3D printing has opened up a whole new world of possibilities when it comes to medical implants, allowing surgeons to equip patients with bespoke devices tailored to perfectly fit their unique anatomy, a courtesy that has even been extended to penguins, turtles and dogs. Scientists at Harvard University have now broken new ground in applying the technology to damaged hearts, providing a potential new tool to keep blocked arteries open and the blood pumping freely.
Heart valve replacement is a relatively common, but tricky, procedure where surgeons will seek to unblock one of the four valves that facilitate the flow of blood in and out of the heart. There are many reason these valves might stop functioning properly, but a particularly common one is the accumulation of calcium on delicate flaps called leaflets, which are supposed to open and close during each heartbeat.
According to the Harvard researchers, more than one in eight people in the US aged 75 and over will endure a moderate-to-severe blockage of the heart's aortic valve. Doctors treat these cases by using a catheter to carefully place an artificial valve inside the aorta, a procedure known as a transcatheter aortic valve replacement (TAVR). But determining the correct size is something of a guessing game.
"If you buy a pair of shoes online without trying them on first, there's a good chance they're not going to fit properly," says James Weaver, Ph.D., a Senior Research Scientist at Harvard University's Wyss Institute. "Sizing replacement TAVR valves poses a similar problem, in that doctors don't get the opportunity to evaluate how a specific valve size will fit with a patient's anatomy before surgery."
And getting it wrong can have serious consequences. Too small and the artificial valve can become dislodged and leaky, too large and it can rupture the heart, potentially in a fatal manner.
Scientists do currently have some tools at their disposal when preparing for these procedures. Patients will typically undergo a CT scan where X-ray images are used to generate a 3D reconstruction of the heart, but this only paints a partial picture. While these will illustrate the outer walls of the aorta along with any calcium buildup, the leaflets are too thin to appear, making it hard to predict how well the artificial valve is going to fit.
Now, the team at Harvard, along with collaborators at Brigham and Women's Hospital, The University of Washington, Massachusetts General Hospital, and the Max Planck Institute of Colloids and Interfaces, have come up with a way to fill in the blanks.
It centers on novel computer software that is able to produce 3D models of the leaflets through parametric modeling by using a set of seven coordinates taken from the CT scan. This, together with the CT data are then used to 3D print a physical model of the heart valve, complete with the aortic walls, calcium deposits and leaflets.
Alongside this model, the team also 3D printed a device described as a "sizer," which can be placed inside the valve model and expanded and contracted until the perfect fit is found, with a thin pressure-sensing film wrapped around it acting as a guide.
The team then tested out the new technology using data from 30 patients who had already undergone TAVR procedures, exactly half of which went on to develop leaks as a result of the valves being undersized. Comparing the ideal valve size for each patient to the ones they actually received revealed that only 60 percent had gotten the correct size, and the team was able to successfully guess the leaky outcomes with a success rate between 60 to 73 percent.
These kinds of insights can help clinicians decide ahead of time which patients will benefit from TAVR procedures, and which are more complex cases that would be better off opting for open-heart surgery.
"Being able to identify intermediate- and low-risk patients whose heart valve anatomy gives them a higher probability of complications from TAVR is critical, and we've never had a non-invasive way to accurately determine that before," says co-author Beth Ripley from the University of Washington. "Those patients might be better served by surgery, as the risks of an imperfect TAVR result might outweigh its benefits."
The team's leaflet modeling software and 3D printing methods are freely available online, with the hope that researchers and clinicians can go ahead and make use of it for better patient outcomes.
The research has been published in the Journal of Cardiovascaulr Computed Tomography.
Source: Wyss Institute