Plastic stents, tubes to keep coronary arteries open, are prone to deforming after an implant, but researchers at the Massachusetts Institute of Technology have worked out how to overcome stent degradation.
Heart disease patients have metal stents to prevent blood clotting in their coronary artery, however, metal can eventually damage the artery.
Surgeons turned to biodegradable polymers to remove the risk of long-term implantation. But after a few years these patients experienced more heart attacks than patients with metal stents, and the polymer stents were taken off the market.
MIT researchers in the Institute for Medical Engineering and Science and the Department of Materials Science and Engineering have now discovered why these stents failed. Their study also reveals why the problems were not uncovered during the development process: The evaluation procedures, which were based on those used for metal stents, were not well-suited to evaluating polymer stents.
MIT Stent Failure
Elazer Edelman, Professor of Health Sciences and Technology at MIT, said: “People have been evaluating polymer materials as if they were metals, but metals and polymers don’t behave the same way. People were looking at the wrong metrics, they were looking at the wrong timescales, and they didn’t have the right tools.”
The degradable stents are made from a polymer called poly-l-lactic acid (pLLA), which is also used in dissolvable sutures. Preclinical testing (studies done in the lab and with animal models) did not reveal any cause for concern. In human patients the stents appeared stable for the first year, but then problems began to arise. After three years, over 10 percent of patients had experienced a heart attack, including fatal heart attacks, or had to go through another medical intervention. That is double the rate seen in patients with metal stents.
After the stents were taken off the market, the team decided to try to figure out if there were any warning signs that could have been detected earlier. To do this, they used Raman spectroscopy to analyse the microstructure of the stents. This technique, which uses light to measure energy shifts in molecular vibrations, offers detailed information about the chemical composition of a material. Ferralis and Grossman modified and optimised the technique for studying stents.
The researchers found that at the microscopic level, polymer stents have a heterogeneous structure that eventually leads to structural collapse. While the outer layers of the stent have a smooth crystalline structure made of highly aligned polymers, the inner core tends to have a less ordered structure. When the stent is inflated, these regions are disrupted, potentially causing early loss of integrity in parts of the structure.
When the stents become deformed, they can block blood flow, leading to clotting and potentially heart attacks. The researchers believe that the information they gained in this study could help stent designers come up with alternative approaches to fabricating stents, allowing them to possibly eliminate some of the structural irregularities.