Acoustic Signals Could Help Identify Knee Osteoarthritis

Wide-band piezoelectric transducers are used in the sensors to detect sound waves with frequencies up to 400 kHz emitted during knee movement, based on the rationale that smooth, optimally lubricated surfaces slide quietly against each other whereas rough, suboptimally lubricated surfaces move unevenly, producing acoustic signals. The inspiration for the device comes from acoustic tools used to detect damage to bearings, such as in jetty loading arms which pump oil and other liquids from ships docked at port.

The researchers found significant differences between healthy and OA knees using statistical analysis of both AE occurrence and distribution of the AE signal features in different movement phases. OA knees were found to produce 6–10 times more acoustic emissions than healthy knees, with amplitudes up to 20 dB higher, and durations up to 10 times longer. A visual representation method of AE feature profiles--based on the multidimensional density of AE hits per repeated movement, and the use of principal component analysis--could lead to an eventual objective assessment of the clinical status of knee joints. The study was published in the May 2010 issue of the Annals of the Rheumatic Diseases.

"We wanted to see whether we could listen to moving joints and use the patterns of sound that are emitted to tell us something about whether the joint is changing,” said lead author John Goodacre, Ph.D. "If we can actually pick the changes up earlier, then that would lay the way open to a whole variety of approaches. ”

Acoustic emission is a naturally occurring phenomenon whereby external stimuli, such as mechanical loading, generate sources of elastic waves. An AE occurs when a small surface displacement of a material is produced, due to stress waves generated when a rapid release of energy occurs in a material or on its surface. The wave generated by the AE source can be used for industrial inspection, quality control, system feedback, process monitoring, and other uses.

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