Physical Diagnostics of Cartilage Degeneration

Summary

We have focused on a new technology for nondestructive measurement of electrical and mechanical properties of articular cartilage via electrodes placed on the tissue surface. The long-term goal of this research is to enable detection of early stages of cartilage degradation based on the sensitivity of cartilage electromechanical properties to damage of the aggrecan-collagen network and loss of the highly charged aggrecan molecules. Ultimately, this technique may find application in early in vivo detection of cartilage degradation via arthroscopy. Experimental and theoretical results [1,2] have shown that an electric current applied to the articular surface of cartilage will produce a current-generated mechanical stress within the bulk of the tissue, via electrokinetic mechanisms, measurable at the surface. Based on this principle, a surface probe has been developed containing electrodes for applying current to the cartilage surface and an overlying piezoelectric sensor for measuring the resulting stress. Small sinusoidal currents applied to cartilage produce sinusoidal surface stresses at the same frequency. Such responses have been observed using disks of bovine articular cartilage, cartilage on intact bovine knee joint surfaces, and, recently, in preliminary in vivo studies in a canine knee joint model. The frequency response of the current-generated stress was found to agree well with trends predicted by poroelastic theory [1]. Because the depth to which current penetrates into the tissue is proportional to the imposed spatial wavelength (twice the electrode spacing), we have developed a multiple wavelength probe to test the possibility that measurement using multiple wavelengths and frequencies could be used to image depth-dependent partial thickness degradation, as occurs in early osteoarthritis.