Now, researchers are hoping to put those cringe-worthy sounds to good use, harnessing them to track the health of patients.
Engineers at the Georgia Institute of Technology have developed a knee band equipped with tiny microphones and sensors that can record the noises our knees make and deliver those recordings electronically to doctors.
Doctors then can listen to those recordings to pinpoint noticeable changes in the sounds, which could help them evaluate damage after a knee injury and track improvements or setbacks in recovery, said Omer Inan, an assistant professor at Georgia Tech who has led research on the new knee band technology.
"We would not want to look at a snapshot of the sounds and try to diagnose if someone has a particular injury. But what we are interested in is looking at a person who already has been diagnosed and then tracking them over time to see if they're getting better or worse," Inan said.
"If an injured person performs exactly the same knee flexion-extension exercise, their knees may produce very different sounds each time," he said. "Whereas, for a healthy person, if they perform the same exercise, we would see very similar signatures. It's very consistent in nature."
Overall, Inan pointed out that a healthy knee without a history of injury tends to sound softer than a knee with a history of injury, which sounds more static-y or crackly.
Here is what my knee, which is healthy and has no history of injury, sounds like:
Here is what Inan's knee sounds like; as a former NCAA discus thrower at Stanford University, he has a history of knee injury:
Knee injuries are common in the military as well as in sports, Inan said. Using sounds to assess the recovery of a soldier's or an athlete's knee could be helpful in deciding when that soldier should return to duty or that athlete should return to competition, he said.
"What we'd like to deliver to a doctor is a health score for the patient, so that score could be something as simple as a red, green and yellow indicator and could then be used by that patient to determine when they can move to different types of exercises during rehab, when they might be able to return to their sport and when they should really back off and maybe work on some more basic exercises to get their knee better," Inan said.
To pair a knee sound with a certain stage in injury recovery, Inan and his colleagues simulated injury and swelling stages in cadaver knees, he said. To simulate swelling, for instance, they put various amounts of saline into the knee capsule and then recorded what sounds that knee produced.
"Quantifying the amount of swelling is a really important question when you look at knee rehabilitation," Inan said. "If we can quantify the level of swelling and then if we can look at if the sounds are abnormal for someone with that level of swelling, it will be really useful information for a doctor."
As a former athlete, Inan said, he hopes the knee band technology could be used to evaluate athletes' knee health. Currently, however, doctors are testing the knee band as a possible marker of joint health for children with arthritis.
A stethoscope for the knees
At any given moment, a pediatric rheumatologist at Children's Healthcare of Atlanta hospitals can be found in a quiet room, listening to the knees of a patient with juvenile arthritis.
The new knee band is being tested and piloted in a trial at the hospital, said Dr. Sampath Prahalad, a professor at Emory University School of Medicine in Atlanta and a pediatric rheumatologist at the hospital.
"We use stethoscopes to hear heart and lung sounds, but we have so far not been able to systematically study joint sounds. This idea was very intriguing, and I felt it will fill a need," Prahalad said of testing the knee band.
"We have compared normal joints with arthritic joints and can find differences," he said. "We also have observed differences in joint sound recordings in the same patient before and after successful treatment of their arthritis."
When listening to a healthy knee in a child, there is almost no sound, or the knee is very quiet, Prahalad said. However, the knee of a child with juvenile arthritis produces more pronounced sounds.
"It's just like, 'Chhh, Chhh, Chhh.' It was interesting," Prahalad said. He added that his colleagues have approached the new technology with a "mixture of curiosity, amusement and promise."
"We all hear patients complain of joints, and we don't know what to tell them," he said. "Now, we can listen for ourselves."
In the future, listening to a knee also can be useful if the knee band device could isolate the exact location from which a noise emanates, said Dr. Armin Tehrany, an orthopedic surgeon, knee specialist and founder of Manhattan Orthopedic Care
in New York, who has not been involved with the knee band technology.
"A patient can often feel and hear sounds coming from the front of the knee or by the kneecap. Other times, patients can also hear sounds coming from the inside or outside of the knee, but they have a difficult time distinguishing where the sound is actually coming from," Tehrany said.
"If that sound can be better pinpointed by a new technology such as this, and if research can be done to see if that sound correlates with arthritis or other pathology, then it can help orthopedic surgeons as a diagnostic tool before MRI scans are necessary," he said, describing the technology as novel. "Knee injuries are very common. The groups that most commonly suffer these injuries are athletes and those with a high body mass index."