Grading of osteoarthritic cartilage: Correlations between histology and biomechanics

We read the manuscript by Waldstein and colleagues,1 appearing in this issue, with great interest and commend them on their nice study that provides the field with an appreciation for cartilage mechanical property changes that correspond to OA grades of the OARSI osteoarthritis cartilage histopathology assessment system.2 Several important findings can be gleaned from the study that have implications to the clinic and laboratory studies alike.

The current study reaffirms what many in the field have long suspected, that gross appearance of cartilage tissue with intact surfaces of normal and early degenerative changes (grade 0 intact surface and grade one with intact but uneven surface of the OARSI scale) may exhibit very different functional properties, as supported by a corresponding significant drop in mechanical properties (equilibrium modulus and dynamic modulus).

The dynamic modulus is most meaningful as it correlates to nearly the full range of the OARSI histology grading scale, decreasing significantly from grade 0 to grade 4. This correlation is remarkable considering the technical challenges associated with testing degenerative tissue, exhibiting variable thickness and regional distribution of properties. The equilibrium modulus, a measure of the solid matrix contribution to load support, appears from this analysis to be insensitive to degenerative changes beyond grade 2. These measurements, however, may have been more susceptible to the adverse testing conditions associated with tissue degradation, since creep indentation testing under a prescribed load can produce increasingly higher compressive strains with advancing tissue degradation, possibly producing nonlinear responses that may confound the analysis. This may have been reflected in the exclusion of a data subset due to poor numerical fit of the indentation response to the biphasic theory.3–5 When using a less refined visual OA grading scheme, the biphasic indentation creep response of human thumb cartilage has shown significant effects in the aggregate modulus between OA and non‐OA tissue6; using biphasic unconfined compression stress‐relaxation under a prescribed compressive strain, human knee cartilage has similarly exhibited significant differences in Young’s modulus.7

While the trend in dynamic modulus provides a guide to the relative change in functional properties for progressively degenerative cartilage across the OARSI histology grade scale, care should be taken not to categorically assign a specific absolute modulus value as reported here to a corresponding histology grade. This cautionary note is made as measured mechanical properties can depend heavily on the type of biomechanical test performed and the particular testing parameters adopted (such as the rate of applied loading or deformation, indenter geometry and shape, constitutive law).8–10

While histological staining reflects qualitative amount and distribution of biochemical constituents in a single plane or dimension, mechanical testing provides a measured parameter(s) that represents the integrated contribution of tissue biochemical content and structural organization. Accordingly, mechanical measurements have the potential for greater sensitivity and discrimination of subtle degenerative changes in cartilage.11