What do rubber tires from the earliest automobiles have in common with fatty atherosclerotic plaques inside blood vessels in the human body? The way they rupture, it turns out, is driven by the same physical processes. In the 1920s, rubber tires exploded, on occasion, due to tiny solid impurities in the rubber. And according to a new PNAS Early Edition paper, the rupture of fibrous caps that cover atherosclerotic plaques in the human body can be attributed to the patterns of microcalcifications embedded in them.
“What we found was just like in rubber tires,” says biomedical engineer Sheldon Weinbaum of the City College of New York, who led the new work. Pairs of closely spaced inclusions in atherosclerotic plaques, he and his colleagues discovered, can produce high enough levels of stress between them to cause a rupture at typical blood pressures.
Weinbaum has been studying the biomechanics of plaque rupture for more than a decade and his work is helping solve a long-standing puzzle surrounding how fibrous caps tear and how to better predict these ruptures.
Atherosclerotic plaques are accumulations of fatty molecules and immune cells that accumulate over time in a person’s arteries. Over the core of the plaque, a thin fibrous cap helps contain the build-up. But when this cap ruptures, it can break loose and send the contents of the plaque careening through the blood stream toward the heart or brain, where they can cause a blockage—and a heart attack or stroke.
Clinicians want to know which of their patients have plaques that are at highest risk of this rupture, so in the late 1980s, biomechanics researchers began studying the process, hoping to find a technique that would pinpoint the riskiest plaques. Models predicted that most of the caps would rupture at their edges, where the supporting structure was the thinnest. But when scientists studied real plaques, more than a third ruptured at their centers. Scientists were stumped as to why.
By 2006, the puzzle still hadn’t been solved, and Weinbaum’s team proposed a solution in a PNAS paper: “We knew that something strange was going on, and we proposed that there were tiny microcalcifications in the caps that nobody was aware of,” he explains. “But the entire field had been focused on macrocalcifications.” The macrocalcifications that clinicians could detect were around 200 microns across—Weinbaum was proposing that there were inclusions under 20 microns wide.
Weinbaum’s team began to develop ways to detect such tiny microcalcifications, and last year published data using a technique that could find inclusions as small as 15 microns across. They found 81 microcalcifications in 9 plaques, proof that small inclusions existed. But the microcalcifications were relatively rare, in only fifteen percent of plaques studied, and the data still didn’t explain how they could lead to such a high percentage of ruptures.
“We made an educated guess that there were still many more we couldn’t see and we were just looking at the tip of the iceberg,” says Weinbaum, “But it’s like looking for a needle in a haystack.” The human coronary artery samples were several centimeters long and the microcalcifications mere microns.
Weinbaum’s team found a way to sort through the haystack though—time on a cutting-edge microCT machine with a resolution of 2 microns.
In their new study, the researchers looked with improved resolution at the same 92 coronary artery samples they’d analyzed in their last paper, and found not 81 microcalcifications, but a whopping 34,408. To sift through the large amounts of data, they needed to develop whole new analysis methods. They first came up with a way to quantify the the local tissue stress that the fibrous cap around each inclusion would be under. Their first key discovery was that the stress in the local tissue was more dependent on the shape of the calcifications than their sizes. Elongated microcalcifications led to significantly higher levels of tissue stress.
Next, Weinbaum’s team observed that the highest stress was found in the small gap between two closely spaced calcifications. So they ran a new algorithm to study all the possible pairs of microcalcifications in their data set. Out of the almost 35 thousand microcalcifications, only 3 pairs were close enough together that their computed “stress concentration factor” could have led to a rupture. Since the plaques studied were all unruptured, Weinbaum’s team wanted to know why those pairs hadn’t caused ruptures. That led to the third part of the finding: the pairs had to be oriented in the right direction in terms of the rest of the tissue’s tension in order to cause enough stress. The three pairs the team had found were either in the wrong orientation or in a region where the cap was too thick, explaining why the plaques were still intact.
The takeaway from the findings is that the fibrous caps of atherosclerotic plaques contain thousands of microcalcifications—a fact that itself has never been appreciated before—but it likely takes the rare combination of properly oriented pairs of inclusions and local cap thickness to cause a rupture.
“The next step is to take plaques inside vessels, pressurize them, and make sure the ruptures definitively occur at the location of these microcalcification pairs,” says Weinbaum. Then, clinicians could begin developing higher resolution imaging methods to detect such pairs in patients to prevent the ruptures.