Knee injuries are absolutely no fun. And the sounds they make can often make the agony worse. But now, that awful crunching and cracking could help doctors determine just how damaged the joint is, how fast it's healing and how much more therapy it needs. Research engineers at the Georgia Institute of Technology are developing a knee band that uses microphones and vibration sensors to listen to and measure the sounds inside the joint. They hope it will lead to a device that will help orthopedic specialists assess damage after an injury and track the progress of recovery.
Omer Inan knows all about knee pain. As a former discus thrower who was a three-time NCAA All-American at Stanford University and now assistant professor of electrical and computer engineering, Inan has long had a personal interest in developing such a device. Discus throwers not only put massive strain on their knees by whirling round (something the knee isn't exactly designed to do), but also participate in weight training that can include squats with 500-pound loads.
"I would always feel like my knee was creaking or popping more if I was putting more stress on it," says Inan. These are precisely the same injuries that many battlefield military personnel experience. So, when the Defense Advanced Research Projects Agency (DARPA), a governmental organization dedicated to discovering technological solutions to issues of defense, put out a call for research proposals on wearable technologies for assisting rehabilitation, Inan's idea seemed a perfect fit.
The first hurdle was to get a recording of the sounds that was of sufficiently useful quality. This was potentially difficult because knee joints are surrounded by a fluid that deadens higher frequency sound waves. Patient movement can also create extraneous noises that can drown out useful sounds. "The fact that the measurement has to occur by definition during movement is a challenge," says Inan, "because you can't just tell the person to be still and avoid motion."
The researchers combined microphones with piezoelectric sensors. Piezoelectrics are incredibly thin, hypersensitive vibration detectors that collect the best sound. However, because they are so sensitive to interference traditional high-quality microphones were also placed against the skin to offer a "second opinion." The micro-electromechanical systems microphones — similar to the ones in mobile phones — are also cheap, costing around 50 cents to a dollar. All this means any new diagnostic tool could be both affordable and effective.
When he heard the quality of the first recordings, Inan was delighted. "It was a lot louder than expected and a lot clearer," he says. "It's a little bit like some kind of Halloween stuff happening. You're listening to your bones rubbing on each other, or maybe cartilage." The team hopes that future medical research will build on the acoustic sensing technology they are designing and be able to decode the sound and so begin to recognize useful patterns.
Currently, the researchers are graphing out the recorded audio and matching it to the knee's range of motion to map exactly where in the movement of the leg creaks and pops occur. The team says shapes in their results have similarities to an electrocardiogram or other physiological signals. The acoustic pattern an injured knee produces is markedly different from that of an intact knee, they say. "It's more erratic," Inan explains. "A healthy knee produces a more consistent pattern of noises."
If paired with medical research, Inan's acoustic device could lead to inexpensive, wearable monitors, which could benefit athletes who have overburdened their knees or elderly patients who have slipped and fallen. But DARPA's greatest interest is in reducing repeat battlefield knee injuries and helping get soldiers back to duty safely. "What most people don't know," Inan says, "is that musculoskeletal injuries of the knees and ankles are among the top reasons for discharge for active duty service members." Backpacks weighing up to 100 pounds are worn as soldiers march for miles over tricky terrain, climb over obstacles and crouch in cramped spots for hours.
Even without a fall or contortion, a soldier can end up requiring surgery and then rehab. Months later, the problem may seem fixed but often it's not and this can lead inevitably to further injury. Like professional athletes, soldiers can be overly eager to leap back into the fray. However, though the knee may feel like new after surgery and therapy, when a soldier jumps back onto it, weaknesses from the injury can re-emerge. As a result, re-injuries are 10 times more frequent than initial ones.
Inan's team say an inexpensive wearable device could give soldiers and clinicians in the future feedback on convalescing knees and so help them avoid major re-injury by refraining from heavy workloads for an adequate period. That could benefit service members in the long run, too. Joint injuries compound over time, setting retired service members up for pain and loss of mobility long into civilian life and resulting in conditions such as osteoarthritis.
The team's research is published in the online journal IEEE Transactions on Biomedical Engineering.