Boston
EAVESDROPPING on the inaudible rumblings of damaged knees could help doctors
diagnose diseases such as osteoarthritis鈥攐r let football managers decide
if an injured player is ready to take to the pitch again.
Many knee conditions involve damage to cartilage, the soft tissue on the ends
of bones that cushions the joint. When a joint is in action, the cartilage gets
compressed and vibrates and some doctors have used stethoscopes or even a small
microphone to amplify the sound it makes. But it鈥檚 hard for doctors to tell the
sounds of healthy and damaged cartilage apart, so they mostly prefer expensive
magnetic resonance imaging for examining joints. According to Canadian
researchers, however, they鈥檝e been listening to the wrong sound frequencies all
along.
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Sridhar Krishnan and his team at Ryerson University in Toronto have applied
for a patent on a diagnostic device that relies on vibration sensors similar to
those in the seismographic stations that listen out for earthquakes. Seismic
signals are normally below 20 hertz, making them inaudible to the human ear. To
use the device, the patient sits on a table and swings the leg in question,
working the knee joint. Vibration sensors taped around the knee relay
low-frequency data to a computer. This shifts the data to a higher frequency, so
it鈥檚 within the range of human hearing, and plays it back more slowly.
In a study carried out with colleagues at the University of Calgary鈥檚 Sport
Medicine Centre, Krishnan found a big difference between the sounds from 19
knees with degraded cartilage, and 18 normal knees, he reports in the current
issue of The Journal of the Acoustical Society of America (vol 110, p
3292). 鈥淚n an abnormal knee you get a mixture of sounds鈥攇rinding and a
sound like a long pop,鈥� says Krishnan. 鈥淏ut in a normal knee you only hear a
clicking sound.鈥� With training, doctors could identify an abnormal knee about 80
per cent of the time, he says. That鈥檚 accurate enough for widespread
screening.
Ian Holte of the University of New Mexico found it difficult to develop an
acoustical diagnostic method for joints at audible frequencies. 鈥淚t鈥檚 just so
straightforward to go in and look at the actual surfaces with an optic fibre
now,鈥� he says. But Krishnan says seismic changes may appear before any visible
signs of knee degradation. 鈥淲e might detect the early stages of osteoarthritis
years before it shows on MRI,鈥� he says.