Dec 11 2018
Over one in eight people aged 75 and older in the US develop moderate-to-severe blockage of the aortic valve in their hearts, commonly caused by calcified deposits that accumulate on the valve’s leaflets and hinder them from fully opening and closing. Most of these older patients are not healthy enough to go through open-heart surgeries; instead, they have artificial valves implanted into their hearts using a procedure known as transcatheter aortic valve replacement (TAVR), which installs the valve via a catheter injected into the aorta. There are difficulties with this procedure, however, including the need to select the perfect-sized heart valve without ever really seeing the patient’s heart: too small, and the valve can dislocate or leak around the edges; too large, and the valve can tear through the heart, carrying a danger of death. Similar to Goldilocks, cardiologists are hunting for a TAVR valve size that is “just right”.
CT scans and a custom parametric modeling process were combined to create multi-material physical models of patients’ aortic heart valves, each with its own unique size, shape, and amount of calcification. (Credit: Wyss Institute at Harvard University)
Scientists at the Wyss Institute for Biologically Inspired Engineering at Harvard University have developed a unique 3D printing workflow that enables cardiologists to assess how different valve sizes will interact with each patient’s distinctive anatomy before the medical procedure is really performed. This protocol uses CT scan data to create physical models of individual patients’ aortic valves, along with a “sizer” device to establish the flawless replacement valve size. The work was carried out in partnership with researchers and physicians from Brigham and Women’s Hospital, The University of Washington, Massachusetts General Hospital, and the Max Planck Institute of Colloids and Interfaces, and is reported in the Journal of Cardiovascular Computed Tomography.
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. 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. Our integrative 3D printing and valve sizing system provides a customized report of every patient’s unique aortic valve shape, removing a lot of the guesswork and helping each patient receive a more accurately sized valve.
James Weaver, Study Co-Author and Ph.D., Senior Research Scientist, Wyss Institute.
When a patient requires a replacement heart valve, they often get a CT scan, which captures a series of X-ray images of the heart to form a 3D reconstruction of its internal anatomy. While the outer wall of the aorta and any related calcified deposits are easily seen on a CT scan, the delicate “leaflets” of tissue that open and close the valve are mostly very thin to appear well.
After a 3D reconstruction of the heart anatomy is performed, it often looks like the calcified deposits are simply floating around inside the valve, providing little or no insight as to how a deployed TAVR valve would interact with them.
James Weaver, Study Co-Author and Ph.D., Senior Research Scientist, Wyss Institute.
To solve that issue, Ahmed Hosny, who was a Research Fellow at the Wyss Institute at the time, developed a software program that uses parametric modeling to produce virtual 3D models of the leaflets using seven coordinates on each patient’s valve that can be seen on CT scans. The digital leaflet models were then combined with the CT data and tweaked so that they fit into the valve fittingly. The resulting model, which includes the leaflets and their related calcified deposits, was then 3D printed into a physical multi-material model.
The team also 3D printed a tailored “sizer” device that fits inside the 3D-printed valve model and expands and contracts to establish what size artificial valve would ideally fit each patient. They then wrapped the sizer using a thin layer of pressure-sensing film to map the pressure between the sizer and the 3D-printed valves and their related calcified deposits, while slowly expanding the sizer.
“We discovered that the size and the location of the calcified deposits on the leaflets have a big impact on how well an artificial valve will fit into a calcified one,” said Hosny, who is presently at the Dana-Farber Cancer Institute. “Sometimes, there was just no way a TAVR valve would fully seal a calcified valve, and those patients could actually be better off getting open-heart surgery to obtain a better-fitting result.”
Furthermore, the multi-material design of the 3D-printed valve models, which integrate flexible leaflets and rigid calcified deposits into a completely integrated shape, could much more accurately imitate the behavior of real heart valves during artificial valve deployment, as well as offer haptic feedback as the sizer is stretched.
The researchers tested their system against data from 30 patients who had already gone through TAVR procedures, 15 of whom had developed leaks from valves that were very small. The team predicted, based on how well the sizer fit into the 3D printed models of their aortic valves, what valve size should each patient have received, and whether they would be any leaks after the procedure. The system was able to effectively predict leak outcome in 60-73% of the patients (based on the type of valve the patient had been provided), and established that 60% of the patients had received the right size of valve.
“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,” said co-author Beth Ripley, M.D., Ph.D, an Assistant Professor in the Department of Radiology at the University of Washington who was a Cardiovascular Imaging Fellow at Brigham and Women’s Hospital when the research was done. “Those patients might be better served by surgery, as the risks of an imperfect TAVR result might outweigh its benefits.” Furthermore, being able to physically mimic the procedure might inform future iterations of valve designs and deployment methods.
The researchers have made their leaflet modeling software and 3D printing protocol freely available online for clinicians or scientists who desire to use them. They hope their project will serve as a catalyst for evolvable biomedical design that can keep abreast with the market’s advancement.
“At the core of the personalized medicine challenge is the realization that one medical treatment will not serve all patients equally well, and that therapies should be tailored to the individual,” said Wyss Institute Founding Director Donald Ingber, M.D., Ph.D., who is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Boston Children’s Hospital, as well as Professor of Bioengineering at Harvard’s School of Engineering and Applied Sciences. “This principle applies to medical devices as well as drugs, and it is exciting to see how our community is innovating in this space and attempting to translate new personalized approaches from the lab and into the clinic.”
Additional authors of the paper include Joshua Dilley, M.D. from Massachusetts General Hospital; Tatiana Kelil, M.D., an Assistant Professor of Radiology at University of California, San Francisco and former Radiology Resident at Brigham and Women’s Hospital; Moses Mathur, M.D. from the University of Washington; and Mason Dean, Ph.D. from the Max Planck Institute of Colloids and Interfaces in Germany.
The Human Frontier Science Program supported this research.