- New Faculty Year: 2010
Chris Hernandez is interested in the mechanics of bone. The human body’s structural underpinnings are subject to the same mechanical laws as bridges and buildings, but unlike steel or concrete, bone is self-repairing.
“If you get some cracks in the bone, some cells will come in and repair it,” explains Hernandez. “That’s how we think most of us can survive our whole lives without ever breaking a bone.”
Testing blood serum can reveal how much of this “bone remodeling” is occurring in an individual. Clinical studies have shown that high levels of remodeling can in some cases predict osteoporosis-related bone fractures—which cost $17 billion per year to treat—better than the conventional indicator of fracture risk. “Why is a measured biological activity telling us more about fracture, which is a mechanical event, than a bone scan?” asks Hernandez. “The hypothesis that we’re working with is actually that the repair process is part of the problem. It doesn’t sound right, but that’s what it looks like.”
When cells are sent in to make repairs, they dig out the old or damaged bone leaving a divot before refilling it with new bone. The entire process can take six months. “This can be viewed as a mechanical flaw that can act as a weakness in the structure,” said Hernandez. “One theory is, all these little patches added up weaken the bone and put someone at increased risk for fracture.”
Women are more vulnerable because bone remodeling goes up at menopause and stays high for the rest of a woman’s life. “Women lose bone mass after menopause, partially because of that increased remodeling,” said Hernandez. “And then it’s also thought that as you get older that refilling is not as complete as it is in a younger individual, so every time that event happens you lose a little bone, and over time, ten years maybe, that results in severe bone loss.”
To determine if his theory is correct, Hernandez has created new imaging approaches to get extremely high resolution images of bone. “This,” he said, holding up a model of a bone showing a cross section, “is a 20 times blow up of this piece of tribecular bone and it was created from data collected at a 10 micron resolution but our system gets down to submicron scale. The divots themselves are much bigger than that but to differentiate them from regular bone curvature, we need a higher resolution.”
Hernandez’s system uses a computer controlled milling machine with a fluorescent microscope strapped on the side. “We take the piece of bone, embed it in plastic, and then the milling machine trims 5 microns off the specimen, moves the specimen to the microscope and then the surface of the specimen is imaged,” he said. “It’s a slow approach, but the best nondestructive imaging techniques cannot even come close.”
In another project, Hernandez’s lab is counting the number of divots in a bone and then measuring how much stress it takes to break it to see if they are correlated with bone strength. “Then we use a fluorescent stain that labels all the cracks in the specimen and see if the cracks are originating at the divots,” he said.
Hernandez came to Cornell after five years on the faculty at Case Western Reserve University in Cleveland. “They made me a great offer here, and there are great possibilities—I couldn’t turn it down,” he said. “There are five faculty in the biomechanics group here and most work in orthopedic biomechanics. And of course the connection with the Hospital for Special Surgery are bonuses that make this an exciting research environment.”
Prof. Hernandez' Web site