Physical diagnostics of cartilage degeneration

Establishing the link between macroscopic electromechanical properties, biochemical composition and ultrastructural organization in cartilage will elucidate the role of mechanical forces in regulating the biosynthetic activity of chondrocytes to maintain a functional extracellular matrix (ECM), and how this process is compromised as osteoarthritis (OA) progresses. Assessment of physical properties of articular cartilage may lead to a better understanding of why certain joints are more prone to OA. Symptomatic OA develops rarely in the ankle (<1%), and the prevalence is independent of age, while in the knee the prevalence increases to 10% in those over 65 years of age. In this study, a protocol was developed to assess the biomechanical properties and biochemical composition of human knee and ankle cartilage. It was found that the ECM of the talar (ankle) cartilage is denser with higher charged glycosaminoglycan content and lower water content, consistent with a higher equilibrium modulus and dynamic stiffness, and lower hydraulic permeability. This denser ECM may be chondroprotective. Its biomechanical properties may endow it with an increased stability to loading, protecting the chondrocyte and making the cartilage less susceptible to OA. These findings demonstrate the utility of diagnostic tools which assess the physical properties of cartilage.

It had been previously shown that damage due to trypsin, which predominately degrades proteoglycan, can be sensitively detected by surface electromechanical spectroscopy measurements. We, therefore, tested the hypothesis that surface electromechanical spectroscopy measurements could sensitively detect degradative changes in cartilage matrix caused by collagenases MMP-1 and MMP-13. We found that MMP-1 induced damage to the collagen-aggrecan network was detected by changes in the current-generated stress response. In addition, the measurement of total tissue impedance using interdigitated electrodes placed on the cartilage surface was capable of detecting superficial (MMP-13) and deeper (MMP-1) lesions caused by collagenase in vitro. The ability of surface electromechanical spectroscopy to detect changes in both electrokinetic and impedance properties enhances its potential diagnostic capabilities in vivo.

Diagnostic applications of surface electromechanical spectroscopy in vivo require the measurement of current-generated stress and impedance non-destructively during arthroscopic or open joint procedures. Therefore, an electrokinetic surface probe has been designed with a 4.5 mm diameter active area. Its size makes it possible to use it within an arthroscopic canula. Its multiple wavelength capability permits the spatial localization of surface cartilage lesions typical of early progressive OA. This a step toward providing physicians with a diagnostic tool for determining cartilage degeneration in the clinic. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)