Inhomogeneous cartilage properties enhance superficial interstitial fluid support and frictional properties, but do not provide a homogeneous state of stress

Abstract

It has been well established that articular cartilage is compositionally and mechanically inhomogenous through its depth. To what extent this structural inhomogeneity is a prerequisite for appropriate cartilage function and integrity is not well understood. The first hypothesis to be tested in this study was that the depth-dependent inhomogeneity of the cartilage acts to maximize the interstitial fluid load support at the articular surface, to provide efficient frictional and wear properties. The second hypothesis was that the inhomogeneity produces a more homogeneous state of elastic stress in the matrix than would be achieved with uniform properties. We have, for the first time, simultaneously determined depth-dependent tensile and compressive properties of human patellofemoral cartilage from unconfined compression stress relaxation tests. The results show that the tensile modulus increases significantly from 4.1 +/- 1.9 MPa in the deep zone to 8.3 +/- 3.7 MPa at the superficial zone, while the compressive modulus decreases from 0.73 +/- 0.26 MPa to 0.28 +/- 0.16 MPa. The experimental measurements were then implemented with the finite-element method to compute the response of an inhomogeneous and homogeneous cartilage layer to loading. The finite-element models demonstrate that structural inhomogeneity acts to increase the interstitial fluid load support at the articular surface. However, the state of stress, strain, or strain energy density in the solid matrix remained inhomogeneous through the depth of the articular layer, whether or not inhomogeneous material properties were employed. We suggest that increased fluid load support at the articular surface enhances the frictional and wear properties of articular cartilage, but that the tissue is not functionally adapted to produce homogeneous stress, strain, or strain energy density distributions. Interstitial fluid pressurization, but not a homogeneous elastic stress distribution, appears thus to be a prerequisite for the functional and morphological integrity of the cartilage.

Figure 1

Contact geometry and finite element mesh in the deformed configuration. The mesh deformation has been scaled up for emphasis in this figure.

Figure 2

Total traction and interstitial fluid pressure at the contact interface between the cylindrical indenter and articular layer, at t = 1 s. Results are shown on the left for the homogeneous models and on the right for the inhomogeneous models. (a) Patellar contact. (b) Femoral contact.

Figure 3

The interstitial fluid pressure field throughout the cartilage layer, at t = 1 s. Results are presented as a contour map together with vector arrows indicating the magnitude and direction of relative fluid flux, for the representative case of the patellar layer with (a) inhomogeneous and (b) homogeneous properties. The mesh deformation is shown to scale.

Figure 4

(a) & (b) Maximum principal normal strain distribution in the articular layer, at t = 1 s, for the inhomogeneous and homogeneous finite element models of the patella, respectively; (c) & (d) maximum principal normal effective stress; (e) & (f) minimum principal normal effective stress; (g) & (h) strain energy density distribution. The mesh deformation is shown to scale.