Differences in Acoustic Properties of Intact and Degenerated Human Patellar Cartilage During Compression

Ultrasound indentation measurements have been shown to provide means to assess cartilage integrity and mechanical properties. To determine cartilage stiffness in the ultrasound indentation geometry, cartilage is compressed with an ultrasound transducer to determine the induced strain from the ultrasound signal using the time-of-flight principle. As the ultrasound speed in cartilage has been shown to vary during compression, the assumption of constant speed generates significant errors in the values of mechanical parameters. This variation in ultrasound speed has been investigated in intact cartilage, however, its existence and significance in degenerated tissue is unknown. In the present study, we investigate this issue with both intact and spontaneously degenerated human tissue. To accomplish this aim, we determined ultrasound speed and attenuation in human patellar cartilage (n = 68) during mechanical loading. For reference, cartilage mechanical properties and proteoglycan, collagen and water contents were determined. The acoustic properties were related to the composition and mechanical properties of the samples. Ultrasound speed showed significant, site-dependent variation and it was significantly associated (r = 0.79-0.81, p < 0.01) with the mechanical properties of cartilage. The compression related decrease in ultrasound speed showed statistically significant variation between different stages of degeneration. Error simulations revealed that changes in ultrasound speed during 2% compression could generate errors up to 15% in the values of elastic moduli of samples with early degeneration, if determined with the ultrasound indentation technique. In samples with advanced degeneration, the error was significantly (p < 0.05) smaller being 2% on average. As the compression related variation in ultrasound speed was lower in more degenerated samples, the mechanical parameters could be diagnosed more reliably in tissue showing advanced degeneration. The present results address the need to consider possible uncertainties in mechano-acoustic measurements of articular cartilage and call for methods to correct the effect of variable sound speed during compression. (E-mail: panu.kiviranta@uku.fi)