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Letters |

Engineering Terms in Joint Disease

Roy E. Brackin, MD
JAMA. 1964;190(4):401. doi:10.1001/jama.1964.03070170142031.
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To the Editor:—  Pentecost, Murray, and Brindley are correct in their explanation of the mechanisms of "stress" fractures ("Fatigue, Insufficiency, and Pathologic Fractures," JAMA187:1001 [March 28] 1964).Engineering terms can also be used to describe subaudible musculoskeletal sound. The knee joint, while in passive motion, generates compound wave forms, as is shown when this acoustic energy is converted to electric energy and recorded. Under certain circumstances, the knee joint at rest generates a wave motion as a pure sinusoidal wave of simple harmonic motion of 28 or 29 cycles per second. These vibrations are recovered from normal knees as periodic vibrations of the menisci vibrating as a system. Under conditions of injury or disease of the menisci this sine wave is absent or distorted because the meniscus vibrates in parts. The natural frequencies of the menisci are 28 cps medially and 29 cps laterally. Their voltage output varies


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