Stethoscope for your Knee

Research engineers at the Georgia Institute of Technology are developing a knee band with microphones and vibration sensors to listen to and measure the sounds inside your knee.

It could lead to a future device to help orthopedic specialists assess damage after an injury and track the progress of recovery.

Omer Inan is an assistant professor of electrical and computer engineering at Stanford University. His group has published a paper on the latest state of development in the journal IEEE Transactions in Biomedical Engineering. The research is being sponsored by the DARPA Biological Technologies Office.

Currently, the researchers are graphing out the recorded audio and matching it to the joint’s range of motion to see where exactly in the leg’s extending and bending the knee creates creaks and pops. The result has peaks and squiggles that resemble an electrocardiogram or other physiological signal.

The acoustic pattern an injured knee produces is markedly different from that of an intact knee. “It’s more erratic,” Inan said. “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, and elderly patients who have slipped and fallen, but DARPA’s interest is to cut down on repeat battlefield knee injuries and help get soldiers back to duty safely.

“What most people don’t know is that musculoskeletal injuries of the knees and ankles are among the top reasons for discharge for active duty service members,” Inan said. Backpacks weighing up to 100 pounds press down on soldiers, as they march for dozens of miles over tricky terrain, climb over obstacles on battlefields, and crouch in cramped spots for hours.

Even without a fall or contortion, a soldier can land in surgery then in rehab. The problem may seem fixed months later, but too often it’s not, and too often that’s because of re-injury.

Like professional athletes, soldiers can be overly eager to leap back into the fray. “They were there in the first place because they wanted to help our country, so they want to get back to it,” Inan said.

After surgery and therapy, that knee may feel like new, but when a fervent soldier jumps back onto it, weaknesses from the injury kick in. As a result, re-injuries are 10 times more frequent than initial ones.

An inexpensive wearable device could give soldiers and clinicians in the future feedback on convalescing knees to help them avoid major re-injury by refraining from heavy workloads when needed.

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. “You can have cases of early osteoarthritis,” Inan said.

The researchers combined microphones with piezoelectric film. The film is a hypersensitive vibration sensor and collects the best sound, but it is very sensitive to interference. The microphones placed against the skin make for an ample backup and for a more practical device.

The knee monitor also takes advantage of a technical advancement you will find in your smart phone. Micro-electromechanical systems microphones, or MEMS, integrate better with current technology than microphones based on previous technologies, and that also makes the microphones downright cheap – 50 cents to a dollar – for a very affordable device.