solution.
The interventional cardiologist are used to metallic stents and prefer to use them,” Waksman said, noting that studies for the Biosolve-II are currently gearing up. “Plastic is not metal it is thick, hard to expand, and tends to break.” Waksman believes that if studies show metallic ioresorbable stents to have comparable efficacy to standard metallic stents then devices like Biosolve will win the day.
While both sides are showing some pros and cons - and clinicians have said they like both options - the ultimate question on clinicians' minds is whether PLLA can ever physically match the properties of metal and whether metal can overcome some of its other challenges to match the safety of PLLA. Only time, and more clinical data, will tell.  
-Chris Wiltz, Associate Editor, MD+DI
[email protected]


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