Though anatomy and biochemistry of cartilage are undeniably important metrics, the sine qua non of cartilage functional integrity is its mechanical properties. Presently, the only available means for in vivo assessment of functional integrity is arthroscopic probing. Unfortunately, the use of arthroscopy is limited because it is inherently invasive and joint anatomy restricts observation to areas accessible to the probe -- again highlighting the need for a noninvasive method for assessing functional integrity. Therefore, another goal of our group was to determine whether the mechanical responses elicited during indentation testing at sites on a human articular surface ex vivo but in situ could be predicted by the average GAG concentration in the cartilage at the sites of indentation. Indentation studies done at locations corresponding to the image planes provided surface "map" showing localized indices of mechanical properties. In a pilot study of three tibial plateau samples, from three different patients, a high correlation was found overall between MRI derived [GAG] and mechanical stiffness (e.g. Figure 1). These results suggest MRI may potentially provide a noninvasive measure of the cartilage mechanical properties.

gag map1gag map 2Figure 1: Example [GAG] map of a section of human tibial plateau: the cartilage surface is superior, subchondral bone inferior. Colormap units are mg/ml. (Note that [GAG] values are valid only in the cartilage). The numbered arrows show the positions of 11 loci where stiffness measurements were performed. MRI-lucent calibration markers are visible beneath the sample. An area of relative GAG depletion can be seen extending from locus 2 to locus 7.