Wearable stress-fracture detection system
4 October 2006
Researchers at the Universities of Purdue and Toledo have collaborated to
create a prototype device that could be used to monitor the formation of
microcracks in bones that can lead to hairline stress fractures unless
detected in time.
(seated), a doctoral student in Purdue's Weldon School of Biomedical
Engineering, and associate professor Ozan Akkus discuss the
prototype device. Source: Purdue News Service/David Umberger
This could help prevent fractures in a wide range of susceptible people
who undergo regular strenuous exercise, such as military recruits, sports
people and dancers. It has even been suggested that the famous metatarsal
fractures of England footballers such as David Beckham, Wayne Rooney, Steven
Gerrard and Michael Owen could have been due to over-training making them
more susceptible to fractures. A high proportion of race horses in training
also suffer from fractures.
The researchers are applying the same basic sound detection technique
seismologists use to measure earthquakes. "The goal is to create a wearable
device that would alert the person when a stress fracture was imminent so
that they could stop rigorous physical activity long enough for the bone to
heal," said Ozan Akkus, an associate professor in Purdue's Weldon School of
The system records "acoustic emission data," or sound waves created by
the tiny bone fissures. The same sorts of acoustic emissions are used to
monitor the integrity of bridges, other structures and mechanical parts like
helicopter turbine blades.
Such a technology could help prevent serious stress fractures in
racehorses and those who perform in situations that cause undue stress to
bones, such as soldiers, athletes and dancers.
The system could be especially useful in preventing fractures in Army
recruits undergoing basic training. "Strenuous military exercises subject
soldiers to prolonged physical activity in which relatively small forces are
repeatedly exerted on bones," Akkus said. "The forces are not initially
strong enough to break a bone, but it's the repetition that poses the most
danger by causing microscopic cracks to accumulate over time and eventually
result in stress fractures."
The cracks form when collagen fibres in bone fail, producing sound waves
that cause a rippling motion on the skin's surface.
"This is the same thing that happens during an earthquake, but on a
microscopic scale and at a higher frequency," Akkus said. "Instead of an
earthquake-size opening, these cracks are about a tenth of a millimeter
Accumulating cracks sometimes cause "spontaneous fractures" that occur
without warning, afflicting the young and old alike, including athletes and
elderly people suffering from osteoporosis.
A major factor in the crack formation is the dynamic process bones use to
continually rebuild themselves. When bone is damaged, specialized cells bore
tunnel-like holes to remove the damaged tissue and then fill in the
resulting cavity with new bone.
"Bone is a very smart material because it can detect and repair damage,"
Akkus said. "That's what keeps your bones young. The repair process digs
tunnels and fills them, digs tunnels and fills them. There is a continuous
renewal, but it takes longer to refill the holes than it does to dig them,
so there is always some porosity, which increases the stress locally in the
most porous portions of bone."
Hard physical activity without rest increases the stress in these porous
areas that are under repair. "The localized stress in the porous portions
then becomes very high, and this can result in a complete stress fracture,"
One reason it's difficult to diagnose the hairline fractures is because
they are caused by the gradual accumulation of microscopic cracks, which are
not detectable with conventional imaging technologies.
"It's really hard to measure stresses in bone without cutting open the
bone to study it," Akkus said. "And there is very little warning because you
don't have horrible pain. You might have some discomfort, but you can keep
exercising or whatever activity you are doing."
From 1 percent to 20 percent of U.S. basic training recruits experience
stress fractures in the femur, commonly referred to as shin splints,
depending on the service branch and type of training, with the highest
incidence in women recruits. In horse racing, 70 percent of young
thoroughbreds experience fractures, Akkus said.
The researchers are developing the monitoring technique by studying crack
formation in pieces of bone from human cadavers that are placed in a machine
that continually bends the bone until it cracks.
Akkus is working with researchers at the University of Toledo to develop
a wearable prototype that will record crack-formation data, which could be
downloaded to a portable digital assistant, or PDA, for review by medical
professionals. Such a device could immediately alert the person by sounding
an alarm, and the data could then be scrutinized by a doctor.
"All of the technology is available, and the sensors exist off the
shelf," he said. "We just have to modify them to work with our system."
Sensors made of a "piezoceramic" material generate electricity when
compressed by a force, such as the vibration created by seismic waves
resulting from crack formation.
"Recently, flexible polymer-based sensors have appeared on the market,
and these could be incorporated into athletic apparel, such as running shoes
and exercise tights to monitor areas most susceptible to fractures," Akkus
said. "Ultimately, we would like to do real-time monitoring of damage
activity and learn how to distinguish between a small crack and a more
structurally threatening defect.
"There are different types of cracks that occur, and it's important to be
able to distinguish among them so that we can determine how serious the
To distinguish the difference between the various types of cracks,
researchers are integrating "pattern recognition" software and earthquake
models, working with Robert Nowak, a Purdue professor of earth and
atmospheric sciences. The multidisciplinary research involves biomedical and
electrical engineering, veterinary medicine, and earth and atmospheric
"One challenge will be to learn when damage is serious enough that you
should stop exercising," Akkus said. "You don't want to give a professional
athlete a premature warning."
Bones most affected are those in the feet, legs and hips, particularly
the ball-and-socket joints that connect the legs to the pelvis.