Microstructural response and fluid flow mechanisms in cartilage loading: New insights using a novel indentation method

Abstract

A novel indentation method was used to investigate the response of articular cartilage in the non-directly loaded region. The indenter contained a relief channel that allowed a tissue bulge to develop within it under load. Healthy bovine tissue samples were statically loaded at a nominal compressive stress of 3.6 MPa. The tissue's equilibrium deformed state was chemically fixed. Differential interference contrast microscopy was used to obtain highresolution images of the deformed microstructure in the region of the tissue bulge.At the submicro-level, the fibrillar resistance to load is highly complex such that regions of relative compression and tension coincide along a single radial direction. This fibrillar level response is manifested as large-scale matrix shear effects within the bulge region. Further, the surface layer, besides being strain-limiting in the tangential direction, has an intrinsic resistance to axial load. Finally, the pattern of load-induced fluid flow is seen to traverse zonal depths and hence suggests an added complexity to the overall permeability in the deformed tissue matrix.

Further Insight into the Depth-Dependent Microstructural Response of Cartilage to Compression Using a Channel Indentation Technique

“…Previously described as a “triple point” [10] this unique pattern of deformation also showed how abrupt the shear was within the directly loaded region as well as between directly loaded and nondirectly loaded regions (refer to regions X, Y, and Z in Figure 11). …”

“…Details of this indenter have been described elsewhere [10]. An Instron 5543 materials testing machine was used for the stress relaxation experiments which were conducted with the samples bathed in 0.15 M saline.…”

“…This optical technique allows fully hydrated sections to be imaged at levels of structural resolution that are highly informative with regard to the pattern of matrix deformation [8]. Second is in the use of a compression indenter incorporating a central channel which creates two distinct sites of structural interest, namely, a directly loaded region and an indirectly loaded relief zone which develops within the channel space [10]. An analysis of the patterns of matrix deformation across these two sites provides new possibilities for investigating fluid flow-related effects, matrix permeability, fibrillar interconnectivity, and the role of the strain-limiting tangential layer.…”

A Microstructural Study of Load Distribution in Cartilage: A Comparison of Stress Relaxation versus Creep Loading

“…Recent studies have attempted to address these visualisation issues and two experimental procedures have been developed by the present researchers that have relevance to the question of how the cartilage matrix, with its coupled fluid phase, responds to deformation [13][14][15]. First is the method by which the compressed state of cartilage-on-bone samples is captured and the related structural response analysed using differential interference contrast (DIC) optical microscopy.…”

“…DIC microscopy enables fully hydrated sections to be imaged at levels of resolution where microstructural details of the cartilage matrix deformation may be imaged [13]. DIC, with polarised light microscopy, is able to delineate between structurally different alignments of fibrillar matrix and is able to clearly show boundaries between zones of different fibrillar orientations [13,14].…”