Spatial localization of cartilage degradation using electromechanical surface spectroscopy with variable wavelength and frequency

This work focuses on developing a new technology for nondestructively measuring electrical and mechanical properties of articular cartilage via electrodes placed on the tissue surface. The long-term goal of this work is to enable detection of early stages of cartilage degradation (e.g., in osteoarthritis) based on the sensitivity of cartilage electromechanical properties to loss of highly charged aggrecan molecules from the tissue. Ultimately, this technique could enable early in vivo detection of cartilage degradation via arthroscopy.

Theoretical analysis has suggested that an electric current applied to the articular surface of cartilage will give rise to a mechanical current-generated stress within the bulk of the tissue, via electrokinetic mechanisms, measurable at the surface. Based on this principle, an electrokinetic surface probe was developed, containing silver/silver chloride electrodes for applying current to the cartilage surface and an overlying piezoelectric polymer film sensor for measuring the resulting stress. Small sinusoidal currents applied to disks of bovine articular cartilage produced sinusoidal surface stresses at the same frequency. The frequency response was found to agree well with a theoretical poroelastic model. Titration of intratissue pH resulted in changes in stress amplitude and phase which paralleled known changes in tissue fixed charge density, with the stress amplitude reaching a minimum near the isoelectric pH of the tissue.

Because the depth to which current penetrates into the tissue is proportional to the imposed spatial wavelength (twice the electrode spacing), it was hypothesized that measurements using multiple wavelengths could enable detection of partial-thickness degradation, as occurs in early osteoarthritis. Thus, a multiple-wavelength probe was developed and tested with full-thickness plugs of calf cartilage. A model system was also developed for producing nonuniform loss of aggrecan, using trypsin, as a model for cartilage degradative changes. Histological and biochemical analysis confirmed that trypsin exposure resulted in progressive loss of aggrecan, proceeding inward from the articular surface. Enzymatic extraction of aggrecan using this model resulted in a relatively greater attenuation of the stress measured using a short-wavelength excitation, in which current remained confined to the degraded region, as compared to long-wavelength excitation, in which current could penetrate the underlying normal tissue as well. These results support the use of wavelength- and frequency-scanning surface spectroscopy for spatially imaging focal regions of cartilage degradation. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)