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Early detection of cartilage diseases

In further experiments, we were able to mimic osteoarthritic disease progression by enzymatic digestion of the GAGs with cathepsin D. At the nanometer scale, i.e. with a sharp nm-size tip, the same difference in the mechanical properties was found as when comparing normal with osteoarthritic cartilage (Fig. 2A). However, at the micrometer scale, i.e. when assessed with a spherical μm-size tip rather than a sharp nm-size tip, no significant hardening was depicted (Fig. 2B). The finding that it is possible to detect alterations at the nanometer scale which are not detectable at larger scales holds a great promise for detecting changes in the cartilage’s biomechanical properties as occurring in osteoarthritis at an early stage of disease progression, hopefully at a stage where the disease might be stopped or even reversed. Moreover, IT AFM should be very helpful for a more systematic analysis of different types of normal, diseased, enzymatically modified as well as engineered cartilage. Such analysis, in turn, will allow us to more rationally understand tissue functionality and, ultimately, will enable us to more effectively repair diseased cartilage, stimulate transplanted cartilage, and design and produce tissue engineered cartilage (TEC) exhibiting long-term mechanical stability and biocompatibility (Langer and Vacanti, 1993).

Fig. 2
The elasticity measurements as presented were taken from the same sample by employing two different indenter sizes, i.e. (A) a nanometer-size sharp pyramidal tip (tip radius = 20 nm) and (B) a micrometer-size spherical tip (tip diameter = 5 micrometer). Each set of the two curves was taken on normal articular cartilage and on cathepsin D digested articular cartilage. For digestion the articular cartilage was treated by cathepsin D at 37°C for two days. Comparison of the two slopes recorded at the micrometer scale exhibited no difference between native and cathepsin D treated cartilage. While at the nanometer scale the cathepsin D treated articular cartilage exhibited a clear stiffening (A), at the micrometer scale the cathepsin D treated cartilage appeared indistinguishable from the normal cartilage (B).

Ultimately, such ex vivo measurements should be performed by direct in vivo inspection of the articular cartilage in a knee or hip joint in a clinical environment (Stolz et al., 2003; Hunziker et al., 2002). To achieve this ambitious goal, we set out to design and build an AFM which can be directly brought to the disease site in a knee joint by an endoscopic approach (Stolz et al., 2003).